The present study continues a series of investigations of hybrid solder joints performed by authors. A major part of these studies is focused on the lead-free Sn-based solder/Cu joints with minor additions of nanosized Fe particles. The nanosized inclusions interacted with Sn-based liquid matrix during soldering process and formed intermetallic compounds (IMCs) which were detected in the analyzed solder joints. According to the binary phase diagrams of Cu-Fe and Fe-Sn, there is practically no dissolution of Fe atoms in Cu- and Sn-matrix in the range of the soldering temperatures. There is no literature data about ternary Cu-Fe-Sn phase diagram focused on the Cu-Sn side at intermediate temperatures. Therefore, it was decided to perform thermodynamic calculations of the ternary Cu-Fe-Sn system at 180 0C focusing on the Cu-Sn side. These data are very important to model the microstructure transformation during the reflow soldering process and predict the phase formation in the solder joints. The calculations were performed using the ThermoCalc package considered ns diffusion coefficients for the liquid, FeSn2, Cu3Sn, Cu6Sn5, and FCC phases. The diffusion coefficients of all other phases, which could exist in the Cu-Sn based ternary alloys were set to zero per default.