Abstract
Sn-Co and Sn-Co-Cu show promise as Pb-free solders. Interfacial reactions in Sn-Co-(Cu)/Ni couples reacted at 180°C and 210°C, with and without current stressing, were examined. In both the Sn-0.05 wt.%Co/Ni and Sn-0.5 wt.%Co/ Ni couples without current stressing, the (Ni,Co)Sn 4 phase and the Ni 3 Sn 4 phase were formed at the interface. After passing an electrical current of 500 A/cm 2 through the sample, the growth rates of both the (Ni,Co)Sn 4 and the Ni 3 Sn 4 phase were enhanced at the cathode side and retarded at the anode side. With longer reaction time and current stressing, the (Ni,Co)Sn 4 phase was not found along the interface at the anode side. If the presence of (Ni,Co)Sn 4 phase is ignored, the results are similar to those for the Sn/Ni couples. Surprisingly, it was found that the effective charge of Co should be negative, which is different from the value found in the literature. In the Sn-0.7 wt.%Cu-0.05 wt.%Co/Ni and Sn-0.7 wt.%Cu-0.5 wt.%Co/Ni couples, the reaction product was the Cu6Sn5 phase in the early stage, and the Ni 3 Sn 4 phase was formed after longer reaction time. After stressing with a 500 A/cm 2 electrical current, growth of Cu6Sn5 was enhanced at the cathode and retarded at the anode. These results are similar to those for Sn-0.7 wt.%Cu/Ni couples. However, the growth of the Ni 3 Sn 4 phase is retarded at the cathode side and nearly unaffected at the anode. The differences in the growth rates of Ni 3 Sn 4 in the Sn-Co-Cu/Ni couples with 500 A/cm 2 current stressing are caused by the competitive formation of Cu6Sn5, which is a secondary effect of electromigration. © 2012 TMS.