Abstract:
A heat spreader ( 10 ) and a method for manufacturing the heat spreader are disclosed. The heat spreader includes a metal casing ( 12 ) and a wick structure ( 16 ) lines an inner surface of the metal casing. The metal casing defines therein a chamber ( 14 ) and includes an evaporating section ( 126 ) and a condensing section ( 127 ). The wick structure is in the form of metal foam and occupies a portion of the chamber. In one embodiment, the wick structure has a pore size gradually increasing from the evaporating section towards the condensing section of the metal casing. The heat spreader is manufactured by electrodepositing a layer of metal coating ( 70 ) on an outer surface of a metal foam framework ( 20 ). The metal coating becomes the metal casing and the metal foam framework becomes the wick structure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for transfer or dissipation of heat from heat-generating components, and more particularly to a heat spreader having a vapor chamber defined therein and a method of manufacturing the heat spreader. 
     DESCRIPTION OF RELATED ART 
     It is well known that heat is generated during normal operations of a variety of electronic components, such as integrated circuit chips of computers. To ensure normal and safe operations, cooling devices such as heat sinks plus electric fans are often employed to dissipate the generated heat away from these electronic components. 
     As progress continues to be made in electronic industries, integrated circuit chips of computers are made to be more powerful while maintaining an unchanged size or even a smaller size. As a result, the amount of heat generated by these chips is commensurately increased. The heat sinks used to cool these chips are accordingly made larger in order to possess a higher heat removal capacity, which causes the heat sinks to have a much larger footprint than the chips. Generally speaking, a heat sink is most effective when there is a uniform heat flux applied over an entire base of the heat sink. When a heat sink with a large base is attached to an integrated circuit chip with a much smaller contact area, there is significant resistance to the flow of heat to the other portions of the heat sink base which are not in direct contact with the chip. 
     Currently, an advantageous mechanism for overcoming the resistance to heat flow in a heat sink base is to attach a heat spreader to the heat sink base or directly make the heat sink base as a heat spreader. In this situation, the heat spreader is configured to have a flat type configuration. Typically, the heat spreader includes a vacuum vessel defining therein a vapor chamber, and a working fluid contained in the chamber. In most cases, a wick structure is provided in the chamber, lining an inside wall of the vessel. As an integrated circuit chip is maintained in thermal contact with and transfers heat to the heat spreader, the working fluid contained in the chamber corresponding to the hot contacting location vaporizes into vapor. The vapor then runs quickly to be full of the chamber, and wherever the vapor comes into contact with a cooler wall surface of the vessel, it releases its latent heat of vaporization and thereafter turns into condensate. The condensate then returns back to the hot contacting location via a capillary force generated by the wick structure, to thereby remove the heat generated by the chip. In the chamber of the heat spreader, the thermal resistance associated with the vapor spreading is negligible, thus providing an effective means of spreading the heat from a concentrated source to a large heat transfer surface. 
     Conventionally, this flat type heat spreader is typically made by connecting two discrete metal plates together. Soldering process is such a method that is widely used to connect the two discrete plates together. However, the heat spreader made by this method is sometimes a little heavier than what is expected, since in the soldering process each of the metal plates is required, in view of the soldering requirements thereof, to have a minimum wall thickness which in some cases may be thicker than normally required. In addition, the reliability of the heat spreader made by the soldering process is also a problem. If the heat spreader is in fact not hermetically sealed in the soldering process, the chamber of the heat spreader will gradually lose its vacuum condition. 
     On the other hand, if the heat spreader is configured to have an elongated configuration, the heat spreader can be used as a heat pipe for spreading heat from one location to another remote location. For example, a first end of the heat pipe is thermally connected to a heat source while a second end of the heat pipe is thermally connected to a plurality of metal fins, thus transferring the heat generated by the heat source to the metal fins where the heat is dissipated. In this situation, the condensate resulted in the second end of the heat pipe has to travel a long distance from the second end to the first end of the heat pipe. The wick structure provided in the heat pipe is expected to provide a high capillary force and meanwhile produce a low flow resistance for the condensate so as to draw the condensate back timely. However, the wick structure provided in the conventional heat pipe generally has a uniform pore size distribution over its entire length. This uniform-type wick structure cannot satisfy this requirement. If the condensate is not timely brought back from the second end, the heat pipe will suffer dry-out problem at the first end thereof. 
     Therefore, it is desirable to provide a method of manufacturing a vapor chamber-based heat spreader which overcomes the foregoing disadvantages of the conventional soldering process. What is also desirable is to provide a vapor chamber-based heat spreader which can draw the condensate back effectively and timely. 
     SUMMARY OF INVENTION 
     The present invention relates, in one aspect, to a method for manufacturing a heat spreader. The method includes the following steps: (1) providing a metal foam framework, the metal foam framework having a plurality of pores and defining therein a major space; (2) filling a material into the pores and the major space of the metal foam framework and solidifying the material in the metal foam framework; (3) electrodepositing a layer of metal coating on an outer surface of the metal foam framework; (4) removing the material from the metal foam framework; and (5) filling a working fluid into the major space in the coating layer and hermetically sealing the coating layer to thereby obtain the heat spreader. The heat spreader has therein a wick structure formed of the metal foam framework and a vapor chamber formed of the major space. By this method, the heat spreader is integrally formed and therefore the reliability thereof improved. Also, the wall thickness of the heat spreader can be easily controlled by regulating the time period and the voltage associated with the electrodeposition step. 
     The present invention relates, in another aspect, to a heat spreader applicable for removing heat from a heat-generating component. The heat spreader includes a metal casing and a wick structure lines an inner surface of the metal casing. The metal casing defines therein a chamber and the wick structure occupies a portion of the chamber. The metal casing includes an evaporating section and a condensing section. The wick structure is in the form of a metal foam and has a pore size gradually increasing from the evaporating section towards the condensing section of the metal casing. Thus, a first section of the wick structure in conformity with the condensing section of the metal casing has a larger pore size and produces a relatively low resistance for the condensate in the condensing section. A second section of the wick structure in conformity with the evaporating section of the metal casing has a smaller pore size and is still capable of maintaining a relatively high capillary force for drawing the condensate back to the evaporating section. As a result, the flow resistance to the condensate is reduced as a whole and the condensate is thereby drawn back to the evaporating section effectively and timely. 
     Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top plan view of a heat spreader in accordance with one embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the heat spreader of  FIG. 1 , taken along line II-II thereof; 
         FIG. 3  is a cross-sectional view of the heat spreader of  FIG. 1 , taken along line III-III thereof; 
         FIG. 4  is a flow chart showing a preferred method of the present invention for manufacturing the heat spreader of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of a wick structure of the heat spreader of  FIG. 1 ; 
         FIG. 6  is a schematic, cross-sectional view of a device applied for filling a filling material into the wick structure of  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of the wick structure of  FIG. 5  after being filled with the filling material; 
         FIG. 8  is a schematic, cross-sectional view of an electrodeposition bath for electrodepositing a layer of metal coating on an outer surface of the wick structure of  FIG. 7 ; 
         FIG. 9  is a view similar to  FIG. 7 , but an outer surface of the wick structure is electrodeposited with the layer of metal coating; 
         FIG. 10  is a radial cross-sectional view of a heat spreader in accordance with an alternative embodiment of the present invention; and 
         FIG. 11  is a longitudinal cross-sectional view of the heat spreader of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-3  illustrate a heat spreader  10  formed in accordance with a method of the present invention. The heat spreader  10  is integrally formed and has a flat type configuration. The heat spreader  10  includes a metal casing  12  with a chamber  14  defined therein. A wick structure  16  is arranged in the chamber  14 , lining an inner surface of the metal casing  12  and occupying a portion of the chamber  14 . The other portion of the chamber  14 , which is not occupied by the wick structure  16  functions as a vapor-gathering region. The wick structure  16  is a porous structure and is in the form of a metal foam. The metal casing  12  is made of high thermally conductive material such as copper or aluminum. The heat spreader  10  has two open distal ends  121  extending from two opposite sides thereof, respectively. A working fluid (not shown) is injected into the chamber  14  through the two open distal ends  121  and then the heat spreader  10  is evacuated and the two distal ends  121  are hermetically sealed. The working fluid filled into the chamber  14  is saturated in the wick structure  16  and is usually selected from a liquid such as water or alcohol which has a low boiling point and is compatible with the wick structure  16 . 
     In operation, the heat spreader  10  may function as an effective mechanism for spreading heat coming from a concentrated heat source (not shown) evenly to a large heat-dissipating surface. For example, a top wall  123  of the heat spreader  10  may be directly attached to a heat sink base (not shown) having a much larger footprint than the heat source in order to spread the heat of the heat source uniformly to the entire heat sink base. Alternatively, a plurality of metal fins may also be directly attached to the top wall  123  of the heat spreader  10 . As a bottom wall  124  of the heat spreader  10  is maintained in thermal contact with the heat source, the working fluid contained in the chamber  14  of the heat spreader  10  evaporates into vapor upon receiving the heat generated by the heat source. The generated vapor enters into the vapor-gathering region of the chamber  14 . Since the thermal resistance associated with the vapor spreading in the chamber  14  is negligible, the vapor then quickly moves towards the cooler top wall  123  of the heat spreader  10  through which the heat carried by the vapor is conducted to the entire heat sink base or the metal fins attached to the heat spreader  10 . Thus, the heat coming from the concentrated heat source is transferred to and uniformly distributed over the large heat-dissipating surface (e.g., the heat sink base or the fins). After the vapor releases the heat, it turns into condensate. In order to bring the condensate back to the bottom wall timely, the wick structure  16  has a plurality of upright ribs  161  connecting the top and bottom walls  123 ,  124  of the heat spreader  10 , for transporting the condensate from the top wall  123  towards the bottom wall  124  where it is again available for evaporation, as particular shown in  FIG. 2 . Also, these ribs  161  provide support for the heat sink attached to the heat spreader  10  and thus improve the mechanical performance of the heat spreader  10 . 
     On the other hand, if the flat type heat spreader  10  is designed to also have an elongated configuration, the heat spreader  10  may function as a plate-type heat pipe for conveying heat from one location to another distant location. For example, if an evaporating section  126  of the elongated heat spreader  10  is thermally attached to a heat source and a cooling device such as a plurality of metal fins is thermally connected to a condensing section  127  of the heat spreader  10 , then the generated vapor in the evaporating section  126  will move toward the condensing section  127  for heat dissipation and the condensate resulting from the vapor in the condensing section  127  will be brought back to the evaporating section  126  via the wick structure  16 . In this situation, the condensate has to travel a long distance as it flows from the condensing section  127  to the evaporating section  126  of the heat spreader  10 . In order to reduce the flow resistance to the condensate, the wick structure  16  is configured to have a pore size that gradually increases from the evaporating section  126  towards the condensing section  127 , as particular shown in  FIG. 3 . Thus, the capillary forces and the flow resistances generated by different sections of the wick structure  16  are different. The general rule is that the larger a pore size a wick structure has, the smaller a capillary force and the lower a flow resistance it provides. Under this rule, a first section of the wick structure  16  in conformity with the condensing section  127  of the heat spreader  10  has a pore size larger than that of a second section of the wick structure  16  in conformity with the evaporating section  126  of the heat spreader  10 . Thus, the first section of the wick structure  16  produces a relatively low resistance for the condensate as it flows in the condensing section  127 , and the second section of the wick structure  16  is still capable of maintaining a relatively high capillary force for drawing the condensate back from the condensing section  127  to the evaporating section  126 . As a result, the flow resistance to the condensate is reduced as a whole and the condensate is drawn back to the evaporating section  126  effectively and timely, thus preventing the potential dry-out problem occurring at the evaporating section  126 . 
     As shown in  FIG. 4 , a method is proposed to manufacture the heat spreader  10 . More details about the method can be easily understood with reference to  FIGS. 5-9 . Firstly, a metal foam framework  20  is provided with a hollow space  22  defined therein, as shown in  FIG. 5 . The metal foam framework  20  is to be formed as the wick structure  16  of the heat spreader  10  and has a configuration substantially the same as that of the wick structure  16 . 
     The metal foam framework  20  may be made of such materials as stainless steel, copper, copper alloy, aluminum alloy and silver. Typically, the metal foam framework  20  is fabricated by expanding and solidifying a pool of liquid metal saturated with an inert gas under pressure. Electroforming is also a typical method for fabricating the metal foam framework  20 , which generally involves steps of providing one kind of porous material such as polyurethane foam, then electrodepositing a layer of metal over the surface of the polyurethane foam and finally heating the resulting product at a high temperature to get rid of the polyurethane foam to thereby obtain a porous metal foam. Another fabrication method for the metal foam, called die-casting process, is also widely used, which generally includes steps of providing one kind of porous material such as polyurethane foam, filling ceramic slurry into the pores of the porous polyurethane foam and then solidifying the ceramic slurry therein, then heating the resulting product at a high temperature to get rid of the polyurethane foam to obtain a matrix of porous ceramic, then filling metal slurry into the pores of the ceramic matrix and finally, getting rid of the ceramic material after solidification of the metal slurry to thereby obtain a porous metal foam. In addition, there are still some other methods suitable for fabrication of metal foam. Fox example, the metal foam can be made by steps of filling a kind of bubble-generating material such as metallic hydride into a metal slurry to generate a large number of bubbles distributing randomly throughout the metal slurry and then solidifying the metal slurry to thereby obtain a metal foam with a plurality of pores therein. The size of the pores of the metal foam framework  20  may be in a wide range, subject to the levels of pressure applied during the fabrication process. If different pressures are applied to different sections of the metal foam framework  20  during the fabrication process, then a metal foam with different pore sizes will be obtained. In the present invention, the pressure is gradually increased along a direction from one end of the metal foam framework  20  toward an opposite end thereof; thus, the pore size is gradually decreased along the direction. Referring to  FIG. 3 , the wick structure  16  formed by the metal foam framework  20  has a pore size gradually decreased from the end neighboring the condensing section  127  towards the end neighboring the evaporating section  126 . 
     Then, a mold  30  with a cavity therein is provided and the metal foam framework  20  is fittingly placed and received in the cavity of the mold  30 , as shown in  FIG. 6 . The cavity of the mold  30  has a configuration substantially the same as that of the chamber  14  of the heat spreader  10  to be formed. A filling material  40  then is filled into the mold  30  via filling tubes  31  connecting to the cavity of the mold  30 . The filling material  40  is selected from such materials that can be easily removed after the heat spreader  10  is formed. For example, the filling material  40  may be paraffin or some kind of plastic or polymeric material that is liquefied when heated. The filling material  40  is filled into the mold  30  when it is at a molten state. The filling material  40  solidifies in the mold  30  when it is cooled. After the filling material  40  in the mold  30  is solidified, the mold  30  is removed. As a result, the pores in the metal foam framework  20  and the space  22  defined by the metal foam framework  20  are filled with the filling material  40 , as shown in  FIG. 7 . 
     Thereafter, the method, as shown in  FIG. 4 , includes an electrodeposition step in order to form the metal casing  12  of the heat spreader  10 . In order to proceed with the electrodeposition, an electrically conductive layer  50  is coated on an outer surface of the metal foam framework  20  filled with the filling material  40 , whereby the outer surface of the metal foam framework  20  is conductive. Then, the metal foam framework  20  with the filling material  40  contained therein is disposed into an electrodeposition bath  60  which contains an electrolyte  61 , as shown in  FIG. 8 . The electrodeposition bath  60  includes a cathode electrode  62  and an anode electrode  63 , both of which are immersed in the electrolyte  61  and are located at opposite sides of the metal foam framework  20 , respectively. After electrodepositing for a specific period of time, the metal foam framework  20  is taken out of the electrodeposition bath  60  and a layer of metal coating  70  is accordingly formed on the outer surface of the metal foam framework  20 , as shown in  FIG. 9 . Then, the filling material  40  in the metal foam framework  20  is removed away from the coating layer  70  by heating the filing material  40  at a temperature above a melting temperature of the filing material  40 . Although it is not shown in  FIG. 9 , it should be recognized that two open ends as illustrated in  FIGS. 1 and 3  are also formed by the coating layer  70  after the electrodeposition step so that the filling material  40  is able to be discharged from the metal foam framework  20  and the coating layer  70 . After the filling material  40  is completely removed, the wick structure  16 , the casing  12  and the heat spreader  10  as shown in  FIGS. 1-3  are obtained. Thereafter, the working fluid is injected into the casing  12  to be saturated in the wick structure  16 . Finally, the casing  12  is vacuumed and the two open ends are sealed. 
     According to the method, the wall thickness of the heat spreader  10  can be easily controlled by regulating the time period and voltage involved in the electrodeposition step. Compared with the conventional soldering method, the reliability of the heat spreader  10  made by the method is also improved since the heat spreader  10  is integrally formed. 
       FIGS. 10-11  show a heat spreader  80  in accordance with an alternative embodiment of the present invention. The heat spreader  80  is elongated and is in the form of a round heat pipe. Similarly, the heat spreader  80  may be made by the foregoing method as shown in  FIG. 4 . The heat spreader  80  includes an elongated metal casing  81  and a wick structure  82  lining an inner surface of the metal casing  81 . The wick structure  82  is in the form of a metal foam and has a pore size gradually increased from an evaporating section  811  towards a condensing section  812  of the heat spreader  80 . 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.