Development of the micro capillary pumped loop for electronic cooling

Electronic devices have been minimized but the performance of those is becoming better and better. Therefore it is needed to develop new cooling methods suitable for a thin packaging structure with high thermal density. The thin flat plate type micro CPL(capillary pumped loop) with the thickness less than 2mm was developed in this study. The proposed micro CPL has two staged grooves in evaporator instead of poles for preventing backflows of the vapor bubble and the simpler structure than that of a micro CPL with the poles. Also a large vapor space from the evaporator to the condenser was constructed in the middle plate therefore flow resistance of the vapor could be reduced. The micro CPL was fabricated using MEMS technology. The micro CPL was composed of lower, middle and upper substrates. The lower substrate was made of silicon and the middle and upper substrates are made of Pyrex glass for visualization. Through a preliminary test it was checked that there was no leakage at the adhesion interface between lower and middle or upper substrates and at the bonding interface between lower substrate and fill tube. Although the experimental studies for the micro CPL have been poor till now, we have obtained the reasonable experimental results in this study. The performance test result has showed 8.5W of the heat transfer rate for the micro CPL and we could observe the operating characteristics of circulating or evaporating and condensing by visualization. Pure distilled water was used as the working fluid.

I. Introduction

Since the electronic communication devices, especially mobile devices, have been toward the trend in miniaturization and slimness, the structural packaging density in the devices is increased. Therefore it has been predicted that a thermal dissipation problem will be issued. The requested thickness of a flat type heat pipe module for packaging in a notebook PC about 8 years ago is 2mm. Recently, a sub-notebook PC or UMPC(Ultra Mobile PC) which emphasizes mobility, smallness and slimness are being offered. While the multi-functions as much as the functions in the notebook PC is requested for the devices, a thinner thickness than 2mm is requested in thermal design using a cooling module. Therefore, it has become extremely difficult to offer an optimal cooling solution for the small mobile devices. The metal spreaders with high thermal conductivity which mainly used in the electronic packaging till now run the risk of capability limitation for increasing heat flux. Therefore development of the cooling methods with lager cooling capacity is needed. The heat pipe with two-phase change mechanism has been used as an alternative of the metal spreader in a decade. The micro heat pipe with polygonal cross section and the flat plate heat pipe developed in ETRI[1] have proper structures and sizes as cooling solutions for the mobile devices. However a cooling method with thinner thickness than 2mm is requested. It is not easy to develop such a thin cooling solution with two-phase change mechanism. Moreover, although the thin cooling solution with two-phase change mechanism is developed, the heat transport capacity of that could not exceed small wattage. The micro CPL(Capillary Pumped Loop) developed in this study has a possibility of large heat transport capacity relatively. The CPL with macro size has been studied from long time ago and has been improved in a view of thermal capacity. However, the first study for the flat type micro CPL which fabricated using MEMS technologies was performed by J. Kirshberg et al. (Univ. of California at Berkeley) in 1999[2]. The micro LHP(Loop Heat Pipe) with a similar concept to the micro CPL was developed by A. Hoelke in 1999[3]. Since then, the micro CPL with two through holes for filling working fluid was developed by Laura Meyer et al.(GE) in 2003[4]. All these days, a study for the micro CPL has not been performed. Moreover, the previous studies for the micro CPL were limited to the area of analytical and fabrication process and the reasonable experimental data for the thermal performance have not been offered. This is due that the design and the fabrication of the micro CPL is not easy. Especially, the more precise technology in the fabrication processes of the filling and controlling of working fluid under vacuum circumstance is requested.