Abstract:
A system and method for coupling a thermal dissipation device to a substrate to be cooled and to an underlying support. The system includes a frame having an aperture, an upper surface abutting at least part of the bottom periphery of the thermal dissipation device, and a lower surface abutting the underlying support. A biasing element is positioned within the aperture of the frame and fastened to the thermal dissipation device. The biasing element urges the substrate into contact with the thermal dissipation device by applying a biasing force thereto, and decouples this biasing force from the force securing the heat sink to the underlying support.

Description:
BACKGROUND  
       [0001]     Electronic substrates, such as integrated circuit chips, chip carriers, and other components, are used with a wide variety of electronic devices to perform various computing and processing functions. Integrated circuits are usually pin-connected, soldered or otherwise connected to an underlying structure, such as a printed circuit board or card, to provide parallel functionality with other circuits and processors. In modern electronic devices, such as personal computers, as processor speeds are increased, integrated circuits require correspondingly more power and thus generate more heat. As a result, various thermal dissipation devices, including active and passive devices, have been developed to provide adequate heat dissipation.  
         [0002]     Passive thermal dissipation devices, often termed ‘heat sinks’, are mounted on top of the substrate to be cooled. The substrate is typically inserted into a socket. In some systems, the socket is attached to an underlying support, such as a circuit board, via a ball grid array. The ball grid array includes a layer of solder balls for making electrical contact with the socket. Loads transmitted through components including the heat sink and substrate, and ultimately applied to the ball grid array by the socket, should be minimized to avoid long-term damage to the ball grid array.  
         [0003]     It is necessary to clamp or otherwise press the heat sink against the substrate to maximize the heat transfer from the substrate. However, when clamping the heat sink and substrate to the circuit board, there is a limit to the clamping force applied to the heat sink that can be tolerated without adversely affecting the integrity of the ball grid array. A number of solutions have been proposed for mounting a heat sink onto the printed circuit card to provide a reliable thermal interface with an integrated circuit or other substrate. One such solution involves using fasteners to secure the heat sink to an underlying circuit card while clamping the integrated circuit tightly between both. However, as the clamping force is increased, progressive deformation of the ball grid array occurs, with the resulting damage to the array being a function of the clamping force. In addition, this mounting method can also cause the circuit card to warp due to bending stresses between the attachment location and the perimeter of the adjacent integrated circuit. Furthermore, the existing clamping force placed on the heat sink-substrate-socket-ball grid array assembly may be exacerbated during shipping and handling of the associated electronic device, when both static and dynamic loads are encountered.  
       SUMMARY  
       [0004]     A system is disclosed for coupling a thermal dissipation device to a substrate to be cooled and to an underlying support. The system includes a frame having an aperture, an upper surface abutting at least part of the bottom periphery of the thermal dissipation device, and a lower surface abutting the underlying support. A biasing element is positioned within the aperture of the frame and fastened to the thermal dissipation device. The biasing element urges the substrate into contact with the thermal dissipation device by applying a biasing force thereto, and decouples this biasing force from the force securing the heat sink to the underlying support.  
         [0005]     A method is also disclosed for coupling a heat sink to a substrate to be cooled and to a circuit board. The method includes the steps of placing the substrate against the heat sink; placing a biasing element against the substrate; fastening the biasing element to the heat sink so that the biasing element urges the substrate against the heat sink; attaching a frame to the heat sink to form a unit; and mounting the unit to the circuit board such that the substrate is in electrical contact with a socket affixed to the circuit board. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a top perspective view showing an exemplary system for coupling a thermal dissipation device to an electronic substrate mounted on a circuit board;  
         [0007]      FIGS. 2 and 3  are cross-sectional views taken along lines  2 - 2  and  3 - 3  in  FIG. 1 , respectively, showing exemplary components of the present system; and  
         [0008]      FIG. 4  is an exploded perspective view showing exemplary individual components of the system shown in  FIGS. 1-3 . 
     
    
     DETAILED DESCRIPTION  
       [0009]      FIGS. 1-4  show an exemplary system  10  for coupling a thermal dissipation device  20  to an electronic substrate  18 , and for mounting the thermal dissipation device and substrate to an underlying support, such as a daughter card or other circuit board  26 .  FIG. 1  is a top perspective view, and  FIGS. 2 and 3  are cross-sectional views taken along lines  2 - 2  and  3 - 3  of  FIG. 1 , respectively, showing exemplary components of the present system  10 .  FIG. 4  is an exploded perspective view showing exemplary components of the system shown in  FIGS. 1-3 .  
         [0010]     As best seen from  FIGS. 2 and 4 , system  10  comprises a biasing element  16  and a frame  12 . System  10  functions with thermal dissipation device  20 , electronic substrate  18 , socket  70 , and circuit board  26 . Substrate  18  includes a device  84  having a support plate  86 . Device  84  is typically an integrated circuit, but may be any type of electronic device, and may or may not include a support plate  86 . As shown in  FIG. 1 , thermal dissipation device  20 , hereinafter ‘heat sink’  20 , is mounted directly onto frame  12 , which is rigidly attached to circuit board  26 . As shown in  FIG. 3 , pin array  90  on substrate  18  interfaces with socket  70 , which may be attached to circuit board  26  via an optional ball grid array  89 .  
         [0011]     As shown in  FIG. 4 , frame  12 , heat sink  20 , and biasing element  16  are registered with respect to one another by aligning bores  62  in tabs  60  on frame  12  with holes  88  in biasing element  16  to accommodate pins  64  attached to heat sink  20 . In an exemplary embodiment, frame  12  is secured to heat sink  20  by locking clips  68 , such as Tinnerman clips, which are pushed onto heat sink pins  64  until the clips  68  make contact with tabs  60  on frame  12  and tabs  56  make contact with surface  91  ( FIG. 3 ) on substrate  18 . As described in detail below, biasing element  16  is attached to heat sink  20  and heat sink  20  is attached to frame  12  independently of the attachment of the heat sink and frame to circuit board  26  to separate the force needed to press substrate  18  against heat sink  20  from the force generated in clamping the heat sink and frame to the circuit board  26 .  
         [0012]     Frame  12  is a generally rectangular structure with a first and second pair of frame members  28 ,  30  defining aperture  14 . The top edge  38  of frame  12  is configured to receive at least part of the bottom periphery of heat sink  20 . The bottom edge  40  of frame  12  is configured to rest on circuit board top surface  42 . In an exemplary embodiment, frame  12  is attached to circuit board  26  by mounting pins  24 , which fit through bores  50  in receiving members  48  in frame  12  and through bores  58  in circuit board  26 . Mounting pins  24  may be any type of fastener, such as a pin or bolt that is pinned or threaded into the circuit board  26  or into a plate (not shown) on the bottom side of the circuit board  26 . Each bore  50  may also be threaded and the mounting pins  24  inserted from the far side of circuit board  26  as well. Frame  12  includes lateral lips or tabs  56  extending from receiving members  48  that locate frame  12  to surface  91  on substrate  18  with minimal force.  
         [0013]     Alternatively, tabs  56  may used to provide the force against substrate  18  necessary for the thermal interface between surface  72  on device  84  and surface  66  of heat sink  12 . In an exemplary embodiment, frame  12  is made from moldable material, such as a hard plastic, such that members  28 ,  30 , receiving members  48 , and tabs  60  all form a unitary body. Frame  12  may be fabricated from other, preferably non-conductive, material.  
         [0014]     The thickness of substrate  18  and the height of frame  12  (i.e., the distance between the top edges  38  of frame members  30  and the top of circuit board  26 ) determine the distance, or separation, between the bottom surface of substrate  18  and the top surface  42  of socket  70  when substrate pin array  90  is inserted into the socket  70 . This substrate-to-socket distance is established for an integrated circuit chip having a particular thickness, and is modified, by changing the height of frame  12 , to accommodate various other chips as a function of their thickness. In an exemplary embodiment, there is a slight separation between the between the bottom surface of substrate  18  and the top surface  42  of socket  70 , to further isolate socket  70  and underlying ball grid array  89  from heat sink  20 . The separation between substrate lower surface  91  and socket top surface  92  eliminates any load from being transferred to socket  70  and ball grid array  89  by forces applied to heat sink  20  when the heat sink is attached to circuit board  26 .  
         [0015]     Biasing element  16  includes two biasing members  76  which provide a compressive, or biasing force, to urge substrate  18  against heat sink  20 , so that the upper surface  72  of the substratescontacts the bottom surface  66  of heat sink  20 . In an exemplary embodiment, biasing members  76  are arched metal strips that function as leaf springs. Each of the biasing members  76  thus forms an arc, a substantial portion of which deforms when pressed against the lower surface  91  of the substrate  18  to urge substrate top surface  72  into contact with heat sink bottom surface  66 . Biasing members  76  may, alternatively, comprise any other type of mechanism for urging substrate  18  into contact with heat sink  20 ; for example, biasing members  76  could be used to compress small coil springs or belleville washers between biasing members  76  and surface  91  of substrate  18 . Optionally, to increase thermal energy transfer, a thermal interface material (not shown), such as thermal grease or other heat-conductive medium, may be applied to the substrate top surface  72  and/or heat sink bottom surface  66 . The thermal interface material is applied in a layer of suitable thickness, for example, about 0.05 to 0.25 millimeters thick.  
         [0016]     In an alternative embodiment, biasing element  16  may be used to hold the substrate  18  in position until frame  12  is fastened to heat sink  20  with a predetermined load. This may be accomplished by applying a predetermined load to frame  12  and then installing clips  68  onto posts  64  until the clips  68  make contact with tabs  60 .  
         [0017]     As shown in  FIG. 4 , biasing members  76  are connected to support members  78  at opposite ends thereof to form a rectangular unit  16  with an aperture through which pin array  90  of substrate  18  passes when the substrate is inserted into socket  70 . In an exemplary embodiment, support members  78  include flanges  82  extending outwardly from the top of members  78 , forming an ‘L’-shaped cross-section. Holes  88  in flanges  82  are aligned with bores  62  in frame tabs  60  to accommodate heat sink pins  64 , which register biasing element  16 , heat sink  20  and frame  12  with respect to each other. In an exemplary embodiment, biasing element  16  is fabricated from metal, such as stainless steel, beryllium copper or phosphor bronze, but may, alternatively, be formed from a material such as fiberglass or fiber-reinforced plastic.  
         [0018]     In an exemplary embodiment, biasing element  16  is attached to heat sink  20  via nuts  74  that are fastened to pins  64  via threads located near the upper end thereof and extending below heat sink lower surface  66 . In an alternative embodiment, biasing element  16  is attached to heat sink  20  by other mounting means, such as four screws (not shown), each of which disposed through a respective bore in one of the flanges  82  on biasing element support members  78 , and fastened to heat sink  20  via a tapped bore therein.  
         [0019]     The forces exerted by biasing members  76  against the bottom surface  91  of substrate  18  maintian good uniform contact between substrate top surface  72  and heat sink surface  66 , thus maximizing heat transfer from substrate  18  to heat sink  20 . In the present configuration, biasing element  16  effectively physically isolates substrate  18 , underlying socket  70 , and ball grid array  89  from loads applied between substrate  18  and heat sink  20 . Therefore, static and dynamic loads placed on heat sink  20  are transferred through frame  12  to circuit board  26 , and are not substantially borne by substrate  18 , nor transferred to socket  70  or ball grid array  89 .  
         [0020]     From the forgoing description, it should be apparent that the present system  10  provides a heat transfer mechanism to prevent an electronic substrate from overheating, while isolating, from the substrate loads applied to the heat sink. Certain changes may be made in the above methods and systems without departing from the scope of the present system. It is to be noted that all matter contained in the above description or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.