Abstract:
A cooling device for an electric component or components, includes a coolant liquid circulation system, a carbon-based heat intake block for transferring heat from said electrical component or components, a top layer on the carbon block for increasing thermal inertia during heat transfer via said layer by system coolant, and means whereby the heated coolant transfers heat to a heat remover.

Description:
[0001]    This application claims priority from provisional application Ser. No. 61/005,012, filed Dec. 3, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The evolution of electronic devices to more compact form factors and, specifically, the migration of semiconductor manufacturing to smaller design processes have increased the power density of modern semiconductors orders of magnitude above that of older designs. Some of the areal power density increase is offset by reduced supply voltages and concurrent reduction in operating current. However, modern semiconductors also operate at much higher frequencies than their predecessors, which counteracts the savings stemming from lower voltages. Power density is equivalent to areal heat dissipation; as a result, the trend towards compact, high speed integrated circuits (ICs) results in higher thermal load and, by extension, increasing challenges for cooling solutions. 
         [0003]    The ideal situation for any cooling device is to maintain a uniform temperature distribution across the entire surface. Uniform temperature distribution is also known as isothermicity and the only way of approaching this is to move heat as quickly and efficiently as possible from the source to any other part of the cooler. Compared to passive heat transfer through any solid material, active transport provides much higher efficacy of heat transport. A well-established example is the liquid cooling system of combustion engines where heat is taken up by water, which is pumped away from the engine to a remote radiator where the heat is then released into the environment. In the case of electronic devices, liquid cooling has been used in specialty designs but never received general acceptance in mainstream consumer devices. Primary reasons for the lack of general acceptance comprise among other factors the inherent risk for spills, life expectancy of pumps, the cost overhead, the complexity of installation which includes routing of tubing and the configuration of more or less bulky radiators. 
         [0004]    Any cooling system can only be as efficient as the primary interface responsible for the removal of thermal energy from the source. In the case of electronics, it appears as if the highest efficiency could be achieved by direct immersion of the semiconductor into the coolant. However, for all practical purposes, in the consumer space, this may not be a viable solution because of the reasons mentioned above. A more feasible solution necessarily entails a self-contained, sealed system. Sealed systems, on the other hand rely on the efficiency of the thermal interface between the semiconductor die and the coolant. In that particular area, many different solutions have been proposed, based on waterblocks machined from copper or silver. However, even copper or silver have a relatively low thermal conductivity compared to carbon structures, for example diamonds. Diamonds, on the other hand are not only too expensive for mainstream cooling devices, they are also close to impossible to machine into a suitable form. Carbon nano tubes (CNT) and carbon nano fibers (CNF) have been discussed as possible thermal conductors but at the present time obtaining pure CNT structures is still cost prohibitive. A superbly thermally conductive material is pyrolytic carbon, which is a carbon material similar to graphite but with additional covalent bonding between the individual graphene sheets. The specific bonding arrangement in form of sheets with additional cross-linking between the sheets results in unique heat transfer distribution characteristics that can be used to increase net thermal transfer from any source. 
       DESCRIPTION OF RELATED ART 
     Carbon-Based Interfaces 
       [0005]    Current approaches to heat transfer away from electronic components have employed a variety of materials, mostly copper or aluminum based as the primary interface. Carbon-based solutions have been used in experimental designs but have not gained wide acceptance. Reasons for the failure in acceptance of carbon materials are found in the lack of three-dimensional transfer of heat, resulting in excellent laminar conduct through the sheets but an almost complete lack of dissipation into the environment. As a result, the surface area at the back end of a graphene-based cooler is essentially the same size as the surface area at the front end, namely the cross sectional surface of each sheet and does not offer any advantage with respect to facilitation of heat dissipation to the environment. Another drawback of carbon-based-solutions is the very low heat capacitance or buffering capability that can cause adverse side effects such as temporary, local boiling of any liquid cooling media on the back end of the carbon interface. 
       Expansion Reservoir 
       [0006]    Most liquid cooling systems used with electronic components rely on a remote reservoir, a pump and more or less elaborate tube connections between the individual components. The reservoir also serves to compensate for the temperature-dependent expansion of the coolant in order to avoid building up of pressure that could eventually break the seals of the system. Expansion reservoirs are usually rather simple, in some closed systems, the plenum is simply not filled completely but contains air bubbles that are compressed with increasing temperature and associated thermal expansion of the liquid coolant. However, any air in the system can cause a breakdown of the cooling efficiency. Within a self-contained compact cooling system partial fills would have the same disadvantages, on the other hand, pressure changes can cause mechanical stress and should be avoided at all means. 
         [0000]    Carbon-Based Waterblock with Heat Exchanger 
         [0007]    The combination of carbon interface machined to contain microchannels with a hermetically sealed, self-contained fluid-cooling system has been disclosed in an earlier patent application (Robinson, 2007). However, the invention described does not address the buffering of fast temperature transients on the fluid back-end of the cooling system, nor does it address the issue of pressure compensation within the closed system. The above mentioned limitations of existing coolers underscore the need for more advanced solutions for the use with high power density electronic components. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a cooling device utilizing the thermal transfer characteristics of pyrolytic carbon for enhanced heat removal from a semiconductor. The high thermal conductivity along the X and Y axes of the sheets can be used to expand the initial contact area towards the heat source (heat absorption area) at least in one dimension. That is, the cleavage plane is typically positioned in normal orientation to the chip interface surface whereas optimal conductance is found in any direction within the sheets parallel to the cleavage plane. This orientation allows for expanding the “release” interface surface area for thermal energy depending on the thickness of the carbon interface block. For the addition of thermal inertia on the release surface, a layer of thermally conductive material is bonded to the carbon block, which also allows for standard processes of machining of any surface increasing structures such as micro or macro channels into the metal layer. The metal layer itself serves as an interface to the liquid coolant that is pumped across its surface. The coolant may then be ducted into a system of pipes that are thermally connected to a cooling fin array. A pump moves the fluid through the channel and pipe system. The entire system may be hermetically sealed, and typically contains a diaphragm to allow for expansion of the fluid as it increases in temperature. In one embodiment, a squirrel cage type fan moves air through the fin array to take up heat and dissipate it into the environment. Because of the high efficiency of the cooler, it is possible to add additional cooling blocks to the main cooler, these satellite coolers can then be ported to the coolant and serve for thermal management of additional components such as chipsets, voltage regulators, power supply transistors or even discrete graphics processors. 
       UTILITY OF THE INVENTION 
       [0009]    In short, the advantages of the current invention can be summarized as follows:
       a) Highest possible heat uptake from the heat source by the carbon interface.   b) Expansion of the heat dissipation area compared to the heat uptake through use of pyrolytic carbon.   c) Combination of carbon with metal increases thermal inertia of the interface to prevent local hot spots on the coolant side.   d) Liquid coolant provides efficient removal of heat from the source.   e) Expansion diaphragm accommodates thermal expansion of the coolant without pressure changes or air in the system.   f) Self-contained cooling system is user-friendly and easy to install.   g) High efficiency of the main cooler allows porting of satellite cooling blocks for additional components.       
 
       DETAILED INITIAL DESCRIPTION OF THE INVENTION 
       [0017]    The present invention provides a self-contained cooling system having extreme efficiency. The self contained, hermetically sealed configuration ensures ease of installation, along with a maintenance free use for the lifespan of the cooling device. The efficiency of the cooling performance stems from a variety of features, each of which is important by itself and which, in combination, work synergistically to remove heat from high power density devices and dissipate it at a high rate into the environment. 
         [0018]    The initial absorption of the heat is achieved through a carbon interface. Pyrolytic carbon has a thermal conductance of approximately 1400 W/m/C along the X and Y directions, parallel to the cleavage plane or planes of the graphite sheets. Since the heat conductance occurs in two dimensions rather than unidirectionally, this circumstance can be used to expand the interface area in an almost lossless manner, which also reduces the power density on the back face of the carbon block. The pyrolytic carbon interface is oriented with the cleavage plane or planes substantially normal to the front and back faces of the carbon block. The expansion of the back face compared to the front face depends on the thickness of the carbon block used and will typically have a 3:1 or greater ratio. 
         [0019]    Pyrolytic carbon has very low thermal capacitance or buffering capability, therefore, fast thermal transients are propagated through the block without much attenuation. In the case of fluid cooling, this can result in boiling of the coolant or else insufficient dissipation into the coolant and either situation can cause transient temperature spikes on the heat source. To avoid these thermal transients, it is of advantage to add a buffer in the form of, for example, copper or aluminum to the back face of the carbon block, thereby forming a hybrid interface block. The increased thermal capacitance results in thermal inertia of the hybrid block, which greatly reduces the thermal fluctuations at the heat source. In addition, it is very easy to machine copper or aluminum to add surface extensions in the form of fins or spikes that facilitate heat transfer to the coolant. 
         [0020]    The cooling apparatus disclosed is typically a single, self contained structure that is mounted onto a standard processor, examples being central processing units as currently manufactured by Advanced Micro Devices (AMD) or Intel, or else graphics processors as manufactured by AMD or nVidia. Those processors have standard mounting brackets associated with each design to allow interchangeable equipment with original and after market cooling devices. In most cases it is a clip that is engaged, alternatively, pegs or screws are commonly employed. Often, a back plate serves to reinforce the printed circuit board in order to avoid flexing of the board caused by the weight of the cooler in situations where the system is transported and possibly subjected to bumps or impacts. 
         [0021]    Because of the self-contained, hermetically sealed nature of the cooler, it is necessary to accommodate the thermal expansion of the coolant that occurs if the processor gives off heat. Different designs are possible to achieve this goal, for example a flexible expansion reservoir can be used with unusual advantage. A variation of this type of reservoir is a concave diaphragm that can flip in or out, depending on the pressure of the coolant in the system. Such a flexible diaphragm is easy to manufacture and implement into the wall of any coolant container. 
         [0022]    The cooler disclosed herein is extremely powerful and scales with size, meaning that any increase of the radiators will increase the amount of heat that can be dissipated into the environment. This allows extension of the cooling apparatus beyond the central processor, and the use of satellite attachments that are ported to the same coolant circulation system to provide thermal management of the voltage regulator modules, the chipset and potentially of discrete graphics as well. None of the mentioned components require any further cooling devices beyond the satellites. 
         [0023]    Most coolers currently used employ axial fans, primarily because of high efficiency and low cost. Axial fans, however, are usually noisier than centrifugal fans also known as squirrel cage fans of similar rating. In the case of the cooling device at hand, a further advantage of the centrifugal fan provided is that there is very little back pressure and the air passes through the cooling fins without being redirected. The combination of the centrifugal fan with a radiator surrounding it results in ultra-quiet operation at very high levels of air movements. 
         [0024]    Remote radiator apparatus may also be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  shows a schematic drawing of the integrated liquid cooler including the carbon interface with the metal overlay for increased thermal inertia, a pump, water pipes with radiator fins, a centrifugal fan and the diaphragm for thermal expansion compensation; 
           [0026]      FIG. 2  is a tilted top view for illustration of the fan arrangement compared to the radiator fins and water pipes;  FIG. 3  shows a tilted bottom view for illustration of the carbon block interface; 
           [0027]      FIG. 4  shows a functional illustration of the action of a thermal expansion compensation diaphragm; 
           [0028]      FIG. 5  schematically shows additional satellite coolers for thermal management of e.g. chipset and voltage regulators connected to the main cooler; 
           [0029]      FIG. 6  shows an alternate heat radiator and liquid cooler configuration; 
           [0030]      FIG. 7  is like  FIG. 6 , but shows a remote radiator; and 
           [0031]      FIG. 8  is a view like  FIG. 1 , showing a modification. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Referring now to preferred cooling apparatus of  FIG. 1 , it includes a housing  10  defining first and second laterally extending liquid coolant flow chambers  11  and  12 , in flow communication via a central passage  13 . That passage may be formed by a pump  14  in the housing and operating to pump fluid centrally from chamber  12  to chamber  11 , as shown by arrows  15 . The flow is directed toward the irregular top surface  16  of a layer  17  to remove or transfer heat from that surface to the coolant flowing in opposite directions in passages  18  and  19  in the housing. From those passages, the coolant flows via pipes  20  and  21  to means indicated generally at  40 , such as fins  41  operating to remove heat from the coolant, and to return the coolant via pipe  42  and  43  to upper chamber  12 , in a highly compact configuration. 
         [0033]    Upper wall  22  of chamber  12  comprises a diaphragm peripherally mounted at  23  to the housing ring  10 a, so as to allow upward flexing of the diaphragm in response to coolant fluid expansion. A housing cover plate  23 ′ extends over the diaphragm and is attached to housing surface  24 , whereby the chambers  11  and  12  and the diaphragm are hermetically sealed. 
         [0034]    An electrical component  124  engages the underside  25   a  of pyrolytic carbon block  25  fitted peripherally in the bounded space formed by housing wall  26 , layer  17  also peripherally fitting in that space. Heat received by block  25 , by conduction from the electrical component, is transferred by conduction to the layer  17  comprising a metal interface block (between water and carbon block  25 ). Its upper surface has irregularity, as for example is provided by recesses  28  in the layer, that increase the surface area in contact with coolant in chamber  12 , for enhanced heat transfer. The structure of block  25  and layer  17 , and their functioning, prevent boiling of the coolant, such as water. 
         [0035]    The planes  30  indicative of molecular cleavage planes in block  25  are directed toward layer  17 , for most efficient heat transfer operation. A centrifugal fan  32  is shown as located in the space  33  between banks  41   a  of fins  41 , to displace cooling air radially in passages  41   b  between fins, for removing heat from the fins. 
         [0036]    Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets. Generally it is produced by heating a hydrocarbon nearly to its decomposition temperature, and permitting the graphite to crystallize (pyrolysis). 
         [0037]      FIG. 5  shows flow ducts  50  and  51  to circulate coolant from  12  to and from a chips at cooler  54 ; and ducts  55  and  56  to circulate coolant from  22  to and from a voltage regulator cooler  57 . 
         [0038]      FIG. 6  incorporates plate  23  and all the structure of  FIG. 1  below that plate. A cover  70  is provided above plate  23  and incorporate passages that connect chamber  12  with a hose or duct  71 , and passages  18  and  19  with a hose or duct  72 . Hoses or ducts  71  and  72  extend to a heat radiator  73 . Fan  32  and fins  41  are eliminated, and the remaining apparatus is simplified. 
         [0039]      FIG. 7  is like  FIG. 6 , excepting that the radiator is remotely located, as is made by the breaks at  71   a  and  72   a  in the hoses or ducts  71  and  72 . 
         [0040]    Cooling fans  74  may be provided to displace air through the radiator. 
         [0041]    In  FIG. 8  the arrangement of elements is generally like that in  FIG. 1 , the same numerals being applied to those elements. 
         [0042]    In  FIG. 8 , the flow passes from space  12  downwardly through central opening  80  and then divides due to operation of the pump  14  to flow downwardly at  81  about pump structure  14   a.  The flow then passes downwardly through central opening  13 , to contact metal interface/water block  17 . The flow then travels laterally at  18  and  19 , as described in  FIG. 1 . Carbon block  25  extends directly beneath and in surface to surface contact with block  17 . Electrical component  124  engages the underside face of block  25 , to transfer heat thereto. Block  17  is in the form of a layer that consists primarily of a material selected from the group that includes aluminum, copper, silver and gold. Carbon block  25  has molecular cleavage planes that extend toward layer  17 . The  FIG. 8  apparatus is preferred. 
         [0043]    Additional compactly arranged elements include:
       an enclosure  10 A extending about the pump and forming passage  13  through which coolant flow is delivered by the pump  14  toward and against the upper irregular surface of block  17 ,   heat radiator fins  41 A and  41 G, spaces  41 B between the fins, and heat exchanger  40 ,   hot water (coolant) pipes  42  and  43 ,   centrifugal fan  32  rotating in a space between inner ends  33  of the fins,   outer housing  10  extending about the pump and supporting housing cover  23 , there being coolant passages  20  and  21  in the cover and communicating with passages  18  and  19  formed between  10  and  10 A,   diaphragm  22  overlying opening  80 , and underlying the fan  32 , the diaphragm carried by the cover  23 .