GB2142477A - Electrical circuit assembly - Google Patents

Abstract

An electrical circuit assembly comprises a mother board (1) and a plurality of printed circuit boards (2) which are mechanically and electrically connected to the mother board. The mother board (1) is provided with a plurality of sets of conductive areas (4) which are interconnected by printed conductors (40). Electrical connection between the printed circuit boards (2) and the corresponding set of conductive areas (4) is by way of elastomeric connectors (3) which press against the conductive areas (4) and the printed conductors on the circuit boards. The arrangement allows partial equipping of a system without the necessity of providing redundant printed circuit board connectors on the mother board. <IMAGE>

Description

SPECIFICATION Electrical circuit assembly The invention relates to an electrical circuit assembly comprising a mother board and one or more printed circuit boards electrically and mechanically connected to the printed circuit board so that the plane of each printed circuit board is substantially perpen circular to that of the mother board.

The electronics industry commonly uses assemblies which comprise a mother board which carries a plurality of edge connectors into which printed circuit boards are plugged, the mother board being provided with printed conductors which interconnect the contacts of the edge connectors in a desired manner. Many systems are sold in a form which allows for a maximum sized installation but which may be only partly equipped. An example of such a system is a private telephone exchange which may have a maximum capacity of say 20 extensions but a particular customer may presently require only 6 extensions.In such a case it would be possible to supply edge connectors only at the positions where printed circuit boards are present but in order to simplify manufacture and to allow for possible future expansion of the system (which may or may not occur) edge connectors are normally provided at all positions on the mother board where a printed circuit board may subsequently be added. This arrangement suffers from the disadvantage of requirting either the provision of relatively expensive redundant parts for many systems, i.e. the edge connectors in positions where no printed circuit boards are to be inserted, or the use of non-standard mother boards, i.e. mother boards with edge connectors only where printed circuit boards are initially to be inserted. The first alternative increases the cost of systems where the maximum capacity is not required, which is likely to be the majority of systems.The second alternative increases manufacturing costs by requiring special instructionsforthe manufacture of each mother board to ensure that the correct number of edge connectors are provided and makes the expansion of already installed systems more difficult.

It is an object of the invention to enable the provision of an electrical circuit assembly in which the effects of these disadvantages are mitigated.

The invention provides an electrical circuitassem- bly as set forth in the opening paragraph characterised in that the mother board is provided with a plurality of electrically conductive contact areas, each printed circuit board is provided with an elastomeric connector clamped to conductors thereon, and means are provided for clamping each printed circuit board against the mother board so that the elastomeric connector is pressed against the conductive contact areas to form an electrical connection between conductors on each printed circuit board and the conductive contact areas.

By providing an elastomeric connector on each printed circuit board and merely providing printed conductive areas on the mother board a low overall system cost can be achieved. The mother board remains standard regardless of the system size, up to the designed maximum, while the provision of separate expensive unused connectors is avoided, the only surplus provision being that of the unused conductive areas which is much less expensive than the individual conventional printed circuit board edge connectors.

A single elastomeric connector may be clamped between two printed circuit boards so as to simultaneously electrically interconnect both printed circuit boards and the mother board.

Cut outs may be provided in the mother board to co-operate with one or more latching mechanisms to clamp each printed board to the mother board. If the latching mechanism is carried by the printed circuit board assembly a quick and simple mechanical and electrical connection may be provided between the mother board and printed circuit boards without requiring access to the rear of the mother board thus removing a design restraint from the assembly.

The electrically conductive contact areas may comprise selectively printed gold areas on the mother board.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a perspective view of an electrical circuit assembly according to the invention.

Figure 2 shows a perspective view of an elastomeric connector for use in the assembly of Figure 1, Figure 3 is a plan view of a printed circuit board assembly for use in the assembly of Figure 1, Figure 4 is a cross-sectional view on line A-A of Figure 3, Figure 5 illustrates a first arrangement for mechanically coupling a printed circuit board to the mother board in an assembly as shown in Figure 1, Figure Gshows a mother board for an electrical circuit assembly according to the invention, the mother board being formed to co-operate with a latching mechanism attached to the printed circuit board, and Figure 7 shows a printed circuit board assembly mechanically latched to the mother board of Figure 6.

Figure 1 shows an electrical circuit assembly comprising a mother board 1, a printed circuit board 2, and an elastomeric connector 3. The mother board 1 has a plurality of sets of electrically conductive contact areas or pads, one of which is indicated by the reference numeral 4, each set being arranged in a straight line and being separated from the next set by a distance which is dependant on the desired spacing of the printed circuit boards. The sets of contact areas 4 are interconnected in a desired pattern by means of electrically conductive tracks 40 printed on the mother board 1.

Figure 2 shows one form the elastomeric connector 3 may take. As shown in Figure 2 the connector 3 comprises an elongate block of elastomeric material having a generally D-shaped cross section with two flat side faces 5 and 6 and a curved face 7. The block may, for example comprise alternating layer of conductive and non-conductive silicone elastomers or may comprise a core of non-conductive silicone elastomer having flat parallel metal wires wrapped round the core. The metal wires or the conductive silicone layers provide the electrical connection while the silicone elastomer core provides the spring force.

Figures 3 and 4 show an assembly of two printed circuit boards 10 and 11 between which an elastomeric connector 12 of the kind shown in Figure 2 is sandwiched. The printed circuit boards 10 and 11 are held a desired distance apart by two spacers 13 and 14 and four bolts 15,16,17,18which pass through holes in the printed circuit boards and spacers and co-operate with respective nuts 19, 20, 21, 22 to compress the edge connector 12. Thus a sufficiently high contact force to enable reliable electrical interconnection to be made between the two printed circuit boards 10 and 11 can be achieved. Further when this assembly is mounted on the mother board 1 the curved face 7 enables electrical connection to be made to the contact areas 4.

Figure 5 illustrates one arrangement for mechanically coupling and electrically connecting the printed circuit board assembly to the mother board. As shown in Figure 5 two studs 23 and 24 are embedded in the spacer 13 and are arranged to pass through holes in the mother board 1 when the printed circuit board assembly is located against the mother board. Corresponding nuts 25 and 26 are tightened onto the studs to bolt the printed circuit board assembly to the mother board. This arrangement requires access to the rear of the mother board 1 is order to assemble the nuts on the studs and to tighten the nuts and the contact force between the elastomeric conductor and the contact areas on the mother board, and hence the contact resistance, is dependent on the degree to which the nuts are tightened. This can lead to an indeterminate contact force and the possibility of unreliable contact.An alternative arrangement which does not require the provision of access to the rear of the mother board and which is capable of providing a comparatively constant contact force without requiring skilled assembly is illustrated in Figures 6 and 7.

Figure 6 shows a mother board 60 having a plurality of rows of electrically conductive contact areas 61-1 to 61-n. In addition the mother board 60 is provided with a plurality of pairs of cut-outs 62-1 to 62-n and 63-1 to 63-n situated one at each end of each row of contact areas. The cut outs 62 and 63 comprise a combination of a rectangular slot and a circular aperture and are designed so that an appropriately dimensioned circular rod having a transverse member can be inserted into the cut-out butwhen rotated through 90" the transverse member prevents withdrawal. The contact areas 61-1 to 61-n are electrically interconnected in a desired pattern by means of printed conductors (not shown) formed on the surface of the mother board 60.

Figure 7 shows a printed circuit board assembly designed to co-operate with the mother board 60.

The assembly shown in Figure 7 comprises two printed circuit boards 70 and 71 between which are sandwiched two spacers 72 and 73 and an elastomeric connector 80 to form an assembly substan tially of the form shown in Figures 3 and 4. The spacers 72 and 73 have circular apertures through which a circular cross-section rod 74 having a handle 75 is inserted. The rod 74 is provided with a cross member 76 adjacent the end opposite the handle 75.

A helical compression spring 77 is arranged round the rod 74 and is fitted between a flange 78 on the rod 74 and the spacer 72.

To produce an electrical circuit assembly accord ing to the invention from the parts shown in Figures 6 and 7 the printed circuit board assembly is offered against the mother board 60 so that the elastomeric connector 80 is brought into contact with a selected row of contact areas 61-1 to 61-n. The printed circuit board assembly is provided with two rods 74 located adjacent to each side of the assembly and these rods are aligned with respective cut-outs 62 and 63 on the mother board 60.The ends of the rods 74 are inserted into the cut outs 62 and 63 against the compressive force of the spring 78 and the rods 74 arethen rotated through 90 about their longitudinal axes to cause the cross members 76 to engage with the rear surface of the mother board 60 thus retaining the printed circuit board assembly against the mother board.

This arrangement allows a quick and simple connection of the printed circuit board assembly to the mother board and does not require the provision of access to the rear of the mother board. Clearly if access to the rear of the mother board is readily available the mechanism described could be reversed so that the rod was inserted from the rear of the mother board and the cross member 76 was arranged to locate on the spacer 72. Depending on manufacturing tolerances achievable and the electrical and mechanical specification required of the assembly it may be possible to dispense with the spring 77 and rely solely on the resilience of the elastomeric connector so to provide the required contact force between the connector 80 and the contact areas 61.

In order to obtain a repeatable and reasonably constant contact pressure overthewhole length of the connector 80 it is desirable to construct the mother board 80 and spacer 72 in such a form that bowing ofthese members is minimised, either by making these members relatively stiff or by providing fixing members at regular intervals. One method of providing the requisite stiffness is to form the mother board from a metallic alloy and to use techniques common in hybrid circuit technology to form the contact areas and the electrical connections therebetween. A suitable material for such a mother board is an iron-chromium-aluminium alloy with yttrium additions which is described in U.K. Patent Application No. 2111312 together with the techni ques for forming hybrid or printed circuits on its surface. Using a material of this type has the further advantage that the contact areas can be selectively gold printed rather than having to gold plate the areas which leads to either the use of gold plated conductors also and hence extravagent use of gold or requires complicated masking procedures which greatly increase the manufacturing costs.

It is not essential to form printed circuit assemblies from two boards with the elastomeric connector sandwiched between them. Single board assemblies can be simply constructed by clamping the elastomeric connector against conductors on the board in any convenient manner.

Claims (5)

1. An electrical circuit assembly comprising a mother board and one or more printed circuit boards electrically and mechanically connected to the mother board so that the plane of each printed circuit board is substantially perpendicular to that of the mother board, characterised in that the mother board is provided with a plurality of electrically conductive contact areas, each printed circuit board is provided with an elastomeric connector clamped to conductors thereon, and means are provided for clamping each printed circuit board against the mother board so that the elastomeric connector is pressed against the conductive contact areas to form an electrical connection between conductors on each printed circuit board and the conductive contact areas.

2. As assembly as claimed in Claim 1, in which a single elastomeric connector is clamped between two printed circuit boards so as to simultaneously electrically interconnect both printed circuit boards and the mother board.

3. An assembly as claimed in Claim 1 or Claim 2, in which cut outs are provided in the mother board to co-operate with one or more latching mechanisms to clamp each printed board to the mother board.

4. As assembly as claimed in any preceding claim in which the electrically conductive contact areas comprise selectively printed gold areas on the mother board.

5. An electrical circuit assembly substantially as described herein with reference to the accompanying drawings.