Liquid cooling in an LTCC-module for a switched mode amplifier

Low Temperature Co-fired Ceramic (LTCC) is a proven packaging technology for microwave and millimetre-wave applications. Advanced low-loss material systems and improved manufacturing technology facilitate economic and highly reliable packaging solutions for automotive, telecom, medical and security applications. The fact that the substrate is hermetic facilitates unique packaging concepts where the LTCC multilayer is part of a hermetic package. However, there is a trade-off for the increasing complexity of these modules: When active devices are involved, the density of power dissipation is also rising. Liquid cooling provides an alternative where (forced air) convection cooling is not possible for lack of space or because of other limitations. With the integration of micro-channels into the LTCC-multilayer-stack the microwave-substrate becomes part of the piping for liquid cooling and brings the coolant in close contact to the heat source. As an example for such a solution a switched mode amplifier shall be presented that uses liquid cooling for the power transistor. Two different approaches for the routing of the cooling channels inside the LTCC are compared with cooling on a metal block at constant temperature. For the exact determination of all parameters, dissipated power and temperature on the die (junction temperature) a thermal test die is used instead of the transistor for the test vehicles.