<p>In response to the demand for high thermal conductivity solder paste in system-in-package technology, a new type of In–15Pb–5Ag solder paste had been successfully developed for low-temperature welding. For the first time, solder powder with particle sizes ranging from 20 to 38 μm, a sphericity of 0.96, and uniform dispersion was prepared using ultrasonic atomization. Its melting point was as low as 148.10 °C, and its microstructure consisted of AgIn₂ phase and In–Pb solid solution. Based on the principle of high-low boiling point compound, adipic acid, and DL-malic acid were selected as activators, with hydrogenated rosin-KE604 serving as the film-forming agent. When the solid content reached 85%, the minimum non-bridging distance of solder paste was 0.2 mm, forming bright solder joints, with an expansion rate of 85.28%, and the flux residue was controlled at 48.13%. The thermal conductivity of solder paste was as high as 48.3946 ± 0.7550 W/(m·K), proving that the material possessed excellent thermal conductivity in low-temperature packaging applications and could be an ideal candidate material for high thermal conductivity system in package.</p>