Abstract: With the rapid development of electronic devices towards higher integration and miniaturization, the interconnection solder joints within these devices must withstand more rigorous thermal, electrical, and mechanical stresses. Consequently, improving the strength and reliability of micro solder joints has become a critical concern. A comparative study on the impact of high temperature and humidity (85 ℃，85%RH) on the microstructural characteristics and mechanical properties of SAC305 solder joints and epoxy resin reinforced SAC305 composite solder joints (SAC305E) was investigated. The study further examined the role of epoxy resin addition in improving the high-temperature and high-humidity reliability of these solder joints. The findings reveal that the epoxy resin reinforcement layer applied to SAC305E solder joints effectively prevents moisture ingress, thereby minimizing the oxidation and corrosion of the solder material. Compared with the SAC305 solder joint, the SAC305E joint exhibits a reduced rate of interfacial layer growth. Additionally, the mechanical strengthening effect of the epoxy resin layer results in improved shear performance of SAC305E solder joints. After undergoing 1000 hours of high temperature and humidity testing, the shear strength of SAC305E solder joints is 21.0% greater than that of SAC305 solder joints.