Abstract
Difficult nucleation of b-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 61C were observed for Sn solidified on
graphite, while lower undercoolings, up to 30C, 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.
