| Summary: | Reliable measurements of the kinetics of β → α allotropic transformation in Sn-based solder underlie the design and development of advanced interconnects for low-temperature electronics. In this paper, a highly-accessible and buoyancy-based method, but different from common dilatometry, was developed to consistently detect a change of the transformed fraction in bulk βSn plates (10 × 10 × 1mm) with time at −20, −40, and −60 °C. Due to the concurrent effects of undercooling temperature and interfacial atomic diffusion, the β → α transformation in Sn plates at −40 °C proceeded most rapidly up to around 70% αSn fraction after 168 h. The transformed fraction versus time curves yielded excellent fits to the classic Johnson-Mehl-Avrami-Kolmogorov model with a constant nucleation rate during transformation process (Avrami exponent, n of 4). Addition of nucleation agent accelerated the transformation by shortening the incubation period, but the nucleation rate decreased to zero in the following transformation (n = 3). Furthermore, coarsening grain size depressed β → α transformation and led to the saturation of nucleation sites in the vicinity of half transformation (n decreasing from 4 to 2). The simple, convenient, and reliable method proposed showed high sensitivity with detection limits of about 2%, and it could be a promising approach to study and predict solid-state phase transformation kinetics. Keywords: Phase transformation, Solder, Bulk Sn, Transformation kinetics, Buoyancy
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