Role of Nominal Stress State on Cyclic Fatigue Durability of SAC305 Grain-Scale Solder Joints

Abstract

Abstract Solder joints in micro-electronic assemblies experience a multiaxial combination of extensional and shear loads due to combinations of thermal expansion mismatch and flexure of printed circuit assemblies during thermal cycling or during vibrational loading of constrained printed circuit assemblies. Although a significant amount of research has been conducted to study cyclic fatigue failures of solder joints under pure-shear loading, most of the current literature on cyclic tensile loading of solders is on long dog-boned monolithic solder coupons. Unfortunately, such specimens do not capture the critical interactions between key microscale morphological features (such as grain orientation, grain boundaries, intermetallic compounds, and substrates) that are believed to play important roles in the fatigue of functional solder joints under life-cycle loading. Therefore, this paper uses a combination of experiments and finite element analysis to investigate the differences in mechanisms of cyclic fatigue damage in Sn-3.0Ag-0.5Cu (SAC305) few-grained (oligocrystalline) microscale solder joints under shear, tensile and multiaxial loading modes at room temperature. Cyclic fatigue durability test results indicate that tensile loads are more detrimental compared to shear loads. Tensile versus shear loading modes are found to cause distinctly different combinations of interfacial damage versus internal damage in the bulk of the solder (transgranular and intergranular damage), which correlates with the differences observed in the resulting fatigue durability. The test results also confirm that the traditional approach of assuming a power-law dependence on equivalent deviatoric strain amplitude is inadequate for modeling cyclic fatigue durability of solder interconnects experiencing multiaxial loading. Instead, multiaxial fatigue damage results are seen to be affected not only by the cyclic equivalent strain amplitudes but also by the severity of the stress-triaxiality, as hypothesized in models such as Chaboche model.