A comparative study of solder fatigue evaluated by microscopic in-situ analysis, on-line resistance measurement and FE calculations

A combined numerical-testing methodology for the evaluation of thermo-mechanical fatigue of small volumes of shear-loaded electronic materials has been developed. Small lap-shear specimens with slightly different thermal expansion are mounted in a loading frame, suffering shear loading of the joint material when subjected to thermal loads. In-situ deformation analysis of the joint surface is an integral part of the procedure. This way the progress of microstructural changes caused by thermal fatigue can be visualized at the surface of the joint at different cyclic states. Fatigue of Sn_95.5Ag_3.8Cu_0.7 solder joints was investigated with this "thermal lap shear test". Different cyclic environments were addressed: Test cycles -40 °C to 125 °C and field cycles 0 °C to 80 °C. During thermal cycling in a microscope temperature chamber, the changes of the microstructure were monitored. When playing these micrographs taken at different temperatures as a video sequence, it becomes obvious that sliding between boundaries of the Sn-rich phases is the dominant deformation mechanism leading to crack propagation at multiple fronts along these "grain boundaries". No principle difference was observed for the field cycle and the test cycle, despite the different maximum stress ranges occuring during the different cycles. It has been shown that the final macroscopic crack starts in the region of highest equivalent creep strain and follows the path along its local maximum, corresponding to the finite element analyses (FEA) results. Fatigue progress is achieved by either conventional thermal shock cycling, during which the electrical resistance changes are recorded, or slow field cycling without electrical measurements. Microstructural degradation progress, electrical resistance changes of the joints and FEA based failure prediction were finally compared for the test cycle.