The initial formation of SnCu intermetallic compounds (IMCs) between a molten SnAgCu alloy and Cu under-bump metallization is observed by removing the liquid solder from the substrate and rapidly cooling just as the interfacial IMCs are forming. The results show that a Cu3Sn layer is formed ahead of the liquid solder on the Cu substrate. The liquid solder subsequently spreads on this existing Cu3Sn layer, forming a Cu6Sn5 layer between the liquid phase and Cu3Sn. © 2008 Acta Materialia Inc.
