Part of a free 2 day presentation in Penang, Malaysia in 2011. Sponsored by the World Gold Council, London. I have to apologise for the content. This was a short version of a more detailed course I give on bonding wire materials. Some of the content requires a reasonable understanding of solid state physics and chemistry and for attendees that don't have that understanding I go to great pains to discuss crystal chemistry and physics and spend a lot of time drawing on white boards and flip charts to explain things in more detail. The white board and flip chart stuff just can't be captured in the presentation because it's 'off the cuff', mainly because I always inform my audience that if there is something they don't understand I am more than willing to spend time to explain stuff because I want them to 'get it', at least to the extent that they can be inspired to go find out more about what I'm presenting.

1. Gold and Copper Wire Processing and
Material Properties
Dr Christopher Breach
ProMat Consultants

2. Agenda
¤ Gold and Copper Wire Processing and Material
Properties
¤ Manufacturing Processes of Au, Cu and Pd-coated Cu
Wires
¤ Wire Chemistry and Analysis
¤ Final Wire Properties
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3. Manufacturing Processes
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4. Process Flow
Continuous(casting(
(copper)
Continuous(casting(&(
doping((gold)
Block(draw Breakdown(draw
Final(draw
Intermediate(annealing
Final(annealing
Spooling
Continuous(casting(
(aluminium)
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5. Continuous Casting
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6. Continuous Casting-Casting Machine
Illustration of a continuous casting machine chamber
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7. Continuous Casting-Principle
There is an uninterrupted liquid –
solid interface
There is a temperature gradient
along and across the exit of the
casting machine
Solid metal must be extracted from
the end of the die slowly enough so
that the solid-liquid interface is not
fractured
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8. Cast Ingot Microstructure
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9. Metal Purity
Wire Purity Classifications
‘N’ % Metal Use Amount of Dopant (parts
per million by weight)
5 99.999 Au: raw material for doping
Cu: usable as a bonding wire
NA
4 99.99 Doped bonding wire. 100 ppm limit
3 99.9 Micro-alloyed or alloyed
bonding wire depending on
composition.
2 99 Stiffer than other wires,
slightly higher electrical
resistivity. Thinner
intermetallics compared to
higher purity wires.
1% by weight limit for alloy
elements including dopants
(if any)
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10. Dopants – Gold Wire
Element Atomic
Radius (Å)
Resistivity
(µΩ-cm)
Role Effect
Ag 1.44 1.59 Dopant
Be 1.13 4 Dopant Increase in yield and
tensile strength
Ca 1.97 3.43 Dopant Increase in yield and
tensile strength
Ce 1.82 73 Dopant Increase in stiffness
La 1.88 57 Dopant
Y 1.81 57 Dopant
Pd 1.37 9.78 Dopant and alloy element In alloy wires helps
reduce intermetallic
thickness
Pt 1.38 9.6 Dopant and alloy element
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11. Dopants- Copper Wire
¤ Almost any metallic element added to copper causes it to
get harder
¤ This is why 5N copper is often touted as easier to bond – it’s
softer because it’s more pure
¤ Dopants are not normally added to copper wire
¤ Adding P has been considered because P is used in
commercial copper ingots and rods as
¤ An oxygen getter
¤ To improve metal fluidity
¤ P also hardens copper
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12. Doping
How can parts per-million levels of dopants be controlled?
Au
(grams)
Dopant
(grams)
+!
Master
alloy
The master alloy is an alloy of known and controlled
composition
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13. Doping during Casting
Adding gram amounts of master alloy to kg amounts
of gold in the casting furnace allows accurate control
of composition
Au (kg)
Master
alloy
(grams)
+
Au alloy
with ppm
dopant
level
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14. Chemical Analysis of Wire: ICP-OES
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15. Chemical Analysis of Wire: ICP-MS
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16. Sensitivity of ICP
¤ The resolution of ICP varies with the element
Chart source: Evans Analytical Group!
ICP-MS is more
sensitive than ICP-
OES i.e. detection
limits are lower
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17. ICP-OES vs. ICP-MS
ICP-MS has detection limit (DL) capability in the parts per-trillion
(ppt) range, ICP-OES has limited DL in the mid ppt range
Source: Agilent’s ICP-MS primer brochure available online at
http://www.chem.agilent.com/en-US/products/instruments/icp-ms
Method
Metals
Approx.
DL
Range
Advantages
Disadvantages
ICP-­‐MS
Most
metals
and
non-­‐
metals
ppt
Rapid
and
sensi=ve
mul=-­‐element
method
with
wide
dynamic
range
and
good
control
of
interferences
Limited
total
dissolved
solids
(TDS)
tolerance
ICP-­‐OES
Most
metals
and
some
non-­‐
metals
mid
ppb
to
mid
ppm
Rapid
mul=-­‐elemental
method
with
high
TDS
tolerance
Complex
interferences
and
rela=vely
poor
sensi=vity
GFAA
Most
metals
but
commonly
Pb,
Ni,
Cd,
Co,
Cu,
As,
Se
ppt
Sensi=ve,
few
interferences
Single
element
technique
with
limited
dynamic
range
Hydride
AA
Hydride
forming
elements
(As,
Se,
Tl,
Pb,
Bi,
Sb,
Te)
ppt
to
ppb
Sensi=ve,
few
interferences
Single
element,
slow,
complex
Cold
Vapour
Mercury
Hg
ppt
Sensi=ve,
simple,
few
interferences
Single
element,
slow,
complex
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18. Drawing
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19. Block Draw
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20. Wire Drawing
Multiple dies and multiple passes with liquid lubricant to reduce friction
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21. Wire Microstructure
¤ Wire drawing is a plastic
deformation process
¤ Plastic deformation breaks large
grains into small grains
¤ Smaller grains cause the material
to become harder
¤ Breaking grains up also created
defects that harden metals
¤ After drawing it is necessary to
heat treat the metal to soften it
¤ The wire then undergoes final
drawing and annealing
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22. Plastic Deformation during Wire
Drawing
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23. Plastic Deformation
¤ Plastic Deformation occurs during wire manufacturing
and bonding
¤ A very basic treatment is given here of plastic
deformation of
¤ Single Crystals
¤ Polycrystals
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24. Single Crystals
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25. Wire Microstructure-Grains
¤ Each grain is a ‘single’ crystal
¤ E.g. Au wire
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26. Crystal Structure
¤ Atoms are arranged within each grain with a specific geometry
¤ The geometry for Au and Cu is the same: Face Centred Cubic
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27. Selected Properties of Pure Metals
Element Neutral free
atom
diameter
(Angstroms)
Unit cell
dimension
a
(Angstroms)
Solid
density
(g/cm3)
Resistivity
(μΩ-cm)
Melting
point
(°C)
Au 1.44 4.079 19.3 2.05 1064
Cu 1.28 3.615 8.96 1.54 1084
Al 1.43 4.05 2.7 2.42 660
27
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28. Crystal Planes & Directions
!
<100> direction and (100) plane
<101> direction and (101) plane
<110> direction and (110) plane
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29. Orientation of Crystals
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30. Anisotropy of Elastic Properties
¤ Solidity Index S S =
3G
4B
Metal Elastic modulus in GPa,
different crystal directions
Bulk
Modulus
(GPa)
Shear
Modulus
(GPa)
G/B S
<111> <110> <100>
Au 115 85 42 171 27.4 0.16 0.12
Cu 75 72 63 138 48 0.35 0.26
Al 191 130 63 75.2 27.8 0.37 0.27
Ir ⎯ ⎯ ⎯ 371 209 0.56 0.42
30
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31. Plastic Deformation
Illustration of a small single crystal under an
applied shear force. Slip is expected in planar
region indicated by the red box
the simplest and smallest distance that
gives rise to permanent deformation
with a distance on the order of
interatomic spacing.
31
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32. Single Crystal Under Tension
τR =
F
A
cosφ cosλ =
F
A
m
Orientation of crystal affects strength
32
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33. Real Data on Single Crystals
0.00 5.00 10.00 15.00 20.00 25.00
Strain 2 ε (%)
0.00
5.00
10.00
15.00
20.00
Stressσ/2(N/mm2)
[100]
m=0.5
Polycrystal
[111]
33
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34. Single Crystals in Tension
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35. Orientation Change with Stress
Tension Compression
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36. Single Slip in a Single Crystal
Single crystal slip on a single slip plane in compression
Plasticity of Micrometer-Scale Single Crystals in Compression. Michael D. Uchic,Paul A. Shade and Dennis M. Dimiduk. Annu.
Rev. Mater. Res. 2009. 39:361–86
36
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37. Dislocations-Defects that Make
Deformation Easier
37
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38. Dislocation Movement and
Microscopic Plastic Deformation
38
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39. Impurities Block Dislocation
Movement
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40. Single Crystal Metal Purity and CRSS
Metal Purity (%) Slip System Critical resolved shear
stress τR
C (MPa)
Au 99.99 {111}<100> 0.9
Cu 99.999 {111}<100> 0.65
99.98 {111}<100> 0.94
Ag 99.999 {111}<100> 0.37
40
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41. Polycrystals
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42. Polycrystal Plastic Deformation
!
Stress
A"polycrystal"is"made"of"single"
crystals"joined"by"grain"boundaries
A"polycrystal"is"made"of"single"
crystals"joined"by"grain"boundaries
Orienta7on"of"each"grain"may"be"
different
Depending"on"orienta7on,"some"crystals"
may"plas7cally"deform"before"others
42
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43. Polycrystal Deformation Behaviour
When single crystals are joined
together as a polycrystal, contact
modifies deformation
Individual grains with different orientation can yield at
different stresses
For example, individual grains may be
differently oriented
43
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44. Polycrystal Deformation Behaviour
Separated, each grain would deform like a single crystal
CO-OPERATIVE deformation occurs due to contact
Joined together, single crystal
behaviour is changed⇓
44
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45. Ashby Model
¤ Ashby suggested that polycrystal deformation can be
broken into steps
1.
M.
F.
Ashby.
Phil.
Mag.
21
(1970)
399.
1. Deform the individual grains (single crystals) so each yields at a
stress given by its orientation
2. Put the grains back together
3. Where there are differences in deformation
(strain) introduce grain boundary dislocations
45
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46. Ashby Model Illustrated
Deform grains by dislocation movement in the grain
Reassemble
Remove overlap with creation
of dislocations at the grain
boundaries
46
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47. Ashby Model Illustrated
Reassemble Remove Overlap
47
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48. Ashby Model Illustrated
Dislocations that cause deformation within the grains are
known as ʻ‘statistically stored dislocationsʼ’ (SSDs)
Because they move through the
grains but can be destroyed by
meeting other dislocations or
they can be randomly trapped
force
48
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49. Ashby Model Illustrated
These types of dislocations are called
‘geometrically necessary dislocations’
Because they are necessary to maintain
co-operative deformation between grains
Creation of these types of dislocations at
grain boundaries can strengthen metals
49
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50. Plastic Deformation SSDs and GNDs
disloca=ons
created
and
moved
inside
grains
all
grains
have
different
orienta=ons
grain
boundaries
can
block
disloca=ons
because
neighbouring
grains
have
different
orienta=ons
50
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51. Plastic Deformation SSDs and GNDs
Trapping dislocations at grain
boundaries requires more stress to
generate more plastic deformation
Movement of dislocations, creation of
new dislocations and dislocation
trapping results in higher strength
51
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52. Dislocations and Strengthening
¤ Dislocations in some metals are created and destroyed
at similar rates
¤ This results in weak strengthening with plastic deformation
¤ In other metals, creation outweighs destruction and
dislocation density increases
¤ Tensile test curves of bonding wires can illustrate this effect
52
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53. Weak and Strong Hardening
At
constant
strain
rate
!σ = Kεγ
Constant
Plastic stress
Strain
Strain hardening index
53
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54. Strain Rate Hardening
¤ Dislocations are generated more rapidly in some
materials by increasing strain rates. e.g. Cu wires
!σ = C εm
Plastic stress
Constant
Strain rate in s-1
Strain rate hardening index
!
54
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55. Final Wire Annealing
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56. Final Annealing
56
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57. Annealing and Recrystallization
Before
After
Intermediate annealing
Post annealing
57
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58. Final wire annealing curve
Change in mechanical properties depends on furnace
temperature as the illustration shows
58
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59. Final Wire Properties
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60. Final Wire Microstructure
Cu generally has a larger grain size
due to more aggressive annealing
Au grains : 300-1000nm
Cu grains : 1000-3000nm
Microstructure can vary across
wires from the centre to the
outside
60
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61. Final Wire Orientation: Au vs. Cu
Au and Cu usually have
the same orientation
after drawing
More aggressive annealing of
Cu wire changes the
orientation from majority <111>
to majority <100>
<111>
<100>
61
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62. Variations in Drawn Microstructure
Variation of microstructure varies across wires changes
mechanical properties
G.
A.
Ber=,
M.
Mon=,
M.
Bietresato,
L.
D’Angelo.
Proc.
NUMIFORM
’07;
Materials
Processing
and
Design:
Modelling,
Simula=on
and
Applica=ons.
American
Ins=tute
of
Physics
(2007).
∅275µm
∅210µm
∅175µm100µm
137.5µm
50µm 20µm
62
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63. Elastic/Plastic Behaviour: Size Effects
The larger the grain size relative
to sample diameter the more
surface grains influence
deformation
Surface may grains deform
like single crystals and inner
grains like polycrystals
G.
Kim,
J.
Ni,
M.
Koç.
J.
Manuf.
Sci.
Eng.
129
(2007)
470.
U.
Engel,
R.
Eckstein.
J.
Mater.
Process.
Technol.
125
(2002)
2245.
63
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64. Wire Grain Size
Au
Cu
64
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65. Mechanical Properties of Finished
Wire
4N Au : flat - small stress required to cause plastic strain
T.
Saraswa=,
Ei
Phyu
Phyu
Theint,
D.
Stephan,
H.
M.
Goh,
E.
Pasamanero,
D.
R.
M.
Calpito,
F.
W.
Wulff,
C.
D.
Breach.
‘High
Temperature
Storage
(HTS)
Performance
of
Copper
Ball
Bonding
Wires’.
Proceedings
of
EPTC
2005
(Electronics
Packaging
and
Technology
Conference),
Grand
Copthorne
Waterfront
Hotel,
Dec
7-­‐9,
Singapore
2005.
5N Au and Cu:
steeper curves show
that stress required to
cause further
elongation increases
as the wire is further
strained
65
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66. Representative Wire Properties
Wire Type
Elastic Modulus
(GPa)
Yield Stress
(MPa)
Ultimate Tensile
Stress (MPa)
Cu wire A
88
172
254
Cu wire B
80
123
212
Cu wire C
96
136
238
Cu wire D
93
98
210
4N Gold
90
190
228
5N Gold
53
48
120
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67. Work or Strain Hardening
Slope of the plastic region of the curve is often described by
!σ = Kεγ
Wire
Yield
strength
(MPa)
UTS
(MPa)
EL
(%)
Strain
hardening
index
γ
K
(MPa)
5N
Au
75
118
4.2
0.18
219
4N
Au
190
228
4.4
0.06
281
Cu
175
250
12
0.15
380
Constant
Plastic stress
Strain
Strain hardening index
67
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68. Strain Rate Hardening
Straining materials at higher speeds can also cause hardening
Higher speeds increase the rate at which dislocations are created
in some materials and plastic stresses increase
!σ = C εm
Plastic stress
Constant
Strain rate in s-1
Strain rate hardening index
68
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69. Strain-Rate Hardening
Tensile tests can be used to measure strain rate sensitivity
Strain rate hardening and sensitivity indices of Cu and Au
bonding wires measured from tensile tests
Wire
Type
Strain
hardening
index
γ
Strain
rate
sensi=vity
m
Cu
wire
1
0.14
0.021
Cu
wire
2
0.25
0.023
Cu
wire
3
0.26
0.018
Cu
wire
4
0.36
0.019
4N
Au
0.06
0.006
The disadvantage of tensile tests is the small range of strain rates
But the results give some idea of the different material
behaviour
69
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70. Strain and Strain Rate Hardening
Wires harden when deformed
Strain
Hardening
Strain
Rate
Hardening
!
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71. Copper Wire Purity & Strength
N Srikanth, J. Premkumar, M. Sivakumar, Y. M. Wong, C. J. Vath III, 9th EPTC 2007, Singapore
71
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72. Effects of Copper Wire Purity
Higher purity leads to
larger grains
Random orientation
N Srikanth, J. Premkumar, M. Sivakumar, Y. M. Wong, C. J. Vath III, 9th EPTC 2007, Singapore
72
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73. Final Wire Properties
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74. Metal Coated Bonding Wires
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75. Coated Wires: Metal Coated Cu
Alternative is to plate Au or Pd on Cu
Au Drill hole Insert Cu rod
Draw
Pd Drill hole Insert Cu rod
Draw
Coating uniformity can be a problem
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76. Coated Wires: Au coated Cu
Spear shaped FABs due to difference in melting points
Au (1064℃ melting)
Cu (1083℃ melting)
Only good for wedge bonding
76
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77. Coated Wires: Pd Coated Cu
Uno et al, ECTC 2009 p1486
77
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78. Metal Coated Wires
Kaimori et al, IEEE Trans Advanced Packaging 29 (2006) 227
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