Embedded Miniature Heat Pipes In Ceramic Cofire Substrates

Prototype miniature heat pipes have been fabricated that are compatible with high and low temperature ceramic multichip module (MCM-C) processing. The heat pipe is fabricated as an integral part of the MCM-C substrate using conventional numerically controlled machining operations. The heat pipe uses an axially grooved wick structure to provide transport of the working fluid from the condenser to the evaporator. The working fluid chosen for the heat pipe was water. Preliminary testing was performed using the heat pipe as a thermal spreader to enhance cooling by convection and for axial transport of heat to the edge of the substrate for cooling by conduction. Up to 16 Watts of heat were transported by the heat pipe with cooling provided along the condenser region by natural convection or by a temperature controlled cooling jacket. In all cases, the temperature variation along the length of the substrate surface was less than 5°C. When a cooling jacket was used, the mean temperature along the substrate surface was less than 5°C above the cooling jacket temperature. The heat pipe was found to have an effective thermal conductivity greater than 10,000 W/m-K, which is over 300 times higher than that of the alumina it replaced within the substrate.