Characterization of SAC Solder Microstructure Evolution During Thermal Cycling with Statistical and In-situ Methods

The evolution of Sn grain microstructure during thermomechanical cycling occurs due to grain boundary movement, as some single crystal solder joints gradually transform to multicrystals. This process has been documented statistically using polarized light and EBSP mapping by counting the numbers of joints with different orientations and microstructures in packages with different aging and thermomechanical cycling histories. The process by which evolution occurs is was examined assessed by comparing diffraction patterns and peak shifts obtained from in-situ measurements from the full volume of a solder ball at different stages of thermal cycling history using synchrotron radiation. Diffraction patterns were used to identify crystal orientations present in the joint, and peak shape and peak shifts were used to assess changes in internal stress in several joints.