Interfacial Reactions in Sn-Co-(Cu)/Ni Couples With/Without Current Stressing

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)Sn4 phase and the Ni3Sn4 phase were formed at the interface. After passing an electrical current of 500 A/cm2 through the sample, the growth rates of both the (Ni,Co)Sn4 and the Ni3Sn4 phase were enhanced at the cathode side and retarded at the anode side. With longer reaction time and current stressing, the (Ni,Co)Sn4 phase was not found along the interface at the anode side. If the presence of (Ni,Co)Sn4 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 Ni3Sn4 phase was formed after longer reaction time. After stressing with a 500 A/cm2 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 Ni3Sn4 phase is retarded at the cathode side and nearly unaffected at the anode. The differences in the growth rates of Ni3Sn4 in the Sn-Co-Cu/Ni couples with 500 A/cm2 current stressing are caused by the competitive formation of Cu6Sn5, which is a secondary effect of electromigration.