Solid-state annealing behavior of two high-Pb solders, 95Pb5Sn and 90Pb10Sn, on Cu under bump metallurgy

The solid-state annealing behavior of two high-lead solders, 95Pb5Sn and 90Pb10Sn (in wt.%), was examined. After reflow, Cu3Sn intermetallics formed on the Cu under bump metallurgy (UBM) for both solder alloys. However, solid-state annealing produced significantly different reaction morphologies for the two solder compositions. The Cu3Sn intermetallics spalled off faster at higher temperatures in the 95Pb5Sn solder. In the case of 90Pb10Sn solder, the Cu3Sn intermetallics continued to grow even after 1500 h at 170 degrees C. The difference was explained by a two-step phenomenon-Sn diffusion from the bulk solder region to the solder/Cu3Sn interface (J(Sn)), and subsequent intermetallic formation (I-Cu3Sn) by interdiffusion of Cu and Sn. For 95Pb5Sn, the relation, J(Sn) < I-Cu3Sn was postulated because of insufficient supply of Sn. The relation, J(Sn) > I-Cu3Sn was suggested for the continuous intermetallic growth of the 90Pb10Sn solder. Although a small difference was expected between the two quantities in both solder alloys, the difference in the solid-state annealing behavior was dramatic.