Investigation of the Phase Equilibria in the Sn-Bi-In Alloy System SEUNG WOOK YOON, BYUNG-SUP RHO, HYUCK MO LEE, CHOONG-UN KIM, and BYEONG-JOO LEE This article presents an investigation of the phase equilibria in the Sn-Bi-In ternary alloy system, performed both by theoretical and experimental methods. Following the regular solution model and a standard thermochemical calculation, a theoretical evaluation of the phase equilibrium in the entire ternary system is conducted. The thermodynamic parameters required for the calculation are initially obtained by fitting the model to existing data available from prior studies. The theoretical results are then validated and further improved by experimental work in which alloys with several critical compositions were chosen and examined. In the experimental work, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) spectroscopy were jointly used to identify the transition temperatures and the phases in the microstructure. The resulting phase diagram agrees well both with the existing data and with the data from the current experiments. However, different from previous findings, this study finds a nonbinary nature of the Sn-BiIn and Sn-BiIn 2 quasi-binaries and nine invariant reactions, one eutectic, six peritectic and two peritectoid. The phase-reaction scheme (Scheil diagram), the liquidus projection, and the phase diagram, covering entire compositional ranges, are established. I. INTRODUCTION The ternary-phase diagram of the Sn-Bi-In system is diffi- cult to determine by experimental techniques alone, as the IN the assembly of electronic devices, solder joints are number of phases to consider is large and the equilibrium an important part of packaging integration because they fields where these phases coexist are usually confined within serve both as electrical contacts and as the mechanical sup- very narrow ranges of temperature and composition. The ports. While solder alloys of lead and tin (Pb-Sn) are the theoretical calculation of the phase diagram, namely, CAL- most commonly used, their application is limited due to the PHAD (calculation of phase diagram), can be helpful in growing demand for an assembly technology less damaging resolving such difficulties and in improving the accuracy of to the environment and to the electronic devices. Due to the phase diagram. [12,13] The CALPHAD software provides this, several Pb-free solder alloys with relatively low melting the theoretical perspective on the phase equilibrium and temperatures have been developed. [1–4] Among these, alloys assists in maintaining the consistency of the data analysis. from the tin-bismuth-indium (Sn-Bi-In) ternary system are It also can suggest specific features to search for in the data, believed to be the most promising. Prior studies indicate which can be overlooked otherwise. Once the thermodyamic that several invariant reactions exist in this alloy system at parameters necessary for the calculation are established, temperatures below 100 °C. [5–11] This allows great flexibility phase equilibria at any composition and temperature can in designing solder alloys for low-temperature applications. be predicted. However, the existing ternary-phase diagram is rather incom- In this article, we report our attempt to construct the Sn- plete and presents several inconsistencies, which hinders full Bi-In ternary-phase diagram using CALPHAD. Following appreciation of their beneficial properties. This study aims the regular solution model, the thermodynamic equation that to resolve these existing issues using theoretical prediction governs the relative stability of a particular phase is defined of the phase equilibria in conjunction with the experimen- for all phases in the system. The model equations contain tal feedback. several unknown thermodynamic parameters, which are extracted by fitting the equations into the pre-existing binary- and ternary-phase diagrams and also into the data obtained SEUNG WOOK YOON, formerly Graduate Student, Department of from our experimental work. The predicted phase diagram Materials Science and Engineering, Korea Advanced Institute of Science resulting from this method fits reasonably well with the pre- and Technology, is Research Staff Member, Package Researching Team, existing data and the data from our experiment. It provides Semiconductor Research Division, Hyundai Electronics Industries Co., Ltd., Kyoungki-do, Korea 467-701. BYUNG-SUP RHO, Graduate Student, and several new insights on the ternary-phase equilibrium, partic- HYUCK MO LEE, Professor, are with the Department of Materials Science ularly on the nature of the invariant reactions. The phase and Engineering, Korea Advanced Institute of Science and Technology, diagrams of several subsystems are presented along with the Taejon, Korea 305-701. CHOONG-UN KIM, Assistant Professor, is with supporting evidence, where experimental data are available. the Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, TX 76019-0031. BYEONG-JOO LEE, Principal They are later combined to show the complete phase dia- Investigator, is with the Microstructural Analysis Division, Materials Evalu- grams and the reaction schemes. The following section con- ation Center, Korea Research Institute of Standards and Science, Taejon, tains a brief review of prior studies on these subsystems, Korea 305-600. Manuscript submitted July 23, 1998. including the three bounding binary systems. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 30A, JUNE 1999—1503