Thermodynamic calculation-aided design Pb-free solders and related experimental research

A new solder alloy with the composition of Sn-3.0Ag-0.5Cu-4.5Bi was developed by means of thermodynamic calculation (CAPHAD) method. The Sn-Ag-Cu eutectic system was further optimized by adding the forth element of Bi in order to improve its melting behavior and mechanical properties; the melting point was reduced by about 5 degree compared to the former system. Its non-equilibrium solidification process was simulated according to the Scheil-Gulliver model and the phase transformation during cooling was predicted; as predicted, the solid phase comes out with β-Sn, η-Cu₆Sn₅, Ag₃Sn and a little Bi-rich phase in sequence. In the experiment procedure, the designed solder was re-melted and prepared using vacuum equipment. Its melting temperature was measured by differential scanning calorimeter (DSC); its microstructure was investigated by means of optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). As observed, the structure of the new alloy consists of matrix β-Sn, eutectic area and small Bi grains which dispersed in the matrix; most Ag₃Sn are fibrous or needlelike, less flaky or granular Cu₆Sn₅ distribute dispersedly; Ag₃Sn and Cu₆Sn₅ mostly present in forms of block at concentrative area. The above calculation and simulation agree with the experimental results well.