Abstract
The limited number of independent β-Sn grain orientations that typically form during solidification of Sn-based solders and the resulting large β-Sn grain size have major effects on overall solder performance and reliability. This study analyzes whether additions of Al to Sn-Cu and Sn-Cu-Ag alloys can be used to change the grain size, morphology, and twinning structures of atomized (as-solidified) and re-melted (reflowed) β-Sn dendrites as determined using scanning electron microscopy and electron backscatter diffraction for as-solidified and reflow cycled (20–250°C, 1–5 cycles) Sn-Cu-Al and Sn-Ag-Cu-Al drip atomized spheres (260 μm diameter). The resulting microstructures were compared to as-solidified and reflow cycled Sn-Ag-Cu spheres (450 μm diameter) as well as as-solidified Sn-Ag-Cu, Sn-Cu, and Sn-Ag microstructures from the literature. Previous literature observations reporting reductions in undercooling and β-Sn grain size with Al micro-alloying additions could not be correlated to the presence of the Cu9Al4 phase or Al solute. The as-solidified spheres displayed no change in β-Sn dendrite structure or grain size when compared to non-Al-modified alloys, and the reflow cycled spheres produced high undercoolings (22–64°C), indicating a lack of potent nucleation sites. The current findings highlighted the role of Ag in the formation of the interlaced twinning structure and demonstrated that with deliberate compositional choices, formation of the alloy’s β-Sn grain structure (cyclical twinning versus interlaced twinning) could be influenced, in both the as-solidified and reflow cycled states, though still not producing the fine-grain sizes and multiple orientations desired for improved thermomechanical properties.
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Acknowledgements
Ames Laboratory (US-DOE), Purdue University, and Nihon Superior supported this work through Ames Lab Contract No. DE-AC02-07CH11358. Additional funding was provided through government support under and awarded by the DoD Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. The research group is grateful for the financial support, advice, and guidance provided by Tetsuro Nishimura and Keith Sweatman of Nihon Superior and for the assistance of Fukuda Co. in producing the drip atomized solder samples presented in this paper. The authors would like to thank Iver Anderson (andersoni@ameslab. gov) of Ames Laboratory and Stephanie Choquette (smc1@iastate.edu) of Iowa State University for their help in facilitating and communicating with Fukuda Co. during the production of the atomized solders and for initial EDS examination of the Sn-Cu-Al as-solidified solder alloy. The authors would also like to thank John Holaday (jholaday@purdue.edu) of Purdue University for calculating the discussed Thermo-Calc solidification path for the Sn-Ag-Cu-Al alloy.
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Reeve, K.N., Handwerker, C.A. Beta-Tin Grain Formation in Aluminum-Modified Lead-Free Solder Alloys. J. Electron. Mater. 47, 61–76 (2018). https://doi.org/10.1007/s11664-017-5819-8
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DOI: https://doi.org/10.1007/s11664-017-5819-8