18 Air or Water Flow Required to Cool a PC Board

In this chapter we will investigate the air and water flow required to cool a printed circuit (PC) board. The components of a PC board generate power, and these components have to be kept at a specified operating temperature. The power generation of these components depends upon the available cooling flow under the PC board. The thermal management of a PC board can be very tricky. A simple PC board is composed of three layers (i.e. a PC board attached by a thin layer of epoxy to an aluminum base plate acting as a heat sink for a uniform conduction thermal path). Let us consider a PC board whose length and width dimensions are 0.25 m by 0.25 m, namely a board area of A = 0.0625 m². The components of the PC board have to be kept at a maximum of 50°C. The cooling fluid is flowing under the aluminum base plate as a laminar flow and/or a turbulent flow over a flat surface. In most PC board designs, about 90% of the heat generated is extracted by the flowing fluid through convective heat transfer. We will consider the fluid flow to be steady, incompressible with constant properties.