Micro Capillary Pumped Loop for Electronic Cooling

Abstract

Electronic devices have been minimized, but their performance is becoming better and better. Their heat flux has been significantly increased and has already exceeded about 100 W/cm2 recently. The insufficient dissipating of the heat flux may lead to performance decrease or failure of the electronic device and components. Heat flux in laptop computers has not been questioned; therefore, only a heat sink has been applied on cooling them. Recently, however, a more powerful cooling solution is sought for high heat flux. Solid materials with high thermal conductivity have been mainly used in low heat flux applications, whereas small-sized heat pipes have been utilized in high heat flux applications. The use of small-sized heat pipes in electronic devices like laptop computers has only been developed recently. For example, the use of heat pipes with diameter of 3–4 became common in laptop and desktop computers only during the early 2000s. Recently, as electronic devices have started to become smaller and thinner, heat pipes with diameter of 3–4 mm have been pressed to fit the form factor to them. However, a lot of problems were encountered in their thermal performance, thus micro heat pipes (MHPs) were developed to solve them. Specifically, a flat plate micro heat pipe (FPMHP) with diameter of less than 1.5 mm was developed by Moon (Moon et al., 2002). FPMHPs are being used mainly in display panel BLU applications and are being prepared to be used in the LED headlight of vehicles. However, in spite of their thermal performance and broad applications, FPMHPs may still show degradation in thermal performance in the case of thinner applications. If we consider phase-change cooling devices like heat pipes that have thermal conductivity that is 500 times larger than copper rods for small-sized and thin electronic devices, there is a need to develop new cooling methods suitable for them. A thin flat plate type micro capillary pumped loop (CPL) with thickness of less than 2 mm was developed by Moon as a trial product. The proposed micro CPL has two-staged grooves in the evaporator, instead of poles, for preventing the backflow of the vapor bubbles; this is a simpler structure compared to that of a micro CPL with poles. A large vapor space from the evaporator to the condenser was also constructed in the middle plate to allow for the reduction of the flow resistance of the vapor. The micro CPL was fabricated using MEMS technology and was composed of lower, middle, and upper substrates. The lower substrate was composed of silicon, while the middle and upper substrates were made from Pyrex glass for visualization. Through a preliminary test, it was verified that there was no leakage at the adhesion interface between the lower and the middle or upper substrates