Anomalous microstructure formed at the interface between copper ribbon and tin-deposited copper plate by ultrasonic bonding

The present paper reports an anomalous microstructure formed at interfaces between Cu ribbons and Sn-deposited Cu plates by ultrasonic bonding. The interface consists of Cu-to-Cu directly bonded part and Sn-dispersed part. In the latter part, Sn is dispersed in the vicinity of the bond interfaces being retained as Sn. The formation process of the interfacial microstructure is discussed on the basis of detailed experimental analysis and theoretical analysis on the solid-state reaction at Cu/Sn interface. The theoretical analysis reveals three important points of the reaction. (1) The formation of Cu6Sn5 precedes that of Cu3Sn. (2) The incubation time for the formation of Cu6Sn5 changes discontinuously at the η/η′ transition temperature. (3) The incubation time for the formation of η′Cu6Sn5 is longer than that for ηCu6Sn5.