WO2003074958A1

WO2003074958A1 - Flat-plate heat-pipe with lanced-offset fin wick

Info

Publication number: WO2003074958A1
Application number: PCT/US2003/001009
Authority: WO
Inventors: Maninder S. Sehmbey; Morris Bowers; Markus Dantinne
Original assignee: Motorola, Inc.
Priority date: 2002-02-28
Filing date: 2003-01-13
Publication date: 2003-09-12
Also published as: US20030159806A1; AU2003202985A1

Abstract

A passive cooling unit for integrated electronics in form of flat-plate heat-pipe device (20) having a shallow cavity base member (26), a cover plate (28), and a lanced-offset fin member (36) and associated porous metal wick material sandwiched therebetween, the fin member (36) being braced to the base member (26) and cover plate (28) to provide structural support and also being coated with the wick material. The resultant flat-plate heat-pipe device (20), when formed of a lightweight metal such as aluminum or titanium, results in a passive cooling device that is lightweight, structurally strong, and of low cost manufacture.

Description

FLAT-PLATE HEAT-PIPE WITH LANCED-OFFSET FIN WICK

Field of the Invention

This invention relates generally to passive cooling for electronic devices, and more particularly to a flat-plate heat-pipe having an internal lanced-offset fin wick structure with associated porous wick material, and a method for forming the same.

Background of the Invention Electronic devices such as power amplifiers, power supplies, integrated circuit chips, multi-chip modules, heat spreaders, electronic hybrid assemblies such as power amplifiers, microprocessors and passive components such as filters, contain heat sources which require cooling during normal operation. Various techniques may be used for cooling electronic devices. Traditionally, electronic devices have been cooled by natural or forced convection which involves moving air past conduction heat sinks attached directly or indirectly to the devices.

Efforts to reduce the size of electronic devices have focused upon increased integration of electronic components. Sophisticated thermal management techniques using liquids, which allow further reduction of device sizes, have often been employed to dissipate the heat generated by integrated electronics.

Two-phase thermosyphons have often been used to provide cooling for electronic devices. Two-phased thermosyphons typically include a two-phase material within a housing. The two-phase material, typically a liquid, vaporizes when sufficient heat density is applied to the liquid in the evaporator section. The vapor generated in the evaporator section moves away from the liquid towards the condenser. In the condenser section, the vapor transforms back to liquid by rejecting heat to the ambient. The heat can also be dissipated to the ambient atmosphere by a variety of means, such as natural convection, forced convection, liquid and other suitable means. Typifying such two-phase thermosyphons is U.S. Patent No. 6,234,242, assigned to the assignee of the present invention.

However, there are significant orientation limitations inherent in the use of such two-phase thermosyphons since they only work in a vertical orientation. This is because thermosyphons need the assistance from gravity to get the condensed working liquid from the condenser section to the evaporator section, i.e., the condenser must always be higher than the evaporator section. Additionally, such two-phased thermosyphon devices are not well suited for very low heat-flux applications.

Another cooling device often used is the so-called flat-plate heat-pipe device ("FPHP"). Such FPHP devices are suitable for low heat-flux and medium heat-flux applications, and thus, are suitable for many electronics applications such as cellular telephonic infrastructure products. FPHP devices operate on the principal of a closed loop of evaporation/boiling and condensation of a fluid. The working liquid evaporates and boils off in the areas where heat is dissipated by electronic components, which components are mounted externally to the FPHP device's walls, and then travels to the condensation section as a vapor. Contrary to the boiling occurring in two-phase thermosyphons, the evaporation in a FPHP device can advantageously occur with a very small temperature rise between the working liquid and the FPHP surface. The vapor spreads evenly in the condensation space and condenses back into liquid form by rejecting heat to the ambient, as often assisted by externally-mounted fin structure to create a heat sink. The condensed liquid travels back to the heated section by a wicking action through porous wick structure formed on the interior surfaces of the FPHP device's cover plates.

Importantly, such FPHP devices have the ability to operate in any orientation. However, commercially available FPHP devices are typically made of thick- walled copper making such FPHP units extremely heavy, and therefore impractical for most electronic cooling situations related to cellular infrastructure products. They typically use a sintered copper wick on the interior faces of the cover plate, such that no other wick substructure is present. Also, due to their method of manufacture, commercially available FPHP devices are very costly and relatively weak. In fact, because they are so structurally weak, they are unable to withstand high internal positive pressures or perform effectively throughout the broad temperature range found in many cellular and other electronic infrastructure products. They are generally limited to operating when internal pressure is lower than 1 atm. Another type of FPHP that has been typically used employs multiple cylindrical heat pipes embedded in a solid plate of aluminum. Typifying such prior FPHP devices are U.S. Patent Nos. 4,880,052, which discloses flat plates with individual heat-pipes embedded within.

Yet a further cooling device used with integrated electronics takes the form of an integral heat pipe, heat exchanger and clamping plate, forming a grid-like pattern of wick-lined channels, such as typified by U.S. Patent No. 5,253,702. There, a base plate functions as an evaporator having a multiplicity of intersecting parallel and perpendicular internal channels extending across the baseplate. A sintered copper thermo wick material is applied to the surfaces of all channels, and there is a series of vertically aligned condenser tubes forming a condenser region terminating at their upper ends in cooling fins. This is a complicated structure formed at great cost, and also has orientation limitations to its use. There is a need for a lightweight and compact flat-plat heat-pipe cooling device that is structurally robust, thermally efficient, and has a low cost of manufacture.

Summary of the Invention A lightweight flat-plate heat-pipe device is constructed as a sealed cavity fabricated by brazing a thin, flat shallow cavity base member with a cover plate to create a thin-shelled hollow-enclosure to contain an evaporative working fluid. The base member includes a number of upstanding bosses which incorporate holes for locating the fasteners used for installing external electronic circuit boards, and other modules, on the outer surfaces of the FPHP device. A lanced-offset fin structure is affixed internally within the cavity to provide structural support to the thin base member and cover plate. The lanced-offset fins have an associated porous wick structure. This overall sandwiched assembly is brazed together to provide a structurally robust sealed unit having very thin external walls, and a small opening for the introduction of a charge of the working fluid. The porous wick structure associated with the lanced offset fins acts as a capillary-type wick for transporting the condensed liquid from the condenser section back to the heated evaporator section. One end of the respective external faces of the FPHP unit, i.e., at the condenser section end, can be configured with appropriate cooling fins to convert the FPHP device into a light-weight heat-sink.

In one embodiment of the present flat-plate heat-pipe invention, the porous wick structure is flame-sprayed directly onto the lanced offset fins. In another embodiment, the porous material is flame-spayed or otherwise applied to the internal facing surfaces of the respective base member and cover plate. In a final embodiment, used for heavy-duty applications, all of the generally transverse surfaces of the lanced-offset fins, i.e. those surfaces extending between the respective base member and cover plate, are encased and all open spaces packed with a sintered metal wick material to create a structurally supportive porous wick which permits capillary movement of the working fluid in all directions and vapor movement in the direction of the fins.

Preferably, the base member, cover plate and lanced-offset wick structure are formed of a lightweight aluminum material or aluminum alloy. Further, the porous wicking material is preferably formed of aluminum powder that has been flame-sprayed or sintered in place.

Brief Description of the Drawings The means by which the foregoing and other aspects of the present invention are accomplished and the manner of their accomplishments will be readily understood from the following specification upon reference to the accompanying drawings, in which:

Fig. 1 depicts a front elevation view of the flat-plate heat-pipe device with lance-offset fin wick structure of the present invention;

Fig. 2 is an end elevation view of the flat-plate heat-pipe device of Fig. 1; Fig. 3 is a perspective view of the base member of the device of Fig. 1; Fig. 4 is a perspective view of the lanced-offset fin structure of the present invention; Fig. 5 is an enlarged cross-sectional view, taken at circled location 5-5 in

Fig. 8, of the present flat-plate heat-pipe device showing the lanced-offset fin structure with associated porous wick material, and showing circuit board structure, and with certain components deleted for better viewing;

Fig. 6 is another cross-sectional view, similar to Fig. 5, but of an alternative form of the flat-plate heat-pipe device of the present invention; Fig. 7 is yet a further enlarged cross-sectional view, similar to Figs. 5 and 6, but of yet a further alternative embodiment for the flat-plate heat-pipe device of the present invention;

Fig. 8 is a side elevation view, similar to Fig. 1, of the present flat-plate heat-pipe device showing a circuit board including heat-dissipating electronic devices, and a clam shell housing; and

Fig. 9 is a side elevation view of the flat-plate heat-pipe device of Fig. 8.

Detailed Description of the Invention Having reference to the drawings, wherein like reference numerals indicate corresponding elements, there is shown in Fig. 1 an illustration of a flat-plate heat- pipe passive cooling device, generally denoted by reference numeral 20. The cooling device 20 includes a flat-plate heat-pipe member 22 and external cooling fins 24, to create a passive cooling heat sink assembly. The FPHP member 22 comprises a base member 26 and a cover plate 28. The base member comprises a base plate 27, a series of upstanding support bosses or shoulders 48 (described later herein), and a peripheral end wall or frame 30. Collectively, the components base member 26 create a shallow cavity generally denoted by reference numeral 34, which when covered off by the cover plate 28 becomes a sealed enclosed cavity. The base member 26 and cover plate 28 are preferably formed of a suitable aluminum material, such as an aluminum-6061 alloy. The base member 26 can be machined from aluminum plate stock to yield cavity 34 and raised bosses 48. So as to create a thin-shelled cooling unit 20, the wall thickness for the aluminum plates 26, 28 is preferably less than 1 mm in thickness, but not less than .5 mm thick, and preferably approximately .7 mm. Alternatively, instead of using an aluminum alloy for the base member 26, cover plate 28, and the lanced-offset fin member 36, a suitable titanium material could be used. This likewise would provide suitable structural support and rigidity to FPHP member 22, yet allow it to remain sufficiently lightweight.

An opening tube 32 (see Figs. 1 and 7), preferably some 4.8 mm in diameter and welded along the end wall 30 (but which may also be welded along the baseplate 27 or cover plate 28, but not shown in either location) can be used to seal off the inner cavity 34. After the charging process, the filler tube 32 is preferably sealed off by means of an ultrasonic welder, which pinches and applies ultrasonic energy to the pinched section of tube 32 to form the needed closure seal.

A lanced-offset fin member, generally denoted by reference numeral 36, is shown in Figs. 4-7. The lanced-offset fin member 36 is preferably made of an aluminum material, such as an aluminum-3033 alloy, and is commercially available from vendors such as Robinson Fin Machines of Kenton, Ohio. This lanced-offset fin member 36 has a series of generally upstanding channel sections 38 and an intervening valley sections 40, with a width therebetween in the range from approximately .5 mm to 2.5 mm, with the preferred width range of approximately 1.0 to 2.0 mm. The transverse wall sections 42 integrally connecting the channel sections 38 and valley sections 40 comprise a staggered or offset series of walls 44 as separated by punched openings 46. Selectively cut clearance holes 47 in fin member 36 permit fin member 36 to fit over and nestle around respective raised bosses 48 within cavity 34.

As will be seen in Figs. 3 and 5, the lanced-offset fin member 36, while formed as a lightweight component, still creates a rigid structural member to support the thin-walled base plate 27 and cover plate 28 particularly against the both significant internal and external pressures that are created within enclosed cavity 34. The material wall thickness for the overall lanced-offset fin member 36 is preferably less than approximately .5 mm thick. Importantly, besides providing structural support, the particular punched-wall configuration of the lanced-offset fin 36 presents numerous surfaces (channel sections 38, valley sections 40, and intervening wall sections 42) to support the associated porous wick material as described below. Other configurations for the lanced-offset fin member 36 can include a wave-type folded fin (shaped like the corrugated layer in cardboard), lanced-offset fins like member 36 but with inclined walls (where the vertical walls are at approximately 120E to the horizontal walls), lanced-offset fins with a fine curved pitch (where the upper and lower horizontal faces are generally curved surfaces instead of flat), and louvered fins (where the vertical faces of a wave-type folded fin have vertically aligned louver openings). However, lanced-offset fin member 36 of Fig. 4 is considered the preferred design because that design permits the best liquid-vapor movement perpendicular to general flow direction, to provide the best thermal performance, such vertical faces provide the best structural strength, and such horizontal faces provide the best high strength brazing bond between the fin 36 and base plate 27, cover plate 28.

Figure 5 depicts how the lanced-offset fin member 36 is fitted into the base member 26 and is sandwiched between the base plate 27 and cover plate 28. During manufacture, the lanced-offset fin 36 can be further shaped by laser cut or EDM to fit into the cavity 34 of base member 26 with needed clearance holes 47 to fit around the raised bosses 48. As is known in brazing operations, a shim of suitable brazing material (not shown), such as aluminum alloy 4004 or 4104, is placed over the interior surface of base plate 27, and the configured lanced-offset fin 36 (with associated porous wick material as described below), is dropped into place on top of the brazing material. The cover plate 28 clad with a suitable brazing material (not shown) is then placed on top of the base member 26 and fin 36, to create a sandwiched assembly. That assembly is then fixtured to provide uniform pressure over its entire span and then placed in a vacuum brazing furnace to seal the cover plate to the base member 26, and to secure affix the fin structure 36 to the interior surfaces of both base plate 27 and cover plate 28. In this manner, the lanced-offset fin member 36 operates to structurally separate and structurally support the respective base member 26 and cover plate 28 to create a thin-shelled flat, rigidly supported FPHP device 22.

Importantly, the use of the lanced-offset fin 36 acts to eliminate the need for having one of the base plate 27 or cover plate 28 punched or dimpled for separating the base and cover plates, such as was required with many prior passive cooling devices. For example, see the Thermacore (Trademark) product sold under the product name Thermabase (Trademark).

The raised bosses 48 (see Fig. 3) each have an opening 50 to receive a threaded fastener 52, which can be used to secure heat dissipating members 54 on associated circuit boards 56 to the FPHP member 22. As seen in Fig. 7, a pair of such circuit boards 56 have been mounted to the respective front and rear external sides of FPHP member 22 via fasteners 52 engaging openings 50 in bosses 48. As seen in Figs. 7 and 8, a large number of heat-dissipating members 54 are mounted to the respective circuit boards 56. Devices 54 can comprise power amplifiers, power supplies, integrated circuit chips, multi-chip modules, heat spreaders, and electronic hybrid assemblies such as power supplies, microprocessors and passive components such as filters. All of these electronic devices contain heat sources which require cooling during normal operation.

As seen in Fig. 2, the FPHP member 22 can have cooling fins 28 mounted to both external surfaces at the upper end thereof to form a complete heat sink assembly. This acts to divide the FPHP unit 22 into generally an evaporator section 58 and a cooling or condensing section 60. Alternatively, simply the FPHP member 22 can be used alone, i.e., without external cooling fins 24, by having the member's condensing section 60 of member 22 in contact with a chassis (not shown) which conducts rejected heat away, such as with a thermal backplane device.

Turning to Fig. 4, the various external surfaces of the lanced-offset fin member 36 (comprising channel sections 38, valley sections 40, and transverse punched wall sections 42) have an appropriate porous metal wicking material, generally denoted by reference numeral 62, applied thereon. The porous metal wicking material 62 forms a porous wick structure to transport fluid medium therealong, and is supported by fin member 36. Material 62 preferably comprises a porous aluminum foam coating, such as powdered aluminum which has been preferably flame-sprayed onto the fin member 36. Advantageously, use of a flame- spraying application method results in a uniform layered coating of wick material 62. The porous wicking material layer 62 is preferably between approximately 1.0 and 2.0 mm thick. Such a porous aluminum wicking material 62 acts as an excellent capillary-type wick to convey the condensed working liquid (such as an evaporative liquid medium; not shown) from the condenser section 60 along the lanced-offset fin member 36 to the evaporator section 58. As an alternate material to use for the porous wicking material 62, it could instead be formed as a sintered copper or sintered bronze material. Either such alternate material can be suitably flame-sprayed or otherwise applied to the lanced-offset fin member 36 to again create a suitable wicking structure for FPHP member 22.

Due to the presence of the porous wicking material 62 as flame-sprayed onto the lanced-offset fin member 36, the present FPHP cooling unit 20 can be utilized in any orientation. That is, the present invention's FPHP device 20 is not gravity- dependent, such as were the prior art FPHP and two-phase thermosyphon devices which were always required to be oriented vertically. Thus, such an "operable-in- all-orientations" feature is a significant improvement over the previously available passive cooling products. In sum, besides acting as a structural support for the base plate 27 and cover plate 28, the lanced-offset fin member 36 further acts as a structural support for the wick surface formed of porous wicking material 62 inside the inner cavity 34 of the FPHP member 22.

Referring to Figs. 1 and 3, using suitable evacuation equipment, the inner shallow cavity 34 can be evacuated via the opening tube 32. Then the desired amount of a suitable working fluid medium, such as acetone, can be introduced through opening tube 32. Once the evaporative liquid medium has been introduced into inner cavity 34, the opening tube 32 can be closed, such as by pinching off opening tube 32, ultrasonic welding, or some other suitable method to seal off filler tube 32 and the inner cavity 34 thereby retaining the working medium within the FPHP member 22. Alternatively, instead of using an aluminum alloy for the base member 26, cover plate 28, and the lanced-offset fin member 36, a suitable titanium material could be used. This likewise would provide suitable structural support and rigidity to FPHP member 22, yet remain sufficiently lightweight.

An alternate embodiment of the present invention is depicted in Fig. 6, where like reference numerals are used for like structural elements relative to the preferred embodiment of Fig. 5. That is, in Fig. 6, the porous wicking material 62 is shown, instead of being applied directly on the lanced-offset fin member 36, as being flame-sprayed onto the interior facing surfaces of the respective base plate 27 and cover plate 28. Thus, in this embodiment there is no porous wicking material 62 present on the lanced-offset fin member 36 at all. Otherwise, this alternate embodiment FPHP member 22' of Fig. 6 is identical to the FPHP member 22 of the preferred embodiment depicted in Fig. 5.

Depicted in Fig. 7 is yet a further alternative embodiment of the present invention, generally denoted by a reference numeral 22". Here, the associated porous wicking material 62 is so formed and configured that the material 62 encases, and packs all open spaces, around the walls 44 of punched wall sections 42, upstanding channel sections 38, and intervening valley sections 40 of lanced- offset fin member 36. Thus, while no porous wicking material 62 is present or formed directly on the inner surfaces of base plate 27 and cover plate 28 so that appropriate brazing occurs, nevertheless there is a substantially larger amount of the porous wicking material 62 present and structurally supported by the lanced- offset fin member 36 in this Fig. 7 embodiment than with the prior two embodiments (of Figs. 5 and 6).

The specific configuration for the porous wicking material 62 found in the embodiment of Fig. 6 is formed by utilizing a pair of stainless steel plates with inwardly-protruding, mating linear fins (none shown) to sandwich the lanced-offset fin member 36 therebetween. The remaining open spaces are packed with a suitable powdered metal material, and then sintered in a furnace. A porous aluminum powdered metal can be used. Then, once suitably adhered to the lanced-offset fin structure 36, the porous wick structure is created with the sintered powdered metal being heat fused together but still essentially retaining to same pore geometry as previously present between the powdered metal particles. The stainless steel plates (not shown) are then removed to result in an integral assembly of lanced-offset fin 36 with sintered wick structure (formed of porous wick material 62) as shown in Fig. 6. Importantly, the surfaces of the lanced-offset fin 36 that face and mate with the base plate 27 and cover plate 28 are free of porous wick material 62, thus allowing a very high strength braze joint between the same. The resulting sintered metal wick structure allows X-Y-Z movement of condensed fluid throughout the structure, while vapor moves in the open areas where the stainless steel fins were during sintering. Due to this particular metal wick structure, a very strong FPHP device 22 is created that is capable of withstanding significant internal and external pressure, e.g., greater than 500psig.

It will be understood that the specially-configured sintered metal wick and lanced-offset fin assembly described above for this alternate embodiment of Fig. 7 can also be beneficially used in the heavy- walled copper type FPHP's of the prior art, so as to reduce their overall resultant weight and cost. In Fig.8 is shown a suitable clam shell-type cover housing 64, which is used to cover off and protect the respective circuit boards 56 and heat dissipating members 54 carried thereon from external environment and potential damage. As needed, the cover housing 64 can be readily removed for suitable installation or repair purposes. Housing 64 can be formed as a cast aluminum or cast magnesium component.

The present FPHP cooling devices are thin-shelled units with lanced-offset fin structure brazed to both cover plates, resulting in a structurally strong sealed unit. They can, for example, be some 406 mm (16 inches) by 254 mm (10 inches) in size. It provides significant weight, size and cost savings over the prior two phase thermosyphons, and prior heavy, thick- walled copper-clad FPHP devices. The present passive cooling FPHP devices are particularly useful in cellular base station equipment, personal computers, supercomputers, power supplies, automotive electronic, and communication infrastructure equipment, e.g., microsite and picosite cellular equipment. The present invention allows use of a low cost passive cooling system in such electronics applications, reduces overall permissible product size and weight, and thus allows a higher overall electronics package density for the end user.

From the foregoing, it is believed that those skilled in the art will readily appreciate the unique features and advantages of the present invention over previous types of passive cooling units for integrated electronic devices. Further, it is to be understood that while the present invention has been described in relation to a particular preferred and alternate embodiments as set forth in the accompanying drawings and as above described, the same nevertheless is susceptible to change, variation and substitution of equivalents without departure from the spirit and scope of this invention. It is therefore intended that the present invention be unrestricted by the foregoing description and drawings, except as may appear in the following appended claims.

Claims

WE CLAIM

1. A flat-plate evaporative-fluid type passive cooling device for use in dissipating heat in electronic applications, comprising in combination: a base member having a shallow cavity formed internally therein, said cavity including a plurality of upstanding support boss members; a cover plate adapted to sealably close off said base member to enclose said shallow cavity to enable containing an evaporative fluid medium therewithin; lanced-offset fin means fitted into said shallow cavity and adapted to support said cover plate and said base member against external and internal pressure; and porous metal wick means associated with said lanced-offset fin means and adapted to permit the evaporative fluid medium to travel therealong by capillary action once condensed from an evaporative state.

2. The device of claim 1, wherein said base member comprises a base plate portion, said plurality of upstanding bosses are formed on said base plate portion, and a peripheral wall portion extends from said base plate portion to form said shallow cavity within said base member.

3. The device of claim 2, wherein said porous metal wick means is applied onto the interior surfaces of said base plate and said cover plate.

4. The device of claim 1, and wherein said lanced-offset fin means, said base member, and said cover plate are formed of metallic material and said lanced-offset fin means are affixed to said base member and said cover plate by brazing.

5. The device of claim 4, wherein said porous metal wick means comprise a powdered metal coating applied directly onto said lanced-offset fin means.

6. The device of claim 4, wherein said porous metal wick means is formed as a sintered metal enclosure covering all non-brazed surfaces of said lanced- offset fm means, whereby said sintered metal enclosure permits the evaporative fluid medium to travel by capillary action in all directions once condensed from an evaporative state.

7. The device of claim 4, wherein said metallic material is formed from one of aluminum and an aluminum alloy.

8. The device of claim 1, and filler opening means adapted to permit evacuation, filling, and sealing of said enclosed shallow cavity.

9. The device of claim 1, and extended cooling fin means affixed to the outer surfaces of at least one end of said respective base member and said cover plate to permit external heat sink dissipation of heat collected and released during condensation of said evaporative fluid medium.

10. A method for providing passive cooling for integrated electronic devices, comprising the steps of: a) forming a shallow-cavity metal base member having a base plate member, a peripheral wall member, and a plurality of upstanding support boss members; b) fitting a lanced-offset metallic fin member into the shallow cavity base member; c) installing porous metal wicking material into said shallow cavity base member; d) enclosing the shallow cavity by affixing a cover plate to the base member; e) brazing the lanced-offset metallic fin member to both the base member and the cover plate; f) forming a fitting opening for the enclosed shallow cavity; g) evacuating the interior of the enclosed shallow cavity through the fitting opening; h) introducing a quantity of evaporative liquid medium into the enclosed shallow cavity through the fitting opening; and i) sealing off the fitting opening.