Plastic housing for receiving an electrical printed circuit board

As, with plastic housings, there is no direct electrical connection between the housing and a metal carrier body, an electrical connection has been made initially to a plug device in the plastic housing in extremely complex fashion, electrical connection to chassis ground being provided outside the plastic housing via a lead to the carrier body. With plastic housings metal inserts are used for mechanically fixing the mounting flange. According to the present invention said inserts can also be electrically connected to an electrical printed circuit board terminal very simply by means of an electrically conducting connecting device.

BACKGROUND OF THE INVENTION
 The invention relates to a plastic housing for receiving an electrical
 printed circuit board according to the preamble of patent claim 1 and a
 procedure for mounting an electronic subassembly on a carrier body
 according to patent claim 5.
 Increasingly, electronic subassemblies are being made with plastic
 housings, especially in the automotive industry, since these are
 considerably lighter, in particular, as well as being relatively easy and
 cheap to manufacture, which in turn reduces fuel consumption. To attach
 the plastic housing to a metal carrier body, the plastic housing generally
 includes a mounting flange with an opening. However, as the plastic
 housing is very susceptible to damage in the area of the opening as a
 result of the forces arising during the mounting operation as when
 tightening screws or riveting, a hole-strengthening metal insert is
 employed, as can be seen, for example, from German patent DE 90 07 621 U1
 (hollow rivet 4 in FIG. 1).
 However, plastic housings have the disadvantage of not being able to
 provide electrical connection to a metal carrier body, as is used in the
 automotive industry, in particular, for connecting to chassis ground. A
 purely friction-grip connection, provided, for example, by screwing the
 printed circuit board to the metal housing carrier body, does not satisfy
 the safety requirements of the automotive industry, as the printed circuit
 board cannot be totally enclosed by the plastic housing and also because a
 friction-grip connection such as this cannot guarantee electrical contact
 over the service life of the product. More particularly, the printed
 circuit board displays a tendency to warp slightly over time in the area
 of the friction-grip connection and the large contact surface required for
 providing electrical contact eventually disappears. Cables passed through
 and welded inside the plastic housing, as can be seen from German patent
 DE 43 30 977 C2, for example, are extremely susceptible to breakage.
 For this reason, electrical wiring to the chassis is currently guided
 through a plug construction in the plastic housing together with the other
 wiring for the printed circuit board and from there connected at the screw
 fastening point to the metal carrier body, and thus to chassis ground,
 during mounting by means of a cable clamp or similar device. A plastic
 housing of this type with a state-of-the-art connection to chassis ground
 via a plug device is shown at FIG. 2. Housing 1 contains printed circuit
 board 2, which has an electrical connection 14 to plug device 15. In plug
 device 15 a grounding pin not further explained is connected to a lead 11
 outside plastic housing 1 and led to flange 10 and mechanical connecting
 means 12 (shown as a screw). However, the cost of having a separate
 grounding pin such as this in the plug device, in particular the cost of
 the parts, and of the mounting operation, including that for lead 11
 outside housing 1, is considerable. German patent DE 297 09 904 U1
 describes a solution similar to this. Additional contact springs for
 grounding purposes are described in German patent DE 91 14 287 U1.
 SUMMARY OF THE INVENTION
 A plastic housing for making a simple electrical connection between a
 printed circuit board and a metal carrier body is disclosed.
 The basic idea is to employ the metal insert, hitherto used solely for
 mechanically stabilizing the mounting flange, as an electrical conductor
 and to connect it electrically with an electrical terminal on the printed
 circuit board. The economic benefits gained by the saving in plug device
 cost can be of decisive market importance. Particularly advantageously,
 the necessary electrical connection between the metal insert and the
 electrical printed circuit board terminal can be realized by running an
 electrical lead such as a metal or wire element bent according to the
 shape of the plastic housing within the plastic housing material from the
 printed circuit board terminal to the metal insert. An electrical
 connecting device such as this is, for example, co-molded directly with
 the plastic material during manufacture of the plastic housing. It may
 also be joined to the metal insert or be an inherent part of it.
 Equally advantageous is the second method of providing electrical
 connection to the printed circuit board terminal by way of an electrically
 conducting layer on the surface of the plastic housing, preferably on the
 inner side. As is commonly known, deposition of a corresponding
 metallization path is very easy to accomplish.
 Generally speaking, electrical connection between the metal insert and the
 printed circuit board can also be made by means of a metal part inside the
 plastic housing such as a screening plate or a metal part of the
 inherently plastic housing.
 In describes a preferred development of the invention the electrical lead
 is run along the inside surface of the plastic housing to a metal
 interference-fit pin that on assembly is pressed into an at least
 partially metallized opening in the plastic housing on the one hand and
 into an opening located in the electrical printed circuit board terminal
 on the other and retained on both sides.
 In a procedure for mounting an electrical subassembly having a metal insert
 in the mounting flange, an additional snap-in means being provided on the
 underside of the plastic housing and an associated snap-in opening on the
 carrier body, by means of which the subassembly is initially locked in
 place and then aligned by rotating the subassembly with respect to the
 snap-in connection so that the metal insert in the subassembly housing
 mounting flange comes to rest over a second opening in the carrier body,
 where the subassembly is retained by a mechanical connecting means such as
 a screw or a rivet. It is particularly advantageous to determine by
 testing the conductivity of the electrical connection between printed
 circuit board and metal carrier body whether the mounting operation has
 been successfully completed and a corresponding electrical connection
 made. The process monitoring of the chassis ground is of particular
 importance, especially for safety-relevant subassemblies, as in the
 automotive industry, for example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is a detailed drawing of an electrically conducting connecting
 device 4 providing electrical connection between metal insert 3 in
 mounting flange 10 and connection zone 5 of printed circuit board 2. Metal
 insert 3 and connection zone 5 of printed circuit board 2 are depicted in
 sketch form in FIG. 1. In this example electrical connecting device 4 is
 fashioned from a metal sheet; it can, however, be accomplished by an
 appropriately bent wire construction. Feasibly, electrically conducting
 connecting device 4 can be permanently joined to metal insert 3 or they
 can be fashioned from one part. Electrically conducting connecting device
 4 is formed according to the shape of the plastic housing and the location
 of connection zone 5 and metal insert 3 in mounting flange 10.
 Electrically conducting connecting device 4 is molded or inserted in the
 plastic housing material, as shown in FIG. 3. Electrically conducting
 connecting device 4 is pressed or soldered into connection zone 5 of
 printed circuit board 2, although with an interference fit it may be
 possible to dispense with soldering insofar as connection zone 5 is
 accordingly metallized also on the inner side of opening 7. FIG. 2 shows
 the state of the art.
 FIG. 3 shows a plastic housing 1 having electrically conducting connecting
 device 4 between metal insert 3 in mounting flange 10 and connection zone
 5 of printed circuit board 2. Therefore, connection to chassis ground is
 not made with connecting lead 17 via plug device 15, but rather directly
 from connection zone 5 of printed circuit board 2 by way of electrically
 conducting connecting device 4 and metal insert 3 to a metal carrier body
 not further shown in FIG. 3, which itself is connected to electrical
 ground.
 Here, however, connecting means 12 may also assume an electrical connecting
 function, should it be of electrically conducting material.
 FIG. 4 depicts an interference-fit pin 6 between electrically conducting
 connecting device 4 and connection zone 5 of printed circuit board 2, the
 latter and metal insert 3 again being shown in sketch form. Here, the
 dimensions of opening 7 in connection zone 5 of printed circuit board 2
 ensure that interference-fit pin 6 is pressed in and rigidly retained
 during assembly.
 As at FIG. 3, FIG. 5 shows a sectional view of a mounted plastic housing 1,
 an interference-fit pin 6 according to FIG. 4 being provided in this
 example between connecting device 4 and connection zone 5 of printed
 circuit board 2. FIG. 5 depicts the mechanical connection of a mechanical
 connecting means (screw) 12 to metal carrier body 8. The embodiment
 featuring the twin-sided interference-fit pin 6, in particular, is also
 suitable for establishing electrical connection to a metal component that
 forms a constituent part of the housing such as a screening plate. It may
 also be used for housings containing metal parts or for all-metal
 housings, it then being possible to dispense with a further metal
 connecting device if there is direct connection with metal insert 3.
 FIG. 6 is a detailed drawing of the snap-in means 13 on the underside of
 the housing by means of which plastic housing 1 is additionally secured on
 carrier body 8.