Low Cycle Fatigue Behavior and Mechanisms of a Lead-free Solder 3.5Ag/96.5Sn(Electronic Devices)

Abstract

Low cycle fatigue tests of as-casted Ag-Sn eutectic solder (3.5Ag/96.5Sn) were carried out using the non-contact strain controlled system at 20 ℃. The fatigue behavior followed the Coffin-Manson equation with the fatigue ductility exponent of 0.76. Without local deformation and stress concentration at contact points between the extensometer and the specimen surface in strain-controlled fatigue tests, crack initiation and propagation behavior during fatigue test was observed on the specimen surface by replication technique. After failure, the longitudinal cross sections as well as fracture surface were examined under an SEM. Steps at the boundary between Sn-dendrite and Ag-Sn eutectic structure, and cavities along the boundaries and especially around the Ag_3Sn paticles, were the initiation sites of microcracks. Stage II crack propagated in mixed manner, i.e. intergranular along Sn-dendrite boundaries, transgranular through Sn-dendrites and Ag-Sn eutectic structure. After fatigue tests, small grains were observed in Sn-dendrites near fracture surface.