Abstract:
A fluid cooled electrical assembly that includes a metal box, having a bottom wall, side walls and a top wall. A set of straight-edged pins, each smaller than 3 mm across in widest dimension, extend down from the top wall and up from the bottom wall. Also, electrical components are mounted on top of the top wall and on bottom of the bottom wall.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 13/189,391, filed Jul. 22, 2011, which is a continuation-in-part of application Ser. No. 12/823,990, filed Jun. 25, 2010, now abandoned. 
    
    
     BACKGROUND 
     Liquid cooling of power electronics is an evolving science. One sort of cooling device used today is formed by casting or machining, by an end mill, an open metal box having protrusions rising from the floor. This box is then closed by welding on a lid, and is turned upside down so that electrical components can be attached to a surface supported by the bottom of what had been the floor. One problem with this technique is that the casting and milling techniques used do not permit the formation of a dense array of thin protrusions, which is most effective at transferring heat into a passing liquid. 
     In another prior art method, sintered copper is molded into a form having narrow pins extending from a planar portion. These pins are, by necessity round. Copper is softer and heavier than is ideally desirable, and rather expensive. A method of making a fluid cooled assembly having thin (less than 3 mm) pins of a harder, stronger and less expensive material is desirable. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
     In a first separate aspect, the present invention may take the form of a method of making a fluid cooled assembly that makes use of a work piece that forms a partial enclosure defining an interior void space and having a top wall having a top surface, and that has a bottom wall having a bottom surface, and that defines at least one top opening through the top wall to the void space and at least one bottom opening through the bottom wall to the void space. The work piece further defines fluid entrance and exit ports into the void space, the top wall and the bottom wall both being made of material that can be friction stir welded. The method also utilizes a top lid having a size and shape substantially conformal to the top opening, having a top surface that at least at the margins and which is formed of a material that can be friction stir welded to the work piece and a similarly constructed bottom lid. The top lid is placed into the top opening so that the top lid top surface is flush with the top surface of the work piece top wall and friction welding the top lid to the work piece and the bottom lid is placed into the bottom opening so that the bottom lid bottom surface is flush with the bottom surface of the work piece bottom wall and friction welding the bottom lid to the work piece. 
     In a second separate aspect, the present invention may take the form of a fluid cooled electrical assembly that includes a metal box, having a bottom wall, side walls and a top wall. A set of straight-edged pins, each smaller than 3 mm across in widest dimension, extend down from the top wall and up from the bottom wall. Also, electrical components are mounted on top of the top wall and on bottom of the bottom wall. 
     In a third separate aspect, the present invention may take the form of a fluid cooled electrical assembly that includes an aluminum alloy metal box, having a bottom wall, side walls and a top wall. A set of pins, each smaller than 3 mm across in widest dimension, extend down from the top wall and are unitary to and made of the same material as the top wall. Another set of similar pins extend up from the bottom wall and are unitary to and made of the same material as the bottom wall. Finally, electrical components are mounted on top of the top wall and on bottom of the bottom wall. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are illustrated in referenced drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
         FIG. 1  is a perspective view of a work piece that is incorporated into a liquid cooled electrical apparatus, according to the present invention. 
         FIG. 2  is a perspective view of a lid that is placed into the work piece of  FIG. 1 , to form a liquid cooled electrical apparatus, according to the present invention. 
         FIG. 3A  is a perspective view of a step in the manufacturing process of liquid cooled electrical apparatus of the present invention, showing the lids of  FIG. 2  positioned so as to be fit into the openings of the work piece of  FIG. 1 . 
         FIG. 3B  is a perspective view of a liquid cooled assembly of the present invention after the lids of  FIG. 2  have been fit in, but before any electrical components have been added. 
         FIG. 4  is a side view of a power module being placed on the work piece shown in  FIG. 3B , to form a final liquid cooled module. 
         FIG. 5  is an exploded perspective view of a two-sided cooling assembly, according to the present invention. 
         FIG. 6  is a sectional view of the cooling assembly of  FIG. 5 , taken along line  6 - 6  of  FIG. 5 . 
         FIG. 7  is a detail view of a portion of the assembly of  FIG. 5 , indicated by circle  7 . 
         FIG. 8  is a perspective view of an alternative embodiment of a two-sided cooling assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , in a preferred embodiment the construction of a liquid cooled assembly includes a work piece  10 , having an inflow port  12  and inflow channel  14  and an outflow channel  16  and outflow port  18 . In the finished assembly, to travel from channel  14  to channel  18 , fluid must flow through one of three flow cavities  20 . Each cavity has a pair of shelves  22  defined on either side. Cavities are separated by ribs  24 . 
     Referring to  FIG. 2 , a lid  40  for work piece  10  includes a planar portion  42 , which supports a number of downwardly extending pins  44 . In one preferred embodiment these pins are formed by sawing into a work piece, which begins as a solid blank of an aluminum alloy. In one embodiment, this sawing is done using ganged saw blades, although it could be done using a single saw blade. In one preferred embodiment pins that are rectangular in cross-section and that have a width of about 0.8 mm to 3 mm are created. 
     Referring to  FIGS. 3A and 3B , lids  40  are placed on shelves  22  and friction stir welded to work piece  10 , to cap the flow cavities  20 . The top of lids  40  and ribs of work piece  10  form a supporting surface for an electrical assembly needing cooling. 
       FIG. 4  shows an electrical power module  56  being placed onto work piece  10 , to form a final assembly  60 . The result is a robust metal structure having excellent cooling characteristics. Comparable prior art structures were formed by using an end mill to machine pins into the bottom of a work piece similar to  10 , and then flipping the assembly upside-down and placing the electrical assembly on this surface. The formation of the pins was hampered by the side walls of the work piece. But in the method of the present invention, the lids  40  do not have comparable side walls, and the metal can be sawn through entirely, from side-to-side, thereby forming a superior array of smaller pins, better suited to carrying heat from the top of each lid  40  into a passing liquid, such as water. 
     Moreover, the final assembly  50 , is made of an aluminum alloy that is hard and strong. In one preferred embodiment aluminum alloy 6061 is used. For the purposes of this application, pure aluminum is considered to be an aluminum alloy. In an alternative preferred embodiment, copper or a copper alloy is used. 
     The above noted materials, however, have a greater coefficient of thermal expansion than the typical power module  56  that would be mounted on the top of assembly  10 . Accordingly, in some embodiments it is advisable to use conductive grease between assembly  10  and module  56 . 
     To achieve a more direct bond, however, the work pieces  10  used to make lids  40  are formed by casting molten aluminum alloy (for example alloy 6061) into a mold having a preformed mesh of silicon carbide, 0.25 to 1 mm high, covering most of the bottom but leaving a circumferential margin, of about 6 mm width, empty. When the molten alloy material fills the mold, it fills the many gaps in the silicon carbide mesh, forming a metal matrix composite (MMC). After removal from the mold, the side opposite the MMC is sawed to form pins  44 , and the lid  40  is flipped over, so that the aluminum filled silicon carbide mesh is on the top, opposite from pins  44 . A circumferential margin, however, does not have the silicon carbide mesh as part of the structure. Alternatively, lids  40  can be made of an aluminum silicon alloy, which can be friction stir welded, and which has desirable thermal expansion characteristics. In one preferred embodiment an aluminum silicon alloy produced by powder metallurgy is used, and contains 40% silicon and 60% aluminum. 
     The aluminum filled silicon carbide mesh portion has a lower coefficient of thermal expansion, depending on the percentage of silicon carbide, than the aluminum alloy, but is also integral to the aluminum alloy that is not intertwined with the silicon carbide. Accordingly, the top of lid  40  (other than the approximately 6 mm circumferential margin) has a coefficient of thermal expansion between 10 and 18 ppm/° C., which is closer to that of power module  56  than is aluminum, so as to make direct soldering practical, by avoiding a degree of thermal mismatch that would destroy the direct solder bond after only a few thermal cycles. Lid  40  as a whole has higher thermal conductivity and is mostly made of a material that is softer and therefore more easily machined. 
     Another embodiment whereby the top surface of the lid  40  can have a lower coefficient of expansion is to use what is known as Cold Spray Technology to spray a layer of a different metal (such as copper, or a copper tungsten alloy) up to 0.2 mm thick, on a portion of lid  40 , after assembly  10  has been fabricated so as to effectively reduce the thermal expansion of the top surface of lid  40 , thereby making direct soldering of power module  56  practical, in like manner to the process described in the above paragraph. 
     Referring to  FIGS. 4-7 , in an alternative preferred embodiment, a two-sided fluid cooled embodiment  110  includes an inflow port  112  and an outflow port  118 . Fluid passageways  116  permit fluid flow through flow cavities  120 , which and thereby from inflow port  112  to outflow port  118 . For each flow cavity, embodiment  110  an upper circumferential seat  122  and a lower circumferential shelf (not shown) to accommodate the placement of a top lid  140  and a bottom lid  141 . Lids  140  and  141  all define a ledge  142  to fit onto seat  122  and a set of pins  144 . Lids  140  and  142 , including pins  144 , are formed and sealed onto work piece  110  as described above with respect to the one sided embodiments.  FIG. 8  shows an alternative embodiment  110 ′ in which an inflow port  112 ′ is located kitty corner from outflow port  118 ′. 
     While a number of exemplary aspects and embodiments have been discussed above, those possessed of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.