The thermal diffusivities of three Sn-based solder alloys (Sn-3%wtAg-0.5%wtCu, Sn-4%wtAg-0.5%wtCu, and Sn-3.8%wtAg-0.7%wtCu), were determined from room temperature to up to 120°C. In addition to the lead-free alloy, the thermal diffusivity of the binary constituent alloys Cu6Sn5 and Ag 3Sn were measured over the same temperature range. For these studies, cylindrical samples of 6cm diameter and 1cm thickness were used. The temperature and the resulting thermal diffusivities were measured at seven different points of each sample in 5--10°C intervals. The heat pulse used leads to a 3--5°C temperature increase during the measurement. The thermal diffusivity between each two points is calculated using the logarithm gradient of the measured temperature difference between each pair versus time. The thermal diffusivity of the binary alloys showed a slight increase in thermal diffusivity with increasing temperature. This is opposite behavior from Sn matrix that has a rather strong decrease in thermal diffusivity with increasing temperature. In the lead-free alloys substantial differences were observed in the thermal diffusivity in both the different locations and sample temperatures. In some cases these differences were close to 300%. COMSOL Multiphysics was used to simulate the experiment and the heat flow in different phases in the sample and the effects of size, distribution and direction of the intermetallic phases and grain boundary mismatch on the overall thermal diffusivity. The experimental differences observed between the thermal diffusivities of different measurement pairs in each sample are explained by the study of the microstructure and the presence of intermetallic compounds and dislocations in the structure.
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