Journal of the Air Pollution Control Association

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The Effects of Air Pollution on Electric Contacts Morton Antler & Jay Gilbert To cite this article: Morton Antler & Jay Gilbert (1963) The Effects of Air Pollution on Electric Contacts, Journal of the Air Pollution Control Association, 13:9, 405-450, DOI: 10.1080/00022470.1963.10468199 To link to this article: http://dx.doi.org/10.1080/00022470.1963.10468199

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Date: 23 January 2017, At: 05:45

The EFFECTS of AIR POLLUTION on ELECTRIC CONTACTS MORTON ANTLER, Deputy Director, Research Division, Burndy Corp., Norwalk, Connecticut and JAY GILBERT, International Business Machines Corp., Poughkeepsie, New York

Introduction

The deterioration of materials by air pollution is well known.1"9 While impossible to calculate precisely, the loss by pollutants probably exceeds one billion dollars annually in the United States.10 Estimates of economic losses by air pollution are usually conservative. Rarely are the costs identified for using expensive materials that are resistant to pollutants, in place of cheaper materials that would be satisfactory in clean air. For example, gold and other precious metals are widely used for electric contacts because of their low chemical reactivity. This paper is concerned with the influence of air pollutants on the behavior of electrical contacts. This is a specialized field of atmospheric corrosion research. It concerns the relationship of environment to film formation on metals, and the influence of these films on the passage of electric current. We consider the role of air pollution in the mechanisms of degradation of certain classes of electric contacts. An approach to environmental test development for contacts, involving both field and laboratory studies is discussed. Finally, specialized tools developed for this work are described.

tacts include slip rings. Static contacts may be separable, like those of the pin-in-socket connector, or permanent, like the solder contact, the evaporated film contact, and the contacts in crimp and wire wrap connections. Air pollution is most concerned with the degradation of separable contacts exposed to the atmosphere, in which closure is made by pressure. Hardware examples are shown in Fig. 1, a relay which switches current, and Fig. 2, a printed circuit board and mating connector. Connector Economics

Electronic gear is rapidly growing in importance. In the last decade, the computer and missiles industries, which are heavily electronic, have mushroomed to giant size. The communications industry has continued steady growth. Electrical instruments monitor and control an increasingly larger share of manufacturing processes. The value of signal transmission hardware (printed circuit boards, relays, etc.) which contain environment-exposed electric contacts manufactured in the United States is of the order of hundreds of millions of dollars a year. The products of which they are an integral part are valued at tens of billions of dollars. The individual

contacts themselves are numbered in the billions. Tarnish and corrosion films on metal surfaces are insulating. Films can be avoided or their effects minimized by fabricating contacts of nonreactive metals, by encapsulation, by the use of air purification devices—such as filters and gas absorbing chemicals, by obtaining redundancy through multiple contact points, by circuit compromises, by using contact designs which give high normal loads at the mating surfaces, or by sliding contacts together on closure so as to break insulating films. These measures, however, are costly, and an economic waste insofar as the contact films themselves can be traced to air pollutants. Trends in Electronic Equipment that Affect Contacts

The problems of contacts and air pollution are becoming worse. Electronic apparatus is growing in complexity, with more component parts. Thus, the individual contact must have even greater reliability so that a satisfactory level of machine performance may be attained. The introduction of solid state devices, such as the transistor, into circuits has also magnified contact problems. These components function

Definitions

A connector is a device which bridges two or more points in an electric circuit without itself having significant resistance. It usually insulates as well, isolating one part of a circuit from another. Connectors take various forms, and range in size from those which serve power distribution lines to the miniature variety of electronic circuits for signal transmission. A contact is the current-carrying part of a connector having an interface, usually solid-solid between metals. Contacts may be primarily static or dynamic, depending on their function while carrying current. Dynamic con* Presented at the 56th Annual Meeting of APCA, Sheraton-Cadillac Hotel, June 9-13, 1963, Detroit, Michigan. September 1 963 / Volume 1 3, No. 9

Fig. 1. Silver wire relays. Right: new. tarnished wire contacts below relay coil.

Left: operated in H2S-polluted field locations.

Note

405

Fig. 2. Printed circuit board (front and rear) and connector. Connector sectioned to show contacts on bottom. Solid gold buttons attached to cantilever springs press on gold plated tabs of board.

at low voltages. To the extent that voltage can break down insulating films to make contacts conductive (1 v. will puncture a film having a thickness of approximately 100 angstroms), older circuits involving electronic tubes were less affected than those of today by film-generating pollutants. Another trend is miniaturization. High contact pressures and large wipes on contact closure cannot as readily be obtained with miniature as with larger connectors. Surfaces, therefore, must be maintained film-free. The difficulty of obtaining clean surfaces, mainly because of air pollution, limits connector miniaturization. Thus, circuit packages, or modules, must be relatively big, i.e., must contain several components mounted on a substrate which is sufficiently large to permit connection. If one component in a module fails, the entire package is discarded. The economic level of pluggability is limited indirectly by dirty surfaces. Optimum connector materials and design selection means avoidance of 406

over-design, if over-design costs money, as well as avoidance of inadequate design. To achieve maximum connector cost-effectiveness, a quantitative knowledge of the effect of environment, and of the effects of air pollutants, is required. In addition to knowledge of the surface behavior of contact metals must be added knowledge of the environmental behavior of insulating materials.

When two metallic surfaces are placed in contact at a light normal load, they touch at only a few small spots, or asperities. Load is concentrated on these spots, and plastic flow readily occurs. As load is increased, more and more asperities come into contact, and the surfaces move together. The true area of contact depends, therefore, on normal load and on the hardness of the metal. The real contact area is only a fraction of the apparent contact area for most metals, except at very heavy loads.11 For example, the ratio of real to apparent contact areas of finely lapped steel flats having an apparent area of 21 cm2, in contact at 2 kg, is 10~5; the true area of contact is 2 X 10~4 cm2. If metallic surfaces are entirely covered by a nonconductive layer, such as an oxide or a sulfide tarnish film, the area of metallic contact will be zero, provided the film is unbroken. If the film is not continuous, or is punctured on making closure, load is borne by bothfilmand metal. The behavior of metallic surfaces as electric contacts at low signal levels, in the millivolt and milliampere range, can be understood from this picture. Current will flow at the contacting spots, except for a small contribution by electron tunneling,12 the latter only for separation distances of several angstroms. Since the usual films on surfaces are substantially thicker than this and have conductivities 10~6 or less than that of metals, conduction for all practical purposes depends only on the area of metallic contact, bulk resistivity of the metal, and to a minor extent on details of the number and distribution of contact spots. Contact resistance is caused by the constriction of current stream through the tiny metallic touching spots. Figure 3 schematically illustrates details of a contact surface partially covered by a film.

Mechanisms of Degradation of Electric Contacts

To fully appreciate the basis for electric contact field and laboratory studies involving air pollutants, it is first necessary to review fundamentals of contact theory and of corrosion. Contact Physics

The surfaces of solids are irregular on a microscopic scale. Even nominally plane, smooth surfaces have a large scale waviness on which is superimposed a surface roughness with peakto-valley distances of several tens of thousands of angstroms.

Fig. 3. Schematic illustration of contact surface. The area of metallic contact, c (solid areas), in most cases is only a tiny fraction of the apparent area. Contact at b (shaded) areas is with insulating contaminant. Area a does not touch. Journal of the Air Pollution Control Association

Figure 4 is a plot of contact resistance against load for clean silver contacts in air, and for silver that has been sulfided by pollutants. Above a few grams there is slight metallic contact in the sulfided samples, which increases with normal load. A reasonable contact resistance for conventional circuits is 0.01 ohm, and this is obtained with clean silver below 1 g and on the sulfided samples only at pressures in excess of 300 g. The differences in contact resistance among the sulfided samples are due to varying film thicknesses and film morphology. When contaminant films are stressed tangentially, i.e., by wiping a clean reference contact on the tarnished surface, the film may be broken. However, continued wiping of a filmcovered surface can also imbed the contaminant. Except at heavy loads, wipedfilmsmay become troublesome. The price we pay in electric contacts for air pollution appears, therefore, in the higher loads, greater wipes, necessity to isolate from environment, or use of more costly, less reactive metals. From a connector design viewpoint, contact configuration becomes more bulky and complex. Larger loads and wipes also give shortened lives, for sliding wear on contact closure may then become a problem. Contact Chemistry

There are many types of environment-induced contaminant films on contact surfaces. Most are caused by air pollutants. Oxide films, however, are an exception, for they normally form on base metals. Thus, easily oxidized hard metals, like chromium plate and molybdenum, are not satisfactory for pressure contacts in low energy circuits at reasonable loads. In polluted environments, contact surface films almost always are thicker and more detrimental than films formed in clean atmospheres. Not only do oxides themselves grow more rapidly, but films form in polluted atmospheres on certain metals which otherwise would be totally unaffected. The ancients considered silver to be as noble as gold, because it remained bright and untarnished for long periods in the atmosphere, which then was relatively sulfur-free.13 Palladium tarnishes only in SO2-containing outdoor atmospheres.14 The present cost of gold used annually in the United States for contacts is approximately 15 million dollars. If silver could replace gold, the saving would be 14.8 million dollars, based on equivalent volumes of metal. The comparable saving with palladium replacing gold is eight million dollars. There are several distinct processes by which air pollutants react with contact surfaces at ambient temperature. September 1963 / Volume 13, No. 9

10'

10'

10 to

e 10 CD O C

o

-I

10

(f)

O O

-2

10

O O

-3

10

io"

4

0

100 200 300 400 contact pressure — grams

500

Fig. 4. Contact resistance versus normal load for silver contacts: determined with 24 carat gold probe, having a 1/g in. diameter hemispherically-ended tip. a-e: tarnished silver, as follows—a: two weeks in laboratory chamber at an average H2S level of 1.6 ppm, 90°F, 8 5 % RH, 50 lineal feet per minute air flow; bi two years out of doors in sheltered location, industrial, Newark, New Jersey, 1 960-1 962; c- one month, indoors, petroleum refinery, 1962; d and e: 3 x /2 months, indoors, office sites, Manhattan, 1962; f: untarnished silver.

These may be differentiated by the physical form and chemical composition of the pollutant, or by special mechanisms which result in surface contamination. (1) Gaseous pollutants that attack metal surfaces :15' 16 (a) Tarnish—Tarnish films are relatively uniform. They can grow at low relative humidity by mechanisms closely related to oxidation. Copper, silver, their alloys, and many other metals are sulfided by H2S and its organic derivatives. Pollutants containing labile halogen also attack these metals.

Some metals form a surface layer of mixed oxide and sulfide. Even high carat gold alloys containing copper or silver, such as the common contact alloy, 69% Au/25% Ag/6% Pt, will tarnish superficially at high sulfide concentrations. (&) Corrosion—"Damp atmospheric corrosion" occurs above a critical relative humidity in the presence of traces of gaseous (or solid) pollutants. A film, usually quite heterogeneous, grows rapidly on many metals at these conditions. Nickel, lead, and tin are quickly coated in SCVpolluted at407

mospheres above 70% relative humidity with sulfates, oxysulfates, and oxides. If water can condense on the contact surface at least part of the time, "wet atmospheric corrosion" may occur. This is common with contacts exposed out-of-doors, and is associated with dewing. The palladium-SO2 reaction probably is one of wet corrosion. In addition to sulfur- and halogencontaining gaseous .pollutants, it is likely that contact metals are degraded by oxidizing agents, such as ozone and the oxides of nitrogen, and by NH3 atmospheres. Contact problems from these other gases, however, have been little identified and studied. (2) Gaseous pollutants that do not attack metal surfaces: A few metals, most notably palladium and other platinum-group members, will catalyze the polymerization of organic gaseous pollutants to resinous solids. These solids accumulate on surfaces and can cause contact opens. They form from all but the most volatile molecules when the surfaces are rubbed, hence the term, "friction polymer."17 Current flow or heating are not required for polymer formation. This is also a serious problem in encapsulated contact-containing systems that include volatiles, such as products from varnish or rubber insulation.18 Polymer forms, although more slowly, at static conditions, i.e., without rubbing.19 (3) Particulate matter: (a) Mechanical effects—Dusts that deposit on surfaces interfere with contact closure.20 Dust may become imbedded in the surface, and can also induce wear by abrasion if the contacts slide. Loosened corrosion and tarnish solids act in the same way as do dust. Dust may accumulate by settling if it is coarse, or may be precipitated on the contact surface from

I

a moving air stream. The latter is common in electronic systems, for contacts are often proximate to components that generate heat, hence moving air currents. Forced air cooling is also much used, particularly for circuits that contain a high population density of components. (6) Chemical effects—Dusts can act chemically as well as mechanically on contact surfaces. If the dust is hygroscopic, water will be attracted to the surface on which the dust has accumulated,21 to form thin electrolyte films. Thesefilmsare corrosive to base metals.22 If the contact members are not of identical composition, galvanic corrosion may arise. Except for dust which originates in ocean salt spray, hygroscopic dusts are usually man-made pollutants. They are formed mainly by the reaction of metal-containing ' dust particles with acidic products of the combustion of fuels. Dusts from coal burning regions tend to be high in sulfate content. Dusts collected in Los Angeles often contain considerable ammonium nitrate, which deliquesces at a lower relative humidity than sulfate-loaded dusts. Nickel-brass connector springs are subject to stress corrosion cracking by aqueous ammonium nitrate films.23 Nonhygroscopic dusts having high surface areas adsorb gaseous pollutants. Such particles can, therefore, concentrate pollutants on contact surfaces to induce degradation reactions which otherwise would proceed more slowly, or not at all. (4) Oily pollutants: Oily pollutants, as aerosols, are common in industrial and urban areas. Clean metal surfaces pick up enough hydrophobic material in a few minutes in such atmospheres so that they cease to be wettable by distilled water. Oily surfaces are serious if the atmosphere is dusty,

1

10 mm

I mm

Fig. 5. Example of contact material contaminated by product of reaction with an air pollutant. Silver plated with gold, 20 X 10~ 5 in. thick, exposed two weeks to H2S, 1.6 ppm (see Fig. 4a): a.- sulfided silver, b: gold plate, c: pore in gold plate, d: gold plate-sulfided silver interface, e: silver sulfide creep.

408

because solids will be entrapped on them. (5) Special cases of contamination: (a) Porous platings—Contact surfaces are often plated with precious metal for atmospheric protection. These platings most often range in thickness from 1 - 20 X 10~5 in., and frequently contain pores. Gaseous air pollutants react with substrate metal at these defect sites, and the zone of contamination may grow to large size if the tarnish and corrosion solids are voluminous. This is illustrated in Fig. 5. (b) Sulfide creep—Silver and copper sulfides grow in sulfiding atmospheres on . their respective parent substrate metals or alloys, and can migrate across a gold interface. Gold is commonly plated on silver and copper contacts to protect them from environmental degradation. In sulfide creep, the efficacy of the plating is lost. Figure 5 shows the creep phenomenon at silver-gold interfaces from exposure in the H2Scontaining laboratory atmosphere described in Fig. 4. (c) Insulation leakage—Hygroscopic dusts on contact insulators can create paths for electrical leakage.24' 25 This is particularly a problem in miniature connectors, because of the close spacing of the current-carrying members and wiring. Studies of Contact Behavior

The mechanisms of. failure of electric contacts, and the influence of environment on contact performance, are known in general terms. However, details of the behavior of individual materials from the point of view of the requirements of a complex product— such as the electronic data processing machine—are less well known. The objective of the program described below is this: to determine materials behavior for low energy contacts exposed to a wide variety of environments, which range from clean to highly polluted. It is a technical study, and part of a larger effort that includes: the continued development of machine packaging concepts and of connector design, the exploration of new manufacturing tools and processes, and a continuing reappraisal of product installation and product field servicing philosophy. The program is split into closely related laboratory and field phases with individual subobjectives. In the field, the environment, and particularly air pollution levels, are defined. Contact materials are exposed and their behavior determined. The laboratory work seeks to develop meaningful accelerated tests based on knowledge of field conditions. These procedures include the simulation of polluted atmospheres. Comparison of field and laboratory results follow. Data correlation is Journal of the Air Pollution Control Association

more than empirical; it is based on an understanding in depth of contact behavior from a phenomenological point of view. Corollary studies, not considered here, but of equal importance to this program, are the long range basic studies made to determine fundamental contact processes. Studies of air pollution and the static contact, and of sliding contacts,, already reported26"28 are made. New tools for surface study are developed.29 New materials may be synthesized, which could be more suitable for contacts than those now available. Methods of Determining Contact Behavior

Exposure Hardware contacts are exposed to the environment basically in either of two ways, in the mated and the unmated conditions. These different conditions often show different patterns of film growth, and certainly different effects of films on contact performance. Accordingly, materials in this program are studied with specimens fabricated into both contact forms. The material, of known chemical composition, metallurgy, geometry, and surface texture is aged "made" as crossed rods. Figure 6 shows several specimen pairs from a rack fixture in which the members are held together by calibrated springs. Several different contact pressures are used. The rods are electrically isolated from each other, except where they touch, and from the rack. Flat samples of contact material, such as IV2 in. sq. of any convenient thickness, are aged "unmade" in racks that permit free airflowover top and bottom surfaces. In addition, connector hardware is exposed, and stressed after exposure, to obtain details of more complex behavior—such as the effects on films of increasing contact pressure with insertion depth, inherent in certain connector designs. Contact Resistance The most important measurement that can be made is, obviously, contact resistance. This is determined periodically with crossed rod specimens by the four-wire method,30 which eliminates lead wire resistances. Voltage is kept to a few millivolts to avoid film breakdown. The variation of contact resistance with time is plotted. As tarnish and corrosion films grow inward toward the point of contact, contact resistance begins to rise, and, provided load is not high, becomes very large when metallic contact is effectively broken.31 The surfaces of the aged-unmade specimens are probed at known presSeptember 1963 / Volume 13, No. 9

sures, voltages, and wipe distances with a precious metal reference contact of controlled geometry, e.g., hemispherically-ended Vs in. diam rods orfinewire loops. Sliding and wear are determined with rubbing testers. Details of these apparatus are given in Appendix A. The contact resistance of a metal depends on surface films, hence, on environment, as well as on the material itself and its method of preparation. If the surface condition of metal specimens is carefully defined, it is probable that contact resistance measurements after exposure can serve as reliable cumulative, or long term, indicators of pollution. The fine wire contact resistance probe (Appendix A) is a simple, inexpensive measuring tool for this purpose. Optical transmission32 and surface resistivity33 of thin evaporated metal films have already been described for pollution indication. Background Measurements Subsidiary studies are made to define surface structure and composition, and changes in them, after exposure to polluted atmospheres. The field of surface measurement is very large, and many tools are valuable in a contact program. Among the more important are: (a) diffraction, for structure: X-ray and electron, (b) microscopy: electron, optical, (c) contour and roughness: profile measurement2 with surface stylus, (d) chemical: film identification by spot tests, X-ray fluorescence, radioisotope studies, electrogra-

phy,34 film transfer,35 and electrolytic reduction36 forfilmthickness. Field Program

Environments differ markedly. Product type and its importance dictate which environments are to be studied. Both industry differences and geographical location affect product environment. Electronic data processing machines are used indoors, and so these environments are emphasized in our study. Published data on air pollution type and concentration is invaluable in the preliminary selection of test sites. Indoor levels, especially at sites that do not themselves contribute significant^ to pollution, are closely related to the outdoor level.37"40 Temperature and relative humidity are the more important environmental differences between indoor and outdoor locations for electric contact work. In this study, we use a series of field test sites. At each, the environment is monitored for pollutants known or suspected to contribute to contact degradation. Gaseous pollutants, suspended particulates, dust fall, temperature, and relative humidity are measured. For materials degradation studies, cumulative pollution monitoring procedures are more useful than those of point-in-time, for materials are required to perform over lengthy periods. Unfortunately, good cumulative monitoring techniques for pollutants have not been extensively developed. Cumulative methods should be specific, simple, inexpensive, require little

Fig. 6. Crossed rod rack and samples for aging contact materials in a prototype mated configuration. Specimens are loaded at known pressure with calibrated springs.

409

attention, and have a long-use span, from weeks to months. The SO2 candle41 is a good example of a cumulative tool which meets these criteria. Similar tools have been described for only a few other pollutants. Procedures for H2S,42 HF,43> 44 and oxidants45- 46 are less well denned. Pointin-time methods are useful to check cumulative devices, as well as for use where no cumulative procedure exists. Point-in-time methods are valueless for field electric contact studies, however, unless many measurements are taken over a long period. Initial surveys are made at a large number of field locations, having a broad range of environments, from those considered "good" to those considered "bad." Information from these surveys is used to select a smaller number of sites suitable for detailed materials studies over a span of years. During this time, the environment is continuously monitored. A variety of air pollution measuring tools are used in the field program, and described in Appendix B. Field environmental chambers have been built to house cumulative air pollution devices and contact test samples. They resemble filing cabinets to conform to customer office furniture. Details of their construction and contents are given in Appendix C. Laboratory Program

Materials degradation in the field is slow. In electric contact studies, as in most others, reliable accelerated laboratory tests are desirable. The laboratory evaluation procedures most used in industry at present have, however, not been designed to realisti-

Table 1—Synthetic Dust for Contact Evaluation Constituent Grams 1.55 KNO 3 KC1 2.12 KF 1.20 NH4CI 3.80 NH4NO3 5.68 Mg(NO 3 ) 2 -6H 2 O 2.26 MgSO 4 -7H 2 O 32.6 CaO-Al 2 O 3 -2SiO 2 17.7 CaSO 4 2.89 TiO 2 0.28 V2O» 0.075 MnO 2 0.82 Fe 2 O 3 7.30 CuSO 4 -5H 2 O 1.33 ZnO 1.07 HgCl 2 0.115 PbCI 2 2.29 PbSO 4 1.26 Paper fiber 14.6 100.0 Particle size: inorganic constituents pass 325-mesh screen. Paper fibers pass 80-mesh screen. Individual fibers have average length-width ratio of 5; 50% by volume longer and 50% shorter than 75 microns. Cold water solubility: 49%. 410

cally simulate air pollution. Salt spray chambers,47 for example, are still widely used for aging low energy connectors, even when marine environments are not a significant factor in connector use. Exposure to a warm, moist atmosphere saturated in sulfur vapor is widely employed for silver-containing contacts. Little is known of the relation between such tests and actual field conditions. Since air pollutants degrade contact materials, it is important that it be possible to simulate and control polluted environments in the laboratory. Gaseous Pollutants Methods for producing polluted atmospheres in test chambers have been surveyed.48- 49 In the present program, two approaches for gaseous pollutants are used. Control of temperature, relative humidity, and air flow are also important. (a) Static method. Sample materials are placed in a sealed chamber, and small measured amounts of atmospheric contaminants and water for humidification are injected through a diaphragm. The advantages of this system are its simplicity and facility with which a number of pollutants can be handled at one time. However, as reactions between pollutants, and of pollutants with test specimens, occur, pollutant levels decrease. Additional charges must be periodically reinjected, which produce a cycling condition. Monitoring of pollutant levels is done externally with gas samples extracted from the chamber. The apparatus shown in Fig. 13 was built for contact studies, and is described in Appendix D. (6) Dynamic method. Samples are exposed in a chamber which is connected to a system that continuously generates known concentrations of an atmospheric contaminant and water vapor. The contaminated air stream flows over the samples. The advantage of this approach is precise control of variables. Also, cycling of pollutant level is eliminated. However, each gas requires a separate generating system, which makes handling more than one pollutant difficult. The equipment developed for dynamic tests is described in Appendix E. Dusts Contact surfaces are innoculated with participates in either of two ways. Gravitational settling in air is used when the solids are coarse. Contact specimens are placed at the bottom of a long tube, and the solids shaken in at the top. A second approach is to suspend solids in a volatile liquid in which they do not dissolve. The contacts are dipped in this mixture, retracted, and dried. If oily pollution is to be simu-

lated at the same time, the volatile liquid is doped with a few tenths of a percent of oil. Following innoculation, the samples are aged in the gaseous chambers. Various dusts have been used for this work. Contact mechanical interference is determined with commercially available graded solids. Chemical effects, e.g., dust-induced corrosion, are determined with a synthetic dust having an inorganic elemental composition which roughly approximates an average for the United States, based on measurements from the National Air Sampling Network.50 Individual dust constituents were chosen because they are representative, or will behave similarly to those in actual dusts. Elements present only in trace amount in the National average are omitted. Shredded paper fiber is added to simulate contamination common in machine accounting rooms. This approach to formulation of synthetic dusts was suggested by Wohlers and Bell,51 although the dust used here is different from the one that they proposed. The formulation of the standard contact test dust is given in Table I. Reagent grade inorganic chemicals are mixed and ground in a ball mill. The dust is then sieved in a dry gloved box, and mixed with paper fiber. At room temperature and 60% relative humidity this formulation deliquesces in part; at 40% relative humidity, it does not. The dust is mildly corrosive to copper at 60%, and severely corrosive at 80% relative humidities. At 40% relative humidity, copper is barely affected by it. The philosophy of laboratory testing. Air pollution at a given field site is variable, depending on level of activity and meteorological conditions, among other things. Materials degradation occurs more rapidly at high than at low pollutant levels. The principle of acceleration in the laboratory programs is this: to maintain pollutants at high levels, but within the range of experience, and to eliminate or reduce the length of time when pollution is low. Second, materials degradation by gases depends largely on the rate of replenishment of reactive pollutant species. Therefore, a high, but again realistic flow rate of polluted air is maintained over the test specimens in the environmental chambers. Third, other environmental factors in contact degradation, such as temperature and relative humidity, are maintained at relatively severe levels. In this way, it is believed the laboratory procedures will yield both significant and rapid results. The tools for detecting contact films are sensitive, and as experience develops, it becomes possible to extrapolate contact behavior from abbreviated tests. Journal of the Air Pollution Control Association

Correlation

This philosophy of laboratory exposure is not without objection. It is possible that low and high pollution levels give films with different properties.52 However, the laboratory effort does not stand alone. A major purpose of the field work is to provide specimens for study, identical in initial composition and similar in exposure to those examined in the laboratory chambers. Since the field environment will be well characterized and the laboratory environment based on it, it is reasonable to expect similarity of results. Surface chemical studies are made to establish film composition relationships between field and laboratory. Contact resistance data, especially those obtained with surface probes, is the common denominator for all exposures. Age acceleration factors are determined from resistance-time-load relationships, such as those in Fig. 4. Conclusions

Air pollution is a major cause of the degradation of electric contacts. Certain materials and connector design types can often minimize the effects of pollutants, but not without considerable added cost. Knowledge of environment and its effects is essential to the development of connector technology. The systematic investigation of air pollution has, however, been largely neglected in the connector field. In the program of this paper, closely related field and laboratory studies are carried out concurrently with the objective of determining the behavior of environment-exposed materials for low energy circuits. Air pollution is measured at field sites in which contact materials and connector hardware are exposed. Laboratory accelerated aging tests involving polluted environments are developed, based onfieldexperience. Contact degradation mechanisms are studied by measuring contact resistance with surface probes, as well as other film-related properties. By combining knowledge of environment, of the basic physics and chemistry of interfaces, and an understanding of the phenomenological response of materials to environment from an electric contact point of view, causeeffect relationships are established which aid in materials selection and the development of connector designs. The end result of this work is a higher reliability product.

ASTAI Committee B-4. The static chamber discussed in Appendix D was built by the American Research Corporation, Farmington, Connecticut. REFERENCES

1. C. P. Larrabee, "Mechanisms of Atmospheric Corrosion of Ferrous Metals," Corrosion, 15: 526t-529t (1959). 2. P. M. Aziz and H. P. Godard, "Mechanisms by which Non-Ferrous Metals Corrode in the Atmosphere," Corrosion, 15: 529t-533t (1959). 3. H. It. Copson, "Design and Interpretation of Atmospheric Corrosion Tests," Corrosion, 15:533t-541t (1959) 4. J. E. Yocom, "Deterioration of Materials in Polluted Atmospheres," Corrosion, 15: 541t-545t (1959). 5. "Symposium on Atmospheric Corrosion of Non-Ferrous Metals," ASTM Special Tech. Pub. No. 175, 1956. 6. P. J. Sereda, "Atmospheric Factors Affecting the Corrosion of Steel," Ind. Eng. Chem., 52: 157-160 (1960). 7. L. Greenburg and M. B. Jacobs, "Corrosion Aspects of Air Pollution," Am. Paint J., 39: 64-78 (1955). 8. R. E. Munn, "Engineering Meteorology: The Weathering of Exposed Surfaces by Atmospheric Pollution," Bull. Am. Meteoral. Soc, 40: 172-178 (1959). 9. J. C. Hudson and J. F. Stanners, "The Effect of Climate and Atmospheric Pollution on Corrosion," ./. Appl. Chem., 3:86-96(1953).

10. E. Le Clerc, Air Pollution, U. S. edition, Columbia U. Press., New York pp. 279-291 (1961). 11. F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Chapter 1, Oxford, Clarendon Press (1954). 12. R. Holm, Electric Contacts Handbook, 3rd ed., Springer-Verlag, Berlin, p. 121 (1958). 13. E. M. Wise, et al., Metals Handbook, Ed., T. Lyman, 8th ed., Am. Soc. Metals, Metals Park, Novelty, Ohio, 1:1175(1962). 14. E. M. Wise, Corrosion Handbook, Ed., H. H. Uhlig, Wiley, New York, p. 308 (1948). 15. U. It. Evans, The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications, Chapter 13, St. Martin's Press, N. Y. (1960). 16. U. R. Evans, "Electrical Contacts, The Effect of Atmospheric Corrosion," Metal Ind., 10-13 (July 21, 1948). 17. H. W. Hermance and T. F. Egan, "Organic Deposits on Precious Metal Contacts," Bell System Tech. J., 37: 739-776 (1958). 18. H. J. Keefer and R. H. Gumley, "Relay Contact Behavior Under Noneroding Circuit Conditions," Bell System Tech. J., 37: 777-814 (1958). 19. S. W. Chaikin, "Mechanics of Electrical-Contact Failure Caused by Surface Contamination," Electro-Technol., 68: 70-75 (1961). 20. J. B. P. Williamson, J. A. Greenwood, and J. Harris, "The Influence of Dust Particles on the Contact of Solids," Proc. Roy. Soc, A237: 560-573 (1956). PROBE LOADING MICROMETER ELECTRICAL CONNECTIONS TO LOOP

INSULATING

PROBE HOLDER SAMPLE MATERIAL INSULATING SAMPLE HOLDER

FINE WIRE LOOP

X-Y TABLE

TABLE MICROMETERS

Acknowledgments

The authors wish to acknowledge the contributions of B. E. Blake, J. Bruce, W. 0. Glasspool, B. C. Henry, and H. B. Ulsh. Specimens described in Table II were provided through the courtesy of September 1963 / Volume 13, No. 9

Fig. 7. Wire probe for determining contact resistance of film covered metals. Clean loop-shaped gold wire is pressed against surface at known load, and potential drop measured with a milliampereand millivolt-limited signal. Circuit shown in insert. One loop connection is for current lead, and other for voltage. Second connections for current and voltage made directly to sample.

411

21. F. P. Bowden and W. II. Throssel, "Adsorption of Water Vapor on Solid Surfaces," Proc. Roy. Soc, A209: 297 ff. (1051). 22. II. St. .1. Preston and B. Sanyal, "Atmospheric Corrosion by Nuclei," J. Appl. Chem., 6: 26-44 (1956). 23. H. W. Hermance, private communication. 24. K. N. Mathes and E. J. McGowan, "Surface Electrical Failure in the Presence of Contaminants: The Inclined-Plane Liquid Contaminant Test," AIEE Trans., 80: Part 1, 281-289 (1961). 25. S. W. Chaikin, et al., "Contamination and Electrical Leakage in Printed Wiring," 51: 305-308 (1959). 26. M. Antler, "Wear, Friction, and Electrical Noise Phenomena in Severe Sliding Systems," ASLE Trans., 5: 297-307 (1962). 27. M. Antler, "Metal Transfer and the Wedge Forming Mechanism," J. Appl. Phys., 34: 438-439 (1963). 28. M. Antler, "The Lubrication of Gold," Wear, 6, 1,44-65(1963). 29. M. Antler, L. V. Auletta, and J. Conley, "An Automated Contact Resistance Probe," to be published by Rev. Set. Instr. 30. R. Holm, Electric Contacts Handbook, 3rd ed., Springer-Verlag, Berlin, p. 42 (1958). 31. A. Fairweather, "The Behavior of Metallic Contacts at Low Voltages in Adverse Environments," Proc. IEE, 100: Part I, 174-182 (1953). 32. J. P. Lodge, Jr., and B. R. Havlik, "Evaporated Metal Films as Indicators of Atmospheric Pollution," Intern. J. Air Pollution, 3 : 249-52 (1960). 33. J. P. Lodge, Jr., and E. R. Frank,

34.

35.

36. 37.

38.

39.

40.

41.

42. 43.

"Evaporated Metal Films as Indicators of Atmospheric PolIution-II. Resistance Measurements," Intern. J. Air Water Pollution, 6: 215-221 (1962) H. W. Hermance and H. V. Wadlow, "Electro Spot Testing and Eleetrography," ASTM Special Tech. Publ. No. 98, 12-34(1950). H. W. Hermance and T. F. Egan, "Examination of Electric Contacts by Plastic Replica Method," AIEE Trans., Part 1, 34: 756-762 (1958). W. E. Campbell and U. B. Thomas, "Tarnish Studies," Trans. Electrochem. Soc, 76: 303 ff. (1939). K. T. Whitby, et al, "The ASHAE Air-Borne Dust Survey," J. Air Pollution Control Assoc, 7: 157-165 (1957). M. B. Jacobs, A. Manoharan, and L. J. Goldwater, "Comparison of Dust Counts of Indoor and Outdoor Air," Intern. J. Air Water Pollution, 6: 205-213 (1962). M. B. Jacobs, L. J. Goldwater, and A. Fergany, "Comparison of Suspended Participate Matter of Indoor and Outdoor Air," Intern. J. Air Water Pollution 6: 377-380 (1962). G. C. R. Carey, et al., "The Effects of Air Pollution on Human Health," Am. Ind. Hyg. Assoc. J., 19: 363-70 (1958). F. W. Thomas and C. M. Davidson, "Monitoring Sulfur Dioxide with Lead Peroxide Cylinders," J. Air Pollution Control Assoc, 11: 24-27 (1961). G. Chanin, J. R. Elwood, and E. H. Chow, Sewage and Ind. Wastes, 26: 1217 ff. (1954). V. L. Miller, et al., "Lime Paper and Indicator Plants in Fluorine Air Pollution Investigations," Agric. Food

Fig. 8. CD tcci resistance probe. Instrument automatically determines contact resistar.ee of filmcovered metals as a function of pressure or voltage. Contact resistance versus wipe distance at a fixed load is determined manually.

412

Chem., J., 1:526-529 (1953). 44. D. F. Adams, "Further Applications of the Lime Filter Paper Technique in Fluorine Air Pollution Studies," J. Air Pollution Control Assoc, 7: 88-91 (1957). 45. R. V. Doughty and D. O. Erisman, "A Reliable Low Cost Instrument for Determining Atmospheric Oxidant Levels," J. Air Pollution Control Assoc, 11: 428-430 (1961). 46. T. Vega and C. J. Seymour, "A Simplified Method for Determining Ozone Levels in Community Air Pollution Surveys," J. Air Pollution Control Assoc, 11 : 28-33, 44 (1961). 47. W. Blum and L. J. Waldron, Corrosion Handbook, Ed. H. H. Uhlig, Wiley, New York, pp. 970-978 (1948). 48. L. Silverman, Air Pollution Handbook, Eds., P. L. Magill, F. R, Holden, and C. Ackley, Section 12, McGraw-Hill, New York, pp. 1-48 (1956). 49. J. P. Lodge, Jr., Air Pollution, Ed., A. C. Stern, Academic Press, New York, 1:496-508(1962). 50. M. Katz, Air Pollution, U. S. edition, Columbia U. Press, New York, pp. 131-134 (1961). 51. H. C. Wohlers and G. B. Bell, Final Report on SRI Project No. SU-1816, Contract No. DA 18-064-404-CML123, Prepared for the Chemical Corps Research and Development Command, Fort Detrick, Frederick, Md. (November 30, 1956). 52. U. R. Evans, The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications St. Martin's Press, New York, pp. 803-818 (1960). 53. R. H. Savage and D. G. Flom, "Exploration of Metal Surfaces with Fine Wires," Ann. N. Y. Acad. Sci., 58: 946-950(1954). 54. H. C. Angus, "Surface Films on Precious Metal Contacts," Brit. J. Appl. Phys., 13: 58-63 (1962). 55. D. G. Flom, "Contact Resistance Measurements at Low Loads," Rev. Sci. Instr., 29: 979-981 (1958). 56. "Air Pollution Measurements of the National Air Sampling Network—• Analvsis of Suspended Participate Samples Collected 1953-1957," Public Health Service Publ. No. 637.U. S. Department of Health, Education, and Welfare, Cincinnati, Ohio, p. 6 (1958). 57. J. Dearie Sensenbaugh and W. C. L. Hemeon, "A Low Cost Sampler for Measurement of Low Concentrations of Hydrogen Sulfide," Air Repair, 4:

Fig. 9. Closeup of automated contact resistance probe. Hemispherically-ended gold rod is pressed against test sample. Journal of the Air Pollution Control Association

(May 1954). 58. P. W. West, "Final Report on the Field Test Kit for Air Pollution Surveys," Contract No. SAph 69656, Prepared for the Department of Health, Education, and Welfare, Public Health Service, Washington, D. C , by Kem-Tech Laboratories, Baton Rouge, La. Appendixes A.

Surface Measuring Instruments

Wire Probe (Fig. 7) Wire probes are used to detect films on surfaces and to determine their contact resistance-load properties. The test surface is touched with a loop-shaped gold or platinum wire, or a quartz fiber to which is attached a precious metal button, and loaded by compression.53"55 The load range that can be studied extends from milligrams to hundreds of grams, depending on wire diameter and loop dimensions. Potential drop of the contact is monitored with a low energy ac or dc signal. Care is necessary to prepare the wire to a state of reproducible surface cleanliness. Automated Probe (Figs. 8 and 9)

A new instrument has been built29 which will automatically determine contact resistance by the four-wire method as a function of dead weight load (pressure mode), the currentvoltage properties of thin films (voltage mode), and contact resistance—wipe distance behavior at fixed load (wipe mode). Probe tip material and geometry can be changed to meet the needs of the experiment. The surfaces which can be studied may be rods or flats, the latter to two inches square in size. This probe is a programmable instrument. It positions the test surface in the XY plane, goes through pressure or voltage mode of operation with recorder read-out, and then automatically repositions and repeats the measurement according to the test schedule. Distance between probings can also be varied. In the pressure mode, load is increased without wipe or vibration to 1000 g or any lesser value, with continuous measuring of constriction resistance in a low energy circuit (open circuit potential is 0.01 or 0.05 v dc). Contact resistance full scale recorder range automatically changes on command from the probe; full scale is varied in decades from 0-1000 ohms to 0-0.01 ohm. Recorder full scale in the pressure range is 1000 g, or can be set to any fraction of this value. In the voltage mode, a predetermined pressure is applied without wipe or vibration, and open circuit potential is automatically increased to 200 v, dc. Film thickness is determined from the voltage required to puncture the film. The wipe mode is a nonprogrammed September 1963 / Volume 13, No. 9

operation. In it, a predetermined pressure is applied, and then the surface moved horizontally. Constriction resistance as a function of wipe distance is recorded automatically. The position of the specimen is also indicated automatically on the chart record. This enables any spot to be located for subsequent microscopic or other examination, if desired. Table II lists typical contact resistance data obtained with this instrument, which illustrate also contact degradation by air pollutants. Copper, gold, and palladium specimens were exposed out-of-doors for two years in an industrial area of Newark, New Jersey. The samples were protected from the rain by a louvered shelter, fitted with coarse spun glass filters at the sides. Voltage mode probing at 10 g (too low a pressure to mechanically break the film) was made with a clean 24 carat gold rod having a diameter of Vs in. and a smooth hemispherical end. Voltages from 36 to 61 were required to puncture the heavily corroded copper sample. Pure gold, which is untarnished in this atmosphere, nevertheless was poor because of heavy dust contamination. Voltages to 198 were required for current flow. Palladium was uniformly coated with a thin tan film and with dust. Voltages in excess of 200 were necessary for film puncture on palladium. The chief advantages of this instrument are: (1) versatility, (2) elimination of most of the operator effort and operator variability in ob-

Table II—DC Voltage Required to Puncture Film, Volts" Observation

Copper

Gold

Palladium

Probe Positive 1 2 3 4

36 43 37

1 2 3 4

58 52 60 61

108 200 200