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The microstructural details of β-Sn precipitation in a 5Sn-95Pb solder alloy

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Abstract

Solders of Pb-rich compositions, such as 5Sn-95Pb, are commonly used in electronic packaging applications, and this demanding use necessitates that the microstructure-property-processing relationships of the solder be understood fully. In this study, the microstructure of 5Sn-95Pb solder was characterized using a variety of metallographic techniques. The effect of cooling rate on the precipitation of β-Sn from supersaturated α-Pb was determined. On slow cooling, β-Sn precipitates discontinuously with resultant β-Sn lamella alternating with the α-Pb. Upon rapid cooling, the β-Sn precipitates with a nominally homogeneous distribution. These discrete precipitates were found to have a platelike shape with a (111)Pb habit plane and an orientation relationship of (111)Pb‖(010)Sn and [011]Pb‖ [001]Sn. Regions of the β-Sn precipitates that curved away from this habit plane formed ledges. Upon heating, the precipitates were found to dissolvevia a ledge mechanism. Prolonged aging of both the fast and slow cooled 5Sn-95Pb resulted in a coarsening of the β-Sn precipitates with a resultant decrease in strength. Furthermore, the strength of the aged alloy was observed to be independent of cooling rate.

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Authors and Affiliations

  1. Sandia National Laboratories, Division 1832, P. O. Box 5800, 87123, Albuquerque, NM, Berkeley, CA, 94720

    D. R. Frear (Technical Staff Member)

  2. Research Triangle Institute, 27709-2194, Research Triangle Park, NC

    J. B. Posthill (Senior Research Engineer)

  3. Department of Materials, Lawrence Berkeley Laboratory, 94720, Berkeley, CA

    J. W. Morris (Professor)

  4. Department of Materials Science, University of California, 94720, Berkeley, CA

    J. W. Morris (Professor)

Authors
  1. D. R. Frear
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  2. J. B. Posthill
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  3. J. W. Morris
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Additional information

D.R. FREAR, formerly Graduate Research Assistant, Lawrence Berkeley Laboratory

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Frear, D.R., Posthill, J.B. & Morris, J.W. The microstructural details of β-Sn precipitation in a 5Sn-95Pb solder alloy. Metall Trans A 20, 1325–1333 (1989). https://doi.org/10.1007/BF02665491

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  • DOI: https://doi.org/10.1007/BF02665491

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