Co Effects upon Intermetallics Growth Kinetics in Sn-Cu-Co/Ni and Sn-Cu-Co/Cu Couples

The effects of minor Co additions in Sn-Cu solders on their interfacial reactions with Ni and Cu substrates were investigated. The growth rate constants and activation energy obtained in aging tests of Sn-0.7wt.% Cu-xCo/Ni and Sn-0.7wt.%Cu-xCo/Cu with x = 0.01, 0.04, 0.1, 0.5, and 1 wt.% have been determined. In Sn-0.7wt.% Cu-xCo/Cu couples, the intermetallics are the Cu6Sn5 and Cu3Sn phases. The total thickness of the reaction phase increases with the amount of Co added. Co facilitates the growth of the Cu6Sn5 phase. In Sn-0.7wt.% Cu-xCo/Ni couples, the intermetallics are either the Cu6Sn5 phase (x = 0.01 wt.%) or both the Cu6Sn5 and Ni3Sn4 phases (x = 0.04-1 wt.%). The total thickness of the reaction phase decreases as the amount of added Co increases. Co facilitates the growth of the Ni3Sn4 phase and thus inhibits formation of the Cu6Sn5 phase. Sn-Cu/Ni and Sn-Cu/Cu are very important soldering joints in electronic packaging, and Co is a widely-used additive in the solders. The kinetic data determined in this study are needed to assess the reliability of Sn-Cu/Ni and Sn-Cu/Cu soldering joints.