Abstract: Composite solders were prepared through mechanical dispersing and the effects of nano-sized Ag reinforced particulates on In-3Ag solder matrix and interfacial IMC(intermetallic compound) layer after multiple reflows were studied. The microstructures of the solder matrix and IMC layer were investigated using SEM, HRTEM, Energy Dispersive Spectrometer(EDS) and Electron probe Micro-Analysis(EPMA). The results indicate that Ag nanoparticulates nucleate grains and the fine AgIn₂ particles in composite solder matrix do not coarsen after multiple reflows. Due to the diffusion path blocking and surface absorption effect, the growth of the interfacial IMC layer is significantly suppressed by Ag nanoparticulates addition. The solder joint with 0.5%(mass fraction) Ag particulates added shows the best mechanical properties. Adding 1% Ag particulates, the particles tend to agglomerate leading to the formation of spherical particles at the interface, which decreases the mechanical properties of solder joint.