Obstructed Growth of Interfacial Intermetallic Compounds on (011) and (111)-Oriented Nanotwinned Cu Substrates

Due to the size effect of solder bumps on the interfacial reaction, the proportion of intermetallic compounds in solder joints increases with the shrunken solder joints, resulting in low reliability of solder joints. The newly developed nanotwinned Cu has received wide recognition in 3D IC packaging on account of high conductivity, high electromigration resistance and excellent mechanical performance of the nanotwinned Cu. In this work, one (011)-oriented nanotwinned Cu film and two (111)-oriented nanotwinned Cu films with different grain size were electroplated and soldered with SAC305 solder balls to form nanotwinned Cu/SAC305 solder joints, and the solder joints were aged for various durations to study the effect of microstructure of nanotwinned Cu on the specific growth behavior of interfacial intermetallic compounds. Roof-type Cu
6
Sn
5
formed at the interface of (111)-oriented nanotwinned Cu /SAC305 solder joints with big grain size of Cu substrate after soldering. During the aging process, the intermetallic compounds in the (111)-oriented nanotwinned Cu/SAC305 solder joints with big grain size of Cu substrate have a slowest growth rate. Since the (111)-oriented nanotwinned Cu has lowest surface energy and a considerable amount of junctions of twin and columnar grain boundaries, the dissolution of Cu atoms and the growth of interfacial intermetallic compounds in the (111)-oriented nanotwinned Cu/SAC305 solder joints were decreased apparently. What's more, no Kirkendall voids formed in the (111)-oriented nanotwinned Cu/SAC305 solder joints after 500 h of aging at a temperature of 150
^o
C, the service reliability of solder joints was increased using (111)-oriented nanotwinned Cu as under pump metallization.