Effect of Microscale Heterogeneities and Stress State on the Mechanical Behavior of Solder Joints

Abstract

SAC305 solder joints consist of a few large coarse highly anisotropic grains, separated by weak grain boundaries. In addition, scalloped brittle layers of intermetallic compounds (IMCs) grow at the interface between the solder and the solder pads, during formation of the solder joints. The interaction between the anisotropic grains, grain boundaries and interfacial IMC layer create localized stress concentrations which affect the degree of vulnerability under different loading conditions.In a previously reported study [1], joint-scale Cu-SAC305-Cu specimens were shown to exhibit different fatigue durability under cyclic tensile vs shear loading, with different dominant failure mechanisms. Tensile loading resulted in a high percentage of brittle damage at/near the interfacial IMC layers, whereas, shear loading caused a lower percentage of brittle interfacial IMC cracks in combination with a considerable degree of ductile fatigue damage in the bulk of the solder (intergranular as well as intragranular). This difference in failure modes is hypothesized to be due to the fact that the stress triaxiality near the IMC-solder interface is much higher in tensile specimens, than in shear specimens. In addition there is additional stress amplification due to grain-scale heterogeneities. Therefore, this study aims to obtain semi-quantitative insights into the effect of grain orientation on failure vulnerabilities in solder joints subjected to tensile and shear loads, using elastic anisotropic grain-scale finite element analysis (FEA), and compared to results of elastic isotropic homogeneous finite element stress analysis that ignores the grain morphology.The takeaways from this work will potentially enhance the understanding of the reasons behind the differences in failure modes in cyclic tensile vs. shear test specimens.