Silver Thick Film Based Insulated Metal Substrates for High Temperature Power Applications

Abstract

Thick film technology is normally applied to ceramic substrates such as Al2O3 and AlN, where conductor thicknesses for Au and Ag-Pd based thick film materials are <20μm for signal processing. For power devices applications, thicker deposits are required to increase the current carrying capacity and thick print Cu and Ag materials have been developed for ceramic substrates. For power applications, this technology still requires attachment of the ceramic substrate to a heat sink, that may be made of aluminium or copper, which needs a thermal interface material. In printed circuit board technology, insulated metal substrates have been developed for power module assembly to eliminate the thermal interface material, where the copper conductor circuit is attached and isolated from the integral heatsink, through an organic based dielectric based on epoxy adhesive material. The organic based material limits the operating temperature of the substrate to around 175°C. This paper covers the development of a thick film based approach to producing insulated metal substrates that can operate at higher temperatures than an organic based dielectric system. Thick film dielectric materials have been developed for direct deposition onto aluminium substrates with firing temperatures of around 500°C, combined with a silver based thick film material, which is used as the conductor to the power devices. SiC die attach to the silver based thick film material has been evaluated using a silver sinter die attach material which provides a mono-metallic die attach system. Samples have been temperature cycled from −40°C to +250°C for over 100 cycles without deterioration and the results are compared with Cu/Si3N4 substrates, which have shown issues with cracking and delamination for the equivalent thermal cycling regime. This work has been carried out under the Innovate UK supported project HITEC, led by Prodrive and involving the University of Warwick, Ricardo, TT Electronics - Semelab, GE Aviation Systems – Newmarket, Diamond Hard Surfaces and GaN Systems.