Abstract: To meet the urgent demand for the packaging and interconnection of high-precision and large-size components driven by the rapid development of highly integrated and multifunctional communication, mechanical, and aerospace fields, and to solve the solder overflow problem in conventional thermocompression soldering processes caused by the synergistic effect of gravity and welding pressure, a composite solder preform composed of silver nanowire aerogel and SAC305 solder was adopted to achieve solder confinement and precise interconnection, and its interfacial metallurgical reactions and confined wetting mechanism were systematically investigated. The results indicate that the silver nanowire aerogel, owing to its highly cross-linked silver nanowire network structure, exhibits both excellent structural flexibility and liquid solder absorption capability and does not inhibit the interfacial reaction between the solder and the substrate while effectively achieving the spatial confinement of the solder. After using the preform, solder overflow is effectively prevented even under a welding pressure of 10 MPa. Moreover, the shear strength of the solder joints under this pressure increases by 92.7% compared with that of conventional pressureless SAC305 solder joints, and its main strengthening mechanism is second-phase strengthening. The composite solder structure proposed in this study provides a new idea for solving the problem of uncontrolled solder overflow under high pressure and has positive significance for promoting the precise integration of large-scale electronic devices and systems.