Silver nanoparticle paste has become a promising candidate for three-dimensional integrated circuit (3D-IC) bonding because of its low process temperature, low contact resistance, and lack of Pb. In addition, interconnects joined by Ag nanoparticles have high heat resistance, high thermal conductivity, and high fatigue resistance. In this study, to enhance mechanical strength, silver nanoparticle-based paste was synthesized from a mixture of a silver precursor and silver nanoparticles. Shear tests followed by a thermo-compression process (250oC, 10MPa, 30min) were carried out to investigate the effects of the addition of the silver precursor on the shear strength, microstructure, and fracture mechanism. After the addition of the precursor, the shear strength increased from 14.0 to 31.2 MPa, microstructure became denser and interlocking joints were observed at the copper-silver interface. Adding a silver precursor thus improves sintering among silver nanoparticles, leading to the formation of interlocking joints at the interface, which increase shear strength.