VAPOR-AUGMENTED HEAT SPREADER DEVICE

TECHNICAL FIELD

[001] This application relates to cooling devices and, more particularly, to a vapor- augmented heat spreader including condensate and vapor grooves, wherein the capillary force generated by the vapor grooves is lower than the capillary force generated by the condensate grooves.

BACKGROUND

[002] As electronic components and devices decrease in size while increasing in operational speed, generated heat becomes a major obstacle to improving performance in electronic devices and systems. The demand for increased performance continues in spite of the continuing need to decrease the size of the semiconductor forming the electronic component. As the size of the semiconductor decreases, the resulting heat flux (heat transfer per unit area, q) increases. The increase in heat flux poses a different challenge to cooling products than a mere increase in total heat, since an increase in heat flux causes overheating on a different time and length scale, which could then lead to electronics failure.

[003] In an attempt to address this problem, a range of solutions has been employed. For example, a metallic lid has been placed on top of the semiconductor to function as a heat spreader. In addition, vapor chambers have been used to enhance the heat spreading effect. While these solutions are somewhat effective, problems still remain. The metallic lid utilizes a conductive mechanism for heat transport; consequently, it needs to be sufficiently thick to ensure appropriate heat spreading. Increased lid thickness, however, results in a semi-finite heat-transfer behavior. Thus, a thicker heat spreader may cause localized, transient overheating. Vapor chambers, although

an improvement over the metallic lid, do not maintain dimensional tolerance under the high vapor pressure generated when the vapor chamber is integrated with the electronics and subjected to the high-temperature reflow processes (e.g., in the context of the vapor chamber operating in the format of a vapor-chamber lid). In addition, vapor chamber wick selection is problematic — the appropriate wick needs to be chosen to both enable a high condensate flow rate and maintain sufficient capillary pressure to overcome the effect of gravity. This becomes more of a problem as the thickness of the vapor chamber is reduced in an effort to replace the function of the metallic lid. Thus, it would be desirable to provide a heat dissipation device that overcomes these and other problems of vapor chambers and metallic lids.

SUMMARY OF THE INVENTION

[004] The present invention is directed toward a vapor-augmented heat spreader device including vapor and condensate channels selectively formed into a pair of fluid transfer elements to enable the selective increase of the device's in-plane thermal spreading ability. Each set of channels may be spaced to form crests configured to provide sufficient joint area to minimize resistances to through-plane heat flow, as well as to ensure an appropriate pressure rating. In one embodiment of the invention, the heat spreader device includes a lower panel and an upper panel. Each panel is generally planar, including a top surface and a bottom surface. The lower panel has condensate channels formed into its top surface, while the upper panel has vapor channels formed into its bottom surface (i.e., the channels are formed on generally opposing interior surfaces). The channels formed into the upper and lower panels are spaced apart such that a plurality of crests is created. With this configuration, when the bottom surface of the upper panel is brought into mechanical contact with the top surface of the lower panel, the crests function as pillars, supporting the sheet against

high pressures existing within the device. Optionally, the crests may be selectively joined (e.g., via welding) to create crest joints. In operation, the device may be evacuated and charged with a vaporizable liquid, which is then sealed inside to create a heat-pipe environment.

[005] The dimensions or shape of the condensate channels may differ from the dimensions or shape of the vapor channels such that the condensate channels can generate a higher capillary force than the vapor channels. For example, the condensate channels may possess dimensions smaller than those of the vapor channels. Alternatively, the condensate channels may possess a shape differing from that of the vapor channels (e.g., the condensate channels may include sharper corners than the vapor channels). The lower panel, furthermore, may include a multi-wick structure, in which wicking power increases with decreasing distance to an evaporation region. The multi-wick structure may also define a boiling-enhanced, multi-wick structure to promote evaporation and minimize boiling superheat.

BRIEF DESCRIPTION OF DRAWINGS

[006] FIG. 1 illustrates a perspective view of a vapor-augmented heat spreader in accordance with an embodiment of the invention.

[007] FIG. 2A illustrates an exploded view of the vapor-augmented heat spreader taken along line A - A of FIG. 1.

[008] FIG. 2B illustrates a cross sectional view of the vapor-augmented heat spreader taken along line A - A of FIG. 1, showing an embodiment including edge joint connections.

[009] FIG. 3 illustrates a cross sectional view of the vapor-augmented heat spreader taken along line A - A of FIG. 1, showing an embodiment including a folded joint connection.

[0010] FIG. 4 illustrates a cross sectional view of the vapor-augmented heat spreader taken along line B - B of FIG. 1, further showing joints around a charging tube.

[0011] FIG. 5 illustrates a plan view of the interior surface of the lower sheet of FIG. 1, showing a groove structure formed into the lower sheet in accordance with an embodiment of the invention.

[0012] FIG. 6 illustrates a plan view of the interior surface of the upper sheet of FIG. 1, showing groove structure formed into the upper sheet in accordance with an embodiment of the invention.

[0013] Like reference numerals have been used to identify like elements throughout this disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0014] FIG. 1 illustrates a perspective view of a vapor-augmented heat spreader in accordance with an embodiment of the invention. The vapor-augmented heat spreader 100 may include a first (upper) fluid transfer element or sheet or panel 110, a second (lower) fluid transfer element or sheet or panel 120, and an optional charging tube 130 disposed between the sheets. Each sheet 110, 120 may be generally planar, including a top surface and a bottom surface. Specifically, as seen in FIG. 2A, the upper sheet 110 includes an exterior surface 200A and an interior surface 205A; similarly, the lower sheet 120 also includes an exterior surface 200B and interior surface 205B (which faces the interior surface of the first sheet).

[0015] Referring back to FIG. 1, in order to allow air to properly escape in the presence of solder flux or thermal interface material, the upper sheet 110 may further include one or more air- grooves. For example, the exterior surface 200A of the upper sheet 110 may include an air groove 140A formed therein; similarly, the exterior surface 200B of the lower sheet 120 may include an air-groove 140B formed therein. The air grooves 140A, 140B may be formed through stamping, selective etching, material removal, cutting, skiving, swaging, scribing, or other processes known in the art. The thickness of the sheets 110, 120 may include, but is not limited to, about 12 μm to about 2 cm. The charging tube 130, which is in fluid communication with the interior chamber of the heat spreader 100, may connect to a vacuum pump and/or liquid supply. In operation, the tube 130 is sealed after gases within the device are evacuated and/or after the device is charged with a vaporizable liquid.

[0016] FIGS. 2A and 2B illustrate cross sectional views of the device taken along lines A-A of FIG. 1, showing the internal structure of the vapor-augmented heat spreader 100 in accordance with an embodiment of the invention. As shown, the upper sheet 110 includes one or more vapor channels or grooves 210A, 210B, 210C formed into its interior surface 205A (i.e., the surface facing the interior surface 205B of the lower sheet 120). The dimensions and/or shape of the vapor grooves 210A - 210C may be any suitable for its described purpose. The vapor grooves 210A - 210C are configured to allow for the condensation and passage of vapor once the condensate (i.e., the vaporizable liquid) is vaporized. The shape of the vapor grooves 210A - 210C may include, but is not limited to, a circular shape 210A, a rectangular shape 210B, and polygonal shape 210C. The rectangular 210B and the polygonal 210C vapor grooves, moreover, may have rounded corners 220 (which may result from the groove formation process). The vapor grooves 210A - 210C may be formed

via stamping, selective etching, material removal, cutting, skiving, swaging, scribing or similar methods known in the art.

[0017] Preferably, the overall vapor groove structure is formed using a combination of groove shapes (e.g., rounded and polygonal). The vapor grooves 210A - 210C may be spaced apart such that crests 225 are selectively disposed along the interior surface 205A of the upper sheet 110 (discussed in greater detail below). The groove structure, moreover, may form predetermined patterns on the surface, including, but not limited to grid patterns and/or a leaf- vein patterns (FIG. 6).

[0018] The lower sheet 120 includes one or more condensate channels or grooves 230A, 230B, 230C, 230D, 230E (also called vaporizable liquid grooves) formed into its interior surface 205B (i.e., the surface facing the interior surface 205A of the upper sheet 110). The condensate grooves 230A - 230E are wicking structures configured to transport vaporizable liquid, e.g., toward the evaporation region of the lower sheet. The condensate grooves 230A - 230E may be formed via stamping, selective etching, machining, material removal, cutting, skiving, swaging, scribing, or similar processes known in the art. The shape of the condensate grooves 230A - 230E includes, but is not limited to, a triangular shape 230A, a rectangular shape 230B, and a circular shape 230C, as well as the shape resulting from forming the grooves using techniques such as skiving 230D (crescent-like shapes) and scribing 230E (e.g., having generally flat, parallel side walls and generally V-shaped base).

[0019] The condensate grooves 230A - 230E possessing a triangular shape 230A and a rectangular shape 230B may further have rounded corners 220 (which may result from the groove formation processes of the grooves 230A - 230E). Preferably, the overall condensate groove structure is formed using a combination shapes (e.g., rounded and polygonal). The condensate grooves 230A - 230E may be spaced apart

such that crests 235 are formed along the interior surface 205B of the lower sheet 120. The condensate grooves 230A - 230E, moreover, may form predetermined patterns on the surface, including, but not limited to grid patterns and/or a leaf-vein patterns (FIG. 5).

[0020] To optimize wicking conditions, the condensate groove structure may further define a multi-wick structure — a structure having wicking power that increases with decreasing distance to the evaporation region (i.e., the region proximate the heat source). Alternatively or in addition to, the condensate groove structure may further incorporate a boiling-enhanced, multi-wick structure 240 to promote evaporation and minimize boiling superheat. For example, the overall condensate groove structure may be formed by selectively combining the boiling-enhanced, multi-wick structure, as well as condensate grooves 230A - 230E having various shapes. Additional information on multi-wick structures (with or without boiling enhancement) is disclosed in U.S. Patent Application No. 11/272,145 (U.S. Published Application No 2006-0060330) and U.S. Patent Application No. 11/164,429 (U.S. Published Application No. 2006-0196640), the disclosures of which are hereby incorporated by reference in their entireties.

[0021] In order to maintain proper wicking conditions such that the vapor grooves 210A - 210C are kept open and that most of the vaporizable liquid stays within the condensate grooves 230A - 230E, the condensate grooves should possess characteristics capable of giving rise to a higher capillary force than the vapor grooves . In other words, the capillary force generated by each vapor groove 210A - 210C may be lower than the capillary force generated by each condensate groove 230A - 230E. This may be achieved by providing the condensate groove 230A - 230E with dimensions (size and/or geometry) that differ from the dimensions (size and/or

geometry) of the vapor groove 210A - 210C. By way of specific example, the condensate grooves 230A - 230E may (1) possess a smaller size than the vapor grooves 210A - 210C and/or (2) possess a geometry/shape with sharper corners than the geometry/shape of the vapor grooves 210A - 210C.

[0022] The upper sheet 110 may be coupled (i.e., functionally joined/sealed) to the lower sheet 120 via processes such as ultrasonic welding, thermosonic welding, TIG welding, plasma welding, laser welding, soldering, brazing or any other methods known in the art. For example, the crests 225, 235, which selectively define contact areas between the interior surface 205 A of the upper sheet 110 and the interior surface 205B of the lower sheet, may be selectively sealed to form crest joints 250. The crest joints 250 effectively form a series of pillars within the chamber, providing strength to the vapor-augmented heat spreader 110 and enabling the device to resist the high level of vapor pressure the heat spreader device 100 has to sustain during reflow process (e.g., pressures created in environments of about 200°C). As shown in FIG. 2A, crest joints 250 may be formed wherever the crests 225, 235 on the two sheets 110, 120 are in mechanical contact with each other. This configuration differs from ordinary vapor chambers having solid internal inserts in that the present crest joints 250 have only one joining interface (although each sheet may itself consist of a plurality of plated and/or coating materials for joint promotion purposes). The number of contact areas that are sealed to form crest joints 250 is not particularly limited. Preferably, at least about 10% of the planar sheet area should be utilized in forming crest joints 250 to provide the heat spreader device 100 with sufficient mechanical strength to withstand the forces generated during the operation of the device.

[0023] Alternatively or in addition to, the vapor-augmented heat spreader device 100 may include an edge joint or seal 260 disposed along the seam of the heat spreader

device 100 (i.e., where the edges of the two sheets 110, 120 come into contact with each other). The edge joints 260 seal the two sheets 110, 120 together, maintaining a fluid tight seal within the device 100.

[0024] Referring to the embodiment illustrated in FIG. 3, the vapor- augmented heat spreader device 100 may also be formed from a single sheet folded over onto itself, creating a structure where the upper sheet or panel 110 and lower sheet or panel 120 are connected along one side via a folded-joint 300. One or more edge joints 260 similar to those discussed above may be utilized to seal the remaining sides of the device 100 together.

[0025] In forming the heat spreader device 100, the edge joints 260 and crest joints 250 may be formed together or separately using the same or different processes. For example, the crest joints 260 and edge joints 250 may be formed simultaneously using thermosonic welding. In operation, the upper 110 and lower 120 sheets may be coupled together, the tube 130 inserted, and then sealed as described above (e.g., via edge joints 260). The heat spreader device 100 may then be evacuated and charged with a vaporizable liquid (via the tube 130). Once charged, the tube 130 may be sealed to maintain the internal conditions of the device 100. FIG. 4 illustrates a cross- sectional view of the vapor-augmented heat spreader 100 taken along lines B-B of FIG. 1. As shown, in the area where the charging tube 130 is connected to the upper sheet 110 and the lower sheet 120, tube edge joints 400 may further be utilized to form a fluid tight seal.

[0026] FIG. 5 illustrates a plan view of the lower sheet 120 in isolation, showing the interior surface 205B of the lower sheet 120 (i.e., FIG. 5 illustrates a top view the device of FIG. 1, with the upper sheet 110 removed for clarity) and a condensate groove structure formed thereon in accordance with an embodiment of the invention.

The condensate groove structure may be formed utilizing one or more predetermined groove patterns. In the illustrated embodiment, the condensate groove structure includes a combination of patterns such as a grid pattern 500, a leaf vein pattern 510, and/or a multi-wick structure pattern 520 (described above). Alternatively or in addition to, the condensate groove structure may include the boiling-enhanced, multi- wick structure 520 within the evaporation region. As explained above, the mechanical contact points 225, 235 of the crests may be utilized to create crest joints 250, e.g., by selectively welding the desired crests together.

[0027] FIG. 6 illustrates a plan view of the upper sheet 110 in isolation (i.e., FIG. 6 illustrates a bottom view of the device of FIGS. 1 and/or 3, with the lower sheet 120 removed for clarity), showing a vapor groove structure formed on interior surface 205A of the upper sheet 110 in accordance with an embodiment of the invention. As with the condensate groove structure, the vapor groove structure may be formed utilizing one or more predetermined patterns including, but not limited to, a grid pattern 600 and a leaf-vein pattern 610. In comparing FIG. 5 (the condensate groove structure) to FIG. 6 (vapor groove structure), it can be seen the groove width (aspect ratio) and spacing of the vapor groove structure is larger and coarser than that of the condensate groove structure.

[0028] With the above groove structure configuration (i.e., the use of both vapor grooves and condensate grooves), the vapor-augmented heat spreader 100 is capable of sustaining the high vapor pressure generated at solder reflow temperatures. Besides providing wicking power to the spreader 100, the groove structures are instrumental in (1) reducing the internal volume of a vapor chamber (thereby decreasing the surface area through which the vapor pressure can act) and (2) increasing the portion/number of directly bonded surfaces, thereby increasing the

structure's mechanical strength. That is, unlike other vapor chambers that explicitly create an internal volume (i.e., an open space) between the two sheets, the internal volume of the present vapor-augmented heat spreader 100 is provided only by the groove structures formed into the sheets 110, 120 (e.g., like the veins of a leaf). By changing the spatial distribution of these groove structures, it is possible to change the surface area that the vapor pressure acts upon — and therefore the forces generated — to balance the generated force against the strength of the crest joints 250 formed between the upper 110 and lower 120 sheets. In this way, the depth/geometry of each groove structure governs its flow property (for example, the wicking power). This ability is independent from the strength of the inter-sheet joints, which is governed by the sparseness of the groove structures. As mentioned above, preferably at least about 10% of the interior sheet surface area should be functionally joined together (e.g., via crest joints 250) to provide the desired strength requirement. Thus, the above groove structure enables the selective increase of the device's in-plane thermal spreading ability, while providing joints sufficient to ensure minimum resistances to through- plane heat flow, as well as ensuring the appropriate pressure rating

[0029] While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. For example, the heat spreader device 100 may possess any shape or have any suitable dimensions. By way of specific example, the device 100 may include a pair of generally rectangular plates brought into contact with each other. As noted above, the bottom plate may further include an evaporation region defined therein.

[0030] The upper 110 and lower 120 sheets may be formed from materials including, but not limited to, metal, metallic composites, polymer (with or without metallic

lining which may be formed by plating, deposition, or any other methods known in the arts), and combinations thereof. The upper 110 and lower 120 sheets may be form separate components of the device 100, or may possess a unitary structure, wherein the sheets are portions of a larger sheet, which are then folded during joining to eliminate the need for one of the edge-joints.

[0031] The vapor grooves 210A - 210C and the condensate grooves 230A - 230E on the sheets 110, 120 could form structured grid-like patterns or irregular patterns like veins on a leaf. The shape of the grooves may be rounded (like semi-circular or semi-elliptic), rectangular, polygonal, triangular, or combinations thereof. The manner of providing the higher capillary force to the condensate grooves may be provided by size differentiation, shape differentiation, or both. For example, the condensate and vapor grooves may possess the same size, but have different geometries (shapes). Alternatively, the condensate and vapor grooves may possess similar geometries, but possess different sizes. For example, a smaller rectangular groove could generate a higher surface tension force than a larger rectangular groove. Also, rectangular grooves of different aspect ratios are well-known to generate different surface tension forces. In addition, there may additionally be pin-grid structures serving the function of a multi-wick boiling enhancement. The surface of a sheet 110, 120 (i.e., the groove structure) may include only one type of groove pattern, or may include a combination of patterns (as illustrated in FIGS. 5 and 6).

[0032] The charging port or tube 130 may be formed out of separate sections of the two sheets 110, 120. Alternatively, the tube 130 may be a solid tube attached to the device 100 to maintain compatibility with standard vacuum pump interfaces. The solid tube 130 may be attached through welding, soldering, or any other similar

methods known in the arts. The vaporizable liquid can be water, alcohol, ammonia, organic liquid or any other similar materials known in the arts.

[0033] The air groove 140A, 140B may possess any suitable shape or possess any suitable dimensions suitable for its described purpose. The number of air grooves, moreover, is not limited — the air groove be formed on the exterior surface 200B of the lower sheet 120 and/or the exterior surface 200A of the upper sheet 110 in order to allow air to properly escape in the presence of solder flux or thermal interface material. The air grooves 140A, 140B may be formed through stamping, selective etching, material removal, cutting, skiving, swaging, scribing, or other processes that are known in the arts.

[0034] Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. For example, it is to be understood that terms such as "left", "right" "top", "bottom", "front", "rear", "side", "height", "length", "width", "upper", "lower", "interior", "exterior", "inner", "outer" and the like as may be used herein, merely describe points of reference and do not limit the present invention to any particular orientation or configuration.