The viscosity and surface tension of metal melt are all sensitive physical properties that relate to the liquid structure and also have a certain correlation between them. For electronic packaging materials, both viscosities and surface tensions are very important parameters affecting the processing properties. In this study, the viscosities of Sn-xCu (x = 0.7, 1.5, 2) solder melts are measured by using a torsional oscillation high-temperature viscometer. Abrupt change in viscosity occurrs in a certain range of temperature. The temperature range can accordingly be divided into a low temperature zone and a high temperature zone. The relationship between viscosity and temperature can fit to the Arrhenius equation very well in each temperature zone. The structure characteristics and evolutions of the liquid solders are then discussed. Meanwhile, the surface tensions of the Sn-xCu solders are calculated according to the viscosity values at the corresponding temperatures. The test results of the wetting angle and the spreading rate are in good agreement with the calculations, indicating that the method of using the viscosity values to calculate the surface tensions of binary lead-free solder alloys and evaluate their wettabilities is feasible.