Directional Solidification and Liquidus Projection of the Sn-Co-Cu System
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HE Pb-free solders have attracted intensive investigation, primarily because of Pb-free regulations and various new applications, such as flip chip soldering and through silicon via technology.[1–18] Sn-based solders usually have significant undercooling, and most of their undercooling can be effectively reduced with a small amount of Co alloying.[5–7] It was reported that the Sn-Co-Cu alloys could be less expensive alternatives to the Sn-Ag-Cu alloys.[8–12] The thermodynamic modeling, mechanical properties, and interfacial reactions of Sn-Co-Cu have already been investigated.[8–12] During soldering processes, solders melt, wet the substrates, and then solidify. Solidification behaviors and the resulting microstructures are crucial to electronic solders. The liquidus projection consists of the lines of liquidus trough, i.e., univariant lines, and their temperature descending directions, and the liquidus projection is important for the understanding and illustration of the alloy’s solidification behaviors.[15–18] This study thus determines the liquidus projection of the Sn-Co-Cu ternary system, since there are no studies presenting experimental results. Liquidus trough lines delineate the boundaries of the primary solidification phases, i.e., the first solid phase coming out of the liquid, and thus the results of primary solidification phases can be used to construct the SINN-WEN CHEN, Professor, JUI-SHEN CHANG, Doctoral Student, KEVIN PAN, Master Student, CHIA-MING HSU, Postdoctoral Researcher, and CHE-WEI HSU, Master Student, are with the Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan, ROC. Contact e-mail: [email protected] Manuscript submitted July 13, 2012. Article published online December 11, 2012 1656—VOLUME 44A, APRIL 2013
liquidus trough lines. The primary solidification phases can be determined from the as-solidified samples, and the phase transformation temperatures are determined using differential thermal analysis (DTA). A liquidus projection of the Sn-Co-Cu system is proposed based on these results and the phase diagrams of the three binary constituent systems.[19–21] Primary solidification phases are determined based on the as-solidified microstructures. However, their identifications are not as obvious for some alloys. Directional solidification is a controlled solidification technique so that solidification occurs from one end of the molten sample progressively toward the other end. Since solidification sequences can be determined much more easily from the directionally solidified microstructures, the directional solidification technique[22,23] is used in this study when the primary solidification phases are difficult to identify from the conventionally solidified samples.
II.
EXPERIMENTAL PROCEDURES
A. Samples Solidified with Conventional Quenching Method Sn-Co-Cu alloys were prepared with pure Sn shots (99.98 pct, Showa, Tokyo), Co shots (99.9 wt pct, Alfa Aesar, Ward Hill, MA), and Cu shots (Sigma-Aldrich, Milwaukee, WI). Proper amounts of constituent elements with a total
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