Effects of indium on the mechanical properties of ternary Sn-In-Ag solders
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NTRODUCTION
TRADITIONAL Pb-Sn solders have been widely used in various levels of electronic packaging. There are serious environmental and health issues associated with Pb-containing solders, because of Pb toxicity. Several Pb-free solders have been developed during the past decade.[1,2] Among these Pb-free solders, the binary Sn-3.5Ag eutectic alloy is the most attractive solder for the electronics industry, which provides better mechanical properties than those of the Sn37Pb eutectic solder.[3,4] However, the high melting temperature of Sn-3.5Ag solder has confined its applications in electronic packaging when using the conventional reflow soldering methods. McCormack et al.[5,6] found that adding small amounts of Zn could decrease slightly the solidus of Sn-3.5Ag eutectic solder; i.e., the melting temperature of Sn-3.5Ag-1Zn was 217 ⬚C. A high-strength and ductile solder with improved creep resistance was obtained. On the other hand, Kariya and Ostuka[7] reported that the addition of zinc up to 2 pct slightly decreased the fatigue life of Sn3.5Ag solder. The melting point of pure indium is only 157 ⬚C. The addition of indium depresses the melting temperature of SnAg solders. The indium-containing solders possess a longer fatigue life than conventional Pb-Sn solders for flip-chip interconnections in thermal-shock tests between room temperature and the temperature of liquid nitrogen.[8] Moreover, Au atoms dissolved much more slowly into the solder when the indium-containing solder was used on a Au substrate.[2,9] However, indium is an expensive metal, which made the indium-containing solder excessively expensive for most applications in electronic packaging.[10] Therefore, it is necessary to further investigate the effects of indium on the mechanical characteristics of ternary Sn-In-Ag solders in order to develop the Pb-free solder for electronic-packaging
M.S. YEH, Associate Professor, is with the Department of Mechanical Engineering, Chung-Hua University, Hsin-Chu, 300, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted June 25, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
applications. In this present study, a series of ternary Sn-InAg solders were tested at a strain rate of 10⫺3 s⫺1 at various homologous temperatures. In addition, the mechanical properties and fracture mold of this alloy were also in-vestigated. II. EXPERIMENTAL In this study, a series of ternary Sn-In-Ag alloys were prepared by melting pure metals (99.99 wt pct) into 100 g ingots within a quartz tube at 600 ⬚C for 12 hours. These ingots were rolled to 1-mm-thick sheets and then homogenized at 95 ⬚C for 100 hours before tests. The thermal analyses for various Sn-In-Ag alloys were conducted with differential scanning calorimetry (DSC) with a heating rate of 10 ⬚C/min under a nitrogen atmosphere. The microstructures were observed with a scanning electron microscope (SEM) after being etched in a 15 vol pct HF-15 vol pct H2O2-70 vol pct H2O solution for 2 seconds. Tensile specimens were cut from the strips with a
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