In comparison to traditional Sn-based solder joints, Cu-cored solder joints exhibit superior thermal conductivity, electrical conductivity and mechanical properties. To elucidate the size effect on its interfacial reaction and shear strength, Cu-cored solder joints ( Cu@Ni-Sn/Cu) were prepared with different Sn coating thickness. The cross-sectional microstructure were observed after different reflow durations to study the liquid-solid interfacial reaction of the Cu-cored micro solder joints under size effect. Subsequently, shear tests were conducted on the Cu-cored micro solder joints, and the fracture mechanism was analyzed by considering the fracture morphology. The interfacial reaction results indicate that when the Cu@Ni-Sn/Cu solder joints are reflowed at 250 ℃, needle-like (Cu, Ni)6Sn5 IMC with higher Ni content is generated at the Sn/Ni interface, while layered ( Cu, Ni)6Sn5 IMC with lower Ni content is generated at the Sn/Cu interface. The shear test results demonstrate that with increasing Sn coating thickness, the shear strength of Cu@Ni-Sn/Cu joints increases initially and then decreases. Due to the direct influence of the Sn coating thickness on the volume of the ( Cu, Ni)6Sn5 IMC at the interface, thicker Sn layer enhances the shear strength of the solder joint. However, due to the offset of the Cu-core position in Cu@Ni-Sn( 60 μm)/Cu micro solder joints, the shear strength decreases slightly.
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