Abstract:
A heat sink includes a heat conducting substrate and a plurality of directive heat elements disposed within the substrate such that a first end of each of the plurality directive heat elements are adapted to be disposed proximate a heat generating device and a second end of each of the plurality of directive heat elements are spaced apart within the substrate to promote the transfer to heat from the heat generating device through the directive elements to an area of the heat conducting substrate which is larger than the area of the heat generating device. In this way, the heat sink transforms a high heat flux density existing at one end of the directive heat elements proximate a device being cooled to a low heat flux density at an opposite end of the directive heat elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of provisional application No. 60/652,383 filed on Feb. 11, 2005 under 35 U.S.C. §119(e) and is incorporated herein by reference in its entirety. 
     
    
     STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     Not applicable.  
       FIELD OF THE INVENTION  
       [0003]     This invention relates generally to heat sinks and more particularly heat sinks having directive heat elements.  
       BACKGROUND OF THE INVENTION  
       [0004]     As is known in the art, certain classes of light emitting diodes (LEDs) are often provided from Group III-IV semiconductor materials such as Gallium-Arsenide (GaAs). Such LEDs can generate between 1-6 watts (W) of energy and consequently generate a substantial amount of heat. Thus, the LEDs are disposed on a heat sink.  
         [0005]     Heat sinks are generally provided from thermally conductive materials such as copper (Cu) or aluminum (Al). Copper has a coefficient of thermal expansion which is relatively large compared with the coefficient of thermal expansion of many Group III-V semiconductor materials such as Gallium-Arsenide (GaAs). Due to the disparity between the coefficients of thermal expansion between the material from which the LED device is provided and the material from which the heat sink is provided, it is sometimes necessary to introduce a so-called “stress shield” between the LED device and the heat sink. Thus, to shield the Group III-V materials from direct contact with the heat sink materials (e.g. Cu), a stress relief plate (e.g. a plate comprised of silicon (Si), for example) is disposed between the LED device and the heat sink.  
         [0006]     In embodiments in which the stress relief plate is comprised of a silicon (Si) substrate, the Si substrate can be provided having one or more connection points (e.g. one or more metalized regions) which allow one surface of the stress plate to be soldered (or otherwise attached) to the heat sink while the LED device is disposed on the opposing surface of the stress plate.  
         [0007]     One problem with this approach is that the junctions between the LED device and the heat sink impede the efficient transfer of heat from the heat generating device (i.e. the LED device) to the heat sink. This limits the amount of power, and thus the amount of light, which the LED can generate without damaging the device. The inability to cool the LED structure results in practical devices being in the 1-5 W range.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the present invention, a heat sink includes a heat conducting substrate and a plurality of directive heat elements disposed within the substrate such that a first end of each of the plurality directive heat elements are adapted to be disposed proximate a heat generating device and a second end of each of the plurality of directive heat elements are spaced apart within the substrate to promote the transfer to heat from the heat generating device through the directive elements to an area of the heat conducting substrate which is larger than the area of the heat generating device.  
         [0009]     With this particular arrangement, a heat sink which transforms a high heat flux density which exists at one end of the directive heat elements proximate a device being cooled to a low heat flux density at an opposite end of the directive heat elements is provided. By closely spacing the end of the directive heat elements proximate the heat generating device and increasing the spacing of the opposite ends the directive heat elements, the heat sink transfers heat from a relatively small area (i.e. the area proximate the heat generating device) of the heat sink to a relatively large area of the heat sink (i.e. an area of the heat sink distal from the heat generating device). By positioning the directive heat elements in the substrate such that they channel heat from the device sought to be cooled to a relatively large, heat sinking area in the substrate, the device can be cooled more rapidly and more efficiently. By providing the directive heat elements from a material having a relatively high heat transfer coefficient, the directive heat elements rapidly channel heat away from the heat generating device and toward a heat sink region having an area larger than the area of the heat source. By directing or channeling the heat from the device to be cooled toward a relatively large heat sinking area, the heat sink can dissipate relatively large amounts of heat and is capable of rapidly dissipating the heat generated by a heat generating device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross-sectional view of a heat sink having directive heat elements disposed in a heat conducting substrate; and  
         [0011]      FIG. 2  is an isometric view of a plurality of directive heat elements. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Referring now to  FIGS. 1 and 2  in which like elements are provided having like reference designations, a heat generating device  12 , is disposed on a first surface  14   a  of a heat sink  14  provided from a heat conducting substrate  15  (also referred to herein as a matrix  15 ) having a plurality of directive heat elements  16  (also referred to herein as heat pipes, fibers, strands or bundles) disposed therein. In this particular embodiment, the heat generating device  12  is shown as two stacked semiconductors  12   a ,  12   b  which can form an LED disposed in a recess region (more clearly visible in  FIG. 2 ) defined by walls  17  projecting from a surface of the substrate  15 .  
         [0013]     The heat generating device  12  may be thermally coupled to the heatsink  14  via a solder connection (e.g. a semiconductor die soldered to the heat sink  14 ), epoxy or via any other connection technique or mechanism now known or unknown to those of ordinaru skill in the art. Electrical signal paths  13   a ,  13   b  may be used to couple device  12  to other circuits (not shown in  FIG. 1 ) as is generally known. In the case where the device  12  corresponds to a semiconductor device, the signal paths  13 ,  13   a  may be provided as bond wires as is generally known. The particular manner in which the signal paths  13   a ,  13   b  are provided is selected in accordance with the particular type of device corresponding to the heat generating element  12  as well as the particular application in which the device  12  is being used.  
         [0014]     The heat sink  14  is provided from a combination of here N thermal directive heat elements  16   a - 16 N, generally denoted  16  and the thermally conductive substrate or matrix  15  through which the directive heat elements  16  are disposed. The directive heat elements  16  may be provided as solid state directive heat elements or as conventional heat pipes (e.g. copper tubes filled with a coolant such as water). In preferred embodiments, the directive heat elements are made from a material having a thermal conductivity higher than the thermal conductivity than the substrate  15 . In one emodiment, the heat pipes  16  are made from graphite fibers. Those of ordocinary skill in the art will appreciate, of course, that other materials may also be used including but not limited to carbon, graphite diamond, Si Carbide, boron nitrude and aluminum nitride. The thermally conductive matrix  15  may, for example, be provided from a material such as copper. Other thermally conductive materials including but not limited to metals such as gold, silver or aluminum may also be used. Alternativley still, a gold-copper eutecctic braze material, or other moderate to higher melting point braze or solder material can also be used. In some embodiments, one criteria to use in selecting a particular material from which to provide the matrix  15  is that the melting point of the matrix material  15  should be higher than that of the solder (or other material) used to attach the device  12  (e.g. a semi-conductor die) to the matrix material and the matrix material should preferaby have a value of K greater than about 20 W/m-K.  
         [0015]     Each of the one or more directive heat elements  16  are arranged in the heat sink matrix  15  in a particular pattern. Since the heat pipes  16  are provided from a material having a higher thermal conductivity than the material from which the substrate  15  is provided, the heat pipes  16  direct heat (or facilitate the conduction of heat) in a particular direction defined by the direction of the neat pipe  16 . Thus, by concentrating one end of the heat pipes in a region proximate the heat generating device and expanding the spacing of the opposite end of the heat pipe throughout the substrate heat is efficiently and rapdily directed away from the heat generating device and dispersed throughout a large region in the substrate  15 .  
         [0016]     In one embodiment, the heat pipes  16  are provided from highly graphitized pitch based graphite fibers that exhibit anisotropic thermal conductivity in excess of that of the matrix material are preferred. Two sources of such fiber bundles or tows are Amoco BP, K1100 and Mitsubishi K13C2U. The K1100, for example is available in tow bundles of 2000 fibers and has a long fiber thermal conductivity of about K=1000 W/m-K. This compares favorably with copper which has a thermal conductivity of about K=345 W/m-K.  
         [0017]     Alternatively, in some embodiments, it may be preferable to provide the heat pipes  16  from bundles of carbon fiber nanotube structures.  
         [0018]     Each of the heat pipes  16   a - 16 N may be provided as a single fiber structure (e.g. provided from a single strand fiber) or as a multi-fiber structure (e.g. a multi-strand fiber). In some embodiments, a combination of single and multi-strand fibers may be used. Significantly, the fibers are positioned in directions in which it is desirable to conduct or channel the heat.  
         [0019]     In one embodiment, the heat pipes  16  are provided as graphite fibers which are arranged in a generally triangular (e.g. pyramidal) or cone shape with a tip of the cone disposed in the portion of the heat sink proximate the heat generating device  12  (e.g. a semiconductor device) which may, for example, be provided as an LED device. The base of the cone is disposed in the heat sink portion distal from the heat generating device. Care should be observed to concentrate the fibers  16  as tightly as can reasonably be achieved in the portion of the heat sink  14  proximate the heat generating device  12  so that the ends of the fibers  16  are exposed to or placed close to (or even in contact with) the heat generating device  12 . In the case where the heat generating device is a semiconductor device, it may be desirable that the ends of the fibers  16  be exposed to or placed close to (or even in contact with) the die location. The ends of the fiber  16  distal from the heat generating device are preferably uniformly distributed over a larger contact area (e.g. corresponding to the base of the triangular or cone shape formed by the fibers  16 ). The fibers  16  may lie along a straight path or they may fan out as shown in  FIG. 1 . Alternatively still, the outer rings of fibers may be bent (e.g. curved) away from a center line  19  of the heat sink  14  to achieve more efficient spreading and dissipation of heat throughout the heat sink  14 . While it is desirable for the fibers  16  to be continuous for best performance, it is not necessary, as long as the fibers are substantially aligned in the direction of desired heat flow.  
         [0020]     By arranging the heat pipes  16  such that a high concentration of heat pipes  16  (per unit area) are disposed proximate the heat generating device  12  and a lower concentration of heat pipes  16  (per unit area) are disposed throughout the heat conducting matrix  15 , the heat sink  14  functions as a heat flux transformer. That is to say, that the heat sink  14  accepts heat at high heat flux density and rejects heat at a lower heat flux density with lower temperature gradient than conventional isotropic heat conduction materials.  
         [0021]     It should be noted that in a cone-like shape or configuration of heat pipes (e.g. a cone, a truncated cone, pyramid or truncated pyramid configuration) their exists a higher concentration of graphite strands per unit area in the tip of the cone (i.e. the portion of the cone proximate the heat generating device) than the base of the cone (i.e. the portion of the cone distal from the heat generating device). If the concentration of fibers is sufficiently high such that the coefficient of thermal expansion in the region of the heat sink proximate the semiconductor device is substantially the same as the coefficient of thermal expansion of the semiconductor device itself, then a stress relief plate between the heat generating device  12  and the heats sink surface  14   a  can be omitted. It should be noted that the effect of reduction of the bulk expansion coefficient is greater than would conventionally be expected from the percentage area of the two materials. This is because the modulus of elasticity of the graphite material is significantly higher than that of the matrix material. Thus, in the case where the heat generating device is a semiconductor device, this allows the semiconductor device to be disposed directly on the surface of the heat sink.  14   
         [0022]     Thus, one advantage gained by including fibers  16  in the substrate  15  is that if the substrate  15  is provided having a relatively large concentration of fibers  16  near the device  12  itself, the device  12  can be connected directly to the substrate  15  (it should be appreciated that the substrate  15  may also sometimes be referred to as a slug or a heat sink block). This approach removes one or more thermal junctions which are typically present in conventional arrangements.  
         [0023]     By removing one or more thermal junctions, the thermal conduction of the die itself can be improved (e.g. from ˜10 c/w to ˜5 c/w) which results in the die being subject to lower stress and thus which allows elimination of any intermediate material (e.g. any intermediate silicon Si material) to act as a stress relief plate. In some embodiments, however, it may not be desirable to entirely omit the stress relief plate, but the stress relief plate can be reduced in size and shape. For example, the stress relief plate could be made thinner. With a thinner relief plate, the temperature gradient across the stress relief plate element would be lower, so that the die could handle more power at the same temperature.  
         [0024]     In one embodiment, the fibers are encased in a matrix material (e.g. like multiple wicks in a wax candle). The best known fibers of this type are made of carbon arranged in a graphite crystal structure. This is a hexagonal structure and the bonds between sheets are very weak. It is possible to roll up the sheets into tubes, called nanotubes. Carbon forms the presently most available and the more general name for these materials is “fullerenes.” It is now beginning to be recognized that other materials may also form these structures.  
         [0025]     Generally the matrix materials are weaker structurally and isotropic (like the wax in a candle). It should be appreciated that a high thermal conductivity matrix that is also strong enough to hold the composite together is desired.  
         [0026]     Thus, the diamond form of carbon, in monolithic form would be an alternative to this embodiment. This approach, however, is presently believed to be relatively expensive. Thus, due at least in part to cost considerations, the approach of using a diamond form of carbon is believed to be too expensive for some applications such as LED lighting applications.  
         [0027]     The substrate  15 , having the heating generating device  12  disposed thereon is disposed over a circuit board  20 . In some embodiments, a thermal epoxy  22  can be disposed between a surface of the substrate  18  and a surface of the circuit board  20 .  
         [0028]     In one embodiment, circuit board  20  can be provided as the type manufactured by Heat Technology, Inc., Sterling Ma under U.S. Pat. Nos. 5,687,062 and 5,774,336 and identified by the name UltraTemp™ circuit boards. In this case, the circuit board  20  can be considered a part of the heat sink  14 .  
         [0029]     Referring now to  FIG. 2 , the heat generating device  12  and substrate  15  are shown in phantom to improve the clarity with which the directive heat elements  16  can be seen. As can be most clearly seen in  FIG. 2 , the directive heat elements  16  are disposed in a cone shape with a first end of the heat pipes disposed in a ring shape (identified by rings  30  in  FIG. 2 ) and second end of the heat pipes disposed in a ring shape (identified by rings  32  in  FIG. 2 ).  
         [0030]     Although the directive heat elements are here shown arranged in a cone shaped pattern within the matrix  15 , it should be appreciated that the directive heat elements  16  may be arranged in any pattern including but not limited to patterns having a rectangular block shape, a square block shape, a pyramidal shape, an egg-shape, a ball shape or even an irregular shape. Also, a mixture of shapes can be used. For example, the first end of the heat pipes  16  may be arranged in a rectangular pattern and the second ends of the heat pipes  16  may be arranged in a circular pattern. Those of ordinary skill in the art will appreciate how to select the particular geometry and shape of the directive heat elements considering a variety of factors including but not limited to the shape of the device being cooled, the shape of the substrate in which the directive heat elements are disposed, the geometry available for the heat sink on a particular circuit board.  
         [0031]     It should be appreciated that the optional circuit board  20  has been omitted from  FIG. 2 , since in some applications the circuit board  20  is not properly a part of the heat sink  14 . Also omitted from  FIG. 2  is the thermal epoxy  22  which is also not properly a part of the heat sink in some applications.  
         [0032]     Having described preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims.  
         [0033]     All publications and references cited herein are expressly incorporated herein by reference in their entirety.