Patent Publication Number: US-7907390-B2

Title: Circuit device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit device and, more particularly, to a circuit device using a metal substrate. 
     2. Description of the Related Art 
     In recent years, as circuit devices incorporated in electronic equipment and the like have become smaller in size, higher-density and more functionally-sophisticated (e.g., multi-functional), the heat generation density per unit volume of a circuit device has been on the rise. To cope with this trend, metal substrates, which excel in their heat release performance, are today often used as substrates for the circuit devices, and at the same time circuit elements, such as ICs (Integrated Circuits) or LSIs (Large Scale Integrated Circuits) are mounted on such metal substrates (See Reference (1) in the following Related Art List, for instance). 
       FIG. 10  is a sectional view schematically illustrating a structure of a conventional circuit device as disclosed in Reference (1). In  FIG. 10 , a metal base plate  103  provided with a first insulating layer  104  and a first circuit pattern  105  on the top thereof constitutes a metal substrate  120 . Power elements  106  are disposed on the metal base plate  103  through the intermediary of the first insulating layer  104  and the first circuit pattern  105 . The power elements  106  and the first circuit pattern  105  are electrically connected with each other via wires  107 . Further, in such a conventional circuit device, the metal base plate  103  is divided into regions thereof, on each of which a power element  106  and a first circuit pattern  105  connected thereto are disposed, and a second insulating layer  108  is disposed between the adjacent regions. 
     3. Related Art List 
     (1) Japanese Patent Application Laid-Open No. Hei11-074452. 
     In the above-cited conventional structure, a plurality of power elements  106  are each disposed on a metal base plate  103  through a first insulating layer  104 . Accordingly, the heat generated by a power element  106  is released to the metal base plate  103  through the first insulating layer  104  which is in the region where the power element  106  is mounted. However, a problem in this setup is that because of a large thermal resistance of the first insulating layer  104 , the heat generated by the power element  106  is not sufficiently released to the metal base plate  103 . 
     Another problem in this setup is that heat is also generated at a portion of the first circuit pattern (pad electrode portion), which is connected to a power element  106  via a wire  107 , but because of a large thermal resistance of the first insulating layer  104  located thereunder, the heat generated is not sufficiently released to the metal base plate  103 . 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing circumstances, and a general purpose thereof is to provide a circuit device which features a superior heat release performance. 
     In order to achieve the above purpose, a circuit device according to one embodiment of the present invention comprises: a metal substrate which is sectioned into a plurality of regions by an insulating film; and a first circuit element and a second circuit element which are electrically connected with the metal substrate wherein the first circuit element and the second circuit element are mounted on one surface of the metal substrate, wherein a first voltage is applied to a connection part of the first circuit element connected with the metal substrate, a second voltage differing from the first voltage is applied to a connection part of the second circuit element connected with the metal substrate, and the first circuit element and the second circuit element are provided on different regions in the plurality of regions, respectively. Here, the circuit element is a semiconductor device represented by an LSI chip where elements are formed in particular on a silicon substrate, a SiGe substrate or a GaAs substrate. The circuit elements may be passive elements that form a passive element circuit on an Al 2 O 3  substrate. 
     According to this embodiment, the circuit elements (the first circuit element and the second circuit element), which have mutually different voltages applied to the respective portions thereof connected to the metal substrate, can be disposed in an electrically-connected state on the metal substrate, so that the heat generated by the circuit elements may be directly conducted to the respective sections of the metal substrate (demarcated regions). This increases the heat release effect, thus enhancing the heat resistance reliability of the circuit device. The insulating film, which exists between the demarcated sections of the metal substrate, absorbs the expansion of the metal substrate caused by the heat conducted from the circuit elements. This reduces warping of the circuit device. Thereby, the drop of reliability of the circuit device due to the heat generated from the circuit elements can be reduced. 
     In the above-described structure, it is preferable that the first circuit element and the second circuit element be each mounted directly on the metal substrate. This structure helps the heat generated from the circuit elements (the first circuit element and the second circuit element) conduct to the metal substrate. This increases the heat release effect, thus further enhancing the heat resistance reliability of the circuit device. 
     In the above-described structure, it is preferable that the regions, on which the first circuit element and the second circuit element are mounted, have their outer edges in positions outside outer edges of the first circuit element and the circuit element, respectively. With this structure, the heat generated by the circuit elements are conducted uniformly into the metal substrate within the planes of the circuit elements, thus achieving effective heat release from the circuit elements. 
     In the above-described structure, the plurality of regions may include a region that functions as an electrode used for wire connection within the metal substrate. In such a case, the wires are directly connected to the regions used as wire connection electrodes, so that the heat generated in these connection portions are conducted directly to the metal substrate. This increases the heat release effect, thus further improving the heat resistance reliability of the circuit device. 
     In the above structure, the insulating film may be made of resin film added with a filler used to accelerate heat transfer. In this case, part of the heat conducted from a circuit element to a metal substrate directly below the circuit element, for instance, is further conducted to a metal substrate of a region adjacent thereto through the insulating film. This increases the heat release performance of a region (the demarcated metal substrate), on which the circuit elements are mounted, thus further improving the heat resistance reliability of the circuit device. 
     In the above structure, the circuit device preferably further comprises a wiring layer, provided on the other surface of the metal substrate, which electrically connects each of the sectioned regions. By employing this structure, the demarcated regions can be easily electrically-connected with each other while maintaining the condition of direct heat conduction to the metal substrate. This increases design freedom concerning the demarcation of metal substrate into sections. As a result, it is possible to achieve area saving of the circuit device while improving the heat release performance thereof. 
     In the above-described structure, it is preferable that the circuit device further comprise a third circuit element whose heat generation rate is less than that of at least one of the first circuit element and the second element and that the third element be mounted on a surface, in both faces of the wiring layer, opposite to the surface on which the metal substrate is provided. In this case, the first circuit element and the second circuit element, which produce relatively much heat, are mounted on one face of a highly heat-radiant metal substrate, and the third circuit elements whose heat generation rate is less than that of at least one of the first circuit element and the second element are mounted on one face of the wiring layer. That is, according to this embodiment, a plurality of circuit elements having different heat generation rates are mounted, as appropriate, on the metal substrate or the wiring layer, thus realizing smaller size by reducing packaging area without impairing the radiation performance. 
     It is to be noted that any arbitrary combinations or rearrangement, as appropriate, of the aforementioned constituting elements and so forth are all effective as and encompassed by the embodiments of the present invention. 
     Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be sub-combination of these described features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of examples only, with reference to the accompanying drawings which are meant to be exemplary, not limiting and wherein like elements are numbered alike in several Figures in which: 
         FIG. 1  is a plan view showing how a metal substrate constituting a circuit device according to a first embodiment of the present invention is demarcated into sections; 
         FIG. 2  is a schematic cross-sectional view of a circuit device taken along the line X-X′ of  FIG. 1 ; 
         FIG. 3  schematically illustrates a distribution of fillers filled in a insulating film of a circuit device according to a first embodiment of the present invention; 
         FIG. 4  is a graph showing a distribution of fillers filled in a insulating film of a circuit device according to a first embodiment of the present invention; 
         FIGS. 5A to 5D  are cross-sectional views for explaining a manufacturing process for a circuit device according to a first embodiment of the present invention; 
         FIGS. 6A to 6D  are cross-sectional views for explaining a manufacturing process for a circuit device according to a first embodiment of the present invention; 
         FIGS. 7A to 7D  are cross-sectional views for explaining a manufacturing process for a circuit device according to a first embodiment of the present invention; 
         FIGS. 8A to 8C  are cross-sectional views for explaining a manufacturing process for a circuit device according to a first embodiment of the present invention; 
         FIGS. 9A and 9B  are cross-sectional views for explaining a manufacturing process for a circuit device according to a first embodiment of the present invention; 
         FIG. 10  is a cross-sectional view schematically illustrating a structure of a conventional circuit device; 
         FIG. 11  is a functional block diagram of a system for driving a fan motor; 
         FIG. 12  is a schematic cross-sectional view of a circuit device according to the second embodiment of the present invention. 
         FIG. 13  is a schematic diagram for explaining a system formed by a circuit element and a part of a metal substrate on which a circuit element is mounted; 
         FIG. 14  is a diagram schematically illustrating a temperature change of a system where the drive of a circuit element is temporarily turned on; and 
         FIG. 15  is a schematic representation of a pulse current that flows through a circuit element according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     Note that the identical components are given the identical reference numerals in all accompanying Figures and the repeated description thereof will be omitted as appropriate. 
     First Embodiment 
     A description will now be given of a circuit device according a first embodiment, with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is a plan view showing how a metal substrate constituting a circuit device  100  according to the first embodiment is demarcated into sections. A metal substrate  20  of this circuit device, which is made of a copper (Cu) plate with high thermal conductivity, is demarcated such that an insulating film  4  to be described later surround copper plates  1   a  and  1   b , which are the regions for mounting circuit elements  10   a  and  10   b , and copper plates  1   c ,  1   d  and  1   e , which are regions for connecting bonding wires  11   a  and  11   b  (regions used as wire connecting electrodes within the metal substrate). 
       FIG. 2  is a schematic cross-sectional view of a circuit device  100  taken along the line X-X′ of  FIG. 1 . This circuit device  100  includes a metal substrate  20 , two circuit elements  10   a  and  10   b , bonding wires  11   a  and  11   b , a sealing resin layer  12 , and a wiring layer  30 . 
     As shown in  FIG. 1 , the metal substrate  20  is formed of an insulating film  4  and copper plates  1   a  to  1   k  (copper plates  1   a  to  1   g  in  FIG. 2 ), which are sections surrounded by the insulating film  4 . The demarcation of the copper plate is done, for instance, for each circuit element mounted on the metal substrate  20  or, more preferably, for each circuit element which has a different voltage applied to the portion thereof connected to the metal substrate  20 . In  FIG. 2 , a circuit element  10   a , which has a predetermined voltage applied to the portion thereof connected to the metal substrate  20 , and a circuit element  10   b , which has a voltage different from that of the circuit element  10   a  applied to the portion thereof connected to the metal substrate  20 , are mounted on the copper plates  1   a  and  1   b , which are the regions demarcated for them respectively. Also, the copper plates  1   a  and  1   b , on which the circuit elements  10   a  and  10   b  are mounted, are provided with larger areas than those of the circuit elements  10   a  and  10   b  respectively, and the copper plates  1   a  and  1   b  have their outer edges in positions outside the outer edges of the circuit elements  10   a  and  10   b , respectively. It is to be noted that according to the present embodiment, the copper plate is also demarcated for the regions where bonding wires are connected. 
     Here, the insulating film  4  is formed in such a manner that it is embedded in a groove  3  which penetrates the copper plate (metal substrate). As shown in  FIG. 3 , the insulating film  4  is an epoxy resin film with a filler  4   a  added to raise the thermal conductivity (a resin film with a heat-conduction-promoting filler  4   a  added). To be more precise, the insulating film  4  is made of an epoxy resin filled, at a weight filling factor of about 75 percent, with a filler  4   a , which contains at least filler  4   a   1  and filler  4   a   2  having their respective particle sizes corresponding to two mutually different kinds of average particle size. The average particle diameters of the filler  4   a   1  and filler  4   a   2  are about 0.7 μm and about 3 μm, respectively. Also, the filler  4   a   1  (average particle size: about 0.7 μm) and the filler  4   a   2  (average particle size: about 3 μm) are added at frequencies of occurrence as shown in  FIG. 4 . More specifically, the compounding ratio of the filler  4   a   1  and filler  4   a   2  are 2:8. In this manner, by mixing fillers having two mutually different kinds of average particle size, the filler  4   a   1  having smaller particle sizes enter the gaps made between the filler  4   a   2  having larger particle sizes, so that the insulating film  4  can be filled more densely with the filler  4   a . This effectively raises the thermal conductivity of the insulating film  4 . Also, the material used for the fillers  4   a   1  and  4   a   2  is alumina (Al 2 O 3 ), which can improve the thermal conductivity of the insulating film  4 . The thermal conductivity of the insulating film  4  will be about 4.4 W/(m·K) with the fillers  4   a   1  and  4   a   2  added as described above. Note that the thermal conductivity of epoxy resin without the addition of filler is about 0.6 W/(m·K). 
     Also, as shown in  FIG. 2 , formed on top of the metal substrate  20  is a solder resist layer  9   a  which covers the insulating film  4  and the copper plates  1   a  to  1   g  and has openings corresponding to part of the copper plates  1   a  and  1   b , where circuit elements are mounted, and part of the copper plates  1   c ,  1   d  and  1   e , which are the regions for wire bonding connection. This solder resist layer  9   a  functions as a protective film for the metal substrate  20 . 
     The circuit elements  10   a  and  10   b  are the circuit elements with mutually different voltages applied to their respective portions connected to the metal substrate  20 . For example, if the two circuit elements perform switching operation, they have mutually different voltages between when the voltage is applied (on state) and when the voltage is not applied (off state). The circuit elements  10   a  and  10   b  are mounted on the demarcated metal substrate  20  via their respective bonding layers (not shown) of conductive material. As a result, the demarcated metal substrate  20  (copper plates  1   a  and  1   b ) are electrically connected to their respective circuit elements  10   a  and  10   b , and the metal substrate  20  is maintained at the voltages applied to the respective circuit elements. 
     The bonding wires  11   a  and  11   b  are, for instance, aluminum or gold wires, which electrically connect the circuit elements  10   a  and  10   b  mounted on the metal substrate  20  to their predetermined regions (regions corresponding to the conventional pad electrode portions) of the metal substrate  20 . 
     The sealing resin layer  12  seals the circuit elements  10   a  and  10   b  mounted on the metal substrate  20 , thereby protecting the circuit elements  10   a  and  10   b  against external influences. The material for the sealing resin layer  12  is, for instance, a thermosetting insulating resin, such as epoxy resin. It should be noted that the sealing resin layer  12  may have a filler added thereto to raise its thermal conductivity. 
     The wiring layer  30 , which is formed on the opposite side of the metal substrate  20  (the face opposite to the one on which the circuit elements  10   a  and  10   b  are mounted), has a structure resulting from alternately laminating insulating films  5  and  7  and conductive films (wiring patterns)  6  and  8 . This wiring layer  30  ensures electrical connection between the demarcated sections of metal substrate  20  and electrical connection with external connection terminals (solder balls)  13 . 
     Also, as shown in  FIG. 2 , formed on the bottom of the wiring layer  30  is a solder resist layer  9   b  which covers the insulating film  7  and the wiring pattern  8  and has openings in positions corresponding to the regions for forming the external connection terminals  13 . This solder resist layer  9   b  functions as a protective film for the wiring layer  30 . 
       FIGS. 5 through 9  are cross-sectional views for explaining a manufacturing process for a circuit device according to the first embodiment of the present invention as shown in  FIG. 2 . Note that  FIGS. 5 through 9  are cross-sectional views of a circuit device taken along the line X-X′ of  FIG. 1  as is the case with  FIG. 2 . Hereinbelow, the manufacturing process for a circuit device according to the present embodiment will be described by referring to  FIG. 2  and  FIGS. 5A-5D  through  FIGS. 9A-9B . 
     Firstly, as illustrated in  FIG. 5A , a resist pattern  2  with the resist removed in the regions where grooves  3  to be described later will be formed is selectively formed by lithography on a copper plate  1  (copper plates  1   a  to  1   g  in a subsequent process). 
     As illustrated in  FIG. 5B , an exposed portion of the copper plate  1  is half-etched and grooves (recesses)  3  are formed by a wet-etching technique before the resist patter  2  is peeled off. 
     As illustrated in  FIG. 5C , a resin containing a filler  4   a  at a predetermined ratio is applied, and then insulating film  4  is formed by filling the grooves (recesses)  3  with the resin by scraping with a squeegee or like means. 
     As illustrated in  FIG. 5D , the back side of the copper plate  1  (the face opposite to the one on which the circuit elements are mounted as described later) is ground off, so that the insulating film  4  filled in the grooves (recesses)  3  are exposed on the back side of the copper plate  1 . This results in the formation of a metal substrate  20  comprised of the insulating film  4  and the copper plates  1   a  to  1   g  demarcated by the insulating film  4 . 
     Next, as illustrated in  FIG. 6A , an epoxy resin is applied to the back side of the metal substrate  20  (copper plate  1 ) to form an insulating film  5 . Note that this insulating film  5  may have a filler added therein to raise the thermal conductivity. After this, a copper foil  6   z  is press-bonded to the surface of the insulating film  5 . It should be noted here that the insulating film  5  and the copper foil  6   z  may be formed by preparing a laminated body of insulating film  5  and copper foil  6   z  in advance and then thermally press-bonding the laminated body to the back side of the metal substrate  20  (copper plate  1 ). 
     As illustrated in  FIG. 6B , the copper foil  6   z  in positions corresponding to the regions where via holes  5   a  (see  FIG. 2 ) are formed is removed by using photolithography and etching techniques. This exposes the regions in the insulating film  5  where via holes  5   a  are formed. 
     As illustrated in  FIG. 6C , the regions from the exposed surfaces of the insulating film  5  to the surface of the metal substrate  20  (copper plate  1 ) are removed by the irradiation thereto of carbon dioxide laser or UV laser from above the copper foil  6   z  (the back side of the metal substrate  20 ). This results in the formation of via holes  5   a  penetrating the insulation film  5 . 
     As illustrated in  FIG. 6D , copper is plated on the surface of the copper foil  6   z  and the inner surface of the via holes  5   a  by electroless plating. Then the surface of the copper foil  6   z  and the inside of the via holes  5   a  are plated by electrolytic plating. Note that in the present embodiment, an inhibitor and an accelerator are added in the plating liquid, so that the inhibitor is adsorbed on the surface of the copper foil  6   z  and the accelerator is adsorbed on the inner surface of the via holes  5   a . Since this makes the copper plating on the inner surface of the via holes  5   a  thicker, it is now possible to embed copper in the via holes  5   a . As a result, as illustrated in  FIG. 6D , a conductive film  6  is not only formed on the insulating film  5  but also embedded in the via holes  5   a . At this time, it is to be noted that a conductive film  6   a  (conductive film  6   a  on the side faces not shown) is also formed on the top surface (the surface on which the circuit elements are mounted as described later) of the metal substrate  20  and on the side faces of the metal substrate  20 . 
     Next, as illustrated in  FIG. 7A , patterning of the conductive film  6  is carried out by using photolithography and etching techniques. This results in the formation of a conductive film  6  which has a predetermined wiring pattern. 
     As illustrated in  FIG. 7B , an epoxy resin made of the same material as the previously-described insulating film  5  in such a manner as to cover the conductive film (wiring pattern)  6 , thus forming an insulating film  7 . Thereafter, a copper foil  8   z  is press-bonded to the surface of the insulating film  7 . It is to be noted here that the insulating film  7  and the copper foil  8   z  may be formed by preparing a laminated body of insulating film  7  and copper foil  8   z  in advance and then thermally press-bonding the laminated body to the back side of the metal substrate  20  (copper plate  1 ). 
     As illustrated in  FIG. 7C , the copper foil  8   z  in positions corresponding to the regions where via holes  7   a  (see  FIG. 2 ) are formed is removed by using photolithography and etching techniques. This exposes the regions in the insulating film  7  where via holes  7   a  are formed. 
     As illustrated in  FIG. 7D , the regions from the exposed surfaces of the insulating film  7  to the surface of the conductive film  6  are removed by the irradiation thereto of carbon dioxide laser or UV laser from above the copper foil  8   z  (the back side of the metal substrate  20 ). This results in the formation of via holes  7   a  penetrating the insulation film  7 . 
     Next, as illustrated in  FIG. 8A , copper is plated on the surface of the copper foil  8   z  and the inner surface of the via holes  7   a  by electroless plating. Then the surface of the copper foil  8   z  and the inside of the via holes  7   a  are plated by electrolytic plating. At this time, an inhibitor and an accelerator are added in the plating liquid, so that the inhibitor is adsorbed on the surface of the copper foil  8   z  and the accelerator is adsorbed on the inner surface of the via holes  7   a . Since this makes the copper plating on the inner surface of the via holes  7   a  thicker, it is now possible to embed copper in the via holes  7   a . As a result, a conductive film  8  is not only formed on the insulating film  7  but also embedded in the via holes  7   a . At this time, it is to be noted that a conductive film  8   a  (conductive film  8   a  on the side faces not shown) is also formed on the top surface side (the surface on which the circuit elements are mounted as described later) of the metal substrate  20  and on the surface of conductive film  6   a  at the side faces of the metal substrate  20 . 
     As illustrated in  FIG. 8B , patterning of the conductive film  8  is carried out by using photolithography and etching techniques. This results in the formation of a conductive film  8  which has a predetermined wiring pattern. As a result, a wiring layer  30  having a structure of the insulating films  5  and  7  and the conductive films (wiring pattern)  6  and  8  laminated alternately is formed on the back side of the metal substrate  20 . 
     As illustrated in  FIG. 8C , after the formation of a protective film (not shown) on the back side of the metal substrate  20 , the conductive film  6   a  and the conductive film  8   a  provided on the top surface and the side surfaces of the metal substrate  20  are removed by etching. And after this etching, the protective film is removed. 
     Next, as shown in  FIG. 9A , a solder resist layer  9   a  is formed which covers the metal substrate  20  (the insulating film  4  and the copper plates  1   a  to  1   g ) and has openings corresponding to part of the copper plates  1   a  and  1   b  of the metal substrate  20 , where circuit elements are mounted, and part of the copper plates  1   c ,  1   d  and  1   e  thereof, which are the regions for wire bonding connection. Further, a solder resist layer  9   b  is formed which covers the insulating film  7  and the wiring pattern  8  and has openings in positions corresponding to the regions for forming the external connection terminals  13 . Note here that the formation of the solder resist layers  9   a  and  9   b  may be carried out by first thermally press-bonding a solder resist film without openings on both sides at the same time and then forming openings in positions corresponding to the above-mentioned regions. Then the circuit elements  10   a  and  10   b  are mounted on top of the copper plates  1   a  and  1   b , respectively, through the medium of a bonding layer (not shown) made of a conductive material. In this manner, the copper plates  1   a  and  1   b  are electrically connected to the circuit elements  10   a  and  10   b , respectively, and thus mutually different voltages are applied to the respective portions of the circuit elements connected to the metal substrate  20 . Then the circuit elements  10   a  and  10   b  are electrically connected to the copper plates  1   c ,  1   d  and  1   e , which are the wire bonding regions of the metal substrate  20  (regions corresponding to the conventional pad electrode portions) by means of bonding wires  11   a  and  11   b , which are, for instance, aluminum or gold wires. 
     As illustrated in  FIG. 9B , a sealing resin layer  12 , made of an epoxy resin, is formed in such a manner as to cover the circuit elements  10   a  and  10   b  on the metal substrate  20  to protect them. 
     Finally, as shown in  FIG. 2 , solder balls  13 , which will function as external connection terminals, are formed on the surface of the conductive film  8  (parts exposed in the openings in the solder resist layer  9   b ) by a solder printing method. To be more specific, a “solder paste”, which is resin and solder in a pasty state, is printed in desired positions through a screen mask, and then the solder balls  13  are formed by heating the paste to the melting point of the solder. 
     A circuit device according to the present embodiment can be manufactured by the process as described above. 
     According to the present embodiment, an operation is performed and advantageous effects thereby are achieved as follows. 
     (1) The circuit elements  10   a  and  10   b , which have mutually different voltages applied to the respective portions thereof connected to the metal substrate  20 , can be disposed in an electrically-connected state on the metal substrate  20  (copper plates  1   a  and  1   b ), so that the heat generated by the circuit elements  10   a  and  10   b  may be directly conducted to the respective sections of the metal substrate  20  (copper plates  1   a  and  1   b ). This increases the heat release performance, thus improving the heat resistance reliability of the circuit device. 
     (2) The insulating film  4 , which exists between the demarcated sections of the metal substrate  20 , absorbs the expansion of the metal substrate  20  (copper plates  1   a  and  1   b ) caused by the heat conducted from the circuit elements  10   a  and  10   b . This reduces warping of the circuit device, thereby reducing the drop of reliability of the circuit device due to the heat release from the circuit elements. 
     (3) The copper plates  1   a  and  1   b , on which the circuit elements  10   a  and  10   b  are mounted, have their outer edges in positions beyond the peripheries of the respective circuit elements, so that the heat generated by the circuit elements are conducted uniformly into the copper plates  1   a  and  1   b  within the respective planes of the circuit elements  10   a  and  10   b . This results in effective heat release from the circuit elements  10   a  and  10   b.    
     (4) The bonding wires  11   a  and  11   b  are directly connected from the circuit elements  10   a  and  10   b  to the copper plates  1   c ,  1   d  and  1   e , which are the regions used as wire connection electrodes (regions corresponding to the conventional pad electrode portions) within the metal substrate  20 , so that the heat generated in these connection portions are conducted directly to the metal substrate  20  (copper plates  1   c ,  1   d  and  1   e ). This increases the heat release performance, thus further improving the heat resistance reliability of the circuit device. 
     (5) The insulating film  4  has a heat-conduction-promoting filler added thereto, so that part of the heat conducted from the circuit element  10   a  to the copper plate  1   a  directly below, for instance, is further conducted to the copper plates  1   c ,  1   d ,  1   h  and  1   j  through the insulating film  4  adjacent to the copper plate  1   a . This increases the heat release performance of the copper plate  1   a , on which the circuit element  10   a  is mounted, thus further improving the heat resistance reliability of the circuit device. 
     (6) The demarcated copper plates can be easily electrically-connected with each other by the wiring layer  30  while maintaining the condition of direct heat conduction to the metal substrate  20 . This increases design freedom concerning the demarcation of metal substrate into sections. As a result, it is possible to achieve area saving of the circuit device while improving the heat release performance thereof. 
     Second Embodiment 
     In a circuit device according to a second embodiment of the present invention, circuit elements are not only mounted on the metal substrate as with the circuit device  100  described in the first embodiment, but they are also mounted on a wiring layer on the opposite side of the metal substrate. This structure allows effective arrangement of a plurality of circuit elements which have varied functions. Hereinbelow, a description will be given of a circuit device capable of driving a small fan motor as an example of such a circuit device. 
       FIG. 11  is a functional block diagram of a system for driving a fan motor. A control unit  130  generates a control signal to run a fan motor  140  based on a signal from a operation unit, such as a temperature sensor or a button, and outputs it to a driver unit  150 . A driver unit  150  amplifies the control signal to such an extent as can drive a power unit  160 . The power unit  160  directly drives the fan motor  140  based on the amplified control signal. 
     The control unit  130  is preferably of a structure capable of advanced control from the viewpoint of noise reduction and low power consumption. To be more specific, the control unit  130  includes parts manufactured by fine CMOS processes, including a signal processor, RAM, flash memory, and the like. Also, the power supply voltage for the control unit  130  is as low as approximately 1.5 to 3 V. 
     The driver unit  150 , which is designed according to the structure of a power device equipped in the power unit  160 , needs to have a function for amplifying the voltage or current to a certain extent. For example, a discrete device, such as a bipolar transistor, is well-suited for the driver unit  150 . 
     The power unit  160  is required to have a drive capability sufficient to drive a load, such as the fan motor  140 , efficiently. For example, a discrete device, such as a MOS transistor, a bipolar transistor, or an insulated gate bipolar transistor, is well-suited for the power unit. 
     To package a system as shown in  FIG. 11 , it is necessary to use packaging technology such as follows: 
     (i) High-Radiation Packaging Technology 
     A power device equipped in the power unit  160  tends to generate more heat due to Joule heat when the equipment load to be driven is large. Hence, it is necessary to use packaging technology suited to high-radiation substrates. 
     (ii) High-Density Packaging Technology 
     A packaging technology applicable to multilayer fine-wiring boards is required to package at high density a control unit  130  which is comprised of parts manufactured by fine CMOS processes. 
     Therefore, in a circuit device according to the present embodiment, devices constituting a driver unit  150  and a power unit  160 , which produce relatively much heat, are mounted on one face of a metal substrate  20 , which is highly heat-radiant, and devices constituting a control unit  130 , which produces relatively little heat, is mounted on one face of a wiring layer  30 , which is of a multilayer wiring structure. 
       FIG. 12  is a schematic cross-sectional view of a circuit device according to the second embodiment of the present invention. The circuit device  200  is markedly different from the circuit device  100  in that the circuit elements are mounted on one face of a wiring layer  30  thereof. In the following description, the same content as explained in the first embodiment will be omitted as appropriate. 
     The circuit device  200  is manufactured by mounting circuit elements  10   c  and  10   d  on a conductive film  8  of the wiring layer  30  through a bonding layer (not shown) made of solder and conductive material when solder balls  13  are formed by a method as shown in  FIG. 2  after the same process illustrated in  FIGS. 5A-5D  through  FIGS. 9A-9B  as the first embodiment. Here, the circuit elements  10   a  and  10   b  function as the aforementioned driver unit  150  and power unit  160 , which produce relatively much heat. And the circuit elements  10   c  and  10   d  function as the aforementioned control unit  130  or passive elements, which produce relatively little heat. In other words, the circuit elements  10   c  and  10   d  produce less heat than the circuit elements  10   a  and  10   b.    
     In this manner, the circuit device  200  can be so configured that the circuit elements  10   a  and  10   b , functioning as a driver unit  150  and a power unit  160 , which produce relatively much heat, are mounted on one face of a highly heat-radiant metal substrate, and the circuit elements  10   c  and  10   d , functioning as a control unit  130 , which produces relatively little heat, are mounted on one face of a high-density-packaging type wiring layer  30 , which is opposite to the side on which the metal substrate is disposed. Therefore, the circuit device  200 , which allows a single packaging of a system as shown in  FIG. 11 , realizes smaller size by reducing packaging area and reduction of manufacturing cost by reducing the number of components used. 
     Also, in the circuit device  200 , the driver unit  150 , the power unit  160 , and the control unit  130  are connected with one another within a single package by the conductive films  6  and  8  of the wiring layer  30  and the copper embedded in the via holes  5   a  and  7   a . Thus, the circuit device  200  can reduce the number of connection pins required compared with the case where the driver unit  150 , the power unit  160 , and the control unit  130  are set in separate packages. Also, the circuit device  200  can shorten the delay in signal transfer compared with the case where the driver unit  150 , the power unit  160 , and the control unit  130  are connected together by wire. 
     Volume of Metal Substrate 
     In the circuit device according to the preferred embodiments so far described, the regions where circuit elements are mounted are demarcated by an insulating film. Hence, it can be assumed that a circuit element and a part of the metal substrate to which the circuit element is jointed forms a closed system.  FIG. 13  is a schematic diagram for explaining a system formed by a circuit element and a part of the metal substrate on which the circuit element is mounted. 
     As illustrated in  FIG. 13 , a circuit element  10   a  is mounted on a copper plate  1   a  of a multiplicity of copper plates constituting the metal substrate  20 . Accordingly, the heat generated by the circuit element  10   a , as it is driven, is preferentially dissipated in the copper plate  1   a  and released therethrough. However, if there is more heat generated by the circuit element  10   a  than the heat released from the copper plate  1   a , then the temperature of the system as a whole will rise gradually. And this may sometimes result in a fault of the circuit element  10   a  or a separation of the circuit element  10   a  from the copper plate  1   a.    
       FIG. 14  is a diagram schematically illustrating a temperature change of a system where the drive of a circuit element is temporarily turned on. As shown in  FIG. 14 , the temperature of the system rises gradually with the drive of the circuit element turned on and drops gradually from the point a little while after the drive of the circuit element is turned off. If the temperature variation at this time is ΔT, then the larger the temperature variation ΔT, the greater the probability will be for the above-mentioned fault or separation of the circuit element. 
     Thus, use of a larger volume V of the copper plate  1   a  is conceivable as a measure for making the temperature variation ΔT smaller while the circuit element  10   a  is energized. Where circuit elements with the same rate of heat generation are mounted, the larger volume V of each copper plate raises the thermal capacity, thus favorably making the temperature variation ΔT of the whole system smaller. Nevertheless, too large a volume V thereof may lead to a larger size of the circuit device. Therefore, an upper limit of the volume V of each copper plate may be selected as appropriate according to the size of the equipment into which the circuit device is incorporated or the amount of heat generated by the circuit element. 
     On the other hand, a lower bound of the volume V of each copper plate may be determined by the following Equation (1): 
     
       
         
           
             
               
                 
                   V 
                   &gt; 
                   
                     Q 
                     
                       
                         ρ 
                         · 
                         C 
                         · 
                         Δ 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Tth 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Equation (1), V[m 3 ] is the volume of metal substrate in the section where a circuit element is mounted; Q[J] is the amount of heat generated by power loss during the energization of a circuit element; ρ[kg/m 3 ] is the density of metal substrate; C[J/kg·K] is the specific heat of metal substrate; and ΔTth[K] is the upper limit of temperature variation where no fault of the system results from the heat generation of a circuit element. 
     That is, a lower limit of the volume V of the metal substrate in the section where the circuit element is mounted is obtained from the amount of heat generation Q of the circuit element while energized, the upper limit ΔTth of temperature variation as determined from the difference between the ambient temperature and the upper bound of operating temperature where the circuit element functions normally, and the density ρ and specific heat C of the material of the metal substrate. The upper limit value ΔTth of temperature variation can be obtained as 125 [K], for instance, if the upper bound of operating temperature of Si is 150° C. and the ambient temperature of the circuit element is 25° C. 
     Hereinbelow is an example of calculation of a lower bound of the volume V of a copper plate.  FIG. 15  is a schematic representation of a pulse current that flows through a circuit element according to the second embodiment. In the example of  FIG. 15 , a current I of 1 [A] is carried to the circuit element for 10 [sec] per cycle. 
     If the ON resistance of a circuit element is Ron=1[Ω] and the energizing time per cycle is t=10[sec], the power loss at the circuit element per cycle will be Q=I 2 ×Ron×t=10[J]. In the case of a copper plate where a circuit element energized by a pulse current as shown in  FIG. 15  is mounted, if the density of copper is ρ=8880[kg/m 3 ], the specific heat of copper is C=0.386[J/kg·K], and the upper limit of temperature variation is ΔTth=125[K], then the volume V can be calculated by Equation (1) as follows.
 
 V= 10/(8880×0.386×125)=2.334×10 −5 [m 3 ]
 
Hence, we know that the aforementioned circuit element should be mounted on a copper plate whose volume is larger than this value.
 
     Thus, the operation reliability of a circuit device may be improved by designing it such that the volume of the metal substrate in the section where each circuit element is mounted satisfies Equation (1). 
     In the above-described embodiments, an example has been presented where two circuit elements with different potentials are mounted within a circuit device. However, the present invention is not limited thereto, and it is also applicable to a circuit device in which three or more circuit elements are mounted. Also, when a plurality of circuit elements with the same voltage applied to the portions connected to the metal substrate are involved, those circuit elements may be mounted on the demarcated and separate metal substrates (copper plates). This may further increase the design freedom concerning the sections of metal substrate. As a result, the area of the circuit device may be reduced while improving the heat radiation performance of the circuit device. The further subdivision of the metal substrate (copper plates) reduces the expansion of the metal substrate (copper plates) due to the heat conducted from the circuit element, thereby reducing the warping of the circuit device. 
     Also, in the above-described embodiments, the metal substrate has a uniform thickness but the present invention is not limited thereto. For example, for higher (thicker) circuit elements to be mounted, the metal substrate (copper plates) in the regions where such circuit elements are mounted may be made thinner. In this manner, the height of the circuit device may be made thinner effectively compared with the circuit device having the metal substrate of uniform thickness. 
     While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.