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 Delta(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) T 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 rho = X-Sn/X-In = 0.85 has been measured. The excess integral free energy change for the mixing, Delta(mix) G(T)(0) (xs), has also been evaluated by making use of both the torsion-effusion experimental and literature data T 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. (C) 2006 Elsevier Ltd. All rights reserved..