Abstract:
The electrical unit has a printed circuit board ( 10 ) supporting the circuit, which includes a power component ( 11 ) which generates heat. In order to dissipate this heat from the power component, the power component rests on a heat conductive layer ( 13 ) which in turn is applied to the upper side ( 12 ) of the printed circuit board. This heat conductive layer further has a portion of the lid ( 18, 19 ) of the housing resting on it, which serves as a cooling area. Alternatively, the cooling area can be a free-standing cooling element. The heat transfer thus takes place via the heat conductive layer ( 13 ) to the cooling element ( 18, 19 ) so that the latter may be applied and formed independently of type and form of the power component.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electrical device having a printed circuit board carrying an electronic circuit and at least one power component to be cooled, particularly a switching or control unit for a motor vehicle. In these known devices, power components which heat up intensely are mounted on cooling elements which are fixed on a printed circuit board or on a housing on which the printed circuit board is secured. These cooling elements consist of special cooling profiles by which the power components are spaced from the printed circuit board or project from it. These cooling elements have the disadvantage that they consist of profiles having a complicated structure, and that the process of assembling the device with the profiles can be automated only with great difficulty. Due to the design of the cooling elements and the associated method of mounting the power components, these electrical devices have to be comparatively large, thus taking up a considerable amount of installation space. 
     In U.S. Pat. No. 4,811,165, an electrical device is described in which the electronic components are arranged on a printed circuit board which carries an electronic circuit. This is made from a flexible material and located on a plate of good thermal conductivity. This has the disadvantage that heat from the electronic components must be dissipated through the printed circuit board to the heat conductive plate. In addition, the (conventional) method of attaching electronic components in a wired structure can be achieved only with considerable effort, since additional insulating provisions vis-à-vis the heat conductive plate are required when the connecting wires or connection electrodes are routed through soldering apertures of the printed circuit board. 
     SUMMARY OF THE INVENTION 
     According to the invention, the electrical device comprises a printed circuit board carrying an electronic circuit and at least one power component to be cooled; a heat conductive layer applied to the printed circuit board at least in the vicinity of the at least one power component, each of the power components resting flat with their largest face in contact with the heat conductive layer; and a free-standing metallic body connected to the heat conductive layer and spaced from the power components to act as a cooling element for dissipation of heat conducted to the free-standing metallic body through the heat conductive layer from the power components. 
     Alternatively, instead of a free-standing metallic body the cooling element can be a part of a housing accommodating the printed circuit board. In a preferred embodiment spring means can be provided which presses the printed circuit board against an interior part of the housing to establish a good heat conduction between the heat conductive layer on the circuit board and the housing. 
     The heat conductive layer can advantageously be a metal cladding, a conductor track or a laminate of the circuit board. It can advantageously provide a screening for improvement of electromagnetic compatibility. 
     In contrast, the electrical device in accordance with the invention has the advantage that a particularly flat form of the electrical device is made possible by the arrangement of the power components. The type and form of the power components has no effect on the form and shape of the cooling elements, so that their type and method of attachment can be determined by other factors. The attachment of the power components in accordance with the invention facilitates good heat dissipation which allows a larger number of power components to be located on the printed circuit board. Good heat dissipation further allows a higher ambient temperature, such as prevails in motor vehicles, for example. The housing of the electrical device can further be automatically equipped and soldered in a small number of production steps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Two embodiment examples of the invention are explained in more detail in the description which follows and in the drawings. 
     FIG. 1 is a longitudinal cross-section cutaway view through a first embodiment of a control unit according to the invention, and 
     FIG. 2 is a top view of a printed circuit board of the control unit of FIG.  1 . 
     FIG. 3 is a top view of a printed circuit board in a second embodiment example of the control unit according to the invention. 
     FIG. 4 is a cross-sectional view of the control unit shown in FIG. 3 taken along the section lines  4 — 4  in FIG.  5 . 
     FIG. 5 is a cross-sectional view of the control unit shown in FIG. 3 taken along the section lines  5 — 5  in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 and 2, the number  10  denotes the printed circuit board of an electronic control unit which carries a circuit which is not shown in any detail. This circuit includes a power component  11  which heats up during operation and from which the heat must be dissipated. 
     The upper side  12  of the printed circuit board  10  has a layer  13  of material with good thermal conductivity applied to it which extends to the edge  14  of the printed circuit board. The layer  13  of heat conductive material is preferably metallic and may be an appropriately designed conductor track, for example, or a screening surface for the improvement of the electromagnetic compatibility (EMC), a laminate, a copper cladding, or similar. 
     This heat conductive layer  13  has the power component  11  mounted to it by its rear face  15  (largest face), and it is affixed in some suitable manner, for example by gluing, soldering, or screw fixing. The attached rear face  15  of the power component  11  is smaller than the base area of the heat conductive layer  13 . 
     The connection electrodes  16  of the power component  11  extend initially parallel to the printed circuit board  10  without touching the heat conductive layer  13 , and they are bent clear of the conductive layer and guided into corresponding soldering apertures  17  of the printed circuit board. The power component is connected to the circuit by soldering. 
     The printed circuit board  10  has on its upper side  12  a hood-shaped cover  18 , preferably a free-standing metallic body, the peripheral flange  19  of which sits on the circuit board at the edge of the circuit board  10  and thus also on the heat conductive layer  13 . The cover  18  or its flange  19  is connected to the printed circuit board  10  in a suitable manner, for example by gluing, screw fixing, or soldering. 
     The cover  18  is constructed, at least in the region of the heat conductive layer  13 , from a heat conductive material, preferably a metal, and it serves as a cooling surface for the power component  11 . 
     The heat to be dissipated from the power component  11  is thus passed via the heat conductive layer  13  directly of to a cooling surface, the cover  18 . There is thus no elaborate mounting of the power component on or to a specifically form adapted cooling surface. The cooling surface is therefore independent of the form of the power component, which means that the form of the power component does not influence either the form or the position of the cooling surface. Furthermore, the power component and the cooling surface do not have to be in close proximity to each other. 
     If the printed circuit board  10 —as shown as hatched in FIG.  1 —is designed as a two-layer circuit board, the underside  20  can also be correspondingly equipped with power components  11 . These then also rest on a heat conductive layer  21 , via which the dissipated heat is passed to a cooling surface, which is not shown. 
     To improve the heat transfer between the heat conductive layer  13  or  21  and the cooling surface, it is possible to structure the surface of the heat conductive layer appropriately. Using suitable methods, it is possible to emboss a soldered layer or some other heat conductive layer in lattice structure, by a reflow soldering process, for example. 
     A large contact area between the heat conductive layer  13  and the cooling surface is of further advantage for improved heat transfer. 
     Where several power components  11  are arranged adjacent to each other on one side  12  or  20  of the printed circuit board, the metallic layers  13  or  21  involved should be insulated from each other, i.e. they should not contact each other. 
     If the cooling surface, as shown in the embodiment example, is on the cover of a housing or a freely accessible—not shown—cooling element (e.g. cooling angle), then to good advantage it is electrically insulated from the heat conductive layer  13 . To this end, the cooling surface may be provided with an insulating layer, an anodized oxide layer, varnish or similar, in the area of contact. The relevant cooling element can then additionally be used for the cooling of several power components. 
     It is further possible to provide the heat conductive layer  13 , at least in the region of the cooling surface, with an insulating layer. 
     In contrast to this method, it may for example, be expedient in certain applications for the improvement of the electromagnetic compatibility, to have an electrically conductive connection between the cooling surface and the heat conductive layer  13 . These are then assembled without any insulation. 
     If, for example, a conductor track of appropriate width or a screening surface is used as a heat conductive layer, this can be constructed thicker (increased layer thickness) in order to improve the heat conductivity. 
     In the wiring area of the printed circuit board, a conductive layer with a thickness of, say, 30 μm is applied. This allows conductive tracks for electrical contact with the components on the circuit board with widths below 200 μm and with similar distances between the conductive tracks to be produced. 
     In the areas in which electrical components are to be connected heat conductively to the conductor tracks, the layer thickness of the conductor track acting as the heat conducting layer  13  should be increased to thicknesses above 50 μm, preferably to layer thicknesses of from 100 μm to 200 μm. 
     To improve the heat conductivity further, the printed circuit board  10  can be provided with heat conductive layers on both sides (side  12  or side  20 ), which are connected to each other by a generally known through-contact method. In this way, parallel heat dissipating paths are created. 
     If the printed circuit board  10  is fitted with SMD components (SMD=surface mounted device) in the area of the heat conductive layer, then the surface of the heat conductive layer  13  and that of the corresponding conductor track section are preferably arranged in one plane, for good mounting and contact. 
     If the cooling surface, as shown in the embodiment example, is on the cover of a housing, this can be designed so that an electromagnetic screening of the covered circuit is effected at the same time (improvement of electromagnetic compatibility, EMC). This cover is then constructed as a metallic or metallized box which covers the appropriate printed circuit board areas. This metallic box can then cover the entire area of the printed circuit board and rests on the edges of the printed circuit board, where electrical and heat conductive contact is made. Alternatively, it is possible to design the box so that it covers the printed circuit board only in that region in which sensitive components are arranged, i.e. components which need to be screened with regard to the electromagnetic compatibility of the circuit. The power components which require cooling can be arranged within or outside the box. 
     In order to connect this box to the printed circuit board in a mechanically stable manner and with good conductive capabilities, connecting tabs can be fitted to its underside, which project through corresponding apertures of the printed circuit board or which are guided past their edges. After soldering the top side of the printed circuit board, the box is placed on it, and the connecting tabs are bent over and soldered. These connecting tabs can also be used to fix a corresponding bottom part (also an EMC box). 
     In the second embodiment example of the electronic control device shown in FIGS. 3 to  5 , a connection between the heat conductive layer and the cooling element is made in a particularly advantageous manner which is easily and securely effected during assembly. The cooling element in this embodiment example is the housing  40  of the control device. Components which are identical to those in the previously described embodiment example are designated with the same reference numbers. 
     The printed circuit board  10  is of rectangular design and carries on its front face  30  a plug strip  31  for contacting the electronic circuit. The upper side  12  of the printed circuit board  10  has the layer  13  of material with good thermal conductivity applied to it. This layer  13  is arranged on the edge regions  32  to  34  in the form of a copper cladding. The layer thickness of this copper cladding or deposit of copper is preferably between 300 μm and 400 μm. In the embodiment example shown here, the heat conductive layer  13  is extended to form a continuous surface on the three free front faces  35  to  37  of the printed circuit board. The width of the heat conductive layer  13  (at right angles to the outer edge of the printed circuit board) is dependent on the power components  11  to be cooled or the dimensions of these. 
     The power components to be cooled (wiring components or SMD=surface mounted devices) are placed on this heat conductive layer  13  such that they are at a certain distance from the front face or from the edge of the printed circuit board. 
     The width of the heat conductive layer  13  is matched to the particular structural shape of the power components, in sections, so that the areas which are in contact are as large as possible in order to achieve good heat transfer, yet still allowing reliable contacting of the particular power component. 
     The housing  40  for the printed circuit board  10  is approximately rectangular and is parallelepiped shaped open on one front face  41 . It is therefore composed of one base part  42 , one lid part  43 , and three side walls  44 ,  45  and to  46 . On each of the inner sides  47 ,  48  and  49 , a flange  50 ,  51  and  52  is formed which extends parallel to the base part  42 . The flanges  50  and  51 ,  51  and  52 , respectively, are continuously connected with each other. 
     On the base part  42 , a wedge  53 ,  54  is formed at the transition to the side walls  44  and  46 , respectively, which extends rising from the open front face  41  to the opposite side wall  45 . 
     The housing  40  is closed on its front face  41  by a front plate  55 . This has an aperture  56 , through which the plug strip  31  projects. On the inner side  57  of the front plate  55 , two wedge shaped spring elements  58 ,  59  are provided. The two spring elements  58 ,  59  are of the same structure, each has a short fixing section  60 , which attaches to the inner side  57  of the front plate  55  and is fixed there. Projecting from this at an approximately right angle is an upper section  61 , the length of which is slightly shorter than that of the flanges  50  and  52 . The upper section  61  merges into a connecting arc  62 , from which a lower section  63  extends. This projects as far as the vicinity of the fixing section  60 . The upper section  61  and the lower section  63  form a wedge, which tapers, starting from the front plate  55 . 
     In the assembled condition of the control unit, the printed circuit board  10  is between the flanges  50 ,  51  and  52  and the base part  42  of the housing. The front plate  55  closes the housing  40 , with the plug strip  31  protruding through the aperture  56 . The spring element  58  rests on the wedge  53 , while the second spring element  59  rests on the wedge  54 . In their spring action and shape, the spring elements  58  and  59  are adapted to the wedges  53  and  54  and their distance to the flanges  50  and  52  so that the printed circuit board is pressed with its upper side against the flanges, causing the heat conductive layer  13  and the underside of the flanges to press against each other. The heat transfer from the power components  11  can thus take place via the heat conductive layer  13  to the flanges  50  to  52  and consequently to the housing  40 . 
     During assembly of the electrical device, the printed circuit board  10  with the plug strip  31  is inserted into the housing  40  together with the front plate  55 . The printed circuit board  10  is then between the flanges  50  and  52  and the base part  42  of the housing. During insertion, the spring elements  58  and  59  rest on the wedges  53  and  54 , respectively. After insertion, each of the wedge shaped spring elements exerts an adequate contact pressure by action of the wedges  53  and  54 , which is required for the heat dissipation. The heat is thus dissipated from the power components via the heat conductive layer to the housing, without any additional cooling body or support frame. Nevertheless, even greater heat outputs can be transferred.