Thermodynamic assessment of phase equilibria in the Sn-Au-Bi system with key experimental verification

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shikazu Takaku, Ikuo Ohnuma, and Kiyohito Ishida Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan (Received 20 April 2009; accepted 19 October 2009)

The phase equilibria at 200  C, 250  C, 300  C, and 400  C and the phase transformation of the Sn-Au-Bi system were investigated by using the electron probe micro-analyzer (EPMA) and differential scanning calorimeter (DSC), respectively. It is found that there is a new ternary intermetallic compound with a possible AuSn structure (called the j phase in the present work), which has a limited solubility of Au in the Au-rich portion, and the j phase decomposes peritectically at about 313  C. Based on the experimental data reported in the previous papers and new experimental data determined by the present work, thermodynamic assessments of the Sn-Au-Bi system were carried out by the calculation of phase diagrams (CALPHAD) method. The thermodynamic parameters for describing the Gibbs free energy of each phase were optimized, and reasonable agreement between the calculated results and experimental data was obtained in the Sn-Au-Bi ternary system.

I. INTRODUCTION

In recent years, because of the health and environmental safety problems caused by conventional Sn-Pb solders, much attention has been given to the development of Pb-free solders for electronic interconnection materials.1–4 Combined with the development of new technologies of soldering in electronics, solders with various melting temperatures are needed. Generally, gold (Au) coatings have been used to protect the conductor surface from oxidation and thereby to promote the solderability.4 In addition, the Sn20Au80 (wt%) alloy is also an important high-temperature solder used in electronic packages and the Bi-based alloys are the new type of high-temperature Pb-free solders.5 Therefore, information on the phase diagram and thermodynamic properties of the Sn-Au-Bi system is useful not only in the design of the Pb-free solders, but also in the research of the interfacial reaction between Bi-based alloys and Au coating. The calculation of phase diagrams (CALPHAD) method has been recognized as an important tool in the design of materials because it significantly decreases the amount of required experimental work.6 Recently, the present a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0056

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J. Mater. Res., Vol. 25, No. 3, Mar 2010 Downloaded: 15 Mar 2015

authors have developed a thermodynamic database for phase diagrams in micro-soldering alloys including the elements of Pb, Bi, Sn, Sb, Cu, Ag, and Zn.7–9 The Sn-Au-Bi ternary system is an important one in both the design of the new solders and the understanding of the interfacial reactions.10–15 In the present work, the phase equilibria at 200  C, 250  C, 300  C, and 400  C in the Au-rich portion of the Sn-Au-Bi system and phase transformation were experimentally investigated, and the phase equilibria in the Sn-Au-Bi system were

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