Patent Publication Number: US-2006011325-A1

Title: Micro-channel heat sink

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application is based on, and claims priority from, U.S. Provisional Patent Appln. No. 60/588,001 filed Jul. 13, 2004, the contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to cooling of electronic components, and more particularly to heat sinks with air cooled fins for cooling electronic components such as integrated circuits.  
     BACKGROUND OF THE INVENTION  
      It is well known that heat can be a problem in many environments, and that overheating can lead to failures of components such as integrated circuits (e.g. a central processing unit (CPU) of a computer) and other electronic components.  
      Heat sinks are a common device used to prevent overheating, and mainly rely on the dissipation of heat from the device using air. However, dissipating heat using a gas, such as air, is difficult because of the poor thermal conductivity of gases. Gases also have a low heat capacity, which causes them to heat up quickly, which retards the rate of heat absorption by decreasing the temperature difference between the gas and the heat sink.  
      Conventional heat sinks have a limited amount of surface area that can be put into a given volume, and as a result, an adequate conventional heat sink must be large in order to provide the necessary convection surface area. Generally, in heat sink designs for cooling a heat source on a substrate, the heat sink dimensions extend substantially perpendicular to the substrate and heat source. Additionally, these heat sink designs do not integrate well with certain types of fluid pump designs.  
      A number of U.S. patents have addressed the problem of heat exchange, including U.S. Pat. No. 6,415,860, U.S. Pat. No. 5,801,442, U.S. Pat. No. 6,712,127, U.S. Pat. No. 6,244,331, U.S. Pat. No. 6,200,536, U.S. Pat. No. 6,705,393, and U.S. Pat. No. 6,675,875. However, these references do not fully solve the problems associated with effective cooling of electronic components as described above.  
      Micro-channels have been described that can create very high convective heat transfer rates, even with gases. In theory, the high convection rates of micro-channels can overcome the poor thermal conductivity issue. However, there are two major obstacles to the practical implementation of a micro-channel concept in a heat sink application. First, micro-channels create a large resistance to fluid flow. The resistance increases as the length (in the direction of flow) increases. Second, the low heat capacity of gases means that they heat up quickly and become ineffective at dissipating heat.  
      Accordingly, it would be desirable if there was a way to overcome these and other obstacles against implementing a micro-channel concept in a heat sink application.  
     SUMMARY OF THE INVENTION  
      An embodiment of the present invention includes a heat sink with an arrangement of micro-fins, spaced apart to form microchannels through which a gas can flow. The heat sink includes a conductive apparatus for conducting heat from a heat source to the arrangement of micro-fins. The conductive apparatus includes a post, with a bottom surface at a proximal end for contact with the heat source. The arrangement extends outward from the post at a distal end in a plane spaced apart from a plane of the bottom surface of the post. In one embodiment, the conductive apparatus includes a plurality of ribs that extend radially outward from the post. Each micro-fin has a length that bridges the space between two ribs. The micro-fins are spaced substantially parallel to each other with a space between them, forming micro-channels for passage of cooling gas. Another embodiment includes a plurality of micro-fins extending radially outward from the post, and also separated to form micro-channels. Another embodiment includes a plurality of micro-fins extending perpendicular to a rectangular post. In operation, heat is conducted from the heat source, through the post to the micro-fins, and into gas around each micro-fin. A fan or other gas pump can be used to force a flow of the gas through the micro-channels and thereby through the arrangement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:  
       FIG. 1A  is a top isometric view of a first embodiment of a heat sink in accordance with the present invention;  
       FIG. 1B  is a bottom isometric view of the first embodiment as shown in  FIG. 1A ;  
       FIG. 2  illustrates the flow of heat and gas facilitated by the heat sink of the present invention, with the gas flow passing through the micro-channels from the top of the arrangement of micro-fins to the bottom of the arrangement;  
       FIG. 3  illustrates the flow of heat and gas facilitated by the heat sink of the present invention, with the gas flowing through micro-channels from the bottom of the micro-fin arrangement to the top of the micro-fin arrangement;  
       FIG. 4  is a close up view of the micro-channels and micro-fins formed and arranged as illustrated in  FIGS. 1A and 1B ;  
       FIG. 5A  illustrates gas flow through a micro-channel;  
       FIG. 5B  illustrates a thermal resistance circuit model of the heat sink of the invention that is useful for determining optimized parameters for implementations of the heat sink;  
       FIG. 6  is a graph illustrating the optimization of the number of fins and ribs under one set of constraints of a heat sink in accordance with the present invention;  
       FIG. 7  illustrates an alternative embodiment of a heat sink in accordance with the present invention;  
       FIG. 8  illustrates an alternative arrangement of micro-channels and fins in accordance with additional embodiments of the present invention;  
       FIG. 9  illustrates an alternative arrangement of micro-channels and fins in accordance with additional embodiments of the present invention; and  
       FIG. 10  illustrates a further alternative arrangement of micro-channels and fins in accordance with additional embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will now be described in detail with reference to the figures of the drawing, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, the present invention is to include multiple components as well as a single component when only one is shown, and vice versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.  
      Generally, the present invention is a heat sink that utilizes an arrangement of micro-fins spaced apart to form microchannels for passage of a gas from one side of the arrangement to an opposite side of the arrangement, wherein the micro-channels create high convection coefficients at the surfaces of the micro-fins. According to a first aspect of the invention, the arrangement of micro-fins is dimensioned to be thin i.e. a small depth, which dimension is in the direction of flow of a gas passing through a micro-channel from one side of the arrangement to an opposite side. The small depth is for maintaining a large heat sink-to-gas temperature difference and for minimizing the pressure drop of gas flowing through the micro-channels. The microchannels are located in a plate-like region which is offset by a distance H from the heat source (as indicated in  FIGS. 2 and 3 ). This allows a gas to flow through the micro-channels in a direction that is substantially perpendicular to (i.e. directly toward or away from) the substrate containing the heat source, thereby cooling the heat source. According to a further aspect of the invention, the microchannels are arranged in a substantially parallel fashion to provide a large amount of surface area in a small volume. The result is a high performance gas-cooled heat sink that is particularly well suited for cooling such prone components as personal computer CPUs and other electronic devices.  
       FIGS. 1A and 1B  are isometric views showing primarily the top and bottom, respectively, of a first preferred embodiment of a heat sink  10  according to the present invention. As shown in  FIGS. 1A and 1B , the heat sink  10  consists of a center post  12 , with a bottom surface  14  ( FIG. 1B ) for contact with a heat source. Micro-fin arrangement portions  16  each include a plurality of micro-fins  18 , spaced apart to form a plurality of micro-channels  20  i.e. the spaces between the micro-fins. The micro-fins as shown are formed in a plate-like portion  22 . In one example of the present invention, the center post  12  and fins  18  are fabricated of the same material such as aluminum. However, the present invention also includes posts and fins constructed of other materials, such as copper, silicon-carbide, graphite, etc. Still further, it is not necessary for the center post and fins to be made from the same material. The micro-fins have a depth “d” equal to the thickness of the portion  22 . The micro-fins  18 , and therefore also the micro-channels  20  in this example have a length “L” in the planar surface.  
      In the embodiment of  FIGS. 1A and 1B  the micro-fins  18  lie substantially on arcs of respective concentric circles each with a center coinciding with the center of the post  12 . Within the portions  16 , therefore, the fins are substantially parallel to each other along the concentric rings. Other shapes and arrangements of the micro-fins are also included in the present invention. The micro-fins  18  extend from one rib  24  to another rib  24  so as to allow heat from a heat source to be conducted through the post and then to the ribs  24  and then to each micro-fin.  
      Various numbers of conducting ribs  24  and/or heat pipes may or may not be present to aid in the conductance of heat from the center post to the outer portions of the heat sink micro-fin arrangement. In other words, although the embodiment of  FIGS. 1A and 1B  shows ribs  24  of solid material, the ribs can also be constructed of other materials, and for example can include heat pipes. The ribs  24  can be integral with the portions  16 , and can have a thickness equal to the depth “d”, or they can have a different thickness. As shown in  FIGS. 1A and 1B , the ribs  24  are thicker than the micro-fins  18  of thickness “d”. The heat sink  10  parts including the post  12 , fins  18  and ribs  24  can all be an integrally formed heat sink of one material, or can be formed from separate parts, each of the same or different materials. A single, integral material construction has an advantage of avoiding heat barriers due to material junctions. The ribs  24  have a larger cross section than the micro-fins, and serve the purpose of conducting heat from the post  12  to the micro-fins  18 .  
      It should be noted that the present invention is not limited to the structure as shown in  FIGS. 1A and 1B  or other figures of the present specification. More generally, the present invention includes an array, arrays or arrangements of a plurality of micro-fins spaced apart to form a plurality of micro-channels providing openings through the array for passage of a cooling gas, and further includes conductive apparatus for conducting heat energy from a heat source to each micro-fin. The apparatus for conducting heat energy to the micro-fins as shown in  FIGS. 1A and 1B  includes the post  12  and the ribs  24 . Other structures for conducting heat from a heat source to the micro-fins will be apparent to those skilled in the art upon reading the present disclosure, and these are to be included in the spirit of the present invention. Similarly, the micro-fins can be configured in various ways that will be apparent to those skilled in the art, and these are also to be included in the spirit of the present invention. In the example in  FIGS. 1A and 1B  wherein ribs  24  are provided, they extend radially from the center post  12 .  
      The flow of heat and gas (e.g. air) through a heat sink according to the present invention such as that shown in  FIG. 1  is shown schematically in  FIGS. 2 and 3 . As shown in both figures, heat flows into the center post  12  by contact with a heat source  26 . For example, the heat source  26  could be an integrated circuit such as a CPU mounted on a substrate or other surface  28 . Next, the heat flows from the post  12  to the micro-fins via any of various conductive structures. As shown in  FIGS. 1A and 1B , the heat flows radially outward through the ribs  24  which could be of various structures such as solid metal, or heat pipes. An important and innovative feature of the embodiment shown is that the portions  16  are offset from the plane of the post bottom  14  and therefore also from a heat source  26 . This allows gas (e.g. air) to freely flow in between the portions  16  and a substrate  28  on which the heat source  26  may reside. Consequently, a gas is able to flow through the micro-channels formed by the micro-fins  18  either from or to the space between the portions  16 , and for example a substrate. This arrangement allows for the use of a large parallel array of channels to be contained in a short structure. In  FIG. 2 , the gas  30  flows toward the portions  16 , through the micro-channels  20  and exhausts radially outwards from the center post  12  in the space between the portions and the substrate.  FIG. 3  shows a flow of gas  32  in the opposite direction. The heat symbolically indicated by arrow  34  is optimally transferred from the heat sink  10  to the gas  30 ,  32  as it passes through the micro-channels  20 .  
      An enlargened view of the portions  16  in the embodiment of  FIGS. 1A and 1B  is shown in  FIG. 4 . Heat is conducted away from the center post  12  by the ribs  24  and is distributed to the micro-channels  20  by the micro-fins  18 . Many geometrical parameters can be optimized such that the heat sink dissipates a maximum amount of heat in the smallest possible volume. The parameters include: the micro-fin length “L”, width “W”, and depth “d”; the number of micro-channels  20 ; the number, width and thickness of the ribs  24 ; and the size of the center post  12 .  
      According to a first aspect of the present invention, the depth “d” of the micro-fins  18 /micro-channels  20  is kept short to minimize the heating of the gas as it passes through, but long enough to provide ample micro-fin surface area for heat transfer to the gas. An optimal micro-fin depth “d” is found by balancing the need for a large convection surface with the desire to minimize the gas flow resistance.  
      According to a further aspect of the present invention, the width “W” of the micro-fins is also optimized. Reducing the width “W” of the micro-fins allows for more micro-channels, but increasing the width “W” provides for better conduction of heat to the micro-channel walls. The cross sectional area and number of ribs is also a critical parameter. A large number of wide ribs conveys heat more efficiently to the outer portions of the heat sink micro-fins; but wider ribs result in less space available for micro-channels.  
      All of the parameters are interrelated and can be optimized using a mathematical model of the heat sink and optimization techniques. The gas flow can be modeled as generated by an external fan or other gas pump that can force the gas through the micro-channels. As illustrated in  FIG. 5A , the total resistance to the gas flow through the micro-channel  36  comes from minor losses at the inlet  38  and outlet  40  of the microchannels and frictional losses inside the channel  36  (see R. Blevins,  Applied Fluid Dynamics Handbook,  Krieger, Malabar Fla., 1992, the contents of which are incorporated by reference herein) (see  FIG. 5A ), and is summarized according to the following formula: 
 
Δ P   system   =ΔP   entrance   +ΔP   friction   +ΔP   exit   (1)
 
      Each of the terms in equation (1) is a function of the gas flow rate. The gas pump also has a relationship between flow rate and pressure drop. Stated mathematically: 
 
 ΔP   system   =f   1 (gas flow rate) and Δ P   pump   =f   2 (gas flow rate)  (2)
 
      The system flow rate is found by equating ΔP system  and ΔP pump . This is the operating point of the pump and determines the system flow rate and pressure drop.  
      Correlations are used to determine the convection coefficient (see W. Kays and M. Crawford,  Convective Heat and Mass Transfer,  McGraw-Hill, New York, 1980, the contents of which are incorporated by reference herein). In general, the convection coefficient h is a function of the gas flow rate, gas properties and channel geometry and is represented mathematically as: 
 
 h=f   3 (gas flow rate, gas properties, channel geometry)
 
      The thermal conduction resistance R of a rib and a fin is modeled as (see F. Incropera and D DeWitt,  Fundamentals of Heat and Mass Transfer,  John Wiley &amp; Sons, New York, 1990, the contents of which are incorporated by reference herein): 
 
 R   rib =length rib/ k   rib Area rib 
 
 R   fin   =f   4 ( h, k   fin , fin geometry)
 
      The terms k rib  and k fin  are the thermal conductivity of the rib and fin materials, respectively. As shown in  FIG. 5B , the heat sink can be thermally modeled as a series-parallel arrangement of resistances of ribs and fins.  
      The equations given above are a system of equations that are solved to determine the overall thermal resistance of the cooling system. This model is used to determine the heat sink geometry, gas pump and heat transport parameters that optimize the cooling system for a given design condition.  
      In one example, the heat sink of the present invention can be used with a typical CPU. In further example, the center post and fins can be fabricated from aluminum, and the thickness of the arrangement i.e. micro-fin/micro channel depth “d” is about 100 to 10,000 microns, the length “L” of the micro-fins and microchannels is about 3 to 50 mm, the width “W” of the micro-fins is about 50 to 2000 microns, the micro-channel spacing “t” is about 100 to 2000 microns, the diameter of the center post is about 5 to 50 mm, and the height of the center post (i.e. the offset between the plate and the heat source) is about 1-10 mm.  
       FIG. 6  further illustrates how the number of micro-fins  18  and ribs  24  can be selected in the above implementation example. As shown in  FIG. 6 , for this particular set of constraints, the optimal design was found to contain approximately 41 micro-fins and 12 ribs (i.e. spokes).  
      As set forth more fully above, one important aspect of the heat sink according to the present invention is an arrangement of a plurality of relatively short micro-fins and corresponding micro-channels located in portions that are offset from the heat source. It should be noted that the micro-fins and micro-channels can have many different shapes and configurations. Although  FIG. 4  shows an embodiment where the micro-fins run azimuthally in concentric circumferential rings, the invention is not limited to this example. An alternative embodiment is shown in  FIG. 7 . In this case the micro-fins  42  and corresponding micro-channels extend radially outward from a center post  44 , and can be formed in a plate-like portion  47 . Ribs  46  are also shown, and are optional.  
       FIG. 8  is a close-up view of the fins  42  of  FIG. 7  and slots/micro-channels  48  and shows a further alternative embodiment wherein shorter slots/micro-channels  49  are interspersed in between longer slots/micro-channels.  
      Another alternative embodiment is shown in  FIG. 9 . In this embodiment  50  a plurality of micro-fins  52  are configured in a plate-like structure portion  53  to form a plurality of micro-channels  54  that extend perpendicularly to a heat conducting apparatus in the form of a rectangular rib  56 .  
       FIG. 10  shows a similar concept, but with a heat pipe  58  joined to a rib portion  60  of the same thickness as the depth “d” of the fins  52 . The heat pipe  58  is used in place of the rectangular rib  56  of  FIG. 9  to deliver heat to the fins  52 , formed in a plate-like structure  62 . Other possible micro-fin and micro-channel geometries include cylindrically shaped fins/channels or irregular shaped fins/channels such as those exhibited by metal foam materials.  
      The heat sink of the present invention is ideally suited for mobile electronics cooling applications. In these cases size and weight are critical. Of particular importance in these applications is the dimension of the heat sink perpendicular to a heat source surface. Because portable devices need to be thin, the low profile heat sink of the present invention is viewed as advantageous.  
      It should be noted that the heat sink of the present invention can work alone or in conjunction with a fan or blower. Although not shown in the above figures, the heat sink can be designed to work with an axial fan directly attached to the plate and either blowing or sucking gas. The heat sink can also be designed for use with a remote fan or blower, provided that proper ducting is used to force air through the heat sink. If an axial fan is used, the arrangement of micro-channels can be designed specifically for use therewith. For example, the micro-channels can be located exclusively in the annulus opposite of the fan blades. This eliminates dead spots in the flow and allows the fan to operate at peak performance.  
      The heat sink also lends itself to being able to work with pumps integrated into the micro-channels. The short micro-channel geometry is advantageous for this type of pumping application.  
      Although the present invention has been particularly described with reference to the preferred embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications.