"Lead-free soldering, originally started as an environmentalissue, is evolving rapidly into a business survival tool for the worldwide electronic industry. Promising lead-free solderalternatives for surface mount assembly applications includeeutectic Sn/Ag, eutectic Sn/Cu, Sn95/Sb5, eutectic Sn/Bi,Sn/Ag/Cu, Sn/Ag/Cu/X, Sn/Bi/Ag/X, Sn/Zn/X, andSn/In/Ag/(X). However, for wafer level area array solderbump interconnects, most of those options fall short in terms of fatigue resistance. Sn/In/Ag/(X) appears to be superiorwhen compared with Sn63/Pb37, as demonstrated bySn/In/Ag/Cu. For applications involving high lead solders, no solder alternatives have been developed yet. While theindustry is advancing toward being finer, smaller, lighter, and faster, wafer level packages using area array solderinterconnects is suffering from the soft error due to alphaemission from the lead in the solders. Although lead-free solder alternatives for eutectic Sn/Pb are virtually free fromalpha emission, the continuous dependence on the use of high-lead solders for C4 applications indicates that the challenge of alpha emission from lead-containing solders will persistregardless of the lead-free move of the industry. Thischallenge is getting tougher with the rapid advancement of IC design toward further miniaturization. Low alpha lead can be obtained from cold lead ore, old lead, and laser isotopeseparation process, with the latter having potential as a long term solution. The price of those low alpha lead is veryexpensive when compared with the regular lead. Due to theincrease in I/O density, requirement on alpha emission levelmay soon move from LC2 to LC3 level. The supply of lowalpha lead for wafer level interconnects does not seem to be an issue.Table 4. Price of low alpha lead or solders.Alpha Emission Rate(count/cm2.hour)Product TypePrice ($/lb)Note<0.5Ingot10LC1<0.05Ingot50-150LC2<0.02Sn63, type 5 & type 6 powder1050-4050LC2<0.01Ingot90-190LC2<0.01Sn63, type 5 & type 6 powder1140-4400LC2<0.005Ingot220-360LC3<0.002Ingot80-150LC3, old Pb<0.001Ingot310-500LC3<0.0006Ingot680-1150LC3"
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ACKNOWLEDGEMENTThe author would like to express sincere gratitude to Dr.Joseph Fjelstad at Tessera, Dr. David Angst at TCAD,Paradiso Coskina at Fraunhofer Institute, Dr. Hugh VanderPlas at Hewlett Packard, and particularly to Dr. Paul Totta at IBM, for their valuable advise and comments on the low alpha solder issues. The author would also like to thank Dr. Benlih Huang at Indium Corporation of America for his extensivework on the Sn/In/Ag/(X) investigation.REFERENCE1. National Center for Manufacturing Sciences, "Lead andthe Electronic Industry: A Proactive Approach", May,1995.2. E. B. Smith III and L. K. Swanger, “Are Lead-free Solders Really Environmental Friendly?”, SMT, p. 64-66, March, 1999.3. N.C. Lee, “Lead free soldering – where the world isgoing”, Advancing Microelectronics, Vol. 26, No. 5,p.29-35, September/October, 1999.4. Frank Gibbs, “Pb Free Interconnect”, NEMI Lead FreeMeeting, Chicago, May 25, 1999.5. B.L. Huang and N.C. Lee, “Prospects of Lead FreeAlternatives For Reflow Soldering”, in Proc. ofIMAPS’99, Chicago, October 28, 1999.6. NEMI internal communication.7. J.A. Slattery, et al., “Lead-free alloy containing tin, silver and indium”, US Patent 5,580,520; Dec. 3, 19968. J.A. Slattery, et al., “Lead-free alloy containing tin, silver and indium”, US Patent 5,256,370; Oct. 26, 19939. S. Yeh, “Fatigue resistant lead free solder”, US Patent5,938,86210. P. Elenius and S. Yeh, “Lead Free Solder for Flip Chipand Chip Scale Packaging (CSP) Applications”, inProceedings of IPCWorks ‘99, Minneapolis, MN, S-03-2-1 to 6, October 23-28, 1999.11. Private communication with Paul Totta, IBM – Fishkill, NY, July 21, 1999. 12. Universal Instruments test data on voiding performance of flip chip underfilling process, 1998.13. J. Lau, T. Krulevitch, W. Schar, M. Heydinger, S.Erasmus and J. Gleason, “Experimental and AnalyticalStudies of Encapsulated Flip Chip Solder Bumps onSurface Laminar Circuit Boards”, Circuit World, Vol. 19, No. 3, 1993.14. M.W. Roberson, “Soft Error Rates in Solder BumpedPackaging”, 4thInternational Symposium and Exhibition on Advanced Packaging Materials, Braselton, GA, March 15-18, 1998.15. Private communication with David Angst, December 20, 1999.16. Peter Bokhan, “Secular Equilibrium”, Pure Technologies literature.17. Electronic Engineering Times, June 21, 1999.18. Private communication with Joseph Fjelstad, May 14,1999. 19. Private communication with Rick Jensen, representativeof SRL, on testing data on no alpha lead for alphaemissions by IBM, IICO, Johnson Matthey and PacificNorthwest National Laboratory, December 9, 1999.20. A. Chance, “A Bizarre Rumor”, internet communications, February 2, 1996.21. Steve. Berry and Sandra Winkler, "Flip Chip MarketExpanding to Meet Speed, Performance Demands", Chip Scale Review, p.6, 11/12, 1999.
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