Abstract: Wetting behavior between the molten Sn and Cu₆Sn₅ and Cu under the temperature 350 ~ 450 ℃ was studied using the modified sessile drop method in the high vacuum. The results show that the IMCs substrates coated with a thin Au film have the better wettability than that of Sn/Cu system under the tested temperatures. The oxide film on the surface of metallic substrate is the key factor for wetting process. The thin passivation Au film on surface after ion-sputtering can be an effective method to improve the wettability and control the IMC thickness at interface. The mechanism of wettability improvement is the reaction between Sn and the oxide film. The precipitated IMC at interface or the melt process of IMC is not the main factor for spreading. All spreading dynamics show the near-linear variation, which can be described by the reaction-limited spreading model. The calculated wetting activation energies are 20.469 kJ/mol and 22.270 kJ/mol for Sn/Cu₆Sn₅ and Sn/Cu, respectively.