Effect Of Microstructure On Electromigration In Pb-free Solder Interconnect

Abstract

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.