Comparison of Electromigration in Tin-Bismuth Planar and Bottom Terminated Component Solder Joints

ABSTRACT Electromigration monitoring of bottom terminated component (BTC) solder joints is limited to electrical resistance measurements of the solder balls. Tracking the microstructural evolution such as bismuth segregation in tin-bismuth solder ball, is typically via metallurgical cross sectioning, a destructive technique. Once cross sectioned, the solder ball is not available for further electromigration current stressing. A novel planar solder geometry has been invented and developed that allows real-time, non-destructive monitoring of solder microstructure, while the progress of electromigration can be concurrently tracked via electrical resistance means. Planar solder joints are easy to fabricate in a typical metallurgical laboratory. If the electromigration behavior of the planar and the BTC solder joints happen to be similar, the planar solder joint approach could greatly aid in the quick development of solder alloys by comparing the rates of electromigration and the metallurgical changes in planar solders of various compositions. In this paper, the electromigration rates and behavior of eutectic Sn-Bi alloy in planar and in BTC solder joints were compared and shown to be similar. This important finding opens the use of planar solder joints for the quick and lowcost development of low-temperature solder alloys.