Effects of Mechanical Cycling on the Microstructure of SAC305 Lead Free Solder

Failures in solder joints are responsible for about 70% of failures in electronic systems. The most common failure mode is fatigue failure due to cyclic thermal loading (e.g. power switching) or cyclic mechanical loading (e.g. vibration). To better design electronic products more strategically for specific applications, it is important to understand the damage accumulation and failure mechanisms of lead-free solders under cyclic loading. However, there have been limited studies on how the microstructures of such solders evolve during cyclic loading. The main purpose of this study was to better understand the microstructural evolution and damage accumulation occurring in SAC305 lead free solder during the mechanical cycling that occurs during fatigue testing. Uniaxial test specimens were first prepared by reflowing solder in circular cross-section glass tubes with a controlled temperature profile. After reflow solidification, the samples were mechanically cycled (strain control) for various durations (e.g. 0, 50, 100, 200, 300 ... cycles) representing various percentages of the fatigue life (N
_f
). The cycled samples were reflowed and polished using the industry standard procedures, and then the microstructures were examined using SEM and the microstructural evolution and damage accumulation were characterized. The SEM images were used to observe features like IMCs and the dendrite structure. The observed microstructure changes were correlated with the degradation of various mechanical properties (elastic modulus, ultimate strength, and creep rate) measured in our prior study. The initial experiments described above allowed us to understand the microstructure evolution taking place in an average sense since different samples (with different levels of cycling) were used for each of the microstructure images. To better visualize and understand the mechanisms of the evolution taking place, additional experiments were performed to examine a fixed region of a single sample subjected to mechanical cycling. An area near the center of the sample was polished, and a small region of interest within the polished area was marked by indents using a Nanoindenter. After various durations of cycling the sample was repeatedly examined using microscopy, and the time history of the microstructural evolution in the fixed region was determined.