Patent Publication Number: US-6985359-B2

Title: Variable-wedge thermal-interface device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to concurrently filed, and commonly assigned U.S. patent application Ser. No. 10/419,386 titled “HEAT SINK HOLD-DOWN WITH FAN-MODULE ATTACH LOCATION,” and to concurrently filed, co-pending, and commonly assigned U.S. patent application Ser. No. 10/419,373 titled “VARIABLE-GAP THERMAL-INTERFACE DEVICE,” the disclosures of which are hereby incorporated herein by reference. This application is further related to 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 disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates to heat transfer and more particularly to a variable-gap thermal-interface device. 
     DESCRIPTION OF RELATED ART 
     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. 
     Accordingly, it would be advantageous to have a thermal-interface device and method that provide high thermal conductivity across a wide range of non-uniform gap thicknesses under moderate compressive loading and high temperature conditions. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a first embodiment disclosed herein, a variable-gap thermal-interface device for transferring heat from a heat source to a heat sink is provided. The device comprises a multi-axis rotary spherical joint comprising a spherically concave surface having a first radius of curvature in slideable contact with a spherically convex surface having the same first radius of curvature. The device further comprises a block having a proximal end rotatably coupled with the heat sink through the rotary spherical joint and having a distal end opposite the proximal end. The device further comprises a wedge having a variable thickness separating a first surface and a second surface opposite and inclined relative to the first surface, such that the first surface is thermally coupled with the distal end of the block, and the second surface is thermally coupled with the heat source. 
     In accordance with another embodiment disclosed herein, a method of transferring heat from a heat source to a heat sink using a variable-gap thermal-interface device is disclosed. The method comprises providing a multi-axis rotary spherical joint, and rotating the multi-axis rotary spherical joint to an orientation to compensate for misalignment between the heat source and the heat sink. The method further comprises providing a wedge having a variable thickness separating a first surface and a second surface opposite and inclined relative to the first surface, where the second surface is thermally coupled with the heat source. The method further comprises offsetting the wedge sufficiently to fill a gap between the heat source and the multi-axis rotary spherical joint. 
     In accordance with another embodiment disclosed herein, a spring clip shaped approximating a deformed rectangular frame is provided. The spring clip comprises a first side and a second side opposite the first side bent inward toward one another. The spring clip is operable to couple an elastic restoring force to the wedge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram representing a conformable thermal-interface device comprising an array of spring-loaded metal pistons sliding inside individual passageways of a thermal spreader; 
         FIG. 2  is a perspective view representing a variable-gap thermal-interface device, in accordance with embodiments disclosed herein; 
         FIG. 3  is a perspective view representing a wedge-socket variable-gap thermal-interface device; 
         FIG. 4  is a perspective view representing a wedge-socket variable-gap thermal-interface device in which the wedge and wedge-socket are held together by a spring clip; 
         FIG. 5A  is an exploded schematic representation of a wedge-ball variable-gap thermal-interface device; 
         FIG. 5B  is a schematic diagram illustrating adjustments that can be performed using a wedge-socket variable-gap thermal-interface device to compensate for a situation where heat source and heat sink base may lie in non-parallel planes and/or where the z-axis distance between heat source and heat sink base is non-uniform; 
         FIG. 6  is a graphic representation comparing the measured heat transfer performance of a wedge-socket variable-gap thermal-interface device with that of an alternative configuration; and 
         FIG. 7  is a schematic diagram illustrating a heat sink hold-down embodiment according to an incorporated disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram representing conformable thermal-interface device  120  comprising array of spring-loaded metal pistons  162   a – 16   c  sliding inside individual passageways  170  of thermal spreader  172 . Compressive load is applied by array of springs  164  to bias pistons  162   a – 162   c  to move along direction  166  in thermal contact with heat source  168  having an uneven surface. Springs  164  compress between spreader  172  and piston head  173  to accommodate the uneven surface of heat source  168 . In some embodiments, retaining element  176  couples with spreader  172 , and pistons  162   a – 162   c  have shoulders  178  that abut retaining element  176  when extended as in piston  162   a . Retaining element  176  forms apertures to accommodate passage of above-shoulder extensions  180  of pistons  162   a – 162   c . Accordingly, the retaining embodiment of  FIG. 1  ensures that pistons  162   a – 162   c  do not completely separate from spreader  172 . Heat sink  174  may optionally couple to spreader  172  to facilitate cooling of heat source  168 . While thermal-interface device  120  solves the problem of thermally contacting an uneven surface, the large relative void area between pistons  162   a – 162   c  reduces the effective thermal conductivity of thermal-interface device  120 . Furthermore, these void areas cause the effective thermal conductivity to be anisotropic, which can degrade heat transfer, particularly from a non-uniform heat source. Additionally, thermal-interface device  120  provides only a limited range of motion. Moreover, devices of this complexity are relatively expensive to produce. For further detail see 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 disclosure of which has been incorporated herein by reference. 
       FIG. 2  is a perspective view representing variable-gap thermal-interface device  20 , in accordance with embodiments disclosed herein. Heat sink extension  21  is a block of high-thermal-conductivity material rigidly attached or held under compression at upper end  22  to heat sink base  23 . Alternatively, heat sink extension  21  can be made as an integral part of heat sink base  23 . Lower end  24  of heat sink extension  21  has an integral spherically convex surface  25  of radius of curvature R. Socket block  26  of high-thermal-conductivity material comprises integral spherically concave socket  27  of matching radius of curvature R at its upper end, operable together in contact with spherically convex surface  25  to provide motion as a multi-axis spherical joint. Radius of curvature R can be any convenient radius, provided that radii of curvature R are matching for both spherically convex surface  25  and spherically concave surface  27 . In some embodiments, convex surface  25  and concave socket  27  can be interchanged, such that convex surface  25  is integral with block  26  and concave socket  27  is integral with heat sink extension  21 . In alternative embodiments, multi-axis spherical joint comprising spherically convex surface  25  and spherically concave surface  27  can be replaced by a single-axis cylindrical joint or by multiply-cascaded cylindrical joints, providing one or more rotational degrees of freedom. 
     Shim  29  is a plate of high thermal conductivity material that contacts flat surface  28  of the lower end of socket block  26 . The high conductivity materials of heat sink extension  21 , socket block  26 , and shim  29  can be either similar or dissimilar, and are typically metals, although they can alternatively be selected from insulators, composite materials, semiconductors and/or other solid materials as appropriate for a specific application. Interface device  20  can be dimensionally scalable over a range potentially from nanometers to meters. Interface device  20  is pressed against heat source  201  under compression from heat sink base  23 . Typically, heat source  201  contains integrated circuit (processor) chip  204  covered by processor lid  203  and mounted on circuit board  205 . Heat source  201  is attached to and supported by bolster plate  206 . The thickness of shim  29  is selected to sufficiently fill a gap between heat source  201  and socket block  26 , thus providing distance compensation between heat sink base  23  and heat source  201 . The interface between spherically convex surface  25  and spherically concave surface  27  forms a rotary joint that compensates for angular misalignment about any combination of axes between the planes of heat sink base  23  and heat source  201 . Thermal-interface material  202  , typically high conductivity grease, is optionally applied to enhance heat conduction and sliding motion at the interfaces between spherically convex surface  25 , spherically concave surface  27 , and shim  29 . 
       FIG. 3  is a perspective view representing a wedge-socket variable-gap thermal-interface device  30 . As in  FIG. 2 , thermal-interface device  30  comprises heat sink extension  21  with flat upper end adjacent heat sink base  23  (not shown in  FIG. 3 ) and lower spherically convex surface  25  of radius R. Wedge-socket  36  has an upper spherically concave surface  27  of radius R in rotational sliding contact with spherically convex surface  25 . For convenience, coordinate axes are shown in  FIG. 3  , such that x, y, and z are orthogonal rectangular axes fixed with respect to wedge-socket  36  and rotating through angular coordinates θ and φ about the common center of curvature of spherically convex surface  25  and spherically concave surface  27 . Wedge-socket  36  has a lower flat face inclined at a wedge angle relative to the x-axis of the xyz rotating coordinate system. 
     Wedge  39  has an upper surface inclined at the same wedge angle and in sliding contact with the lower inclined flat face of wedge-socket  36 . Although the lower flat face of wedge  39  can be inclined at any angle relative to the xyz rotating coordinate system, for convenience it is oriented parallel to the rotating xy plane. Wedge  39  contacts heat source  201  and provides heat transfer from heat source  201  through solid, high thermal-conductivity material of wedge-socket  36  and heat sink extension  21  to heat sink base  23  (not shown in  FIG. 3 ). The interface between wedge  39  and wedge-socket  36  may be filled with a thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and friction. Heat source  201  as shown in  FIG. 3  typically comprises the same layers as shown in  FIG. 2 , namely processor chip  204 , processor lid  203 , and circuit board  205 . 
       FIG. 4  is a perspective view representing wedge-socket variable-gap thermal-interface device  40 , comprising wedge-socket variable-gap thermal-interface device  30  in which wedge  39  and wedge-socket  36  are spring-loaded in the x-direction by spring clip  41 . In one variation, spring clip  41  is shaped approximating a deformed rectangular frame. Two opposite sides  42   a ,  42   b  may be but need not be straight and parallel as shown in  FIG. 4 . Two remaining opposing sides  43   a ,  43   b  are bent inward toward one another and are tempered to exert a compressive squeezing force toward one another. In wedge-socket variable-gap thermal-interface device  40 , spring clip  41  is aligned, so that a first inwardly bent side, for example side  43   a , presses against the largest area vertical surface (normal to the x-axis) of wedge  39 , and a second inwardly bent side, for example side  43   b , presses against the largest area vertical surface (also normal to the x-axis) of wedge-socket  36 . Compressive forces applied by spring clip  41  generate shear force components along the incline of wedge  39 , causing the contacting inclined surfaces of wedge  39  and wedge-socket  36  to slide across one another, thereby extending the length of the z-axis wedge-socket variable-gap thermal-interface device  40  to fill the available gap between heat sink extension  21  and heat source  201 . This simultaneously drives the wedge components to become offset relative to one another along the x-axis, reducing the inclined contact area. When the gap is filled, z-axis compressive forces prevent further offset between wedge  39  and wedge-socket  36 . Spring clip  41  can be used similarly to apply shear forces to sliding wedge elements in other applications, including heat transfer and non-heat transfer applications. 
     The socket end of wedge-socket  36  is spherically concave with radius of curvature R in the present example, and contacts a surface of heat sink extension  21  which is spherically convex in the present example with the same radius of curvature R. This provides adjustment in angle about three axes. Again, the interfaces between wedge-socket  36  and heat sink extension  21  and between contacting inclined surfaces of wedge  39  and wedge-socket  36  may be filled with a thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and sliding friction. Wedge-socket variable-gap thermal-interface devices  30  and  40  are potentially scalable dimensionally over a range from nanometers to meters. 
       FIG. 5A  is an exploded schematic representation of wedge-ball variable-gap thermal-interface device  50 , which is a variation of wedge-socket variable-gap thermal-interface device  40 . In the example of  FIG. 5A , heat sink extension  51  has a lower spherically concave socket of radius of curvature R rotationally matching spherically convex ball of radius R on the upper surface of wedge-ball  56 . Wedge-ball  56  has a flat inclined lower surface configured to slide across the top inclined surface of wedge  39 . Spring clip  41  is disposed to spring-load wedge-ball  56  and wedge  39  with a shear force. As shown in the example of  FIG. 5A , spring clip  41  can be secured to wedge-ball  56  using set screw  55  or other traditional fastener. As in previously described examples, the interfaces between wedge-ball  56  and heat sink extension  51  and between contacting inclined surfaces of wedge  39  and wedge-ball  56  may be filled with a thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and sliding friction. 
       FIG. 5B  is a schematic diagram illustrating adjustments that can be performed using wedge-socket variable-gap thermal-interface device  40  to compensate for a situation where heat source  203 – 204  and heat sink base  23  may lie in non-parallel planes and/or where the z-axis distance between heat source  203 – 204  and heat sink base  23  is non-uniform. Heat source  203 – 204  is supported by bolster plate  206 . All adjustments are performed under compressive loading between heat sink base  23  and bolster plate  206 . Spring clip  41  generates a shear force, that causes the wedged surfaces of wedge-socket  36  and wedge  39  to slide across one another. To compensate for tilt angle α between heat source  203 – 204  and heat sink base  23 , wedge-socket  36  is rotated relative to the spherically convex surface of heat sink extension  21  through rotation angle α. As illustrated, this is accompanied by a corresponding offset of wedge-socket  36  relative to heat sink extension  21 . Although for simplicity of illustration, tilt angle α is shown in the xz-plane, in the general case, tilt angle α can lie in any plane containing the common center of curvature of the spherically convex surface of heat sink extension  21  and the spherically concave surface of wedge-socket  36 . 
     To compensate for a z-axis gap of width h, compressive loading by spring clip  41  between heat sink base  23  and bolster plate  206  generates a shear force component that drives an offset perpendicular to the z-axis between the wedged components of wedge  39  and wedge-socket  36 . Because of the wedge geometry, this extends the z-axis length of combined wedge  39  and wedge-socket  36 . When the z-axis extension reaches an incremental length h, then the gap is filled, and the corresponding offset between the wedged components wedge  39  and wedge-socket  36  is δ, where the ratio h/δ is just the incline slope of the wedge. Compressive z-axis loading between heat sink base  23  and bolster plate  206  then prevents further sliding offset between wedge  39  and wedge-socket  36 . 
       FIG. 6  is a graphic representation comparing the measured heat transfer performance of a wedge-socket variable-gap thermal-interface device, for example wedge-socket variable-gap thermal-interface device  40 , with that of an alternative configuration similar to that illustrated in  FIG. 1 . The vertical axis plots specific thermal resistance in relative units normalized per unit area, as a function of compressive load in arbitrary normalized pressure units along the horizontal axis. Pressure is applied uniformly across the respective heat transfer surfaces. Curve  61  represents the performance of a configuration similar to wedge-socket variable-gap thermal-interface device  40 , curve  62  represents performance of a device similar to that of  FIG. 1 , in which the piston is all copper, and curve  63  represents performance of a device similar to that of  FIG. 1 , in which the piston is all aluminum. In accordance with the data plotted in  FIG. 6 , curve  61  advantageously shows a relatively lower thermal resistance that is reached at lower applied pressures than exhibited in either of curves  62  or  63 . 
     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 concurrently filed, 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. 7  is a schematic diagram illustrating heat sink hold-down device  70  according to the incorporated disclosure. Bolster plate  206  supports heat source  201 . Heat sink  73  includes heat sink base  23  attached to central post  74 , and finned structure  72 . Cage  75  is attached with clips to bolster plate  206  and supports lever spring  76  through clearance slots. Cap  77  rigidly attached to cage  75  using screws or other fasteners  78  presses downward on the ends of lever spring  76 , which transfer the load through a bending moment to central post  74 . Central post  74  is disposed to distribute the load symmetrically across the area of heat sink base  23 . 
     In some embodiments, heat sink extension  71  transfers the compressive loading between heat sink base  23  and heat source  201 . Alternatively, a variable-gap thermal-interface device in accordance with the present embodiments, for example variable-gap thermal-interface device  20  or wedge-socket variable-gap thermal-interface device  40 , is coupled thermally and mechanically with heat sink hold-down device  70 , replacing heat sink extension  71  in its entirety. In this configuration, heat sink hold-down device  70  applies the loading that holds variable-gap thermal-interface device  20 ,  40  under compression against heat source  201 . 
     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.