A FE-study of solder fatigue compared to microstructural damage evaluation by in-situ laser scanning and FIB microscopy

A combined numerical-testing methodology was developed for microscopic in-situ observation of fatigue failure of small material volumes thermo-mechanically loaded in shear, which was applied to Sn95.5Ag3.8Cu0.7 (SAC) solder. Different temperature cyclic environments were investigated: test cycles of -40°C to 125°C and field cycles of 0°C to 80°C. Fatigue testing was accompanied by FE-modeling. For all cycles the FE-analyses revealed a non-constant shear strain distribution, which showed local maxima at the interface edges between solder and pads and a region of relatively constant amplitudes at the central part of the joints. By laser scanning microscopy the local deformation behavior and the fatigue progress could be visualized. Microcracking along strain incompatibilities, e.g. small angle grain boundaries, was the preferred mechanism of damage initiation. From comparisons with FE simulations of the tests it became obvious that this damage path usually coincides with the regions where the local maximum creep strains are calculated. However, it was also observed that inhomogeneities, e.g. intermetallic platelets or voids, are additional preferred locations for damage initiation, which are usually not considered in FEA. Locations of strain incompatibility appeared particularly critical for test cycle loading. Focused ion etching and related FIB microscopy allowed insight in further localized microstructural degradation processes down to the nanoscale. Failure prediction is finally compared to the failures observed for both cyclic regimes.