Localized Pool Boiling and Condensation Experiments over Functional CPU: Optimizing the Overall Thermal Resistance via Different Heat Transfer Scenarios

Pool boiling cooling systems are one of the most promising candidates to address the increase of electronics power consumption. This cooling technique still exhibits many challenges to be fully adopted, such as the high-reliability risk associated with the full immersion of electronic components in dielectric liquids and the film boiling phenomena. This paper reports an investigation of the effects of multiple boiling scenarios on the overall thermal resistance of a close two-phase cooling system, mounted directly over a functional microprocessor. Two dielectric fluids (Novec 649 and 7000 from the 3M Corporation) were tested over nickel and copper processor surfaces. A better overall thermal resistance was achieved when boiling the Novec 7000 on top of the copper exposed processor surface. Degassing the setup to remove non-condensable gases lowered the absolute pressure inside the system and reduced the overall thermal resistance. Moreover, partially immersing inward heat sink pins into the dielectric liquid was observed to also lower thermal resistance. The best boiling scenario was achieved while using Novec 7000 and combining all other improvements. A (0.38±0.01) °C/W minimum overall thermal resistance was calculated from junction to air at a (130±4) W power consumption and a (73±0.4) °C maximum junction temperature. This minimum overall thermal value was 30% lower than the one associated with the best boiling scenario using Novec 649.