A comparative study of thermal fatigue life of Eutectic Sn-Bi, Hybrid Sn-Bi/SAC and SAC solder alloy BGAs

Low melting temperature solders are becoming a hot topic in Pb-free solder studies. Among these solders, Sn[sbnd]Bi has attracted more attention because it can not only reduce the energy for reflow, but more importantly, increase the reliability of joints by minimizing warpage. In this study, the board-level reliability of the assembly of BGA components was investigated. Three types of assemblies are compared: mixed (hybrid) Sn-Bi/SAC305 assembly, which is made by soldering BGAs with SAC305 solder balls using eutectic Sn[sbnd]Bi solder pastes so that the low reflow temperature avoids the SAC305 solder joints from being molten; homogeneous eutectic Sn[sbnd]Bi assembly, which is made with solder balls and pastes that are both eutectic Sn-Bi; and homogenous SAC305 assembly as a control group. An accelerated thermal cycling test shows that homogeneous eutectic Sn[sbnd]Bi assembly exhibits the best cycling reliability while the mixed assembly has the shortest fatigue life. The failure mechanism of the mixed assembly is also different from the other two types. Based on the experimental fatigue life results, the parameters of constitutive equation of eutectic Sn[sbnd]Bi are calculated to evaluate the fatigue life of the solder joints. The study shows the better thermal cycling reliability of eutectic Sn[sbnd]Bi assembly and help predict the fatigue life of different assemblies.