<p>This study examined the influence of TiO₂ nanoparticles on the thermomechanical fatigue reliability of SAC305 solder joints, with the aim of linking their microstructural evolution, interfacial stability, and electrical performance degradation. Solder joints with and without 0.5 wt.% TiO₂ reinforcement were fabricated by reflowing between copper substrates and subjected to 1000 thermal cycles from −40°C to +125°C. Microstructural changes were characterized using scanning electron microscopy, X-ray diffraction analysis, and geometrically necessary dislocation (GND) density mapping, while electrical reliability was assessed through resistance monitoring, current–voltage characterization, and electrical noise analysis. Results revealed that TiO₂ nanoparticles refine the morphology of intermetallic compounds (IMCs), suppress coarsening, and preserve smoother interfacial layers during cycling. GND mapping demonstrated that reinforced joints exhibit reduced strain localization and fewer dislocation hotspots, indicating more uniform plastic strain accommodation. These microstructural improvements translate into superior electrical performance: TiO₂-doped joints showed a markedly slower resistance increase, maintained near-ohmic <i>I</i>–<i>V</i> characteristics after cycling, and exhibited significantly lower low-frequency electrical noise. Together, these findings establish a direct mechanistic correlation between nanoparticle-induced microstructural refinement and enhanced electrical reliability, highlighting TiO₂ reinforcement as an effective strategy for improving the durability of solder interconnections in advanced electronic packaging.</p>