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MTD
REFLOW PROCESS CONTROL
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Date:
14-Feb-2005
Prepared by:
(Also subject responsible if other)
Alejandro
Rodriguez
Reviewed by:
(Also subject responsible if other)
Guadalupe Villarreal
Approved by:
(Also subject responsible if other)
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Table of Contents:
1. Objective..................................................................................................................................................2
2. Introduction............................................................................................................................................2
3. Project Basic Information......................................................................................................................3
4. Scope (Defined with EMS Corporate).................................................................................................3
4.1. Agreements (Defined with corporate) .......................................................................................3
5. Test vehicle selected ..............................................................................................................................4
6. Status of the process control at the beginning of the project...........................................................5
6.1. Controls at the beginning of the project ....................................................................................5
6.2. Quality levels .................................................................................................................................6
6.2.1. Quality rate from SMT...........................................................................................................6
6.2.2. Criteria from IPC-A-610 C Acceptability for Electronic Assemblies ..............................8
6.2.3. Criteria Standards for Inspection in EMS Monterrey .......................................................8
6.2.4. Profile used by Production Line...........................................................................................9
7. Process Control Project Flow..............................................................................................................11
7.1. Machine settings..........................................................................................................................11
7.2. Process Flow ................................................................................................................................12
7.3. Tests Description.........................................................................................................................12
7.3.1. Experimental Set up of Reflow...........................................................................................12
7.3.2. Validation of First DOE .......................................................................................................17
7.3.3. Second DOE...........................................................................................................................17
7.3.4. Validation of Second DOE ..................................................................................................21
7.3.5. Conclusion of profile recommended.................................................................................21
7.3.6. Operation window for each variable.................................................................................22
8. Other variables of SMT affecting solder beading............................................................................23
8.1.1. What solder beading is ........................................................................................................23
8.1.2. How solder beading happens.............................................................................................24
8.1.3. Why Solder Beading is problematic ..................................................................................24
8.1.4. Some possible causes of solder beading............................................................................25
8.1.5. Possible Solutions for solder beading................................................................................25
9. Control Process Options .....................................................................................................................26
9.1. Introduction to Process Control Charts ...................................................................................26
9.2. Control Chart Selection for Reflow Variables.........................................................................30
9.3. Final variables selected to be monitored .................................................................................32
9.4. Profiling Frequency: ...................................................................................................................32
9.5. Monitoring Equipment Options ...............................................................................................32
9.5.1. Real Time profile monitoring device (Short description)...............................................33
10. Project Conclusions ...........................................................................................................................33
MTD
REFLOW PROCESS CONTROL
Document code:
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Date:
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1. Objective
The basic purpose of the Process Control Project in the Reflow Oven Operation is to
determine which are the main variables of the reflow profile that have significant
influence on the output (defect rate, failures, quality of the solder joints, etc.) and
determine and implement a process control for them.
Another objective of this project is to find some results or general highlights that can
be used as a reference for other EMS factories to have a better process control for
reflow.
Finally, this project will help to Engineering team in EMS Monterrey to understand
better why process control is required to improve the quality of our products.
2. Introduction
In the Industry’s ongoing drive to reach higher levels of quality in surface mount
assembly, electronics manufacturers have focused on Process Control. Establishing
and maintaining performance limits for each assembly process has been viewed as
critical to achieving output that is consistent, require minimal rework or has few or,
ideally zero defects.
The process control is vital to obtain a continuous quality performance in our
products; the control process of the most important variables must be implemented
to assure this performance. Eventually, the Statistical Process Control has taken a big
increasing in all the companies due to the control that can be approached in the main
selected process variables. It is important to determine and select all the variables
that must be monitored in the normal process to achieve a continuous quality
performance. Those variables must be working with the best parameters determined
by tests. DOE’s and statistical analysis should be used to improve analysis and
parameters selection.
In the SMA process steps, machines are used and they have several process
parameters to properly be adjusted. The process parameters are subject to complex
interactions of different factors, determined not by the particular type of process but
also by the design of the products to be processed.
Parameters can be categorized to three groups,
1) Machine Settings: for example conveyor speed.
2) Machine Parameters: conveyor speed (for example) as a measured value.
3) Process Parameters: top temperature of the joint during soldering.
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To meet the quality challenge, equipment suppliers have designed machines that
perform with greater accuracy and repeatability. Software has been developed to
define and monitor a tight process window, ensuring that output remains within
stated limits. About the Process Control in the Reflow Oven, several kind of software
can be used to help on this.
3. Project Basic Information
Define which parameters have more effect on the output (defects), DOE’s and other
statistical analysis shall be developed to success with the project.
We are going to use an existing product (the most problematic on reflow process) to
perform the DOE; we will purchase extra material in case scrap is generated.
These are the outputs to be considered in the project: testing result, quantity of
defects in SMT (cold solder, dull joints, poor coalescence, flux residue, dark residues,
component cracking, solder balls tombstoning, etc.). Parameters to be evaluated will
be: Ramp-up rate, soaking time, soaking temperature, reflow time, peak temperature,
peak time & ramp-down rate.
4. Scope (Defined with EMS Corporate)
Develop required tests, analysis & DOE’s to:
1. Find the most significant input variables.
2. Define the process window (lower & upper limits) for each variable.
3. Know what is the effect on the output when the parameters move within the
process window.
4. Measure the improvement on the quality (defect rate) once the parameters are
controlled.
5. Reduction of defects due reflow process based on a proposed profile.
4.1. Agreements (Defined with corporate)
Next comments were discussed with corporate, and was decide to include them in the
projet:
• Make the tests with only one product and only one solder paste type. This would
reduce the DOE matrix size and cost of the project.
• Do not use the microstructure analysis to evaluate the process. This does not give a
representative output due to low samples quantity. Besides the high cost of the
tests.
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• At the end of the project, be able to give recommendations of the frequency for
ovens profiling.
5. Test vehicle selected
The test vehicle selected was a wireless product from NMP basically for three
reasons:
a) This product platform is similar than other products assembled in different
Elcoteq plants, so there is more chance to use some of the conclusions from
this project at least as a reference. Besides, the paste used for this product is
the approved by EMS for Lead Free applications (LF300 from Loctite
Henkel).
b) This was one of the products with higher DPU rate.
c) The cost of the product would allow us to perform more tests (buying extra
materials) based on the budget assigned by Corporate.
The following figures represent the test vehicle chosen for this project:
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6. Status of the process control at the beginning of the project
6.1. Controls at the beginning of the project
This control plan explains what kind of controls are used for this product in Reflow
process:
For the first control described in the document above, the visual inspection is made
by using a magnification lens after reflow, 100 % of the product is inspected by this
method but one problem is that the operator just inspect the areas of the panel that
are not covered by shield cans (Figure 1) and just 10% of the products are inspected
completely using the same method (magnification lens).
For the second control described, an X-Ray inspection is done for 1 panel every 2
hours and a SPC control chart for attribute is used.
Size
Frequen
cy
ReflowOven Omniflo7 51
Solder
Joint
QA014
(workmanship)
Visual
Inspection
100%
each
panel
Defect Report Format
(typeof defect,
reference, DPUand
Yieldper hour are
collected)
Stopthelineif 3equal
defects or 5different
defects arefoundwithinan
hour
52
Solder
Joint on
BGA
QA014
(workmanship)
X-Ray
machine
1panel
every2
hours
AttributeControl chart
Stopthelineif 3equal
defects or 5different
defects arefoundwithinan
hour
110
Set Up
Verification
Format withOven
parameters
Once
Each
shift
Check List andwork
instruction
Verify/ Adjust theoven
parameters andsendto
repair thenon
conformanceproduct,
previuslyidentified
Control Method
Methods
ReactionPlanProduct / Process /
Specification/
Tolerance
Evaluation/
Measurement
Technique
Sample
Product Process
Characteristics
Special
Char
Class
Process
Name/
Operation
Description
Machine,
Device, Jig,
Tools for
manufacturing
No.
Figure 1
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The third control described is just a verification of the oven parameters, it consists
just in looking in the monitor of the oven and see if the machine parameters are the
correct ones.
There are some controls out of the documented Control Plan, which show
opportunities, and we will recommend include. As an example: process technicians
perform weekly profiles and only Peak Temperature & Reflow Time are evaluated,
and there is no history of them.
If Peak Temperature & Reflow Time are within specs, the profile is saved, but there
are no monitoring and tendency analysis.
The current controls for reflow process are basically based on attribute data by using
SPC charts or just a simple table of defects in a documented format. The rate or
quantity of defects are taken as the only process control.
The current control method is good to measure the quality of the product and also is
helpful to compare the level of quality with different products, but it is not adequate
to measure the ability or behavior of the machine and the process itself. Only outputs
or Y’s are measured instead of the inputs or X’s. The measurement of the inputs
process variables or X’s will lead us to have more revealing information and
prevention of the problems or defects.
6.2. Quality levels
6.2.1. Quality rate from SMT
The quality levels obtained from the production line had not been validated with an
Attribute gage R & R for inspectors. Another difficulty is that the areas of the panels
that are underneath the shields are not inspected 100 % but 10% and the ones
inspected are qualified just with a magnification lens (10x), so small solder beading
or solder balling are not easy to detect. However, data from SMT and also from
diagnostic was taken as a reference for the project as the main issues to solve.
As we didn’t find significant data from production line, whether SMT data or
diagnostic data, we inspected a bunch of panels form different lines and different
shifts taking off the shield cans and by using not only magnification glass but also
magnifiers, we saw a lot of solder beading present on the panels so we increased the
sample for inspection without shields and with the same method.
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Below is the DPU and some pictures of the defect or process indicator:
DPU = Quantity of defects / Units produced
DPU = 20
Component Size:
2.8 mm x 2.2mm
Component Size:
1.52 mm x 0.76mm
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6.2.2. Criteria from IPC-A-610 C Acceptability for Electronic Assemblies
6.2.3. Criteria Standards for Inspection in Elcoteq Monterrey
There is a workmanship standard based on IPC-A-610C used for the inspectors as a
reference to determine if there are defects on the product.
This point can generate
controversy because how
can we be sure that the
solder ball is not going to
become dislodged during
normal service.
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REFLOW PROCESS CONTROL
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6.2.4. Profile used by Production Line
A linear profile within Abcde specifications has been used so far. A normal product
board with three of four thermocouples attached on different locations along the
board is utilized to set up and optimize the settings until acceptable profile is
achieved, once the profile fulfill the requirements and several measurements are
done for validation, a calibration board is used to monitor the profile with a certain
frequency (once a week). Below there are two tables with Abcde specifications, the
first one is for the product board and the second one is for calibration board.
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A profile obtained with the calibration board for Terminator product and the
measured variables are shown below:
Calibration board is a square
board made with FR4, one
thermocouple is connected to
a screw and the second one
is just on the air
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7. Process Control Project Flow
7.1. Machine settings
As a reference, this is the list of the machine settings that affect directly the process
conditions or process parameters:
• Conveyor speed
• Temperature settings of the zones
• Convection (Low, Med and High)
• Cooling (Low, Med and High)
• Separation between consecutives PCB’s going into the oven can affect the
variables in some cases.
• Temperature setting of the rails
Exhaust is not a machine setting but it can influence some of the process parameters.
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7.2. Process Flow
7.3. Tests Description
7.3.1. Experimental Set up of Reflow
A first DOE was developed to examine several important factors of the reflow
process, in order to determine which variables had significant effect on the defects
(mainly solder beading). The variables examined in the DOE were soak time, reflow
time, peak temperature and cooling slope. The variation on each factors are shown in
the Table 1. All the variations are within the solder paste specifications and customer
specifications except the soaking time which the maximum value in the DOE is
longer than the one required by the customer (according to the gradient
recommended from 70°C to 180°C, the soaking would be from 35 to 43 sec).
PROCESS
INPUT OUTPUTREFLOW
PROCESS
INPUT OUTPUT
- Preheat Slope
- Soaking time
- Soaking
Temperature
- Peak
Temperature
- Peak Time
- Reflow Time
- Cooling slope
- Yield
- DPU
- Defects
- Testing failures
- Visual Inspection
- X-Ray
- Voids
- Tombstoning
- Solder beading
- Poor coalescence
- Flux charring
- Reliability of the
solder joints
- Cold joints
- Component cracks
- Excessive
Intermetallics
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For this DOE, the printing and placement processes were held constant.
The Design of Experiment was defined as follows:
• Full Factorial experiment
• 4 Factors
• 2 Levels
• 2 replicates
• 2 central points
Output of the DOE was Defect quantity or defect rate per panel.
The four variables used in the study were set up into a DOE that required 34 runs in
total because of the 2 replicates and the 2 central points. Metrics of visual inspection
under magnification lens and microscope were used, X-Ray was used for BGA’s to
detect any problem (mainly voiding).
For the results of the DOE, a 95 % or higher confidence interval was required for a
factor to be considered statistically significant.
After inspect the 34 panels and test them through functional station, the only defect
noticed was again solder beading. The quantity of solder beading per panel was 8.5.
Below you can see the statistical analysis of the DOE, soaking time was the only
statistically significant factor for solder beading according to the P-value obtained (P-
value lower than 0.05).
Factor Low High
Soak Time
38 - 42
seconds
98 - 102
seconds
Time Above Liquidous
30 - 32
seconds
58 - 60 seconds
Peak Temperature 235 - 237 C 248 - 250 C
Cooling Slope 2.4 - 2.6 C/sec 3.4 - 3.6 C/sec
Table 1 - Variables included in DOE
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Factorial Fit: Defectos SMT versus Peak temp, Ramp Down, ...
Estimated Effects and Coefficients for Defectos SMT (coded units)
Term Effect Coef SE Coef T P
Constant 14.344 0.7276 19.71 0.000
Peak temp -0.187 -0.094 0.7276 -0.13 0.899
Ramp Down -0.938 -0.469 0.7276 -0.64 0.528
Reflow Time 2.937 1.469 0.7276 2.02 0.060
Soak Time 4.937 2.469 0.7276 3.39 0.003
Peak temp*Ramp Down -1.312 -0.656 0.7276 -0.90 0.380
Peak temp*Reflow Time -0.188 -0.094 0.7276 -0.13 0.899
Peak temp*Soak Time -0.687 -0.344 0.7276 -0.47 0.643
Ramp Down*Reflow Time 2.562 1.281 0.7276 1.76 0.096
Ramp Down*Soak Time 0.562 0.281 0.7276 0.39 0.704
Reflow Time*Soak Time 1.187 0.594 0.7276 0.82 0.426
Peak temp*Ramp Down*Reflow Time 0.437 0.219 0.7276 0.30 0.767
Peak temp*Ramp Down*Soak Time -0.562 -0.281 0.7276 -0.39 0.704
Peak temp*Reflow Time*Soak Time -1.187 -0.594 0.7276 -0.82 0.426
Ramp Down*Reflow Time*Soak Time 2.562 1.281 0.7276 1.76 0.096
Peak temp*Ramp Down*Reflow Time* -1.812 -0.906 0.7276 -1.25 0.230
Soak Time
Ct Pt -5.844 3.0000 -1.95 0.068
Analysis of Variance for Defectos SMT (coded units)
Source DF Seq SS Adj SS Adj MS F P
Main Effects 4 271.375 271.375 67.84 4.00 0.018
2-Way Interactions 6 84.188 84.187 14.03 0.83 0.564
3-Way Interactions 4 67.875 67.875 16.97 1.00 0.434
4-Way Interactions 1 26.281 26.281 26.28 1.55 0.230
Curvature 1 64.281 64.281 64.28 3.79 0.068
Residual Error 17 288.000 288.000 16.94
Pure Error 17 288.000 288.000 16.94
Total 33 802.000
Term
Sta nda rdiz e d Effe ct
A
A C
A BC
BD
A BD
A D
B
C D
A C D
A B
A B C D
B C D
BC
C
D
3.53.02.52.01.51.00.50.0
2.110
F a ctor
S oak T im e
N a m e
A P e ak te m p
B R a m p D ow n
C R e flow T im e
D
P are to Chart of the S tandar dize d Effe cts
(response is Defe ctos S M T , A lpha = .05)
This chart shows that soaking time has significant effect on
the solder beading
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Standardized Effect
Percent
43210-1-2-3
99
95
90
80
70
60
50
40
30
20
10
5
1
F actor
S oak T im e
N ame
A P eak tem p
B Ramp D ow n
C Reflow T ime
D
Effect Ty pe
Not Significant
SignificantD
Normal Probability Plot of the Standardized Effects
(response is Defectos SMT, Alpha = .05)
This chart shows that only soaking time has significant effect
on the solder beading
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The Figure Below represents the combination of the factors and their results in
quantity of defects:
An optimization of the first DOE was performed and these are the results:
The values in red are the ones recommended to minimize the quantity of solder
beading. The quantity of solder beading with this set up is 9. More replicates per set
up would be recommended but one big obstacle to do so was that the budget was
limited.
10045
60
30
3.5
2.5
250235
S oak Time
Reflow T ime
Ramp D ow n
P eak temp
8.5
18.5
17.016.0
24.0
12.0
18.017.0
12.0
13.5
13.513.5
10.5
8.5
13.010.5
12.0
Centerpoint
Factorial Point
Cube Plot (data means) for Defectos SMT
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7.3.2. Validation of First DOE
A validation run was run with 15 panels and the quantity of solder beading per panel
increased a little bit (11 solder beading per panel), again just solder beading was
detected as an issue.
7.3.3. Second DOE
A second DOE was developed in order to reduce the quantity of solder beading per
panel, we know that there are more possible causes and not only the reflow profile
that can lead to have solder beading but they are going to be mentioned in a separate
point of this report. The variables examined in this DOE were soaking time and
preheat slope. Peak Temperature, Reflow Time and Cooling slope were held constant
with central points values from first DOE. The variation of the factors for the second
DOE is shown in Table 2. The values of the other reflow variables are shown in
Table 3.
Best Set Up form DOE: 9 defects
per panel
Validation: 11 defects per panel
Factor Low High
Soak Time
38 - 42
seconds
78 - 82
seconds
Preheat Slope
(50-140 C)
0.7- 0.9
C/sec
1.1 - 1.3
C/sec
Table 2 - Variables included in second DOE
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Like in the first DOE, the printing and placement processes were held constant.
The Design of Experiment was defined as follows:
• Full Factorial Experiment
• 2 Factors
• 2 Levels
• 5 Replicates
Output of the DOE was Defect quantity or defect rate per panel.
Table 4 shows the series of experiments performed in the DOE.
StdOrder RunOrder
Soaking Time
(°C)
Preheat slope
(°C/sec)
19 1 40 1.2
18 2 80 0.8
16 3 80 1.2
14 4 80 0.8
1 5 40 0.8
8 6 80 1.2
4 7 80 1.2
10 8 80 0.8
20 9 80 1.2
17 10 40 0.8
15 11 40 1.2
7 12 40 1.2
3 13 40 1.2
9 14 40 0.8
5 15 40 0.8
12 16 80 1.2
11 17 40 1.2
13 18 40 0.8
6 19 80 0.8
2 20 80 0.8
Table 4 Set Up of Second DOE
Factor Low
Time Above Liquidus
45
seconds
Peak Temperature 243 C
Cooling Ramp Down -3.0 C/sec
Table 3 - Constant Factors for second DOE
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Metrics of visual inspection under magnification lens and microscope were used, X-
Ray was used for BGA’s to detect any problem (mainly voiding).
For the results of the DOE, a 95 % or higher confidence interval was required for a
factor to be considered statistically significant.
After inspect the 34 panels and test them through functional station, the only defect
noticed was again solder beading. The quantity of solder beading per panel was 8.5.
Below you can see the statistical analysis of the DOE, soaking time and the 2-way
interaction between soaking time and preheat slope were statistically significant for
solder beading according to the P-value obtained (P-value lower than 0.05).
Factorial Fit: Quantity of defects versus Soaking Time, Preheat slope
Estimated Effects and Coefficients for Quantity of defects (coded units)
Term Effect Coef SE Coef T P
Constant 11.3000 0.2398 47.12 0.000
Soaking Time 5.6000 2.8000 0.2398 11.68 0.000
Preheat slope 0.8000 0.4000 0.2398 1.67 0.115
Soaking Time*Preheat slope -1.4000 -0.7000 0.2398 -2.92 0.010
Analysis of Variance for Quantity of defects (coded units)
Source DF Seq SS Adj SS Adj MS F P
Main Effects 2 160.000 160.000 80.000 69.57 0.000
2-Way Interactions 1 9.800 9.800 9.800 8.52 0.010
Residual Error 16 18.400 18.400 1.150
Pure Error 16 18.400 18.400 1.150
Total 19 188.200
Term
Standardized Effect
B
AB
A
121086420
2.12
F actor N ame
A S oaking Time
B P reheat slope
Pareto Chart of the Standardized Effects
(response is Quantity of defects, Alpha = .05)
This chart shows that soaking
time and 2-Way interaction
between factors have
significant effect on the solder
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An optimization of the first DOE was performed and these are the results:
Standardized Effect
Percent
12.510.07.55.02.50.0-2.5-5.0
99
95
90
80
70
60
50
40
30
20
10
5
1
Factor Name
A Soaking Time
B Preheat slope
Effect Type
Not Significant
Significant
AB
A
Normal Probability Plot of the Standardized Effects
(response is Quantity of defects, Alpha = .05)
This chart shows that soaking
time and 2-Way interaction
between the 2 factors have
significant effect on the solder
1.2
0.8
8040
Preheat slope
Soaking Time
13.8
14.47.4
9.6
Cube Plot (data means) for Quantity of defects
Combination of
factors with
lower quantity
of defects
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An optimization of the first DOE was performed and these are the results:
The values in red are the ones recommended to minimize the quantity of solder
beading. The quantity of solder beading with this set up is 7.
7.3.4. Validation of Second DOE
A validation run was run with 15 panels and the quantity of solder beading per panel
was 9. Again, just solder beading was detected as an issue.
7.3.5. Conclusion of profile recommended
After the two DOE’s we concluded that soaking time has the more significant effect
on solder beading (on this specific product and with this solder paste). Table 5 shows
the recommended values for the profile to reduce the quantity of solder beading.
Factor Low
Time Above Liquidus 45 seconds
Peak Temperature 243 C
Cooling Ramp Down -3.0 C/sec
Soaking Time 40 seconds
Preheat Slope
(50-140 C)
0.8 C/sec
Table 5 - Parameters Recommended after two
DOE's
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7.3.6. Operation window for each variable
Next table (Table 6) contains the window recommended for each of the process
variables of the reflow profile selected in this project, this parameters are
recommended to have the best results for solder beading and good quality in general
for the solder joints and assembly. The chart illustration shows the phases of the
profile.
0
50
100
150
200
250
300
0 90 180 270 360
Time (sec)
Temperature(ºC)
Prehat Phase Soaking Time Reflow Phase Cooling Phase
Peak Temp
Liquidus Temp = 217ºC
0
50
100
150
200
250
300
0 90 180 270 360
Time (sec)
Temperature(ºC)
Prehat Phase Soaking Time Reflow Phase Cooling Phase
Peak Temp
Liquidus Temp = 217ºC
Profile Feature Large Body Small Body
Preheat Ramp up Rate
(From 70 to 140 ºC)
Soaking Time
(From 130 to 165 ºC)
Time Above Liquidus (217 ºC)
Peak Temperature
Cooling Ramp Down -2.7 to -3.3 ºC/sec
0.8 - 0.9 ºC/sec
37 - 43 sec
42 - 48 seconds
240 - 246 ºC
Table 6 - Recommended process window for each variable
Reflow Profile
The Scale in X & Y axis
and the curve are just
examples for a typical
profile, the curve is not
the necessarily the
recommended
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The values of the variables recommended come from the two DOE’s performed and
the validation of them, the values are almost in the center of customer specifications
(except the preheat ramp up rate which is in the minimum specification).
The operational window recommended for each variable might seem very tight, but
is the one that gave better results. The specifications from the customer can be used
as well as an operational window but the quantity of solder beading expected would
be higher, other than that the quality of the assembly was acceptable (visual
inspection by certified inspector, X-Ray Inspection and testing) according to the
pieces that we assembled for the trials in DOE’s.
There are some other actions that can lead us to eliminate completely the solder
beading, they are going to be explained in the point 8.
Table 7 shows again customer specifications for the variables mentioned in this point
(7.3.6)
8. Other variables of SMT affecting solder beading
As we couldn’t avoid completely the problem of the solder beading with the reflow
profile, we analyzed the problem taking in account other possible causes in SMT.
8.1.1. What solder beading is
Profile Feature Large Body Small Body
Preheat Ramp up Rate
(From 70 to 140 ºC)
Soaking Time
(From 130 to 165 ºC)
Time Above Liquidus (217 ºC)
Peak Temperature
Cooling Ramp Down
0.8 - 1.0 ºC/sec
35 - 43 sec
35 - 60 seconds
232 - 250 ºC
-2 to -5 ºC/sec
Table 7 - Customer Specifications
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Solder beading is a special phenomenon of solder balling when using solder paste in
certain SMT applications. In brief, solder beads are large solder balls near
components with very low stand-off.
Often confused with solder balling, solder beading is a defect recognized by one or a
few larger balls, generally located around chip caps and resistors.
8.1.2. How solder beading happens
a) Solder paste is printed on the pads of a circuit board.
b) During component placement some solder is squeezed underneath the body of the
component and broken off from the solder on the pads.
c) During reflow, the solder trapped underneath the component does not flow back
to the solder pads. Contrarily, its cohesive properties (surface tension) cause it to
form a large ball (bead).
d) The surface tension of the cooling solder draws the component closer to the pads.
As the body of the component is drawn down, the solder bead squeezes out the side
and remains there.
8.1.3. Why Solder Beading is problematic
Basically, solder beads may form a “bridge” of solder that runs from one component
termination to another, thus causing an electrical connection that was not designed to
be there. This poses the threat of resulting in a short circuit. This may occur where the
bead was originally formed or elsewhere on the assembly if vibration causes the bead
to break loose and move around. While the above may not necessarily occur if the
solder beads are present, solder beading obviously remains a defect that should be
minimized or eliminated if possible.
Solder beading could create an unwanted electrical bridge if they are located between
two adjacent parts of the PCB. Additionally, they could be dislodged during
handling and affect the performance of adjoining assemblies or components.
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Therefore, solder beading that are formed need to be detected visually and removed
manually. These steps add to the labor costs associated with the assembly process. If
a solder micro-ball eludes detection and adversely affects the performance in service,
or results in customer returns, then the economic impact can be even greater. The
obvious answer to this problem is to eliminate solder micro-ball formation, which
can be difficult, because solder micro-balls can be caused by a variety of sources.
8.1.4. Some possible causes of solder beading
There are many causes that can lead to have solder beading but we listed the ones
that we thought are more important in this specific case:
- Bad Stencil Aperture 1:1 with the pad
- High value of stencil thickness
- Incorrect Snap Off: A gap between the stencil and PWB can increase the
paste volume deposited on the assembly
- Solder Paste tend to generate solder balling (this feature can be tested
following IPC test .
- Too slow or too fast preheat ramp rate on Reflow
- Incorrect speed and squeegee pressure on printing process
- High humidity in the environment.
- Oxidation of solder powder.
- Incompatibility between board finish (OSP) and solder paste flux.
8.1.5. Possible Solutions for solder beading
Some recommendations to prevent solder beading are listed below:
- Stencil Aperture Size / Shape - Reduce the aperture size (home plate, bow
tie or D shape can be options).
- Stencil Thickness – Reduce the stencil thickness to reduce the paste deposit.
- Snap Off – Use on contact or zero snap off (top side of PWB on contact with
bottom side of stencil).
- Solder paste viscosity – Keep the viscosity of the solder paste within
specifications, the higher viscosity, the better results for solder beading.
- Reflow Profile – Improve the preheat ramp rate (too slow or too fast ramp
rate can lead to solder beading). This optimization was already made in the
project.
Solder beading problem appeared mostly in components 0402, 0603 and some mid
size components. As with the current stencils the aperture is 1:1 with the pad, we
recommended some changes in the aperture for the most problematic components.
One stencil was ordered and the changes on the stencil were applied on the more
recurrent references of the product. The results were pretty good, we assembled 30
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panels with this stencil and we didn’t find any solder beading on the references
where the changes on the aperture were applied. Table 6 shows the
recommendations of the apertures:
Next step is to change the stencil apertures for all 0402 & 0603 references (apertures
for problematic mid size components were already implemented) on the board.
This change is on going because we are waiting for approval from customer.
9. Control Process Options
This section is intended to determine which controls can be proposed to perform the
Process Control of the selected reflow variables.
9.1. Introduction to Process Control Charts
Next section is related to the theoretical structure of the control of the process
variables using Control Chart.
Ongoing monitoring is typically managed with a control chart.
Uses of control Charts
• Determine appropriate managerial action in response to the value of a data point from a
particular process
COMPONENT APERTURE SHAPE DRAWING
0402 D-Shape
0603 Home Plate
MID SIZE
COMPONENT
Reduction in inner
part of the
component
Table 6 - Recommended aapertures to reduce solder beading
problem
Yellow means Land
pattern area
Area in White means
recommended
stencil aperture
The squares in blue
represent just other
references
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- To see if high or low points are due to special causes.
• Understand and predict process capability (expected ranges of future values) for
planning purposes.
• Identify Root causes of variations by differentiating between special and common
causes of variation in the data.
• See whether intentional changes in the process had a desired result.
• Monitor key processes and identify shifts or changes quickly to help hold the gains
made from an improvement project.
Common-Cause Variation
Common causes are the process inputs and conditions that contribute to the regular,
everyday variation in a process.
• Common causes are a part of the process.
• They contribute to output variation because they themselves vary.
• Each common cause contributes a small part of the total variation.
• By looking at a process over time, we know how much variation to expect from
common causes.
• The process is stable, or predictable, when all the variations is due to common
causes.
Special-Cause Variation
Special causes are factors that are not always present in a process but that appear
because of some particular circumstance.
• Special causes are not usually present.
• They may come and go sporadically; may be temporary or long-term.
• A special cause is something special or specific that has a pronounced effect on the
process.
• We can’t predict when a special cause will occur or how it will affect the process.
• The process is unstable, or unpredictable, when special causes contribute to the variation.
Tests for special causes
• 8 or more points in a row of the same side of the median indicates a process
shift.
• If the data are symmetrical, it’s Ok to use the average as the central line
instead of the median.
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• 6 or more points in a row continuously increasing or decreasing indicates a
trend.
• Start counting at the point where the direction changes.
• Too few runs indicates a shift in the process average, a cycle, or a trend.
• Too many runs indicates sampling from two sources, overcompensation, or a
bias.
• 14 or more points in a row alternating up and down indicates bias or sampling
problems.
• One or more points outside the control limits indicates that something is
different about those points.
Individual Charts
Because they can be used with any data that is time-ordered, and in general and very
versatile, individual charts are the most frequently used type of control charts.
However with particular kinds of data or situations, they are sometimes slower to
signal special causes than other kind of charts, so it’s best to understand other types
of control charts as well.
SPECIFICATION LIMITS Vs CONTROL LIMITS
Specification Limits
• Come from engineering or customer requirements.
• Represent what someone wants a process to do.
• Can sometimes be changed by changing the requirements of the product or service.
Control Limits
• Come from calculations of the process data.
• Represent what a process is actually capable of doing
• Can only be changed by changing the process.
When to calculate new control limits?
You should calculate new control limits when:
• You know there was a change in the process based on
- Statistical Evidence, such as 8 continuous points above or below the centerline.
- You have determined why the change occurred (based on your process
knowledge).
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- You are confident the process will stay changed
• You are confident the process will stay changed.
- The change was not temporary.
- The change has become a standard part of the process.
Calculate the new limits when you have enough data points to see a change. Call the
new limits temporary until you get at least 24 new data points.
Assumptions for individual charts
• Data are roughly normal (data might need to be transformed)
• Data points are independent
What to look for when using Control Charts
• A good control chart is one that is being used concurrently with the process.
- Charts should be posted or be readily at hand.
- Charts should be up-to-date.
- Charts should look well-used.
• Comments should be written on charts
- Dates of process changes.
- Notes on events that might cause problems later.
- Confirmation of verified special causes.
- Actions taken to eliminate special causes (only rarely should the chart indicate
that the cause could not be identified).
Common mistakes when using control charts
- Chart not created correctly
- Wrong formula used to calculate “3 sigma” limits (st. dev. used instead of moving
ranges).
- Wrong type of charts used based on type of date collected.
- Missing, poor or erroneous measurements.
• Chart not regularly updated
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- Data on charts are not current
- Process adjustments have not been noted.
- Control limits and average have not been updated.
• Actions taken are
- Rewards given for “good points” or explanations sought for “bad points” even
though they are not signaled as special.
- Special-cause signals ignored.
- Non-random patterns or cycles not studied to determine specific causes.
- Spec limits or goals are placed on chart instead of control limits
9.2. Control Chart Selection for Reflow Variables
As a part of the project is required to propose an option to measure, monitor and control
the final variables selected on all the previous analysis. The process to select the
appropriate graph was carried based on next flow chart.
Selecting an Appropriate Control Chart
Can you mistake
proof the defect?
Do not use SPC;
implement PM
Output or input? Improvement or
run/stop?
Continuous data? Pre-control
Tracking System
Individual and Moving
Range Charts
Process in
Control?
Past as important
as present?
EWMA
chart
CuSum
chart
Is subgroup size >
8 and sigma easily
Can you measure
more than one defect
per unit?
Xbar and
sigma
chart
X bar and
Range
chart
% defective
(bad parts)
Defect/unit
Sample size
constant?
Sample size
constant?
nP
chart
P
chart
C
chart
U
chart
Input
No
Yes
Automatic Manual
Yes
No Yes
No Yes
No Yes No Yes
No Yes
Run/Stop
Improvement
Output
Yes
No
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This is a sample of the “Individual-X & Moving Range Chart” proposed to monitor
and control the selected variables in the Reflow Oven profile.
E:INDIVIDUAL-X &
MOVING RANGES CONTROL CHART.xls
Individual-X & Moving Range Charts are a set of control charts for variables data
(data that is both quantitative and continuous in measurement, such as a measured
dimension or time). The Individual-X chart monitors the process location over time,
based on the current subgroup, containing a single observation. The Moving Range
chart monitors the variation between consecutive subgroups over time.
The Individual chart on top shows each reading and is used to analyze Central
Location. The Moving Range chart at bottom is used to study system variability.
Why it is used?
To analyze a subgroup size that cannot be more than one for any reason such as
costs, expense involved, difficulty to collect data or large size collection is not
practical or possible.
When it is used?
1. When you need to assess the stability of system.
2. When the data is variable.
3. When the size of subgroups is one.
4. When the time order of subgroups preserved.
Advantages
The charts are sensitive and hence can detect even a small variation in normality.
Highly reliable and easy to use.
Disadvantages
Constant and thorough study of chart is necessary
Slow in detecting sudden jumps in average values
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9.3. Final variables selected to be monitored
Based on results, soak time was the only significant variable obtained.
Every time the profile is evaluated, the Data pack can be programmed and all data
can be obtained and graphed. Even though only soaking time was the main variable
obtained in all results, there is no restriction to increase the control of all the list of
variables mentioned:
• Soak Time
• Peak Temp
• Ramp Down
• Reflow Time
• Cooling slope
9.4. Profiling Frequency:
Actually there is a process instruction which states that the profile must be evaluated
each week. This can give enough data of how the oven is working. Gauge R&R to the
reflow oven shows that repeatability and reproducibility are good, due to this, it is
not recommendable to decrease the profiling period (i.e. from one week, to two
weeks or one month), until we obtain enough data in a period of at least 6 months.
After that, we would be able to decrease frequency.
9.5. Monitoring Equipment Options
Data logger Real Time Reflow Profiler
Gold M.O.L.E Oven watch
Data Pack
Slim KIC 2000 KIC 247
The proposed graph in section 9.2, is based on the use of the “Gold M.O.L.E.” or
“Data Pack” data loggers, used at the moment in the Monterrey Plant. A “Real Time
Reflow Profiler” device is recommended as a better option to control the variables.
This is an option online, which can give the profile for each PCB and result on a
better monitoring.
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9.5.1. Real Time profile monitoring device (Short description)
The automated real-time thermal process monitoring system, is an equipment that
provides and records real-time thermal process data for every product, as opposed to
the conventional practice of only periodically checking oven performance. This
allows this equipment to automatically catch potential defects before they happen,
rather than discovering actual defects during “Inspection”. It utilizes thermocouple
sensors permanently embedded in the oven at the process level. The key feature is
the creation and recording of a Virtual Profile. By associating the dynamics within
the oven with the profiled data collected using profile data logger, the real-time
monitoring device, automatically calculates a unique product profile including all
pertinent process data for every product that exits the oven.
It utilizes real-time process data for real-time SPC charting. Process data is
automatically charted for all critical process specs: peak temperature, soak time, time
above liquidus, etc. The data is plotted on real-time control charts and Process
Capability (Cpk) is calculated for each spec. Any the process drift outside of control
limits will bring an immediate alarm. The process engineer also has the option of
setting a warning limit on the Cpk. Real-time Cpk tracking enables the system to flag
an out of control process before the oven has produced a single defect.
10. Project Conclusions
• Control process is required to assure the continuous quality performance in our
product, a Control Chart is proposed to control main variables on the oven profile.
Eventually, changing to 100% monitoring of the profile or “real time” is
recommended.
• Individual values and Moving Range (I-MR) charts are recommended as SPC for
soaking time, preheat ramp up, reflow time, peak temperature and cooling slope.
• The use of control chart applies not only for the variables mentioned above but also
for all significant variables at any kind of process. The more variables monitored,
the better process control.
• Solder Beading defect can be categorized as a process indicator or defect based on
IPC criteria. This was found as an opportunity as an output of the DOE’s. Several
recommendations were done related to this.
• Requesting for theoretical information, solder beading is mainly caused due to
soaking time, same as our results, this is a validation of the results.
• Historically, the profiling period has been 1 profile per week. Give a frequency for
profiling depends pretty much on the repeatability of the machine (oven); again, a
100 % or real time monitoring of the profile is more recommended but Budget for
investment has to be analyzed.
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34 of 34
Having a 100% or real time monitoring of the profile is recommended but not
mandatory to have a good process control, we can have a good process control by
determining the right frequency of profiling and analyzing the data properly.
We have seen that the repeatability among ovens varies; Analyze how repeatable
your oven is and how small your process window is will lead you to determine the
right frequency for profiling. Of course SPC and good analysis of it is going to be
helpful.
• During DOE’s tests, it was shown that profile specification limits are correct, based
on visual inspection (IPC certified operator), X-ray inspection, and electrical-testing
results.
• The final proposed profile is basically in the center of all the customer specification
limits. This is giving us a confirmation that we are running under the best profile,
considering only few changes at the beginning of the profile.

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Final report reflow process control

  • 1. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 1 of 34 Table of Contents: 1. Objective..................................................................................................................................................2 2. Introduction............................................................................................................................................2 3. Project Basic Information......................................................................................................................3 4. Scope (Defined with EMS Corporate).................................................................................................3 4.1. Agreements (Defined with corporate) .......................................................................................3 5. Test vehicle selected ..............................................................................................................................4 6. Status of the process control at the beginning of the project...........................................................5 6.1. Controls at the beginning of the project ....................................................................................5 6.2. Quality levels .................................................................................................................................6 6.2.1. Quality rate from SMT...........................................................................................................6 6.2.2. Criteria from IPC-A-610 C Acceptability for Electronic Assemblies ..............................8 6.2.3. Criteria Standards for Inspection in EMS Monterrey .......................................................8 6.2.4. Profile used by Production Line...........................................................................................9 7. Process Control Project Flow..............................................................................................................11 7.1. Machine settings..........................................................................................................................11 7.2. Process Flow ................................................................................................................................12 7.3. Tests Description.........................................................................................................................12 7.3.1. Experimental Set up of Reflow...........................................................................................12 7.3.2. Validation of First DOE .......................................................................................................17 7.3.3. Second DOE...........................................................................................................................17 7.3.4. Validation of Second DOE ..................................................................................................21 7.3.5. Conclusion of profile recommended.................................................................................21 7.3.6. Operation window for each variable.................................................................................22 8. Other variables of SMT affecting solder beading............................................................................23 8.1.1. What solder beading is ........................................................................................................23 8.1.2. How solder beading happens.............................................................................................24 8.1.3. Why Solder Beading is problematic ..................................................................................24 8.1.4. Some possible causes of solder beading............................................................................25 8.1.5. Possible Solutions for solder beading................................................................................25 9. Control Process Options .....................................................................................................................26 9.1. Introduction to Process Control Charts ...................................................................................26 9.2. Control Chart Selection for Reflow Variables.........................................................................30 9.3. Final variables selected to be monitored .................................................................................32 9.4. Profiling Frequency: ...................................................................................................................32 9.5. Monitoring Equipment Options ...............................................................................................32 9.5.1. Real Time profile monitoring device (Short description)...............................................33 10. Project Conclusions ...........................................................................................................................33
  • 2. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 2 of 34 1. Objective The basic purpose of the Process Control Project in the Reflow Oven Operation is to determine which are the main variables of the reflow profile that have significant influence on the output (defect rate, failures, quality of the solder joints, etc.) and determine and implement a process control for them. Another objective of this project is to find some results or general highlights that can be used as a reference for other EMS factories to have a better process control for reflow. Finally, this project will help to Engineering team in EMS Monterrey to understand better why process control is required to improve the quality of our products. 2. Introduction In the Industry’s ongoing drive to reach higher levels of quality in surface mount assembly, electronics manufacturers have focused on Process Control. Establishing and maintaining performance limits for each assembly process has been viewed as critical to achieving output that is consistent, require minimal rework or has few or, ideally zero defects. The process control is vital to obtain a continuous quality performance in our products; the control process of the most important variables must be implemented to assure this performance. Eventually, the Statistical Process Control has taken a big increasing in all the companies due to the control that can be approached in the main selected process variables. It is important to determine and select all the variables that must be monitored in the normal process to achieve a continuous quality performance. Those variables must be working with the best parameters determined by tests. DOE’s and statistical analysis should be used to improve analysis and parameters selection. In the SMA process steps, machines are used and they have several process parameters to properly be adjusted. The process parameters are subject to complex interactions of different factors, determined not by the particular type of process but also by the design of the products to be processed. Parameters can be categorized to three groups, 1) Machine Settings: for example conveyor speed. 2) Machine Parameters: conveyor speed (for example) as a measured value. 3) Process Parameters: top temperature of the joint during soldering.
  • 3. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 3 of 34 To meet the quality challenge, equipment suppliers have designed machines that perform with greater accuracy and repeatability. Software has been developed to define and monitor a tight process window, ensuring that output remains within stated limits. About the Process Control in the Reflow Oven, several kind of software can be used to help on this. 3. Project Basic Information Define which parameters have more effect on the output (defects), DOE’s and other statistical analysis shall be developed to success with the project. We are going to use an existing product (the most problematic on reflow process) to perform the DOE; we will purchase extra material in case scrap is generated. These are the outputs to be considered in the project: testing result, quantity of defects in SMT (cold solder, dull joints, poor coalescence, flux residue, dark residues, component cracking, solder balls tombstoning, etc.). Parameters to be evaluated will be: Ramp-up rate, soaking time, soaking temperature, reflow time, peak temperature, peak time & ramp-down rate. 4. Scope (Defined with EMS Corporate) Develop required tests, analysis & DOE’s to: 1. Find the most significant input variables. 2. Define the process window (lower & upper limits) for each variable. 3. Know what is the effect on the output when the parameters move within the process window. 4. Measure the improvement on the quality (defect rate) once the parameters are controlled. 5. Reduction of defects due reflow process based on a proposed profile. 4.1. Agreements (Defined with corporate) Next comments were discussed with corporate, and was decide to include them in the projet: • Make the tests with only one product and only one solder paste type. This would reduce the DOE matrix size and cost of the project. • Do not use the microstructure analysis to evaluate the process. This does not give a representative output due to low samples quantity. Besides the high cost of the tests.
  • 4. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 4 of 34 • At the end of the project, be able to give recommendations of the frequency for ovens profiling. 5. Test vehicle selected The test vehicle selected was a wireless product from NMP basically for three reasons: a) This product platform is similar than other products assembled in different Elcoteq plants, so there is more chance to use some of the conclusions from this project at least as a reference. Besides, the paste used for this product is the approved by EMS for Lead Free applications (LF300 from Loctite Henkel). b) This was one of the products with higher DPU rate. c) The cost of the product would allow us to perform more tests (buying extra materials) based on the budget assigned by Corporate. The following figures represent the test vehicle chosen for this project:
  • 5. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 5 of 34 6. Status of the process control at the beginning of the project 6.1. Controls at the beginning of the project This control plan explains what kind of controls are used for this product in Reflow process: For the first control described in the document above, the visual inspection is made by using a magnification lens after reflow, 100 % of the product is inspected by this method but one problem is that the operator just inspect the areas of the panel that are not covered by shield cans (Figure 1) and just 10% of the products are inspected completely using the same method (magnification lens). For the second control described, an X-Ray inspection is done for 1 panel every 2 hours and a SPC control chart for attribute is used. Size Frequen cy ReflowOven Omniflo7 51 Solder Joint QA014 (workmanship) Visual Inspection 100% each panel Defect Report Format (typeof defect, reference, DPUand Yieldper hour are collected) Stopthelineif 3equal defects or 5different defects arefoundwithinan hour 52 Solder Joint on BGA QA014 (workmanship) X-Ray machine 1panel every2 hours AttributeControl chart Stopthelineif 3equal defects or 5different defects arefoundwithinan hour 110 Set Up Verification Format withOven parameters Once Each shift Check List andwork instruction Verify/ Adjust theoven parameters andsendto repair thenon conformanceproduct, previuslyidentified Control Method Methods ReactionPlanProduct / Process / Specification/ Tolerance Evaluation/ Measurement Technique Sample Product Process Characteristics Special Char Class Process Name/ Operation Description Machine, Device, Jig, Tools for manufacturing No. Figure 1
  • 6. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 6 of 34 The third control described is just a verification of the oven parameters, it consists just in looking in the monitor of the oven and see if the machine parameters are the correct ones. There are some controls out of the documented Control Plan, which show opportunities, and we will recommend include. As an example: process technicians perform weekly profiles and only Peak Temperature & Reflow Time are evaluated, and there is no history of them. If Peak Temperature & Reflow Time are within specs, the profile is saved, but there are no monitoring and tendency analysis. The current controls for reflow process are basically based on attribute data by using SPC charts or just a simple table of defects in a documented format. The rate or quantity of defects are taken as the only process control. The current control method is good to measure the quality of the product and also is helpful to compare the level of quality with different products, but it is not adequate to measure the ability or behavior of the machine and the process itself. Only outputs or Y’s are measured instead of the inputs or X’s. The measurement of the inputs process variables or X’s will lead us to have more revealing information and prevention of the problems or defects. 6.2. Quality levels 6.2.1. Quality rate from SMT The quality levels obtained from the production line had not been validated with an Attribute gage R & R for inspectors. Another difficulty is that the areas of the panels that are underneath the shields are not inspected 100 % but 10% and the ones inspected are qualified just with a magnification lens (10x), so small solder beading or solder balling are not easy to detect. However, data from SMT and also from diagnostic was taken as a reference for the project as the main issues to solve. As we didn’t find significant data from production line, whether SMT data or diagnostic data, we inspected a bunch of panels form different lines and different shifts taking off the shield cans and by using not only magnification glass but also magnifiers, we saw a lot of solder beading present on the panels so we increased the sample for inspection without shields and with the same method.
  • 7. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 7 of 34 Below is the DPU and some pictures of the defect or process indicator: DPU = Quantity of defects / Units produced DPU = 20 Component Size: 2.8 mm x 2.2mm Component Size: 1.52 mm x 0.76mm
  • 8. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 8 of 34 6.2.2. Criteria from IPC-A-610 C Acceptability for Electronic Assemblies 6.2.3. Criteria Standards for Inspection in Elcoteq Monterrey There is a workmanship standard based on IPC-A-610C used for the inspectors as a reference to determine if there are defects on the product. This point can generate controversy because how can we be sure that the solder ball is not going to become dislodged during normal service.
  • 9. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 9 of 34 6.2.4. Profile used by Production Line A linear profile within Abcde specifications has been used so far. A normal product board with three of four thermocouples attached on different locations along the board is utilized to set up and optimize the settings until acceptable profile is achieved, once the profile fulfill the requirements and several measurements are done for validation, a calibration board is used to monitor the profile with a certain frequency (once a week). Below there are two tables with Abcde specifications, the first one is for the product board and the second one is for calibration board.
  • 10. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 10 of 34 A profile obtained with the calibration board for Terminator product and the measured variables are shown below: Calibration board is a square board made with FR4, one thermocouple is connected to a screw and the second one is just on the air
  • 11. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 11 of 34 7. Process Control Project Flow 7.1. Machine settings As a reference, this is the list of the machine settings that affect directly the process conditions or process parameters: • Conveyor speed • Temperature settings of the zones • Convection (Low, Med and High) • Cooling (Low, Med and High) • Separation between consecutives PCB’s going into the oven can affect the variables in some cases. • Temperature setting of the rails Exhaust is not a machine setting but it can influence some of the process parameters.
  • 12. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 12 of 34 7.2. Process Flow 7.3. Tests Description 7.3.1. Experimental Set up of Reflow A first DOE was developed to examine several important factors of the reflow process, in order to determine which variables had significant effect on the defects (mainly solder beading). The variables examined in the DOE were soak time, reflow time, peak temperature and cooling slope. The variation on each factors are shown in the Table 1. All the variations are within the solder paste specifications and customer specifications except the soaking time which the maximum value in the DOE is longer than the one required by the customer (according to the gradient recommended from 70°C to 180°C, the soaking would be from 35 to 43 sec). PROCESS INPUT OUTPUTREFLOW PROCESS INPUT OUTPUT - Preheat Slope - Soaking time - Soaking Temperature - Peak Temperature - Peak Time - Reflow Time - Cooling slope - Yield - DPU - Defects - Testing failures - Visual Inspection - X-Ray - Voids - Tombstoning - Solder beading - Poor coalescence - Flux charring - Reliability of the solder joints - Cold joints - Component cracks - Excessive Intermetallics
  • 13. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 13 of 34 For this DOE, the printing and placement processes were held constant. The Design of Experiment was defined as follows: • Full Factorial experiment • 4 Factors • 2 Levels • 2 replicates • 2 central points Output of the DOE was Defect quantity or defect rate per panel. The four variables used in the study were set up into a DOE that required 34 runs in total because of the 2 replicates and the 2 central points. Metrics of visual inspection under magnification lens and microscope were used, X-Ray was used for BGA’s to detect any problem (mainly voiding). For the results of the DOE, a 95 % or higher confidence interval was required for a factor to be considered statistically significant. After inspect the 34 panels and test them through functional station, the only defect noticed was again solder beading. The quantity of solder beading per panel was 8.5. Below you can see the statistical analysis of the DOE, soaking time was the only statistically significant factor for solder beading according to the P-value obtained (P- value lower than 0.05). Factor Low High Soak Time 38 - 42 seconds 98 - 102 seconds Time Above Liquidous 30 - 32 seconds 58 - 60 seconds Peak Temperature 235 - 237 C 248 - 250 C Cooling Slope 2.4 - 2.6 C/sec 3.4 - 3.6 C/sec Table 1 - Variables included in DOE
  • 14. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 14 of 34 Factorial Fit: Defectos SMT versus Peak temp, Ramp Down, ... Estimated Effects and Coefficients for Defectos SMT (coded units) Term Effect Coef SE Coef T P Constant 14.344 0.7276 19.71 0.000 Peak temp -0.187 -0.094 0.7276 -0.13 0.899 Ramp Down -0.938 -0.469 0.7276 -0.64 0.528 Reflow Time 2.937 1.469 0.7276 2.02 0.060 Soak Time 4.937 2.469 0.7276 3.39 0.003 Peak temp*Ramp Down -1.312 -0.656 0.7276 -0.90 0.380 Peak temp*Reflow Time -0.188 -0.094 0.7276 -0.13 0.899 Peak temp*Soak Time -0.687 -0.344 0.7276 -0.47 0.643 Ramp Down*Reflow Time 2.562 1.281 0.7276 1.76 0.096 Ramp Down*Soak Time 0.562 0.281 0.7276 0.39 0.704 Reflow Time*Soak Time 1.187 0.594 0.7276 0.82 0.426 Peak temp*Ramp Down*Reflow Time 0.437 0.219 0.7276 0.30 0.767 Peak temp*Ramp Down*Soak Time -0.562 -0.281 0.7276 -0.39 0.704 Peak temp*Reflow Time*Soak Time -1.187 -0.594 0.7276 -0.82 0.426 Ramp Down*Reflow Time*Soak Time 2.562 1.281 0.7276 1.76 0.096 Peak temp*Ramp Down*Reflow Time* -1.812 -0.906 0.7276 -1.25 0.230 Soak Time Ct Pt -5.844 3.0000 -1.95 0.068 Analysis of Variance for Defectos SMT (coded units) Source DF Seq SS Adj SS Adj MS F P Main Effects 4 271.375 271.375 67.84 4.00 0.018 2-Way Interactions 6 84.188 84.187 14.03 0.83 0.564 3-Way Interactions 4 67.875 67.875 16.97 1.00 0.434 4-Way Interactions 1 26.281 26.281 26.28 1.55 0.230 Curvature 1 64.281 64.281 64.28 3.79 0.068 Residual Error 17 288.000 288.000 16.94 Pure Error 17 288.000 288.000 16.94 Total 33 802.000 Term Sta nda rdiz e d Effe ct A A C A BC BD A BD A D B C D A C D A B A B C D B C D BC C D 3.53.02.52.01.51.00.50.0 2.110 F a ctor S oak T im e N a m e A P e ak te m p B R a m p D ow n C R e flow T im e D P are to Chart of the S tandar dize d Effe cts (response is Defe ctos S M T , A lpha = .05) This chart shows that soaking time has significant effect on the solder beading
  • 15. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 15 of 34 Standardized Effect Percent 43210-1-2-3 99 95 90 80 70 60 50 40 30 20 10 5 1 F actor S oak T im e N ame A P eak tem p B Ramp D ow n C Reflow T ime D Effect Ty pe Not Significant SignificantD Normal Probability Plot of the Standardized Effects (response is Defectos SMT, Alpha = .05) This chart shows that only soaking time has significant effect on the solder beading
  • 16. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 16 of 34 The Figure Below represents the combination of the factors and their results in quantity of defects: An optimization of the first DOE was performed and these are the results: The values in red are the ones recommended to minimize the quantity of solder beading. The quantity of solder beading with this set up is 9. More replicates per set up would be recommended but one big obstacle to do so was that the budget was limited. 10045 60 30 3.5 2.5 250235 S oak Time Reflow T ime Ramp D ow n P eak temp 8.5 18.5 17.016.0 24.0 12.0 18.017.0 12.0 13.5 13.513.5 10.5 8.5 13.010.5 12.0 Centerpoint Factorial Point Cube Plot (data means) for Defectos SMT
  • 17. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 17 of 34 7.3.2. Validation of First DOE A validation run was run with 15 panels and the quantity of solder beading per panel increased a little bit (11 solder beading per panel), again just solder beading was detected as an issue. 7.3.3. Second DOE A second DOE was developed in order to reduce the quantity of solder beading per panel, we know that there are more possible causes and not only the reflow profile that can lead to have solder beading but they are going to be mentioned in a separate point of this report. The variables examined in this DOE were soaking time and preheat slope. Peak Temperature, Reflow Time and Cooling slope were held constant with central points values from first DOE. The variation of the factors for the second DOE is shown in Table 2. The values of the other reflow variables are shown in Table 3. Best Set Up form DOE: 9 defects per panel Validation: 11 defects per panel Factor Low High Soak Time 38 - 42 seconds 78 - 82 seconds Preheat Slope (50-140 C) 0.7- 0.9 C/sec 1.1 - 1.3 C/sec Table 2 - Variables included in second DOE
  • 18. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 18 of 34 Like in the first DOE, the printing and placement processes were held constant. The Design of Experiment was defined as follows: • Full Factorial Experiment • 2 Factors • 2 Levels • 5 Replicates Output of the DOE was Defect quantity or defect rate per panel. Table 4 shows the series of experiments performed in the DOE. StdOrder RunOrder Soaking Time (°C) Preheat slope (°C/sec) 19 1 40 1.2 18 2 80 0.8 16 3 80 1.2 14 4 80 0.8 1 5 40 0.8 8 6 80 1.2 4 7 80 1.2 10 8 80 0.8 20 9 80 1.2 17 10 40 0.8 15 11 40 1.2 7 12 40 1.2 3 13 40 1.2 9 14 40 0.8 5 15 40 0.8 12 16 80 1.2 11 17 40 1.2 13 18 40 0.8 6 19 80 0.8 2 20 80 0.8 Table 4 Set Up of Second DOE Factor Low Time Above Liquidus 45 seconds Peak Temperature 243 C Cooling Ramp Down -3.0 C/sec Table 3 - Constant Factors for second DOE
  • 19. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 19 of 34 Metrics of visual inspection under magnification lens and microscope were used, X- Ray was used for BGA’s to detect any problem (mainly voiding). For the results of the DOE, a 95 % or higher confidence interval was required for a factor to be considered statistically significant. After inspect the 34 panels and test them through functional station, the only defect noticed was again solder beading. The quantity of solder beading per panel was 8.5. Below you can see the statistical analysis of the DOE, soaking time and the 2-way interaction between soaking time and preheat slope were statistically significant for solder beading according to the P-value obtained (P-value lower than 0.05). Factorial Fit: Quantity of defects versus Soaking Time, Preheat slope Estimated Effects and Coefficients for Quantity of defects (coded units) Term Effect Coef SE Coef T P Constant 11.3000 0.2398 47.12 0.000 Soaking Time 5.6000 2.8000 0.2398 11.68 0.000 Preheat slope 0.8000 0.4000 0.2398 1.67 0.115 Soaking Time*Preheat slope -1.4000 -0.7000 0.2398 -2.92 0.010 Analysis of Variance for Quantity of defects (coded units) Source DF Seq SS Adj SS Adj MS F P Main Effects 2 160.000 160.000 80.000 69.57 0.000 2-Way Interactions 1 9.800 9.800 9.800 8.52 0.010 Residual Error 16 18.400 18.400 1.150 Pure Error 16 18.400 18.400 1.150 Total 19 188.200 Term Standardized Effect B AB A 121086420 2.12 F actor N ame A S oaking Time B P reheat slope Pareto Chart of the Standardized Effects (response is Quantity of defects, Alpha = .05) This chart shows that soaking time and 2-Way interaction between factors have significant effect on the solder
  • 20. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 20 of 34 An optimization of the first DOE was performed and these are the results: Standardized Effect Percent 12.510.07.55.02.50.0-2.5-5.0 99 95 90 80 70 60 50 40 30 20 10 5 1 Factor Name A Soaking Time B Preheat slope Effect Type Not Significant Significant AB A Normal Probability Plot of the Standardized Effects (response is Quantity of defects, Alpha = .05) This chart shows that soaking time and 2-Way interaction between the 2 factors have significant effect on the solder 1.2 0.8 8040 Preheat slope Soaking Time 13.8 14.47.4 9.6 Cube Plot (data means) for Quantity of defects Combination of factors with lower quantity of defects
  • 21. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 21 of 34 An optimization of the first DOE was performed and these are the results: The values in red are the ones recommended to minimize the quantity of solder beading. The quantity of solder beading with this set up is 7. 7.3.4. Validation of Second DOE A validation run was run with 15 panels and the quantity of solder beading per panel was 9. Again, just solder beading was detected as an issue. 7.3.5. Conclusion of profile recommended After the two DOE’s we concluded that soaking time has the more significant effect on solder beading (on this specific product and with this solder paste). Table 5 shows the recommended values for the profile to reduce the quantity of solder beading. Factor Low Time Above Liquidus 45 seconds Peak Temperature 243 C Cooling Ramp Down -3.0 C/sec Soaking Time 40 seconds Preheat Slope (50-140 C) 0.8 C/sec Table 5 - Parameters Recommended after two DOE's
  • 22. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 22 of 34 7.3.6. Operation window for each variable Next table (Table 6) contains the window recommended for each of the process variables of the reflow profile selected in this project, this parameters are recommended to have the best results for solder beading and good quality in general for the solder joints and assembly. The chart illustration shows the phases of the profile. 0 50 100 150 200 250 300 0 90 180 270 360 Time (sec) Temperature(ºC) Prehat Phase Soaking Time Reflow Phase Cooling Phase Peak Temp Liquidus Temp = 217ºC 0 50 100 150 200 250 300 0 90 180 270 360 Time (sec) Temperature(ºC) Prehat Phase Soaking Time Reflow Phase Cooling Phase Peak Temp Liquidus Temp = 217ºC Profile Feature Large Body Small Body Preheat Ramp up Rate (From 70 to 140 ºC) Soaking Time (From 130 to 165 ºC) Time Above Liquidus (217 ºC) Peak Temperature Cooling Ramp Down -2.7 to -3.3 ºC/sec 0.8 - 0.9 ºC/sec 37 - 43 sec 42 - 48 seconds 240 - 246 ºC Table 6 - Recommended process window for each variable Reflow Profile The Scale in X & Y axis and the curve are just examples for a typical profile, the curve is not the necessarily the recommended
  • 23. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 23 of 34 The values of the variables recommended come from the two DOE’s performed and the validation of them, the values are almost in the center of customer specifications (except the preheat ramp up rate which is in the minimum specification). The operational window recommended for each variable might seem very tight, but is the one that gave better results. The specifications from the customer can be used as well as an operational window but the quantity of solder beading expected would be higher, other than that the quality of the assembly was acceptable (visual inspection by certified inspector, X-Ray Inspection and testing) according to the pieces that we assembled for the trials in DOE’s. There are some other actions that can lead us to eliminate completely the solder beading, they are going to be explained in the point 8. Table 7 shows again customer specifications for the variables mentioned in this point (7.3.6) 8. Other variables of SMT affecting solder beading As we couldn’t avoid completely the problem of the solder beading with the reflow profile, we analyzed the problem taking in account other possible causes in SMT. 8.1.1. What solder beading is Profile Feature Large Body Small Body Preheat Ramp up Rate (From 70 to 140 ºC) Soaking Time (From 130 to 165 ºC) Time Above Liquidus (217 ºC) Peak Temperature Cooling Ramp Down 0.8 - 1.0 ºC/sec 35 - 43 sec 35 - 60 seconds 232 - 250 ºC -2 to -5 ºC/sec Table 7 - Customer Specifications
  • 24. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 24 of 34 Solder beading is a special phenomenon of solder balling when using solder paste in certain SMT applications. In brief, solder beads are large solder balls near components with very low stand-off. Often confused with solder balling, solder beading is a defect recognized by one or a few larger balls, generally located around chip caps and resistors. 8.1.2. How solder beading happens a) Solder paste is printed on the pads of a circuit board. b) During component placement some solder is squeezed underneath the body of the component and broken off from the solder on the pads. c) During reflow, the solder trapped underneath the component does not flow back to the solder pads. Contrarily, its cohesive properties (surface tension) cause it to form a large ball (bead). d) The surface tension of the cooling solder draws the component closer to the pads. As the body of the component is drawn down, the solder bead squeezes out the side and remains there. 8.1.3. Why Solder Beading is problematic Basically, solder beads may form a “bridge” of solder that runs from one component termination to another, thus causing an electrical connection that was not designed to be there. This poses the threat of resulting in a short circuit. This may occur where the bead was originally formed or elsewhere on the assembly if vibration causes the bead to break loose and move around. While the above may not necessarily occur if the solder beads are present, solder beading obviously remains a defect that should be minimized or eliminated if possible. Solder beading could create an unwanted electrical bridge if they are located between two adjacent parts of the PCB. Additionally, they could be dislodged during handling and affect the performance of adjoining assemblies or components.
  • 25. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 25 of 34 Therefore, solder beading that are formed need to be detected visually and removed manually. These steps add to the labor costs associated with the assembly process. If a solder micro-ball eludes detection and adversely affects the performance in service, or results in customer returns, then the economic impact can be even greater. The obvious answer to this problem is to eliminate solder micro-ball formation, which can be difficult, because solder micro-balls can be caused by a variety of sources. 8.1.4. Some possible causes of solder beading There are many causes that can lead to have solder beading but we listed the ones that we thought are more important in this specific case: - Bad Stencil Aperture 1:1 with the pad - High value of stencil thickness - Incorrect Snap Off: A gap between the stencil and PWB can increase the paste volume deposited on the assembly - Solder Paste tend to generate solder balling (this feature can be tested following IPC test . - Too slow or too fast preheat ramp rate on Reflow - Incorrect speed and squeegee pressure on printing process - High humidity in the environment. - Oxidation of solder powder. - Incompatibility between board finish (OSP) and solder paste flux. 8.1.5. Possible Solutions for solder beading Some recommendations to prevent solder beading are listed below: - Stencil Aperture Size / Shape - Reduce the aperture size (home plate, bow tie or D shape can be options). - Stencil Thickness – Reduce the stencil thickness to reduce the paste deposit. - Snap Off – Use on contact or zero snap off (top side of PWB on contact with bottom side of stencil). - Solder paste viscosity – Keep the viscosity of the solder paste within specifications, the higher viscosity, the better results for solder beading. - Reflow Profile – Improve the preheat ramp rate (too slow or too fast ramp rate can lead to solder beading). This optimization was already made in the project. Solder beading problem appeared mostly in components 0402, 0603 and some mid size components. As with the current stencils the aperture is 1:1 with the pad, we recommended some changes in the aperture for the most problematic components. One stencil was ordered and the changes on the stencil were applied on the more recurrent references of the product. The results were pretty good, we assembled 30
  • 26. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 26 of 34 panels with this stencil and we didn’t find any solder beading on the references where the changes on the aperture were applied. Table 6 shows the recommendations of the apertures: Next step is to change the stencil apertures for all 0402 & 0603 references (apertures for problematic mid size components were already implemented) on the board. This change is on going because we are waiting for approval from customer. 9. Control Process Options This section is intended to determine which controls can be proposed to perform the Process Control of the selected reflow variables. 9.1. Introduction to Process Control Charts Next section is related to the theoretical structure of the control of the process variables using Control Chart. Ongoing monitoring is typically managed with a control chart. Uses of control Charts • Determine appropriate managerial action in response to the value of a data point from a particular process COMPONENT APERTURE SHAPE DRAWING 0402 D-Shape 0603 Home Plate MID SIZE COMPONENT Reduction in inner part of the component Table 6 - Recommended aapertures to reduce solder beading problem Yellow means Land pattern area Area in White means recommended stencil aperture The squares in blue represent just other references
  • 27. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 27 of 34 - To see if high or low points are due to special causes. • Understand and predict process capability (expected ranges of future values) for planning purposes. • Identify Root causes of variations by differentiating between special and common causes of variation in the data. • See whether intentional changes in the process had a desired result. • Monitor key processes and identify shifts or changes quickly to help hold the gains made from an improvement project. Common-Cause Variation Common causes are the process inputs and conditions that contribute to the regular, everyday variation in a process. • Common causes are a part of the process. • They contribute to output variation because they themselves vary. • Each common cause contributes a small part of the total variation. • By looking at a process over time, we know how much variation to expect from common causes. • The process is stable, or predictable, when all the variations is due to common causes. Special-Cause Variation Special causes are factors that are not always present in a process but that appear because of some particular circumstance. • Special causes are not usually present. • They may come and go sporadically; may be temporary or long-term. • A special cause is something special or specific that has a pronounced effect on the process. • We can’t predict when a special cause will occur or how it will affect the process. • The process is unstable, or unpredictable, when special causes contribute to the variation. Tests for special causes • 8 or more points in a row of the same side of the median indicates a process shift. • If the data are symmetrical, it’s Ok to use the average as the central line instead of the median.
  • 28. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 28 of 34 • 6 or more points in a row continuously increasing or decreasing indicates a trend. • Start counting at the point where the direction changes. • Too few runs indicates a shift in the process average, a cycle, or a trend. • Too many runs indicates sampling from two sources, overcompensation, or a bias. • 14 or more points in a row alternating up and down indicates bias or sampling problems. • One or more points outside the control limits indicates that something is different about those points. Individual Charts Because they can be used with any data that is time-ordered, and in general and very versatile, individual charts are the most frequently used type of control charts. However with particular kinds of data or situations, they are sometimes slower to signal special causes than other kind of charts, so it’s best to understand other types of control charts as well. SPECIFICATION LIMITS Vs CONTROL LIMITS Specification Limits • Come from engineering or customer requirements. • Represent what someone wants a process to do. • Can sometimes be changed by changing the requirements of the product or service. Control Limits • Come from calculations of the process data. • Represent what a process is actually capable of doing • Can only be changed by changing the process. When to calculate new control limits? You should calculate new control limits when: • You know there was a change in the process based on - Statistical Evidence, such as 8 continuous points above or below the centerline. - You have determined why the change occurred (based on your process knowledge).
  • 29. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 29 of 34 - You are confident the process will stay changed • You are confident the process will stay changed. - The change was not temporary. - The change has become a standard part of the process. Calculate the new limits when you have enough data points to see a change. Call the new limits temporary until you get at least 24 new data points. Assumptions for individual charts • Data are roughly normal (data might need to be transformed) • Data points are independent What to look for when using Control Charts • A good control chart is one that is being used concurrently with the process. - Charts should be posted or be readily at hand. - Charts should be up-to-date. - Charts should look well-used. • Comments should be written on charts - Dates of process changes. - Notes on events that might cause problems later. - Confirmation of verified special causes. - Actions taken to eliminate special causes (only rarely should the chart indicate that the cause could not be identified). Common mistakes when using control charts - Chart not created correctly - Wrong formula used to calculate “3 sigma” limits (st. dev. used instead of moving ranges). - Wrong type of charts used based on type of date collected. - Missing, poor or erroneous measurements. • Chart not regularly updated
  • 30. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 30 of 34 - Data on charts are not current - Process adjustments have not been noted. - Control limits and average have not been updated. • Actions taken are - Rewards given for “good points” or explanations sought for “bad points” even though they are not signaled as special. - Special-cause signals ignored. - Non-random patterns or cycles not studied to determine specific causes. - Spec limits or goals are placed on chart instead of control limits 9.2. Control Chart Selection for Reflow Variables As a part of the project is required to propose an option to measure, monitor and control the final variables selected on all the previous analysis. The process to select the appropriate graph was carried based on next flow chart. Selecting an Appropriate Control Chart Can you mistake proof the defect? Do not use SPC; implement PM Output or input? Improvement or run/stop? Continuous data? Pre-control Tracking System Individual and Moving Range Charts Process in Control? Past as important as present? EWMA chart CuSum chart Is subgroup size > 8 and sigma easily Can you measure more than one defect per unit? Xbar and sigma chart X bar and Range chart % defective (bad parts) Defect/unit Sample size constant? Sample size constant? nP chart P chart C chart U chart Input No Yes Automatic Manual Yes No Yes No Yes No Yes No Yes No Yes Run/Stop Improvement Output Yes No
  • 31. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 31 of 34 This is a sample of the “Individual-X & Moving Range Chart” proposed to monitor and control the selected variables in the Reflow Oven profile. E:INDIVIDUAL-X & MOVING RANGES CONTROL CHART.xls Individual-X & Moving Range Charts are a set of control charts for variables data (data that is both quantitative and continuous in measurement, such as a measured dimension or time). The Individual-X chart monitors the process location over time, based on the current subgroup, containing a single observation. The Moving Range chart monitors the variation between consecutive subgroups over time. The Individual chart on top shows each reading and is used to analyze Central Location. The Moving Range chart at bottom is used to study system variability. Why it is used? To analyze a subgroup size that cannot be more than one for any reason such as costs, expense involved, difficulty to collect data or large size collection is not practical or possible. When it is used? 1. When you need to assess the stability of system. 2. When the data is variable. 3. When the size of subgroups is one. 4. When the time order of subgroups preserved. Advantages The charts are sensitive and hence can detect even a small variation in normality. Highly reliable and easy to use. Disadvantages Constant and thorough study of chart is necessary Slow in detecting sudden jumps in average values
  • 32. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 32 of 34 9.3. Final variables selected to be monitored Based on results, soak time was the only significant variable obtained. Every time the profile is evaluated, the Data pack can be programmed and all data can be obtained and graphed. Even though only soaking time was the main variable obtained in all results, there is no restriction to increase the control of all the list of variables mentioned: • Soak Time • Peak Temp • Ramp Down • Reflow Time • Cooling slope 9.4. Profiling Frequency: Actually there is a process instruction which states that the profile must be evaluated each week. This can give enough data of how the oven is working. Gauge R&R to the reflow oven shows that repeatability and reproducibility are good, due to this, it is not recommendable to decrease the profiling period (i.e. from one week, to two weeks or one month), until we obtain enough data in a period of at least 6 months. After that, we would be able to decrease frequency. 9.5. Monitoring Equipment Options Data logger Real Time Reflow Profiler Gold M.O.L.E Oven watch Data Pack Slim KIC 2000 KIC 247 The proposed graph in section 9.2, is based on the use of the “Gold M.O.L.E.” or “Data Pack” data loggers, used at the moment in the Monterrey Plant. A “Real Time Reflow Profiler” device is recommended as a better option to control the variables. This is an option online, which can give the profile for each PCB and result on a better monitoring.
  • 33. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 33 of 34 9.5.1. Real Time profile monitoring device (Short description) The automated real-time thermal process monitoring system, is an equipment that provides and records real-time thermal process data for every product, as opposed to the conventional practice of only periodically checking oven performance. This allows this equipment to automatically catch potential defects before they happen, rather than discovering actual defects during “Inspection”. It utilizes thermocouple sensors permanently embedded in the oven at the process level. The key feature is the creation and recording of a Virtual Profile. By associating the dynamics within the oven with the profiled data collected using profile data logger, the real-time monitoring device, automatically calculates a unique product profile including all pertinent process data for every product that exits the oven. It utilizes real-time process data for real-time SPC charting. Process data is automatically charted for all critical process specs: peak temperature, soak time, time above liquidus, etc. The data is plotted on real-time control charts and Process Capability (Cpk) is calculated for each spec. Any the process drift outside of control limits will bring an immediate alarm. The process engineer also has the option of setting a warning limit on the Cpk. Real-time Cpk tracking enables the system to flag an out of control process before the oven has produced a single defect. 10. Project Conclusions • Control process is required to assure the continuous quality performance in our product, a Control Chart is proposed to control main variables on the oven profile. Eventually, changing to 100% monitoring of the profile or “real time” is recommended. • Individual values and Moving Range (I-MR) charts are recommended as SPC for soaking time, preheat ramp up, reflow time, peak temperature and cooling slope. • The use of control chart applies not only for the variables mentioned above but also for all significant variables at any kind of process. The more variables monitored, the better process control. • Solder Beading defect can be categorized as a process indicator or defect based on IPC criteria. This was found as an opportunity as an output of the DOE’s. Several recommendations were done related to this. • Requesting for theoretical information, solder beading is mainly caused due to soaking time, same as our results, this is a validation of the results. • Historically, the profiling period has been 1 profile per week. Give a frequency for profiling depends pretty much on the repeatability of the machine (oven); again, a 100 % or real time monitoring of the profile is more recommended but Budget for investment has to be analyzed.
  • 34. MTD REFLOW PROCESS CONTROL Document code: NOT IN USE Classification: Confidential Date: 14-Feb-2005 Prepared by: (Also subject responsible if other) Alejandro Rodriguez Reviewed by: (Also subject responsible if other) Guadalupe Villarreal Approved by: (Also subject responsible if other) Version: 0.0 Page/pages: 34 of 34 Having a 100% or real time monitoring of the profile is recommended but not mandatory to have a good process control, we can have a good process control by determining the right frequency of profiling and analyzing the data properly. We have seen that the repeatability among ovens varies; Analyze how repeatable your oven is and how small your process window is will lead you to determine the right frequency for profiling. Of course SPC and good analysis of it is going to be helpful. • During DOE’s tests, it was shown that profile specification limits are correct, based on visual inspection (IPC certified operator), X-ray inspection, and electrical-testing results. • The final proposed profile is basically in the center of all the customer specification limits. This is giving us a confirmation that we are running under the best profile, considering only few changes at the beginning of the profile.