Thermomechanical reliability of Ag flake paste for die-attached power devices in thermal cycling

The developments of SiC power devices have been dramatically advanced for the last several years, and are gaining much attention as a key technology for the low carbon society. These SiC power devices are expected to work under the high temperature condition of 250-300°C because of the wide band-gap energy and the high heat resistance. To meet the high temperature operations, the die-bonding materials must have sufficient reliability under the extreme thermal environments. The present study hence focuses on the thermal reliability of the die-attach technology with using Ag flake paste, which can be sintered at about 200°C. For Si die-attachment on the Cu substrate, the Ag paste displays excellent reliability in the thermal cycles from -40°C to 180°C, and no serious degradation such as crack growth or interfacial debonding happens at either Si/Ag or Ag/Cu interfaces. The tested SiC die-attach samples maintained the high die-bonding strength up to 750 thermal cycles of -40°C and 250°C. However, thermal cracks and many voids appeared after 1000 cycles, and thus these damages due to thermal fatigue decreased the joining strength. These results indicate that the sintered Ag flake paste can withstand at the high operating temperature of new power devices, and our Ag paste can be employed as die-attach materials for both Si and SiC semiconductor power devices.