Abstract

Solder joints in microelectronic assemblies experience a combination of extensional, shear and multiaxial loads due to printed circuit board (PCB) flexure during thermal cycling or during vibrational loading of constrained PCBs. Although, a significant amount of research has been conducted to study 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 micro-scale morphological features (such as grain orientation, grain boundaries, IMCs 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 microscale solder joints under shear, tensile and multiaxial loading modes at room temperature. The fatigue durability test results indicate that tensile loads are more detrimental compared to shear loads. Tensile vs. shear loading modes are found to cause distinctly different combinations of interfacial damage vs. 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 fatigue durability is 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.

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