Measurement of Variable Conductance Ultrathin Vapor Chambers to Double the Capillary Limit During Transient Startup

Ultrathin vapor chambers are passive heat spreading devices, which are seeing wide deployment in mobile electronics. A major drawback of vapor chambers is their capillary limit, which prevents operation beyond a certain power. The capillary limit occurs when the capillary force of the wick can no longer overcome viscous drag of the vapor flow and liquid flow-this capillary limit may be high for higher temperature operation, but lower for low temperature operation, due to temperature dependence of fluid viscosity and density. As a result, when rapidly heating from ambient temperature, the capillary limit of a vapor may be less than half as high as a similar vapor chamber heated by slowly increasing the power. In other words, rapid heating can cause maximum power to drop by 50%, because of effects at low temperature. This paper explores a method to introduce non-condensable gas (NCG) in order to circumvent the failure of such an ultrathin vapor chamber at low temperatures. NCG block convection of internal liquid and vapors, and shorten the length of fluid flow thereby decreasing the viscous drag and increasing the capillary limit. Furthermore, the region of NCG is large at low temperature but shrinks as the VC heats due to the vapor pressure of water. In this paper, an ultrathin vapor chamber which can reach 18 W when the vapor chamber is heated slowly is measured to fail due to the capillary limit at only 7.6 W when heated in transient; on the other hand when 0.03 smL of air is introduced as NCG, the same vapor chamber is measured to fail at 15 W in transient-doubling its transient performance.