Patent Publication Number: US-2005135061-A1

Title: Heat sink, assembly, and method of making

Description:
FIELD OF THE INVENTION  
      The present invention relates to the field of thermal management devices and, in particular, to heat sinks for convectively cooling electrical devices and components, to assemblies utilizing these heat sinks, and to methods of making such heat sinks and assemblies.  
     BACKGROUND OF THE INVENTION  
      Semiconductors and other electrical components generate heat as a by-product of their operation. As technology has advanced, the amount of heat to be dissipated from many of these components has risen dramatically, while the acceptable cost of heat dissipating devices has remained constant or, in many cases, has dropped. Consequently, the art of heat sinking to cool heat dissipating components has continually evolved to meet these new market requirements.  
      One current need involves the cooling of IGBT semiconductors, which often have power dissipation requirements of over 500 Watts. Until now, liquid cooled heat sinks have been the only effective means for cooling many of these high power devices and, consequently, these types of heat sinks have become the fastest growing segment of the power heat sink industry. Unfortunately, liquid cooling is a last resort due to its high cost and potential for catastrophic failure in the event that leaks occur. Therefore, many designers have eschewed liquid cooling and, instead have accepted reduced performance from these devices in order to allow them to be cooled by forced air convective heat sink assemblies.  
      Forced air convective heat sink assemblies have typically used finned metal heat sinks to dissipate heat generated by electrical components. These finned metal heat sinks generally include a substantially rectangular base plate to which the heat generating device or devices are mounted, and a plurality of fins projecting from the base plate for dissipating the generated heat. In many applications, a fan is attached to the assembly in order to force cooling air across the fins of the heat sink and enhance cooling from the heat sinks. In these applications, the amount of heat that may be dissipated from a heat sink of given volume at a given air velocity is directly related to the efficiency of the heat sink.  
      Heat sink efficiency is defined as thermal performance generated per given volume. An efficient heat sink provides substantial cooling, while consuming a small physical volume. In general, the more surface area the heat sink has, the more heat you can typically transfer from the component. However, in many applications, other factors come into play that can limit the effectiveness of any increase in heat sink surface area. One such factor is the flow profile of the fluid at its interface with the heat dissipating surfaces. In many cases, the fluid flowing is along fins of a finned heat sink will form a boundary layer having substantially laminar flow. As fluid flowing in this fashion is relatively poor at removing heat, these boundary layers tend to increase in temperature, with heat being primarily dissipated by the turbulent air flowing adjacent to this layer. Boundary layers are especially troublesome when fins are spaced closely together, as the boundary layers formed on adjacent fins tend to overlap along the bottom portion of the trough created by the adjacent fins and the base, causing what is commonly referred to as “choking”. This choking limits the surface area of the boundary layer that is in contact with the flow of turbulent fluid and, consequently, limits the overall thermal performance of the heat sink.  
      One common means of reducing the effect of choking in finned heat sinks has been to utilize a plurality of “pin fins”, which extend from the base and have spaces therebetween that act to break up any boundary layers that would be formed on long, straight fins. Pin fin heat sinks come in many forms and may or may not appear as individual pins. For example, some heat sinks utilize traditional finned extrusions that are cross cut to produce short finned sections broken up by spaces. Others are cast to have substantially cylindrical extending pins. Others are impact extruded to create a variety of unique configurations. Still others are manufactured through skiving and broaching operations, or by fully machining the desired profile. Regardless of their particular configuration, the common thread is that the spaces between the pins, sections of fin, etc. act to reduce the thickness of boundary layers about each pin and increase the amount of turbulent air flowing there between. This reduction in boundary layer thickness generally allows pins to be more densely spaced than straight fins, without choking, resulting in increased effective surface area and increased heat sink efficiency.  
      Unfortunately, pin fin type heat sinks also have distinct limitations. The most significant of these limitations is caused by conduction losses from the heat source though the pins. Conduction is the process of transferring heat through a specific medium without perceptible motion of the medium itself. When applied to heat sinks, this conduction occurs through molecule to molecule contact and, accordingly, can be said to follow a substantially linear path from the heat source to the tips of the fins or pins. At each of these molecules along the way, the amount of heat transferred from one molecule to the other is dependent upon the thermal conductivity of the material. Materials having high thermal conductivities tend to transfer heat more efficiently, meaning that the adjacent molecule becomes hotter than it would were the material a poor conductor. However, even the best conducting metals are not perfect conductors and, therefore, the temperature of a metal heat sink will always be higher at its base than it is at the tips of its fins. Because heat transfer is higher when the temperature difference between the air and the hot surface is greater, and the fins or pins are incrementally cooler the further they are from heat source, any increase in fin or pin height will have an incrementally reduced effect upon the thermal performance of the heat sink, and consequently, will result in a decrease in heat sink efficiency.  
      Therefore, there is a need for a heat sink that will efficiently cool heat-generating equipment. It is likewise recognized that, to increase heat sink efficiency, there is also a need to reduce the thickness of boundary layers between fins or pins and to reduce conduction losses through the fins or pins.  
     SUMMARY OF THE INVENTION  
      In its most basic form, the present invention is a heat sink having a base from which a plurality of heat pipes extends to form the surfaces from which heat is convected.  
      A heat pipe is a simple heat-exchange device that relies upon the boiling and condensation of a working fluid in order to transfer heat from one place to another. The basic principle behind all heat pipes is that a large amount of heat is required in order to change a fluid from a liquid to a gas. The amount of heat required to effect this phase change in a given fluid is referred to as the “latent heat of vaporization”. Similarly, because the second law of thermodynamics states that energy may not be lost, but may only be transferred from one medium to another, the energy that is absorbed by the fluid during its change to a gas is subsequently released when the gas is condensed back into a liquid. Because the latent heat of vaporization is usually very high, and the vapor pressure drop between the portion of the heat pipe in which the fluid is boiled and the portion where is it condensed is very low, it is possible to transport high amounts of heat from one place to another with a very small temperature difference from the heat source to the location of condensation. In fact, at a given temperature difference, a heat pipe is capable of conducting up to one hundred and fifty times as much heat as a solid copper pipe of equal cross section, and as much as three hundred times as much heat as an aluminum member of equal cross section. Therefore, heat pipes have traditionally been used to efficiently transfer heat from one point to another in applications where there is limited physical space to effect such cooling proximate to the heat source.  
      The present invention uses heat pipes in a manner in which they have not heretofore been utilized; i.e. as the primary convective surfaces of the heat sink. As noted above, the basic embodiment of the heat sink of the present invention includes a base and a plurality of heat pipes that extend from the base. The base is dimensioned and shaped to promote good thermal contact with the heat source, and the heat pipes are attached thereto in such a manner as to promote good thermal contact to the working fluid. Each heat pipe includes an outer surface and an inner surface that form a condenser portion from which from heat is transferred during condensation of the working fluid. In some embodiments, each heat pipe is a closed system that includes its own working fluid and an evaporator portion that is in contact with the heat sink base. However, in other embodiments the heat pipes share a common reservoir of working fluid, preferably located within the base plate, and do not include individual evaporator portions  
      The type, number, and layout of the heat pipes extending from the base are largely a function of the application in which the heat sink is to be used. For example, in forced convection applications, where the velocity of the air tends to reduce the thickness of the boundary layers surrounding the heat pipes, the pipes are spaced more closely together. Conversely, in natural convection applications, in which airflow is not forced over the heat pipes and boundary layers surrounding each pipe are thicker, the heat pipes are preferably spaced farther apart from one another. Regardless of their application, heat sinks in accordance with the present invention will always include a plurality of heat pipes that each convect heat from a substantial portion of their outer surface area. These heat pipes are spaced primarily to maximize conduction based upon the conductivity of the base, allowing pins to be spaced such that they are placed were they are needed; ex. directly above high heat sources. In addition, heat pipes need not be the only convective surfaces and may be augmented through the use of additional metal fins, pins, or other art recognized convective surfaces.  
      In some embodiments of the invention, the heat pipes are merely pressure vessels having a working fluid disposed therein that simply exploits gravitational forces to return condensed fluid flow to the evaporator portion thereof. In these embodiments, the heat sink assembly is dimensioned for mounting such that, in operation, the heat source is at a lower elevation than the condenser portions of the heat pipes. In other embodiments, however, the heat pipes utilize wicks or other fluid transport means for transporting the condensed fluid to their evaporator portions. In these embodiments, the relationship between the assembly and the heat source is irrelevant, allowing the heat sink to be mounted in a variety of orientations.  
      The outer surfaces of each heat pipe are preferably sized and shaped to maximize heat transfer therefrom. In some embodiments, these outer surfaces have dimples, bumps, grooves, or other means for reducing the thickness of the boundary layer formed thereon. In other embodiments, appendages, such as fins, are affixed to the outer surfaces of the heat pipes in order to increase the surface area thereof. The preferred appendages are merely a plurality of flat cylindrical fins that extend from the outer surface of each heat pipe. However, other embodiments include appendages that extend between, and are affixed to, at least two heat pipes. Regardless of their number and orientation, it is recognized that each appendage is attached to an outer surface of the heat pipe in such a way as to promote good thermal contact and, thereafter, is considered to be a part of the heat pipe itself.  
      In some embodiment of the system, other heat convective surfaces are disposed upon and extend from the base plate in order to augment the cooling provided by the heat pipes. These convective surfaces may be pins, fins, or other art recognized means for convecting heat from a heat sink and are preferably located upon the base plate in a location in which conduction losses will not significantly affect their efficiency.  
      The basic embodiment of the heat sink assembly of the present invention includes the basic embodiment of the heat sink discussed above and a means for forcing air over the heat pipes. The means for forcing air over the heat pipes is preferably a fan or blower that is mounted directly to the heat sink in a desired orientation. In some embodiments, the fan is mounted to the heat sink by attaching a pair of side plates to the outside edges of the base plate and attaching a fan to these side plates. It is preferred that the fan be mounted to the side plates such that air flows in a direction parallel to the plane formed by the base plate. In these embodiments of the assembly, is preferred that appendages, such as fins, be disposed from the outer surfaces of the heat pipes. However, in some embodiments of the assembly, the fan is mounted such that air flows perpendicular to, and impinges upon, the base plate. In these embodiments, outer surfaces having bumps, dimples, grooves or the like are preferred over those having fins or other appendages.  
      In some embodiments of the assembly, the heat source is an integral part thereof. Accordingly, the present invention contemplates heat sink assemblies in which components are mounted to the base plate, or the base plate forms part of the heat generating device or component itself. For example, the base plate could form an integral part of the housing of a power supply, be laminated to a printed circuit board, or otherwise integrated with the heat source itself.  
      The present invention also includes a method for making the heat sinks described above. The first step in this method is to obtain a base plate having good thermal conductivity. A plurality of heat pipe receiving details is formed within the base plate. These details may be depressions into, holes through, or other details within the base plate that are dimensioned to allow a heat pipe to be received thereby. Heat pipes of sufficient quantity and size to be received by all receiving details are obtained and are disposed within these details. The heat pipes are then secured with the receiving details such that the heat pipe is in good thermal contact with the base plate. In some embodiments, this securing step involves press fitting the heat pipe into the receiving detail with a suitable thermal interface material, such as thermal grease, disposed therebetween. In others, the heat pipes are fixtured after they are disposed within the receiving details and secured by epoxy bonding, soldering, or other art recognized means.  
      Some embodiments of the method further include the step of disposing at least one appendage about the outer surface of at least one heat pipe. Others include forming a reservoir within the base plate and in communication with at least two heat pipes and disposing a working fluid therein. In these embodiments, it is preferred that the base plate include two portions that are affixed together and sealed after the reservoir is formed therebetween.  
      Therefore, it is an aspect of the present invention to provide a heat sink that uses air convection to cool electrical devices and components, such as SCR&#39;s , Transistors, Diodes, IGCT&#39;s and IGBT&#39;s, having power dissipation requirements of over 100 Watts.  
      It is a further aspect of the present invention to provide a highly efficient heat sink that minimizes conduction losses, and hence temperature differences, between the heat sink base and its conductive surfaces.  
      It is a further aspect of the present invention to provide a heat sink and method of making that allow the heat sink to be manufactured from standard, “off the shelf”, heat pipes and base plate stock.  
      It is a still further aspect of the present invention to provide a heat sink that is capable of distributing high heat loads.  
      It is a still further aspect of the present invention to provide a heat sink that allowing a matching of heat sources and heat sinks with differing thermal characteristics.  
      It is a still further aspect of the present invention to provide a heat sink capable of reducing overall system size and costs from those currently available.  
      It is a still further aspect of the present invention to provide a heat sink assembly that does not require active liquid cooling to dissipate large amounts of power from a heat generating component or device.  
      It is a still further aspect of the present invention to provide a heat sink assembly that may be used in forced air and forced liquid convection cooling systems.  
      It is a still further aspect of the invention to provide a heat sink and heat sink assembly in which additional convective surfaces may be used in order to reduce cost and tailor performance of the heat sink to a particular application.  
      These aspects of the invention are not meant to be exclusive and other features, aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side view of one embodiment of the heat sink of the present invention.  
       FIG. 2  is a top view of the heat sink of  FIG. 1 .  
       FIG. 3  is as cut away side view of one embodiment of a heat pipe used in connection with the heat sink of the present invention demonstrating its operation.  
       FIG. 4  is a side view of an embodiment of the heat sink of the present invention that includes heat pipes from which a plurality of fins extends.  
       FIG. 5  is a top view of the heat sink of  FIG. 4 .  
       FIG. 6  is a top isometric view of an alternative embodiment of the heat sink of the present invention in which U-shaped heat pipes are disposed and secured with the base plate.  
       FIG. 7  is a cut away end view of the heat sink of  FIG. 6 .  
       FIG. 8  is a top isometric assembly view of an alternative embodiment of the heat sink of the present invention in which the pipes have profiled end that are disposed and secured within recesses in the base plate using compressive mounting plates.  
       FIG. 9  is a cut away side view of an alternative embodiment of the heat sink of the present invention in which all heat pipes are in fluid communication with a central reservoir of working fluid disposed within the base plate.  
       FIG. 10A  is a top isometric view of one embodiment of the heat sink assembly of the present invention showing the fan mounted such that air if moved in substantially parallel relation to the base plate.  
       FIG. 10B  is a side view of the heat sink assembly of  FIG. 10A .  
       FIG. 10C  is a top view of the heat sink assembly of  FIGS. 10A and 10B .  
       FIG. 11  is a bottom isometric view of another embodiment of the heat sink assembly of the present invention utilizing two fans. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring first to  FIGS. 1 and 2 , one embodiment of the heat sink  10  of the present invention is shown. The heat sink  10  includes a base plate  12  and a plurality of heat pipes  14  that extend from the top surface  15  of the base plate  12 . The base plate  12  has a bottom surface  13  that is dimensioned and shaped to promote good thermal contact with the heat source (not shown). The base plate  12  is manufactured of a material, such as copper or aluminum, that has relatively good thermal conductivity, and should be of sufficient thickness to efficiently spread the heat from a heat source (not shown) disposed upon its bottom surface  13  to the heat pipes  14  extending from its top surface  15 . In many of the embodiments shown herein, the base plate  12  is portrayed as a substantially solid rectangular plate. However, it is recognized that base plates  12  having different shapes and/or cross sections may be utilized and the present invention should not be viewed as being limited to heat sinks  10  having rectangular base plates  12 .  
      The heat pipes  14  may take many forms, and virtually any type of heat pipes  14  currently available could be joined to the top surface  15  of the base plate  12  to form the heat sink  10  of the present invention. As shown in  FIG. 3 , one type of heat pipe  14  that could be used includes a closed pressure vessel  20  having an outer surface  22  and an inner surface  24 , and in which a working fluid, in the form of a liquid  26 , is disposed. The liquid  26  is disposed in the evaporator portion  30  of the vessel, where it is heated and changes phase into a gaseous working fluid  34 . The gaseous working fluid  34  then fills the remaining interior of the vessel  20 , which forms the condenser portion  32  thereof. Because the outer surface  22  of the vessel  20  surrounding the condenser portion  32  is cooler then the interior of the vessel  20 , heat flows from the inner surface  24  to the outer surface  22 , where is it convectively removed from the system. This transfer of this heat is accomplished through condensation of the gaseous working fluid  34 , which releases the latent heat of vaporization from the fluid  34  and forms droplets of condensate  36  along the inner surface  24  of the vessel  20 . The condensate  36  is then transported by gravitational forces back into the evaporator portion  30  of the vessel  20  and mixes with the liquid  26 , where the cycle is repeated.  
      As demonstrated by the above description, the vessel  20  isolates the working fluid  26 ,  34 ,  36  from the outside environment. By necessity, the vessel  20  must be leak-proof, maintain the pressure differential across its walls, and enable transfer of heat to take place from and into the working fluid. Selection of a fabrication material for the vessel  20  depends on many factors including chemical compatibility, strength-to-weight ratio, thermal conductivity, ease of fabrication, porosity, etc. Once filled with the working fluid  26 , the vessel  20  is preferably evacuated to eliminate any pockets of air that might otherwise prevent the flow of the gaseous working fluid  34  to substantially the entire inner surface  24  of the condenser portion  32  of the vessel  20 .  
      Working fluids  26  are many and varied. The prime consideration is the selection of the working fluid  26  is operating vapor temperature range. Often, several possible working fluids  26  may exist within the approximate temperature band. Various characteristics must be examined in order to determine the most acceptable of these fluids for the application considered such as good thermal stability, compatibility with wick and wall materials, vapor pressure relative to the operating temperature range, high latent heat, high thermal conductivity, liquid phase viscosities and surface tension, and acceptable freezing or pour point, to name a few. The selection of the working fluid  26  must also be based on thermodynamic considerations, which are concerned with the various limitations to heat flow occurring within the heat pipe such as, viscous, sonic, capillary, entrainment and nucleate boiling levels. Many conventional heat pipes use water and methanol as working fluid, although other more exotic materials, such as fluorocarbons, may also used.  
      The heat pipe  14  described in connection with  FIG. 3  is a basic design that requires the heat sink  10  to be orientated such that gravity will return the condensate  36  to the evaporator portion  30 . However, other embodiments of the invention utilize heat pipes  14  having internal wicks (not shown), or other fluid transport means for transporting the condensate  36  to their evaporator portions  30 . A typical wick is a porous structure, made of materials like steel, aluminum, nickel or copper in various pore size ranges. Wicks are typically fabricated using metal foams, and more particularly felts, with the latter being more frequently used. By varying the pressure on the felt during assembly, various pore sizes can be produced. By incorporating removable metal mandrels, an arterial structure can also be molded in the felt. The prime purpose of the wick is to generate capillary pressure to transport the condensate  36  from the condenser portion  32  of the vessel to the evaporator portion  30  proximate to the heat source (not shown). It must also be able to distribute the liquid  26  around the evaporator portion  30  to any area where heat is likely to be received by the heat pipe  14 . Often these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid. However, such-selection is an art unto itself and, therefore, is not discussed herein.  
      Referring again to  FIGS. 1 and 2 , regardless of their type, the heat pipes  14  are preferably arranged such that the boundary layers formed thereon will not overlap at the airflows and working temperatures anticipated for a given application. As shown in  FIGS. 1 and 2 , the heat pipes  14  are arranged in a rectangular four by four pattern forming rows and columns of spaces between heat pipes  14 . This arrangement is a good one for use in natural convection environments, and is also preferred in applications using impingement air flow, as the rows and columns reduce the pressure drop created by airflow, promoting good airflow away from the heat sink. However, in other embodiments, such as those in which the airflow is parallel to the base plate, the heat pipes  14  may be arranged in a staggered arrangement in order to induce additional turbulence to the airflow and decrease the thickness of the boundary layers upon the outer surface of each heat pipe  14 .  
      As described herein, the heat pipes  14  may be attached to the base plate  12  in many ways. For example, in the embodiment of  FIG. 1 , the heat pipes  14  are simply press fit into holes  17  bored through the base plate  14  such that the evaporator portion thereof is in sufficient thermal contact with the base plate to promote boiling of the working fluid disposed therein.  
       FIGS. 4 and 5  show an alternative embodiment of the heat sink  10  for use in applications in which airflow is disposed parallel to the top surface  15  of the base plate  12 . This embodiment includes a similar base plate  12 , having top and bottom surfaces  15 ,  13 , and a similar arrangement of heat pipes  14 , as the embodiment of  FIGS. 1 and 2 . However, in this embodiment, each of the heat pipes  14  includes a plurality of fins  16  that extend from the outer surface  22  of the condenser portion  32  thereof. These fins  16  are preferably manufactured of a conductive material, such as copper or aluminum, and are affixed to the outer surface  22  of the heat pipe  13  in such a manner as to promote good heat flow therefrom such that the fins  16  can be said to form an integral part of each heat pipe  14 . This may be accomplished through a number of art recognized processes, including brazing, soldering, epoxy bonding, press fitting, mechanical or other means. The fins  16  are spaced apart from one another a distance that is determined by the nature of the airflow between these spaces.  
       FIGS. 6 and 7  show a similar embodiment of the heat sink to that shown and described with reference to  FIGS. 4 and 5 . However, in this embodiment, the heat pipes  14  are substantially U-shaped such that two condenser portions  32  are in communication with a single evaporator portion  30  at the bottom of the U-portion of the heat pipe  14 . The evaporator portions  30  of each heat pipe  14  may be affixed to the base plate  12  in a number of ways. As shown in  FIGS. 6 and 7 , this is accomplished by forming mating grooves  44  in the top surface  15  of the base plate  12 , disposing the U-portion of the each heat pipe  14 , and securing the heat pipes into the grooves  44  via mechanical fasteners, such as a bar  42  and screws  43 . However, in other such embodiments, the U-portions of the heat pipes  14  are affixed by soldering, brazing, press fitting, epoxy bonding, or other art-recognized means for securing a U-shaped object into a flat plate.  
      Referring now to  FIG. 8 , another embodiment of the heat sink  10  is shown. This heat sink is similar to that of  FIGS. 4 and 5 , as it includes a similar base plate  12 , having top and bottom surfaces  15 ,  13 , and a similar arrangement of heat pipes  14  from which a plurality of fins  16  extend. However, the base plate  12  of this embodiment includes a plurality of bores  50  having shaped inner surfaces  52  machined in its top surface  15 , and the heat pipes  14  each include base evaporator portions  30  that are formed with outer surfaces  31  shaped to mate with the inner surfaces  52  of the bores  50 . The interface between the outer surfaces  31  of the evaporator portions  30  and the interior surfaces  52  of the bores  50  may be enhanced through the use of known thermal interface materials, thermally conductive epoxy or the like. However, in some applications, such as where the base plate and heat pipes are manufactured of copper or other soft materials, no interface material is used and, instead, the deformation of the two surfaces  31 ,  52  together forms a highly conductive interface. Regardless of how the interface is made, the heat pipes  14  of this embodiment are held into place, at least during assembly, by hold down plates  56  having bores  64  therethrough of a larger diameter than the body of the heat pipe  14  and smaller diameter than the evaporator portions  30  thereof. The plates  56  are compressed against the evaporator portions  30  by screws  58 , which are secured into mating threaded bores  60  in the top surface  15  of the base plate  12 , and act to exert downward pressure causing the interface surfaces  31 ,  52  to be drawn together.  
      Referring now to  FIG. 9 , still another embodiment of the heat sink  10  is shown. In this embodiment, each heat pipe  14  is linked to a common evaporator portion  30  within the base plate  12 , which contains the liquid working fluid  12 . In this embodiment, the base plate  12  is preferably manufactured of two pieces that are joined together such that the will withstand the pressure generated by the evaporation of the working fluid  26 . The evaporator portion  30  is preferably proximate to the bottom surface  13  of the base plate and is preferably filled with liquid  26  to a level such that provides an open space between the level of the liquid and the openings leading to the condenser portion  32  of each heat pipe  14 . The condenser portions  32  of each heat pipe are embedded into the top surface  15  of the base plate  12  and are sealed thereto such that they will likewise withstanding the working pressure of the system. In operation, the heat pipes  14  will function in the same manner as described above. However, by eliminating the interface between the base plate  12  and liquid  26  within the heat pipe  14 , the overall efficiency of the heat sink  10  is enhanced.  
      Referring now to  FIGS. 10A-10C , one embodiment of a heat sink assembly  100  of the present invention is shown. The heat sink assembly  100  is similar in all essential respects as those described above and includes a heat sink  10  having the same base plate  12  from which heat pipes  14  extend. Further, the heat pipes  14  each have the extending fins  16  that were described with reference to  FIGS. 4 and 5 . However, the heat sink  10  in this case also includes a plurality of fins  102  that likewise extend from the top surface  15  of the base plate  12 .  
      The fins  102  provide additional cooling capacity at lower cost than could be achieved using all heat pipes  14 . Here, the fins  102  are disposed directly below three heat-generating components  104 ,  106 ,  108 , which are mounted to the bottom surface  13  of the base plate  12 . A fourth heat-generating component  110  is also mounted to the bottom surface  13  of the base plate  12  proximate to the location of the heat pipes  14 . For purposes of this embodiment, the fins  102  and heat pipes  14  are disposed in their respective locations upon the base plate  12  because the fourth component  110  has a high power dissipation requirement, while the three others  104 ,  106 ,  108  do not. In such an embodiment, this arrangement is preferred as the heat pipes  14  are most useful when in close proximity to the high heat source, here the fourth component  110 , while the location of both directly proximate to the air outlet  122  from the fan  120  insures a maximum temperature difference between the air flowing from the fan  120  and the surfaces of the heat pipes  14  and fins  16 . However, other arrangements are possible, including those with multiple groups of heat pipes  14  and fins  102 , provided the heat pipes  14  are disposed in closer proximity to the highest heat sources than the fins.  
      The heat sink assembly  100  of this embodiment includes a pair of side panels  130 ,  132  attached to the sides of the base plate  12 . The side panels  130 ,  132  are dimensioned to extend beyond the end of the base plate  12  and attach to the fan  120 . The base plates  130 ,  132  are dimensioned for mounting to a chassis or other surface such that the side panels  130 ,  132 , base plate  12  and the surface form a duct through which air is blown by the fan  120 . However, in other embodiments, the fan  120  is mounted such that it blows air downward in an impingement arrangement. In these embodiments, the fins  16  are eliminated from the heat pipes  14  and may or may not be replaced by other surface enhancements that are effective in impingement cooling applications.  
      Referring now to  FIG. 11 , still another embodiment of the heat sink assembly  100  is shown. This embodiment utilizes the same side panels  130 , 132  and fan  120  as the assembly of  FIGS. 10A-10C , but has no fins and utilizes a second fan  120  at the other end of the assembly  100 . This fan  122  preferably moves air in the same direction as the other fan  120 , creating a push/pull effect upon the air passed over the heat pipes  14 . As was the case with the heat pipes of  FIGS. 10A-10C , the heat pipes  14  of this embodiment likewise utilize radial fins  16  disposed parallel to the direction of flow.  
      Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions would be readily apparent to those of ordinary skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.