Microstructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying |
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Authors: | I. E. Anderson B. A. Cook J. Harringa R. L. Terpstra |
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Affiliation: | (1) Metal and Ceramic Sciences Program, Ames Laboratory (U.S. Department of Energy), Iowa State University, 50011 Ames, IA |
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Abstract: | Slow cooling (1–3°C/sec) of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens, made by hand soldering, simulated reflow in a surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shear-strength testing, using Sn-3.5Ag (wt.%) as a baseline. Consistent with predictions from a recent Sn-Ag-Cu ternary phase-diagram study, either Sn dendrites, Ag3Sn primary phase, or Cu6Sn5 primary phase were formed during solidification of joint samples made from the selected near-eutectic Sn-Ag-Cu alloys. Minor substitution of Co for Cu in Sn-3.7Ag-0.9Cu refined the joint-matrix microstructure by an apparent catalysis effect on the Cu6Sn5 phase, whereas Fe substitution promoted extreme refinement of the Sn-dendritic phase. Ambient-temperature shear strength was reduced by Sn dendrites in the joint microstructure, especially coarse dendrites in solute poor Sn-Ag-Cu, e.g., Sn-3.0Ag-0.5Cu, while Sn-3.7Ag-0.9Cu with Co and Fe additions have increased shear strength. At elevated (150°C) temperature, no significant difference exists between the maximum shear-strength values of all of the alloys studied. |
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Keywords: | Sn-Ag-Cu solder Sn-Ag eutectic solder joint microstructure joint shear strength Cu-Sn intermetallics Sn dendrites |
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