Sn-3.5Ag solder on Cu foil and electrodeposited films were reflowed at 260?C for Imin. After reflow process, all specimens showed typical scallop shape of Cu6Sn5 at the Cu/solder interface. After thermal aging at 150?C, the formation and growth of Kirkendall void and two IMCs, Cu3Sn and Cu6Sn5 showed quite differently on Cu foil and electrodeposited Cu films. In the case of Kirkendall voids, the electrodeposited Cu film showed much more at the IMCs than OFHC Cu foil. The voids in electrodeposited Cu with an additive were clearly much more than that without additive. And also, most voids of electrodeposited Cu with an additive were distributed at the interface between Cu3Sn and Cu, while the voids of electrodeposited Cu without additive were randomly distributed in Cu3Sn layer. For the growth kinetics of Cu6Sn5 and Cu3Sn layer, total thickness and relative fractions of IMCs were measured after thermal aging at 150?C. The ratio of Cu3Sn layer to total IMC was about 50% except for specimen of electrodeposited Cu film with additive. For the electrodeposited Cu film with additive, ratio of Cu3Sn to total IMC decreased with the aging time. It is clear that electrodeposition condition of Cu film influences interfacial reaction and void formation behavior between Cu and solder. The drop impact test showed that electrodeposited Cu film with additive degraded drastically with aging time. Fracture occurred at the Cu/Cu3Sn interface, where a lot of voids existed.