Patent Document

CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based on and incorporates herein by reference Japanese Patent Applications No. 2000-305228 filed on Oct. 4, 2000 and No. 2001-385791 filed on Dec. 19, 2001.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a semiconductor device, in which heat is released from two sides of a semiconductor chip accommodated therein.  
           [0003]    As that kind of device, a semiconductor device shown in FIG. 1 is proposed. As shown in FIG. 1, semiconductor chips  101 ,  102  and couplers  103 ,  113  are located between a first heat radiation plate  106  and a second heat radiation plate  105 . Each semiconductor chips  101 ,  102  and corresponding coupler  103 ,  113 , each semiconductor chips  101 ,  102  and the second heat radiation plate  105 , and each coupler  103 ,  113  and the first heat radiation plate  106  are respectively electrically connected to each other by solders  104 .  
           [0004]    Therefore, the two semiconductor chips  101 ,  102  are electrically connected in parallel using the couplers  103 ,  113  and the first and second heat radiation plates  106 ,  105 . Mold resin  109  is also located between the first and second heat radiation plates  106 ,  105  and in contact with a coating resin film  110 , which is located on surfaces of the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the first and second heat radiation plates  106 ,  105 .  
           [0005]    The semiconductor chips  101 ,  102  are respectively, for example, an IGBT chip  101 , which is an insulated gate bipolar transistor, and an FWD chip  102 , which is a fly-wheel diode. Each semiconductor chip.  101 ,  102  has an element formation surface  101   a ,  102   a , or a front surface  101   a ,  102   a  and a back surface  101   b ,  102   b , which is opposite to the front surface  101   a ,  102   a . Each coupler  103 ,  113  is located on corresponding front surface  101   a ,  102   a.    
           [0006]    The coupler  103  located on the front surface  101   a  of the IGBT chip  101  forms a space for wirebonding a bonding wire  108 , which is described later, above the front surface  101   a  of the IGBT chip  101 . The coupler  103  located on the front surface  102   a  of the FWD chip  102  adjusts the distance between the FWD chip  102  and the first heat radiation plate  106  such that the first heat radiation plate  106  becomes substantially parallel to the second heat radiation plate  105 .  
           [0007]    The second heat radiation plate  105  is electrically connected to the back surface  101   b  of the IGBT chip  101 , which is a collector electrode, and the back surface  102   b  of the FWD chip  102 , which is a cathode. The first heat radiation plate  106  is electrically connected to the front surface  101   a  of the IGBT chip  101 , which is an emitter electrode, and the front surface  102   a  of the FWD chip  102 , which is an anode.  
           [0008]    The couplers  103 ,  113  and the first and second heat radiation plates  106 ,  105  release the heat that is generated by the semiconductor chips  101 ,  102  while functioning as electric wiring for the semiconductor chips  101 ,  102 . Therefore, the solders  104  need to have a relatively high electric conductance and a relatively high thermal conductance.  
           [0009]    Although not illustrated, a gate electrode is located at a predetermined position on the front surface  101   a  of the IGBT chip  101 . The gate electrode is electrically connected to a control terminal  107  with the bonding wire  108 . The semiconductor chips  101 ,  102 , the couplers  103 ,  113 , the first and second heat radiation plates  106 ,  105 , the control terminal  307 , and the bonding wire  108  are integrally molded with a molding resin used for forming the molding resin  109  such that a back surface  105   b  of the second heat radiation plate  105 , a front surface  106   b  of the first heat radiation plate  106 , and a portion of the control terminal  307  are exposed, as shown in FIG. 1.  
           [0010]    Although not illustrated, cooling members, which cool the first and second heat radiation plates  106 ,  105 , are located in contact with the back surface  105   b  of the second heat radiation plate  105  and the front surface  106   a  of the first heat radiation plate  106 , so heat is efficiently released from the first and second heat radiation plates  106 ,  105 .  
           [0011]    In the semiconductor device shown in FIG. 1, the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105  are respectively different in thermal expansion coefficient from the molding resin  109 . Therefore, a relatively great stress is generated in the vicinity of the boundary between the molding resin  109  and each of the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105  when the semiconductor device experiences thermal cycles. When the thermally generated stress overcomes the adhesion between the molding resin  109  and any of the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105 , the molding resin  109  peels off. The greater the difference in temperature of the thermal cycles, the smaller the number of the cycles that cause the peeling.  
           [0012]    A stress is also generated in each solder  104  during the thermal cycles due to the difference in thermal expansion coefficient between the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105 . However, the stress in each solder  104  is suppressed by the molding resin  109  because the molding resin  109  restrains the thermal expansions of the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105 . Therefore, if the coating resin film  110  did not exist and the molding resin  109  peeled off any of the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105 , the stress in each solder  104  would increase and the solders  104  would deteriorate at an undesirably high rate. As a result, any solder  104  would crack, and the electric resistance of the solder  104  would increase.  
           [0013]    The coating resin film  110  has a relatively high adhesion with the molding resin  109  and any of the semiconductor chips  101 ,  102 , the couplers  103 ,  113 , and the heat radiation plates  106 ,  105 , so the molding resin  109  is prevented from peeling off during the thermal cycles.  
           [0014]    Nevertheless, in the manufacturing process of the semiconductor device shown in FIG. 1, the solders  104  spread and adhere to any side surface of the semiconductor chips  101 ,  102  and the couplers  103 ,  113 , as illustrated in FIG. 2. In that case, a portion of the solders  104 , which is mechanically relatively weak, exists between the side surface and the coating resin film  110 . If the semiconductor device having the portion of the solders  104  between the side surface and the coating resin film  110  experiences thermal cycles, the portion of the solders  104  peels off the side surface.  
           [0015]    In other words, the molding resin  109  is disconnected from the side surface. In that case, as described above, the stress in that solder  104  increases and that solder  104  deteriorates at an undesirably high rate. In addition, in the case that two types of solders, which have a different melting point from each other, are used, the solders might be mixed with each other, and as a result, eutectic solder having a melting point much lower than those of the two types of solders might be formed to fuse at the temperature for the molding using the molding resin  109 .  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention has been made in view of the above aspects with an object to provide a semiconductor device in which a molding resin is prevented from peeling off to assure the durablity in its electric performance.  
           [0017]    In the present invention, a semiconductor device includes a first conductive member, a second conductive member, a semiconductor chip, which is located between the conductive members, a bonding member, which is located between the first conductive member and the semiconductor chip, and another bonding member, which is located between the second conductive member and the semiconductor chip.  
           [0018]    The semiconductor device further includes a molding resin, which is located between the first and second conductive members to seal the semiconductor chip, and a bonding member anti-sticking means, which is located between the molding resin and a surface of one member selected from the group consisting of the semiconductor chip and the conductive members. The bonding member anti-sticking means prevents the bonding members from sticking to the surface in the manufacturing process. As a result, the otherwise insufficient connection due to the sticking between the molding resin and the surface is improved, and the semiconductor device becomes more durable in its electric performance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    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:  
         [0020]    [0020]FIG. 1 is a schematic cross-sectional view of a proposed semiconductor device;  
         [0021]    [0021]FIG. 2 is a partially enlarged view of the semiconductor device of FIG. 1;  
         [0022]    [0022]FIG. 3 is a schematic cross-sectional view of a semiconductor device according to a first embodiment of the present invention;  
         [0023]    [0023]FIGS. 4A to  4 C are cross-sectional views showing the steps for manufacturing the semiconductor device of FIG. 3;  
         [0024]    [0024]FIG. 5 is a cross-sectional view of a semiconductor device according to a second embodiment;  
         [0025]    [0025]FIG. 6 is a schematic cross-sectional view of a semiconductor device according to a third embodiment;  
         [0026]    [0026]FIGS. 7A to  7 C are cross-sectional views showing the steps for manufacturing the semiconductor device of FIG. 6;  
         [0027]    [0027]FIG. 8 is a schematic cross-sectional view of a semiconductor device according to a third embodiment;  
         [0028]    [0028]FIG. 9 is a schematic cross-sectional view of a semiconductor device according to a fourth embodiment; and  
         [0029]    [0029]FIG. 10 is a schematic cross-sectional view of a semiconductor device according to a fifth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The present invention will be described in detail with reference to various embodiments.  
         [0031]    First Embodiment  
         [0032]    A semiconductor device shown in FIG. 3 includes two semiconductor chips  1 ,  2 , a first conductive member  3 ,  6 , which includes two couplers  3  and a first heat radiation plate  6 , and a second conductive member  5 , or a second heat radiation plate  5 . The semiconductor chips  1 ,  2  are electrically connected in parallel using the couplers  3 , the first heat radiation plate  6 , and the second heat radiation plate  5 . The semiconductor chips  1 ,  2  and the couplers  3  are located between the first and second heat radiation plates  6 ,  5 . A bonding member anti-sticking means  14 , or a first coating resin film  14 , is located on each side surface of the couplers  3 , as illustrated in FIG. 3. A second coating resin film  15  is located on surfaces of the semiconductor chips  1 ,  2 , the first and second heat radiation plates  6 ,  5 , and the first coating resin film  14 . Mold resin  9  is located between the first and second heat radiation plates  6 ,  5  and in contact with the second coating resin film  15 .  
         [0033]    The semiconductor chips  1 ,  2  are respectively an IGBT chip  1 , which is an insulated gate bipolar transistor, and an FWD chip  2 , which is a fly-wheel diode. Each semiconductor chip  1 ,  2  is made of substantially silicon and has a thickness of about 0.5 mm. Each semiconductor chip  1 ,  2 , has an element formation surface  1   a ,  2   a , or a front surface  1   a ,  2   a , in which a region making up a transistor is located, and a back surface  1   b ,  2   b , which is opposite to the front surface  1   a ,  2   a . Each coupler  3  is located on corresponding front surface  1   a ,  2   a . Although not illustrated, an emitter electrode and a gate electrode are located on the front surface  1   a  of the IGBT chip  1 , and a collector electrode is located on the back surface  1   b  of the IGBT chip  1 .  
         [0034]    Each front surface  1   a ,  2   a  of the semiconductor chips  1 ,  2  is bonded to corresponding back surface  3   b  of the couplers  3  with bonding members  4 , or solders  4 , which have a relatively high electric conductance and a relatively high thermal conductance. The first coating resin film  14 , which is located on the side surfaces of the couplers  3 , is made of a resin such as a polyamide resin, a polyimide resin, and an amide resin.  
         [0035]    The coupler  3  located on the front surface  1   a  of the IGBT chip  1  forms a space for wirebonding a bonding wire  8 , which is described later, above the front surface  1   a  of the IGBT chip  1 . The coupler  3  located on the front surface  2   a  of the FWD chip  2  adjusts the distance between the FWD chip  2  and the first heat radiation plate  6  such that the first heat radiation plate  6  becomes substantially parallel to the second heat radiation plate  5 .  
         [0036]    The area of the coupler  3  at which the coupler  3  is bonded to the IGBT chip  1  is substantially equal to the dimension of the emitter electrode of the IGBT chip  1 . Therefore, the coupler  3  is preferably in contact with the emitter electrode with the maximum area while being prevented from undesirably contacting an area outside the emitter electrode. If the IGBT chip  1  contacted the area outside the emitter electrode, the area outside the emitter electrode would undesirably become equipotential with the emitter electrode.  
         [0037]    The back surfaces  1   b ,  2   b  of the semiconductor chips  1 ,  2  are bonded and electrically connected to a front surface  5   a  of the second heat radiation plate  5  with solders  4 . Front surface  3   a , which is opposite to the back surfaces  3   b  of the couplers  3 , is boned and electrically connected to a back surface  6   b  of the first heat radiation plate  6  with solders  4 . The coupler  3  and the first and second heat radiation plates  6 ,  5  are made of a metal having electrical conductivity. Specifically, the couplers  3  are made of copper, and the first and second heat radiation plates  6 ,  5  are made of copper alloy.  
         [0038]    Although not illustrated, a gate electrode is located at a predetermined position on the front surface  1   a  of the IGBT chip  1 . The gate electrode is electrically connected to a control terminal  7  with the bonding wire  8 . The semiconductor chips  1 ,  2 , the couplers  3 , and the first and second heat radiation plates  6 ,  5 , the control terminal  7 , and the bonding wire  8  are molded en bloc with the molding resin  9  such that a back surface  5   b  of the second heat radiation plate  5 , a front surface  6   a  of the first heat radiation plate  6 , and a portion of the control terminal  7  are exposed, as shown in FIG. 3. For example, an epoxy based resin can be used as the molding resin  9 . Although not illustrated, a pair of molds is used for the molding.  
         [0039]    The second coating resin film  15  improves the adhesion between the molding resin  9  and each semiconductor chip  1 ,  2  and the adhesion between the molding resin  9  and each of the first and second heat radiation plates  6 ,  5 . The second coating resin film  15  is made of a resin such as a polyamide resin, a polyimide resin, and an amide resin.  
         [0040]    In the semiconductor device shown in FIG. 3, the heat generated by the semiconductor chips  1 ,  2  is transmitted to the couplers  3  and to the first and second heat radiation plates  6 ,  5  through the solders  4 , and the heat is released outward from the back surface  5   b  of the second heat radiation plate  5  and the front surface  6   a  of the first heat radiation plate  6 . Although not illustrated, cooling members, which cool the first and second heat radiation plates  6 ,  5 , are located in contact with the back surface  5   b  of the second heat radiation plate  5  and the front surface  6   a  of the first heat radiation plate  6 , so heat is efficiently released from the first and second heat radiation plates  6 ,  5 .  
         [0041]    In the manufacturing process of the semiconductor device shown in FIG. 3, the first coating resin film  14  is formed to cover the side surfaces of the couplers  3 . Therefore, even if any solder  4  spreads along the side surfaces of the couplers  3  when the semiconductor chips  1 ,  2 , the couplers  3 , and the heat radiation plates  6 ,  5  are integrated with the solders  4 , no solders  4  stick to any side surface. In addition, substantially no solders  4  stick to the first coating resin film  14  because the solders  4  dewet the first coating resin film  14 .  
         [0042]    Therefore, the side surfaces of the couplers  3  and the molding resin  9  are firmly connected by the first and second coating resin films  14 ,  15 . Thus, even when the semiconductor device of FIG. 3 experiences thermal cycles, the molding resin  9  is prevented from peeling off to be disconnected from the couplers  3 . Accordingly, the stress in each solder  4  is prevented from increasing, and each solder  4  is prevented from deteriorating. In addition, even if two types of solders, which have a different melting point from each other, are used, the solders are not mixed with each other. Therefore, eutectic solder having a melting point much lower than those of the two types of solders is not formed to fuse at the temperature for the molding using the molding resin  9 .  
         [0043]    The semiconductor device of FIG. 3 is manufactured as follows. First and second heat radiation plates  6 ,  5  are stamped out of plates made of copper alloy and so on. A resin such as a polyamide resin, a polyimide resin, and an amide resin is coated on surfaces of copper plates to form couplers  3  having a first coating resin film  14 .  
         [0044]    Then, as shown in FIG. 4A, an IGBT chip  1  and an FWD chip  2  are bonded to a front surface  5   a  of the second conductive member  5  using a solder  4 . Next, each coupler  3  is bonded to corresponding front surface  1   a ,  2   a  of the semiconductor chip  1 ,  2  using a solder  4  to form a work  10 , as shown in FIG. 4A. Then, although not illustrated, the IGBT chip  1  is connected to a control terminal  7  by a bonding wire  8 .  
         [0045]    Next, as shown in FIG. 4B, the first heat radiation plate  6  is mounted on a jig  11  such that a back surface  6   b  of the first heat radiation plate  6  faces upward, and solders  4  are placed on predetermined positions of the back surface  6   b . Then, the work  10  is turned over. The work  10  is aligned with and placed on the first heat radiation plate  6 .  
         [0046]    Then, a plate-shaped weight  12  is placed on a back surface  5   b  of the second heat radiation plate  5 . Spacers  13  having a predetermined length are placed between the jig  11  and the second heat radiation plate  5  for adjusting the distance between the first and second heat radiation plates  6 ,  5  to a predetermined value, as shown in FIG. 4C. The body of the FIG. 4B is placed en bloc in a heating furnace to permit the solders  4  to reflow. During the reflowing, the work  10  is pressed by the weight  12 , so the solders  4  are thinned. As a result, as shown in FIG. 4C, the distance between the back surface  6   b  of the first heat radiation plate  6  and the front surface  5   a  of the second heat radiation plate  5  becomes equal to the length of the spacers  13 . The degree of parallelization between the first and second heat radiation plates  6 ,  5  is substantially determined by the spacers  13 .  
         [0047]    In the manufacturing process of FIGS. 4A to  4 C, the semiconductor chips  1 ,  2  and the second heat radiation plate  5  are bonded. Next, the couplers  3  and the semiconductor chips  1 ,  2  are bonded. Finally, the first heat radiation plate  6  and the couplers  3  are bonded. However, the order of the above bonding steps may be changed. For example, the following order is possible. The couplers  3  and the first heat radiation plate  6  are bonded with solders  4 . Then, the couplers  3 , the semiconductor chips  1 ,  2 , and the second heat radiation plate  5  are bonded together with solders  4  at the same time. Alternatively, the semiconductor chips  1 ,  2 , the couplers  3 , and the first and second heat radiation plates  6 ,  5  can be stacked and bonded together with solders  4  at the same time.  
         [0048]    Subsequently, a resin such as a polyamide resin, a polyimide resin, and an amide resin is coated on surfaces of the semiconductor chips  1 ,  2 , the first and second heat radiation plates  6 ,  5 , and the first coating resin film  14  for forming the coating resin film  15 . The resin may be coated by immersing the soldered body shown in FIG. 4C in the resin solution. Alternatively, the resin may be coated by drizzling or spraying the resin from a dispense nozzle. It is preferred that the control terminal  7  and the bonding wire  8  be coated with the resin. Finally, the semiconductor chips  1 ,  2 , the couplers  3 , and the first and second heat radiation plates  6 ,  5 , the control terminal  7 , and the bonding wire  8  are molded en bloc with molding resin  9  to complete a semiconductor device of FIG. 3.  
         [0049]    Second Embodiment  
         [0050]    A semiconductor device shown in FIG. 5 includes two semiconductor chips  1 ,  2 , a first conductive member  33 ,  6 , which includes two couplers  33  and a first heat radiation plate  6 , and a second conductive member  5 , or a second heat radiation plate  5 . The semiconductor device shown in FIG. 5 does not include the same bonding member anti-sticking means, or the first coating resin film  14 , as the one used in the semiconductor device shown in FIG. 3. Instead, in the semiconductor device shown in FIG. 5, a flange is located at the side surface of each coupler  33 , at which each coupler  33  is connected to a molding resin  9  by a second coating resin film  15 , as a bonding member anti-sticking means. In that aspect, the semiconductor device shown in FIG. 5 is different from the semiconductor device shown in FIG. 3.  
         [0051]    Therefore, even if any solder  4  spreads along the side surfaces of the couplers  33  when the semiconductor chips  1 ,  2 , the couplers  33 , and the first heat radiation plate  6  are bonded with solders  4 , no solders  4  stick to, at least, the top surface of each flange. That is, the top surface and the molding resin  9  are firmly connected by the second coating resin film  15 . Thus, even when the semiconductor device of FIG. 5 experiences thermal cycles, the molding resin  9  is prevented from peeling off to be disconnected from the top surface. Accordingly, the stress in each solder  4  is prevented from increasing, and each solder  4  is prevented from deteriorating.  
         [0052]    Third Embodiment  
         [0053]    A semiconductor device shown in FIG. 6 includes two semiconductor chips  301 ,  302 , a first conductive member  303 ,  306 , which includes two plate-like couplers  303  and a first heat radiation plate  306 , and a second conductive member  305 , or a second heat radiation plate  305 . The semiconductor chips  301 ,  302  are respectively, an IGBT chip  301 , which is an insulated gate bipolar transistor, and an FWD chip  302 , which is a fly-wheel diode. The semiconductor chips  301 ,  302  are made of substantially silicon and have a thickness of about 0.5 mm.  
         [0054]    Each semiconductor chip  301 ,  302  has an element formation surface  301   a ,  302   a , or a front surface  301   a ,  302   a , in which a region making up a transistor is located, and a back surface  301   b ,  302   b , which is opposite to the front surface  301   a ,  302   a . Each coupler  303  is located on corresponding front surface  301   a ,  302   a . Although not illustrated, an emitter electrode is located on the front surface  301   a  of the IGBT chip  301 , and a collector electrode is located on the back surface  301   b  of the IGBT chip  301 .  
         [0055]    Each front surface  301   a ,  302   a  of the semiconductor chips  301 ,  302  is bonded to corresponding back surface  303   b  of the couplers  303  with a first bonding member  304 , or a first solder  304 , that has a relatively high electric conductance and a relatively high thermal conductance. The area of the coupler  303  at which the coupler  303  is bonded to the IGBT chip  301  is substantially equal to the dimension of the emitter electrode of the IGBT chip  301 .  
         [0056]    Therefore, the coupler  303  is preferably in contact with the emitter electrode with the maximum area while being prevented from undesirably contacting an area outside the emitter electrode, where elements such as a guard ring are located. If the IGBT chip  301  contacted the area outside the emitter electrode, the area outside the emitter electrode would undesirably become equipotential with the emitter electrode.  
         [0057]    The back surfaces  301   b ,  302   b  of the semiconductor chips  301 ,  302  are electrically connected to a front surface  305   a  of the second heat radiation plate  305  with second bonding members  304 , or second solders  304 . Front surfaces  301   a ,  303   a , which are opposite to the back surfaces  301   b    303   b  of the couplers  303 , are electrically connected to a back surface  306   a  of the first heat radiation plate  306  with third bonding members  304 , or third solders  304 . The couplers  303  and the first and second heat radiation plates  306 ,  305  are made of a metal having electrical conductivity. Specifically, the couplers  303  are made of copper, and the first and second heat radiation plates  306 ,  305  are made of copper alloy.  
         [0058]    A step  303   c , which is defined by a flange  303   d , is located around each coupler  303 , as shown in FIG. 6. Therefore, the front surface  303   a  of each coupler  303 , at which each coupler  303  is connected to the first heat radiation plate  306 , is smaller than the back surface of each coupler  303 , at which each coupler  303  is connected to corresponding semiconductor chip  301 ,  302 .  
         [0059]    Although not illustrated, plated Ni layers are located on the front and back surfaces of each coupler  303  for improving the wettability of the first and third solders  304  to the surfaces. An oxide layer is located on the side surface of each coupler  303  and a surface of each flange  303   d . Each radiation plate  306 ,  305  has a thickness of about 1 mm. Each coupler  303  has a thickness of 1 mm, and the flange  303   d  has a thickness of about 0.4 mm.  
         [0060]    Although not illustrated, a land is located on the front surface  301   a  of the IGBT chip  301 , and is electrically connected to a control terminal  307  of a lead frame with a bonding wire  308 . The semiconductor chips  301 ,  302 , the couplers  303 , the flanges  303   d , the second heat radiation plate  305 , the first heat radiation plate  306 , and the control terminal  307  are molded en bloc with the molding resin  309  such that a back surface  305   b  of the second heat radiation plate  305 , a front surface  6   a  of the first heat radiation plate  6 , and a portion of the control terminal  7  are exposed, as shown in FIG. 6. For example, an epoxy based resin can be used as the molding resin  309 .  
         [0061]    In the semiconductor device shown in FIG. 6, the heat generated by the semiconductor chips  301 ,  302  is transferred to the couplers  303  and to the first and second heat radiation plates  306 ,  305  through the solder  304 , and the heat is released outward from the back surface  305   b  of the second heat radiation plate  305  and the front surface  306   a  of the first heat radiation plate  306 . Although not illustrated, cooling members, which cool the first and second heat radiation plates  306 ,  305 , are located in contact with the back surface  305   b  of the second heat radiation plate  305  and the front surface  306   a  of the first heat radiation plate  306 , so heat is efficiently released from the first and second heat radiation plates  306 ,  305 .  
         [0062]    The couplers  303  and the first and second radiation plates  306 ,  305  form electric current paths for the semiconductor chips  301 ,  302 . That is, the electrical communication with the collector electrode of the IGBT chip  301  is permitted through the second heat radiation plate  305 , while the electrical communication with the emitter electrode of the IGBT chip  301  is permitted through the first radiation plate  306  and corresponding coupler  303 .  
         [0063]    In the semiconductor device of FIG. 6, the flanges  303   d  are less rigid than the couplers  303 . Therefore, the flanges  303   d  can conform to the deformation of the resin  309  that is connected to the flanges  303   d  to decrease the stress thermally generated at the boundary between each of the semiconductor chips  301 ,  302  and corresponding coupler  303  when the semiconductor device experiences thermal cycles.  
         [0064]    Furthermore, the front surface  303   a  of each coupler  303  is smaller than the back surface of each coupler  303 . Because the bonding strength decreases as the bonding areas of each coupler  303  for the heat radiation plates  305 ,  306  decreases, the third solder  304 , which is located between each coupler  303  and the first heat radiation plate  306 , cracks more readily than the first solder  304 , which is located between each coupler  303  and corresponding semiconductor chip  301 ,  302 , when the semiconductor device of FIG.  6  experiences thermal cycles.  
         [0065]    If the third solder  304  cracks, the stress thermally generated in the third solder  304  relaxes. At the same time, the stress thermally generated in the first solder  304  relaxes. Therefore, at least, the first solder  304  can be prevented from cracking. In addition, the couplers  303  and the first heat radiation plate  306  include copper as a main component, so the couplers  303  and the first heat radiation plate  306  are similar to each other in the deformation caused by the thermal cycle.  
         [0066]    Therefore, even if the third solder  304  cracks, the cracking of the third solder  304  proceeds relatively slowly. In addition, the current path between each coupler  303  and the first heat radiation plate  306  is formed by the entire area at which each coupler  303  and the first heat radiation plate  306  are connected. Therefore, even if the cracking proceeds, the electric resistance at the area does not steeply increase locally or as a whole.  
         [0067]    The oxide layer is located on the side surface of each coupler  303  and the surface of each flange  303   d . Therefore, the adhesion between the molding resin  309  and each coupler  303  and the adhesion between the molding resin  309  and the surface of each flange  303   d  is relatively high. As a result, the molding resin  309  conforms to the deformation of the coupler  303 , which is caused by the thermal cycle, without peeling off, and the stress thermally generated in the solders  304  decreases. Incidentally, the adhesion between copper alloy and the molding resin  309  more increases by plating nickel on the surface of the copper alloy. Therefore, each surface of the first and second radiation plates  306 ,  305  is plated with nickel instead of being oxidized.  
         [0068]    As shown in FIG. 6, the step  303   c  helps to increase the distance from the surface of the semiconductor device to the first solder  304  along the interface between the first heat radiation plate  306  and the molding resin  309 , the interface between each coupler  303  and the molding resin  309 , and the inter face between each flange  303   d  and the molding resin  309 . Therefore, the step  303   c  helps to prolong the time until a peeling of the molding resin  309  that is generated at the surface of the semiconductor device reaches the first solder  304  along the interfaces.  
         [0069]    The semiconductor device of FIG. 6 underwent a thermal cycle test. In the thermal cycle test, the semiconductor device was alternately exposed to a temperature of −40° C. for 60 minutes and a temperature of 125° C. for 60 minutes. Then, the resistance between the first heat radiation plate  306  and the control terminal  307  was measured, and the resistance change rate was calculated using the initial resistance value as a reference. It was confirmed that the resistance change rate did not increase steeply until 200 cycles and the semiconductor device of FIG. 6 is more durable than the proposed device of FIG. 1.  
         [0070]    The semiconductor device of FIG. 6 is manufactured as follows. A pair of metal plates is stamped out of a board made of copper alloy and so on. Then, the entire surface of each plate is plated with nickel to complete a second heat radiation plate  305  and a first heat radiation plate  306 .  
         [0071]    A copper board for forming the couplers  303  is plated with nickel at its front and back surfaces. After that, a pair of copper plates is stamped out of the copper board. Then, each copper plate is pressed to form a flange  303   d , which defines a step  303   c , and a coupler  303 . Each coupler  303  included nickel layers only at front and back surfaces  303   a ,  303   b . No nickel layer is located on the side surface of each coupler  303  or the top surface of each flange  303   d , which is exposed by the stamping. No nickel layer is located on the surface of the step  303   c  because the plated nickel layer peels off from the surface when the step  303   c  is formed by the pressing.  
         [0072]    As shown in FIG. 7A, the semiconductor chips  301 ,  302 , which are an IGBT chip  301  and an FWD chip  302 , are bonded to a back surface  305   a  of the second heat radiation plate  305  with second solders  304 . Next, the couplers  303  are bonded to the semiconductor chips  301 ,  302  with first solders  304  to form a work  310 , as shown in FIG. 7A. The first and second solders  304  have a relatively high melting point. For example, a high melting point solder, which includes 10 weight % of tin (Sn) and 90 weight % of lead (Pb) and has a melting point of 320° C., can be used for the first and second solders  304 .  
         [0073]    Next, as shown in FIG. 7B, the first heat radiation plate  306  is mounted on a jig  311  such that a back surface  306   b  of the first heat radiation plate  306  faces upward, and third solders  304  are placed on predetermined positions of the back surface  306   b . Then, the work  310  is turned over. The work  10  is aligned with and placed on the first heat radiation plate  6 . The third solders  304  have a melting point lower than that of the high melting point solder. A low melting point solder, which includes tin (Sn) more than 90 weight % and has a melting point of 240° C., can be used for the third solders  304 .  
         [0074]    Then, a plate-shaped weight  312  is placed on the back surface  305   b  of the second heat radiation plate  305 . Spacers  313  having a predetermined length are placed between the jig  311  and the second heat radiation plate  305  for adjusting the distance between the first and second heat radiation plates  306 ,  305  to a predetermined value, as shown in FIG. 7C. The body of the FIG. 7B is placed en bloc in a heating furnace to permit the third solders  304  to reflow. During the reflowing, the work  310  is pressed by the weight  312 , so the third solders  304  are thinned. As a result, as shown in FIG. 7C, the distance between the back surface  306   b  of the first heat radiation plate  306  and the front surface  305   a  of the second heat radiation plate  305  becomes equal to the length of the spacers  313 . The degree of parallelization between the first and second heat radiation plates  306 ,  305  is substantially determined by the spacers.  313 .  
         [0075]    The third solder  304  includes the low melting point solder, and the first and second solders  304  include the high melting point solder. Therefore, when the couplers  303  are bonded to the first heat radiation plate  306 , the first and second solders  304  do not melt. Therefore, the positional relation between each coupler  303  and corresponding semiconductor chip  301 ,  302  remains unchanged. Incidentally, when the melting point of the first and second solders  304  is 320° C. and that of the third solder  304  is 240° C., the reflowing temperature is preferably 250° C.  
         [0076]    After that, although not illustrated, the IGBT chip  301  is electrically connected to a control terminal  307  by a bonding wire  308 . Finally, the semiconductor chips  301 ,  302 , the couplers  303  and the first and second heat radiation plates  306 ,  305 , the control terminal  307 , and the bonding wire  308  are molded en bloc with molding resin  309  to complete a semiconductor device of FIG. 6. A molding resin having a temperature of about 180° C. is injected for the molding, so an oxide layer of the couplers  303  is formed during the molding.  
         [0077]    The nickel plating for forming the couplers  303  could be done after corresponding copper plates are formed from a copper board without plating the copper board. In that case, the copper plates would be immersed in a plating bath to form a nickel layer on the copper plates. As a result, the entire surface of each copper plate would be plated. In that case, the first and second solders  304  could easily wet and spread to the side surface of each coupler  303 , which needs to be connected to the molding resin  309 .  
         [0078]    In addition, the thickness of each coupler  303  is as thin as about 1 mm, so the third solders  304 , which has a lower melting point, and the first solders  304 , which has a higher melting point, are separated with a small distance of 1 mm from each other. Therefore, if the entire surface of each copper plate would be plated, the first and third solders  304  might be mixed with each other. In that case., eutectic solder having a melting point much lower than those of the third solder might be formed to fuse at the temperature for the molding using the molding resin  309 , which is, for example, 180° C.  
         [0079]    However, in the semiconductor device of FIG. 6, no nickel layer is located on the side surface of each coupler  303 . Instead, the oxide layer, which is dewetted by the solders  304 , is located on the side surface to separate the third solders  304  and the first solders  304 . Therefore, neither the third solders  304  nor the first solders  304  spreads to the side surface of each coupler  303  and mix with each other.  
         [0080]    Fourth Embodiment  
         [0081]    As shown in FIG. 8, a semiconductor device according to the fourth embodiment includes a first heat radiation plate  306  that differs in shape from the first heat radiation plate  306  of the semiconductor device in FIG. 6. In other aspects, the two semiconductor devices are substantially the same.  
         [0082]    The first heat radiation plate  306  in FIG. 8 includes a step  306   c  defined by a flange portion  306   d  on a front surface  306   a , at the side of which the first heat radiation plate  306  is exposed. As shown in FIG. 8, the flange portion  306   d  is covered with a molding resin  309 . Therefore, the step  306   c  further helps to increase the distance from the surface of the semiconductor device to the first solder  304  along the interface between the first heat radiation plate  306  and the molding resin  309 , the interface between each coupler  303  and the molding resin  309 , and the interface between each flange  303   d  and the molding resin  309 . Therefore, the step  306   c  further helps to prolong the time until a peeling of the molding resin  309  that is generated at the surface of the semiconductor device reaches the first solder  304  along the interfaces. As a result, the first solder  304  is further prevented from cracking.  
         [0083]    Incidentally, the distance increases as the area covered with the molding resin  309  on the front surface  306   a  of the first heat radiation plate  306  increases. However, as the covered area increases, the exposed area of the front surface  306   a , or the heat radiation capability of the first heat radiation plate  306 , decreases. Therefore, the first heat radiation plate  306  needs to be covered with the molding resin  309  taking the heat radiation capability of the first heat radiation plate  306  into consideration.  
         [0084]    Fifth Embodiment  
         [0085]    As shown in FIG. 9, a semiconductor device according to the fifth embodiment includes two additional couplers  314 . In that aspect, the semiconductor device of FIG. 9 differs from the semiconductor device of FIG. 6. Therefore, the semiconductor device of FIG. 9 has the effect described before in addition to the same effects as the semiconductor device of FIG. 6.  
         [0086]    Each additional coupler  314  is located between each semiconductor chip  301 ,  302  and a second heat radiation plate  305 . As shown in FIG. 9, each additional coupler  314  has a front surface  314   a  and a back surface  314   b , which is opposite to the front surface  314   a , and each semiconductor chip  301 ,  302  has a front surface  301   a  and a back surface  301   b , which is opposite to the front surface  301   a . Each front surface  314   a  of the additional couplers  314  has approximately the same dimensions as corresponding back surface  301   b ,  302   b  of the semiconductor chips  301 ,  302 .  
         [0087]    Each front surface  314   a  of the additional couplers  314  is bonded to corresponding back surface  301   b ,  302   b  of the semiconductor chips  301 ,  302  with a solder  304 . Each back surface  314   b  of the additional couplers  314  is bonded to a front surface  305   a  of the second heat radiation plate  305  with another solder  304 .  
         [0088]    The dimensions of the second heat radiation plate  305  are usually large in comparison with its thickness enough to warp relatively readily. When a curved second heat radiation plate  305  is pressed by a mold during the injection molding for forming a molding resin  309 , the additional couplers  314  are unevenly pressed by the curved second heat radiation plate  305 . However, the locally concentrated force due to the uneven pressing is cushioned by the additional couplers  314 , and the semiconductor chips  301 ,  302  are evenly pressed by the additional couplers  314 . Therefore, in the semiconductor device of FIG. 9, the additional couplers  314  prevent the semiconductor chips  301 ,  302  from breaking during the molding.  
         [0089]    Other Embodiments  
         [0090]    The bonding member anti-sticking means of FIGS. 3, 5, and  6  may be eclectically combined with each other. For example, the first coating resin film  14  shown in FIG. 3 and the flanges shown in FIG. 5 may be combined to create another semiconductor device. Alternatively, the flanges shown in FIG. 5 and the oxide layers on the side surface of the couplers  303  shown in FIG. 6 may be combined to create another semiconductor device.  
         [0091]    In the manufacturing process of the semiconductor device shown in FIG. 3, the second coating resin film  15  is formed after each semiconductor chips  1 ,  2  and corresponding coupler  3 , each semiconductor chips  1 ,  2  and the second heat radiation plate  5 , and each coupler  3  and the first heat radiation plate  6  are respectively bonded with the solders  4 . However, the second coating resin film  15  may be formed before the bonding steps. In that case, the second coating resin film  15  itself functions as a bonding member anti-sticking means, so even if any solder  4  spreads along the side surfaces of the couplers  3  when the couplers  3  is bonded to the semiconductor chips  1 ,  2  and the heat radiation plates  6 ,  5  with the solders  4 , no solders  4  stick to the side surface of the couplers  3 . Therefore, the side surfaces of the couplers  3  and the molding resin  9  are firmly connected by the second coating resin film  15  without the first coating resin film  14 .  
         [0092]    In the semiconductor devices shown in FIGS. 3, 5, and  6 , the bonding member anti-sticking means is located only on the side surfaces of the couplers  3 ,  33 ,  303 . However, the bonding member anti-sticking means may be formed on the side surfaces, which need to be connected to the molding resin  9 ,  309 , of the semiconductor chips  1 ,  2  and the first and second heat radiation plates  6 ,  5 .  
         [0093]    In the semiconductor device shown in FIG. 3, the second coating resin film  15  is located on the first coating resin film  14 . However, the first and second coating resin films  14 ,  15  are made of the same kind of resin, so the second coating resin film  15  does not necessarily need to be located on the first coating resin film  14 .  
         [0094]    In the semiconductor devices shown in FIGS. 3, 5, and  6 , the solder  4 ,  304  are used as a bonding member. However, other materials such as silver paste may be used instead of the solders  4 . Moreover, each semiconductor chips  1 ,  2 ,  301 ,  302  and corresponding coupler  3 ,  33 ,  303 , each semiconductor chips  1 ,  2 ,  301 ,  302  and the second heat radiation plate  5 ,  305 , and each coupler  3 ,  33 ,  303  and the first heat radiation plate  6 ,  306  are respectively bonded with bonding members that are different in type from each other.  
         [0095]    In each semiconductor device shown in FIGS. 3, 5, and  6 , the semiconductor chips  1 ,  2  are respectively, an IGBT chip  1 , which is an insulated gate bipolar transistor, and an FWD chip  2 , which is a fly-wheel diode. However, the semiconductor chips  1 ,  2  may be other types of semiconductor. For example, instead of the FWD chip  2 , each semiconductor device in FIGS. 3, 5, and  6  may includes a MOSFET having the same function as the FWD chip  2 .  
         [0096]    The semiconductor device shown in FIG. 8 may also include additional couplers  314  to prevent the semiconductor chips  301 ,  302  from breaking by a curved second heat radiation plate  305  during the molding.  
         [0097]    In each semiconductor device shown in FIGS. 6, 8, and  9 , the step  303   c  is located around each front surface  303   a  of the couplers  303 , which faces the first heat radiation plate  306 . However, as shown in FIG. 10, the step  303   c  may be located around each back surface  303   b  of the couplers  303 , which faces corresponding semiconductor chip  301 ,  302 . As described, the flanges  303   d  are less rigid than the couplers  303 , and the flanges  303   d  can conform to the deformation of the resin  309  that is connected to the flanges  303   d  to decrease the stress thermally generated at the boundary between each of the semiconductor chips  301 ,  302  and corresponding coupler  303  when the semiconductor device experiences thermal cycles. Therefore, in the semiconductor device shown in FIG. 10 as well, the thermally generated stress is reduced by the flange  303   d.    
         [0098]    In each semiconductor device shown in FIGS. 6, 8,  9 , and  10 , each step  303   c  is located all around each coupler  303 . However, the molding resin  309  starts to peel off the second radiation plate  306  at a surface of the semiconductor device. Therefore, the flanges  303   d  may not be located between the semiconductor chips  301 ,  302 . The reason is that the distance from the surface of the semiconductor device to the first solder  304  along the interface between the second heat radiation plate  306  and the molding resin  309 , the interface between each coupler  303  and the molding resin  309 , and the interface between the flange  303   d  and the molding resin  309  is long enough without forming the couplers  303  between the semiconductor chips  301 ,  302 .

Technology Category: 5