Experimental and thermodynamic assessment of Sn-Ag-Cu solder alloys

Sn-rich alloys in the Sn-Ag-Cu system are being studied for their potential as Pb-free solders. Thus, the location of the ternary eutectic involving L, (Sn), Ag sub(3)Sn and Cu sub(6)Sn sub(5) phases is of critical interest. Phase diagram data in the Sn-rich corner of the Sn-Ag-Cu system are measured. The ternary eutectic is confirmed to be at a composition of 3.5 wt.% Ag, 0.9 wt.% Cu at a temperature of 217.2 plus or minus 0.2 degree C (2 sigma ). A thermodynamic calculation of the Sn-rich part of the diagram from the three constituent binary systems and the available ternary data using the CALPHAD method is conducted. The best fit to the experimental data is 3.66 wt.% Ag and 0.91 wt.% Cu at a temperature of 216.3 degree C. Using the thermodynamic description to obtain the enthalpy-temperature relation, the DTA signal is simulated and used to explain the difficulty of liquidus measurements in these alloys.