CA1121011A

CA1121011A - Dendritic electrical contacts and connectors

Info

Publication number: CA1121011A
Application number: CA000301230A
Authority: CA
Inventors: Robert Babuka; Robert E. Heath; George J. Saxenmeyer, Jr.; Lewis K. Schultz
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-04-15
Filing date: 1978-04-17
Publication date: 1982-03-30
Also published as: DE2816328A1; IT1112620B; DE2816328C3; DE2816328B2; IT7822260A0

Abstract

DENDRITIC ELECTRICAL CONTACTS
AND CONNECTORS
Abstract of the Disclosure An electrical contact is provided by forming on a contact pad or contact surface a bunch of tiny resilient metal projections by a dendritic growth thereon of con-ductive metal crystals. A separable or disconnectable electrical connection is provided by urging the dendritic projections on mating contacts into intermeshing or interwedging engagement. Good quality submillimeter size electrical contacts are readily fabricated and are particularly useful for providing low cost space-efficient multipoint connector systems for large-scale integration (LSI) circuit modules, printed circuit cards and boards and other modern day electronics hardware.

Description

-,, ~ackground of the Invention 22 This invention relates to electricàl contacts and 23 electrical connectors and to methods of manufacturing 24 the same.
This invention is particularly useful in those 26 applications where it is desired to provide a relatively 27 large number of separable or disconnectable connections 28 in a relatively sma~l space. One example of such an 29 application i5 a connector system for connecting large-scale integration (LSI) circuit modules to printed 31 circuit cards and boards. The method commonly employed :: - .~:: . . .

- ~ :

. . ~ . . . . ,: . -:
, . .

1 at the present time i5 to construct each LSI module so

2 that it has an array of contact pins protruding from

3 the bottom side of the module. The printed circuit

4 card or board includes a like array of connector assemblies for receiving the contact pins on the module 6 when it is plugged into the card or board. Each such 7 connector assembly includes some form of spring mecha-8 nism for applying a substantial amount of contact force g to its corresponding module contact pin. While this provides satisfactory performance, the card or board 11 mounted connector assemblies are relatively expensive 12 to manufacture and frequently take up more space than 13 is desired. In ~act, such space requirements are 1 L~ generally the limiting factor on the number of contact pins that can be provided for any given module. It 16 would be desirable, therefore, to have a connector 17 ~ystem which would make possible the provision of a 18 much greater number o~ contact points in a given size 19 area on a printed circuit card or board. This would allow construction of L5I modules having a much greater 21 number of input/output connections.
22 This invention is also particularly useful in 23 those applications where it is desired to provide 24 separable multipoint connections at a lower cost per connection point. One example of such an application 26 is the case where it is desired to connect the conduc-27 tors in a flat multiconductor cable to a printed circuit 28 card or board. A common approach currently in use is 29 to solder the cable conductors to conductive elements on a so-called interposer card. These conductive 31 elements are connected by interposer ca~d wiring to :
-- 2 ~

1 individual spring mechanisms mounted on the in-terposer 2 card, there being one spring mechanism ~or each cable 3 conductor. These spring mechanisms are used to engage 4 contact pins mounted on the primary printed circuit board to which it is desired to make the electrical 6 connections. Such an arrangement provides satisfactory 7 performance, but is somewhat cumbersome and relatively 8 expensive. It would be desirable, therefore, to pro-9 vide a cable-to-board connector system which does not require the use of an interposer card and does not 11 require the use of an individual spring mechanism for 12 each connectiorl point. Among other things, this would 13 provide a substantial reduction in the cost of the 14 connector system.
Other examples could be given, but the foregoing 16 should suffice to show the need for electrical contact 17 systems which cost less and which use less space.
18 Summary of the Invention 19 It is an object of the invention, therefore, to provide a new and improved electrical contact construc-21 tion for providing good quality electrical connections 22 at lower cost per connection point.
23 It is another object of the invention to provide a 24 new,and improved electrical contact construction for '~
providing reliable low resistance connections requiring 26 only a minimal amount of clamping force.
27 It is a further object of the invention to provide 28 a new and improved electrical contact construction for 29 providing good quality multipoint electrical connections with a minimum amount of connector hardware.
31 It is an additional object of the~invention to , -1 provide a new and improved electrical contact construc-2 tion for providing good quality multipoint electrical 3 connections without use of an individual spring assembly 4 or spring ~echanism for each connection point.
It is yet another object of the invention to 6 provide a new and improved electrical contact construc-7 tion for providing very small contact connections which 8 are not adversely affected to any appreciable degree by 9 the presence of dust particles or other commonplace contaminants.
11 It is a further object of the invention to provide 12 a new and improved electrical contact construction for 13 providing a greater number of electrically independent 14 contact points in a given area.
It is an additional object of the invention to 16 provide a new and improved electrical contact construc-17 tion which enables many different sizes and shapes and 18 arrangements of multipoint contact arrays to be readily 19 fabricated at relatively low cost.
It is yet another object of the invention to 21 provide a new and improved electrical contact construc-22 tion for providing reliable low-cost pad-on-pad connec-23 tors for printed circuit cards and boards and the 24 hardware associated therewith.
In accordance with ~he present invention, an 26 electrical contact is provided by forming on a contact 27 pad or contact surface a bunch of tiny resilient metal 28 projections by a dendritic growth thereon of conductive 29 metal crystals. A separable or disconnectable electrical connection is provided by urging the dendritic projec-31 tions on mating contacts into intermeshing or - ~Z~

l interwedging engagement, This interwedging en~a~ement provides 2 a self~locking action~ Thus~ a minimal amount of clamping force 3 is required to mainta;n the connection~ The self~wiping action 4 of the dendritic projections as they intermesh with one another and the contact forces built up by the interwedging action 6 provide a reliable low~resistance connection. The conductive 7 dendritic metal crystal projections are preformed on the contact 8 pad or surface prior to any intermeshing of the contact structure 9 with another like contact structure. The dendritic metal proj-ections are grown on the contact pad or contact surface by ll electroplating same at a higher than normal current density with 12 a plating solution having a lower than normal concentration of 13 metal ions, The contact area covered by a single bunch of 14 dendritic projections may be submillimeter in size. Hence, a relatively large number of contacts can be provided in a rel-16 atively small area.
17 For a better understanding of the present invention, 18 together with other and further objects and features thereof, 19 reference is made to the following description taken in con-nection with the accompanying drawings, the scope of the 21 invention being pointed out in the,appended claims.
~2 srief Descri~tion of the Drawings 23 Referring to the drawings:
24 FIG. l i6 a greatly enlarged view of a pair of mating dendritic contacts constructed in accordance with the 26 present invention;
27 FIG. 2 is a plan view of a printed circuit card or board 28, having a plurality of LSI modules mounted thereon and having 29 different widths of flat multiconductor cable connected thereto and is used to explain representative applications of the 31 present invention;

D

1 FIG~ 3 is a side elevational view of the printed 2 circuit card of FIG. 2;
3 FIG. 4 shows an enlarged cross-sectional view of a 4 portion of the printed circuit card of FIG. 2 and an enlarged elevational view of a portion of one of the 6 LSI modules of FIG. 2, together with an enlarged view 7 of some of the mating dendritic.contacts which provide 8 electrical connections therebetween; .
9 FIG. 5 is an exploded view showing in greater detail the manner in which the flat cables are con-11 nected,to the printed circuit card in FIG. 2;
12 FIG. 6 is an enlarged upside down view of one of 13 the Elat cables of FIG. 2 and shows the dendritic 14 contacts for making electrical connections to the printed circuit card of FIG. 2;
16 FIG. 7 is an enlarged view of one corner of the 17 printed circuit card of FIG. 2 and show$ the dendritlc 18 contacts thereon which mate with the cable contacts 19 shown in FIG. 6; and FIGS. 8 and 9 are scanning electron microscope 21 photographs showing representative dendritic contact 22 structures on a greatly magnified scale.
23 Description of the Preferred Embodiments . . _ ~
2LI Referring to FIG. 1, there is shown to an enlarged scale, a pair of mating dendritic contacts shown just 26 prior to engagement. In particular, there is shown a 27 first electrical contact 10 comprised of a conductive 28 member 11 and an array or bunch 12 of tiny closely-29 spaced submillimeter size resilient metal projections 13 formed on the conductive member 11.by a dendritic 31 growth thereon,of conductive metal ,cry~,talsO The -.. . ' , q " ,., ; -1 conductive member 11 includes a conductive element or 2 substrate 14 having a thin surface plating layer 15 of 3 noble metal plated on the surface region in which 4 elec-trical contact is to be established. The metal projections or dendritic structures 13, which are also 6 composed of a noble metal, are formed or grown on the 7 exposed surface of this thin layer 15 by elèctroplating 8 under conditions which promote the formation of den-9 drltic structures.
There is further shown a second electrical contact 11 20 comprised of a conductive member 21 and a bunch 22 12 of tiny closely-spaced submillimeter size resilient 13 metal projections 23 formed on the conductive member 21 14 by a dendritic growth thereon of conductive metal crystals. The conductive member 21 includes a conduc-16 tive element or substrate 24 having a thin surface 17 plating layer 25 of noble metal plated on the surface 18 region in which electrical contaet is to be established.

. . .
19 The metal projections or dendritic structures 23 are also composed of a noble metal and are formed or grown 21 on the exposed surface of this thin layer 25 by electro-22 plating under conditions whieh promote the formation of 23 dendritic structures.

24 The two electrical contaets 10 and 20, when used `

in the cooperative manner shown in FIG. 1, provide a 26 separable or diseonneetable eleetrieal eonneetor.

27 Electrieal conneetion is established by urging the 28 metal projeetions 13 on the first conductive member 11 29 into an intermeshing or interwedging engagement with the metal pro~ections 23 on the seeond eonductive 31 member 21. As engagement takes place, the metal .

1 projections 13 and 23 slide past one another and a 2 wedging action builds up, eventually creating relatively 3 high stresses on various of the crystal surfaces. This 4 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 oE contact points 7 insures high reliability and low contac-t resistance.
8 Any dust particles that may be present are physically 9 displaced and pushed out of the way and do not adversely affect the quality of the electrical connection.
11 It is important to note that intermeshing engage-12 ment of the metal projections 13 and 23 is accomplished 13 by applying only very low forces. The metal projections 14 13 and 23 slide past one another fairly easily. When connected, the structure is quite stable because of the 16 self~locking properties of the interwedged metal projec-17 tions and only a minimal amount of clamping force is 18 required to maintain the connection. Note also that 19 because of the dendritic structure, a small amount of misalignment or tolerance errors along any spatial axis 21 can be accommodated without serious effect on the 22 contact properties.
23 The choice of metal to use for the dendri-tic 24 projections 13 and 23 and the surface plating layers 15 and 25 will be discussed at greater length hereinafter.
26 A good e~ample of a suitable metal for both purposes is 27 l palladium. Typically, the substrates 14 and 24 will be 28 made of copper or other metal commonly used for elec-29 trical conductors. Assuming the choice of palladium, the surface plating layers 15 and 25 are formed by 31 electroplating onto the substrate surfacçs, 14 and 24 i 1 respectively, a thin layer of palladium under plating 2 conditions which do not promote -the formation of den-3 dritic structures. The metal projections 13 and 23 are 4 then grown on the exposed surfaces of layers 15 and 25 by electroplating with palladium under conditions which 6 do promote the formation of dendritic s-tructures.
7 The metal projections 13 and 23 formed in this 8 manner are quite small in size. The maximum height of 9 a projection (dimension H in FIG. 1~ is in the range of 0.1 to 0.15 millimeters. In the representative applica-11 tions to be described herein, the width of the entire 12 bunch for a single contact (dimension ~ in FIG. 1) is 13 in the range of 0.5 to O.B millimeters. ,As indicated 14 in FIG. 1, the projections making up any given bunch will be of various different sizes and shapes inter-16 mingled in a more or less irregular manner.
17 Photographs of actual dendritic arrays 12 or 22, 18 formed of palladium in a manner to be hereinafter 19 described, are shown in FIGS. 8 and 9. These photo- ~;
graphs were made with a scanning electron mi'croscope 21 and the dendritic projections 13 or 23 illustrated 22 therein have been magnified by a factor of approximately 23 one thousand relative to their actual size. A typical 24 array 12 will have from three thousand to twenty thou-sand dendritic projections per square millimeter, the 26 actual number in any situation being a ~unction of the 27 particular application.
28 The dendritic projections 13 will range in height ' 29 from a minimum of 10 to a maximum of 150 microns. In addition, the aspect ratio of the dendritic projections 31 13, the height,of a projection divided"by its maximum 9 -- ..

1 diameker, will vary between a minimum of 4 and a ma,ximum 2 of 10. Further, the silhouette ratio o~ an array ~2, the 3 total area for all projections.in an array determined, 4 for example, by perpendicularly illuminating the array from above, divided by the pad area on which the array 6 lS formed, will vary between 0.10 and 0.39. .
7 Referring now to FIGS. 2 and 3, there will be 8 described representative uses of dendritic electrical 9 contacts of the kind shown in FIG. 1 to provide new and~
improved multiple contact el.ectrical connector sys-tems 11 for electronic apparatus. FIGS. 2 and 3 show a printed 12 circuit card or board 30 having a plurality of large 13 scale integration (LSI) circuit modules mounted thereon. I
14 Three such LSI modules 31, 32, and 33 are shown in place on the board 30. A fourth module 34 (FIG. 4) is 16 mountable at location 35 on the board 30, but has been 17 removed to show an array 36 of small dendritic contacts 18 formed on the board 30 for purposes of making multi-19 point electrical connections to the LSI module 34. A
like array 37 (FIG. 4) of small dendritic contacts are 21 located on the underside of the LSI ~nodule 34 for 22 individually mating with the corresponding contact in 23 the array 36. Elements 38 are guide pin holes in the 24 board 30 for receiving matching guide pins 39 (FIG. 4) which protrude downward f~om the underside of the LSI
26 module 34.
27 No distinctions are made or intended herein between 28 the terms "card" and "board". Both of these terms 29 refer to the same type of physical structure and are used herein in a synonymous or interchangeable manner.
31 Each dendritic cont~ct.36a, 36b., 36c, etc., in the - 10 - , , q 1 board mounted array 36 is of the same construction as 2 described for the contact 10 in FIG. 1. Each dendritic 3 contact 37a, 37b, 37c, etc.~ in the module mounted 4 array 37 is of the same construction as described for the contact 20 in FIG. 1. Thus, multiple electrical 6 connections are established by urging the module 34 7 downward toward the board 30 and causing the metal 8 projections on the mating contacts to slide into inter-g meshing or interwedging engagement wlth one another.
By way of example, the array 36 in FIG. 2 may ~l 11 include a total of 400 individual dendritic contacts 12 located in a 2.54 centimeter by 2.54 centimeter square 13 area. In such case, the conductive substrate for each 14 contact may take the form of a circular pad having a diameter of 0.5 millimeters, with a spacing of 1.27 16 millimeters between the centers of adjacent pads. The 17 matching array on the underside of the module 34 would, 18 of course, have these same dimensions. This figure of 19 400 contacts should be compared with the less than 100 (typically 70 to 90) contacts which can be provided in 21 this same size area using commonly employed existing 22 techniques.
~3 As indicated in FIG. 4, the board 30 is a multi- ~ -24 layer printed circuit board. Inner layers of copper foil 40 and 41 are sandwiched between layers of elec-26 trically nonconductive material 42, 43, and 44. An 27 additional layer of copper may be deposited on the 28 upper surface of upper insulating layer 42 and a fur-29 ther layer of copper deposited on the lower surface of lower insulating layer 44. The copper layers are 31 selectively etched in the known manner to provide the -1 desired electrlcal conductor patterns, each of the 2 inner layers 40 and L11 being etched before -the covering 3 insulation layer is placed over it. The contact pads 4 on which the dendritic projections are grown are formed in this same manner. In other words, the contact pads 6 or conductive substrates for the contacts 36a, 36b, 7 36c, etc., are formed on the upper surface of insulating 8 layer 42 by depositing thereon a thin layer of copper 9 and then etching away the undesired copper.
By way of example only, the pad for contact 36b i9 11 shown as being electrically connected to a conductor in 12 the foil layer 40 and the pad for contact 36c is shown 13 as being electrically connected to a conductor in the 14 foil layer 41. This is accomplished by drilling small holes through the surface pads and appropriate insu-16 lating layers and thereafter filling the holes with 17 conductive material. The connecting conductive materia]
18 for the contact 36b pad is indicated at 45, while the 19 connecting conductive material for the contact 36c pad is indicated at 46. These pad to inner foil connections 21 are made before the dendritic projections are grown on 22 the pads. Electrical connection to the pad for contact 23 36a is made by way of copper on the surface of insu-24 lating layer 42 but, for simplicity of illustration, i5 not shown.
26 Multipoint electrical connections for the other 27 modules 31-33 are provided in the same manner as just 28 described for the module 34.
29 As indicated in FIGS. 2 and 3, the modules 31-34 are held in place on the board 30 by means of elongated 31 bars 47 which are attached to the board 30 and cross ~ z~

1 bars 48, the ends of which are secured to the elon~ated 2 bars L17 by small screws 4~. As indicated in FIG. 3, 3 the cross bars 48 bear against the tops of the modules 4 to prevent them from working free of the board 30 as a result of vibration or the like. I'hus, this mechanism 6 maintains the metal pro~ections on the mating dendritic 7 contacts in interwedged engagement with one another.
8 The force applied by the cross bars 48 is, however, 9 more in the nature of a retaining force, as opposed to a clamping force. As such, it is of relatively small 11 value.
12 FIGS. 2 and 3 also illustrate the use of the 13 dendritic contacts for purposes of connecting flat 14 multiconductor electrical cables to the printed circuit card or board 30. The board ends of four diffe~ent 16 flat cables 50, 51, 52 and 53 are shown in FIG. 2. As 17 seen from the exploded view of FIG. 5, these flat 18 cables 50-53 are clamped near the edge of the board 30 19 by means of U-shaped pressure pads 54-57, respectively, and U-shaped spring clamps S8-61, respectively.
21 Pressure pads 54-57 are made of rubber of other pliable ~ ;
22 nonconductive material.
23 As lndicated in the upside down view of FIG. 6 for 24 the case of flat cable 50, each of these flat cables is comprised of a goodly number of flat electrical con-26 ductors 62 arranged in a side-by-side manner and sand-27 wiched between two layers of nonconductive material.
28 For example, the conductors 62 may be thin strips of 29 copper foil embedded in a flexible plastic material. A
pair of guide pin holes 63 are provided near the ends 31 of each cable for mating with corrssponding yuide pins 1 64 mounted on and protruding a short distance above the 2 surface of the board 30. FIG. 7 is an enlarged view of 3 that por-tion of the board 30 to which the flat cable 50 4 is connected. It is to the same scale as the enlarged FIG. 6 view of flat cable 50.
6 A multipoint connector system for connecting the 7 individual conductors in the cable 50 to a corresponding 8 set of conductors on the printed circuit board 30 is 9 provided by forming a linear array of dendritic con-tacts directly on the cable 50 and by forming a corre-11 sponding linear array of dendritic contacts near the 12 edge of the board 30. In particular, small electrical 13 contact areas 65 are formed near the end of the cable 14 50 by scraping away or otherwise removing the insu-lating material covering the conductors 62. A thin 16 surface plating of nob]e metal is then plated onto each 17 of these e~posed contact areas h5, after which dendritic 18 me'cal projections are grown on t:his surface plating in 19 each such contact area 65. For simplicity of illustra-tion, the dendritic projections are not shown in FIG. 6.
21 Mating dendritic contacts are formed on a corre-22 sponding array of small contact areas 66 located on the 23 board 30 as shown in FIG. 7. Each such contact area 66 24 is comprised of a copper foil contact pad affixed to the surface of the board 30. In each case, a thin 26 layer of noble metal is plated on the upper surface of 27 the copper and a bunch of dendritic metal projections 28 are grown on the upper surface of the noble metal 29 plating. Again, for simplicity of illustration, the dendritic projections are not shown in FIG. 7.
31 The cable 50 is connected to the board 30 by u~

1 ta]cing the cable 50 as shown in FIG. 6 and turnin~ it 2 upside down to give it the orientation shown in FIG. 5 3 and then placing the guide pin holes 63 onto the ~uide 4 pins 64. The pressure pad 54 is then sliyped over the overlapplng portions of cable 50 and board 30 in the 6 manner indicated in FIG. 3, after which the spring 7 clamp 58 is slipped over the pressure pad 54 and moved 8 to its final position as also shown in FIG. 3. This 9 mechanism maintains the dendritic projections on the contact areas 65 of cable 50 in interwedged engagement 11 with the dendritic projections on the corresponding 12 ones of the contact areas 66 on the board 30.
13 Each of the contact areas 66 on board 30 is elec-14 trically connected to a different one of copper foil conductors 67 formed on or within the printed circuit 16 board 30. For the case of the multilayer board being 17 considered, some of these conductors 67 may lie on the 18 surface of the board 30, while others may be located at 19 the different inner foil levels within the board 30.
In the latter cases, electrical connections are made 21 thereto by drilling small holes and filling them with 22 conductive material in the manner as described in 23 connection with FIG. 4. The individual contact axeas 24 66 on the board 30 are the same size as the individual contact areas 65 on the cable 50. The width of each 26 such contact area 65 or 66 is the same as the width o~
27 one of the cable conductors 62, which width may be, for 28 example, 0.254 millimeters. The length of each contact 29 area 65 or 66 may be about twice that value or, in other words, about 0.5 millimeters.
31 The connector systems for the other flat cables 1 51-53 are of the same construction as that just de-2 scribed for the flat cable 50, except that different 3 numbers of dendritic contacts are provided in accordance 4 with the di~fering numbers of electrical conductors in the other cables 51-53.
6 As seen from the foregoing examples, electrical 7 contacts constructed in accordance with the present 8 invention can be used to provide very space efficient 9 multipoint connector systems of the separable or dis-connectable type. No individual spring assemblies are 11 required for each connection point and the contacts can 12 be made very small in size. This savings in space can 13 be used, as illustrated by the LSI module to board 14 connector system described above, to provide a greater number of independent electrical connections in a given 16 size area.
17 Also of considerable significance is the fact that 18 electrical connector systems constructed in accordance 19 with the present invention are lower in cost than those constructed with presently used techniques. This 21 results primarily from the elimination of the individual 22 springs and contact pins for the various connection 23 points and the labor involved in assembling connectors 24 which use the same. A further sa~ings occurs for the case of flat cable from the improved cable usage effi-26 ciency which results from being able to cut the cable 27 to any desired width so as to include any desired 28 number of conductors. The number of board located 29 contacts can be easily varied to accommodate any number of cable conductors.
31 The fabrication of an individual electrical contact ~z~

1 constructed in accordance with the present invention2 will now be considered in greater detail using, where 3 appropriate, the contact 10 of FIG. 1 as the model for 4 purposes of discussion. With this in mind, the tiny submilllmeter size metal projections 13 which actually 6 make the electrical connection are preferably composed 7 of a metal which is electrically conductive, mechanically 8 resilient and does not tarnish at room temperature in a g normal room environment. By "tarnish" is meant the formatlon on the exposed surface of the metal of an 11 adherent corrosion film such as an oxide film, a sulfide 12 film or the like. By "resilient" is meànt that the `
13 metal is springy or elastic in character such that it 14 is capable of recovering its size and shape after applied stresses are removed. As a consequence, the 16 metal projections 13 are springy in character such that 17 they can be bent or deformed somewhat when intermeshed 18 with a mating contact and then resume their original 19 shape and position when the mating contact is removed.
20 In accordance with these requ1rements, the metal -21 projections 13 are preferably composed bf a noble metal 22 selected from the group consistlng of palladium, 23 platinumj rhodium, iridium,-ruthenium and osmium. Each 24 of these metals is electrically conductive, does not tarnish àt room temperature and can be fabricated to be 26 mechanically resilient. Because of i*s somewhat lower 27 cost, palladium is probably the more attractive choice.
28 The dendritlcally grown structures represented by 29 the metal projections 13 are sometimes referred to as "dendrites". They can assume somewhat different shapes, 31 depending upon the particular electrQplating~parameters - 17 ~

1 under which they are grown. Probably the better shape 2 for most connector purposes is the needlelike or spear-3 like acicular shape shown in FIG. 1~ A somewhat modi-4 fied shape which also appears to be attractive for some purposes is obtained by allowing some of the needlelike 6 structures to form small mushroomlike knobs at their 7 outer ends. This provides somewhat enhanced self-8 locking or self-retaining characteristics, where such 9 is desirable.
The thin surface plating layer 15 is also preferably 11 composed of a noble metal. The purpose of the la~er 15 12 is to Eorm a dense, compact, pore-free, corrosion-proof 13 surface upon which to grow the dendritic projections 14 13. This provides a better bond and suppresses any corrosion or undesired electrolytic actions. This 16 layer 15 may be Eormed of any of the metals yiven above 17 as being preferred for -the dendritic projections 13.
18 In the case of the layer 15, however, it is not neces-19 sary that the metal be resilient in character. Thus, a non-resilient noble metal, such as gold, may also be 21 used for the layer 15.
22 The conductive element or conductive substrate 14 23 is, or is part of, the electrical circuit element or 24 circuit hardware structure upon which the contact is to be provided. As such, it may take the form of a con-26 ductive wire, pin, rod, bar, pad, plate, sheet or the 27 like. Typically, the substrate 14 will be made of 28 çopper, though it may be made of any metal commonly 29 used for electrical circuit conductors. If by chance the substrate 14 should be made of a nontarnishing 31 noble metal havlng the desired surface characteristics, ~z~

1 then the thin surface plating layer 15 may be elimi-2 nated.
3 The dendritic structures or projections 13 are 4 preferably grown by electroplating under non-normal conditions. During normal commercial type electro-6 plating operations, considerable care is exercised to 7 prevent the formation of dendritic structures. For 8 present purposes, the normal electroplating rules are 9 deliberately violated in order to promote the growth of dendritic structures. In particular, the dendritic 11 structures or projections 13 for the present invention 12 are grown by electroplating at a higher than normal 13 current density witn a plating solution having a lower 14 than normal concentration of metal ions. By "normal"
is meant those values which give a dense, compact, 16 pore-free surface.
17 By way of contrast, the surface plating layer 15 18 is Eormed by electroplating under normal conditions ;
19 which do not promote the formation of dendritic struc-tures.
21 A plating solution which has been found useful for 22 growing dendritic projections formed of palladium is a 23 solution of water (H2O), ammonia (NH3), ammonium 24 chloride (NH4Cl) and palladosammine chloride ( Pd(NEl3)2Cl2 ). The consti-tuent concentrations found 26 useful are in the range:
27 Pd~2 at 5 to 50 millimolar, 28 Cl at 2 to 5 molar, and 29 NH3 to hold pH in the range 9.0 to 10.5.
sy way of comparison, a normal concentration of palladium 31 ions would be on the order of 100 millimolar, as opposed ~ 19 -, q 1 to the 20-50 millimolar range given above.
2 Assuming that the surface plating layer 15 has 3 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 6 electrically connected so as to form the cathode elec-7 trode for the plating;operation. Any conductive surface 8 of the specimen which is not to have dendritic projec-9 tions grown thereon is covered with a layer or film of insulating material before placement of the specimen in 11 the plating bath. A source of direct current is then 12 connected between the cathode formed by the specimen 13 and an anode electrode also located in the plating 14 bath. ~n electrical current is then passed through the plating solution at a higher than normal current density 16 and the dendritic me-tal projections are thereby grown 17 on the exposed conductive surface of the specimen.
18 A current density value on the order of 100 milli-19 amperes per square centimeter has been found useful for this purpose. By way of contrast, the normal current 21 density for plating with palladium without the growth 22 of dendritic structures is on the order oE 10 milli-23 amperes per square centimeter. In bo-th cases, these 24 current density values are for the current density measured at the workpiece or cathode surface.
26 When it is desired to simultaneously grow dendritic 27 projections on a plurality of separate contact areas, 28 it is necessary that each of these areas be electrically 29 connected to the negative terminal of the direct current source during the performance of the plating operation.
31 For the case of the flat cable shown in,,FIG,. 6, this 1 can be accomplished by connecting the far ends of the 2 conductors 62 to the ne~3a~ive direct current terminal 3 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 6 FIG. 4 or FIG. 7, simultaneous dendritic growth can be 7 accomplished by leaving copper foil connections be-tween 8 the contac-t areas for commoning purposes and subse-9 quently etching away these commoning connections after the growth of the dendritic projections on the desired 11 contact areas.
12 Based on our knowledge and experience to date, 13 electroplating under the novel conditions described 14 above is helieved to be the preferred method for forming the dendritic structures. It is not intended, however, 16 that this be taken as an implied limitation in those of 17 the appended claims which made no mention of the method 18 of forming the dendri-tic structures because other 19 methods also appear to be feasible for this purpose.
Such other methods include electroless plating (reduction 21 done chemically without use of electrodes and electric 22 current), electroforming and chemical vapor deposition. ;~
23 While there have been described what are at present 24 considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that 26 various changes and modifications may be made therein 27 without departing from the invention, and it is, there-28 fore, intended to cover all such changes and modi~ica-29 tions as fall within the true spirit and scope of the invention.

, ~ -

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrical contact adapted to intermesh with other like electrical contacts, each contact having preformed thereon a contact surface consisting of a multitude of tiny resilient metal projections created by a dendritic growth thereon of conductive metal crystals.

2. The eletrical contact according to claim 1 wherein said projections comprise an array of projections extending generally away from a surface of said electrical contact.

3. The electrical contact according to claim 2 wherein the shape of said projections is acicular.

4. The electrical contact according to claim 1 wherein said projections are composed of a metal selected from the group consisting of palladium, platinum, rhodium, iridium, ruthenium and osmium.

5. The electrical contact according to claim 4 wherein said projections comprise an array of projections extending generally away from a surface of said electrical contact.

6. The electrical contact according to claim 5 wherein the shape of said projections is acicular.

7. The electrical contact according to claim 1 wherein said projections are composed of a material which will not develop an adherent film at room temperature in a normal room atmosphere.

8. The electrical contact according to claim 7 wherein said projections comprise an array of projections extending generally away from a surface of said electrical contact.

9. The electrical contact according to claim 8 wherein the shape of said projections is acicular.

22 established, said layer of noble metal being suitable for hosting the formation of said metal projections thereon.

11. The electrical contact according to claim 10 wherein said projections comprise an array of projections extending generally away from a surface of said electrical contact.

12. The electrical contact according to claim 11 wherein the shape of said projections is acicular.

13. The electrical contact according to claim 12 wherein said projections are composed of a metal selected from the group consisting of palladium, platinum, rhodium, iridium, ruthenium and osmium.

14. The electrical contact according to claim 13 wherein said projections are composed of a material which will not develop an adherent film at room temperature in a normal room atmosphere.

15. The electrical contact according to claim 10 wherein said projections are composed of a metal selected from the group consisting of palladium, platinum, rhodium, iridium, ruthenium and osmium.

16. The electrical contact according to claim 10 wherein said projections are composed of a material which will not develop an adherent film at room temperature in a normal room atmosphere.

17. The electrical contact according to claim 2 wherein said projections have an aspect ratio of between 4 and 10.

18. The electrical contact according to claim 2 wherein said projections have a silhouette ratio relative to said surface of said electrical contact of at least 0.10.

19. The electrical contact according to claim 2 wherein said projections have a silhouette ratio relative to said surface of said electrical contact of between 0.10 and 0.39.

20. The electrical contact according to claim 2 wherein said projections number between three and twenty thousand for each square millimeter of area of surface of said electrical contact.

21. The electrical contact according to claim 17 wherein said projections have a silhouette ratio relative to said surface of said electrical contact of at least 0.10.

22. The electrical contact according to claim 17 wherein said projections number between three and twenty thousand for each square millimeter of area of surface of said electrical contact.

23. The electrical contact according to claim 18 wherein said projections number between three and twenty thousand for each square millimeter of area of surface of said electrical contact.

24. The electrical contact according to claim 21 wherein said projections number between three and twenty thousand for each square millimeter of area of surface of said electrical contact.

25. An electrical contact as defined in claim 1, said contact capable of reliably carrying current levels of at least ten amperes per square millimeter of surface area of said electrical contact.