In a series of articles, we consider the effects of the microstructure and the associated anisotropy of Sn grains on electromigration in lead-free solder bumps. In this article, we carried out experiments to characterize microstructures of tin-based solder bumps and to investigate their effects on electromigration-induced failure. Accelerated electromigration tests and electron backscatter diffraction (EBSD) tests are performed on tin-based solder bumps in wafer-level chip-scale packages (WLCSPs). EBSD analysis, together with the results of time to failure from accelerated electromigration tests, provides valuable information on the effects of the microstructure on electromigration-induced failure that can be used for predictive modeling. For Ni-based metallization, we find that tin self-diffusion is the dominant degradation mechanism and a strong effect of the grain orientation in the solder on the time to failure.