Overview of highly accelerated life test (halt)

OVERVIEW OF HIGHLY
OVERVIEW OF HIGHLY
ACCELERATED LIFE
ACCELERATED LIFE
TEST
Chet Haibel
©2011 ASQ & Presentation Chet
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OVERVIEW OF HIGHLY
ACCELERATED LIFE TEST

Chet Haibel
Hobbs Engineering Corporation
www.hobbsengr.com (303) 465-5988

Chet Haibel ©2012 Hobbs Engineering Corp.

What Is Reliability?

CLASSICAL DEFINITION

Reliability is the probability that a component,
subassembly, instrument, or system will perform
its specified function for a specified period of time
under specified environmental and use conditions.

1

What is a Product Failure?

Failure is the inability of a device to perform its intended functions
under stated environmental conditions for a specified time.
Failures are classified into three types based on time:
• Early-Life (Infant Mortality)
• Useful-Life (Random-in-time)
• Wear-Out (End of useful life)
Each failure type has different kinds of causes and therefore different
tests to discover them and different methods of correction / prevention.

2

What is a Product Failure?

Failures are also classified into three types based on their persistence:
• Hard Failure (Persistent)
Typically a component must be replaced, but trouble-shooting may be
done at room temperature with no vibration or other stimulus

• Soft Failure (Temporary)
Often merely removing the environmental stimulus clears the problem,
but sometimes it is necessary to cycle power, clear fault logs, etc.
Product must be stressed to duplicate and trouble-shoot soft failures
Many very important reliability issues are SOFT FAILURES.

• Intermittent Failure (Elusive)
This is permanent but the failure mode must be put into a detectable state

3

What Causes Product Failure?

A component fails when applied load exceeds design strength.

Applied Load Design Strength

Failure

Units of Applied Load, Strength

4

Applied Loads

Examples of applied load might be:
 Force  Voltage

 Torque  Current

 Tension  Wattage

 Shear  Clock Speed
 Pressure  Electrostatic
Discharge
 Electromagnetic
Interference

5

Design Strength

Examples of design strength:
 Torque rating of a bolt
 Voltage rating of a capacitor
 Current rating of a diode
 Power rating of a resistor
 Shear strength of solder
 Tensile rating of plastic
 Temperature rating of transformer insulation

6

Load / Strength Interference

Desirable
Load Strength

Obvious
Strength Load

More Subtle
Load Strength

7

Load / Strength Interference

Early-Life Load Strength

Useful-Life Load Strength

with time

Wear-Out Load Strength

8

Bathtub Curve

Early-Life Wear-Out
Hazard Rate - h(t)

Failures Failures

Useful-Life
Failures

Random-in-Time Failures

Life to the Beginning of Wear-Out Operating Time (t)

9

Wear-Out Failures

with time
Load Strength

Increasing Hazard Rate h(t)
Failures due to cycle fatigue
Corrosion Hazard
Rate
Frictional wear
Shrinkage, cracking in plastic components Time

Typical of mechanical systems

10

Cycle Fatigue

Stresses: Cycled by:
• Pressure • Product Operation
• Tension • Thermal Cycling
• Torsion • Vibration
• Shear • Shock
• Etc. • Etc.

Use up Fatigue Life

11

Observed Failure Behavior

For a given stress level, the number of cycles to failure in a sample
will occur in a distribution due to specimen variation
16

14

12

10

8

6

4

2

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Cycles to Failure

12


Higher stress level requires fewer cycles to failure
Higher Stress Lower Stress
16

14

12

10

8

6

4

2

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Cycles to Failure

13


For the same failure mode, stress level and the number of cycles
to failure are related by a straight line on log scales
S - N Diagram
1.6
1.5
Log S, Stress

1.4 
S1 N1
1.3
1.2 S2 N2
1.1 

1.0
0.9
0 1 2 3 4 5
Log N, Cycles to Failure

14

One Failure Mode: Fatigue Damage
Vibration Analysis of Electronic Equipment by Dave Steinberg, Wiley, 1973

D  n  b, where

• D is the Miner’s Criterion fatigue damage accumulation,

• n is the number of cycles of stress,

•  is the stress in force per unit area,

• b is the negative, inverse slope of the S-N diagram for the material.

For wrought Aluminum, doubling the stress decreases the
fatigue cycles by a factor of 1000 b is approximately 10

15

S-N Diagram for 7075 Aluminum
Vibration Analysis of Electronic Equipment by Dave Steinberg, Wiley, 1973

O
O

~ 2 thousand cycles at 80 KSI, but at 40 KSI it takes 2 million cycles
16

Fatigue Damage from Vibration

Assume a resonance at 1,000 Hz

At 40,000 psi, failure would occur at 2 million cycles
2 million ÷ 1 kHz = 2000 seconds or 33 minutes

At 80,000 psi, failure would occur in 2 seconds

Doubling the G rms level would achieve a time compression
factor of 1,000.

This TIME COMPRESSION is normal for HALT

17

Time Compression
Reference: GE Lighting, private telecon with Jim Harsa in 2000

Dtvb Increased voltage stress shortens
time to see the same dominant
t is time Wear-Out failure mode

v is the voltage

b =13 for incandescent lights

b = 8 for fluorescent lights

18

Discovering Wear-Out Failures Without Using HALT

If possible, set up a repetitive “cycle test” which removes the “dead
time” between cycles. But brainstorm what test artifact may be
added and / or what the test may be concealing
Test until a minimum of five failures are produced [Haibel’s rule]
Use Weibull Analysis to fit a distribution to the failure data
If life is not sufficient, determine the reservoir of material and the
process consuming the reservoir. Increase the reservoir of material
and / or slow down the process consuming it
If necessary, replace the reservoir of material periodically with a
scheduled preventive maintenance program

19

Discovering Wear-Out Failures Without Using HALT

Electromigration
(photo courtesy Alcatel-Lucent)

Standard test for
electromigration in
MIL-STD-883 is
Dynamic Burn-In:
125°C for 160 hours
with all voltages,
currents, and clock
speed maximized

20

Useful-Life Failures

Load Strength

Constant Hazard Rate h(t)
Random-in-time failures
Hazard
Parts are new until they fail
Rate
Strength-Load interference
Insufficient design margin Time
Typical of electronic hardware

21

Quantifying Strength / Load Interference

Subtracting two Normal
distributions produces
another Normal
distribution whose mean
is the difference of the
means, but whose
standard deviation is the
root-sum-square of the
two standard deviations

We define Safety Margin
MS  ML
SM 
( S   L )1/ 2
2 2

22

Useful-Life Failures

Load Strength

For simple mechanical products with few parts, we can calculate
reliability one part at a time using Safety Margin for Normal
distributions, or using Monte Carlo simulations for non-Normal
distributions.

For electro-mechanical products with thousands of components
(each of which may have several relevant strength characteristics),
we need an efficient technique to catch the few component
applications that have marginal strength / load relationships. So far,
the most efficient technique is Highly Accelerated Life Test (HALT).
23

HALT
Highly Accelerated Life Test

Used in the Design Phase

24

HALT Finds Useful-Life Failures

Load Strength

Load Strength
constantly
increasing load

Increase probability of seeing an existing failure mode

25

HALT

HALT is the method of seeing the existing failure modes
with the minimum number of prototypes (4 or 8)
in the minimum time (typically a week)
By experience with early prototypes or with similar
products, determine which environmental factors will
“stimulate” the relevant failure modes
Many failure modes in typical electromechanical
products are well stimulated by temperature and
rapid temperature cycling simultaneous with six
degree-of-freedom random vibration

26

Temperature (Celsius) Goal “limit of technology”
80

60

ENV2
40
ENV1

20 G rms
5 10 15 20 25 30

0

-20

-40 Goal “limit of technology”
27

HALT

Every stimulus of potential value is used during New Product
Development to find the weak links in the product design
These stresses are not meant to simulate field environments but to find
the weak links in the design using only a few units in a very short
period of time
Stress levels are taken well beyond the normal mission profile
Sometimes one kind of stress will produce a failure mode in HALT,
but a different kind of stress will produce that same failure mode
in the hands of customers
Crossover Effect
Focus on fixing the failure mode, don’t focus on the stimulus

28

Crossover Effect

29

Stimulus-Flaw Precipitation Relationships
Reference: “Flaw-Stimulus Relationships”, G. K. Hobbs, Sound and Vibration, August 1986

All Combined
Vibration

High Temp
Burn in

Thermal Voltage
Cycle Cycle

Margining

30

Perhaps a Different Order

More than one failure mode may be affected by the same stress

Failure modes will not necessarily be exposed according to the field
Pareto chart, but maybe in some other order

Field HALT
Pareto Order

The time compression factor for the failure modes will be different

31

Failure % by Stress Type
“Summary of HALT and HASS Results at an Accelerated Reliability Test Center” by Mike Silverman

Based on 49 products from 19 different industries

Order of application and discovery:
Cold Step Stress 14%
Hot Step Stress 17%
Temperature Transition 4%
6-Axis Vibration 45%
Combined Temp and Vibe 20%
Without simultaneous, all axis vibration,
65% would have been missed!

32

“Our Path to Reliability Using HALT”
Chuck Laurenson, Parker Hannifin 1999 Hobbs Engineering ARTS USA Award Winning Paper

Where Design Flaws Were Discovered

Cold Step Stress 10%
Hot Step Stress 12%
Rapid Thermal Cycling 4%
Vibration Step Stress 43%
Combined Temp and Vibe 31%
74% of the flaws would have been missed
without simultaneous, all axis vibration!

33

Let’s Focus on Vibration
Swept Sine, Single Axis
Random, Single Axis
Six Degree of Freedom

34

ElectroDynamic Shaker

35

Z-Axis Mode of Vibration

36

Driven Harmonic Motion

d 2z dz
M 2  D  Kz  A cos 2ft
dt dt

Transfer Function
10

1

0.1

Z-axis 0.01

excitation
A cos 2πft 0.001
1 10 100 1000
Shaker frequency in Hz

37

Swept Sine Vibration

Essentially one frequency at a time,
sweeping at one octave per minute
Typically uses a Hydraulic shaker (limited upper frequency) or an
ElectroDynamic shaker (high powered voice coil)
Using a Stroboscope, one can observe behavior at resonance
But can only see one resonance at a time, in one translation
axis at a time; must mount the product for X, Y, & Z
Miss interactions between resonances at different
frequencies or in different directions
No guarantee of stimulating rotational resonances at all !
38

Voice Coil Can be Rotated to Drive the Slip Table for X or Y

39

An Oil Bearing Supports the Slip Table

40

Random Vibration

Broadband, Pseudo Random (noise-like)
vibration generated by a computer

Typically uses an ElectroDynamic shaker, therefore one translation
axis at a time; still have to mount the product three times for X, Y,
& Z and that doesn’t stimulate rotational resonances very well
But this is a major improvement to see all frequencies at once,
therefore see the interaction of resonances in one direction
Crest factor (ratio of peak to average acceleration) is around 3
Major advantage is to shape the spectrum for qualifying to some
external standard (e.g., RCTA/DO-160D Category U Helicopter)

41

Random Vibration Shaped Spectrum
1.000

Vertical axis is
Power Spectral
Power Spectral Density

0.100
Density in units
of g2/Hz
g2/Hz

To convert to G
0.010
rms, integrate
the power (g2)
over frequency
0.001 and take the
10 100 1000
Freqency (Hz) square root

Shown is approximately 5G rms
42

TIME COMPRESSORTM TC-1 Ocelot by
HALT & HASS Systems Corporation

43

Features of the TC-1 Ocelot

Temperature change rates of plus or minus 120 Celsius degrees per
minute, the highest in the industry, from -100°C to +200°C
Vibration will start and run anywhere from 0.1 to 150 G rms
Low G levels are important for executing Modulated Excitation™
which is a breakthrough for detecting intermittent failures
X, Y, and Z acceleration balance is near 1:1:1
Sound level is only 50 dBA at 30 G rms, the lowest in the industry,
no ear protection is necessary, can be used on production lines
Will operate on 110 volts, 50-60 Hz with reduced heating for trouble
shooting – this is important for duplicating soft failures

44

TC-1 Ocelot Vibration System
These are
pneumatically-
driven pistons
which generate
six-axis (6 DoF)
vibration from
approximately
20Hz to 10kHz

(one spring is
removed to
show the table
Bottom View construction
detail)
45

Repetitive Shock Spectrum
T d

Mathematically, a string of
rectangular pulses of period T and
duration d in the Time Domain

Time in seconds
1

Transforms into a “comb” of 0.1

frequencies whose fundamental 0.01

frequency is 1/T with harmonics 0.001

Sin df
weighted by in the 0.0001
πdf
Frequency Domain 0.00001
Frequency in Hz
46

Six-Axis Random Vibration

Using several pneumatic pistons, with air flow modulated in a
proprietary fashion, produces overlapping smeared spectrums
The different angles of the pneumatic pistons generate a feedback
controlled, broadband level of random vibration in X, Y, and Z
translational directions and yaw, pitch, and roll angular directions
Feedback for the control system is provided from one z-direction
accelerometer on the bottom (piston side) of the table
This results in all frequencies in all directions, simultaneously
exciting all resonances for complete failure mode stimulus
The Crest Factor, the ratio of peak to average acceleration is ~10,
which rapidly precipitates design and manufacturing flaws

47

Some Defects Precipitated by Vibration

Poorly mounted components
Poorly formed leads
Poor solder joints
Fretting Corrosion
Loose hardware
Loose wires
Adjacent parts contacting
Wires over sharp edges
Stacked resonances

48

Some Defects Precipitated by Vibration

49

Some Defects Precipitated by Thermal Cycling

Poorly matched expansion coefficients
• Boards and components should match
• Structures should match
Poor solder joints
Improperly formed leads
Improper crimps
PCB shorts, opens
Plated through hole defect

50

Some Defects Precipitated by Thermal Cycling

51

Effect of Temperature Rate on Number of Cycles
“Effective and Economics-Yardsticks for ESS Decisions”, S. A. Smithson, IES, 1990

52

Time Compression for Data from the Previous Slide
Calculations by G. K. Hobbs

At a Ramp Rate of 5⁰C per minute, 400 cycles with a range of 165⁰C
(with no dwells) would take 440 hours

At a Ramp Rate of 25⁰C per minute, 4 cycles with a range of 165⁰C
(with no dwells) would take less than 60 minutes

(At a Ramp Rate of 40⁰C per minute, 1 cycle with a range of 165⁰C
(with no dwells) would take less than 10 minutes)

This is real TIME COMPRESSION !

53

Stresses Used in HALT

Wide range temperature Humidity
High rate temp. cycling Dimensional parameters
All axis random vibration Viscosity of a fluid
Power cycling Vary pH of a fluid
Power voltage and frequency Salinity of a fluid
Secondary voltage Add particulates to the fluid
Digital clock frequency Back Pressure

54

More Stresses Used in HALT

Vary magnetic tape thickness Inject electrical noise
Vary gear diameter Mistune the channel
Off axis alignment Radiation (E & M)
Mismatch / Overload Nuclear radiation
Imbalance Multiple sterilizations
Off-track Whatever else makes
Higher RPM sense for the
particular product
55

Crossover Effect

A flaw may be exposed by a different stress in HALT than the
stress which exposes the flaw in the field environment

Focus on the failure modes and mechanisms, not the stresses
used to expose them or the margin beyond field environment

Focusing on margin may lead to missing an opportunity for
improvement followed by field failures of the same mode

This is a frequent, serious mistake in HALT!

56

What Level of Stresses to Use

57

Product Response is of Prime Importance, the Inputs Are Not

Vibration
• All modes excited
• Second modes are very important
Thermal
• All sites reach the desired temperatures
• All sites reach the desired rates of change
Voltage
Humidity
Current density
Other stresses or parameters

58

What Level of Stresses to Use
In HALT, one must go beyond customer-specified stress level to
compress the time to see the dominant failure modes
Stress level has been substituted for sample size!
This is one of the MAJOR BENEFITS of HALT
We do not need many units to HALT (four is good)
We can HALT a few at each stage of development and manufacturing.
• Prototype (as early as feasible)
• Pre-production (after corrections)
• Early production (after design transfer)
• Ongoing production (re-HALT)

59

Understand First

Again, the key is to focus on the failure mode, not the stress type
used, or the margin beyond the field environment
Through failure analysis, gain root cause understanding first and
then decide if the weakness would cause field failures or whether
the weakness would put limitations on manufacturing screening

It’s often easier to fix it than prove it’s not a customer issue!

60

HALT Attitude

Every weakness found represents an opportunity for improvement
HALT is proactive, but no action means no improvement
We try to break the product in order to find its weak links
This is discovery testing compared to qualification (success) testing

This is a total paradigm shift!
Opportunities not taken will probably lead to field failures much
more expensive than the improvement would have been. This fact
has been documented in thousands of cases

If you find it in HALT, it is probably relevant !

61

Example of Success
Ed Minor, Boeing, in a presentation at a Hobbs Engineering Seminar

Boeing 777 was the first
commercial airplane
ever certified for Extended
Twin-engine Operations
(ETOPS) at the outset of
service

“Dispatch reliability after only two months of service was
better than the next best commercial airliner after six years”
63

Some Product Types Successfully Improved by HALT

Accelerometers Magnetic Resonance Scanners
Analysis &Test Equipment Medical Products
ASICs / Processors / Drives Military / NASA (mixed)
Land / Air / Water Craft Monitors / Displays / TVs
A/V Products & Systems Ovens
Avionics / Aerospace Pneumatic Vibration
Compressors/Generators Point of Sale Systems
PCs to Mainframes Power Supplies
Lipstick Radar / GPS Systems
Electronics / Electrical Telecommunications
Gears / Transmissions Thermal Controls
Instruments / Gauges Jet Engines / Missiles

64

The Complete HALT Process

HALT consists of:
• Precipitation
– Stresses
– Stress Levels
• Detection All must be present or no
– Detectable State improvement happens !
– Coverage
• Failure Analysis
• Corrective Action
– Corrective Action Verification

65

The First Part of Detection

Achieve a Detectable State, the “Magic Level” or the “Sweet Spot”
where the intermittent is detectable
• Detection Screens are a well established technique commonly
practiced by the experts
• Requires equipment designed for HALT and HASS for best results
• Modulated ExcitationTM frequently improves detection by two
orders of magnitude, sometimes even more

66

Detection Excellence

Some damage from the HALT stresses may not be
immediately discernable – it may be LATENT !
HAST (Highly Accelerated Stress Test -- Pressure Cooker) may
precipitate latent damage, making it patent -- discernable
• Cracked component bodies (e.g. MLCC)
• Other long term failure modes not yet completed
If feasible, expose all HALT units to HAST
Or perform a biased (power on with signals toggling) exposure
to 60°C and 90% RH for one week

67

Multi-Layer Ceramic Capacitor
CALCE Electronic Products and Systems Center, University of Maryland

PCBA Flexing

68

Equipment Required

Combined all-axis, broad-band vibration and high-rate thermal
cycling. Low frequencies must be present in sufficient amplitude
to precipitate the defects.

Electrical stressing (power supply, clock frequency, loads)

Monitoring with high coverage is absolutely essential

Temperature, pressure, and humidity (HAST) equipment
Traditional 85/85 takes 1,000 to 5,000 hours
HAST takes only 48 hours!

Other stressors (such as corrosive atmosphere or radiation) as
appropriate for the product and its environments

69

Appreciating HALT

To Appreciate
HALT, let’s look at
prototype test
quantities required
under normal
conditions


Reasonable Example

Suppose an R&D project has a product reliability
goal to have less than 5% Annual Failure Rate.

(this is not a lofty goal)

How many prototype units would have to be put
on test to have 70% probability of seeing all the
problems that must be resolved to be successful?

71

Infinite, Decreasing, Geometric Series

Mathematical Model for a Pareto
2 3
F1 , F1R , F1R , F1R , ...

Sum = F1 / (1 - R) 0<R<1

Example:
If sum = 5%, R = 0.8, solve for F1
Answer:
(Sum)(1 - R) = F1 = (5%)(0.2) = 1%
72

Infinite, Decreasing, Geometric Series

4
“allowed”
3
PERCENT

failure
modes
2

1

0
A B C D E F G H I J K L M N O P
FAILURE MODE

73

70% Chance of Seeing Failures for 5% Annual Failure Rate

1000

Number of units on test 0.50 0.70 0.90 0.99

100 O

10
0.001 0.01 0.1
Failure mode's failure probability

74

Minimum Prototypes and Time

To see the failure modes that must be eliminated
for even mediocre reliability (5% AFR),

Test 120 units for a year
at normal mission (customer, field) conditions,
or

HALT 4 units for a week

75

How to Prove that HALT Works

There are “Accelerated Reliability Test Centers” where you can take
some products to try a HALT chamber

The persons at the ARTC will run the chamber, but you have to
run your product using diagnostic software

Take an existing (currently shipping) product for which you know
the failure modes experienced by your customers

This is an excellent way to prove that HALT will find the relevant
failure modes in YOUR product

76

HALT WORKSHOP

Preparing to HALT a Product
Preparing a Product for HALT

77


In any test we have to stimulate the product and look for a response
from it. HALT is no different, we need inputs and outputs which
we can control and observe from outside the HALT chamber.

Ideally, we want to check all functions of the product so we can see
any (soft) failures.

We often figure out a “quick test” which we can run at each condition
of voltage, temperature, vibration, etc. This might be the power-on
self-test (POST), so we power cycle the product at each condition.

Then occasionally, we will take the time to do a thorough checkout.

78


Many products (especially software driven products) detect power
supply voltage and will shut down outside an upper and lower limit.

Some products detect temperature and will shut down outside an
upper and lower limit.

These protections must be disabled, either with special HALT
software (firmware) or by modifying the hardware (supplying a
stable voltage to the temperature and / or voltage comparators).

We want to see the underlying (raw) performance of the circuits.

These voltage and temperature limits will improve design margin.

79


Some products have rubber feet on them to reduce skidding and
scratching, and take out minor irregularities in the support surface.

These will tend to dampen the vibration we are trying to drive into the
product. We must overcome this dampening by removing the feet
or supporting the product next to the feet on the chassis.

Similarly, inside the product there may be elastomer material to dampen
vibration. These dampeners must be defeated to transmit vibration.

80


Most products have covers to protect the electronics from foreign
(conductive) material and protect the user from coming in contact
with live voltages.

Some products have fans to circulate air to cool the hot components
(and heat the cool components).

These covers and fans will get in the way of the turbulent airflow in the
HALT chamber, which is trying to impose a temperature on the
components. It makes a convection oven look tame!

Unless these covers are structural, they should be removed. If they are
structural, they must have holes drilled in them to let the airflow in.

81

Overview of highly accelerated life test (halt)

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Overview of highly accelerated life test (halt)