It is known that pure tin will undergo an allotropic transformation below 13°C where it becomes a semiconductor with a 26% [1,2] volume increase, and in appearance turns from a bright shiny metallic material, white β tin, to a dark blue/grey dust, α tin. Such a transformation for an electrical interconnect is disastrous, if it were to occur in any of the high tin lead-free alloys it would be a catastrophe. The elimination of lead, one of the best elements to arrest the transformation process, has resulted in a number of high tin content alloys about which the potential to transform is unknown. Environmental factors that may enhance or arrest the rate and the incubation period of the transformation processes are also unknown. Due to the optimal transformation temperature of approximately -35°C and the long time required for the transformation, a direct observation of the phenomenon has not been possible. This study proposes a new method for observing the β/α transformation in situ using a time-lapse photographic technique. This study concentrates on pure tin, but the applicability of the method opens new possibilities for studying the phenomenon for other tin alloys, such as the two commonly encountered eutectics of SnCu and SnAgCu. The transformation progressed radially from the inoculation point, starting at the surface. Propagation into the bulk occurred by peeling; with the external layers tending to "roll out" due to the volume expansion of the internal layers. In the meantime, cracks parallel to the propagation direction formed. Typically a pure tin sample would completely transform in just over 24 hours.
展开▼
