Abstract:
A method for producing an electronic assembly in which an electronic component supported on a wiring support is encapsulated with an encapsulation material, the method including: -arranging the electronic component on the wiring support in such a manner that a stress applied onto the electronic component by the encapsulation material falls below a predetermined value; and -encapsulating the electronic component with the encapsulation material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Application of PCT International Application PCT/EP2013/076134, filed Dec. 10, 2013, which claims priority to German Patent Application No. 10 2012 223 982.0, filed Dec. 20, 2012, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for producing an electronic assembly and to an electronic assembly. 
       BACKGROUND OF THE INVENTION 
       [0003]    An electronic assembly having an encapsulated electronic component (referred to in the document as an encapsulated structure), which is held on a substrate in the form of a printed circuit board having an electrical circuit and is interconnected via bond wires with the electrical circuit, is known from DE 10 2007 060 931 A1, which is incorporated by reference. The substrate having the electronic component is fixed in a housing, which is potted using an embedding material. The potted housing is closed using a cover which is used as a shield. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the present invention improves the known electronic assembly. 
         [0005]    According to one aspect of the invention, a method for producing an electronic assembly, in which an electronic component supported on a wiring carrier is encapsulated using an encapsulation material, comprises the steps of arranging the electronic component on the wiring carrier such that a stress introduction through the encapsulation material onto the electronic component falls below a predetermined value, and encapsulating the electronic component using the encapsulation material. 
         [0006]    The specified method is based on the consideration that the electronic assembly mentioned at the outset is dimensioned very large, which is because the electronic component is first independently encapsulated and subsequently must be embedded using a further material for the fixing in a housing. Accordingly, the structures require a very large amount of installation space, whereby the electronic assembly of the type mentioned at the outset is unsuitable for technical applications having little installation space, for example, as a surface-mounted device, referred to hereafter as an SMD. 
         [0007]    In contrast, the specified method is based on the idea of using the embedding of the electronic component directly as a housing, so that an extra housing around the embedding can be omitted. This would noticeably reduce the required installation space for the entire electronic assembly. 
         [0008]    However, this concept is based on the finding that the embedding must have a specific elasticity and/or viscosity to prevent an excessively high mechanical stress introduction onto the electronic assembly and/or its electrical connections such as bond wires, because this mechanical stress introduction could electrically and/or mechanically damage the electronic assembly and/or the electrical connections, and thus result in corresponding malfunctions. 
         [0009]    It is therefore the concept of the specified method, when preparing the electronic assembly to be produced, to take the stress introduction into consideration in its dimensioning and to take suitable precautions so that the stress introduction remains below a limiting value, at which the function of the entire electronic assembly can no longer be reliably ensured. Examples of how the stress introduction can be taken into consideration in the dimensioning of the electronic assembly are specified in the dependent claims. 
         [0010]    The predetermined value for the stress introduction can result directly or indirectly during the dimensioning in this case. A direct specification can be implemented, for example, in the scope of a simulation, in which the predetermined value is directly predefined. An indirect specification can be implemented, for example, on the basis of an array of comparison tests, in the scope of which only one result to be expected, for example, a specific failure safety, is predefined and from which the predetermined value then implicitly results. 
         [0011]    In a refinement of the specified method, the encapsulation material has a coefficient of thermal expansion which is in the range of a coefficient of thermal expansion of the electronic component, so that the encapsulation material and the electronic assembly encapsulated thereby expand in the same manner and therefore apply no or only minimal mechanical stress in relation to one another. 
         [0012]    The encapsulation material can comprise a curable resin, in particular an epoxy resin, so that the encapsulation of the electronic component can be carried out in a manner known to one skilled in the art by means of transfer molding. 
         [0013]    In another refinement of the specified method, the electronic component can be arranged at a zero position of the wiring carrier, at which the thermal movements of the encapsulation material cancel each other out. A stress introduction can also be reduced in this manner. 
         [0014]    The electronic component can be an analysis circuit for analyzing measurement signals from at least two encoder elements in this case, wherein the two encoder elements can be arranged symmetrically around the analysis circuit. In this manner, a stress introduction onto the encoder elements and a possible measurement error linked thereto can be taken into consideration subsequently on the basis of the known symmetry. 
         [0015]    In yet another refinement of the specified method, for the encapsulation of the electronic component using the encapsulation material, an encapsulation geometry can be selected, in the case of which a thermal movement of the encapsulation material in at least one direction falls below a predetermined value. This geometry can be selected in an arbitrary manner, wherein the encapsulation geometry is particularly preferably selected such that a wall thickness of the encapsulation material on at least one wall of the electronic component falls below a predetermined value, so that the mechanical stress applied by the wall remains limited to a stress value dependent on this predetermined value. 
         [0016]    In yet another refinement, the specified method comprises the step of enveloping the electronic component with a mechanical decoupling material before the encapsulation. The mechanical decoupling material further reduces the stress introduction into the electronic assembly and thus reduces the mechanical strain of the electronic component. 
         [0017]    In a special refinement, the mechanical decoupling material is at least made thixotropic. In this manner, the mechanical decoupling material may be poured in a simple manner onto the electronic component, but has a sufficient stability in the poured-on state to completely envelop the electronic component. 
         [0018]    In an additional refinement, the wiring carrier comprises a terminal for electrically contacting the electronic component with a printed circuit board and an electrical shielding layer, wherein the specified method comprises the step of arranging the electronic component on the wiring carrier such that the electronic component is accommodated, viewed in at least one direction of the electrical assembly, between the electrical shielding layer and the terminal. In this manner, the wiring carrier can simultaneously be used for improving the electromagnetic compatibility (EMC), without an extra electrical shielding layer being necessary. 
         [0019]    According to a further aspect, the invention comprises an electronic assembly, an electronic component supported on a wiring carrier, which is encapsulated using an encapsulation material, wherein the electronic component is arranged on the wiring carrier such that a stress introduction through the encapsulation material onto the electronic component falls below a predetermined value. 
         [0020]    The specified electronic assembly can be expanded with features which correspond in meaning to the dependent claims of the specified method. 
         [0021]    In a refinement of the specified electronic assembly, the specified electronic assembly is designed as an inertial sensor. 
         [0022]    The specified electronic assembly can preferably be produced using a specified method in this case. 
         [0023]    According to a further aspect of the invention, a vehicle comprises a specified electronic assembly, in particular for detecting vehicle dynamics data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above-described properties, features, and advantages of this invention and the manner in which they are achieved will become clearer and more comprehensible in conjunction with the following description of the exemplary embodiments, which will be explained in greater detail in conjunction with the drawings, wherein: 
           [0025]      FIG. 1  shows a schematic view of a vehicle having a vehicle dynamics controller, 
           [0026]      FIG. 2  shows a schematic view of an inertial sensor from  FIG. 1  designed as an SMD part, 
           [0027]      FIG. 3  shows a schematic view of an intermediate product of an inertial sensor from  FIG. 1 ; 
           [0028]      FIG. 4  shows a schematic view of an inertial sensor from  FIG. 1  on a printed circuit board; and 
           [0029]      FIG. 5  shows a further schematic view of the inertial sensor from  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Identical technical elements are provided with identical reference signs in the figures and are only described once. 
         [0031]    Reference is made to  FIG. 1 , which shows a schematic view of a vehicle  2  having a vehicle dynamics controller known per se. Details of this vehicle dynamics controller can be inferred, for example, from DE 10 2011 080 789 A1, which is incorporated by reference. 
         [0032]    The vehicle  2  comprises a chassis  4  and four wheels  6 . Each wheel  6  can be decelerated in relation to the chassis  4  via a brake  8  which is fastened in a stationary manner on the chassis  4 , to decelerate a movement of the vehicle  2  on a road (not shown in greater detail). 
         [0033]    In this case, it can occur in a manner known to a person skilled in the art that the wheels  6  of the vehicle  2  lose their ground contact and the vehicle  2  even moves away from a trajectory, which is predefined for example via a steering wheel (not shown in greater detail), due to understeer or oversteer. This is prevented by control circuits known per se such as antilock braking systems (ABS) and electronic stability programs (ESP). 
         [0034]    In the present embodiment, the vehicle  2  has speed sensors  10  on the wheels  6  for this purpose, which detect a speed  12  of the wheels  6 . Furthermore, the vehicle  2  has an inertial sensor  14 , which detects vehicle dynamics data  16  of the vehicle  2 , from which, for example, a pitch rate, a roll rate, a yaw rate, a lateral acceleration, a longitudinal acceleration, and/or a vertical acceleration can be output in a way known per se to a person skilled in the art. 
         [0035]    Based on the detected speeds  12  and vehicle dynamics data  16 , a controller  18  can determine in a way known to a person skilled in the art whether the vehicle  2  is slipping on the roadway or is even deviating from the above-mentioned predefined trajectory and can react thereto accordingly using a controller output signal  20  known per se. The controller output signal  20  can then be used by a positioning unit  22  to activate actuators, such as the brakes  8 , by means of positioning signals  24 , which react to the slipping and the deviation from the predefined trajectory in a manner known per se. 
         [0036]    The controller  18  can be integrated, for example, in an engine controller, known per se, of the vehicle  2 . The controller  18  and the positioning unit  22  can also be designed as a shared control unit and can optionally be integrated in the above-mentioned engine controller. 
         [0037]    In  FIG. 1 , the inertial sensor  14  is shown as an external unit outside the controller  18 . In such a case, one refers to an inertial sensor  14  designed as a satellite. However, in the present embodiment, the inertial sensor  14  is to be constructed as an SMD part, so that it can also be integrated in a housing of the controller  18 , for example. 
         [0038]    The inertial sensor  14  comprises at least one micro-electromechanical system  26  (MEMS  26 ), as a measurement pickup, which, in a manner known per se, outputs a signal (not shown in greater detail) dependent on the vehicle dynamics data  16  via bond wires  28  to two signal analysis circuits  30  in the form of application-specific integrated circuit  30  (ASIC  30 ). The ASIC  30  can then generate the vehicle dynamics data  16  based on the received signal, which is dependent on the vehicle dynamics data  16 . 
         [0039]    The MEMS  26  and the ASIC  30  are supported on a printed circuit board  32  and are electrically contacted with various electrical circuits  34  formed on the printed circuit board  32 . Alternatively, the printed circuit board  32  can also be designed as a lead frame, which will be described in greater detail hereafter. Of the lines  34 , only a single line  34  is visible in section in  FIG. 2 . The contacting can be performed directly in this case, for example, via a flip chip connection known per se or, as shown in  FIG. 2 , via a bond wire  28 . 
         [0040]    The MEMS  26  and the ASIC  30  can furthermore be enveloped by a mechanical decoupling material  36 , known as globe top mass  36 , which can in turn be encapsulated jointly with the MEMS  26  and the ASIC  30  in a transfer molding material  38 , for example, an epoxy resin  38 . 
         [0041]    The transfer molding material  38  could therefore already be used alone as the housing of the inertial sensor  14  and protect the circuit components accommodated therein. 
         [0042]    Finally, corresponding contact options, such as solder beads  40  shown in  FIG. 2 , are provided on the inertial sensor  14  designed as an SMD part for electrically contacting a circuit of the controller  18 . Alternatively, the contact options of the inertial sensor  14  designed as an SMD part could, however, be designed as gullwing solder terminals or J-lead solder terminals known per se, if the substrate is designed as a lead frame  32 , which is known per se and is shown in  FIG. 3 . 
         [0043]    Reference is made to  FIG. 3 , on the basis of which the internal structure of the inertial sensor  14  is to be explained. To illustrate the manifold options for how the inertial sensor  14  can be implemented as an SMD part, instead of solder beads  40  as contact options, contact pins  40  are formed as contact options in  FIG. 3 . 
         [0044]    In  FIG. 3 , instead of on a printed circuit board  32  as a wiring carrier, the MEMS  26  and the ASIC  30  are supported on a lead frame  32  as a wiring carrier, on which contact pins  40  are formed, via which the MEMS  26  and the ASIC  30  can be electrically connected to the above-mentioned circuit of the controller  18 . For the sake of comprehensibility, not all of these contact pins are provided with a reference sign in  FIG. 3 . 
         [0045]    The MEMS  26  and the ASIC  30  can be fixed on the lead frame  32  by soldering, gluing, and/or adhesive films, such as DAF tapes known per se. The electrical contacting can be produced in this case via the fixing and/or via bond wires  28 , not all of which are provided with a reference sign for the sake of comprehensibility. 
         [0046]    In the present embodiment, a further MEMS  26  can alternatively or additionally be supported on the ASIC  30 , which can either also be used as an encoder element for detecting the vehicle dynamics data  16  or as an alternative encoder element, for example, for detecting a temperature or another physical variable. Further electrical components, for example, passive components, could also be interconnected on this lead frame in a manner which is not shown. 
         [0047]    Observed into the plane of the image of  FIG. 3 , the ASIC  30  and the MEMS  26  can be supported on a metal layer  46 , which is used as an electrical shielding layer  46  and thus increases the EMC of the fusion sensor  14 . In this manner, the EMC of the fusion sensor  14  can be noticeably improved by supporting the entire electronic assembly on a printed circuit board  48  shown in  FIG. 4 , on the one hand, and by the electrical shielding layer  46 , on the other hand. 
         [0048]    To envelop the ASIC  30  and the MEMS  26  with the above-mentioned decoupling material  36  and/or to encapsulate them using the transfer molding material  38 , tabs  50  are formed on the lead frame  32 , on which the lead frame  32  can be mechanically fixed during the enveloping and/or encapsulation procedure. 
         [0049]    Since the tabs  50  represent a penetration of the lead frame  32  through the transfer molding material  38  to the outside, the tab offers an access for moisture to the ASIC  30  and the MEMS  26 . To make it more difficult for this moisture to penetrate, recesses  51  are formed on the tabs  50 , which reduce the cross section of the above-mentioned penetration of the lead frame  32  through the transfer molding material  38  to the outside. At the same time, the transfer molding material  38  and therefore the encapsulation of the ASIC  30  and the MEMS  26  are further anchored in the recess  51 . 
         [0050]    The individual contact pins  40  have anchoring elements in this case, which are formed as notches  52  in the present embodiment, for example, at which the contact pins  40  are tapered. Not all of these notches are provided with a reference sign in  FIG. 3  for the sake of comprehensibility. The notches  52  can be enclosed by the transfer molding material  38 , to prevent the contact pins  40  from subsequently being pulled out of the transfer molding material  38 . 
         [0051]    In addition, the anchoring elements, which are formed as notches  52  as an example, limit the movement of the contact pins  40  as a result of thermal expansion, vibration, etc. within the transfer molding material  38 , which prevents damage to the bond wires  28  or the electrical connection thereof to the contact pins  40 , for example, due to shear forces arising in this manner. 
         [0052]    The finished fusion sensor  14  is shown supported on the printed circuit board  48  in  FIG. 4 , on which the controller  18  can be wired (not shown further), for example. 
         [0053]    The volume of the transfer molding material  38  which is formed as the encapsulation material as an example, observed from the lead frame  32 , minus the volume of the ASIC  30  and the MEMS  26 , is to be equal or deviate at most by 30% from one another on the upper side  54  and the lower side  56  as shown in  FIG. 5 . In this manner, an unequal shrinkage and reduced warping are achieved, which keeps the internal tensions of the cast transfer molding material  38  low and therefore further reduces or even avoids mechanical strains of the ASIC  30  and the MEMS  26  and also the bond wires. 
         [0054]    To ensure demolding in a tool during the production of the fusion sensor  14 , demolding bevels  58  of at least 5° are to be formed, of which only one is provided with a reference sign in  FIG. 5  for the sake of comprehensibility.