Abstract:
A sensor unit with at least one magnetic field sensor element, at least one application specific integrated circuit (ASIC) and at least one magnet are arranged in a capsule housing on at least one leadframe. The capsule housing is surrounded by a sensor encapsulation and is intended to have smaller dimensions so as to allow it to be positioned in the encapsulating mold with less effort and at the same time in the exact position. For this purpose, it is provided that the capsule housing has a number of fixing aids adapted in their outer dimensions to an injection mold assigned to the sensor encapsulation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase application of PCT International Application No. PCT/EP2008/067135, filed Dec. 9, 2008, which claims priority to German Patent Application No. 10 2007 060 604.6, filed Dec. 13, 2007, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a sensor element with at least one Application Specific Integrated Circuit (ASIC) and at least one magnet on at least one leadframe, all the elements being arranged in a capsule housing that is surrounded by a sensor encapsulation. 
       BACKGROUND OF THE INVENTION 
       [0003]    One of the application areas for magnetic field sensor elements is, for example, that of motor vehicles. Among their tasks in this area is use in air springs for determining the level of the vehicle in a level measuring device. DE 100 23 622 B4, which is incorporated by reference, describes in this respect the use of the magnetic field strength of a permanent magnet, sensed by a magnetic field sensor element, for level determination. 
         [0004]    Similarly, magnetic field sensor elements may be used for controlling a reciprocating-piston internal combustion engine, such as that typically used in a motor vehicle. DE 10 2005 035 881 A1 and EP 1 310 634, which are incorporated by reference, show the use of magnetic field sensor elements for sensing the rotational angular position of a camshaft, from which the necessary values for adjusting the camshaft in relation to the crankshaft can be calculated. 
         [0005]    Magnetic field sensors may also be used for sensing the surroundings of the motor vehicle as a component part of a safety device. In this case, items of image information, such as those described in DE 102 12 483 A1, which is incorporated by reference, from the surroundings of the motor vehicle, for example of stationary and/or movable objects and/or road users, are recorded and assessed, the assessment possibly leading to triggering of one or more airbags, dependent on the seating occupancy in the motor vehicle. Coupling with a parking system is also possible. 
         [0006]    Magnetic field sensor elements are used, for example, for determining the wheel speed, that is to say the rotational speed of a wheel. These are known as wheel speed sensors. 
         [0007]    The speed determination typically takes place by using three elements: 
         [0000]    a sensor with a sensor head
 
a magnetic track (encoder)
 
an air gap between the two hardware elements.
 
         [0008]    As a result of the way in which they are designed, magnetic field sensor elements have only a small range. Therefore, to function well, they should be positioned as close as possible to the recording magnetic track, known as the encoder. 
         [0009]    Wheel speed sensors usually operate with two different types of encoder: 
         [0010]    Ferromagnetic Encoder 
         [0011]    The ferromagnetic encoder influences the magnetic field that is generated by the magnet of the sensor element. 
         [0012]    Magnetic Encoder 
         [0013]    The magnetic encoder generates its own magnetic field. It typically consists of elastomer with a so-called filler comprising a ferrite content of 20%. 
         [0014]    The advantages of a magnetized encoder are that it can be easily integrated in other mechanical components and/or subassemblies, preferably in the seal of the wheel bearing. It can, however, also assume an additional, independent function as a seal of the wheel bearing. On account of their technology, the sensor elements for the magnetized encoder require little space, and therefore make a particularly small sensor head possible. 
         [0015]    There are known wheel speed sensors that have their housed magnetic field sensor elements, that is to say bridges, dies, ASICs, magnets or leadframes, encapsulated in epoxy, and for positionally exact positioning have to be fitted in prefabricated plastic or plastic-metal inserts known as carriers, or in a so-called pre-molds performing the task of pre-encapsulation. 
         [0016]    These carriers or pre-molds are held in the encapsulating mold exactly in position for the sensor unit with the magnetic field sensor and (read head), in order to ensure the positional exactness of the sensor element during the encapsulation to form the finished sensor. The necessary sealing of the sensors in the region of the carrier holding points is achieved here by the plastics material bonding for the encapsulation of the sensor unit as well as by the forming of a number of melt ribs at the holding points. 
       SUMMARY OF THE INVENTION 
       [0017]    The invention relates to providing a prefabricated magnetic field sensor element of the stated type which, with smaller dimensions, can be positioned in the encapsulating mold with less effort and at the same time in the exact position. 
         [0018]    An object of the present invention is achieved according to aspects of the invention by the capsule housing having a number of fixing aids adapted in their outer dimensions to an injection mold assigned to the sensor encapsulation. 
         [0019]    The invention is based on the idea that, for the final encapsulation of the sensor element, it is necessary to have fixing aids which ensure the exact position of the sensor element by supporting themselves in the mold of the final sensor encapsulation. 
         [0020]    In order to achieve production at the lowest possible cost, it is intended to dispense with unnecessary intermediate production steps or components for positioning the housed sensor element. A standard component such as the carrier used for positionally exact positioning can be replaced by the complete sensor unit being fixed in the encapsulating mold in that a number of fixing aids, which comprise at least one sealing component for the final encapsulation, advantageously being arranged on the capsule housing. 
         [0021]    In order that a standard component without arranged fixing aids can be used in the production for the sensor unit, whereby the costs incurred can be lowered, said fixing aids are advantageously subsequently placed on or adhesively attached to the capsule housing in the manner of pairs of shells. 
         [0022]    To allow a standard component to be used for the sensor unit, and consequently to allow costs to be saved and/or not to be tied to a predetermined design of the housed sensor element in the possible event of problems in terms of space, the fixing aids are advantageously not arranged on the capsule housing itself but on the leadframe connected to the capsule housing. 
         [0023]    In order to increase the service life of the cable connection between the magnetic field sensor element and the assigned electronics in the motor vehicle and, to the greatest extent, avoid loading of the contacts by tensile loading, for example during maintenance work or as a result of external influences in road traffic, as well as to provide alternative fixing for the encapsulation, the leadframe and/or the cable for the connection of the sensor connector pin advantageously comprises a tension relief, it being possible for this also to be a fixing aid that is formed as a tension relief. 
         [0024]    In order that a sealed connection can be achieved between the coating of the capsule housing and the later encapsulating compound of the capsule housing, and that no moisture penetrates at the contact surfaces, the same material is advantageously provided for the coating of the capsule housing and for the sensor encapsulation, this material expediently being polyamide (PA), polyurethane (PU) or PBT. 
         [0025]    The advantages achieved with the invention are, in particular, that the capsule housing is positioned by the integrated positioning aids directly and in the exact position without further auxiliary means, which could comprise additional components and/or supporting elements on the mold, or any corresponding intermediate encapsulation in the sensor encapsulating mold, and consequently a sensor with smaller dimensions can be used. Furthermore, the integrated positioning aids provide good preconditions for sealing of the sensor, since it is completely sealed by the encapsulation, and the contacting area of the encapsulated sensor unit with the mold contour of the outer encapsulation is particularly small. Therefore, no external protection against moisture and/or dirt is necessary. Servicing and/or maintenance work as well as exchange or removal of the wheel bearing are also possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention is best understood from the following detailed description when read in connection with the accompanying drawings. Included in the drawings is the following figures: 
           [0027]      FIGS. 1   a )- 1   c ) shows the construction of a wheel speed sensor and sensor head of a conventional type in different, schematic views, 
           [0028]      FIG. 2  shows the sensor element (according to aspects of the invention) with positioning aids in section, 
           [0029]      FIG. 3  shows an alternative arrangement of the elements in section, 
           [0030]      FIG. 4  shows alternative fixing of the leadframe in section, and 
           [0031]      FIG. 5  shows the wheel speed sensor with the sensor head in section. 
       
    
    
       [0032]    The same parts are provided with the same designations in all the figures. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]      FIGS. 1   a ) to  c ) schematically show, by way of example, a wheel speed sensor system  1  of a conventional type of design from various views. Between the encoder  2 , which may be configured as a magnetized encoder or as a ferromagnetic encoder, and the sensor head  4  with the sensor unit  5 , which comprises the magnetic field sensor element  6  (read head) and the application specific integrated circuit(ASIC)  7 , there is an air gap  8 . Since the sensor unit  6  has only a limited range, it should be positioned as close as possible to the magnetic track, that is to say the encoder  2 . 
         [0034]    In  FIG. 1   a ), a sensor head  4  of a standard type of design is represented as an example. It comprises the sensor unit  5  with a magnetic field sensor element  6 , a number of electrical conductors  12  from/to the magnetic field sensor element  6 , which are led out from the sensor head  4  in a protective sheathing  14 , and a carrier  16 , which is intended to ensure the positionally exact positioning of the magnetic field sensor element  6  in the sensor head  4 . 
         [0035]    The electrical conductors  12 , which are usually led out as cables, are connected to the carrier  16 , for example by means of crimp contacts  10 . As a result, tension relief is achieved, since no forces can act on the sensor unit  5  even when the electrical conductors  12  led out from the sensor head  4  are subjected to tensile loading, for example during servicing work and/or exchange, but instead these forces are already intercepted at the carrier  16 . 
         [0036]    The sensor unit  5  with the magnetic field sensor element  6 , the carrier  16  and the number of electrical conductors  12  are surrounded by an encapsulation  18 , which serves for protecting the sensor head  4  and the sensor unit  5  from moisture and dirt. 
         [0037]    During the injection of the enclosing material into the mold for the encapsulation of the sensor unit  5 , it is important to take care that the sensor unit  5  is positioned exactly in the sensor head  4 . As  FIG. 2  shows, the sensor head  4  is fastened to the carrier  16 . However, the carrier  16  also requires a positioning aid in the mold for the encapsulation  18 , in order to ensure the positional exactness of the sensor unit  5  in relation to the encoder  2 . For this purpose, according to the current state of the art, the carrier  16  is held by means of pins  19  for the duration of the encapsulation. These pins  19  are withdrawn after encapsulation has taken place. 
         [0038]      FIG. 2  illustrates how the sensor unit  5  can be positioned in the sensor head  4 . However, by contrast with the prior-art versions that are represented in  FIGS. 1   a ) to  c ), the sensor head  4  is already the version according to aspects of the invention with a magnetic field sensor element  6  and an ASIC  7  integrated in the sensor head  4 . 
         [0039]    The sensor unit  5  is in a capsule housing  20  and usually comprises a number of sensor elements, ASICs, leadframes, magnets and, for certain applications, capacitors, this capsule housing  20  being positioned exactly in the sensor head  4  in order to set the distance between the encoder  2  (not represented here) and the magnetic field sensor  6  (read head) of the sensor unit  5  as optimally as possible for reading. For this purpose, fixing aids  22  are provided on the housing  20 . These fixing aids may be molded onto the capsule housing  20 , that is to say provided in the same method step as the production of the housing  20 . 
         [0040]    Alternatively, the fixing aids  22  may be adhesively attached to the housing  20  as a separate housing part, in the manner of pairs of shells, or be provided as an insert in the injection mold for the encapsulation of the sensor unit  5 . 
         [0041]    In order to achieve sealing of the encapsulation  18  from external influences during the operation of a motor vehicle, the capsule housing  20  with arranged fixing aids  22  should be coated with a suitable material, for example with PU. This is so since the capsule housing  20  usually consists of epoxy resin, so that later encapsulation requires coating of the capsule housing  20  in order to achieve sealing between the capsule housing  20  and the encapsulation  18  consisting of thermoplastic material. This coating should preferably be of the same material or a softer material than the later encapsulation  18 , in order that a kind of fusion can be achieved between the two materials. 
         [0042]    Furthermore, the encapsulation  18  should enclose the fixing aids  22  with an amount of play of at most 2/10 mm between the outer end of the fixing aids  22  and the delimitation of the encapsulation  18  or a very small oversize of the encapsulation  18  with respect to the fixing aids  22 , that is to say with elastic deformation of the fixing aids  22 , in such a way that pressure exertion and/or pressing of the sensor unit  5  is avoided when the encapsulating mold is closed. For this purpose, a conical, outwardly tapering shape of the fixing aids  22  may also be provided, so that the outward contact locations are also kept as small as possible. This is so since hard pressing could otherwise damage or destroy sensitive electronics inside the encapsulation. 
         [0043]    Furthermore, with an amount of play between the fixing aids  22  and the encapsulating mold it is ensured that complete encapsulation, including of the fixing aids  22 , is secured at least on one side of the capsule housing  20 , and sealing, for example against moisture penetration, is achieved at this location by the encapsulation  18 . 
         [0044]    On the opposite side, the fixing aid  22  could be pressed against the wall of the mold by the encapsulating material that is led into the mold under pressure during the encapsulating operation, whereby contact with the mold, and consequently a leakage point, may occur at this location. Therefore, coating is likewise advantageous for such a case. 
         [0045]    Suitable methods for ensuring sealing of the later component as a whole by means of coating of the capsule housing  20  may be: 
         [0000]    coating with PU (polyurethane) by dipping or spraying,
 
mounting of PU shells or in the form of a tube, or
 
mounting as a mesh, neither of the variants of the use of PU shells being represented here, or
 
use of the material used later for the encapsulation  18  of the sensor unit  5  as coating material instead of PU, for example polyamide.
 
         [0046]    Alternatively, or in addition to the fixing aids  22  arranged on the capsule housing  20 , a fixing aid  26  may also be arranged on the leadframe  24 . This fixing aid  26  may be provided as a finished component or may be molded on, adhesively attached or clamped on. For this purpose, it may, for example, be pushed on from the side. As a result of the arrangement of the fixing aid  26  on the leadframe  24 , a low-cost standard encapsulation could be chosen for the capsule housing  20 , even if it is does not have the fixing aids  22  arranged on the capsule housing  20 . Since the leadframe  24  is fixedly connected to the capsule housing  20  of the sensor unit  5 , the fixing aid  26  satisfies the preconditions that are required for positionally exact positioning of the sensor unit  5  in the sensor head  4 . 
         [0047]    Furthermore, a fixing aid  28  on the leadframe  24  is conceivable. This may likewise be arranged on the leadframe  24  by molding on, adhesive attachment or clamping on. The geometrical form of the fixing aids  28  satisfies two requirements: it is formed in such a way that it has receptacles for mold pins  30  and it is held by means of these mold pins  30  exactly in position for the encapsulating operation. The mold pins  30  are withdrawn again after the encapsulating operation has taken place. 
         [0048]    Furthermore, after the external encapsulation has taken place, the fixing aid  28  ensures tension relief, even under tension on the cable lead  12  led out from the sensor head  4  and/or on the leadframe  24 . Even if the leadframe  24  is provided with a contact connector for the connection to the systems of a motor vehicle, tensile and compressive loading in the longitudinal direction of the leadframe  24  may occur during servicing work and/or an exchange, such loading being kept away from the sensor unit  5  by the shaping of the fixing aid  28 . 
         [0049]      FIG. 3  shows an arrangement of the components that is an alternative to the arrangement shown in  FIG. 2 , further embodiments and forms of arrangement being conceivable. 
         [0050]    In  FIG. 4 , a fixing aid  34  for arrangement on the leadframe  24  that is an alternative to the fixing aid  28  is described. Elevations  32  are provided in the encapsulating mold, with the effect of creating a clearance in the encapsulation  18  during encapsulation. The fixing aids  34  represented in  FIG. 4  ensure that the elevations  32  of the encapsulating mold have the effect that the leadframe  24 , and consequently the sensor unit  5 , is positioned and held exactly in position transversely in relation to the longitudinal direction of the leadframe  24  during the encapsulating operation. 
         [0051]      FIG. 5  schematically represents the interplay of the individual elements. The sensor head  4  is positioned in such a way that it is separated from the encoder  2  by a suitably dimensioned air gap  8 . The sensor unit  5  is suitably positioned on the capsule housing by means of the fixing aids  22 . The fixing aids  28  on the leadframe  24  ensure both positionally exact positioning in the production process and tension relief on the finished component. 
         [0052]    However, after withdrawal of the mold pins  30  from the encapsulation  18 , a leakage point could now occur at the contact locations between the fixing aid  28  and the mold pins  30 . Therefore, sealing should be achieved for said contact locations by coating of the sensor unit  5 . In the event that this coating does not achieve sealing, the openings  40  produced by the means for leading the mold pins  30  should be closed by a suitable measure.