Abstract:
An apparatus for spreading heat over a plurality of fins is provided. The apparatus includes a heat dissipating member composed of metal and having a plurality of fins on a first side of the heat dissipating member. The apparatus also includes a plurality of strips of thermal material having a thermal conductivity in a direction parallel to the heat dissipating member higher than a thermal conductivity of the heat dissipating member, the plurality of strips disposed on a side of the heat dissipating member opposite of the first side and configured to spread heat along the heat dissipating member.

Description:
BACKGROUND 
       [0001]    For many devices removing heat is essential in order to keep the device operating effectively. Often, to aid in removal of heat, a heat sink is coupled to the device. The heat sink is generally a metal component with a flat base on one side and a number of fins on the other. The flat base is coupled to the device and the fins extend out from the base into the surrounding environment. The fins increase the surface area available for heat dissipation and aid in drawing air past the heat sink. Often, however, heat from the electronic device does not propagate evenly from the heat generating device to all areas of the heat sink. This results in localized “hot spots” which generally occur near where the heat is coupled into the heat sink. Because of the thermal conductivity of the metal heat sink, generally the heat does not adequately spread from the hot spot. This results in some areas on the heat sink being unused, or dissipating heat only minimally. Thus, the heat sink is not cooling up to its potential, because the heat is being dissipated from only a portion of the surface area on the heat sink. 
         [0002]    The heat dissipation problems are increased when using heat sinks with electronic devices, because many electronic devices generate a large amount of heat in a small area. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an apparatus and method for improving the heat dissipation of a finned surface. 
       SUMMARY 
       [0003]    The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. In one embodiment, an apparatus for spreading heat over a plurality of fins is provided. The apparatus includes a heat dissipating member composed of metal and having a plurality of fins on a first side of the heat dissipating member. The apparatus also includes a plurality of strips of thermal material having a thermal conductivity in a direction parallel to the heat dissipating member higher than a thermal conductivity of the heat dissipating member, the plurality of strips disposed on a side of the heat dissipating member opposite of the first side and configured to spread heat along the heat dissipating member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The present invention can be more easily understood, and further advantages and uses thereof are more readily apparent, when considered in view of the detailed description and the following figures in which: 
           [0005]      FIG. 1  is an exploded view of one embodiment of a heat sink that spreads heat over a finned surface; 
           [0006]      FIG. 2A  is a cross-sectional view of the heat sink of  FIG. 1 ; 
           [0007]      FIG. 2B  is an enlarged cross-sectional view of the heat sink of  FIG. 2A ; and 
           [0008]      FIG. 3  is a perspective view of one embodiment of the heat sink of  FIG. 1 . 
       
    
    
       [0009]    In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present invention. 
       DETAILED DESCRIPTION 
       [0010]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the method and system may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
         [0011]    Embodiments of the present invention provide for an apparatus for improving the heat dissipation of a finned surface. The apparatus includes a finned surface and a plurality of strips of a thermal material for spreading the heat across the finned surface. Some embodiments provide for a finned surface with fins oriented parallel to each other. Strips of thermal material are oriented parallel with the fins and are positioned such that heat released from the thermal material flows easily into a nearby fin. Thus, the strips of thermal material aid in spreading heat along each fin. Other embodiments provide for a cross strips of thermal material in addition to the parallel strips of thermal material. The cross strips aid in spreading heat across different fins. Some embodiments of the apparatus have a generic heat spreading design which allows the apparatus to be applied to many different layouts of heat generating devices. 
         [0012]      FIG. 1  is an exploded view of one embodiment of a heat sink  100  that spreads heat across a finned surface. Heat sink  100  includes a finned section  102 , a thermal material  104 , and a base  106 . Finned section  102  has a plurality of fins  108  projecting out normal to the surface of heat sink  100 , extending along one side of finned section  102 , and having a pointed ridge shape. In the embodiment shown, fins  108  are oriented parallel to each other. In an alternative embodiment, fins  108  are oriented in an asterisk shape such that one end of each fin is near the center of finned surface  102  and fins  108  extend outward in different directions from the center. In other embodiments, fins  108  are oriented in a parallel diagonal manner or in other patterns as known to those skilled in the art. In still other embodiment, fins  108  are obelisk type structures, or other shapes as known to those skilled in the art. 
         [0013]    In one embodiment, finned section  102  is composed of solid aluminum. In an alternative embodiment, finned section  102  is composed of steel. In other embodiments, finned section  102  is composed of other conductive materials or a combination of conductive materials. 
         [0014]    In the embodiment shown in  FIG. 1 , thermal material  104  is disposed on finned section  102  on a side opposite of fins  108 . Thermal material  104  aids in distributing heat throughout finned section  102 . Thermal material  104  is a material having a higher thermal conductivity than that of finned section  102 , thus heat propagates through thermal material  104  more easily than through finned section  102 . In one embodiment, thermal material  102  comprises a plurality of strips  110 . Each strip  110  of material is spaced from adjacent strips and extends in a direction parallel with fins  108  of finned section  102 . 
         [0015]    Finned section  102  comprises a plurality of grooves  112  to house each strip  110  of thermal material  104 . Grooves  112  are defined in finned section  102  on a side opposite of fins  108  and extend in a direction parallel to fins  108 . In the embodiment shown in  FIG. 1 , cross grooves  116  are also defined in finned section  102  and will be described in more detail below. Each strip  110  of thermal material  104  is housed in a groove  112  and base  106  covers each strip  110  of thermal material  104 . A plurality of fasteners  114  secure base  106  to finned section  102 , thereby securing thermal material  104  between base  106  and finned section  102 . In one embodiment, fasteners  114  are screws. In alternative embodiments, fasteners  114  are rivets, clamps, or other structures as known to those skilled in the art. 
         [0016]    Thermal material  104  is thermally coupled to finned section  102 . Thus, heat can propagate into thermal material  104 , spread across heat sink  100  and couple into finned section  102 . In one embodiment, heat sink  100  dissipates heat from one or more heat generating devices (not shown) which are thermally coupled to base  106 . As the devices generate heat, the heat is coupled into base  106 . The heat propagates from base  106  into finned section  102  and thermal material  104 . Due to the high thermal conductivity of thermal material  104 , the heat in thermal material  104  can easily propagate from one area along the particular strip  110  of thermal material  104  to another area, thus spreading the heat across heat sink  100 . Each strip  110  of thermal material  102  acts as a corridor through which heat can propagate along finned section  102 . In one embodiment, the heat generating devices are electronic devices mounted on base  106 . 
         [0017]    In one embodiment, thermal material  104  is thermal pyrolytic graphite (TPG), which is commercially available from Momentive Performance Materials in Wilton, Conn. TPG may be referred to as highly oriented pyrolytic graphite (HOPG), or compression annealed pyrolytic graphite (CAPG). In any case, TPG refers to graphite materials consisting of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers or a high degree of preferred crystallite orientation, with an in-plane (a-b direction as shown in  FIG. 1 ) thermal conductivity greater than 1000 W/m-K. In one embodiment, the TPG has an in-plane thermal conductivity of approximately 1,500 W/m-K. 
         [0018]    Although TPG has a high thermal conductivity in the a-b direction, its thermal conductivity in a direction normal to that plane (c direction) is low. For example, in one embodiment, TPG has a c direction thermal conductivity of less than 20 W/m-K. TPG, therefore, rapidly spreads heat in the a-b direction, but resists heat flow in the c direction. 
         [0019]    In one embodiment, TPG is formed as described in U.S. Pat. No. 5,863,467 which is hereby incorporated herein by reference. Briefly, to manufacture heat sink  100  with TPG, pyrolytic graphite is deposited in grooves  112 , base  106  is positioned overtop, and heat sink  100  is heat treated to form the pyrolytic graphite into a crystal structure. The resulting crystal structure, TPG, has a high in plane conductivity. 
         [0020]    Referring now to  FIGS. 2A and 2B , a cross-sectional view of one embodiment of heat sink  100  in which TPG is used as thermal material  104 .  FIGS. 2A and 2B  illustrate a plurality of vias  202 . Vias  202  are a portion of finned section  102  adjacent thermal material  104 . In this embodiment, vias  202  are formed between grooves  112  of finned section  102  and are aligned with fins  108 . Vias  202  provides thermal coupling between finned section  102  and thermal material  104  as explained below. 
         [0021]    TPG is oriented in strips  110  such that the plane of high thermal conductivity (a-b plane  204 ) is parallel to base  106 . Thus, the TPG propagates heat along the a-b plane and thermal transfer occurs where edges  206  of a-b plane of thermal material  104  come into contact with finned section  102 . Since TPG has a low thermal conductivity in the c direction  208 , vias  202  provide a c-direction path for heat to travel between finned section  102  and thermal material  104 . Vias  202  are aligned with fins  108 , thus heat propagating through strips  110  has a direct path to fins  108 . In an alternative embodiment, strips  110  of thermal material  104  are aligned with fins  108 . 
         [0022]    In operation, TPG acts to increase the efficiency of fins  108  through passive heat spreading. Heat accumulated by base  106  is transferred to finned section  102  through surface to surface contact with vias  202  as shown in  FIGS. 2A and 2B . Once the heat reaches vias  202  of finned section  102 , some of the heat propagates directly to fins  108 . Additionally, some of the heat propagates from vias  202  into thermal material  104  through contact with edges  202 . Once the heat enters thermal material  104 , the heat rapidly propagates along thermal material  104 . As heat travels along each strip  110  of thermal material  104 , heat is transferred back into finned section  102 . The heat then propagates through finned section  102  into fins  108  through vias  202 . From fins  110 , the heat is dissipated to the environment. 
         [0023]    As shown in  FIGS. 2A and 2B  strips  110  of thermal material  104  have generally rectangular cross-sections, which allows for easy manufacture with an adequate edge  204  size. In other embodiments, however, the cross-section of strips  110  has a triangular, semi-circular, or other shape. Further, although  FIG. 2B  illustrates vias  202  as being substantially similar in width to fins  108 , other embodiments of heat sink  100  include vias  202  having widths large or smaller than that of fin  108 , due to wider, narrower, or irregularly spaced strips  110 . Finally, the depth of strips  110  can be modified according the amount of heat propagation needed or desired for a particular application. 
         [0024]    Referring back to  FIG. 1 , to further improve heat spreading, in one embodiment, heat sink  100  also includes at least one cross strip (not shown) of thermal material  104  perpendicular to strips  110 . Finned section  102  as shown in  FIG. 1  has a plurality of cross grooves  116  for housing a plurality of cross strips of thermal material  104 . Cross strips of thermal material  104  thermally couple adjacent strips  110  of thermal material  104 . Cross strips, therefore, facilitate lateral heat transfer across finned section  102 , and between different fins  108  of finned section  102 . Without cross strips of thermal material  104 , strips  110  of thermal material  104  allow heat to propagate  1 -dimensionally along finned section  102  in a direction parallel with fins  108 . Cross strips of thermal material  104 , therefore, allow heat to propagate between strips  110  and thus facilitate  2 -dimensional spreading across finned section  102 . In one embodiment, thermal material  104  includes a plurality of cross strips which, along with strips  110 , form a grid of thermal material  104 . 
         [0025]    As heat propagates through strips  110  and comes into contact with a cross strip of thermal material  104 , the heat can propagate along the cross strip to other strips  110  of thermal material and along other fins  108 . Lateral heat propagation can be increased by increasing the number of cross strips. Thus, although four (4) cross grooves  116  are shown in  FIG. 1 , the number of cross grooves  116 , and associated cross strips can be increased or decreased to increase or decrease the lateral propagation of heat. 
         [0026]      FIG. 3  illustrates a cross-sectional view of one embodiment of heat sink  100 .  FIG. 3  shows one example of the heat spreading process of heat sink  100 . A hot spot  302  in finned section  102  is formed by, for example, an electronic device emitting heat to finned section  102 . Some heat from hot spot  302  dissipates directly through finned section  102  and into fins  304 ,  306 ,  308 . The rest of the heat is transferred into adjacent thermal material  104 . Once the heat enters thermal material  104 , the heat rapidly propagates along each strip  110  of thermal material  104 . The heat is then transferred back into finned section  102  along edge  202  between the strips  110  of thermal material  104  and finned section  102 . Generally, the heat from thermal material  104  transfers into finned section  102  at a point having less heat than hot spot  302 . Thus, thermal material  104  passively spreads the heat from warmer to cooler points along finned section  102 , increasing the efficiency of fins  108  by evenly distributing the heat along fins  108 . 
         [0027]    Advantageously, the design of heat sink  100  is economical to manufacture. For example, the orientation of grooves  112  and fins  108  as parallel to each other allows finned section  102  to be made through an extrusion process and include both grooves  112  and fins  108 . As shown in  FIG. 1 , only the apertures for fasteners  114  and cross grooves  116  require machining. Thus, in an alternative embodiment, cross grooves  116  are not included in finned section  102  to reduce the cost of manufacturing. Additionally, since finned section  102  is formed by extrusion, heat sink  100  can easily be manufactured at different sized for different applications. This is because the length of finned section  102  can be cut at any length according the needed or desired application. 
         [0028]    As described above, some embodiments of heat sink  100  provide further economical benefits, because heat sink  100  has a generic thermal design that can be applied to many different layouts of heat generating devices. For example, one embodiment of heat sink  100  the plurality of strips  110  are positioned on substantially all of finned section  102 . Further, in this embodiment, finned section  102  is substantially larger than an area of heat generated by one or more devices. Since strips  110  are positioned on substantially all of finned section  102 , strips  110  will spread heat from one or more heat generating devices regardless of where the devices are coupled to base  106 . To further improve the ability to accommodate varying locations of heat devices, an alternative embodiment of heat sink  100  includes cross strips  116 , such that thermal material  104  forms a grid. The grid of thermal material  104  enables heat spreading regardless of the location of one or more heat generating devices. Advantageously, the more closely spaced that the cross strips  116  are, the more likely it is that a device mounted on base  106  will be near to a cross strip  116 . Thus, the more closely spaced the cross strips  116 , the more accommodating heat sink  100  is to differing device layouts. The space between cross strips  116  is limited, however, by the requirement for vias  202  to transfer heat between finned section  102  and thermal material  104 . 
         [0029]    Although  FIGS. 1 ,  2 A,  2 B, and  3  illustrate an embodiment of heat sink  100  having strips  110  of thermal material  104  oriented parallel with fins  108 , other orientations of strips  110  could be used. For example, in an alternative embodiment, strips  110  of thermal material  104  are parallel to each other, and are angled at 45 degrees with respect to fins  108 . Thus, each strip  110  facilitates propagation of heat partially in a lateral direction across multiple fins, and also facilitates propagation partially along the length of fins  108 . In another alternative embodiment, strips  110  are parallel with fins  108  as shown in  FIG. 1 , and cross cuts  116  run diagonal across strips  110 . In yet other embodiments, the placement and orientation of strips  110  and/or cross strips  116  is customized to match a particular fin design and/or design layout. 
         [0030]    Although for some embodiments, thermal material  104  has been described as TPG, the present invention is not intended to be so limited and can include other thermal materials. For example, in one embodiment, thermal material  104  is copper. Copper allows equal heat propagation in all directions. Thus, heat can propagate along the strips  110  of copper and can couple into vias  202  as well as into finned section  102  above strips  110 . When copper is used, for example, strips  110  may be aligned with fins  108 . In another embodiment, thermal material  104  is a heat pipe. Alternatively, thermal material  104  can be other materials having a high in-plane conductivity, such as diamond-like-carbon (DLC) or diamond or any material having thermal conductivities higher than the material of finned section  102 . 
         [0031]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to base any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.