Abstract:
In one embodiment, an apparatus includes a phase change material, a plurality of particles intermixed with the phase change material, and a conductive structure encapsulating the phase change material. The conductive structure includes a cavity including a cone shape. In one embodiment, a method includes forming a conductive structure having a cavity, injecting a phase change material into the cavity, injecting a plurality of spheres into the cavity, and sealing the cavity.

Description:
This application is a divisional of U.S. patent application Ser. No. 10/716,269, filed Nov. 17, 2003 now U.S. Pat. No. 7,316,265, which is a divisional of U.S. patent application Ser. No. 09/525,173, filed Mar. 14, 2000, now issued as U.S. Pat. No. 6,672,370, the disclosures of which are incorporated herein by reference. 
    
    
     FIELD 
     This invention relates to removing heat from a system, and more particularly to removing heat from an integrated circuit die. 
     BACKGROUND 
     An integrated circuit, such as a processor circuit, produces heat in a circuit die or substrate. Failure to efficiently remove the heat from the die results in failure of the circuit. One method of removing heat from a die includes thermally coupling a finned heat sink to the die and forcing air across the fins using a fan. Unfortunately, forced air cooling is not practical for cooling integrated circuits in hand held communication devices or in personal digital assistants. In addition, forced air cooling has a number of significant disadvantages, even when used in cooling systems, such as servers and engineering workstations, that have traditionally used forced air cooling. 
     One disadvantage associated with forced air cooling is that it is expensive. Customers who purchase systems composed of integrated circuits, such as computers, are interested in reducing the operating costs of those systems. A fan moves air by driving a fan blade with a motor. The motor requires energy to operate. Using a fan to provide forced air cooling in these systems increases the operating costs of the systems. 
     As the circuit density on a die increases, more heat is produced on the die and the die needs to be cooled quickly to avoid circuit failure. To cool the die quickly, the rate at which air is forced across the heat sink is increased. Increasing the rate at which air is forced across the heat sink generally requires a fan having a larger blade and a larger motor. The larger motor consumes more power and increases the system operating costs. 
     Another significant disadvantage associated with forced air cooling is that it is not effective for cooling hot spots on a substrate. Heat is not generated uniformly over the surface of an integrated circuit substrate. This uneven generation of heat produces hot spots in the substrate. In some systems, hot spots may be cooled sufficiently to prevent immediate catastrophic failure of the circuit using forced air cooling, but over time, failure to adequately cool hot spots leads to premature failure of the circuits fabricated near the hot spots. 
     For these and other reasons there is a need for the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of example embodiments of the present invention showing a heat sink thermally coupled to an integrated circuit. 
         FIG. 1B  is a perspective view of example embodiments of the present invention showing a heat sink thermally coupled to an integrated circuit package. 
         FIG. 2  is a cross-sectional side view of example embodiments of the heat sink of the present invention. 
         FIG. 3  is a perspective view of example embodiments of the present invention showing a heat sink thermally coupled to an integrated circuit included in a wireless communication device. 
     
    
    
     DESCRIPTION 
     In the following detailed description of the invention reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. 
     A heat sink and a method of fabricating and using the heat sink are described herein. The heat sink is thermally coupled to an integrated circuit die and dissipates heat produced in the integrated circuit die. The heat sink includes a cavity containing a phase change material and a number of particles for enhancing convection in the heat sink during the cooling of an integrated circuit die. The heat sink is highly reliable and exhibits enhanced cooling characteristics when compared with a traditional finned heat sink. Efficient manufacturing of the heat sink is achieved by a symmetrical arrangement of the heat sink structures. 
       FIG. 1A  is a perspective view of example embodiments of the present invention showing heat sink  100  thermally coupled to integrated circuit die  103 . Heat sink  100  is not limited to operation in connection with a particular type of integrated circuit fabricated on die  103 . Digital circuits, such as processors, digital signal processors, and communication circuits are all suitable for use in connection with heat sink  100 . Similarly, analog circuits, such as amplifiers, power amplifiers, radio-frequency amplifiers, phase-locked loops, and frequency filters are also suitable for use in connection with heat sink  100 . Thermally conductive layer  105  is formed on die  103  and fabricated from a thermally conductive gel, paste, tape or other thermally conductive material. When heat sink  100  is affixed to thermally conductive layer  105 , thermally conductive layer  105  provides a thermal path to heat sink  100  from integrated circuit die  103 . In operation, heat sink  100  dissipates heat produced in integrated circuit die  103 . 
     Example embodiments of the present invention, as described above, are effective in dissipating heat from an integrated circuit directly coupled to heat sink  100 . However, the present invention is not limited to such embodiments. The present invention is also effective in dissipating heat from a packaged die.  FIG. 1B  is a perspective view of example embodiments of the present invention showing heat sink  100  thermally coupled to integrated circuit package  107 . Heat sink  100  is coupled to package  107  by the same process used to couple heat sink  100  directly to integrated circuit die  103  shown in  FIG. 1A . Thermally conductive layer  105  is formed on package  107  and heat sink  100  is affixed to thermally conductive layer  105 . The heat flow from packaged integrated circuit die  103  is coupled to package  107  by the ambient air surrounding die  103  in package  107  or is directly coupled to package  107  by a heat spreader. Heat sink  100  includes a number of structures and materials that participate in the dissipation of the heat produced by integrated circuit die  103 . These structures and materials are described in more detail below. 
       FIG. 2  is a cross-sectional side view of example embodiments of heat sink  100 . Heat sink  100  comprises body  201  including a number of fins  203  and cavity  205  including cavity surfaces  207 - 209 , phase change material  211  encapsulated in cavity  205 , and a number of particles  213  intermixed with phase change material  211 . 
     Body  201  is fabricated from a thermally conductive material, preferably a metal, such as copper or aluminum or an alloy of copper or aluminum. External surface  215  of body  201  is a substantially flat surface suitable for thermally coupling to a surface of integrated circuit die  103  as shown in  FIG. 1A  or package  107  shown in  FIG. 1B . The flat surface is formed on heat sink  100  by machining or other material shaping process. External surface  215  has a footprint that is preferably significantly larger than the surface area of integrated circuit die  103  or package  107 . The remaining surface area of body  201  preferably provides a large area for transferring heat from body  201  to the surrounding ambient environment. In one embodiment, a number of fins  203  are formed on the remaining surface area of body  201  by machining or other material shaping process. Alternatively, the number of fins  203  are fabricated as a separate unit and attached to the outer surface of heat sink  100  by brazing, soldering or welding. After machining or attachment, fins  203  provide a large surface area for transferring heat to the surrounding ambient environment. The dissipation of heat in phase change material  211  allows the fins  203  of the present invention to have a height that is about 10% to 20% less than the height of fins fabricated on a comparable heat sink intended for use in a forced air cooling system. This creates a lower profile package which is useful in cooling circuits in a variety of devices, such as mobile phones and personal digital assistants. Alternatively, if the height of fins  203  is not reduced, then cooling using heat sink  100  is greater than cooling produced by a standard size package. 
     Cavity  205  is included in body  201  to hold phase change material  211 . Cavity  205  has a volume that is sufficient to allow convection currents  217  to arise in phase change material  211  during the operation of integrated circuit  103 . Convection currents  217  assist in cooling integrated circuit die  103 . Cavity  205  is positioned in body  201  to enhance the heat transfer from phase change material  211  to the number of fins  203 . Thus, cavity  205  is generally centered with respect to the number of fins  203  located on the side surfaces of body  201 . The thickness of the wall between cavity surfaces  207  and  209  and the number of fins  203  is preferably made as thin as possible without compromising the structural integrity of heat sink  100 . 
     In one embodiment, body  201  is fabricated from a pair of symmetrical structures  100 A,  100 B. Each structure includes a cavity having a volume equal to one-half of the volume of cavity  205 . Coupling the pair of symmetrical structures  100 A,  100 B together forms heat sink  100  including cavity  205  formed in the interior of heat sink  100 . Coupling is accomplished by brazing, welding, soldering, or any other suitable metal fusing process. 
     In an alternate embodiment, a bottom section of body  201  is formed having cavity  205  including cavity surfaces  208  and  209 . A top section of body  201  is formed having surface  207 . Finally, the top section of body  201  is coupled along the dashed lines  219  and  221  to the bottom section of body  201  to form heat sink  100  including body  201  having cavity  205 . Coupling is accomplished by brazing, welding, soldering, or any other suitable metal fusing process. 
     Cavity surfaces  207 - 209  are shaped to enhance the formation of convection currents  217  in phase change material  211  during the operation of integrated circuit  103 . Cavity surface  208  slopes upward toward cavity surface  209  from low area  223  located near the center of surface  208 . Low area  223  is preferably positioned in body  201  such that when body  201  is coupled to integrated circuit die  103 , low area  223  is positioned directly above the hot spot of integrated circuit die  103 . Such positioning of low area  223  enhances the flow of convection currents  217  in phase change material  211 . For most integrated circuits, the hot spot is located approximately in the center of the die. The shape of low area  223  is not limited to a particular shape. In some embodiments, low area  223  is a point or a small flat rectangular area and surface  208  has the shape of a pyramid or a flat top pyramid. In alternate embodiments, low area  223  is an approximately circular area or a point and surface  208  has the shape of a flat top cone or a cone. In still another alternate embodiment, low area  223  is a wedge running the length of cavity  201 . 
     Cavity surface  207  also contributes to enhancing the flow of convection currents  217  in cavity  205 . Cavity surface  207  slopes upward toward cavity surface  209  from second low area  225  located near the center of surface  207 . Second low area  225  is preferably positioned in body  205  such that when body  205  is coupled to integrated circuit die  103 , second low area  225  is positioned directly above the hot spot of integrated circuit die  103 . Such positioning of second low area  225  enhances the flow of convection currents  217  in phase change material  211  as described below. 
     The shape of second low area  225  is not limited to a particular shape. In some embodiments, second low area  225  is a point or a small flat rectangular area and surface  207  has the shape of a pyramid or a flat top pyramid. In alternate embodiments, second low area  225  is an approximately circular area or a point and surface  207  has the shape of a flat top cone or a cone. In still another alternate embodiment, second low area  225  is a wedge running along the length of cavity  205 . 
     Cavity surface  209  forms an outer wall for cavity  205 . As convection currents  217  are induced by heating at surface  215  and flow up from low area  223 , the convection currents  217  are split at second lower area  225  and routed toward cavity surface  209 . Cavity surface  209  directs convection currents  217  down and along surface  208  toward low area  223 . In one embodiment, cavity surface  209  comprises a number of substantially flat surfaces. In an alternate embodiment, cavity surface  209  comprises a substantially curved surface. 
     Phase change material  211  in the solid phase only partially fills cavity  205 . The unfilled portion of cavity  205  provides room for expansion of phase change material  211  during the operation of integrated circuit  103 . Heat sink  100 , including phase change material  211 , exhibits superior heat dissipation properties when compared with a heat sink that does not include phase change material  211 . Phase change material  211  absorbs heat and provides conductive cooling during the power up cycle of integrated circuit  100 . Phase change material  211  is especially effective in cooling local hot spots and thermal transients that occur on die  103  during the power up cycle. In addition, phase change material  211 , by circulating in a liquid state, provides convective cooling during the steady state operation of integrated circuit  103 . In one embodiment, phase change material  211  comprises TH58. A phase change material designated as TH58 has a melting temperature of about 58 degrees centigrade, an average density of about 1500 kg/m 3 , and a latent heat of between about 175 kJ/kg and about 225 kJ/kg. Phase change materials suitable for use in connection with the present invention include hydrated salts, eutectic salts and paraffins. Other phase change materials may also be suitable for use in connection with the present invention. 
     A number of particles  213  are intermixed with phase change material  211 . The number of particles  213  enhance fluid mixing during the heating of phase change material  211  without relying on moving parts. By not relying on moving parts, heat sink  100  is highly reliable. Enhancing the fluid mixing inside cavity  205  increases the ability of heat sink  100  to dissipate heat through convection in phase change material  211 . Each particle in the number of particles  213  preferably has a density about equal to the density of phase change material  211 . Each particle in the number of particles  213  also preferably has an approximately spherical shape. In one embodiment, the number of particles  213  are solid SiO 2  spheres. In an alternate embodiment, the number of particles  213  are hollow SiO 2  spheres. In still another alternate embodiment, the number of particles  213  are particles of sand. The quantity of particles intermixed with phase change material  211  is selected to be large enough to enhance the convective cooling in phase change material  211 , but the number of particles  213  should not be so large as to significantly decrease the conductive cooling provided during a phase change of phase change material  211 . 
     The number of particles  213  also provide a secondary thermal dissipation effect. As the number of particles assist in mixing phase change material  211 , they move through areas of cavity  205  having different temperatures. As the number of particles  213  move through areas having a lower temperature, phase change material  211  congeals on the number of particles  213 . The congealing of phase change material  211  on the number of particles  213  increases the weight of the particles and causes them to fall to warmer surface  208 . After falling to warmer surface  208 , the number of particles  213  are recirculated within cavity  205 . 
     Efficient manufacturing of heat sink  100  is achieved by maintaining symmetry in the design of heat sink  100 . Symmetry is maintained by arranging the number of fins  203  and cavity  205  symmetrically about a dividing line  101  that separates heat sink  100  into two substantially identical halves  100 A,  100 B. Since the halves  100 A,  100 B are identical, a machining process for cutting the number of fins  203  and cavity  205  is developed for producing only one half. After two halves  100 A,  100 B are fabricated, the halves  100 A,  100 B are joined by a brazing, welding, soldering or other suitable metal fusing process to form body  201 . Injection hole  227  is drilled in body  201  and a mixture of phase change material  211  and a number of particles  215  is injected into cavity  205 . The injection hole is sealed, and the fabrication of heat sink  100  is complete. The volume of the injected mixture, if injected at a temperature less than the operating temperature of heat sink  100 , does not completely fill cavity  205 . Some volume in cavity  205  is reserved for expansion of phase change material  211  during the operation of heat sink  100 . 
       FIG. 3  is a perspective view of example embodiments of the present invention showing heat sink  100  thermally coupled to integrated circuit  301  included in wireless communication device  303 . In one embodiment, wireless communication device  303  is a cell phone, In an alternate embodiment, wireless communication device  303  is a personal digital assistant. Heat sink  100  is capable of cooling integrated circuit  301  in wireless communication device  303  without forced air. Integrated circuit  301 , in one embodiment, is a processor. In an alternate embodiment integrated circuit  301  is a digital signal processor. Portable devices, such as wireless communication device  303  produce a significant amount of thermal energy and heat sink  100  is also capable of dissipating the heat without increasing the height of the heat sink, which permits wireless communication device  303  to have a low profile package. 
     Heat sink  100  is not limited to use in connection with wireless devices. Since heat sink  100  is more efficient at dissipating thermal energy than a standard heat sink, heat sink  100  is especially useful for dissipating thermal energy in integrated circuits used in computing devices, such as laptop computers, servers, and engineering workstations, that generate a large amount of thermal energy. 
     A heat sink and a method for fabricating and using the heat sink has been described. The heat sink is thermally coupled to an integrated circuit die. The heat sink includes a cavity containing a phase change material and a number of particles for enhancing convection in the heat sink during the cooling of the integrated circuit die. The heat sink is highly reliable and exhibits enhanced cooling characteristics when compared to a heat sink that does not include a cavity containing a phase change material and a number of particles. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.