A Comparative Study of Sn3.5Ag and Sn3.8Ag0.7Cu for BGA Spheres on Ni/Au Finish

A comparative study was conducted with Sn3.5%Ag and Sn3.8%Ag0.7%Cu 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 under any stress condition. 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 similar performance as SAC387. The reason for the similarity was also explained by examining the microstructure change that occurred during board mounting process.