GB2112419A

GB2112419A - Coated electrical connection elements

Info

Publication number: GB2112419A
Application number: GB08235461A
Authority: GB
Inventors: Raymond Chapelot
Original assignee: Labinal SA; Precision Mecanique Labinal SA
Current assignee: Safran Electrical and Power SAS; Precision Mecanique Labinal SA
Priority date: 1981-12-28
Filing date: 1982-12-13
Publication date: 1983-07-20
Also published as: DE8234750U1; GB2112419B; DE3245781A1; FR2519201A1; IT8268474A0; JPS58150204A; FR2519201B1; IT1156394B

Abstract

The contacts of such elements made of highly electrically conductive material have a coating of tin or a tin alloy with a further very thin coating of chromium. The chromium coating preserves solderability whilst maintaining the ductility of the tin coating.

Description

SPECIFICATION Improvements relating to the surface treatment of electrical connection elements The present invention relates to the electrical connection interface between different electrically conductive elements. These latter may be: electrical conductors such as insulated cables with a single-strand or multi-strand conductor core, -non-insulated strands, lugs of electrical components, exposed ends of insulated conductors, detachable connecting elements such as cable thimbles, terminals, pluggable contact elements such as male/female connections of cylindrical cross section, male/female connections of flat cross section, connections of pluggable components, which may be permanently interconnected by mounting or welding, or temporarily interconnected by elastic insertion or mechanical pressure.

The invention relates more particularly to electrical connection elements manufactured in large quantities.

Conductive elements are generally made of a material of high electrical conductivity such as copper and its alloys or aluminium and its alloys.

As these various materials are liable to suffer corrosion from the atmosphere, the latter may under extreme circumstances cause the conductor elements to break (as in the case of the strands of conductors) and in any event lead to deterioration of the electrical connection with other elements, owing to increase in the electrical resistance of the contact, and it is thus usual to protect the materials in question by providing them with a coating of a metal of high electrical conductivity.

Among the coating methods most frequently applied are the following: cadmium-bichromium, almost completely confined to termninal boards; nickel coating, used for detachable connections and pluggable connections; such coatings can only be deposited on finished components when the contact zones are accessible to the chemical or electroplating bath (in order to preclude the formation of blind holes) or when the connection elements do not have the soldered; silvent coating, used on conductor (strands), detachable conductors, connections of components and sometimes printed circuits; despite their advantages, i.e. malleability and solderability, silver coatings, owing to the cost of this material, are nowadays almost entirely confined to high-frequency connections in which surface conductivity is of prime importance;; precious metals (such as gold or palladium) applied to detachable or pluggable connecting elements and also to the connection zones of printed circuits by depositing them on suitable substrates.

Coatings of this kind are increasingly expensive, their application being consequently less and less frequent.

The coatings most widely used are metal coatings of tin or stannic alloys. These coatings afford satisfactory protection and, thanks to their exceptional ductility, are advantageously applied in the continuous treatment of conductor wires and strips which after being cut, bent and shaped as required, are intended as detachable or pluggable connection devices. The fact is that, in the course of the various machining operations involved, the basic layer of tine, or other coatings, follows the different deformations of the substratum without cracking and also does not erode the tools employed.

In addition to the fact that the cost of such coatings with a tin base is tenable, they have the advantage of enabling soft-brazing solder to be used.

Attempts have nevertheless been made to provide, as a provisional measure, a layer of oxide of low conductivity between contacts treated with a coating having a tin base, so that for the passage current of low or very low intensity a very high contact pressure has to be exerted to enable the current to pass through the said oxide layer.

This contact pressure, which in the case of microcurrents may reach several hundred grams per contact element, is hardly compatible with the technology or miniaturized connectors, particularly when the latter comprises a large number of contact elements.

It has also been found that the solderability of such coatings is reduced in the course of time, and this naturally raises a serious problem in the case of parts which sometimes may have to be stored for a considerable period.

The purpose of the present invention is to remedy the various drawbacks described, and it is aimed at a surface treatment process for electrical connection elements of a highly conductive material such as copper or aluminium or alloys of either of these metals which has undergone a surface treatment with a tin base. In the process to which the present invention relates the said surface coating with a tin base is to be provided with a layer of chromium deposited thereon.

This additional treatment ensures the reduction of the stannic oxides already present on the surface to be treated and improved surface protection for the entire assembly. It also increases the surface hardness of the contact element, and, as it is extremely thin, it does not detract from the ductility of solderability of the protection with a tin base.

In preference, the layer of chromium has an approximate thickness of 1/100 micron.

The invention will now be described in greater detail by reference to certain particular embodiments thereof, given solely as examples and illustrated in the accompanying drawings, wherein: Fig. 1 is an enlarged partial transversal section, in the direction shown by the arrows I of Figure 2, through a conductor treated in accordance with the invention; Fig. 2 is a view, in perspective, of a rigid insulated conductor from the end of which the insulation has been removed; Fig. 3 is a view in perspective of a flexible multi-strand cable, the end being similarly exposed; Fig. 4 is a sectional view in the direction indicated by the arrows IV of Figures 2 or 3; Fig. 5 is a view in perspective of a flexible multi-strand cable of which the end is exposed and which has received a subsequent tin-coating treatment; Fig. 6 is a sectional view in the direction shown by the arrows VI of Fig. 5;; Fig. 7 illustrates two pluggable contact elements prior to insertion; Fig. 8 is a view of the contact elements of Fig.

7 in the insertion position; Fig. 9 is a sectional view in the direction shown by the arrows IX of Fig. 8, showing the interconnection of the two elements of Figs. 7 and 8; Fig. 10 is a view showing a female electrical connection element and an edge of printed circuit card before the operation of plugging it in; Fig. 1 1 is a similar view to Fig. 10, showing the two elements interconnected; and Fig. 12 is a partial sectional view in the direction shown by the arrows XII of Fig. 1 1.

Fig. 1 shows an enlarged partial section through the conductor of Fig. 2, and constitutes a detail drawing of the protection provided. The substrate or base metal of high electrical conductivity is marked 1, and a coating has been deposited according to the invention on its surface 2, the coating being marked 3 as a whole and consisting of a layer 4 of an alloy with a tin base, on which the thin layer 5 of chromium is superimposed.

The surface 2 may have a layer of material capable of preventing the migration of certain components of the substrate 1 to the coating 4 or of assisting the adhesion of the said coating 4.

Fig. 2 shows an insulated rigid conductor 6 comprising a solid wire 7 embedded in an insulant 8. One of the ends of the said conductor 7 is exposed and has a coating 3 in accordance with the invention, deposited on a substrate 1, which consists of a material of high electrical conductivity (see Fig. 4 and Fig. 1).

Fig. 3 shows a flexible multi-strand conductor 8 comprising seven single strands 9 accommodated in an insulating sheath 10. One of the ends of said conductor 8 is exposed, showing the strands 9, each of which has a coating in accordance with the invention, deposited on a substrate 1 of a material of high electrical conductivity (see Fig. 4 and Fig. 1).

Fig. 4 shows an enlarged cross section in the direction indicated by the arrows IV of Figs. 2 and 3, illustrating in detail the single conductor 7 of Fig. 2 or a single strand 9 of Fig. 3 and showing the protection 3 on the substrate 1.

Fig. 5 shows a flexible multi-strand conductor 8a similar to the conductor 8 of Fig. 3 and comprising a number of single strands 9a accommodated in an insulating sheath 1 ova. One of the ends of the conductors 8a is exposed, showing the single strands 9a, which have no individual coating but which, after being laid bare, have been provided with a tin coating 11 comparable to the coating 4 deposited as shown in Fig. 1 and then covered with a thin layer of chromium 5 similar to that of Fig. 1 (see Fig. 6).

Figs. 7 and 8 show an assembly of pluggable electrical contacts comprising a female element 12 and a male element 13 of cylindrical shape, one of the ends of which is provided with clips 14 to enable a flexible conductor 15 to be attached thereto. The male element 13 is made from a strip of material of good electrical conductivity, both surfaces of said strip being covered with the protections 3, consisting of a layer of tin 4 on the one hand and a thin layer of chromium 5 on the other (see Fig. 1). The female element 12 is likewise produced from a strip of material of good electrical conductivity of which at least the internal surface has been coated in advance with the protections 3 of Fig. 1. Both the final shape of the male element 13 and that of the female element 12 can be obtained by the cutting, rolling and stamping of the strip of metal treated in advance.

The female element 12 is provided at its free end with a tubular portion of substantially cylindrical shape into which has been stamped three oblong depressions 17 parallel to the axis of the cylinder, their convex sides being directed towards the interior of the said cylinder in order to interact with the cylindrical surface of the male element 13. The contact pressure is ensured by means of a slit 16 which enables the tubular portion to open elastically over its entire length when the male element is introduced.

The electrical contact is ensured by the stamped depressions 17 of the female element and the cylindrical part of the male element, which have received the same treatment 3, consisting of a deposit 4 with a tin base, covered with a thin layer 5 of chromium.

Figs. 10 and 1 1 show, separate from each other and in the interconnected position respectively, a female contact element 18 connected to a conductor 1 5a by means of mounting clamps 19 and 20, and also the edge of a single-face printed circuit card, the example marked 21 being coated with a conductive track 22 and functioning as the male contact element.

The printed circuit is produced from a little plate 23 of insulating material, provided on one of its surfaces with the conductive track 22, generally of copper, on which has been deposited the coating 3 of Fig. 1, consisting of a layer of alloy 4 with a tin base, covered with a thin layer of chromium 5.

The female contact element 18, of a known type, is produced from a strip of material of a high electrical conductivity, on both surfaces of which, after machining, the coating 3 of Fig. 1 has been deposited, as in the case of the element 13 shown in Fig. 7, 8 and 9.

The pluggable part of the element 18 consists of two limbs 24 and 25 substantially symmetrical in respect of the connecting plane, of which the free ends 28 and 29 are bent backwards, forming convex portions 28 and 29 facing towards each other. The elasticity of the limbs 28 and 29 and of the bent free ends 26 and 27 creates, on a level with the contact points 28 and 29, and after the insertion of the printed circuit 21, a pressure P (Fig. 12) on the side having the conductive track 22, counterbalanced by an opposing pressure P' on the side having the insulating surface 21 of the said printed circuit.

The surface treatment in accordance with the invention is highly advantageous inasmuch as it first of all enables the ductility of the coating with a tin base to be maintained, thus enabling contact elements to be produced by the cutting, folding, bending and stamping of a strip of metal treated in advance on one or both surfaces, and thus produced continuously in an economical manner.

In certain cases it is desirable to effect the treatment before the component is constructed in order to ensure a uniform coating over zones which would be inaccessible if the treatment were carried out on the finished parts. It also enables the deterioration which the electrical performances of the electrical contact elements are liable to undergo in the course of time to be considerably reduced, as they are thus protected, whether in an ambient temperature, in dry heat, in damp heat, in a saline misty atmosphere or in industrial surroundings.

The tests performed have shown that the considerable reduction of the surface oxidation enables use to be made of electrical connecting elements designed to carry microcurrents (of the order of 10 yA) with a far lower contact pressure than that required in order to obtain a similar result with a coating of the known type with a tin base. A coating of this kind can be applied in a completely satisfactory manner to miniature connectors even if they have a large number of contacts.

In view of the results obtained with the invention the surface treatment can be applied generally to a wide variety of different electrical connection devices, covering in particular an intensity range from 1/A to about 100 A.

Test have also shown that the electrical connection devices coated with a thin layer of chromium can remain in storage for a considerable period without any appreciable deterioration in their solderability.

This particular advantage offered by the invention also applies to: the electrical conductors of Figs. 2, 3, and 4 treated continuously over their entire length; the electrical conductors of Figs. 5 and 6 given subsequent additional treatment on their exposed ends; the soldering lugs of electronic components; the tin-coated soldering contacts of printed circuits, so that they no longer have to be protected by a deposit of storage varnish.

The thin layer of chromium can be deposited by any suitable method, e.g. by a chemical electroplating process or by evaporation in a vacuum.

The choice of the process to be employed, i.e.

the continuous treatment of a wire, a new untreated strip, a strip cut to shape in advance, the bulk treatment of finished parts, or the subsequent application of additional treatment to finished parts, will depend on the nature of the electrical connection device to be treated.

Needless to say, the invention is not limited to the embodiments described in the foregoing, and numerous modifications can be made thereto in matters of detail without thereby departing from the scope of the invention.

Claims

1. A process for the surface treatment of electrical connection elements of the type of which the portion designed to ensure the conduction of current is made of a material of high electrical conductivity, the said material being provided with a coating of tin or a tin alloy, the said process comprising the operation of depositing a very thin layer of chromium on the said coating.

2. A process according to claim 1 wherein the material of high electrical conductivity is selected from copper, copper alloys, aluminium and aluminium alloys.

3. A process according to claim 1 or 2 wherein the layer of chromium has a thickness of approximately 1/100 micron.

4. A process according to claim 1, 2 or 3 wherein the coating of tin or tin alloy and/or the deposition of chromium are carried out on the connection elements.

5. A process according to claim 1, 2 or 3 wherein the electrical connection elements are manufactured employing material of high electrical conductivity which has been provided with said coating of tin or tin alloy and/or said deposit of chromium.

6. An electrical connection element at least a portion of which is designed to ensure the conduction of current and is made of a material of high electrical conductivity, the material being provided with a coating of tin or a tin alloy on which has been deposited a very thin layer of chromium.

7. An element according to claim 6 wherein the layer of chromium has a thickness of approximately 1/100 micron.

8. A process according to claim 1 substantially as described herein.

9. An electrical connection element substantially as shown in Figures 1, 2 and 4, Figures 3 and 4, Figures 5 and 6, Figures 7, 8 and 9 or Figures 10, 1 1 and 12 and described herein with reference thereto.

10. Electrical connection elements when made by the process of any of claims 1 to 5 or 8.