Sizing of Heat Spreaders Above Dielectric Layers

The thermal resistance of thin dielectric layers between high power- dissipation devices and thermal ground is substantial. Therefore, power devices are frequently mounted on a thin heat spreader (generally copper) which spreads the thermal load over a larger area of the dielectric through which it conducts. Besides the material properties of the heat spreader and dielectric layer, the thermal resistance between the device and thermal ground depends primarily on the geometry (thickness and footprint) of the heat spreader. Proper sizing of heat spreaders is therefore necessary to minimize the material required to manufacture them and the size of their footprint while achieving the necessary thermal performance. Herein a means to properly size heat spreaders is developed by representing the heat spreader and dielectric as a thermal resistance network. Generalized formulae and non- dimensional charts to optimize their thickness and footprint are represented. The power device and heat spreader are assumed to be (concentric) rectangular solids of arbitrary length, width and thickness and the non-dimensional results are validated by finite element analysis (FEA). Examples are given which demonstrates the utility of the methodology to thermal design engineers.