Abstract:
A heatsink assembly is disclosed that includes a holder slidably engaging a circuit board component to be cooled and a heatsink secured to the heatsink holder. A method of cooling a circuit board component is also disclosed that involves the steps of providing a bracket having a central portion and first and second end portions directed toward one another, sliding the bracket over a portion of a circuit board component to be cooled such that the central portion faces a first surface of the circuit board component and the first and second end portions face opposing edges of the circuit board component, providing a heatsink, and securing the heatsink to the bracket in a manner that retains the heatsink and bracket on the circuit board component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims the benefit of U.S. Provisional Patent Application No. 60/524,040, filed Nov. 24, 2003, the entire contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention is directed toward a heatsink assembly and a method of cooling a component, and, more specifically, toward a heatsink assembly including a holder that mechanically engages a component to be cooled, a heatsink secured to the holder, and a method of cooling a component. 
   BACKGROUND OF THE INVENTION 
   Dissipation of excess heat is a problem that must often be addressed by equipment designers. Various electronic components, microprocessors, for example, need to be cooled in order to operate properly. As increasingly large numbers of components are formed in small areas, problems caused by excess heat become more pronounced. Various methods and devices have therefore been developed to cool components. 
   A popular method for dissipating heat is convection dissipation, especially via cooling fans in the vicinity of components to be cooled. In this method, moving air carries thermal energy from the heated component and dissipates it to the surrounding atmosphere. While this is sufficient for many purposes, it has been found that fan cooling alone is often insufficient. 
   Another common method of heat dissipation uses a combination of cooling by conduction and by radiation. Devices, known as heatsinks are mounted in direct contact with components to be cooled so that the thermal energy may be conducted, in accordance with equilibrium principles, from the component to the heatsink device. Most heatsinks have a substantial surface area that is not in contact with the component so that the absorbed thermal energy can dissipate into the air. Many heatsink devices include dissipation fins of various configurations, while others rely principally on a substantial heatsink mass to absorb heat, especially transient heat. 
   Heatsinks are sometimes secured to the top surface of electronic components by wires, clips, or the like. These wires or clips may snap over the heatsink to engage either the electronic device package or the socket into which the electronic device package is installed. Alternately, a heatsink may be connected to either the electronic component or the circuit board on which the component is mounted using screws or similar connectors. In addition, some heatsinks may be chemically bonded to the electronic component to be cooled; however, over time chemical bonds tend to break, resulting in decreased thermal conduction from the component to the heatsink. In extreme cases, the chemically bonded heatsink may detach completely from the component to be cooled. 
   Heatsinks that attach to electronic components or to surrounding circuit boards often require the use of mounting holes in either the component or the surrounding circuit board which complicates the installation process. Moreover connecting a heatsink to a circuit board can sometimes stress or damage the electronic connections between the circuit component and the circuit board, especially in the case of ball grid array (BGA) mounted components. It is therefore desirable to provide a heatsink that mounts to an electronic component in a manner unlikely to damage the electronic component and that does not necessarily require mounting holes in either the electronic component or the circuit board on which it is mounted. 
   SUMMARY OF THE INVENTION 
   These problems and others are addressed by the present invention which comprises, in a first embodiment, a heatsink assembly comprising a holder slidably engaging a circuit board component to be cooled and a heatsink secured to the heatsink holder. 
   Another aspect of the present invention comprises a heatsink assembly comprising a holder slidably engaging a circuit board component to be cooled and a heatsink secured to the heatsink holder, where the holder comprises a planar, bar shaped central portion having a first side and a second side, a first leg and a second leg depending from the first side, a first tab projecting from the first leg and a second tab projecting from the second leg toward the first leg, and a boss projecting from the second side. The holder is used with a heatsink that includes a first side having projections for increasing the surface area of the heatsink and a second side, the second side including a depression adapted to receive a portion of the component to be cooled and a bore receiving the holder boss. A fastener extends through an opening in the heatsink first side and into the boss to secure the heatsink to the holder. 
   An additional aspect of the invention comprises a heatsink assembly made up of a heatsink device for dissipating heat and a retaining device connected to the heatsink device for mechanically securing the heatsink device to a circuit board component to be cooled. 
   A further aspect of the invention comprises a heatsink assembly that includes a holder comprising first and second spaced channels for receiving first and second opposing portions of a circuit board mounted component to be cooled, a heatsink having a first side including a depression for receiving a portion of the circuit board mounted component to be cooled and a portion of the holder, and a connector connecting the heatsink to the holder. 
   Another aspect of the invention comprises a method of cooling a circuit board component that involves providing a bracket having a central portion and first and second end portions directed toward one another and sliding the bracket over a portion of a circuit board component to be cooled. The bracket slides over the component so that the central portion faces a first surface of the circuit board component and the first and second end portions face opposing edges of the circuit board component. A heatsink is then secured to the bracket in a manner that retains the heatsink and bracket on the circuit board component. 
   An additional aspect of the invention comprises a method of cooling a circuit board component that involves providing a circuit board component having a top surface lying in a first plane, connecting a holder to the circuit board component in a manner that allows the holder to slide in a second plane substantially parallel to the first plane, and fixing a heatsink to the holder in a manner that fixes the holder in a first position with respect to the circuit board component. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These aspects of the invention and others will be better appreciated after a reading of the detailed description that appears below in connection with the following drawings wherein: 
       FIG. 1  is a partially exploded perspective view of a circuit board including an component to be cooled and a heat sink assembly including a heat sink and a holder according to a first embodiment of the present invention; 
       FIG. 2  is a top plan view of the heat sink of  FIG. 1 ; 
       FIG. 3  is a sectional side elevational view taken along line  3 — 3  in  FIG. 2 ; 
       FIG. 4  is a bottom plan view of the heat sink of  FIG. 1 ; 
       FIG. 5  is a perspective view of the holder of  FIG. 1 ; 
       FIG. 6  is a side elevational view of the holder of  FIG. 1 ; 
       FIG. 7  is a sectional side elevational view of the heatsink assembly of  FIG. 1  mounted to a first type of circuit board component; 
       FIG. 8  is a sectional side elevational view of the heatsink assembly of  FIG. 1  mounted to a second type of circuit board component; 
       FIG. 9  is perspective view of a heat sink according to a second embodiment of the present invention; 
       FIG. 10  is a top plan view of the heat sink of  FIG. 9 ; 
       FIG. 11  is a sectional side elevational view taken along line  11 — 11  in  FIG. 10 ; 
       FIG. 12  is a side elevational view of the heat sink of  FIG. 9  taken in the direction of line  12 — 12  in  FIG. 10 ; 
       FIG. 13  is a bottom plan view of the heat sink of  FIG. 9 ; and 
       FIGS. 14   a – 14   f  illustrate a method of attaching the heat sink assembly of the second embodiment of the present invention to the circuit board component shown in  FIG. 8 . 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings, wherein the showings are for the purpose of illustrating preferred embodiments of the invention only, and not for the purpose of limiting same,  FIG. 1  illustrates a circuit board  10  having a plurality of devices  12  mounted thereon including a component  14 , such as a microprocessor, that requires cooling. A heatsink assembly  16 , comprising a holder  18 , a heatsink  20  and a fastener  22 , described in detail hereinafter, attaches to component  14  to cool component  14  by a combination of conduction and convection. 
   As used herein, directional terms such as “upper” and “lower” will refer to components arranged and positioned as viewed in  FIG. 1 , that is with circuit board  10  forming a horizontal support and the remaining elements positioned thereabove, it being understood that the circuit board could be oriented vertically, at an angle to the horizontal or even upside down, without affecting the operation of heatsink  16 . 
     FIGS. 2 through 4  illustrate heatsink  20  in greater detail. Heatsink  20  includes a body  24  formed from a material with a high heat conductivity such as copper or aluminum. Aluminum is generally preferred for its lighter weight and lower cost, although copper conducts heat somewhat better. Body  24  includes a first, or top side  26  having a plurality of cooling fins  28  projecting therefrom and a raised, centrally disposed, cylindrical platform  30  having a top wall  32  with a through opening  34  leading to a hollow interior  35 . Body  24  further includes a second, or bottom side  36 , a front face  38 , a rear face  40 , first and second side faces  42  connecting front face  38  and rear face  40  and a channel  44  in bottom side  36  running between front face  38  and rear face  40  about midway between first and second side faces  42 . Channel  44  includes an inner wall  46  and first and second side walls  48  and further includes a cylindrical bore  50  in inner wall  46  that extends into the interior  35  of cylindrical platform  30 . Bottom side  36  is substantially surrounded by a flange  52  depending from side faces  42  and front and rear faces  38 ,  40  except at the locations where channel  44  meets the front and rear faces, the flange defining a region  54  that may be referred to hereinafter as a depression  54 . 
     FIGS. 5 and 6  illustrate holder  18  which comprises a bar-shaped body portion  60  having a top side  62 , a bottom side  64  and first and second ends  66 . A cylindrical boss  68  projects from a central portion of top side  62  and includes a top wall  70  having a threaded bore  72 . First and second legs  74 ,  76  extend from first and second ends  66 , and each leg includes a tab  78  projecting inwardly with respect to body portion  60 , that is, the tab  78  formed on first leg  74  projects toward second leg  76  and the tab  78  on second leg  76  projects toward first leg  74 . Tabs  78 , first and second legs  74 ,  76  and bottom side  64  define two channels  79  which together define a slot  80  having a generally rectangular cross section for receiving an object such as a substrate or holder for a chip as discussed hereinafter. 
     FIG. 7  shows holder  18 , heatsink  20  and fastener  22 , in this case a screw, assembled and attached to an component  14  to be cooled. Component  14  comprises a substrate  82  which includes solder balls  84  forming a ball grid array for attachment to a suitable socket  84  in circuit board  10 , and a chip  86  having a top surface  88  mounted on substrate  82 . Chip  86 , substrate  82  and solder balls  84  are sometimes referred to as a BGA package or BGA device. As will be appreciated from  FIG. 8 , holder  18  is configured so that slot  80  receives opposing sides of substrate  82  to allow holder  18  to slide over the BGA device. Tabs  78  extend under the substrate  82  to secure the holder  18  against removal in a direction normal to top surface  88  of the chip  86  while allowing the holder  18  to slide with respect to the chip  86 . Preferably, the length of legs  74  and  76  is selected such that bottom  64  of holder  18  engages top surface  88  of chip  86  or rests on a thin layer of thermal compound  98  (shown in  FIG. 14   b ) that may be applied over top surface  88  in a well known manner to improve heat transfer from the chip  86  to the heatsink assembly  16 . 
   Heat sink  20  is mounted on holder  18  so that boss  68  of holder  18  is received in cylindrical bore  50  in channel inner wall  46  and projects into the interior  35  of cylindrical platform  30 . The height of channel side walls  48 , illustrated in  FIG. 3 , is selected to be greater than or equal to the thickness of holder body  60  so that the bottom  36  of the heatsink  20  rests on top surface  88  of chip  86  (or on a layer of thermal compound applied thereto) when the heatsink assembly  16  is attached. Opening  34  in top wall  32  of cylindrical platform  30  aligns with threaded bore  72  in top wall  70  of boss  68  so that fastener  22 , when inserted through opening  34  and into bore  72  can be tightened to secure heatsink  20  to holder  18 . Flange  52  extends over the edges of chip  86  to provide additional contact area between the heatsink  20  and the chip  86 , and depression  54  receives the top portion of chip  86 . 
   Heatsink  20  could be secured to holder  18  using other fasteners, such as clips or snaps, or by partially melting the material of the heatsink  20  to bond it to the holder; however, the use of a fastener  22  such as a screw provides for simple and inexpensive installation while also allowing the heatsink to be removed and reused if desired. Beneficially, the use of a fastener  22  such as a screw also allows the contact pressure of heatsink  20  on top surface  88  of chip  86  to be adjusted—tightening screw  22  increases the pressure exerted by heatsink  20  on top surface  88  thereby improving the thermal connection between these elements. And, since the fastener  22  is centrally disposed in the present embodiment, contact pressure is applied substantially evenly by heatsink  20  across top surface  88  of chip  86  and is not concentrated at, for example, the corners of the chip  86  as may occur when using conventional heatsink clips. 
   Moreover, this mechanical connection between the heatsink  20  and chip  86  will not release on its own and is unlikely to fail, unlike some chemical adhesives, and furthermore, does not require the provision of mounting holes in the circuit board. Attaching a heatsink to the BGA device directly also avoids stressing the solder connections between the BGA device and the circuit board. 
     FIG. 8  shows the heatsink assembly  16  of  FIGS. 1–6  mounted on a second type of BGA device. Like the first type of BGA device illustrated in  FIG. 7 , the second type of device of  FIG. 8  includes a substrate  90  and a chip  92  and, in addition, includes a chip cover  94 . Heatsink assembly  16  can be attached to the substrate  90  of this second type of BGA device in the same manner described above. Alternately, and as illustrated in  FIG. 8 , it can be also be attached to chip cover  94  so that tabs  78  are received in a gap between chip cover  94  and chip substrate  90 . In fact, heatsink assembly  16  could be mounted to substantially any component that requires cooling as long as opposed slots, such as the ones formed by the gap between chip cover  94  and substrate  90  or the gap between substrate  82  and circuit board  10  are present in either the component to be cooled, a support for the component to be cooled, or a combination of both, without exceeding the scope of the claimed invention. 
   A second embodiment of the present invention is disclosed in  FIGS. 9 through 13 , wherein the same reference numerals are used to identify elements common to both embodiments.  FIG. 9  is a perspective view of a heatsink  120  having a body  124 , formed from a material with a high degree of heat conductivity such as copper or aluminum. Body  124  includes a first, or top side  126  having a plurality of cooling fins  128  projecting therefrom and a raised, centrally disposed, cylindrical platform  130  having a top wall  132  with a through opening  134  that leads to a hollow interior  135 . 
   Body  124  further includes a second, or bottom side  136 , a front face  138 , a rear face  140 , first and second side faces  142  connecting front face  138  and rear face  140  and a channel  144  in the bottom side  136  extending across bottom side  136  between but stopping short of first and second side faces  142 . Channel  144  includes an inner wall  146  and first and second side walls  148  and first and second end walls  149 . Channel  144  further includes a cylindrical bore  150  in inner wall  146  that extends into the interior  135  of circumferential platform  130 . 
   Bottom side  136  is completely surrounded by a flange  152  depending from side faces  142  and front and rear faces  138 ,  140 , the flange  152  defining a region  154  that may be referred to hereinafter as a depression  154 . In this embodiment, holder  18  is received within heatsink  120 ; legs  74 ,  76  are not exposed when heatsink  120  is mounted on holder  18 . This differs from heatsink  20  which includes openings in the front and rear faces  38 ,  40  that expose legs  74 ,  76 . This second embodiment of a heatsink assembly allows for the provision of a heatsink larger that the circuit board component to be cooled, which may be desirable when a large amount of cooling is required. 
   A method of attaching the heatsink  120  to the second type of BGA device will now be described with reference to  FIGS. 14   a – 14   f .  FIG. 14   a  illustrates a BGA package including a substrate  90  and chip cover  94  (covering a chip  92  seen only in  FIG. 8 ) mounted on a circuit board  10 . 
     FIG. 14   b  illustrates the application of an optional layer of thermal compound  98  to the top surface  96  of chip cover  94 . The thermal compound helps to fill small gaps between the heatsink and the component to be cooled and improves the transmission of thermal energy from the component to the heatsink. Such thermal compounds are useful when an component to be cooled has a metal chip cover as shown in this embodiment and are even more useful to improve heat conduction when no metal cover is present; this is because thermal conduction from the metal chip cover to a metal heatsink is generally better that thermal conduction between a ceramic chip and a metal heatsink. Many thermal compounds comprise, for example, a mixture of silicone and zinc oxide; however, the present invention is not limited to the use of any particular thermal compound. As these compounds are well known in the art and commonly available, they will not be described further herein. 
   In  FIG. 14   c , holder  18  has been slid over chip cover  94  so that tabs  78  are received in the gap between chip cover  94  and substrate  90  in the manner illustrated in  FIG. 8 . Holder  18  is thus substantially fixed against movement normal to top surface  96  of the chip cover but is free to move in a plane parallel to the top surface. Once properly positioned in the center of chip cover  94 , as seen in  FIGS. 14   d  and  14   e , heatsink  120  is placed over holder  18  and chip cover  94  so that boss  68  of holder  18  is received into the interior  135  of cylindrical platform  130  of the heatsink  20 , body  60  is received in channel  144 , and part or all of chip cover  94  is received in depression  154 . Fastener  22  is then placed through the aligned openings in the heatsink  120  and holder  18  as previously described and tightened, as illustrated in  FIG. 14   f , to firmly secure heatsink  120  to chip cover  120  with bottom side  136  of heatsink  120  in good thermal contact with the top  96  of chip cover  94  via the layer of thermal compound  98 . 
   As will be apparent from the foregoing description, neither holder  18  nor heatsink  120  is directly connected to the BGA device. Rather, holder  18  remains free to slide relative to the surface of chip cover  94  until heatsink  120  is connected thereto. Fastener  22  secures the heatsink  120  to the holder  18  and prevents the heatsink from being moved away from the component to be cooled. Once the top of chip cover  94  is received in depression  154 , and the heatsink  120  is connected to the holder  18 , neither heatsink  120  nor holder  18  is free to move parallel to the surface of chip cover  94 . Thus, a method of fixing a heatsink to a component is disclosed in which the only fastener used holds a heatsink to a holder. 
   The invention has been described herein in terms of several embodiments; however, obvious modifications and additions to these embodiments will become apparent to those skilled in the relevant arts upon a reading of the foregoing description. It is intended that all such obvious modifications and additions form a part of the present invention to the extent that they fall within the scope of the several claims appended hereto.