Abstract:
The invention relates to a printed circuit board arrangement, more particularly a multilayer printed circuit board. The printed circuit board arrangement comprises at least two printed circuit boards which are arranged parallel to one another and connected to one another. According to the invention, in the case of the printed circuit board arrangement of the type mentioned initially, at least one surface region of one printed circuit board is connected to another printed circuit board of the printed circuit board arrangement by means of an element embodied in an elastic and/or damping fashion in such a way that an oscillatory system, more particularly a spring-mass system, an oscillatory bending strip or a flexurally oscillatory board is formed by means of the surface region of the printed circuit board and the element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a printed circuit board arrangement, in particular a multilayer printed circuit board. The printed circuit board arrangement comprises at least two printed circuit boards arranged mutually parallel and connected to one another. 
         [0002]    In the case of printed circuit boards known from the prior art, which are equipped with electronic components, for example with sensors, the problem has been identified that the printed circuit boards together with the components arranged thereon may be excited in natural oscillations which can impair a detection result to be received by the sensor. This problem arises, for example, in the case of sensors which are formed as oscillation receivers, for example in the case of microphones, pressure sensors or acceleration sensors. Furthermore, the problem may consist in an oscillation frequency of the printed circuit board lying in a detection frequency range of the sensor, and thus leading to a detection result which is not in causal relationship with the measurement quantity to be detected. 
       SUMMARY OF THE INVENTION 
       [0003]    According to the invention, in the case of a printed circuit board arrangement of the type mentioned in the introduction, at least one surface region of one printed circuit board is connected to another printed circuit board of the printed circuit board arrangement, by means of an element formed elastically and/or so as to be damping, in such a way that an oscillatory system, in particular a spring-mass system, an oscillatory flexion strip or a flexurally oscillatory plate, is formed by means of the surface region of the printed circuit board and the element. 
         [0004]    In this way, the surface region and a component connected to the surface region, for example an oscillation receiver which is formed in order to generate, as a function of a received oscillation, a sensor signal representing the oscillation, in particular a pressure sensor, a rotation rate sensor or an acceleration sensor, can oscillate together with the surface region on the elastically formed element. 
         [0005]    Preferably, the elastically formed element is additionally formed so as to be damping, and also form internal damping during the oscillation. In this way, a spring-mass system with an oscillation damper can advantageously be formed. 
         [0006]    In the case of a multilayer-design printed circuit board arrangement, comprising at least two, preferably a plurality of printed circuit board layers, the elastically formed element, which is more preferably formed so as to be damping, may advantageously be arranged between two printed circuit boards. The printed circuit boards each form a layer of the multilayer-design printed circuit board arrangement. 
         [0007]    The printed circuit board is, for example, a multilayer-design HDI printed circuit board (HDI=High-Density Interconnect), or an ECP printed circuit board (ECP=Embedded-Component Packaging). Interconnects of the individual printed circuit boards of the printed circuit board arrangement, in particular a multilayer printed circuit board, may preferably be connected to one another by means of through-contacts, also referred to as vias. By means of the vias, thermal connections may also be formed between printed circuit boards adjacent to one another. Preferably, the surface region is connected to an electronic component. The electronic component is, for example, a microphone, an acceleration sensor, a rotation rate sensor or a pressure sensor. 
         [0008]    An acceleration sensor may, for example, comprise a proof mass and a piezo element connected to the mass. In another embodiment, the acceleration sensor is a micromechanical sensor, in particular a MEMS sensor (MEMS=Micro-Electro-Mechanical Sensor), which is formed in order to capacitively detect a deflection of a sensor mass. The sensor mass weighs, for example, a few micrograms and comprises webs which are connected to beams and are separated from one another along a beam longitudinal direction, which can oscillate between two electrodes forming a capacitor. For example, the sensor mass is maintained in particular in oscillation in an electric field. The oscillation of the sensor mass can be detuned in frequency and/or amplitude by an acceleration acting on the sensor mass, so that the frequency and/or amplitude of the acceleration can be detected as a function of the detuning. 
         [0009]    In a rotation rate sensor, a mass of an acceleration sensor can be deflected as a function of a centrifugal force of a rate of a rotation, and the acceleration can thus be detected as a function of the deflection, in particular a capacitance change or piezo voltage proportional to the deflection. 
         [0010]    The pressure sensor is preferably formed in order to detect a static air pressure, in particular a change in the air pressure, and to generate a sensor signal which represents the air pressure or the change in the air pressure. To this end, a pressure-sensitive membrane of a container may be connected to a piezo element or to a capacitive element, which can detect a deflection of the membrane. 
         [0011]    By means of the electronic component, an additional mass can advantageously be formed, which can oscillate together with the surface region on the elastically formed element. Another electronic component may also be envisaged, as a sensor, for example a microprocessor, which is vibration-sensitive. 
         [0012]    Preferably, a resonant frequency of the spring-mass system, comprising the surface region of the printed circuit board, the electronic component and the elastically formed element, is less than a frequency which can be detected by the sensor. Good damping of the vibrations can advantageously be achieved in this way. Also advantageously, the sensor can be decoupled from a resonance, in particular the flexural oscillation resonance, of the printed circuit board, which lies in the frequency detection range of the sensor. 
         [0013]    In a preferred embodiment, the surface region is decoupled from oscillations, in particular flexural oscillations, of a surface of the printed circuit board next to the surface region, the printed circuit board comprising a recess for the surface region and the surface region being arranged in the recess. For example, the surface region is decoupled and thus separated from the surface region surrounding the surface region by means of slits, in particular sawed slits. The decoupling may, for example, be generated by means of a saw or by means of laser cutting. By means of the decoupling formed in this way, the surface region can oscillate freely on the elastic element relative to the surface region surrounding the surface region of the printed circuit board. 
         [0014]    Preferably, the surface region is connected to the surface of the printed circuit board, in particular the printed circuit board surface surrounding it, by means of a film, in particular a cover film. Preferably, the film has a smaller thickness dimension than the printed circuit board. The intermediate space, in particular a gap, between the printed circuit board and the oscillatory surface region of the printed circuit board, can thus advantageously be sealed. By virtue of the sealing, advantageously no contamination can enter the intermediate space between the printed circuit boards. The film is, for example, coated with adhesive on at least one side. Such a film, in particular self-adhesive film, may also advantageously close the gap, formed for the decoupling, between the surface region and the printed circuit board surface surrounding it, after the printed circuit board has been fitted with components and also after the electronic components have been soldered to the printed circuit board. 
         [0015]    In a preferred embodiment, the elastically formed element comprises silicone. By virtue of a silicone element, the element can advantageously have a damping property in addition to the elastic property. More advantageously, the silicone element is substantially resistant to various chemicals. 
         [0016]    In another embodiment, the elastically formed element comprises a plastic foam, in particular a polyurethane foam. By virtue of the plastic foam, the elastic element can advantageously be provided economically. 
         [0017]    Preferably, the element formed elastically and/or so as to be damping is inserted into a recess of the printed circuit board by means of injection onto the other printed circuit board. For example, the elastically formed element may be incorporated into an interlayer by means of an embedding method. Thus, the other printed circuit board, which is arranged next to and parallel to the printed circuit board, can form an oscillation counter-bearing and fastening point for the elastic element and the surface region, which is connected to the elastic element. 
         [0018]    In a preferred embodiment, the element is arranged between the planes formed by the printed circuit board and the other printed circuit board. In this way, the surface region with the printed circuit board surrounding it can advantageously be arranged in the same plane. By virtue of the decoupling by means of the elastic element, in a multilayer printed circuit board assembly no more installation space in the height direction is used up by constructing a spring-mass system. 
         [0019]    The invention also relates to a circuit arrangement having a printed circuit board according to the type described above. In the circuit arrangement, at least some of the components of the circuit arrangement are connected mechanically and/or electrically to a surface region of the printed circuit board, which forms an oscillatory system, in particular a spring-mass system, an oscillatory flexion strip or a flexurally oscillatory plate. 
         [0020]    The surface region preferably has a surface size on which the sensor can be fastened. For example, a square surface region of a spring-mass system has an edge length of between one centimeter and five centimeters. An oscillatory system in the micro-range may also be envisioned, so that in the case of thin printed circuit boards, for example having a printed circuit board thickness of between 100 and 150 micrometers of the multilayer printed circuit board arrangement, a MEMS sensor with a square dimension having an edge length of 50 micrometers without electronic interconnection of the MEMS sensor may be less than one millimeter, preferably less than 500 micrometers, depending on the resonant frequency to be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  shows an exemplary embodiment of a printed circuit board arrangement having an oscillatory system formed by a surface region of a printed circuit board and a spring element; 
           [0022]      FIG. 2  shows a plan view of the printed circuit board arrangement represented in a sectional view in  FIG. 1 ; 
           [0023]      FIG. 3  shows an exemplary embodiment of a printed circuit board arrangement having an oscillatory system formed by a surface region of a printed circuit board and a damping element in plan view; 
           [0024]      FIG. 4  shows the printed circuit board arrangement of  FIG. 3  in a sectional representation; 
           [0025]      FIG. 5  shows an exemplary embodiment of a printed circuit board arrangement having an oscillatory system formed by a surface region of a printed circuit board and a damping element, the surface region forming a leaf spring having a freely oscillating end; 
           [0026]      FIG. 6  shows the printed circuit board arrangement of  FIG. 5  in a sectional representation; 
           [0027]      FIG. 7  shows a diagram of two transfer functions of the printed circuit board arrangement in  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 1  shows an exemplary embodiment of a printed circuit board arrangement  1 . The printed circuit board arrangement  1  comprises a printed circuit board  5  and a printed circuit board  7  connected to the printed circuit board  5 . The printed circuit boards  5  and  7  are arranged mutually parallel, an interlayer  33  which connects the printed circuit boards  5  and  7  to one another being arranged at least locally between the printed circuit boards  5  and  7 . The printed circuit boards  5  and  7  are for example fiber-reinforced, in particular glass fiber-reinforced epoxy resin printed circuit boards. In this exemplary embodiment, the printed circuit board arrangement  1  forms a multilayer printed circuit board, the printed circuit boards  5  and  7  respectively forming a layer which enclose the interlayer  33 , also referred to as an inner layer, between them. For example, the interlayer  33  is formed by an epoxy resin layer. The printed circuit boards  5  and  7  each have a copper sheet on at least one side. The copper sheet may, for example, be formed by an RCC sheet (RCC=Resin-Coated Copper). In this exemplary embodiment, the printed circuit board arrangement  1  also comprises a plurality of electronic components. Electronic components  14  and  16  are represented, which are respectively arranged on one side of the printed circuit board  7 . In this exemplary embodiment, the components  14  and  16  are formed by an integrated circuit. The printed circuit board arrangement  1  also comprises electronic components  20  and  22 , which are respectively formed as capacitors. The components  20  and  22  are respectively arranged in the interlayer  33 , and therefore between the printed circuit boards  5  and  7 . An electronic component  18  is also represented, in this exemplary embodiment an integrated circuit, which is arranged on one side of the printed circuit board  5  and is connected to the printed circuit board  5 , in particular to copper tracks of the printed circuit board  5 . 
         [0029]    The printed circuit board  7  comprises a surface region  9  which—for example by means of sawing—has been separated from the printed circuit board  7 . The surface region  9  therefore forms a kind of cover, which is enclosed by a gap  25  extending around the cover. The gap  25  is produced, for example, by sawing or laser cutting of the printed circuit board  7 . Advantageously, the gap may have been produced before connection of the printed circuit board  7  to the printed circuit board  5 . In the case of thin printed circuit boards, a plurality of printed circuit boards may be placed on one another and laser-cut or sawed together. In this way, the thin printed circuit boards cannot break, and more advantageously a plurality of printed circuit boards can be produced economically in one method step. 
         [0030]    The surface region  9  of the printed circuit board  7  is connected to the printed circuit board  5  by means of a spring element  11 . In this exemplary embodiment, the spring element  11  is formed by a silicone rubber. In this exemplary embodiment, the spring element  11  also has damping properties. 
         [0031]    A sensor  13  is also arranged on the surface region  9  of the printed circuit board  7 . The sensor  13  is for example a pressure sensor, in particular a microphone, an acceleration sensor or a rotation rate sensor. The sensor  13  is connected to the printed circuit board  7 , and there to at least one copper interconnect, by means of bonding wires  35  and  37 . In another embodiment, the sensor is electrically connected by means of flip-chip technology or by means of through-contacts, also referred to as vias. 
         [0032]    Together with the surface region  9  of the printed circuit board  7 , the sensor  13  forms a mass of an oscillatory system, in particular of a spring-mass system. In this exemplary embodiment, the spring element  11  is arranged between the printed circuit boards  5  and  7 . In the region of the spring element  11 , the interlayer  33  comprises a recess, so that a cavity is formed in which the spring element  11  is arranged. The spring element  11  is therefore arranged in a cavity extending between the printed circuit boards  7  and  5 , in particular the surface region  9  and the printed circuit board  5 . 
         [0033]    By means of the oscillatory system formed in this way, the sensor  13  can advantageously be decoupled from mechanical oscillations of the printed circuit board  5  and of the components connected to the printed circuit board  5 . 
         [0034]    In this embodiment, the circumferential gap  25  is covered by an adhesive film  30 . The adhesive film  30  is, for example, a polyimide film which covers the gap  25  and advantageously has additional damping properties. When the sensor  13  together with the surface region  9  oscillates on the spring element  11 , the film  30  can move elastically together with the oscillation movements and generate damping in addition to the damping properties of the spring element  11  by means of the elastic deformations of the to-and-fro movements of the oscillation movement. 
         [0035]    A resonant frequency f=ω/2Π of the spring-mass system formed in this way is calculated according to the formula: 
         [0000]    
       
         
           
             ω 
             = 
             
               
                 s 
                 m 
               
             
           
         
       
     
         [0036]    with ω=angular frequency 
         [0037]    s=spring stiffness of the spring element 
         [0038]    m=mass of the sensor and the surface element. 
         [0039]      FIG. 2  shows a plan view of the printed circuit board arrangement represented in a sectional view in  FIG. 1 . The printed circuit board  7 , which comprises an opening, is represented. 
         [0040]    The surface element  9 , which has smaller dimensions than the opening, is arranged in the opening, so that a circumferential gap  25  which encloses the surface region  9  is formed between the surface region  9  and the printed circuit board  7  in the opening. The sensor  13  is arranged on the surface region  9  of the printed circuit board  7  and is connected thereto. 
         [0041]      FIG. 3  shows an exemplary embodiment of a printed circuit board arrangement  2 . In this embodiment, the printed circuit board arrangement  2  comprises two printed circuit boards  5  and  7 . The printed circuit board  7  comprises an opening, extending lengthwise in a printed circuit board plane of the printed circuit board  7 , which forms a gap  29 . Transversely with respect to the gap  29 , a further gap  28  extends in the region of one end of the gap  29 . In the region of the other end of the gap  29 , a gap  24  extends in the same direction as the gap  28 . The ends of the gaps  24  and  28  which lie next to the gap  29  are respectively separated from the gap  29  by webs, the webs being formed in the printed circuit board  7 . The webs are, for example, formed by printed circuit board material of the printed circuit board  7  remaining during the sawing of the gaps  28 ,  29  and  24 . The webs are therefore formed integrally onto the printed circuit board  7 . A gap  27  extends parallel to the gap  29  and in the region of the ends of the gaps  24  and  28  away from the gap  29 . A surface region  8  of the printed circuit board  7  which is connected to the printed circuit board  7  by means of the webs is therefore enclosed by means of the gaps  24 ,  28  and  27 . The surface region  8  can therefore—suspended from the webs—oscillate to-and-fro transversely in the printed circuit board  7  with respect to a printed circuit board plane of the printed circuit board  7 . The to-and-fro oscillation of the surface region  8  causes deformation of the surface region  8 , which, for example in the case of a first oscillation mode, describes a kind of cushion shape toward the end regions of an oscillation amplitude. 
         [0042]    A sensor  13  is arranged on the surface region  8 . The sensor  13  is for example a pressure sensor, an acceleration sensor or a microphone. 
         [0043]      FIG. 4  shows the printed circuit board arrangement  2  represented in a plan view in  FIG. 3  in a sectional representation. The section extends through the gap  27 . In this way, the multilayer structure of the multilayer printed circuit board arrangement, comprising three layers in this exemplary embodiment, namely the printed circuit board  5  and the printed circuit board  7  and an interlayer  32  arranged between them. In this exemplary embodiment, the interlayer  32  comprises a cavity, in which a damping element  12  is arranged. The damping element  12  is formed for example by a plastic foam, in particular polyurethane foam, by a silicone rubber or by a gel element, in particular a silicone gel element. 
         [0044]    The sensor can oscillate to-and-fro together with the surface region  8  transversely with respect to a printed circuit board plane of the printed circuit board  7 , the surface region  8  being suspended from the webs and connected to printed circuit board  7  by means of the webs. 
         [0045]    The oscillation of the surface region  8 , in particular a resonant frequency, is therefore determined substantially by a flexural stiffness of the printed circuit board material of the printed circuit board  7  and its size. The printed circuit board material is for example fiber-reinforced, in particular glass fiber-reinforced, epoxy resin. In this exemplary embodiment, the oscillation amplitude of the oscillation of the surface region  8  is determined both by the damping properties of the printed circuit board  7 , in particular of the surface region  8  of the printed circuit board  7 , itself, and by the damping properties of the damping element  12  which connects the surface region  8  to the printed circuit board  5  and is arranged in the intermediate space between the surface region  8  and the printed circuit board  5 . The resonant frequency of the oscillatory system may, for example, with a predetermined flexural stiffness of the surface element  8 , be established by a length of the gaps  24 ,  27 ,  28  and  29 . The resonant frequency may, for example, be determined empirically. 
         [0046]    A frequency profile of the oscillation of the surface region  8  is represented in  FIG. 7 . 
         [0047]      FIG. 5  shows an exemplary embodiment of a printed circuit board arrangement  3 . In this exemplary embodiment, the printed circuit board arrangement  3  comprises three layers, of which one layer are respectively formed by a fiber-reinforced printed circuit board, namely in this exemplary embodiment a printed circuit board  5  and a printed circuit board  7 . The printed circuit boards  5  and  7  are connected to one another by an interlayer  34 . The interlayer  34  is formed, for example, by an epoxy resin layer. The interlayer  34  comprises a cavity  31 , in which a damping element  15  is arranged. In this exemplary embodiment, the damping element  15  fills the cavity  31  only partially. In this exemplary embodiment, the printed circuit board  7  has a U-shaped opening, which forms by the U-shaped gap  26 . 
         [0048]    The U-shaped gap  26  encloses a surface region  10  of the printed circuit board  7 , and the cavity  31  extends between the surface region  10  of the printed circuit board  7  and the printed circuit board  5 . In this exemplary embodiment, the surface region  10  has an elongate shape. The surface region  10  can therefore oscillate, starting from a line  39  which joins the ends of the U branches of the gap  26 , into the cavity  31  and out of the printed circuit board surface  7 . 
         [0049]    The surface region  10  may in this case—in a similar way to a springboard—flex in the manner of a spring along its lengthwise extent. The spring properties, in particular a resonant frequency of the oscillatory system formed in this way, are essentially determined by the flexural stiffness of the surface element  10  along its lengthwise extent, and, with a fixed width of the surface element  10 , by the freely oscillating length of the surface element  10 . 
         [0050]    A damping element  15  is arranged in the region of one end of the surface element  10 . The damping element  15  connects an end region of the surface element  10  to the printed circuit board  5 . The damping element  15  is arranged in the cavity  31  between the printed circuit board  5  and the surface region  10 . By means of the damping element  15 , the oscillation movement of the surface region  3  transversely to the printed circuit board plane of the printed circuit board  7  can be damped. The damping element  15  is formed for example by a silicone rubber or a gel element, in particular a silicone gel element. 
         [0051]      FIG. 6  shows a plan view of the printed circuit board arrangement  3  already represented in  FIG. 5 . The sensor  13  is represented, which is connected in the region of one end of the lengthwise-extending surface region  10 , which forms a printed circuit board section of the printed circuit board  7 , to the surface region  10 . The gap  26  enclosing the surface region  10  is also represented. The gap  26  may, for example, have been produced by means of sawing or laser cutting. 
         [0052]      FIG. 7  shows a transfer function of an oscillation of the surface region  8 , represented in  FIG. 4 , of the printed circuit board  7 . The sensor  13  is connected to the surface region  8 . Together with the surface region  8 , the sensor  13  therefore forms an oscillatory system.  FIG. 7  shows a diagram  40 . The diagram  40  has a frequency axis  42  and an amplitude axis  44 . On the amplitude axis  44 , the transfer value of the oscillation is plotted in decibels. The diagram  40  shows a transfer function  46  of an undamped oscillation of the surface region  8 . In the case of the undamped oscillation of the surface region  8 , the printed circuit board arrangement  2  does not have a damping element  12 . A pronounced resonant frequency  50  of the surface element  8  can be seen. The transfer function  46  falls off strongly toward higher frequencies. There is therefore damping of the oscillatory system toward higher frequencies. The oscillatory system can therefore advantageously be decoupled toward higher frequencies. 
         [0053]    A transfer function  48  is also represented. The transfer function  48  represents an oscillation of the surface region  8  which is connected to the printed circuit board  5  by means of the damping element  12 . A resonant frequency  52  is also represented, which is greater than the resonant frequency  50 . A smaller oscillation amplitude in the region of the resonance  52  than in the region of the resonance  50  of the transfer function  48  can be seen. The smaller amplitude is advantageously caused by the damping of the damping element  12 . Starting from the resonant frequency  52 , the transfer function falls off steeply toward higher frequencies, so that oscillation decoupling can thereby effectively be produced. In this exemplary embodiment, the resonant frequency  50  is at about 5000 Hertz, and the resonant frequency  52  is at about 7000 Hertz. It can be seen that, starting at frequencies greater than 10,000 Hertz, the sensor  13  is effectively decoupled from oscillations. The sensor  13  can therefore be effectively decoupled by oscillations, in particular flexural oscillations, of the printed circuit boards  5  and  7  of the printed circuit board arrangement. In this way, advantageously, a detection range of the sensor, in particular a frequency range of the detection range of the sensor  13 , which lies above the resonant frequency  50  or  52 , cannot be impaired, or can be impaired only slightly, by oscillations of the printed circuit board arrangement.