Thin diffusion bonded flat loop heat pipes for electronics: Fabrication, modelling and testing

Abstract

Diffusion bonding process was used to manufacture three different thin flat loop heat pipes (LHP), two with 1.52 mm and one with 0.92 mm of thickness. The proposed LHPs were designed to be candidates for thin electronics cooling applications, such as smartphones. The devices attended to the following heat transfer requirements, based on recent electronic technologies: dissipation of both low (2 W/cm2) and high heat fluxes (8 W/cm2) at natural air convection. A workbench, capable of evaluating the LHP thermal performance, was developed to simulate the operating condition of a chip processor with 1 cm2. The LHPs were investigated experimentally using ethanol as the working fluid in three orientations: horizontal, gravity-assisted and against-gravity. As the main problem of operating thin LHPs with low heat fluxes is the heat leakages from the evaporator, a theoretical model was proposed to predict the conduction heat transfer between the evaporator and the liquid line. The minimum thermal resistance of 0.2 °C/W was achieved for the 1.52 mm thick device at 8 W/cm2. Experimental results confirmed that the thickness reduction of heat pipes leads to a decrease in their thermal performance. The heat leakage of the thinner LHP to the liquid line was approximate 17% of the total heat transferred, while the thicker LHP showed a heat leakage of 10%. Although still demands development, two of the three proposed loop heat pipes showed good heat transfer capacity, keeping the evaporator temperature, supposedly in contact with the electronic component in actual applications, below 100 °C. This device is shown to be suitable for the thermal management of thin electronic devices, especially mobile phones.