The effects of the fourth elements, i.e., Ni and Co, on microstructural features of Sn-3wt%Ag-0.5wt%Cu lead-free solder and Cu joints were investigated. The interfacial phases of Sn-3Ag-0.5Cu/Cu joints are typical Cu6Sn5 scallops. Sn-3Ag-0.5Cu (-0.1wt%X; X = Ni and Co)/Cu joints form very fine Sn-Cu-Ni and Sn-Cu-Co scallops at interface. The growth kinetics of total ternary compound layers was examined. The apparent activation energies of Sn-Cu-Ni and Sn-Cu-Co layers were 64 and 112 kJ/mol, respectively.