<p>To improve the thermo-mechanical reliability of Sn–58Bi (SB) eutectic solder, a SB/Sn–3.0Ag–0.5Cu (SAC) composite solder was developed. The thermo-mechanical reliability of this composite solder was assessed via thermal shock testing (− 55 to 125&#xa0;°C, 1000 cycles) on solder joints fabricated from both SB and SB/SAC solders. Before thermal shock exposure, the SB/SAC composite exhibited superior mechanical bonding strength relative to pure SB solder, attributable to precipitation hardening and dispersion strengthening effects arising from Ag₃Sn, Cu₆Sn₅, and Bi-rich phase particles uniformly distributed within the joint. Following thermal shock testing, both SB and SB/SAC solder joints exhibited reduced mechanical properties, primarily due to the growth and flattening of the interfacial intermetallic compound (IMC) layer, as well as microstructural coarsening. Nonetheless, the SB/SAC composite maintained higher mechanical reliability than the SB solder. This improvement is ascribed to its initially enhanced bonding strength, achieved through reinforcement by combining SB and SAC alloys, and to the mitigation of microstructural coarsening and suppression of IMC layer growth and flattening, resulting from compositional modifications in the composite solder.</p>