Patent Publication Number: US-9905541-B2

Title: Semiconductor module with bus bar including stacked wiring layers

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is a U.S. national stage application of International Patent Application No. PCT/JP2015/001758 filed on Mar. 26, 2015 and is based on Japanese Patent Application No. 2014-91148 filed Apr. 25, 2014, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a semiconductor module including a semiconductor switching element. 
     BACKGROUND 
     Patent Literature 1 suggests a structure that can reduce an influence of noise in a power converter including a semiconductor module. The power converter has a structure that connects input and output terminals and a contact terminal drawn out from the semiconductor module to a control substrate at the shortest distances. Accordingly, an inductance of a main circuit that passes the input and output terminals (main circuit inductance) and a control terminal path inductance that passes the control terminal are reduced, and the influence of noise is reduced. 
     However, in the semiconductor module described in Patent Literature 1, although the distance between the input and output terminals and the control substrate and the distance between the control terminal and the control substrate set to be the shortest, a distance between the input and output terminals and a distance between the input and output terminals and the control terminal are not considered. For example, when a distance between a positive electrode terminal and a negative electrode terminal that become a main terminal corresponding to an input terminal for performing current input in the input and output terminals, the main circuit inductance increases. In addition, when a distance between the main terminal and an alternating-current input terminal for connecting with a load such as a motor, which corresponds to the output terminal in the input and output terminals, is long, this path becomes a noise source. Due to the above-described factors, a reduction of an inductance of a large current path for supplying power to the load is insufficient, and a surge associated with a high-speed operation cannot be restricted. In addition, for example, when distances between the input and output terminals and the control terminal are short, a surge may cause a malfunction, for example, turning on and off of a semiconductor switching element by error (hereafter, referred to as an erroneous-on and an erroneous-off). 
     PATENT LITERATURE 
     Patent Literature 1: JP 2012-157161 A 
     SUMMARY 
     An object of the present disclosure is to provide a semiconductor module that can reduce an inductance so as to restrict a surge associated with a high speed operation and to restrict a malfunction of a semiconductor switching element. 
     A semiconductor module according to an aspect of the present disclosure includes upper arms and lower arms for three phases, heat sinks, a main circuit side bus bar, an output terminal side bus bar, a control terminal, and a resin mold portion. Each of the upper arms and the lower arms includes a semiconductor chip in which a semiconductor switching element is formed, and the semiconductor chip has a front surface and a rear surface. The heat sinks are respectively disposed to the front surface and the rear surface of the semiconductor chip in each of the upper arms and the lower arms. 
     The main circuit side bus bar forms a main circuit including a positive electrode wiring layer, a positive electrode terminal, a negative electrode wiring layer, and a negative electrode terminal. The positive electrode wiring layer is connected to positive electrode sides of the semiconductor chips in the upper arms. The positive electrode terminal electrically connects the positive electrode wiring layer and a positive electrode side of an external power source. The negative electrode wiring layer is disposed opposite to the positive electrode wiring layer via an insulating layer and is connected to negative electrode sides of the semiconductor chips in the lower arms. The negative electrode terminal is electrically connected with the negative electrode wiring layer. 
     The output terminal side bus bar includes an output wiring layer and an output terminal. The output wring layer is connected to negative electrode sides of the semiconductor chips in the upper arms and positive electrode sides of the semiconductor chips in the lower arms so as to be connected to middle potential points of the upper arms and the lower arms. The output terminal electrically connects the output wiring layer and a load. 
     The control terminal becomes a signal line of the semiconductor switching elements. The resin mold portion covers the upper arms and the lower arms while exposing one surface of each of the heat sinks, an end portion of the main circuit side bus bar adjacent to the positive electrode terminal and the negative electrode terminal, an end portion of the output terminal side bus bar adjacent to the output terminal, and an end portion of the control terminal. 
     The output wiring layer includes a U-phase wiring layer, a V-phase wiring layer, and a W-phase wiring layer connected to the middle potential point of the upper arm and the lower arm in each of the three phases. The U-phase wiring layer, the V-phase wiring layer, and the W-phase wiring layer are disposed opposite to each other via an insulating layer. The output terminal includes a U terminal, a V terminal, and a W terminal electrically connecting each of the U-phase wiring layer, V-phase wiring layer, and the W-phase wiring layer and the load. A stacked layer number of the U-phase wiring layer, the V-phase wiring layer, and the W-phase wiring layer is set to be an even number. 
     In this way, in the semiconductor module, the stacked layer number of the U-phase to W-phase wiring layers is set to be an even number. Accordingly, an inductance of the semiconductor module can be reduced, a surge associated with a high speed operation can be restricted, and a malfunction of the semiconductor switching elements can be restricted. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a circuit diagram of a three-phase inverter circuit included in a semiconductor module according to a first embodiment of the present disclosure; 
         FIG. 2  is a perspective view of the semiconductor module; 
         FIG. 3  is a perspective view showing a state in which both surfaces of the semiconductor module are sandwiched by cooling devices; 
         FIG. 4  is an exploded perspective view of components of the semiconductor module; 
         FIG. 5  is a top layout diagram of the semiconductor module; 
         FIG. 6  is a perspective view of a multilayer wiring bus bar; 
         FIG. 7A  is a cross-sectional view viewed from an arrow direction of VIIA-VIIA in  FIG. 6 ; 
         FIG. 7B  is a cross-sectional view viewed from an arrow direction of VIIB-VIIB in  FIG. 6 ; 
         FIG. 7C  is a cross-sectional view viewed from an arrow direction of VIIC-VIIC in  FIG. 6 ; 
         FIG. 7D  is a cross-sectional view viewed from an arrow direction of VIID-VIID in  FIG. 6 ; 
         FIG. 7E  is a cross-sectional view viewed from an arrow direction of VIIE-VIIE in  FIG. 6 ; 
         FIG. 7F  is a cross-sectional view viewed from an arrow direction of VIIF-VIIF in  FIG. 6 ; 
         FIG. 7G  is a cross-sectional view viewed from an arrow direction of VIIG-VIIG in  FIG. 6 ; 
         FIG. 7H  is a cross-sectional view viewed from an arrow direction of VIIH-VIIH in  FIG. 6 ; 
         FIG. 7I  is a cross-sectional view viewed from an arrow direction of VIII-VIII in  FIG. 6 ; 
         FIG. 7J  is a cross-sectional view viewed from an arrow direction of VIIJ-VIIJ in  FIG. 6 ; 
         FIG. 7K  is a cross-sectional view viewed from an arrow direction of VIIK-VIIK in  FIG. 6 ; 
         FIG. 8  is a cross-sectional view viewed from VIII-VIII in  FIG. 2 ; 
         FIG. 9A  is a cross-sectional view showing flow of electric currents at a position where a positive electrode wiring layer and a negative electrode wiring layer are staked when the semiconductor module is used; 
         FIG. 9B  is a cross-sectional view showing flow of electric currents at a position where U-phase to W-phase wiring layers are formed into a stacked structure when the semiconductor module is used; 
         FIG. 10  is a circuit diagram showing an operation of the three-phase inverter circuit; 
         FIG. 11  is a waveform chart of electric currents Iu, Iv, Iw that flow to respective phases of the three-phase inverter circuit; 
         FIG. 12  is a perspective view of a stacked structure of conductive layers used as a specimen; 
         FIG. 13  is a graph showing a result of inventing a change of inductance with respect to a change of a size of each portion of a staked structure of the conductive layers; 
         FIG. 14A  is a cross-sectional view of a stacked structure of U-phase to W-phase wiring layers used as a specimen; 
         FIG. 14B  is a cross-sectional view of a stacked structure of U-phase to W-phase wiring layers used as a specimen; 
         FIG. 14C  is a cross-sectional view of a stacked structure body of U-phase to W-phase wiring layers used as a specimen; 
         FIG. 15A  is a graph showing results of investing inductances at a timing ( 6 ) in  FIG. 11  using the specimens shown in  FIG. 14A  to  FIG. 14C ; 
         FIG. 15B  is a graph showing results of investing inductances at a timing ( 2 ) in  FIG. 11  using the specimens shown in  FIG. 14A  to  FIG. 14C ; 
         FIG. 15C  is a graph showing results of investing inductances at a timing ( 4 ) in  FIG. 11  using the specimens shown in  FIG. 14A  to  FIG. 14C ; 
         FIG. 16A  is a circuit diagram showing a state of a series-arm short circuit; 
         FIG. 16B  is a circuit diagram showing a state of an output short circuit; 
         FIG. 17A  is a graph showing results of investing self-inductances L 1 , L 2  with respect to a distance Ls 1 ; 
         FIG. 17B  is a graph showing results of investing a mutual inductance M 12  with respect to the distance Ls 1 ; 
         FIG. 18  is a perspective view of a specimen used for the investigation in  FIG. 17A  and  FIG. 17B ; 
         FIG. 19A  is a graph showing results of investing self-inductances L 1 , L 2  with respect to a distance Ls 2 ; 
         FIG. 19B  is a graph showing results of investing a mutual inductance M 12  with respect to the distance Ls 1 ; 
         FIG. 20  is a perspective view of a specimen used for the investigation in  FIG. 19A  and  FIG. 19B ; 
         FIG. 21  is a simple model diagram of a circuit to which a bridge circuit J 3  constituted of a semiconductor module is applied; and 
         FIG. 22  is a timing diagram showing a state at switching of a semiconductor switching element J 1  in the bridge circuit J 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Before describing embodiments of the present disclosure, circumstances of how the inventors arrived at present disclosure will be described by exemplifying a case in which a semiconductor module is used as a three-phase inverter circuit. 
     As a simple model shown in  FIG. 21 , the three-phase inverter circuit includes bridge circuits J 3 , in which a parallel connection of a semiconductor switching element J 1  such as an IGBT and a freewheel diode (hereafter, referred to as FWD) J 2  are provided for each of an upper arm and a lower arm, for three phases. The three-phase inverter circuit is connected with a load J 4  such as a motor, includes a smoothing capacitor J 5  in parallel with the upper arm and the lower arm, and drives the load J 4  by power supply from a direct-current power source J 6 . Specifically, the inverter circuit switches on and off the semiconductor switching elements J 1  in the upper arm and the lower arm to convert a direct current supplied from the direct-current power source J 6  to an alternating current, and supplies the alternating current to the load J 4 . A high side terminal (a positive electrode terminal) in the upper arm and a low side terminal (a negative electrode terminal) in the lower arm in this figure corresponds to input terminals of input and output terminals in Patent Literature 1. In addition, an output terminal that performs a current supply to the load J 4  corresponds to an alternating-current input terminal in Patent Literature 1, and a gate terminal of the semiconductor switching element J 1  provided for each of the arms corresponds to a control terminal of Patent Literature 1. 
     A drain-source current Ids, a drain-source voltage Vds, and a switching loss Esw when the current supply is performed to the load J 4  are shown in  FIG. 22 . 
     In the above-described circuit configuration, a short-circuit loop of the upper arm and the lower arm, which is shown by the arrow in  FIG. 21 , is formed. When the switching element J 1  in the lower arm is switched from on to off, a dl/dt change occurs in the short-circuit loop. 
     As shown in  FIG. 22 , a surge voltage ΔVsur is generated at switching. The surge voltage ΔVsur is indicated by the following mathematical expression. In the following mathematical expression, L indicates an inductance of the short-circuit loop.
 
Δ V sur= L·dl/dt   (Math. 1)
 
     The surge voltage ΔVsur tends to increase due to a promotion of a large current and a high switching speed in recent years. A surge protection is realizable when an element breakdown voltage is set to be high. However, an on-resistance that is in a trade-off relation increases, and an increase of a stationary loss is caused. In addition, there are requirements of reducing the switching loss Esw and reducing a device size, and an improvement of dl/dt and a high frequency are necessary to meet the requirements. Thus, a reduction in inductance of the short-circuit loop is necessary to improve dl/dt without increasing the surge voltage ΔVsur. 
     Specifically, it is necessary to reduce a main circuit inductance when a path that passes through the upper and lower arms and returns to the smoothing capacitor J 5  as shown by the arrow in  FIG. 21  is set as a main circuit and to reduce an output terminal induction that is a current supply path to the load J 4 . In the main circuit, the upper and lower arms are controlled so as not to be turned on at the same time in an inverter control. However, because an upper and lower arm short-circuit occurs from the viewpoint of dl/dt change at switching, when the inductance of the main circuit that becomes an upper and lower short-circuit loop is large, a large surge is generated. At the output terminal, the control circuit works so as to interrupt an electric current of the inverter when an abnormality occurs in the motor. However, when an inductance of an output terminal portion is large, the output terminal portion becomes a noise source and causes a malfunction of the control circuit. Thus, it is necessary to reduce the output terminal inductance as well as the main circuit inductance. 
     Therefore, in view of the above-described points, it is an object of the present disclosure to reduce an inductance of a semiconductor module and to provide a semiconductor module that can restrict a surge associated with a high-speed operation and can restrict a malfunction of a semiconductor switching element. 
     The following describes embodiments of the present disclosure with reference to the drawings. In each of the following embodiments, the same or equivalent parts will be described with being designated by the same reference numerals. 
     First Embodiment 
     The following describes a first embodiment of the present disclosure. In the present embodiment, as an application example of a semiconductor module according to an embodiment of the present disclosure, a semiconductor module that includes a three-phase inverter circuit driving, for example, a three-phase alternating-current motor will be described. 
     First, with reference to  FIG. 1 , a configuration of a three-phase inverter circuit  1  included in the semiconductor module will be described. As shown in  FIG. 1 , the three-phase inverter circuit  1  drives a three-phase alternating-current motor as a load  3  based on a direct-current power source (external power source)  2 . The three-phase inverter circuit  1  is connected in parallel with a smoothing capacitor  4  so as to form a constant power source voltage by reducing a ripple at switching and restricting an influence of noise. 
     The three-phase inverter circuit  1  has a configuration in which serially-connected upper and lower arms  51 - 56  are connected in parallel for three phases. The three-phase inverter circuit  1  applies each of middle potentials between the upper arms  51 ,  53 ,  55  and the lower arms  52 ,  54 ,  56  to each of a U-phase, a V-phase, and a W-phase of the three-phase alternating-current motor as the load  3  while switching in turn. Specifically, each of the upper and lower arms  51 - 56  includes a semiconductor switching element  51   a - 56   a  such as an IGBT and a MOSFET and a rectifier (one-side conductive element)  51   b - 56   b  such as a FWD for back flow current. On-off states of the semiconductor switching elements  51   a - 56   a  of each of the phases are controlled so as to supply three-phase alternating currents having different periods to the three-phase alternating-current motor. Accordingly, the three-phase alternating-current motor can be driven. 
     In the present embodiment, semiconductor chips in which the semiconductor switching elements  51   a - 56   a  and the rectifiers  51   b - 56   b  respectively constituting six upper and lower arms  51 - 56  that constituting the three-phase inverter circuit  1  are formed are modulized to be integrated. That is, the three-phase inverter circuit  1  is configured as the semiconductor module having a 6-in-1 structure in which six arms are integrated. 
     Next, a detailed structure of the semiconductor module that includes the three-phase inverter circuit  1  having the above-described circuit configuration will be described with reference to  FIG. 2  to  FIG. 6 . 
     A semiconductor module  6  shown in  FIG. 2  includes semiconductor chips  10 , an upper heat sink  11 , a lower heat sink  12 , a multilayer wiring bus bar  13  in which various terminals and wirings are integrated, control terminals  14 , element relay electrodes  15 , and plates  16 ,  17  as shown in  FIG. 4 . In the above-described components, the semiconductor chip  10 , the control terminal  14 , the element relay electrode  15 , and the plates  16 ,  17  are set as a component block for one arm, and six component blocks are provided. The six components blocks are sandwiched with the multilayer wiring bus bar  13  by the upper heat sink  11  and the lower heat sink  12 , and are covered by a resin mold portion  18 . In this way, the semiconductor module  6  is configured as a plate-shape member having a predetermined thickness as shown in  FIG. 2 . Both sides in the thickness direction of the semiconductor module  6  are sandwiched by cooling devices  19  so that the semiconductor module  6  drives the load  3  while releasing heat. Cooling pipes that form coolant passages not shown are respectively inserted into through holes  19   a  provided at two points of each of the cooling devices  19 . Accordingly, the cooling devices  19  are cooled, and thereby the semiconductor module  6  is used with an efficient cooling. 
     The following describes a detailed structure of the semiconductor module. Although detailed structures of the component blocks for six arms covered by the resin mold portion  18  are slightly different from each other, basic structures are similar. First, each of components that constitute the basic structures of the component blocks covered by the resin mold portion  18  will be described. 
     The semiconductor chips  10  shown in  FIG. 4  have front surfaces and rear surfaces, and the semiconductor switching elements  51   a - 56   a  and the rectifiers  51   b - 56   b  that constitute the upper arms  51 ,  53 ,  55  are the lower arms  52 ,  54 ,  56  are formed in the semiconductor chips  10 . For example, the semiconductor chips  10  are formed using semiconductor substrates such as Si, SiC, GaN as base substrates. In the present embodiment, the semiconductor switching elements  51   a - 56   a  and the rectifiers  51   b - 56   b  are formed as vertical elements that let electric current flow in a vertical direction of the substrate. Various electrodes (pads) are formed on the front surfaces and the rear surfaces of the semiconductor chips  10  and electrical connections are enabled via the electrodes. In a case of the present embodiment, the rear surface of each of the semiconductor chips  10  is electrically and physically connected to a front surface of the lower plate  17  via a joining material  20  made of a high-temperature conductive material such as a solder or an Ag sintered material. A rear surface of the lower plate  17  is joined to the lower heat sink  12  via a joining material  21  made of a high-temperature conductive material such as a solder and an Ag sintered material. 
     The front surface of the semiconductor chip  10  is connected to the element relay electrode  15  that is made of, for example, Cu, Al, or Fe as a base material via a joining material  22  made of a high-temperature conductive material such as a solder or an Ag sintered material. Furthermore, the element relay electrode  15  is electrically and physically connected to the rear surface of the upper plate  16  via a joining material  23  made of a high-temperature conductive material such as a solder or an Ag sintered material. The upper plate  16  is joined to the upper heat sink  11  via a joining material  24  made of a high-temperature conductive material such as a solder or an Ag sintered material. 
     By the above-described structure, each of the semiconductor chips  10  is sandwiched by the upper heat sink  11  and the lower heat sink  12 . 
     In the present embodiment, the semiconductor chip  10  has a structure in which elements constituting each of the arms  51 - 56 , such as the semiconductor switching elements  51   a - 56   a  and the rectifiers  51   b - 56   b , are formed together in one chip. However, this is merely one example, and elements constituting each of the arms  51 - 56 , such as the semiconductor switching elements  51   a - 56   a  and the rectifiers  51   b - 56   b , may be formed in different chips. 
     Each of the upper heat sink  11  and the lower heat sink  12  corresponds to a heat sink and is made of a high-temperature conductive material mainly including, for example, Cu, Al, or Fe. One surface of each of the upper heat sink  11  and the lower heat sink  12  faces the semiconductor chips  10  and the other surface is exposed from the resin mold portion  18  as shown in  FIG. 2 . The upper heat sink  11  and the lower heat sink  12  are insulated from the semiconductor chips  10  because the upper plates  16  and the lower plates  17  are partially made of insulating materials. However, because the upper plates  16 , the lower plates  17 , and the element relay electrodes  15  are made of the high-temperature conductive materials, the upper heat sink  11  and the lower heat sink  12  can release heat from the semiconductor chips  10  with a high thermal conductivity. Specifically, the front surface of the upper heat sink  11  and the rear surface of the lower heat sink  12 , that is, both surfaces opposite from the surfaces to which the semiconductor chips  10  are disposed are exposed from the resin mold portion  18 , and the heat is released from the exposed portion. 
     The multilayer wiring bus bar  13  is a portion that constitutes various wirings and various terminals in the semiconductor module  6  according to the present embodiment. In the present embodiment, the multilayer wiring bus bar  13  is made of a plate-shaped and rod-shaped member and is disposed so as to pass between the semiconductor chips  10  that constitute the upper arms  51 ,  53 ,  55  of the respective phases and the semiconductor chips  10  that constitute the lower arms  52 ,  54 ,  56 . For example, the multilayer wiring bus bar  13  includes a positive electrode side wiring that connects the upper arms  51 ,  53 ,  55  and a plus terminal of the direct-current power source  2 , a negative electrode side wiring that connects the lower arms  52 ,  54 ,  56  and a minus terminal of the direct-current power source  2 , and an output wiring that connects each of the arms  51 - 56  and the load  3 . In addition, the multilayer wiring bus bar  13  includes various connection terminals  13   a - 13   e  for connecting wirings to the direct-current power source  2  and the load  3 . The multilayer wiring bus bar  13  is a portion that constitutes a main feature of the present application. A detailed structure of the multilayer wiring bus bar  13  will be described later. 
     The control terminals  14  are signal terminals that constitute various signal lines such as gate wirings of the semiconductor switching elements  51   a - 56   a . For example, the control terminals  14  are electrically connected to electrodes that are connected to the gates of the semiconductor switching elements  51   a - 56   a  formed to the front surfaces of the semiconductor chips  10  via bonding wires  25  (see  FIG. 4 ). End portions of the control terminals  14  opposite from the semiconductor chips  10  are exposed from the resin mold portion  18  as shown in  FIG. 2 , and a connection with an external device is performed via the exposed portions. In  FIG. 4 , the control terminals  14  are described in such a manner that the control terminals  14  are integrated in a lead frame state and are also integrated with the lower heat sink  12 . However, the control terminals  14  are divided when becoming a final product, and each of the signal lines becomes an independent state. 
     The element relay electrodes  15  are members electrically connected to the upper plate  16  while the surfaces of the element relay electrodes  15  adjacent to the semiconductor chips  10  are electrically connected with the electrodes on the front surfaces of the semiconductor chips  10 . The element relay electrodes  15  are provided so as to make a space in which the bonding wires  25  are disposed between the semiconductor chips  10  and the upper plate  16 . The element relay electrodes  15  are made of a high-temperature conductive member mainly including, for example, Cu. 
     The upper plates  16  serve a function of insulating the semiconductor chips  10  and the upper heat sink  11  while the surfaces of the upper plates  16  adjacent to the semiconductor chips  10  are electrically connected with the electrodes on the front surfaces of the semiconductor chips  10  via the element relay electrodes  15 . Similarly, the lower plates  17  serve a function of insulating the semiconductor chips  10  and the lower heat sink  12  while the surfaces of the lower plates  17  adjacent to the semiconductor chips  10  are electrically connected with the electrodes on the rear surfaces of the semiconductor chips  10 . 
     The surfaces of the upper plates  16  and the lower plates  17  adjacent to the semiconductor chips  10  are made of a high-temperature conductive material including Cu or Al as a base material so as to enhance a thermal conductivity in addition to an electrical connection. Layers of the upper plates  16  and the lower plates  17  located on opposite sides from the surfaces adjacent to the semiconductor chips  10  are made of, for example, inorganic or organic insulating material so as to enhance a thermal conductivity while insulating. For example, the upper plates  16  and the lower plates  17  can be formed by sticking conductive plates that include Cu as a base material to both surfaces a ceramic insulating body such as Si 3 N 4 , AlN, or Al 2 O 3 . The upper plates  16  and the lower plates  17  can also be formed by sticking a Cu plate on which an insulating sheet is stuck and an adhesive sheet having an insulation adhesive function with a die bond plate made of a conductive material such as Cu. 
     By the above-described configuration, the upper plates  16  are connected with front surface electrodes of the semiconductor switching elements  51   a - 56   a  (for example, sources of MOSFETs or emitters of IGBTs) and first electrodes of the rectifiers  51   b - 56   b  (for example, anodes of FWDs). In addition, the upper plates  16  are also electrically connected with electrodes included in the multilayer wiring bus bar  13 . Similarly, the lower plates  17  are connected with rear surface electrodes of the semiconductor switching elements  51   a - 56   a  (for example, drains of MOSFETs or collectors of IGBTs) and second electrodes of the rectifiers  51   b - 56   b  (for example, cathodes of FWDs). In addition, the lower plates  17  are also connected with electrodes included in the multilayer wiring bus bar  13 . Thus, the upper plates  16  and the lower plates  17  constitute a part of the positive electrode side wiring, the negative electrode side wiring and the output wiring with respect to each of the arms  51 - 56 . 
     As described above, the front surface of the upper heat sink  11  and the rear surface of the lower heat sink  12 , that is, the surfaces opposite from the surfaces to which the semiconductor chips  10  are disposed are exposed from the resin mold portion  18 , and the heat is released from the exposed portion. These heat sinking planes are in contact with the cooling devices  19  as shown in  FIG. 3 . However, because the upper heat sink  11  and the lower heat sink  12  are insulated from the semiconductor chips  10  by the upper plates  16  and the lower plates  17 , a generation of a current leakage to outside through the upper heat sink  11  and the lower heat sink  12  can be prevented. 
     The resin mold portion  18  is a sealing resin that is formed by filling a resin in a molding tool after disposing the above-described components in the molding tool, and has, for example, a quadrangular plate shape. The resin mold portion  18  is made of a resin that has an insulation property and has a linear expansion coefficient and a Young&#39;s modulus lower than conductive portions such as the upper heat sink  11  and the lower heat sink  12 . For example, the resin mold portion  18  can be mainly made of an organic resin such as epoxy or silicone. The ends of the control terminals  14  and both ends of the multilayer wiring bus bar  13  are exposed from sides of the quadrangular plate shape of the resin mold portion  18  so as to be electrically connected with external devices. Specifically, the control terminals  14  of the upper arms  51 ,  53 ,  55  and the control terminals  14  of the lower arms  52 ,  54 ,  56  are exposed from two opposed sides of the resin mold portion  18  having the quadrangular plate shape, that is, to opposite directions sandwiching the resin mold portion  18 . In addition, the both ends of the multilayer wiring bus bar  13  are respectively exposed from two opposed sides of the resin mold portion  18  having the quadrangular plate shape, that is to opposite directions sandwiching the resin mold portion  18 . In addition, the upper heat sink  11  and the lower heat sink  12  are respectively exposed from front and rear surfaces of the quadrangular plate shape so that the heat can be released excellently. 
     Specifically, each of the above-described parts is mounted on the front surface of the lower heat sink  12  in a lead frame state to which the control terminals  14  are integrated. After the semiconductor chips  10  and the control terminals  14  are electrically connected with bonding wires  25 , the upper heat sink  11  is mounted thereon. These are disposed in the molding tool in this state, and the resin is inject into the molding tool and is molded to form the resin mold portion  18 . Because the resin mold portion  18  covers portions except for exposed portions of the control terminals  14  and the multilayer wiring bus bar  13  in addition to the surfaces of the upper heat sink  11  and the lower heat sink  12 , the semiconductor chips  10  and the like are protected. 
     The semiconductor module  6  according to the present embodiment has the above-described structure. Next, a detailed structure of the multilayer wiring bus bar  13  will be described with reference to  FIG. 6 ,  FIG. 7A  to  FIG. 7K . 
     As described above, the multilayer wiring bus bar  13  constitutes the various wirings and the various terminals of the semiconductor module  6  and is formed by stacking a plurality of conductive layers while sandwiching insulating layers. In a case of the present embodiment, as shown in  FIG. 6 , the multilayer wiring bus bar  13  is formed of a plate-shaped and rod-shaped member having a longitudinal direction in one direction. As shown in  FIG. 2 , one end and the other end of the multilayer wiring bus bar  13  are respectively exposed from the two opposed sides of the resin mold portion  18  having the approximately quadrangular shape. 
     As shown in  FIG. 6 , at the one end of the multilayer wiring bus bar  13 , a positive electrode terminal  13   a  that connects the upper arms  51 ,  53 ,  55  and the plus terminal of the direct-current power source  2  and a negative electrode terminal  13   b  that connects the lower arms  52 ,  54 ,  56  and the minus terminal are formed. At the other end of the multilayer wiring bus bar  13 , a U terminal  13   c , a V terminal  13   d , and a W terminal  13   e  corresponding to output terminals that connect middle potential points of the upper and lower arms  51 - 56  of each of the U-phase, the V-phase, and the W-phase, and the load  3  are provided. The positive electrode terminal  13   a , the negative electrode terminal  13   b , the U terminal  13   c , the V terminal  13   c , the W terminal  13   e  are exposed from the resin mold portion  18  as shown in  FIG. 2 . By the above-described configuration, the semiconductor module  6  is electrically connected with the direct-current power source  2  and the smoothing capacitor  4  via the positive electrode terminal  13   a  and the negative electrode terminal  13   b . In addition, electrical connections with the U-phase, the V-phase, and the W-phase of the three-phase alternating-current motor as the load  3  are provided via the U terminal  13   c , the V terminal  13   d , and the W terminal  13   e.    
     As shown in  FIG. 7A , the positive electrode terminal  13   a  is constitute of a through hole  13   ac . On an inner wall and a periphery of an opening portion of the through hole  13   ac , a penetrating inner portion  13   aa  and a surface conductive layer portion  13   ab  made of, for example, a Cu-plating are formed. The penetrating inner portion  13   aa  in the positive electrode terminal  13   a  is connected with a positive electrode wiring layers  131  having a multilayer structure constituted of internal layer conductors that are stacked while being sandwiched by insulating layers  130 . The positive electrode wiring layer  131  constitutes wirings connected to positive electrodes  137   a - 137   c  (see  FIG. 7G  to  FIG. 7H ) connected to the high side of each of the upper arms  51 ,  53 ,  55 . In the present embodiment, the stacked layer number of the positive electrode wiring layer  131  is two. When a thickness t of one positive electrode wiring layer  131   a  is set to 0.5, a thickness t of the other positive electrode wiring layer  131   b  is set to, for example, 0.25 which is a half of the thickness t of the positive electrode wiring layer  131   a.    
     As shown in  FIG. 7B , the negative electrode terminal  13   b  is constituted of a through hole  13   bc . On an inner wall and a periphery of an opening portion of the through hole  13   bc , a penetrating inner portion  13   ba  and a surface conductive layer portion  13   bb  made of, for example, a Cu-plating are formed. The penetrating inner portion  13   ba  in the negative electrode terminal  13   b  is connected with a negative electrode wiring layers  132  having a multilayer structure constituted of internal layer conductors that are stacked while being sandwiched by insulating layers  130 . The negative electrode wiring layer  132  constitutes wirings connected to negative electrodes  139   a - 139   c  (see  FIG. 7I  to  FIG. 7K ) connected to the low side of each of the lower arms  52 ,  54 ,  56 . In the present embodiment, the stacked layer number of the negative electrode wiring layer  132  is two. When a thickness t of one negative electrode wiring layer  132   a  is set to 0.25, a thickness t of the other negative electrode wiring layer  132   b  is set to, for example, 0.5 which is a double of the thickness t of the negative electrode wiring layer  132   a.    
     The above-described positive electrode wiring layer  131  and the negative electrode wiring layer  132  are arranged at the one end of the multilayer wiring bus bar  13  in such a manner that the positive electrode wiring layer  131   a , the negative electrode wiring layer  132   a , the positive electrode wiring layer  131   b , the negative electrode wiring layer  132   b  are arranged in this order from one surface to the other surface of the multilayer wiring bus bar  13 . Thus, the negative electrode wiring layer  132   a  and the positive electrode wiring layer  131   b  having equal thickness are sandwiched by the positive electrode wiring layer  131   a  and the negative electrode wiring layer  132  having equal thickness. 
     As shown in  FIG. 7C , the U terminal  13   c  is constituted of a through hole  13   cc . On an inner wall and a periphery of an opening portion of the through hole  13   cc , a penetrating inner portion  13   ca  and a surface conductive layer portion  13   cb  made of, for example, a Cu-plating are formed. The penetrating inner portion  13   ca  in the U terminal  13   c  is connected with a U-phase wiring layer  133  constituted of internal layer conductors that are stacked while being sandwiched by the insulating layers  130 . The U-phase wiring layer  133  constitutes a wiring connected to the middle potential point of the upper and lower arms  51 ,  52  of the U-phase and the U terminal  13   c . The stacked layer number of the U-phase wiring layer  133  is one and the U-phase wiring layer  133  has a thickness of 0.5. In the present embodiment, the U-phase wiring layer  133  is constituted of the same internal conductor as the positive electrode wiring layer  131   a.    
     As shown in  FIG. 7D , the V terminal  13   d  is constituted of a through hole  13   dc . On an inner wall and a periphery of an opening portion of the through hole  13   dc , a penetrating inner portion  13   da  and a surface conductive layer portion  13   db  made of, for example, a Cu-plating are formed. The penetrating inner portion  13   da  in the V terminal  13   d  is connected with a U-phase wiring layer  134  constituted of internal layer conductors that are stacked while being sandwiched by the insulating layers  130 . The V-phase wiring layer  134  constitutes a wiring connected to the middle potential point of the upper and lower arms  53 ,  54  of the V-phase and the V terminal  13   d . The stacked layer number of the V-phase wiring layer  134  is one and the V-phase wiring layer  133  has a thickness of 0.5. In the present embodiment, the V-phase wiring layer  134  is constituted of the same internal conductor as the negative electrode wiring layer  132   b.    
     As shown in  FIG. 7E , the W terminal  13   e  is constituted of a through hole  13   ec . On an inner wall and a periphery of an opening portion of the through hole  13   ec , a penetrating inner portion  13   ea  and a surface conductive layer portion  13   eb  made of, for example, a Cu-plating are formed. The penetrating inner portion  13   ea  in the W terminal  13   e  is connected with W-phase wiring layers  135  having a multilayer structure constituted of internal layer conductors that are stacked while being sandwiched by the insulating layers  130 . The W-phase wiring layers  135  constitute a wiring connected to the middle potential point of the upper and lower arms  55 ,  56  of the W-phase and the W terminal  13   e . The stacked layer number of the W-phase wiring layers  135  is two and the W-phase wiring layers  135  sandwich the U-phase wiring layer  133  and the V-phase wiring layer  134 . Each of the W-phase wiring layers  135  has a thickness of 0.25. In the present embodiment, one W-phase wiring layer  135   a  is constituted of the same internal conductor as the negative electrode wiring layer  132   a  and the other W-phase wiring layer  135   b  is constituted of the same internal conductor as the positive electrode wiring layer  131   b.    
     The above-described U-phase to W-phase wiring layers  133 - 135  are arranged at the other end of the multilayer wiring bus bar  13  in such a manner that the W-phase wiring layer  135   a , the U-phase wiring layer  133 , the V-phase wiring layer  134 , the W-phase wiring layer  135   b  are arranged in this order from the one surface to the other surface of the multilayer wiring bus bar  13 . Thus, the U-phase wiring layer  133  and the V-phase wiring layer  134  having thickness are sandwiched by the W-phase wiring layer  135   a  and the W-phase wiring layer  135   b  having equal thickness. 
     As shown in  FIG. 7F , on the positive electrode terminal  13   a  and the negative electrode terminal  13   b  side of a front side of the paper at a middle position in the longitudinal direction of the multilayer wiring bus bar  13  shown in  FIG. 6 , a U electrode  136   a  is formed on the one surface (the front surface) and the positive electrode  137   a  is formed on the other surface (the rear surface). 
     The U electrode  136   a  corresponds to one of first output electrodes and is electrically connected with the U-phase wiring layer  133 . In the present embodiment, the U electrode  136   a  is constituted of a surface electrode layer  136   aa  having a rectangular upper-surface shape and a blind via  136   ab  penetrating the multilayer wiring bus bar  13  from the one surface to the U-phase wiring layer  133  through the insulating layer  130 . The U electrode  136   a  is electrically connected with the low side of the upper arm  51  of the U-phase. Specifically, as shown in  FIG. 4 , the rear surface of the upper plate  16  corresponding to the semiconductor chip  10   a  of the upper arm  51  is electrically connected to the U electrode  136   a  via a joining material  26 . Accordingly, the front surface electrode of the semiconductor chip  10   a  of the upper arm  51  is electrically connected to the U electrode  136   a  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     The positive electrode  137   a  is electrically connected with the positive electrode wiring layer  131 . In the present embodiment, the positive electrode  137   a  is constituted of a surface electrode layer  137   aa  having a rectangular upper-surface shape and a blind via  137   ab  penetrating the multilayer wiring bus bar  13  from the other surface to the positive electrode wiring layer  131   a  through the insulating layer  130 . Specifically, as shown in  FIG. 4 , the front surface of the lower plate  17  corresponding to the semiconductor chip  10   a  of the upper arm  51  is electrically connected to the positive electrode  137   a  (not shown in  FIG. 4 ) via a joining material  27 . Accordingly, the rear surface electrode of the semiconductor chip  10   a  of the upper arm  51  is electrically connected to the positive electrode  137   a  via the joining material  20 , the lower plate  17 , and the joining material  27 . 
     As shown in  FIG. 7G , a V electrode  136   b  is formed on the one surface and the positive electrode  137   b  is formed on the other surface on the front side of the paper at the middle position in the longitudinal direction of the multilayer wiring bus bar  13 . 
     The V electrode  136   b  corresponds to one of the first output electrodes and is electrically connected with the V-phase wiring layer  134 . In the present embodiment, the V electrode  136   b  is constituted of a surface electrode layer  136   ba  having a rectangular upper-surface shape and a blind via  136   bb  penetrating the multilayer wiring bus bar  13  from the one surface to the V-phase wiring layer  134  through the insulating layer  130 . The V electrode  136   b  is electrically connected with the low side of the upper arm  53  of the V-phase. Specifically, as shown in  FIG. 4 , the rear surface of the upper plate  16  corresponding to the semiconductor chip  10   c  of the upper arm  53  is electrically connected to the V electrode  136   b  via the joining material  26 . Accordingly, the front surface electrode of the semiconductor chip  10   c  of the upper arm  53  is electrically connected to the V electrode  136   b  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     The positive electrode  137   a  is electrically connected with the positive electrode wiring layer  131 . In the present embodiment, the positive electrode  137   b  is constituted of a surface electrode layer  137   ba  having a rectangular upper-surface shape and a blind via  137   bb  penetrating the multilayer wiring bus bar  13  from the other surface to the positive electrode wiring layer  131   b  through the insulating layer  130 . Specifically, as shown in  FIG. 4 , the front surface of the lower plate  17  corresponding to the semiconductor chip  10   c  of the upper arm  53  is electrically connected to the positive electrode  137   b  (not shown in  FIG. 4 ) via the joining material  27 . Accordingly, the rear surface electrode of the semiconductor chip  10   c  of the upper arm  53  is electrically connected to the positive electrode  137   a  via the joining material  20 , the lower plate  17 , and the joining material  27 . 
     As shown in  FIG. 7H , on the U terminal  13   c , the V terminal  13   d  and the W terminal  13   e  side of the front side of the paper at the middle position in the longitudinal direction of the multilayer wiring bus bar  13  shown in  FIG. 6 , a W electrode  136   c  is formed on the one surface and the positive electrode  137   a  is formed on the other surface. 
     The W electrode  136   c  corresponds to one of first output electrodes and is electrically connected with the W-phase wiring layer  135 . In the present embodiment, the W electrode  136   c  is constituted of a surface electrode layer  136   ca  having a rectangular upper-surface shape and a blind via  136   cb  penetrating the multilayer wiring bus bar  13  from the one surface to the W-phase wiring layer  135   b  through the insulating layer  130 . The W electrode  136   c  is electrically connected with the low side of the upper arm  55  of the W-phase. Specifically, as shown in  FIG. 4 , the rear surface of the upper plate  16  corresponding to the semiconductor chip  10   e  of the upper arm  55  is electrically connected to the W electrode  136   c  via the joining material  26 . Accordingly, the front surface electrode of the semiconductor chip  10   e  of the upper arm  55  is electrically connected to the W electrode  136   c  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     The positive electrode  137   c  is electrically connected with the positive electrode wiring layer  131 . In the present embodiment, the positive electrode  137   c  is constituted of a surface electrode layer  137   ca  having a rectangular upper-surface shape and a blind via  137   cb  penetrating the multilayer wiring bus bar  13  from the other surface to the positive electrode wiring layer  131   b  through the insulating layer  130 . Specifically, as shown in  FIG. 4 , the front surface of the lower plate  17  corresponding to the semiconductor chip  10   e  of the upper arm  55  is electrically connected to the positive electrode  137   c  (not shown in  FIG. 4 ) via the joining material  27 . Accordingly, the rear surface electrode of the semiconductor chip  10   e  of the upper arm  55  is electrically connected to the positive electrode  137   c  via the joining material  20 , the lower plate  17 , and the joining material  27 . 
     As shown in  FIG. 7I , on the positive electrode terminal  13   a  and the negative electrode terminal  13   b  side of the back side of the paper at the middle position in the longitudinal direction of the multilayer wiring bus bar  13  shown in  FIG. 6 , a U electrode  138   a  is formed on the other surface (the rear surface) and the negative electrode  139   a  is formed on the one surface (the front surface). 
     The U electrode  138   a  corresponds to one of second output electrodes and is electrically connected with the U-phase wiring layer  133 . In the present embodiment, the U electrode  138   a  is constituted of a surface electrode layer  138   aa  having a rectangular upper-surface shape and a blind via  138   ab  penetrating the multilayer wiring bus bar  13  from the other surface to the U-phase wiring layer  133  through the insulating layer  130 . The U electrode  136   a  is electrically connected with the high side of the lower arm  52  of the U-phase. Specifically, as shown in  FIG. 4 , the front surface of the lower plate  17  corresponding to the semiconductor chip  10   b  of the lower arm  52  is electrically connected to the U electrode  138   a  (not shown in  FIG. 4 ) via the joining material  27 . Accordingly, the rear surface electrode of the semiconductor chip  10   b  of the lower arm  52  is electrically connected to the U electrode  138   a  via the joining material  20 , the lower plate  17 , and the joining material  27 . 
     The negative electrode  139   a  is electrically connected with the negative electrode wiring layer  132 . In the present embodiment, the negative electrode  139   a  is constituted of a surface electrode layer  139   aa  having a rectangular upper-surface shape and a blind via  139   ab  penetrating the multilayer wiring bus bar  13  from the one surface to the negative electrode wiring layer  132   a  through the insulating layer  130 . Specifically, as shown in  FIG. 4 , the front surface of the upper plate  16  corresponding to the semiconductor chip  10   b  of the lower arm  52  is electrically connected to the negative electrode  139   a  via the joining material  26 . Accordingly, the front surface electrode of the semiconductor chip  10   b  of the lower arm  52  is electrically connected to the negative electrode  139   a  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     As shown in  FIG. 7G , a V electrode  138   b  is formed on the other surface and the negative electrode  139   b  is formed on the one surface on the back side of the paper at the middle position in the longitudinal direction of the multilayer wiring bus bar  13  shown in  FIG. 6 . 
     The V electrode  138   b  corresponds to one of the second output electrodes and is electrically connected with the V-phase wiring layer  134 . In the present embodiment, the V electrode  138   b  is constituted of a surface electrode layer  138   ba  having a rectangular upper-surface shape and a blind via  138   bb  penetrating the multilayer wiring bus bar  13  from the one surface to the V-phase wiring layer  134  through the insulating layer  130 . The V electrode  138   b  is electrically connected with the high side of the lower arm  54  of the V-phase. Specifically, as shown in  FIG. 4 , the front surface of the lower plate  17  corresponding to the semiconductor chip  10   d  of the lower arm  54  is electrically connected to the V electrode  138   b  (not shown in  FIG. 4 ) via the joining material  27 . Accordingly, the rear surface electrode of the semiconductor chip  10   d  of the lower arm  54  is electrically connected to the V electrode  138   b  via the joining material  20 , the lower plate  17 , and the joining material  27 . 
     The negative electrode  139   b  is electrically connected with the negative electrode wiring layer  132 . In the present embodiment, the negative electrode  139   b  is constituted of a surface electrode layer  139   ba  having a rectangular upper-surface shape and a blind via  139   bb  penetrating the multilayer wiring bus bar  13  from the one surface to the negative electrode wiring layer  132   b  through the insulating layer  130 . Specifically, as shown in  FIG. 4 , the front surface of the upper plate  16  corresponding to the semiconductor chip  10   d  of the lower arm  54  is electrically connected to the negative electrode  139   b  via the joining material  26 . Accordingly, the front surface electrode of the semiconductor chip  10   d  of the lower arm  54  is electrically connected to the negative electrode  139   b  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     As shown in  FIG. 7K , on the U terminal  13   c , the V terminal  13   d  and the W terminal  13   e  side of the back side of the paper at the middle position in the longitudinal direction of the multilayer wiring bus bar  13  shown in  FIG. 6 , a W electrode  138   c  is formed on the other surface and the negative electrode  139   a  is formed on the one surface. 
     The W electrode  138   c  corresponds to one of second output electrodes and is electrically connected with the W-phase wiring layer  135 . In the present embodiment, the W electrode  138   c  is constituted of a surface electrode layer  138   ca  having a rectangular upper-surface shape and a blind via  138   cb  penetrating the multilayer wiring bus bar  13  from the other surface to the W-phase wiring layer  135   a  through the insulating layer  130 . The W electrode  138   c  is electrically connected with the high side of the lower arm  56  of the W-phase. Specifically, as shown in  FIG. 4 , the front surface of the lower plate  17  corresponding to the semiconductor chip  10   f  of the lower arm  56  is electrically connected to the W electrode  138   c  (not shown in  FIG. 4 ) via the joining material  27 . Accordingly, the rear surface electrode of the semiconductor chip  10   f  of the lower arm  56  is electrically connected to the W electrode  138   c  via the joining material  20 , the lower plate  17 , and the joining material  27 . 
     The negative electrode  139   c  is electrically connected with the negative electrode wiring layer  132 . In the present embodiment, the negative electrode  139   c  is constituted of a surface electrode layer  139   ca  having a rectangular upper-surface shape and a blind via  139   cb  penetrating the multilayer wiring bus bar  13  from the one surface to the negative electrode wiring layer  132   a  through the insulating layer  130 . Specifically, as shown in  FIG. 4 , the front surface of the upper plate  16  corresponding to the semiconductor chip  10   f  of the lower arm  56  is electrically connected to the negative electrode  139   c  via the joining material  26 . Accordingly, the front surface electrode of the semiconductor chip  10   f  of the lower arm  56  is electrically connected to the negative electrode  139   c  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     The multilayer wiring bus bar  13  is formed by the above-described structure. Wiring portions of the three-phase inverter circuit  1  are formed using the multilayer wiring bus bar  13 , and components included in the three-phase inverter circuit  1  are electrically connected. For example, as shown in  FIG. 8 , in the V-phase, the front surface electrode of the semiconductor chip  10   c  of the upper arm  53  is electrically connected to the V electrode  136   b  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . In addition, the rear surface electrode of the semiconductor chip  10   c  of the upper arm  53  is electrically connected to the positive electrode  137   b  via the joining material  20 , the lower plate  17 , and the joining material  27 . In addition, the rear surface electrode of the semiconductor chip  10   d  of the lower arm  54  is electrically connected to the V electrode  138   b  via the joining material  20 , the lower plate  17 , and the joining material  27 . Furthermore, the front surface electrode of the semiconductor chip  10   d  of the lower arm  54  is electrically connected to the negative electrode  139   b  via the joining material  22 , the element relay electrode  15 , the joining material  23 , the upper plate  16 , and the joining material  26 . 
     In this way, in a cross section of cutting the multilayer wiring bus bar  13  in a width direction (a cross section in  FIG. 8 ), the V electrode  136   b  and the V electrode  138   b  are disposed   one diagonal, and the positive electrode  137   b  and the negative electrode  139   b  are disposed on the other diagonal. Then, the V electrode  136   b  and the V electrode  138   b  are electrically connected via the internal layer wiring, the front surfaces and the rear surfaces of the semiconductor chips  10   c ,  10   d  of the upper and lower arms  53 ,  54  can be disposed so as to face the same directions. In this way, the components of the semiconductor module  6  are electrically connected via the multilayer wiring bus bar  13 . Although the V-phase is taken as an example in  FIG. 8 , the U-phase and the W-phase have similar cross-sectional structure. 
     In the multilayer wiring bus bar  13  formed as described above, a stacked layer number of the internal layer conductors constituting the positive electrode wiring layer  131 , the negative electrode wiring layer  132 , and the U-phase to W phase wiring layers  133 - 135  are an even number. In the present embodiment, the multilayer wiring bus bar  13  has four-layer structure. That is, in a case of driving the three-phase alternating current motor, a wiring layer of each of the phases connected to the three phases of the U-phase, the V-phase, and the W-phase needs to be formed of one layer. However, because the stacked layer number becomes an odd number, the W-phase wiring layers  135  are divided into two layers so that the stacked layer number becomes an even number. Then, the U-phase wiring layer  133  and the V-phase wiring layer  134  are disposed between the W-phase wiring layers  135  divided into two layers. 
     In addition, in the internal layer conductors, the thickness of two outer layers are set to be the same while the thicknesses of the two inner layers are set to be the same. Accordingly, the total thickness of the W-phase wiring layers  135  is made correspond to the thicknesses of the U-phase wiring layer  133  and the V-phase wiring layer  134  so that resistance values when electric current flow in the W-phase wiring layers  135 , the U-phase wiring layer  133 , and the V-phase wiring layer  134  are approximated even when the W-phase wiring layers  135  are divided into two layers. However, because of dividing into two layers, the resistance value of the W-phase may be larger than the resistance values of the U-phase and the V-phase. In such a case, the total thickness of the W-phase needs to be larger than the thicknesses of the U-phase and the V-phase to reduce the resistance value. Thus, the total thickness is not always made correspond. 
     Each of the positive electrode wiring layer  131  and the negative electrode wiring layer  132  needs to be formed of one layer. However, each of the positive electrode wiring layer  131  and the negative electrode wiring layer  132  is divided into two layers so as have four-layer structure in accordance with the stacked layer number of the U-phase to W-phase wiring layers  133 - 135 . Because of the above-described structure, effects of reducing the main circuit inductance and the output terminal inductance, restricting a surge due to a high-speed operation, and restricting a malfunction of the semiconductor switching elements can be obtained. The following describes reasons for obtaining the above-described effects with reference to experimental results and the like. 
     In the multilayer wiring bus bar  13  having the above-described structure, basically, the U-phase to W phase wiring layers  133 - 135  have the stacked structure, and the U-phase wiring layer  133  and the V-phase wiring layer  134  are disposed between the W-phase wiring layers  135  divided into two layers. In addition, the positive electrode wiring layers  131  and the negative electrode wiring layers  132  have stacked structure. 
     In this configuration, for example, as shown by arrows in  FIG. 9A , at a position where the positive electrode wiring layers  131  and the negative electrode wiring layers  132  are stacked, electric currents flow in opposite directions in the positive electrode wiring layers  131  and the negative electrode wiring layers  132  when the semiconductor module  6  is used. Specifically, in the positive electrode wiring layers  131 , ⅔ of the total electric current Ir flows in the positive electrode wiring layer  131   a , and ⅓ of the total electric current Ir flows in the positive electrode wiring layer  131   b . In the negative electrode wiring layers  132 , ⅓ of the total electric current Ir flows in the negative electrode wiring layer  132   a , and ⅔ of the total electric current Ir flows to the negative electrode wiring layer  132   b . Thus, in the main circuit that passes the main terminals as the positive electrode terminal  13   a  and the negative electrode terminal  13   b , directions of diverted electric currents are opposite to each other, magnetic fluxes counteract, and a mutual inductance is reduced. Then, the electric current is carried to outside while keeping the relation. Accordingly, the main circuit inductance can be reduced. 
     Similarly, as shown by the arrows in  FIG. 9B , at a position where the U-phase to W-phase wiring layers  133 - 135  have the stacked structure, electric currents flow in opposite directions in the wiring layers. Specifically, in the three-phase inverter circuit  1  that drives the three-phase alternating-current motor as the load  3  as shown in  FIG. 10 , electric current that respectively flow in the U-phase to the W-phase of the three-phase alternating-current motor are set to Iu, Iv, Iw. In this case, when a current value that flows into the relay point of the three-phase alternating-current motor is indicated as positive and a current value that flows out from relay point is indicated as negative, Iu+Iv+Iw=0 is satisfied at all timings, and on-off states of the arms  51 - 56  are controlled so as to draw alternating-current waveforms as shown in  FIG. 11 . 
     For example, the arrows A 1 , A 2  in  FIG. 10  show flows of electric currents at timing ( 2 ) in  FIG. 11 . At timing ( 2 ), the lower arm  56  of the W-phase is turned on while the upper arms  51 ,  53  of the U-phase and the V-phase are turned on, and the other arms  52 ,  54 ,  55  are turned off. 
     In the three-phase inverter circuit  1  for driving the three-phase alternating-current motor, a mode of operation is classified into three modes. In the first mode, while two of the upper arms  51 ,  53 ,  55  are turned on, one of the lower arms  52 ,  54 ,  56  which corresponds to the other of the upper arms  51 ,  53 ,  55  which is not turned on is turned on (timings ( 6 ), ( 2 ), ( 4 ) in  FIG. 11 ). In the second mode, while one of the upper arms  51 ,  53 ,  55  is turned on, two of the lower arms  52 ,  54 ,  56  which correspond to the other two of the upper arms  51 ,  53 ,  55  which are not turned on are turned on (timings ( 1 ), ( 3 ), ( 5 ) in FIG.  11 ). In the third mode, while one of the upper arms  51 ,  53 ,  55  is turned on, one of the lower arms  52 ,  54 ,  56  which corresponds to another of the upper arms  51 ,  53 ,  55  which is not turned on is turned on (timing ( 7 ) in  FIG. 11 ). The three-phase inverter circuit  1  drives the three-phase alternating-current motor by switching these modes, and the electric current Iu, Iv, Iw draw alternating-current waveforms of the first to three phases (phases 1-3) in which phases are shifted each other by 120°. 
     Then, when electric currents that flow into the relay point of the three-phase alternating-current motor and electric currents that flow out from the relay point are considered, electric currents flow in opposite directions in all the first to three phases (see  FIG. 9B ). Thus, an inductance of a large current path for supplying the electric current to the load  3  can be reduced. 
     Here, in order to confirm the effects of the stacked conductors, specimens having a two-layer structure in which conductive layers  28   a ,  28   b  are disposed opposite to each other while providing a space therebetween, and ends of the conductive layers  28   a ,  28   b  are connected by a connecting portion  28   c  made of a conductor are prepared. Then, an electric current is supplied so as to flow from one end of the conductive layer  28   a , pass the connecting portion  28   c  provided at the other end, and pass an end of the conductive layer  28   b  opposite from the connecting portion  28   c . Then, a case where a thickness t, a length L, a width W, and a space Sp of each of the conductive layers  28   a ,  28   b  and the connecting portion  28   c  are predetermined values are set as reference values, and inductances of the specimens are examined while changing the thickness t, the length L, the width W, and the space Sp. Specifically, the reference values of the thickness t, the length L, the width W, and the space Sp are set to 1, and the inductances of the specimens are examined while changing a ratio of one of the values into 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and maintaining the other values at the reference values. As a result, results shown in  FIG. 13  are obtained. 
     As shown in this figure, the inductance increases as the length L or the thickness t increases. Although it is natural that the mutual inductance increases in accordance with the length L, it can be understood that the inductance increases as the thickness t increases and the inductance can be reduced as the width W increases. From this, it can be said that it is more effective to increase the width W than to increase the thickness t for reducing the inductance in a case where cross-section areas of portions where an electric current passes are the same. 
     Then, in the present embodiment, the positive electrode wiring layers  131  and the negative electrode wiring layers  132  are formed into plate shapes opposite to each other, and the U-phase to W-phase wiring layers  133 - 135  are also formed into plate shapes opposite to each other. Thus, as described above, a structure in which a dimension of the width W is larger than the thickness t is provided, and the inductance can be reduced. 
     In addition, also in a case where the positive electrode wiring layer  131  and the negative electrode wiring layer  132 , and the U-phase to W-phase wiring layers  133 - 135  are formed into the four-layer structure like the present embodiment, not a two-layer structure shown in  FIG. 12 , the above-described effect of reducing the inductance is confirmed. Like the present embodiment, an effect of reducing an inductance is evaluated for a case where thicknesses of two outer layers of internal layer conductors are set to 0.5 with respect to thicknesses 1 of two inner layers. 
     Specifically, a change of an inductance is investigated for three structures shown in  FIG. 14A  to  FIG. 14C . In  FIG. 14A , the U-phase to W-phase wiring layers  133 - 135  are formed into a three-layer structure. In  FIG. 14B , like the present embodiment, the W-phase wiring layer  135   a , the U-phase wiring layer  133 , the V-phase wiring layer  134 , the W-phase wiring layer  135   b  are arranged in order to form a four-layer structure. In  FIG. 14C , the U-phase wiring layer  133 , the W-phase wiring layer  135   a , the W-phase wiring layer  135   b , the V-phase wiring layer  134  are arranged in order to form a four-layer structure. For each of the structures, a change of the inductance is investigated at each of timings ( 2 ), ( 4 ), ( 6 ) in  FIG. 11 . A change due to an inductive load of the motor is omitted. As a result, results shown in  FIG. 15A  to  FIG. 15C  are obtained. The length L, the with W, the thickness t of each of the wiring layers are set to the reference values described in  FIG. 12 , and the thicknesses of the W-phase wiring layers  135   a ,  135   b  divided into two layers are set to ½ of the thicknesses t of the U-phase wiring layer  133  and the V-phase wiring layer  134 . 
     As known from  FIG. 15A  to  FIG. 15C , basically, when large opposite direction currents flow between adjacent wiring layers, a magnetic-flux counteracting effect is large, and thereby the inductance becomes small. For example in  FIG. 15C , while both inductances Luv, Lvw are low in the structure in  FIG. 14A , the inductances Luv, Lwu are large in the structure in  FIG. 14A . In the structure in  FIG. 14A , the magnetic-flux counteracting effect is likely to work in a case where a large electric current (ratio 1.0) flows into the V-phase in the middle layer (sandwiched position). However, in a case where a large electric current (ratio 1.0) flows into the U, W-phases at both ends, the magnetic flux counteracting effect is less likely to work. Thus, it is difficult to reduce the inductance in any phase state only by simply stacking the wiring layers. In the structure in  FIG. 14C , the inductances of Lvu and Luv in  FIG. 15B ,  FIG. 15C  are high. Thus, the inductance reduction effect depends on stacking ways even in four-layer structures. 
     As compared with this, in  FIG. 14B , the inductance reduction effect becomes the maximum by employing the four-layer structure and the optimal stacking way. The inductances become small at all timings and all electric current paths, and the change amounts are small. Because the magnetic fluxes generated by electric currents that flow separately are small, as a result, the mutual inductance M also becomes small. 
     In this way, the four-layer structure can reduce the inductance, and like the present embodiment, a structure in which one of the wiring layers divided into two layers (e.g., the W-phase wiring layer  135   a ), another wiring layer (e.g., the U-phase wiring layer  133 ), another wiring layer (e.g., the V-phase wiring layer  134 ), and the other of the wiring layers divided into two layers (e.g., the W-phase wiring layer  135   b ) are arranged in order can especially reduce the inductance. Accordingly, the output terminal inductance of the electric current supply path to the load  3  using the U to W terminals  13   c - 13   e  as output terminals can be reduced. 
     For example, due to an erroneous-on by a failure of a driving circuit that drives the semiconductor module  6  from outside or unexpected noise, as shown by the arrow in  FIG. 16A , the upper and lower arms  51 - 56  in the same phase are turned on at the same time, and a series-arm short circuit that forms a short-circuit path not through the load  3  may be generated. In this case, because an excessive electric current flows when short circuits of the upper and lower arms  51 - 56  occur, if the main circuit inductance is large, an excessive surge (noise) is generated, and a further malfunction or failure may be caused. 
     In addition, due to a dielectric breakdown of the inductive load  3 , as shown by the arrow in  FIG. 16B , an output short circuit that forms a short-circuit path at somewhere before passing through the load  3  may occur. In this case, although the upper and lower arms  51 - 56  are normally driven, because an electric current flows without passing through the load  3 , an excessive electric current may be generated in a manner similar to the series-arm short circuit. A fixed time is required before the excessive electric current is detected in the driving circuit of the semiconductor module  6  and the upper and lower arms  51 - 56  are turned off, and if the output terminal inductance is large, a generated magnetic flux (noise) causes a malfunction of the driving circuit, and as a result, a breakdown of the semiconductor switching elements  51   a - 56   a  is caused. 
     On the other hand, in the semiconductor module  6  according to the present embodiment, the main circuit inductance and the output terminal inductance can be reduced. Thus, because the main circuit inductance is reduced, a generation of an excessive surge can be restricted when a series-arm short circuit occurs, and a further malfunction or failure can be restricted. In addition, because the output terminal inductance is reduced, a malfunction of the driving circuit due to the magnetic flux (noise) generated when an output terminal short-circuit occurs can be restricted, and the semiconductor switching elements  51   a - 56   a  can be protected from breakdown. 
     Furthermore, in the semiconductor module  6  according to the present embodiment, the positive electrode wiring layers  131  and the negative electrode wiring layers  132  are formed into the stacked structure and the U-phase to W-phase wiring layers  133 - 135  are formed into the stacked structure by using the multilayer wiring bus bar  13 . In addition, the control terminals  14  of the upper arms  51 ,  53 ,  55  and the control terminals  14  of the lower arms  52 ,  54 ,  56  are exposed from the two opposed sides of the resin mold portion  18  formed into the quadrangular plate shape. Then, the both ends of the multilayer wiring bus bar  13  are exposed from the other two opposed sides of the resin mold portion  18  formed into the quadrangular plate shape so that the control terminals  14  and the both ends of the multilayer wiring bus bar  13  are kept at distances. 
     Thus, compared with a case in which the control terminals  14 , the positive and negative electrode terminals  13   a ,  13   b , and the U to W terminals  13   c - 13   e  are arranged laterally, a noise can be reduced between the multilayer wiring bus bar  13  in which a large current flows and the control terminals  14  that transmits various signals. The following describes about this with reference to  FIG. 17  to  FIG. 20 . 
       FIG. 17A  and  FIG. 17B  show results of investing an inductance when power terminals  29   a ,  29   b  in which a large electric current flows are arranged opposite to each other in a vertical direction and a distance Ls 1  between the power terminals  29   a ,  29   b  and control terminals  29   c  is changed as shown in  FIG. 18  as a specimen. In  FIG. 17A , L 1  indicates a self-inductance of the power terminals  29   a ,  29   b  (corresponding to the main circuit inductance), and L 2  indicates a self-inductance of the control terminals  29   c  as signal lines. In addition, M 12  in  FIG. 17B  indicates a mutual inductance of the power terminals  29   a ,  29   b  and the control terminals  29   c.    
     As clear from these figures, because the power terminals  29   a ,  29   b  are stacked, the self-inductance L 1 , which becomes a noise source, is small. In addition, even if the power terminals  29   a ,  29   b  are disposed adjacent to the control terminals  29   c , an influence is small. This means that because a magnetic flux on an L 1  path becomes small by stacking the power terminals  29   a ,  29   b , an induced electromotive force of an L 2  path is less likely to be generated. Thus, the mutual inductance M 12  as its index is small, and the influence to the control terminals  20   c  is small even if the power terminals  29   a ,  29   b  are disposed adjacent to the control terminals  29   c.    
       FIG. 19A  and  FIG. 19B  show a result of investing an inductance when the power terminals  29   a ,  29   b  in which a large electric current flows are arranged at a distance so as not to be opposed to each other as a specimen, and a distance Ls 2  between the power terminals  29   a ,  29   b  and the control terminals  20   c  is changed as shown in  FIG. 20  as a specimen. L 1 , L 2  in  FIG. 19A  and M 12  in  FIG. 19B  are similar to those in  FIG. 17A  and  FIG. 17B . 
     As clear from these figures, because the power terminals  29   a ,  29   b  are not stacked, the self-inductance L 1 , which becomes a noise source, is large. In addition, an influence is large when the power terminals  29   a ,  29   b  are disposed adjacent to the control terminals  29   c . On the other hand, because the power terminals  29   a ,  29   b  are not stacked, the mutual inductance M 12  is also large, and the influence to the control terminals  29   c  is large if the power terminals  29   a ,  29   b  are disposed adjacent to the control terminals  29   c . Thus, if a state in which a large current is turned on and off with the power terminals  29   a ,  29   b  occurs, an unintentional electromotive force is generated in the driving circuit and a malfunction is caused. 
     In order to reduce the inductance, it is effective to form each of the components of the positive electrode wiring and each of the components of the negative electrode wiring by parallel conductors so that electric currents flow in opposite directions in the positive electrode and the negative electrode. Accordingly, a magnetic offsetting occurs between the positive electrode wiring and the negative electrode wiring, and the inductance can be reduced. 
     In the semiconductor module  6  according to the present embodiment, the positive electrode wiring layers  131  and the negative electrode wiring layers  132  are formed into the stacked structure, and the U-phase to W-phase wiring layers  133 - 135  are formed into the stacked structure by using the multilayer wiring bus bar  13 . Thus, the self-inductance which becomes the noise source can be reduced, the influence to the control terminals  14  is reduced, and a malfunction of the driving circuit for driving the semiconductor module  6  can be restricted. 
     In addition that the influence to the control terminals  14  can be reduced by the stacked structure, the control terminals  14  and the both ends of the multilayer wiring bus bar  13  are drawn out from the four sides of the resin mold portion  18 . Thus, these components can be kept away from each other, and a malfunction of the driving circuit for driving the semiconductor module  6  can be further restricted. 
     Other Embodiments 
     The present disclosure is not limited to the above-described embodiments and may be suitably modified. 
     For example, in a case where each of the positive electrode wiring layers  131  and the negative electrode wiring layers  132  that constitute the main circuit are divided into two layers, and the thicknesses of the negative electrode wiring layer  131   a  and the negative electrode wiring layer  132   b  are set to 1, the thicknesses of the positive electrode wiring layer  131   b  and the negative electrode wiring layer  132   a  are set to 0.5. However, this is merely an example, and an object is to set the stacked layer number to be an even number. Thus, the thickness of the layers are optional. For example, in a case where the thicknesses of the positive electrode wiring layer  131   a  and the negative electrode wiring layer  132   b  are set to 1, the thicknesses t of the positive electrode wiring layer  131   b  and the negative electrode wiring layer  132   a  may be set to 0.6. However, it is preferable to set cross-section areas of portions in the U-phase to W-phase wiring layers  133 - 135  in which electric currents pass to be the same. Thus, in a case where the positive electrode wiring layer  131  and the negative electrode wiring layer  132  are formed by the same internal layer wiring as the U-phase to W-phase wiring layers  133 - 135 , it is preferable to set the thicknesses of the above-described embodiment. 
     In addition, in the above-described embodiment, the U-phase to W-phase wiring layers  133 - 135  are formed into the four-layer structure while the positive electrode wiring layers  131  and the negative electrode wiring layers  132  are formed into the four-layer structure. However, because the stacked layer number only have to be an even number, for example, each of the U-phase to W-phase wiring layers  133 - 135  may be divided into two layers so that the total stacked layer number is six. In this case, for example, a structure in which the U-phase to W-phase wiring layers  133 - 135  are stacked in order can be employed. 
     In addition, the multilayer wiring bus bar  13  in which a main circuit side bus bar forming the positive and negative electrode terminals  13   a ,  13   b , the positive electrode wiring layer  131 , and the negative electrode wiring layer  132  and an output terminal side bus bar forming the U to W terminals  13   c - 13   e  and the U-phase to W-phase wiring layers  133 - 135  are integrated is taken as an example. However, the main circuit side bus bar and the output terminal side bus bar may be formed by separate bus bars. 
     In the above-described embodiment, a mode in which the positive electrode side of the direct-current power source  2  that becomes the external power source and the positive electrode terminal  13   a , and the negative electrode side of the direct-current power source  2  and the negative electrode terminal  13   b  are directly connected. However, the positive electrode terminal  13   a  is a terminal to which a voltage is applied from the external power source, and the negative electrode terminal  13   b  is a terminal connected to a low potential point. Thus, an element such as a resistor may be provided between the positive electrode terminal  13   a  and the external power source or the negative electrode terminal  13   b  and the ground potential point.