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.

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.

DETAILED DESCRIPTION

FIG. 1is a schematic diagram representing conformable thermal-interface device120comprising array of spring-loaded metal pistons162a–16csliding inside individual passageways170of thermal spreader172. Compressive load is applied by array of springs164to bias pistons162a–162cto move along direction166in thermal contact with heat source168having an uneven surface. Springs164compress between spreader172and piston head173to accommodate the uneven surface of heat source168. In some embodiments, retaining element176couples with spreader172, and pistons162a–162chave shoulders178that abut retaining element176when extended as in piston162a. Retaining element176forms apertures to accommodate passage of above-shoulder extensions180of pistons162a–162c. Accordingly, the retaining embodiment ofFIG. 1ensures that pistons162a–162cdo not completely separate from spreader172. Heat sink174may optionally couple to spreader172to facilitate cooling of heat source168. While thermal-interface device120solves the problem of thermally contacting an uneven surface, the large relative void area between pistons162a–162creduces the effective thermal conductivity of thermal-interface device120. 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 device120provides 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. 2is a perspective view representing variable-gap thermal-interface device20, in accordance with embodiments disclosed herein. Heat sink extension21is a block of high-thermal-conductivity material rigidly attached or held under compression at upper end22to heat sink base23. Alternatively, heat sink extension21can be made as an integral part of heat sink base23. Lower end24of heat sink extension21has an integral spherically convex surface25of radius of curvature R. Socket block26of high-thermal-conductivity material comprises integral spherically concave socket27of matching radius of curvature R at its upper end, operable together in contact with spherically convex surface25to 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 surface25and spherically concave surface27. In some embodiments, convex surface25and concave socket27can be interchanged, such that convex surface25is integral with block26and concave socket27is integral with heat sink extension21. In alternative embodiments, multi-axis spherical joint comprising spherically convex surface25and spherically concave surface27can be replaced by a single-axis cylindrical joint or by multiply-cascaded cylindrical joints, providing one or more rotational degrees of freedom.

Shim29is a plate of high thermal conductivity material that contacts flat surface28of the lower end of socket block26. The high conductivity materials of heat sink extension21, socket block26, and shim29can 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 device20can be dimensionally scalable over a range potentially from nanometers to meters. Interface device20is pressed against heat source201under compression from heat sink base23. Typically, heat source201contains integrated circuit (processor) chip204covered by processor lid203and mounted on circuit board205. Heat source201is attached to and supported by bolster plate206. The thickness of shim29is selected to sufficiently fill a gap between heat source201and socket block26, thus providing distance compensation between heat sink base23and heat source201. The interface between spherically convex surface25and spherically concave surface27forms a rotary joint that compensates for angular misalignment about any combination of axes between the planes of heat sink base23and heat source201. Thermal-interface material202, typically high conductivity grease, is optionally applied to enhance heat conduction and sliding motion at the interfaces between spherically convex surface25, spherically concave surface27, and shim29.

FIG. 3is a perspective view representing a wedge-socket variable-gap thermal-interface device30. As inFIG. 2, thermal-interface device30comprises heat sink extension21with flat upper end adjacent heat sink base23(not shown inFIG. 3) and lower spherically convex surface25of radius R. Wedge-socket36has an upper spherically concave surface27of radius R in rotational sliding contact with spherically convex surface25. For convenience, coordinate axes are shown inFIG. 3, such that x, y, and z are orthogonal rectangular axes fixed with respect to wedge-socket36and rotating through angular coordinates θ and φ about the common center of curvature of spherically convex surface25and spherically concave surface27. Wedge-socket36has a lower flat face inclined at a wedge angle relative to the x-axis of the xyz rotating coordinate system.

Wedge39has an upper surface inclined at the same wedge angle and in sliding contact with the lower inclined flat face of wedge-socket36. Although the lower flat face of wedge39can be inclined at any angle relative to the xyz rotating coordinate system, for convenience it is oriented parallel to the rotating xy plane. Wedge39contacts heat source201and provides heat transfer from heat source201through solid, high thermal-conductivity material of wedge-socket36and heat sink extension21to heat sink base23(not shown inFIG. 3). The interface between wedge39and wedge-socket36may be filled with a thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and friction. Heat source201as shown inFIG. 3typically comprises the same layers as shown inFIG. 2, namely processor chip204, processor lid203, and circuit board205.

FIG. 4is a perspective view representing wedge-socket variable-gap thermal-interface device40, comprising wedge-socket variable-gap thermal-interface device30in which wedge39and wedge-socket36are spring-loaded in the x-direction by spring clip41. In one variation, spring clip41is shaped approximating a deformed rectangular frame. Two opposite sides42a,42bmay be but need not be straight and parallel as shown inFIG. 4. Two remaining opposing sides43a,43bare bent inward toward one another and are tempered to exert a compressive squeezing force toward one another. In wedge-socket variable-gap thermal-interface device40, spring clip41is aligned, so that a first inwardly bent side, for example side43a, presses against the largest area vertical surface (normal to the x-axis) of wedge39, and a second inwardly bent side, for example side43b, presses against the largest area vertical surface (also normal to the x-axis) of wedge-socket36. Compressive forces applied by spring clip41generate shear force components along the incline of wedge39, causing the contacting inclined surfaces of wedge39and wedge-socket36to slide across one another, thereby extending the length of the z-axis wedge-socket variable-gap thermal-interface device40to fill the available gap between heat sink extension21and heat source201. 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 wedge39and wedge-socket36. Spring clip41can 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-socket36is spherically concave with radius of curvature R in the present example, and contacts a surface of heat sink extension21which 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-socket36and heat sink extension21and between contacting inclined surfaces of wedge39and wedge-socket36may 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 devices30and40are potentially scalable dimensionally over a range from nanometers to meters.

FIG. 5Ais an exploded schematic representation of wedge-ball variable-gap thermal-interface device50, which is a variation of wedge-socket variable-gap thermal-interface device40. In the example ofFIG. 5A, heat sink extension51has a lower spherically concave socket of radius of curvature R rotationally matching spherically convex ball of radius R on the upper surface of wedge-ball56. Wedge-ball56has a flat inclined lower surface configured to slide across the top inclined surface of wedge39. Spring clip41is disposed to spring-load wedge-ball56and wedge39with a shear force. As shown in the example ofFIG. 5A, spring clip41can be secured to wedge-ball56using set screw55or other traditional fastener. As in previously described examples, the interfaces between wedge-ball56and heat sink extension51and between contacting inclined surfaces of wedge39and wedge-ball56may be filled with a thermal-interface material, typically thermal grease or paste, to reduce both thermal resistance and sliding friction.

FIG. 5Bis a schematic diagram illustrating adjustments that can be performed using wedge-socket variable-gap thermal-interface device40to compensate for a situation where heat source203–204and heat sink base23may lie in non-parallel planes and/or where the z-axis distance between heat source203–204and heat sink base23is non-uniform. Heat source203–204is supported by bolster plate206. All adjustments are performed under compressive loading between heat sink base23and bolster plate206. Spring clip41generates a shear force, that causes the wedged surfaces of wedge-socket36and wedge39to slide across one another. To compensate for tilt angle α between heat source203–204and heat sink base23, wedge-socket36is rotated relative to the spherically convex surface of heat sink extension21through rotation angle α. As illustrated, this is accompanied by a corresponding offset of wedge-socket36relative to heat sink extension21. 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 extension21and the spherically concave surface of wedge-socket36.

To compensate for a z-axis gap of width h, compressive loading by spring clip41between heat sink base23and bolster plate206generates a shear force component that drives an offset perpendicular to the z-axis between the wedged components of wedge39and wedge-socket36. Because of the wedge geometry, this extends the z-axis length of combined wedge39and wedge-socket36. When the z-axis extension reaches an incremental length h, then the gap is filled, and the corresponding offset between the wedged components wedge39and wedge-socket36is δ, where the ratio h/δ is just the incline slope of the wedge. Compressive z-axis loading between heat sink base23and bolster plate206then prevents further sliding offset between wedge39and wedge-socket36.

FIG. 6is 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 device40, with that of an alternative configuration similar to that illustrated inFIG. 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. Curve61represents the performance of a configuration similar to wedge-socket variable-gap thermal-interface device40, curve62represents performance of a device similar to that ofFIG. 1, in which the piston is all copper, and curve63represents performance of a device similar to that ofFIG. 1, in which the piston is all aluminum. In accordance with the data plotted inFIG. 6, curve61advantageously shows a relatively lower thermal resistance that is reached at lower applied pressures than exhibited in either of curves62or63.

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. 7is a schematic diagram illustrating heat sink hold-down device70according to the incorporated disclosure. Bolster plate206supports heat source201. Heat sink73includes heat sink base23attached to central post74, and finned structure72. Cage75is attached with clips to bolster plate206and supports lever spring76through clearance slots. Cap77rigidly attached to cage75using screws or other fasteners78presses downward on the ends of lever spring76, which transfer the load through a bending moment to central post74. Central post74is disposed to distribute the load symmetrically across the area of heat sink base23.

In some embodiments, heat sink extension71transfers the compressive loading between heat sink base23and heat source201. Alternatively, a variable-gap thermal-interface device in accordance with the present embodiments, for example variable-gap thermal-interface device20or wedge-socket variable-gap thermal-interface device40, is coupled thermally and mechanically with heat sink hold-down device70, replacing heat sink extension71in its entirety. In this configuration, heat sink hold-down device70applies the loading that holds variable-gap thermal-interface device20,40under compression against heat source201.

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.