A Journey Into the Emotions of Software Developers
The Materials Science of Ballbonding
1. The Materials Science of Ballbonding
AB i fO
Brief Overview
i
1Christopher Breach & 2Frank Wulff
Christopher Breach & Frank Wulff
10th EPTC 2008 Dec 9‐12th, Grand Copthorne Watefront Hotel, Singapore
1 Vice President, Wire Bond Strategic Business Unit, Oerlikon Esec Assembly Equipment Pte Ltd, 1
Science Park Road, Singapore Science Park 2, #03‐10 Capricorn Building, Singapore 117528
2Materials Characterization Consultant Finnentrop Wilhelm Busch Straβe 2 Germany
Consultant, Finnentrop, Wilhelm‐Busch‐Straβe 2,
3. Ball – wedge bonding is a materials joining
process
Ballbonds and wedge bonds are solid state welds between bonding wires
and metallization on microelectronic chips and substrates respectively
Wirebonds provide electrical conduction from chip to substrate
Cu ballbond on Al – 1%Si – Cu wedge bond on Au plated
0.5% Cu Al bondpad Cu lead
Cu wire
Au plated
Cu lead
Wedge bond on BGA Substrate
Ballbond at Chip
Page 3
with Au leads
4. These are illustrations of the ball –wedge
bonding process
Wire clamps closed
Ultrasonic vibrations
FAB formation
Wire clamps open scrub ball against
Impact of ball
FAB descends towards bondpad
with bondpad
bondpad Vertical force deforms ball
The process begins with ball
The process begins with ball
formation and then bond formation
Capillary moves upwards Loop formation complete
and start to form and shape Formation of 2nd bond
the loop
Formation of the loop is next
Tail bond formation Tail bond termination FAB formation
And finally the wedge bond
is formed
Page 4
5. The physics and chemistry of the
ballbonding process are not well known
Elevated temperature enhances the process but welds can be
made at very low temperature
dt l t t
suggests thermal and athermal contributions
The microscopic mechanism is not really understood
There are several macroscopic or phenomenological theories
There are several macroscopic or phenomenological theories
of how it occurs
There are some microscopic theories
8. Theories of thermosonic bonding /3
Microscopic Theories
Interfacial plastic deformation without slip at the interface
p p
defect generation, point defects, dislocations, enhanced diffusion
try to explain ultrasonic and/or thermosonic welding
Interface plastic deformation due to sliding
generates mixing via dislocations
tries to explain new phase formation during cryomilling, sliding
friction, fretting
Alternative
Athermal and thermal contributions?
Defect generation and mixing at low T?
Defect generation and mixing at low T?
Thermal (diffusive) contribution as T increases?
9. Experimental characterization /1
Temperature rise at the ball – substrate interface
From Tsujino et al
F T ji tl
Temperature measurements
Show 80‐100°C temperature rise at
short times*, †
short times
Show very high temperature rise at
much longer times‡
much longer times
Suggests that at a bonding
temperature of 170°C, diffusion is
p ,
important
* K. C. Joshi. Welding Journal 50 (1971) 840.
†A. Schneuwly, P. Gröning, L. Schlapbach, G. Müller. J. Electronic Materials. 27 (1998) 1254.
‡J. Tsujino, H. Yoshihara, K. Kamimoto, Y. Osada. Ultrasonics 36 (1998) 59.
11. The Role of Ultrasonics / 1
Ultrasonics soften metals significantly
Higher ultrasonics with same bond force
Critical Strain Amplitude results in flatter balls – ultrasonic softening
Metals may deform under ultrasonic
radiation if the ultrasonic amplitude
radiation if the ultrasonic amplitude
exceeds the critical stress amplitude
Deformation is due to resonance and
movement of dislocations
Dislocations resonate in the kHz to MHz
range
16. An Alternative View / 4
Low Temperature (even sub‐ambient) Bonding
Plastic deformation by interfacial shear/mixing
dominant, athermal
Defect generation by ultrasonics
Moderate Temperature
Same mechanism
Thermal component and enhanced diffusivity due to
Thermal component and enhanced diffusivity due to
defect generation
High Temperature
High Temperature
Thermal component dominates
17. Capillary geometry influences metal
flow and hardening
The capillary causes metal flow degree
Work hardening occurs and depends
on capillary geometry and material
properties
20. Intermetallics formed between Al and
Au or Al and Cu result in a weld
The processes described in the previous section leads to the
formation of a new phase
formation of a new phase
The new phase joins the wire to the bondpad
Intermetallic coverage is a term used to describe the amount of
new phase formed during bonding
It strictly refers to phases of specific chemistry with narrow / no
solubility range
Intermetallics are typically strong and brittle
Not all wires form intermetallics – Au on Pd or Ni metal forms a
Not all wires form intermetallics Au on Pd or Ni metal forms a
random alloy
Page 20
23. This is an example of a poor
wedge bond
Page 23
Capillary drawing courtesy of Jimmy Castaneda & Mary Ong of SPT Singapore
24. The wire properties results from the wire
drawing process
The wire drawing process for Cu and
Au wires is essentially the same
There are intermediate heat
treatments to soften the wire
There is a final annealing treatment
to control the final properties of the
wire
Page 24
Drawing from Aristo Tec http://www.aristo‐tec.com
25. Annealing changes the grain size and
orientation
Au Wire ‐ Annealed
Au Wire ‐ Drawn
Page 25
From F. Wulff, C. D. Breach, K. Dittmer, J. Mater. Sci. Lett., 22(19) 1373 (2003).
26. Different Au wire types show strong
<111> texture
Type B Au Wire
Type A Au Wire
Double‐fibre texture
<111> dark
<100> bright
<100> bright
Page 26
From F. Wulff, C. D. Breach, K. Dittmer, J. Mater. Sci. Lett., 22(19) 1373 (2003).
27. Gold and copper wires are made
up of many grains
There are significant differences in
Cu and Au wire grain size
Au wires show strong <111> texture
along the wire axis
Grain size is very fine, 0.1‐1µm
Cu wires show strong <100>
Cu wires show strong <100>
alignment along the wire axis
Grain size is >1µm
Grain size is >1µm
Images from Saraswati; Ei Phyu Phyu Theint; D. Stephan, H. M. Goh, E. Pasamanero, D. Calpito,
F. Wulff, C. D. Breach. Proceedings of 7th Electronic Packaging Technology Conference, 2005,
Page 27
2, 7-9 Dec. 2005.
28. Wedge bond shape and consistency is
affected by the way the grains deform
Metals usually consist of grains
Each grain is a single crystal
The deformation behaviour of
The deformation behaviour of
polycrystalline metals depends on the
deformation of the individual grains
Polycrystalline deformation is not
simply an average of single crystal
deformation
Schmid Factor (single crystal) and Taylor
Factor (polycrystal)
Factor (polycrystal)
Page 28
Drawing from P. Haasen. Physical Metallurgy. Cambridge University Press (1996).
29. Orientation of Au wire in
transverse direction can vary
There is <111> orientation along the
wire axis
Perpendicular to the wire axis, the
orientation of individual grains can
vary
Page 29
Capillary drawing courtesy of Jimmy Castaneda & Mary Ong of SPT Singapore
30. Orientation of Cu wire in
transverse direction can vary
There is <100> orientation along the
wire axis
Perpendicular to the wire axis, the
orientation of individual grains can
vary
Page 30
33. Orientation of single grains affects
the mechanical response
The orientation of a single crystal
affects the load required to cause
deformation
Schmid Factor
τR
= cos φ cos λ
σ
Cu and Au are FCC metals
They deform by slip along {111}
planes in [110] directions
Page 33
34. Identical single grains of metal: different
strength with orientation
Graph of flow stress versus
Graph of flow stress versus
single crystal orientation for
copper showing how the flow
stress depends on slip plane
t d d li l
orientation
Page 34
Drawing from P. Haasen. Physical Metallurgy. Cambridge University Press (1996).
35. Cu wire wedge bonds can vary in dimensions
compared with Au wedge bonds
The variations can be due to grain size
and orientation
In Cu wires a smaller number of larger
grains carry the plastic deformation
Variations in transverse orientation of
the grains can affect the amount of slip
In addition, there is the wire direction
relative to the capillary bond force
Ultrasonic are also superimposed on
the bond force
Page 35
Drawing from P. Haasen. Physical Metallurgy. Cambridge University Press (1996).
36. Consider an example of just one wire
direction with Au
Distribution of orientations in
the transverse direction
The average deformation is the
summation over a large
number of small grains
Page 36
37. Consider an example of just one wire
direction with Cu
Distribution of orientations in
the transverse direction
The average deformation is the
summation over a small
number of large grains
number of large grains
The orientation of the large
grains may have more influence
on the amount of plastic
deformation
Therefore more influence on
final dimension
Page 37
38. Premature termination of the 2nd
bond is known as ‘tailing’
Au and Cu respond differently
From Calpito DRM, Alcala D, Tirtonady A. Tail lift off solutions for fine Image/drawing courtesy of Jimmy Castaneda & Mary Ong of SPT
Page 38
pitch applications. Semicon Singapore 2004. Singapore
39. Tailing may also be due to the
deformation behaviour of wire
As in compressive deformation
Au and Cu respond differently
Au and Cu respond differently
in tensile deformation
Page 39
42. Intermetallic growth in ballbonds is
rarely considered in detail
Ballbonds are diffusion couples
semi‐infinite due to the large amount of Au and finite
amount of Al
Common assumptions are that
Every void is a Kirkendall void
Every void is a Kirkendall void
Al diffuses faster than Au
Page 42
43. When a random alloy forms there is a large
concentration gradient
Diffusion of either species can occur by
vacancies
i
Vacancies not tied to a particular species
Diffusion driven by concentration gradient
more strictly, a chemical
potential gradient
potential gradient
Page 43
44. When an intermediate compound forms
there is a small concentration gradient
Chemical compounds AXBY
Some solubility of A in B or vice versa
Overall chemistry must be maintained
O ll h i t tb iti d
Elements form individual lattices with
their own vacancies
their own vacancies
Diffusion of ‘wrong’ atom on ‘wrong’
lattice can occur with limitations
lattice can occur with limitations
Page 44
45. When an line compound forms there is a
effectively no concentration gradient
Chemical compounds AXBY
Limited/no solubility of A in B or vice
versa
Overall chemistry must be maintained
Elements form individual lattices with
Elements form individual lattices with
their own vacancies
⎡ ∂ ln γ i ⎤ ⎡ ∂ ln ai ⎤
Di = Di* ⎢1 + = Di* ⎢ = Di* Φ
⎥ ⎥
Diffusion of wrong atom on wrong
Diffusion of ‘wrong’ atom on ‘wrong’
∂ ln Ni ⎦ ∂ ln Ni ⎦
⎣ ⎣
lattice difficult without disrupting local
chemistry
⎡ ∂ ln aA ⎤
D = (NAD + NBD ) ⎢
* *
⎥
∂ ln NA ⎦
B A
Driving force for diffusion is activity
⎣
gradient
Page 45
47. Effective heat of formation is an
indicator of which phases may form
Gold Aluminides Copper Aluminides
Effective Heat of Effective Heat of Formation
Compound Compound
Formation (kJ/g at) (kJ/g at)
Au4Al ‐18.5 Cu9Al4 ‐3.20
Au8Al3 ‐20 Cu3Al2 ‐4.25
Au2Al ‐19.8 Cu4Al3 ‐4.77
AuAl ‐16.3 CuAl ‐5.44
AuAl2 ‐10.2 CuAl2 ‐6.13
48. General features of intermetallic growth / 1
The first product layer forms and
p y
becomes a diffusion barrier
Further growth of the compound occurs
at two interfaces
Different elements diffuse to each
interface to form the same compound
i f f h d
With line compounds elements cannot
diffuse freely – diffusion occurs on the
diffuse freely diffusion occurs on the
lattice of each element
Page 48
54. Comparing Cu – Al with Au – Al compounds / 2
Charge carriers in Cu – Al intermetallics are mainly holes
Hall Co‐efficient (x 10‐11 m3 A‐1 s‐1)
Common
Phase Charge Carrier
notation
Cu with minor Al ≈‐5 Electrons
Cu9Al4 γ2 ≈0 – 10 Holes
generally positive and varies with
composition
Cu3Al2 δ ≈10 Holes
Cu4Al3 ξ2 ≈7 Holes
CuAl η2 ≈5 Holes
CuAl2 θ ≈‐1 Electrons (nearly free)
Al with minor Cu ≈‐2 Electrons
55. Are all voids Kirkendall voids?
96 hrs @ 175°C
These are well aged ballbonds Au
ballbonds on Al
The Al bondpad is consumed
The voids cannot be Kirkendall voids
200 hrs @ 175°C
Page 55
56. Voids can also be caused by stress
Stress gradients can change the thermodynamic driving force for diffusion
Thermodynamic factor
Stresses can alter atomic mobilities
S l i bili i
Diffusion tends to occur from regions of compressive to regions of
tensile stress
Stresses can be due to different specific volumes of growing phases
Au4 Al(2)
()
Au4 Al(1)
Au8 Al3
Gold ball
AuAl2
Si Chip
Si Chi
Page 56
Voids due to shear
Shear stresses
58. In Summary……………
Some largely unproven ideas regarding wirebond materials
science have been presented
science have been presented
The microscopic mechanism of welding of the ball and wire to bondpad or
leads is considered to be governed by plastic deformation processes
leads is considered to be governed by plastic deformation processes
Wire properties are suggested to have an influence on wedge bond
consistency and short tail behaviour, texture influences the behaviour
y
Intermetallic growth is viewed differently, more in line with solid state
materials physics and chemistry
Al does not necessarily diffuse faster than Au in gold ballbonds
Structure of Cu – Al compounds may account for the slow
intermetallic growth
it t lli th
