Abstract: High reliability application fields often require specialized devices, which have slower iteration updates and higher costs. COTS devices have a fast update speed and good quality consistency, but their applicability and reliability in specialized fields have not been fully verified. Therefore, a reliability method for large-size COTS device solder joints based on solder mask layer design is proposed. Firstly, solder joint morphology simulation is used to obtain solder joint morphology under different solder mask layer parameters; Furthermore, plate level thermal cycling simulations were conducted on devices with different parameters. The results showed that the design of the solder mask layer would affect the shape of the solder joints, thereby affecting their stress-strain state during the thermal cycling process. The optimized parameters were obtained by calculating the expected lifespan of the solder joints, and the predicted lifespan of the optimized solder joints was increased by 118%. This proved the feasibility of improving the reliability of COTS device solder joints from the perspective of packaging design, and promoted the application process of COTS devices in the field of high reliability.