Thermodynamic Properties of Liquid Ag-Bi-Sn Alloys ZUOAN LI, 1 SABINE KNOTT, 1 and ADOLF MIKULA 1,2 1.—Institut fu ¨ r Anorganische Chemie/Materialchemie, Universita ¨ t Wien, A-1090 Wien, Austria. 2.—E-mail: adolf.mikula@univie.ac.at As a promising lead-free solder, the thermodynamic properties of the liquid ternary Ag-Bi-Sn system were investigated. Using an appropriate galvanic cell, the partial free energies of Sn in liquid Ag-Bi-Sn alloys were determined as a function of concentration and temperature. Thermodynamic properties were obtained for 27 alloys. Their composition was situated on three cross sections with the constant ratios of Ag:Bi = 2:1, 1:1, and 1:2. The integral Gibbs free energy and the integral enthalpy for the ternary system at 900 K were calculated by Gibbs–Duhem integration. Key words: Silver-bismuth-tin, lead-free solders, liquid alloys, thermodynamic properties, electromotive force measurements INTRODUCTION Lead-tin base solders have long been the most popular materials for electronic packaging because of their low cost and superior properties required for interconnecting electronic components. However, the toxic nature of lead and the increasing awareness of its adverse effect on environment and health have led to the pressing need for development of lead-free solders in recent years. New lead-free solders, which will be used as the alternative of Sn-37Pb solder, must meet some required material properties, such as low melting temperature, good wettability, and excellent mechanical properties. Thermodynamic data are of great importance for the accurate calcu- lation of phase diagrams, for the development of lead- free solder database, for the design of new lead-free solders, and for the prediction of physical and chemical properties of lead-free solders, such as surface tension and viscosity. 1,2 In our group, the thermodynamic properties of a series of lead-free solders including Ag-Sn-Zn, Cu-Sn-Zn, In-Sn-Zn, and Al-Sn-Zn have been investigated completely. 3–6 Previous investigations have proposed the Ag-Bi- Sn alloy as a promising lead-free solder, because it is superior to other candidates with respect to melting properties, wettability, and mechanical proper- ties. 7,8 Unfortunately, the data of thermodynamic properties and the phase diagram are scarce for the ternary Ag-Bi-Sn system. In the present investiga- tion, the thermodynamic properties of tin were measured with an electromotive force (emf) method at three cross sections with a constant Ag:Bi molar ratio of 2:1, 1:1, and 1:2. A Gibbs–Duhem integration was carried out to determine the integral thermo- dynamic properties of the entire ternary system. EXPERIMENTAL PROCEDURE The ternary alloys were prepared from starting materials of high-purity 5 N metals (from Johnson Matthey GmbH, Karlsruhe, Germany). In order to remove the oxide layer from the surface, Sn was polished with a fine emery paper, Bi was cleaned prior to its use by melting it under vacuum and fil- tering it through quartz wool under a purified argon atmosphere, and Ag was heated in a carbon crucible for 10 min at 973 K to remove Ag 2 S. The metals were weighed and sealed in quartz tube and melted at 873 K for 5 days. Afterward, the sam- ples were quenched in cold water. Approximately 2 g of each alloy was used for the emf measurements. The liquid electrolyte for the emf measurements was a eutectic mixture of KCl and LiCl. The prep- aration of electrolyte and the assembling of the cell is described in Ref. 9. The 0.5 mol.% of potential- forming Sn +2 ion salt—dehydrated SnCl 2 —could not be added directly to the KCl-LiCl because the chlorine gas would oxidize the Sn +2 to Sn +4 , and it was added directly into the emf cell before the measurements. Tungsten wire was used for the (Received March 17, 2005; accepted June 27, 2005) Journal of ELECTRONIC MATERIALS, Vol. 36, No. 1, 2007 Regular Issue Paper DOI: 10.1007/s11664-006-0002-7 Ó 2006 TMS 40