Micro-structural changes and crack propagation in solder joints due to vibration

The effects of vibration on the micro-structure of the solder alloy SAC305 (96.5 wt.-% tin, 3 wt.-% silver and 0.5 wt.-% copper) were investigated. Chip resistors were soldered onto printed circuit boards and exposed to constant sinusoidal vibration at room temperature. The investigation was focused on the interaction between surface morphology and micro-structure of the solder joint and crack propagation. It is shown that vibration load leads to micro-structural changes of soldered interconnections. In the initial state solder joints usually consist of few grains. Under vibration-induced aging, the micro-structure changes towards smaller grain sizes and also micro-cracks are initiated. After critical crack length is reached, the crack propagation accelerates. This leads to cracks through the whole solder joint and finally to failure of the assembly. It was observed that a characteristic crack path is formed. In contrast to temperature shock tests the crack is initiated at the surface of the solder joint meniscus instead of its standoff. Furthermore, crack propagation due to vibration is inter-granular as well as trans-granular, while in temperature shock tests only inter-granular cracks develop.