Reliable WBG Devices Packaging by Forming Nano-gradient Structures on Silver-plated Substrates

The coefficient of thermal expansion (CTE) mismatch between the chip, sintered silver paste and substrate leading to the formation of voids and cracks, and ultimately to device failure. In this paper, nano-gradient structure is formed on substrate through the combination of sputtering and annealing process, which effectively increases the shear strength of die-attach samples from 19.9 MPa to 31.4 MPa. High reliability is achieved by nano-gradient substrate, the shear strength is reduced from 31.4 MPa to 25.8 MPa after 1000 cycles thermal shock test (TST), the overall shear strength degradation was only 17.8%. For die-attach samples after 1000 h of thermal aging test, the failure occurred at the interface of electroplating rather than the bonding interface between nano-gradient silver layer and silver paste. The upper deformation-refractory nanocrystalline particles in substrate suppressed fatigue cracks initiation, while the lower coarse-crystalline interior effectively prevented the cracks propagation, allowing the high reliability of die-attach samples. The formation of nano-gradient structures on the substrate enables the development of WBG devices with superior reliability in rigorous operating environment.