Thermal design considerations on wire-bond packages

Due to their excellent microwave behaviour (low or moderate dielectric loss, highly conductive inks) and their hermeticity, LTCC materials are being widely used for MMIC packaging. The latter is often required to protect bare dies against the impact of harsh working conditions on the field. Several applications, such as front end modules for mobile communication or transmit modules for point-to-point systems, require packages with very low thermal resistance, since up to 70% of the supplied power is converted into heat. Although glass ceramic materials have a better thermal conductivity compared to organic boards, it is still insufficient. Thermal vias are one way to locally improve the thermal conductivity. However, their dimensions, bulk material, arrangement and number of layers have a strong impact on the resulting performance. Heat sources (hot spots) on the MMIC, the MMIC thickness and optional heat spreaders need to be included in the evaluation as well. Previous simulations showed that as many thermal vias as possible should be placed below the heat source. In addition, thermal vias should be directly located under the hot spot. The whole system with the environment from the bare die to the heat sink has to be taken into account, as well the heat path in the package. In this investigation various constructions are considered. The principal build-up consists of an Al-plate as heat sink (thermal ground) with a glued PCB on the top, the mounted package and the bare die as the heat source in the package. A simulation model of a power amplifier TGA9083-SCC from TriQuint was used as a bare die to realize a realistic scenario. The paper considers different thermal concepts by means of simulation. Variables in the studies are LTCC metallization materials, PCB designs and mounting materials for wire-bond packages. The optimal system for lowest thermal resistance is a LTCC-housing with an inserted MoCu-plate which is directly mounted onto the board heat sink.