Using FEA to Analyze the Effects of Paste-to-Ball Volume Ratio and Package Design Parameters on Thermal Cycling Performance of Hybrid SAC-LTS Solder Joints

Hybrid SAC-LTS solder joints are widely adopted in SMT processes due to their ability to lower reflow temperatures, thereby limiting package warpage and improving reliability. However, accurately predicting their thermomechanical performance remains challenging due to the complex relation between dynamic Bi diffusion during thermal exposure and paste-to-ball volume ratio, as well as the effects of package design parameters such as die size, die thickness, substrate and PCB thickness. Prior FEA models attempted to capture Bi diffusion effects but often oversimplified Bi diffused layer geometries or did not capture plasticity while modeling those layers. Moreover, the influence of key package design parameters was not explored. This study refines FEA modeling by incorporating curved Bi diffusion layers and a viscoplastic Anand model to better represent their mechanical behavior. It evaluates the effects of paste-to-ball volume ratio, die size, die thickness, PCB thickness, and component substrate thickness on plastic work accumulation in the critical solder joints, enabling a more precise thermal fatigue life (TFL) assessment. Results confirm that curved Bi diffusion layers can effectively capture plastic work distributions and failure locations. Also, it was found that a paste-to-ball volume ratio of 0.50 maximizes TFL - which aligns with a prior experimental study. Moreover, the study reveals that die size and thickness are dominant factors in TFL. These findings refine the approach of the reliability assessments of hybrid SAC-LTS solder joints using FEA and provide insights for optimizing next-generation packaging solutions with hybrid joints.