Pb-free solder alloys for flip chip applications

In addition to the environmental issue regarding the use of Pb-bearing solders in microelectronics applications, there is another issue associated with using Pb-bearing solders in interconnections, like flip chip solder interconnections in an advanced CMOS technology, that are near active circuits. In order to minimize the soft error rate due to alpha particle emission from Pb-bearing solder alloys, Pb-free solder alloys were studied as possible replacements for the Pb-based solders that are presently used in flip chip interconnections. A large number of solder compositions was selected for evaluation. Since all the candidate alloys were Sn-based, alternatives for the ball-limiting metallurgy (BLM) were also investigated. The physical, chemical, mechanical and electrical properties of the alloys were determined by thermal analysis, wettability testing, microhardness measurement, electrical resistivity measurement, interfacial reaction study and others. Test vehicles were also built with some selected Pb-free solder alloys with the proper BLM to evaluate integrity of the flip chip solder bump structure. Based on this study, a few candidate solder alloys were selected with a proper BLM barrier layer for flip chip applications.