Thermally optimised millimeter wave package on an LTCC ceramic board

High power density within very small regions is a typical feature of advanced millimeter wave devices and components. Sufficient cooling has to be guaranteed during operation also under unfavorable ambient conditions. In the research project "Multifunctional Micro- and Millimeterwave Modules (4M)", funded by the BMBF of Germany, a 28 GHz communication unit was developed using low temperature cofired ceramic (LTCC) multi layer substrates as a basis for the millimeter-wave multi chip module (MCM). The TX and LO signal amplifier GaAs chips produce a major part of the dissipated power that has to be transferred to the base plate of the system. Thermal vias were implemented into the LTCC multilayer in order to overcome the poor thermal conductivity of the substrate material. The via diameter and pitch as well as the different possibilities of die assembly had to be optimised with regard to thermal and cost aspects. The authors explain their approach in thermal via and die assembly optimisation. It will be described how the best suited thermal via arrangement was found by means of finite element (FE) submodeling, which enabled a very detailed thermal analysis for a wide variety of thermal via arrangements. Two assembly technologies, soldering and gluing, were studied in order to characterise the thermal resistances of the different layers between die and case. The heat sharing of the several paths could be quantified. An optimised assembly process could be proposed which ensures sufficient cooling and minimum Ag or AgPd volume. The comparison of calculated and measured surface temperatures allows for the influence of inaccurate parameters in the FE model to be diminished and leads to a more realistic FE modeling of prospective electronic assemblies.