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, X Bi = 0.20 , and 565 K behaves as a non-ideal system with exothermic Δ mix H T 0 passing through a minimum at X In = 0.58 and X Sn = 0.22 . In the whole range of the quantity X Sn / ( X Sn + X In ) 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 X Bi and fixed ratio ρ = X Sn / X In = 0.85 has been measured. The excess integral free energy change for the mixing, Δ mix G T 0 ( x s ) , 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.