The transition to 800V architectures and silicon carbide (SiC) inverters is pushing the power limits of electric vehicle (EV) drivetrains. In these high-current environments, the thermal management of electrical connections, specifically contact rivets, is a critical engineering challenge. Excessive heat not only degrades the contact material but can also lead to catastrophic failure of the surrounding power electronics.
Sources of Heat in Inverter Contacts
Heat in inverter contact rivets primarily stems from two sources: Joule heating (I²R) and contact resistance heating. As current flow increases, even a few micro-ohms of additional resistance can result in significant temperature rises. In high-frequency switching environments, skin effect and proximity effect can further concentrate current and increase localized heating. Managing these factors requires a combination of high-conductivity materials and optimized mechanical design.
The Bimetal Advantage: Ag/Cu Rivets
For high-current EV inverters, bimetal rivets (silver alloy face on a copper shank) offer the best thermal performance. Silver (or its alloys like AgSnO2 or AgNi) provides the necessary low contact resistance at the switching interface, while the oxygen-free copper (OFC) shank provides maximum thermal conductivity (approx. 390 W/m·K). This allows the rivet to act as a heat pipe, pulling thermal energy away from the sensitive contact interface and dissipating it into the busbar or heat sink.
Temperature Rise Testing (IEC 60947-1)
To validate the thermal design of inverter contacts, engineers perform rigorous temperature rise tests according to standards like IEC 60947-1. The contacts are subjected to their rated continuous current until the temperature stabilizes. The maximum allowable temperature rise (typically 65K or 70K above ambient) ensures that the material does not reach its annealing point, which would cause a loss of mechanical strength and a subsequent increase in contact resistance.
Metallurgical Bonding and Interface Quality
In high-current applications, the quality of the bond between the silver alloy and the copper shank is paramount. At WEUP, we utilize cold-heading and specialized bonding techniques to ensure a metallurgical bond with zero voids. Voids at the interface act as thermal insulators, significantly increasing the temperature rise of the contact. We use ultrasonic testing and cross-sectional analysis to verify the integrity of our high-current rivets.
Conclusion
Thermal management is the defining challenge of modern EV inverter design. By selecting high-performance bimetal contact rivets and ensuring the highest levels of manufacturing quality, engineers can build power electronics that are both compact and reliable. For data on thermal performance and material conductivity for your SiC inverter project, contact our technical support team today.
