Effects of Aging on the Damage Accumulation in SAC305 Lead Free Solders Subjected to Cyclic Loading

In this study, we have examined the effects of prior aging on damage accumulation occurring in SAC305 solder materials subjected to mechanical cycling (fatigue testing). Uniaxial samples have been prepared and polished so that the microstructural changes could be tracked after the initial aging, and then subsequently with mechanical cycling. In particular, we examined the microstructural changes that occurred in small fixed regions in the solder samples, rather than using several different regions. Regions of interest near the center of the sample were marked using small indents formed with a nanoindentation system. Samples were then subjected to aging at 125 °C for various durations to produce several different initial microstructures. Polarized light optical microscopy (OM) and scanning electron microscopy (SEM) were used to investigate the aging induced microstructural changes in the regions of interest in the solder sample. After aging, the samples were then subjected to mechanical cycling. After various durations of cycling (e.g. 0, 10, 25, 50, 75, 100, 200, 300 cycles) that were below the fatigue life of the materials, the regions of interest were again examined using OM and SEM. Using the recorded images, the microstructural evolutions in the fixed regions were observed, and the effects of the initial aging on the results were determined. It was found that the number of IMC particles decreased while the average diameter of the particles increases significantly due to the initial aging. After the cyclic loading was performed on the sample, it was observed that the cycling induced damage consisted primarily of small intergranular cracks forming along the sub-grain boundaries within the β-Sn dendrites. These cracks continued to grow in size as the cycling continued, resulting in a weakening of the dendrite structure, and eventually resulting in the formation of large transgranular cracks. The distribution and size of the intermetallic particles in the inter-dendritic regions were observed to remain essentially unchanged with the application of the mechanical cyclic load. Relative to the non-aged samples, there were significant differences observed in the rate and intensity of the micro crack growth occurring in the heavily aged samples that began with much coarser microstructures.