The phase equilibria of the Cu-In-Sn system were investigated by means of the diffusion couple method, differential scanning calorimetry (DSC) and metallography. The isothermal sections at 110-900 degreesC, as well as vertical sections at 10wt.%Cu-70wt.%Cu were determined. It was found that there are large solubilities of In in the epsilon (Cu3Sn), delta (Cu41Sn11), and eta phases in the Cu-Sn system, and large solubilities of Sn in the gamma, eta, and delta (Cu7In3) phases in the Cu-In system. The eta phase was found to continuously form from the Cu-In side to the Cu-Sn side, and a ternary compound (Cu2In3Sn) was found to exist at 110 degreesC. Thermodynamic assessment of the Cu-In-Sn system was also carried out based on experimental data of activity and phase equilibria using the CALPHAD method, in which the Gibbs energies of the liquid, fcc and bcc phases are described by the subregular solution model and that of compounds, including two ternary compounds, are represented by the sublattice model. The thermodynamic parameters for describing the phase equilibria were optimized, and agreement between the calculated and experimental results was obtained
