Investigation of Sn3.5Ag and Sn3.8Ag0.7Cu Pb-free Alloys for BGA application on Ni/Au Finish

Abstract

Sn3.5Ag and Sn3.8Ag0.7Cu (SAC387) Pb-free solder alloys on Ni/Au BGA pad finishing were studied for manufacturability, component mechanical robustness and board level reliability. Reflow profile windowing study showed that both alloys would form solder joint properly with profiles with peak temperature over 230°C. At individual solder joint level, Sn3.5Ag showed significantly lower intermetallic brittle failure rate compared to SAC387 in cold ball pull test. 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 compared to SAC387.