High reliability design Ag sinter joining on softened Ni-P /Pt/Ag metallization substrate during harsh thermal cycling

The demand using power electronics technology of wide bandgap (WBG) power semiconductors such as SiC and GaN has increased for high output density and high temperature for power converters. At present, the use of silver (Ag)-based sinter die attached bonding technology is mainly being considered for high-temperature operation. However, recently, a bonding structure by Ag sintered on a DBC (Direct Bonded Copper) substrate, a large wavy deformation was generated on the Cu surface of DBC, and cracks were generated at the vertical direction from inside the Ag sintered layer during a harsh thermal cycle test. In this study, in order to improve the reliability of Ag sinter joint structure, a softened low phosphorus electroless Ni-P/Pt /Ag plating layers on DBC was applied, and bonding with Ag sinter paste in low temperature pressure less condition. The bonding interface and interface evolution was analyzed as bonded and a thermal shock test from -50°C to 250°C. The shear strength slightly decreased after the thermal shock test of 100 cyc and 500 cyc, but the development of the softened electroless Ni-P/Pt /Ag plated substrate that does not form cracks and suppresses the undulating deformation of the substrate surface. The shear strength also was improved after thermal shock test comparing with a hard medium phosphorus electroless Ni-P case. A possible deterioration mechanism was proposed based on the SEM observation of the cross-sectional of sinter Ag joint structure after shock thermal test.