Thermal analyses of composite copper/porous graphite spreaders for immersion cooling applications

The performance of composite spreaders cooled by nucleate boiling of FC-72 liquid and made of top porous graphite layer (>= 0.4 mm or Delta(PG) >= 0.2), for enhancing nucleate boiling, and copper substrate (<= 1.6 mm), for efficient spreading of the thermal power dissipated by underlying chips is investigated. The spreaders are of the same thickness (2.0 mm) and have square surface areas that are sized depending on the composition and cooling condition of the spreaders. The 10 x 10 mm and 15 x 15 mm chips considered in the analysis are assumed to uniformly dissipate the thermal power. With composite spreaders (Delta(PG) = 0.2) cooled by 30 K subcooled nucleate boiling the removed total dissipation thermal powers of 160.3 W and 98.4 W from the 10 x 10 mm and 15 x 15 mm chips, respectively, are attainable at total thermal resistances of 0.29 and 0.48 degrees C/W, respectively. When these spreaders are cooled by saturation nucleate boiling, however, the removed dissipation thermal powers (85.6 W and 53.4 W, respectively) and the total thermal resistances (0.12 and 0.20 degrees C/W, respectively) are much lower. For same cooling conditions, the removed dissipation thermal powers by the porous graphite spreaders (Delta(PG) = 1.0) are much lower and the thermal resistances are much higher than those with composite spreaders, because of the relatively low and anisotropic thermal conductivity of porous graphite. Results also showed that the surface temperatures of the chips are not uniform. The maximum chip temperatures at the highest removed dissipation powers by composite spreaders are < 71 degrees C and the temperature differentials across the chips are < 8 degrees C. Results demonstrated that composite spreaders are an attractive option for cooling high power computer chips at relatively low chip temperature.