CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to EP Application No. 12 182 566.5-1806, filed Aug. 31, 2012, the disclosure of which is incorporated in its entirety by reference herein. 
       TECHNICAL FIELD 
       [0002]    Disclosed herein is a circuit board system as can be used, for instance, in an infotainment head unit or in a telematics unit, and a method for producing a circuit board system. 
       BACKGROUND 
       [0003]    Modular circuit board systems with two or more electrically interconnected circuit boards are becoming more and more popular as the modularity of such arrangements allows swiftly amending or adapting existing technologies. In many conventional circuit board systems, the connections between individual circuit boards are implemented as plug-in connections. However, plug-in technology is expensive as it requires plugs and sockets. Aside from that, plug and socket technology occupies a lot of space on the circuit boards. If, on the other hand, soldered connections are used instead of plug-in technology, the operating temperature of the system is limited in range and/or cycles. At increased temperatures and temperature cycles, the failure probability is significantly higher than for systems that use plug-in technology. Hence there is a need for an improved circuit board system. 
       SUMMARY 
       [0004]    In a method for producing a circuit board system is provided a first circuit board with a top side, a bottom side, a top metallization layer arranged at the top side, and a bottom metallization layer arranged at the bottom side. The bottom metallization layer includes a number of soldering pads. A first solder is applied over the soldering pads and a second solder is applied over the top metallization layer. Also included are a number of electronic components, and a metallic or metallized shielding frame that has an opening. The number of electronic components and the shielding frame are arranged on the applied second solder and soldered to the top metallization layer by melting the second solder and by subsequently cooling of the second solder to below its solidification temperature. After cooling, a thermal interface material is applied through the opening over the top side of at least one of the electronic components. 
         [0005]    Optionally, a housing cover with an electrically conductive surface may be attached to the first circuit board such that the contact springs electrically contact the housing cover and that the thermal interface material mechanically contacts the housing cover and at least one of the electronic components. 
         [0006]    In order to further produce a circuit board arrangement, a second circuit board with a top side, a bottom side and a top metallization layer arranged at the top side is provided. The soldering pads are electrically and mechanically connected to the top metallization layer of the second circuit board by soldering. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Various specific examples are described in more detail below based on the principles illustrated in the figures of the drawings. Unless stated otherwise, similar or identical components are labeled in all of the figures with the same reference numbers. 
           [0008]      FIG. 1  is a cross-sectional view of a circuit board system which is a land grid array (LGA) module; 
           [0009]      FIG. 2  is a bottom view of a circuit board of the circuit board system of  FIG. 1 ; 
           [0010]      FIG. 3  is an exploded view of the circuit board system of  FIG. 1 ; 
           [0011]      FIGS. 4-9  illustrate different steps of a method for producing the circuit board system as explained above with reference to  FIGS. 1 to 3 ; 
           [0012]      FIGS. 10-16  illustrate different steps of a method for producing a circuit board arrangement; 
           [0013]      FIG. 17  is an enlarged section of the circuit board of the circuit board system as explained above with reference to  FIG. 1 ; 
           [0014]      FIG. 18  is an enlarged section of the circuit board of the circuit board system as explained above with reference to  FIG. 1 , wherein a first solder is applied over the soldering pads and a second solder is applied over the top metallization layer; 
           [0015]      FIG. 19  is a another exploded view of the circuit board system of  FIG. 1 ; 
           [0016]      FIG. 20  is an exploded view of a circuit board system explained above with reference to  FIGS. 1 to 3  prior to being thermally coupled to a housing lid; 
           [0017]      FIG. 21  is a perspective view of a first circuit board populated with a number of electronic components and a shielding frame that includes openings; 
           [0018]      FIG. 22  is a top view of the first circuit board of  FIG. 21 ; 
           [0019]      FIG. 23  is a top view of the first circuit board of  FIGS. 21 and 22  with a thermal interface material applied through some of the openings over the top sides of some electronic components; and 
           [0020]      FIG. 24  is a top view of the first circuit board of  FIGS. 21 to 23  that schematically illustrates the openings formed in the shielding frame. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  illustrates a circuit board system  100  with a first circuit board  1 . The first circuit board  1  has a top side  11  and a bottom side  12 . A top metallization layer  13  is arranged at the top side  11  and a bottom metallization layer  14  is arranged at the bottom side  12 . In the bottom metallization layer  14 , a number of soldering pads  141  are formed. A first solder  16  is arranged on the soldering pads  141  such that the first solder  16  completely covers the soldering pads  141 . A number of electronic components  17  and a shielding frame  18  are soldered to the top metallization layer  13  using a second solder  15 . A spring frame  19 , which has an electrically conductive surface and several electrically conductive contact springs  191 , is snapped onto the shielding frame  18 . 
         [0022]    In order to form a more complex circuit board arrangement, such a circuit board system  100  may be, prior to or after snapping the spring frame  19  onto the shielding frame  18 , soldered together with other electronic components to a second circuit board. For instance, such a circuit board arrangement may form an infotainment head unit or a telematic unit of a vehicle. However, the invention is not limited to infotainment head units or telematic units and/or the use in vehicles. 
         [0023]      FIG. 2  is a bottom view of the first circuit board  1  of the circuit board system  100 . In this case, no first solder  16  is applied over the soldering pads  141 . The soldering pads  141  are formed in the bottom metallization layer  14 . Also illustrated in  FIG. 2  is the cross-sectional plane E1 which corresponds to the view of  FIG. 1 . Except for the soldering pads  141 , the bottom metallization layer  14  is protected with a solder mask  10 . In the arrangement of  FIG. 1 , the solder mask  10  is present but is not shown in  FIG. 1 . The bottom metallization layer  14  also includes conductor lines for electrically connecting the soldering pads  141 . However, the conductor lines are covered by the solder mask  10  and therefore hidden in  FIG. 2 . 
         [0024]    As also illustrated in  FIG. 2 , the soldering pads  141  may have different sizes. The size may depend on the required capacity &amp; inductance. A first subset of the soldering pads  141  may have a first pitch p1, for instance at least 2.4 mm, and a second subset of the soldering pads  141  may have a second pitch p2, for instance at least 3 mm. The total number of soldering pads  141  has a total surface area A141tot. In this regard, the surface of a soldering pad  141  is the area of the footprint of the pad  141 . 
         [0025]    The first circuit board  1  itself has a footprint of w1 (length)×w2 (width) which may be, for instance, less than or equal to 70 mm×70 mm. However, the footprint may also exceed the size of 70 mm×70 mm, that is, the length w1 and the width w2 may each be more than 70 mm. 
         [0026]    If during the soldering of the first circuit board  1  to a substantially plane second circuit board a significant warp age of the first circuit board  1  occurs, it may happen that some of the soldering pads are not wetted with solder. As a maximum of war page predominantly occurs in the corner regions of the first circuit board  1 , the corner regions of the first circuit board  1  may optionally be free of soldering pads  141 , which is also illustrated in  FIG. 2 . 
         [0027]    For the sake of clearness,  FIG. 3  shows an exploded view of the circuit board system  100  of  FIG. 1 . As illustrated in  FIG. 3 , the top and bottom metallization layers  13 ,  14  form integral parts of the first circuit board  1 . In order to produce a circuit board system  100 , a first solder  16  (e.g., a soldering paste) is applied over the soldering pads  141 , for instance by means of dispensing, screen printing or mask printing. The result is illustrated in  FIG. 4 . Then, according to  FIG. 5 , the first circuit board  1  with the first solder  16  applied over the soldering pads  141  is arranged in a reflow soldering machine  300  in which the first solder  16  is melted and subsequently cooled to below its solidification temperature. As a result, the soldering pads  141  are plated with the first solder  16 , for instance a solder depot, such that the first circuit board  1  can be handled without the risk of wearing the soldering paste away. Applying the first solder  16  may be carried out using screen printing, stencil printing or jet printing. 
         [0028]    Subsequently, the first circuit board  1  is removed from the reflow soldering machine  300  and a second solder  15  (e.g., a soldering paste) is applied over the top metallization  13 , for instance by means of dispensing, screen printing or mask printing. The result is illustrated in  FIG. 6 . Then, a number of electronic components  17  and a shielding frame  18  are arranged on the applied second solder  15 , that is, on the soldering paste, see  FIG. 7 . For example, the shielding frame  18  may consist of or include nickel (Ni) and/or silver (Ag), or consist of or include an alloy of both nickel (Ni) and silver (Ag). 
         [0029]    Then, according to  FIG. 8 , the first circuit board  1  with the number of electronic components  17  and the shielding frame  18  arranged on the applied second solder  15  (i.e., the soldering paste) is arranged in the same or a further reflow soldering machine  300  in which the number of electronic components  17  and the shielding frame  18  are soldered to the top metallization layer  13  by melting the second solder  16  and subsequently cooling the melted solder  16  to below its solidification temperature. 
         [0030]    Subsequently, the first circuit board  1  with the number of electronic components  17  and the shielding frame  18  soldered to the top metallization layer  13  may be removed from the reflow soldering machine  300 . The result is an electronic subunit  100  which may be soldered to a second circuit board in order to form a circuit board arrangement. Prior to or after soldering the electronic subunit to a second circuit board, the spring frame  19  is snapped onto the shielding frame  18 .  FIG. 9  shows the subunit  100  with the spring frame  19  snapped onto the shielding frame  18 . 
         [0031]    As will now be explained with reference to  FIGS. 10 to 16 , using a subunit  100  as explained above, either with or without the spring frame  19  snapped onto the shielding frame  18 , a circuit board arrangement  200  ( FIG. 16 ) can be produced by soldering. To this end, a second circuit board  2  as exemplarily illustrated in  FIG. 10  is provided. The second circuit board  2  has a top side  21  and a bottom side  22 . A top metallization layer  23  is arranged at the top side  21  and an optional bottom metallization layer  24  is arranged at the bottom side  12 . 
         [0032]    In order to produce a circuit board arrangement  200 , a third solder  25  (e.g., a soldering paste) is applied over the top metallization layer  23 , for instance by means of dispensing, screen printing or mask printing. The result is illustrated in  FIG. 11 . Then, according to  FIG. 12 , the subunit  100  as described above and one ore more further arbitrary electronic components  27  are arranged on the applied third solder  25 , for instance in a pick-and-place process. Subsequently, the second circuit board  2  with the third solder  25  and the subunit  100  and the further arbitrary electronic components  27  arranged on the third solder  25  are arranged in one of the previous or a further reflow soldering machine  300  (e.g., see  FIG. 13 , in which the third solder  25  is melted and subsequently cooled to below its solidification temperature). As a result, the subunit  100  and the further arbitrary electronic components  27  are fixedly connected to the second circuit board  2 . After the soldering process, the soldered arrangement may be removed from the reflow soldering machine  300  and optionally be mounted in a bottom housing  32 , see  FIG. 14 . 
         [0033]    Optionally, after the first solder  16  and the second solder  15  are cooled to below their respective solidification temperatures, a subunit which includes the first circuit board  1  with the top metallization layer  13  and the bottom metallization layer  14 , the first solder  16 , the second solder  15  and the number of electronic components  17  but not the shielding frame  18 , the thermal interface material  4  and the housing cover  31 , may have a weight such that the ratio between the mass of the subunit and the total surface area A141 tot  is less than or equal to 0.08 g/mm 2 . This allows soldering further electronic components to the bottom metallization  24  of the bottom metallization layer  24  of the second circuit board  2  with the subunit already soldered to the top metallization layer  23  and with the second circuit board  2  top side  23  down (i.e., facing towards earth) in a reflow soldering process. Due to the low weight of such a subunit, it will not drop down from the second circuit board  2 . 
         [0034]    In any case, a thermal interface material  4 , for instance a thermal interface paste or a self-adhesive thermal interface pad, is arranged on the top side of at least one of the electronic components  17 , that is, on that side of the respective at least one electronic component  17  facing away from the first circuit board  1  (e.g., see  FIG. 15 ). The thermal interface material  4  is applied after the last of all above mentioned soldering processes, as far as such soldering processes are used. 
         [0035]    Regardless of whether the soldered arrangement is mounted in a bottom housing  32  or not, the thermal interface material  4  may be arranged on the respective electronic components  17  prior to or after snapping the spring frame  19  onto the shielding frame  18 . 
         [0036]    Finally, as illustrated in  FIG. 16 , a housing cover  31  with an electrically conductive surface is attached to the first circuit board  1  such that the contact springs  191  of the spring frame  19  electrically contact the housing cover  31 , and that the thermal interface material  4  mechanically contacts the housing cover  31  and at least one of the electronic components  17 . Hence, the electronic components  17  provided with the thermal interface material  4  are efficiently cooled by the housing cover  31 . That is, the housing cover  31  acts both as a heat sink and, due to its electrically conductive surface, as an electromagnetic shielding. For example, the housing cover  31  may be made of a bent thin sheet-metal with a thickness D19 (see  FIG. 3 ), for instance 0.1 mm to 0.2 mm (e.g., about 0.15 mm). 
         [0037]      FIG. 17  is an enlarged view of a section Si of the first circuit board  1  illustrated in  FIG. 3 . The bottom metallization layer  14  includes soldering pads  141  and conductive lines  142 . The conductive lines  142  serve to electrically connect the soldering pads  141 . A solder mask  10  covers the conductive lines  142  but not the soldering pads  141 . Metallic vias  134  are used to electrically interconnect different metallization layers  13 ,  14  of the first circuit board  1 . Optionally, the first circuit board  1  may include one or more additional metallization layers between the top metallization layer  13  and the bottom metallization layer  14 . For instance, a first circuit board  1  may have, including the top and bottom metallization layers  13 ,  14 , four or eight metallization layers, one above the other. In cases where a first circuit board  1  has three or more metallization layers, metallic vias  134  may be used to electrically interconnect any two of the metallization layers of the first circuit board  1 . 
         [0038]    According to one embodiment, none of all metallic vias  134  of the first circuit board  1  is formed in the area of a soldering pad  141  of the bottom metallization layer  14 . In other words, any electric connection between a metallic via  134  of the first circuit board  1  and a soldering pad  141  of the bottom metallization  14  is realized via a conductor line  142  formed in the bottom metallization layer  14 . There is no direct electric contact between a metallic via  134  of the first circuit board  1  and a soldering pad  141 . 
         [0039]    Alternatively, a via may be formed in the area of a soldering pad  141 , then filled with electrically conductive material, and grinded in order to achieve a flat surface of the soldering pad  141 . 
         [0040]      FIG. 18  is an enlarged view of a section S 2  of the first circuit board  1  illustrated in  FIG. 6 . The sections S 1  and S 2  are identical, however, in the arrangement of  FIGS. 6 and 18  (S 2 ) there is a first solder  16  applied over the soldering pads  141  and a second solder  15  applied over the top metallization layer  13 . Although the second solder  15  is still present as a paste, the first solder  16  has already undergone a reflow and a subsequent cooling and solidification process as described above with reference to  FIGS. 4 and 5 . Optionally, the (structured) first solder layer  16  may have an average thickness D16 of at least 100 μm. According to a further option which is independent from an average thickness of the first solder layer  16 , there may exist a plane E2 which does not intersect the first solder  16  with the secondary condition that for each of the soldering pads  141 , the distance d2 between the plane E2 and that section of the first solder  16  arranged above the respective soldering pad  141  is less than 100 gm. 
         [0041]      FIG. 19  again shows an exploded view of a circuit board system  100  as explained above. From this view it can be seen that the shielding frame  18  may include a ring-shaped section and a cross-like section  181  arranged inside the ring-shaped section. The cross-like section  181  may be used in a pick-and-place process, in which the shielding frame  18  is placed on the second solder  15  as described above with reference to  FIG. 7 . The cross-like section  181  may include a point of application which is used by a pick-and-place machine in the pick-and-place process. 
         [0042]      FIG. 20  shows the arrangement of  FIG. 19  in the assembled state, with a thermal interface material  4  applied over the top sides of some of the electronic components  17  and with a housing cover  31  to be mounted on the circuit board system  100 . 
         [0043]      FIG. 21  is a perspective view of a further example of a first circuit board  1  that is populated with a number of electronic components. A shielding frame  18  with several openings  20  is soldered to the first circuit board  1 . Underneath one or more of the openings  20 , electronic components such as processors, power transistors etc. with flat large area top sides (e.g., at least 30 mm 2  each) are arranged.  FIG. 22  is a top view of the first circuit board of  FIG. 21 . 
         [0044]    In order to provide for sufficient cooling of the electronic components, a thermal interface material  4  is applied through some of the openings  20  over the top sides of some of the electronic components  17  underneath the openings  20 . The thermal interface material  4  allows thermally coupling the respective electronic components to a heat sink or a housing cover. As the thermal interface material  4  is applied after the soldering process in which the first circuit board  1  is populated with the electronic components and the shielding frame  18 , the thermal interface material  4  (e.g., a thermal interface paste or a self-adhesive thermal interface pad) is not exposed to the high temperatures of the soldering process. Through this, degradation of the thermal interface material  4  is avoided. 
         [0045]    In  FIG. 24 , the openings  20  are schematically shown in a hatched manner. In order to achieve a sufficient shielding effect, the total footprint area of all openings  20  may be less than 30% of the footprint area of the first circuit board  1 . Hereby, the footprint area of the openings  20  is taken in the plane of the top side  11  of the first circuit board  1 . The footprint area of the first circuit board  1  is basically the area of its top side  11 . 
         [0046]    According to a further option also illustrated in  FIGS. 21 to 24 , the shielding frame  18  may be provided with a bar code. For this, the bar code may be directly printed onto the shielding frame  18 , or attached to the shielding frame  18  by means of an adhesive label that shows the bar code. 
         [0047]    A circuit board system or arrangement as described above may be used in any arbitrary technical field, for instance in automotive applications such as in a head unit of an infotainment system or in a telematic unit. Such a telematic unit may include, for instance, one or—in any combination—some or all of the following functions: emergency call; modem; wireless hot spot; stolen vehicle tracking; global positioning system (GPS). 
         [0048]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.
Methods for producing a circuit board system and a circuit board arrangement are disclosed. One method for producing a circuit board system includes: providing a first circuit board including a top side, a bottom side, a top metallization layer arranged at the top side, and a bottom metallization layer arranged at the bottom side, wherein the bottom metallization layer comprises a number of soldering pads; applying a first solder over the soldering pads; and applying a second solder over the top metallization layer. The method further includes providing a number of electronic components and a metallic or metallized shielding frame; arranging the number of electronic components and the shielding frame on the applied second solder; and soldering the number of electronic components and the shielding frame to the top metallization layer with the second solder.