Abstract
Abstract Development of lead-free solders is an important subject in the electronic industry. Present investigations of lead-free solders focus on the wettability, interfacial reaction and mechanical properties. Thermodynamic properties of lead-free solder alloys which are important but only few information is available, are investigated in this study. Thermodynamic properties, such as melting temperatures, fusion enthalpies and activities, of Sn-Bi, Sn-Zn, Sn-Bi-Zn lead-free solders materials are determined. The melting temperatures and fusion enthalpies of alloys are measured by differential scanning calorimetry. Reliable electromotive force method (EMF method) is developed to measure the activities of Sn or Zn. EMF method measures the open circuit potential between the Sn (or Zn) and alloys in liquid state. The equilibrium potentials of the difference of the red-ox ability of pure element and the alloy samples are determined. The relationship of the potentials and the activities and other thermodynamic properties are calculated by using the thermodynamic and electrochemical equilibrium functions. The mixing enthalpies are very small in the Sn-Bi alloys. The activities of Sn in Sn-Bi alloys are nearly ideal and show slightly positive deviation. The activities of Zn in Sn-Bi-Zn and Sn-Zn alloys show positive deviation; in the same content of Zn and temperature, the activities of Zn in Sn-Bi-Zn alloys are larger than in Sn-Zn alloys, but mixing Gibbs free energy of Sn-Bi-Zn alloys are lower than Sn-Zn alloys. The invariant temperature of the class I: L ↔ (Sn) + (Bi) + (Zn) phase transformation in Sn-Bi-Zn alloys is 134 oC.