Wirebonding  Tutorial

Jonathan  Harris  CMC  Laboratories,  Inc.

Basic  Wirebonding  Mechanism

Bond  formation  process • Material  from  bonding  wire  and  

pad  material  in  intimate  contact • Apply  energy:  thermal,  

mechanical  (ultra-­‐sonic) • A  bond  is  formed  between  the  

two  materials

Types  of  Bonds  that  Form  in  Wirebonding

Type  of  Bond Example  Systems Comments

Intermetallic  compounds  (IMC)  between  the  wire  and  pad  form  bond

Au/Al,  Al/Ni,  Cu/Al,  (AuAg)/Al

Distinct  interfacial  layer.  Properties  of  that  layer  determine  bond  reliability.  

Diffusion  bond  forms  where  wire  material  and  pad  material  inter-­‐diffuse.  

Au/Ag,  (AuAg)/Au No  distinct  interface.  Bonding  layer  is  diffuse  extending  into  both  the  pad  and  the  wire.  This  type  of  bond  is  characterized  by  very  high  reliability.  

Atomic  welding-­‐  direct  atom  to  atom  bonding  at  microscopically  clean  interfaces

Au/Au No  distinct  interface  but  narrower  distribution  than  diffusion  bond.  Characterized  by  very  high  reliability.

Diffusion  Bonds

Basic  Wirebonding  Mechanism

What  is  the  bonding  layer? • If  the  wire  and  pad  are  the  same  (Au/

Au)  or  very  similar  (Au/Ag)  material–  a  diffusion  bond  forms

• Diffuse  region  where  atoms  from  the  two  materials  are  mixed.  No  distinct  interface

Region  where  Au  from  the  ball  and  Au  from  the  pad  are  mixed  together–  called  a  diffusion  bond

Wirebonding  with  Ag  and  Au

Properties  of  Ag

• Nobility  (tendency  to  oxidize)  Ag  is  in  between  Ni  and  Au  

• Very  ductile  •  Unlike  Cu,  Ag  will  not  work  harden  •  Forms  a  “continuous  solid  solution”  with  Au  • Ag  does  form  a  sulfide  (Ag2S)  

–  Sulfur  from  air  pollution  –  Sulfide  layer  acts  like  an  oxide–  will  inhibit  wirebond  formation  

– Some  organic  additives  or  coatings  inhibit  sulfide  formation  and  do  not  effect  wirebonding

Au/Ag  Solid  Solution• Au  and  silver  atoms  are  

similar  in  size  •  Au  and  Ag  have  the  same  

crystal  structure  (FCC)

Au/Ag  Solid  Solution• Au  and  silver  atoms  are  

similar  in  size  •  Au  and  Ag  have  the  same  

crystal  structure  (FCC)  • Continuously  soluble  in  

each  other  • No  intermetallic  

compounds  are  formed

Au/Ag  Solid  Solution• Au  and  Ag  atoms  are  similar  in  

size  •  Au  and  Ag  have  the  same  

crystal  structure  (FCC)  • Continuously  soluble  in  each  

other  • No  intermetallic  compounds  are  

formed  • Au  and  Ag  in  contact–  Au  will  

diffuse  into  the  Ag  layer  and  the  Ag  will  diffuse  into  the  Au  layer  

• Distinct  interface  between  the  two  layers  disappears  

• Eventually  one  layer  is  formed  with  Au  and  Ag  evenly  distributed

Au Pad Silver Wire

Diffuse Bond extends into pad and wire Silver in pad Au in wire

Au  Wire  on  Ag  Pad

• Very  common  type  of  wirebond  for  power  applications  

• Au  ball  bond  on  Al  (device)  followed  by  secondary  Au  stitch  bond  on  a  Ag  plated  lead-­‐frame

IMC  Bonding  Mechanism

Basic  Wirebonding  MechanismWhat  is  the  bonding  layer? • Bond  occurs  by  forming  a  distinct  

reacted  layer  between  the  wire  and  pad

• Atomic  constituents  to  form  this  layer  comes  from  both  materials

• Reacted  layer(s)  can  be  one  or  more  different  compounds

• These  layers  are  called  Inter-­‐metallic  Compounds  or  IMCs

• The  IMCs  form  the  bond  between  the  pad  and  wire

• Details  of  the  IMC  composition  and  morphology  determine  the  bond  reliabilityAl

Region  where  Au  from  the  ball  and  Al  from  the  pad  have  reacted  to  form  a  AuAl  IMC  compound(s)

Reacted  Intermetallic  Compound  (IMC)  Bonding  Layer

Cu Wire Wire

Cu WireCuAl IMC bonding Layer formed by reaction and inter-diffusion

Al Bond Pad

Bonding Layer

F. W. Wulff, C. D. Breach, D. Stephan, Saraswati and K.J. Dittmer Materials & Applications Centre Kulicke & Soffa (S.E.A.) Pte. Ltd #04-05

TECHplace II Block 5002 Ang Mo Kio Ave 5 Singapore 569871

Key  Characteristics  for  IMC  in  Wirebonding

• The  IMC  is  the  bonding  phase  so  the  presence  of  an  IMC  layer  is  required  to  form  a  bond  for  some  material  systems  (Ni/Al,  Au/Al,  Cu/Al)  

• Ideal  characteristics  for  wirebonding  IMC:  –  High  coverage  of  the  interface  (60%  minimum  for  a  reliable  bond)  

–  Thin    •  Key  consideration:  IMC  thickness  increases  with  subsequent  temperature  exposure  –  Device  burn-­‐in  –  Encapsulation  material  curing

Why  is  a  Thin  IMC  Preferred

• IMC  are  mechanically  brittle–  so  thicker  IMC  layers  can  fracture  

• IMC  materials  have  high  electrical  resistance–  so  a  thick  IMC  can  add  electrical  resistance  to  the  wirebond  

•  As  IMCs  grow  thicker–  vacancies,  impurities  and  other  defects  coalesce  to  form  bulk  defects  such  as  voids  or  cracks  which  decrease  bond  reliability  

Au  and  Cu  wirebonding  to  Al  Pads

Ball  Bonding  Process  with  Cu

• Ball  formed  in  a  plume  of  N2/H2.    – N2  prevents  oxidation  of  Cu  during  EFO  by  flushing  away  air  – Small  amount  of  H2  will  reduce  Cu2O  à Cu  metal:  Cu2O  +  H2 à

H2O  +  2Cu  

•  Impact  of  Cu2O  formation  on  ball  surface:  –  Cu2O  will  prevent  bond  formation=  no  sticks  –  Cu2O  will  diffuse  into  Cu  making  it  harder  =  cratering  of  substrate  

(discussed  later)  • Ball  bonding  parameters  such  as  Bond  Force,  Ultrasonic  energy  

must  be  optimized  for  Cu  in  a  DOE  

Impact  of  Cu  Ball  Formation  Atmosphere

Au  and  Cu  Wirebonding  to  Al  Pads

Al Pad

Cu Wire Au Wire Au  wire  on  Al  Pad

AuAu

Au

AuAl

AuAl

Cu  wire  on  Al  Pad

CuAlCuAl

Cu

Cu

Cu

Both  systems  can  form  5  different  IMC  compounds  with  Al  Metal

IMC  Formation  Reactions-­‐  Heat  of  Formation  of  IMC

•  –ΔH  for  a  reaction  means  the  reaction  will  occur  (exothermic–  heat  is  given  off)  

•  Example:  oxidation  of  a  metal  like  Cu  or  Ti  has    –ΔH    • Example  of  a  wirebonding  reaction:  4Au  (wire)  +  1Al  (pad)  à

Au4Al    –ΔH  =  18.5J/gram.  AuAl  IMC  formation  is  very  favorable.  •  18.5  J  of  energy  are  given  off  for  each  gram  of  Au4Al  that  is  created  •  Magnitude  of    –ΔH  is  a  measure  of  how  favorable  a  reaction  will  be  

–  Large    –ΔH  means  very  favorable  –  Small    –ΔH    means  it  will  occur  but  not  very  aggressively  –    +ΔH  means  the  reaction  will  not  occur  unless  external  heat  or  energy  is  

applied

Au  and  Cu  Wirebonding  to  Al  Pads

Al Pad

Cu Wire Au Wire Au  wire  on  Al  

Δ(J/g)  

Au -­‐18.5

Au -­‐20

Au -­‐19.8

AuAl -­‐16.3

AuAl -­‐10.2

Cu  wire  on  Al  

ΔH  (J/g)

CuAl -­‐6.13

CuAl -­‐5.44

Cu -­‐4.77

Cu -­‐4.25

Cu -­‐3.2

-­‐ΔH  is  much  larger  for  Au-­‐Al  IMC  compared  to  Cu-­‐Al  IMC  so  AuAl  formation  is  highly  favorable  compared  to  Cu-­‐Al  IMC  formation.

Au  and  Cu  Wirebonding  to  Al  Pads As-­‐Bonded  IMC  Thickness

Al Pad

Cu Wire Au Wire

As bonded IMC layer is much thinner for Cu/Al system compared to Au/Al

Growth  of  IMC  Thickness  at  Elevated  Temperatures

• Two  key  factors  •  Favorability  of  reaction  (magnitude  of  –ΔH  )-­‐  chemical  species  that  are  reacting  

• Diffusion  kinetics  of  wire  material  and  bond  pad  material  through  the  IMC  layer-­‐  specific  composition  and  morphology  of  the  IMC  layer

Comparison  of  IMC  Growth  on  Al  Pads:  Au  and  Cu

• Aging  time  of  5  hours • IMC  thickness  vs.  temperature •  AuAl  lMC  layer  is  much  thicker  than  

CuAl  at  same  temperature • At  350C,  CuAl  is  8µm,  AuAl  is  >45µm

Cu and Al diffusion through CuAl compounds are much slower than Au and Al diffusion through AuAl compounds.

IMC  thickness  vs.  time  for  fixed  temperature

Deley et al, Semicon Singapore 2005

Cross  Section  of  Cu  Wirebond  after  HTS—Very  Thin  IMC  Layer

250C 200 hrs

250C 1000 hrs

Au/Al  Wire  Bond  Microstructure

Au Wire

Au-Al IMC Phases

Al Pad

Karpel et al.Adi Karpel, Giyora Gur, Ziv Atzmon, and Wayne D. Kaplan Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel Kulicke & Soffa Bonding Tools, Yokneam Elite, 20692 Israel

Au/Al  Microstructural  Changes  During  HTS

2 hours at 175C Isolated Voids

Center Region of Wire Bond

Karpel et al.Adi Karpel, Giyora Gur, Ziv Atzmon, and Wayne D. Kaplan Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel Kulicke & Soffa Bonding Tools, Yokneam Elite, 20692 Israel

Au/Al  Microstructural  Changes  During  HTS

24.hours at 175C Line of voids has formed at the interface. Some voids are large.

Center Region of Wire Bond

Karpel et al.Adi Karpel, Giyora Gur, Ziv Atzmon, and Wayne D. Kaplan Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel Kulicke & Soffa Bonding Tools, Yokneam Elite, 20692 Israel

Au/Al  Microstructural  Changes  During  HTS

24.hours at 175. Voids have coalesced to form cracks which become initiation sites for wirebond failure

Edge Region of Wire Bond

Adi Karpel, Giyora Gur, Ziv Atzmon, and Wayne D. Kaplan Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel Kulicke & Soffa Bonding Tools, Yokneam Elite, 20692 Israel

Au/Al  Microstructural  Changes  During  HTS

HTS à AuAl IMC growth à void formation à Void coalesce à crack formation à potential for wirebond failure

Adi Karpel, Giyora Gur, Ziv Atzmon, and Wayne D. Kaplan Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel Kulicke & Soffa Bonding Tools, Yokneam Elite, 20692 Israel

Au  on  Al  Wirebond  Reliability77% IMC coverage. Voids only at edges of wire bond. Shear strength of 6.5 grams/mil2 . Wire pull of 10 grams with neck failure.

63% IMC coverage. Small voids at center and larger voids at edges of wire bond. Shear strength of 5.5 grams/mil2. Wire pull of 10 grams and neck failure.

34% IMC coverage. Voids throughout the interface region. Shear strength <4.5 grams/mil2. Wire pull of 4 grams with lift-off failure mode.

Jamin  Ling,  Ziv  Atzmon,  Dominik  Stephan,  Murali  Sarangapani      Kulicke  &  Soffa,    Semicon  Singapore  2008

Au  on  Al  Wirebond  Reliability

Osborne, M., J.Ling, et al., IMAPS Dev Pkg Conf, March 2005

Au  on  Al  Wirebond  Reliability77% IMC coverage. Voids only at edges of wire bond. Shear strength of 6.5 grams/mil2 . Wire pull of 10 grams with neck failure.

63% IMC coverage. Small voids at center and larger voids at edges of wire bond. Shear strength of 5.5 grams/mil2. Wire pull of 10 grams and neck failure.

34% IMC coverage. Voids throughout the interface region. Shear strength <4.5 grams/mil2. Wire pull of 4 grams with lift-off failure mode.

Because AuAl IMC grows thick with time/temperature, it is critical to have high IMC coverage as-bonded to maximize bond reliability

Jamin  Ling,  Ziv  Atzmon,  Dominik  Stephan,  Murali  Sarangapani      Kulicke  &  Soffa,    Semicon  Singapore  2008

Summary  of  IMC  Growth  for  Cu  on  Al

• Initial  IMC  is  extremely  thin  • Growth  rate  of  CuAl  IMC  during  high  temperature  storage  is  much  slower  than  AuAl  

•  Thin  IMC  should  provide  a  more  robust  and  reliable  wirebond

Summary  of  IMC  Growth  for  Au  on  Al

• AuAl  IMC  grows  more  rapidly  than  CuAl  IMC  • Thick  AuAl  IMC  results  in  a  tendency  to  form  large  voids  and  cracks  at  the  Au  wire/  pad  interface  

• High  coverage  initial  bond  is  critical  to  maximize  IMC  reliability

Another  Key  Consideration–  Mechanical  Properties  of  Au  and  Cu  Wirebonds  on  Al

Concept  of  Work  Hardening

• When  metals  are  exposed  to  repeated  mechanical  stress,  they  become  harder  

• This  is  why  you  can  break  a  ductile  copper  wire  by  flexing  it  back  and  forth  many  times–  the  flex  point  becomes  hard  and  brittle  and  the  “ductile”  wire  will  crack  

• Deformation  and  ultrasonic  energy  during  wirebonding  will  significantly  work  harden  the  wire    

• Cu  is  harder  than  Au  • Cu  will  work  harden  more  than  Au

Work  Hardening  in  a  Ball  Bond-­‐  FEA• FEA  simulation  of  wirebond  process  

showing  expected  high  stress  areas  (susceptible  to  work  hardening)

• More  metal  stress  on  edges  of  ball  where  more  deformation  of  the  metal  occurs  during  bonding

Hong Meng Ho1*, ,Yee Chen Tan1, Wee Chong Tan3, Heng Mui Goh2, Boon Hoe Toh1, Jonathan Tan1, and Zhao Wei Zhong3 1Kulicke & Soffa Pte. Ltd, 2Kulicke & Soffa (S.E.A.) Pte. Ltd, 3Nanyang Technological University * 6, Serangoon North Ave 5, #03-16, Singapore 554910

Micro-­‐hardness  Analysis  of  Au  and  Cu  Wire  after  Bonding

• Micro-­‐hardness  for  Au  and  Cu  ball  bonds  vs.  position

• Much  higher  hardness  for  Cu  vs.  Au

•  Work  hardening  for  both  Cu  and  Au  on  edges  of  ball  as  predicted  by  the  FEA  stress  model

• Degree  of  work  hardening  is  much  larger  for  Cu  compared  to  Au

• At  the  edge  of  the  Cu  ball,  hardness  of  130  Hv–  compared  to  75-­‐80  Hv  for    Au

Hong Meng Ho1*, ,Yee Chen Tan1, Wee Chong Tan3, Heng Mui Goh2, Boon Hoe Toh1, Jonathan Tan1, and Zhao Wei Zhong3 1Kulicke & Soffa Pte. Ltd, 2Kulicke & Soffa (S.E.A.) Pte. Ltd, 3Nanyang Technological University * 6, Serangoon North Ave 5, #03-16, Singapore 554910

Cu  Hardness  Implications

• More  stress  is  transmitted  to  the  underlying  substrate  during  bonding  for  harder  wire    –  More  stress  for  Cu  vs.  Au  – More  stress  for  heavily  work  hardened  Cu  compared  to  less  work  hardened  Cu  

•  Can  result  in  fracture  of  the  underlying  substrate  during  bonding  (cratering)  

•  Can  result  in  micro-­‐cracking  of  the  substrate  during  bonding  which  subsequently  fractures  during  post  bond  stress  (such  as  temperature  exposure)

Crater  in  a  Si  Substrate  from  Cu  Ball  Bond

Cu  Wirebonding  Summary

• Cu  wire  on  Al  pads  form  an  ideal  IMC  structure  –  Very  thin  –  Very  slow  growth  during  temperature  exposure  –  This  is  due  to  the  fundamental  chemistry  of  the  Cu/Al  reaction  (small  –ΔH,  slow  diffusion  of  Cu  and  Al  through  the  IMC)  

–  Cu/Al  wirebonds  rarely  fail  due  to  defective  IMC  structure  which  is  typical  of  Au/Al  bonds

Cu  Wirebonding  Summary

• Cu  wire  is  much  harder  than  Au  and  will  work  harden  during  bond  formation  –  Work  hardening  is  a  basic  property  of  Cu  –  Will  be  worse  with  dissolved  Cu2O  –  Balls  are  harder  on  edges  where  they  are  deformed  under  the  capillary  

–  Implication  of  harder  Cu  is  more  cratering  during  initial  bonding  

–  Cratering  can  also  occur  after  HTS  from  growth  of  micro-­‐cracks  in  the  substrate  formed  during  bonding  

–  Cratering  is  most  common  failure  mode  for  Cu  wirebonded  to  Al

Au  on  Al  Wirebonding  Summary

• Au  and  Al  IMC  grows  faster  and  thicker  than  an  ideal  IMC  structure  

• Potential  for  void  growth  and  subsequent  wirebond  failure  

• Potential  for  failure  increases  with  decreasing  initial  IMC  coverage  

•  Reliability  testing  indicates  initial  coverage  should  be  >66%  for  reliable  bonding  

•  Au  work  hardening  is  minor  reducing  probability  of  cratering

Wirebond  Pad  Material  Considerations

Pad  Deposition  and  Metallurgy

Deposition  Technology Metallurgy Comments

Thin  Film Al,  Au Sputtering  or  evaporation  at  wafer  fab

Plated Au,  Ni,  Ag,   Electrolytic  or  electroless  

Thick  Film Au,  Ag Al2

Pad  Contamination  and  Impact  on  Wirebond  Reliability

Type  of  Contamination Impact Source

Metal  oxide  (CuO,  NiO) Poor  bonding  strength Diffusion  from  underlying  plated  layers

Organic  contamination Poor  bonding  strength Plating  bath  contamination  (spent  additives,  dissolved  photo-­‐resist)

Halogen  contamination Degradation  of  bond  strength  over  time

Cleaning  chemicals,  resins  or  epoxies

Ni  Impurity  on  Au  Pad

• Au/Au  wirebond  reliability  • Ni  diffuses  to  Au  surface  and  then  Ni  à NiO  

• Small  concentration  of  NiO  dramatically  increases  bond  lifting

Harman, Wirebonding in Microelectronics

Impurity  Concentration  during  Wirebonding

Initial bond

Impurities in pad

Impurity  Concentration  during  Wirebonding

Initial bond

Impurities in pad

HTS

IMC growsà pad impurities get concentrated in IMC layerà amplifies the impact of small impurity concentration

Plasma  Treatment• Impurities  on  the  wirebond  pad  can  decrease  wire  pull  strength  and  

wire  pull  consistency  –  Residual  organic  from  die  attach  –  Organic  contamination  from  plating  – Ni  oxide  contamination  on  Au  from  plating  

•  Plasma  can  be  used  to  remove  these  contaminants  –  Oxygen  plasma  for  organics  –  Argon  plasma  for  non-­‐organics  (Ni-­‐oxide)  – Note  that  oxygen  plasma  can  only  be  used  for  non-­‐oxidizing  metal  pads  such  as  

Au  – For  Al  or  Ni  pads,  Ar  plasma  must  be  used  – For  very  thin  Au  (over  Pd),  non-­‐sputtering  oxygen  plasma  must  be  used  to  avoid  

removing  the  Au  layer

Plasma  Cleaning  Impact

Mean  PullCpK

Bonding  Wire  Considerations

Mechanical  Deformation  of  Wire  Bond  Wire

• Alteration  of  mechanical  properties  from  U/S  exposure  •  Deformation  from  capillary  or  wedge  • Plastic  deformation  to  form  loop  • Resistance  to  viscous  force  from  molding  compound  during  

packaging  (transfer  molded  device)  • Mechanical  fatigue  from  temperature  and  power  cycling  

during  device  operation

Key  Considerations

• Stress  state  of  wire  • Resistance  to  metal  fatigue  • Corrosion  resistance  • Elongation  • Breaking  load  • Electrical  conductivity  • High  Frequency  characteristics  • Size  of  heat  affected  zones  (HAZ)  after  bonding

Elongation  and  Breaking  Load  Trade-­‐Offs

Smooth  Loop  formation  No  wire  breaks  during  bonding

No  wire  deformation  or  sagging  after  formation.

Wire  deformation  and  sagging  after  formation

Wire  breaks  during  bonding

Low elongation, high breaking load

High elongation, low breaking load

Gold  Wire  Alloying  Effects

Additives to 99.99% pure Au wire. Be and Ca increase the break strength and increase the re-crystallization temperature which decreases the HAZ after bonding. Data from Williams Advanced Materials (Bond wire supplier)