Ultra-thin vapor chambers are passive thermal management systems which employ the evaporation of an encapsulated liquid to lift heat from a hot spot, convection of the hot vapor to spread the heat, condensation of the vapor to reject heat at a condenser, and capillary pumping of the liquid to return it to the hot spot. By utilizing phase change and convection, they can spread heat with effective thermal conductivities several times higher than copper or graphite, at a fraction of their mass. This is especially useful in mobile electronics applications, but mobile systems demand ever shrinking thermal solutions.
We have developed vapor chambers with total thickness as low as 0.15 mm, capable of dissipating 5W of power. The maximum power that a vapor camber can dissipate at such size scales is limited by the capillary effect: the capillary pumping power of the wick must overcome viscous drag of the circulating liquid and vapor. As the thickness decreases, the viscous drag increases. Dense wick structures can increase the capillary effect, but there is a design trade-off, as dense wicks increase the drag on the liquid. Using optimization to select an ideal density wick, as well as vapor cavity and cladding structure, we find a minimum theoretical vapor chamber thickness is 125 μm, for 5W applications. However, a wick only needs its maximum capillary pressure near the heat source, so a better solution may be a wick with varying density: high near the heat source to provide capillary pressure, but lower further away to reduce liquid drag. By designing an ideal inhomogeneous wick, the minimum theoretical vapor chamber thickness is 75 μm. This presentation will cover such design methods.