Patent Document

CROSS-REFERENCE TO A RELATED APPLICATION 
     The invention described and claimed hereinbelow is also described in German Patent Application DE 10 2007 031 980.2 filed on Jul. 10, 2007. This German Patent Application, whose subject matter is incorporated here by reference, provide the basis for a claim of priority of invention under 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to a connection unit for a pressure measuring cell according to the general class of the independent claim. Publication DE 10 2004 033 846 A1 makes known a valve comprising an integrated pressure sensor. A pressure sensor is integrated in a valve for controlling fluids, e.g., for brake systems of a motor vehicle, in order to measure the pressure of the fluid that is controlled by the valve. The pressure of the hydraulic fluid is transferred to the measuring diaphragm of the measuring cell via a bore hole in the pole core of the valve dome. Several through-bores are formed in a printed circuit board, through which the electrical contacts of the pressure sensor and/or the electrical part of the valve are guided. 
     A braking device comprising an integrated pressure sensor module is made known in DE 101 22 330 A1. It includes an assembled control device, in the case of which a first pluggable housing unit—which mainly encloses the electronic components on one or more component carriers—is plugged together via a block-shaped solid part to a first surface of the solid part in order to establish a magnetic and electrical connection, the solid part including magnetically actuated hydraulic valves for controlling brakes and hydraulic lines. Pressure sensors which are integrated in the assembled control device are provided for measuring the pressure in the hydraulic lines at suitable measuring points. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a connection unit (4), a pressure measuring cell, which facilitates miniaturization while retaining the use of conventional production methods. 
     The connection unit, according to the present invention, for a pressure measuring cell having the features of the independent claim has the advantage that it is particularly compact in design. The assembly is therefore mechanically very robust, which has positive consequences in particular in terms of replacing a built-on unit that is connected to the pressure measuring cell. Furthermore, the assembly results in a separable connection between the pressure measuring cell and the built-on unit. The compact design of the connection unit for the pressure measuring cell may also reduce manufacturing costs. 
     In an advantageous further development, the contacting between the pressure measuring cell and the circuit carrier, and/or between the circuit carrier and the printed circuit board/electronic components is established via conductive adhesive points. In particular, the design of the contacting using conductive adhesive domes ensures that the conductive adhesive contacting is secure even under fluctuating operating temperatures. 
     In an advantageous further development, the circuit carrier includes an opening above the pressure measuring cells, into which a sealing means, preferably a gel, may be applied. This sealing means is used as a passivation measure in order to protect the measuring bridge of the pressure measuring cell from contamination and corrosion. A gel that is elastic and does not affect the sensitivity of the measuring diaphragm of the pressure measuring cell is particularly well suited for this purpose. 
     In an advantageous further development, guide means which support an exact positioning of a printed circuit board on the circuit carrier are provided. Appropriate segments on the circuit carrier that engage in the associated recesses or openings in the printed circuit board are well suited in particular for this purpose. Mainly, this simplies the installation of very small components. A fixing adhesive may be applied to the top side of these guide means, which are designed, e.g., as a segment, to permanently attach the printed circuit board to the circuit carrier. The conductive adhesive domes are located next to the centering segment, via which the printed circuit board is contacted in an electrically conductive manner to the pressure measuring cell and/or the connection contact points. The conductive adhesive domes and the underside of the printed circuit board are therefore separated by a defined distance. 
     In an advantageous further development, the circuit carrier preferably includes three-dimensional, external current carrying traces. Via these current carrying traces, the through-contacting between the pressure measuring cell to the printed circuit board and from the printed circuit board to the connection contacts is established. This dual function of the circuit carrier, i.e., to accommodate the printed circuit board, and to contain current carrying traces, further facilitates the miniaturization of the connection unit for a pressure measuring cell. The circuit carrier is preferably manufactured using MID-2K technology, i.e., the injection-molded circuit carrier (“molded interconnect device”) is composed of two components, e.g., a galvanizable plastic that is partially enclosed in a coating of a second, non-galvanizable plastic applied via injection molding. The partially projecting surfaces of the preliminary injection-molded article are coated with a metallic surface using a galvanic process, thereby producing the external current carrying traces. The use of an injection-molded circuit carrier is particularly well-suited for use in the present application since it facilitates the miniaturization of the connection unit for the pressure measuring cell, thanks to the improved freedom in terms of shape, and thanks to the integration of electrical and mechanical functions. 
     Alternatively, this circuit carrier may also be manufactured using laser direct structuring MID. In this case, the circuit carrier is composed of an injection-molded part, in which the locations of the current carrying traces are structured using a laser, which are then coated with a metallic surface using a galvanic process, thereby producing the external current carrying traces. 
     According to an advantageous further development, bonded contact means, preferably contact rivets, which are used as contact surfaces for a separable electrical connection are attached using conductive adhesive. This simplifies the manufacture, and makes it possible to create a detachable connection of the pressure sensor and the built-on control device. 
     In an advantageous further development, a preferably metallic protective sleeve is provided, which, as mechanical protection, prevents the pressure measuring cell from being bent, in particular when it is replaced, and it diverts overvoltages (e.g., ESD pulses) to the fastening flange. This increases the robustness of the system both mechanically and electrically, to equal extents. 
     Further advantageous refinements result from the further dependent claims and the description. 
     An embodiment of the connection unit for a pressure measuring cell is depicted in the drawing and is described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a fastening flange with a welded-on pressure measuring cell, 
         FIG. 2  shows a circuit carrier in a view from below, 
         FIG. 3  shows a printed circuit board with the components installed, before it is bonded to the circuit carrier, 
         FIG. 4  shows the back side of the printed circuit board with the associated circuit carrier, in another view, 
         FIG. 5  shows the circuit carrier with the printed circuit board bonded to it, 
         FIG. 6  shows the centering segment for fixing the printed circuit board in position on the circuit carrier, and 
         FIG. 7  shows a cross section through the connection unit, with the protective sleeve in place. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The hydraulic pressure of a solenoid-valve controlled fluid in a brake system is converted to an electrical signal via a pressure measuring cell  14 . To this end, pressure measuring cell  14  is placed on a fastening flange  17  in such a manner that a diaphragm of pressure measuring cell  14  is deformed depending on the pressure of the fluid. The deformation of the diaphragm is detected by a measuring bridge  19 . Pressure measuring cell  14  is composed of a carrier  15  which is substantially tubular in design. Furthermore, carrier  15  includes a flange in its middle section, which is primarily annular in design. To connect additional components in correct positions, at least one section of the flange of carrier  15  includes a preferably rectangular projection. On the side of pressure measuring cell  14  far from the pole core when in the installed state, four electrical contact points  18  are provided on measuring bridge  19 , via which it is possible to tap the output signals from measuring bridge  19 . Carrier  15  of pressure measuring cell  14  is designed such that its inner side interacts in a form-fit manner with the top side of the pole core and with fastening flange  17 . Carrier  15  is placed on fastening flange  17  and is connected thereto in a pressure-tight manner via welding, preferably laser welding  16 . 
     A circuit carrier  20  is placed on pressure measuring cell  14 . Circuit carrier  20  is substantially cylindrical in design and has a rectangular middle section for receiving printed circuit board  32 . The inner contour of the lower section of circuit carrier  20 —as indicated above with reference to FIG.  2 —is designed to be mechanically connected to the outer contour of pressure measuring cell  14 , and it encloses it at least partially. For this purpose, circuit carrier  20  includes a shoulder in the lower region of the cylindrical contour, which is situated on the flange of carrier  15  when in the installed state. Four domes  31  are provided for the purpose of establishing electrical contact between contact points  18  of pressure measuring cell  14  and current carrying traces  47  of circuit carrier  20 ; conductive adhesive is applied to the top side of domes  31  for establishing electrical contact between pressure measuring cell  14  and circuit carrier  20  and mechanically fastening them in position. The lower cylindrical region of circuit carrier  20  is closed by two diametrically opposed centering segments  37  which overhang in the axial direction opposite to pressure measuring cell  14 . In addition, an opening  60  is formed in the lower region of the lower cylindrical section of circuit carrier  20 , into which a sealing means  59 , e.g., a gel, may be applied, in order to protect measuring bridge  19  located underneath it against environmental influences. In the upward direction, the cylindrical region of circuit carrier  20  transitions into a rectangular region, on one side of which a central segment  41  is provided in the middle section in the axial direction, on the external side of which a fixing adhesive  43  may be applied for attaching printed circuit board  32  to circuit carrier  20 . In addition, eight conductive adhesive domes  45  are provided on the side of the rectangular region of circuit carrier  20  on which printed circuit board  32  is accommodated. Domes  45  project slightly outwardly relative to the rectangular body of circuit carrier  20 . They are coated with conductive adhesive to establish electrical contact between printed circuit board  32  and current carrying traces  47 . 
     Circuit carrier  20  becomes cylindrical in shape toward the top. Four openings  63  are provided, which are used to receive contact means  51 , which are designed in the form of rivets. Openings  63  are provided with conductive adhesive surfaces  49  in order to establish contact between contact means  51  and particular current carrying traces  47  using conductive adhesive. A central centering segment  38  is provided on the top, cylindrical region of circuit carrier  20 , as an extension of central segment  41 ; central centering segment  38  interacts with a central centering aid  36 , i.e., a preferably U-shaped recess in printed circuit board  32 . 
       FIG. 3  shows printed circuit board  32  with components installed, on which electronic components  26 , such as an integrated circuit, resistors, capacitors, etc., are located. In the lower region, printed circuit board  32  includes lateral recesses  34  which interact with external centering segments  37  of circuit carrier  20 . A recess which is used for centering purposes is also provided in the lower, middle region of printed circuit board  32 . 
       FIG. 4  shows the back side of printed circuit board  32 . Eight conductive adhesive surfaces  53  are provided, via which four incoming signals from pressure measuring cell  14  are guided, as are the four outgoing signals that are forwarded to contact means  51 . Current carrying traces  47 , which are external in design, are shown in this view, in the rectangular region of circuit carrier  20 . Via current carrying traces  47 , conductive adhesive surfaces  53  of printed circuit board  32  are connected to contact points  18  of measuring bridge  19 , and the four other conductive adhesive surfaces  53  are connected above them to four contact means  51  and their conductive adhesive surfaces  49  in an electrically conductive manner. 
       FIG. 5  shows printed circuit board  32  in the installed state.  FIG. 5  also shows opening  60  which is used to receive sealing means  59 , e.g., a gel. Contact means  51  are also inserted into appropriate openings  63  in circuit carrier  20 . 
       FIG. 6  shows more clearly how middle centering segment  38  interacts with middle centering aid  36  of printed circuit board  32 , and how outer centering segments  37  interact with external centering aids  34  on printed circuit board  32 .  FIG. 6  also shows that middle centering segment  38  increases in terms of its outer circumference in the direction toward the rectangular section of circuit carrier  20 , thereby making it easy to insert printed circuit board  32 . At the end stop, the exact position of printed circuit board  32  relative to circuit carrier  20  is reached. 
       FIG. 7  shows the components in the installed state. In this state, circuit carrier  20  is mechanically and fixedly connected to fastening flange  17  via a fixing adhesive  43 . Pressure measuring cell  14  encloses fastening flange  17  toward the top, and it is located on a shoulder thereof in such a manner that the fluid pressure directed through a bore is converted into appropriate output signals of measuring bridge  19  via the diaphragm (not depicted) of the pressure measuring cells. Circuit carrier  20  encloses pressure measuring cell  14  at least partially from the top. Opening  60  is located in circuit carrier  20 , pointing upwardly, and it closes the cylindrical lower region of circuit carrier  20 . A sealing means  59  is inserted into opening  60 . Furthermore, contact points  18  of measuring bridge  19  are each contacted in an electrically conductive manner via conductive adhesive  55  to an external current carrying trace  47  of circuit carrier  20 . Printed circuit board  32  is connected via fixing adhesive  43  to middle segment  41  of circuit carrier  20 . The electrical contacting of printed circuit board  32  to current carrying traces  47  takes place via conductive adhesive  55  that is applied to the top sides of domes  45 . Four openings  63  are provided in the upper cylindrical region of circuit carrier  20  for receiving rivet-shaped contact means  51 . Contact means  51  are connected to circuit carrier  20  via conductive adhesive  55 . A current carrying trace  47  is directed to each of the four contact means  51 , and the electrically conductive contact is established via conductive adhesive surfaces  49  using conductive adhesive  55 . Furthermore, a protective sleeve  57  is provided; protective sleeve  57  encloses circuit carrier  20  with printed circuit board  32  on which components have been installed. Protective sleeve  57  is connected via laser welding  16  to fastening flange  17 . Contact means  51  are still accessible, however. Installation aids  61  extend in the axial direction on the top side of circuit carrier  20 . Installation aids  61  make it easier to slide protective sleeve  57  over circuit carrier  20  via a guide that is beveled appropriately. 
     The connection unit for a pressure measuring cell  14  depicted in the figures functions as described below. The hydraulic pressure in a brake system is converted to an electrical signal via pressure measuring cell  14 , which is installed, e.g., on the solenoid valve. The electrical signal is prepared by an integrated circuit as an example of an electronic component  26 , and it is amplified and forwarded to a control device. Pressure measuring cell  14  includes a diaphragm that detects the pressure of the fluid. A fluid-filled channel located in the interior of fastening flange  17  is formed in the interior of the pole core of the valve. This channel is closed in a pressure-tight manner via the diaphragm. The pressure conditions of the diaphragm are converted into appropriate electrical signals by measuring bridge  19 . Fastening flange  17  is permanently mounted in the hydraulic block using a self-clinch method. 
     Circuit carrier  20  is preferably designed using MID-2K technology. It is manufactured as a plastic injection-molded circuit carrier  20  using MID (molded interconnect device) technology, in particular using two-shot molding. Using MID technology, it is possible to realize three-dimensional circuit structures. A preliminary injection-molded article composed of a galvanizable plastic is partially covered with a second, non-galvanizable plastic via injection molding. The partially projecting surfaces of the preliminary injection-molded article are coated with a metallic surface using a galvanic process, thereby producing three-dimensional, external current carrying traces  47 . Circuit carrier  20  is characterized by a great deal of freedom in terms of shape, in particular via the integration of electrical and mechanical functions. As a result, the connection unit for pressure measuring cell  14  may be particularly compact in design. 
     Three-dimensional, external current carrying traces  47  are now used to establish contact between contact points  18  of measuring bridge  19  and printed circuit board  32 . The electrical contact between particular current carrying traces  47  and contact points  18  is established using a conductive adhesive  55 , as indicated in  FIG. 7 . Particular current carrying traces  47  are contacted in an electrically conductive manner to conductive adhesive surfaces  53  on the back side of printed circuit board  32  via conductive adhesive domes  45 . In the same manner, the output signals from the integrated circuit are guided from printed circuit board  32  to contact means  51 . In turn, the output signals are contacted to conductive adhesive domes  45  using conductive adhesive  55  via corresponding conductive adhesive surfaces  53 . External current carrying trace  47  which is electrically contacted to particular dome  45  is directed on the middle section in the direction toward the particular openings in upper, cylindrical region of circuit carrier  20 . In turn, the outsides of openings  63  are designed as conductive adhesive surfaces  49 , in order to ensure that electrical contacting exists between particular current carrying traces  47  and contact means  51 . In turn, the electrical contacting between contact means  51  and conductive adhesive surfaces  49  is established via conductive adhesive  55 . Contact means  51 , which are designed as contact rivets, are the contact surfaces for a separable electrical connection. The counterpart, which is not shown, is composed, e.g., of four contact springs or spring pins which are anchored in the electrical control device. 
     Welded-on protective sleeve  57  is composed of metal and is used to prevent pressure measuring cell  14  from being bent; it may also divert overvoltages, e.g., ESD pulses, to fastening flange  17 . Three pyramid-shaped installation aids  61  are used as installation-centering tools to ensure that protective sleeve  57  is joined well. 
     The above-described connection unit for a pressure measuring cell  14  is suited for use in particular in brake systems in motor vehicles, although it is not limited thereto. A large number of pressure sensors is used where installation space is limited, in premium-class brake systems in particular. A pressure sensor that requires a minimum of installation space is therefore well suited for use for this application.

Technology Category: b