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Basics of Bonding Wire Manufacturing

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Part of a free 2 day presentation in Penang, Malaysia in 2011. Sponsored by the World Gold Council, London. ...

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.

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Basics of Bonding Wire Manufacturing Basics of Bonding Wire Manufacturing Presentation Transcript

  • 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 2 15/9/11
  • Manufacturing Processes 3 15/9/11 View slide
  • Process Flow Continuous(casting( (copper) Continuous(casting(&( doping((gold) Block(draw Breakdown(draw Final(draw Intermediate(annealing Final(annealing Spooling Continuous(casting( (aluminium) 4 15/9/11 View slide
  • Continuous Casting 15/9/11
  • Continuous Casting-Casting Machine Illustration of a continuous casting machine chamber 6 15/9/11
  • 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 7 15/9/11
  • Cast Ingot Microstructure 8 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 11 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Chemical Analysis of Wire: ICP-OES 14 15/9/11
  • Chemical Analysis of Wire: ICP-MS 15 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Drawing 15/9/11
  • Block Draw 19 15/9/11
  • Wire Drawing Multiple dies and multiple passes with liquid lubricant to reduce friction 20 15/9/11
  • 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 15/9/11
  • Plastic Deformation during Wire Drawing 22 15/9/11
  • 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 15/9/11
  • Single Crystals 24 15/9/11
  • Wire Microstructure-Grains ¤  Each grain is a ‘single’ crystal ¤  E.g. Au wire 25 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Crystal Planes & Directions ! <100> direction and (100) plane <101> direction and (101) plane <110> direction and (110) plane 28 15/9/11
  • Orientation of Crystals 29 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Single Crystal Under Tension τR = F A cosφ cosλ = F A m Orientation of crystal affects strength 32 15/9/11
  • 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 15/9/11
  • Single Crystals in Tension 34 15/9/11
  • Orientation Change with Stress Tension Compression 35 15/9/11
  • 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 15/9/11
  • Dislocations-Defects that Make Deformation Easier 37 15/9/11
  • Dislocation Movement and Microscopic Plastic Deformation 38 15/9/11
  • Impurities Block Dislocation Movement 39 15/9/11
  • 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 15/9/11
  • Polycrystals 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Ashby Model Illustrated Deform grains by dislocation movement in the grain Reassemble Remove overlap with creation of dislocations at the grain boundaries 46 15/9/11
  • Ashby Model Illustrated Reassemble Remove Overlap 47 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Weak and Strong Hardening At  constant  strain  rate   !σ = Kεγ Constant Plastic stress Strain Strain hardening index 53 15/9/11
  • 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 15/9/11
  • Final Wire Annealing 15/9/11
  • Final Annealing 56 15/9/11
  • Annealing and Recrystallization Before After Intermediate annealing Post annealing 57 15/9/11
  • Final wire annealing curve Change in mechanical properties depends on furnace temperature as the illustration shows 58 15/9/11
  • Final Wire Properties 15/9/11
  • 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 15/9/11 
 

  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11 

  • Wire Grain Size
 
 Au Cu 64 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Strain and Strain Rate Hardening Wires harden when deformed Strain  Hardening   Strain  Rate  Hardening   ! 70 15/9/11
  • Copper Wire Purity & Strength N Srikanth, J. Premkumar, M. Sivakumar, Y. M. Wong, C. J. Vath III, 9th EPTC 2007, Singapore 71 15/9/11
  • 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 15/9/11
  • Final Wire Properties 15/9/11
  • Metal Coated Bonding Wires 74 15/9/11
  • 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 15/9/11
  • 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 15/9/11
  • Coated Wires: Pd Coated Cu Uno et al, ECTC 2009 p1486 77 15/9/11
  • Metal Coated Wires Kaimori et al, IEEE Trans Advanced Packaging 29 (2006) 227 78 15/9/11