Patent Publication Number: US-11398695-B2

Title: Plug connector with improved shielding and method of producing the same

Description:
FIELD 
     The present invention relates to a plug connector system and to a method for manufacturing a plug connector system. 
     BACKGROUND INFORMATION 
     Electrical plug connectors, for instance for automotive applications, are available in the related art. A plug connector of this kind can be installed on a housing, for instance, in a shielded fashion, components that are electrically connected to the plug connector being disposed in the interior of the housing. Such components can be, for instance, control device components such as a circuit board having electronic or electrical constituents (ASICs, resistors, capacitors, coils, etc.) disposed thereon, or data processing modules that output large data flows (e.g. more than 100 Mbit/s) via the electrical connection, or high-current terminals, e.g. contacts of an inverter of an electrically operated motor vehicle; currents of more than 10 A or more than 50 A can flow, for example, via such high-current terminals, and voltages of more than 12 V or more than 45 V or more than 100 V can be reached. The combination of plug connector and housing can be referred to as a “plug connector system.” It can be necessary to keep an external environment of the housing and of the plug connector as free as possible of electromagnetic radiation from the interior of the housing, or conversely it can be advisable to shield the interior of the housing as much as possible from irradiation of electromagnetic radiation from the external environment. 
     When the plug connector of the plug connector system is plug-connected to a complementary counterpart plug connector, that assemblage of a housing, plug connector, and counterpart plug connector can be referred to as a “plug connector assemblage.” 
     SUMMARY 
     In accordance with an example embodiment of the present invention, the housing of a plug connector system is advantageously constituted from a favorable and highly thermally conductive and shielding material, e.g., a metal, for instance aluminum. It can be advantageous to electrically contact a shielding element, for instance a sheet-metal shield of the plug connector, to the, for instance, electrically conductive housing, so as thereby to achieve continuous shielding. It can be advantageous in this context to furnish an electrical contact resistance between the housing and the shielding element which is as low as possible, since large shielding currents can occur. 
     Aluminum can, however, generally form, on its surface, aluminum oxide that does not have particularly good electrical conductivity. It can therefore be advantageous to use copper, or a material that predominantly encompasses copper, for such a shielding element of the plug connector. 
     The following problems can occur with an exemplifying material pairing of this kind: on the one hand the poorly conductive aluminum oxide layer of the housing can produce an elevated contact resistance. On the other hand, because of the differing electrochemical standard electrode potentials of aluminum and copper, so-called “contact corrosion” can occur, for example when even a low level of atmospheric humidity is present, at the interface between the housing and the shielding element. This can locally increase the contact resistance. In addition, a soft material such as aluminum can creep when a large pressure is applied to it. These two effects (contact corrosion and creep in response to excessive contact force or point load) can cause a decrease over time in that proportion of the contact surface through which a low contact resistance exists. The contact resistance can therefore undesirably rise. 
     A need can therefore exist for furnishing a plug connector system that is simple to manufacture, can be manufactured using inexpensive materials, reliably maintains shielding between the housing and a shielding element of the plug connector over a planned service life of the plug connector system, and in which the contact resistance between the housing and the shielding element of the plug connector remains as low as possible over the service life. This need can exist in particular with single-wire shielding. 
     This need can be met by an example embodiment of the present invention. Advantageous embodiments of the present invention are described herein. 
     According to a first aspect of the present invention, a plug connector system is provided. In accordance with an example embodiment of the present invention, the plug connector system has:
         a plug connector;   a housing on which the plug connector is installed or attached or fastened or disposed.       

     The housing has an opening having a first inner wall, the plug connector having a sheet-metal shield that projects at least in portions into the opening. A ring is disposed in the opening and, with an outer wall, electrically contacts the first inner wall of the opening, the sheet-metal shield electrically contacting, with a further outer wall of the sheet-metal shield, a second inner wall of the ring. 
     In other words, if an axial direction is defined by a center axis of the opening, what results is then the following sequence of elements when viewed radially externally: firstly the inner wall of the opening, then, adjacently inward, the ring having an outer wall and second inner wall, then, adjacently as an innermost element, a first portion of the sheet-metal shield which, with the further outer wall of the sheet-metal shield, contacts the inner wall of the ring. These three elements (inner wall of the opening, ring, sheet-metal shield) can be disposed, for instance, directly adjacently, e.g. with zero clearance, i.e., in direct, gap-free contact. A particularly large contact area, and thus a particularly low contact resistance, can result therefrom. 
     Particularly simple installation can thereby advantageously be brought about. With further advantage, a particularly large contact area, and thus a particularly low contact resistance, can thereby be brought about, since the ring, constituting a relatively small and easily fabricated part, can be fabricated substantially more simply and less expensively with high precision in terms of its dimensions. The opening in the housing can thus be fabricated relatively coarsely, i.e., with a relatively large tolerance, for instance, for a diameter of the opening, and/or can be, for instance, very rough on the first inner wall, for instance directly from an injection molding process or from a melt casting method or from a die casting process (e.g., aluminum die casting). Thanks to the disposition of the ring in the opening, on the one hand a good and large contact area with respect to the first inner wall of the opening can be brought about, and at the same time, thanks to more precise tolerances for the ring, it is possible to ensure that the sheet-metal shield always securely and reliably contacts the second inner wall of the ring. Lastly, the ring or the material of the ring can be selected so that contact corrosion does not occur either at its contact surface with the housing or at its contact surface with the sheet-metal shield. 
     The plug connector can represent, as described above, a kind of external electrical interface for components that are disposed in an interior of the housing. 
     The plug connector can have, for instance, a plug connector housing and/or a contact pin or contact blade. 
     The plug connector can initially be embodied as an element separate from the housing, and can be installed on, for instance placed onto or into, the housing only in the course of a manufacturing operation. It can then be, for instance, bolted or riveted on, or adhesively bonded on. The plug connector housing can be configured, for instance, from a plastic, for instance injection-molded from a thermoplastic, for instance from polyamide, polyethylene, polypropylene, or other plastics. 
     The sheet-metal shield can be embodied in the manner of a crown. In other words, the sheet-metal shield can have a base element from which a plurality of arms can project oppositely from an insertion direction of a counterpart plug connector into or onto the plug connector. The insertion direction can proceed, for instance, parallel to the axial direction. The base element can be embodied in a continuous ring shape. It can constitute a first portion of the sheet-metal shield which protrudes into the opening of the housing. The further outer wall of the sheet-metal shield, which electrically contacts the inner wall of the ring, can belong to the base element. For better and more durable or more reliable contacting, contact points can be embodied, for instance by embossing, on the further outer wall. Upon installation of the plug connector on the housing the sheet-metal shield can be disposed in, for instance pressed into, the ring, for instance, by way of a press fit or by nonpositive or frictional engagement. 
     The housing can also have a plurality of at least two openings in each of which, for instance, a ring is disposed, and the plug connector has, correspondingly thereto, a corresponding number of (or fewer) sheet-metal shields that each protrude in portions, at least with a first portion, into one of the openings. 
     The fact that the ring is fastened in the opening by nonpositive or frictional engagement, or the ring being pressed into the opening, advantageously ensures particularly reliable contacting between the ring and housing over the service life, with a large contact area. At the same time, this manner of fastening ensures that any nonconductive surface that might be present, for instance oxide layers or dirt, oil, grease, etc., are broken through, thereby lowering the contact resistance between the ring and the housing. Lastly, penetration of fluid media, e.g. air or oxygen, into the contact point between the ring and the first inner wall of the opening is thereby impeded or prevented, so that the electrical connection permanently has a low resistance. 
     The advantageous result of the fact that an average first roughness of the outer wall of the ring is equal to less than 1.0 μm, preferably less than 0.5 μm, particularly preferably less than 0.35 μm, and very particularly preferably less than 0.3 μm is that a particularly large contact area, and thus a particularly low contact resistance, is produced between the ring and the first inner surface of the opening. A further advantageous result that can be achieved by way of the smooth surface of the outer wall of the ring thereby created is that a possibly rough(er) first surface of the opening, for instance having an average roughness of more than 10 μm, is, as it were, shaved off or ground down by the first outer wall of the ring, and particularly good contact, and a particularly low contact resistance, are thereby produced. The reason is that when, for instance, the radially inwardly projecting “peaks” of the first inner wall of the opening, which contribute to a high average roughness, become abraded, any surface contamination or any oxide layer then also thereby becomes abraded, and the electrical contact resistance decreases. The low roughness of the outer wall of the ring which is thereby selected furthermore minimizes the risk of damaging or scratching a coating of the first inner wall of the opening which may possibly be present. The ring can thus also be used, for instance, for systems in which a first inner surface has a high-grade coating (e.g., using a noble metal) and must not be scratched. The ring can thus be utilized in modular fashion, and effects of scale with regard to manufacture can occur. 
     The average roughness is usually referred to using the abbreviation R a . 
     The advantageous result of the fact that an average second roughness of the second inner wall of the ring is less than 1.0 μm or less than 0.5 μm or less than 0.35 μm or less than 0.3 μm is to decrease the risk of damaging or destroying or scratching a possible coating of the sheet-metal shield. A further result is to produce a smooth contact surface of maximum size between the second inner wall of the ring and the sheet-metal shield, and thus a low electrical contact resistance. 
     The advantageous result of the fact that an average third roughness of the further outer wall of the sheet-metal shield is less than 1.0 μm or less than 0.5 μm or less than 0.35 μm or less than 0.3 μm is to lower the risk of damaging or destroying or scratching the second inner wall of the ring. A further result is to produce a smooth contact surface of maximum size between the second inner wall of the ring and the sheet-metal shield, and thus a low electrical contact resistance. 
     Because the further outer wall of the sheet-metal shield has a coating, it is advantageously possible to permanently minimize the electrical contact resistance of the sheet-metal shield and/or to protect the sheet-metal shield from corrosion, for instance as a consequence of exposure to air. The insertion force can also be reduced. It is also advantageously possible, depending on the coating, for the surface to be hardened as compared with the surface of the uncoated sheet-metal shield. A coating of this kind can be embodied, for instance, in multiple plies. This advantageously makes possible particularly good adhesion of the outermost layer onto the sheet-metal shield (and/or onto the ring) by the fact that one or several plies of material that can serve, for instance, as adhesion promoters is/are applied between the outermost layer and the sheet-metal shield material (and/or ring material). A difference in standard electrode potential between the sheet-metal shield material and the outermost layer or the ring, and then also with respect to the housing, can thereby also be decreased gradually, which lowers the risk of contact corrosion. 
     For example, the coating can encompass, for instance predominantly, a material that is selected from the group: silver, gold, platinum, palladium, nickel, tin. For example, the coating can encompass, considered from inside to outside, firstly nickel (directly on the sheet-metal shield or on the ring), and then (facing outward) silver, i.e. applied onto the nickel. 
     Provision can also be made in principle, alternatively or additionally, that an inner surface and/or an outer surface of the ring has a coating. This also makes it possible, with a suitable coating sequence, to decrease the difference in standard electrode potentials between the ring material and the outer and/or inner contact partners. 
     Particularly simple, inexpensive manufacturing is enabled, and particularly good shielding in the region of the opening is ensured, by the fact that the first inner wall of the opening encompasses or predominantly encompasses aluminum. 
     For example, the predominant portion of the housing can be produced from aluminum or from alloys having aluminum, or can predominantly encompass aluminum. 
     The advantageous result of the fact that the first inner wall of the opening exhibits, before installation of the ring, an average fourth roughness that is more than 10 μm, or is between 10 μm and 50 μm, or is between 15 μm and 30 μm, is that the housing can be used directly after manufacture without further processing, for example directly after injection molding or melt casting or die casting of the housing. Because no further processing then needs to be necessary performed, the housing can thus be manufactured particularly inexpensively. Thanks to the arrangement of the ring in the opening, a layer having poor electrical conductivity which may possibly be present on the surface of the first inner wall can advantageously be shaved off or abraded particularly effectively or at a particularly large number of locations. After the ring is placed in (or pressed into) the opening, a large contact area is then present which does not have any electrically insulating layers (e.g. oxide or dirt). The probability of such an abrasion process at least at some points is greatly increased because of the high roughness. At the same time, the ring thus does not need to exhibit high roughness on its outer wall and does not need to scratch the surface of a smooth first inner surface. The ring can thus also be used, for instance, for systems in which a first inner surface has a high-grade coating (e.g., using a noble metal) and must not be scratched. 
     Particularly simple and inexpensive manufacture of the plug connector or of the sheet-metal shield is made possible by the fact that that portion of the sheet-metal shield which projects into the opening of the housing encompasses or predominantly encompasses copper. Copper (or an alloy of copper) furthermore has particularly good electrical conductivity and thus particularly low electrical resistance. Copper furthermore has particularly good thermal conductivity. Even large shielding currents can thus be dissipated without difficulty with no occurrence of excessive heating of the shielding. 
     The risk of contact corrosion is advantageously decreased by the fact that the ring is constituted from a material that has a standard electrode potential that is between the standard electrode potential of the first inner wall of the opening of the housing and the standard electrode potential of that portion of the sheet-metal shield, in particular having no coating, which projects into the opening. In other words, the risk of an increase over time in the contact resistance between the housing, or the first inner wall of the opening, and the sheet-metal shield can thereby be reduced. Especially with a material pairing of aluminum (housing) and copper (sheet-metal shield), the durability of the shielding can be considerably increased by the interposition of the ring. 
     Particularly inexpensive manufacture is made possible by the fact that the ring is constituted from steel or stainless steel, or the ring predominantly encompasses steel or stainless steel. Stainless steel or steel has a standard electrode potential that is, for instance, between that of aluminum and that of copper, or between aluminum and silver. Stainless steel or steel can furthermore be manufactured without corrosion, so that corrosion of the ring as such can be suppressed even in a context of high relative humidity. Lastly, steel or stainless steel is a relatively hard material, so that a ring produced in that manner can be permanently introduced into the opening, e.g., using a pressing operation, without difficulty. The opening of the housing can also, for instance, have relatively wide tolerances in terms of its inside diameter; thanks to the hard material of the ring, the latter can have, for instance, an outside diameter that is larger than or equal to the largest first inside diameter (within a production tolerance band) of the opening. If the housing then encompasses material that is relative soft compared with the ring, the ring can always be securely and fixedly disposed or installed or fastened in the opening, for instance using a pressing-in operation. 
     According to a second aspect of the present invention, a method for manufacturing a plug connector system is provided. An example method in accordance with the present invention includes the following steps:
         furnishing a housing having an opening that has a first inner wall;   furnishing a ring having an outer wall and a second inner wall;   disposing the ring in the opening in such a way that the ring, with its outer wall, electrically contacts the first inner wall of the opening;   furnishing a plug connector having a sheet-metal shield, the sheet-metal shield having a further outer wall;   disposing the plug connector on or in or at least in portions in the housing, in such a way that the sheet-metal shield projects at least in portions into the opening and, with the further outer wall, electrically contacts the second inner wall of the ring.       

     The advantageous result is to make possible particularly simple production of the plug connector system, to bring about a particularly low electrical contact resistance, and to create the possibility of preventing contact corrosion. 
     The fact that the ring is pressed into the opening advantageously brings about particularly durable seating of the ring in the opening. A particularly large contact area between the ring and the first inner surface of the opening is furthermore created. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will be apparent to one skilled in the art from the description below of exemplifying embodiments with reference to the figures, which are nevertheless not to be construed as limiting the present invention. 
         FIG. 1  is a perspective and partly sectioned depiction of a plug connector assemblage in the non-plug-assembled state in accordance with an example embodiment of the present invention. 
         FIG. 2  is a perspective view of a detail of a housing of the plug connector system of the plug connector assemblage of  FIG. 1 . 
         FIG. 3  is a perspective view of an exploded depiction of the plug connector system of  FIG. 1 . 
         FIG. 4 a    is a cross section through the plug connector system of  FIG. 1 . 
         FIG. 4 b    is an enlarged detail of  FIG. 4   a.    
         FIG. 5  is a cross section through the plug connector assemblage of  FIG. 1  in the plug-assembled state. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
       FIG. 1  is, by way of example, a partly sectioned perspective depiction of a plug connector assemblage  200  in accordance with an example embodiment of the present invention, in the non-plug-assembled state. Plug connector assemblage  200  is constituted from a plug connector system  100  and a counterpart plug connector  60  which can be inserted in an insertion direction E onto or into a plug connector  1  of plug connector system  100 . 
     All that is depicted of plug connector  1  here is a plug connector housing  2  having an outer wall  3 . Two studs  4  facing away from each other project from plug connector housing  2  in a radial direction R transversely to an axial direction A that here extends parallel to insertion direction E. A circumferential direction U extends around axial direction A. 
     Counterpart plug connector  60  has a counterpart plug connector housing  61  on which a lever  62  is disposed in rotatably mounted fashion. Lever  62  has on both sides, in the region of its shaft, a respective gate  63  into which a respective stud  4  of plug connector  1  can engage upon plug-assembly of counterpart plug connector  60  with plug connector  1  in insertion direction E. By rotating lever  62  it is thus possible to convert the rotary motion into a plugging motion in insertion direction E, and thus to reduce the force that an installer needs to apply for plug assembly. 
     Two electrical leads  70  project from counterpart plug connector  60  oppositely to insertion direction E. In the interior of counterpart plug connector  60 , an inner conductor  71  (not visible here) of lead  70  is connected to a contacting element  78  (not visible here) for each lead  70 . This contacting element  78  is embodied suitably for contacting a corresponding contact element  5  (not visible here) of plug connector  1  (see e.g.  FIG. 5 ). 
     Plug connector system  100  has:
         the aforementioned plug connector  1 , which here has, for example, a plug connector housing  2  made of plastic;   a housing  20  on which plug connector  1  is installed, e.g. inserted, and then bolted on or fastened releasably or nonreleasably by way of a riveted connection or an adhesively bonded or welded connection.       

     Housing  20  is merely indicated here, and is shown in section to allow better depiction of the further components or elements of plug connector system  100 . Housing  20  can be embodied here, for example, from aluminum or an aluminum alloy. 
     In the exemplifying embodiment, housing  20  has two openings  21  each having a first inner wall  22 . It is also possible, however, for only exactly one opening  21  to be provided in housing  20 ; or more than two openings  21  can be provided. 
     Housing  20  has an interior  28  in which further components, e.g. a circuit board, a control device, a data processing module, electrical terminals of an inverter, etc., can be disposed, interior  28  of housing  20  preferably being electrically shielded with respect to an external environment  29  of the housing. Distal first ends  5   a  of two contact pins  5  (see  FIGS. 3 to 5 ), which each engage through one of the two openings  21  and can be electrically connected or contacted at a second distal end  5   b  (see  FIGS. 4 and 5 ) respectively to one of electrical leads  70 , also project into interior  28  of housing  20 . Contact pins  5  can be connected, in interior  28  of housing  20 , for example to electrical terminals for data leads or high-current applications. They can have, for instance, a cross section of at least 1 mm 2  or at least 10 mm 2  or at least 20 mm 2  in order to be able to transfer currents of at least 1 A or at least 10 A or at least 50 A. 
     Plug connector  1  has a sheet-metal shield  40  (not visible in this Figure) that projects at least in portions into opening  21  (see  FIGS. 3 to 5 ). Disposed in opening  21  is a ring  30  that, with an outer wall  31  of ring  30 , electrically contacts first inner wall  22  of opening  21 . Sheet-metal shield  40  (not visible here) electrically contacts, with a further outer wall  41  of sheet-metal shield  40 , a second inner wall  32  of ring  30 . Continuous shielding from housing  20  to sheet-metal shield  40  is thereby ensured. This shielding can then be embodied uninterruptedly by an electrical connection of the sheet-metal shield to a counterpart plug connector sheet-metal shield  90  (see  FIG. 5 ) and from there to a shielding conductor  73  of lead  70 ; what exists here, for instance, is a single-wire shield or single-lead shield, and not a collective shield. Contact pins  5  are each individually shielded by a sheet-metal shield  40 . 
     Ring  30  is embodied here, by way of example, as a press-in part. It can be disposed in or pressed into opening  21 , for instance, by nonpositive or frictional engagement, and as a result can exhibit a large contact area with respect to first inner wall  22  of opening  21 , thus resulting in a low electrical contact resistance. 
     Ring  30  can be embodied, for instance, from steel or stainless steel. Housing  20  or first inner wall  22  of opening  21  can be embodied, for instance, from aluminum or an aluminum alloy. The use of this material pairing reduces the risk of contact corrosion. The electrical contact resistance thus remains low over the service life. 
     An average first roughness RA 1  of outer wall  31  of ring  30  is equal to less than 1.0 μm, preferably less than 0.5 μm, particularly preferably less than 0.35 μm, and very particularly preferably less than 0.3 μm. Outer wall  31  of ring  30  is thus very smooth. 
     First inner wall  22  of opening  21  has, for example, before installation of ring  30 , an average fourth roughness RA 4  that is equal to more than 10 μm or is equal to between 10 μm and 50 μm or is equal to between 15 μm and 30 μm. For example, inner wall  22  of opening  21  is constituted directly from the casting of the housing, i.e. without machining. 
     When ring  30 , with its smooth outer wall  31  and its hard material, is then pressed into opening  21 , it shaves off or abrades off all the “peaks” on first inner wall  22  of opening  21 , and thus also any nonconductive surface layer such as oxide layers or greases or oils or contaminants. The result is to bring about a particularly large contact area, and thus a particularly low and permanently low contact resistance. 
       FIG. 2  is a perspective view of a detail of housing  20  of plug connector system  100  of plug connector assemblage  200  of  FIG. 1 . Disposed in the housing, around the two openings  21  and between the two openings  21 , are a total of five fastening openings  25  onto or into which plug connector housing  2  can be fastened, for example via a threaded connection or rivets. 
     Housing  20  has the two openings  21  in one housing wall  24 . Housing wall  24  is penetrated in each case completely, in the manner of a channel, at the locations of the two openings. The rim of openings  21 , however (embodied here, merely by way of example, circularly), is embodied in two steps when considered along insertion direction E or axial direction A. In other words, viewed from external environment  29 , an opening  21  here firstly has a first diameter D 1 . The diameter of opening  21  then decreases slightly, for instance by an amount equal to a ring thickness DR of ring  30 , to a second diameter D 2 . The ring thickness can be, for instance, in a range between 100 μm and 3 mm, preferably between 500 μm and 1.5 mm. The result is to create in opening  21  a pedestal on which ring  30 , inserted or pressed into opening  21  or disposed in opening  21 , rests. This makes possible particularly simple installation of ring  30  in opening  21 , since ring  30  cannot fall into interior  28  of housing  20 . A third diameter D 3  (not depicted here), constituting an outside diameter of ring  30 , can be, for instance at room temperature (in particular under light pressure), identical in size to first diameter D 1  of opening  21 , or it can be, at room temperature, slightly larger than first diameter D 1  of opening  21 , for example 1 μm to 1000 μm larger, preferably 10 μm to 500 μm larger, and very particularly preferably 20 μm to 250 μm larger. Ring  30  can thus be held by a press fit in opening  21 . 
     This  FIG. 2  clearly illustrates outer wall  31  and second inner wall  32  of ring  30 , as well as first inner wall  22  of opening  21 . 
       FIG. 3  is a perspective exploded view of plug connector system  100  of  FIG. 1 . Contact pins  5  here have not yet been inserted into plug connector housing  2 . Two receptacles  6 , each for one fastening element  7 , are embodied at an end of plug connector housing  2  which faces toward interior  28  of housing  20 . 
     Fastening element  7  is depicted here by way of example as a threaded nut. Fastening element  7  is inserted into receptacle  6 ; contact pin  5  is then slid with its first distal end  5   a  into plug connector housing  2 . An electrical terminal of, for instance, an inverter, or another electrical contact from interior of  28  of housing  20 , can be bolted onto fastening element  7  when plug connector  1  is disposed on housing  20 . 
     A sealing element  50  in the form of a sealing ring is also disposed between housing  20  and plug connector housing  2 . Penetration of fluid media from external environment  29  into interior  28  of housing  20  can thereby be prevented. The two rings  30 , initially constituting parts separate from housing  20 , are inserted into the two openings  21  of housing  20  or installed therein, for instance pressed thereinto. It is understood that it is also possible for only a single opening  21 , or more than two openings  21 , to be provided in housing  20 . 
     Lastly,  FIG. 3  depicts two sheet-metal shields  40 , one sheet-metal shield  40  for each contact pin  5 . Sheet-metal shield  40  here has, merely by way of example, the shape of a crown. In other words, it has an annularly continuous base element  44 . Disposed on base element  44  is a plurality of, for instance, eight arms  43  that, looking oppositely from insertion direction E, extend from base element  44  and each have a free end  46 . The result of the plurality of arms  43  is on the one hand to ensure contacting redundancy, since shielding with respect to counterpart plug sheet-metal shield  90  ( FIG. 5 ) is ensured even in the event of failure of one arm  43 . At the same time, the plurality of arms  43  results in a plurality of parallel current paths to counterpart plug sheet-metal shield  90 , so that in accordance with Kirchhoff&#39;s law, the electrical contact resistance decreases as compared with only a single contact point. 
     In a state installed in plug connector housing  2  and in plug connector system  100 , base element  44  projects at least in portions into the associated opening  21  of housing  20 . This can be, for instance, portion  45 , for instance the entire base element  44 , or a front portion (considered in insertion direction E) of base element  44 . Base element  44  has further outer wall  41 , or a further outer wall with which sheet-metal shield  40  is contacted to second inner wall  32  of ring  30 . Contact points  42  are embodied on further outer wall  41  of sheet-metal shield  40 , for example by a boss that protrudes radially outward from further outer wall  41 . Contacting with respect to second outer wall  32  of ring  30  can thereby be ensured particularly reliably. In addition, the electrical contact resistance with respect to ring  30  can be decreased by the plurality of contact points  42 , since what is produced here is a kind of parallel circuit having a number of current paths that corresponds to the number of contact points  42  (Kirchhoff&#39;s law). 
     Further outer wall  41  of sheet-metal shield  40  can have, for instance, a coating (e.g., resulting from electrodeposition or a CVD or PVD process). A coating of this kind can, for instance, reduce an electrical contact resistance. It can also serve as a kind of corrosion protection since, for instance, the material of sheet-metal shield  40  could tarnish in contact with air. A coating can, for instance, also bring about hardening of the surface and can thus protect the surface upon insertion. Lastly, a coating of this kind can bring about a reduction in insertion force. A coating of this kind can encompass, for instance, in particular predominantly, a material that is selected from the group: silver, gold, platinum, palladium, nickel, tin. Such a coating can be embodied, for instance, in multiple plies. For instance, the material of sheet-metal shield  40  can have a layer of nickel applied onto it, and a layer of silver onto that. 
     In principle, alternatively or additionally, ring  30  can also have a coating of this kind on outer wall  31  and/or on second outer wall  32 . 
     Second outer wall  32  of ring  30  is preferably embodied to be smooth. It can have, for instance, an average second roughness RA 2  that is less than 1.0 μm or less than 0.5 μm or less than 0.35 μm or even less than 0.3 μm. This minimizes the risk of scratching a surface of shielding panel  40  upon insertion of sheet-metal shield  40  into opening  21 . 
     An average third roughness RA 3  of further outer wall  41  of sheet-metal shield  40  can be, for example, less than 1.0 μm or less than 0.5 μm or less than 0.35 μm or less than 0.3 μm. The insertion force upon insertion into opening  21 , or upon contacting of second inner wall  32  of ring  30 , can thereby be reduced. The risk of damaging the surface of sheet-metal shield  40  can furthermore thereby be reduced. 
       FIG. 4 a    is a cross section through plug connector system  100  of  FIG. 1 . It is apparent that, looking radially from outside in the region of opening  21 , firstly housing  20 , then ring  30 , and then sheet-metal shield  40  follow one another and are in electrical contact. Also clearly apparent, looking in an axial direction A, is the pedestal in housing  20  in the region of the rim of opening  21 . 
     Contact pin  5  has, at its second distal end  5   b , a contact guard  8  that is constituted, for instance, in the manner of a cap made of an electrically nonconductive plastic. This can be important, for instance, for high-current or high-power or high-voltage applications. Plug connector housing  2  has, located radially externally, an insertion opening  9  for counterpart plug connector  60 . 
       FIG. 4 b    is an enlarged detail of  FIG. 4 a   , counterpart plug connector  60  here being inserted into plug connector  1 . This is apparent from counterpart plug connector sheet-metal shield  90 , which projects into insertion opening  9  and contacts an inner surface of arm  43  of sheet-metal shield  40 . 
       FIG. 5  is a cross section through the plug connector assemblage of  FIG. 1  in the plug-assembled state. 
     It is evident that electrical lead  70  has an inner conductor  71  that, when looking radially outward, is surrounded by an inner insulator  72  that in turn is surrounded by a shielding conductor  73  that, lastly, is surrounded externally by an outer insulator  74 . 
     The Figure schematically depicts the manner in which a contacting element  78  establishes the electrical connection between contact pin  5  (which can also be embodied as a contact blade or a contact element, etc.) and inner conductor  71  of electrical lead  70 . 
     Counterpart plug connector sheet-metal shield  90  is in turn connected, by way of a crown-like connecting element  80 , to shielding conductor  73  of electrical lead  70 . 
     Plug connector assemblage  200  is thus radially externally electrically shielded everywhere when viewed in axial direction A or along the current path, and thus exhibits particularly good electromagnetic compatibility (EMC).