Abstract:
A rotating electrical machine control embodying a circuit including semiconductor devices mounted on a conductive pattern formed on a metal substrate without using heat sinks. Performance is improved as is durability by matching the linear expansion coefficient of the resin used to seal the semiconductor chips with that of the conductive pattern formed on the metal substrate.

Description:
BACKGROUND OF INVENTION 
     This invention relates to a circuit comprised of an insulated metal substrate on which a semiconductor bare chip is mounted and more particularly to an improvement in thermal dissipation from the bare chip. 
     Circuits embodying semiconductor devices are used as control circuits for various types of electronic equipment and devices and in vehicle and industrial equipment and devices. The circuit takes the form of a bare chip is mounted on the electrodes or circuit pattern formed on an insulated substrate. This improves performance by reducing resistance by shortening the wire length. It also permits higher efficiency in the manufacturing process and higher mounting density to reduce size. Generally the bare chip is soldered to the electrodes or circuit pattern on the substrate and then sealed with resin. 
     In such semiconductor circuits, thermal stress is generated by the difference in thermal expansion and/or shrinkage between the semiconductor chip and the substrate. This is generated by the heat developed in the semiconductor chip itself and temperature cycling of the ambient environment. To reduce such thermal stress, the semiconductor chip is soldered to the substrate via a heat sink formed from a highly conductive plate member, such as copper. However, use of such a heat sinks increase the number of parts, cause the structure to become complicated, reduce the density of chip mounting and increase the difficulty and expense in assembly process. 
     A circuit including a semiconductor device intended to reduce thermal stress without such a heat sink is disclosed in Japanese Publication 07-249714. The semiconductor device described in that publication is a complex circuit embodying a semiconductor device. The circuit comprises an aluminum substrate on which a conductive pattern is formed via an insulating layer. A semiconductor chip is directly soldered on the conductive pattern, and then sealed with resin having a coefficient of thermal expansion smaller than that of the aluminum substrate. 
     However, the resin for sealing the semiconductor chip has a lower coefficient of thermal expansion that of the aluminum substrate. Therefore, it is not always possible to obtain sufficient reduction in thermal stress for all types of semiconductor chips and of materials of conductive patterns. 
     It is therefore a principle object of this invention to provide a circuit embodying a semiconductor device which reduces the thermal stress in the semiconductor chip and without a heat sink. 
     SUMMARY OF INVENTION 
     This invention is adapted to be embodied in a circuit comprising a metal substrate, an insulating layer on the substrate and a conductive pattern formed on the insulating layer. A semiconductor bare chip is mounted directly onto the conductive pattern without a heat sink. A sealing body is formed over the semiconductor bare chip. The sealing body is formed from a material having a thermal expansion coefficient approximately equal to that of the conductive pattern. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross sectional view of a circuit constructed in accordance with the invention taken through the center of one of the mounted semiconductor devices. 
     FIG. 2 is a top plan view of an aluminum substrate to which the present invention is applied. 
     FIG. 3 is a top plan view, in part similar to FIG. 2, but showing the chips and other components mounted on the aluminum substrate. 
     FIG. 4 is a side elevational view of the structure shown in FIG.  3 . 
     FIG. 5 is top plan view of a motor control unit for an electric-powered vehicle incorporating the invention. 
     FIG. 6 is a side elevational view of the motor control unit. 
     FIG. 7 is an end elevational view of the motor control unit. 
     FIG. 8 is a top plan view in part similar to FIG. 5, but with the potting compound removed and showing the contained components in solid lines. 
     FIG. 9 is a side elevational view in part similar to FIG. 6, but with the potting compound removed and showing the contained components in solid lines. 
     FIG. 10 is an end elevational view in part similar to FIG. 7, but with the potting compound removed and showing the contained components in solid lines. 
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings and initially to FIGS. 1-4 a circuit board on which a printed circuit and solid state components such as semiconductor chips as well as other components are mounted is shown and indicated generally by the reference numeral  21 . In the above embodiment, the circuit board  21  comprises, as shown in FIG. 2, an aluminum (Al) substrate  22  of about 2-3 mm thickness, an insulating layer  23  of 75-100 μm thickness made of, for example, epoxy resin, and a conductor pattern  24  of a copper (Cu) film on the insulating layer  23 . 
     The conductor pattern  24  is coated with the solder resist  25 , which is patterned to be opened to form a land pattern  26  in the position where a semiconductor chip will be mounted. 
     A semiconductor bare chip  27  is jointed directly by solder (eutectic solder or lead-free solder)  28  onto the conductor pattern  24  being exposed in the land pattern  26 . The semiconductor bare chip  27  is, for example, a bare chip of power devices such as electric power diodes or power transistors for power conversion, through which a large amount of electric current flows. Because of the large current flowing through the conductive pattern  24 , the cross-sectional area of the pattern is thickened. A thickness of between 300 to 500 μm is chosen. That normally used with bare chips is in the order of 75 to 105 μm in thickness. 
     In the present invention, when the semiconductor chip is soldered, the amount of the solder used is the one sufficient for the whole chip bottom surface to be covered. The excess the molten solder can flow out in the middle of escapes formed in the land pattern. Thus, the effects of the land pattern constitution of the invention of the above can be gained by appropriately selecting the amount of the solder. 
     After being soldered, the semiconductor bare chip  27  is sealed or potted by resin  29  such as epoxy having particular thermal expansion characteristics. The linear thermal expansion coefficient of the resin  29  is approximately equal to that of the conductive pattern  28 . In this case copper which has a linear thermal expansion coefficient of 16.7×10 −6 /° C. Therefore, matching the linear expansion coefficient of the epoxy resin  29  with that of the conductive pattern  28  allows to effectively reduce the thermal stress generated by the temperature cycling acting on the semiconductor bare chip  27 . This has been empirically confirmed. 
     In this connection, the linear expansion coefficient of the aluminum substrate  22  is 23×10 −6 /° C. Alternatively, a ceramic substrate (linear expansion coefficient=2.4×10 −6 /° C.) or an iron substrate (linear expansion coefficient=about 12×10 −6 /° C.) may be adopted. In either case, the linear expansion coefficient of the resin  29  should be adjusted to be approximately equal to that of copper used to constitute the conductive pattern  28 . 
     FIGS. 2-4 show an embodiment where a substrate embodying the invention can be used as a motor control unit for driving an electric-powered vehicle. A conductor pattern of copper (not shown) is formed on an aluminum substrate  31  and coated with a resist  32 . By patterning the resist  32 , diode land patterns  33  and FET land patterns  34  constituting portions of a motor control circuit are formed. Output terminals  35   a,    35   b,  and  35   c  of the control circuit are formed at three places on the aluminum substrate  31  each having two output terminal holes  36 , respectively. At the four corners of the aluminum substrate  31  are disposed mounting holes  37  for fixing a casing body as will be described later by reference to FIGS. 7-12. Further on the substrate is provided a gate resistance  38  forming a further component of the drive circuit. 
     Diodes  39  are soldered within the respective diode land patterns  33 , and FETs  41  are soldered within the respective FET land patterns  34 . Each diode  39  is respectively sealed or potted with a resin  42 . Each FET  41  is sealed with resin  43  along with a connector  44 . A commercially available liquid sealing material or resin of linear expansion coefficient of (15 to 30)×10 −6 /° C. can be selected to use as the potting material for sealing such diodes  39  and FETs  41 . (For example, sealing materials of linear expansion coefficient of 15×10 −6 /° C. and 22×10 −6 /° C. approximate to those of copper and aluminum respectively are easily available in the market.) Referring now to FIGS. 5-10, these figures show how a complete motor control unit incorporating the aluminum substrate  31  of FIGS. 2-4 can be constructed in accordance with a further feature of the invention. The motor control unit, indicated generally by the reference numeral,  45  includes a drive control circuit configured of the aluminum substrate  31  as previously described by reference to FIGS. 2-4 in a casing body  46 . 
     The casing body  46  is formed by the extrusion of a metallic material of aluminum or aluminum alloy. The casing body  46  is of a cylindrical shape with both ends open. A plurality of aligned parallel ribs  47  are formed to project from its outer circumferential surface. The ribs  47  increase the surface area of the casing body  46 , resulting in the increase of heat radiation as well as the rigidity and strength of the casing body  46 . 
     On the aluminum substrate  31  are mounted further devices constituting a drive control circuit such as an electrolytic capacitor  48  constituting a drive control circuit (FIG.  8 ). Also terminal attaching plates  49   a,    49   b,  and  49   c  are connecting to the aforementioned output terminals  35   a,    35   b,  and  36   c.  Each signal wire of the control circuit is connected through an electric cable  51  and a coupler  52  to switches and other drive or control parts on the vehicle side. Output terminals  53  passing through the aforementioned output terminal holes  37  project from the lower surface of the aluminum substrate  31 . Such an aluminum substrate  31  and the electronic parts mounted thereon are accommodated in the casing body  46  and sealed or potted with resin  54  having the aforenoted thermal expansion characteristics. 
     As described, by matching the linear expansion coefficient of the resin used to seal the semiconductor chips with that of the conductive pattern formed on the metal substrate reduces thermal stress and mechanical stress caused by the temperature cycling. This achieves improvement in its durability and prevention from deterioration with a simple configuration that does not require the use a heat sink. Of course the embodiment described is a preferred embodiment of the invention and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.