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This paper investigates the effect of solders with different grain sizes (5–15 μm, 2–15 μm, 2–11 μm) on the mechanical and thermal reliability of flip-chip LED chip Au/Sn Ag Cu/Cu solder joints during reflow soldering. The lead-free solder SAC305 was selected as the solder. The microstructure of the IMC interface and the inferred surface of the solder joint is observed, and the microstructure evolution of the solder joint is analyzed. The void ratio of the solder joints under different grain sizes is tested to characterize the influence of the contact area between the chip and the solder joints on the shear stress. In addition, the solder joints were aged for 1000 h under a relative humidity of 85 °C/85%. The photoelectric thermal performance of the FC-LED filament and the influence of high temperature and high humidity aging on the reliability of the filament were tested and analyzed. The results show that when the size of the solder paste is small and the uniformity is poor within a certain range of solder size, the voids in the flux layer are large and concentrated, and the void ratio is significantly higher, which leads to a decrease in the mechanical reliability of the solder joints. The thermal resistance test results show that the cavity will cause excessive thermal resistance, poor heat dissipation, a significant increase in junction temperature, and a decrease in thermal reliability, which in turn leads to severe light aging and ultimately changes in photoelectric performance. When the grain size is 5–15 μm, the uniformity of the particle size is good. After the filament is aged for 1000 h at high temperature and humidity, the light maintenance rate remains at 65.6%, and the filament performance is stable and reliable. It has certain reference value in the actual production process.