WLCSP CTE failure mitigation via solder sphere alloy

Extensive alloy composition development continues in order to optimize solder joint performance in electronic packaging and componentry. Although the general use tin, silver and copper, or “SAC”, alloy in the SAC305 composition remains most prevalent throughout the electronics industry, much work has been performed in order to improve upon lead free metallurgies specific to solder spheres. Challenges with device miniaturization and reliability have led to ever increasing alloy variant proliferation. The SAC305 alloy remains a viable solution for many applications. More recently, the packaging industry has moved to decrease silver content from 3.0% to 1.0~2.5% in order to enable greater bulk joint ductility, enhance sphere performance against drop shock type failures and reduce cost. As die sizes continue to expand, lose thickness and require packaging methods having greater complexity, new stresses are becoming apparent. WLCSP developments have driven changes in the package structure and layup. New under bump metallurgy (UBM) and passivation design have shown current alloys to be lacking in uniform reliability. This paper will focus on current research in solder sphere alloy development, with special focus on compositions intended to decrease failures prominent with CTE mismatch in wafer level chip scale packages. The paper will provide information related to the basic strategy for bulk alloy joint sphere structure optimization through solid solution strengthening and intermetallic compound (IMC) refining. Comparisons will be made between common alloys now in use and various modified compositions. The modified alloys will target improvements in high speed shear testing, solder wetting ability and drop testing capability, while maintaining thermal cycle performance similar to existing alloy technologies.