The interface microstructure and shear strength of Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints under thermal-cycle loading were investigated with scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and physical and chemical tests. The results show that an intermetallic compound (IMC) layer of Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints evolved gradually from the scalloped into larger wavy forms with increasing number of thermal cycles. The roughness and average thickness of IMC increased with thermal-cycle loading. However, at longer thermal-cycle loading, the shear strength of the joints was reduced by about 40%. The fracture pathway of solder joints was initiated in the solder seam with ductile fracture mechanism and propagated to the solder seam/IMC layer with ductile-brittle mixed-type fracture mechanism, when the number of thermal cycles increased from 100 to 500 cycles. By adding 0.05 wt.% Ni, the growth of the joint interface IMC could be controlled, and the roughness and average thickness of the interfacial IMC layer reduced. As a result, the shear strength of joints is higher than those without Ni. When compared to joint without Ni, the roughness and average thickness of 0.05 wt.% Ni solder joint interface IMC layer reached the minimum after 500 thermal cycles. The shear strength of that joint was reduced to a minimum of 36.4% of the initial state, to a value of 18.2 MPa.

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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.