Failure Mechanism and Reliability Research of Solder Layer Tilt in Double-Sided Cooling Power Modules

The double-sided cooling (DSC) module introduces greater thermomechanical stress compared to single-sided cooling (SSC) modules, posing a significant threat to reliability. The manufacturing process is complex, requiring multiple sintering or reflow operations. Due to gravitational factors, this results in uneven thickness in the solder layer, further exacerbating the reliability issues. This article investigates the failure mechanism of the middle solder layer (SAC305) in flip-chip double-sided cooling (FCDSC) modules under thermal cycling conditions using a thermomechanical coupled model. The results indicate that when the solder layer tilt angle reaches 1.53°, the lifetime is reduced by 99.3%. Local viscoplastic strain in the solder at stress concentration areas is identified as a key factor in solder layer fatigue failure. Subsequent experiments confirm that fatigue cracks occur on the thinner side of the solder layer. There, the coarsening of the Ag3Sn eutectic phase is more severe, leading to reduced tensile strength, thus becoming a crack initiation site. Finally, the protrusions-spacer technique is proposed to control the evenness of the solder layer, with experiments demonstrating an average reduction in solder layer tilt by 79.7%.