Prediction of Crack Propagation in Large-Area Sintered Ag Joints for Bonding Power Modules and Coolers in Electric Vehicle Inverters

Sintered Ag Joints are promising technology that significantly reduces thermal resistance compared to conventional solder joints and have been widely applied to chip-level and small-package interfaces. However, their application to large-area interfaces in electric vehicle (EV) inverters remains limited due to reliability issues associated with thermally induced stresses, which lead to crack propagation and performance degradation. This study proposes a design-oriented crack-propagation prediction methodology for large-area sintered Ag joints used in power module–to–cooler bonding. Large-area joint samples (53 mm × 42 mm), consisting of a power module bonded to a metal plate, were fabricated and subjected to temperature cycling tests. Crack propagation behavior was experimentally evaluated using scanning acoustic tomography, and lifetime was defined as the number of cycles required for a crack length corresponding to a specified increase in thermal resistance. Finite element analysis was conducted to calculate thermally induced strain, and the strain amplitude, including plastic and creep components, was used as the crack-driving parameter. A strong correlation between strain amplitude and crack-propagation lifetime was obtained, and a Coffin–Manson-type model with a high coefficient of determination (R² > 0.97) was established. Based on this model, three inverter cooler structures were designed to reduce strain amplitude. Prototype evaluations confirmed that reducing strain amplitude effectively suppresses crack propagation. The proposed methodology enables quantitative lifetime prediction and provides practical design guidelines for improving the reliability of large-area sintered Ag joints in EV inverter applications.