Abstract:
The present invention provides a compound heat sink, mainly comprising a graphite layer and a metal layer. The graphite layer has a first embedding structure on a surface; the metal layer has a second embedding structure on a surface and corresponding to the first embedding structure. The graphite layer and the metal layer are bonded by the first and second embedding structures for increasing the bonding strength between two heterogeneous materials as well as reducing the interfacial heat resistance. Thereby, the stability of heat dissipation performance can be improved.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a compound heat sink, and particularly to a compound heat sink having excellent thermal conduction property in all of the X-, Y-, and Z-axis. 
       BACKGROUND OF THE INVENTION 
       [0002]    Owing to the developments in technologies and the trend of demands in the consumer market, electronic produces have been developing in the direction of high performance, high speed, and compact size, and hence increasing the relative density of electronic devices. Nonetheless, because electronic devices generate a great deal of heat during operation. how to enable electronic products have excellent heat dissipating efficiency given limited device volume for guaranteeing normal operation of the electronic products and thus extending their lifetime has become the primary challenge for modern electronic products. 
         [0003]    Because metal sheets have excellent thermal conduction property in all of the X-, Y-, and Z-axis, metals having high thermal conductivity, such as copper and aluminum, are usually used for manufacturing heat sinks currently for leading out the heat generated during device operations. Nonetheless, compared with copper and aluminum, graphite owns the advantages of lighter weight and higher anisotropic thermal conductivity in X and Y directions. Thereby, nowadays, graphite has been regarded as a superior heat conducting material for solving the heat dissipation problem for modern electronic products. 
         [0004]    Nevertheless, graphite sheets are weak and their thermal conductivity is inferior in the Z-axis. These problems limit the application of graphite sheets in heat dissipation. The current solution is to bond a graphite sheet with a metal sheet using a glue layer to form a compound heat dissipating material in hope of reinforcing the thermal conduction performance of the graphite sheet in the Z-axis by the metal sheet. However, under such a bonding method, the existence of the glue layer introduces substantial thermal resistivity between the graphite sheet and the metal sheet, leading to unpromising performance of the compound heat dissipating material in thermal conduction. 
         [0005]    Accordingly, the present invention provides a novel compound heat sink for solving the problems described above. 
       SUMMARY 
       [0006]    An objective of the present invention is to provide a compound heat sink with superior thermal conductivity in the X-, Y-, and Z-axis, which bonds a first embedding structure of the first layer and a second embedding structure of the second layer for improving the bonding strength and stability between two heterogeneous materials. 
         [0007]    Another objective of the present invention is to provide a compound heat sink. When the first layer is an artificial graphite sheet and the second layer is copper or aluminum, the thermal conductivity of the compound heat sink according to the present in the X-, Y-, and Z-axis can reach above 400 W/m ° C. 
         [0008]    Still another objective of the present invention is to provide a lightweight and thin compound heat sink. 
         [0009]    For achieving the objectives described above, the present invention provides a compound heat sink mainly comprising a graphite layer and a metal layer. The graphite layer has a first embedding structure on a surface. The metal layer has a second embedding structure on a surface and corresponding to the first embedding structure. The graphite layer and the metal layer are bonded firmly by the first and second embedding structures. 
         [0010]    The present invention discloses another compound heat sink, which comprises a metal layer, a graphite layer, and a graphite bonding layer composed of graphite powder located between the metal layer and the graphite layer for bonding the metal layer and the graphite layer. 
         [0011]    The graphite bonding layer is manufactured by vermicular graphite powder or by mixing vermicular graphite powder and glue. 
         [0012]    Moreover, the present invention further discloses a metal oxide layer can be formed on the surface of the metal layer described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1(   a ) shows a schematic diagram of the compound heat sink according a first embodiment of the present invention; 
           [0014]      FIG. 1(   b ) shows a partially enlarged diagram of  FIG. 1(   a ) according to the present invention; 
           [0015]      FIG. 1(   c ) shows a flowchart for manufacturing the compound heat sink of  FIG. 1(   a ) according an embodiment of the present invention; 
           [0016]      FIG. 2  shows a flowchart for manufacturing the compound heat sink of  FIG. 1(   a ) according another embodiment of the present invention; 
           [0017]      FIG. 3(   a ) shows a schematic diagram of the compound heat sink according another embodiment of the present invention; 
           [0018]      FIG. 3(   b ) shows a flowchart for manufacturing the compound heat sink of  FIG. 3(   a ) according an embodiment of the present invention; 
           [0019]      FIG. 4(   a ) shows a schematic diagram of the compound heat sink according another embodiment of the present invention; 
           [0020]      FIG. 4(   b ) shows a flowchart for manufacturing the compound heat sink of  FIG. 4(   a ) according an embodiment of the present invention; 
           [0021]      FIG. 5(   a ) shows a schematic diagram of the compound heat sink according another embodiment of the present invention; 
           [0022]      FIG. 5(   b ) shows a flowchart for manufacturing the compound heat sink of  FIG. 5(   a ) according an embodiment of the present invention; 
           [0023]      FIG. 6(   a ) shows a thermal image of the heat dissipation experiment of the copper layer/glue/artificial graphite sheet compound heat sink according to the prior art to a heat source; 
           [0024]      FIG. 6(   b ) shows a thermal image of the heat dissipation experiment of the artificial graphite sheet without compound copper layer to a heat source; 
           [0025]      FIG. 6(   c ) shows a thermal image of a heat dissipation experiment of the copper layer/artificial graphite sheet compound heat sink according to the present invention to a heat source; 
           [0026]      FIG. 7  shows a schematic diagram of the experimental architecture in  FIG. 6(   a ) to  FIG. 6(   c ), 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    In order to make the structure and characteristics as well as the effectiveness of the present invention to be further understood and recognized, the detailed description of the present invention is provided as follows along with embodiments and accompanying figures. 
         [0028]    The spirit of the present invention is to provide a compound heat sink with superior thermal conductivity in the X-, Y-, and Z-axis. The compound heat sink comprises a graphite layer, a metal layer, and a bonding structure located between the graphite layer and the metal layer. The bonding structure can reinforce the bonding strength of the graphite layer and the metal layer. 
         [0029]    According to an embodiment, the bonding structure includes a first embedding structure on a surface of the graphite layer and a second embedding structure on a surface and corresponding to the first embedding structure. 
         [0030]    The first embedding structure described above can be the material of the graphite layer or formed by surface processing. 
         [0031]    In the following, several embodiments are used for describing the present invention. However, the present invention is not limited to the following structure types, materials, or manufacturing methods. 
         [0032]    Please refer to  FIG. 1(   a ),  FIG. 1(   b ), and  FIG. 1(   c ), which show a schematic diagram of the compound heat sink according a first embodiment of the present invention, a partially enlarged diagram of  FIG. 1(   a ) according to the present invention, and a flowchart for manufacturing the compound heat sink of  FIG. 1(   a ) according an embodiment of the present invention, respectively. 
         [0033]    According to the present embodiment, the first layer is an artificial graphite sheet; the material of the second layer is copper or aluminum. In the following, copper is used as an example. 
         [0034]    First, as shown in the step S 11 , provide an artificial graphite sheet  10 . Then, as shown in the step S 12 , coat copper glue (not shown in the figures) on the artificial graphite sheet  10 . Next, as shown in the step S 13 , sinter the artificial graphite sheet  10  coated with copper glue at approximately 1100□ for removing the glue in the copper glue. Finally, as shown in the step S 14 , a compound heat sink  14 , which is a copper layer  12  on the artificial graphite sheet  10  shown in  FIG. 1(   a ), is given. 
         [0035]    According to the present embodiment, because the artificial graphite sheet  10  is composed of multiple stacked and interlaced layers of laminated graphene  16 , there are many voids and gaps among graphene. These gaps are then used as the embedding structure  18 . Copper glue is formed by mixing copper powder and glue. When coating copper glue on the artificial graphite sheet  10 , copper powder will flow into the gaps along with the glue. After the sintering process, the glue will solidify and the copper powder will crystallize and bond during the sintering process, forming the crystal structure embedded in the gaps. The crystal structure is used as the embedding structure  20  corresponding to the embedding structure  18 , as shown in  FIG. 1(   b ). 
         [0036]      FIG. 2  shows a flowchart for manufacturing the compound heat sink of  FIG. 1(   a ) according another embodiment of the present invention. According to the present embodiment, copper powder is used for replacing the copper glue. First, as shown in the step S 21 , provide an artificial graphite sheet. Then, as shown in the step S 22 , spray the copper powder on the artificial graphite sheet for forming a copper powder layer. Next, as shown in the step S 23 , perform a high-pressure sintering process on the copper powder layer at the pressure of 80 kg/cm 2  and at the temperature of approximately 1100□. Finally, the compound heat sink as shown in  FIG. 1(   a ) is given. 
         [0037]    Because the artificial graphite sheet is composed of multiple stacked and interlaced layers of laminated graphene, there are many gaps on the surface of the artificial graphite sheet. These gaps are then used as the embedding structure. The copper powder will fill into the gaps after the high-pressure sintering process. In addition, the copper powder will crystallize and bond during the sintering process, forming the embedding structure embedded in the gaps. 
         [0038]    Besides, graphite powder, such as vermicular graphite powder, can be mixed in the copper powder described above for reinforcing the bonding strength between the copper powder and the artificial graphite sheet. 
         [0039]    Please refer to  FIG. 3(   a ) and  FIG. 3(   b ), which show a schematic diagram of the compound heat sink according another embodiment of the present invention and a flowchart for manufacturing the compound heat sink, respectively. 
         [0040]    According to the present embodiment, the first layer adopts an artificial graphite sheet; the material of the second layer is copper. As shown in the step S 31 , provide an artificial graphite sheet  22 . Then, as shown in the step S 32 , perform surface process on the artificial graphite sheet  22  for forming rugged microstructure on the surface and used as an embedded structure  24 . The surface processing methods include pressing the artificial graphite sheet directly using a mold having rugged veins, wet etching, or laser surface processing. As shown in the step S 33 , form a copper layer  26  on the artificial graphite sheet  22 . The copper layer  26  has an embedded structure  27  corresponding to the embedded structure  24 . Finally, as shown in the step S 34 , the compound heat sink  28  ash shown in  FIG. 3(   a ) is given. 
         [0041]    Moreover, the methods for forming the copper layer  26  described above can be a plating process or coating copper glue first and then sintering. Alternatively, the pressing bonding method can be adopted for forming a copper powder layer first and then performing sintering, in which the copper powder layer can be mixed with graphite powder as well. Alternatively, the copper layer  26  can be formed by disposing a copper foil on the surface of the artificial graphite sheet  26  having the embedding structure  24  and then performing press-bonding sintering. By using the press-bonding sintering, the copper foils melts and fills into the gaps in the embedding structure  24 , and thus forming the embedding structure matching the embedding structure  24 . The related process parameters are described above, and will not be repeated again. 
         [0042]    In the following embodiments, the bonding structure is the graphite bonding layer manufactured by vermicular graphite powder. 
         [0043]    Please refer to  FIG. 4(   a ) and  FIG. 4(   b ), which show a schematic diagram of the compound heat sink according another embodiment of the present invention and a flowchart for manufacturing the compound heat sink, respectively. According to the present embodiment, first, as shown in the step S 41 , provide a graphite sheet  30 . Then, as shown in the step S 42 , spray a vermicular graphite powder layer  32  on the graphite sheet  30 . Next, as shown in the step S 43 , place a copper foil  34  on the vermicular graphite powder layer  32 . Finally, perform a press-bonding sintering process to give a compound heat sink  36  bonding the copper foil  34  and the graphite sheet  30  using a graphite bonding layer  35  as shown in  FIG. 4(   a ). 
         [0044]    According to the present embodiment, the vermicular graphite powder is used for filling the voids or gaps among graphene. In addition, during the press-bonding sintering process, the copper foil melts, flows into the gaps among vermicular graphite powder, crystallizes, and bonds to form the crystal structure embedded in the gaps. 
         [0045]    Besides, the vermicular graphite powder layer can be mixed with glue, as described in the following embodiment. 
         [0046]    Please refer to  FIG. 5(   a ) and  FIG. 5(   b ), which show a schematic diagram of the compound heat sink according another embodiment of the present invention and a flowchart for manufacturing the compound heat sink, respectively. According to the present embodiment, first, as shown in the step S 51 , provide a copper foil  40 . Then, as shown in the step S 52 , spot coat glue  42  on the copper foil  40 . Next, as shown in the step S 53 , form a vermicular graphite powder layer  44  covering the glue  42  on the surface of the copper foil. Finally, as shown in the step S 54 , dispose an artificial graphite sheet  46  on the vermicular graphite powder layer  44  and perform a press bonding process to give the compound heat sink as shown in  FIG. 5(   a ). 
         [0047]    The press bonding process according to the present invention includes the thermal press bonding process. Thereby, there will be no matching problem of thermal expansion for heterogeneous materials. Not only the stability is enhanced, the interfacial thermal resistivity between two heterogeneous materials can be reduced as well. 
         [0048]    Moreover, an oxide layer can be further formed by anode processing on the surface of the metal layer not contacting the graphite layer. 
         [0049]    When the artificial graphite sheet and the copper layer (copper foil) are adopted for compounding according to the present invention, the ratio of the thickness of the copper layer to the thickness of the artificial graphite sheet can be between 1:1 and 20:1 for achieving better heat dissipating effect. In addition, by selecting these materials, the thermal conductivity of the compound heat sink according to the present invention in the X-, Y-, and Z-axis can all reach above 400 W/m□ with superior stability and light weight. Thereby, it can be applied extensively to heat dissipation of many electronic products in the market, such as portable electronic products including mobiles phones and tablet computers. 
         [0050]    Please refer to  FIG. 6(   a ),  FIG. 6(   b ), and  FIG. 6(   c ), which show thermal images of the heat dissipation experiment of the copper layer/glue/artificial graphite sheet compound heat sink according to the prior art, the artificial graphite sheet without compound copper layer, and the copper layer/artificial graphite sheet compound heat sink according to the present invention to a heat source, respectively. The diagram of the experimental architecture is shown in  FIG. 7 . A 4-Watt, 20×20 mm 2  LED die is used as the heat source  50  disposed at the center of the heat sink  52 . The area of the heat sink  52  is 100×100 mm 2 . The temperature sensing point is selected to be the central point T1 and the edge point T2; the spacing between T1 and T2 is 50 mm. 
         [0051]    As shown in the figure, the temperature at the center of the copper layer/glue/artificial graphite sheet compound heat sink according to the prior art reaches 70.7□; for the artificial graphite sheet without compound copper layer, the temperature at the center is 56.3□; and for the copper layer/artificial graphite sheet compound heat sink according to the present invention, the temperature at center is 55.4□. Thereby, the compound heat sink according to the present invention has superior thermal conducting effect. The existence of glue contrarily makes the thermal conducting effect of the artificial graphite sheet inferior. 
         [0052]    Accordingly, the present invention conforms to the legal requirements owing to its novelty, nonobviousness, and utility. However, the foregoing description is only embodiments of the present invention, not used to limit the scope and range of the present invention. Those equivalent changes or modifications made according to the shape, structure, feature, or spirit described in the claims of the present invention are included in the appended claims of the present invention.