Low Temperature Bonding Technology for 250 °C-Operating SiC Power Modules Using Nano-Composite Cu/Sn Paste

A low temperature bonding technology for SiC power devices bonded to an active metal brazed copper on Si3N4 substrate is successfully realized using a transient liquid phase sintering (TLPS) method with a nano-composite Cu/Sn paste. The bonding process of the paste is composed of two steps. The first step is the TLPS reaction of the molten Sn with Cu powder. The second step is the solid state reaction between Cu6Sn5 and Cu which forms Cu3Sn. Differential scanning calorimetry (DSC) analysis is used to evaluate the reaction temperature of the TLPS process. The solid state reaction of the Cu3Sn phase formation is investigated with X-ray diffraction (XRD) analysis. It is found that the completion of both the TLPS reaction and the solid state reaction is realized at 280 deg C for 20 min. The microstructure change of each reaction step is analyzed with a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). Based on the results mentioned above, the bonding process conditions of temperature and time is determined. SiC devices are bonded to an active metal brazed copper (AMC) on Si3N4 ceramic substrate using the nano-composite Cu/Sn paste. The shear strength of the bond is 39 MPa, which is about 6 times higher than the IEC standard. Since these reactions are completed at the bonding process temperature of 280 deg C, the shear strength is not affected by the process temperature over 280 deg C. The shear strength at 400 deg C is also measured and 18 MPa is obtained even the devices are bonded at 280 deg C. The proposed bonding method with the nano-composite Cu/Sn paste is a promising technology for 250 deg C-operating SiC power modules, because the process temperature is close to the operating temperature of the devices.