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Herein, we prove that alloying of reactive nanoparticles can improve nanoparticles/β-Sn interfacial bonding and influence the solder alloy's melting point. Furthermore, the alloying effect can enhance the creep resistance of solder alloy. 0-0.5 wt% Ag nanoparticles (NPs) were added to Sn3.0Ag0.5Cu (SAC305) solder paste by mechanical stirring for the preparation of composite solder alloys, which were then extracted from the composite solder pastes. The joints were observed by scanning electron microscope and transmission electron microscope. The elementary composition of the prepared alloys was analyzed by electron dispersive spectrometer. The melting properties of alloys were tested by differential scanning calorimeter. The influences of Ag NPs as reactive nanophase on solder microstructure and melting properties were studied. The results indicated the existence of transition layer at the interface between Ag NPs and β-Sn. Additionally, the interfacial bonding improved after Ag NPs converted to Ag3Sn NPs. Due to Ag NPs alloying and solder alloy microstructure refinement being affected, melting points of alloys increased with 0-0.2 wt% addition of Ag NPs and decreased with 0.2–0.5 wt% addition.