Flexible Thermal Ground Planes Fabricated With Printed Circuit Board Technology

Abstract

Thermal ground planes (TGPs) are passive thermal management devices that utilize the phase-change of a working fluid to achieve high thermal conductivity and low thermal resistance. TGPs are flat, two-dimensional heat pipes—similar to vapor chambers—in which liquid is held within a capillary wick, and vapor is held in a sealed vapor layer. Heat is absorbed at an evaporator region, causing the liquid to evaporate. The heated vapor in the vapor core is carried via convection to a condenser region where it condenses as the heat is expelled from the TGP to an external heat sink. The condensed liquid is then pulled back to the evaporator via capillary forces in the wick. In numerous applications, mechanical flexibility of the TGP is required, as is low-cost manufacturing and viable integration routes with electronics. This work describes a flexible TGP (FTGP) fabricated using printed circuit board (PCB) technology, in which commercially available copper-cladded polyimide sheets are used as the casing material. The wick is composed of three layers of fine copper mesh electroplated or sintered together and coated with atomic layer deposited TiO2. A coarse nylon or polyether ether ketone (PEEK) mesh defines the vapor transport layer, and water is used as the working fluid. The perimeter of the device is heat-sealed with flouroethylene propylene (FEP), which has been found to provide a near-hermetic seal for several months and is suitable for flexible applications. This architecture allows the TGP to function with minimal reduction in heat transfer performance while bent by 90 deg, and full functionality is returned when the device is returned to its flat configuration. The FTGP's measured thermal resistance is about half that of an equivalent copper reference for input heat fluxes of 3–6 W/cm2. More than 30 copper-cladded polyimide FTGPs were fabricated and characterized using both simple qualitative and more involved quantitative test setups. The results show that the fabrication and assembly processes developed in this work are repeatable and the devices are durable.