Lead-Free Soldering and Low Alpha Solders for Wafer Level Interconnects

Lead-free soldering, originally started as an environmental issue, is evolving rapidly into a business survival tool for the worldwide electronic industry. Promising lead-free solder alternatives for surface mount assembly applications include eutectic Sn/Ag, eutectic Sn/Cu, Sn95/Sb5, eutectic Sn/Bi, Sn/Ag/Cu, Sn/Ag/Cu/X, Sn/Bi/Ag/X, Sn/Zn/X, and Sn/In/Ag/(X). However, for wafer level area array solder bump interconnects, most of those options fall short in terms of fatigue resistance. Sn/In/Ag/(X) appears to be superior when compared with Sn63/Pb37, as demonstrated by Sn/In/Ag/Cu. For applications involving high lead solders, no solder alternatives have been developed yet. While the industry is advancing toward being finer, smaller, lighter, and faster, wafer level packages using area array solder interconnects is suffering from the soft error due to alpha emission from the lead in the solders. Although lead-free solder alternatives for eutectic Sn/Pb are virtually free from alpha 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 persist regardless of the lead-free move of the industry. This challenge 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 isotope separation process, with the latter having potential as a long term solution. The price of those low alpha lead is very expensive when compared with the regular lead. Due to the increase in I/O density, requirement on alpha emission level may soon move from LC2 to LC3 level. The supply of low alpha lead for wafer level interconnects does not seem to be an issue.