Sn-Cu films were electroplated on Cu seed layers of various thicknesses. The velocity of the diffusion of Cu atoms from a seed layer into the electroplated Sn-Cu film was investigated, focusing on the effect of microstructural properties of the film. It was found that the diffusion velocity of Cu from a Cu seed layer into the electroplated Sn-Cu film increased with a decrease in the thickness of the Cu seed layer. Moreover, the film microstructure was influenced by the thickness of the Cu seed layer. When the crystalline quality of the Cu seed layer was inferior, a large lattice mismatch developed at the interface between the electroplated Sn-Cu film and the Cu seed layer. As a result of this lattice mismatch, many lattice defects and stress may exist at the interface between the electroplated Sn-Cu films and the Cu seed layers. Therefore, the lattice mismatch was considered to be related to the high-diffusion velocity of Cu from the Cu seed layer. Furthermore, the high-diffusion velocity of Cu was found to cause whisker formation. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3090179] All rights reserved.