Influence of Sn grain size and orientation on the thermomechanical response and reliability of Pb-free solder joints

The size and orientation of Sn grains in Pb-free, near eutectic SAC solder joints were examined. A clear dependence of the thermomechanical response of these solder joints on Sn grain orientation was observed. Solder balls with Sn grains of particular orientation (a-axis perpendicular to the substrate) were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn along the a-axis is half the value along the c-axis; joints observed to be damaged had maximum mismatch in the coefficient of thermal expansion between solder and substrate at the joint interface, as well as tensile stress modes during the hot part of the cycle