Comparison of electromigration behaviors of SnAg and SnCu solders

Two commonly used Pb-free solders, SnAg and SnCu, are studied for electromigration (EM) reliability. Two major EM failure mechanisms are identified in Sn-based Pb-free solders, which is mainly due to the differences in microstructures and Sn-grain orientation. In general, the EM damage in SnCu solder is driven by the fast interstitial diffusion of Ni and Cu away from solder/UBM interface and leads to 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 current density power law exponent is 2 for SnAg and 1.2 for SnCu, respectively. Blech effect is observed in the solders with Sn-self diffusion dominated failures. The roles of Ag and Cu on EM performance will be discussed.