High Pressure Electrical Contacts

Abstract

High pressure electrical contacts are provided coated with a deformable ductile white metal, for example, white metals selected from the group consisting of Sn, Pb, Cd, Zn, Bi, In, alloys of at least two of these metals with each other and alloys of at least one of these metals with Sb. The coating metal advantageously provides an easily separable and reusable low contact resistance junction at low loads when the coated male member of the contact is in electrical contact with an electrically conductive element siliarly coated, whereby the two coatings mutually deform one into the other to provide a gas tight contact at the region of coating deformation.

Description

This invention relates to high pressure electrical connectors or contacts and, in particular, to electrical contacts characterized by very low electrical resistance between a male contact member and an electrically conductive target surface receiving said male contact. The invention is particularly applicable to the production of separable and reusable connectors.

STATE OF THE ART AND THE PROBLEM

For the purpose of this invention, electrical connectors or contacts are defined as those which provide separable junctions in an electrical circuit, such as in low energy circuits, in which the open circuit voltage is less than 20 mv. High resistances cannot be tolerated in such circuits and, therefore, the contact resistances must be extremely low, for example, desirably not more than about several milliohms.

A mere tarnish on the surface of an electrically conductive element forming part of an electrical circuit can cause a high contact resistance depending upon the applied pressure of the male contact member and the open circuit voltage. When the voltage is greater than 20 mv, it will normally break down the tarnish film electrically. In circuits less than 20 mv, the breakdown must be done mechanically. The function of an electrical connector is to repeatedly or continuously interconnect circuits without gross changes in contact resistance. It is very desirable that electrical contact resistance remain constant regardless of the environment for the entire useful life of the circuit. Up to the present, precious metal platings, which resist tarnishing and the formation of oxide films, have been used to maintain a stable resistance at the contact interface; however, these materials are very expensive and, therefore, have their economic limitations.

The term "high pressure contact" used herein is meant to cover those contacts in which the pressure required is sufficient to establish a level of plastic deformation at the contact interface, the extent of penetration of the contact being such as to provide resistance to corrosion by certain corrosive gases.

Recent experimental work has indicated that the stability of a high force connection improves with higher contact forces until a cold weld junction is achieved. Yet, a high pressure contact does not necessarily mean a high load must be applied, since a low load applied to a male contact member having a sharp point will provide high pressure.

However, it would be desirable to provide pressure contacts requiring only the application of very low loads to assure proper contact accompanied by very low contact resistance. Up to now, it has not been possible to utilize the high pressure contact principle in separable miniature and micro-miniature connectors. In the past, such connectors required (1) providing resistance to wear, (2) avoiding prow formation of debris at the contact interface, (3) assuring fit within small contact center-to-center spacing, for example, as low as 0.05 inch to 0.1 inch, and (4) providing means for separating multicontact connectors without using high withdrawal forces.

It has now been discovered that separable and reusable electrical connectors can be provided for low energy circuits characterized by low resistance at the contact interface without the use of precious metals.

OBJECTS OF THE INVENTION

It is thus the object of the invention to provide an electrical connector characterized by very low contact resistance at the connecting interface.

Another object of the invention is to provide a connector for use in coupling electrical circuits in which the contact is achieved by penetration to provide nascent or fresh metal contact at the coupling joint.

A further object of the invention is to provide an electrical connection characterized by low contact resistance.

These and other objects will more clearly appear when taken in conjunction with the following disclosure and the accompanying drawing, wherein:

FIG. 1 is a schematic representation of an experimental high pressure connector comprised of a male contact member and a target contact-receiving member or substrate of electrical conducting metal;

FIGS. 2A to 2C and 3A to 3C are illustrative of embodiments of male contact members having chisel and conically pointed penetrating ends, respectively;

FIGS. 4A and 4B are illustrative of one embodiment of a connector plug comprising a spring-loadable male member, the loading being predetermined in accordance with the amount of spring deflection to provide a particular pressure during electrical contact of the member with a circuit-making element;

FIGS. 5A and 5B are further embodiments of a male contact member made by die-forming a metal strip to provide an element thereof with a chisel-like point, FIG. 5B being a perspective view of FIG. 5A;

FIGS. 6A and 6B are another embodiment of a male contact member produced similarly from a metal strip of contact metal, FIG. 6B being a perspective view of FIG. 6A;

FIGS. 7A and 7B are still further embodiments of a male contact member made by die-forming a metal strip to form a redundant connector (a male member with more than one contact point, FIG. 7B being a perspective view of FIG. 7A;

FIG. 8 shows in amplified cross-section a male contact member about to make contact with a circuit-making element; while FIG. 9 shows the resulting electrical joint after contact has been made.

STATEMENT OF THE INVENTION

As one embodiment, the invention provides in combination an electrical connector comprising a plug contact portion having at least one pointed male contact member of a non-ferrous electrically conductive metal selected from the group consisting of copper and copper-base alloys, coated with a thin layer of a deformable ductile white metal. Examples of such metals are white metals selected from the group consisting of Sn, Pb, Cd, Zn, Bi, In, alloys of at least two of these metals with each other and alloys of at least one of these metals with Sb; and a receiving target or base member having a predetermined array of electrically conductive circuit-making elements (e.g. printed circuit board or other suitable substrate) of also said electrically conductive non-ferrous metals cooperable with at least one of said male contacts, the elements being also coated with one of said white deformable metals. When contact is made between the at least one pointed male contact member and said circuit-making element at a predetermined force, a low resistance contact is obtained at the contacting interface by virtue of the deformable coatings which are mutually deformed one against the other at the point of contact. Thus, when the sharply pointed male member penetrates the surface of the coated element, a fresh metal seal is formed by the mutually deformed coatings characterized by a rim of displaced white metal substantially circumjacent or around the contact area. The displaced coatings together form a fillet, so to speak, at the electrical joint formed at the contact area. This will be apparent from the drawing to be described later.

With regard to the male contact member, a mathematical study indicated that small radii (e.g. 0.0005 inch to 0.00015 inch) spheres and cylinders should achieve very high contact pressures with low contact forces. Points that small and lines that sharp resemble sharp needles and knife blades. However, by coating such members with a coating of deformable white metal referred to hereinabove, deep penetration need not be required into the coated metal substrate, and thus low contact forces can be applied and still obtain the advantages of a high pressure contact with low contact resistance at the point of penetration.

A study of the slope angles of asperities of the pointed male member generally indicated that the uncoated point of the male member may tend to be fragile unless the point is backed by a mass configuration. Thus, points with an included angle of over 90.degree. would be preferred, since the larger the included angle of the point, e.g., 120.degree., the greater is the mass of metal backing up the point and, therefore, the stronger the point. A chisel point or cone with an included angle of about 120.degree. is particularly desirable, the chisel point which in effect is a sharp knife edge being particularly preferred.

DETAILS OF THE INVENTION

Tests were conducted using coated target substrates made of O.F.H.C. copper (i.e., oxygen-free, high conductivity copper), beryllium copper, phosphor bronze, nickel-silver and the like. The penetrator contact used had a diameter of about 0.030 inch and was chisel pointed (note FIGS. 2A to 2C) with an included angle of about 120.degree.. The penetrator was made of copper and various alloys of copper and the line contack point had a radius ranging from about 0.0003 to 0.0005 inch.

Both the penetrator (male contact member) and the target substrate were coated with various metals which included the white metals tin electroplate and a lead-tin solder, and also a gold electroplate over a nickel plate over the metal substrate in question.

The assembly employed in carrying out the tests is shown schematically in FIG. 1 comprising an upper contact-carrying element 10 which is fitted via shaft 11 extending from body portion 12 to a machine element (not shown) by means of which a predetermined force is applied axially of the contact-carrying element, the contact being the cylindrical member 13 extending downwardly from the body portion, the end 14 of the member being ground to provide a knife edge with an included angle of about 120.degree. (note FIGS. 2A to 2C). Terminals 15 and 16 are provided for connection to a contact resistance probe device of the type described in the Review of Scientific Instruments (Vol. 34, No. 12, December, 1963, pps. 1317-1322).

The lower member comprises a target head 17 having a substrate 18 on which the contact measurement is made, the target head having a downwardly extending shaft 19 which fits into a supporting base (not shown). Two terminals 20 and 21 are provided for connection to the contact resistance probe device referred to hereinabove. The probe device may be programmed to determine the contact resistance sequentially in steps across the face of the substrate being tested, if desired. The contact loading to shaft 11 is varied from zero to any predetermined maximum load. The loads were applied in the test at increments of 100 grams, up to 500 grams and the resistance measurements in milliohms automatically determined by the probe device, at a typical current flow of about 47 milliamps.

The following results were obtained:

Table 1 __________________________________________________________________________ Resistance in Milliohms on Substrate of O.H.F.C. Copper __________________________________________________________________________ Male 300 .+-. 100 M.I.* Sn 200 .+-. 50 M.I.* of Contact Over 60 Sn - 40 Pb over of copper 200 .+-. 100 M.I.* Cu Substrate __________________________________________________________________________ 100 300 500 100 300 500 grs. grs. grs. grs. grs. grs. __________________________________________________________________________ 300 .+-. 100 M.I.* 1.07 0.70 0.60 0.89 0.51 0.44 Tin Over 200 .+-. 100 M.I.* Cu 40 .+-. 10 M.I.*Au 1.25 0.76 0.57 0.89 0.52 0.45 over 250 .+-. 10 M.I.*Ni __________________________________________________________________________ * M.I. stands for microinches.

Table 2 __________________________________________________________________________ Resistance in Milliohms on Substrate of O.H.F.C. Copper __________________________________________________________________________ Male 300 .+-. M.I.* Sn 200 .+-. 50 M.I. of Contact of Over 60 Sn - 40 Pb over Phosphor 200 .+-. 100 M.I. Cu Substrate Bronze 100 300 500 100 300 500 grs. grs. grs. grs. grs. grs. __________________________________________________________________________ 300 .+-. 100 M.I. Tin over 200 .+-. 100 M.I. Cu 3.15 1.05 0.85 1.45 0.72 0.62 40 .+-. 10 M.I. Au over 250 .+-. 10 M.I. Ni 3.90 2.17 1.85 2.20 1.82 1.08 __________________________________________________________________________ * M.I. stands for microinches

The gold plating was produced from an alkaline bath while the tin plate was a bright acid tin electroplate. The 60 percent tin-40 percent lead coating was produced as a solder electroplate.

It will be noted from Tables 1 and 2 that consistent low resistances were obtained with the combination of a tin-coated male contact and a tin and solder-coated (60 Sn-40 Pb) metal substrate. While the tin and solder-coated metal substrates also gave low resistance with the gold plated male contact, it will be noted that the tin coated male contact compared favorably with the gold plated contact on O.H.F.C. copper substrate and on phosphor bronze.

The deformable soft white metals as coating material appeared to give consistently better results. Microscopic examinations indicated this to be due to the deformable characteristics of the soft white metal. This will be apparent by referring to FIGS. 8 and 9. In FIG. 8, the male contact portion 25 of copper with a coating of tin 26 is shown approaching a printed circuitboard 27 or target comprising an electrically conductive circuit-making element 28 of copper coated with a deformable layer of solder 29 (60 percent Sn-40 percent Pb). Upon striking the target, the coatings 26 and 29 (note FIG. 9) mutually deform one against the other as the point of the male contact penetrates and makes contact with the coating of the copper substrate 28 of the printed circuitboard. As will be noted from FIG. 9, a substantially gas tight electrical joint is formed by the mutually deformed soft metal coatings which provide a fillet-like structure at 30, comprising a rim of displaced soft metal substantially circumjacent or around the contact area.

Tests have indicated that the contacts can be separated and repeatedly used and still provide low resistance. While the straight line chisel type edge is preferred for the male contact, the conically shaped edge or point 31 shown in FIGS. 3A to 3C for male member 32 also gives good results.

Various embodiments of straight line chisel type edges may be employed as male contact members. Illustrative embodiments are those shown in FIGS. 5A, 5B, 6A, 6B, 7A and 7B. Such edges can be easily produced by dieforming metal strip.

Thus, FIG. 5A shows a strip 45 which has been pierced intermediate its side edges by a die to provide a chisel edge 46 which is shown more clearly in the perspective of FIG. 5B. The advantages of a strip are that it can be mounted as a spring contact member with the desired amount of spring which, when compressed against a target surface, will provide the desired load or force to produce a good electrical joint.

FIG. 6A is a strip of electrically conductive metal 47 which has been die-formed wholly across its width to provide a sharp bend or chisel edge 48 which is also shown in perspective in FIG. 6B.

In order to assure electrical contact of a male element in a connector, a redundant male connector of the type shown in FIGS. 7A and 7B may be provided. By redundant connector is meant that the male element has a plurality of contact points so as to assure that at least one point of the male element makes contact with the target surface. A metal strip having a plurality of triangular ears extending laterally and alternately along opposite sides thereof may be employed, the ears being then formed downwardly as shown in FIGS. 7A and 7B. Thus, referred to FIGS. 7A and 7B, a strip 49 of electrically conductive metal is shown depicting oppositely disposed ears 50, 50A which have been bent downwardly at right angles to the plane of the strip, each of the ears having a sharp contact point 51 and 51A.

It will be noted from Tables 1 and 2 that low resistances are obtainable with the use of deformable white metal coatings at low applications loads of, for example, about 100 to 300 grams comparable or superior to those obtained with gold platings. This is important economically and is also important since such light loads are desirable in order to avoid destroying the insulation between two separate opposing circuits, particularly in miniature circuits. Low loads are also essential for achieving practical multi-contact connectors, whereas high loads tend to produce high stresses on plastic parts, e.g., printed circuitboards.

The advantages of the coated contact over the uncoated contact will be apparent by comparing a chisel point copper-to-copper system with the coated system at a load of 100 grams. The uncoated assembly exhibited a resistance of 2 to 5 milliohms while the system in Table 1 in which the male element was tin coated and the target substrate tin or solder coated exhibited resistances generally 1 milliohm or below. The additional advantage is that the coated system assures a gas tight electrical joint against corrosion and also assures increased contact area at the deformed region of the white metal coatings.

One method of assuring a predetermined low contact force between the male contact member and the target surface is to design the male element as a spring with a particular stiffness capable of being loaded during contact to the maximum force to be tolerated. One example of a spring-loadable electrical connector in the form of a leaf spring is shown in FIGS. 4A and 4B in which the male element is depicted coacting with a target substrate comprising a printed circuitboard 35 with an electrically conductive circuit-making element of copper 36 with a coating of soft white metal such as tin which is exaggerated in thickness to make it visible.

The plug carrying the male contact element is shown in fragmented cross section with the element in the form of spring member 38 which is also depicted in dotted lines in its freely extending unstressed position 39 before loading against the target surface. The contact and spring are mounted in a plastic body 40 provided with barrier walls 41 to isolate each male contact element from the other. Each element is connected via posts 43A to lead wires 43. The contact element is coated with a layer of deformable white metal, the thickness of which has been exaggerated to make it visible. The point 38A of the male element is formed by die piercing to produce the type illustrated in FIGS. 5A and 5B.

Thus, when the connector plug is coupled to the circuitboard 35 by means not shown against the action of the male contact spring member, a load depicted by arrow 42 is applied to the male contact member predetermined not to exceed a fixed maximum load to effect penetration of the male contact element in the manner shown in FIG. 9. The point should preferably be a chisel edge with a large included angle in excess of 60.degree. and preferably in excess of 90.degree., such as 120.degree..

As stated hereinabove, the metal coating is preferably selected from the group consisting of the deformable soft or ductile white metals Sn, Pb, Cd, Zn, Bi, In, alloys of at least two of these metals with each other and alloys of at least one of these metals with Sb. Soft ductile alloys of the foregoing metals are well known in the art. Examples of deformable soft white metals are as follows:

Table 3 ______________________________________ % Sn % Pb % Cd % Zn % Bi % In % Sb ______________________________________ 100.0 -- -- -- -- -- -- 60 40 -- -- -- -- -- 19 36 9.5 -- 35.5 -- -- 60 -- -- -- 40 -- -- 14.5 28.5 -- -- 48 -- 9.0 91 -- -- 9 -- -- -- 37.5 37.5 -- -- -- 25 -- -- -- -- -- -- 100 -- -- 87 -- -- -- -- 13 10 -- -- -- -- 90 -- 8.3 22.6 5.3 -- 44.7 19.1 -- 80 20 -- -- -- -- -- 90 10 -- -- -- -- -- 70 -- -- 30 -- -- -- ______________________________________

The term "deformable ductile white metals" used herein is understood to cover generally the metals and alloys described hereinbefore. Such white metals are defined on page 41 of the "Metals Handbook" (Vol. 1, 8th Ed., 1961, American Society of Metals) as comprising white-colored metals of relatively low melting points (e.g., lead, bismuth, tin, cadmium, zinc, etc.) and of alloys based on these metals.

Generally, but not necessarily, the male contact member of the chisel point type will have a length dimension of the knife edge ranging from about 0.010 to 0.030 inch and preferably have a chisel point, although a conical point can be employed. While the included angle of the point may be in excess of 30.degree., it is preferred that the included angle be much larger, for example, in excess of 90.degree., such as up to about 120.degree. or 130.degree..

The coating on both the male contact member and the target substrate making up the other part of the connector may have a metal coating thickness of at least about 0.0002 inch and may range up to about 0.015 inch, e.g. about 0.0002 to 0.0006 inch. Generally, the thickness of the contact may range from about 0.005 to 0.015 inch.

Examples of electrically conductive metals and alloys which may be employed in the connector are as follows:

1. Electrolytic copper

2. O.F.H.C. copper

3. Copper-base alloys such as:

A. cupro-nickel (88.5 Cu - 10 Ni - 1.5 Fe)

B. cupro-nickel (69.5 Cu - 30 Ni - 0.5 Fe)

C. nickel-silver (65 Cu - 17 Zn - 18 Ni)

D. nickel-silver (55 Cu - 27 Zn - 18 Ni)

E. beryllium-copper (3 Be - 0.25 Co or 0.35 Ni, bal Cu)

F. phosphor bronze (95 Cu - 10 Ni - 2 Sn)

H. phosphor copper (99.98 Cu - 0.02 P)

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

Claims

What is claimed is:

1. In combination in an electrical connector for effecting multiple, readily separable low energy, signal level connections, comprising a plug contact connector characterized by a plurality of extending spring elements, each having a male contact point, said elements and said contact points being made of an electrically conductive metal coated with a layer of a deformable white metal, and a receiving member comprising a substrate having thereon a plurality of electrically conductive elements made of an electrically conductive metal, said elements being also coated with a deformable ductile white metal, said plug connector with said coated male contact points being adapted to effect a separable spring-loaded contact connection with said coated elements of said receiving member, such that when the said plurality of male contact points is in spring-loaded penetrating contact against the coated elements of said receiving member, the white metal coatings at the area of contact are mutually plastically deformed at the point of contact, to provide a low resistance contact between the coated male contact points and the said coated electrically conductive elements.

2. A combination as in claim 1, wherein said spring elements, said male contact points, and said substrate electrically conductive elements are made from a non-ferrous electrically conductive metal selected from the group consisting of copper and copper base alloys.

3. A combination as in claim 1 comprising means for bringing said male contact points of said contact connector into electrical contacting engagement with said electrically conductive elements on said substrate in a non-sliding manner.

4. The electrical connector of claim 1, wherein the deformable ductile white metal is selected from the group consisting of Sn, Pb, Cd, Zn, Bi, In, alloys of at least two of these metals with each other and alloys of at least one of these metals with Sb.

5. The connector of claim 4, wherein the coated pointed male contact member is characterized by a chisel point.

6. The connector of claim 5, wherein the chisel point is formed with an included angle of over about 60.degree..

7. The connector of claim 6, wherein the metal coating is tin.

8. The connector of claim 6, wherein the metal coating is a tin-lead alloy.

9. The connector of claim 6, wherein the coating metal is selected from the group consisting of indium and indium-base alloys.