Bismuth activity in lead-free solder Bi-In-Sn alloys

The thermodynamics of Bi-In-Sn lead-free solder alloys has been studied by two different experimental techniques: torsion-effusion and differential scanning calorimetry. The results show that this ternary system at fixed Bi composition, XBi=0.20, and 565 K behaves as a non-ideal system with exothermic ΔmixHT0 passing through a minimum at XIn=0.58 and XSn=0.22. In the whole range of the quantity XSn/(XSn+XIn) and at 1050 K the average value of the bismuth activity is 0.14±0.02 which implies an average activity coefficient of Bi equal to 0.70±0.01. The Bi activity in ternary alloys at 1000 K with variable XBi and fixed ratio ρ=XSn/XIn=0.85 has been measured. The excess integral free energy change for the mixing, ΔmixGT0(xs), has also been evaluated by making use of both the torsion-effusion experimental and literature data for Bi-Sn and Bi-In binary systems. Under these conditions, the ternary system is not a regular solution and the entropy contribution to the free energy of mixing is dominating the Gibbs energy.