Method of fabrication for a thermal spreader using thermal conduits

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.

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.

DETAILED DESCRIPTION

Referring now toFIG. 1, an exemplary embodiment of a thermal spreading device is shown generally at10and is hereinafter referred to as “thermal spreader10.” Thermal spreader10is a conduction medium that provides for thermal communication between electronic circuitry (e.g., a chip) and an environment to which thermal spreader10is exposed. The thermal communication is effectuated by the conduction of heat across a substrate12to a heat sink (shown with reference toFIG. 5). Because the materials from which substrate12are fabricated are generally of an anisotropic nature, substrate12is oftentimes characterized by a marked disparity in thermal conductivities in orthogonal directions. In particular, the thermal conductivity of substrate12in a direction shown by an arrow16(Z direction), which is normal to the interface of thermal spreader10and the circuitry (not shown), may be substantially less than thermal conductivities in the directions shown by an arrow18(X direction) and an arrow19(Y direction) along the same interface of thermal spreader10and the circuitry. Due to such disparities, the thermal resistance across substrate12(in the direction of arrow16) is increased, and the rate of heat transfer (flux) across thermal spreader10varies dramatically from the flux in the direction (as shown by arrows18and19) that the interface extends.

In order to enhance the thermal communication across thermal spreader10, substrate12is configured to include thermal conduits14. The materials from which thermal conduits14are fabricated generally have thermal conductivity values that are substantially higher than the thermal conductivity values in the Z direction of the material from which substrate12is fabricated. Because the flux through conduits14is greater than the flux in the same direction across the surrounding substrate12, heat conduction is enhanced across substrate12in the direction shown by arrow16(Z direction), viz., in the direction in which conduits14extend. Heat transfer is thereby optimized through substrate12via conduits14.

Conduits14are 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, conduits14are 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.

Substrate12provides an anchor into which conduits14are 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 spreader10on the chip. Exemplary materials from which substrate12can be fabricated include, but are not limited to, carbon and carbon composites. As noted above with respect to conduits14, the carbon materials may be fibrous or particulate in structure.

The configuration of thermal spreader10is generally such that conduits14are arranged to be parallel to each other, as is shown inFIG. 1. Furthermore, conduits14generally extend linearly between opposing surfaces of substrate12. As shown, the architecture of thermal spreader10is further defined by a substantially uniform spatial positioning of conduits14over any randomly selected section of substrate12. The even distribution of conduits14facilitates and improves the conduction of heat from a first face20disposed adjacent the chip and an opposingly-positioned second face24disposed adjacent the heat sink. Such a distribution provides for the effective transfer of heat longitudinally through conduits14while maintaining the substantially uniform transfer of the heat in the directions radial to the surfaces of conduits14.

When thermal spreader10is mounted between a chip (shown with reference toFIG. 5) and a heat sink (also shown with reference toFIG. 5), conduits14enable heat generated during the operation of the chip to be communicated from first face20of thermal spreader10through conduits14across substrate12to second face24of thermal spreader10. Although the material of which substrate12is fabricated allows for some degree of thermal conduction between faces20,24, the anisotropic nature of the material causes heat generated by the chip and transferred to thermal spreader10to be more substantially dissipated through substrate12in the directions shown by arrows18and19. Dissipation of heat in the directions shown by arrows18and19allows for the heat to be conducted to a larger number of conduits14, which further allows for the more effective transfer of heat from the chip to the heat sink.

Referring now toFIGS. 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 conduits14are defined into an array, which is shown generally at30inFIG. 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 inFIG. 2B. The hardened and cured molding material forms a block, shown generally at32, having thermal conduits14extending between first face20and opposing second face24thereof. Block32is then sawed or otherwise made into sheets34, as is illustrated inFIG. 2C. Each sheet34is of a thickness tS, which is slightly in excess of the desired thickness of the finished thermal dissipating device. Sheets34are then polished on at least one face thereof to bring thicknesses tSwithin the allowable tolerances of final product. Polishing of the sheets on both sides further provides sheets34with surface textures conducive to a more effective transfer of heat between the chip and the heat sink. Finally, sheets34are cut into individual thermal spreaders10of the desired length and width.

In another exemplary process of the fabrication of the thermal spreader, thermal conduits14may be press-fitted into substrate12, as is shown inFIGS. 3A and 3B. Referring toFIG. 3A, holes28are drilled, punched, or otherwise formed in block32. The cross sectional geometries of holes28correspond with the cross sectional geometries of conduits14insertable into holes28. Referring now toFIG. 3B, conduits14are inserted into holes28under a compressive force Cfeffectuated by a press (not shown) or a similar apparatus. The mechanical tolerances of conduits14are such that when conduits14are received in holes28, a tight fit is maintained between the inner surfaces of holes28and the outer surfaces of each conduit14, thereby allowing effective thermal communication to be maintained between the material of block32and conduits14. Block32may 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 conduits14may be shrink-fitted into substrate12, as is shown inFIG. 4. In the shrink-fitting process, holes28are again drilled, punched, or otherwise formed in block32, as was described above. Block32is heated to a temperature that causes block32(and subsequently holes28) to expand. Upon expansion, conduits14are inserted into holes28with little effort such that space is defined by inner surfaces34of holes and outer surfaces36of conduits14. Block32is then cooled to cause the material of fabrication of block32to contract, thereby constricting holes28and eliminating the spaces defined between the inner surfaces of holes28and the outer surfaces of conduits14. Once constricted, conduits14are securely retained within block28. Block32may 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 toFIG. 5, a thermal conduction package is shown generally at38. In thermal conduction package38, thermal spreader10is shown as it would be disposed between the chip40disposed in electronic communication with its associated circuitry through substrate42and the heat sink44. Thermal spreader10is adhered to chip40with an adhesive48, which may be a solder or an epoxy material applied to chip40as a thin layer upon which thermal spreader10is placed. A layer of thermal paste50, which is typically a natural or synthetic oil-based compound with thermally conductive particle filler, is applied to the exposed surface of thermal spreader10upon which heat sink44is mounted. Both adhesive48and thermal paste50facilitate the transfer of heat between chip40and thermal spreader10and thermal spreader10and heat sink44respectively, thereby enhancing the conduction of heat across thermal spreader10.

As is shown with reference toFIGS. 6 and 7, another exemplary embodiment of a thermal dissipating device is shown generally at110. Thermal spreader110is substantially similar to thermal spreader10as illustrated above with reference toFIGS. 1 through 5. Thermal spreader110, however, includes an arrangement of variably spaced conduits114disposed within a dissipating substrate112. The arrangement of variably spaced conduits114is configured to define regions150in which the density of conduits114is greater than the density of conduits114in adjacently positioned regions152of the same substrate112. The high-density regions150are positioned on substrate112to register with areas of high heat flux on a chip upon assembly of the thermal conduction package.

Referring now toFIG. 8, the engagement of the thermal spreader with the chip is illustrated generally at138. When thermal spreader110is placed in communication with chip140, the high-density regions150register with the areas of high flux160on chip140. Such a placement allows for the increased transfer of heat from the areas of high flux160on chip140to high-density regions150of thermal spreader110while simultaneously providing a thermally adequate transfer of heat from the areas of chip140from which lower heat flux is realized. The disparities in the densities of the conduits in each region150,152are engineered to provide for the removal of heat from each portion of chip140and the transfer of heat to the heat sink to minimize disparity in heat build up at the interface of chip140and thermal spreader110. Minimization of such disparity may provide improved operability of chip140and increase the useful life thereof. Fabrication of thermal spreader110is effectuated in a batch process substantially similar to that illustrated inFIGS. 2A through 4for thermal spreader10.