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 eta-Cu(6)Sn(5) and epsilon-Cu(3)Sn phases are formed in Sn-57 wt.%Bi/Cu couples reacted at 80 degrees C, 100 degrees C, and 100 degrees C, whereas only eta-Cu(6)Sn(5) is formed at 160 degrees C. The formation of epsilon-Cu(3)Sn is suppressed and only eta-Cu(6)Sn(5) 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 eta-Cu(6)Sn(5) and the dissolution rate of the Cu substrate increase with Co addition. The morphology of eta-Cu(6)Sn(5) is also altered with Co addition, becoming a porous structure with solder trapped in the voids.