Two different Sn-Cu/Co and Sn/Co-Cu reaction couples were examined at temperatures ranging from 150 degrees C to 200 degrees C to examine the Cu alloying effect by varying Cu contents. Two reaction phases, CoSn(3) and (Cu,Co)(6)Sn(5), were observed simultaneously at the interfaces of the Sn-Cu/Co couples (0.1 wt%-3 wt%Cu). The CoSn(3) layer was much thicker than (Cu,Co)(6)Sn(5), and CoSn(3) exhibited a linear growth rate at the initial reaction stage. Remarkably, minor Cu addition into the solders resulted in strongly suppressing the layer growth. On the other hand, the potential Co-Cu alloy substrate was also considered because of its impact on decreasing the growth rate. Sn/Co-Cu (less than 10 wt%Cu) interfacial reactions were carried out. Only one CoSn(3) phase layer was produced. In addition, the temperature dependence of the reaction layer growth was also examined. Compared with the binary Sn/Co couple, the layer growth rates in the Sn-Cu/Co and Sn/Co-Cu couples are relatively lower and more temperature-sensitive. (C) 2011 Elsevier B.V. All rights reserved.