Parametric investigation of a graphite foam evaporator in a thermosyphon with fluorinert and a silicon CMOS chip

High thermal conductivity graphitic foam was utilized as the evaporator in a modified thermosyphon. The foam was soldered directly to the back of a silicon CMOS die and mounted in a standard PGA. Fluorinert FC-87 and FC-72 were evaluated as the working fluids of choice and a variety of variables on the foams were explored. It was found that the density of the foam evaporators affected the thermal performance of the system. However, the fluid level and fluid type had very little effect on the overall performance in the system, making fabrication of a commercial device less challenging. The most significant effect on performance was the modifications to the foam structure. Slotted patterns were found to enhance the rate of return of fluid to the foam closest to the die, thus improving performance. With a slotted foam evaporator, a heat flux of 150W/cm² resulted in wall superheats of only 11°C. The experimental setup used in this research gives accurate measurements of the actual active layer in the chip and temperatures less than 71°C have been achieved at heat fluxes of 150 W/cm². This performance is significantly better than any prior literature data. In fact, the graphite foam thermosyphons were shown to outperform spray cooling. In addition, it was found that critical heat flux was not reached in these experiments with graphite foam evaporators at heat fluxes as high as 150 W/cm².