IMECE 2011-64944 DEVICE PACKAGING TECHNIQUES FOR IMPLEMENTING A NOVEL THERMAL FLUX METHOD FOR FLUID DEGASSING AND CHARGING OF A PLANAR MICROSCALE LOOP HEAT PIPE

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 Reservoir Inlet port Outlet port Liquid microchannels Condenser Vapor microchannels Wick Evaporator Thermal barrier (a) mLHP Prototype I (b) mLHP Prototype II Figure 1. 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]. 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 100 −150oC temperature range [17]. Anhydrous ammonia and acetone are also very good candidates for the mLHP due to good heat transfer efficiencies in the 50− 100o 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. 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 −20μmwide) rectangular channels, and this is where the liquid to vapor phase change happens. The wick is supplied with liquid by the liquid supply channels (150μm 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 5×10−6 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 Rectangular wick (width ~ 10-20 microns) Liquid supply channels Reservoir supply channel Region I Region II Bubbles 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 (∼ 350−400oC) 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 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 The end of the pipe is closed using a solder or by brazing