Ultrasonic-assisted sintering of power electronic components in ambient atmosphere on copper surfaces utilizing oxidation-protected silver-copper pastes

• Applicants: Dr.-Ing. Jens Twiefel; Professor Dr.-Ing. Marc Christopher Wurz
• Subject Area: Materials in Sintering Processes and Generative Manufacturing Processes ; Production Automation and Assembly Technology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546457721

Power electrical components are of major importance for the ongoing transformation of industry, mobility and energy. These applications define the requirements for future power modules, including the need for high robustness even in harsh environments and especially at very high operating temperatures. The bond between the active elements and the direct bonded copper (DBC) plays an important role in ensuring this. Silver compound sintering is an accepted solution. However, long process times and the high cost of silver are significant drawbacks and limit its use. We have shown that the use of ultrasound can reduce the process time by two thirds. The use of copper paste is the obvious step to replace silver, but this introduces the problem of oxidation before, during and after the process. Therefore, an inert gas atmosphere is used to eliminate oxidation during sintering. To reduce the silver requirement for a room atmosphere sintering process, we propose Protected Silver Copper Pastes, where the copper particles are encapsulated with a thin layer of silver. This has the potential to reduce the silver volume by approximately 80%. The use of ultrasound allows the porosity to be almost eliminated, thus significantly reducing the oxidation susceptibility after sintering. Using this advantage, we aim to neglect the silver coating on the DBC and chip to reduce the volume by a further approximately 10% of the original volume. Combining the two will reduce the silver volume by 90%. Our previous work has shown that ultrasound has a very positive effect on the process. However, the mechanism behind this has not been understood. We were able to disprove our previous hypothesis that the additional friction was the main driver for the process improvement. Using the in-house developed temperature sensors integrated into the Chip, we have shown an insignificant temperature increase (low friction work). This leads us to our new working hypothesis that during the main sintering process, ultrasonic softening and enhanced diffusion are the main drivers for process improvement. As the necking starts during the hot drying of the paste, large relative movements between the particles are already blocked. Therefore, densification is a result of plastic deformation, which is where ultrasonic softening comes in. As the contact surfaces increase, diffusion connects the particles and this is accelerated by the ultrasound. Our analysis shows a second time period to use ultrasonic vibration, immediately after screen printing and during hot drying, where the particles are unconnected and the vibration allows the layer to compact. This has the potential to significantly improve the initial sintering situation. We aim to support our new hypotheses with appropriate experiments where the effects are separated in the proposed project.

• DFG Programme: Research Grants