Improvement of High-temperature Reliability of Power Modules by Multi-step Temperature Bonding Process of Zn-Al Solder

In this paper, we describe the bonding process and bonding stability of Zn-Al solder and the reliability results of temperature cycle tests. XPS analysis showed the presence of oxides that could not be reduced by formic acid on the surface of Zn-Al solder. The reliability test results of power modules assembled using Zn-Al solder were unstable because of it was difficult to form good joints with Zn-Al solder oxides. We have found that diffusion proceeds at the interface by holding the Zn-Al solder and copper in contact at a temperature below the melting point. By adding a process using this diffusion, the bonding state of Zn-Al solder has been improved. This solder joining process was applied to the assembly process of the SiC 2-in-1 power module. This module showed no increase in thermal resistance due to the significant degradation of the bonding layer in transient thermal analysis after 1000 cycles of -40deg C / 250deg C temperature cycling.