Investigation of Innovative Cooling Concepts for Functional Ceramic Circuits

Abstract

Low temperature co-fired ceramics (LTCC) technology offers many outstanding advantages when used as functional ceramic in microwave applications due to the possibility to integrate passive components into the LTCC body, reliability aspects, as well as excellent physical properties, especially in the high frequency range. The limited thermal conductivity of the LTCC, however, requires innovative approaches for the cooling of heat critical devices. State-of-the art solutions use passive concepts with thermal vias, heat spreaders and heat sinks to achieve adequate system cooling, but these components are tailored to the customer-related design and suffer from a poor efficiency. Therefore, future module designs will need active cooling concepts to combine all benefits of functional ceramic circuit technology together with the required demands to dissipate the thermal power losses produced by active devices. To establish a practicable workbench for future layouts, test structures were fabricated to investigate experimentally in combination with theoretical considerations, the impact of water cooling when implementing a micromachined channel system in the LTCC body. To simulate the thermal power losses of active devices a thick film resistor was realized on top of the ceramic substrate. To measure the cooling efficiency as a function of flow conditions such as flow rate and flow direction, a waterfilled channel system was placed directly below in the inner layers of the ceramic. Basically, a good correspondence is achieved between FEM (finite element method) simulations and IR-based measurements with respect to the corresponding decrease in surface temperature. All in all, it can be concluded that active cooling systems offer a remarkable potential to meet the requirements for future circuit designs, in particular where local hot spots may cause problems and the applicability of only passive cooling concepts is not sufficient.