Abstract:
A power semiconductor device having a power element sealed into a package, which comprises the package, the power element fixed in the package, a bonding wire connected to the power element, and a gel insulator for covering the power element, characterized in that foam material is further charged so as to fill a cavity left inside of said package.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    A related patent application is a commonly assigned Japanese Patent Application No. 2001-257798 filed on Aug. 28, 2001, which is incorporated by reference into the present patent application.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a power semiconductor device, and more particularly to a power semiconductor device provided with vibration-proof.  
           [0004]    2. Description of the Related Art  
           [0005]    [0005]FIG. 3 is a sectional view of a conventional power semiconductor device indicated in its entirety as  500 . Power semiconductor device  500  includes a heat spreader plate  1 . Onto the heat spreader plate  1 , an insulation substrate  2  is connected via a solder layer  6 . The insulation substrate has wiring patterns  4  and  5  formed on both sides of a ceramic substrate  3 . Onto the wiring pattern  4 , power elements  7  are connected via a solder layer  8 .  
           [0006]    On the heat spreader plate  1 , a case  9  is provided so as to surround the insulation substrate  2 . The case  9  has a terminal  10 . The terminal  10  and wiring pattern  4 , as well as the terminal  10  and power elements  7  are connected via bonding wires  11 . Silicone gel  12  is charged inside of the case  9  so as to embed the insulation substrate  2 , the power elements  7 , and the bonding wires  11 . On the other hand, the part that is not filled with silicon gel  12  is left as hollow part  20 .  
           [0007]    In addition, a shield board  13  and a control board  14  are fixed inside of the case  9 . On the control board  14 , control elements  15  are provided and coupled to the power elements  7  via a relay terminal  16 . On the control board  14 , a signal terminal  17  is provided.  
           [0008]    Over the case  9 , a cover  18  is provided. The signal terminal  17  projects outside from a hole (not shown) provided through the cover  18 . The heat spreader plate  1 , the case  9 , and the cover  18  form a package.  
           [0009]    When the power semiconductor device  500  is mounted on a train or an automobile and used therein, external vibrations at frequencies of approx. 20 Hz to 2,000 Hz are applied to the power semiconductor device  500 . Therefore, resonance occurs in soft silicone gel  12 , and the shield board  13  and the control board  14  both screwed onto the case  9 , at predetermined respective characteristic frequencies thereof. As a result, the amplitude of the vibration of the silicon gel  12  or the like is larger than those of the external vibrations applied thereto. This phenomenon poses a problem, such as breakage of bonding wires  11  in the silicon gel  12 , and failure of the control board  14  or the relay terminal  16 .  
         SUMMARY OF THE INVENTION  
         [0010]    Therefore, the present invention aims to provide a power semiconductor device preventing the constituent components from failure caused by the resonance thereof when the device is used under such conditions that external vibrations are applied thereto.  
           [0011]    In accordance with one aspect of the present invention, there is provided a power semiconductor device having a power element sealed into a package. The power semiconductor device includes: the package; the power element fixed in the package; a bonding wire connected to the power element; and a gel insulator for covering the power element. The power semiconductor device is characterized in that foam material is charged so as to fill a cavity left inside of the package.  
           [0012]    In accordance with another aspect of the present invention, there is provided another power semiconductor device having a power element sealed into a package. The power semiconductor device includes: the package; the power element fixed in the package; and a bonding wire connected to the power element. In addition, the power semiconductor device is characterized in that foam material is charged so as to fill a cavity in the package. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a cross sectional view of a power semiconductor device in accordance with a first preferred embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is a cross sectional view of a power semiconductor device in accordance with a second preferred embodiment of the present invention; and  
         [0015]    [0015]FIG. 3 is a cross sectional view of a conventional power semiconductor device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Preferred Embodiment  
       [0016]    [0016]FIG. 1 is a sectional view of a power semiconductor device in accordance with this embodiment indicated in its entirety as  100 . In the drawing, reference numerals like those shown in FIG. 3 refer to similar or corresponding elements.  
         [0017]    The power semiconductor device  100  includes heat spreader plate  1 . The heat spreader plate  1  is made of copper or aluminum. Onto the heat spreader plate  1 , an insulation substrate  2  is connected via a solder layer  6 . The insulation substrate has wiring patterns  4  and  5  formed on both sides of a ceramic substrate  3 . The ceramic substrate  3  is made of aluminum nitride, alumina, or other materials. Each of wiring patterns  4  and  5  is made of a thin layer of copper or aluminum. Onto the wiring pattern  4 , power elements  7 , such as an insulated gate bipolar transistor (IGBT) and a diode, are connected via solder layers  8 .  
         [0018]    On heat spreader plate  1 , a case  9  is provided so as to surround an insulation substrate  2 . In the case  9 , terminal  10  for external elicitation is provided. The terminal  10  is integrated into the case  9  when the case  9  is formed. The connections of the wiring pattern  4  or the power elements  7  with the terminal  10 , are formed via bonding wires  11  made of aluminum, for example. So are the wiring pattern  4  or the power elements  7  with the relay terminal  16 .  
         [0019]    In addition, a shield board  13  and a control board  14  are fixed inside of the case  9 . On the control board  14 , control elements  15  are provided and coupled to the power elements  7  via the relay terminal  16 . The control board  14  and the relay terminal  16  are connected by solder, for example. Provided on the control board  14  is a signal terminal  17  for inputting external control signals into the control elements  15 .  
         [0020]    Over the case  9 , a cover  18  is provided. The signal terminal  17  projects outside from a hole (not shown) provided through the cover  18 .  
         [0021]    Inside of the case  9 , silicone gel  12  is charged so as to cover the power elements  7 . Covering the power elements  7  with the highly insulating silicone gel  12  in this manner can maintain the insulating properties between the terminals of the power elements  7 .  
         [0022]    In addition, polyurethane foam  19  is charged so as to fill the cavity above the silicon gel  12 . The polyurethane foam  19  is charged so as to cover bonding wires  11 , the shield board  13 , the control board  14 , and the relay terminal  16 , and other components.  
         [0023]    The polyurethane foam  19  has a low density ranging from approx. 25 kg/m 3  to 50 kg/m 3 . For this reason, the characteristic frequency of the polyurethane foam  19  largely differs from the frequencies (approx. 20 Hz to 2,000 Hz) of external vibrations applied to the power semiconductor device  100  when the device is mounted on an automobile. Thus, the resonance does not occur in the device even when external vibrations are applied thereto.  
         [0024]    Furthermore, because the polyurethane foam  19  is of closed-cell type, the insulating properties thereof can be maintained higher than those of an open-cell type.  
         [0025]    In addition, the shield board  13  and other components are in contact with the polyurethane foam  19 . This increases the rigidity of the shield board  13  and other components. Therefore, the characteristic frequencies of the shield board  13  and other components can be excluded from a frequency range of external vibrations applied to the device and the resonance of the shield board  13  and other components can be prevented or reduced.  
         [0026]    Even when the characteristic frequencies of the shield board  13  and other components cannot be excluded from the frequency range of external vibrations applied to the device, the amplitudes of the shield board  13  and other components can be reduced.  
         [0027]    Moreover, the vibration of the silicon gel  12  is inhibited by the contact thereof with the polyurethane foam  19 .  
         [0028]    Especially in this embodiment, as many of the bonding wires  11  as possible are covered with the polyurethane foam  19 . Thus, even when the silicon gel  12  vibrates, the vibration of bonding wires  11  is inhibited and the breakage of bonding wires  11  can be prevented.  
         [0029]    As mentioned above, with the power semiconductor device  100  in accordance with this embodiment, even when it is used under such conditions that external vibrations are applied, resonance of the constituent components thereof can be inhibited and the failure thereof can be prevented.  
         [0030]    JP, 3-112153, A discloses a memory module having foam material charged inside of a case thereof. However, the formed material is used as cushioning material for absorbing external shocks or vibrations applied to the device. Unlike the power semiconductor device  100 , the formed material does not aim to inhibit or prevent the resonance. Especially, unlike the power semiconductor device  100 , the memory module is not intended to be used under such conditions that continuous vibrations are applied.  
       Second Preferred Embodiment  
       [0031]    [0031]FIG. 2 is a sectional view of a power semiconductor device of this embodiment indicated in its entirety as  200 . In the drawing, reference numerals like those as shown in FIG. 1 refer to similar or corresponding elements.  
         [0032]    With power semiconductor device  200 , the surroundings of power elements  7  or other components are embedded with polyurethane foam  19  only, and no silicon gel is used.  
         [0033]    While the use of the polyurethane foam  19  instead of silicon gel slightly reduces insulation characteristics of the device, it can improve vibration absorption characteristics. For example, when the interval of the electrodes for high voltage application is wide, the use of the polyurethane foam  19  instead of the silicone gel can provide sufficient insulating characteristics.  
         [0034]    In First and Second Preferred Embodiments, the power semiconductor devices  100  and  200  having the shield board  13  and the control board  14  are described. The present invention can be implemented in a power semiconductor device that does not include these components. In this case, the damage to bonding wires caused by vibrations can be prevented.