Cyclic softening of the Sn-3.8Ag-0.7Cu lead-free solder alloy with equiaxed grain structure

Low-cycle fatigue behavior of the Sn-Ag-Cu ternary-eutectic alloy was investigated under a fully reversed loading condition. The solder alloy exhibited cyclic softening early in the fatigue life and continued to soften as the number of fatigue cycles increased. Following cyclic loading, numerous microcracks were found in the microstructure. Most of the microcracks were located along the grain boundaries in the areas with finer grains. The areal density of the microcracks increased with both strain amplitude and cycle number. By combining percolation theory with microcracking analysis, the cycle-dependent softening behavior was shown to result from accumulation of microcrack density with fatigue cycles.