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Section I: Basic and Applied Research

Thermodynamic Modeling of the Ag-Au-Sb Ternary System E. Zoro, C. Servant and B. Legendre

(Submitted March 14, 2006) A thermodynamic assessment of the Ag-Au-Sb ternary system has been carried out using the CALPHAD method based on new thermodynamic and phase equilibria data that we have recently determined. The Ag-Sb binary system has also been reoptimized. The calculated phase diagram and thermodynamic properties of the ternary system Ag-Au-Sb show satisfactory general agreement with the experimental data. The liquidus projection and monovariant valleys of the ternary system are well reproduced by the optimized thermodynamic model parameters. Additional experiments to confirm the proposed reaction scheme, which involves two ternary transition peritectic reactions, are suggested.

Keywords

Ag-Au-Sb (silver-gold-antimony) system, Calphad method, COST 531, optimization, phase diagram, solder, thermodynamics

1. Introduction Due to environmental and health concerns, a substantial amount of research is currently being carried out to develop new solder alloys capable of replacing the traditional leadbased solder alloys.[1] To design suitable lead-free substitute solders, it is necessary to evaluate several properties of the various candidate alloy systems. The properties required to be known include the phase diagram and thermodynamic properties since these provide basic information on the phase stability of each phase involved in a stable or metastable equilibria, liquidus and solidus temperatures, phase compositions and phase fractions at any given alloy composition and temperature. In addition, the thermodynamic approach will help to understand and predict the various possible interfacial reactions between the solder and the substrate materials. Therefore, a detailed knowledge of the thermodynamic properties and phase equilibria for these alloy systems obtained via critical thermodynamic evaluations based on sound thermodynamic models is an essential first step in the successful design of new solder alloy systems. The components of the Ag-Au-Sb ternary system are among the selected elements defined by the COST 531 (COope´ration Scientifique et Technique)[2] program as leadfree solder candidates. The only previous experimental E. Zoro and C. Servant, Laboratoire de Physico-Chimie de lÕEtat Solide, ICMMO, UMR 8182, Baˆt 410-415, Orsay Cedex 91405, France; E. Zoro and B. Legendre, Laboratoire de Chimie Physique Mine´rale et Bioinorganique, EA401, Fac. Pharm., Universite´ de ParisSud XI, 5, rue JB Cle´ment, Chaˆtenay Malabry 92296, France; Contact e-mail: [email protected]

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study of the Ag-Au-Sb ternary system is that published by Burkhard et al.[3] These authors showed that the ðeÞ Ag3Sb phase extends into the Ag-Au-Sb system as a single phase up to about 10 at.% Au. In our more recent paper,[4] the Ag-Au-Sb phase diagram was studied experimentally using x-ray diffraction (XRD) analysis, electron-probe microanalysis (EPMA), and differential scanning calorimetry (DSC).