Patent Publication Number: US-7911790-B2

Title: Electronic assemblies with high capacity curved and bent fin heat sinks and associated methods

Description:
DIVISIONAL APPLICATION 
     The present application is a divisional of U.S. patent application Ser. No. 10/716,764, filed on Nov. 19, 2003 now U.S. Pat. No. 7,120,020, which is a divisional of U.S. patent application Ser. No. 09/950,100, filed on Sep. 10, 2001, now U.S. Pat. No. 6,671,172, which are both incorporated herein by reference. 
    
    
     RELATED APPLICATIONS 
     The present application is related to the following applications that are assigned to the same assignee as the present application: 
     Ser. No. 09/716,510, now U.S. Pat. No. 6,633,484, entitled “Heat Dissipating Devices, Systems, and Methods with Small Footprint”; 
     Ser. No. 09/766,757, now U.S. Pat. No. 6,535,385, entitled “High-Performance Heat Sink Configurations For Use In High Density Packaging Applications”; 
     Ser. No. 09/800,120, entitled “Radial Folded Fin Heat Sink”; 
     Ser. No. 09/860,978, now U.S. Pat. No. 6,479,895, entitled “High Performance Air Cooled Heat Sinks Used In High Density Packaging Applications”; 
     Ser. No. 10/047,101, entitled “Heat Sinks and Methods of Formation” 
     Ser. No. 09/950,898, now U.S. Pat. No. 6,705,144, entitled “A Manufacturing Process for a Radial Fin Heat Sink”; 
     Ser. No. 09/950,101, now U.S. Pat. No. 6,657,862, entitled “Radial Folded Fin Heat Sinks and Methods of Making and Using Same”; and 
     Ser. No. 10/656,968, entitled “Electronic Assemblies with High Capacity Heat Sinks and Methods of Manufacture.” 
     TECHNICAL FIELD 
     The inventive subject matter relates generally to electronics packaging and, more particularly, to several embodiments of an electronic assembly that includes a high-performance electronic component and a high capacity heat sink, and to manufacturing methods related thereto. 
     BACKGROUND INFORMATION 
     Electronic components, such as integrated circuits (ICs), are typically assembled into packages by physically and electrically coupling them to a substrate, such as a printed circuit board (PCB), to form an “electronic assembly”. The “electronic assembly” can be part of an “electronic system”. An “electronic system” is broadly defined herein as any product comprising an “electronic assembly”. Examples of electronic systems include computers (e.g., desktop, laptop, hand-held, server, Internet appliance, etc.), wireless communications devices (e.g., cellular phones, cordless phones, pagers, etc.), computer-related peripherals (e.g., printers, scanners, monitors, etc.), entertainment devices (e.g., televisions, radios, stereos, tape and compact disc players, video cassette recorders, MP3 (Motion Picture Experts Group, Audio Layer 3) players, etc.), and the like. 
     In the field of electronic systems there is an incessant competitive pressure among manufacturers to drive the performance of their equipment up while driving down production costs. This is particularly true regarding the packaging of ICs on substrates, where each new generation of packaging must provide increased performance, particularly in terms of an increased number of components and higher clock frequencies, while generally being smaller or more compact in size. 
     As the internal circuitry of ICs, such as processors, operates at higher and higher clock frequencies, and as ICs operate at higher and higher power levels, the amount of heat generated by such ICs can increase their operating temperature to unacceptable levels, degrading their performance or even causing catastrophic failure. Thus it becomes increasingly important to adequately dissipate heat from IC environrments, including IC packages. 
     For this reason, electronic equipment often contains heat dissipation equipment to cool high-performance ICs. One known type of heat dissipation equipment includes an impinging fan mounted atop a heat sink. The heat sink comprises a plurality of radial fins or rods formed of a heat-conductive material such as copper or aluminum formed around a core. The bottom surface of the core is in thermal contact with the IC to conduct heat from the IC to ambient air. The fan moves air over the fins or rods to enhance the cooling capacity of the heat dissipation equipment. However, with high-performance ICs consuming ever greater amounts of power and accordingly producing greater amounts of heat, heat dissipation equipment must have higher heat dissipation capability than that heretofore obtained. 
     In order to offer higher capacity heat transfer, new heat dissipation equipment must be more efficient. It is difficult for air-cooled heat sinks to grow in size, because equipment manufacturers are under tremendous competitive pressure to maintain or diminish the size of their equipment packages, all the while filling them with more and more components. Thus, competitive heat dissipation equipment must be relatively compact in size and must perform at levels sufficient to prevent high-performance components from exceeding their operational heat specifications. 
     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 significant need in the art for apparatus and methods for packaging high-performance electronic components in an electronic assembly that minimize heat dissipation problems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art electronic assembly including a heat sink attached to an IC package; 
         FIG. 2  is a top view of a prior art radial fin heat sink; 
         FIG. 3  is a top view of the portion within dashed rectangle  22  of  FIG. 2 , showing an air flow pattern within fins of a prior art radial fin heat sink; 
         FIG. 4  is a side view of a section, taken between dashed line segments  24  and  25  of  FIG. 2 , of a prior art radial fin heat sink positioned upon an IC package; 
         FIG. 5  illustrates a perspective view of a curved fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 6  illustrates a top view of the curved fin heat sink shown in  FIG. 5 ; 
         FIG. 7  illustrates a perspective view of an electronic assembly including a curved fin heat sink positioned upon an IC package, in accordance with an embodiment of the inventive subject matter; 
         FIG. 8  illustrates a perspective view of a portion of an electronic assembly including an axial flow fan atop a curved fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 9  illustrates a top view of the portion within dashed rectangle  56  of  FIG. 6 , showing an air flow pattern within fins of a curved fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 10  illustrates a side view of a section of the curved fin heat sink shown in  FIG. 6 , taken between dashed line segments  51  and  53 ; 
         FIG. 11  illustrates a perspective view of a bent fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 12  illustrates a top view of a bent fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 13  illustrates a perspective view of an electronic assembly including a bent fin heat sink positioned upon an IC package, in accordance with an embodiment of the inventive subject matter; 
         FIG. 14  illustrates a schematic view of a fan, including its tangential and axial air flow components, and a side view of a bent fin heat sink as positioned upon a sectioned IC package on a substrate, in accordance with an embodiment of the inventive subject matter; 
         FIG. 15  illustrates a perspective view of a curved-bent fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 16  illustrates a top view of a curved-bent fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 17  illustrates a perspective view of an electronic assembly including a curved-bent fin heat sink positioned upon an IC package, in accordance with an embodiment of the inventive subject matter; 
         FIG. 18  illustrates an air flow pattern for a prior art radial fin heat sink; 
         FIG. 19  illustrates an air flow pattern for a bent fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 20  illustrates an air flow pattern for a curved-bent fin heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 21  illustrates a flow diagram of a method of fabricating a heat sink, in accordance with an embodiment of the inventive subject matter; 
         FIG. 22  illustrates a flow diagram of a method of fabricating an electronic assembly, in accordance with an embodiment of the inventive subject matter; and 
         FIG. 23  is a block diagram of an electronic system incorporating at least one electronic assembly with at least one high capacity heat sink, in accordance with an embodiment of the inventive subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of some exemplary embodiments of the inventive subject matter, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration, but not of limitation, some specific embodiments in which the inventive subject matter may be practiced, including a preferred embodiment. These embodiments are described in sufficient detail to enable those skilled in the art to understand and practice them, and it is to be understood that other embodiments may be utilized and that structural, mechanical, compositional, and procedural changes may be made without departing from the spirit and scope of the inventive subject matter. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of embodiments of the inventive subject matter is defined only by the appended claims. Such embodiments of the inventive subject matter may be referred to, individually and/or collectively, herein by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. 
     The inventive subject matter provides a solution to thermal dissipation problems that are associated with prior art packaging of integrated circuits that have high circuit density and that operate at high clock speeds and high power levels, by employing a high capacity heat sink. Various embodiments are illustrated and described herein. 
     In one embodiment, the heat sink comprises a thermally conductive core. The core has a number of thermally conductive fins projecting from it in a substantially radial fashion. The core can have a central cavity into which a thermally conductive material is inserted. The heat sink fins can be formed in various shapes. In one embodiment, the fins are curved. In another embodiment, the fins are bent. In yet another embodiment, the fins are curved and bent. 
     In one embodiment, the heat sink can be used in an electronic assembly having an impinging fan, e.g. an axial flow fan, directing air onto an upper face of the heat sink. The lower face of the heat sink is in thermal contact with a heat-generating electronic component such as a high performance IC. The heat sink is structured to capture air from the fan and to direct the air to optimize heat transfer from the heat sink. 
     Various methods of fabricating heat sinks and electronic assemblies are also described. 
       FIG. 1  is a perspective view of a prior art electronic assembly  1  including a heat sink  2  attached to an IC package  5 . Electronic assembly  1  comprises a plurality of electronic components  5 - 9  mounted upon a printed circuit board (PCB)  3 . Heat sink  2  comprises a relatively thick, flat base plate  12  and an array of fins  11  extending to the edge of and substantially perpendicular to the base plate  12 . Although the fins  11  shown in  FIG. 1  are folded fins, other prior art heat sinks do not have folded fins. For example, it is known in the prior art to use brazed, machined, or extruded solid fins. Base plate  12  is clamped to IC package  5  through an attachment device  13 . Base plate  12  is often formed of solid copper, and it contributes a significant amount of cost and mass to electronic assembly  1 . 
     While the sizes of packaged, high performance ICs are decreasing, the amount of heat generated by these components per unit volume is increasing. Increasing the heat dissipation capabilities of the prior art heat sink  2  would require enlarging the surface area of the base plate  12  and/or the array of fins  11 . This in turn would result in consuming more PCB real estate, which is generally not a viable option in an environment where system packaging densities are increasing with each successive, higher performance, product generation. 
     Prior art heat sink  2  illustrated in  FIG. 1  can be used in conjunction with an axial flow fan (not shown in  FIG. 1 ) to increase heat dissipation from the array of fins  11 . An axial flow fan has a spinning impeller that is generally shaped like an airfoil. One component of the air flow emanating from an axial flow fan moves parallel to the axis about which the impeller rotates, and this “axial component” is directed normal to the array of fins  11  of the heat sink  2 , i.e. perpendicular to the PCB  1 . (Refer to axial component  132  in  FIG. 14 .) 
     Another component of the air flow from an axial flow fan is tangential to the impeller&#39;s direction of rotation. This “tangential component” results in air swirling about the impeller&#39;s axis of rotation. (Refer to tangential component  130  in  FIG. 14 .) The ratio of air being moved by the axial component versus the tangential component varies with the particular fan blade geometry. For example, low angles of attack in the fan blade generally result in a higher ratio of axial flow, while high angles of attack generally result in a higher ratio of tangential flow. In some axial flow fans, the ratio is 1:1. 
     When an axial flow fan is mounted facing downward on prior art heat sink  2 , its axial component of air flow provides substantially all of the cooling effect, because very little of the tangential component of air flow is captured by the straight vertical fins  11 . 
       FIG. 2  is a top view of a prior art radial fin heat sink  20 . Heat sink  20  is referred to as a “radial fin heat sink”, because its fins  21  emanate radially from a central core  41 . Fins  21  are substantially straight, and the base of each fin  21  is attached to core  41  parallel to a central axis  42  (refer to  FIG. 4 ). Referring back to  FIG. 2 , core  41  can have a central cavity  23 , and a thermal plug  40  of thermally conductive material can reside within cavity  23  to enhance thermal dissipation. 
       FIG. 3  is a top view of the portion within dashed rectangle  22  of  FIG. 2 , showing an air flow pattern within fins of the prior art radial fin heat sink  20  shown in  FIG. 2 . In  FIG. 3 , a tangential air flow component  29  from an axial flow fan (not shown) impinges upon fins  26  and  27 . 
     Before discussing tangential air flow component  29 , it should be first noted that fins  26  and  27  are substantially perpendicular to core  41 , and that fins  26  and  27  diverge considerably as they emanate from core  41 . The radius  43  at the base of fins  26  and  27  is substantially smaller than the fin tip distance  28  at the tips of fins  26  and  27 . 
     Tangential air flow component  29  impinges against the fins of prior art radial fin heat sink  20 , such as fins  26  and  27 . A major portion  30  of tangential air flow component  29  moves outwardly towards the tips of fins  26  and  27 . A smaller portion  33  of tangential air flow component  29  moves inwardly towards the bases of fins  26  and  27 . 
     Due to the diverging geometry of fins  26  and  27 , air flow from the tangential component  29 , as well as air flow from the axial component (not seen in  FIG. 3 ), moves towards the fin tips to escape the region between adjacent fins  26  and  27 , and thus little air flow reaches the hottest part of fins  26  and  27  near core  41 . This results in inefficient thermal dissipation. Consequently, a more powerful and noisier fan must be substituted, or the electronic component will not be sufficiently cooled to avoid performance degradation or catastrophic failure. 
       FIG. 4  is a side view of a section, taken between dashed line segments  24  and  25  of  FIG. 2 , of a prior art radial fin heat sink  20  positioned upon an IC package  34 . Fins  31  and  32  are on opposite sides of heat sink  20 . The lower surface of thermal plug  40  is in thermal contact with the upper surface of a heat-producing IC package  34 . Heat, represented by arrows  35 , is transferred from IC package  34  into thermal plug  40 . From thermal plug  40 , heat is transferred through sidewall  38  of cavity  23  to fin  31  (the heat sink core has been omitted to simplify this illustration), and through sidewall  39  of cavity  23  to fin  32 . The hottest part of fins  31  and  32  is nearest the thermal plug  40 . 
     A group  36  of air flow vectors is schematically shown to represent an axial air flow component produced by an axial flow fan (not shown) downward between adjacent fins, including fin  31 , of prior art radial fin heat sink  20 . It will be seen that little if any air flow moves against the hottest part of fin  31  nearest thermal plug  40 . 
     Likewise, another group  37  of air flow vectors represents an axial air flow component produced by the axial flow fan (not shown) downward between adjacent fins, including fin  32 . Again, little if any air flow moves against the hottest part of fin  32  nearest thermal plug  40 . 
     In addition, it is not readily apparent from  FIGS. 3 and 4 , but only an insubstantial amount of air flow from the tangential component produced by a typical axial flow fan is captured by the prior art radial fin heat sink. This is illustrated further below regarding  FIG. 18 . 
     It should be apparent that what is needed is a heat sink structure that significantly increases the amount of air impinging upon the hottest part of the heat sink, and that significantly increases the volume and velocity of air moving through the heat sink fins, including significantly increasing the amount of the tangential component of an axial flow fan that is captured by the heat sink. 
       FIG. 5  illustrates a perspective view of a curved fin heat sink  50 , in accordance with an embodiment of the inventive subject matter. Curved fin heat sink  50  comprises a plurality of cooling fins  52  arranged about a core  55 . Fins  52  are formed of a material having high thermal conductivity such as a thermally conductive metal. In one embodiment, fins  52  are formed of aluminum; however, they could also be formed of copper or any other suitable thermally conductive metal or metal alloy. 
     Core  55  has a central axis  58 . Core  55  can optionally have a central cavity  54  for insertion of a thermal plug (not shown). Each fin  52  has a base and a tip. The base of each fin  52  is coupled to core  55  substantially parallel to central axis  58 . It will be seen from  FIG. 5  that the tips of fins  52  define the periphery of a face to face the component (e.g. IC package  64 ,  FIG. 7 ), and that the face comprises inter-fin openings in the form of spaces between individual fins  52 . Each fin  52  is curved in the same relative direction. As will be seen from the description below, the fins  52  of curved fin heat sink  50  are shaped to capture the tangential component of air from an axial flow fan (not shown in  FIG. 5 ). Fins  52  are also shaped to direct a relatively large volume and relatively high velocity of air flow to contact substantially the entire surface of each fin  52 , including the hottest portion of each fin  52  adjacent the core  55 . 
       FIG. 6  illustrates a top view of the curved fin heat sink  50  shown in  FIG. 5 . An explanation will now be given as to how curved fin heat sink  50  is shaped in order to maximize the number of cooling fins  52  for a desired “semi-rectangular” shape of curved fin heat sink  50  while maintaining a substantially uniform aspect ratio among all of cooling fins  52 . “Semi-rectangular” is defined herein to mean a geometrical figure having four straight or slightly curved (either concave or convex) sides that meet at corners that are perpendicular, rounded, and/or otherwise different from perpendicular. 
     A semi-rectangular shape was chosen for one embodiment of curved fin heat sink  50 , because that shape most closely matched the footprint of a high performance IC package on which curved fin heat sink  50  was mounted. A further constraint on the shape of curved fin heat sink  50 , in this embodiment, was a “keep-out area” on the circuit board around the IC package, due to the necessity of mounting other components in the keep-out area and of minimizing the overall physical size of the circuit board. 
     The semi-rectangular shape of curved fin heat sink  50  can be seen in  FIG. 6 , in that curved fin heat sink  50  comprises two slightly convex-curved sides of length  61  and two slightly convex-curved ends of length  62 . Each side meets a respective end at a rounded corner such as corner  57 . 
     Fins  52  are fabricated, in one embodiment, through an extrusion process. By using an extrusion process, heat sinks can be made at a significant savings in manufacturing costs as compared with a process, for example, in which fins are machined from a heat sink core, or brazed or soldered onto a heat sink core. Using high volume manufacturing techniques, extrusions several feet long can be quickly formed and then cut into individual curved fin heat sinks, each having a plurality of curved fins and, optionally if desired, a central cavity to accommodate a thermal plug. 
     However, the extrusion process for curved fins is currently subject to several process constraints. One constraint is that for extruding aluminum, for example, the aspect ratio of a curved fin  52 , i.e. the ratio of the length of a fin  52  to the average width of the gap between two adjacent fins  52 , cannot exceed about 10:1 to 12:1. Another constraint is that the radius at the base of the fins cannot be less than about 1.0 to 1.2 millimeters. 
     Yet another constraint is to provide as many fins  52  as possible (subject to the above-mentioned radius constraint), with each fin  52  as long as possible (subject to the above-mentioned aspect ratio constraint), in order to provide as great a total heat dissipation surface as possible. In the situation where the heat sink is being used to cool an IC, the heat dissipation from the heat sink must be at least sufficient to maintain a junction temperature within the IC at or below a predetermined maximum value. 
     In view of the above-mentioned process constraints, the core  55  is shaped to substantially match the shape or footprint of curved fin heat sink  50 , which in the embodiment shown in  FIG. 6  is a semi-rectangular shape. Thus, core  55  comprises two slightly convex-curved sides of length  71  and two slightly convex-curved ends of length  72 . Each side meets a respective end at a rounded corner such as corner  77 . As a result, the aspect ratio of fins  52  can be maintained substantially uniform around the entire periphery of curved fin heat sink  50 . Some variation in aspect ratio of fins  52  around the periphery of curved fin heat sink  50  is acceptable, so long as the maximum aspect ratio of approximately 10:1 to 12:1 is not exceeded for any fin  52 . It will be understood that with advances in extrusion technology the upper end of the aspect ratio range can be expected to rise; however, the same principles of the disclosure will nonetheless be applicable to heats sinks extruded with more advanced extrusion technology. 
       FIG. 7  illustrates a perspective view of an electronic assembly  60  including a curved fin heat sink  50  positioned upon an IC package  64 , in accordance with an embodiment of the inventive subject matter. IC package  64  is shown mounted upon a circuit board  63 , which can be of similar or identical type to the prior art circuit board illustrated in  FIG. 1 ; however, circuit board  63  can be of any type. The lower face of curved fin heat sink  50  is in thermal contact with IC package  64 . 
     An axial flow fan  65  is shown schematically positioned over the upper face of curved fin heat sink  50 . Fan  65  comprises a plurality of fan blades or impellers  66  that rotate, in the direction indicated by arrow  68 , about an axis  67  that is substantially perpendicular to the upper face of curved fin heat sink  50 . 
     Because heat sink  50  is considerably less expensive to fabricate, and has considerably less mass, than the prior art heat sink  2  illustrated in  FIG. 1 , electronic assembly  60  is more commercially desirable than the prior art electronic assembly  1  illustrated in  FIG. 1   
       FIG. 8  illustrates a perspective view of a portion of an electronic assembly including an axial flow fan  70  atop a curved fin heat sink  50 , in accordance with an embodiment of the inventive subject matter. Fan  70  comprises a plurality of curved blades  74  disposed about an axis  69  that is substantially perpendicular to the upper face of curved fin heat sink  50 . Blades  74  are attached to a hub  84  that is driven, in the direction of rotation indicated by arrow  75 , by fan motor  73 . A hold-down mechanism  76  is used to clamp fan  70  and curved fin heat sink  50  to the upper surface of a heat-producing IC (not shown) on a circuit board (not shown) underlying curved fin heat sink  50 . 
       FIG. 9  illustrates a top view of the portion within dashed rectangle  56  of  FIG. 6 , showing an air flow pattern within fins  81  and  82  of curved fin heat sink  50 , in accordance with an embodiment of the inventive subject matter. In  FIG. 9 , a tangential air flow component  79  from an axial flow fan (not shown) impinges upon fins  81  and  82 . Each fin, such as fin  81  or  82 , is curved towards, or faces, counter to the direction of rotation  75  of fan blades  74  ( FIG. 8 ). 
     Before discussing tangential air flow component  79 , it should be first noted that the base regions of fins  81  and  82  are substantially perpendicular to core  55 . From their bases, fins  81  and  82  curve substantially away from the perpendicular. However, fins  81  and  82  diverge only slightly as they emanate from core  55 . The radius  78  at the base of fins  81  and  82  is only slightly smaller than the fin tip distance  88  at the tips of fins  81  and  82 . This geometry provides significantly improved air flow between fins  81  and  82 . It provides a more constricted path towards the tips of the fins, thus retaining more of the air flow between the fins, where it can dissipate heat from the fins. 
     Tangential air flow component  79  impinges against the fins of curved fin heat sink  50 , such as fins  81  and  82 . A relatively small portion  80  of tangential air flow component  79  moves outwardly towards the tips of fins  81  and  82 . A significantly larger portion  83  of tangential air flow component  79  moves inwardly towards the bases of fins  81  and  82 . Thus, significantly more air flow is directed towards the hottest part of heat sink, i.e. core  55  and particularly the base portions of fins  81  and  82  near core  55 . Because air flow is directed inwardly toward the core, in some embodiments a fan shroud, which would block air flow from exiting out the tips of the fins, may be dispensed with, thus offering significant cost, mass, and reliability advantages. 
       FIG. 10  illustrates a side view of a section of the curved fin heat sink  50  shown in  FIG. 6 , taken between dashed line segments  51  and  53 . Fins  91  and  92  are on opposite sides of curved fin heat sink  50 . The lower surface of thermal plug  90  is in thermal contact with the upper surface of a heat-producing IC package  94 . Heat, represented by arrows  95 , is transferred from IC package  94  into thermal plug  90 . From thermal plug  90 , heat is transferred through sidewall  98  of cavity  54  to fin  91  (the heat sink core has been omitted to simplify this illustration), and through sidewall  99  of cavity  54  to fin  92 . The hottest part of fins  91  and  92  is nearest the thermal plug  90 . 
     A group  96  of air flow vectors is schematically shown to represent an air flow component produced by an axial flow fan (not shown) downward between adjacent fins, including fin  91 , of curved fin heat sink  50  ( FIG. 6 ). Still referring to  FIG. 10 , it will be seen that substantially more air flow moves against the hottest part of fin  91  nearest thermal plug  90  than in the prior art radial fin heat sink  20 , as was discussed earlier regarding  FIG. 4 . The increase in air flow is produced by the curved fin geometry, which not only curves the fins to capture both the normal and tangential components of the air flow from the axial flow fan, but which also has an inter-fin space of near uniform width to allow air to move down between the fins at a higher volume and higher speed than if the fins widened towards their tips, as in the prior art heat sink  20  shown in  FIG. 2 . 
     Still referring to  FIG. 10 , another group  97  of air flow vectors represents an air flow component produced by the axial flow fan (not shown) downward between adjacent fins, including fin  92 . Again, substantially more air flow moves against the hottest part of fin  92  nearest thermal plug  90 . 
     In addition, although it is not readily apparent from  FIGS. 9 and 10 , a substantial amount of air flow from the tangential component produced by a typical axial flow fan is captured by the fins of curved fin heat sink  50  ( FIG. 6 ). This again is achieved by the curved fin geometry that curves the fins towards the tangential component of air flow. 
     Thus, the curved fin heat sink  50  ( FIG. 6 ) significantly increases the amount of air impinging upon the hottest part of the curved fin heat sink  50 , and it significantly increases the volume and velocity of air moving through the curved fin heat sink  50 , including significantly increasing the amount of the tangential component of an axial flow fan that is captured by the curved fin heat sink  50 . 
     In addition, an axial flow fan used in conjunction with curved fin heat sink  50  can have a relatively low rotational speed, thus keeping fan noise to a minimum, while nonetheless producing sufficient air flow to dissipate heat from a heat-generating component in an electronic assembly. 
       FIG. 11  illustrates a perspective view of a bent fin heat sink  100 , in accordance with an embodiment of the inventive subject matter. Bent fin heat sink  100  comprises a plurality of cooling fins  102  arranged about a core  105 . Fins  102  are formed of a thermally conductive metal. In one embodiment, fins  102  are formed of aluminum; however, they could also be formed of copper or any other suitable thermally conductive metal or metal alloy. 
     Core  105  has a central axis  101 . Core  105  can optionally have a central cavity  106  for insertion of a thermal plug (not shown). Each fin  102  has a base and a tip. The base of each fin  102  is coupled to core  105  substantially parallel to central axis  101 . 
     Each fin  102  comprises a vertical portion  107  and an angled portion  108 . The angled portion  108  of each fin  102  is bent in the same relative direction. As will be seen from the description below, the fins  102  of bent fin heat sink  100  are shaped to capture the tangential component of air from an axial flow fan (not shown in  FIG. 11 ). They are also shaped to direct a relatively large and relatively high velocity air flow to contact substantially the entire surface of each fin  102 , including the hottest portion of each fin  102  adjacent the core  105 . 
     According to one embodiment of a bent fin heat sink  100 , after forming (e.g. by extrusion) a plurality of straight unbent fins emanating radially from core  105 , the upper portion of the heat sink  100  is counterbored to produce a counterbore  104 , in which part of the base of each fin  102  is sheared from core  105  in the vicinity only of angled portion  108 . This allows angled portion  108  of each fin  102  to be bent in a subsequent operation. 
     In one embodiment, the angle that the angled portion  108  of each fin makes with the vertical portion  107  is approximately 150 degrees. In other embodiments, different angles could be used, depending upon the air flow characteristics of the particular axial flow fan being used in conjunction with the bent fin heat sink. 
     Instead of counterboring the upper portion of heat sink  100 , a hole saw or other tool could be utilized to make a groove in the upper portion of heat sink  100  of sufficient depth to enable the angled portion  108  of each fin  102  to be bent. 
     It will be noted that for certain fins in the “corner” regions of bent fin heat sink  100 , their upper tips  109  are slightly clipped to fit into a desired “semi-rectangular” (as earlier defined) footprint. However, in other embodiments, such clipping could be omitted. 
       FIG. 12  illustrates a top view of a bent fin heat sink  100 , in accordance with an embodiment of the inventive subject matter. Bent fin heat sink  100  is shaped in order to maximize the number of cooling fins  102  for a desired “semi-rectangular” shape of curved fin heat sink  100 . 
     The semi-rectangular shape of curved fin heat sink  100  can be seen in  FIG. 12 , in that curved fin heat sink  100  comprises two substantially straight sides of length  111  and two substantially straight ends of length  112 . Each side meets a respective end at a rounded corner such as corner  114 . 
     Fins  102  are fabricated, in one embodiment, through an extrusion process. The extrusion process for bent fins is currently subject to basically the same process constraints as for the curved fin heat sink described in  FIG. 6 , except that the aspect ratio of the fins  102  can be slightly greater than for curved fins, ranging up to approximately 14:1 to 16:1. 
     In view of the fact that the fabrication of the angled portions  108  of the fins  102  of bent fin heat sink  100  requires counterboring a counterbore  104 , the shape of core  105  is maintained generally circular in the embodiment shown in  FIG. 12 . However, in another embodiment, the shape of core  105  could be semi-rectangular, as in the embodiment shown in  FIG. 6 . 
     The trimmed upper tips  109  of certain fins  102  near the corners of heat sink  100  can be seen in  FIG. 12 . 
       FIG. 13  illustrates a perspective view of an electronic assembly  120  including a bent fin heat sink  100  positioned upon an IC  124  package, in accordance with an embodiment of the inventive subject matter. 
     IC package  124  is shown mounted upon a circuit board  122 , which can be of similar or identical type to the prior art circuit board illustrated in  FIG. 1 ; however, circuit board  122  can be of any type. 
     An axial flow fan  125  is shown schematically positioned over bent fin heat sink  100 . Fan  125  comprises a plurality of fan blades or impellers  126  that rotate, in the direction indicated by arrow  128 , about an axis  127  that is substantially perpendicular to the upper face of bent fin heat sink  100 . Bent fin heat sink  100 , in this embodiment, comprises a thermal plug  123 . Thermal plug  123  can be formed of any suitable thermally conductive material. In one embodiment, thermal plug  123  is made of copper; however, aluminum or a copper or aluminum alloy could also be used. 
       FIG. 14  illustrates a schematic view of a fan  135 , including its tangential air flow component  130  and its normal air flow component  132 , and a side view of a bent fin heat sink  100  as positioned upon a sectioned IC package  150  on a substrate  160 , in accordance with an embodiment of the inventive subject matter. 
     Fan  135  can be similar or identical to fan  70  shown in  FIG. 8 . Fan  135  is an axial flow fan having a plurality of fan blades  136 , rotating in a direction indicated by arrow  138 , and disposed about an axis of rotation  137 . 
     Fan  135 , when rotating about axis  137 , produces an air flow that can be analyzed as having two different components. A tangential component  130  comprises a plurality of angular vectors  131  generally increasing towards the fan blade periphery. An axial component  132  comprises a plurality of downward vectors  133 , again generally increasing towards the fan blade periphery. 
     Because the fins  102  of bent fin heat sink  100  are angled towards, or face, the tangential component  130 , a relatively greater air flow, represented by arrows  140 , is captured and flows downward between fins  102 , exiting in the direction of arrows  142  beneath bent fin heat sink  100 . 
     Thermal plug  123  of bent fin heat sink  100  is in thermal contact with an IC package  150 . IC package  150 , illustrated in cross-section, includes a die  154  mounted on a package substrate  152  and covered with a lid or integrated heat spreader (IHS)  158 . A thermal grease or phase change material  156  can be used between die  154  and IHS  158 . Likewise, a thermal grease or phase change material (not shown) can be used, if desired, between IHS  158  and thermal plug  123 . Some of the relative dimensions of the structures shown in  FIG. 14  are exaggerated or diminished, and they are not drawn to scale. For example, in a different embodiment the thermal plug  123  could be as wide as IHS  150 , with bent fin heat sink  100  accordingly widened to accommodate an IHS  150  of such width. 
       FIG. 15  illustrates a perspective view of a curved-bent fin heat sink  200 , in accordance with an embodiment of the inventive subject matter. Curved-bent fin heat sink  200  comprises a plurality of cooling fins  202  arranged about a core  205 . Fins  202  are formed of a thermally conductive metal. In one embodiment, fins  202  are formed of aluminum; however, they could also be formed of copper or any other suitable thermally conductive metal or metal alloy. 
     Core  205  has a central axis  201 . Core  205  can optionally have a central cavity  206  for insertion of a thermal plug (not shown). Each fin  202  has a base and a tip. The base of each fin  202  is coupled to core  205  substantially parallel to central axis  201 . Each fin  202  is curved between its base and its tip, and the curve of each fin  202  is towards the same relative direction. In the embodiment shown in  FIG. 15 , each fin  202  is curved towards, or faces, a counterclockwise direction, opposite to the direction of rotation of an axial flow fan to be used in conjunction with heat sink  200 . 
     Each fin  202  comprises a vertical portion  207  and an angled portion  208 . The angled portion  208  of each fin  202  is bent in the same relative direction. As will be seen from the description below, the fins  202  of curved-bent fin heat sink  200  are shaped to capture the tangential component of air from an axial flow fan (not shown in  FIG. 15  but shown in  FIG. 17 ). They are also shaped to direct a relatively large and relatively high velocity air flow to contact substantially the entire surface of each fin  202 , including the hottest portion of each fin  202  adjacent to the core  205 . 
     According to one embodiment of a curved-bent fin heat sink  200 , after forming a plurality of curved unbent fins emanating substantially radially from core  205 , for example using an extrusion process, the upper portion of the heat sink  200  is counterbored to produce a counterbore  204  in which part of the base (i.e. inner portion) of each fin  202  is sheared from core  205  in the vicinity only of angled portion  208 . This allows angled portion  208  of each fin  202  to be bent in a subsequent operation. 
     In one embodiment, the angle that the angled portion  208  of each fin makes with the vertical portion  207  is approximately 150 degrees. In other embodiments, different angles could be used, depending upon the air flow characteristics of the particular axial flow fan being used in conjunction with the bent fin heat sink. 
       FIG. 16  illustrates a top view of a curved-bent fin heat sink  200 , in accordance with an embodiment of the inventive subject matter. Curved-bent fin heat sink  200  is shaped in order to maximize the number of cooling fins  202  for a desired “semi-rectangular” shape of curved-bent fin heat sink  200 . 
     The semi-rectangular shape of curved-bent fin heat sink  200  can be seen in  FIG. 16 , in that curved-bent fin heat sink  200  comprises two slightly convex-curved sides of length  211  and two slightly convex-curved ends of length  212 . Each side meets a respective end at a rounded corner such as corner  214 . 
     Fins  202  are fabricated, in one embodiment, through an extrusion process followed by a counterboring process and then a bending process. The extrusion process for curved-bent fins is currently subject to basically the same process constraints as for the curved fin heat sink described in  FIG. 6 . For this reason, the core  205  is shaped to substantially match the shape or footprint of curved-bent fin heat sink  200 , which in the embodiment shown in  FIG. 16  is a semi-rectangular shape. 
     Thus, core  205  comprises two slightly convex-curved sides of length  231  and two slightly convex-curved ends of length  232 . Each side meets a respective end at a rounded corner such as corner  234 . As a result, the aspect ratio of the fins can be maintained substantially uniform around the entire periphery of curved-bent fin heat sink  200 . Some variation in aspect ratio of the fins around the periphery of curved-bent fin heat sink  200  is acceptable, so long as the maximum aspect ratio of approximately 10:1 to 12:1 is not exceeded for any fin. 
       FIG. 17  illustrates a perspective view of an electronic assembly  220  including a curved-bent fin heat sink  200  positioned upon an IC package  224 , in accordance with an embodiment of the inventive subject matter. 
     IC package  224  is shown mounted upon a circuit board  222 , which can be of similar or identical type to the prior art circuit board illustrated in  FIG. 1 ; however, circuit board  222  can be of any type. 
     An axial flow fan  225  is shown schematically positioned over curved-bent fin heat sink  200 . Fan  225  comprises a plurality of fan blades or impellers  226  that rotate, in the direction indicated by arrow  228 , about an axis  227  that is substantially perpendicular to the upper face of curved-bent fin heat sink  200 . Curved-bent fin heat sink  200 , in this embodiment, comprises a thermal plug  223 . 
       FIG. 18  illustrates an air flow pattern  250  for a prior art radial fin heat sink. Straight, vertical, radially-attached fins  251  each receive an air flow vector  255  from an axial flow fan (not shown) above the heat sink. As mentioned earlier, an axial flow fan produces an air flow having both an axial component directed substantially perpendicular to the upper face of the heat sink, and a tangential component in the direction of rotation of the fan blades. 
     In  FIG. 18 , substantially all of the tangential component  256  of air flow vector  255  is deflected away from the opening between adjacent fins  251 . The predominant component of air flow into the space between adjacent fins  251  is the axial component  257 . However, a portion of axial component  257  is also deflected away and does not go between adjacent fins  251 , due to the vertical geometry of the fins. For this fin geometry, there is increased air pressure between the fins, resulting in reduced mass flow and decreased heat dissipation performance. 
       FIG. 19  illustrates an air flow pattern  260  for a bent fin heat sink, in accordance with an embodiment of the inventive subject matter. Bent, radially-attached fins  261  each receive an air flow vector  265  from an axial flow fan (not shown) above the heat sink. 
     In  FIG. 19 , substantially all of the tangential component of air flow vector  265  is captured by the angled portions  269  of fins  261  and goes into the space between adjacent fins  261 , including vertical portions  268 , which are the hottest portions of fins  261 . Only a small component  266  of the tangential component is deflected away. In addition, little of the axial component  267  is deflected away, as occurs with the heat sink fin geometry of the prior art straight, radial fin heat sink illustrated in  FIG. 18 , and most of axial component  267  goes between adjacent fins  261 . 
       FIG. 20  illustrates an air flow pattern  270  for a curved-bent fin heat sink, in accordance with an embodiment of the inventive subject matter. Curved-bent, radially-attached fins  271  each receive an air flow vector  275  from an axial flow fan (not shown) above the heat sink. 
     In  FIG. 20 , substantially all of the tangential component of air flow vector  275  is captured by the angled portions  279  of fins  271  and goes into the space between adjacent fins  271 , including vertical portions  278 , which are the hottest portions of fins  271 . Only a small component  276  of the tangential component is deflected away. In addition, little of the axial component  277  is deflected away, as occurs with the heat sink fin geometry of the prior art straight, radial fin heat sink illustrated in  FIG. 18 , and most of axial component  277  goes between adjacent fins  271 . 
     In addition, the curvature of fins  271  assists in directing the air flow inward towards the heat sink core (not shown, but in this view it would be behind fins  271 ). Because substantial air flow from the fan (not shown) is captured by the curved-bent heat sink, and because the captured air flow is directed inward towards the heat sink core and the hottest part of fins  271  (next to the core), the curved-bent heat sink is capable of dissipating a significant amount of heat from a heat-producing electronic component with which it is used. 
     In summary, for the fin geometries of the bent fin heat sink and the curved-bent fin heat sink, there is decreased air pressure between the fins, resulting in increased mass flow and increased heat dissipation performance. 
       FIG. 21  illustrates a flow diagram of a method of fabricating a heat sink, in accordance with an embodiment of the inventive subject matter. The method begins at  300 . 
     In  302 , a billet of thermally conductive metal, such as aluminum or copper, is obtained. 
     In  304 , a plurality of fins are formed from the billet, for example by an extrusion or micro-forging process. The fins extend outwardly from a core in an asymmetric pattern (in the case of curved fins). The core has a central axis, and each fin has a base that is coupled to the core substantially parallel to the central axis. If desired, a central cavity can be formed in the core. The central cavity can be formed in any suitable manner, for example as part of the extrusion operation. 
     In  306 , if the fins are to be bent, the process goes to  308 ; otherwise, it goes to  312 . 
     In  308 , the portions of the fins to be bent are separated from the core, for example by forming a cavity (e.g. by counterboring) or channel (e.g. by machining or sawing) into the core a predetermined distance along the central axis, from the top of the heat sink. 
     In  310 , a portion of each fin is bent in substantially the same relative direction. In one embodiment, the upper portion of each fin is bent down approximately 30 degrees from vertical, so that the angled portion of the fin forms an angle of approximately 150 degrees with the vertical portion of the fin. 
     In  312 , which is optional depending upon whether a central cavity was formed in  304 , a thermal plug is inserted into the central cavity to provide increased thermal dissipation from the IC through the heat sink core to the heat sink fins. The process ends at  314 . 
       FIG. 22  illustrates a flow diagram of a method of fabricating an electronic assembly, in accordance with an embodiment of the inventive subject matter. The process begins at  400 . 
     In  402 , an electronic component is mounted on a circuit board. 
     In  404 , an axial flow fan is provided. The axial flow fan is capable of moving air having a component normal to the electronic component and a component tangential to the electronic component. 
     In  406 , a heat sink is mounted between the electronic component and the axial flow fan. The heat sink includes a number of cooling fins that are arranged about a core having a central axis. Each cooling fin has a base coupled to the core substantially parallel to the central axis. The cooling fins are shaped to capture both components of air, i.e. the axial component and the tangential component. A first face of the heat sink is in thermal contact with the electronic component and has a semi-rectangular periphery. A second face of the heat sink faces the fan and has a semi-rectangular periphery. The second face is substantially opposite the first face. The core is shaped to maximize the number of cooling fins while maintaining a substantially uniform aspect ration in the cooling fins. The method ends at  408 . 
     The operations described above with respect to  FIGS. 21 and 22  could be performed in a different order from those described herein. Also, although the flow diagrams of  FIGS. 21 and 22  are shown as having a beginning and an end, they can be performed continuously. 
       FIG. 23  is a block diagram of an electronic system  501  incorporating at least one electronic assembly  502  with at least one high capacity heat sink, in accordance with an embodiment of the inventive subject matter. Electronic system  501  is merely one example of an electronic system in which embodiments of the inventive subject matter can be used. In this example, electronic system  501  comprises a data processing system that includes a system bus  504  to couple the various components of the system. System bus  504  provides communications links among the various components of the electronic system  501  and can be implemented as a single bus, as a combination of busses, or in any other suitable manner. 
     Electronic assembly  502  is coupled to system bus  504 . Electronic assembly  502  can include any circuit or combination of circuits. In one embodiment, electronic assembly  502  includes a processor  506  which can be of any type. As used herein, “processor” means any type of computational circuit, such as but not limited to a microprocessor, a microcontroller, a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, a graphics processor, a digital signal processor (DSP), or any other type of processor or processing circuit. 
     Other types of circuits that can be included in electronic assembly  502  are a chip set  507  and a communications circuit  508 . Chip set  507  and communications circuit  508  are functionally coupled to processor  506 , and they can be configured to perform any of a wide number of processing and/or communications operations. Other possible types of circuits (not shown) that could be included within electronic assembly  502  include a digital switching circuit, a radio frequency (RF) circuit, a memory circuit, a custom circuit, an application-specific integrated circuit (ASIC), an amplifier, or the like. 
     Electronic system  501  can also include an external memory  512 , which in turn can include one or more memory elements suitable to the particular application, such as a main memory  514  in the form of random access memory (RAM), one or more hard drives  516 , and/or one or more drives that handle removable media  518  such as floppy diskettes, compact disks (CDs), digital video disks (DVDs), and the like. 
     Electronic system  501  can also include a display device  509 , one or more speakers  510 , and a keyboard and/or controller  520 , which can include a mouse, trackball, game controller, voice-recognition device, or any other device that permits a system user to input information into and receive information from the electronic system  501 . 
       FIGS. 1–20  and  23  are merely representational and are not drawn to scale. Certain proportions thereof may be exaggerated, while others may be minimized.  FIGS. 5–17 ,  19 ,  20 , and  23  are intended to illustrate various implementations of the inventive subject matter that can be understood and appropriately carried out by those of ordinary skill in the art. 
     The inventive subject matter provides for a heat sink and an electronic assembly that minimize thermal dissipation problems associated with high power delivery, and to methods of manufacture thereof. An electronic system and/or data processing system that incorporates one or more electronic assemblies that utilize the inventive subject matter can handle the relatively high power densities associated with high performance integrated circuits, and such systems are therefore more commercially attractive. 
     By substantially increasing the thermal dissipation from high performance electronic assemblies, such electronic equipment can be operated at increased clock frequencies. Alternatively, such equipment can be operated at reduced clock frequencies but with lower operating temperatures for increased reliability. 
     As shown herein, the inventive subject matter can be implemented in a number of different embodiments, including a heat sink, an electronic assembly, an electronic system, and various methods, including a method of fabricating a heat sink, and a method of fabricating an electronic assembly. Other embodiments will be readily apparent to those of ordinary skill in the art. The elements, materials, geometries, dimensions, and sequence of operations can all be varied to suit particular packaging and heat-dissipation requirements. 
     While certain operations have been described herein relative to “upper” and “lower” surfaces, it will be understood that these descriptors are relative, and that they would be reversed if the relevant structure(s) were inverted. Therefore, these terms are not intended to be limiting. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for a specific embodiment shown. This application covers any adaptations or variations of the inventive subject matter. Therefore, it is manifestly intended that embodiments of this subject matter be limited only by the claims and the equivalents thereof.