Thick film pastes for nitride ceramics for high power applications

Aluminum and silicon nitride ceramics show high potent as substrate materials for thick film- / hybrid applications in the field of power electronics and microwave technology. A main advantage of AlN is its very high thermal conductivity. Si3N4 has also a thermal conductivity comparable to AlN but higher thermal shock resistance and fracture toughness. Nevertheless, the use of Si3N4 in power electronic packages or heater applications is not achieved due to the lack of suitable connection technology such as thick film pastes. The main challenge for the respective pastes is the adhesion on the substrate. The low thermal expansion (CTE) of about 2.8 ppm/K of Si3N4 and material interactions with the thick film components must be considered in the paste development. Therefore, new glasses and glass-ceramic composites are required. Reason development was performed towards AgPd based thick film heater pastes for Si3N4. The pastes consists of AgPd in a 1:1 ratio, an inorganic filler and a glass phase adapted to the CTE of Si3N4. By variation of the content of inorganic filler and glass content three pastes with a sheet resistivity of 0.1 ohm/sq, 0.8 ohm/sq, and 7.9 ohm/sq and temperature coefficient of the resistance (TCR) between −100 and 100 ppm/K were developed. The performance of heater films prepared from these pastes under electric pulse load was studied. An electrical power of up to 85 W can be applied without a significant change of the resistivity (ΔR/R) of 0.1%. As reference comparable investigations on AlN were performed with adapted pastes. The film structures were analyzed in FESEM studies.