Study of new alloy composition for solder balls - Identifying material properties as key leading indicators toward improved board level performance

The quest for improved board level reliability (BLR) in wafer level packages (WLPs) motivates a characterization of novel alloys, and their impact on BLR. Previous studies on lead free solders have shown the effects of alloy composition on silver precipitation, creep behavior, and IMC formation in SnAgCu (SAC) alloys. Studies have shown that the solder ball alloy needs to have ductility to absorb the stresses related to CTE mismatch, drop shock (DS), and temperature cycle test (TCT). Depending on alloy composition, it has been widely accepted that there has to be necessary tradeoff between mechanical properties that are needed to have robust drop shock characteristics versus temperature cycle robustness. Studying solder balls of new alloy compositions should help assist the packaging engineer to identify what material properties could be key leading indicators to help improve BLR. Existing SAC alloys like SAC305, SAC405, SAC105, etc have properties that have been widely accepted to have a necessary tradeoff qualities in mechanical properties built-in to the alloy system to achieve the right drop shock characteristics versus temperature cycling requirements. [1-7] The “tradeoff” is primarily due to inherent ductile property of regular SnAgCu (SAC) alloy that could potentially have a reverse effect to the drop shock and temperature cycling performance of a package.