Direct Submount Cooling of High-Power LEDs

Rapidly increasing light emitting diode (LED) heat fluxes necessitate the development of aggressive thermal management techniques that can intercept the dissipated heat directly in the submount. Microgap coolers, which eliminate solid-solid thermal interface resistance and provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, offer a most promising approach for cooling high-power LEDs. This paper focuses on the two-phase thermofluid characteristics of a dielectric liquid, FC-72, flowing in an asymmetrically heated chip-scale microgap channel, 10 mm wide × 37 mm long, with channel heights varying from 110 μm to 500 μm and channel wall heat fluxes of 200 kW/m
²
. The experimental two-phase, area-averaged heat transfer coefficients of FC-72 reached 10 kW/m
²
·K, significantly higher than the single-phase FC-72 values, thus providing cooling capability in the range associated with water under forced convection. Data obtained for single-phase water yielded very good agreement with predictions for the convective heat transfer coefficients and served to validate the accuracy of the experimental apparatus and measurement technique. It is shown that this two-phase cooling approach could be used to dissipate in excess of 600 kW/m
²
in the submount of high-power LEDs.