Abstract:
A thermal spreading device disposable between electronic circuitry and a heat sink includes a substrate having parallel first and second faces and conduits extending through the substrate between the faces. The substrate material has a first thermal conductivity value in a direction parallel to the faces and a second thermal conductivity value in a direction normal to the faces, with the second thermal conductivity value being less than the first thermal conductivity value. The conduit material has a thermal conductivity value associated with it, with the thermal conductivity value being greater than the second thermal conductivity value of the substrate. One method of fabricating the thermal spreading device includes disposing a molding material radially about the rods and hardening the material. Other methods include press fitting and shrink fitting the rods into a substrate material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 09/965,489 filed Sep. 27, 2001, now U.S. Pat. No. 6,821,625 the contents of which are incorporated by reference herein in their entirety. 

   BACKGROUND 
   In nearly every sector of the electronics industry, electronic circuitry involves the interconnection of an integrated chip (hereinafter “chip”) and a surface or device upon which the chip is supported. During operation of the circuitry, heat is generated and a heat flux is established between the chip and its environment. In order to remove heat more effectively to ensure the proper functioning of the circuitry, the heat flux is disseminated across a surface area larger than the surface area of the chip and transferred to an attached heat sink device. Once the heat is transferred to the heat sink device, it can be removed by a forced convection of air or other cooling means. 
   In some applications, multiple processors and their associated control and support circuitry are arranged on a single chip. Such arrangements may result not only in a further increase in the heat flux, but also in a non-uniform distribution of the heat flux across the surface of the chip. The non-uniformity of the distribution of the heat flux is generally such that a higher heat flux is realized in the processor core region and a significantly lower heat flux is realized in the region of the chip at which the control and support circuitry is disposed. The high heat flux in the processor core region may cause devices in this region to exceed their allowable operating temperatures. The resulting disparity in temperature between the two regions, which may be significant, may contribute to the stressing and fatigue of the chip. 
   A thermally conductive heat spreading device is oftentimes disposed between the chip and the heat sink device to facilitate the dissemination of heat from the chip. Such heat spreading devices are generally plate-like in structure and homogenous in composition and fabricated from materials such as copper, aluminum nitride, or silicon carbide. Newer carbon fiber composites exhibit even higher thermal conductivities than these traditional thermal spreader materials; however, they tend to be anisotropic in nature, exhibiting wide variations in thermal conductivity between a major axis normal to the face of the structure (in the Z direction) and the axes orthogonal to the major axis (in the X and Y directions). Moreover, the lower thermal conductivity in the direction along the major axis tends to have the effect of increasing the thermal resistance of the heat spreading device, thereby inhibiting the dissemination of heat from the device. 
   SUMMARY 
   A thermal spreading device disposable between electronic circuitry and a heat sink is disclosed. The device includes a substrate having a first face and a second face and a plurality of conduits extending through the substrate from the first face to the second face. The two faces of the substrate are disposed in a parallel relationship. The material of which the substrate is fabricated has a first thermal conductivity value in a direction parallel to the faces and a second thermal conductivity value in a direction normal to the faces, with the second thermal conductivity value being less than the first thermal conductivity value. The material of which each conduit is fabricated has a thermal conductivity value associated with it, with the thermal conductivity value of each conduit being greater than the second thermal conductivity value of the substrate. 
   One method of fabricating the thermal spreading device includes arranging a plurality of thermally conductive rods such that the rods extend longitudinally in a common direction, disposing a molding material radially about the longitudinally extending rods, hardening the molding material around the plurality of thermally conductive rods, and cutting the hardened molding material into slices in a direction perpendicular to the direction in which the rods longitudinally extend. Other methods of fabrication include press fitting or shrink fitting the thermally conductive rods into holes in the substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present disclosure will be better understood by those skilled in the pertinent art by referencing the accompanying drawings, where like elements are numbered alike in the several FIGURES, in which: 
       FIG. 1  is a perspective cutaway view of a thermal spreading device; 
       FIGS. 2A through 2C  are perspective views of a batch process of the fabrication of a thermal spreading device; 
       FIGS. 3A and 3B  are perspective views of a batch process of the fabrication of a thermal spreading device in which conduits are press fitted into the substrate; 
       FIG. 4  is a sectional view of a step in a batch process of the fabrication of a thermal spreading device in which conduits are shrink fitted into the substrate; 
       FIG. 5  is a sectional view of the engagement of a thermal spreading device with a chip and a heat sink; 
       FIGS. 6 and 7  are plan and cross sectional views of an alternate exemplary embodiment of a thermal spreading device; and 
       FIG. 8  is an exploded perspective view of the engagement of the thermal spreading device of  FIGS. 6 and 7  with a chip. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , an exemplary embodiment of a thermal spreading device is shown generally at  10  and is hereinafter referred to as “thermal spreader  10 .” Thermal spreader  10  is a conduction medium that provides for thermal communication between electronic circuitry (e.g., a chip) and an environment to which thermal spreader  10  is exposed. The thermal communication is effectuated by the conduction of heat across a substrate  12  to a heat sink (shown with reference to  FIG. 5 ). Because the materials from which substrate  12  are fabricated are generally of an anisotropic nature, substrate  12  is oftentimes characterized by a marked disparity in thermal conductivities in orthogonal directions. In particular, the thermal conductivity of substrate  12  in a direction shown by an arrow  16  (Z direction), which is normal to the interface of thermal spreader  10  and the circuitry (not shown), may be substantially less than thermal conductivities in the directions shown by an arrow  18  (X direction) and an arrow  19  (Y direction) along the same interface of thermal spreader  10  and the circuitry. Due to such disparities, the thermal resistance across substrate  12  (in the direction of arrow  16 ) is increased, and the rate of heat transfer (flux) across thermal spreader  10  varies dramatically from the flux in the direction (as shown by arrows  18  and  19 ) that the interface extends. 
   In order to enhance the thermal communication across thermal spreader  10 , substrate  12  is configured to include thermal conduits  14 . The materials from which thermal conduits  14  are fabricated generally have thermal conductivity values that are substantially higher than the thermal conductivity values in the Z direction of the material from which substrate  12  is fabricated. Because the flux through conduits  14  is greater than the flux in the same direction across the surrounding substrate  12 , heat conduction is enhanced across substrate  12  in the direction shown by arrow  16  (Z direction), viz., in the direction in which conduits  14  extend. Heat transfer is thereby optimized through substrate  12  via conduits  14 . 
   Conduits  14  are defined by rods or wires having substantially circular cross sectional geometries, as is shown. Rods or wires having substantially circular cross sectional geometries enable a substantially uniform transfer of heat to be maintained in the directions radial to the circular cross section. Other cross sectional geometries that may be used include, but are not limited to, elliptical, square, flat, multi-faced, and configurations incorporating combinations of the foregoing geometries. Regardless of the cross sectional geometry, conduits  14  are formed from materials having high thermal conductivities. Such materials include, but are not limited to, copper, aluminum, carbon, carbon composites, and similar materials that exhibit a high thermal conductivity along the conduit axis. The carbon materials may be fibrous or particulate in structure. 
   Substrate  12  provides an anchor into which conduits  14  are disposed while further providing a medium for the transfer of heat in directions along and parallel to the interface defined by the positioning of thermal spreader  10  on the chip. Exemplary materials from which substrate  12  can be fabricated include, but are not limited to, carbon and carbon composites. As noted above with respect to conduits  14 , the carbon materials may be fibrous or particulate in structure. 
   The configuration of thermal spreader  10  is generally such that conduits  14  are arranged to be parallel to each other, as is shown in  FIG. 1 . Furthermore, conduits  14  generally extend linearly between opposing surfaces of substrate  12 . As shown, the architecture of thermal spreader  10  is further defined by a substantially uniform spatial positioning of conduits  14  over any randomly selected section of substrate  12 . The even distribution of conduits  14  facilitates and improves the conduction of heat from a first face  20  disposed adjacent the chip and an opposingly-positioned second face  24  disposed adjacent the heat sink. Such a distribution provides for the effective transfer of heat longitudinally through conduits  14  while maintaining the substantially uniform transfer of the heat in the directions radial to the surfaces of conduits  14 . 
   When thermal spreader  10  is mounted between a chip (shown with reference to  FIG. 5 ) and a heat sink (also shown with reference to  FIG. 5 ), conduits  14  enable heat generated during the operation of the chip to be communicated from first face  20  of thermal spreader  10  through conduits  14  across substrate  12  to second face  24  of thermal spreader  10 . Although the material of which substrate  12  is fabricated allows for some degree of thermal conduction between faces  20 ,  24 , the anisotropic nature of the material causes heat generated by the chip and transferred to thermal spreader  10  to be more substantially dissipated through substrate  12  in the directions shown by arrows  18  and  19 . Dissipation of heat in the directions shown by arrows  18  and  19  allows for the heat to be conducted to a larger number of conduits  14 , which further allows for the more effective transfer of heat from the chip to the heat sink. 
   Referring now to  FIGS. 2A through 2C , an exemplary batch process illustrating the fabrication of the thermal spreader is illustrated. The process comprises arranging the rods or wires by which conduits  14  are defined into an array, which is shown generally at  30  in  FIG. 2A . The rods are arranged such that the longitudinal axes of the rods are parallel to each other and held fast by a jig (not shown) or other device configured to maintain the rods in their proper alignment. Molding material of which the substrate is formed is then disposed around the rods, hardened, and cured, as is shown in  FIG. 2B . The hardened and cured molding material forms a block, shown generally at  32 , having thermal conduits  14  extending between first face  20  and opposing second face  24  thereof. Block  32  is then sawed or otherwise made into sheets  34 , as is illustrated in  FIG. 2C . Each sheet  34  is of a thickness t S , which is slightly in excess of the desired thickness of the finished thermal dissipating device. Sheets  34  are then polished on at least one face thereof to bring thicknesses t S  within the allowable tolerances of final product. Polishing of the sheets on both sides further provides sheets  34  with surface textures conducive to a more effective transfer of heat between the chip and the heat sink. Finally, sheets  34  are cut into individual thermal spreaders  10  of the desired length and width. 
   In another exemplary process of the fabrication of the thermal spreader, thermal conduits  14  may be press-fitted into substrate  12 , as is shown in  FIGS. 3A and 3B . Referring to  FIG. 3A , holes  28  are drilled, punched, or otherwise formed in block  32 . The cross sectional geometries of holes  28  correspond with the cross sectional geometries of conduits  14  insertable into holes  28 . Referring now to  FIG. 3B , conduits  14  are inserted into holes  28  under a compressive force C f  effectuated by a press (not shown) or a similar apparatus. The mechanical tolerances of conduits  14  are such that when conduits  14  are received in holes  28 , a tight fit is maintained between the inner surfaces of holes  28  and the outer surfaces of each conduit  14 , thereby allowing effective thermal communication to be maintained between the material of block  32  and conduits  14 . Block  32  may then be sawed or otherwise formed into sheets and polished and cut to the desired lengths and widths. 
   In yet another exemplary process of the fabrication of the thermal spreader, thermal conduits  14  may be shrink-fitted into substrate  12 , as is shown in  FIG. 4 . In the shrink-fitting process, holes  28  are again drilled, punched, or otherwise formed in block  32 , as was described above. Block  32  is heated to a temperature that causes block  32  (and subsequently holes  28 ) to expand. Upon expansion, conduits  14  are inserted into holes  28  with little effort such that space is defined by inner surfaces  34  of holes and outer surfaces  36  of conduits  14 . Block  32  is then cooled to cause the material of fabrication of block  32  to contract, thereby constricting holes  28  and eliminating the spaces defined between the inner surfaces of holes  28  and the outer surfaces of conduits  14 . Once constricted, conduits  14  are securely retained within block  28 . Block  32  may then be sawed or otherwise formed into sheets and polished and cut to the desired dimensions in manners similar to those described above to form the final product. 
   Referring now to  FIG. 5 , a thermal conduction package is shown generally at  38 . In thermal conduction package  38 , thermal spreader  10  is shown as it would be disposed between the chip  40  disposed in electronic communication with its associated circuitry through substrate  42  and the heat sink  44 . Thermal spreader  10  is adhered to chip  40  with an adhesive  48 , which may be a solder or an epoxy material applied to chip  40  as a thin layer upon which thermal spreader  10  is placed. A layer of thermal paste  50 , which is typically a natural or synthetic oil-based compound with thermally conductive particle filler, is applied to the exposed surface of thermal spreader  10  upon which heat sink  44  is mounted. Both adhesive  48  and thermal paste  50  facilitate the transfer of heat between chip  40  and thermal spreader  10  and thermal spreader  10  and heat sink  44  respectively, thereby enhancing the conduction of heat across thermal spreader  10 . 
   As is shown with reference to  FIGS. 6 and 7 , another exemplary embodiment of a thermal dissipating device is shown generally at  110 . Thermal spreader  110  is substantially similar to thermal spreader  10  as illustrated above with reference to  FIGS. 1 through 5 . Thermal spreader  110 , however, includes an arrangement of variably spaced conduits  114  disposed within a dissipating substrate  112 . The arrangement of variably spaced conduits  114  is configured to define regions  150  in which the density of conduits  114  is greater than the density of conduits  114  in adjacently positioned regions  152  of the same substrate  112 . The high-density regions  150  are positioned on substrate  112  to register with areas of high heat flux on a chip upon assembly of the thermal conduction package. 
   Referring now to  FIG. 8 , the engagement of the thermal spreader with the chip is illustrated generally at  138 . When thermal spreader  110  is placed in communication with chip  140 , the high-density regions  150  register with the areas of high flux  160  on chip  140 . Such a placement allows for the increased transfer of heat from the areas of high flux  160  on chip  140  to high-density regions  150  of thermal spreader  110  while simultaneously providing a thermally adequate transfer of heat from the areas of chip  140  from which lower heat flux is realized. The disparities in the densities of the conduits in each region  150 ,  152  are engineered to provide for the removal of heat from each portion of chip  140  and the transfer of heat to the heat sink to minimize disparity in heat build up at the interface of chip  140  and thermal spreader  110 . Minimization of such disparity may provide improved operability of chip  140  and increase the useful life thereof. Fabrication of thermal spreader  110  is effectuated in a batch process substantially similar to that illustrated in  FIGS. 2A through 4  for thermal spreader  10 . 
   While the disclosure has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.