Effect of Ag Addition on the Ripening Growth of ${\rm Cu}_{6}{\rm Sn}_{5}$ Grains at the Interface of Sn-xAg-0.5Cu/Cu During a Reflow

The ripening growth (or lateral growth) of Cu
₆
Sn
₅
grains that form between molten Sn-rich solders and Cu under-bump metallurgies (UBMs) was investigated with Sn-0.5Cu, Sn-1.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu solders. After reactions with Cu UBMs, the size and morphologies of Cu
₆
Sn
₅
grains were compared with each other through the observation of the top views. The Cu
₆
Sn
₅
intermetallic compounds (IMCs) that form at the interface of Sn-3.0Ag-0.5Cu solders exhibit much smaller grains, though the thickness of the grains is similar to that of the grains that form in Sn-0.5Cu and Sn-1.0Ag-0.5Cu solders. In other words, the ripening growth of Cu
₆
Sn
₅
grains during a reflow is reduced in Sn-3.0Ag-0.5Cu solders. The effects of Ag on the ripening growth of Cu
₆
Sn
₅
grains were examined by comparing the angles of two neighboring Cu
₆
Sn
₅
grains that formed at the interface of each solder, in addition, the interfacial energies between the Cu
₆
Sn
₅
grains and the molten solders are discussed. The effect of Ag on the ripening growth of the Cu
₆
Sn
₅
grains depends on the solder volume: as the volume increases, the ripening growth of the Cu
₆
Sn
₅
IMCs at the interface is reduced more effectively, and small Cu
₆
Sn
₅
grains are produced during the reflow. The tendency of the way smaller Cu
₆
Sn
₅
grains to reduce the formation of Cu
₃
Sn IMCs during thermal aging is also discussed.