Pb-free solders are studied for electromigration (EM) reliability. Two major EM failure mechanisms are identified in Sn-based Pb-free solders, due to the differences in microstructures and Sn-grain orientation. Rapid depletion of intermetallic-compounds and Under-Bump-Metallurgy are caused by fast diffusion of Cu and Ni along the c-axis of Sn crystals. When c-axis of Sn-grain is not aligned with the current direction, electromigration damage is dominated by Sn self-diffusion, which takes longer to occur. In general, the EM damage in SnCu solder is driven by the fast interstitial diffusion of Ni and Cu away from solder/UBM interface resulting in early fails; while the damage in SnAg solders is mostly dominated by Sn-self diffusion resulting in longer lifetime. The effective activation energy is 0.95 eV for SnAg solder and 0.54 eV for SnCu solder. The power exponent is 2 for SnAg and 1.2 for SnCu. Blech effect is observed only in the solders with Sn-self diffusion dominated failures, not in fast diffusion dominated failures. Therefore, optimizing and control solder microstructure is important to the solder reliability. © 2010 American Institute of Physics.