Patent Document

BACKGROUND 
   The present invention relates to removing excess heat from electronic circuit housings and, more particularly, to the removal of such heat through use of heat transporting coolants. 
   The operation of electronic circuits inevitably involves the dissipation of electrical energy into heat in the electronic and passive circuit device structures used in forming such circuits. These circuits are typically provided for use by being housed in some kind of housing arrangement. In those situations in which the generated heat is sufficiently removed simply through convection or conduction to the surrounding atmosphere, or both, to thereby avoid raising temperatures to values that could lead to failures in the circuits or the housings in which they are positioned, nothing more is needed for cooling such circuit housing arrangements. 
   However, some kinds of electronic circuits are used to control relatively large amounts of electrical power and are often provided in large power dissipation electronic module assemblies. Correspondingly, such circuits dissipate relatively large amounts of electrical power in the controlling of these much larger amounts of electrical power, and so result in the generation of too much heat in the housing arrangements for removal by just atmospheric based convection and conduction means. In these situations, further housing cooling provisions must be made to avoid temperatures in those housings being raised to values that lead to failures in the circuits or their housings. A typical such further provision is a duct, in which a coolant can be caused to flow to form a heat exchanger, and this duct is thermally coupled to one side of the electronic circuit housing being cooled thereby. 
   Such a duct provision for circuit housings, while an improvement over relying on just atmospheric based conduction and convection, is only a limited improvement for circuit housings in which the supported circuits generate sufficiently large amounts of heat primarily because of the thermal resistivity encountered over the transport paths. That is, although the cooling duct removes heat well from the circuit and housing portions relatively close thereto, heat that must be transported thereto from more remote portions thereof, usually mostly by conduction, is limited often by the relatively small thermal conductivities of the materials present along those transport paths. Those thermal conductivities can be improved by use of alternative materials along the transport paths or by adding more parallel paths through broadening the heat sources such as by using heat generating components having larger sizes or lateral extents, or by enlarging the housing to have more area abutting the cooling duct. Such remedies, however, either substantially add to the cost or the weight of the circuit housings, or both. Thus, there is a desire for better arrangements to sufficiently remove excessive heat generated in housed circuit devices during their operation. 
   SUMMARY 
   The present invention provides a cooling structure for electrical circuit devices dissipating energy as heat during operation of electrical circuits in which such devices are electrically connected with the cooling structure having a pair of cooling ducts. Each cooling duct has a passageway therein formed by passageway walls thereabout through which a coolant can selectively be caused to flow and an electrical circuit device housing formed by a pair of spaced apart heat transfer plates each joined to a housing side extending between them to provide a sealed housing space. An electrical circuit device is positioned in that sealed housing space and each of the pair of heat transfer plates is positioned adjacent to, and thermally coupled to, a corresponding one of the pair of cooling ducts. An electrically insulative heat transfer material is provided in the sealed housing space so as to be capable of being in contact with the electrical circuit device also therein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross-sectional side view diagrammatic representation of an electronic circuit housing arrangement embodying the present invention, 
       FIG. 2  shows a cross-sectional side view diagrammatic representation of another electronic circuit housing arrangement embodying the present invention, and 
       FIG. 3  shows a cross-sectional side view diagrammatic representation of yet another electronic circuit housing arrangement embodying the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention provides a second coolant transport duct that can form a heat exchanger on the opposite side of the electronic circuit housing from the first coolant transport duct described above but is thermally coupled to the circuit device in which heat is generated differently than is the first duct. The first duct is thermally coupled to the heat generating circuit devices primarily by conduction. Those devices are supported on a sequence of thermally conductive material layers each in contact with its adjacent neighbor or neighbors in the circuit housing structure thereby providing a direct thermally conductive path between the heat generating circuit devices and the first duct. However, the presence of electrical circuit interconnections of various sorts on the sides of the heat generating circuit devices opposite the sides thereof nearest the first duct prohibits use of solid material layers forced against these opposite sides for good conductive thermal contact, especially use of any electrically conductive material layers which are often also the choice in other circumstances for good thermal conductivity materials. Thus, an alternative heat transfer arrangement is needed between the heat generating circuit devices and the second duct. 
     FIG. 1  shows a cross-sectional side view of a diagrammatic representation for such an alternative electronic circuit housing arrangement,  10 . A first duct,  11 , is formed of two separated aluminum plate-like structures with the upper one having a thermal grease,  12 , thereon to keep it well thermally coupled to the electronic circuit component enclosure structure adjacent thereto. The separation space between the two separated aluminum plate-like structures in first duct  11  provides the passageway through which a coolant liquid,  13 , represented by directed arrows, is caused to flow to transport away with it heat absorbed thereby from the plate-like structures in duct  11 . Various structures are brazed together (not shown) in forming duct  11  with its plate-like structures and the passageway connections therein through which coolant  13  is supplied to flow through the separation space passageway between its plate-like structures when cooling is desired. Finned structures (not shown) can be provided in this passageway thermally coupled (typically through brazing) to the plate-like structures forming same to aid in the transfer of heat transferred to these structures to coolant  13 . Coolant  13 , although possibly a gas in some circumstances, is typically a liquid such as polyalphaolefin or a propylene glycol and water mixture which alternatives both have relatively large thermal conductivities and specific heats for transferring heat but also have relatively low viscosity to thereby keep pressure drops relatively small along the passageway during coolant flows. 
   A first thermal transfer, or base, plate,  14 , formed of copper, is fastened to first duct  11  with thermal grease  12  between a first side of that plate and duct  11 . This fastening typically is provided by some sort of fastening screws (not shown), and the resulting arrangement provides for conducting heat from plate  14  to coolant  13 . A component enclosure structure,  15 , in electronic circuit housing arrangement  10  is provided based on plate  14 , and so the opposite side of base plate  14  has supported directly thereon, and bonded thereto, a plurality of mounting pedestals,  16 , each having a lower direct bond copper layer,  17 , and another upper direct bonded segmented copper layer,  17 ′, bonded to opposite sides of an electrical insulator layer,  17 ″, formed of either aluminum oxide or nitride. The segments of upper copper layer  17 ′ in each mounting pedestal  16  form electrical interconnections for a corresponding one of a plurality of substantial electrical power management semiconductor material transistor chips,  18 , also mounted on a segment of layer  17 ′ of that pedestal, and which chip so mounted is electrically connected to such pedestal segments by a corresponding set of wire bonds,  19 . Heat generated in operating the electrical circuits containing chips  18  is in part conducted from each of those chips through the corresponding one of mounting pedestals  16  to plate  14 , and subsequently conducted to flowing coolant  13  in duct  11  to thereby be transported away. 
   Pedestals  16  and power transistor chips  18  are shown in  FIG. 1  submerged in a dielectric liquid,  20 , contained in the enclosed space in component enclosure structure  15 , in which bubbles containing vapors of that liquid (shown as circles in liquid  20  in  FIG. 1 ) form due to part of the heat generated in chips  18  during operations of the circuits containing them (boiling of liquid  20 ) as a single component working fluid in a two phase system. These bubbles buoyantly float up to the surface of liquid  20  to join the accumulating vapor of that liquid in a space,  21 , above that liquid and accumulate there about a thermally conductive finned structure,  22 , in that enclosed space. Heat is taken up by liquid  20  undergoing a phase change from liquid to vapor  21  thereby accomplishing the absorption of significant amounts of heat in providing the latent heat of vaporization needed to convert small amounts of liquid to the vapor shown as the circles in liquid  20  in the figure. Accumulated vapor  21  condenses on the fins of finned structure  22  so that the latent heat of vaporization released in doing so is taken up in those fins and conducted from there to second heat transfer plate  24 . Liquid  20  in such a single component, two phase working fluid system can be taken from the general fluid classes of Novec™ hydrofluoroether (HFE), such as HFE-7100 fluid, or Fluorinert™ (FC), such as FC-72 or FC-84 fluid, all sold by the 3M Co. 
   Alternatively, dielectric liquid  20  can be provided as a single component working fluid in just a single phase by providing liquid  20  in a quantity sufficient to fill, or essentially fill, the enclosed space of the component enclosure structure (not shown in  FIG. 1 ). This arrangement will thereby eliminate, or nearly eliminate, vapor space  21  so that liquid  20  is in direct contact with both transistor chips  18  and finned structure  22  thereby allowing heat from the former to be conducted to the latter. Liquid  20  in this latter arrangement can be a silicone cooling fluid such as Dow Corning® 550 sold by the Dow Corning Corporation. However, such a single component, single phase working fluid leads to a greater thermal gradient along the heat transport path in the electronic circuit housing structure than does a single component, two phase working fluid, and more of liquid  20  for the single component, single phase working fluid is needed to fill or essentially fill the electronic circuit housing structure thereby adding to both the cost and the weight of housed circuit components. 
   Both liquid  20  and vapor  21  (if present) are, as indicated, contained in the enclosed space provided in component enclosure structure  15  in electronic circuit housing arrangement  10  that results from base plate  14  therein being bonded at a surface thereof, opposite that surface adjacent to first duct  11 , to one end of an enclosing housing side,  23 , and by a second thermal transfer plate,  24 , being bonded at a first side thereof to the opposite end of housing side  23 . Housing side  23  is formed of formed of a high temperature plastic with a temperature coefficient of expansion not too different from that of base plate  14 . Such a material can be any of 25 to 40% fiber reinforced polyphenyl sulphide (PPS) having a maximum operating temperature of 210° C. and a temperature coefficient of 29×10 −6 /° C., 25 to 40% fiber reinforced polyether sulfone (PES) having a maximum operating temperature of 210° C. and a temperature coefficient of 23×10 −6 /° C., or 25 to 40% fiber reinforced polyester having a maximum operating temperature of 150° C. and a temperature coefficient of 20×10 −6 /° C. as circuit and housing operating temperatures allow. Bonding can be either adhesive bonding or ultrasonic bonding. 
   Second heat transfer plate  24  is formed of aluminum with finned structure  22  being thermally coupled thereto at the first side thereof. This coupling can come about through finned structure  22  being integral to that plate by being formed from a starting aluminum plate that is thicker than the final thickness of plate  24  from which starting plate the material initially between the resulting fins has been removed such as by etching or sawing. Alternatively, finned structure  22  can be formed as a separate structure subsequently fastened to plate  24  in a thermally coupled manner such as by brazing them together. 
   Thermal grease,  25 , on a second side of second heat transfer plate  24  thermally couples the component enclosure structure to a second duct,  26 , in electronic circuit housing arrangement  10  which duct is formed as is first duct  11  of two separated aluminum plate-like structures forming the walls of a passageway in the separation space therebetween. Here, too, thermal grease  25  is located between a second side of plate  24  and duct  26  to transfer heat from component enclosure structure  15  to this duct, and they are fastened to each other typically again with some sort of fastening screws (not shown). Again, various structures are brazed together (not shown) in forming duct  26  with its plate-like structures and the passageway connections therein through which a coolant,  27 , again represented by directed arrows and typically of the same substance as that used for coolant  13 , is supplied to flow through the separation space passageway between those plate-like structures when cooling is desired. As before, finned structures (not shown) can be provided in this passageway thermally coupled to the plate-like structures forming same (typically by brazing) to aid in the transfer of heat, that has been transferred to these structures, to coolant  27 . 
   Thus, a heat portion represented by directed arrows,  28 , that has been taken up by finned structure  22  from chips  18  and liquid  20  and its vapors, is then conducted to second heat transfer plate  24  and from there conducted through thermal grease  25  to duct  26  and then to flowing coolant  27  to thereby be transported away. Another portion of the heat generated in chips  18  is conducted through pedestals  16  and liquid  20  to base plate  14 , as described above, and represented by directed arrows,  29 , and this portion is conducted to coolant  13  for removal. 
   As a further alternative to just having dielectric liquid  20  being provided as a single component working fluid in only a single phase, as described above, there is shown in  FIG. 2  a cross-sectional side view of a diagrammatic representation of another electronic circuit housing arrangement,  10 ′. In addition to providing a fill in the unoccupied space in electronic component enclosure  15  based on the same kind of liquid as liquid  20  as was provided in this space in this electronic circuit housing arrangement (as in the alternative described above for  FIG. 1  leaving no vapor space  21 ), a very large number of electrically insulative, thermally conductive nanoparticles are added as the dispersed phase to that liquid serving as the continuous phase in a liquid particle suspension mixture, or colloidal system like a sol,  20 ′. Liquid particle suspension mixture  20 ′ has a larger effective thermal conductivity as a result of being a mixed phase heat transporting working fluid than does that of liquid  20  used alone. 
   Powders of boron nitride (BN), optimally distributed in some sense in being dispersed in the liquid phase, can serve as the nanoparticles, but nanoparticles of alumina (Al 2 O 3 ) are a suitable alternative. Finned structure  22  is shown present in component enclosure  15  in arrangement  10 ′ as this will enhance conductive heat transfer from liquid particle suspension mixture  20 ′ to coolant  27  through plate  24 , thermal grease  25  and duct  26 , but this finned structure is not as necessary here as it is in arrangement  10  of  FIG. 1  because here there need not be surface for condensing a vapor, and so this structure could be omitted. 
   In yet another alternative, there is shown in  FIG. 3  a cross-sectional side view of a diagrammatic representation of another electronic circuit housing arrangement,  10 ″, with another kind of colloidal system based on a soft matter, or compliant, electrically insulative encapsulant material such as a gel that can be used in place of liquid  20  at least in part for a continuous phase. Again, electrically insulative, thermally conductive nanoparticles, such as the kinds indicated above, are dispersed therein to increase the thermal conductivity of the resulting mixture. A silicone gel is suitable for this purpose. The compliance of the resulting gel based mixture,  20 ″, reduces any stress that may otherwise occur on wire bonds  19 . Finned structure  22  is again shown in component enclosure space  15  thermally coupled to plate  24  to enhance heat transfer but again could be omitted. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Technology Category: h