EP0074630A2

EP0074630A2 - Apparatus including electrical contacts

Info

Publication number: EP0074630A2
Application number: EP82108330A
Authority: EP
Inventors: John Travis Plewes; Murray Robbins; Tom Daniel Schlabach
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1981-09-11
Filing date: 1982-09-09
Publication date: 1983-03-23
Also published as: ES8401818A1; ES515607A0; EP0074630A3; JPS58501434A; GB2110197A; GB2110197B; EP0088123A1; WO1983000945A1; EP0088123A4

Abstract

This invention relates to apparatus including mating electrical contacts at least one of which comprises a substrate and a contact layer in intimate contact with the said substrate. The contact layer includes a crystallographic phase produced by in situ reaction using energies associated with conventional ambient and involving a reactant present in a substrate together with one introduced externally. The resulting layered chemical compound is a suitable replacement for electroplated gold and serves in make/break switches, connectors, sustained contact elements, etc.

Description

Background ot the Invention

1. Filed of the Invention

The invention is concerned with apparatus including electrical contacts whicn depend on mechanical contact for closure. A category of included apparatus may oe characterized as "low current" or "nonarcing" and has traditionally relied upon gold. The category includes electronic devices such as wire and cable connectors, printed wiring boards, and integrated circuits.

2. Description of the Prior Art

The unique position of gold contact surfaces in tne electrical arts has been secure from the beginning.
While gold is classified as a member of the class of "noble metals", it alone is truly nonreactive with commonly encountered ambient constituents. Only by this nonreactivity has the integrity of required contact properties been maintained. With recent increases in prices, efforts to replace gold have increased. Studies have been successful generally in terms of reduction in gold rather than in eliminating. significant cost savings nave resulted from reduction in gold layer thickness by attention to deposition techniques, e.g., gold plating conditions, and also to substrate surface composition and condition.
Electrical contact structures in which gold is replaced by other "noble metals" nave realized some degree of success. platinum-group metals, platinum and palladium, tor example, sometimes alloyed with silver have been successfully used. However, even such materials have been round to be somewhat reactive with common ambient constituents. The indisrensability ot gold is illustrated by a recent development in which a palladium-silver alloy contact is coated witn a very thin gold layer to improve reliability.
In dircumstances where arcing results upon make or break a vallety of non-noble metals, notably Cu- containing alioys have founu use. Here, reliance may be had on arcing to "punch through" high resistance surface compouids produceu by reaction with ambient constituents.
A number of chemical compounds characterized by metallic or near metallic conductivity have been reported in the literature. Films of such materials have sometimes been formed by deposition of the compound, e.g., by sputtering. See for example, Proceedings of the Electroctiemical Society, 00, page 216 (1979) . Generally, such work has been of a fundamental research nature, although observed electrical properties have provoked conmtent.

Summary or tne Invention

The invention provides for contact surfaces which depend tor their electrical characteristics primarily upon chemical compounds. Illustrative compounds, generally well characterized in the literature, are chemically grouped as silicioes, carbides, nitrides, phosphides, _Dorides, sulfides and selenides. Since a main objective of the invention is economic, it is compounds of platinum-group metals as well as precious metals (ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold) that are excluded.
From the thermodynamic standpoint, like gold, appropriate compounds in accordance with the invention are generally at true equilibrium. Under usual ambient circumstances compounds of the invention do not react with ambient constituents at least on prolonged exposure so that chemical change at tile free contact surface is avoided.
From the electrical standpoint, contacts in accordance witn the invention have low contact resistance. Resistivity as measured by a tour point probe is no greater than about 10^-3 ohm-centimeter (Ωcm). For usual structures, contact resistance is generally below about 10 milliohms. This resistance level is seen after high humidity-temperature cycling under test conditions used for qualitying telephone equipment.
The invention contemplates the formation of thin layers of compound contact material (layers of the order of 10 micrometers or less in thickness) produced by in situ reaction with substrate material. Contemplated reactions which may involve vapor phase or liquid phase reactant involve at least one constituent -- usually a metallic constituent -- of the substrate. It is a significant aspect of the invention that such in situ formed layers may be directly substituted for gold layers deposited, for example, by electroplating, sputtering, etc. The invention, therefore, contemplates formation ot contact layers on partially tabricated or on otherwise completed devices.
Thin-film compound contacts of the invention are generally useful in nonarcing applications which have traditionally been served by gold. While the lower melting compounds, e.g., sulfides and selenides, are generally limited to such use, others are not so limited.

I. Glossary

A. Contact Compound

Tnese are the in situ formable materials characterized by bulk resistivity of 10^-3 Ωcm or less. All such compounds are generally stable to the extent that resistivity does not increase beyond such maximum value under test conditions applied to relevant gold devices. In situ formation involves at least one constituent -- usually a metallic element -- contained in the substrate upon wnich the layer is formed. Appropriate constituents do not include platinum-group metals or other precious metals (pronibited elements are ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold).

B. Nonarcing

The meaning nere is consistent with the usually recognized meaning in that normal operation does not result in formation of a luminous region at the contact upon make or break. Contemplated conditions under which tne characteristic is satisfied are "dry circuit" conditions. Contemplated devices in accordance with the invention are generally nonarcing under contemplated operating conditions, as well.

C. Electronic Devices

These are generally small current devices such as electronic tubes, transistors, etc., as used, e.g., in communications computers, as distinguished from large current devices, such as those used for power applications.

D. Small Current

Tnis is the circuit current normally passing through the compound contact layer and its mating surface resulting in circuit closure. This corresponds with an open circuit voltage of 50 millivolts or less. Use of the terminology does not require actual measurable open circuit voltage as described since contemplated devices include those in which continuous closure is required. Resulting currents are usually below 100 ma or, for many devices, below 50 ma.

E. Contact Compound Layer

Tnis is the in situ formed contact compound in the form of a layer having a thickness of the order of 10 pm or less. Contact resistance under a load of 100 grams is generally below 10 milliohms before or after appropriate testing, e.g., for some.purposes, at high numidity (90 percent relative humidity air at 90 degrees Fahrenheit) and temperature cycling between -40 degrees C and +140 degrees C.

F. Contact

Again, the term is used in its traditional context. It contemplates a tree surface of contact compound in the mechanical sense. Ordinarily open and closed contact conditions are contemplated, out use may also involve Susuained closure. Pressure on closure typically does not exceed 100 grams. The front or back electrode in the usual carbon granule transmitter is illustrative or a normally closed contact.

2. Examples

The following examples were conducted on structures of similar or identical configurations. Structures consisted of a flat compound contact contacted by a hemispherical probe. All hemispherical probes were surtaced with soft gold (99.99 weight percent). Experiments with compound contact probes yielded approximately the same contact resistance measurements. Open circuit voltages were less than 20 millivolts resulting in currents of less than 10 milliamperes upon closure.
The probe structure was parabolic in cross- section with the contacting region approximately defining hemisphere having a diameter of one millimeter.
Tests were conducted in normal air ambient at room temperature.
Electrical measurements were made in the conventional four point manner with current and voltage contacts to the probe and tne flat contact.
measurements conducted at 100 grams in each instance were reliably reproduced and tell well within the bounds required for most device designs. For comparison purposes the identical measurements conducted with a flat and dome both have 99.99 weight percent gold yielded a resistance value of 0.7 milliohms under the test conditions. Layer thickness was estimated from weight gain. Composition was determined by X-ray analysis.

Example 1

Composition - Nickel Phosphide, NiP.
preparation - Nickel was exposed to phosphorus vapor at a temperature of 44U degrees C for 2 hours thereby resulting in a compound layer of an approximate thickness of 5 µm. Resistance - 18 milliolims at 100 grams.

Example 2

Composition - Titanium Nitride, TiN.

preparation - Titanium was exposed to ammonia gas at a temperature of 1100 degrees C for a period of 1 hour resulting in a layer thickness of approximately 8 µm. Resistance - 50 milliohms at 100 grams.

Example 3

Composition - Tantalum Nitride, TaN.

Composition - Tantalum metal was reacted in ammonia for

1 nour at a temperature of 1100 degrees C to yield a layer of a thickness of about 10 µm. Reslstance - 150-200 milliohms at 100 grams.

Example 4

Composition - Titanium Carbide, TiC.

preparation - Titanium metal was exposed to acetylene for a period of 1 half hour at a temperature of 950 degrees C, yielding a layer of a thickness of about 2 um. Resistance - 90 milliohms at 100 grams.

Example 5

Composition - Copper Selenide, CuSe.

Preparation - A 90 percent copper, 10 percent nickel substiate was exposed to a 0.3 percent solution of selenium in dichlorobenzene at 179 degrees C for 10 minutes. This resulted in a coating thickness of 5 µm.
Resistance - 10 milliohms at 100 grams.

Example 6

Composition - Copper Sulfide, CuS. Preparation - A 80 percent copper, 20 percent cobalt substrate was exposed to sulfur vapor at 187 degrees C using a nitrogen carrier gas for 2.5 hours. This resulted in a coating thickness of 10µm.
Reslstance - 3.U and 7.8 millionms at 100 and 15 grams, respectively.
Variations on the examples included different reactant states. So, for example, the Cube of Example 5 was reproduced by reacting a similar surface with selenium vapor using a nitrogen carrier to yield substantially identical electrical properties. The material of Example 6 was produced by an alternative technique in whicn reaction was with molten sulfur as well as by use of diculorobenzene solvent. Electrical properties were substantially identical.

Example 7

Cowposition - Titanium Silicide, Tisi.

Preparation - A titanium substrate was exposed to an atmosphere composed of 10 percent silane, 90 percent N₂ gas at a temperature of 900 degrees C for a period of 1 hour. Coating thickness was approximately 3.5 µm.
Resistance - 50 to 60 millionms at 100 grams.

2. Contact Composition

It has been stated that contact composition in accordance with the invention avoids the use of gold, silver, and metals of the platinum group (Ru, Rh, Pd, Re, Os, Ir, Pt). Generally, exclusion is based on economics wnicn, after all, is the major thrust of the invention. Functional material in accordance with the invention is produced by in situ reaction involving at least one reactant which is present as a substrate constituent and another reactant introduced externally. The externally introduced reactant is generally in fluid form, either vapor or liquid. Introduction may involve a carrier, for example, to introduce the external reactant in the vapor phase but permit reaction at a temperature below its vaporization temperature.
An aspect of the inventive teaching depends upon the concept of replacing a thin layer of conventional contact material - generally gold - with a layer of a cnemical compound. An important teaching permits substitution of the one layer for the other and thereby minimizes or avoids device redesign. Formation of gold or gold-containing layers in prior art devices is by disposiition - e.g., by electroplating. Formation of the compounds of the present invention is accomplished by in situ reaction.
The invention does not depend upon designation of compound composition. The technical literature includes reports of compoumds with measured electrical properties and Known stability in some encountered ambients so, for example, titanium nitride has been studied to result in a literature reference, 1980 Proceedings of the Electrochemical Society, 316 (1979), reporting bulk resistivity values of 50µΩcm. The compound is known to be stable in usual air ambient over usually prescribed temperature ranges of operation. Similar information is available for a variety of compounds including silicides of titanium, zirconium, vanadium, niobium, tantalum, iron, coualt, e.g., TiSi₂, TiSi, ZrSi, VSi₂, NbSi₂, TaSi₂, FeSi, CoSi₂, carbides of titanium, tantalum, tungsten, molybdenum, chromium, niobium, vanadium, hafnium, zirconium, lanthanum-group metals, e.g., TiC, TaC, WC, W₂C, MoC, Mo₂C, Cr₃C₂, NbC, VC, HfC, ZrC, YC₂, LaC2, CeC₂, PrC₂, NdC₂, SmC₂, GdC₂, TbC₂, D^yC₂, ErC₂, Y₂C₃, La₂C₃, ^Ce ₂C₃, Nd₂C₃, nitrides of titanium, zirconium, niobium, tantalum, chromium, tungsten, hafnium, e.g., TiN, ZrN, NbN, TaN, Ta₂N, Cr₂N, CrN, WN₂, HfN, borides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, e.g., TiB, TiB₂, ZrB₂ HfB₂, V₃B₂, V₃B₄, NbB, NbB₂, TaB, TaB₂, Cr₃b₂,CrB₂, Mo₂B, MoB₂ and phosphides of a variety of materials.
Whereas compounds with requisite electrical properties are generally of known stability in air ambient, i.e., in O₂, N₂, and H20, other considerations may well require screening. Compounds tested in the work which led to this disclosure have manifested stability in the presence of sulfur bearing ambient material, H₂0, SO₂, SO₃, H₂S, as well as in Cl₂ and hCl.
in general, electrical properties of contacts produceu in accordance with the invention are primarily due to the compounds noted. Reaction may involve more than two reactants, e.g., may proceed by reaction of an external reactant with two or more substrate constituents, e.g., where both yield compounds otherwise appropriate for the practice of the invention. It is expected that contacts produced in accordance with the invention will at least in initial stages contain little if any material in addition to the contemplated compound/s. However, structures may include unintentional material. Examples include substrate ingredients which under fabrication conditions migrate into the compound layer sometimes to the free contact surface. Resulting mixture, again consistent with prior experience, may result in improvement ot the contact surface. In accordance with example 6, analysis of the free contact surface has revealed presence of 2 weignt percent cobalt. Studies of the nominal composition CuS showed increased resistivity upon inclusion of tin, iron, zinc, manganese, titanium, chromium, nickel, aluminum, silicon, antimony and cobalt in solid solution (all were in the range of 1-15 weight percent) .
The contact-compound rich phase is desirably composed of at least 85 percent of compound/s in accordance with the inventive teaching. Second phase modifications, on the other hand, which may involve elements or compounds to the extent not dissolved in the contact compound rich phase have only a linear effect on contact resistance and may be tolerated in larger amounts. Such second phase may be unintentional or may be introduced deliberately in order to modify physical characteristics. In general, such second phase should be present only in an amount to occupy up to 30 percent of the free surface area. To a first approximation under usual circumstances, this limit may be expressed as weight percent.
Intentional modification of composition includes constituents added after formation of the compound. Examples include gold diffusion to produce a graded structure of guod electrical and mechanical properties. btructures may also be graded by altering reactant composition during reaction.

4. procedure

Appropriate procedures for forming compounds suitable for the practice of the invention are described in the examples. -In general, external reactant/s are brought into contact with supstrate reactant/s in a fluid form - either vapor or liquid. Expedient processing to result in, e.g., desired layer thickness, sometimes suggests use of a carrier to permit reaction at temperatures at which external reactant might otherwise not be fluid/vapor.
It has oeen found desirable to limit compound formation to layers of a thickness no greater than approximately 25µm. From the electrical standpoint, compounds of the nature described usually have greater bulk resistivity than substrates on which formed so that greater thickness unnecessarily increases series resistance. In addition, compound layers of some included compositions show cohesion values which are less than adhesion to the supstrate. The observable phenomenon is an effective integrity which varies inversely as layer thickness.
All compounds tested with the exception of CuS and CuSe had sufficient adhesion to permit most demanding aevice operation as produced by reaction with substrate composed solely of reactant. CuS and CuSe, however, showed substantially improved adhesion when prepared on substrate containing additional ingredients. Examples 5 and 6 exemplify satisfactory device requirements. CuS had improved adhesion when prepared on a substrate containing greater than 15 weight percent cobalt. Adhesion of CuSe is increased by variants of example 6, e.g., by use of substrates in whicn copper is alloyed with nickel.
In like manner, procedural variations, e.g., variation in temperature, pressure, etc., may result in desired structures by compositional grading.
Detailed discussion nas been restricted largely to creation of cue contact surface itself. Fabrication of the entire apparatus may place other demands so, for example, substrate composition may be chosen with a view to ductility to permit fabrication uy coldworking. Minimum thickness is determined on the basis of substrate protection. Monolayers are generally inadequate. Substrate protection improves dramatically to layer thicknesses of up to about 3 micrometers.

brief Description of the Drawings

FIGS. lA, 1B, 2A, 2B, 3 and 4 are perspective views of nonarcing contacts incorporating in situ formed compound contact layers of the invention.

Detailed Description

FIGS. 1A and 18 are cross-sectional elevational views of a wire connector ot a design used for telephone handset interconnection. It consists of wires 10 contained in recess 11 and molded in detail 12 to terminate in spring portions 13 provided with contacting surface 14 produced in accordance with the invention. Mating wires 15 are mechanically fixed in position by locking inserts 16 and 17 ana terminate in spring portions 18, also surfaced with a compound contact layer of the invention. Upon inserting portion 1B in 1A as shown by arrows 19, spring portions 14 and 18 are brought into mechanical and electrical contact.
FIGS. 2A and 2B are sectional and elevation views depicting a circuit board connector. FIG. 2A shows a printed circuit board receptacle 20 provided with a spring contact 21 having a substrate 22 and contact layer 23 in accordance with the invention. In installation, circuit board 25 of FIG. 28 is inserted into recess 24 of receptacle 20. Contact is completed when compound contact 26 engages contact layer 23. Contact 26 is produced by in situ reaction with substrate 27.
FIG. 3 is a sectional view uf a momentary makebreak contact in use. Compound contact regions 30 and 31 produced on substrates 32 and 34 _dre brought into compressive contact upon distortion of element 33 from the configuration shown in solid outline to the configuration shown in phantom.
FI_G. 4 is an elevation view of a wire wrapped connector consisting of post 40 provided with compound contact layer 41. Wire 42 may be conventional gold plated, or surfaced with a layer in accordance with the invention.

Claims

1. Apparatus comprising mating electrical contacts at least one ot which comprises a substrate and a contact layer in intimate contact with the said substrate, the said contact layer having a first mating surface wnerein physical contact with the said mating surface results in electrical connection, CHARACTERIZED IN THAT the said contact layer comprises a first crystallographic phase that is produced by a procedure comprising in situ reaction using energies associated with conventional ambient to produce a chemical compound of wnich the said first crystallographic pnase is comprised, the said reaction involving reactants including a first reactant which is an integral part of the said substrate, the said first reactant being uesignated substrate reactant and a second reactant which is introduced during reaction.

2. Apparatus of claim 1 in which the said first crystallographic phase consists of of at least 85 percent by weight of the said chemical compound.

3. Apparatus of claim 2 in which the said first crystallographic phase constitutes at least 50 percent of the area of the said first mating surface.

4. Apparatus of claim 3 in which said contact layer includes a second phase which contains material introduced subsequent to the said reaction.

5. Apparatus of claim 2 in which the said crystallographic phase constitutes substantially the entirety of the said area.

6. Apparatus of any of claims 1, 2, 3, 4 or 5 in wnich the said chemical compound is selected from the group consisting of sulfides, selenides, carbides, nitrides, borides, phosphides and silicides.

7. Apparatus of claim 6 in which the said substrate reactant is an element selected from the group consisting of copper, titanium, tantalum, molybdenum, chromlum, niobium, vanadium, hafnium, zirconium, yttrium, lautuanum-group metals, cobalt, nickel and iron.

8. Apparatus of claim 6 in which the average thickness ot the said contact layer is less than 12U micrometers.

9. Apparatus of claim 8 in which the bulk resistivity of the said contact layer is a maximum of 1 x 10^-3Ω-cm.

10. Apparatus of claim 5 in which the said compound is selected from the group consisting of a sultides, selenides, carbides, nitrides, borides, phosphides and silicides of copper, titanium, tantalum, molybdenum, chromium, niobium, vanadium, hafnium, zirconium, yttrium, lanthanum-group metals, cobalt, nickel and iron.

11. Apparatus of claim 1 in which normal operation contemplates continued closure between the said mating surfaces.

12. Apparatus of claim 1 in which normal operation contemplates intermittent closure between the said mating suriaces.

13. Apparatus of claim 1 in which normal operation contemplates an electrical contact current of a maximum of 50 milliamperes.

14. Apparatus of claim 11 in which the said current does not exceed 20 milliamperes.

15. Apparatus of claim 12 in which the said current does not exceed 10 milliamperes.

16. Apparatus of claim 1 in which the said second reactant is introduced in the vapor phase.

17. Apparatus of claim 15 in which the said second reactant is above its vaporization temperature.

18. Apparatus of claim 15 in which the said second reactant is below its vaporization temperature and is introduced with a carrier.

19. Apparatus of claim 1 in which the said second reactant is introduced in a liquid phase.

20. Apparatus of claim 18 in which the said second reactant is molten.

21. Apparatus of claim 18 in which the said second reactant is in solution.

₂2. Apparatus of claim 1 in which normal contemplated operation is nonarcing.