Investigation of the phase equilibria in the Sn-Bi-In alloy system
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I. INTRODUCTION
IN the assembly of electronic devices, solder joints are an important part of packaging integration because they serve both as electrical contacts and as the mechanical supports. While solder alloys of lead and tin (Pb-Sn) are the most commonly used, their application is limited due to the growing demand for an assembly technology less damaging to the environment and to the electronic devices. Due to this, several Pb-free solder alloys with relatively low melting temperatures have been developed.[1–4] Among these, alloys from the tin-bismuth-indium (Sn-Bi-In) ternary system are believed to be the most promising. Prior studies indicate that several invariant reactions exist in this alloy system at temperatures below 100 8C.[5–11] This allows great flexibility in designing solder alloys for low-temperature applications. However, the existing ternary-phase diagram is rather incomplete and presents several inconsistencies, which hinders full appreciation of their beneficial properties. This study aims to resolve these existing issues using theoretical prediction of the phase equilibria in conjunction with the experimental feedback. SEUNG WOOK YOON, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is Research Staff Member, Package Researching Team, Semiconductor Research Division, Hyundai Electronics Industries Co., Ltd., Kyoungki-do, Korea 467-701. BYUNG-SUP RHO, Graduate Student, and HYUCK MO LEE, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, Korea 305-701. CHOONG-UN KIM, Assistant Professor, is with the Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, TX 76019-0031. BYEONG-JOO LEE, Principal Investigator, is with the Microstructural Analysis Division, Materials Evaluation Center, Korea Research Institute of Standards and Science, Taejon, Korea 305-600. Manuscript submitted July 23, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The ternary-phase diagram of the Sn-Bi-In system is difficult to determine by experimental techniques alone, as the number of phases to consider is large and the equilibrium fields where these phases coexist are usually confined within very narrow ranges of temperature and composition. The theoretical calculation of the phase diagram, namely, CALPHAD (calculation of phase diagram), can be helpful in resolving such difficulties and in improving the accuracy of the phase diagram.[12,13] The CALPHAD software provides the theoretical perspective on the phase equilibrium and assists in maintaining the consistency of the data analysis. It also can suggest specific features to search for in the data, which can be overlooked otherwise. Once the thermodyamic parameters necessary for the calculation are established, phase equilibria at any composition and temperature can be predicted. In this article, we report our attempt to construct the SnBi-In ternary-phase diagram using
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