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

Abstract

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.