Cooling of a PWB With Irregular Populated Chips

Abstract

A simple analysis and series of 3-D CFD simulations are carried out to estimate the temperature for the 208-pin QFP chips on a PWB. The main feature of the problem is the low primary velocity (0.09∼0.36 m/s), which implies that 3D effects, buoyancy force, PWB thermal conductivity and radiation will have a big impact on the temperature profile. This is verified by using commercial CFD simulation codes. The results show CFD tools are ideally suited for these classes of problems. For the PWB under study, the worst case consists of an inlet air velocity of 0.09 m/s, no conduction in the board, and the board is laid horizontally, for which the buoyancy force is not well utilized). The resulting highest case temperature will be 110∼120C, which is close to the allowable case temperature 125C. Several heat spreader (heat sink) configurations are studied. One of them reduces the highest case temperature below 100C. However, if the board conductivity is not too low (>0.5 W/mK), or if. the board is laid vertically and air is blown from bottom to top, there is no need for extra heat spreaders.