The In-Situ Observation of microstructure, grain orientation evolution and its effect on crack propagation path in SAC305 under extreme temperature changes

Microstructures and grain structure evolution of solder joints under extreme temperature changes considerably affects its thermomechanical response and lifetime. Thermal shock tests were conducted to study the thermomechanical responses in Sn-3.0Ag-0.5Cu BGA solder bumps. In this work, the in-situ observation was carried out by EBSD and SEM to examine the microstructure and grain structure evolution during thermal shock tests, which provides the accuracy and continuity of the observation. For the Sn-based solder joint containing limited grains, early failure always occurs in certain solder joints where orientation of grain in contact with substrates exhibited relatively high CTE values parallel to the substrates. Elaborated examination of different stages of recrystallization in the same bump identified localized recrystallization occurred at the place with high CTE mismatch. Moreover, the grain structure had changed from cyclic twin to small random oriented grains, which provides additional degradation mechanism. The grain orientation was critical to reliability and lifetime. Inherent anisotropic properties of ß-Sn grain play a vital effect on thermomechanical responses, leading to a change in the initiation propagation path of crack and corresponding reliability problems.