Application of a numerical tool for thermal evaluation of printed boards in communication systems

The authors explain how they use a commercial computational fluid dynamics (CFD) tool to evaluate printed circuit boards in the design of an advanced communication system. The computation volume is restricted to one or two chimneys. The environment, external to the computation volume, is taken into account by means of experimentally determined boundary values of flow and temperature. First, temperature measurements (junction, board, air) are performed on a test PBA (printed board assembly) under well-defined conditions: sub-rack chimney inlet air velocity and temperature, as well as component power dissipation are kept constant over the inlet area. These temperatures are used to calibrate the mathematical model by comparing the simulation results with the measured temperatures. Then measurements are performed under real-life conditions: the inlet area air velocity and temperature distributions are evaluated in the actual rack configuration. In the simulations, this inlet area is characterized by either global worst-case values or by semi-local values, assumed constant over a large portion of the chimney entrance. Practical examples are discussed