Thermodynamic study of phase equilibria in the Sn-Ag-Zn system

Due to its toxicity, the development of a new Pb-free solder that would replace the traditional Pb-Sn alloys becomes the pressing need. The present study was undertaken to examine the phase equilibria in the Sn-Ag-Zn ternary system using the CALPHAD (CALculation of PHAse Diagrams) approach, which is one of the new Pb-free solder alloys. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. A set of thermodynamic parameters for the individual phases was achieved by combining the experimental results obtained by differential scanning calorimetry and electron probe microanalysis in the present study with literature data on phase boundaries and thermochemical properties. The optimized parameters reproduced the experimental results satisfactorily. The liquidus surface of the ternary system shows that the ternary eutectic point falls at 3.7%Ag, 0.9%Zn and 216 degrees C. The melting point is lowered up to 193 degrees C by an addition of Zn, and therefore the adjustment of the melting temperature might enable determination of the appropriate composition of a new Pb-free solder.