Effect of Electrodeposition Conditions on Kirkendall Void Formation between Electrodeposited Cu Film and Sn-3.5Ag Solder

Sn-3.5Ag solder on Cu foil and electrodeposited films were reflowed at 260degC for 1 min. After reflow process, all specimens showed typical scallop shape of Cu₆Sn₅ at the Cu/solder interface. After thermal aging at 150degC, 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 150degC. 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. In contrast, Cu foil showed much more reliable although brittle fracture occurred at the Cu₆Sn₅/Cu₃Sn interface. Therefore, voids occupied at the Cu/Cu₃Sn interface are shown to be degraded seriously drop reliabilities of solder joints.