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The Ni nanoparticle‐decorated reduced graphene oxide (Ni‐rGO) reinforced Sn2.5Ag0.5Cu lead‐free composite solder, as well as the interfacial intermetallic (IMC) and the shear strength of soldering joints, are investigated with scanning electron microscopy (SEM), X‐ray diffraction (XRD), and transmission electron microscope (TEM). The results show that Ni‐rGO can be synthesized by the thermal decomposition method, and the adsorption type between Ni atoms and rGO is chemisorption. When the amount of Ni‐rGO addition reaches 0.05 wt%, the high strength and toughening Ni‐rGO reinforced lead‐free composite solder is obtained, with a tensile strength and elongation of 58.0 MPa and 32.3%, respectively. The growth of the interfacial IMC layers is restrained by the addition of Ni‐rGO, and the highly reliable soldering joints are obtained accordingly. When the amount of Ni‐rGO addition reaches 0.05 wt%, the shear strength of the Ni‐rGO reinforced lead‐free composite solder is 28.1 MPa, which is 26.6% higher than that of the plain Sn2.5Ag0.5Cu soldering joints. With the increase in the amount of Ni‐rGO, the fracture pattern is transforme from a ductile‐brittle mixed fracture to a ductile fracture. Additionally, the fracture pathway shifts from a transition zone, consisting of soldering seam and interfacial IMC, to the direction of the soldering seam.

Dynamic observation of the microstructure evolution of Sn2.5Ag0.7Cu0.1RE/Cu solder joints and the relationship between the interfacial intermetallic compound (IMC) and the mechanical properties of the solder joints were investigated during isothermal aging. The results showed that the original single scallop-type Cu6Sn5 IMC gradually evolved into a planar double-layer IMC consisting of Cu6Sn5 and Cu3Sn IMCs with isothermal aging. In particular, the Cu3Sn IMC grew towards the Cu substrate and the solder seam sides; growth toward the Cu substrate side was dominant during the isothermal aging process. The growth of Cu3Sn IMC depended on the accumulated time at a certain temperature, where the growth rate of Cu3Sn was higher than that of Cu6Sn5. Additionally, the growth of the interfacial IMC was mainly controlled by bulk diffusion mechanism, where the activation energies of Cu6Sn5 and Cu3Sn were 74.7 and 86.6 kJ/mol, respectively. The growth rate of Cu3Sn was slightly faster than that of Cu6Sn5 during isothermal aging. With increasing isothermal aging time, the shear strength of the solder joints decreased and showed a linear relationship with the thickness of Cu3Sn. The fracture mechanism of the solder joints changed from ductile fracture to brittle fracture, and the fracture pathway transferred from the solder seam to the interfacial IMC layer.

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