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United States Patent Application	20040035558
Kind Code	A1
Todd, John J. ;   et al.	February 26, 2004

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Claims

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What is claimed is:

1. A heat transfer device for dissipating heat from a heat source, the device comprising: a heat pipe including a vessel to be placed in thermally conductive relation to the heat source, the heat pipe comprising thermally conductive material at least at an evaporator part and at a condenser part that are in fluid communication with one another and contain a heat transfer fluid for movement in a cycle between the evaporator and the condenser; a base plate for at least partly supporting the heat pipe, the base plate having a side to be directed toward a heat source, and at least one through opening leading into a receptacle on the side of the base plate directed toward the heat source; wherein the at least a part of the heat pipe extends into the through opening to the receptacle and is positioned for contact with the heat source.

2. The heat transfer device of claim 1, further comprising a heat sink disposed opposite from the side of the base plate directed toward the heat source.

3. The heat transfer device of claim 2, wherein the evaporator is disposed in the receptacle and the condenser is in thermal transfer relation with the heat sink.

4. The heat transfer device of claim 1, further comprising a wicking material in the vessel, capable of supporting a capillary flow of the heat transfer fluid at least for part of the cycle between the condenser to the evaporator.

5. The heat transfer device of claim 1, wherein the receptacle forms a channel on the side of the base plate directed toward the heat source and the vessel comprises an elongated tubular structure disposed in the channel.

6. The heat transfer device of claim 5, wherein the base plate has a thickness greater)than an outside of the elongated tubular structure, and wherein the receptacle is dimensioned to complement an outside, shape of the elongated tubular structure, such that the elongated tubular structure rests substantially in surface contact with the base plate at the receptacle.

7. The heat transfer device of claim 6, wherein the elongated tubular structure is flattened along a surface coextensive with a surface of the base plate on the side directed toward the heat source.

8. The heat transfer device of claim 7, wherein the base plate has at least one further through opening leading into the receptacle on the side of the base plate directed toward the heat source, and wherein the elongated tubular structure forms a U-shape or L-shape with legs traversing the through openings and a bottom disposed in the receptacle.

9. The heat transfer device of claim 8, wherein the legs comprise parallel sections extending from the base plate and a transverse bend adjacent to the bottom.

10. The heat transfer device of claim 9, wherein the receptacle forms a U-shape or L-shape in a plane perpendicular to the leg sections and the vessel is press fit into the U-shape or L-shape of the receptacle.

11. The heat transfer device of claim 10, further comprising at least one heat sink mounted on the leg sections.

12. The heat transfer device of claim 11, comprising a plurality of heat transfer fins carried on said legs sections and spaced along the leg sections from the base plate.

13. The heat transfer device of claim 8, comprising at least one further said heat pipe comprising an elongated tubular structure forming a further U-shape or L-shape affixed in a further receptacle and having further leg sections.

14. The heat transfer device of claim 13, further comprising a plurality of heat transfer fins spaced from the base plate, wherein the leg sections and further leg sections form spaced parallel columns supporting the heat transfer fins.

15. The heat transfer device of claim 14, wherein the elongated tubular structures are compressed in surface contact with the base plate along a surface of the receptacles and are flattened coextensive with a surface of the base plate on a side facing the heat source.

16. The heat transfer device of claim 15, wherein the elongated tubular structures comprise tubing having a round cross section with a flattened side.

17. The heat transfer device of claim 15, wherein the elongated tubular structures have at least two flattened surfaces.

18. The heat transfer device of claim 15, wherein the vessels are affixed to the base plate by at least one of a press fit, a potting compound, an adhesive and a solder.

19. The heat transfer device of claim 18, wherein the heat transfer fins are affixed to the leg sections by at least one of a press fit, a potting compound, an adhesive and a solder.

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Description

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CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority from co-pending Provisional Patent Application Serial No. 60/388,779, filed Jun. 14, 2002, and entitled MULTIPLE HEAT PIPE TOWER--THERMAL ENHANCEMENT FEATURE.

FIELD OF THE INVENTION

[0002] The invention relates to heat exchangers, and in particular to a heat dissipation tower arrangement for transferring heat energy away from a thermal source, such as an integrated circuit package, into the ambient air. At least one, and preferably a plurality of transfer heat pipe conduits are fit into complementary channels extending through a base plate and along a surface in thermal contact with the source. The heat pipe conduits transfer heat energy along their length and serve as supporting columns for stacked heat transfer fins. The heat transfer pipes can be arranged in double ended U-shapes or single ended L-shapes with transverse bends to optimize support and heat transfer surface contact.

BACKGROUND OF THE INVENTION

[0003] Certain semiconductor devices in electrical and electronic circuits, such as large scale integrated circuits, voltage regulators, current switching devices, high current drivers and other similar devices, generate heat that is deleterious to their operation and must be dissipated. An individual semiconductor junction may be subject to thermal runaway current conduction leading to further heating and damage. In large scale digital integrated circuits, operation at or above the maximum rated temperature can result in spurious switching operations and functional failure.

[0004] The power dissipation or rate of generation of heat per unit of time, is a matter of resistive or Joule heating resulting from conduction of current through semiconductors that have a corresponding resistance, the relationship being W=I.sup.2R. In a highly integrated semiconductor device such as a computer processor, a single semiconductor switching transistor may conduct little concurrent on its own, but is densely mounted with many other transistors. A single integrated device may generate heat energy of a hundred Watts or more, and require supplemental cooling arrangements in addition to convective cooling by heat driven circulation of ambient air.

[0005] Some heat energy may be dissipated by conduction from the integrated circuit package into the adjacent air, circulating by convection. There is also some thermal conduction through circuit lands and the like. These minimal means for thermal conduction often are not adequate, and maintaining operational temperatures within design ranges can be a problem. Thermally conductive heat sink devices, normally of cast or sheet metal and potentially having a substantial surface area exposed to the air, are mounted so as to bear physically against the heat generating circuit element.

[0006] In highly integrated computer processor circuits and similar devices, a clamping mounting may be provided to press a finned heat exchanger block down against the circuit package when mounted, e.g., in a snap-in mounting on a motherboard. The heat exchanger has a base pressed against the integrated circuit and may include a mounting for a small electrically powered fan to force air over the heat exchanger. This spreads out the heat energy within the housing of the associated device. Another fan may be provided to circulate air between the housing and the ambient room air.

[0007] Integrated circuit devices are available according to more or less demanding temperature specifications, but devices that have a relatively wider temperature range also are more expensive. Standard commercial computer processor components, for example, may be rated up to 70.degree. C. The most durable military application devices may be rated up 125.degree. C. Within these constraints, it is often necessary to provide supplemental cooling.

[0008] In order to assist in the movement of thermal energy from an integrated circuit or other localized heat source, toward a remote area or toward a structure that carries the heat away, it is necessary to rely on one or more of thermal conduction, convection and radiation. Conduction of heat energy requires contact between thermally conductive masses and proceeds at a rate that depends in part on the difference in temperature between the masses. Convection requires movement of a heat transfer fluid (gas or liquid) and involves differences in fluid density due to differences in fluid temperature.

[0009] Heat transfer arrangements can involve passing a current of cooler air or other heat transfer fluid over a hotter surface to be cooled. In a heat pipe or thermal siphon arrangement, a captive heat transfer fluid is provided in closed volume and is arranged to circulate. The fluid is heated by a source of heat energy that is in heat transfer relationship with one part of the closed volume. A heat sink is arranged in heat transfer relationship with another part of the closed volume. The heat transfer fluid advantageously undergoes cyclic phase changes, each such phase change storing or releasing a quantity of heat energy due to the latent thermal energy involved in the phase change itself.

[0010] In this way, a liquid phase change heat transfer fluid can be evaporated (vaporized) at the heat source and condensed again at the heat sink. Different techniques can be used to return the condensed liquid from the condenser to the evaporator, which need not be powered by outside energy sources. A return path is possible, for example, over a gravity flow path. Apart from gravity, a return path for the condensed liquid can be provided by lining the vessel confining the heat transfer fluid with a wicking material that supports capillary flow, such as a sintered particulate or powder lining wire mesh screen, felt or grooves. In either gravity or capillary flow return, the heat source and the heat sink can be thermally coupled to remote parts of a vessel of simple shape such as a cylinder or other such shape. There is no requirement for complex shapes and flow paths.

[0011] Assuming that the heat transfer fluid is confined in an integral metal vessel, some thermal conduction from the heat source to the sink can occur. It is desirable on grounds of efficiency to separate the evaporator and condenser sections by a distance or to interpose a thermal barrier, so that the dominant thermal transfer phenomena are heat transfer from the source to the fluid at the evaporator and from the fluid to the sink at the condenser, rather than conduction along the vessel walls from the source to the sink. Nevertheless, phase change heat exchange circuits as described can operate with a very modest temperature difference between the source and the sink and can efficiently move heat energy to assist in heat dissipation.

[0012] There are a number of design considerations for thermal transfer arrangements, sometimes known as heat pipes. In addition to the ability to handle the necessary flow of thermal energy to keep the heat source within desired temperature limits, the evaporator and the condenser should have a good heat transfer coupling with the heat source and sink, respectively. The thermal transfer characteristics of the heat pipe structures, the various dimensions and quantities, is etc. need to operate over the range of expected temperatures. Preferably the device is compact and does not interfere unduly with necessary access to structures associated with the heat source and sink.

[0013] A number of heat pipe arrangements according to the foregoing general description are available from Thermacore International, Inc., Lancaster, Pa., and are disclosed in US patents assigned to their licensor, Thermal Corp., Georgetown, Del.

[0014] It would be advantageous if thermal efficiency, mechanical complexity and production ease could be maximized in a finned heat pipe arrangement. The production of a heat pipe, of course, is more involved than mounting a tube to a heat source and a heat sink at different points. Assuming that the relative dimension issues have been decided, and in addition to the mechanical affixations that will be needed during assembly, the heat pipe envelope needs to be charged with the working fluid. The vessel typically is evacuated and back-filled with a small quantity of working fluid, for example enough liquid coolant to ensure saturation of the wick. The vessel is sealed, which must be done while the vessel is accessible.

[0015] The liquid and vapor phases of the heat transfer medium in a heat pipe reach an equilibrium in the absence of temperature differences and remain substantially stagnant. When heat energy is then added at the evaporator, vaporization of the heat transfer medium leads to increased local vapor pressure in that area. The added vapor expands and a portion arrives at the condenser. The condenser is at a slightly lower temperature. The vapor is cooled by contact with the condenser and condenses, releasing the latent heat energy of vaporization. The condensed liquid phase heat transfer medium flows back to the evaporator due to capillary forces developed in the wick structure, and the cycle can repeat. Where there is a positive temperature difference between the evaporator (e.g., warmed by an electrical circuit element) and the condenser (e.g., cooled by convection, forced air, contact with a thermal sink, etc.) the cycle can continue indefinitely, moving heat energy. The technique is operative at low thermal gradients. The operation is passive in that it can be driven wholly by the heat energy that it transfers.

[0016] U.S. Pat. Nos. 6,381,135--Prasher; 6,389,696--Heil; and 6,382,309--Kroliczek teach additional heat dissipation apparatus intended for cooling integrated circuit devices and the like, as described. These references are hereby incorporated for their teachings of heat pipe or thermal siphon devices.

[0017] A stacked-fin heat sink device for a large scale integrated circuit or processor chip package is disclosed in U.S. Pat. No. 6,061,235--Cromwell et al. In that device, a mounting fixture is attached to the motherboard or other circuit card to surround the processor, and the fixture receives a spring biased mounting that presses a thermally conductive plate into full-surface mechanical and thermal contact with the processor package. A heat pipe is contained in a cylindrical vessel disposed centrally on and longitudinally extending perpendicular to the thermally conductive plate. A plurality of heat transfer fins are disposed parallel to one another and perpendicular to the extension of the cylindrical vessel. In this patent, which is hereby incorporated in this disclosure, the thermally conductive plate at the bottom end of the heat pipe vessel can function as the evaporator, having a slightly higher temperature than the finned sidewalls of the vessel remote from the bottom, which maintain a lower temperature and can function as the condenser. In the standing configuration shown, gravity can power the return path. In other orientations, a wicking material can be provided so that capillary action drives the return path.

[0018] The Cromwell arrangement represents a straightforward application of a heat pipe to the known sort of finned heat exchanger blocks that often are clamped to processor and VLSI chips. However there is room for improvement.

[0019] The thermal plate arranged to contact the heat source (the IC package) is an integral and continuous over the area of contact. This would appear to provide good thermal coupling, but as a result, the mounting of the heat pipe must be accomplished by affixing the bottom of the cylindrical vessel to the flat opposite face of the thermal plate. There are exacting production steps involved to produce a cut cylinder bottom that matches the thermal plate and to solder or otherwise securely affix the cylinder to the plate in a manner that also seals the vessel to confine the heat transfer fluid.

[0020] The spaced air-contact fins in Cromwell also present a potential assembly demand. Whereas the fins are rectangular and the heat pipe is a cylinder, there are issues respecting vertical, horizontal and rotational alignment of the plates to the one another, and attachment to the cylinder in good thermally conductive contact. These problems appear to have been addressed by affixing the fins to opposed side plates, thus requiring additional parts and assembly while affecting the extent of available air circulation. Air circulation characteristics and heat transfer characteristics are also affected by the relative size of the heat pipe and the fins.

[0021] A mounting plate arrangement has certain potentially useful aspects in connection with a heat transfer device. A plate is useful to present a large surface area for contact with a heat source having a planar surface, such as a processor or VLSI circuit. The rate of heat transfer by conduction is partly a function of the area and intimacy of contact. The plate can have a reasonably substantial thickness, which provides a thermal storage capacity and leads to rapid heat transfer throughout the material of the plate. Apart from these benefits, the drawbacks include the complications associated with mounting the plate to the heat source, the need to mount the thermal siphon vessel to the plate or to form a vessel using the plate, and complication of attaching heat dissipation structures such as fins for convective or forced air contact.

[0022] In U.S. Pat. No. 5,826,645--Meyer, a thermal siphon vessel comprises a tubular vessel wherein one end of the tubular vessel forms the evaporator and is affixed in a channel in a thick plate. A manufacturing challenge is to obtain intimate contact between the plate and the tubular vessel for good thermal energy transfer. In that patent, the problem is addressed by forming thin tabs at the surface of the channel and bending the tabs against the vessel to press the vessel against the bottom of the channel. This arrangement provides for good contact between the tube and the bottom of the channel, at the expense of contact elsewhere. It would be advantageous to improve on such a structure both as to thermal energy transfer efficiency and ease of manufacture.

[0023] It would be advantageous, to adapt the idea of thermal siphon devices to dissipating unwanted concentrations of thermal energy, in a way that optimally maximizes the efficiency of thermal transfer, but minimizes the complexity and expense of such devices.

SUMMARY OF THE INVENTION

[0024] It is an object of the invention concurrently to improve the thermal energy transfer efficiency of a heat dissipation device and the ease of manufacture of the device.

[0025] It is an object to employ at least one and preferably a plurality of heat pipe vessels as structural support elements that function to mount an air-exchange heat transfer fins on a source-contact heat transfer base.

[0026] It is another object to minimize the number and complexity of parts needed to construct a heat dissipation device.

[0027] It is still another object to modify structural aspects of a heat pipe for a heat dissipation device, and a base for mounting the device, to enable contact between the heat pipe and a heat source directly from the source to the heat pipe as well as through the base as a thermally conductive element.

[0028] These and other objects are met in a heat transfer device such as a heat sink having one or more heat pipe tubes mounted in a base plate. The heat pipe tubes have a working fluid in a vessel with a wicking material between an evaporator and condenser. The heat pipe traverses a through opening in the base plate and extends along a receptacle in the base plate facing the heat source, this portion preferably defining the heat pipe evaporator. The heat pipe has legs extending perpendicularly from the base plate, that preferably hold spaced heat transfer fins, the legs forming the condenser part of a stacked tower of fins on the base plate. Preferably two or more heat pipes are provided in the form of U-shaped or L-shaped tubes that are flattened along the underside of the base plate to bear against the heat source.

[0029] According to an inventive aspect, the leg(s) of one or more U-shaped or L-shaped tubular heat pipes form the structural columns that carry a column of stacked fins. According to another aspect, these legs are rigidly held in position due to a transverse bend formed in between the legs and the portions of the heat pipes that extend through the openings in the base plate and along the receptacle on the side of the base plate facing the heat source.

[0030] The device as thus configured is easily and inexpensively manufactured. The heat pipes can be charged and sealed before assembly or afterwards, because the ends of the U-shapes or L-shapes remain accessible. Although not excluded, no supplemental fasteners are needed to arrange and support the assembled parts. The base plate can be clamped with spring clips or the like to a computer processor or VLSI chip to form an effective convection heat dissipation device that actively moves heat into the ambient air and can be scaled larger or smaller or coupled to fan for additional cooling capacity as needed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiments of the invention, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:

[0032] FIG. 1 is a perspective view of a heat dissipation tower for circuit devices according to an embodiment of the invention having two dual heat pipes;

[0033] FIG. 2 is a perspective view of the invention as shown in FIG. 1, turned over to show details of the underside;

[0034] FIG. 3 is a perspective view illustrating a different embodiment of the invention and showing the manner of assembly;

[0035] FIG. 4 is a perspective illustration of one form of dual heat pipe for use with the invention;

[0036] FIG. 4A is a partially broken-away, partially cross-sectional view of the dual heat pipe shown in FIG. 4, as taken along lines 4A-4A;

[0037] FIG. 4B is a broken-away cross-sectional view of the dual heat pipe shown in FIG. 4, as taken along lines 4B-4B;

[0038] FIG. 5 is an elevation view along lines 5-5 in FIG. 4, showing the contour of the heat pipe including a flattened bottom portion;

[0039] FIG. 6 is a perspective view showing an alternative heat pipe structure with a transverse bend;

[0040] FIG. 6A is a partially broken-away, partially cross-sectional view of the dual heat pipe shown in FIG. 6, as taken along lines 6A-6A;

[0041] FIG. 7 is an elevation view along lines 7-7 in FIG. 6;

[0042] FIG. 8 is an elevation view, partly in section, showing mounting of the heat dissipation tower on a heat source such as an integrated circuit;

[0043] FIG. 9 is an exploded perspective view in which the invention is applied to a single post heat pipe; and

[0044] FIG. 10 is an elevation view of the embodiment of FIG. 9 as assembled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] This description of preferred embodiments is intended to be read in connection with the accompanying drawings, together forming the description of the invention and illustrating certain nonlimiting examples. The drawing figures are not necessarily to scale and certain features are represented in schematic form in the interest of clarity and conciseness.

[0046] Spatial and relative terms denoting an overall orientation, such as "horizontal," "vertical," "up," "down," "top" and "bottom" as well as their derivatives (e.g., "horizontally," "downwardly," "upwardly," etc.) are intended to refer to the orientation as then described or as shown in the drawing figure under discussion. These terms are used for convenience of description and are not intended to require a particular orientation unless that is clear in the context.

[0047] Likewise, internally relative terms such as "inwardly" versus "outwardly," "longitudinal" versus "lateral" and the like are to be interpreted relative to one another or relative to an axis of elongation, rotation, assembly or the like, as appropriate to the description.

[0048] Terms stating relationships of attachment, coupling and the like, such as "connected" and "interconnected," refer to a relationship wherein the structures can be attached, coupled, connected (etc.) directly or indirectly through intervening structures. Such attachments, couplings and the like can be movable or rigid attachments, unless the description indicates otherwise. Where elements are "operatively" connected, attached, or coupled, that connection, attachment or coupling is intended to denote a connection or the like that allows the pertinent structures to operate as stated, by virtue of such relationship.

[0049] Insofar as the description and claims recite means-plus-function clauses or elements are defined by their function, those elements are intended to encompass the structures described, suggested, or obvious in view of the written description and/or drawings for performing the recited function.

[0050] Referring to FIGS. 1 through 10, a heat transfer device 20 according to the invention includes a heat pipe 22 forming a vessel 24 to be placed in thermally conductive relation to a heat source 25 (shown in FIG. 8). The heat pipe 22 has thermally conductive material at least at an evaporator part 27 and at a condenser part 29 that are in fluid communication with one another, namely by being connected to one another as different locations in the same vessel 24. A heat transfer fluid 31 (shown in FIGS. 4a, 6a) is contained in the vessel 24, and moves between the evaporator part 27 and the condenser part 29 for transferring heat energy.

[0051] Preferably the cyclic movement of the heat transfer fluid 31 is driven substantially by the heat energy of the heat source 25, using a phase change cycle. The heat energy of the source 25 warms and vaporizes the heat transfer fluid 31 at the evaporator 27, thus storing latent heat energy. The vaporized heat transfer fluid diffuses through the vessel 24. Latent heat energy is given up when vaporized heat transfer fluid 31 is condensed due to cooling at the condenser 29. The condensed heat transfer fluid 31 is returned to the evaporator 27 and the cycle repeats. Return of the condensed heat transfer fluid preferably involves capillary flow through a wicking material 32 provided on the inside walls of the vessel 24 as shown in FIGS. 4a, 6a.

[0052] A base plate 33 at least partly supports the heat pipe 22 and preferably forms a substantial part of the thermally conductive path from the heat source 25 to the heat transfer fluid 31 in vessel 24. The base plate 33 has a side 35 that can be directed toward the heat source 25, preferably being held against the heat source. At least one through-opening or passage 36 through the base plate 33 leads from a recess or opening 42 forming a receptacle for the evaporator part 27 of the heat pipe 22. The recess or receptacle 42 positions the evaporator 27 so as to absorb heat energy from the heat source 25.

[0053] The heat pipe vessel 24 forms a passage coupling through the base plate 33, namely from the evaporator part 27 at the receptacle 42 on the heat source side of the base plate 33, through the base plate 33 to a heat sink 43 that, in the example shown, is defined by a number of spaced fins 44 in thermal contact with the evaporator part 29 on the opposite side of the base plate 33 from the heat source 25. Part of the heat pipe 22 in the area of the evaporator 27 preferably is in direct contact with the heat source 25. It is also possible for at least part of the heat pipe functioning as the evaporator part 27 to be in contact or to have an intervening structure (not shown) that couples heat energy to the evaporator on a side of the base plate 33 opposite from the heat transfer fins 44 or other heat sink 43. In any event, the heat transfer fluid 31 at the evaporator 27 is heated, preferably but not necessarily by a close contact thermal relationship, with the heat source 25.

[0054] The heat transfer device 20 a heat sink 43 thermally coupled to the heat pipe 22 apart from the evaporator 27 and base plate 33, where the heat pipe is cooled by convection or by another thermal path at which heat energy is dissipated. The heat sink 43 in the embodiment shown comprises a stack of thermally conductive fins 44 in contact with ambient air that may be forced air or may be circulated by convection due to heating from fins 44. The fins 44 are in thermal contact with the heat pipe 22 and dissipate heat energy from the condenser part 29 of the heat pipe 22, at a location that is relatively spaced from the evaporator 27 on the side of the base plate 33 that is directed toward the heat source. Other relative locations are possible, but the condenser 29 at least is sufficiently distinct and/or distant from the evaporator 27 that the condenser 29 maintains a lower temperature than the evaporator 27.

[0055] The fins 44 can be subjected simply to convection air or currents due to localized heating. Alternatively, the fins 44 can be in a forced air path. Other heat dissipation structures are also possible, such as a heat exchange relationship with a liquid medium as opposed to air. It is also possible to employ more than one form of heat dissipation at the same time, in parallel or serial heat energy transfer paths.

[0056] In the embodiment shown in the drawings, the receptacle 42 under the base plate 33 forms a channel or other recess that apart from the through opening to the condenser extends only part way through the thickness of the base plate 33, on the side or base plate 33 directed toward the heat source 25. In the embodiment of FIGS. 1 and 2, for example, the vessel 24 defines the evaporator 27 and the condenser 29 at different longitudinal positions along an elongated tubular structure. The evaporator part can be at an end or at an intermediate point disposed in the receptacle channel 42 of the base plate. The condenser part 29 forms a hollow supporting column 52 protruding at the opposite side of the base plate 33 and structurally supporting the fins 44.

[0057] The base plate 33 could have a thickness that is less than or equal to that of the evaporator 27. Preferably, however, the base plate 33 is thicker than an outside diameter or thickness of the elongated tubular vessel or similar structure including the evaporator 27. The receptacle 42 on the base plate 33 is dimensioned to complement an outside shape of this elongated tubular vessel structure, which preferably can be press fit or otherwise intimately and securely fitted so as to support thermal energy transfer. For optimal heat transfer contact, the heat pipe 22 and its tubular structure rest substantially in surface contact with the base plate 33 at the receptacle 42, or are potted in the receptacle by a material (not shown) having good heat transfer characteristics, such as a metal solder or thermally conductive adhesive or resin.

[0058] It is convenient and inexpensive to use a heat pipe structure based on a cylindrical tube (i.e., a tube with a round cross section), which can be formed or bent somewhat more easily than other cross sections, such as rectangular tubes. However, forming a rectangular channel to complement a rectangular tube is likely to be an easier manufacturing job than forming a channel with a rounded-bottom U-shaped or L-shaped contour for a round tube. According to one aspect of the invention, a U-shaped or L-shaped or preferably half-round contour in the base plate 33 can be used for the receptacle 42 carrying a heat pipe tube with a round cross section of complementary size, providing substantially full surface contact between the heat pipe and the base plate over part the sides of the receptacle 42. The tube can be press fit into the receptacle. However the side 54 of the round heat pipe tube facing toward the heat source 25 is flattened along a surface that is substantially flush with the outside of the base plate 33. This structure, shown in FIGS. 4a and 6a, provides a substantially continuous surface oriented toward the heat source 25, preferably contacting the heat source 25.

[0059] At least part of the flattened surface 54 corresponds with is the evaporator part 27 of the heat pipe 22. The walls of the heat pipe 22 can be relatively thin compared to the thickness of the base plate, and as a result, heat energy can be coupled efficiently into the evaporator 27 to vaporize the heat transfer fluid 31.

[0060] In the respective drawings, several alternative arrangements are shown for a heat dissipation device as described. Referring to FIG. 1, one embodiment includes a base plate 33 comprising a thermally conductive material and several tubular heat pipe columns 52 carried on the base plate 33 and in turn supporting parallel spaced air-contact heat transfer fins 44. FIG. 2 shows, however, that the several heat pipe columns 52 (four being shown) can be paired columns associated with two dual heat pipe vessels. That is, each of the two vessels 24 has an evaporator portion 27 along a central part of the vessel 22 disposed parallel to and in the recess 42 of the base plate 33 and exposed along a bottom side of the base plate 33. Each of these vessels 22 has two opposite ends that are diverted from the plane of the base plate 33 and the exposed bottom side. These opposite ends are turned upwardly, the opposite ends forming the columns 52 standing on the base plate 33 and providing structural support for the air-transfer fins 44.

[0061] FIGS. 1 and 2 show an embodiment with U-shaped dual heat pipe vessels 62 in which the lowermost horizontal portion 64 of the heat pipe vessel is disposed on the side of the base plate 33 that is to face the heat source 25 when mounted as shown in FIG. 8. This evaporator or central part 64 is disposed in a groove on the underside of the baseplate 33 such that the evaporator is inset in the groove and resides substantially flush with the surface of the base plate 33 on its underside. The columns are formed by the legs 66 of the U-shape or L-shape at the ends of the vessel 22, which are turned upwardly from the plane of the base is plate 33, extending through the openings in the base plate 33 to support the fins 44. This provides a good structural connection of the heat pipe 22 to the base plate 33, for supporting the base plate, heat pipe and air transfer fins in fixed relative positions.

[0062] There are a number of specific shapes possible wherein one or more heat pipes 22 extends from an evaporator 27 exposed on the underside of a base plate 33, through the base plate to support a heat exchanger 43 such as a stack of air contact heat transfer fins or plates 44. The four heat pipe columns in FIG. 1, which are the dual condensers on the opposite ends of heat pipe vessels with central evaporators, are generally U-shaped. The bottom 64 of each U-shape (the evaporator) is generally parallel to the base plate 33 and exposed on the underside of the base plate. The sides or legs 66 of the U-shapes are perpendicular to the base plate.

[0063] The receptacle or slot 42 in the underside of the base plate 33 is straight for the embodiments in FIGS. 3-5. The receptacle or slot in FIG. 2, however, also is U-shaped in a plane parallel to that of the base plate, by virtue of a transverse bend 72 in the bottom 64, namely a bend a plane that is perpendicular to the plane of the U-shape that includes the legs 66. This transversely curved slot receives a heat pipe vessel wherein a part of the vessel, specifically the evaporator 27 in the embodiment shown, forms a U-shape in a plane perpendicular to the plane of the leg sections. The vessel nonetheless can be press fit into the U-shape of the receptacle 42. Additionally, a solder or thermally conductive resin or potting formulation can fill any spaces between the material of the base plate 42 and that of the evaporator vessel 27. The bottom side 54 of the evaporator 27, which preferably is flattened and disposed flush with the base plate surface, can be treated to enhance thermal conductivity by contact with the housing of the circuit package. For example, the evaporator surface can be fly-cut so as to be flat and smooth, for example to a local dimensional flatness tolerance of 0.001", and thus complement a flat and incompressible circuit package surface. Alternatively, if the circuit passage is compressible, the evaporator surface can be roughened or patterned to increase the surface area of contact.

[0064] The transverse bends shown in FIG. 2, wherein the legs 66 and bottom 64 on the one hand and the U-bottoms 64 on the other hand, form separate perpendicular U-shapes. This shape with a transverse bend in the bottom, where the heat pipe vessels 24 are inserted into and engaged by the base plate 33, also shown apart from the base plate in FIG. 6, avoids play or freedom of movement that could enable the heat pipe vessel to become displaced. By comparison, the shape of FIG. 4, for example, wherein the U-bottom is in the same plane as the perpendicular legs 66, could be subject to a tendency to rotate relative to the axial center of the receptacle or slot holding the evaporator of the heat pipe, at least within the range of any clearance. The embodiment of FIG. 2, having a heat pipe vessel with a transverse bend that in this case forms a U-shape in a plane parallel to the base plate, provides an inherently rigid assembly when the heat pipe vessel and the base plate are assembled, and is preferred. This structure in turn forms a rigid and durable support for the air contact heat transfer fins that are stacked on the condenser columns protruding on the opposite side of the base plate.

[0065] FIG. 3 illustrates an arrangement in which a plurality of heat pipe vessels 24 are provided, each of the heat pipe vessels in this embodiment has an evaporator 27 and a condenser 29 in a right angle arrangement. The base plate 33 has slots or receptacles 42 along an underside to be oriented toward a heat source (not show in FIG. 3). These slots can be precisely complementary to the evaporator ends of the heat pipes that are to reside along the underside of the base plate when oriented as shown. Alternatively and as also shown, the slots can extend across the full width between the opposite edges of the base plate. Preferably, any portion of the slots that is not occupied by the evaporators is filled with a potting compound 77 or the like, to improve thermal transfer.

[0066] The evaporator parts of the heat pipes 22 are set into the slots 42 and the condenser parts 29 of the heat pipes extend through the base plate 33 to engage and support the heat dissipation fins 44. As in the embodiment of FIGS. 1 and 2, the L-shaped arrangement of FIG. 3 comprises heat pipes wherein the condenser ends are turned up and passed through openings that are at least partly perpendicular to the plane of the base plate. In this way, the heat pipes not only provide for a thermal transfer route to pass heat energy from a heat source at the evaporator area to the air contact heat dissipation fins, but moreover, the heat pipes also form the columns that support the assembly as a unit. As shown in FIG. 4 in perspective and FIG. 5 in end elevation, the L-shaped arrangement of FIG. 3 can be replaced with a dual arrangement in which two standing condenser columns 52 are coupled in a U-shaped arrangement with a bottom evaporator portion 27 connecting between them.

[0067] In each of these embodiments, the evaporator parts of the heat pipes are arranged to transfer heat efficiently by contact with a heat source disposed under the base plate 33, such as an integrated circuit package or the like against which the base plate is clamped (not shown in FIG. 3). For best efficiency, the contact is as intimate as possible between the heat source and the heat transfer fluid inside the heat pipe vessel in the area of the evaporator. Thus the wall of the heat pipe in the area of the evaporator should be thin and constructed of a thermally conductive metal or the like. Furthermore, according to an inventive aspect, the heat pipe vessel is flattened at the evaporator as shown in FIG. 4a, as compared to the preferred shape of the columns 52, shown in FIG. 4b and preferably round. That is, the elongated tubular structure of the heat pipe is flattened along a surface 54 coextensive with a surface of the base plate 33 on the side directed toward the heat source. This increases the surface area and provides more direct transfer of heat energy into the heat transfer fluid, than does a round evaporator tube cross section.

[0068] A round cross section is possible for the evaporator, but the evaporator is carried in a receptacle slot having a downwardly opening U-shaped contour, so a round evaporator contour is characterized by a very limited area of surface contact between the evaporator and the heat source (assuming that the heat source is typically flat). Such an embodiment could provide a gap in the area of contact, where the heat source and either the evaporator or the base plate are spaced, particularly at the lateral edges of the slots 42. The embodiments shown in FIG. 4, 4a, 4b and 5 have round column condensers and flattened bottoms 54 on the evaporators 27 to avoid such a gap. This provides a wide area of contact between the evaporators 27 and the heat source 25, and also substantially fills the available area between the lateral sides of the slots. In this way, there is intimate contact and good heat transfer efficiency between the source and the heat transfer fluid in the evaporator. Alternatively, a solder or potting compound can close the gap.

[0069] As an alternative, the heat pipe vessel 22 can be formed of square tubing (not shown) at least in the area of contact with the heat source and with portions of the base plate. Square tubing has a flat bottom side that can contact the heat source. A squared channel as the receptacle for the evaporator part 27 of the heat pipe is relatively easy to manufacture by machining or otherwise providing a squared-side channel. The engagement of a squared tubing form (or a tubing form that has at least two flattened faces bearing respectively on the heat source and on at least one complementary flat surface of the receptacle in the base plate) is also an inherently rigid structural connection, i.e., one that unlike a tubular connection is inherently held from rotating.

[0070] FIGS. 6, 6a and 7 illustrate that the flat bottom evaporator configuration also is applicable to other specific configurations, most notably including the embodiments of FIGS. 1 and 2, wherein the heat pipes comprise dual condenser tubes with U-shapes having at least one transverse bend. The embodiments shown are structurally simple examples in which a U-shaped planar configuration is provided with two bends of about 40 degrees, in the same is direction, forming a symmetrical shape. Other specific bends are possible, such as bends in opposite directions forming an S-shape or zigzag, bends that are less discrete and form curves or arcs, closed shapes such as polygons, etc.

[0071] In its general form, the heat pipe vessel extends through the base plate so as to place the evaporator on the exposed underside of the base plate and the condenser column extending from the opposite side. The heat pipe vessel 22 also can be formed in part from the structure of the base plate 33, for example having an evaporator defined by a slot on the bottom of base plate 33 with a cover closing the vessel 9 (not shown). In that case, tubes for columns 52 can be inserted into sockets at openings communicating with the evaporator through the base plate 33 from the opposite side.

[0072] However in the preferred arrangement, the heat pipe vessel 22 is a discrete tube that is affixed to the base plate. In addition to the opening at which the vessel extends through the base plate 33 to couple the evaporator 27 to one condenser column 52, the preferred base plate has at least one further through opening leading to a second condenser column 52. The evaporator is exposed on the side of the base plate directed toward the heat source. The elongated tubular structure of the vessel forms a U-shape with legs traversing two spaced through openings connecting through the base plate to the condensers. The preferred orientation of the legs is perpendicular to the plane of the base plate. The legs comprise parallel sections extending from the base plate, with a bend to join perpendicularly with the evaporator at the bottom of the U-shape, and preferably at least one transverse bend at the bottom or the U-shape.

[0073] FIG. 8 illustrates the relationship between the heat source 25, which in this example comprises an integrated circuit 82 in a packaged housing 84, and the heat transfer device 20 of the invention. The integrated circuit comprises an active semiconductor device in a housing that can be plastic or ceramic and is thermally conductive. The housing is typically mounted by snap fit into a receptacle 86 capable of making the necessary electrical connections with leads that couple signal and power lines to the circuit 82. The base plate 33 of the heat transfer device is clamped directly in contact with the housing 84 of the semiconductor device, for example via spring clips 88. In this way the evaporator 27 on the underside of the base plate 33 takes up thermal energy by direct contact with the housing 84 of the semiconductor device and also by indirect thermal transfer from the housing to the base plate and then to the evaporator. The thermal transfer from the base plate to the evaporator can be enhanced by appropriate choice of potting compound or solder to affix the evaporator in the receptacle on the underside of the base plate.

[0074] The heat transfer fins attached to the columnar condenser parts of the heat pipe vessels provide a heat sink apparatus in that the heat released by the heat transfer fluid in the heat pipe vessels is coupled by thermal conduction to the fins 44. The fins can be press fit, soldered, or epoxied to the columnar condenser parts 52. The fins 44 can be wholly flat sheets with openings that slide onto the condenser columns 52. Alternatively, the fins 44 can be stamped to include flanges or collars of metal surrounding the openings for the columnar condensers (not shown). Insofar as such flanges extend for a short distance perpendicular to the plane of the fins, the flanges improve the rigidity of structural connection and also increase the intimacy of thermally conductive contact between the condenser parts and the fins. The flanges can also assist in obtaining equal close spacing of the fins along the spaced parallel columns of the condensers.

[0075] The respective dimensions of the heat pipe, base plate and fins are subject to variations as necessary for the circumstances. In a typical exemplary application, the heat source is a packaged highly integrated processor circuit chip that may produce heat at a rate of 100 Watts, but in a typical ambient air temperature up to 40.degree. C. may need to be maintained at or below 70.degree. C. for dependable operation. Such a chip may be 9 to 11 mm thick and up to 30 mm on a side.

[0076] The heat dissipation device of the invention can have a base plate with the same area or footprint as the circuit package, so as to be attachable to bear against the circuit package as shown in FIG. 8. The heat pipe columns are arranged so that the evaporator parts that are exposed on the underside of the base plate occupy a central area of the chip package, namely the area that aligns with the semiconductor element that actually produces the heat. The dimensions and capacity of the heat pipes is then chosen to provide the necessary rate of heat dissipation.

[0077] An advantage of the invention is that in addition to functioning as structural columns, the use of several heat pipes of relatively smaller diameter produces greater surface area per unit volume than a single larger diameter structure (or perhaps a smaller number, such as two columns instead of four. As a is result, there is a comparable heat transfer efficiency achieved in a smaller heat pipe volume.

[0078] Another advantage of the invention is that the exposure of the evaporator part of the heat pipe on the lower face of the base plate provides a more direct thermal conduction path than a comparable arrangement in which an evaporator is coupled to the top surface of a base plate, or even an arrangement in which the baseplate forms a relatively thick bottom wall of an evaporator. This advantage of direct thermal contact can be achieved in an arrangement having fewer than four heat pipe structural columns, for example two or three, or even a single column 92 as shown in FIGS. 9 and 10. In this embodiment, the single column 92 is provided by a heat pipe tube that has a relatively wider evaporator section 94 on the bottom of a coaxial cylindrical column 96 forming the condenser. Of course a non-round cross sectional shape is also possible. The evaporator resides in a receptacle or recess 98 that is complementary with the evaporator part, namely round in this example. The condenser column, as in the previous embodiments, is connected to the evaporator part by a part of the heat pipe extending through an opening in the base plate. The evaporator part is thereby positioned for direct contact with the heat source (not shown in FIGS. 9, 10). The condenser part forms a structural support column for the air heat exchange fins.

[0079] The embodiment of FIGS. 9 and 10, which has a single column heat pipe, generally results in a more substantial obstruction to air flow than a plurality of smaller width heat columns as in the previous embodiments. This larger obstruction is not preferred in a forced air situation, such as an installation in which is a fan (not shown) directs a flow of air over the fins, in a direction perpendicular to the longitudinal extension of the condenser. For such installations, a larger number of smaller columns are preferred for providing good structural support for the fins on the base plate, with optimal surface area of contact between the condensers and the fins at points that are generally distributed over the fins as opposed to concentrated. The attachment of the evaporator(s) in the recess in the base plate, and the attachment of the condenser(s) in the fins, can be made by a press fit, a potting compound, an adhesive, a solder or other specific connections that are capable of conveying heat energy across the attachment.

[0080] The internal arrangements of the heat pipe in the overall heat dissipation or heat sink device can otherwise incorporate a number of the aspects of known heat pipes. The heat pipe vessel(s) form an envelope containing a working fluid, and are either oriented for gravity return of condensed fluid to the evaporator, or have a wicking material along the inside walls so as to return the condensed fluid by capillary action. The wick can be structured as sintered particles, fibers or the like, and in a preferred arrangement includes micro-encapsulated phase change particles that are adhered to the inside surfaces of the walls of the vessel. The vessel is vacuum tight and may be formed from a sealed tube of thermally conductive material, e.g., aluminum, copper, titanium alloy, tungsten, etc. Although shown as substantially tubular with flattened surfaces for contact with the heat source, the heat pipe vessels can take other shapes.

[0081] The working thermal transfer fluid can be selected from a variety of well known two phase fluids depending upon expected operational conditions such as the operating temperature range over which the heat transfer device will operate. Appropriate fluids may include, for example, one or more of water, Freon, ammonia, acetone, methanol, ethanol and the like. The prime requirements for a suitable working fluid are compatibility with the materials forming wick and the envelope wall, good thermal stability, ease of wetting of the wick and wall materials as well as viscosity and surface tension attributes suitable for capillary flow.

[0082] The working fluid can be charged into the heat pipe vessels before or after the assembly with the base plate and heat transfer fins, because the arrangement is characterized by access to the heat pipe vessel after assembly, at least at an end located at the uppermost fin. In that case, the vessel is first shaped and attached, but is unsealed at a limited point such as a charging tube as shown in FIG. 9. The working fluid is added, usually after partially evacuating the air in the vessel, and the charging tube is then plugged by adhesive, soldering and/or crimping operations.

[0083] The pressure and working fluid charge are arranged to obtain an operating vapor pressure in the vessel over the working temperature range, within vapor pressure limits that permit evaporation and condensation to occur at different points in the vessel (i.e., at the evaporator and condenser parts) when maintained at design temperature differences. For optimal results, at all points within the temperature range, the working fluid has advantageous characteristics including high latent heat storage capacity, high thermal conductivity, low liquid and vapor viscosities, high surface tension and an acceptable freezing or pour point. Preferably, the quantity of working fluid in the vessel is at least enough to saturate any wick material provided, or to support a gravity flow in a circulating manner in the absence of a wick.

[0084] In a preferred arrangement, the heat pipe vessel comprises one or more metals such as silver, gold, copper, aluminum, titanium or their alloys. Polymeric materials are also useful, including materials known in the electronics industry for heat transfer applications, such as thermoplastics (crystalline or non-crystalline, cross-linked or non-cross-linked), thermosetting resins, elastomers or blends or composites thereof. Some illustrative examples of useful thermoplastic polymers include, without limitation, polyolefins, such as polyethylene or polypropylene, copolymers (including terpolymers, etc.) of olefins such as ethylene and propylene, with each other and with other monomers such as vinyl esters, acids or esters of unsaturated organic acids or mixtures thereof, halogenated vinyl or vinylidene polymers such as polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride and copolymers of these monomers with each other or with other unsaturated monomers, polyesters, such as poly(hexamethylene adipate or sebacate), poly(ethylene terephthalate) and poly(tetramethylene terephthalate), polyamides such as Nylon-6, Nylon-6,6, Nylon-6,10, Versamids, polystyrene, polyacrylonitrile, thermoplastic silicone resins, thermoplastic polyethers, thermoplastic modified cellulose, polysulphones and the like.

[0085] Examples of some useful elastomeric resins for potting and adhesive aspects include, without limitation, elastomeric gums and thermoplastic elastomers, natural or synthetic. The term "elastomeric gum", refers to polymers which are noncrystalline and which exhibit after cross-linking rubbery or elastomeric characteristics. The term "thermoplastic elastomer" refers to materials which exhibit, in various temperature ranges, at least some elastomer properties. Such materials generally contain thermoplastic and elastomeric moieties. For purposes of this invention, the elastomer resin can be cross-linked or non cross-linked when used in the inventive compositions.

[0086] Illustrative examples of some suitable elastomeric gums for use in this invention include, without limitation, polyisoprene (both natural and synthetic), ethylene-propylene random copolymers, poly(isobutylene), styrene-butadiene random copolymer rubbers, styrene-acrylonitrile-butadie- ne terpolymer rubbers with and without added copolymerized amounts of unsaturated carboxylic acids, polyacrylate rubbers, polyurethane gums, random copolymers of vinylidene fluoride and, for example, hexafluoropropylene, polychloroprene, chlorinated polyethylene, chlorosulphonated polyethylene, polyethers, plasticized poly(vinyl chloride), substantially non-crystalline random co- or ter-polymers of ethylene with vinyl esters or acids and esters of unsaturated acids, silicone gums and base polymers, for example, poly(dimethyl siloxane), poly(methylphenyl siloxane) and poly(dimethyl vinyl siloxanes).

[0087] Some illustrative examples of thermoplastic elastomers suitable for use in the invention include, without limitation, graft and block copolymers, such as random copolymers of ethylene and propylene grafted with polyethylene or polypropylene side-chains, and block copolymers of -olefins such as polyethylene or polypropylene with ethylene/propylene or ethylene/propylene/diene rubbers, polystyrene with polybutadiene, polystyrene with polyisoprene, polystyrene with ethylene-propylene rubber, poly(vinylcyclohexane) with ethylene-propylene rubber, poly(-methylstyrene) with polysiloxanes, polycarbonates with polysiloxanes, poly(tetramethylene terephthalate) with poly(tetramethylene oxide) and thermoplastic polyurethane rubbers.

[0088] Examples of some thermosetting resins useful herein include, without limitation, epoxy resins, such as resins made from epichlorohydrin and bisphenol A or epichlorohydrin and aliphatic polyols, such as glycerol, and which can be conventionally cured using amine or amide curing agents. Other examples include phenolic resins obtained by condensing a phenol with an aldehyde, e.g., phenol-formaldehyde resin. Other additives can also be present in the composition, including for example fillers, pigments, antioxidants, fire retardants, cross-linking agents, adjuvants and the like.

[0089] It is to be understood that the invention is not limited only to the particular constructions herein disclosed and shown in the drawings, but also encompasses modifications or equivalents within the scope of the appended claims.

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