Abstract:
A liquid cooled power electronics assembly configured to use electrically conductive coolant to cool power electronic devices that uses dielectric plates sealed with a metallic seal around the perimeter of the dielectric plates to form a device assembly, and then forms another metallic seal between the device assembly and a housing. The configuration allows for more direct contact between the electronic device and the coolant, while protecting the electronic device from contact with potentially electrically conductive coolant. Material used to form the dielectric plates and the housing are selected to have similar coefficients of thermal expansion (CTE) so that the reliability of the seals is maximized.

Description:
TECHNICAL FIELD OF INVENTION 
     This disclosure generally relates to a liquid cooled power electronics assembly, and more particularly relates to an assembly that uses dielectric plates attached to an electronic device and a metallic seal along the perimeter of the plates to protect the electronic device from contamination or operational interference by electrically conductive coolant such as automotive engine coolant. 
     BACKGROUND OF INVENTION 
     It is a continuing desire to increase power dissipation ratings of electronics, and put those electronics into smaller packages. One industry where this is especially true is the transportation industry, especially in view of the advent of electric or hybrid automobiles. Such automobiles are propelled, all or in-part, by electric motors that rely on transistors and other devices to switch electrical power to the electric motors. The power controlled by these transistors may have voltage potentials ranging from 100 Volts to 2400 Volts, and may switch currants range from 50 Amperes to 600 Amperes. Any increase in the efficiency by which heat is removed from transistors can increase the reliability or power rating of the electronics. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment, a liquid cooled power electronics assembly is provided. The assembly is configured to tolerate the use of electrically conductive coolant to cool power electronic devices. The assembly includes a housing, an electronic device, a lead frame, a first dielectric plate, a second dielectric plate, a first metallic seal and a second metallic seal. The housing is configured to define an inlet, an outlet, and a cavity configured to contain coolant within the cavity between the inlet and the outlet. The electronic device is located within the cavity. The electronic device is characterized as being substantially planar in shape and so defines a first planar side, a second planar side opposite the first planar side, and a device perimeter between the first planar side and the second planar side. The lead frame is electrically coupled to the electronic device and extends outside the cavity through an opening in the housing. The first dielectric plate is attached to the first planar side. The first dielectric plate has a first plate perimeter that extends beyond at least a portion of the device perimeter. The second dielectric plate is attached to the second planar side. The second dielectric plate has a second plate perimeter that extends beyond at least a corresponding portion of the device perimeter. The first metallic seal is formed between the portion of the first plate perimeter and the corresponding portion of the second plate perimeter. The first metallic seal is effective to isolate the electronic device from the coolant. The first dielectric plate, the second dielectric plate, and the first metallic seal cooperate to form a device package. The second metallic seal is formed between the device package and the opening effective to prevent coolant from passing out of the cavity through the opening. 
     Further features and advantages will appear more clearly on a reading of the following detailed description of the preferred embodiment, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention will now be described, by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a cutaway perspective view of a liquid cooled power electronics assembly in accordance with one embodiment; 
         FIG. 2A  is a perspective view of a partially assembled device package used in the assembly of  FIG. 1  in accordance with one embodiment; 
         FIG. 2B  is a perspective view of a fully assembled device package used in the assembly of  FIG. 1  in accordance with one embodiment; 
         FIG. 3  is a perspective view of a combination lead frame/perimeter frame optionally used in the device package of  FIG. 2B  in accordance with one embodiment; 
         FIG. 4  is a perspective view of a device package used in the assembly of  FIG. 1  in accordance with one embodiment; 
         FIG. 5  is a perspective view of part of the assembly of  FIG. 1  in accordance with one embodiment; 
         FIG. 6  is a device package used in the assembly of  FIG. 1  in accordance with one embodiment; and 
         FIG. 7  is a device package used in the assembly of  FIG. 1  in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a non-limiting example of a liquid cooled power electronics assembly, hereafter the assembly  10 . In general, the assembly  10  described herein is a sealed electronics assembly that immersion cools a power dissipating electronic device  12  ( FIG. 2 ) that is inside a device package  14  with coolant  16 , such as a mixture that includes water and ethylene glycol. It should be appreciated that such water-based coolant may be electrically conductive, and so the assembly  10  described herein must sufficiently isolate the electronic device  12  from the coolant  16  in order to prevent electrical shorting or unexpected operation of the electronic device  12 . It should also be appreciated that it is not a requirement that the coolant  16  be electrically conductive, and so a non-electrically-conductive coolant would be suitable if it has adequate heat transfer characteristics. 
     The assembly  10  generally includes a housing  18 . The housing  18  may be formed of polymeric material such as a glass gilled nylon marketed as Zytel™ by Dupont (part number 70G25HSLR BK099), or may be formed of metal such as aluminum. The housing  18  may include an inlet  20  and an outlet  22  configured to make a fluidic sealed connection to, for example, hoses (not shown) providing a fluidic connection to a heat exchanger (not show) that transfers heat from the coolant to, for example, ambient air. In general, the inlet  20  receives relatively lower temperature coolant for the assembly  10 , and the outlet  22  removes coolant warmed by power dissipated by the electronic device  12 . The housing  18  also generally defines a cavity  24  inside the housing  18  that contains the coolant  16  as it passes from the inlet  20  to the outlet  22 . The size and shape of the cavity  24 , the inlet  20 , and the outlet  22  are determined based on the number and size of the device package  14 , the amount of power dissipated by the device package  14 , and expected coolant inlet temperatures using know engineering rules and design practices. 
       FIGS. 2A and 2B  illustrate a device package  14  that is partially assembled and fully assembled, respectively. The electronic device  12  is typically a wafer level device, meaning that the electronic device  12  is generally described as unpackaged, typically with exposed surfaces of silicon, passivation, or thin film metallization. The electronic device  12  may be a solid state electronic switch such as a transistor, for example, a metal oxide semi-conductor field effect transistor (MOSFET), or insulated gate bipolar transistor (IGBT), or a diode. By way of example and not limitation, a typical electronic device is formed predominately of silicon having dimensions of 12 millimeters (mm) by 12 mm by 0.075 mm. The electronic device  12  may be generally characterized as being substantially planar in shape, and so generally defines a first planar side  26  (not specifically shown), and a second planar side  28  opposite the first planar side  26 . The planar shape of the electronic device  12  may also generally define a device perimeter  30 , or edge, between the first planar side  26  and the second planar side  28 . 
     The device package  14  may also include a first dielectric plate  32  attached to the first planar side  26 . The first dielectric plate  32  generally defines a first plate perimeter  34  that extends beyond at least a portion of the device perimeter  30 . The first dielectric plate  32  is preferably a ceramic material, for example aluminum nitride, aluminum oxide, or silicon dioxide. Ceramic material is preferred because the coefficient of thermal expansion (CTE) typically more closely matches that of the electronic device  12 , and so is believed to generally improve the reliability of the attachment of the first dielectric plate  32  to the electronic device  12 . As the outer surface of the device package  14  will be exposed to the coolant  16 , ceramic based materials are also an excellent choice as they are generally impervious to fluids that may be used as the coolant  16 . By way of example and not limitation, suitable dimensions for the first dielectric plate  32  for the typical electronic device suggested above are 28 mm by 20 mm by 2 mm. If the first dielectric plate  32  is too thin, then it may be too delicate to reliably handle and process as described herein. If the first dielectric plate  32  is too thick, then it may undesirably increase thermal resistance between the electronic device  12  and the coolant  16 . 
     The device package  14  may also include a lead frame  36  electrically coupled to the electronic device  12  and extending beyond the first plate perimeter  34 . The lead frame may be formed of copper or a copper alloy, and may be fabricated by folding, coining, and/or shearing as will be known by those in the art. The lead frame  36  may advantageously formed of a metal that has a CTE that closely matches the material selected for the first dielectric plate  32 . Closely matched CTE&#39;s are desirable for the same reasons of improved reliability given above. The lead frame  36  illustrated has several leads or fingers coupled together by a joining section for the purpose of simplifying the assembly of the lead frame  36  to the first dielectric plate  32 . It will be recognized by those in the art that all or part of the joining section may be cut off along the dashed line  42  after the device package  14  is assembled so that individual connections to the several legs are not electrical shorted together. 
     The device package  14  may also include a second dielectric plate  38  attached to the second planar side  28 . The second dielectric plate  38  generally defines a second plate perimeter  40  that extends beyond at least a corresponding portion of the device perimeter  30  that corresponds to at least a portion of where the first plate perimeter  34  extends beyond the device perimeter  30 . The attachment of the first dielectric plate  32  and the second dielectric plate  38  to the electronic device  12  may be by way of soldering, sintering, or conductive adhesive as will be recognized by those in the art.  FIG. 2B  does not show the second dielectric plate  38  overlaying the lead frame  36  only for the purpose of simplifying the illustration. It is recognized that the second dielectric plate  38  may be extended to overlay all or part of the lead frame  36  for the purpose of making an electrical connection between the lead frame  36  and the second planer side  28  of the electronic device  12 . Preferably, the second dielectric plate has the same thickness as the first dielectric plate  32  so that stresses on the electronic device  12  are substantially balanced after the device package  14  is assembled. 
     Further details regarding the assembling of the electronic device  12 , the first dielectric plate  32 , the second dielectric plate  38 , and the lead frame  36  may be found in U.S. Pat. No. 6,812,553 issued to Gerbsch et al. on Nov. 2, 2004, and U.S. Pat. No. 7,095,098 issued to Gerbsch et al. on Aug. 22, 2006. The entire contents of both patents are hereby incorporated herein by reference. It is noted that the electronic package described in these patents would not be suitable for immersion cooling because of the gap between the dielectric plates that would allow coolant to contact the electronic device therebetween. The benefit of having the first plate perimeter  34  and the second plate perimeter  40  extend correspondingly beyond the device perimeter  30  will become apparent as sealing of the device package  14  to protect the electronic device  12  from the coolant  16  is described below. 
     To avoid the problem of coolant contacting the electronic device  12 , the device package  14  includes a first metallic seal  44  formed between a portion of the first plate perimeter  34  that extends beyond the device perimeter  30 , and a corresponding portion of the second plate perimeter  40 . In general, the first metallic seal  44  isolates the electronic device  12  from the coolant  16 . Preferably, the first metallic seal  44  formed by one of silver sintering or soldering. A sintered seal is formed using heat and pressure and is thought to be stronger and more reliable than a soldered seal. However, a soldered seal joint can be formed without the added complexity of applying pressure to the device package  14 , but a soldered seal may be less reliable because of intermetallic alloys between the solder and base metal on the dielectric plates, for example, see metalized region  46  and metallization layer  54  described below. Sintering may also be preferable because it would not be affected if the device package was subjected to a subsequent soldering operation, for example when the device package is installed into the housing  18 . As such, the first dielectric plate  32 , the second dielectric plate  38 , and the first metallic seal  44  cooperate to form the device package  14 . 
       FIG. 2A  illustrates a non-limiting example of a metalized region  46  that defines the location of the first metallic seal  44 . A suitable width of the metalized region  46  is 3 mm. The metalized region  46  may include a layer of thick-film ink printed and fired onto the first dielectric plate  32 . Alternatively the metalized region  46  may be built upon a foundation of thin film metal deposited onto the first dielectric plate  32 . If the electronic device  12  is thin enough, for example less than 0.12 mm, then the metalized region  46  may be made sufficiently thick on the first dielectric plate  32 , and a corresponding metalized region (not shown) may be established on the second dielectric plate  38  so that the first metallic seal  44  can be formed. In one embodiment, the metalized region  46  may have a plating thickness about equal to or slightly greater than (e.g. 1 mil) half of a device thickness of the device so that when the metalized regions are joined, the electronic device  12  is closely coupled to the first dielectric plate  32  and the second dielectric plate  38 . 
       FIG. 3  illustrates a non-limiting example of a perimeter frame  48 . If the electronic device  12  is thicker than about 0.12 mm, then it may be preferable to use the perimeter frame  48  sized and shaped to overlay the metalized region  46  in order to provide additional material thickness that properly fills and seals a gap  60  ( FIG. 4 ) between the first dielectric plate  32  and the second dielectric plate  38 . In one embodiment, the lead frame portion of the perimeter frame  48  may have a different or variable thickness relative to the portion of the perimeter frame  48  overlying the metalized region  46  so that the thickness of the lead frame portion is appropriate for a desired stiffness of the leads, and the portion overlying the metalized region is appropriate to allow for single layer printing to dispense material onto the metalized region  46  for forming the first metallic seal  44 . 
     It is recognized that other types of seals between the first dielectric plate  32  and the second dielectric plate  38  are possible, for example dipping the assembly of the first dielectric plate  32 , second dielectric plate  38  and electronic device  12  into a polymeric coating material, or spraying a similar material. However, it is believed that such non-metallic seals would not provide sufficiently reliable seals in view of the typical negative forty (−40) to positive one hundred twenty five (+150) degree Celsius (° C.) operating temperatures for automotive applications. Furthermore, any additional coating over the exposed surfaces of the first dielectric plate  32  and/or the second dielectric plate  38  would likely reduce heat transfer from dielectric plates to the coolant  16 . Applying a polymeric or epoxy material into the gap  60  between the first dielectric plate  32  and the second dielectric plate  38  is also considered undesirable as it is considered to be less reliable than a metallic type seal. 
     Referring again to  FIG. 1 , the housing  18  generally defines an opening  50  through which the device package  14  may be inserted and secured in place by forming a second metallic seal  52  between the device package  14  and the opening  50 . The second metallic seal  52  is preferably formed by soldering, as opposed to using only a polymeric material. Soldering is preferred in order to reliably prevent the coolant  16  from passing out of the cavity  24  through the opening  50 , as described above with regard to the first metallic seal  44 . The second metallic seal  52  may include a metallization layer  54  ( FIG. 2B ,  FIG. 4 ) applied to the first dielectric plate  32  and the second dielectric plate  38 . Like the metalized region  46 , the metallization layer  54  may be a direct bond metal layer such as copper directly bonded to the dielectric plate, or may be another form of patterned metal suitable for cooperating with other materials to form the second metallic seal  52 . 
     In one embodiment, the metallization layer  54  may extend around the edges of the first dielectric plate  32  and the second dielectric plate  38  so that the second metallic seal  52  goes all the way around the device package  14 , including extending over the first metallic seal  44 . 
     With this arrangement, an electronic device  12  may be located within the cavity  24  in order to make intimate contact with the coolant  16 , and the lead frame  36  is used to electrically coupled to the electronic device and extend outside the cavity  24  through the opening  50  in the housing  18  so that an electrical connection to the lead frame  36  can be readily made. Preferably, the device package  14  is oriented to protrude into the coolant such that the plane of the device package  14  or electronic device  12  is substantially parallel to a flow direction  56  of the coolant  16  from the inlet  20  to the outlet  22 . By making the device package  14  substantially parallel to the flow direction  56 , it is believed that the electronic device  12  is cooled more uniformly than other orientations. 
     Continuing to refer to  FIG. 1 , it may be preferable to form most of the housing  18  of one material, a polymeric compound for example, but form the portion of the housing  18  that includes the opening  50  of a solderable material so the second metallic seal  52  is readily formed. As such, the assembly  10  or housing  18  may include a carrier plate  58  that defines the opening  50 , and is coupled to the housing  18  in a manner effective to form a fluidic seal  62  between the carrier plate  58  and the housing  18 . The fluidic seal  62  may include a sealant such as silicone adhesive, or an ethylene propylene diene monomer (EPDM) gasket. The carrier plate  58  may be secured to the housing  18  by the sealant, or may be secured using fasteners such as screws. Alternatively, if the materials used to form the carrier plate  58  and the housing  18  are compatible, the carrier plate  58  may be attached to the housing  18  using the known process of friction stir welding. 
       FIG. 5  illustrates a non-limiting example of a carrier plate  58  having several of the device package  14  installed and preferably secured to the carrier plate  58  by the second metallic seal  52  (not specifically shown). The carrier plate  58  is preferably formed of a material having a CTE similar to that of the device package  14  for the same reasons give above with regard to the first metallic seal  44 . By way of example and not limitation, the carrier plate  58  may be formed of nickel iron alloy # 42 . 
       FIG. 5  further illustrates an optional feature of a sealant  72  applied to encapsulate the lead frame  36  proximate to where the lead frame  36  protrudes from the device package and proximate to the opening  50 . The sealant  72  may be a room temperature vulcanization (RTV) type material, or it may be a curing epoxy type. Applying the sealing  72  is believed to help protect the lead frame  36  for vibration induced failures, and provide another barrier against coolant leaking. 
       FIG. 4  illustrates additional non-limiting features that may be part of the device package  14 . The device package  14  may include complementary metallization  64  on each side of both the first dielectric plate  32  and the second dielectric plate  38 . As used herein, complementary metallization means that a metallization pattern on one side of a particular dielectric plate is generally mirrored on the other side of that dielectric plate. While not subscribing to any particular theory, it is believe that by providing the complementary metallization  64 , the risk or tendency of the particular dielectric plate to warp as temperature varies is reduced. By reducing the tendency to warp, the complementary metallization  64  helps to balance stress in the device package  14  and thereby improve reliability and manufacturability. The same theory may be applied to the attachment of the first dielectric plate  32  and the second dielectric plate  38  to the electronic device  12 . In this case, the first planar side  26  ( FIG. 1 ) defines a first attachment region for the electronic device  12 , and the second planar side  28  defines a second attachment region such that when the electronic device  12  is attached to the first dielectric plate  32  and the second dielectric plate  38 , stress on the electronic device  12  is substantially balanced, meaning that stresses that may try to warp the electronic device  12  are minimized. 
       FIG. 6  illustrates additional non-limiting features of the device package  14 . The first dielectric plate  32  and/or the second dielectric plate  38  may include features on an exterior surface  66  of the device package  14  that increase the surface area of the device package  14 , for example micro-channels  68  formed on the dielectric plates. Increasing the surface area of the device package  14  is generally believed to improve heat transfer from the device package  14  to the coolant  16 . The micro-channels  68  may be formed by extrusion of the material used to form the dielectric plate, for example aluminum nitride. the aluminum nitride may be dry pressed and fired using known manufacturing methods of forming various “3D” shapes of ceramic. 
       FIG. 7  illustrates additional non-limiting features of the device package  14 . The first dielectric plate  32  and/or the second dielectric plate  38  may include a metal heat sink  70  attached to each dielectric plate. The metal may be copper or other suitable material, and the metal heat sink may be attached by soldering, sintering, gluing, or other methods known to those in the art. 
     Accordingly, a liquid cooled power electronics assembly  10  is provided. The assembly  10  makes use of metallic seals ( 44 ,  52 ) to provide robust seals suitable for automotive application that prevent the coolant  16  from leaking out of the housing  18 , and/or contacting the electronic device  12  within the device package  14 . The assembly  10  is particularly advantageous because it minimized the thermal path between the heat generating electronic device  12  and the coolant  16  when compared to other arrangements. Testing has demonstrated that the assembly  10  described herein has a power dissipating rating of 0.11 degrees Celsius per Watt (0.11° C./W), while an in-production assembly market by Delphi Inc. of Troy, Mich. under the moniker Viper has a less desirable rating of 0.15° C./W. While well suited for the automotive industry, the teachings set forth herein are applicable to other industries. 
     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.