Long-term joint reliability of sic power devices at 330ºc

SiC power devices were die bonded to a AlN/Cu/Ni(Au) direct bonded copper (DBC) substrate with a Au-Ge eutectic solder in a vacuum reflow system. The long term joint reliability of the bonded chips was evaluated at 330oC in air for up to 1600 hours. The bonded samples were inspected with a micro focus X-ray TV system. The microstructure of the samples was observed and analyzed by the scanning electron microscope (SEM) equipped with an energy dispersed X-ray analyzer (EDX). The mechanical and electrical properties of the bonded samples were evaluated before and after high temperature aging. After reflow, the die shear strength of the bonded samples reached up to 72MPa. In the initial aging stage, the die shear strength sharply decreased with aging time. This was due to the formation of NiGe intermetallic compound (IMC) at the solder/DBC substrate. The NiGe IMC was resulted from the reaction of Ge in the solder and and Ni in the DBC substrate. After 400 hrs-aging, little change was observed. After aging for 1600 hrs, the die shear strength decreased to 22 MPa, which was about 3 times higher than the standard value 6 MPa (IEC 749, Japan). Little change was observed in the electrical resistance between the cathode of the chip and the DBC substrate. Oxidation of the DBC substrate was also observed.