Reliability of homogeneous Sn-Bi and hybrid Sn-Bi/SAC BGAs

The low melting temperature solder is becoming a hot topic due to the energy-saving benefits during reflow, yet its reliability, as well as other properties like constitutive relations are not well understood. In this paper, homogeneous Sn42Bi58 packages along with homogeneous Sn-Ag-Cu (SAC305) packages and hybrid Sn42Bi58/SAC packages are assembled and tested. All these three different types of assemblies are put under the same thermal cycling condition to investigate their reliability. Based on the test results, the homogenous Sn-Bi assemblies exhibit best reliability performance during cycling test, homogenous SAC305 assemblies become second, and hybrid Sn-Bi/SAC assemblies show the least cycling resistance due to different failure mechanisms. The crack of both homogeneous assemblies initiated at the substrate side of the package. It will further propagate along the intermetallic layer with the proceeding of cycling. The hybrid assembly, on the other hand, will fail at the interface of SAC and Sn-Bi solder. Except for experiment results, the constitutive equation of Sn-Bi solder is also studied. Based on Garofalo model and creep test data, the constants of the solder are determined by curve fitting. With the help of such relationship, we are able to simulate the energy accumulation distribution of the solder. As a result, the fatigue life of each different type of assembly can be predicted by applying the accumulated energy to the fitted Darveaux and Syed model.