Abstract:
A semiconductor device includes a semiconductor chip generating heat, a pair of heat sinks, which face each other, to conduct heat from both surfaces of the chip, a pair of compressible insulating sheets, and a mold resin covering the chip, the heat sinks, and the sheets such that the sheets are exposed from the surface of the resin. The mold resin is prevented from covering the outer surfaces of the heat sinks, which are pressed by mold parts, and breakage of the chip is avoided during molding. The plates are insulated by the sheets, so no dedicated insulating sheets for the heat sinks are needed after the device is completed.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-127516, which was filed on Apr. 25, 2001.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device in which a semiconductor chip generating heat is located between a pair of heat sinks and to a method for manufacturing the device.  
         [0003]     A semiconductor chip such as a power IC, which controls a large amount of electric power and current, generates so much heat that a semiconductor device using the chip requires a heat sink to dissipate heat. As shown in  FIG. 4 , in a proposed semiconductor device  1 , a semiconductor chip  4  and a heat sink coupler  5  are located between a pair of heat sinks  2 ,  3 . One heat sink  2  and the chip  4  are soldered together, the chip  4  and the coupler  5  are soldered together, and the coupler  5  and the other heat sink  3  are soldered together. A mold resin  7  covers the chip  4 , the heat sink coupler  5 , and the heat sinks  2 ,  3  such that the heat sinks  2 ,  3  are exposed from the surface of the resin  7 . Because the resin  7  covers neither of the outer surfaces of the heat sinks  2 ,  3 , heat is efficiently transferred from the heat sinks  2 ,  3  from both sides of the chip  4 .  
         [0004]     The semiconductor device  1  is molded in a mold  8  that includes a lower mold  8   a,  an upper mold  8   b,  and a movable mold  8   c,  as shown  FIG. 4 . The movable mold  8   c  presses the device  1  during the molding to prevent the resin  7  from covering the outer surfaces of the heat sinks  2 ,  3  while adjusting the pressure applied to the device  1  during molding. Therefore, the structure of the mold  8  is relatively complicated and the production cost of the mold  8  is relatively high. Thus, the production cost of the semiconductor device  1  is relatively high. Although the pressure applied to the device  1  is controlled, the semiconductor chip  4  tends to break during the molding because there is nothing to limit the force the chip  4  receives from the mold  8 .  
         [0005]     In addition, the outer surfaces of the heat sinks  2 ,  3  need to be insulated when the completed device  1  is inspected, screened, or assembled into a unit such as an inverter unit. For example, as shown in  FIG. 5 , when the device  1  is located between a pair of cooling plates  9 , two heat conducting sheets  10  made of insulating material are inserted between the plates  9  and the outer surfaces of the heat sinks  2 ,  3 .  
         [0006]     Moreover, a coating resin such as polyamide resin is applied to the surface of the heat sinks  2 ,  3  prior to the molding to create a desired adhesion between the heat sinks  2 ,  3  and the mold resin  7 . However, in the semiconductor device  1 , the distance between the heat sinks  2 ,  3  is in the range between 1 and 2 mm, so it is difficult to apply the coating resin on the inner surfaces of the heat sinks  2 ,  3 . It is also difficult to inspect the coating quality.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has been made in view of the above problems with an object to provide a semiconductor device that makes it possible to use a mold having a simple structure without damaging a semiconductor chip during molding, makes it unnecessary to insulate both outer surfaces of the heat sinks using dedicated insulating sheets after the device is completed, and makes it easier to apply coating resin on a pair of heat sinks in the manufacturing process of the device.  
         [0008]     Insulating sheets, which are made of compressively deformable material, are adhered to the outer surfaces of the heat sinks, and the device is molded in a mold while the sheets are located between the mold and the heat sinks. The sheets are compressively deformed during the molding to prevent the chip from being broken, even if an economical mold having a simple structure is used. The plates are insulated by the sheets to make dedicated insulating sheets unnecessary when the completed device is inspected, screened, or assembled into a unit such as an inverter unit. The coating resin is coated by immersing the chip and the heat sinks in a solution containing the resin to simplify the coating. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
         [0010]      FIG. 1  is a cross-sectional view of a semiconductor device in a mold according to the present invention;  
         [0011]      FIG. 2A  is a diagrammatic exploded perspective view showing a process stage of a method for making the semiconductor device of  FIG. 1 ;  
         [0012]      FIG. 2B  is a diagrammatic perspective view showing a process stage of a method for making the semiconductor device of  FIG. 1 ;  
         [0013]      FIG. 2C  is a diagrammatic perspective view showing a process stage of a method for making the semiconductor device of  FIG. 1 ;  
         [0014]      FIG. 2D  is a diagrammatic exploded perspective view showing a process stage of a method for making the semiconductor device of  FIG. 1 ;  
         [0015]      FIG. 2E  is a diagrammatic cross sectional view showing a further process stage of a method for making the semiconductor device of the invention;  
         [0016]      FIG. 3  is a cross-sectional view of a semiconductor device located between a pair of heat-transfer members;  
         [0017]      FIG. 4  is a cross-sectional view of a proposed semiconductor device; and  
         [0018]      FIG. 5  is an exploded cross-sectional view of the proposed semiconductor device located between a pair of heat-transfer plates. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     As shown in  FIG. 1 , a semiconductor device  11  includes a semiconductor chip  12 , which generates heat, a lower heat sink  13  and an upper heat sink  14 , which conduct the heat generated by the semiconductor chip, and a heat sink coupler  15 . The lower surface of the chip  12  and the upper surface of the lower heat sink  13  are connected by solder  16 , the upper surface of the chip  12  and the lower surface of the heat sink coupler  15  are soldered together, and the upper surface of the heat sink coupler  15  and the lower surface of the upper heat sink  14  are soldered together. The heat generated by the chip  12  is conducted through the heat sinks  13  and  14 .  
         [0020]     In this embodiment, the semiconductor chip  12  is a power semiconductor such as an IGBT (Insulated Gate Bipolar Transistor) and a thyristor in the shape of a thin rectangular plate, as shown in  FIG. 2A . The lower heat sink  13 , the upper heat sink  14 , and the heat sink coupler  15  are made of metal having high heat conductivity and electric conductivity, such as copper or aluminum. The lower heat sink  13  and the upper heat sink  14  are connected electrically to a pair of electrodes such as collector and emitter electrodes by the solder  16 .  
         [0021]     The lower heat sink  13  is a rectangular plate and is integrated with a terminal  13   a  which is a rectangular plate extending rearward in  FIG. 2A . The heat sink coupler  15  is a rectangular plate and is a little smaller in area than the semiconductor chip  12 . The upper heat sink  14  is a rectangular plate and is integrated with another terminal  14   a,  which is a rectangular plate extending rearward in  FIG. 2D . The terminals  13   a,    14   a  do not face one another; that is, they are offset from one another as shown in  FIG. 2D . The distance between the heat sinks  13 ,  14  is in the range between 1 and 2 mm.  
         [0022]     As shown by a bold line in  FIG. 1 , a coating resin  17  made of polyamide resin is applied on the surfaces of the heat sinks  13 ,  14 , the chip  12 , and the heat sink coupler  15  to improve adhesion between a mold resin  19  and the heat sink members  13 ,  14 . In this embodiment, the resin  17  is coated by immersion. Two insulating sheets  18 , which are made of a material that deforms when compressed and is highly heat conductive, are adhered to the outer surface of the heat sinks  13 ,  14 . The insulating sheets  18  are adhered using the resin  17  as adhesive, so no dedicated adhesive is needed.  
         [0023]     The insulating sheets  18  used in this embodiment are made of silicon rubber and have the same thickness and the same surface characteristics. The heat conductivity of the sheets  18  is higher than that of the resin  19 . However, the insulating sheets may differ in material, thickness, and surface characteristics from each other. For example, one of those insulating sheets  18  may be made of the same material as the other but may be thinner than the other to improve heat conduction or may have an uneven surface to improve damping efficiency. One of the sheets  18  may be made of a material having superior heat conductivity and inferior damping efficiency and the other may be made of a material having inferior heat conductivity and superior damping efficiency.  
         [0024]     In general, the heat conducting sheets  18  are made of elastic polymer material that includes a heat conductive filler, and heat conductivity is improved at the expense of elasticity. Therefore, heat conductivity takes priority if one side of the chip is particularly hot. For example, the sheet  18  adhered to the lower heat sink  13  may be given greater conductivity than the upper sheet  18  if the lower heat sink corresponds to a collector electrode, which requires greater cooling.  
         [0025]     As shown in  FIG. 1 , the mold resin  19 , which is made of a material such as epoxy resin, fills the space between the insulating sheets  13 ,  14  to surround the chip  12  and the heat sink coupler  15 . A mold  20  is used to mold the resin  19  to a stack that includes the semiconductor chip  12 , the heat sink coupler  15 , the heat sinks  13 ,  14 , and the insulating sheets  18 . The mold  20  has a simple structure of a lower mold  21  and an upper mold  22 , so the production cost of the semiconductor device  11  is relatively low. When the stack is held in a cavity  23  of the mold  20 , the insulating sheets  18  are pressed between the mold  20  and the heat sinks  13 ,  14  and compressively deformed by about 10 to 40%. Therefore, when the molten mold resin  19  is injected into the cavity  23 , the resin  19  is prevented from covering the insulating sheets  18  on the surface pressed by the mold  20 . In addition, the force applied to the chip  12  while the stack is pressed by the mold  20  is dampened and distributed by the sheets  18 , so the chip  12  is not broken.  
         [0026]     The outward surfaces of the heat sinks  13 ,  14  are insulated by the sheets  18 . Thus, no dedicated heat conducting sheets made of insulating material (like the sheets  10  in  FIG. 5 ) are needed when the completed device  11  is inspected, screened, assembled into a unit such as an inverter unit, or located between cooling plates  28 ,  29  as shown in  FIG. 3 .  
         [0027]     The thickness of the semiconductor device  11 , that is, the distance between the outer surfaces of the lower and the upper heat sinks  13  and  14 , is nominally determined by the manufacturing method described later. However, the thickness varies due to deviation in various factors such as the size of each member and the flatness and inclination of the heat sinks  13 ,  14 . Assuming that the deviation is 0.1 mm and the sheets  18  are compressively deformable by 15%, the minimum thickness of the sheets  18  is calculated by the following equation. 
 
0.1 mm÷0.15÷2=0.33 mm 
 
         [0028]     Thus, the sheets  18  must have a thickness of at least 0.33 mm. The minimum thickness must to be adjusted in response to the thickness deviation of the semiconductor device  11  on a case-by-case basis.  
         [0029]     The semiconductor device  11  is manufactured as shown in  FIGS. 2A  to  2 E. First, the semiconductor chip  12 , the heat sink coupler  15 , and the lower heat sink  13  are soldered together. To be specific, the chip  12 , the coupler  15 , and a pair of solder foils  24  are stacked on the upper surface of the lower heat sink  13 , as shown in  FIGS. 2A and 2B . Then, the stack is heated by a solder-reflowing apparatus to fuse the solder foils  24 . As shown in  FIG. 2C , a pair of control electrodes of the chip  12 , which may include a gate pad, are wire bonded to lead frames  25   a  and  25   b  using wires  26  made of metal such as aluminum or gold. Afterward, the upper heat sink  14  and the solder foil  24  are stacked on the coupler  15 , as shown in  FIG. 2D . The stack is heated by a solder-reflowing apparatus to fuse the solder foil  24 . While the stack is heated, the stack is pressed by a weight  27 , and a spacer (not illustrated) is located between the heat sinks  13 ,  14  to retain a predetermined distance between the heat sinks  13 ,  14 , as shown in  FIG. 2E .  
         [0030]     Before the solder foils  24  are fused, the distance between the heat sinks  13  and  14  is greater than the predetermined distance. When the solder foils  24  are fused, the fused solder foils  24  become thinner, because the foils  24  are pressed by the weight  27 , and the predetermined distance is retained by the spacer between the heat sinks  13  and  14 . The fused foils  24  mechanically and electrically connect the heat sink  13  and the chip  12 , the chip  12  and the coupler  15 , and the coupler  15  and the heat sink  14 . In this embodiment, the solder foils  24  are used. However, instead of the foils  24 , solder paste or conductive adhesive may also be used.  
         [0031]     Subsequently, the coating resin  17 , which is made of polyamide resin, is homogeneously coated on the surfaces of the heat sinks  13 ,  14 , the chip  12 , and the heat sink coupler  15  by immersing the soldered stack in polyamide resin solution. In this immersion, the coating resin  17  is also applied to the surfaces of the wires  26  and the lead frames  25   a,    25   b.  Instead of the immersion, the coating may be done by dripping or spraying the resin  17 . In that case, it is preferred to insert a nozzle dispensing the resin  17  into the space between the heat sinks  13 ,  14  and drip or spray the resin  17  from the tip of the nozzle.  
         [0032]     In this embodiment, the insulating sheets  18  are adhered to the heat sinks  13 ,  14  before the resin  17  dries, so no dedicated adhesive is needed. That is, the resin  17  doubles as adhesive. However, the sheet  18  may be adhered using an adhesive that has good heat conductivity after the resin  17  is thoroughly dried.  
         [0033]     As shown in  FIG. 1 , the stack, which is coated with the resin  17 , is put in the cavity  23  of the mold  20 , which has the lower mold  21  and the upper mold  22 , and is partially covered with the mold resin  19 . The molten mold resin  19  is injected into the cavity  23  fill the space between the insulating sheets  18  and to surround chip  12 , the heat sink coupler  15 , and the heat sinks  13 ,  14 , shown in  FIG. 1 . After the resin  19  is set, the completed device is ejected from the mold  20 .