Effects of Co Alloying and Size on Solidification and Interfacial Reactions in Sn-57 wt.%Bi-(Co)/Cu Couples

Solidification and interfacial reactions in Sn-57 wt.%Bi-(Co)/Cu couples are investigated. The addition of 0.05 wt.% Co and 0.5 wt.% Co and changes between 2 g and 20 mg solder sizes have no significant effects on the undercooling of Sn-57 wt.%Bi solders. Both η-Cu₆Sn₅ and ε-Cu₃Sn phases are formed in Sn-57 wt.%Bi/Cu couples reacted at 80°C, 100°C, and 100°C, whereas only η-Cu₆Sn₅ is formed at 160°C. The formation of ε-Cu₃Sn is suppressed and only η-Cu₆Sn₅ is found in the couple with 0.05 wt.% Co and 0.5 wt.% Co addition in Sn-57 wt.%Bi solder. Both the growth rate of η-Cu₆Sn₅ and the dissolution rate of the Cu substrate increase with Co addition. The morphology of η-Cu₆Sn₅ is also altered with Co addition, becoming a porous structure with solder trapped in the voids.