Abstract:
A heat pipe with superior heat transfer between the heat pipe and the heat source and heat sink is provided. The heat pipe is held tightly against the heat source by mounting holes which penetrate the structure of the heat pipe but are sealed off from the vapor chamber because they each are located within a sealed structure such as a pillar or the solid layers of the casing surrounding the vapor chamber. Another feature of the heat pipe is the use of a plurality of particles joined together by a brazing compound such that fillets of the brazing compound are formed between adjacent ones of the plurality of particles so as to form a network of capillary passageways between the particles of the wick.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 09/852,322, filed May 9, 2001, which is a continuation of U.S. patent application Ser. No. 09/310,397, filed May 12, 1999, now U.S. Pat. No. 6,302,192.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to active solid state devices, and more specifically to a heat pipe for cooling an integrated circuit chip, with the heat pipe designed to be held in direct contact with the integrated circuit.  
         [0003]     As integrated circuit chips decrease in size and increase in power, the required heat sinks and heat spreaders have grown to be larger than the chips. Heat sinks are most effective when there is a uniform heat flux applied over the entire heat input surface. When a heat sink with a large heat input surface is attached to a heat source of much smaller contact area, there is significant resistance to the flow of heat along the heat input surface of the heat sink to the other portions of the heat sink surface which are not in direct contact with the contact area of the integrated circuit chip. Higher power and smaller heat sources, or heat sources which are off center from the heat sink, increase the resistance to heat flow to the balance of the heat sink. This phenomenon can cause great differences in the effectiveness of heat transfer from various parts of a heat sink. The effect of this unbalanced heat transfer is reduced performance of the integrated circuit chip and decreased reliability due to high operating temperatures.  
         [0004]     The brute force approach to overcoming the resistance to heat flow within heat sinks which are larger than the device being cooled is to increase the size of the heat sink, increase the thickness of the heat sink surface which contacts the device to be cooled, increase the air flow which cools the heat sink, or reduce the temperature of the cooling air. However, these approaches increase weight, noise, system complexity, and expense.  
         [0005]     It would be a great advantage to have a simple, light weight heat sink for an integrated circuit chip which includes an essentially isothermal surface even though only a part of the surface is in contact with the chip, and also includes a simple means for assuring intimate contact with the integrated circuit chip to provide good heat transfer between the chip and the heat sink.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is an inexpensive heat pipe heat spreader for integrated circuit chips which is of simple, light weight construction. It is easily manufactured, requires little additional space, and provides additional surface area for cooling the integrated circuit and for attachment to heat transfer devices for moving the heat away from the integrated circuit chip to a location from which the heat can be more easily disposed of. Furthermore, the heat pipe heat spreader is constructed to assure precise flatness and to maximize heat transfer from the heat source and to the heat sink, and has holes through its body to facilitate mounting.  
         [0007]     The heat spreader of the present invention is a heat pipe which requires no significant modification of the circuit board or socket because it is held in intimate contact with the integrated circuit chip by conventional screws attached to the integrated mounting board. This means that the invention uses a very minimum number of simple parts. Furthermore, the same screws which hold the heat spreader against the chip can also be used to clamp a finned heat sink to the opposite surface of the heat spreader.  
         [0008]     The internal structure of the heat pipe is an evacuated vapor chamber with a limited amount of liquid and includes a pattern of spacers extending between and contacting the two plates or any other boundary structure forming the vapor chamber. The spacers prevent the plates from bowing inward, and therefore maintain the vital flat surface for contact with the integrated circuit chip. These spacers can be solid columns, embossed depressions formed in one of the plates, or a mixture of the two. Porous capillary wick material also covers the inside surfaces of the heat pipe and has a substantial thickness surrounding the surfaces of the spacers within the heat pipe, thus forming pillars of porous wick surrounding the supporting spacers. The wick therefore spans the space between the plates in multiple locations, and comprises a plurality of particles joined together by a brazing compound such that fillets of the brazing compound are formed between adjacent ones of the plurality of particles so as to form a network of capillary passageways between the particles.  
         [0009]     The spacers thus serve important purposes. They support the flat plates and prevent them from deflecting inward and distorting the plates to deform the flat surfaces which are required for good heat transfer. The spacers also serve as critical support for the portions of the capillary wick pillars which span the space between the plates provide a gravity independent characteristic to the heat spreader, and the spacers around which the wick pillars are located assure that the capillary wick is not subjected to destructive compression forces.  
         [0010]     The spacers also make it possible to provide holes into and through the vapor chamber, an apparent inconsistency since the heat pipe vacuum chamber is supposed to be vacuum tight. This is accomplished by bonding the spacers, if they are solid, to both plates of the heat pipe, or, if they are embossed in one plate, bonding the portions of the depressions which contact the opposite plate to that opposite plate. With the spacer bonded to one or both plates, a through hole can be formed within the spacer and it has no effect on the vacuum integrity of the heat pipe vapor chamber, from which the hole is completely isolated.  
         [0011]     An alternate embodiment of the invention provides the same provision for mounting the heat pipe spreader with simple screws even when the heat pipe is constructed without internal spacers. This embodiment forms the through holes in the solid boundary structure around the outside edges of the two plates. This region of the heat pipe is by its basic function already sealed off from the vapor chamber by the bond between the two plates, and the only additional requirement for forming a through hole within it is that the width of the bonded region be larger than the diameter of the hole. Clearly, with the holes located in the peripheral lips, the heat pipe boundary structure can be any shape.  
         [0012]     Another alternative embodiment of the invention provides for improved heat transfer between the integrated circuit chip and the heat pipe heat spreader. This is accomplished by using a different capillary wick material within the heat pipe at the location which is directly in contact with the chip. Instead of using the same sintered copper powder wick which is used throughout the rest of the heat pipe, the part of the wick which is on the region of the heat pipe surface which is in contact with the chip is constructed of higher thermal conductivity sintered powder. Such powder can be silver, diamond, or many other materials well known in the art. This provides for significantly better heat transfer in the most critical heat transfer area, right at the integrated circuit chip.  
         [0013]     The present invention thereby provides a heat pipe superior heat transfer characteristics, and the simplest of all mounting devices, just several standard screws. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     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:  
         [0015]      FIG. 1  is a cross-sectional view of one embodiment of a flat plate heat pipe with through holes through its vapor chamber and in contact with a finned heat sink;  
         [0016]      FIG. 2  is a cross-sectional view of the flat plate heat pipe shown in  FIG. 1 , with the finned heat sink removed for clarity of illustration;  
         [0017]      FIG. 3  is a plan view of the flat plate heat pipe shown in  FIGS. 1 and 2 ;  
         [0018]      FIG. 4  is an exploded and enlarged view of a portion of the wick structure formed in accordance with the present invention;  
         [0019]      FIG. 5  is a representation of a brazed wick formed in accordance with one embodiment of the present invention; and  
         [0020]      FIG. 6  is a representation of another brazed wick formed in accordance with a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     Heat pipe  10  is constructed by forming a boundary structure by sealing together two formed plates, contact plate  18  and cover plate  20 . Contact plate  18  and cover plate  20  are sealed together at their peripheral lips  22  and  24  by conventional means, such as soldering or brazing, to form heat pipe  10 . Heat pipe  10  is then evacuated to remove all non-condensible gases and a suitable quantity of heat transfer fluid is placed within it. This is the conventional method of constructing a heat pipe, and is well understood in the art of heat pipes.  
         [0022]     The interior of heat pipe  10  is, however, constructed unconventionally. While contact plate  18  is essentially flat with the exception of peripheral lip  24 , cover plate  20  includes multiple depressions  26 . Depressions  26  are formed and dimensioned so that, when contact plate  18  and cover plate  20  are joined, the flat portions of depressions  26  are in contact with inner surface  28  of contact plate  18 . Depressions  26  thereby assure that the spacing between contact plate  18  and cover plate  20  will be maintained even through pressure differentials between the inside volume of heat pipe  10  and the surrounding environment might otherwise cause the plates to deflect toward each other.  
         [0023]     Heat pipe  10  also includes internal sintered metal capillary wick  30  which covers the entire inside surface of contact plate  18 . As is well understood in the art of heat pipes, a capillary wick provides the mechanism by which liquid condensed at the cooler condenser of a heat pipe is transported back to the hotter evaporator where it is evaporated. The vapor produced at the evaporator then moves to the condenser where it again condenses. The two changes of state, evaporation at the hotter locale and condensation at the cooler site, are what transport heat from the evaporator to the condenser.  
         [0024]     In the present invention, heat pipe  10  also has capillary wick pillars  32  which bridge the space between contact plate  18  and cover plate  20 . Pillars  32  thereby interconnect cover plate  16  and contact plate  14  with continuous capillary wick. This geometry assures that, even if heat pipe  10  is oriented so that cover plate  16  is lower than contact plate  14 , liquid condensed upon inner surface  34  of cover plate  20  will still be in contact with capillary pillars  32 . The liquid will therefore be moved back to raised surface  28  which functions as the evaporator because it is in contact with a heat generating integrated circuit (not shown). Capillary pillars  32  are wrapped around and supported by depressions  26 , which prevents the structurally weaker capillary pillars  32  from suffering any damage.  
         [0025]      FIG. 1  also shows frame  36  which is typically used to surround and protect heat pipe  10 . Frame  34  completely surrounds heat pipe  10  and contacts lip  24  of contact plate  18 . When heat pipe  10  is used to cool an integrated circuit chip (not shown) which is held against contact plate  18 , cover plate  20  is held in intimate contact with fin plate  38 , to which fins  16  are connected. The entire assembly of heat pipe  10 , frame  34 , and fin plate  38  is held together and contact plate  18  is held against an integrated circuit chip by conventional screws  40 , shown in dashed lines, which are placed in holes  42  in fin plate  38  and through holes  12  in heat pipe  10 , and are threaded into the mounting plate (not shown) for the integrated circuit chip.  
         [0026]     Holes  12  penetrate heat pipe  10  without destroying its vacuum integrity because of their unique location. Holes  12  are located within sealed structures such as solid columns  44 , and since columns  44  are bonded to cover plate  20  at locations  46 , holes  12  passing through the interior of columns  44  have no affect on the interior of heat pipe  10 .  
         [0027]     The preferred embodiment of the invention has been constructed as heat pipe  10  as shown in  FIG. 1 . This heat pipe is approximately 3.0 inches by 3.5 inches with a total thickness of 0.200 inch. Cover plate  20  and contact plate  18  are constructed of OFHC copper 0.035 inch thick, and depressions  26  span the 0.100 inch height of the internal volume of heat pipe  10 . The flat portions of depressions  26  are 0.060 inch in diameter. Capillary wick  30  is constructed of sintered copper powder and averages 0.040 inch thick. Columns  44  have a 0.250 inch outer diameter, and holes  12  are 0.210 in diameter.  
         [0028]      FIG. 2  is a cross section view of an alternate embodiment of the flat plate heat pipe  11  of the invention with through holes  48  located within peripheral lips  22  and  24  of the heat pipe and hole  50  shown in another sealed structure, one of the depressions  26 . The only requirement for forming hole  50  within a depression  26  is that the bottom of depression  26  must be bonded to inner surface  28  of contact plate  18  to prevent loss of vacuum within the heat pipe. Of course, the region of the peripheral edges is also a sealed structure since bonding between lips  22  and  24  is inherent because heat pipe  11  must be sealed at its edges to isolate the interior from the outside atmosphere.  
         [0029]     The only differences between heat pipe  11  of  FIG. 2  and heat pipe  10  of  FIG. 1  are that finned heat sink  16  is not shown in  FIG. 2 , lips  22  and  24  are slightly longer in  FIG. 2  to accommodate holes  48 , and hole  50  is shown. In fact, through holes  12  shown in  FIG. 12  are also included in  FIG. 2 . Although it is unlikely that holes  12 , holes  48 , and hole  50  would be used in the same assembly, manufacturing economies may make it desirable to produce all the holes in every heat pipe so that the same heat pipe heat spreader can be used with different configurations of finned heat sinks. The unused sets of holes have no effect on the operation or benefits of the invention.  
         [0030]      FIG. 3  is a plan view of the internal surface of the contact plate  18  of the heat pipe  10  of the invention showing region  31  of capillary wick  30 . Region  31  is constructed of sintered silver powder. While the balance of capillary wick  30  is conventional sintered metal such as copper, region  31  of capillary wick  30 , which is on the opposite surface of contact plate  18  from the integrated circuit chip (not shown), is formed of powdered silver. The higher thermal conductivity of silver yields significantly better heat conduction through region  31  of the wick  30 , and thereby reduces the temperature difference between the integrated circuit chip and the vapor within heat pipe  10 . This reduction of temperature difference directly affects the operation of heat pipe  10 , and essentially results in a similar reduction in the operating temperature of the chip.  
         [0031]     In one embodiment of the present invention, a brazed wick  65  is located on the inner surface of contact  18 . Brazed wick  65  comprises a plurality of metal particles  67  combined with a filler metal or combination of metals that is often referred to as a “braze” or brazing compound  70 . It will be understood that “brazing” is the joining of metals through the use of heat and a filler metal, i.e., brazing compound  70 . Brazing compound  70  very often comprises a melting temperature that is above 450° C.-1000 C but below the melting point of metal particles  67  that are being joined to form brazed wick  65 .  
         [0032]     In general, to form brazed wick  65  according to the present invention, a plurality of metal particles  67  and brazing compound  70  are heated together to a brazing temperature that melts brazing compound  70 , but does not melt plurality of metal particles  67 . Significantly, during brazing metal particles  67  are not fused together as with sintering, but instead are joined together by creating a metallurgical bond between brazing compound  70  and the surfaces of adjacent metal particles  67  through the creation of fillets of re-solidified brazing compound (identified by reference numeral  73  in  FIGS. 5 and 6 ). Advantageously, the principle by which brazing compound  70  is drawn through the porous mixture of metal particles  67  to create fillets  73  is “capillary action”, i.e., the movement of a liquid within the spaces of a porous material due to the inherent attraction of molecules to each other on a liquid&#39;s surface. Thus, as brazing compound  70  liquefies, the molecules of molten brazing metals attract one another as the surface tension between the molten braze and the surfaces of individual metal particles  67  tends to draw the molten braze toward each location where adjacent metal particles  67  are in contact with one another. Fillets  73  are formed at each such location as the molten braze metals re-solidify.  
         [0033]     In the present invention, brazing compound  70  and fillets  73  create a higher thermal conductivity wick than, e.g., sintering or fusing techniques. This higher thermal conductivity wick directly improves the thermal conductance of the heat transfer device in which it is formed, e.g., heat pipe, loop heat pipe, etc. Depending upon the regime of heat flux that, e.g., region  31 , is subjected to, the conductance of brazed wick  65  has been found to increase between directly proportional to and the square root of the thermal conductivity increase. Importantly, material components of brazing compound  70  must be selected so as not to introduce chemical incompatibility into the materials system comprising flat plate heat pipe  10 .  
         [0034]     Metal particles  67  may be selected from any of the materials having high thermal conductivity, that are suitable for fabrication into brazed porous structures, e.g., carbon, tungsten, copper, aluminum, magnesium, nickel, gold, silver, aluminum oxide, beryllium oxide, or the like, and may comprise either substantially spherical, oblate or prolate spheroids, ellipsoid, or less preferably, arbitrary or regular polygonal, or filament-shaped particles of varying cross-sectional shape. For example, when metal particles  67  are formed from copper spheres ( FIG. 5 ) or oblate spheroids ( FIG. 6 ) whose melting point is about 1083° C., the overall wick brazing temperature for flat plate heat pipe  10  will be about 1000 C. By varying the percentage brazing compound  70  within the mix of metal particles  67  or, by using a more “sluggish” alloy for brazing compound  70 , a wide range of heat-conduction characteristics may be provided between metal particles  67  and fillets  73 .  
         [0035]     For example, in a copper/water heat pipe, any ratio of copper/gold braze could be used, although brazes with more gold are more expensive. A satisfactory combination for brazing compound  30  has been found to be about six percent (6)% by weight of a finely divided (−325 mesh), 65%/35% copper/gold brazing compound, that has been well mixed with the copper powder (metal particles  67 ). More or less braze is also possible, although too little braze reduces the thermal conductivity of brazed wick  65 , while too much braze will start to fill the wick pores with solidified braze metal. One optimal range has been found to be between about 2% and about 10% braze compound, depending upon the braze recipe used. When employing copper powder as metal particles  67 , a preferred shape of particle is spherical or spheroidal. Metal particles  67  should often be coarser than about 200 mesh, but finer than about 20 mesh. Finer wick powder particles often require use of a finer braze powder particle. The braze powder of brazing compound  70  should often be several times smaller in size than metal particles  67  so as to create a uniformly brazed wick  65  with uniform properties.  
         [0036]     Other brazes can also be used for brazing copper wicks, including nickel-based Nicrobrazes, silver/copper brazes, tin/silver, lead/tin, and even polymers. The invention is also not limited to copper/water heat pipes. For example, aluminum and magnesium porous brazed wicks can be produced by using a braze that is an aluminum/magnesium intermetallic alloy.  
         [0037]     Brazing compound  70  should often be well distributed over each metal particle surface. This distribution of brazing compound  70  may be accomplished by mixing brazing compound  70  with an organic liquid binder, e.g., ethyl cellulose, that creates an adhesive quality on the surface of each metal particle  67  (i.e., the surface of each sphere or spheroid of metal) for brazing compound  70  to adhere to. In one embodiment of the invention, one and two tenths grams by weight of copper powder (metal particles  67 ) is mixed with two drops from an eye dropper of an organic liquid binder, e.g., ISOBUTYL METHACRYLATE LACQUER to create an adhesive quality on the surface of each metal particle  67  (i.e., the surface of each sphere or spheroid of metal) for braze compound  70  to adhere to. A finely divided (e.g., −325 mesh) of braze compound  70  is mixed into the liquid binder coated copper powder particles  67  and allowed to thoroughly air dry. About 0.072 grams, about 6% by weight of copper/gold in a ratio of 65%/35% copper/gold brazing compound, has been found to provide adequate results. The foregoing mixture of metal particles  67  and brazing compound  70  are applied to the internal surfaces of flat plate heat pipe  10 , for example the inner surface contact plate  18  and heated evenly so that brazing compound  70  is melted by heating metal particles  67 . Molten brazing compound  70  that is drawn by capillary action, forms fillets  73  as it solidifies within the mixture of metal particles  67 . For example, vacuum brazing or hydrogen brazing at about 1020 C for between two to eight minutes, and preferably about five minutes, has been found to provide adequate fillet formation within a brazed wick. A vacuum of at least 10 −5  torr or lower has been found to be sufficient, and if hydrogen furnaces are to be used, the hydrogen furnace should use wet hydrogen. In one embodiment, the assembly is vacuum fired at 1020° C., for 5 minutes, in a vacuum of about 5×10 −5  torr or lower.  
         [0038]     It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims. For example, through holes could also penetrate heat pipe boundary structures with curved surfaces or heat pipe boundary structures with offset planes which create several different levels for contact with heat sources or heat sinks.