SUPPORT STRUCTURE FOR PLANAR COOLING DEVICES AND METHODS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Republic of Korea patent application No. 10-2006-0034917, filed on Apr. 18, 2006, and U.S. patent application No. 11/410,772, filed Apr. 24, 2006, which are incorporated herein by reference in entirety.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

[0002] The invention relates generally to heat transfer and more specifically to cooling devices and methods capable of operating in small spaces.

BACKGROUND INFORMATION

[0003] Today, memory chips, central processing units, and embedded chips are used in numerous types of electronic devices. To ensure reliable operation of these devices, the devices have to be prevented from overheating. Keeping various electronic devices cool enough for optimal performance has become more challenging as the circuit density increases and many of these devices are made smaller and lighter. Furthermore, many consumer electronic devices today include additional heat sources such as optical display components. For example, laptops, personal digital assistants (PDA), and mobile phones may include liquid crystal display (LCD) panels using lamps or light emitting diodes (LEDs) as light sources. With electronic devices getting more compact yet including more heat sources, the traditional methods of dealing with heat dissipation, such as open packaging or cooling fans, do not provide an adequate solution to heat generation in these devices.

[0004] Recently, cooling devices that use evaporation and condensation, generally called "heat pipes," have become popular for dissipating heat in electronic devices. An advantage of heat pipes over cooling fans is that they can be made smaller, in line with the general size trend of consumer electronics.

[0005] FIGs. IA and IB are exploded perspective view and a cross-sectional view, respectively, of a conventional planar heat pipe 1. As shown, the heat pipe 1 has an open box 5 and a lid 2 that are designed to be combined. The open box 5 includes a base 6 and

sidewalls 7. The lid 2 is a plate having a thick portion 3 near its center. As shown, the thick portion 3 is located such that it is placed inside the cooling device 1 when the open box 5 is combined with the lid 2.

[0006] A plurality of support structures 8, 10 are formed inside the heat pipe 1. The support structure 8 is of such height that it extends between the base 6 and the part of the lid 2 that is not the thick portion 3. The support structure 10 extends between the base 6 and the thick portion 3. The outer layers of the support structures 8, 10 are made with capillary wicks, thus enhancing the circulation of fluid (e.g., coolant) through capillary force.

[0007] If a heat source (e.g., CPU) is attached to the lid 2 of the heat pipe 1 , the fluid that was absorbed in the capillary wick layer of the support structures 8, 10 evaporate by absorbing the heat. The gas-phase fluid moves toward the base 6 and condenses as heat is dissipated through the sidewalls 5. The condensed fluid is absorbed back by the capillary wick layer of the support structures 8, 10, and the evaporation-condensation cycle repeats.

[0008] The heat pipe 1 shown in FIGs. IA and IB has numerous disadvantages. First, the design of the support structures 8, 10 cause the cooling device 1 to be thicker than what is acceptable for many applications. The overall thickness cannot be decreased without compromising the cooling efficiency. Second, the heights of the support structures 8, 10, which span the distance between the base 6 and the lid 2, have to be exact for the cooling device to function properly. A small manufacturing error that changes the height of one of the support structures 8, 10 could prevent the cooling device 10 from being properly assembled, possibly adversely affecting its performance.

[0009] FIG. 2 is an exploded perspective view of another conventional heat pipe 10. The heat pipe of FIG. 2 includes an upper member 20 and a lower member 30 that can be combined to form a coolant space. Similarly to the heat pipe 1 of FIGs. IA and IB, the lower member 30 has a base 40 and sidewalls 50. However, unlike the cooling device 1 , the cooling device 10 has a capillary sheet 60 that fits in the space defined by the sidewalls 50. The support structure 70 are also different from the support structures 8, 10 of the heat pipe 1 because it is designed to fit with holes 80 on the capillary sheet 60.

[0010] The heat pipe 10 is not without shortcomings. For example, each support structure 70 is manufactured separately and placed in the holes 80 before it is placed in the

heat pipe 10. These steps complicate the fabrication process. Furthermore, because the capillary sheet 60 and the support structures 70 contain ceramic or rnetal, the cost of fabrication becomes higher. Therefore, the heat pipe 10 does not lend itself to cost-effective fabrication of thin, light-weight apparatuses.

[00111 A need exists for a small, planar heat pipe having a relatively simple structure that can be made cost-effectively.

SUMMARY OF THE INVENTION

[0012] In one aspect, the invention is a support structure for a cooling device. The support structure includes a flat plate with a portion cut out to form an opening and a spacer on the flat plate. The spacer is made by bending the cut-out portion of the flat plate. The support structure may be useful for a planar, thin cooling device.

[0013] In another aspect, the invention is a cooling device including a planar casing. A wick layer and the above support structure are positioned inside the casing.

[0014] In yet another aspect, the invention is a method of making a cooling device. The method entails forming a support structure by providing a flat plate, cutting a portion of the flat plate to create an opening, and forming a spacer on the flat plate. The spacer is formed by bending the cut portion of the flat plate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. IA and IB show a conventional planar heat pipe.

[0016] FIG. 2 shows another conventional planar heat pipe.

[0017] FIG. 3 is an exploded perspective view of the cooling device in accordance with a first embodiment of the invention.

[0018] FIG. 4 is a cross-sectional view of the cooling device of FIG. 3.

[0019] FIG. 5 is a close-up view of a portion of the support structure in the cooling device of FIG. 3.

[0020] FIG. 6 is a close-up view of a portion of an embodiment of the support structure usable with the cooling device of the invention.

[0021] FIG. 7 is a close-up view of a portion of another embodiment of the support structure usable with the cooling device of the invention.

[0022] FIG. 8 is an exploded perspective view of a second embodiment of the cooling device in accordance with the invention.

[0023] FIG. 9 is a third embodiment of the cooling device in accordance with the invention.

[0024] FIG. 10 is an open perspective view of a fourth embodiment of the cooling device in accordance with the invention.

[0025] FIG. 11 is an exploded perspective view of the cooling device attached to a heat source.

[0026] FIG. 12 is a cross-sectional view of the cooling device of FIG. 11 illustrating its operation.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention provides a planar cooling device that is structurally simpler than the above-described conventional heat pipes. In the planar cooling device of the invention, a bent portion is incorporated into the support structure to maintain the coolant travel path at a desired size. The design allows for increased coolant travel space, allowing efficient coolant circulation and improved cooling. The cooling device of the invention may be made as thin as 0.5 — 5 mm.

[0028] An additional benefit of the structure of the invention is that the support structure imparts rigidity to the device, making the cooling device more reliable.

[0029] Due to its simplified structure, the invention can be produced more cost- effectively than the conventional heat pipes without compromising the cooling power.

[0030] As used herein, "planar" structure is a structure having a flat portion.

[0031] FIG. 3 is an exploded perspective view of the cooling device in accordance with a first embodiment of the invention, FIG. 4 is a cross-sectional view of the cooling device of FIG. 3, and FIG. 5 is a close-up view of a portion of the support structure in the cooling device of FIG. 3.

[0032] As shown in FIG. 3, the cooling device of the invention includes a casing made of a first plate 200 and a second plate 210. The first plate 200 and the second plate 210 may be combined to form an enclosed space between the two plates 200, 210. The two plates 200, 210 may be sealed together to form the enclosed space so that a coolant fluid (e.g., water, methanol, ethanol) is contained in the enclosed space. The seal may be formed around the edges so as to not intrude upon the enclosed space. Either or both of the first and second plates 200, 210 is designed so that it can be attached to a printed circuit board (PCB) or a chip. For example, the first plate 200 may have a flat surface that can easily be attached to a PCB or a chip, and an adhesive layer may be placed on the flat surface.

[0033] The first plate 200 and the second plate 210 are made of a material that has sufficient rigidity to protect the structures in the space between the two plates. The material may be, for example, aluminum, titanium, plastic, metallized plastic, graphite, copper, or any plastic combination capable of transferring heat effectively. Preferably, the first plate 200 and the second plate 210 are prepared from one large sheet of material. The first plate 200 and the second plate 210 may be laminated to facilitate their attachment to a heat source.

[0034] Between the first plate 200 and the second plate 210 is a capillary wick 220. The capillary wick 220, which is attached to the first plate 200, is capable of absorbing the coolant fluid by capillary action. The capillary wick 220 is preferably made of a hydrophilic wick. A hydrophilic wick, which is well-known, contains a plurality of micro- fibers and each micro-fiber is capable of absorbing and retaining the coolant fluid. In some embodiments, the capillary wick 220 may be attached to the second plate 210.

[0035] A support structure 230 is also located between the first plate 200 and the second plate 210. In the particular embodiment of FIG. 3, the support structure 230 is located between the wick layer 220 and the second plate 210. The support structure 230 supports the capillary wick 220 and forms the coolant fluid travel path by maintaining a certain distance between itself and the second plate 210. The support structure 230 has openings 231 through which the gaseous coolant can travel from one side of the support structure 230 to the other side (e.g., from the side closer to the first plate 200 to the side closer to the second plate 210).

[0036] To form the openings 231 , the support structure 230, which initially starts out as a solid plate, is cut to form one or more "flaps" that are partially attached to the main body of the solid plate. For example, an "H" may be cut into the solid plate so that two flaps are formed for each opening 231. The flaps are then bent or folded to open up the openings 231, and the bent/folded portion of the flap forms spacers 235 located near the openings 231. The folded portion is bent by more than a 90°-angle such that the flap touches the flat plate.

[0037] The side of the cooling device that has the capillary wick 220 is the side that is closer to the heat source. The coolant fluid is absorbed by the capillary wick 220. When the cooling device receives heat from the heat source, the coolant fluid in the capillary wick 220 evaporates by absorbing the latent heat. The gas passes through the openings 231 to the other side of the support structure 230, where it cools down and condenses.

[0038] The spacers 235, 236 may be formed so that some of them (spacers 235) are bent toward the first plate 200 and others (spacers 236) are bent toward the second plate 210. As illustrated in FIG. 4, the spacers 235 "push" the capillary wick 220 toward the first plate 200, eliminating any unevenness in the contact area between the capillary wick 220 and the first plate 200 and enhancing the efficiency of heat absorption and the effect of capillary force. The spacers 236 push against the second plate 210 to maintain a substantially uniform gap between the support structure 230 and the second plate 210, thereby maintaining an unimpeded travel path for the coolant fluid. The support structure 230 is made of a material that is rigid enough that the spacers 236 prevent the coolant travel path from being deformed by pressure applied to the cooling device from the outside.

[0039] The support structure 230 may be made from a similar list of materials as the first and the second plates 200, 210, such as aluminum, titanium, plastic, metalized plastic, graphite, copper, any plastic combination.

[0040] FIG. 6 is a close-up view of a portion of a support structure 330 usable with the cooling device of the invention. Unlike in the embodiment of FIGs. 3, 4, and 5, all the spacers 335 are bent in the same direction in this embodiment. The support structure 330 is particularly suited for use with a multi-wick embodiment of the cooling device such as the one illustrated below in FIG. 8.

[0041] FIG. 7 is a close-up view of a portion of a support structure 430 usable with the cooling device of the invention. As shown, the spacers 435, 436 are bent to form a substantially 90° angle in this embodiment, unlike in the embodiment of FIGs. 3, 4, and 5 where the cut-out flaps were "folded."

[0042] In yet another embodiment, it is possible to form one spacer per opening instead of two spacers per opening as in the above embodiments. In this case, the support structure 230 would be formed by cutting lines through a solid plate such that the lines form three sides of a rectangle, a "C" shape, or a "U" shape (instead of an "H" shape as in the above embodiments). The flap would then be bent/folded to create the opening and a spacer.

[0043] The spacers and the openings do not always have to be formed together. For example, the solid plate of the support structure may be cut with lines that form multiple rectangles so that the piece that is cut out is completely detached from the solid plate. The cut-out piece can then be formed into a desired shape and attached to a desired spot on the solid plate so that it can maintain a separation distance between the support structure and an adjacent layer.

[0044] The support structure 230/330/430 may be attached to one of the first and second plates 200, 210 by any well-known attachment method, such as welding (tig welding, plasma welding, seam welding, high frequency welding, etc.) or soldering a surface of the support structure 230/330/430 to the plate. Attaching the support structure to one of the plates 200, 210 improves heat exchange and enhances general reliability of the cooling device.

[0045] FIG. 8 is an exploded perspective view of a second embodiment of the cooling device in accordance with the invention. As shown, the cooling device of the multi-wick embodiment includes the first plate 200, the second plate 210, and the support structure 330. In addition, the cooling device includes a first wick layer 220a and a second wick layer 220b positioned between the support structure 330 and one of the plates 200, 210, respectively. The support structure 330 is, therefore, located between the two wick layers 220a, 220b. Details of the support structure 330 were provided above in reference to FIG. 6. The cooling device of FIG. 8 may be made with the support structure 230 or 430 as well. The multi-wick embodiment of the cooling device works particularly well when there are two heat sources, one near each of the two plates 200, 210. In the case where there are two

heat sources, the parts of the cooling device that are closest to the two heat sources act as evaporation areas, and condensation occurs in the other parts of the cooling device.

[0046] FIG. 9 is a third embodiment of the cooling device in accordance with the invention. The finned-embodiment that is shown is substantially similar to the embodiment of FIG. 3 except that it has fins 213 on the lower plate 211 to increase the heat exchange surface. In this embodiment, the first plate 210 is designed so it can be positioned adjacent to a heat source. The coolant fluid evaporates on the side of the support structure 230 that is closer to the first plate 200, then expands into the side that is closer to the second plate 210 through the openings 231. Optionally, each of the fins 213 is designed with a hollow space 215 so that the coolant fluid can travel inside the fins 213 for a more effective heat dissipation. However, the fins 213 being designed to hold coolant fluid is not a requirement of the invention. The presence of the fins 213, even without the hollow spaces 215, aids heat dissipation by increasing the heat exchange surface.

[0047] FIG. 10 is an open perspective view of a fourth embodiment of the cooling device in accordance with the invention. Unlike the embodiment shown in FIG. 3, the casing for this embodiment does not use a first plate and a second plate. Instead, the casing is formed with a planar tube 600. The planar tube 600 may be prepared by obtaining a cylindrical pipe and pressing it down to a planar-shape. Methods that may be used to press the pipe to form a planar tube are well known, and depends on the material from which the cylindrical pipe is made. The capillary wick 220 is attached to an inner surface of the planar tube 600, and the support structure 230 is positioned inside the planar tube. Once the capillary wick 220 and the support structure 230 are placed in the tube 600, the open ends of the tube 600 are sealed. Although FIG. 10 shows the support structure 230 as being used, support structures of other designs (e.g., support structures 330, 430) may be used with the tube embodiment. The tube embodiment is cost-effective because it is simple to manufacture.

[0048] The tube 600 may be made of the same material as the first and the second plates 200, 210, such as aluminum, titanium, plastic, copper, metalized plastic, graphite, plastic combinations, and other suitable metals with good heat conduction properties.

[0049] FIG. 11 is an exploded perspective view and FIG. 12 is a cross-sectional view illustrating the operation of the cooling device of the invention. In the example that is

illustrated, a PCB 550 with a chip 500 (a heat source) is attached to the cooling device. The first plate 200 of the cooling device is adjacent to the PCB 550, so that the heat that is generated from the chip 500 reaches the first plate 200 through the PCB 550. The heat that reaches the first plate 200 evaporates at least some of the coolant fluid that was absorbed in the wick layer 220. As some of the coolant fluid in the wick layer 220 evaporates, more liquid-phase coolant is pulled to fill the space by capillary force (see dark arrows). Due to the thinness of the cooling device, the weight of the water vapor is low enough that gravitational effects do not interfere with the liquid absorption by capillary force.

[0050] In the hot part of the cooling device, the coolant fluid absorbs latent heat to change its phase from liquid to gas, making the heat absorption that occurs at the interface between the cooling device and the PCB 550 highly efficient.

[0051] Some of the evaporated gas-phase coolant fluid expands in the space between the first plate 200 and the wick layer 220. Some gas-phase coolant fluid crosses over to a cooler portion of the cooling device through the openings 231 in the support structure 230, as shown by the light arrows. The coolant fluid that passes through the openings 231 expands in all directions to fill the space between the support structure 230 and the second plate 210. As the coolant fluid expands in the cool portion of the cooling device and dissipates heat through the second plate 210, condensation occurs. The condensed liquid is absorbed back by the wick layer 220 through capillary force and repeatedly goes through the evaporation-condensation cycle. The spacers 235 maintain the distance between the support structure 230 and the first plate 200, while the spacers 236 maintain the distance between the support structure 230 and the second plate 210. Thus, with the wick layer 220 and the space created by the support structure 230, there is an open path through which the coolant fluid can circulate unimpeded between the hot and cool portions of the cooling device.

[0052] The cooling device of the invention simplifies manufacturing and therefore allows cooling devices to be made cost-effectively. The support structure 230 "pushes" the wick layer 220 close to the surface of the cooling device that is closest to the heat source, improving heat transfer. At the same time, the support structure 230 maintains a circulation space for the coolant fluid to aid heat dissipation. Furthermore, the support structure 230 makes the cooling device rigid, and therefore more reliable. Without the support structure,

the cooling space could be deformed (e.g., the two plates collapse toward each other) easily upon impact, impeding the coolant circulation path and disturbing the flow of the coolant fluid. The simple design of the spacers 235, 236 and the openings 231 allow the support structure to perform all these functions without raising the manufacturing cost.

[0053] Although the invention has been described with reference to the above example, it will be understood that modifications and variations are encompassed within the spirit and scope of the invention. For example, the shape and number of the spacers are not limited to the particular embodiments shown here. Any structure that may be formed from a solid plate to maintain a certain distance between the support structure and an adjacent plate would be within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims.