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The morphological evolution of Cu6Sn5 grains formed at the interfaces of “Sn-3.0Ag-0.5Cu/Poly-crystal Cu (PC-Cu)” and “Sn-3.0Ag-0.5Cu/Single crystal Cu (SC-Cu)” under different reflow conditions was investigated. Reflow experiments were performed at different peak temperatures of 230, 260 and 290°C corresponding to the dwelling time of 20s, 60s and 600s, respectively. The results show that Cu6Sn5 grains formed at the interface of Sn-3.0Ag-0.5Cu/PC-Cu show two kinds of morphologies. The round scallop-type Cu6Sn5 grains present under reflow conditions of 230°C/20s-10min, 260°C/20s and 290°C/20s, while the faceted ones appear at 260°C/60s, 260°C/600s, 290°C/60s and 290°C/600s. The morphology transition of Cu6Sn5 was interpreted by the Jackson's parameter, which is smaller than 2 for a rough surface while being larger than 2 for faceted grains. In the meantime, long prismatic grains, namely the whiskers, can be found on the existing interfacial grains after 60s dwelling time, which generate during the subsequent solidification process. At the Sn-3.0Ag-0.5Cu/SC-Cu interface, prismatic Cu6Sn5 grains appear and then grow in three directions with an angle of 60°. Also, the Cu6Sn5 whiskers can be found under a reflow condition of 290°C/20s, along the longest axis of the existing prism-type grains.