Solderability and interfacial reaction between 43Sn-57Bi-X solders and Cu-substrate were studied. The solderability of 43Sn-57Bi-X solders on Cu substrate was examined as a function of soldering temperature, soldering time and types of flux. Cu/solder joints were subjected to aging treatments for up to 60days to see interfacial reaction at 100℃ and then examined changes of microstructure and interfacial compound by optical microscopy and EDS. Addition of small amount of Cu, Ag and Zn, respectively, to solder result in decreased spread area on Cu substrate with RMA-flux treatment. but addtion of Ag and Zn showed increased spread area with R-flux treatment. According to the result of EDS, it was analyzed that the interfacial zone between solder and Cu-substrate was composed of Cu₃Sn and Cu_6Sn_5. But addition of Zn resulted in suppression of Cu_xSn_y, compound formation because of superior tendency to Cu_xZn_y compound formation.