Performance of Isotropic and Anisotropic Heat Spreaders

Abstract

Anisotropic heat spreaders (flexible graphite and continuous carbon fiber polymer-matrix composite) and isotropic heat spreaders (copper and aluminum) have been evaluated numerically in terms of thermal resistance. Anisotropic ones are attractive for their through-thickness thermal insulation ability. Flexible graphite is superior to carbon fiber composite in providing lower thermal resistance. Carbon fiber composite is advantageous in its superior through-thickness thermal insulation ability and its smaller critical thickness (the optimal thickness for maximizing heat spreading while minimizing thickness). The isotropic heat spreaders are superior to the anisotropic ones in providing low thermal resistance, provided that the thickness is large, but they do not have the through-thickness thermal insulation ability. A higher value of the in-plane thermal conductivity enhances the effectiveness of flexible graphite. As the heat source area decreases, the thermal resistance increases while the critical thickness decreases. For the same heat source area, a greater in-plane dimension of the heat source perpendicular to the intended heat spreading direction decreases the thermal resistance and critical thickness. Flexible graphite is comparatively more advantageous when the thickness is smaller and when the heat source area is larger. For the same thickness below 2 mm, flexible graphite with in-plane conductivity of 1500 W/(m K) is superior to copper and that with in-plane conductivity of 600 W/(m K) is superior to aluminum. The highest thermal conductance obtained is 6.1 × 104 W/(m2 K) when the thermal interfacial resistance is neglected and 5.1 × 104 W/(m2 K) when this resistance is included. The conductance increases with decreasing heat source area and with decreasing heat spreader length.