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 Cu₆Sn₅ 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 Cu₆Sn₅ grains were compared with each other through the observation of the top views. The Cu₆Sn₅ 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 Cu₆Sn₅ grains during a reflow is reduced in Sn-3.0Ag-0.5Cu solders. The effects of Ag on the ripening growth of Cu₆Sn₅ grains were examined by comparing the angles of two neighboring Cu₆Sn₅ grains that formed at the interface of each solder, in addition, the interfacial energies between the Cu₆Sn₅ grains and the molten solders are discussed. The effect of Ag on the ripening growth of the Cu₆Sn₅ grains depends on the solder volume: as the volume increases, the ripening growth of the Cu₆Sn₅ IMCs at the interface is reduced more effectively, and small Cu₆Sn₅ grains are produced during the reflow. The tendency of the way smaller Cu₆Sn₅ grains to reduce the formation of Cu₃Sn IMCs during thermal aging is also discussed.