Abstract:
A leaded semiconductor power module includes a first heatsink, an electrically insulated substrate thermally coupled to the first heatsink, one or more semiconductor chips, a leadframe substrate, and a second heatsink thermally coupled to the leadframe substrate, the assembly being overmolded with an encapsulant to expose the first heatsink, the second heatsink and peripheral terminals of the leadframe substrate. The semiconductor chips are electrically and structurally coupled to both the insulated substrate and the leadframe substrate, and conductive spacers electrically and structurally couple the insulated substrate to the leadframe substrate.

Description:
TECHNICAL FIELD 
   The present invention relates to a power semiconductor power module that includes power and control leads, and that is configured for both top-side and bottom-side cooling. 
   BACKGROUND OF THE INVENTION 
   Semiconductor modules used in power switching applications can include one or more current control devices such as power IGBTs or power MOSFETs and power diodes. For example, a common configuration for inverter and converter applications includes two power transistors and two power diodes in the form of a half-H bridge circuit. In order to optimize reliability and heat rejection, manufacturers have tended to minimize or eliminate wire bonding and to design the modules for double sided cooling. For example, the U.S. Pat. No. 6,873,043 to Oman discloses a surface-mount or lead-less package configuration in which a power transistor die having top-side and bottom-side contacts is sandwiched between a pair of copper-clad substrates bonded directly to the die contacts. The direct electrical and structural attachment eliminates wire bonds, and heat generated by the die is rejected through both substrates. The object of the present invention is to achieve these same objectives in a leaded semiconductor power module. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a leaded semiconductor power module including a first heatsink, an electrically insulated substrate thermally coupled to the first heatsink, one or more semiconductor chips, a leadframe substrate, and a second heatsink thermally coupled to the leadframe substrate, the assembly being overmolded to expose the first heatsink, the second heatsink and peripheral terminals of the leadframe substrate. The semiconductor chips are direct bond coupled to both the insulated substrate and the leadframe substrate, and conductive spacers electrically and structurally couple the insulated substrate to the leadframe substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a first isometric view of an un-finned leaded semiconductor power module according to the present invention; 
       FIG. 1B  is a second isometric view of the leaded semiconductor power module of  FIG. 1A ; 
       FIG. 1C  is an isometric view of a finned leaded semiconductor power module according to the present invention; 
       FIG. 1D  is a circuit diagram for the leaded semiconductor power modules of  FIGS. 1A-1C ; 
       FIG. 2A  is an isometric view of a lower heatsink and insulated substrate for the leaded semiconductor power module of  FIGS. 1A-1B ; 
       FIG. 2B  is an isometric view of the lower heatsink and insulated substrate of  FIG. 2A , with semiconductor chips mounted on the insulated substrate; 
       FIG. 2C  is an isometric view of a leadframe substrate for the leaded semiconductor power modules of  FIGS. 1A-1C ; 
       FIG. 2D  is an isometric view of an assembly including the lower heatsink and insulated substrate of  FIG. 2B , and the leadframe substrate of  FIG. 2C ; and 
       FIG. 2E  is an isometric view of the semiconductor power module of  FIGS. 1A-1B  prior to overmolding. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the drawings,  FIGS. 1A-1D  depict exemplary half-H bridge versions of a leaded semiconductor power module according to this invention. Other circuit variants such as a single switching device, parallel-connected switching devices, and a full-H bridge are also included within the scope of the invention.  FIGS. 1A-1B  depict a first version of the power module, designated generally by the reference numeral  10 ;  FIG. 1C  depicts a second version of the module, designated generally by the reference numeral  10 ′; and  FIG. 1D  is a diagram of the half-H bridge circuit. 
   Referring first to  FIG. 1D , the half-H bridge circuit includes upper and lower insulated-gate power bipolar transistors (IGBTs)  12  and  14 , and associated anti-parallel diodes  16  and  18 . The circuit components  12 - 18  are coupled to a set of module terminals, including gate terminals G 1  and G 2  respective connected to the gates of IGBTs  12  and  14 , direct voltage terminals DC+ and DC− respectively connected to the collector of IGBT  12  (and cathode of diode  16 ) and the emitter of IGBT  14  (and anode of diode  18 ), and an AC phase terminal ACPH connected to the emitter of IGBT  12  (and anode of diode  16 ) and the collector of IGBT  14  (and cathode of diode  18 ). In an alternate implementation, the IGBTs  12  and  14  may be replaced with power MOSFETs, power diodes, or some other vertical integrated circuit devices. 
   Referring to  FIGS. 1A-1C , the power modules  10 ,  10 ′ are overmolded with an encapsulant  20 ,  20 ′ such that the aforementioned terminals G 1 , G 2 , DC+, DC−, ACPH and upper and lower heatsinks  22 ,  22 ′ and  24 ,  24 ′ remain exposed. The exposed terminals G 1 , G 2 , DC+, DC−, ACPH are electrically coupled to external circuitry and/or bus bars to define a DC-to-AC inverter, for example. In the configuration of  FIGS. 1A-1B , the upper and lower heatsinks  22  and  24  are planar, and in a typical application are thermally coupled to planar regions of a housing to dissipate heat generated by the module  10 . In the configuration of  FIG. 1C , the upper and lower heatsinks  22 ′ and  24 ′ are finned to promote efficient heat transfer to air or a fluid coolant. Other heatsink variants are possible, of course; for example, one of the heatsinks may be planar and the other finned, and so forth. The significant aspect is that the modules  10 ,  10 ′ are configured for double-sided (i.e., top-side and bottom-side) cooling. 
   As further explained below, the terminals G 1 , G 2 , DC+, DC−, ACPH are defined by the outboard ends of an equal number of metal leadframe segments  26 ,  28 ,  30 ,  32 ,  34  initially joined by metal bridge segments  36  that are removed after the overmolding process. The bridge segments  36  are located so that they are exposed—that is, not covered by the encapsulant  20 ,  20 ′—and then severed so that the leadframe segments formerly joined by the bridge segments  36  become electrically distinct terminals. 
     FIGS. 2A-2E  illustrate several successive stages in the manufacture of module  10  of  FIGS. 1A-1B .  FIG. 2A  illustrates an assembly of lower heatsink  24  and an insulated substrate generally designated by the reference numeral  40 .  FIG. 2B  illustrates the assembly of  FIG. 2A , with the IGBTs  12 ,  14  and diodes  16 ,  18  (all shown in bare die form) bonded to the insulated substrate  40 .  FIG. 2C  illustrates the inboard side of a leadframe substrate, generally designated by the reference numeral  42 , and  FIG. 2D  illustrates the assembly of  FIG. 2B , with the leadframe substrate  42  bonded to insulated substrate  40  and the top faces of IGBTs  12 ,  14  and diodes  16 ,  18 . And finally,  FIG. 2E  illustrates the assembly of  FIG. 2D , with the upper heatsink  22  mounted on the leadframe substrate  42 . 
   Referring to the  FIGS. 2A-2B , the insulated substrate  40  includes a set of copper pads  44 ,  46 ,  50 ,  52  that provide mounting surfaces  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  for the IGBTs  12 - 14 , diodes  16 - 18 , and leadframe substrate  42 . In the illustrated embodiment, the lower heatsink  24  is fabricated of a conductive material such as aluminum, and the copper pads  44 ,  46 ,  50 ,  52  are insulated from the lower heatsink by an insulation layer  84  deposited on the its inboard face. The copper pads  44 ,  46 ,  50 ,  52  could be fabricated as a single copper conductor, such as a leadframe substrate, with the metal bridge segments removed after overmolding as described above. In applications where the lower heatsink  24  is fabricated of an electrically insulative material such as ceramic, the insulation layer  84  may be omitted. In the illustrated embodiment, the mounting surfaces  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  are defined by solder paste deposited using a conventional plating and masking process, as can seen in  FIG. 2A . The mounting surfaces  56  and  58  correspond to the emitter and gate contacts of IGBT  12 ; the mounting surfaces  60  and  62  correspond to the emitter and gate contacts of IGBT  14 ; and the mounting surfaces  64  and  66  correspond to the anode contacts of diodes  16  and  18 . The mounting surfaces  68 ,  70 ,  72  and  74  are aligned for engagement with the leadframe substrate  42 , and are augmented in height by a set of copper spacers  90 ,  92 ,  94 ,  96  having a thickness that matches that of IGBTs  12 - 14  and diodes  16 - 18 , so that the upper surfaces of IGBTs  12 - 14 , diodes  16 - 18  and spacers  90 - 96  are substantially co-planar. 
   Referring to  FIGS. 2C-2D , the leadframe segments  26 ,  28 ,  30 ,  32 ,  34  of leadframe substrate  42  are provided with mounting surfaces  100 - 114  that align with the IGBTs  12 - 14 , diodes  16 - 18  and spacers  90 - 96 . The mounting surfaces  100 - 114  may be formed in the same fashion as the mounting surfaces  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  of the insulated substrate  40 . The mounting surface  100  formed on leadframe segment  26  mates with the copper spacer  94  so that the outboard end of leadframe segment  26  defines the gate terminal G 1  of the power module  10 . Similarly, the mounting surface  102  formed on leadframe segment  28  mates with the copper spacer  96  so that the outboard end of leadframe segment  28  defines the gate terminal G 2 . The mounting surfaces  104  and  106  formed on leadframe segment  30  respectively mate with the upper face (collector) of IGBT  12  and the upper face (cathode) of diode  16  so that the outboard end of leadframe segment  30  defines the DC+terminal of the power module  10 . The mounting surface  108  formed on leadframe segment  32  mates with the copper spacer  90  so that the outboard end of leadframe segment  32  defines the DC-terminal of the power module  10 . And finally, the mounting surfaces  110 ,  112  and  114  formed on leadframe segment  34  respectively mate with the copper spacer  92 , the upper face (collector) of IGBT  14  and the upper face (cathode) of diode  18  so that the outboard end of leadframe segment  34  defines the ACPH terminal of the power module  10 . 
   Referring to  FIG. 2E , the upper heatsink  22  is mounted atop the leadframe substrate  42 . In the illustrated embodiment, the upper heatsink  22  is fabricated of a conductive material such as aluminum, and an insulation layer (not shown) is deposited on its inboard face to electrically insulate heatsink  22  from the leadframe substrate  42 . As with the lower heatsink  24 , the upper heatsink  22  may alternately be fabricated of an electrically insulative material such as ceramic, in which case the insulation layer may be omitted. The assembly depicted in  FIG. 2E  is then selectively overmolded to form the power module depicted in  FIGS. 1A-1B , and the exposed bridge segments  36  of leadframe substrate  42  are removed to complete the fabrication process. 
   In summary, the leaded semiconductor power module  10 / 10 ′ of the present invention is characterized by its ease of assembly, low-cost, improved reliability due to the elimination of wire bonds, and double-sided cooling. The overmolded encapsulant  20  seals around the components and solder joints, and mechanically isolates the components and solder joints from terminal stress during removal of the bridge segments  36  and subsequent installation in a product. 
   While the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the copper spacers  90 - 96  may be formed on the leadframe substrate  42  instead of the insulated substrate  40 , the insulated substrate  40  may be formed on the upper heatsink  22  instead of the lower heatsink  24 , and so on. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.