Microstructure and In Situ Observations of Undercooling for Nucleation of β-Sn Relevant to Lead-Free Solder Alloys
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Difficult nucleation of β-Sn during solidification of tin and tin-based lead-free solder alloys can result in high degrees of undercooling of the liquid prior to solidification. The undercooling can produce solder joints with large grains, anisotropic behavior, and undesirable mechanical properties. This paper describes our examination of the amount of undercooling of tin on both graphite (non-wetting) and copper (wetting) surfaces using in situ x-ray diffraction. The microstructure was further characterized by optical microscopy, scanning electron microscopy, and electron backscattering diffraction imaging microscopy. Undercoolings as high as 61°C were observed for Sn solidified on graphite, while lower undercoolings, up to 30°C, were observed for Sn solidified on copper. The microstructure of the high purity Sn sample solidified on graphite showed very few grains in the cross-section, while the commercially pure Sn sample solidified with only one grain and was twinned. Tin solidified on copper contained significant amounts of copper in the tin, intermetallic phase formation at the interface, and a eutectic microstructure.
Key wordsIn situ x-ray diffraction solidification nucleation undercooling twinning grain boundaries tin lead-free solders cooling rate microstructure wetting
The authors would like to thank Mr. Jackson Go of the Lawrence Livermore National Laboratory (LLNL) for performing the optical metallography, Mr. Edwin Sedillo of LLNL for performing the SEM and EBSD characterization, Mr. Mike Santella of Oak Ridge National Laboratory (ORNL) for assisting with the x-ray diffraction analysis software, and Jenia Karapetrova of the APS for assisting with the synchrotron beam-line setup and operation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and by Oak Ridge National Laboratory under contract DE-AC05-00OR22725. Much of this work was supported by the Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Science and Engineering. In situ experiments were performed on 34-BM-C at the APS, which is supported by the U.S. DOE, Basic Energy Sciences, Office of Science under contract no. W-31-109-ENG-38.
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