Influences of original solder grain orientation on thermal fatigue damage and microstructure evolution of the SnAgCu/Cu solder joints revealed by in-situ characterization

To intuitively reveal the influences of original solder grain orientation on thermal fatigue damage mechanisms of the SnAgCu/Cu solder joints at relatively high temperature, in this study the solder joints were prepared, and their thermal fatigue damage and microstructure evolution processes were in-situ characterized. It was found that most of the solder joints are single crystal, and the others have 2 or 3 solder grains. Under the thermal cycling of 20 °C ~ 140 °C, the interfacial strain concentration is more serious than that cycled at relatively low temperature, and the damage accumulation rate is higher. The original solder grain orientation determines the activation of slip systems and the plastic deformation degree, especially at the interfacial strain concentration zone. Due to softening of the solder and the high recovery rate, deformation of the solder not close to the joint interface is not very serious. The slip bands are not straight, and the activated slip systems are the {110}<001 > and {100}<001 > systems, but different slip systems can be activated at different zones within one solder grain. For the solder joints with 2 or 3 solder grains, deformation of the grain with higher Schmidt factor is restricted by the other grains, and the deformation mismatch resulting in grain boundary (GB) sliding. The high temperature recovery and relative torsion between different parts of the solder grains form new low-angle GBs. For a group of solder joints, the joint most susceptible to deformation determines the life of the device.