Reinforcement materials such as carbon nanotubes (CNTs), have been demonstrated to be beneficial in improving composite-solder reliability through their super electrical, mechanical and thermal properties. However, interfacial interaction weakness still exists between CNTs and solder alloys. In this study, we managed to incorporate nickel-coated multi-walled carbon nanotubes (Ni-CNTs) into Sn3.0Ag0.5Cu solder matrix with various weight percentages of 0.01 wt%, 0.05 wt%, and 0.1 wt%. Microstructures, intermetallic compound (IMC) layers, mechanical properties including micro-hardness and shear testing, have been implemented to investigate the solderability of composite solders with the utilization of scanning electron microscope and energy dispersive X-ray spectrometer analysis. In comparison with Ni-CNT doping method, CNT doping is easily getting saturated and hereafter arduously to be incorporated due to their physical and chemical limitations. With 0.05 wt% doping Ni-CNTs, fine Ag3Sn strips and scallop-shaped (Cu, Ni)6Sn5 IMC layer are formed at the solder-subtract interface, resulting in the maximum improvement of 24.3% and 14.9% in hardness and shear strength, respectively. The existence of micro dopants in the composite solder act as impurity centers and can effectively retard the diffusion of atoms. The increase in strengthening effects in solder joints can be attributed to the combination of (a) impeding effects of uniformly distributed second-phase particles and (b) the consumption of Ni by intermetallic reactions.