GB1568464A

GB1568464A - Electrical contacts

Info

Publication number: GB1568464A
Application number: GB14518/78A
Authority: GB
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-04-15
Filing date: 1978-04-13
Publication date: 1980-05-29
Also published as: ES468796A1; BE864830A; SE429278B; BR7802377A; JPS5637654B2; JPS53133763A; FR2387529A1; NL7803606A; SE7804118L; FR2387529B1

Description

(54) ELECTRICAL CONTACTS (71) We, INTERNATIONAL BUSINESS MACHINES CORPORATION, a corporation organized and existing under the laws of the State of New York in the United States of America, of Armonk, New York 10504, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to electrical contacts and electrical connectors and to methods of manufacturing same.

This invention is particularly useful in those applications where it is desired to provide a relatively large number of separable or disconnectable connections in a relatively small space. One example of such an application is a connector system for connecting large-scale integration (LSI) circuit modules to printed circuit cards and boards. The method commonly employed at the present time is to contruct each LSI module so that it has an array of contact pins protruding from the bottom side of the module. The printed circuit card or board includes a like array of connector assemblies for receiving the contact pins on the module when it is plugged into the card or board. Each such connector assembly includes some form of spring mechanism for applying a substantial amount of contact force to its corresponding module contact pin. While this provides satisfactory performance, the card or board mounted connector assemblies are relatively expensive to manufacture and frequently take up more space than is desired. In fact. such space requirements are generally the limiting factor on the number of contact pins that can be provided for any given module. It would be desirable, therefore. to have a connector system which would make possible the provision of a much greater number of contact points in a given size area on a printed circuit board or board. This would allow construction of LSI modules having a much greater number of input/output connections.

This invention is also particularly useful in those applications where it is desired to provide separable multipoint connections at a lower cost per connection point. One example of such an application is the case where it is desired to connect the conductors in a flat multiconductor cable to a printed circuit card or board. A common approach currently in use is to solder the cable conductors to conductive elements on a so-called interposer card. These conductive elements are connected by interposer card wiring to individual spring mechanisms mounted on the interposer card. there being one spring mechanism for each cable conductor. These spring mechanisms are used to engage contact pins mounted on the primary printed circuit.

The invention provides an electrical contact the contact zone of which comprises a dendritic structure of conductive material formed on a conducting contact member and capable of intermeshing, inter-wedging or other interfitting engagement with a similar dendritic structure formed on a mating or co-operating electrical contact.

Preferably the dendritic structure comprising a dendritic growth of conducting metallic crystals.

Preferably the dendritic structure is composed of a noble metal e.g. palladium, platinum, rhodium, iridium, ruthenium or osmium.

The invention also provides an electrical contact comprising a conductive member and a cluster or group of closely spaced. submillimeter sized resilient. metal projections formed on the conductive member by a dendritic growth thereon of crystals of a metal which is electrically conductive, mechanically resilient and does not tarnish at room temperature.

The invention includes an electrical connector system comprising two electrical contacts, each as aforesaid, having their dendritic structures of projections in inter-fitting or otherwise inter-twined engagement with each other in intimate electrical contact, and a mechanism for maintaining the dendritic structures or projections in the aforesaid engagement.

The invention also includes a method of manufacturing an electrical contact, as aforesaid, which method comprises placing a conductive member which is to form part of the contact in a plating solution having a lower than normal concentration of metal ions: connecting a source of direct current between the conductive member and a plating solution electrode, and passing an electrical current through the plating solution at a higher than normal current density; whereby a dendritic structure is formed on the exposed surface of the conductive member.

The invention will now be more particularly described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a greatly enlarged view of a pair of mating dendritic contacts embodying the present invention; Figure 2 is a plan-view of a printed circuit card or board having a plurality of LSI modules mounted thereon and having different widths of flat multiconductor cable connected thereto and is used to explain representative applications of the present invention; Figure 3 is a side elevational view of the printed circuit card of Figure 2: Figure 4 shows an enlarged cross-sectional view of a portion of the printed circuit card of Figure 2 and an enlarged elevational view of a portion of one of the LSI modules of Figure 2, together with an enlarged view of some of the mating dendritic contacts which provide electrical connections therebetween; Figure 5 is an exploded view showing in greater detail the manner in which the flat cables are connected to the printed circuit card in Figure 2: Figure 6 is an enlarged upside down view of one of the flat cables of Figure 2 and shows the dendritic contacts for making electrical connections to the printed circuit card of Figure 2; Figure 7 is an enlarged view of one corner of the printed circuit card of Figure 2 and shows the dendritic contacts thereon which mate with the cable contacts shown in Figure 6; and Figures 8 and 9 are scanning electron microscope photographs showing representative dendritic contact structures on a greatly magnified scale.

Description of the preferred embodiments Referring to Figure 1, there is shown to an enlarged scale a pair of mating dendritic contacts shown just prior to engagement. In particular. there is shown a first electrical contact 10 comprised of a conductive member 11 and a bunch 12 of tinv closely-spaced submillimeter size resilient metal projections 13 formed on the conductive member 11 bv a dendritic growth thereon of conductive metal crystals. The conductive member 11 includes a conductive element or substrate 14 having a thin surface plating layer 15 of noble metal plated on the surface region in which electrical contact is to be established. The metal projections or dendritic structures 13, which are also composed of a noble metal. are formed or grown on the exposed surface of this thin layer 15 by electroplating under conditions which promote the formation of dendritic structures.

There is further shown a second electrical contact 20 comprised of a conductive member 21 and a bunch 22 of tiny closely-spaced submillimeter size resilient metal projections 23 formed on the conductive member 21 by a dendritic growth thereon of conductive metal crystals. The conductive member 21 includes a conductive element or substrate 24 having a thin surface plating layer 25 of noble metal plated on the surface region in which electrical contact is to be established. The metal projections or dendritic structures 23 are also composed of a noble metal and are formed or grown on the exposed surface of this thin layer 25 by electroplating under conditions which promote the formation of dendritic structures.

The two electrical contacts 10 and 20. when used in the cooperative manner shown in Figure 1. provide a separable or disconnectable electrical connector. Electrical connection is established by urging the metal projection 13 on the first conductive member 11 into an intermeshing or interwedging engagement with the metal projections 23 on the second conductive member 21. An engagement takes place, the metal projections 13 and 23 slide past one another and a wedging action builds up. eventually creating relatively high stresses on various of the crystal surfaces. This is accompanied by a wiping action of metal on metal, thus assuring intimate metal contact and a gas tight seal at many points. The redundancy of contact points insures high reliability and low contact resistance. Any dust particles that may be present are physically displaced and pushed out of the way and do not adversely affect the quality of the electrical connection.

It is important to note that intermeshing engagement of the metal projections 13 and 23 is accomplished by applying only very low forces. The metal projections 13 and 23 slide past one another fairly easily. When connected, the structure is quite stable because of the self-locking properties of the interwedged metal projections and only a minimal amount of clamping force is required to maintain the connection. Note also that because of the dendritic structure a small amount of misalignment or tolerance errors along any spatial axis can be accommodated without serious effect on the contact properties.

The choice of metal to use for the dendritic projections 13 and 23 and the surface plating layers 15 and 25 will be discussed at greater length hereinafter. A good example of a suitable metal for both purposes is palladium. Typically, the substrates 14 and 24 will be made of copper or other metal commonly used for electrical conductors. Assuming the choice of palladium, the surface plating layers 15 and 25 are formed by electroplating onto the substrate surface a thin layer of palladium under plating conditions which do not promote the formation of dendritic structures. The metal projections 13 and 23 are then grown on the exposed surfaces of layers 15 and 25 by electroplating with palladium under conditions which do promote the formation of dendritic structures.

The metal projections 13 and 23 formed in this manner are quite small in size. The maximum height of a projection (dimension H in Figure 1) is in the range of 0.1 to 0.15 millimeters. In the representative applications to be described herein, the width of the entire bunch for a single contact (dimension D in Figure 1) is in the range of 0.5 to 0.8 millimeters. As indicated in Figure 1, the projections making up any given bunch will be of various different sizes and shapes intermingled in a more or less irregular manner.

Referring now to Figures 2 and 3, there will be described representative uses of dendritic electrical contacts of the kind shown in Figure 1 to provide new and improved multiple contact electrical connector systems for electronic apparatus. Figures 2 and 3 show a printed circuit card or board 30 having a plurality of large scale integration (LSI) circuits modules mounted thereon. Three such LSI modules 31, 32 and 33 are shown in place on the board 30. A fourth module 34 (Figure 4) is mountable at location 35 on the board 30, but has been removed to show an array 36 of small dendritic contacts formed on the board 30 for purposes of making multipoint electrical connections to the LSI module 34. A like array 37 (Figure 4) of small dendritic contacts are located on the underside of the LSI module 34 for individually mating with the corresponding contact in the array 36. Elements 38 are guide pin holes in the board 30 for receiving matching guide pins 39 (Figure 4) which protrude downward from the underside of the LSI module 34.

No distinctions are made or intended herein between the terms "card" and "board".

Both of these terms refer to the same type of physical structure and are used herein in a synonymous or interchangeable manner.

Each dendritic contact 36a, 36b, 36c, etc., in the board mounted array 36 is of the same construction as described for the contact 10 in Figure 1. Each dendritic contact 37a. 37b, 37c, etc., in the module mounted array 37 is of the same construction as described for the contact 20 in Figure 1. Thus, multiple electrical connections are established by urging the module 34 downward toward the board 30 and causing the metal projections on the mating contacts to slide into intermeshing or interwedging engagement with one another.

By way of example, the array 36 in Figure 2 may include a total of 400 individual dendritic contacts located in a 2.54 centimeter by 2.54 centimeter square area. In such case. the conductive substrate for each contact may take the form of a circular pad having a diameter of 0.5 millimeters, with a spacing of 1.27 millimeters between the centers of adjacent pads.

The matching array on the underside of the module 34 would, of course, have these same dimensions. This figure of 400 contacts should be compared with the less than 100 (typically 70 to 90) contacts which can be provided in this same size area using commonly employed existing techniques.

As indicated in Figure 4, the board 30 is a multilayer printed circuit board. Inner layers of copper foil 40 and 41 are sandwiched between layers of electrically nonconductive material 42, 43 and 44. An additional layer of copper may be deposited on the upper surface of upper insulating layer 42 and a further layer of copper deposited on the lower surface of lower insulating layer 44. The copper layers are selectively etched in the known manner to provide the desired electrical conductor patterns. each of the inner layers 40 and 41 being etched before the covering insulation layer is placed over it. The contact pads on which the dendritic projections are grown are formed in this same manner. In other words. the contact pads or conductive substrates for the contacts 36a. 36b. 36c. etc.. are formed on the upper surface of insulating layer 42 by depositing thereon a thin layer of copper and then etching away the undesired copper.

By way of example only, the pad for contact 36b is shown as being electrically connected to a conductor in the foil layer 40 and the pad for contact 36c is shown as being electrically connected to a conductor in the foil layer 41. This is accomplished by drilling small holes through the surface pads and appropriate insulating layers and thereafter filling the holes with conductive material. The connecting conductive material for the contact 36b pad is indicated at 45, while the connecting conductive material for the contact 36c pad is indicated at 46. These pad to inner foil connections are made before the dendritic projections are grown on the pads. Electrical connection to the pad for contact 36a is made may way of copper on the surface of insulating layer 42 but, for simplicity of illustration, is not shown.

Multipoint electrical connections for the other modules 31-33 are provided in the same manner as just described for the module 34.

As indicated in Figures 2 and 3, the modules 31-34 are held in place on the board 30 by means of elongated bars 47 which are attached to the board 30 and cross bars 48, the ends of which are secured to the elongated bars 47 by small screws 49. As indicated in Figure 3, the cross bars 48 bear against the tops of the modules to prevent them from working free of the board 30 as a result of vibration or the like. Thus. this mechanism maintains the metal projections on the mating dendritic contacts in interwedged engagement with one another.

The force applied by the cross bars 48 is. however, more in the nature of a retaining force, as opposed to a clamping force. As such, it is of relatively small value.

Figures 2 and 3 also illustrate the use of the dendritic contacts for purposes of connecting flat multiconductor electrical cables to the printed circuit card or board 30. The board ends of four different flat cables 50, 51, 52 and 53 are shown in Figure 2. As seen from the exploded view of Figure 5. these flat cables 50-53 are clamped near the edge of the board 30 by means of U-shaped pressure pads 54-57, respectively, and U-shaped spring clamps 58-61. respectively. Pressure pads 54-57 are made of rubber or other pliable nonconductive material.

As indicated in the upside down view of Figure 6 for the case of flat cable 50, each of these flat cables is comprised of a goodly number of flat electrical conductors 62 arranged in a side-by-side manner and sandwiched between two layers of nonconductive material. For example, the conductors 62 may be thin strips of copper foil embedded in a flexible plastic material. A pair of guide pin holes 63 are provided near the ends of each cable for mating with corresponding guide pins 64 mounted on and protruding a short distance above the surface of the board 30. Figure 7 is an enlarged view of that portion of the board 30 to which the flat cable 50 is connected. It is to the same scale as the enlarged Figure 6 view of flat cable 50.

A multipoint connector system for connecting the individual conductors in the cable 50 to a corresponding set of conductors on the printed circuit board 30 is provided by forming a linear array of dendritic contacts directly on the cable 50 and by forming a corresponding linear array of dendritic contacts near the edge of the board 30. In particular. small electrical contact areas 65 are formed near the end of the cable 50 by scraping away or otherwise removing the insulating material covering the conductors 62. A thin surface plating of noble metal is then plated onto each of these exposed contact areas 65. after which dendritic metal projections are grown on this surface plating in each such contact area 65. For simplicity of illustration. the dendritic projections are not shown in Figure 6.

Mating dendritic contacts are formed on a corresponding array of small contact areas 66 located on the board 30 as shown in Figure 7. Each such contact area 66 is comprised of a copper foil contact pad affixed to the surface of the board 30. In each case. a thin layer of noble metal is plated on the upper surface of the copper and a bunch of dendritic metal projections are grown on the upper surface of the noble metal plating. Again. for simplicity of illustration, the dendritic projections are not shown in Figure 7.

The cable 50 is connected to the board 30 by taking the cable 50 as shown in Figure 6 and turning it upside down to give it the orientation shown in Figure 5 and then placing the guide pin holes 63 onto the guide pins 64. The pressure pad 54 is then slipped over the overlapping portions of cable 50 and board 30 in the manner indicated in Figure 3. after which the spring clamp 58 is slipped over the pressure pad 54 and moved to its final position as also shown in Figure 3. This mechanism maintains the dendritic projections on the contact areas 65 of cable 50 in interwedged engagement with the dendritic projections on the corresponding ones of the contact areas 66 on the board 30.

Each of the contact areas 66 on board 30 is electrically connected to a diffent one of copper foil conductors 67 formed on or within the printed circuit board 30. For the case of the multilayer board being considered, some of these conductors 67 mav lie on the surface of the board 30. while others may be located at the different inner foil levels within the board 30. In the latter cases. electrical connections are made thereto by drilling small holes and filling them with conductive material in the manner as described in connection with Figure 4. The individual contact areas 66 on the board 30 are the same size as the individual contact areas 65 on the cable 50. The width of each such contact area 65 or 66 is the same as the width of one of the cable conductors 62, which width may be. for example, 0.254 millimeters. The length of each contact area 65 or 66 may be about twice that value or, in other words, about 0.5 millimeters.

The connector system for the other flat cables 51-53 are of the same construction as that just described for the flat cable 50, except that different numbers of dendritic contacts are provided in accordance with the differing numbers of electrical conductors in the other cables 51-53.

As seen from the foregoing examples, electrical contacts constructed as described in those examples can be used to provide very space efficient multipoint connector systems of the separable or disconnectable type. No individual spring assemblies are required for each connection point and the contacts can be made very small in size, This savings in space can be used, as illustrated by the LSI module to board connector system described above, to provide a greater number of independent electrical connections in a given size area.

Also of considerable significance is the fact that electrical connector systems described as examples herein are lower in cost than those constructed with presently used techniques.

This results primarily from the elimination of the individual springs and contact pins for the various connection points and the labour involved in assembling connectors which use same. A further savings occurs for the case of flat cable from the improved cable usage efficiency which results from being able to cut the cable to any desired width so as to include any desired number of conductors. The number of board located contacts can be easily varied to accommodate any number of cable conductors.

The fabrication of an individual electrical contact will now be considered in greater detail using, where appropriate, the contact 10 of Figure 1 as the model for purposes of discussion. With this in mind, the tiny submillimeter size metal projections 13 which actually make the electrical connection are preferable composed of a metal which is electrically conductive, mechanically resilient and does not tarnish at room temperature in a normal room environment. By "tarnish" is meant the formation on the exposed surface of the metal of an adherent corrosion film such as an oxide film. a sulfide film or the like. By "resilient" is meant that the metal is springy or elastic in character such that it is capable of recovering its size and shape after applied stresses are removed. As a consequence, the metal projections 13 are springy in character such that thev can be bent or deformed somewhat when intermeshed with a mating contact and then at least tend to resume their original shape and position when the mating contact is removed.

In accordance with these requirements. the metal projections 13 are preferably composed of a noble metal selected from the group consisting of palladium, platinum, rhodium, iridium, ruthenium and osmium. Each of these metals is electrically conductive, does not tarnish at room temperature and can be fabricated to be mechanically resilient. Because of its somwhat lower cost, palladium is probably the more attractive choice.

The dentritically grown structures represented by the metal projections 13 are sometimes referred to as "dendrites". They can assume somewhat different shapes. depending upon the particular electroplating parameters under which they are grown. Probably the better shape for most connector purposes is the needlelike or spearlike acicular shape shown in Figure 1. A somewhat modified shape which also appears to be attractive for some purposes is obtained by allowing some of the needlelike structures to form small mushroomlike knobs at their outer ends. This provides somewhat enhanced self-locking or self-retaining characteristics, where such is desirable.

The thin surface plating layer 15 is also preferably composed of a noble metal. The purpose of the layer 15 is to form a dense. compact. pore-free. corrosion-proof surface upon which to grow the dendritic projections 13. This provides a better bond and suppresses any corrosion or undesired electrolytic actions. This layer 15 may be formed of any of the metals given above as being preferred for the dendritic projections 13. In the case of the layer 15, however, it is not necessary that the metal be resilient in character. Thus, a non-resilient noble metal, such as gold, may also be used for the layer 15.

The conductive element or conductive substrate 14 is. or is part of. the electrical circuit element or circuit hardware structure upon which the contact is to be provided. As such, it may take the form of a conductive wire, pin. rod. bar. pad. plate, sheet or the like.

Typically, the substrate 14 will be made of copper, though it mav be made of any metal commonly used for electrical circuit conductors. If bv chance the substrate 14 should be made of nontarnishing noble metal having the desired surface characteristics. then the thin surface plating layer 15 may be eliminated.

The dendritic structures or projections 13 are preferably grown by electroplating under non-normal conditions. During normal commercial type electroplating operations, consid erable care is exercised to prevent the formation of dendritic structures. For present purposes, the normal electroplating rules are deliberatelv violated in order to promote the growth of dendritic structures. In particular. the dendritic structures or projections 13 are grown by electroplating at a higher than normal current density with a plating solution having a lower than normal concentration of metal ions. By "normal" is meant those values which give a dense, compact, pore-free surface.

By way of contrast, the surface plating layer 15 is formed by electoplating under normal conditions which do not promote the formation of dendritic structures.

A plating solution which has been found useful for growing dendritic projections formed of palladium is a solution of water (H2O), ammonia (NH,). ammonium chloride (NH4Cl) and palladosammine chloride (Pd(NH3)2Cl2). The constituent concentrations found useful are in the range: Pud+2 at 20 to 50 millimolar CL at 2 to 5 molar NH3 to hold pH in the range 9.0 to 10.5 By way of comparison, a normal concentration of palladium ions would be on the order of 100 millimolar, as opposed to the 20-50 millimolar range given above.

Assuming that the surface plating layer 15 has already been formed. the specimen on which the palladium dendritic structures are to be grown is placed in a bath of the above-specified plating solution and is electrically connected so as to form the cathode electrode for the plating operation. Any conductive surface of the specimen which is not to have dendritic projections grown thereon is covered with a layer or film of insulating material before placement of the specimen in the plating bath. A source of direct current is then connected between the cathode formed by the specimen and an anode electrode also located in the plating bath. An electrical current is then passed through the plating solution at a higher than normal current density and the dendritic metal projections are thereby grown on the exposed conductive surface of the specimen.

A current density value on the order of 100 milliamperes per square centimeter has been found useful for this purpose. By way of contrast. the normal current density for plating with palladium without the growth of dendritic structures in on the order of 100 milliamperes per square centimeter. In both cases, thes current density values are for the current density measured at the workpiece or cathode surface.

When it is desired to simultaneously grow dendritic projections on a plurality of separate contact areas. it is necessary that each of these areas be electrically connected to the negative terminal of the direct current source during the performance of the plating operation. For the case of the flat cable shown in Figure 6. this can be accomplished by connecting the far ends of the conductors 62 to the negative direct current terminal and placing the end with the contact areas 65 in the plating bath. For the case of multiple contact areas on a printed circuit board, like those shown in either Figure 4 or Figure 7, simultaneous dendritic growth can be accomplished by leaving copper foil connections between the contact areas for commoning purposes and subsequently etching away these commoning connections after the growth of the dendritic projections on the desired contact areas.

Figures 8 and 9 are photographs of actual dendritic structures grown by electroplating with palladium in the manner described above. These photographs were made with a scanning electron microscope and the dendritic structures shown therein are enlarged or magnified by a factor of approximately 1000 relative to their actual size.

Based on our knowledge and experience to date. electroplating under the novel conditions described above is believed to be the preferred method for forming the dendritic structures. It is not intended, however. that this be taken an as implied limitation in those of the appended claims which made no mention of the method of forming the dendritic structures because other methods also appear to be feasible for this purpose. Such other methods include electroless plating (reduction done chemicallv without use of electrodes and electric current), electroforming and chemical vapor deposition.

Conveniently the projections have an aspect ratio of b typical array 12 will have from these thousand to twenty thousand dendritic projections per square millimeter the actual number in any situation being a function of the particular application. The dendritic projections 13 will range in height from a minimum of 10 to a maximum of 150 microns. In addition the aspect ratio of the dendritic projections 13, the height of a projection divided by its maximum diameter, will vary between a minimum of 4 and a maximum of 10. Further, the silhouette ratio of an array 12. the total area for all projections in an array determined, for example, by perpendicularly illuminating the array from above, divided by the pad area on which the array is formed, will vary between 0.10 to 0.39.

WHAT WE CLAIM IS: 1. An electrical contact the contact zone of which comprises a dendritic structure of conductive material formed on a conducting contact member and capable of intermeshing, interwedging or other inter-fitting engagement with a similar dendritic structure formed on a mating or co-operating electrical contact.

2. A contact as claimed in claim 1, in which the dendritic structure comprising a dendritic growth of conducting metallic crystals.

3. A contact as claimed in claim 1 or 2, in which the dendritic structure comprises a multiplicity of projections.

4. A contact as claimed in claim 3. in which a majority of the projections are of needlelike, spearlike or other acicular shape.

5. an electrical contact as claimed in claim 3 or 4, in which the projections are resilient in character.

6. An electrical contact as claimed in claim 3 or 4. in which the projections are springy in character.

7. An electrical contact as claimed in claim 3. 4, 5 or 6. in which the projections ar submillimeter in size.

8. An electrical contact as claimed in anyone of claims 3 to 7. in which the projections are closely spaced.

9. An electrical contact as claimed in anyone of claims 1 to 8. in which the dendritic structure is composed of a metal which does not tarnish at room temperature.

10. An electrical contact as claimed in anyone of claims 1 to 8. in which the dendritic structure is composed of a metal which does not develop an adherent corrosion film at room temperature in a normal room atmosphere.

11. An electrical contact as claimed in anyone of claims 1 to 10. in which the dendritic structure is composed of a noble metal.

12. An electrical contact as claimed in anyone of claims 1 to 11, in which the dendritic structure is composed of palladium. platinum, rhodium. iridium, ruthenium or osmium.

13. An electrical contact as claimed in anyone of claims 1 to 12. in which the dendritic structure is grown by electroplating under conditions which promote the formation of dendritic structures on the surface being plated.

14. An electrical contact as claimed in claim 13. in which the electroplating is carried out at a higher than normal current density with a plating solution having a lower than normal concentration of metal ions.

15. An electrical contact as claimed in anyone of claims 1 to 14, in which the conducting contact member includes a conductive element having a thin layer of noble metal plated on the surface region to which electrical connection is to be established and in which the dendritic structure is formed on the exposed surface of this thin layer of noble metal.

16. An electical contact comprising a conductive member and a cluster or group of closely spaced, submillimeter sized. resilient. metal projections formed on the conductive member by a dendritic growth thereon of crystals of a metal which is electrically conductive, mechanically resilient and does not tarnish at room temperature.

17. An electrical contact as claimed in claim 16. in which the projections are composed of palladium, platinum, rhodium. iridium. rutheniunm or osmium.

18. An electrical contact as claimed in claim 16 or 17. in which the conductive member includes a conductive element having a thin layer of noble metal plated on the surface region in which electrical contact is to be established and in which the metal projections are formed on the exposed surface of this thin laver of noble metal.

19. A method of manufacturing an electrical contact as claimed in anyone of claims 1 to 18, which method comprises placing a conductive member which is to form part of the contact in a plating solution having a lower than normal concentration of metal ions; connecting a source of direct current between the conductive member and a plating solution electrode, and passing an electrical current through the plating solution at a higher than normal current density; whereby a dendritic structure is formed on the exposed surface of the conductive member.

20. An electrical connector system comprising two electrical contacts, each as claimed

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. typical array 12 will have from these thousand to twenty thousand dendritic projections per square millimeter the actual number in any situation being a function of the particular application. The dendritic projections 13 will range in height from a minimum of 10 to a maximum of 150 microns. In addition the aspect ratio of the dendritic projections 13, the height of a projection divided by its maximum diameter, will vary between a minimum of 4 and a maximum of 10. Further, the silhouette ratio of an array 12. the total area for all projections in an array determined, for example, by perpendicularly illuminating the array from above, divided by the pad area on which the array is formed, will vary between 0.10 to 0.39. WHAT WE CLAIM IS:

1. An electrical contact the contact zone of which comprises a dendritic structure of conductive material formed on a conducting contact member and capable of intermeshing, interwedging or other inter-fitting engagement with a similar dendritic structure formed on a mating or co-operating electrical contact.

in any one of claims 1 to 18, having their dendritic structures or projections in inter-fitting or otherwise inter-twined engagement with each other in intimate electrical contact, and a mechanism for maintaining the dendritic structures or projections in the aforesaid engagement.

21. A multiple contact electrical connector system for electronic apparatus comprising: a first component having an array of small electrical contacts formed thereon; a second component having a like array of small electrical contacts formed thereon: each electrical contact of both arrays being as claimed in any one of claims 1 to 18 and the dendritic structures of the contacts of the respective components interfitting or otherwise inter-twining with each other in intimate electrical contact: and a mechanism for maintaining the dendritic structure in electrical contact as aforesaid, whereby an array of reliable, readily disconnectable, electrically independent. low resistance electrical connections are made in a very small space.

22. An electrical contact substantially as hereinbefore described with reference to. and as shown in, Figure 1, or Figure 8 or Figure 9 of the accompanying drawings.

23. An electrical assembly substantially as hereinbefore described with reference to and as shown in Figures 2, 3 and 4 of the accompanying drawings, which assembly comprises electrical contacts, each as claimed in claim 22.

24. An assembly as claimed in claim 23, having a flat cable connected thereto substantially as hereinbefore described with reference to and as shown in Figures 5. 6 and 7 of the accompanying drawings.

25. A method of manufacturing an electrical contact as claimed in any one of claims 1 to 18 or claim 22, which method comprises growing the dendritic structures by electroplating substantially as hereinbefore described.