Balancing Thermal and Electrical Packaging Requirements for GaN Microwave and Millimeter-Wave High Power Amplifier Modules

A method for balancing thermal and electrical packaging requirements for gallium nitride (GaN) high power amplifier (HPA) modules is presented. The goal is to find a design approach that minimizes the junction temperature of the GaN so that it is reliable and has interconnects that meet electrical performance requirements. One benefit of GaN is that it can simultaneously achieve high power density and operate at microwave and millimeter-wave frequencies. However, the power density can be so high that the necessary thermal solutions can have negative impact on electrical performance. This is especially a concern for the electrical interconnects required for the input/ output ports on high power amplifier devices. This is because the signal interconnects must operate at GHz frequencies, which means that special care must be taken to avoid problems such as undesired signal coupling and ground path inductance. Therefore, this work focuses on GaN packaging and its integration into a module. The results show that an optimum thickness for the GaN heat spreader exits for thermal performance but the electrical design is impacted negatively if the optimum thermal design is chosen. Therefore, a balanced design is chosen which meets overall system level requirements.

A Simple Closed Form Solution to Single Layer Heat Spreading Angle Appropriate for Microwave Hybrid Modules

A simple equation for heat spreading angle is derived which is useful for cases with a single layer thermal spreader. The derivation starts with Fourier’s heat transfer law. Heat spreading in two dimensions is then introduced which results in a quadratic equation relative to spreading angle. The result is a closed form equation for heat spreading angle. Calculations using the equation are compared to 3D finite element simulations which show agreement acceptable for most practical applications and over a wide range of physical dimensions and thermal conductivities. A norma-lized dimensional parameter is defined which is used to generate a curve fit equation of the spreading angle. A three step procedure is then presented which allows the calculation of the spreading angle and temperature rise in the thermal spreader. The result has application for initial calculations of temperature rise in microwave hybrid modules and electronic packages such as heat sinks for high power amplifiers. This is because it is common for these types of modules and packages to use a single layer heat spreader in copper-tungsten (CuW) or copper-molybde-num (CuMo) connected to a cold plate. An important benefit of this method is that it allows microwave hybrid designers and high power amplifier packaging engineers a method to quickly perform trade studies to determine the maximum mounting temperature for integrated circuits.