Abstract:
A power module for converting direct current to alternating current, the power module including: a semiconductor switching circuit device, a substrate onto which said switching circuit device is physically and electrically coupled, at least one secondary substrate with the semiconductor switching circuit device being physically and electrically coupled to the at least one secondary substrate such that the semiconductor switching circuit device is formed between the substrate and the at least one secondary substrate, at least one thermal mass attached to a respective secondary substrate of the at least one secondary substrate, and a cover at least partially disposed about said power module, said cover including an opening exposing a bottom side of the substrate.

Description:
FIELD OF THE INVENTION 
     This invention relates to a power module for converting high voltage direct current (DC) to high voltage alternating current (AC), such as, but not necessarily limited to, power modules used in hybrid vehicles and purely electric vehicles. 
     BACKGROUND OF THE INVENTION 
     In the past, power modules for hybrid or electric automobiles have often provided cooling on a single side of an electronic device, such as a power MOSFET (metal oxide semiconductor field effect transistor), IGBT (insulated gate bipolar transistor), or other component. Due to the placement of such power modules on heat sinks, lead frame terminals of such devices may come in close proximity to the heat sinks. Further, past power modules typically have used wire bonds to one or more sides of the power module device. The use of wire bonds creates problems with high assembly time and capital equipment costs, as well as high parasitic inductances that cause voltage overshoots. Still further, wire bonds can lead to failures due to repetitive power cycling. 
     However, in practice, it can be difficult to achieve double-sided cooling due to mechanical tolerances of the various components making up the power module. Such modules with double-sided cooling may include two DBC (direct bond copper) substrates, each made up of two copper layers and a ceramic layer, and each with a thickness tolerance, two solder layers and a power semiconductor device sandwiched between the two DBC substrate layers. Required tolerances on power module thicknesses can make it difficult to provide heat sinking, especially if trying to heat sink two adjacent devices, each with their own thickness and flatness tolerances. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention provide a thermal mass with good thermal conductivity that is added to a power module to improve double-sided cooling. These and other features provide a power module with improved transient thermal performance and lowered thermal impedance (bottom side cooling). 
     According to an aspect of the present invention, a power module for converting direct current to alternating current comprises a semiconductor switching circuit device, a substrate onto which the semiconductor switching circuit device is physically and electrically coupled, at least one secondary substrate with the semiconductor switching circuit device being physically and electrically coupled to the at least one secondary substrate such that the semiconductor switching circuit device is formed between the substrate and the at least one secondary substrate, and a cover. The cover includes an opening exposing a bottom side of the substrate. The semiconductor switching circuit device is also coupled to the substrate and to the at least one secondary substrate by a soldered or a sintered connection. A thermal mass is also added above each of the at least one secondary substrate. A thermal mass may be attached to a corresponding upper surface of each of the at least one secondary substrate using a thermally conductive layer. A thermal mass may also be attached to an upper surface of a secondary substrate using a variety of other methods, such as thermally conductive adhesive, solder, sintering and laminated foils. 
     In particular embodiments the cover is disposed over a top side of the power module and includes at least one cover aperture exposing a top side of a thermal mass attached to a corresponding secondary substrate. The cover may also include a plurality of cover apertures with each cover aperture exposing a top side of a respective thermal mass that is attached to a corresponding secondary substrate. 
     According to other aspects of the invention, the semiconductor switching circuit device includes at least one switching circuit, each at least one switching circuit comprising an insulated gate bipolar transistor and a diode. The substrate and/or secondary substrate may include a ceramic layer having a top side and a bottom side, a first copper layer coupled to said top side of said ceramic layer, and a second copper layer coupled to said bottom side of said ceramic layer. Alternatively, the substrate and/or secondary substrate may include a copper layer, an aluminum oxide layer, and an aluminum plate, with said aluminum oxide layer being formed on said aluminum base plate and said copper layer being applied over said aluminum oxide layer. A thermal mass is therefore attached to a top side of a first copper layer of a secondary substrate. 
     According to still other aspects of the invention, each switching circuit of the semiconductor switching circuit device is physically and electrically coupled to the substrate and to a corresponding second substrate, such that a plurality of switching circuits are physically and electrically coupled to the substrate and to corresponding second substrates. 
     According to still other aspects of the invention, the at least one cover aperture in the cover is formed by a process whereby a portion of a top surface of the cover is subjected to a grinding process to remove a portion of the cover and to reveal a top surface of the at least one thermal mass, such that each cover aperture reveals a corresponding thermal mass. 
     A power module for converting direct current to alternating current includes a semiconductor switching circuit device, a substrate, at least one secondary substrate, and may be employed with a cooling unit. The switching device may include one or more MOSFETs, IGBTs, or other suitable switching components, including die-up or flip-die IGBTs. The cooling unit may be physically coupled to the switching circuit device and the substrate and secondary substrate by way of a pressure fit, with the cooling unit including a first portion and a second portion spaced away from the first portion wherein the first and second portions are adapted to sandwich the substrate, switching circuit device, and secondary substrate therebetween. The first and second portions of the cooling unit may include hollow cavities adapted to allow a cooling liquid to flow therethrough. The switching circuit device may include a plurality of insulated gate bipolar transistors and diodes. The module may be constructed such that no separate fasteners are used to couple the cooling unit to the substrate, switching circuit device, and secondary substrate. The first portion of the cooling unit may make contact with a top side of a secondary substrate attached to a switching circuit of the semiconductor switching circuit device at a location aligned with the switching circuit inside the semiconductor switching circuit device, and the second portion of the cooling unit may make contact with a bottom side of the substrate that is aligned with the first portion. 
     These and other objects, advantages, purposes, and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section view of the power module of  FIG. 1  without molding or copper slugs; 
         FIG. 2  is a cross-section view of the power module of  FIG. 1  with a copper slug; 
         FIG. 3  is an isometric view of the power module of  FIG. 2  without molding; 
         FIG. 4  is a cross-section view of the power module of  FIG. 2  with copper slug and molding; 
         FIG. 5  is an isometric view of the power module of  FIG. 4  without top side grinding; 
         FIG. 6  is an isometric view of the power module of  FIG. 5  showing an exposed back side copper layer; 
         FIG. 7  is a simplified cross-section view of an over-molded power module that contains no copper slugs; 
         FIG. 8  is a simplified cross-section view of the over-molded power module that contains copper slugs; 
         FIG. 9  is a simplified cross-section view of the over-molded power module of  FIG. 8  after milling to expose the copper slugs; 
         FIG. 10  is an isometric view of the power module of  FIG. 5  with top side grinding to expose copper slugs; 
         FIG. 11  is a simplified cross-section view of a portion of an over-molded power module that contains copper slugs with irregularities in height and planarity; 
         FIG. 12  is a simplified cross-section view of a portion of the over-molded power module of  FIG. 11  with exposed copper slugs after a grinding operation to remove irregularities; 
         FIG. 13  is a flow diagram of the steps to a computer implemented process for grinding a power module cover to expose thermal slugs and to remove height/planarity irregularities for double-sided cooling of the power module; and 
         FIG. 14  is a schematic diagram of a semiconductor switching circuit device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying figures, wherein the numbered elements in the following written description correspond to like-numbered elements in the figures. 
     As discussed in detail herein, a thermal mass (also referred to as a slug) with good conductivity (e.g., plated or unplated copper) attached to a top surface of a top DBC (direct bonded copper) substrate layer in a power module improves transient thermal performance and lowers thermal impedance (bottom side cooling). In addition, as also discussed herein, if a post-mold grinding or milling operation is used, it is possible to achieve a consistent module thickness and flatness, which facilitates efficient double-sided cooling. 
     Exemplary embodiments of the present invention provide processes that improve these metrics without affecting the cooling path through the bottom of the power module (bottom side cooling). As described herein, a thermal mass may be added by means of a thermally conductive attachment to a back copper plane of the uppermost DBC substrate. As also discussed herein, the exemplary thermal mass needs to have a good heat capacity and a high thermal conductivity. Copper is an example of a suitable material, however, other substances may be used. 
     A portion of a power module  100  according to one embodiment of the present disclosure, is illustrated in  FIG. 1 . The exemplary power module  100  may be used to implement a switching circuit  120  which may be used in a variety of different applications. Such applications may include the conversion of DC electricity to AC electricity inside of either a purely electric vehicle, or a hybrid vehicle. An AC electrical output from a power module  100  may be used in powering an AC motor inside a vehicle, or which may be used to power other components of a vehicle, as well as in non-vehicle applications. Exemplary power modules are discussed in detail in U.S. patent application Ser. No. 13/880,553, titled “POWER MODULE FOR CONVERTING DC TO AC,” by James D. Tomkins, dated Oct. 19, 2011, which is herein incorporated by reference. 
     In various embodiments, a power module  100  may comprise a plurality of switching circuits  120 . In one embodiment, a power module  100  may comprise four switching circuits  120 . Other embodiments may also include other quantities of switching circuits  120 . In one embodiment, also illustrated in  FIG. 1 , an exemplary switching circuit  120  comprises one or more switching modules  125 , which may also be referred to as power silicon members  125 . In one embodiment as shown in  FIG. 14 , power silicon members  125  are insulated gate bipolar transistors (IGBTs) and diodes  127 . Each switching circuit  120  or switching module  125  may be a commercially available switching circuit marketed by companies such as International Rectifier of El Segundo, Calif. 
       FIG. 1  also illustrates the layered construction of an exemplary power module  100  that is assembled in a “thermal stack.” As illustrated in  FIG. 1 , the power module  100  comprises a primary substrate  110 , a switching circuit  120 , a secondary substrate  130 , and various solder connections  10 . 
     Transient thermal impedance (Zth) and steady state thermal impedance (Rth) are key metrics in the design of a power module. The primary substrate  110  includes an outer copper layer  112 , a central ceramic layer  114 , and an inner discontinuous copper layer  116 , with primary substrate  110  thus comprising a direct bonded copper (“DBC”) substrate. Correspondingly, secondary substrate  130  comprises an outer copper layer  136 , a central ceramic layer  134 , and an inner copper layer  132  such that secondary substrate  130  also comprises a DBC substrate. As also illustrated in  FIG. 1 , between substrates  110  and  130  are positioned power silicon members  125 , such as either an IGBT or a diode, with various solder connections  10  formed between the substrates  110  and  130  and the power silicon members  125 . 
     In one embodiment, solder connections  10  may alternatively be sintered connections. The use of sintered connections, such as silver based sintering, provides higher melt temperatures relative to soldered connections  10 . Sintering, thus, provides a greater delta difference relative to the operating temperatures of the switching devices  120  and, in turn, may increase reliability in view of the cyclic temperature cycling of the power module  100  in operation. Further still, formation of sintered connections  10  via a sintering process employing the applications of both temperature and pressure may be used to promote flatness of switching devices  120 . 
       FIG. 2  illustrates a power module  100 , with an added thermal mass or slug  210  in accordance with a feature of the present invention. As illustrated in  FIG. 2 , the thermal mass or slug  210  may be attached to the outer copper layer  136  of the secondary DBC substrate  130  using a thermally conductive layer  215 . A thermally conductive layer  215  may include thermally conductive adhesives, soldering, sintering, and laminated foils. A type of material used for the thermal mass/slug  210  and the material  215  of the thermally conductive layer used in the attachment process will determine whether a plating of the DEC substrate copper layer  136  and/or mating surface of the slug  210  is required. For example, if a silver epoxy is used as a thermal adhesive, then silver plating of the DBC substrate&#39;s copper layer  136  and of the copper slug  210  would be required. It is important to minimize the thickness of the thermally conductive layer  215  to improve the thermal impedance between the DBC substrate copper layer  136  and the copper slug  210 . A size and thickness may be varied to meet application needs and budget. 
     In one embodiment, as illustrated in  FIG. 3 , a power module  100  comprises four switching circuits  120  sandwiched between the substrate  110  and four corresponding secondary substrates  130 .  FIG. 3  also illustrates that each secondary substrate  130  is also paired with a respective copper mass/slug  210 . While a single copper mass  210  may be placed above the four switching circuits  120  for thermal cooling, irregularities in the soldering connections  10  and in the power silicon members  125  themselves may result in one or more DBC substrate copper layers  136  not making adequate contact with the single thermal mass/slug  210 . Instead, by using individual thermal masses/slugs  210 , each thermal mass/slug  210  need only deal with a single DBC substrate (and its individual height and planarity irregularities), and would therefore ameliorate issues related to variations in the other components of the other thermal stacks. The power module also includes lead frame terminals  150  and  152 . Lead frames may be joined to the primary substrate  110  in various manners such as laser welding, ultrasonic welding, and by sintering. The lead frames include power leads associated with the battery terminals and circuit elements of the power module. 
       FIGS. 4 and 5  illustrate an embodiment of the power module  100  illustrated in  FIGS. 2 and 3  with the addition of an over molded plastic cover  410  encapsulating or covering a side or portion of the slugs  210 . In one embodiment, the molded plastic cover  410  may be made as thin as possible. The power module  100  illustrated in  FIGS. 4 and 5  may be used with single-sided cooling operations. As illustrated in  FIG. 6 , the underside of the power module  100  will not be encased by the plastic molded cover  410 . The molded power module  100  illustrated in  FIG. 6  may be placed onto a heat sink through the exposed outer copper layer  112  of the substrate  110 . The use of such heat sinks are discussed in detail by James D. Tomkins in the previously incorporated U.S. patent application, titled, “POWER MODULE FOR CONVERTING DC TO AC.” 
       FIGS. 7 and 8  illustrate simplified cross-sectional views of over-molded power modules. In  FIG. 7 , the power module does not contain a thermal mass/slug  210  and therefore contains a thicker layer of plastic over the secondary DBC substrate  130 . Such an embodiment may be used, when only single-sided cooling is desired. Since double-sided cooling isn&#39;t desired, the additional thermal mass/slug  210  may be omitted. However, similar to the embodiment illustrated in  FIG. 4 , a thermal mass/slug  210  may be attached to the outer copper layer  136  of the secondary DBC substrate  130 , as illustrated in  FIG. 8 . As illustrated in  FIGS. 4 and 8 , a thickness of the thermal mass/slug  210  may be selected such that the thickness of the cover  410  over the thermal mass/slug  210  will be relatively thin. In one embodiment, the height of the cover  410  over the power module  100  will be the same height as that in  FIG. 7  so that either embodiment (with and without thermal mass/slug  210 ) will have the same package height. In one embodiment, by molding the plastic cover  410  to be as thin as possible over the thermal mass/slug  210 , the thermal mass/slug  210  may be as thick as possible (while still providing for a uniform thickness of the power module  100 ). 
     As illustrated in  FIGS. 9 and 10 , when double-sided cooling of the power module  100  is desired, a top surface  412  of the molded plastic cover  410  may be subjected to a grinding or milling operation to remove a portion of the molded plastic cover  410  over the thermal mass/slugs  210 , to reveal the thermal mass/slugs  210 , such that the heat sink discussed herein may be coupled to the exposed thermal mass/slugs  210  for double-sided cooling of the power module  100 . 
     As illustrated in  FIG. 11 , one of the problems that must be contended with when double-sided cooling is desired is the ability to attach a heat sink to both the top and the bottom of the power module  100 , where such double-sided cooling results in mechanical structures contacting components on both sides of the power module  100 . For optimal cooling efficiency, there are necessarily very tightly controlled soldering and assembly parameters to realize the desired module thickness and planarity requirements. Such module thickness and planarity requirements may be very difficult to consistently achieve. As illustrated in  FIG. 11 , the two thermal masses/slugs  210  on the right side of the figure have irregularities in height and/or planarity. For the sake of illustration and clarity, the structures illustrated are simplified and not drawn to scale. While such irregularities are not seen when only single-sided cooling is desired (and the molded plastic cover  410  is intact), the irregularities would be exposed when the top surfaces of the thermal masses/slugs  210  are exposed for double-sided cooling. 
     In one embodiment, to ensure that thickness and planarity requirements are able to be met when double-sided cooling is to be performed during a grinding or milling operation, a portion of the molded plastic cover  410  may be removed during the grinding/milling operation as well as milling/grinding a portion of the thermal masses/slugs  210  such that the irregularities are removed whereby the exposed surfaces are substantially planar with regard to each other. For example, as illustrated in  FIG. 11 , if a thermal mass/slug  210   a  were attached to an outer copper layer  136  of a secondary DBC substrate  130  with a thicker solder joint at one end than on the other, the thermal mass/slug  210   a  would be titled. As also illustrated in  FIG. 11 , if a thermal mass/slug  210   b  were attached to a thermal stack of a secondary DBC substrate, switching circuit and DBC substrate with an irregular height, the thermal mass/slug  210   b  would have an irregular height when compared to the other thermal masses/slugs  210 . As illustrated in  FIG. 12 , during the grinding/milling operation that could be used to remove the portion of the molded plastic cover over the thermal masses/slugs  210 , the grinding/milling operation may also be used to remove height and planarity irregularities. 
     As illustrated in  FIGS. 9 and 12 , even when thickness and planarity irregularities are present in a power module  100 , the grinding/milling operation can be used to remove the thickness and planarity irregularities, such that a desired power module height and planarity (with respect to the underside of the power module  100 ) can be realized. As illustrated in  FIG. 12 , after the grinding/milling operation, a desired height and planarity regularity can be achieved. As noted above, for the sake of clarity, the structures are simplified and not drawn to scale. 
       FIG. 13  illustrates an exemplary flow diagram for a grinding/milling process for when a power module  100 , such as illustrated in  FIGS. 4 and 8  (that contain thermal masses/slugs  210 ), is to be used with double-sided cooling. As discussed herein, and illustrated in  FIGS. 11 and 12 , an exemplary grinding/milling operation/process may be used to remove any height/planarity irregularities of the thermal masses/slugs  210  of the power module  100 . 
     In step  1302  of  FIG. 13 , when a power module  100  is to be used with double-sided cooling, a grinding/milling operation/process is used to remove a selected thickness of molded plastic from an upper portion of the cover  410  of the power module  100 . In one embodiment, a selected thickness of molded plastic to be removed may be defined by an average thickness of the molded plastic over the thermal masses/slugs  210 . 
     In step  1304  of  FIG. 13 , when the initial grinding/milling is completed, a determination is made as to whether there are any height and/or planarity irregularities in the exposed thermal masses/slugs  210  of the power module  100 . In one embodiment, the initial grinding/milling will remove the layer of molded plastic of the cover  410  over the thermal masses/slugs  210 . When there are no detected height or planarity irregularities in the exposed thermal masses/slugs  210  of the power module  100 , the process continues on to step  1306  of  FIG. 13  and the grinding/milling process is complete. 
     When there are detected height and/or planarity irregularities in the exposed thermal masses/slugs  210  of the power module  100 , the process continues on to step  1308  of  FIG. 13 . In step  1308  of  FIG. 13 , a second grinding/milling operation/process is performed to remove a portion of one or more thermal masses/slugs  210  to remove any detected irregularities in the exposed thermal masses/slugs  210  of the power module  100 . In one embodiment, the amount of additional grinding/milling is defined by detected height and/or planarity irregularities. After the additional grinding/milling operation/process has completed, the process continues back to step  1304  of  FIG. 13  for a determination as to whether there are still height/planarity irregularities in the exposed thermal masses/slugs  210  of the power module  100 . In one embodiment, steps  1308  and  1304  may be repeated several times. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the present invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.