Abstract:
Aviation fuel is used as the coolant for a microchannel heat sink for airborne electronics. The use of preexisting aviation fuel as a coolant medium eliminates the weight, size and expense of a separate liquid system dedicated solely to electronics cooling. The heat sink is comprised of a body of material having high thermal conductivity and in which are formed a set of parallel closed-ended microchannels on the order of 0.001 in. by 0.004 in. separated by a distance of 0.001 in.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to copending application Ser. No. 08/681,344 entitled, &#34;Microchannel Cooling of High Power Semiconductor Devices&#34;, Robin E. Hamilton et al, filed on Jul. 22, 1996. This application is assigned to the assignee of the present invention and is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the coolant medium utilized in conjunction with heat sinks for electronic devices and more particularly to a coolant medium utilized in connection with microchannel heat sinks for relatively high power airborne electronic devices. 
     2. Description of the Prior Art 
     The greatest limitation in the design of low cost, light weight, high power airborne electronics is the reliable cooling of densely packaged components. Current state of the art electronic packaging typically utilizes redundant parts and excess substrate areas which increases system cost, size and weight. All of this results from the inherent limitations in the cooling schemes employed. For example, in solid state transmitters, RF transistors are regularly designed to operate below their power output capability and are spaced relatively far apart from one another in order to provide cooling which complies with military standards. Conventional heat sinks utilizing typical coolants such as ethylene glycol water, are not adequate for practical RF circuit densities. 
     SUMMARY 
     Accordingly, it is a primary object of the present invention to provide an improvement in the extraction of heat from electronic devices. 
     It is another object of the invention to provide an improvement in the cooling of semiconductor devices. 
     It is a further object of the invention to provide an improvement in the cooling of power semiconductor devices which are utilized in airborne electronics equipment. 
     Briefly, the foregoing objects are achieved by convection cooling apparatus for airborne electronics systems devices which include heat generating devices and a heat sink therefor, and wherein the heat sink comprises a body of material having relatively high thermal conductivity and in which is formed a plurality of close-ended liquid coolant conduction microchannels which are located as close as possible to the heat source and in which the coolant medium comprises the aviation fuel used to power the aircraft. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a diagram generally illustrative of the inventive concept of this invention; 
     FIG. 2 is a perspective view partially cut away, of a microchannel heat sink in accordance with this invention; and 
     FIG. 3 is an exploded view of the embodiment shown in FIG. 2 mounted on a ground plane. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and more particularly to FIG. 1, shown thereat is an aircraft identified by reference numeral 10 which includes one or more power plants 12 which may be, for example, jet engines fueled from one more fuel tanks 14. In this invention, aviation fuel from fuel tank 14 is used as the coolant medium for extracting heat from electronic circuitry 16 located on board the aircraft. The electronic circuitry 16 is cooled by a microchannel forced convection heat sink configuration which is shown in FIG. 2 and identified by reference numeral 18. The heat sink 18 is eutectically bonded to a ceramic substrate 20 which in turn is braised to a metal carrier number 22 referred to as a flange. The flange 22 is additionally shown including a pair of open-ended U-shaped slots 24 which are used for attachment to a ground plane 26 shown in FIG. 3. 
     The heat sink 18 is formed from solid material, e.g. metal (aluminum) or semiconductor (silicon/silicon carbide) having a high thermal conductivity and on which a plurality of semiconductor devices 28 1 , 28 2 , 28 n  are located or integrated therewith. The heat sink 18 further includes a plurality of mutually parallel close-ended microchannels 30 of rectangular cross section formed in the block of material from which the heat sink 18 is formed. 
     Each microchannel 30 comprises an elongated linear groove ranging in width between 0.001 in. and 0.004 In., a depth ranging between 0.004 in. and 0.01 in. with a spacing ranging between 0.001 and 0.003 in. 
     The rectangular spacing sections 32 act as fins. As shown in FIG. 2 where the semiconductor device 28 is thermally in contact with the top of the heat sink 18, the heat generated in the devices 28 is transferred through the solid upper portion of the heat sink 18 and the fins 32 by conduction and is thereafter transferred to a coolant, not shown, flowing through the microchannels 30. 
     As shown in FIG. 3, coolant 33 which comprises the aviation fuel for powering the engines 12, is coupled to and from the microchannels 30 by means of an input duct 34 and output duct 36 formed in the ground plane member 26 upon which the semiconductor package 16 is mounted. The input duct 34 is coupled to the fuel tank 14 (FIG. 1), while the output duct 36 is coupled back to the fuel feedline(s) 15 coupled to the engines 12. Also as shown in FIG. 3, the fuel coolant 33 is fed to and from the microchannels through pairs of manifolds 38, 40 and 42, 44 formed in the ceramic substrate 20 and the metal flange 22, respectively. With such a structure, conduction and thermal radiation are typically negligible compared to convection and the transfer of heat to the coolant. 
     For maximum effectiveness, the heat generating devices which comprise the semiconductor devices 28, are formed directly on the heat sink material, eliminating the thermal resistance of the joint between the device and the heat sink. The microchannels 30 can be simply etched, for example, using standard photolithographic processes on the back side of a transistor chip. The ceramic substrate 20 acts to seal the bottom close-ended microchannels 30. 
     This packaging approach can be utilized with different components other than transistors and even with integrated circuits up to a wafer scale level of integration. It can also be used with various semiconductors forming the heat sink 18 such as silicon, silicon carbide as noted above, but also germanium and gallium arsenide. The ceramic substrate 20 is typically comprised of beryllium oxide and aluminum nitride but is not constrained thereto. 
     The small size of a 0.5 in.×0.5 in. microchannel heat sink 18 is ideal for direct cooling of hot components. The thermal efficiencies in large mass typical with traditional heat transfer from a device through a circuit board, to a heat exchanger, is now eliminated. A microchannel heat sink 18, such as shown in FIG. 2, is many times more efficient than conventional heat sinks and is due to the dimensions of the microchannels 30. The use of microchannels 30 enhances heat transfer in two ways. First, narrow channels can be closely spaced, giving a large number of fins 32 with a combined surface area much greater than the &#34;footprint&#34; of the heat sink 18 itself. Secondly, the small hydraulic diameters of the narrow passages 30 result in relatively high convective heat transfer coefficients. Since the thermal conductance of a heat sink is proportional to the product of the convective heat transfer coefficient in the surface area, small channels permit an increase in the maximum power density for a given operating temperature. 
     This superior performance makes aviation fuel, even with fluid conductivity and specific heat less than that of water, a desired medium 33 for high heat density coolant applications. For example, as outlined in Table I below, CHEVRON® 100LL aviation fuel used in a 0.5 in.×0.5 in. microchannel heat sink for dissipating 50 watts, outperforms a conventional water cooled heat sink. 
     
                                           TABLE I__________________________________________________________________________       heat sink     Temp delta                           Temp deltafluidheat sink type       geometry               flow rate                     (fluid-fin)                           (fluid)__________________________________________________________________________waterlanced-offset       0.5&#34; × 0.5&#34; base,               4.01 gm/sec                     38.8 deg C.                           3 deg C.aluminum       0.125&#34; high fins.finstock       0.006&#34; thk fins.       12 fins/inchChevronlanced-offset       0.5&#34; × 0.5&#34; base               7.49 gm/sec                     64.6 deg C.                           3 deg C.100LLaluminum       0.125&#34; high fins.finstock       0.006&#34; thk fins       12 fins/inchChevronaluminum       0.5&#34; × 0.5&#34; base               0.90 gm/sec                      0.8 deg C.                           25 deg C.100LLmicrochannel       0.001&#34; wide fins.       0.001&#34; wide       channels       0.010&#34; tall fins__________________________________________________________________________ 
    
     The fluid to fin temperatures Δ within the microchannel heat sink 18, is drastically reduced, while using even less flow rate. Similar performance can be achieved using jet fuel JP-5. Not unexpectedly, 100LL used in a conventional heat sink performs worse than water as shown in Table I. 
     The volume and flow rate of fuel needed for cooling will vary with electronics applications and/or aircraft. Typically, the required flow rate will equal or be less than the burn rate of the aircraft 10. Use of a microchannel heat sink 18 as shown in FIG. 2 permits the volume and flow rate of required coolant 33 to be reduced by orders of magnitude over conventional cooling techniques, thus making aviation fuel a practical coolant. In this case, a predetermined volume of the fuel can be routed through the electronics circuitry 16 (FIG. 2) first as shown in FIG. 1, and which is then fed to the engine(s) 12. This approach would minimize plumbing and pumping requirements and would not require a heat exchanger. Alternatively, a secondary pumping system could supply coolant 33 to the electronics circuitry 16 and return the fuel via a heat exchanger, not shown, back to the fuel tank(s) 14. An aircraft with fuel tanks 14 integral with the wings can capitalize on the &#34;free&#34; heat exchanger provided by such a configuration, in that forced air flowing over the wing, fuel line and fuel tank combination would lower the temperature of the heated fuel returning from the electronics down to near ambient. 
     Microchannel cooling using aviation fuel is particularly suitable to remotely piloted vehicles (RPVs) since malfunction of coolant fuel lines 15 or electronic equipment 16 as may happen in a battle environment, would not present a safety hazard to aircraft personnel. 
     Thus what has been shown and described is the concept of utilizing aviation fuel as a coolant medium for use in connection with a heat sink for heat generating electronic devices where the heat sink includes a series of close-ended microchannel grooves which act as the cooling paths for the aviation fuel fed to and from the heat sink. 
     The invention being thus disclosed, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications that would be obvious to one skilled in the art are intended to be included within the scope of the following claims.