Selecting capillary structures for heat pipes in multilayer printed circuit boards

Heat pipes are increasingly used to manage heat dissipations in electronic products. In a novel design approach for these products, a flat miniature heat pipe has been integrated inside the laminated structure of a printed circuit board (PCB). The capillary wick structure, required to facilitate the fluid transport, in combination with the orientation of the embedded heat pipe determines, to a large extent, the heat pipe's performance. Important design parameters are the wick's effective pore size and permeability, together with production and assembly requirements. A narrow pore gives great capillary pressure, however more friction is encountered. In general, as the gravitation assisted fluid flow decreases, so does the design freedom to select a type of wick. For gravity assisted orientations an open wick structure, such as microgrooves, channels and arteries, is preferred, whereas against gravity a more compact wick, for instance a sintered metal wick, is required. The optimal capillary structure should be selected for the intended application and production volume, thus obtaining sufficient heat transport, at the lowest manufacturing cost.