Abstract:
A variable-height thermal-interface device is provided for transferring heat from a heat source to a heat sink. The device comprises a first uniaxial rotary cylindrical joint comprising a first cylindrically concave surface in slidable contact with a first cylindrically convex surface. The first cylindrically concave surface and the first cylindrically convex surface share a common first radius of curvature relative to a common first cylinder axis. The first cylindrically concave surface is operable to rotate about the common first cylinder axis relative to the first cylindrically convex surface to compensate for uniaxial angular misalignment between the heat source and the heat sink.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to concurrently filed, co-pending, and commonly assigned U.S. Patent Application [Attorney docket 200300041-1], titled “METHOD OF ASSEMBLY OF A WEDGE THERMAL INTERFACE TO ALLOW EXPANSION AFTER ASSEMBLY”; co-pending and commonly assigned U.S. patent application Ser. No. 10/419,386, titled “HEAT SINK HOLD-DOWN WITH FAN-MODULE ATTACH LOCATION,” filed Apr. 21, 2003; co-pending and commonly assigned U.S. patent application Ser. No. 10/419,373, titled “VARIABLE-GAP THERMAL-INTERFACE DEVICE,” filed Apr. 21, 2003; co-pending and commonly assigned U.S. patent application Ser. No. 10/419,406, titled “VARIABLE-WEDGE THERMAL-INTERFACE DEVICE,” filed Apr. 21, 2003; and co-pending and commonly assigned U.S. patent application Ser. No. 10/074,642, titled THERMAL TRANSFER INTERFACE SYSTEM AND METHODS,” filed Feb. 12, 2002; the disclosures of all of which are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to heat transfer and more particularly to a variable height thermal interface.  
       DESCRIPTION OF THE RELATED ART  
       [0003]     There are circumstances in which a heat sink is fixed at a set distance above a heat source, for example a processor or other active electronic device. Due to variations in thickness of the parts, primarily the active device, a gap of unknown height may exist between the heat sink and the active device. There is then a need for a thermal interface to fill the gap and concurrently provide good heat transfer properties.  
         [0004]     Traditionally, heat has been transferred between a heat source and a heat sink across non-uniform width gaps through the use of “gap pads,” or silicone-based elastic pads. For example, The Bergquist Company (see web page http://www.bergquistcompany.com/tm_gap_list.cfm and related web pages) offers a range of conformable, low-modulus filled silicone elastomer pads of various thickness on rubber-coated fiberglass carrier films. This material can be used as a thermal-interface, where one side of the interface is in contact with an active electronic device. Relative to metals, these pads have low thermal conductivity. Furthermore, large forces are generally required to compress these pads. Moreover, silicone-based gap pads cannot withstand high temperatures.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     In accordance with an embodiment disclosed herein, a variable-height thermal-interface device is provided for transferring heat from a heat source to a heat sink. The device comprises a first uniaxial rotary cylindrical joint comprising a first cylindrically concave surface in slidable contact with a first cylindrically convex surface. The first cylindrically concave surface and the first cylindrically convex surface share a common first radius of curvature relative to a common first cylinder axis. The first cylindrically concave surface is operable to rotate about the common first cylinder axis relative to the first cylindrically convex surface to compensate for uniaxial angular misalignment between the heat source and the heat sink.  
         [0006]     In accordance with another embodiment disclosed herein, a variable-height thermal-interface device is provided for transferring heat from a heat source to a heat sink. The device comprises a first wedge interface having a first planar surface in slidable contact with a second planar surface. The slidably contacting first and second planar surfaces are inclined diagonally relative to the z-axis parallel to the shortest distance between the heat source and the heat sink. The first wedge interface is operable to provide z-axis expansion of the variable height thermal interface device. The device further comprises a second wedge interface having a third planar surface in slidable contact with a fourth planar surface. The slidably contacting third and fourth planar surfaces are inclined diagonally relative to the z-axis. The second wedge interface is operable to provide z-axis expansion of the variable height thermal interface device.  
         [0007]     In accordance with yet another embodiment disclosed herein, a method of transferring heat from a heat source to a heat sink using a variable-height thermal-interface device is provided. The method comprises providing a first uniaxial rotary cylindrical joint comprising a first cylindrically concave surface in slidable contact with a first cylindrically convex surface, the first cylindrically convex surface and the first cylindrically concave surface sharing a common first radius of curvature relative to a common first cylinder axis. The method further comprises sliding the first cylindrically concave surface relative to the first cylindrically convex surface, causing filling of gaps between the heat source and the heat sink. The method further comprises applying compressive loading between the heat source and the heat sink through the first uniaxial rotary cylindrical joint, and transferring heat from the heat source through the first uniaxial rotary cylindrical joint to the heat sink. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1A  is a perspective view of an embodiment illustrating a variable-wedge thermal-interface device  
         [0009]      FIG. 1B  is a perspective view of an embodiment illustrating a spring-loaded variable-wedge thermal-interface device, in which a spring clip is added to the thermal-interface device of  FIG. 1A ;  
         [0010]      FIG. 2  is a perspective view of an embodiment illustrating a cascaded variable-wedge thermal-interface device, in which two or more wedge structures similar to variable-wedge thermal-interface devices depicted in  FIGS. 1A and 1B  are stacked or cascaded in the z-direction;  
         [0011]      FIG. 3A  is a perspective view of an embodiment illustrating an assembled variable-height thermal-interface device, including at least one single-axis rotary cylindrical joint;  
         [0012]      FIG. 3B  is an exploded perspective view of an embodiment illustrating the variable-height thermal-interface device of  FIG. 3A ; and  
         [0013]      FIG. 4  is a schematic diagram of an embodiment illustrating a heat sink hold-down device, in accordance with a disclosure incorporated herein. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The embodiments disclosed herein describe a system and method for creating a thermal interface that will fill a variable gap and concurrently provide efficient heat transfer properties.  
         [0015]      FIGS. 1A and 1B  show a wedge-based variable gap thermal interface, as disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 10/419,406, the disclosure of which has been incorporated herein by reference.  
         [0016]      FIG. 1A  is a perspective view of an embodiment illustrating variable-wedge thermal-interface device  110 . Thermal-interface device  110  comprises heat sink extension  106  with flat upper end  107  mechanically and thermally coupled to a heat sink base (not shown in  FIG. 1A ). Alternatively, heat sink extension  106  may be fabricated as an integral part of the heat sink or heat sink base. For convenience, coordinate axes are shown in  FIG. 1A , such that x, y, and z are orthogonal rectangular axes fixed with respect to heat sink extension  106  and rotating through angular coordinates θ and φ about the respective z and y axes. Heat sink extension  106  has a lower flat face inclined at a wedge angle relative to the x-axis in the example of  FIG. 1A .  
         [0017]     Lower wedge element  105  has an upper surface inclined at the same wedge angle and making sliding contact with the lower inclined flat face of heat sink extension  106 . Although the lower flat face of lower wedge element  105  can be inclined at any angle relative to the xyz rotating coordinate system, for convenience in the example of  FIG. 1A  it is oriented parallel to the rotating xy plane. Likewise, although the lower flat face of lower wedge element  105  can be inclined at any angle relative to flat upper end  107  of heat sink extension  106 , for convenience in the example of  FIG. 1A  it is oriented parallel to flat upper end  107 . Lower wedge element  105  is coupled thermally and mechanically to heat source  101  and thus provides efficient heat transfer from heat source  101  through solid, high thermal-conductivity material of lower wedge element  105  and heat sink extension  106  to the heat sink base. The sliding-contact interface between lower wedge element  105  and heat sink extension  106  may be filled with a conformal thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and friction. Heat source  101 , as shown in the example of  FIG. 1A , includes processor chip  104 , processor lid  102 , and circuit board  103 .  
         [0018]      FIG. 1B  is a perspective view of an embodiment illustrating spring-loaded variable-wedge thermal-interface device  120 , in which spring clip  141  is added to thermal-interface device  110  of  FIG. 1A . A wedge-based thermal-interface device including a spring clip is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 10/419,406, the disclosure of which has been incorporated herein by reference. In the example of  FIG. 1B , wedge element  105  and heat sink extension  106  are spring-loaded together in the x-direction by spring clip  141 . In one variation, spring clip  141  is shaped approximating a deformed rectangular frame. Two opposing sides  142   a ,  142   b  may be but need not necessarily be straight and parallel as shown in  FIG. 1B . Two alternating opposing sides  143   a ,  143   b  are typically bent inward toward one another and are pre-stressed to exert a compressive force toward one another.  
         [0019]     In spring-loaded variable-wedge thermal-interface device  120 , spring clip  141  is aligned, so that a first inwardly bent side, for example side  143   a , presses against the largest area vertical surface (aligned normal to the x-axis) of wedge element  105 , and a second inwardly bent side, for example side  143   b , presses against the largest area vertical surface (also aligned normal to the x-axis) of heat sink extension  106 . The combined compressive forces applied by spring clip  141  to wedge element  105  and heat sink extension  106  generate shear force components across the inclined interface between wedge element  105  and heat sink extension  106 , urging the contacting inclined interface surfaces of wedge element  105  and heat sink extension  106  to slide relative to one another, thereby driving wedge element  105  to expand the z-axis length of spring-loaded variable-wedge thermal-interface device  120  to fill the available gap between heat sink extension  106  and heat source  101 . This simultaneously drives wedge element  105  along the x-axis to become offset relative to heat sink extension  106 , thereby somewhat reducing the inclined surface contact area. When the z-axis gap is filled, z-axis compressive forces prevent further offset between wedge element  105  and heat sink extension  106 . Spring clip  141  may be used similarly to apply shear forces to sliding wedge elements in other applications, including both heat transfer and non-heat transfer applications. Optionally, spring clip  141  may be attached to one of the wedge elements using a screw, bolt, or other traditional fastener.  
         [0020]      FIG. 2  is a perspective view of an embodiment illustrating cascaded variable-wedge thermal-interface device  200 , in which two or more wedge structures similar to variable-wedge thermal-interface devices  110  and  120  depicted in  FIGS. 1A and 1B  are stacked or cascaded in the z-direction. For purposes of illustration, in the example of  FIG. 2  are depicted two such wedge structures having inclined wedge interfaces  215  and  216  oriented at a 90-degree rotation angle about the z-axis relative to one another. In other implementations, arbitrary numbers of wedge structures may be stacked at arbitrary orientations relative to one another. For most applications, however, there is little or no advantage achieved by increasing the number of cascaded wedge structures beyond two.  
         [0021]     In the example depicted in  FIG. 2 , wedge interface  215 , formed between lower wedge element  205  and second wedge element  206 , is inclined to provide offset motion along the x-direction, whereas wedge interface  216 , formed between second wedge element  206  and heat sink extension  207 , is inclined to provide offset motion along the y-direction. Lower surface  204  of lower wedge element  205  is typically flat and is coupled thermally and mechanically with a heat source, for example heat source  101  in  FIGS. 1A and 1B , whereas heat sink extension  207  typically has a flat upper surface, but is typically coupled thermally and mechanically with a heat sink (not pictured). The upper surface of heat sink extension  207  is typically but not necessarily parallel to lower surface  204 . Alternatively, heat sink extension  207  may be fabricated as an integral part of a heat sink or heat sink base. Multiply-cascaded wedge thermal interfaces, for example wedge interfaces  215  and/or  216 , may be spring-loaded under shear force, for example using spring clips, as represented by spring clip  141  in  FIG. 1B .  
         [0022]     Co-pending and commonly assigned U.S. patent application Ser. No. 10/419,406, the disclosure of which has been incorporated herein by reference, discloses a variable-wedge thermal-interface device that includes a multi-axis rotary spherical joint. This implementation is particularly advantageous for multi-axis angular adjustment in a situation in which the heat source and the heat sink may lie in non-parallel planes and/or where the distance between heat source and heat sink is non-uniform. This situation arises frequently when attempting to conduct heat from multiple heat sources to a single heat sink.  
         [0023]      FIG. 3A  is a perspective view of an embodiment illustrating assembled variable-height thermal-interface device  300 , including at least one single-axis rotary cylindrical joint  315   a - 315   b ,  316   a - 316   b .  FIG. 3B  is an exploded perspective view of an embodiment illustrating variable-height thermal-interface device  300 .  FIGS. 3A-3B  depict a variable-height thermal-interface device  300  including two cascaded cylindrical joints  315   a - 315   b  and  316   a - 316   b  oriented orthogonally relative to one another about the z-axis and having respective cylinder axes  325  and  326  each inclined relative to the x-y plane. In general, variable-height thermal-interface devices, in accordance with the disclosed embodiments, may contain from one cylindrical joint to any number of cascaded cylindrical joints, each of which may be oriented at any angle(s) about the z-axis relative to any other cylindrical joint, and each of which may have a cylinder axis oriented parallel with the x-y plane or inclined at any angle relative to the x-y plane.  
         [0024]     In the example embodiment depicted in  FIG. 3A  and/or  3 B, cylindrical joint  315   a - 315   b  is formed at the sliding interface between concave upper surface  321  of lower element  305  and convex lower surface  322  (hidden in  FIG. 3B ) of second element  306 . Concave surface  321  and convex surface  322  have radii of curvature matched to one another, represented by broken-line arrow  335 , centered at cylinder axis  325 . Concave surface  321  is rotatably slidable relative to convex surface  322  about cylinder axis  325 , as represented by curved arrows α 1 , providing single-axis bending capability in variable-height thermal-interface device  300 . Orthogonally, concave surface  321  is linearly slidable parallel to cylinder axis  325  relative to convex surface  322 , as represented by linear arrows ±Δ 1 , providing single-axis translation capability in variable-height thermal-interface device  300 .  
         [0025]     Cylindrical joint  316   a - 316   b  is similarly formed at the sliding interface between concave upper surface  323  of second element  306  and the convex lower surface  324  (hidden in  FIG. 3B ) of heat sink extension  307 . Concave surface  323  and convex surface  324  have radii of curvature matched to one another, represented by broken-line arrow  336 , centered at cylinder axis  326 . Concave surface  323  is rotatably slidable relative to convex surface  324  about cylinder axis  326 , as represented by curved arrows α 2 , providing single-axis bending capability in variable-height thermal-interface device  300 . Orthogonally, concave surface  323  is linearly slidable, parallel to cylinder axis  326  relative to convex surface  324 , as represented by linear arrows ±Δ 2 , providing single-axis translation capability in variable-height thermal-interface device  300 .  
         [0026]     Radii of curvature  335  and  336  may be but need not necessarily be matched between different joints of the same variable height thermal interface device. Cylinder axes  325 ,  326  may be parallel to the x-y plane or may be oriented or inclined at any angle relative to the x-y plane and/or relative to one another. Cylindrical joints having cylinder axes so inclined may interface wedged elements, such that relative translation between interfacing elements provides z-axis expansion of the variable height thermal interface device. Interfacing elements of a cylindrical joint may optionally be spring-loaded for shear force across the interface, facilitating z-axis expansion in a manner similar to spring-loaded variable-wedge thermal-interface device  120  depicted in  FIG. 1B . As in the case of variable height thermal interface devices previously described herein, the interfaces between contacting cylindrical surfaces may be filled with a thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and sliding friction.  
         [0027]     Two stacked or cascaded orthogonally oriented cylindrical joints provide the same degrees of bending motion as those provided by a single rotary spherical joint. Advantages of a cylindrical-joint variable thermal interface implementation include:  
         [0028]     First, a cylindrical surface is much easier to fabricate than a sphere. A cylindrical surface can be machined using many methods, including any of the following methods: 
        Horizontal form milling;     Crush-form grinding;     Diamond dress grinding (traditional method of grinding bearing raceways);     Fly-cutting, where the path of the part is at an oblique angle to the axis of the fly-cutter. This will in reality create a surface that is not quite cylindrical, but rather elliptical. Modeling has shown that the deviation between the surfaces can be less than 1.5 nanometers (nm), when the rotation range required for heat source tilt is considered.        
 
         [0033]     The cost of machining a bearing raceway is $0.05 to $0.10 per cut. If all three elements of a variable height thermal interface device were made of copper, about 32 grams of copper would be required, at a total material cost of about $0.22. The cost of machining each of the six required cuts is ˜$0.60. An assembly could then cost less than a dollar.  
         [0034]     Second, with two stacked inclined cylindrical joints, the vertical travel can be taken up by both of the effective wedges. This doubles the vertical travel range of the variable height thermal interface. In accordance with the embodiments disclosed herein, a variable height thermal interface device may include from one to any larger number of stacked cylindrical joints, spherical joints, wedge interfaces, or any combination of these three structures. A cylindrical or spherical joint provides respectively uniaxial or multi-axial compensation for misalignment between a heat source and a heat sink, whereas a wedge interface provides variable height z-axis gap compensation between the heat source and heat sink. An inclined-axis cylindrical joint provides hybrid capabilities of a cylindrical joint combined with a wedge interface.  
         [0035]     Wedge-based variable thermal-interface devices, for example variable thermal interface devices  200  and  300  are potentially scalable dimensionally over a range from nanometers (nm) to meters.  
         [0036]     In practice, the compressive load between the heat sink base and bolster plate in any of the embodiments disclosed herein can be provided by any of a variety of heat sink hold-down devices. An advantageous configuration of such a hold-down device is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 10/419,386, the disclosure of which has been incorporated herein by reference.  FIG. 4  is a schematic diagram of an embodiment illustrating heat sink hold-down device  40 , in accordance with the above-incorporated disclosure. Bolster plate  49  supports heat source  101 . Heat sink  43  includes heat sink base  401  attached to central post  44 , and finned structure  42 . Cage  45  is attached with clips to bolster plate  49  and supports lever spring  46  through clearance slots. Cap  47  rigidly attached to cage  45  using screws or other fasteners  48  presses downward on the ends of lever spring  46 , which transfer the load through a bending moment to central post  44 . Central post  44  is disposed to distribute the load symmetrically across the area of heat sink base  401 .  
         [0037]     In some embodiments, heat sink extension  41  transfers the compressive loading between heat sink base  401  and heat source  101 . Alternatively, a variable-height thermal-interface device in accordance with the present embodiments, for example variable-height thermal-interface device  110 ,  120 ,  200  or cylindrical joint variable-height thermal-interface device  300 , is coupled thermally and mechanically with heat sink hold-down device  40 , replacing at least in part heat sink extension  41 . In this configuration, heat sink hold-down device  40  applies the loading that holds variable-height thermal-interface device  110 ,  120 ,  200  or cylindrical joint variable-height thermal-interface device  300  under compression against heat source  101 .  
         [0038]     Embodiments disclosed herein address the problem of minimizing the thermal resistance between a heat source and a heat sink for a situation in which the heat source and the heat sink may lie in non-parallel planes and/or where the distance between heat source and heat sink is non-uniform. This is a problem that arises especially when attempting to conduct heat from more than one heat source to a single heat sink.