Co alloying and size effects on solidification and interfacial reactions in the Sn–Zn–(Co)/Cu couples

Sn–Zn based alloys are promising as Pb-free solders, and Cu is commonly used in electronic products. Solidification and interfacial reactions of Sn–8.8wt%Zn, Sn–8.8wt%Zn–0.05wt%Co, and Sn–8.8wt%Zn–0.5wt%Co solders on Cu substrates are investigated. Two different masses of solders are used. The degrees of undercooling increase with increasing Co additions in the Sn–8.8wt%Zn alloys. The reaction products evolve with reaction time, and the timing of different reaction stages is influenced by both the minor Co alloying and the mass of solders. In the initial reaction stage, two reaction phases, γ-Cu₅Zn₈ and ε-CuZn₅, are observed in the Sn–8.8wt%Zn/Cu and Sn–8.8wt%Zn-0.05wt%Co/Cu couples, and only the γ-Cu₅Zn₈ phase is found when the Co addition is up to 0.5 wt%. The reaction layers are thinner with higher Co alloying. The addition of Co into the Sn–Zn alloys consumes Zn, and this depletion of Zn in the Sn–Zn solders is the primary reason for the changes of reaction products and the thinner reaction layers.