Study on changes in intermetallic compounds and whisker formation over time in Sn/Cu plating

In Sn/Cu plating, whiskers are formed because of the formation of intermetallic compounds (IMCs) at the Sn/Cu interface and along the Sn grain boundaries. Many previous studies have focused on the mechanism of whisker formation and the change in whisker density over time. However, only a few studies have focused on the formation and growth behavior of IMCs and the correlation between IMCs and whiskers. Furthermore, no studies have quantitatively investigated the time-dependent changes in the formation and growth behavior of IMCs and whiskers using mathematical formulas. In this study, Sn/Cu plating was applied to a 7–3 brass substrate, and the formation and growth of IMCs at the Sn/Cu interface and the behavior of whisker formation were studied. An approximate equation was derived based on the reaction kinetics to quantitatively express the time-dependent changes in both parameters. Three differences were observed between the formation behaviors of IMCs and whiskers. First, no incubation time t_th was observed for IMC formation, but it was for whiskers. Second, the whisker density increased until t = 10 d and then saturated. However, the IMC density increased until t = 70 d and then became saturated. Third, the IMC density in the cross section is 7–10 times higher than the whisker density, and the rate constant of the IMCs is smaller than that of the whiskers.