Abstract:
The subject invention provides a method of manufacturing an all-metal cold plate heat exchanger assembly for removing excess heat from a heat generating electronic component. The cold plate assembly includes a base plate having micro-channels and associated micro-fins, a manifold plate having alternating channels, and a manifold cover, wherein the manifold plate cooperates with the micro-channels to form an engineered pathway for coolant flow. This method assures a tight interface sealing between the contact surfaces of the alternating channels of the manifold plate and the coplanar surfaces of the micro-fins that would prevent coolant by-pass flow, but without clogging or jeopardizing the flow through the micro-channels. This method utilizes a layer of recast metallic particulates, which is a natural by-product of laser machining, as a compliant gasket material between the coplanar edges of the alternating channels and the coplanar edges of the micro-fins.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    This invention relates to a method of manufacturing an all-metal cold plate heat exchanger assembly. 
       BACKGROUND OF INVENTION 
       [0002]    Forced air-cooling is typically used to remove excess heat generated by computing processing units (CPUs); however, forced air-cooling alone is no longer sufficient to meet the cooling needs of increasingly faster and hotter CPUs. An alternate to forced air-cooling is a recirculating closed-loop liquid cooling system. Recirculating closed-loop liquid cooling systems are known for their higher efficiency and capacity for excess heat removal. 
         [0003]    A schematic of a typical recirculating closed-loop liquid cooling system for cooling a heat generating electronic component is shown in  FIG. 1 . Illustrated is a recirculating closed-loop liquid cooling system  10  that includes a reservoir tank  15 , a coolant pump  22 , a cold plate heat exchanger assembly  25 , a radiator  35 , and a fan  40 . The cold plate heat exchanger assembly  25  is in thermal conductive contact with the heat generating electronic component  30 . It is understood that the heat generating electronic component  30  can represent a CPU. 
         [0004]    The coolant pump  22  transfers a liquid coolant from the reservoir tank  15  to the cold plate heat exchanger assembly  25 . Within the cold plate heat exchanger assembly  25  are engineered flow channels, which provide a tortuous path for flow of liquid coolant in order to optimize heat transfer from the heat generating electronic component  30  to the coolant. After exiting the cold plate heat exchanger assembly  25 , the heated coolant continues to the radiator  35  where the heat is released to the ambient air by convection with the aid of a fan  40  blowing a stream of cooler air across the radiator  35 . The cooled coolant then returns to the reservoir tank  15  to repeat the heat transfer process. 
         [0005]    U.S. patent application Ser. No. 11/221,526 discloses an all-metal cold plate heat exchanger assembly with engineered flow channels. The disclosed cold plate assembly includes a base plate of copper having a flat exterior surface that is adapted to thermally bond to a heat generating electronic component. Located on the interior surface of the copper base plate are a series of micro-channels and associated micro-fins with coplanar edges. The base plate is assembled to a manifold cover and the interior surface of the manifold cover has manifold channels with corresponding co-planer surfaces that cooperate with the smaller coplanar edges of the micro-fins to define a path for coolant flow. 
         [0006]    When the manifold cover is engaged to the base plate, the coplanar surfaces of the manifold channel are in intimate contact with the coplanar edges of the micro-fins of the base plate forming a checkerboard pattern for fluid flow, providing more effective and efficient heat extraction. The contact between the coplanar surfaces of the manifold channels and the coplanar edges of the micro-fins must be sufficiently tight, to prevent flow from bypassing the manifold channels, which would impair the regularity of the flow pattern and result in reduced heat transfer efficiency. 
         [0007]    One known method of manufacturing a cold plate assembly having good sealing characteristics between the contact surfaces of the manifold channels and the micro-fins is to utilize materials such as solder, braze cladding, or adhesives to provide a gasket material between the coplanar surfaces of the manifold channels and the coplanar edges of the micro-fins. A drawback for such materials is the tendency during the assembly process for such materials to seep into and clog the micro-channels. 
         [0008]    Another known method of manufacturing a cold plate assembly having good sealing characteristics between the contact surfaces of the manifold channels and the coplanar edges of the micro-fins is resistance welding, which is disclosed in U.S. patent application Ser. No. 11/221,526. The drawbacks to resistance welding are the cost of materials and complexity of the manufacturing operation. Resistance welding requires the use of highly pure, oxygen free copper that is both electronically and thermally conductive. In addition to the material requirements, the joining surfaces of the cold plate assembly require precision machining to exact specifications. 
         [0009]    There exists a need for an economical method of manufacturing an all-metal micro-channel cold plate heat exchanger assembly that assures a tight interface seal between the contact surfaces of the manifold channels and the micro-fins without clogging the micro-channels. 
       SUMMARY OF THE INVENTION 
       [0010]    The subject invention provides a method of manufacturing an all-metal cold plate heat exchanger assembly for removing excess heat from a heat generating electronic component. The method assures a tight interface seal between the contact surfaces of the alternating channels of the manifold plate and the coplanar edges of the micro-fins which prevents coolant by-pass flow, but does not clog or jeopardize the coolant flow through the micro-channels, and is economical to manufacture and assemble. This method utilizes recast metallic particulates, a natural by-product of laser machining, as a compliant gasket material between the coplanar surfaces of the alternating channels and the coplanar edges of the micro-fins. 
         [0011]    The method includes providing a base plate formed of a material suitable for laser machining, preferably copper. A beam of laser energy is provided to machine alternating substantially parallel micro-channels and associated micro-fins into the interior surface of the copper base plate. During the laser machining process, the laser-machined surface of the copper base plate is vaporized into microscopic aerosol particulates, which are then cooled and condensed onto the coplanar edges of the micro-fin to form a layer of recast metal. The recast layer is microscopic and cannot be seen without magnification. 
         [0012]    A manifold cover and a manifold plate, which may be integrated with the interior surface of the manifold cover, are provided. The manifold plate has alternating inlet-outlet channels with at least one face having coplanar edges. The manifold cover is arranged onto the base plate with the manifold plate in between, such that one face of inlet-outlet channel coplanar edges is disposed adjacent to the micro-fin edges with the recast layer in between. The manifold cover is then pressed toward the base plate to compress the recast layer to form a compliant gasket between the contact surfaces of the alternating inlet-outlet channels and micro-fins. The exterior joining surfaces of the base plate and manifold cover are hermetically sealed. 
         [0013]    One advantage of the present invention is that it utilizes the vapor-deposited copper particulate that is a natural by-product of the laser machining process used in the formation of the micro-channels to form a compliant gasket that facilitates the intimate contact between the micro-channels and the manifold channels, thus preventing flow by-pass and increasing thermal performance. 
         [0014]    Another advantage of the present invention is the elimination of the need to remove the recast layer prior to assembly; this, simplifies the manufacturing process and reduces cost. 
         [0015]    Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of the preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]    This invention will be further described with reference to the accompanying drawings in which: 
           [0017]      FIG. 1  is a schematic diagram of a closed-loop liquid cooling system for removing heat from a heat producing electronic component. 
           [0018]      FIG. 2  is an exploded perspective view of the various components of a cold plate heat exchanger assembly. 
           [0019]      FIG. 3  is a perspective view of the bottom of the manifold cover with an integrated manifold plate. 
           [0020]      FIG. 4  is an enlarged perspective view of a portion of the bottom surfaces of the manifold plate&#39;s channels crossing the top edges of the micro-fins. 
           [0021]      FIG. 5  is a schematic view of the flow pattern enabled by the completed cooling assembly. 
           [0022]      FIG. 6  is a cross section of the base cold plate as micro-channels are being machined onto the interior surface. 
           [0023]      FIG. 7  is an enlargement of detail circle shown in  FIG. 6  showing a layer of recast metallic particulates on the coplanar edges of the micro-fins. 
           [0024]      FIG. 8  is a portion of the cold plate having a layer of recast metallic particulate materials on the coplanar edges of the micro-fins as the manifold cover is pressed onto base plate. 
           [0025]      FIG. 9  is an enlargement of detail circle shown in  FIG. 8  showing the recast metal deforming and forming a compliant gasket between the top edges of the micro-fins and bottom surfaces of the manifold channels. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0026]    In accordance with a preferred embodiment of this invention, referring to  FIGS. 1 through 9 , shown is an all-metal micro-channel cold plate assembly. The cold plate assembly utilizes what is normally an undesirable by-product of laser machining, a layer of recast metallic particulates, as a compliant gasket between the contact surfaces of the top edges of the base cold plate micro-fins and bottom surfaces of the manifold channels. 
         [0027]    In reference to  FIG. 2 , shown is an exploded view of a preferred embodiment of the current invention, which is indicated generally at  20 . The primary components of the cold plate assembly  20  are a base plate  100 , a manifold cover  200 , a manifold plate  300 , and inlet/outlet pipes  400 . Manifold plate  300  is shown as a separate component; however, manifold plate  300  may be manufactured as an integral part of the interior face  210  of manifold cover  200 . All components are manufactured of a heat conducting metal, preferably pure substantially oxygen free copper. The components are assembled into an integral unit where the jointing surfaces are hermetically sealed by methods known in the art for joining metallic components that include, but not limited to, brazing, welding, chemical bonding, or resistance welding. 
         [0028]    Base plate  100  is substantially circular in shape and is approximately 34 mm in outer diameter with a thickness of approximately 1.0 mm. Base plate  100  has a perimeter wall  110  that is approximately 3.0 mm in height demarcating an inner surface  120  from outer surface  130  of base plate  100 . Inner surface  120  and outer surface  130  are machined to a high degree of flatness. Outer surface  130  is adapted to be thermally bonded to a heat producing electronic component. Inner surface  120  is laser machined to form a fin-channel pattern  135  that includes a dense pattern of extremely thin structures, alternating micro-channels  140  and micro-fins  150 . These are shown in greater detail in  FIG. 4 . Fin-channel pattern  135  is substantially rectangular in outline and measures approximately 17 mm by 24 mm. In laser machining fin-channel pattern  135 , a beam of laser energy is used to cut the pattern of micro-channels  140  and micro-fins  150 . Shown in  FIGS. 6 and 7 , micro-channels  140  are approximately 350 to 450 microns deep and approximately 60 microns wide. Micro-fins  150  have substantially flat coplanar edges  160  that are approximately 50 microns in width. 
         [0029]    Manifold cover  200  is also substantially circular in shape having an outer diameter of approximately 42 mm and is approximately 2 mm thick. Manifold cover  200  is adapted to engage perimeter wall  110  of base plate  100  and form a hermetically sealed structure. Located between the inner surface  120  of base plate  100  and interior face  210  of manifold cover  200  is a manifold plate  300 , which may be formed as an integral part of interior face  210  of manifold cover  200  as shown in  FIG. 3 . Also attached to manifold cover  200  are inlet/outlet pipes  400 , which may be composed of the same materials as manifold cover  200  and base plate  100 . The joining surfaces of inlet/outlet pipe  400  are joined to manifold cover  200  by suitable methods such as welding or brazing. 
         [0030]    In reference to  FIGS. 3 and 4 , manifold plate  300  includes a sinuous wall  310  that forms a substantially rectangular pattern measuring approximately 14 by 21 mm, which is slightly smaller than fin-channel pattern  135 . The rectangular shape of manifold plate  300  is formed by the natural shape of sinuous wall  310  as it alternates forming a regular pattern of side-by-side alternating channels  320 . Sinuous wall  310  has opposing faces of coplanar surfaces  330  that are approximately 0.44 mm in width, and the width of the corresponding channel is approximately 0.50 mm. 
         [0031]    In reference to  FIG. 3 , one face of coplanar surfaces  330  may be integral with interior face  210  of manifold cover  200  while the opposing face of coplanar surfaces  330  is exposed. The exposed coplanar surfaces  330  are substantially parallel to coplanar edges  160  of micro-fins  150  on base plate  100  and are adapted to be positioned to engage coplanar edges  160  in a crossing pattern. The height of sinuous wall  310  is approximately 2.0 mm, which is substantially the same as the interior distance between inner surface  120  of base plate  100  and interior face  210  of manifold cover  200 , when assembled, to provide close mechanical contact between coplanar surface  330  of manifold plate  300  and coplanar edges  160  of micro-fins  150  without significant bending or deformation of micro-fins  150 . To prevent by-pass leaks created by less than solid contact between coplanar surfaces  330  and coplanar edges  160 , a compliant gasket is applied, which will be discussed herein below in detail. 
         [0032]    In reference to  FIG. 5A , once the manifold cover  200  is assembled to base plate  100  with manifold plate  300  there between, the coplanar surfaces  330  of manifold plate  300  are in intimate contact with coplanar edges  160  of micro-fins  150  forming a checkerboard flow pattern. Coolant enters into alternating channels  320  of manifold plate  300  indicated by “X”, then down into the micro-channels  140  to flow under and across coplanar surfaces  330 , and then exits up into adjacent alternating channels  320  of manifold plate  300 , indicated at ‘O’, towards the outlet. 
         [0033]    As discussed herein above, inner surface  120  of base plate  100  is machined to a high degree of flatness. The inner surface  120  is then laser machined to form a fin-channel pattern  135  that includes a dense pattern of extremely thin structures that include micro-channels  140  and micro-fins  150 , which have corresponding coplanar edges  160  that are substantially planer to inner surface  120 . In reference to  FIGS. 6 and 7 , the vaporized metallic particles from the laser machining process are controlled so as to condense and remain on the coplanar edges  160  of micro-fins  150  to be used as a compliant metallic gasket  510  as shown in  FIGS. 8 and 9 ; the method of which will be discussed herein below. 
         [0034]    The process of laser machining micro-channels onto a flat metallic surface is generally known in the art. As a high-powered laser is focused on a targeted material, the mass of the material is removed by vaporization caused by the intense heat generated by the laser beam at the point of contact. The amount of material removed is determined by the pulse duration, energy, and wavelength of the laser, as well as by the number of passes by the laser beam. These variables can be controlled according to the need of the material targeted to be removed. The desired setting of the laser should be adequate to remove the material by vaporization and not so high as to cause liquid ejection or phase explosion of the targeted material. 
         [0035]    In reference to  FIG. 6 , a beam of laser energy  520  from a laser device  560  is provided to cut substantially parallel micro-channels  140  into inner surface  120  of base plate  100  in a predefined fin-channel pattern  135 . When a pulse of laser energy  520  makes contact with inner surface  120 , the extremely high-energy input ablates the metallic base plate  100  at the point of contact by vaporizing the metallic mass into a plume  530  of aerosol microscopic particulate matter  540 . An inert blanket gas (not shown), such as nitrogen, is typically used to cover the surface of the work piece to prevent oxidation of the work piece and to cool plume  530 , thereby assisting in controlling the rate of precipitation of the microscopic particulate matter  540 . 
         [0036]    The blanket of inert gas cools plume  530  as it expands outward. The aerosol particulate matter  540  collides with each other and coalesces into larger particulates in the range of nanometers. The larger particulate matter condenses and settles onto the coplanar edges  160  of the micro-fins  150  forming a layer of recast metal. Contrary to the teachings of the prior art to use an acid solution to remove the recast layer, the recast layer is allowed to remain on top of the coplanar edges  160  of micro-fins  150 . Recast layer  500  consists of the same metallic material as base plate  100  and is at a consistency that is malleable enough to form a compliant gasket  510  between coplanar edges  160  of micro-fins and coplanar surfaces  330  of manifold plate  300 . 
         [0037]    As discussed herein above, it is critically important to insure intimate contact between the coplanar edges  160  of micro-fins  150  and coplanar surfaces  330  of manifold plate  300  to prevent fluid bypass. The recast layer  500  aids in the critical seal by deforming on a microscopic level to average out any inconsistencies in the micro-channel peak-to-peak dimensions and facilitates the required intimate contact. 
         [0038]    The height and percentage coverage of the recast layer can be controlled by varying the intensity of the laser, temperature of the inert cover gas, and duration of cut. It is preferable that the temperature is below the dew point of the vaporized metallic particulates. 
         [0039]    The height of the recast layer  500  is defined by the distance between the coplanar edges  160  of micro-fins  150 , which is substantially planar with inner surface  120  of base plate  100 , and plateau  550  of recast layer  500  shown as distance ‘X’ in  FIG. 7 . The ratio of coverage is defined by the width of coplanar edge  160  of micro-fin  150  that is occupied by recast layer  500 , indicated as distance ‘A’, divided by the total width of coplanar edge  160  shown as ‘B.’ The optimum height ‘X’ of the recast layer  500  on the micro-fins coplanar edges  160  is determined through laboratory analysis and testing to be approximately 30 to 64 microns; and the preferred ratio of coverage is 0.3 to 1.0. Any height greater than 64 microns may result in the excess recast layer  500  deforming into micro-channels  140  to obstruct coolant flow. 
         [0040]    Once recast layer  500  has been formed on coplanar edges  160  of micro-fins  150 , a manifold cover  200  having a manifold interior face  210  and manifold plate  300  is arranged over base plate  100 , with the channels  320  of manifold plate  300  substantially perpendicular to micro-channels  140  of base plate  100 . Manifold cover  200  is then positioned onto base plate  100  such that the coplanar surfaces  330  of manifold plate  300  are disposed adjacent to the micro-fin coplanar edges  160  with recast layer  500  in between. Manifold cover  200  is pressed toward base plate  100  to compress recast layer  500  to form a compliant gasket  510  in between. 
         [0041]    The exterior joining surfaces of the base plate  100  and manifold cover  200  are hermetically sealed by any of the known methods in the art; however, brazing is preferable. At temperatures favorable to brazing, recast layer  500  becomes more ductile and lends itself readily to act as a conformable layer to reduce fluid bypass. 
         [0042]    While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow. Dimensions are only presented to illustrate the diminutive size of cold plate heat exchanger assembly  20  and are not intended to be limiting. Those skilled in the art can adjust the dimensions of cold plate assembly  20  to accommodate specific heat transfer requirements. 
         [0043]    Furthermore, the function of cold plate assembly  20  has been described as removing excess heat from a heat generating electronic component; those skilled in the art can recognize that cold plate assembly  20  can also function by adding heat to a component by pumping preheated coolant through the cold plate assembly  20 . 
         [0044]    Still furthermore, cold plate assembly has been described as being all-metal. Those skilled in the art can substitute alternative materials other than metal for components that are not crucial to the making or using of the present invention. Therefore, it will be understood that it is not intended to limit the method of the invention to just the embodiment disclosed.