Basics of Bonding Wire Manufacturing

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

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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Manufacturing Processes
3
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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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Continuous Casting-Casting Machine
Illustration of a continuous casting machine chamber
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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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Cast Ingot Microstructure
8
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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)
9
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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
10
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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
11
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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
12
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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
13
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Chemical Analysis of Wire: ICP-OES
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Chemical Analysis of Wire: ICP-MS
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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
16
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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

17
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Wire Drawing
Multiple dies and multiple passes with liquid lubricant to reduce friction
20
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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
21
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Plastic Deformation during Wire
Drawing
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Plastic Deformation
¤  Plastic Deformation occurs during wire manufacturing
and bonding
¤  A very basic treatment is given here of plastic
deformation of
¤  Single Crystals
¤  Polycrystals
23
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Wire Microstructure-Grains
¤  Each grain is a ‘single’ crystal
¤  E.g. Au wire
25
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Crystal Structure
¤  Atoms are arranged within each grain with a specific geometry
¤  The geometry for Au and Cu is the same: Face Centred Cubic
26
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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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Crystal Planes & Directions
!
<100> direction and (100) plane
28
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Orientation of Crystals
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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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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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Single Crystal Under Tension
τR =
F
A
cosφ cosλ =
F
A
m
Orientation of crystal affects strength
32
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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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Single Crystals in Tension
34
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Orientation Change with Stress
Tension Compression
35
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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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Dislocations-Defects that Make
Deformation Easier
37
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Dislocation Movement and
Microscopic Plastic Deformation
38
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Impurities Block Dislocation
Movement
39
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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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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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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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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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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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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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Reassemble Remove Overlap
47
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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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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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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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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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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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Weak and Strong Hardening
At
constant
strain
rate

!σ = Kεγ
Constant
Plastic stress
Strain
Strain hardening index
53
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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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Annealing and Recrystallization
Before
After
Intermediate annealing
Post annealing
57
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Final wire annealing curve
Change in mechanical properties depends on furnace
temperature as the illustration shows
58
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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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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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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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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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Wire Grain Size 
 
Au
Cu
64
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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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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

66
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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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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
15/9/11

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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Strain and Strain Rate Hardening
Wires harden when deformed
Strain
Hardening
Strain
Rate
Hardening

!
70
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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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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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Metal Coated Bonding Wires
74
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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
75
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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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Coated Wires: Pd Coated Cu
Uno et al, ECTC 2009 p1486
77
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Metal Coated Wires
Kaimori et al, IEEE Trans Advanced Packaging 29 (2006) 227
78
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Basics of Bonding Wire Manufacturing

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