Substrate surface finish-dependent strength evolution and reliability of ultrasonic bonded power module terminal joints

Terminals in electric-vehicle (EV) power modules are critical components responsible for delivering high voltage and current; therefore, ensuring the electrical performance and long-term reliability of their joints is essential. As reliability degradation under high-temperature environments has become a major concern, ultrasonic bonding—which allows rapid joining without filler materials or flux while maintaining excellent thermal stability—has emerged as a promising alternative. In this study, Cu terminals were bonded to active metal brazed (AMB) substrates with Cu, electroless Ag, and electroless nickel-immersion gold (ENIG) surface finishes using both reflow soldering and ultrasonic bonding. Reflow soldering was employed as a reference joining process to benchmark the high-temperature reliability of ultrasonic bonded joints. The joint reliability was evaluated under thermal aging at 150 °C for up to 1008 h. For reflow soldered joints, the shear strength progressively decreased with increasing thermal aging time across all substrate types. In contrast, ultrasonic bonded joints—except those on bare Cu—exhibited increased bonding strength during thermal aging while maintaining stable performance. Specifically, Ag- and ENIG-based ultrasonic joints showed an increase in shear strength from approximately 76–77 MPa to 86–88 MPa and sustained this level up to 1008 h. Furthermore, the strengthening mechanisms responsible for this reliability enhancement were elucidated. These findings demonstrate that ultrasonic bonding is a highly promising joining technology for next-generation SiC- and GaN-based power modules that demand high thermal stability.