Microstructure, Wetting, and Tensile Behaviors of Sn-Ag Alloy Reinforced with Copper-Coated Carbon Nanofibers Produced b
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INTRODUCTION
IN the past few years, considerable efforts have been directed toward the development of composite solders as Pb-free alternatives to satisfy Pb-free regulations and joint-strength requirements.[1–3] These composite solders are primarily Sn-based binary and ternary alloys.[4] The popular binary alloys include Sn-Cu, Sn-Ag (SA), Sn-Bi, and Sn-Zn, and the ternary alloys include Sn-Ag-Cu, Sn-Ag-Bi, and Sn-Zn-Al.[4–8] Among these, Sn-3.5Agbased solders have been recommended as replacements for Sn-Pb solders. The melting point (221 C) of SA is slightly higher than that of the popular SAC305 (219 C).[1–8] SA alloys have a reasonable creep resistance and viscoplastic/bending fatigue characteristics.[4,9,10] Ag improves the wetting, adhesion, and impact reliability. In general, a Ag content of ‡ 3 pct is beneficial for wetting to Cu conductors, but a high Ag content of > 3.5 wt pct may reduce the reliability owing to the formation of large Ag3Sn.[11] Despite these benefits, ASHUTOSH SHARMA and BYUNGMIN AHN are with the Department of Materials Science and Engineering and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea. Contact e-mail: [email protected] ASHOK K. SRIVASTAVA is with the Department of Metallurgical Engineering, SOE, OP Jindal University, Raigarh (C.G.), India. Manuscript submitted January 13, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
there have been few studies on the mechanical behavior of SA solders due to a coarse b-Sn dendritic microstructure. SA alloys are continuously being improved to effectively refine the microstructure of these alloys.[4–6,9,10] Various researchers have attempted to control the intermetallic compound (IMC) shape and distribution of composite solders by adding either metallic[12–16] or nonmetallic/ceramic nanoparticles.[17–29] Nadia et al. used Cu nanoparticles in an SA solder and reported improvements in the wetting and strength.[12] Nishikawa et al. studied the addition of Ni and Co to an SA solder. They showed that Ni and Co refined the interfacial compounds significantly.[14] Intermetallic or ceramic nanoparticles have also been used frequently, as they are not reactive to the matrix. Choi et al. added Cu6Sn5, Ni3Sn4, and FeSn2 IMCs to an SA alloy. They found a remarkable improvement in the creep resistance when the FeSn2 IMC was added to the alloy.[9,10] Sharma et al. showed that CeO2 is effective for the mitigation of whiskers in the solder finish and protects against adverse environmental conditions.[5] Shen et al. significantly refined the Ag3Sn IMCs and improved the microhardness up to 50 pct by adding ZrO2 nanoparticles to SA alloy.[4] Similarly, Babaghorbani et al. found 18 and 31 pct improvement in yield strength and tensile strength after addition of SnO2 nanoparticles in SA alloy.[20] Additionally, it has been reported that the ductility and strength can be enhanced if reinforcements are introduced at an optimum content.[24]
The latest additions to the category of ceramic additives are C nanomaterials, e.g., C nanotubes (CNTs),
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