wb c2Copper Wire Ball Bonding

The Cutting Edge of Copper Wire Ball Bonding 2 Agenda Growth of copper wire bonding Challenges and solutions Pd Coated Copper Wire Summary 3 Copper Wire - The Journey Engineers had been experimenting with alternate wire to gold for more than 20 years Until recently, the only alternative bonding wire is aluminum which is usually wedge bonded Copper is chosen because of the excellent conductivity and a commonly used metal for electrical circuits Initial work with copper wire found that forming a FAB is relatively easier - K&S had patented an earlier version of the copper kit in 1999 Early Cu Wire Bond adoption cost saving for heavy wire in 1990s First used on discretes and Power ICs which have a robust pad structures Cu kits designed via trial and error Wire size usually thick than 2.0mil Renewed interest to convert 1mil wire to Cu 2006 Gold prices began to shot up significantly Effort was focus on understanding the fundamental knowledge for Cu wire bonding Engineering models and characterizations were initiated to study the fundamentals Today Gold prices continuing to rise dramatically Cu Process Knowledge increased dramatically and reliability had been proven Fine Pitch/High performance apps entering Cu Wire production at a rapid pace A few large IDMs are transitioning to Cu very aggressively Subcons Competing for Cu Business (aggressively adding capacity) 4 Copper Wire Penetration Estimated 15% automatic ball bonders worldwide are installed with a copper kits and running regular production By end 2010, estimate about 1215% units are bonded with copper wire, compared to less than 2% before 2008 Prismark projects that copper wire penetration will grow from just over 5% in 2009 to almost 30% in 2014 IC applications now account for more than 70% of the copper wire volume. Taiwan and China lead the world in transition to copper Copper kit installed by K&S 5 Challenges And Solutions 6 Controlled enviroment for EFO firing Design based on Computational Fluid Dynamics analysis and Gas Flow Visualization Copper Kit Example 7 Intensive use of gas flow modeling to determine design to exclude oxygen in the EFO firing process Contour plots showing mass fraction of oxygen (levels corresponds to previously reported measured oxygen) Predicts trade-off between forming gas usage (flow rate) and enviroment robustness (size of usable low oxygen region) FAB Region Ceramic Tube EFO Wand Range for good FAB formation Flow rate: 0.2 lpm EFO Wand FAB Region Ceramic Tube Range for good FAB formation Flow rate: 0.6 lpm Designing the Kit 8 Establishing the right level of gas flow Malformed and Oxidized Low forming gas flow Visible oxide layer Pointed Flow is too high Oxide layer not visible but surface tension disrupted Round and clean Ideal forming gas flow Making The Right Free-Air-Ball 9 Copper is a Harder Material ANSYS 10.0 NODAL SOLUTION STEP=9 SUB =46 TIME=9 EPTOX (AVG) RSYS=1 PowerGraphics EFACET=1 AVRES=Mat DMX =.219E-03 SMN =-.459073 SMX =.48208 1 -.48 -.32 -.16 -.555E-16 .16 .32 .48 ANSYS 10.0 NODAL SOLUTION STEP=9 SUB =46 TIME=9 EPTOX (AVG) RSYS=1 PowerGraphics EFACET=1 AVRES=Mat DMX =.192E-03 SMN =-.421537 SMX =.279271 1 -.48 -.32 -.16 -.555E-16 .16 .32 .48 Au Total Radial Strain Cu Bare copper wire is about 40% harder than gold 85Hv compared to 60Hv To achieve the same ball height, 2025% higher force is needed on the copper wire compared to gold The higher bond force causes stress on the pad and the underlying layer Pad peel and crack are the common failures 10 Dealing With First Bond Issues Critical 1st bond issues are Pad crack Aluminum splash Material supplier had developed softer copper wire that is up to 1015% softer in term of Hv hardness Equipment maker develop processes to minimizes pad crack and aluminium splash, features like Various mode of table scrub Segmented bonding And lately, dedicated bonder for copper wire (IConn ProCu from K&S) The next step is to develop a complete solution that involves Optimized material - wire and capillary Optimized copper bonder Pad structure design Pad SplashPad Crack 11 Ball Shear Requirement Higher USG power generally gives a better ball shear but also create higher level of pad crack and Al splash Copper wire requires slightly more than 2x the energy to increase 1 unit of bonded ball size Need to review spec to limit upper limit, balance between lifted ball and Al splash, ex. Not more than 8 gm/mil2 The danger of too high USG power causes potential reliability failure as the Al under the ball is too thin - potential stress on underlying layer Ball Diameter vs. Current Factor for Au and Cu Wire 40.00 45.00 50.00 55.00 60.00 65.00 0.750.80.850.90.9511.051.11.151.21.25 Copper) Au 1 um / 20% 1 um / 8% Al splash vs SUA 48 50 52 54 56 58 60 67891011 Shear per Unit Area (g.) 12 Improve Al Pad Splash Extensive research was accomplish to study ho