Abstract:
A planar heat sink, using heat pipe principals, is constructed by encapsulating a metalized heat fugitive plastic mandrel in a copper electroform bath and removing the plastic mandrel. The heat pipe chamber of the heat sink is constructed with a plurality of cruciform shaped vanes, wicking structures, for improved wetting and to prevent the formation of droplets. The plastic mandrel is injection molded having opposing negative front and back panels containing negative vanes. The core and cavity for the injection mold tool are formed by electroforming a machined aluminum plate which is etched by laser with the vane pattern.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Microprocessor chips continue to be manufactured with increased capacity and speed. These chips contain densely packed microcircuits demanding increased power consumption. The engineering and use of such chips have reached a point where the dissipation of heat from these units is a limiting factor. A need has, therefore, arisen for heat dissipating modules with greater efficiencies of operation. One promising approach is the adaptation of heat pipe concepts for the purpose of heat removal from microprocessor chips.  
           [0002]    A heat pipe comprises a closed system of continuous evaporation and condensation of an operating fluid such as methyl alcohol (methanol). The heat pipe is positioned with one end near a source of heat and the other at a cooler ambient temperature. The methanol coats the internal surfaces of the heat pipe and, as the temperature differential increases between the ends of the heat pipe, the methanol at the hotter end vaporizes. The methane vapor flows towards the cooler end where it condenses and releases its heat of vaporization. In this manner a current of liquid is set up on the walls of the heat pipe towards the hotter end with the gas passing to the cooler end via a central channel. This circulation provides an efficient mechanism for removing heat.  
           [0003]    In order to take advantage of this process in a device for dissipating heat from miniature components such as semiconductor chips, there is a requirement for a cost effective method of constructing heat sinks which will use this heat pipe cooling effect. An effort to develop such heat pipes was conducted at Sandia Laboratories using photolithographic etching of pure silicon wafers to construct a heat pipe having wicking structures in the heat pipe channel. The wicking structures are in the form of cruciform shaped vanes extending into the heat pipe channel to encourage wetting of the interior surfaces of the heat pipe with the methanol and eliminate the formation of isolated droplets. This will facilitate the capillary action required by the heat pipe heat transfer process.  
           [0004]    It is the object of this invention to construct a heat sink suitable for dissipating heat from semiconductor chips and other devices in which the heat sink uses the heat transfer mechanism of a heat pipe. In particular it is an object of this invention to construct a heat pipe having a planar orientation for adaptation into environments having limited space.  
         SUMMARY OF THE INVENTION  
         [0005]    A miniature heat sink is designed to dissipate heat from a semiconductor chip or other small device. The heat sink utilizes the heat transfer capabilities of a heat pipe. Flat walls are joined to form a planar heat pipe having an interior heat transfer chamber in which a heat transfer medium such as methanol is inserted. Small vanes, in the range of a tenth of a millimeter to a millimeter in size, are constructed on the interior surfaces of the chamber to encourage capillary action and minimize beading of the fluid medium. The vanes are formed by electoforming over a metalized molded plastic mandrel. The heat sink comprises a pair of flat walls formed about the mandrel and joined about their periphery to enclose a internal heat transfer chamber. The chamber is evacuated and partially filled with a liquid, such as methanol, which has a high latent heat of vaporization. One end of the heat sink is provided with external fins extending outward from the heat sink to promote the dissipation of heat. Another end of the heat sink is constructed with provision for engagement with the component with which it is designed to engage and cool. As is well known, the methanol will evaporate as the heat of the component rises. The methanol vapor will travel towards the cool end through the chamber where it condenses into a liquid. The liquid travels by capillary action along the interior surfaces of the opposing walls of the heat sink. In this manner an effective flow of liquid and gaseous methanol is set up in which the fluid medium absorbs heat at the warm end of the heat sink and dissipates it at the cool end for as long as there is a temperature differential sufficient to initiate the flow. The cruciform shaped vanes which extend into the interior of the chamber effectively eliminate the formation of droplets of liquid on the walls which will hamper effective flow along the wall surface.  
           [0006]    In order to manufacture large production quantities of the heat sinks of this invention at a reasonable price, it is necessary to produce low cost tools for electroforming the heat sinks. A plastic mandrel is constructed by injection molding a planar tool having appropriate vane cavities formed on both sides. The plastic mandrel is then metalized to allow the deposit of copper through electroforming.  
           [0007]    Since the plastic mandrel is formed by injection molding, appropriate tooling must first be constructed for the injection molding process. To build the tools for forming the plastic mandrel, a tool of sheet aluminum is machined to the desired dimensions and laser marked with a series of mesoscale cruciform shaped vanes. This forms a negative element onto which a hard nickel layer is electroformed. The nickel layer is removed from the aluminum mandrel and backed with a structural material, for example an aluminum filled epoxy. This step generates a positive mold element for one side of the injection molded plastic mandrel.  
           [0008]    The opposite side of the plastic mandrel requires a separate tool in which the vanes are offset from the vanes of the first side. Again a tool is constructed starting with a machined aluminum sheet laser etched with an offset pattern of vanes. The etched aluminum sheet is coated with nickel to form the mating mold element for forming the plastic mandrel. The two mold elements thus formed are assembled in a mold fixture for use in an injection molding process.  
           [0009]    The resulting molded part is the plastic mandrel which is metalized in preparation for the final electroforming step. The metalized plastic mandrel is placed in an electroforming bath to allow its encapsulation by copper to form the heat sink. An appropriate sprue remains uncoated to allow the removal of the plastic mandrel, evacuation of the chamber formed thereby, and the insertion of an operational amount of a working fluid. In this manner a heat pipe style heat sink is constructed in an efficient and cost effective manner for a variety of applications. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention is described in more detail below with reference to the attached drawing in which:  
         [0011]    [0011]FIG. 1 is a side view of the heat sink of this invention;  
         [0012]    [0012]FIG. 2 a  is a sectional view of the heat sink along section lines  2 - 2  in FIG. 1;  
         [0013]    [0013]FIG. 2 b  is an enlarged view of a portion of the heat sink shown in FIG. 2 a;    
         [0014]    [0014]FIG. 3 a  is an enlarged view showing the offset of the vanes of the heat sink of this invention;  
         [0015]    [0015]FIG. 3 b  is an enlarged view showing the dimensions of the vanes of the heat sink of this invention;  
         [0016]    [0016]FIG. 4 a  partial view of the aluminum mandrels constructed in the process of this invention;  
         [0017]    [0017]FIG. 4 b  is partial view of the injection molding tool constructed in the process of this invention;  
         [0018]    [0018]FIG. 5 a sectional view of a portion of the injection mold constructed in the process of this invention;  
         [0019]    [0019]FIG. 6 is a perspective view of the plastic mandrel constructed in the process of this invention; and  
         [0020]    [0020]FIG. 7 is a block diagram of the steps of the method of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    A heat sink  1  of this invention is shown in FIG. 1 in direct heat conductive association with a semiconductor chip  2 . The heat sink  1  includes fins  3  to assist in radiating heat away from the heat sink  1 . A nipple  4  is provided to allow access to the interior of the heat sink  1  and the insertion of a working fluid  6 . In FIG. 2 a , an internal chamber  5  is shown defined by joined walls  9  and  10 . A working fluid  6 , such as methanol, is inserted into the chamber  5  which is then sealed. The working fluid  6  wets the interior surface of the chamber  5  and is distributed over such surfaces by capillary action. In the preferred embodiment the walls  9  and  10  of the heat sink are constructed as flat panels of thin flexible heat conductive metal, such as copper. The finished heat sink will be planar in shape and flexible.  
         [0022]    In operation the heat sink  1  is subjected to heat generated during the operation of semiconductor chip  2 . As the temperature differential between the ends of the heat sink  1  increases, an amount of methanol begins to evaporate at the high temperature end. The methanol vapor  6  migrates towards the cooler end, as shown by dotted arrows  8  in FIGS. 2 a  and  2   b . As the methanol vapor cools, it condenses and flows down the walls  9  and  10  of the heat sink  1 , as shown by the arrows  7  in FIGS. 2 a  and  2   b . In this manner, the heat sink operates as a heat pipe with all the heat dissipation advantages of such devices. The working fluid  6  absorbs its heat of vaporization at the heated end and releases it at the cooling end.  
         [0023]    The efficient operation of the heat pipe depends on a consistent flow of fluid along the interior of walls  9  and  10 . To insure this function, wicking vanes  11  and  12  are dispersed in offset patterns over the interior surfaces of walls  9  and  10  respectively. The wicking vanes  11  and  12  create a tortuous path for the fluid on the walls  9  and  10  and effectively prevent the formation of droplets which significantly impede the fluid flow and hinder the operation of the heat sink  1  as a heat pipe.  
         [0024]    It has been found through research conducted at Sandia Laboratory that wicking structures formed in the shape of a cruciform, as shown in FIGS. 3 a  and  3   b , and having a depth (d) of from 8 to 10 microns, a width (w) of around 20 microns and a length (l) of approximately 100 microns are particularly effective for this purpose. The work at Sandia, however, stopped short of a cost effective method of manufacturing these heat dissipating devices.  
         [0025]    In the method of this invention a combination of electroforming and injection molding processes is used. The object of this method is to generate cost effective tools from which the heat sink can be economically constructed by electroforming. For this purpose a disposable plastic mandrel  13  is formed by injection molding. To achieve the pattern of outward extending vanes in the electroformed heat sink, the plastic mandrel  13  must be a negative tool.  
         [0026]    Initially a set of injection molding tools is constructed by a first electroforming process. The tool for the injection molding of the disposable plastic mandrel therefore begins with the construction of a first electroform mandrel. As shown in FIG. 4 a , an aluminum sheet  14  is machined to the desired size and laser etched with a pattern of multiple cavities  15  in the shape of cruciform vanes. Electroform mandrel  16 , formed in this manner is subjected to the deposition of a nickel layer in an electroform bath.  
         [0027]    Since walls  9  and  10  are constructed with differing patterns, a mandrel  16   a  is constructed in the same manner with a pattern of cavities  15   a , laser etched into an aluminum sheet  14   a , as shown in FIG. 4 a . The patterns are offset a predetermined distance to eventually generate the sequence of opposing patterns of wicking structures, as illustrated in FIG. 3 a.    
         [0028]    The mandrels  16  and  16   a  are placed in an electroforming bath in which a layer of nickel  17  is deposited which will eventually form the active surface of the injection molding tools. The layer of nickel  17  is removed from mandrels and made rigid by the application of an epoxy impregnated with aluminum. In this manner injection molding tools  18  and  18   a  are formed having nickel active surfaces  17  and  17   a  and structural backing  19  and  19   a.    
         [0029]    To form the plastic mandrel  13 , the tools  18  and  18   a  are arranged in a fixture  20  having all of the components required for injection molding. A plastic material such as low density polyethylene is injected into the assembled mold to construct plastic mandrel  13 . In this manner large numbers of plastic mandrels may be constructed on a production basis at minimal cost.  
         [0030]    Plastic mandrel  13  is shown in FIG. 6 and is constructed with a tab  21  to assist in subsequent steps and to insure a means of entry into the internal chamber  5  of the heat sink  1 . Consistent with the molding process and the tools  18  and  18   a , plastic mandrel  13  will have a pattern of cavities (negative) in the shape of tiny cruciform vanes. These patterns are molded on both sides  22  and  23  of the mandrel  13  slightly offset to create the labyrinth type path for the fluid as it flows, through capillary action, from the cool end of the heat sink to the warmer end. To prepare the plastic mandrel  13  for the electroform bath, the mandrel must be metalized. This can be accomplished on a batch basis by dipping the mandrels in a silver nitrate solution and subsequently in a reducing agent. A coating of silver nitrate, approximately 1 micrometer in thickness, is applied.  
         [0031]    As illustrated in the block diagram of FIG. 7, the method of this invention, involves a somewhat convoluted, but effective, combination of steps to generate a series of mandrels and tools to construct the heat sink  1 . With the end product being the interior chamber  5 , the process begins with negative aluminum mandrels  16  and  16   a . Using the mandrels  16  and  16   a , injection mold core and cavity  18  and  18   a  are formed by a first electroforming process. The first electroforming process coats mandrels  16  and  16   a  with active surface layers  17  and  17   a . The active surface layers are removed from the mandrels  16  and  16   a  and are structurally backed with aluminum filled epoxy layers  19  and  19   a.  At this point, the injection molding tools  18  and  18   a  are positive representations of the final product. After assembling the tools  18  and  18   a  in a mold fixture, heat fugitive plastic mandrel  13  may be produced in quantity and present a negative active surface for the final electroforming step.  
         [0032]    To complete the process, a batch of mandrels  13  are placed in a continuous copper electroforming bath to encapsulate the mandrel in a copper coating. The electroforming process is maintained for sufficient time to allow the deposition of a copper coating of between 0.015 to 0.030 inches, approximately 10 to 20 hours. A portion of the tab  21  is masked to provide an entry to the interior of the heat sink  1 . Since the mandrel  13  is plastic, it is readily removed from the electroformed heat sink structure by subjecting the heat sink to further heat. This may be accomplished by placing the assembly in a hydrogen reduction furnace and raising the temperature to 500° C. A copper heat sink is thus formed having walls which define an interior heat transfer chamber into which liquid methanol is injected by means of a syringe or other device. Other working fluids which may be used are ethanol and isopropyl alcohol. After the chamber is sealed the heat sink is complete and will function as a planar heat pipe.  
         [0033]    Although the invention is described for use with semiconductor components, it will be adaptable to many different uses. The planar shape of the resulting heat sink and its method of manufacture will allow the generation of a line of thin flat heat sinks which will be flexible in the thicknesses obtainable. This is especially true of heat sinks made of copper according to the method of this invention. The heat sink of this invention is essentially a low cost, flexible heat plate having the operational characteristics of a heat pipe and will have many different uses, for example, thermal camouflage, clothing, electronic systems cooling, among others.  
         [0034]    In this manner large production quantities of the thin, planar, flexible heat sinks, which employ heat pipe principals, can be manufactured in an inexpensive may.