Abstract:
A power semiconductor device includes: a mold unit that includes a power semiconductor element, a base plate, and a mold unit, the power semiconductor element being mounted on one surface of the base plate, a convex portion being formed on an other surface of the base plate, the convex portion including a plurality of grooves, the mold unit having a mold resin with which the power semiconductor element is sealed in such a manner as to expose the convex portion; a plurality of radiation fins inserted into the grooves, respectively, and fixedly attached to the base plate by swaging; and a metal plate that includes a opening into which the convex portion is inserted, the metal plate being arranged between the mold unit and the radiation fins with the convex portion inserted into the opening, wherein the metal plate includes a protrusion that protrudes from an edge of the opening and that digs into a side surface of the convex portion when the convex portion is inserted into the opening.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2012/069487 filed Jul. 31, 2012, the contents of which are incorporated herein by reference in their entirety. 
     FIELD 
     The present invention relates to a power semiconductor device. 
     BACKGROUND 
     Conventionally, when a heat sink is attached for cooling electronic components (power semiconductor elements) that generate a large amount of heat such as a CPU (central processing unit) and a power transistor, application of thermal grease is widely performed to fill a minute gap between a contact surface of the electronic components and that of the heat sink to improve heat dissipation performance. 
     The thermal conductivity of the thermal grease is quite lower than those of metals. Accordingly, a radiation-fin-integrated power semiconductor device having radiation fins integrated with a base plate of a metallic part of the power semiconductor device without using the thermal grease is also realized to further improve the heat dissipation performance. In the radiation-fin-integrated power semiconductor device, grooves for joining the radiation fins are provided in the base plate, resin molding is performed in a state of exposing a part of the surface of the base plate including a portion in which these grooves are formed, and the radiation fins are inserted into the grooves of the base plate and then swaged to be fixedly attached to the grooves, thereby integrating the base plate with the radiation fins to improve the heat dissipation performance. 
     In relation to the power semiconductor device adapted to improve the heat dissipation performance, it is known that radiation noise from power semiconductor elements and malfunction is suppressed by inserting a metal plate between the radiation fins and the base plate and by causing this metal plate to function to connect the power semiconductor device to an earth potential (see Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open No. 2012-49167 
       
    
     SUMMARY 
     Technical Problem 
     In the conventional technique, heating and cooling is carried out at a time of sealing the power semiconductor elements with resin. This often causes a module of the power semiconductor device to be completed in a warped state or causes the metal plate to be inserted itself to be warped. Therefore, the conventional technique has a problem that a gap is generated between the metal plate and the base plate, which increases the electrical resistance between the metal plate and the base plate. Furthermore, an oxide film is formed in the atmosphere on the surface of the base plate that is a metal, and the oxide film on the metal is higher than the metal itself in the electrical resistance. With the conventional technique, an electrical contact between the metal plate and the base plate is realized by placing the metal plate between the radiation fins and the base plate. Accordingly, the metal plate and the base plate are electrically connected to each other via the oxide film formed on the surface of the base plate except for minute regions where the oxide film on the surface of the base plate is damaged as a result of the contact of the metal plate with the base plate and where the metal is exposed. For this reason, even if neither the metal plate nor the base plate is warped and the base plate comes in surface contact with the metal plate, a ratio of portions that are made conductive as a result of the contact of the metals is low, disadvantageously resulting in a high electrical resistance between the base plate and the metal plate. 
     The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a power semiconductor device having an enhanced effect of suppressing radiation noise from a power semiconductor element and malfunction. 
     Solution to Problem 
     There is provided a power semiconductor device comprising: a mold unit that includes a power semiconductor element, a base plate, and a mold resin, the power semiconductor element being mounted on one surface of the base plate, a convex portion being formed on an other surface of the base plate, the convex portion including a plurality of grooves, the power semiconductor element being sealed with the mold resin in such a manner as to expose the convex portion; a plurality of radiation fins inserted into the grooves, respectively, and fixedly attached to the base plate by swaging; and a metal plate that includes a cut-off portion into which the convex portion is inserted, the metal plate being arranged between the mold unit and the radiation fins with the convex portion inserted into the cut-off portion, wherein the metal plate includes a protrusion that protrudes from an edge of the cut-off portion and that digs into a side surface of the convex portion when the convex portion is inserted into the cut-off portion. 
     Advantageous Effects of Invention 
     The power semiconductor device according to the present invention can reduce the electrical resistance between a base plate and a metal plate and enhance the effect of suppressing radiation noise from power semiconductor elements and malfunction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of a configuration of a power semiconductor device according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the power semiconductor device according to the first embodiment. 
         FIG. 3  is a cross-sectional view of a mold unit in the power semiconductor device according to the first embodiment. 
         FIG. 4  is a plan view of a metal plate in the power semiconductor device according to the first embodiment. 
         FIG. 5  is an enlarged cross-sectional view of a portion in which the metal plate contacts a base plate. 
         FIG. 6  is an enlarged cross-sectional view of the portion in which the metal plate contacts the base plate. 
         FIG. 7  is an example of the metal plate from which a notch is cut off as a cut-off portion. 
         FIG. 8  is a plan view of a metal plate in a power semiconductor device according to a second embodiment of the present invention. 
         FIG. 9  is an enlarged cross-sectional view of a portion in which the metal plate contacts a base plate. 
         FIG. 10  is an enlarged cross-sectional view of the portion in which the metal plate contacts the base plate. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of a power semiconductor device according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. 
     First Embodiment 
       FIG. 1  is an exploded perspective view of a configuration of a power semiconductor device according to a first embodiment of the present invention.  FIG. 2  is a cross-sectional view of the power semiconductor device according to the first embodiment. A power semiconductor device  100  according to the first embodiment includes a mold unit  1 , radiation fins  2 , and a metal plate  3 . 
       FIG. 3  is a cross-sectional view of the mold unit in the power semiconductor device according to the first embodiment. The mold unit  1  includes power semiconductor elements  11 , a base plate  12  having one surface on which the power semiconductor elements  11  are mounted, and a mold resin  13  with which the power semiconductor elements  11  are sealed. The mold unit  1  is formed by integrally molding the base plate  12  on which the power semiconductor elements  11  are mounted with the mold resin  13 . A convex portion  121  is formed on a surface of the base plate  12  opposite to the surface on which the power semiconductor elements  11  are mounted, and the convex portion  121  protrudes from the mold resin  13 . A plurality of grooves  122  are provided in the convex portion  121 . Peripheral edges of the convex portion  121  form a flat surface  123 . The base plate  12  is formed using, as a material, a metal (such as aluminum) softer and higher in thermal conductivity than the metal plate  3 . 
       FIG. 4  is a plan view of the metal plate in the power semiconductor device according to the first embodiment. A substantially rectangular opening  31  is cut off from the metal plate  3  as a cut-off portion, and the convex portion  121  can be inserted into the opening  31 . The substantially rectangular shape mentioned in this case includes a shape having rounded shape portion to prevent concentration of a stress on the corners. Protrusions  32  are provided on edges of two opposed sides (two short sides in this case) of the opening  31 . The distance L 2  between tip ends of the two protrusions  32  is smaller than the width L 1  of the convex portion  121  on the base plate  12 . The metal plate  3  is formed using a metal harder than the material of the base plate  12 . For example, a steel plate can be applied as the metal plate  3 . 
     The radiation fins  2  are thin-plate fins and as many radiation fins  2  as the grooves  122  provided in the convex portion  121  are prepared. The radiation fins  2  are inserted into the respective grooves  122  of the convex portion  121 , swaged in such a manner as to be pressed from left and right sides, and fixed to the base plate  12 . 
       FIGS. 5 and 6  are enlarged cross-sectional views of a portion in which the metal plate contacts the base plate.  FIG. 5  depicts a state before the convex portion  121  is inserted into the opening  31  and  FIG. 6  depicts a state after the convex portion  121  is inserted into the opening  31 . Because the distance L 2  between the tip ends of the protrusions  32  is smaller than the width L 1  of the convex portion  121  on the base plate  12 , the protrusions  32  scrapes side surfaces  124  of the convex portion  121  when the convex portion  121  on the base plate  12  is inserted into the opening  31 , and the convex portion  121  is fitted into the opening  31  in a state in which the protrusions  32  dig into the side surfaces  124  of the convex portion  121 . At that time, an oxide film on the surface of the base plate  12  is damaged, an internal non-oxidized metal is exposed, and electrical connection between the metal plate  3  and the base plate  12  is realized. By allowing the protrusions  32  to act as portions in which the metals contact each other as a whole, an electrical resistance between the metal plate  3  and the base plate  12  can be reduced. 
     If the protrusions  32  are too small, it is difficult to secure a sufficient area of the portions made conductive as a result of the contact of the metals between the metal plate  3  and the base plate  12 . If the protrusions  32  are too large, a gap between the metal plate  3  and the base plate  12  becomes large to hinder the downscaling of the power semiconductor device  100 . When the size of each protrusion  32  is set to about 0.5 millimeter to 1.5 millimeters, it is possible to secure the area of the portions made conductive as a result of the contact of the metals between the metal plate  3  and the base plate  12  without increasing the power semiconductor device  100  in the size. However, this range is given only as an example and the present invention is not limited to this range. 
     The metal plate  3  of a shape having the protrusions  32  on the edges of only the two short sides of the opening  31  has been described by way of example. Alternatively, the protrusions  32  can be provided on edges of only long sides of the opening  31  or provided on the edges of both the short sides and the long sides thereof. When the protrusions  32  are provided on the edge of only one of two opposed sides of the opening  31 , a side surface of the metal plate  3  on the side on which no protrusions  32  are provided is pressed against the side surface  124  of the convex portion  121 . This can reduce the electrical resistance between the metal plate  3  and the base plate  12  as compared with a conventional structure that provides conduction on the flat surface  123 . Furthermore, the number of protrusions  32  provided on the edge of each side of the opening  31  is not limited to two but can be two or more, or two or less. Further, the number of protrusions  32  can be set differently among the edges of the respective sides of the opening  31 . 
     It has been described above that the opening  31  into which the convex portion  121  is inserted is cut off from the metal plate  3  as the cut-off portion. Alternatively, a notch  33  instead of the opening  31  can be cut off as the cut-off portion to form the metal plate  3  into a substantially U-shape.  FIG. 7  is an example of the metal plate from which the notch is cut off as the cut-off portion. When the notch  33  into which the convex portion  121  is inserted is cut off as the cut-off portion, the protrusions  32  can be formed on two opposed sides across an open side, thereby scraping the side surfaces  124  of the convex portion  121  when the convex portion  121  is inserted into the notch  33 . By providing the cut-off portion into which the convex portion  121  can be inserted to have a notch shape, it is possible to reduce an amount of the material used for the metal plate  3 . 
     The power semiconductor device according to the first embodiment can reduce the electrical resistance between the base plate and the metal plate and enhance the effect of suppressing radiation noise from the power semiconductor elements and malfunction. 
     Second Embodiment 
     A power semiconductor device according to a second embodiment is similar to that according to the first embodiment and includes the mold unit  1 , the radiation fins  2 , and the metal plate  3  as shown in  FIGS. 1 and 2 . As shown in  FIG. 3 , the mold unit  1  includes the power semiconductor elements  11 , the base plate  12  on which the power semiconductor elements  11  are mounted, and the mold resin  13  with which the power semiconductor elements  11  are sealed.  FIG. 8  is a plan view of the metal plate in the power semiconductor device according to the second embodiment. The opening  31  is cut off from the metal plate  3  as the cut-off portion and the convex portion  121  can be inserted into the opening  31 . A metal foil  34  is bonded to edges of four sides of the opening  31 . As the metal foil  34 , a foil such as a copper foil or an aluminum foil high in malleability and made of a softer metal than the metal plate  3  is applicable. 
       FIGS. 9 and 10  are enlarged cross-sectional views of a portion in which the metal plate contacts the base plate.  FIG. 9  depicts a state before the convex portion  121  is inserted into the opening  31  and  FIG. 10  depicts a state after the convex portion  121  is inserted into the opening  31 . When the convex portion  121  on the base plate  12  is inserted into the opening  31 , the metal foil  34  deforms to conform to a shape of the gap between the base plate  12  and the metal plate  3 . By causing the metal foil  34  to fill the gap between the base plate  12  and the metal plate  3 , it is possible to increase a contact area in which the metal plate  3  contacts the base plate  12  and to reduce the electrical resistance between the metal plate  3  and the base plate  12 . 
     If the metal foil  34  is too thin, the metal foil  34  cannot sufficiently fill the gap between the base plate  12  and the metal plate  3 , and it is difficult to secure a sufficient contact area in which the metal plate  3  contacts the base plate  12 . If the metal foil  34  is too thick, the metal foil  34  does not easily deform, and it is difficult to secure a sufficient contact area in which the metal plate  3  contacts the base plate  12 . When the thickness of the metal foil  34  is set to about 0.1 millimeter to 0.3 millimeter, the contact area in which the metal plate  3  contacts the base plate  12  can be easily secured. However, this range is given only as an example and the present invention is not limited to this range. 
     It has been described above that the power semiconductor device is configured to use the metal plate  3  having the metal foil  34  bonded to the edges of the four sides of the opening  31  by way of example. It suffices to bond the metal foil  34  to the edge of at least one side of the opening  31 . However, it is desirable to arrange the metal foil  34  on the edges of all the sides of the opening  31  in view of increasing the contact area in which the base plate  12  contacts the metal plate  3 . Similarly to the first embodiment, the notch  33  instead of the opening  31  can be cut off from the metal plate  3  as the cut-off portion. 
     Similarly to the first embodiment, the power semiconductor device according to the second embodiment can reduce the electrical resistance between the base plate and the metal plate and enhance the effect of suppressing radiation noise from the power semiconductor elements and malfunction. 
     INDUSTRIAL APPLICABILITY 
     As described above, the power semiconductor device according to the present invention is useful in a feature that it can enhance the effect of suppressing radiation noise from power semiconductor elements and malfunction. 
     REFERENCE SIGNS LIST 
       1  mold unit,  2  radiation fin,  3  metal plate, power semiconductor element,  12  base plate,  13  mold resin,  31  opening,  32  protrusion,  33  notch,  34  metal foil,  100  power semiconductor device,  121  convex portion,  122  groove,  123  flat surface,  124  side surface.