<p>SnBi-based lead-free solders are widely used for low-temperature interconnections due to their low melting point and favorable mechanical properties. However, SnBi solder joints are prone to high brittleness and a coarse Bi phase after aging, resulting in reduced joint stability. To address these limitations, in this study, a 7-wt.% composite solder was prepared by doping and stirring Cu@Ag core–shell particles into liquid SnBiAg solder. The melting characteristics and microstructure of the composite solder, as well as the microstructure and mechanical properties of the solder joints, were then investigated. The microstructure of Cu/7-wt.% /Cu solder joints before and after aging was examined using scanning electron microscopy. The elemental distribution of the solder joints before and after aging was analyzed via energy-dispersive spectroscopy. The melting behavior of the composite solder was evaluated using a differential scanning calorimeter, while the phase composition of the solder and joints (both before and after aging) was analyzed using X-ray diffraction. The shear strength of the solder joints was measured with a universal testing machine. The results show that doping with 7-wt.% Cu@Ag particles improves the mechanical properties of solder joints at low temperatures. Notably, compared to unaged joints, the joint aged for 120&#xa0;h did not exhibit a decline in shear strength. Moreover, the shear strength of the aged joint increased by 16.1% compared to that of joints without particle reinforcement.</p>