Failure investigation of a Ag sinter-joining SiC power device under harsh cycling conditions

Abstract

Silicon-Carbide (SiC) power devices have become the prime candidates for future high-performance energy conversion on account of their superior performance over traditional silicon power devices. The superior electro-thermal properties of SiC power devices permit higher temperature of operation and enable higher power density. Nevertheless, the high operation temperature has introduced new changes in die attachment packaging of the SiC power device, which is one of the most vulnerable places due to a direct connection to the SiC chip. Traditional Pb-free solder-joining is hardly to withstand high temperatures because of the low melting point of solder materials. Besides, severe temperature swings of the SiC chip can introduce severe deterioration in the solder layer due to brittle alloy compounds. Currently, Ag sinter-joining is an ideal connection technique for high reliable power electronics packaging. Due to excellent thermal stability and chemical resistance, the Ag sinter-joining is applicable to various harsh conditions, such as extreme temperatures, high humidity, high heavy current, etc., showing promising application scenarios in die attachment packaging. In this work, we applied Ag sinter-joining to die attachment of a molded SiC device and focused reliability of Ag sinter joining under harsh thermal cycling and power cycling conditions. The SiC device was assembled by a SiC diode and direct bonding copper (DBC) substrate via the Ag sinter-joining. Ribbon bonding was used for the topside connection of the SiC device. An imide resin was applied to mold the SiC device. The thermal cycling test was conducted at a temperature range from -50 °C to 200 °C and the power cycling test was conducted at an initial junction of 200 °C. The structures inspection, thermal resistance variations, and voltage-current curve were investigated to understand failures in the SiC device. The thermal cycling can introduce crazing cracks in the sintered layer due to drastic thermal stress caused by a huge temperature swing. However, the thermal resistance, of the SiC device measured by T3ster, was not significantly affected by the crazing pattern. The Ag sinter-joining also shows an excellent resistance during the power cycling test, while the ribbon lift-off will introduce a final failure of the SiC device. Figure 1 shows a cross section of the structure of the SiC power device with and without a power cycling test. Obvious cracks can be differentiated at the ribbon bonding interface after cycling, while there is no deterioration happened in the sintered Ag layer. Base on the present results, it can be concluded that the Ag sinter-joining can withstand a harsh cycling condition either in huge temperature swings or heavy currents. Figure 1