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Flexible Thermal Ground Planes with Thicknesses Below Quarter-mm
Keywords: Thermal Ground Plane, Heat Pipe, Ultra-thin
The trend in smartphones toward thinner configurations presents challenges for thermal management. For future smartphones with thickness reduced from 6 mm to 3 mm or 1 mm, highly effective ultra-thin heat spreading technology is required, in order to remove hot spots and control the maximum skin temperature. Flexible thermal ground planes (FTGPs), which employ the physics of a heat pipe or vapor chamber, have proven to be successful thermal management solutions, with effective thermal conductivities in the range of 1,000 to 4,000 W/m-K—up to an order of magnitude above copper. Such FTGPs achieve high effective thermal conductivity by employing phase change heat transfer and convection, in which liquid evaporates over a hot spot and heat is carried away in the vapor phase. The vapor condenses over a cooler region, and capillary forces pull the condensed liquid back to the hot spot, thereby completing the closed loop. Typical thin TGPs have a thickness around 0.5 to 0.75mm. However, emerging thin mobile systems will require thinner FTGPs. Recently we have developed FTGPs with thicknesses down to 0.25 mm. These devices have active areas of 5 cm x 10 cm. When tested with a 0.8 mm x 0.8 mm evaporator and a 2 cm x 5 cm condenser, the FTGPs show thermal resistance lower than 1/3 that of an equivalent reference copper heat spreader—giving the FTGP an effective thermal conductivities greater than 1,200 W/m-K. When tested with distributed cooling by natural convection across the entire surface, the FTGP show surface temperature uniformity above 5x that of equivalent copper sample. Fabrication is based on printed circuit board (PCB) manufacturing techniques, allowing low-cost, high-volume development. The measured thermal results are analyzed in terms of a series-network of thermal resistances, which can be used to understand the effect of different geometries. This thermal model can also be used to guide further thickness reduction down to 0.1mm.
Ryan Lewis, Research Associate
University of Colorado at Boulder
Boulder, CO

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