Phase Equilibria of Sn-Co-Cu Ternary System
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DUCTION
IN addition to the commonly used Sn-Ag, Sn-Cu, and Sn-Ag-Cu solders, there are various kinds of promising Pb-free solders.[1–10] Sn-Co-Cu alloys have been studied as potential Pb-free solders[6–8] primarily because it has been found that a minor amount of Co addition in the Sn-based solder can significantly reduce the undercooling effect and effectively refine the solder microstructures.[9,10] Knowledge of Sn-Co-Cu phase equilibria is fundamentally important for the development and applications of the Sn-Co-Cu solders. However, there are very few reports on the Sn-Co-Cu phases[6–8] and no experimental determinations of the Sn-Co-Cu phase diagrams. In this study, Sn-Co-Cu ternary alloys are prepared and equilibrated at 523 K, 1073 K, and 1273 K (250 °C, 800 °C, and 1000 °C), and the phases formed are determined. Three isothermal sections of the Sn-Co-Cu ternary system of the respective temperatures are then constructed based on the ternary phase equilibria data determined in this study and those in the literature, as well as the phase diagrams of the three binary constituent systems, Sn-Co,[11,12] Sn-Cu,[13,14] and Co-Cu.[15] II.
EXPERIMENTAL PROCEDURES
Sn-Co-Cu alloys were prepared with pure Sn shots (99.985 wt pct, Showa, Tokyo), Co foil (99.9 wt pct, YU-KAI CHEN, formerly Master Student, Department of Chemical Engineering, National Tsing Hua University, Hsin-chu 300, Taiwan, R.O.C., is now an Engineer with VIA, Taipei 231, Taiwan, R.O.C. CHIA-MING HSU, Postdoctoral Research Fellow, SINN-WEN CHEN, Professor, and CHIH-MING CHEN, Master Student, are with the Department of Chemical Engineering, National Tsing Hua University. Contact e-mail: [email protected] YU-CHIH HUANG, formerly Doctoral Student, Department of Chemical Engineering, National Tsing Hua University, is now a Researcher with TSMC, Hsin-chu 300, Taiwan, R.O.C. Manuscript submitted November 22, 2011. Article published online June 6, 2012 3586—VOLUME 43A, OCTOBER 2012
Alfa Aesar, Ward Hill, MA), and Cu shots (99.99 wt pct, Sigma Aldrich, St. Louis, MO). These pure elements were weighed in proper proportions using an electronic balance (Mettler Toledo AG245, Toledo, OH) and then alloyed together by arc melting. The weight of each alloy was about 1 gram. For homogenization, each alloy was melted and turned upside down repeatedly at least five times. The alloys were weighed before and after arc melting to ensure the mass losses were less than 0.5 wt pct. The alloys were encapsulated in quartz tubes in a 10 5 bar vacuum. The sample was heat treated at 1273 K (1000 °C) for 5 days and quenched in water. The quenched sample was equilibrated at 1273 K, 1073 K, and 523 K (1000 °C, 800 °C, and 250 °C) for 20, 45, and 390 days, respectively. The equilibrated samples were then metallographically analyzed. The microstructures were determined by optical microscopy (Olympus, Tokyo) and scanning electron microcopy (JEOL,* JSM5600). The compositions were *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
determined by electron probe microanalysis (EPMA,
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