In-situ solder fatigue studies using a thermal lap shear test

A combined numerical-testing methodology has been developed for the evaluation of thermo-mechanical fatigue of small volumes of electronic materials loaded in shear. Small lap-shear specimens are mounted in a loading frame with slightly different thermal expansion, causing 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. Fatigue of Sn95.5Ag3.8Cu0.7 solder joints was investigated with this "thermal lap shear test". 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, which leads to crack propagation at multiple fronts along these "grain boundaries". However, it is 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 conventional thermal shock cycling, during which the electrical resistance changes are recorded. Microstructural degradation progress, electrical resistance changes of the joints and FEA based failure prediction are finally compared.