Abstract:
The invention relates to a printed circuit board arrangement ( 20 ) having at least two printed circuit boards ( 11; 12 ′), which are located alongside one another on a common, electrically conductive base plate ( 13 ′), have two mutually opposite edges, forming an intermediate space ( 14 ), and are connected to one another, bridging the intermediate space ( 14 ) by means of electrical connections ( 21 ), in order to transmit extremely high frequencies. In the case of a printed circuit board arrangement such as this, RF-compatible electrical connections are made between the printed circuit boards in a simple and space-saving manner when the printed circuit boards are fitted on the base plate and without any further necessary additional tasks or reworking, in that the electrical connections are each made via at least one contact-making element ( 21 ), via which the printed circuit boards make detachable electrical contact with one another when mounted on the base plate ( 13 ′).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of electronic circuitry in the extremely high frequency region, in the range of operating frequencies up to several GHz. It relates in particular to a printed circuit board arrangement according to the precharacterizing clause of claim  1 . 
     In modern communications technology and in other fields which operate using extremely high frequency, electronic circuits are formed on printed circuit boards (PCBs) which have one or more conductor layers on an electrically insulating supporting board, which conductor layers are structured at least in places and form conductor tracks or surfaces which connect electronic components which are fitted and/or soldered to the supporting board to one another and/or to the outside world. 
     In the case of relatively large appliance units such as base stations for mobile radio technology, a plurality of separate printed circuit boards are accommodated in one appliance and must be electrically connected to one another after or during installation in the appliance. Many of the required connections must in this case be RF-compatible, in order to allow signals to be reliably passed on from one printed circuit board to the other at high operating frequencies in the GHz range. In principle, such retrospective connections can be soldered, but this involves a considerable amount of labour, particularly when a large number of such connections must be made. It is also possible to make the connections by short lines with plug connectors, although this is likewise associated with considerable labour and in addition occupies a not insignificant space. 
     2. Discussion of Related Art 
     If the printed circuit boards to be connected are arranged one above the other or at least partially overlapping, pressure-contact, preferably coaxial RF connections can be used, for example as disclosed in the documents U.S. Pat. No. 6,231,352, U.S. Pat. No. 7,416,418, U.S. Pat. No. 7,210,941, EP-A2-1 289 076 or WO A1 2007/009549. 
     However, a different situation results with printed circuit board arrangements as shown in  FIG. 1  or  FIG. 2 . In the case of the printed circuit board arrangement  10  shown in  FIG. 1 , two printed circuit boards  11  and  12  are arranged alongside one another on a common, relatively thick base plate  13  which is used in particular for heat dissipation and, for example, may be composed of aluminium. The two printed circuit boards  11  and  12  are aligned with two opposite edges parallel to one another, and separated, thus forming an intermediate space  14  between them, which must be bridged by RF-compatible electrical connections  15 , which are represented schematically by double-headed arrows in  FIG. 1 . The intermediate space  14  is not absolutely essential, and instead the edges of the printed circuit boards  11 ,  12  can also directly abut on one another. 
     In the printed circuit board arrangement  20  shown in  FIG. 2 , the situation is similar, with the difference that one of the printed circuit boards, specifically the printed circuit board  12 ′, is designed specifically for a high-power circuit and is therefore additionally provided on the lower face with a dissipation plate  16  which is intended to directly absorb and distribute the heat which is created in the components (a so-called “heat spreader plate”), before it is then passed on to the base plate  13 ′. By way of example, the dissipation plate  16  may be composed of sintered aluminium. This makes the printed circuit board  12 ′ thicker than the printed circuit board  11 . A corresponding thickness stage is provided for compensation in the base plate  13 ′. 
     Connection apparatuses which operate by means of pressure have already been proposed for the electrical connection of printed circuit boards arranged alongside one another, in U.S. Pat. No. 6,612,851, but these act on the upper face and lower face of the printed circuit boards and are therefore not suitable for printed circuit boards arranged on a base plate. 
     Furthermore, U.S. Pat. No. 6,464,510 has disclosed microwave connectors which connect both printed circuit boards on the upper face by means of pressure contacts. However, the connectors must be installed retrospectively and must be screwed to one or both printed circuit boards, which is tedious and susceptible to faults. 
     Finally, U.S. Pat. No. 3,591,834 discloses a type of plug connector system, in which the opposite side edges of the two printed circuit boards must be introduced from the side, making contact, into an apparatus which is provided for this purpose. This connection technique is also unsuitable for printed circuit boards which rest on a base plate. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to provide a printed circuit board arrangement of the type mentioned initially, in which RF-compatible electrical connections are made between the printed circuit boards in a simple and space-saving manner when the printed circuit boards are fitted on the base plate, and without any need for further additional tasks or reworking, and which electrical connections are suitable for use as signal lines up to the extremely high frequency range of several GHz. 
     The object is achieved by the totality of the features of claim  1 . One major factor for the solution according to the invention is that the electrical connections are each made via at least one contact-making element, via which the printed circuit boards make detachable electrical contact with one another when fitted on the base plate. 
     One refinement of the apparatus according to the invention is characterized in that the electrical connections between the two printed circuit boards are made and are maintained by exerting mutual pressure between the printed circuit boards and/or by pressure between the printed circuit boards and the base plate. In this case, the contact is therefore not made in a clamping manner but by pressure exerted by the printed circuit boards. 
     The pressure can therefore act at right angles to the plane of the printed circuit boards. This is the case when the printed circuit boards press against a contact element arranged underneath them. 
     However, the pressure can also act parallel to the opposite edges of the printed circuit boards. This is the case when the printed circuit boards have contact apparatuses which overlap in the intermediate space. 
     According to a further refinement of the invention, at least one contact-making element is provided in order to make the electrical connections, is arranged in a recess in the base plate underneath the two printed circuit boards and detachably makes contact with and connects to one another the two printed circuit boards, from underneath. 
     One development of this refinement of the invention is distinguished in that the contact-making element has a contact rocker which is arranged and held with a centre piece in an insulating supporting body and projects upwards from the supporting body with a contact tongue, which is bent upwards, at each of the ends, and in that the supporting body is seated with the contact rocker in the associated recess in the base plate such that the contact tongues simultaneously press from underneath, making contact, against the printed circuit boards which are lying on the base plate. 
     The supporting body may be composed of widely differing insulating materials. However, the supporting body is preferably composed of an elastomer and is held in a clamping manner in the recess, with the contact rocker being held in a clamping manner in a depression on the upper face of the supporting body. 
     In this context, it has been proven for the recess to be in the form of an elongated hole, and for the edge contour of the supporting body to be matched to the edge contour of the recess. 
     A further refinement of the invention is characterized in that a contact surface is in each case provided on the lower face of both printed circuit boards for contact to be made by the contact tongues of the contact rocker, which contact surface is connected via a plated-through hole to a conductor track on the upper face of the printed circuit board. 
     According to another refinement, means are provided in order to hold down at least one of the printed circuit boards on the base plate, against the pressure of the contact rocker, in the area of the contact tongues. 
     A further refinement of the apparatus according to the invention is distinguished in that an intermediate space remains free between the two printed circuit boards, in that the contact rocker runs underneath the plane of the upper face of the base plate in the area of the intermediate space, and in that a vertical separating wall of an electrically conductive housing part is placed on the base plate, making contact with it, in the area of the intermediate space, such that the space above the two printed circuit boards is subdivided into two, separate, screened spaces, which are associated with the printed circuit boards. 
     However, it is also feasible for the contact-making element to have a further printed circuit board, on which contact posts are arranged in the area of the printed circuit boards and are conductively connected to one another via the further printed circuit board, and in that the further printed circuit board is seated with the contact posts in the associated recess in the base plate such that the contact posts press simultaneously, from underneath and making contact, against the printed circuit boards which are resting on the base plate. Contact posts such as those known from U.S. Pat. No. 6,231,352, for example, can be used for this purpose. 
     A refinement which does not require any action on the base plate is characterized in that an intermediate space remains free between the two printed circuit boards in that two contact-making elements are in each case provided in order to make the electrical connections and are arranged on the upper face of the two printed circuit boards such that they project from both sides into the intermediate space, make detachable contact, and connect the two printed circuit boards to one another. 
     The two contact-making elements are preferably the same, wherein each contact-making element is in the form of a stamped and bent part, which can be soldered on, and has a U-shaped body from which a sprung contact arm projects at the side, and is provided at its free end with a contact section, and wherein the contact sections of the two contact-making elements overlap in the intermediate space and press against one another in a sprung manner. 
     In particular, contact-making elements such as these are characterized in that they each have solder feet which are integrally formed on the body, in that an inclined element, which is bent obliquely outwards, is in each case integrally formed at the top in the contact sections, in that a conductor element, which is bent at right angles outwards, is in each case integrally formed at the bottom in the contact sections, and in that the contact sections enter the intermediate space from above. 
     Another refinement of the printed circuit board arrangement according to the invention is characterized in that the contact-making element has a contact body which is arranged and held with a base body in an insulating supporting body and at each of the ends has a contact tongue, which is bent back upwards, and projects upwards out of the supporting body, and in that the supporting body is seated with the contact body in the associated recess in the base plate such that the contact tongues at the same time press from underneath, making contact, against the printed circuit boards which are resting on the base plate. 
     In particular, the supporting body is composed of an elastomer and is held in a clamping manner in the recess. 
     The recess is preferably in the form of an elongated hole, and the edge contour of the supporting body is matched to the edge contour of the recess. 
     Advantageously, a contact surface is in each case provided on the lower face of the two printed circuit boards in order to make contact by means of the contact tongues of the contact body, and is connected via a plated-through hole to a conductor track on the upper face of the printed circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawing, in which: 
         FIG. 1  shows a schematic illustration of the RF connection of two printed circuit boards, which rest on a common base plate, in a first configuration, on which the present invention is based; 
         FIG. 2  shows a schematic illustration of the RF connection of two printed circuit boards, which rest on a common base plate, in a second configuration, on which the present invention is based; 
         FIG. 3  shows a section illustration of a printed circuit board arrangement having two printed circuit boards, which are RF-connected via a contact-making element inserted into the base plate, according to a first exemplary embodiment of the invention, 
         FIG. 4  shows a perspective illustration of the contact-making element, which can be inserted into the base plate, as shown in  FIG. 3 ; 
         FIG. 5  shows an exploded illustration of the contact-making element from  FIG. 4 , above the base plate which is provided with a corresponding recess; 
         FIG. 6  shows a perspective view from above of the electrical connecting path in the arrangement shown in  FIG. 3 , with the supporting material for the printed circuit boards being omitted, for the sake of visibility; 
         FIG. 7  shows a printed circuit board with a soldered-on contact-making element for a printed circuit board arrangement according to a second exemplary embodiment of the invention, 
         FIG. 8  shows a section illustration, comparable to  FIG. 3 , of a printed circuit board arrangement having two printed circuit boards, which are RF-connected via contact-making elements as shown in  FIG. 7 , according to the second exemplary embodiment of the invention; 
         FIG. 9  shows a perspective view from above of the printed circuit board arrangement as shown in  FIG. 8 ; 
         FIG. 10  shows a third exemplary embodiment of the invention, comparable to  FIG. 3 , in which contact posts which are known per se are used; 
         FIG. 11  shows the printed circuit board arrangement as shown in  FIG. 3 , as part of a screened RF assembly. 
         FIG. 12  uses a plurality of sub- FIGS. 12(   a ),  12 ( b ) and  12 ( c ) to show various views (perspective side view, plan view from above, longitudinal section) of a further exemplary embodiment of the invention; 
         FIG. 13  shows an exploded illustration ( 13 ( a )) and an enlarged detail ( 13 ( b )) of the printed circuit board arrangement shown in  FIG. 12 ; 
         FIG. 14  shows a perspective view obliquely from above on one of the printed circuit boards shown in  FIG. 12 ; and 
         FIG. 15  shows a perspective view obliquely from underneath of the printed circuit board shown in  FIG. 14 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 3  shows a section illustration of a printed circuit board arrangement having two printed circuit boards, which are RF-connected via a contact-making element inserted into the base plate, according to a first exemplary embodiment of the invention. The printed circuit board arrangement  20 , which has a base plate  13 ′ of stepped thickness based on the principle illustrated in  FIG. 2 , comprises two printed circuit boards  11  and  12 ′ of different thickness, with the thicker printed circuit board  12 ′ being designed for higher thermal loads by means of a thermal dissipation plate  16 , as already mentioned initially, on the lower face. The two printed circuit boards  11  and  12 ′ rest, with a separation between them, on the base plate  13 ′ arranged underneath, such that an intermediate space  14  remains free, which is bridged by a contact-making element  21  for each signal line, for the electrical connection of the two printed circuit boards  11 ,  12 ′. 
     The contact-making element  21  in the exemplary embodiment shown in  FIG. 3  is arranged underneath the two printed circuit boards  11 ,  12 ′ and is integrated in the base plate  13 ′. For this purpose, a recess  25 , which passes through underneath the intermediate space  13  and is in the form of an elongated hole with vertical side walls, is provided in the base plate  13 ′, the elongated hole for example being machined or incorporated by casting in the base plate  13 ′ (see also  FIG. 5 ). The recess  25  accommodates the contact-making element  21  and holds it firmly, in a clamping manner. On the upper face, the contact-making element  21  contains an electrically conductive, sprung contact rocker  22  with a horizontal centre piece  31  ( FIG. 5 ), which merges on opposite sides into contact tongues  23 ,  24  which are bent upwards. 
     The contact-making element  21  is designed and matched to the recess  25  such that, in the inserted state, it projects with the contact tongues  23 ,  24  of the contact rocker  22  so far upwards out of the recess  25  that the contact tongue  23  presses against the lower face of the printed circuit board  11 , making contact, and the contact tongue  24  presses against the lower face of the printed circuit board  12 ′, making contact, in an area in which the thermal dissipation plate  16  has been removed or omitted ( FIG. 3 ). The contact rocker  22  is mounted in a depression  27  on the upper face of an electrically insulating supporting body  26 . The supporting body  26  is composed of an electrically insulating material, preferably an elastomer, and forms a dielectric between the signal line (inner conductor), which is produced by the contact rocker  22 , and the earth plane (outer conductor), which is formed by the base plate  13 ′. 
     The supporting body  26  tapers slightly conically downwards in order to allow it to be inserted more easily from above into the recess  25  (for example by means of a placement robot). The supporting body  26  is held in a clamping manner in the recess  25 , by virtue of its elastic characteristics. In this case, two groove-like cutouts  29  and  30 , which run in the longitudinal direction on the lower face of the supporting body  26 , have a supporting effect. The contact rocker  22  is a stamped and bent part, is preferably manufactured from spring bronzes, and has surface finishing, for example silver coating. 
     The depression  27  on the upper face of the supporting body  26  is matched in the edge contour to the contact rocker  22 . The depression  27  in the area of the broadened centre piece  31  of the contact rocker  22  is designed such that the contact rocker  22 , which is pushed into the depression  27 , is pressed in with the side edges of the centre piece  31  at the side into the supporting body  26 , and is therefore held securely in the supporting body  26 . This interlock between the centre piece  31  and the supporting body  26  is assisted by broadened areas  28  of the supporting body  26 , which are fitted at the side and produce additional pressure inwards on insertion into the recess  25 . 
     Contact surfaces  18  (contact pads) are arranged on the lower faces of the printed circuit boards  11  and  12 ′ in order to make contact with the contact rocker  22 , and are electrically connected via plated-through holes  17  (vias) to conductor tracks  19  on the upper face of the printed circuit boards  11 ,  12 ′ (see also  FIG. 6 ). The conductor tracks  19 , the plated-through holes  17 , the contact surfaces  18  and the contact rocker  22  thus form the inner conductor of an RF-compatible signal line, via which the two printed circuit boards  11 ,  12 ′ are connected to one another after their installation. 
     In the situation in which the printed circuit boards  11 ,  12 ′ do not withstand, in the long term, the pressure exerted from underneath by the contact rocker  22  and become deformed, a mechanically robust holding plate  39 , which extends transversely over the contact-making area, can be provided, as shown in  FIG. 6 , on upper face of the respective printed circuit board  11  or  12 ′, and is screwed to the base plate  13 ′ via attachment holes  40  which penetrate as far as the base plate  13 ′. This may be necessary in particular for the printed circuit board  12 ′, because the lack of the stabilizing dissipation plate  16  in the area of the contact-making element  21  means that the remaining layer structure is less robust. 
     The contact-making element  21 , which is integrated in the base plate  13 ′ underneath the printed circuit boards  11 ,  12 ′, advantageously makes it possible, as shown in  FIG. 11 , to place the vertical separating wall  42  of an electrically conductive housing part  41  directly in the intermediate space  14  and over the complete area onto the base plate  13 ′, thus subdividing the space above the printed circuit board arrangement  20  into two screened spaces  43  and  44 , such that the circuit on the printed circuit board  11  is RF-isolated from the circuit on the printed circuit board  12 ′, and they are connected to one another only by the contact-making elements  21 . 
     For complete electromagnetic screening of the contact rocker even without housing part  41 , the base plate  13  may contain a further depression at the upper edge of the recess  25 , which depression holds a screening body  48 . This screening body  48 , preferably a screening plate, is designed such that it projects under the respective front edge of the printed circuit boards  11 ,  12 , and thus produces a coherent electrically conductive connection to the metallization on the printed circuit boards. For this purpose, the structure of the base plate  13  and the arrangement of the printed circuit boards  11 ,  12  can preferably be chosen to result in a horizontal plane, in order to avoid a step in the screening body  48  (the screening body  48 , with its layer, is shown by way of example by dashed lines in  FIG. 1 ). The printed circuit board arrangement shown in  FIGS. 3-6  and  11  is distinguished by the following characteristic properties and advantages:
         The connection between the printed circuit boards has very good RF characteristics.   As a result of the direct contact with the base plate, it allows optimum screening of the individual printed circuit boards ( FIG. 11 ).   No contact arms project at the side from the densely fitted printed circuit boards.   The connection is very position-tolerant: the contact force is independent of movements of the printed circuit boards on the board plane (x-y plane); the contact rocker and supporting body contribute to the build-up of the contact forces, because of their elastic characteristics.   The contact rocker and the supporting body can, to a certain extent, compensate for height tolerances of the printed circuit boards.   The standard rules can be applied without any restriction for the design of the circuitry of the printed circuit boards, and in particular the RF characteristics can be optimized.   The capability to transmit direct current via the contact-making elements  21  can be enhanced by the use of a plurality of vias on each side.   The mechanical robustness of PTFE printed circuit boards can be enhanced by the screwed-on holding plates  39 .   Different lengths of the connections can be implemented.   The capability to transmit direct current by the connections can be selected virtually without restriction.   The connections represent a virtually ideal 50-Ohm environment, without RF band limiting, up to the coaxial frequency limit (for example 4 GHz).   The connections are insensitive to any change in the position of the printed circuit boards during fitting, and there is therefore no need for any guide pins; tolerances can easily be coped with by appropriate design of the contact pads  18 .   The arrangement is mechanically robust.   The contact rocker is a simple stamped and bent part.   Because of the self-clamping, the fitting of the contact-making element  21  in the base plate is simple.   Before fitting, the contact-making elements  21  can be protected in a simple manner with a covering cap.   The printed circuit boards can be handled and fitted particularly easily: putting in place and screwing tight.   No specific placement sequence is required during construction of the printed circuit boards.   The modules can be repaired and replaced easily and quickly.   The effort required to provide the necessary recesses in the base plate and in the dissipation plate is minor.   Because of the earth line above the base plate  13  or  13 ′, no screening cover is required to test the connection (housing part  41  in  FIG. 11 ).       

     However, as is indicated by way of example in FIG.  10 —a contact-making element  21 ′ can also be integrated in the base plate  13  (or  13 ′) instead of the contact-making element  21  equipped with the contact rocker  22 , which contact-making element  21 ′ uses contact-making posts  45 ,  46 , for example as described in the initially cited document U.S. Pat. No. 6,231,352, and as commercially available. The two contact-making posts  45 ,  46  are soldered by the solder face onto a further printed circuit board  47 , which is in the form of a strip, and are thus electrically connected to one another. The unit comprising the printed circuit board  47  and the contact-making posts  45 ,  46  is inserted into the recess  25  such that the pressure contacts of the contact-making posts  45 ,  46  are pushed from underneath against the printed circuit boards  11  and  12 , making contact, when they are placed onto the base plate  13 . 
     Another embodiment of the printed circuit board arrangement according to the invention is illustrated in  FIGS. 7 to 9 : in this case, two identical contact-making elements  32  or  32   a  and  32   b  are used which are soldered, in each case rotated through 180°, onto a corresponding conductor track on the upper face of the printed circuit boards  11  and  12 ′, and project with a respective contact arm  35  or  35   a  and  35   b , which is sprung on the board plane, overlapping one another, into the intermediate space  14  ( FIGS. 8 ,  9 ). The contact-making elements  32 , which are preferably in the form of stamped and bent parts and are produced from spring bronze, have a U-shaped body  33  or  33   a ,  33   b , on whose limbs solder feet  38  are integrally formed. Starting from the body  33 , the contact arm  35  first of all extends to the rear, makes a 180° bend, and is passed out through the interior of the body  33  forwards, in order to allow an adequate length for the arm. A contact section  34  is formed at the free end of the contact arm  35 , comprising an approximately square contact surface to which an inclined element  36  which is bent outwards obliquely (through 30°-45°) is connected at the top, and a conductor element  37 , which is bent outwards at right angles, is connected at the bottom. 
     The contact arms  35   a ,  35   b  of the two contact-making elements  32   a ,  32   b  of one connection overlap and make contact with one another by pressure, which is exerted parallel to the board edge, in the intermediate space  14  with the square contact surface which, by virtue of its expansion, allows tolerance for position errors of the printed circuit boards  11 ,  12 ′. The inclined elements  36  make it easier to insert the two contact-making elements  32   a ,  32   b  into one another during the fitting of the printed circuit boards  11 ,  12 ′. The conductor elements  37  improve the electrical characteristics of the connection in conjunction with the outer conductor, which is passed over the base plate  13 ′. The recess in the area of the intermediate space in the contact arms  35   a ,  35   b  is also used for the same purpose. In the same way as other components, the contact-making elements  32   a ,  32   b  can be mounted on the printed circuit boards  11 ,  12 ′ using conventional surface mounted device solder technology, and can be provided on belts for appropriate automatic devices. They can be used to transmit signals up to a frequency of 4 GHz. Holding pins can be provided in order to increase the mechanical strength of the arrangement, and are plugged into corresponding holes on the printed circuit board. These holding bolts can either directly integrated in the stamped and bent body, or are integrated in a surrounding insulating body, in order to improve the RF characteristics. This insulating body can preferably be designed using plastic injection-moulding insert technology. 
     The characteristic features and advantages of this embodiment are as follows:
         The contact-making elements are very small and do not occupy much space.   The contact-making elements are symmetrical, and are the same for both printed circuit boards.   The contact-making elements are simple stamped and bent parts.   They can be partially plated.
           They do not require any changes to the base plate or the dissipation plate.   
           Because of the earth line which is provided above the respective base plate  13  or  13 ′, no screening cover is required for testing the connection (analogously to the housing part  41  in  FIG. 11 ).       

     A further exemplary embodiment of the invention is illustrated in  FIGS. 12-15 . The printed circuit board arrangement  50  in this exemplary embodiment is based on a rectangular base plate  53 , on whose upper face two printed circuit boards  51 ,  52  are detachably mounted (screwed on). The two printed circuit boards  51 ,  52  are designed to be of the same type, but are mounted on the base plate  53  in mirror image form with respect to one another. On the sides with which they abut against one another forming an intermediate space  54 , the two printed circuit boards  51 ,  52  are connected to one another for RF purposes by means of a contact-making element  60  which is arranged recessed in the base plate  53 . Each of the two printed circuit boards  51 ,  52  is designed as illustrated in  FIG. 14  and  FIG. 15 . An externally accessible connecting socket  57 ,  58  is arranged on the respective outer narrow faces of the printed circuit boards  51 ,  52 , and a corresponding line can be connected to these connecting sockets  57 ,  58 . A central conductor track  55 ,  56 , which runs in the longitudinal direction, leads to the other narrow face of the printed circuit board from the connecting socket  57 ,  58  on the upper face of the printed circuit board  51 ,  52 , on which narrow face it is conductively connected by means of a plated-through hole ( 56   a  in  FIGS. 14 ,  15 ) to a contact surface (pad)  56   b  which is provided on the lower face of the printed circuit board. The contact-making element  60 , which is arranged under the printed circuit board  51 ,  52 , makes contact with the contact surfaces  56   b  of the two printed circuit boards  51 ,  52 , producing a conductive connection between the two printed circuit boards  51 ,  52 . 
     The contact-making element  60  is designed as shown in  FIG. 13(   b ). A contact body  67  is provided within the contact-making element  60  for making contact with and conductive connection of the two pads ( 56   b ) on the printed circuit boards  51 ,  52 , which contact body  67  is in the form of a stamped-and-bent part and has a base body  67   a  from which two contact tongues  67   b ,  67   c , which are bent back upwards, extend in mirror-image form on opposite sides. The contact body  67  is mounted in an electrically isolated form and fixed (not rocking) in a supporting body  66  which is in the form of a trough and is itself inserted into a corresponding recess  65  in the base plate  53 . 
     The two printed circuit boards  51 ,  52  are each provided with holes  63 , which are arranged in a distributed manner, and which correspond with threaded holes  64  in the base plate  53 . The printed circuit boards  51 ,  52  are mounted on the base plate  53  by means of a plurality of screws  59 , which are passed through the holes  63  in the printed circuit boards  51 ,  52  and are screwed into the threaded holes  64 . Appropriate recesses  61 ,  62  are provided on the base plate  53  in order to create space for the connecting socket  57 ,  58  which projects downwards out of the printed circuit boards  51 ,  52 . 
     The geometry of the arrangement  50  is chosen such that the contact surfaces  56   b  which are located on the lower face of the printed circuit boards  51 ,  52  which are mounted on the base plate  53  press in a sprung manner downwards against the contact tongues  67   b, c  of the contact-making element  60  and thus create a secure and permanent electrical contact via the contact body  67  between the two printed circuit boards  51 ,  52 , to be precise the two conductor tracks  55 ,  56 . 
     International Patent Application PCT/EP2010/056733, filed 17 May 2010 and Swiss Patent Reference 838/09, filed 2 Jun. 2009, the priority documents corresponding to this invention, to which a foreign priority benefit is claimed under Title 35, United States Code, Section 119, and their entire teachings are incorporated, by reference, into this specification.