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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.