MICROSTRUCTURAL IMPROVEMENTS OF SAC ALLOYS WITH BI ADDITIONS DURING ACCELERATED THERMAL CYCLING

摘要

Tin based Pb-free solders continue to present challenges in high reliability applications, particularly when exposed to accelerated thermal conditions. Bi as an alloying element has been found to have beneficial properties, both by reducing the overall process temperature and through intrinsic material characteristics. This paper explores the microstructural evolution of three Pb-free solders; two have a near-eutectic composition in a quaternary system, while a third has a composition along the eutectic valley of a ternary compound. The properties and microstructures of each are compared to SAC305. Reduced amount of Ag, including one alloy with no Ag, are also examined. Changes to the bulk microstructure and the interfacial IMC between time of solidification and exposure to accelerated thermal cycling are explored and compared with reliability data. Two thermal cycling conditions are considered, 0 to 100°C and - 55 to 125°C.