Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011 November 11-17, 2011, Denver, Colorado, USA

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1 Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011 November 11-17, 2011, Denver, Colorado, USA IMECE DEVICE PACKAGING TECHNIQUES FOR IMPLEMENTING A NOVEL THERMAL FLUX METHOD FOR FLUID DEGASSING AND CHARGING OF A PLANAR MICROSCALE LOOP HEAT PIPE Navdeep S. Dhillon Department of Mechanical Engineering University of California, Berkeley Berkeley, California dr.ndhillon@gmail.com Jim C. Cheng Albert P. Pisano University of California, Berkeley Berkeley, California appisano@me.berkeley.edu ABSTRACT A novel two-port thermal flux method is implemented for degassing a microscale loop heat pipe (mlhp) and charging it with a working fluid. The mlhp is fabricated on a silicon wafer using standard MEMS micro-fabrication techniques, and capped by a Pyrex wafer, using anodic bonding. For these devices, small volumes and large capillary forces render conventional vacuum pump-based methods quite impractical. Instead, we employ thermally generated pressure gradients to purge noncondensible gases from the device, by vapor convection. Three different, high-temperature-compatible, MEMS device packaging techniques have been studied and implemented, in order to evaluate their effectiveness and reliability. The first approach uses O-rings in a mechanically sealed plastic package. The second approach uses an aluminum double compression fitting assembly for alignment, and soldering for establishing the chip-totube interconnects. The third approach uses a high temperature epoxy to hermetically embed the device in a machined plastic base package. Using water as the working fluid, degassing and filling experiments are conducted to verify the effectiveness of the thermal flux method. NOMENCLATURE P Pressure. Address all correspondence to this author. INTRODUCTION There are two clear trends in the microelectronics industry: the miniaturization of electronic devices resulting in increasing densities of transistors on a single chip, and increasingly compact mounting of these devices on circuit boards through stacking and other high density packaging techniques [1 3]. Conventional cooling technologies, such as conduction and convection, can no longer handle the heat flux densities that are emitted by today s electronic devices both in the arena of consumer electronics, and the defense and space sectors. The problem is further exacerbated by the fact that thermal hotspots on chips can have thermal flux densities roughly 3 to 8 times the average value for the whole chip [2]. As a result, localized cooling solutions are required close to the source of heat generation in an electronic system; This will minimize the intermediary thermal resistances, which can considerably increase the junction temperature. A number of new and improved technologies are being investigated in response to the thermal management problems being faced by the electronics industry. Loop heat pipes (LHP) belong to a class of passive, two-phase, high-heat-flux thermal transport systems, which includes heat pipes and capillary pumped loops. Whereas competing technologies like microchannel heat sinks, jet impingement, and spray cooling require external pumping power, loop heat pipes are completely passive and hermetically sealed systems, where capillary surface tension forces drive the two-phase fluid flow loop. Loop heat pipes have several design advantages over heat pipes that enable them to carry larger heat fluxes over greater distances [4, 5]. However, 1 Copyright c 2011 by ASME

2 Evaporator Inlet port Wick Reservoir Vapor microchannels Thermal barrier Condenser Liquid microchannels Outlet port (a) mlhp Prototype I Fig. 1 shows two microscale Loop Heat Pipe (mlhp) prototypes that have been fabricated. Degassing and controlled fluid charging is one of the major challenges to the successful operation of these devices. The working fluid is chosen according to the desired device operating temperatures water, ammonia, acetone, methanol, toluene, freon-11, freon-152a are among the many available options [17, 18]. We have chosen to work with water due to its good wetting properties, high latent heat value, and good efficiency in the o C temperature range [17]. Anhydrous ammonia and acetone are also very good candidates for the mlhp due to good heat transfer efficiencies in the o C temperature range [6]. The loop heat pipe needs to be completely degassed to remove all non-condensible gases (NCGs), before it is filled with the working fluid. This is essential because the presence of NCGs can lead to flow blockages due to the formation of bubbles in the wick structure as well as the transport microchannels. This affects device performance and can also lead to a general failure of the capillary evaporator. In this paper, we present high-temperature-compatible MEMS device packaging techniques for implementing a novel two-port degassing and fluid filling setup for microscale loop heat pipes. Figure 1. (b) mlhp Prototype II Microscale Loop Heat Pipe (mlhp) prototypes fabricated on silicon substrates using standard MEMS microfabrication techniques, and capped by anodically bonded transparent Pyrex wafers. conventional LHPs are bulky devices fabricated out of cylindrical metal pipes; therefore, they usually operate on the periphery of electronics modules. For this reason, they cannot eliminate the rise in junction temperatures due to the various contact and spreading resistances, and are generally unable to provide any sort of localized thermal management. Many groups are working on miniaturizing these metalpiping-based LHP designs, in order to enable them to fit inside modern, more compact consumer electronic devices, such as computers and laptops [6 10]. Although these devices can interface relatively well with standalone heat generating components, such as microprocessors in laptops, they are unsuited for cooling densely packed electronic substrates, which are usually stacked on top of each other [11]. Out-of-plane heat dissipation by conduction is severely restricted in these modules, and thin, high-conductivity substrates are required to enable the lateral dissipation of heat. MEMS-based loop heat pipes have been proposed towards the development of an ultra-conductive, thin, planar substrate that can be interfaced directly with electronic chips [5, 12 16]. STATEMENT OF THE PROBLEM Fig. 2 illustrates why degassing is so important in loop heat pipes, and especially so in its MEMS versions. It provides a close-up view of the entrance region to the capillary wicking structure in the mlhp prototype-i (shown in Fig 1). The wicking structure consists of very narrow (10 20 µm wide) rectangular channels, and this is where the liquid to vapor phase change happens. The wick is supplied with liquid by the liquid supply channels (150 µm wide) and a reservoir supply channel. Due to the presence of non-condensible gases, we can see bubbles both at the entrance to the wicking structure (Region I) and at the end of the liquid supply channels (Region II). These bubbles prevent the supply of liquid for evaporation during device operation, causing wick dryout and subsequent device failure. In macroscale pipe-based designs, the loop heat pipes are first evacuated by establishing a secure connection to an external vacuum pump, and then the working fluid is introduced into the device in a controlled manner. Vacuum levels as low as Torr are required to minimize the presence of noncondensible gases in the device [19]. The amount of the liquid in the device needs to be strictly controlled, since the device can only be partially filled; the volume of liquid in the device is dictated by the size of the compensation chamber and that of the device itself [20]. Similarly, in the design and fabrication of mlhp prototypes (Fig. 1), the degassing and filling issues are deferred until after the fabrication of the entire device. This is currently the only option, since the available microfabrication equipments do not provide for in-situ liquid filling of MEMS devices. Since the last 2 Copyright c 2011 by ASME

3 Bubbles The end of the pipe is closed using a solder or by brazing Rectangular wick (width ~ microns) Liquid in introduced from one end of the heat pipe The vapor generated by heating the liquid purges the air from the heat pipe and saturates it completely Region I Region II Heating flame Reservoir supply channel Liquid supply channels Figure 3. A simple degassing and filling method employed in the fabrication of large size cylindrical heat pipes. The working fluid should have a low boiling point, so that it readily vaporizes upon heating. Figure 2. Microscale loop heat pipe components susceptible to blockage due to the presence of non-condensible gases (NCGs) in the working fluid. Bubbles formed at the entrance to the rectangular wicking structure (region I) and at the end of the liquid supply channels can block the supply of liquid to the wick, causing immediate device failure. step in mlhp device fabrication [21] is the anodic bonding of silicon and pyrex wafers at only moderately high temperatures ( o C) in a Karl Suss Anodic Bonder opportunities exist for the industrial level modification of this equipment to enable injection of a degassed liquid into the device before the bonding process. But, this will require significant capital investment and substantial modifications to the current MEMS anodicbonding technology. The remaining alternative, which we have pursued, is to provide fill holes in the fabricated mlhp devices, which are connected to an external degassing and filling system. This requires high-vacuum-grade tube-to-chip connections and connecting tubing, and an ultra-high-vacuum pumping system that can provide vacuum levels as low as 10 6 Torr. This will, in practice, be a large system with a big pump, and is extremely impractical when evacuating devices whose internal volumes are on the order of microliters. In the following section, we present a novel thermal-flux based degassing and fluid-filling system that does not require a vacuum pump. We then present three different device packaging techniques that were implemented in order to interface the mlhp device with the external filling system. The effectiveness and reliability of each of these techniques, as well as their advantages and disadvantages, will be explored in detail. THERMAL-FLUX DEGASSING AND FILLING SYSTEM In contrast to the vacuum-pump based system for degassing cylindrical loop heat pipes, as described in the previous section, there is an alternative approach that is commonly employed for heat pipes (HP) when the working fluid has a low boiling point, for example acetone. As shown in Fig. 3, a small amount of liquid is introduced through the open end of the cylindrical heat pipe. The liquid settles at the bottom of the heat pipe, which is then heated with a flame to cause evaporation. The vapor rises through the pipe and in the process purges the air from the pipe. When the pipe is completely saturated with vapor, its open end is sealed shut using soldering or brazing. This method works best for heat pipes since they require only a small amount of working fluid for operation, with most of the space occupied by vapor. On the other hand, loop heat pipes are mostly filled with liquid; moreover, the MEMS versions have large capillary forces, which prevent air bubbles from escaping the device. We have extended the above concept to design a two-step, thermal flux-based degassing and fluid filling process, using water as a working fluid, as outlined in Fig. 4. The inlet port of the mlhp device is connected, using steel capillary tubing, to an external reservoir flask containing water, which has been pre- 3 Copyright c 2011 by ASME

4 water vapor are removed from the system due to convection. In step 2, the outlet port tube exit is closed, and the reservoir flask is inverted to let water into the connecting steel tubing. A valve is used to control the flow of liquid into the device, the liquid being pushed into the device by thermally pressurizing the reservoir flask. In this way the mlhp can be partially filled with a working fluid devoid of all non-condensible gases. (a) Step 1: Superheated vapor purges system Valve Heater DEVICE PACKAGING TECHNIQUES FOR IMPLEMENT- ING THE THERMAL FLUX FILLING METHOD In order to implement the above system to degas and fill individual micro loop heat pipes, we need high-temperature compatible, high-vacuum grade chip-to-tube interconnects. Currently, only epoxy-based plastic microfluidic interconnects are available in the market, and there are no commercial solutions for reliably connecting metal microtubing to MEMS wafer-scale devices. Further, any microfluidic connections to the mlhp device must be able to withstand the stresses that can propagate through the stiff metal tubing. We have implemented three different mlhp device packaging techniques that enable use to degas, fill, and experimentally test these wafer scale devices. Method I: Mechanically Sealed Plastic Package with O- rings and Interference-fitted Microtubing Figure 4. (b) Step 2: Degassed liquid bled into the device Thermal-flux based mlhp degassing and filling setup: (a) Superheated vapor, generated by heating the liquid in a flask, is used to purge NCGs from the mlhp. This also removes any dissolved gases from the liquid in the reservoir flask. (b) The flask is inverted and raised above the device; Using a valve the liquid is controllably bled into the device, taking advantage of pressure differential created by heating the flask. viously boiled to remove dissolved air. The outlet port of the mlhp is open to the ambient. The process operation is as follows: In step 1, the water in the flask is boiled vigorously using a digital heater. This extracts any dissolved gases from the water, and further, the steam generated in the process creates a high pressure inside the flask. The pressure gradient between the flask and ambient drives the steam through the capillary tubing, into the mlhp device, and then out to the ambient through the exit tubing connected to the outlet port. In this way all gases except Fig. 5 shows a packaging technique that employs a mechanically sealed polycarbonate (PC) package to establish a lowvacuum-grade, high-temperature-resistant connection between the mlhp prototype-i device and 1/16 inch OD steel microtubing. The package consists of a bottom polycarbonate plate, which has 3/64 inch diameter holes drilled into it. The steel tubing is interference fitted into these holes to provide a leak proof and mechanically stable connection. At the top of the holes, nylon O-rings are placed inside a slightly bigger cavity, half as deep as the thickness of the O-rings. The mlhp device, with two 1 mm 1mm square holes on its bottom side, is aligned with the O-rings. A top polycarbonate plate, forced down with screws attached to both plates, sandwiches the mlhp device, and establishes a secure O-ring connection between the device fill holes and the steel tubing. Observation windows are also machined in the top and bottom plates for visual data collection and heating/cooling of the device components respectively. This packaging approach is a good way to enable the experimental testing of mlhp devices in the lab. The package is reusable and can be easily assembled/disassembled to accommodate different device prototypes, with the only limitation being size and placement of fill holes on the devices. 4 Copyright c 2011 by ASME

5 Nylon O-rings Top compression blocks secure fill tubing Bottom PC plate (a) Screws Bottom compression blocks sucure mlhp device Copper fill tubing Top PC plate mlhp Prototype I Nuts Top screws clampinbottom screwsg screws Solder or epoxy connection (b) Steel inlet and outlet tubing Figure 5. Mechanically sealed plastic packaging (Method I): (a) The bottom polycarbonate (PC) plate has drilled holes containing O-rings, which align with the fill holes etched into the mlhp device. Steel microtubing is interference-fitted into these holes. A window in the bottom plate allows for the application of heat to the device evaporator section. (b) A top polycarbonate plate (with an observation window) is used to mechanically seal the package by applying pressure on the sandwiched mlhp device using screws. mlhp Prototype II Figure 6. Aluminum double compression fitting assembly (DCFA) packaging (Method II): Copper microtubing is mechanically and securely placed on top of the mlhp fill holes using a machined aluminum setup. The tubing can be either epoxied or soldered onto the device. In the latter case a thin metal film has to be deposited on the wafer. Method II: Aluminum Double Compression Fitting Assembly (DCFA) employing Solder connections Fig. 6 shows a different packaging approach, which relies on a double compression fitting technique for the relative mechanical positioning of the mlhp device and the metal tubing, while employing solder or epoxy based sealing solutions to create a hermetic connection between them. The package is machined out of three different aluminum blocks, which are connected to each other with the help of screws. The mlhp device is fixed in place by sandwiching it between the middle and bottom blocks. The top and middle blocks grip the 1/4 inch OD inlet and outlet copper tubing, and thus help position it over the device fill holes. They also provide mechanical protection for the chip-tube connections by neutralizing any external system stresses that can migrate through the stiff copper tubing. The connection between the tubing and the mlhp fill holes can be established by either using a high-temperature compatible epoxy paste, or by solder- ing the copper tube to a thin metal film that has been sputtered onto the device. The DCFA package is entirely reusable and, unlike in the previous approach, the positioning of fill holes on the device is not critical, due to the available flexibility in positioning of copper tubing. Some manual labor, however, is expended in shaping the copper tubing, to interface it correctly with the device fill holes; moreover, this has to be done for every new device that is to be filled and tested. Method III: Cold-Welded Plastic Package with Interference-fitted Microtubing Fig. 7 shows a packaging approach that was implemented to create compact, standalone packaged mlhp devices; this approach makes it easy to fill and test devices in the laboratory environment. Only a single plastic base is required, which can be machined according to the geometric peculiarities of a device 5 Copyright c 2011 by ASME

6 Purge step Refill step Heater cavity Inlet port cavity Outlet port cavity Cooling fluid cavity (a) Fill setup Inlet hole Bubbly flow Outlet hole (a) Cold-weld epoxy (b) Device purging at low thermal pressures Inlet tubing Outlet tubing Pure vapor flow (c) Device purging at high thermal pressures (b) Figure 7. Cold-welded plastic packaging (Method III): (a) A base is machined out of plastic, with cavities that can interface with the mlhp fill holes. The cavities lead to smaller holes, which are interference-fitted with copper microtubing. (b) A cold-weld epoxy is spread around the cavities, and the mlhp device is placed on top of the plastic base, making sure the device fill holes align with the base cavities. More epoxy is used on the top to increase the mechanical strength of the package. prototype. The plastic base has machined cavities, leading to interference fitted 1/16 inch OD copper microtubing. These inlet and outlet filling cavities are meant to align with smaller fill holes etched on the mlhp device. Separate cavities are also machined in the plastic base in order to provide for conductive heating of the evaporator section and convective liquid cooling of the condenser section of the mlhp device. JB-Weld epoxy paste is first spread around the cavities on the plastic base, whose surface has been roughened in order to improve bond strength. The mlhp device is now placed on top of the plastic base making sure the device fill holes align with the corresponding base cavities and is pressed slightly. Some more epoxy is applied at the device edges in order to improve the mechanical strength of the bond. The epoxy is allowed to cure for 24 hours at room temperature. This packaging approach is not reusable but allows for the simultaneous preparation of multiple packaged mlhp proto- Figure 8. Bubbles completely absent (d) Degassed and filled mlhp device Results for the packaging method I: (a) Implementation of the mlhp filling setup using the mechanically sealed plastic packaging; (b) Two phase flow is observed in the device during the initial stages of thermal-flux-purging of the system (step 1); (c) With rise in the system temperature and pressure, pure vapor flow is observed in the device; (d) After the completion of the thermal-flux-refilling step (step 2), the device is found to completely fill with wafer with vapor bubbles completely absent. types for filling and testing. RESULTS AND DISCUSSION The thermal-flux degassing and filling system was implemented using all the three mlhp packaging techniques discussed above. Water was used as the working fluid, because of its good device compatibility and lack of associated safety issues. Fig. 8 shows the degassing and filling results obtained using the mechanically sealed plastic packaging (Method I). Dur- 6 Copyright c 2011 by ASME

7 Copper tubing mlhp prototype I Valves Solder connection Valves Heated Flask mlhp prototype II (a) (a) 2-phase flow water leakage (b) Fluid leakage at solder connection Additional epoxy seal Figure 9. (b) Results for the packaging method II: (a) Implementation of the mlhp filling setup using the DCFA packaging; (b) The solder connections get compromised and leak water under high pressure, indicating the need for improving tube-to-chip soldering techniques. ing the thermal-flux purging step (step 1), water in the flask was boiled and the vapor forced through the device. During the initial stages of this process, a two-phase bubble flow was observed in the device (see Fig. 8b). This happened because the vapor, as it traveled from the hot flask to the device, partially condensed in the steel tubing. As the temperature and pressure in the flask was further increased, pure vapor flow was observed in the device (see Fig. 8c). During this process, however, parts of the device still remained occupied with water due to the large capillary forces in the device. But this water was mostly degassed due to its high temperature. In Fig. 8d we see that the device got completely filled with water after the thermal-flux refilling step (step 2), and no bubbles were observed in any part of the device. This proves that the thermal-flux technique is successful in completely removing all non-condensible and dissolved gases, both Figure 10. (c) Results for the packaging method III: (a) Implementation of mlhp filling setup using the cold-welded plastic packaging; (b) At low purge pressures, two-phase flow is observed through the device without any leaking; but at higher pressures, the epoxy seal is compromised at the inlet port by the superheated steam; (c) Additional high-temperature epoxy, employed to plug the possible leak, is also found to delaminate from the wafer surface. from the external fluid reservoir as well as the mlhp device. As the external fluid reservoir is cooled down to room temperature, over several days, small air bubbles begin to enter the reservoir through the steel tubing. This indicates that some part of the system, either the compression-fitted microtubing connections or the mlhp package itself, is leaking air into the system. As is illustrated in Fig. 9, the DCFA packaging (Method II) needs some more work before it can be successfully implemented for the filling setup. Although the epoxy sealing method showed no signs of leakage during the thermal-flux purging step, no flow was observed through the mlhp prototype-ii due to a device design flaw, leading to the possible blockage of the microchannels in the device. The solder-based connection, imple- 7 Copyright c 2011 by ASME

8 mented by sputtering a gold layer on the device, underwent failure at high system pressures, as shown in Fig. 9b. Soldering experiments need to be conducted to characterize the sputtered metal thin film requirements and the allowable soldering temperatures. Fig. 10 shows the performance of the cold-welded plastic packaging (Method III) under the high temperatures and pressures encountered during the thermal-flux vapor purging process. The packaging showed good integrity under moderately high purge pressures, and vapor flow was observed through the device. After continuous two hours of operation, and upon a further increase in the external reservoir temperature, some water leakage was observed near the inlet filling hole (see Fig. 10b). This area was already a weak point: the inlet cavity in the machined base was exposed around the edge of the device, and had been plugged with excess epoxy paste at this point. Additional epoxy paste was reapplied in this region and allowed to cure. But the new joint failed again, with the cured epoxy ultimately delaminating from the surface of the wafer. Since the outlet port remained intact, it is possible that in the absence of the defect at the inlet port the package might have performed well. Nevertheless, it seems that the temperature limit of the epoxy ( 400C) is severely limited by the latent heat of condensation of the incoming steam. CONCLUSIONS A two-port thermal flux method was implemented to remove non-condensible gases from a planar microscale loop heat pipe (mlhp), and to charge this device with a degassed working fluid, in order to make it operational. Due to the small device volumes involved, this method has several advantages over conventional vacuum pump based evacuation techniques. In this method, an external fluid reservoir is connected to the mlhp device, and thermally generated pressure gradients are used to purge the device with the working fluid vapor. After the purge step is complete, the entire system is sealed, and the degassed liquid inside the hermetically sealed system is thermally leaked into the device. Three different, high temperature compatible device packaging techniques were implemented in order to interface the mlhp device with the external filling setup. The first approach, using O-rings to mechanically seal the device in a plastic package, proved highly effective in hermetically sealing the device during the filling operation, and helped demonstrate the effectiveness of the thermal-flux method. But since this approach was primarily designed for device testing in the lab environment, alternative soldering and epoxy based packaging approaches were also implemented. Although currently facing some reliability issues, these approaches will lead to the development of hermetic sealing solutions for standalone and fully operational mlhp devices. Acknowledgments Defense Advanced Research Projects Agency, Microsystems Technology Office (MTO), Program: Thermal Ground Plane (TGP), Issued by DARPA/CMO under Contract No: HR C REFERENCES [1] Dhillon, N. S., Pisano, A. P., Hogue, C., and Hopcroft, M. A., Mlhp: A high heat flux localized cooling technology for electronic substrates. ASME Conference Proceedings, 2008(48746), pp [2] Krishnan, S., Garimella, S. V., Chrysler, G. M., and Mahajan, R. V., Towards a thermal Moore s law. IEEE Transactions on Advanced Packaging, 30(3), AUG, pp [3] Faulkner, D., Khotan, M., and Shekarriz, R., Managing electronics thermal management. Heat Transfer Engineering, 25(2), MAR, pp [4] Feldman Jr., K., and Noreen, D., Design of heat pipe cooled laser mirrors with an inverted meniscus evaporator wick.. AIAA Paper. [5] Liepmann, D., Design and fabrication of a micro-cpl for chip-level cooling. In American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD, Vol. 369, pp [6] Pastukhov, V., Maidanik, Y., Vershinin, C., and Korukov, M., Miniature loop heat pipes for electronics cooling. Applied Thermal Engineering, 23(9), JUN, pp [7] Maydanik, Y. F., Evaporation chamber of a loop heat pipe. Russian Patent [8] Maydanik, Y., Vershinin, S., Korukov, M., and Ochterbeck, J., Miniature loop heat pipes - A promising means for cooling electronics. IEEE Transactions on Components and Packaging Technologies, 28(2), JUN, pp [9] Pastukhov, V. G., and Maydanik, Y. F., Low-noise cooling system for PC on the base of loop heat pipes. Applied Thermal Engineering, 27(5-6), APR, pp [10] Singh, R., Akbarzadeh, A., and Mochizuki, M., Thermal Potential of Flat Evaporator Miniature Loop Heat Pipes for Notebook Cooling. IEEE Transactions on Components and Packaging Technologies, 33(1), MAR, pp [11] Popova, N., Schaeffer, C., Sarno, C., Parbaud, S., and Kapelski, G., Thermal management for stacked 3d microelectronic packages. In PESC Record - IEEE Annual Power Electronics Specialists Conference, pp [12] Kirshberg, J., Liepmann, D., and Yerkes, K. L., Micro-cooler for chip-level temperature control. In 8 Copyright c 2011 by ASME

9 Aerospace Power Systems Conference, SAE Technical Paper Series, SAE International. [13] Kirshberg, J., Yerkes, K. L., and Liepmann, D., Demonstration of a micro-cpl based on mems fabrication technologies. In Proceedings of the Intersociety Energy Conversion Engineering Conference, Vol. 2, pp [14] Kirshberg, J., Yerkes, K., Trebotich, D., and Liepmann, D., Cooling effect of a mems based micro capillary pumped loop for chip-level temperature control. ASME. [15] Yerkes, K. L., Pettigrew, K., Smith, B., Gamlen, C., and Liepmann, D., Development and testing of a planar, silicon mini-capillary pumped loop. In Space Technology and Applications International Forum-STAIF, M. El-Genk, ed., American Institute of Physics, pp [16] Meyer, L., Dasgupta, S., Shaddock, D., Tucker, J., Fillion, R., Bronecke, P., Yorinks, L., and Kraft, P., A silicon-carbide micro-capillary pumped loop for cooling high power devices. In Annual IEEE Semiconductor Thermal Measurement and Management Symposium, pp [17] Maydanik, Y., Loop heat pipes. Applied Thermal Engineering, 25(5-6), pp [18] Riehl, R., Camargo, H., Heinen, L., and Bazzo, E., Experimental investigation of a capillary pumped loop towards its integration on a scientific microsatellite. In American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD, Vol. 372, pp [19] Bazzo, E., and Riehl, R., Operation characteristics of a small-scale capillary pumped loop. Applied Thermal Engineering, 23(6), pp [20] Ghajar, M., Darabi, J., and Crews Jr., N., A hybrid cfd-mathematical model for simulation of a mems loop heat pipe for electronics cooling applications. Journal of Micromechanics and Microengineering, 15(2), pp [21] Dhillon, N. S., Hogue, C., Hopcroft, M. A., and Pisano, A. P., Geometric control of the fluid-transport meniscus in a passive phase-change microfluidic electronics cooling device. In Proceedings of Power MEMS, pp Copyright c 2011 by ASME