Pb-free BGA Solder Joint Reliability Improvement with Sn3.5Ag Solder Alloy on Ni/Au Finish

In this work, Sn3.5Ag solder alloy was being studied for the purpose of Pb-free solder joint reliability improvement over conventional Sn3.8AgO.7Cu solder balls on Ball Grid Array (BGA) packages with Ni/Au pad finishing. The study was carried out in different levels. At individual solder joint level, Sn3.5Ag showed no intermetallic brittle failure in cold ball pull test even up to 6times multiple reflow and 168 hrs high temperature storage for TBGA & TePBGA, and 504 hrs high temperature storage for PBGA Spanish Oak. In contrast, 70-100% of the failure mode of SAC387 was brittle failure. At package level, Sn3.5Ag survived 8~10x more drop cycles than SAC387 in tray drop and packing drop tests. These results indicate that the mechanical strength of Sn3.5Ag on Ni/Au pad is considerably stronger than that of SAC387. The difference in mechanical strength between the two alloys was correlated to their microstructures. At the same time, board level solder joint reliability tests such as thermal cycling and mechanical bend test were carried out. Sn3.5Ag showed better or similar performance as SAC387.