Abstract:
The present invention discloses a heat spreader and method for making the heat spreader. The heat spreader comprises: a hollow metallic housing including an upper cover having an inner surface and a lower cover having an inner surface, the upper and lower covers being bonded together along their perimeters defining a cavity; a capillary structure in a form of metallic meshes bonded to the inner surfaces of the upper and lower covers of the metallic housing; a plurality of reinforcing members disposed in the cavity and bonded between the inner surfaces of the upper and lower covers of the metallic housing; and a working fluid receive in the cavity; wherein bonded surfaces between the metallic meshes and the inner surfaces of the metallic housing, the upper cover and the lower cover, and the reinforcing members and the inner surfaces of the metallic housing all are diffusion-bonded interfaces.

Description:
[0001]     This application is a divisional application of pending U.S. application Ser. No. 11/138,478 filed on May 27, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a heat spreader and method for fabricating same, and more particularly, to a bendable heat spreader with metallic meshes-based microstructure, for example, copper meshes, and method for fabricating same.  
       BACKGROUND OF THE INVENTION  
       [0003]     Recent electronic appliances such as personal computers, communication devices, TFT-LCD, etc. adopt a variety of electronic devices which generate heat in operation. These devices inevitably generate more heat than before, particularly under the demand of high-speed computation. Therefore, preventing electronic devices from degrading performance due to overheating becomes extremely important, and a variety of cooling devices and methods are developed for this purpose.  
         [0004]     For example, a cooler with heat pipe(s) attached to a copper plate has been used in the industry. However, since this kind of heat pipe cannot be used independently, another kind of independent plate type heat pipe, also called a “heat spreader”, has been developed. Due to its ability of allowing independent usage and having good cooling efficiency, the heat spreaders have been widely used in the industry recently.  
         [0005]     Generally, the heat spreader is a sealed, hollow housing formed by copper plates. The interior of the housing is evacuated to an extent of vacuum and then filled with a working fluid. A capillary structure is formed on inner walls of the housing. Under vacuum condition, the working fluid absorbs heat from a heat-absorbing side of the housing and vaporizes rapidly. The vaporized working fluid is cooled back to the original liquid phase at a heat-radiating side of the housing where the heat absorbed is radiated out and then the working liquid is directed back to the heat-absorbing side of the housing via the capillary structure to proceed the heat absorbing-radiating cycle repeatedly.  
         [0006]     Typically, the capillary structure of a heat spreader can be formed by micro-trench machining or copper-powder sintering. However, it is not so easy to form trenches on a micro scale over the copper plate. In contrast, though copper-powder sintering would be easier for forming the capillary structure, it is difficult to control the final sintering quality, which results in higher scrap rates and thus higher fabrication costs. In addition, in case where bending the heat spreader is needed, the capillary structure formed by copper-powder sintering would be damaged due to the bending.  
         [0007]     Moreover, the traditional heat spreader is formed by sealing two half housings into one by, for example, soldering or welding. A known structure of the heat spreader, such as the plate type heat pipe with supports disclosed in Taiwan Utility Model Patent Publication No. 577538, has its supports fixed inside the housing by means of soldering. However, this approach only allows soldering each of the supports at one end while the other end of the supports cannot be soldered after the housing has been sealed. This may result in deformation of the housing due to the heat generated during the soldering operation.  
         [0008]     In view of the above mentioned drawbacks, the present invention provides a fabrication method for heat spread that enables the fabrication of heat spreader at lower costs and easier formation of the capillary structure of the heat spreader, and that would less thermally deform the heat spreader housing during the fabricating process. The present invention also provides a heat spreader that can be bended in use and will not easily deform due to absorbing heat.  
       SUMMARY OF THE INVENTION  
       [0009]     A method for fabricating a heat spreader according to an embodiment of the present invention comprises: 
        providing an upper cover and a lower cover, each of the covers being a sheet of metallic material and having a perimeter and an inner surface;     attaching metallic meshes to the inner surface of the upper cover and the inner surface of the lower cover using diffusion bonding so as to form a capillary structure on the respective inner surface;     interposing a plurality of reinforcing members between the inner surfaces of the upper cover and the lower cover to which the metallic meshes are attached;     bonding the upper cover, the lower cover and the reinforcing members together using diffusion bonding, such that the inner surfaces of the upper cover and the lower cover define a cavity and the reinforcing members are bonded therebetween;     evacuating the cavity to form a vacuum;     filling a working fluid into the evacuated cavity; and     sealing the cavity with the working fluid therein.        
 
         [0017]     A heat spreader according to an embodiment of the present invention comprises: 
        a hollow metallic housing including an upper cover having an inner surface and a lower cover having an inner surface, the upper and lower covers being bonded together along their perimeters defining a cavity;     a capillary structure in a form of metallic meshes bonded to the inner surfaces of the upper and lower covers of the metallic housing;     a plurality of reinforcing members disposed in the cavity and bonded between the inner surfaces of the upper and lower covers of the metallic housing; and     a working fluid receive in the cavity;     wherein bonded surfaces between the metallic meshes and the inner surfaces of the metallic housing, the upper cover and the lower cover, and the reinforcing members and the inner surfaces of the metallic housing all are diffusion-bonded interfaces.        
 
         [0023]     According to a preferred embodiment of the present invention, the material forming the metallic housing and reinforcing members is copper or aluminum and the reinforcing members is in the shape of post or strip. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     Following figures only depict the correlations between elements, not conforming to the proportion of real dimension. In addition, like numerals in the drawings present like elements or features.  
         [0025]      FIG. 1  is a perspective view of a heat spreader in accordance with the present invention;  
         [0026]      FIG. 2  is a cross sectional view of the heat spreader of  FIG. 1  taken on line  2 - 2  hereof;  
         [0027]      FIG. 3  is an exploded perspective view of the spreader of  FIG. 2 ;  
         [0028]      FIG. 4  is a cross sectional view of a second embodiment in accordance with a heat spreader of the present invention;  
         [0029]      FIG. 5  is an exploded perspective view of the spreader of  FIG. 4 ;  
         [0030]      FIG. 6A  is a schematic cross sectional view illustrating using a fixture to bond the upper cover and the copper mesh of a heat spreader of the present invention;  
         [0031]      FIG. 6B  is a schematic cross sectional view illustrating using a fixture to bond the lower cover and the copper mesh of a heat spreader of the present invention;  
         [0032]      FIG. 7  is a schematic cross sectional view illustrating use of a fixture to bond the whole heat spreader of  FIG. 2 ;  
         [0033]      FIG. 8  is a graph illustrating the profile of temperature and pressure against time for the diffusion bonding of copper upper cover, copper lower cover, copper meshes and copper posts or strips of the heat spreader of the invention;  
         [0034]      FIG. 9  is a list of data of the temperature, pressure and time period on which the graph of  FIG. 8  is plotted;  
         [0035]      FIG. 10  is a graph illustrating the profile of temperature and pressure against time for the diffusion bonding of aluminum upper and lower covers, and copper meshes of the heat spreader of the invention;  
         [0036]      FIG. 11  is a list of data of the temperature, pressure and time period on which the graph of  FIG. 10  is plotted;  
         [0037]      FIG. 12 a  graph illustrating the profile of temperature and pressure against time for the diffusion bonding of aluminum upper and lower covers, and aluminum posts or strips of the heat spreader of the invention; and  
         [0038]      FIG. 13  is a list of data of the temperature, pressure and time period on which the graph of  FIG. 12  is plotted. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]      FIG. 1  illustrates a heat spreader  10  in accordance with the present invention. The heat spreader  10  comprises an upper cover  12 , a lower cover  14  and a filling pipe  16 . The material used for these components generally is copper, while any other metals, such as aluminum, can also be used as long as they having good heat-dissipating capability.  
         [0040]      FIGS. 2 and 3  illustrate the heat spreader  10  in accordance with a first embodiment of the present invention. The upper cover  12  and lower cover  14  each have a perimeter  12   a,    14   b,  respectively. A portion within the perimeter  12   a  of upper cover  12  protrudes slightly. A hermetically sealed cavity  13  is formed by diffusion bonding between the perimeters of the upper cover  12  and lower cover  14 . An appropriate volume of working fluid (e.g., pure water, not shown) can be filled in the cavity  13  via the filling pipe  16 , one end of which is in fluid communication with the cavity  13 , while the other end is sealed after the cavity  13  has been filled with the working fluid.  
         [0041]     According to one aspect of the present invention, copper meshes  18  are attached to the inner surface of the heat spreader so as to form a capillary structure. In addition, some openings  18   a  are formed in the meshes, which allow two ends of copper posts  20  to pass through and to be diffusion bonded between the upper cover  12  and lower cover  14 , respectively. These copper posts  20  form a housing-reinforcing structure for the heat spreader to prevent it from deforming as a result of vaporization pressure of the working fluid during absorbing heat.  
         [0042]     Note that the contour dimensions of the copper meshes  18  are somewhat smaller than those of the upper cover  12  and lower cover  14  so as to ensure that no part of copper meshes extends into the region where the upper cover  12  and lower cover  14  are bonded together. Copper mesh  18  can be obtained by cutting or stamping a commercially available  200  mesh micro copper mesh. The openings  18   a  are not necessarily required, while it is preferable to have them in this embodiment in order to advantageously position the copper posts  20  used to reinforce the housing structure. The openings  18   a  can be stamped out at the same time the copper meshes  18  are stamped out so as to obtain an identical layout of the openings  18   a  on each piece of copper meshes  18 . In this embodiment, there are two pieces of copper meshes  18 , one attached to the inner surface of the upper cover  18 , the other attached to the inner surface of the lower cover  14 ; however, the number of the copper meshes is not limited to two, more pieces of copper meshes  18  may be used in a stacked manner as well.  
         [0043]      FIGS. 4 and 5  show a heat spreader  10 ′ in accordance with a second embodiment of the present invention. The major distinction between the first and the second embodiments reside in that the latter employs copper strips  20   a  as a reinforcing structure in place of the copper posts  20 . In this embodiment, copper meshes  18 ′ do not have openings  18   a  as in the first embodiment, but openings (not shown) matching the contour of the copper strip  20   a  can also be made, if desired.  
         [0044]     The copper strip  20  is substantially in an elongated shape. In order to more stably place the copper strip  20  and thus facilitate the subsequent bonding operation, a plurality of separated, enlarged portions arranged are formed on the copper strip  20 . With these enlarged portions, the copper strip  20  will not easily incline during pre-assembly of the heat spreader. Generally, either copper strip  20   a  or copper post  20  is used solely, while both of them may be used together, if desired.  
         [0045]     It is advantageous that the above-mentioned copper posts or strips may be formed by from sintering copper powders so that fine apertures naturally generated therein by sintering can serve as capillary structure. In addition, it can also be taken into account that capillary structure may be formed by wrapping a copper mesh on a non-sintered copper post or strip (not shown).  
         [0046]     A significant advantage of the present invention is that the copper-mesh micro structure, which is used to replace the copper-powder-sintering micro structure, is not only easy to make and cost-effective, but also particularly useful when the heat spreader ( 10 ,  10 ′) is needed to be bended to conform to the contour of a device (heat source) to be cooled or to accommodate itself to other conditions without damaging the micro structure.  
         [0047]     Nevertheless, for the internal structure as shown in  FIGS. 2 and 3 , in order to avoid reducing the integrity of the reinforcing structure of the copper posts  20 , it is preferred that the heat spreader is bended at a region where there are no copper posts  20 . Therefore, it is possible to arrange the layout of copper posts in advance such that the copper posts can be kept away from the region where the heat spreader is to be bended to conform to the contour of the heat source. However, the reinforcing structure as shown in  FIGS. 4 and 5  will not be subject to such limitation, because the elongated copper strip  20   a  allows the heat spreader  10 ′ to be bended almost at anywhere without damaging its reinforcing structure.  
         [0048]     A method for fabricating the heat spreader  10  in accordance with the first embodiment of the present invention will be described in detail as follows.  
         [0049]     Step  1 : Forming the Upper Cover and Lower Cover  
         [0050]     The upper cover  12  and lower cover  14  can be formed by a sheet of copper material. Typically, there are many processing approaches that may be used, such as stamping, forging or machining, etc. However, for sake of cost consideration, it is preferable to adopt the stamping approach. As shown in  FIG. 2 , the portion within the perimeter  12   a  of upper cover  12  is raised by stamping process, while the lower cover  14  is of a flat shape (or of a shape similar to the upper cover  12 ). The upper cover  12  and lower cover  14  are also stamped, respectively, to form protrusions  12   b,    14   a  ( FIG. 3 ). Then the upper cover  12  and lower cover  14  are washed to eliminate scraps thereon.  
         [0051]     Step  2 : Attaching the Metallic Meshes onto the Upper Cover and Lower Cover (Diffusion Bonding  1 )  
         [0052]     Referring to  FIGS. 6A and 6B , one copper mesh  18  is first placed on one bonding fixture  30  (e.g., a tool steel fixture), and then the copper mesh  18  is overlapped by the upper cover  12 ; another copper mesh  18  is placed on a bonding fixture  32 , and then the another copper mesh  18  is overlapped by the lower cover  14 . Then the sub-combinations as shown in  FIGS. 6A and 6B  are sent into a vacuum heat pressing furnace to diffusion bond the copper meshes onto the upper cover and lower cover, respectively.  
         [0053]     Diffusion bonding provides bonding between components or materials by properly controlling several bonding parameters such as the temperature, pressure and time duration, such that they can be bonded at a temperature lower than their melting points. For the diffusion bonding of copper material, the temperature and pressure are generally specified as, for example, from 450° C. to 900° C. and from 2 MPa to 20 MPa, respectively, for over 30 minutes (preferably within 3 hours).  
         [0054]     The temperature, pressure and duration for the diffusion bonding specified in the embodiment of the present invention are shown in  FIGS. 8 and 9 , wherein the diffusion bonding mainly precedes at a temperature of 700° C. and a pressure of 2.0 MPa for about 80 minutes (i.e., from 80 th  minutes to 160 th  minutes in the transverse axis).  
         [0055]     Now, tow pieces of copper meshes are attached onto the inner surfaces of the upper cover and lower cover, respectively, wherein both the bonded surface a ( FIG. 6A ) between the copper mesh  18  and the inner surface of the upper cover  12 , and the bonded surface b ( FIG. 6B ) between the copper mesh  18  and the inner surface of the lower cover  14  are diffusion-bonded interfaces.  
         [0056]     Step  3 : Bonding the Upper Cover Having Copper Mesh, Lower Cover Having Copper Mesh and Reinforcing Posts (Diffusion Bonding  2 )  
         [0057]     Referring to  FIG. 7 , the upper cover  12  having copper mesh  18 , lower cover  14  having copper mesh  18 , and the copper posts  20  are pre-assembled together, such that the upper cover  12  and lower cover  14  are aligned each other and the copper posts  20  are sandwiched therebetween, with two ends of the copper post  20  passing openings  18   a  to avoid tipping. Then the pre-assembled unit is clamped in a bonding fixture  34 , and sent into the vacuum heat pressing furnace again to precede the diffusion bonding process with the same bonding parameters as specified in Step  2 .  
         [0058]     Now, all of the resulted interface c formed between the upper cover  12  and the lower cover  14 , the resulted interfaces d, e, and f formed between the upper cover  12  and the copper posts  20 , and the resulted interfaces g, h, and i formed between the lower cover  14  and the copper posts  20  are diffusion-bonded interfaces.  
         [0059]     Step  4 : Soldering the Pipe  
         [0060]     The filling pipe  16  is soldered onto an opening formed by the protrusions  12   b,    14   a  of the upper cover  12  and lower cover  14  ( FIG. 1 ).  
         [0061]     Step  5 : Testing Pressure and Leakage  
         [0062]     The heat spreader  10  is then filled with a testing gas (e.g., nitrogen) to test the pressure resistance and hermetical capability for the structure. If qualified, then the cavity  13  or interior of heat spreader  10  is evacuated to form a vacuum of about between 10 −3  torr to 10 −7  torr.  
         [0063]     Step  6 : Filling a Working Fluid  
         [0064]     An appropriate volume of a working fluid, for example, pure water, is filled into the cavity  13  of the heat spreader  10  via the filling pipe  16 . Other working fluids such as methyl alcohol or coolant, etc., may be used as well.  
         [0065]     Step  7 : Sealing the Pipe  16   
         [0066]     The open end of the pipe  16  is sealed (e.g., by soldering) after the working fluid has been filled.  
         [0067]     In the above-described embodiments, copper is used as the material for the upper cover, lower cover, reinforcing structure and capillary structure. However, in a further embodiment for the present invention, aluminum is used in place of copper for the upper cover, lower cover and reinforcing structure, while the capillary structure remains using copper material. In this case, except using another set of bonding parameters due to different material, all of the structure, fabrication method and invention efficacy can refer to the descriptions of the former embodiment. Therefore, the bonding parameters for this embodiment will be further illustrated as bellows.  
         [0068]     In this embodiment, the first diffusion bonding is directed to the bonding between the aluminum upper cover and lower covers and copper meshes. Generally, the temperature and pressure can be set from 300° C. to 600° C. and from 0.6 MPa to 1.0 MPa, respectively, for about 30 minutes to about 4 hours. The preferred diffusion bonding temperature, pressure and time duration are illustrated in  FIGS. 10 and 11 . More preferably, the diffusion bonding is carried out at a temperature of about 450° C. and under a pressure of about 0.6 MPa for about 80 minutes.  
         [0069]     In this embodiment, the second diffusion bonding is directed to the bonding between the aluminum upper and lower covers and the aluminum posts or strips. The preferred diffusion bonding temperature, pressure and time duration are illustrated in  FIGS. 12 and 13 . More preferably, the diffusion bonding is carried out at a temperature of about 550° C. and under a pressure of about 0.6 MPa for about 80 minutes.  
         [0070]     By means of the steps disclosed in the above two embodiments, no matter using copper or aluminum material, a heat spreader having the aforementioned diffusion-bonded interfaces a to i can be obtained. Accordingly, another advantage of the present invention is that because the diffusion-bonded interfaces do not include any foreign interfaces (e.g., solder interfaces), the respective material characteristics of copper and aluminum can be maintained, thereby reducing heat stress and facilitating bending applications for the heat spreader.  
         [0071]     While several particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications and fall within the true spirit and scope of the invention.