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 3 Sn and Cu 6 Sn 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 ± 0.2 °C (2σ). 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 °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.

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This paper presents experimental results and theoretical calculations to evaluate the effects of Pb contamination on the solidification behavior of Sn -x Bi alloys (x = 5, 10, and 58 mass %). The pasty (mushy) range, the type of solidification path, and the fraction of the ternary eutectic are described. The experimental results are obtained from thermal analysis and quantitative metallography, and the solidification calculations are performed using Lever and Scheil assumptions. The experimental results agree with the Scheil calculations. The freezing range of Pb contaminated Sn-Bi solders is greatly increased due to the formation of a ternary eutectic reaction at (95.3 ± 0.5)°C. This increase is a likely cause of porosity in contaminated solder joints. The results provide an example of an analysis method for use in solder alloys in general.