Patent Publication Number: US-6668912-B2

Title: Heat exchanger cast in metal matrix composite and method of making the same

Description:
This application is a divisional of application Ser. No. 09/721,724 filed Nov. 27 2000, now U.S. Pat. No. 6,460,598. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the dissipation of heat from high power electronic devices, and more particularly to the casting of an improved heat exchanger structure within a metal matrix composite. 
     BACKGROUND OF THE INVENTION 
     Heat dissipating plate and shell structures having desirable combinations of thermal conductivity, thermal expansion, and stiffness are in great demand for critical electronic applications. The heat generated in electronic components requires, in many cases, that heat sinks be used to remove the heat from the electronic device or component. Enhanced component reliability is achieved through use of heat sinks with good thermal conductivity to transfer heat away from high-density electronic components and devices such as integrated circuits. When heat sinks are not used, various other devices have been designed for direct attachment to a heat dissipating plate which, in turn, will act as a heat transfer element. 
     Moreover, the rapidly increasing density of integrated circuits, large-scale integrated circuits, power diodes and the like continues to create the need for improved thermal management solutions. Consequently, heat-dissipating structures have undergone continuous improvements as the electronics industry has developed. In these situations, it is often desirable to form internal cooling channels within a heat dissipating structure. Circulating fluids can then be used to more effectively remove heat from the heat-generating device mounted to the structure. This would allow for enhanced cooling efficiency and thermal stability in situations where the thermal conductivity of the heat dissipating structure alone is insufficient. 
     When the electronic components are attached to a heat dissipating structure, even one of the more advanced structures available today to provide good thermal conductivity, the different coefficients of thermal expansion for the various elements often cause problems. For example, the heat sink and the electrically insulating substrate, to which the electronic component is attached, may have different coefficients of thermal expansion. Similarly, different coefficients of thermal expansion may exist between the electronic component and the heat sink. When there is significant difference in coefficients of thermal expansion between components, temperature changes arising from soldering during assembly and heat generated in the systems during operation can cause large thermal stresses due to the differences in relative expansion of the materials. Such stresses may cause, in turn, premature component failure. Traditional thermal dissipation materials such as Al and Cu have CTE values that are much greater than IC devices or substrate materials. Therefore, the IC package typically includes a marginally compatible CTE interface, such as a high thermal resistivity Al 2 O 3  substrate, to provide a stress-reducing interface. The solution presents a thermal penalty and often a reliability penalty in the form of cracked Al 2 O 3  substrates. Traditional CTE-compatible packaging materials, such as Kovar and various other Ni—Fe alloys, have low thermal conductivity. More advanced packaging materials such as CuMo and CuW offer high thermal conductivity and compatible CTE values but are heavy and costly to fabricate in all but simple flat plate shapes. Aluminum Silicon Carbide (AlSiC) packages offer desirable packaging attributes of compatible CTE value and high thermal conductivity. An AlSiC heat dissipating structure is lightweight making it desirable for portable devices and airborne, spaceborne or any other weight sensitive application. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to a fluid flow heat exchanger cast in a metal matrix composite and method of making the same. The method comprises the steps of 1.) positioning an insert within a mold chamber of a closed mold 2.) filling the closed mold with a preform reinforcement material such that the insert is completely encased or “overmolded” within the preform material, and 3.) infiltrating the preform and insert with a liquid metal and allowing for the metal to solidify to form a metal matrix composite. The process of filling the closed mold containing the insert with preform reinforcement material allows for intimate contact between the insert and the reinforcement material. Enhanced thermal and mechanical properties are realized by eliminating voids or gaps at this reinforcement material-to-tube interface. In one embodiment, the insert comprises a stainless steel hollow core closed at both ends. After the metal infiltration process is complete the closed ends of the tubes are opened to allow for liquid fluid flow through the metal matrix composite. In one embodiment, the metal matrix composite encasing the insert is comprised of an Aluminum Silicon Carbide (AlSiC) housing offering a unique set of material properties that are suited to high performance advanced thermal management packaging designs. AlSiC is a composite material of Al-metal and SiC particulate. Thermal conductivity values for Al/SiC materials are similar to Al metal and the AlSiC CTE values are compatible with direct device attachment. Furthermore, AlSiC CTE can be designed to fit the requirements of the specific application. Changing of the Al/SiC ratio and/or the Al-metal composition can modify the CTE to be compatible with the CTE of the heat-generating device mounted thereon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawings, which illustrate an embodiment of the present invention: 
     FIG. 1 is a top view of the bottom  1  of the preform reinforcement mold cavity containing a tube structure  2  mounted on “cradles”  3  located therein. 
     FIG. 2 is a bottom view of the top  4  of the preform reinforcement mold cavity illustrating “hold downs”  5  located therein. 
     FIG. 3 is a top view of the bottom  6  of the infiltration mold cavity illustrating the preform reinforcement material with an insert contained therein. 
     FIG. 4 is a top view of the top  7  of the infiltration mold cavity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is directed to a highly efficient thermal management system utilizing liquid flow through cooling channels cast in a metal matrix composite. Referring now to the drawings, and more particularly to FIG. 1, an insert  2  is positioned on the bottom  1  of preform reinforcement mold cavity, resting on cradles  3  integrally connected to bottom  1  of the interior of the mold cavity. The cradles  3  are raised to allow for no contact between the insert and the bottom  1  of the mold cavity. The inserts can be in the form of complex tube structures or a closed box structure containing baffles or pin fins integrated therein. The insert material can be made of stainless steel, refractory metals, or other metals coated such as not to dissolve or corrode when in contact with molten metal. Inserts made of graphite, titanium and ceramic can also be utilized. The insert is CTE compatible with the metal matrix composite and is preferably thermally conductive. The insert is sealed at its ends prior to being positioned in the reinforcement mold cavity  1  to prevent it from being filled with preform reinforcement material. This also prevents the insert from being filled with molten metal during the metal infiltration process. The reinforcement mold cavity  1  is typically prepared from a metal cavity with the dimensions defined to produce a preform structure containing the insert. FIG. 2 illustrates the top  4  of the mold cavity with hold-downs  5  integrally connected thereto and positioned over cradles  3 . The hold-downs are raised to allow for no contact between the insert  2  and the top  4  of the mold cavity. When bottom  1  and top  4  of the mold cavity are joined together insert  2  is rigidly secured between cradles  3  below and hold-downs  5  above forming a closed cavity suitable for injection molding. Next, a preform reinforcement material such as Silicon Carbide (SiC) is injected into the closed mold. While SiC is the embodiment illustrated, other suitable reinforcement materials include Aluminum Nitride, Al 2 O 3 , Boron Nitride, Diamond, Molybdenum, tungsten and graphite powder. The preform reinforcement material is injection molded in a well-dispersed slurry consisting essentially of at least about 30 vol. % inorganic sinterable particulates, a non-aqueous liquid vehicle having a volume change on freezing of not greater than about +/−10%, and a dispersant. The process of forming the reinforcement material is described in U.S. Pat. No. 5,047,182, incorporated by reference. The preform comprises a microstructure designed to have a predetermined fraction of void volume or open structure that is to be subsequently filled with molten metal. The volume fraction of preform material is determined by its type and by the final CTE requirement of the finished metal matrix composite. For example, in the case of a SiC particulate preform infiltrated with molten aluminum, the volume fraction of SiC is in the range of 0.20 to 0.70 and is sufficient to obtain composite CTE values in the range of 6 to 13 or more ppm/degree Celsius when exposed to temperatures in the range of −50 to 150 degree celsius. In a structure having graphite fiber reinforcement, the volume fraction of 0.60 graphite fibers is sufficient enough to produce CTE values of less than 5 ppm/degree Celsius. The thermal conductivity of an aluminum matrix composite such as described above, will have a range of 160-200 Wm/K at room temperature. As the preform material is injected into the closed mold, the mold cavity is chilled to a mold temperature of less than the freezing temperature of the vehicle, as described in U.S. Pat. No. 5,047,182 incorporated by reference. The injected preform material comes into intimate contact with the surface of the insert thereby eliminating voids or gaps at the preform to insert interface. Subsequent to the preform material solidifying the material is demolded or removed from the closed mold. The preform containing the insert is next dried by the process of non-destructive evaporation/sublimination as described in U.S. Pat. No. 5,047,182 incorporated by reference. After drying, any crevices or openings created by the mold cavity cradles  3  or hold-downs  5  are backfilled with preform material. Next, the preform material containing the insert is placed into an infiltration mold cavity  6 . FIG. 3 illustrates a tube insert  2 , completely encased within a Silicon Carbide (SiC) preform reinforcement material, placed in the infiltration mold cavity  6 . FIG. 3 further illustrates the crevices or openings  3 A, which are backfilled with SiC reinforcement material. Infiltration mold cavity  6  and top  7  are joined together to form a closed mold cavity suitable for molten metal infiltration casting. The porous preform material surrounding the insert is next infiltrated with molten aluminum to form a dense hermetic metal matrix composite in the desired product shape geometry. While molten aluminum is the embodiment illustrated other suitable metals include aluminum alloys, copper, and magnesium. The liquid metal infiltration process is described in U.S. Pat. No. 3,547,180 incorporated by reference. After aluminum infiltration, the metal matrix composite containing the insert is demolded or removed from the closed mold. Next, the inserts are connected to the external environment by machining a hole at the inlet and outlet of the insert. Appropriate fittings are placed on the inlet and outlet to allow for a cooling liquid source connection. 
     It should be understood that the preceding is merely a detailed description of one embodiment of this invention and that numerous changes to the disclosed embodiment can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.