Abstract:
An apparatus for transferring heat from a heat spreader is provided. The apparatus includes a heat dissipating member and a heat spreading member adjacent the heat dissipating member, the heat spreading member configured to spread heat laterally across the heat dissipating member, the heat spreading member defining a heat conduction plane. The apparatus also includes a base adjacent to the heat spreading member, wherein the heat spreading member is between the base and the heat dissipating member, and at least one thermal via within the heat conduction plane, the thermal via thermally coupled to the heat spreading member, the at least one thermal via thermally coupled to the heat dissipating member and the base through surface-to-surface contact.

Description:
BACKGROUND 
   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. The fins increase the surface area available for heat dissipation to the surrounding environment. 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” near where the heat is coupled into the heat sink. Because the heat does not adequately spread from the hot spot, some areas on the heat sink may be 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. 
   To solve this problem, some devices use a heat spreader to aid in spreading the heat throughout the heat sink. Heat spreaders are structures that have a higher thermal conductivity than their surrounding structure. The heat spreaders are generally positioned between a heat generating device and a heat dissipating member of the heat sink, and are oriented such that heat entering the heat spreader travels lateral to the heat dissipating member. Thus, as heat enters the heat spreader, the heat is allowed to easily propagate across (lateral to the surface of) the heat dissipating member. 
   To effectively spread heat across a heat dissipating member, many heat spreaders are made of materials having a high thermal conductivity in one direction or plane. The high thermally conducting plane is generally oriented parallel with the heat dissipating surface, such that the heat can propagate easily lateral to the heat dissipating surface. The materials used to obtain a high planar thermal conductivity, however, often have a very low thermal conductivity in a direction normal to that plane. Thus, although the heat spreader effectively spreads heat laterally, the heat spreader does not allow good heat conduction between the adjacent heat dissipating member and the heat spreader. Vias, therefore, are generally included to aid in transferring heat between the heat spreader and the heat dissipating member. Conventional vias are metallic projections from the heat dissipating surface or from an opposing surface which extend through the heat spreader and contact the heat dissipating surface. These vias are thermally coupled to the heat spreader in the high thermal conductivity plane. Thus, heat easily propagates to the via from the heat spreader. Once the heat enters the vias, the heat can propagate up through the vias and into the rest of the heat dissipating member. 
   Manufacturing heat sinks having vias and heat spreaders such at those discussed above, however, often requires expensive processes which add to the cost of the heat sink. 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 cost effectively transferring heat from a heat spreader. 
   SUMMARY 
   An apparatus for transferring heat from a heat spreader is provided. The apparatus includes a heat dissipating member and a heat spreading member adjacent the heat dissipating member, the heat spreading member configured to spread heat laterally across the heat dissipating member, the heat spreading member defining a heat conduction plane. The apparatus also includes a base adjacent to the heat spreading member, wherein the heat spreading member is between the base and the heat dissipating member, and at least one thermal via within the heat conduction plane, the thermal via thermally coupled to the heat spreading member, the at least one thermal via thermally coupled to the heat dissipating member and the base through surface-to-surface contact. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is an exploded view of one embodiment of a heat sink that effectively transfers heat from a heat spreader; 
       FIG. 2A  is a side view of one embodiment of the heat sink of  FIG. 1 ; 
       FIG. 2B  is a cross-sectional view of one embodiment of the heat sink of  FIG. 2A ; 
       FIG. 2C  is an enlarged cross-sectional view of one embodiment of the heat sink of  FIG. 2B ; 
   

   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 
   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. 
   Embodiments of the present invention provide for an apparatus that effectively transfers heat from a heat spreader. Some embodiments of the apparatus include a heat spreader between a heat dissipating member and a base. The apparatus also includes at least one thermal via which is an insert placed with a plane of the heat spreader, between the heat dissipating member and the base. The thermal via, therefore, has strong thermal coupling with the heat spreader and thermally couples with the base and the heat dissipating member through surface-to-surface contact. In one embodiment, a fastener used to secure the base to the heat dissipating member extends through an aperture in the thermal via. The fastener secures the thermal via in place and ensures solid contact between the thermal via and the heat spreading member and base. 
     FIG. 1  is an exploded view of one embodiment of a heat sink  100  that effectively transfers heat from a heat spreader. Heat sink  100  includes a heat dissipating member  102 , a heat spreader  104 , a base  106 , and a plurality of thermal vias  108 . Heat sink  100  dissipates heat from heat generating devices (not shown) to the surrounding environment. In the embodiment shown in  FIG. 1 , a plurality of fins  112  aid in dissipating heat from heat dissipating surface  102 . In one embodiment, heat generating devices are thermally coupled to base  106 . For example, in one embodiment, heat generating devices are electronic devices which are mounted to base  106 . Heat from the heat generating devices propagates from base  106  through thermal vias  108  into heat spreader  104 . Heat spreader  104  spreads the laterally and the heat then couples back through thermal vias  108  and into heat dissipating member  102 . Heat dissipating member  104  dissipates the heat into the surrounding environment. 
   Base  106  and heat dissipating member  102  are composed of metal to provide adequate thermal conductivity, as well as the structural integrity necessary to support heat sink  100 . For example, in one embodiment, base  106  and heat dissipating member  102  are composed of aluminum. In an alternative embodiment, finned section  102  and base  106  are composed of steel. In other embodiments, finned section  102  and/or base  106  are composed of other conductive materials or a combination of conductive materials. 
   Heat spreader  104  distributes heat across heat dissipating member  102 . Heat spreader  104  is composed of a thermal material having a higher thermal conductivity than heat dissipating member  102 , but does not posses the structural strength needed for heat sink  100  and is therefore, placed between heat dissipating member  102  and base  106  for structural integrity. Fasteners  110  hold heat dissipating member  102  to base  106  and secure heat spreader  104  between heat dissipating member  102  and base  106 . 
   Heat spreader  104  is a material having a high thermal conductivity in a plane parallel with heat dissipating member  102 . For example, in one embodiment, heat spreader  104  is composed of thermal pyrolytic graphite (TPG), which is commercially available from Momentive Performance Materials in Wilton, Connecticut. 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) 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. As shown in  FIG. 1 , the in-plane (a-b direction) of heat spreader  104  is parallel to heat dissipating member  102 . 
   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 between base  106  and heat dissipating member  102 , 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. 
   In a direction normal to the a-b plane however, (c-direction) heat spreader  104  has a low thermal conductivity and thus, does not propagate heat efficiently. In this embodiment, the a-b plane is parallel to heat dissipating member  102 . Thus, heat spreader  104  efficiently propagates and distributes heat parallel to heat dissipating member  102 , but does not efficiently propagate heat between heat dissipating member  102  and heat spreader  104 . 
   To aid in heat propagation to and from heat dissipating member  102  and base  106 , heat sink  100  includes a plurality of thermal vias  108 . Thermal vias  108  are separate components from both heat dissipating member  102  and base  106 . Thermal vias  108  are located between heat dissipating member  102  and base  106  within the plane of heat spreader  104 . Thermal vias  108  are thermally coupled to the high heat conductive plane of heat spreader  104 . Thermal vias  108  are also thermally coupled to heat dissipating member  102  and base  106 . The coupling between thermal vias  108  and heat dissipating member  102  and base is by surface-to-surface contact. Unlike heat spreader  104 , thermal vias  1 O 8  are composed of metal and propagate heat evenly in all directions. Thus, heat traveling laterally in heat spreader  104  travels vertically through thermal vias  108  and is coupled to heat dissipating member  102 . In one embodiment, thermal vias  108  are composed of copper. 
   In one embodiment, each fastener  110  extends through one of the plurality of thermal vias  108 . Fasteners  110  aid in securing thermal vias  108  in place and ensuring solid surface-to-surface contact between thermal vias  108  and heat dissipating member  102  and base  106 . In the embodiment shown in  FIG. 1 , fasteners  110  are bolts. In other embodiments, fasteners  110  are rivets, screws, or other devices as known to those skilled in the art. 
   Referring now to  FIGS. 2A ,  2 B, and  2 C. As shown, thermal vias  108  have a washer type shape composed of an outer circular surface and an inner circular surface. The inner circular surface of each thermal via  108  is aligned such that one of the fasteners  110  is placed therethrough and into base  106 . Fasteners  110  improve thermal coupling between heat dissipating member  102  and via  108  by forcing heat dissipating member  102  against via  108 . Likewise, fasteners  110  improve thermal coupling between base  106  and via  108  by forcing base  106  against via  108 . In order to maintain contact with both heat dissipating member  102  and base  106 , thermal vias  108  are substantially similar in height to heat spreader  104 . 
   Advantageously, positioning thermal vias  108  around fasteners  110  allows for good thermal coupling between thermal vias  108  and base  106  and thermal vias  108  and heat dissipating member  102 . This is because fastener  110  provides force most effectively nearby the location of fastener  110 , and thus strongly forces heat dissipating member  102  and base  106  against thermal vias  108 . 
   In alternative embodiments, thermal vias  108  are located nearby or partially around fasteners  110 , thereby providing force upon thermal vias  108 . In other embodiments, thermal vias  108  are located within the plane of heat spreader  104 , but away from fasteners  110  to provide heat transfer in a specific area. Additionally, although thermal vias  108  are shown as cylindrical washer shapes surrounding fasteners  110 , in other embodiments, thermal vias  108  are square washer shapes, or other shapes surrounding fastener  110 . 
   The amount of thermal energy (heat) that can transfer into/out of thermal vias  108  is dependent upon the surface are of the thermal vias  108 . Thus, the area on the top and bottom surfaces  202  of thermal via controls the amount of heat transferred between thermal via  108  and heat dissipating member  102  and base  106  respectively. Thus, the size of thermal via  108  can be determined based on the amount of thermal conduction between heat dissipating member  102  and heat spreader  104 . For example, if a higher thermal conduction is needed or desired for a particular application, a larger diameter thermal via  108  can be used. Likewise, if higher thermal conduction is not needed and a larger heat spreader  102  is needed or desired, a smaller diameter thermal via  108  can be used. Since the contact between thermal via  108  and heat dissipating member  102  and base  106  is metal surface-to-surface, in one embodiment, a thermal paste is applied between each of the surfaces to improve heat conduction. 
   Additionally, the surface area of the lateral side  204  of thermal via  108  affects the amount of heat transferred between thermal via  108  and heat spreader  104 . Thus, to increase or decrease the amount of heat transfer between thermal via  108  and heat spreader  104 , the lateral surface area of thermal via  108  can be increase/decreased respectively. 
   Advantageously, using thermal vias  108  which are separate from both heat dissipating member  102  and base  106  enables heat sink  100  to be manufactured economically. For example, as shown in  FIG. 1 , heat dissipating member  102  and base  106  can be formed through an extrusion process, with the only slight machining necessary to put apertures in for the fasteners  110 . Conventional systems with thermal vias that are integral with one of the sides of the heat sink, however, often require that the component having the thermal vias be manufactured through a more expensive process (e.g. casting, powder metallurgy, forging), or go through a large amount of machining after an extrusion process. Thus the use of detached thermal vias  108  allows for flexible, and thus economical manufacturing of a heat sink, because the components of the heat sink can be formed through extrusion, or other process as needed or desired, and the thermal vias  108  can be added after extrusion. Additionally, the use of detached thermal vias  108  enables heat sink  100  to be easily adapted to different layouts of heat generating devices by placing thermal vias  108  in different locations. Detached thermal vias  108  also enable heat sink  100  to be adaptable to different magnitudes of thermal energy transferred by using different sized thermal vias  108 . 
   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.