The melting point of SnAgBi system solders exceeds that of SnPb solder. Adding In reduces the melting point without reducing the mechanical strength of the joints. In this study, different amounts of In were added to SnAgBi solder systems to investigate the formation of intermetallic compounds at the interfaces and in the bulk solders under reflows at various temperatures for various durations. Unlike in the reactions between SnIn solders and Cu substrates, the compounds formed at the interfaces were Cu6Sn5 and Cu3Sn, without the substitution of Sn by In. The kinetics of the growth of Cu6Sn5 was analyzed. The intermetallic compounds formed in the bulk solders were identified as Cu6Sn5 and zeta solid solution, which is different from the information provided by the phase diagrams. The exact compound compositions were determined by EPMA and a thermodynamic explanation was provided. (C) 2008 Elsevier B.V. All rights reserved.