Thermodynamic Calculation of Phase Equilibria in the Sn-Ag-Cu-Ni-Au System

Sn-Ag-Cu base solders are the most promising candidates to substitute for Sn-Pb eutectic solder. Gold (Au) coatings are used to protect conductor surfaces from oxidation and thereby to promote solderability, and Ni is often used as a diffusion barrier layer between lead-free solders and substrates to restrict the growth of intermetallic compound layers. In the present work, thermodynamic calculations of phase equilibria in the Sn-Ag-Cu-Ni-Au system, which is of importance for developing lead-free solders, are carried out using the calculation of phase diagrams (CALPHAD) method. Substitutional solution and sublattice models are used to describe the solution and intermediate phases, respectively. Some examples of thermodynamic calculation are presented, and it is shown that the phase diagrams, liquidus projection, and thermodynamic properties can be predicted based on the present calculations.