Thick Film Materials for High Power Hybrid Circuits on Aluminum Nitride

Abstract

Abstract For over 40 years, the substrate of choice for designing and fabricating traditional hybrid circuits has been alumina. It has provided the required mechanical strength, electrical resistivity, and thermal performance needed for proper circuit operation. Over the past several years however, we have experienced a shift in hybrid technology towards electronic devices with highly complex, dense circuit configurations that produce more power and consequently, more heat than previous designs. This requires the use of a substrate with a higher thermal conductivity to properly manage the heat transfer and dissipation in order to maintain optimum performance and functionality of the end device. The thermal properties displayed by aluminum nitride provide design engineers with a reliable alternative to traditional alumina. While creating new and exciting possibilities, the use of aluminum nitride also creates a different set of challenges for thick film suppliers and circuit fabricators. Due to the thermal expansion mismatch, as well as the chemical changes that occur to the substrate which affect adhesion during the firing process, thick film pastes previously suitable for alumina are typically not compatible with aluminum nitride. To overcome this challenge and the performance demands of high power, high reliability circuit applications, a new line of RoHS and REACH compliant thick film pastes has been developed. The following conductors are available: silver, silver/palladium, silver/platinum, copper, and gold. In addition, we have developed two resistor pastes and a compatible overglaze. This paper will discuss the aforementioned thick films and their critical performance properties before and after reliability testing. This includes adhesion for the conductors, resistance and TCR for the resistors.