I
SAUL W. CHAIKIN, JOAN JANNEY,' FRANKLIN M. CHURCH, and CHARLES W. McCLELLAND2 Analytical Chemistry Section, Stanford Research Institute, Menlo Park, Calif.
Silver Migration and Printed Wiring Silver migration, long a cause of dielectric failure in electronic equipment, can be either eliminated or predicted
IN
SILVER MIGRATION, a n electric field causes silver to move from a metallic conductor either into or along the surface of a n adjoining dielectric body. Such deposits of metallic silver lower dielectric strength and can ultimately cause dielectric failure. I n printed wiring silver is also employed extensively in a variety of ways.
Experimental Procedure
For accelerating silver migration a standard test period of 65 hours was used. T h e test sample, in a vessel maintained at 96% relative humidity by a saturated potassium sulfate solution ( 8 ) ,\vas placed in an oven held at 40' 5 0.5' C. .I 480-volt direct current potential was applied, through a 1megohm resistor, to the anode and cathode leads attached to the sample. The sample consisted of insulating material in contact with silver! either as paint (Du Pont conducting silver paint S o . 4817) or as 0.010-inch fine silver foil (Figure 1). Rectangular insulator samples, 6 8 X 1 ' ' 4 inches. were prepared. Each electrode was a l,'4 X '3 inch rectangle with a small tab on one long edge for attachment of a lead. Tbvo electrodes were placed flat on each sample \vith their untabbed long edges parallel and spaced l , ' 4 inch apart. Because Teflon is immune to silver migration, the sample was mounted betlveen a base and a smaller rectangle, both made the Teflon. The assembly \vas held in position by screiv pressure horn above. applied through brass pressure plares. In the silver paint method. electrodes jvere applied by spraying a circular clectrode pattern through a two-piece steel mask lying on the flat sample. Tubular samples were also used. Two silver rings. painted '4 inch apart, served as electrodes on these samples, which \Yere suspended in the chamber by beryllium-copper spring connectors. T h e Characteristic appearance of migrated silvrr of X X X P (a phenolicpaper laminate) under these conditions is sholvn in Figure 2. Emission spectrographic analysis sho\ved that the dark fibrous deposits consisted of silver. Vnder the microscope it was clear that Present address, 3404B Urban, Los .4lamos, N. M. * Present address, Varian Associates, Palo Alto: Calif.
the silver had deposited in the paper fibers of the laminate, which gave the deposit its characteristic appearance. I n nvo series of experiments first voltage (at 967, relative humidity) and then relative humidity (at 480 volts direct current) were varied while the temperature and duration of the run were unchanged. Experiments \vith 480 volts produced extensive migration (Figure 2). 22.5 volts caused moderate migration (about one tenth as much as sho\vn in Figure 2), and 6 volts caused faint migration (barel>-visible Ivithout a microscope). T h e experiments at relative humidities 75, 49, and 32% revealed only slight migration on XXXP a t 75% and none at 49 and 32%;. T h e stated relative humidities were obtained with salt solutions, as before (8). To determine the effect of polarity on migration from a silver electrode, one silver electrode was used as the anode on one sample of X X X P and another silver electrode as the cathode on another sample. The nonsilver electrode was copper. Heavy migration occurred only when silver \vas the anode; the silver deposit formed in the region of the anode. .4 \'cry slight dark deposit (migrated copper) formed when copper \\-as the anode. Tests ivere run using alrernating instead of direct current. .Alternating voltage produces far less silver migration than a undirectional potential. The effects of 60 and 400 cycles seem to differ-only 60 cycles permitted detectible silver migration. Susceptibility of Base Materials
L-sing the standard conditions described, the susceptibility to silver migration of a number of printed wiring and other base materials \vas studied (Table I and Figures 3 and 4 ) . The contrasting behavior of nylon and N-1. a nylon fiber-filled phenolic? seemed strange; pure nylon \vas the \vorst performer in regard to silver migration and K-1 was among the best. T h e chemically discrete types of nylon are Nylon 6 (polycaprolactam), Nylon 66 (condensed adipic acid and hexamethylenediamine): and Nylon 610 (condensed sebacic acid and hexamethylenediamine). Domestic nylon fiber is made of Nylon 66, and presumably
Figure holder
1.
Silver
migration sample
this was present in the S-1. The susceptibility of thi: simple nylons to migration (Figure 5) increased in the order 610, 66, 6 ; Nylon 6 \vas far kvorse than the others. This order of susceptibility to silver migration parallels the tendency of these nylons to absorb water. The interpolymer (grade 6503) \vas as susceptible as the \\orst of rhe simple polymers, N>lon 6 . Silver Migration in Printed Wiring
Conducting Silver I n k a n d Paint. I n one direct technique for making printed circuits, a conductive ink is screened directly onto a base material. An air-drying D u Pont silver conductive paint was found to undergo silver migration on many niaterials. .4norher brand of silver paint shoived significanrly less silver migration on X X X P . Silver-Bearing Printed Circuit Solders. T o test the tendenq- of the silver in silver-bearing tin-lead solders to undergo miSration, penny-sized electrodes were prepared by compressing solder pellets in a hydraulic press.
Literature Background Subject
Retereirce
Silver migration failures on paperbase phenolic laminate in telephone switching equipment occurred in 4 years. Migration i s favored by high humidity and high d.c. potential Mica and ceramic dielectrics in silvered capacitors failed through silver migration Linen-phenolic panel boards in Whirlwind I Computer at M I T broke down a s result of silver migration Ceramics exhibited extensive silver migration, some within 24 hours, at high humidity with 135 volts d.c. O n wet ceramics silver shorted across a ''~e-inch gap in 15 seconds with 2 volts d.c. No migration was observed from chemically deposited silver film on printed circuit Printed circuits foimed from silver powder hot-pressed into plastic laminate present no migration problem i n home radio and T V sets
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Figure 2. Silver migra- Figure 3. Silver migration on XXXP produces tion on epoxy-glass this pattern (transmitted light)
Leads were attached to the electrodes and silver migration !vas measured on XXXP. S o migration occurred from either solder under conditions in which a pure silver anode produced a serious amount of migration. X-ray diffraction analysis showed that silver was present as an intermetallic compound rvith tin (.4gsSn), and hence was unavailable for migration. Silver-Plated P r i n t e d Circuits. Silver plate on copper is used in two {vays. -4s an etching resist it is plated on the clad laminate in the conductor patrern areas. Etching tvith ferric chloride then removes the copper only where it is unprotected by silver. For electronic purposes this process has certain advantages as a printed circuit fabrication method. As a method of making switch plates, for example, it yields a surface lvhich has l o ~ vcontact resistance and high corrosion resistance. its sulfide is conducting, and it has adequate wear life. At first thought it seems unlikely that such a circuit could undergo silver migration? because (Figure 6) the silver should not actually touch the base laminate. .4ctual contact of the silver with a n insulator is necessary for occurrence of silver migration. Hoirever? it seemed possible that undercutting of the silver by the etchant might leave a weak silver overhang Ivhich could be bent during normal handling operations or during operation of the rotor of a switch, in such a way that the silver would touch the base material. A commercial silver-
Figure 4. Silver migration on steatite (reflected light)
plate circuit \vas found to undergo migration (Figure 7). A stereoscopic microscope shoived undercutting of the copper by the etching solution. Consequently. normal handling of the board would almost una\,oidably bend some overhanging silver down until it touched the plastic. Many sites of silver migration could be traced to an area ivhere silver touched the plastic. At other sites. a dust particle or fiber \vhich touched both the silver and the base material permitted silver to migrate. Silver plate may also be applied to copper after the conductor pattern has been formed. I n this t p e of printed circuit the silver is in direct contact
Table I. Materials Have Differing Susceptibilities to Silver Migration (Silver paint or foil used) Material
Poly(dially1 phthalate)glass Kel-F N-1 Polystyrene Polyethylene Poly(viny1 chloride) Cellulose acetatebutyrate Silicone-glass Epoxy-glass Steatite, glazed, grade L-5 Teflon-glass
XXXP ADHESIVE
,SILVER
PLATE
Melamine-glass Glass-bonded mica
\BASE
Figure 6.
300
LAMINATE
Nylon, grade 6503
Silver-plated etched circuit
INDUSTRIAL AND ENGINEERING CHEMISTRY
Silrer Migration
None detectable
Figure 5. Nylons differ in susceptibiliiy to silver migration Silver foil electrodes o r e removed
with the laminate. As expected. cxperiments showed a considerable amounr of migration Lvith XXXP. Unexpectedly, epoxy-glass base material also supported extensive migration (Figure 8). .ipparently. immersion of the board in the alkaline cyanide silver-plating bath predisposed the surface to migration. Pressed Silver Powder. One type of printed circuit is manufacrured by pressing the circuit pattern into the base material with a hot die acting on a layer of silver poivder. The unpressed silver po\vder is removed mechanically. leaving the silver-printed circuit embossed in the base material. Migration of a commercial XXXP circuit made by this process \vas very extensive and caused dielectric failure in the sample tested. Plated Copper on Chemically Deposited Silver. Still another type of printed circuit employs a conductor pattern buil: u p horn hare base material throtigh an extremely thin film of silver chemically deposited on an adhesive (70). Copper is plared o n top of this
None detectable None detectable None detectable None detectable None detectable Barely detectable None detectable Slight: on surface and in fibers Slight ; surface only Slight; in glass fibers Heavy; in cellulose fibers Heavy: in glass fibers Heavy; surface only Very extensive : surface only
Figure 7. Silver migrates in a silverplated etched circuit (transmitted light)
PRIHTED W I R I N G 4 Figure 8. Silver migrates in a silverplated epoxy-glass printed circuit
b Figure 9. Cross section of circuit through a conductor
printed
Silver layer is about lo-’ inch thick
silver fi!m in the desired conductor pattern. .A cross section of a commercial circuit of this type tvas prepared (Figure 9). Tests \i,ith three samples of such circuits produced very slight silver migration a t localized places along the edge of the conductor selected as the anode. However, in t\vo of the three samples tested, there Tvas a single spot ivhere a greater amount of migration occurred, with silver almost completely bridging the 16-inch gap between conductors. Dielectric failure did not occur, although the possibility of failure \vas apparent (Figure 10).
Silver Migration in Electronic Components
Closely associated with silver migration in printed wiring is migration in the miniature and subminiature electronic components frequently used in printed circuits. Tube Sockets. T h e behavior of several tube sockets with silver-plated terminals was studied: a miniature %lazed
Figure 10. Silver migrates in a printed circuit made by electroplating copper on chemically deposited silver
steatite socket, a subminiature socket of mica-filled Bakelite, and t\vo Xfycalex 410 subminiature tube sockets (one Lvith silver-plated terminals and the other with a gold flash on silver-plated terminals). Electrical connections !\-ere made to tivo adjacent socker terminals. If the glazed steatite tube socket supported migration, it \vas not extensive. T h e slight black staining of the walls of the pin receptacles could not be distinguished from rubbed-off metal observed on the \calls of pin receptacles to Ivhich no voltage \vas applied. Silver migration on the mica-filled Bakelite socket \vas readily visible without a microscope both on the surface and on the underside (Figure 11) between the terminals. Seither hfycalex tube socket supported migration. This contradicts the results of the silver migration study with sheet Mycalex. T h e sheet Mycalex is made from hlycalex 400, a machinable grade, Lvhile the tube sockets are made of Mycalex 410: an injection molding grade. T h e mold release agent may have protected the tube sockets from absorbing moisture a t high humidity. T h e resistance of Mycalex 410 at 97% relative humidity \vas 108 megohms; that for sheet Mycalex \vas about 10 megohms. Therefore, the immunity of the Mycalex tube sockets to silver migration was probably due to the grade of Mycalex or to some sFecial moistureresistant surface condition which did not exist on the Mycalex sheet. Quartz Frequency Control Crystals.
Figure 11. Silver migrates on subminiature tube socket of mica-filled Bakelite
Quartz frequency control crystals I\ ith silver electrodes arc normally usrd in sealed cans. Hoicever. leaks sometimes develop so that ambient conditions pievail inside the can. An exposed quartz cr:-stal: \vith plated silver electrodes, was subjected to the usual condirions for accelerated silver migration. Definite signs of migration \\.ere observed all around the edge of the anode. I t \cas not extensivr. but could easily be seen without a microscope. Jacks, Switches, and Cable Connectors. Fourteen other electronic components containing silver-plated parrs were tested for silver migration under the usual conditions. They includcd jacks, switches, a cable connector, a Jones socket, capacitors, a relay, and a coil form. Silver migration occurred on half of the components tested. T h e susceptible dielectrics Lvere unglazed steatite, paperbase phenolic, and nylon. Mechanism
Chemical View. Kohman, Hermance, and Downes ( J ) offer the simple explanation of silver migration that silver ions leave the anode in thc adsorbcd water film on the dielectric. Silver hydroxide is precipitated in the alkaline region near the anode and immediately
Figure 12. silver
Copper migrates less than
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Figure 13.
Silver migration on glass-fiber-filled epoxy is confined to the surface Ovals a t bottom a r e cross sections of a bundle of fibers running almost a t right angles to the u p p e r fibers
Figure 14.
Silver migrates both on the surface and within glass fiber-filled melamine Figure
The irregular objects a r e p a p e r fibers.
decomposes to silver oxide. Subsequently. the silver oxide may be reduced to metallic silver by light. reducing agents in the insulating materials (such as organics in the plastics), or cellulose. For XXXP, the most-studied material in the present work, a direct test was made of the step involving the conversion of silver ion to the dark, nonmobile form found following migration. -4 droplet of dilute silver nitrate was placed on the surface of a sample of X X X P . which was then stored in a shielded container at 9770 relative humidity. The folloiving day, using transmitted light under the microscope, unmistakable signs of the dark, fibrous structure characteristic of silver migration were detected in the laminate under the droplet. Several days later the darkening \vas clearly visible Lvithout a microscope and was still of precisely the same character as an authentic example of migration. Thus silver ions, which may be formed electrochemically, are capable of silver migration by a nonelectrical process. The results of additional experiments on the effect of silver nitrate solution on a variety of plastics correlated well with the tendency of these plastics to support silver migration. Polystyrene, siliconeglass. epoxy-glass, X X X P , melamineglass, and nylon (interpolymer) behaved similarly in both tests. Only N-1 and Teflon-glass were anomalous. IYith N-1 silver appeared only in surface scratches in the silver nitrate treatment. IYith Teflon-glass, the laminate might not have had any reducing properties and therefore would not produce a black deposit from silver nitrate, while cathodic reduction would yield dark deposits. Migration of Other Metals. T o learn whether ability to migrate is peculiar to silver, other metals used in electronic applications were studied with X X X P as the base material. Other than silver, only copper, tin, and gold, in decreasing order, migrated in detectable quantities. Lead! nickel, indium, aluminum, platinum. cadmium, and rhodium did not migrate. A photomicrograph of the copper-migrated sample is shown in Figure 12. This should be compared with the silver-migrated sample (Figure 2). In work a t the Bell Telephone Laboratories (-1) the unique position of silver
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15. Silver migration in sectioned XXXP
A. Silver rests on the surface a n d fills one fiber; 6. silver i s l a r g e l y below the surface a n d wilhin the fibers
has been ascribed to lack of development of a passivating oxide film around the metal. Metallography. .A metallographic technique \vas applied to silver-migrated samples of epoxy-glass, melamine-glass, and X X X P laminates. The epoxy sample (Figure 13) shoivs migration on the surface only and very near glass fibers bvhich are almost a t the surface. The melamine sample (Figure 14) shotvs migration at the surface and in the volume of the laminate, ivhere it appears only at the glass-resin interface. T h e photograph also clearly sho\vs the voids-spaces not filled with resin during lamination. T h e voids stand out because they become filled Lvith grinding compound during the polishing of the specimen. It cannot be said ivhether silver is present in the voids. Figure 15 shoivs t\vo vie\vs of silver migration in a sectioned sample of X X X P . T h e horizontal boundary in each is the interface between the laminate and the potting resin in which the sample is cast to facilitate polishing.
Prevention
Coating the Silver. A gold flash electroplated on silver foil was almost conipletely successful as a silver migration preventive. T h e normal “moderate” amount of migration {vhich occurs on XXXP was reduced by gold flashing to a “very slight” amount (barely detectable with unaided eye). I n another experiment, cadmium plating of silver electrodes \vas shown to eliminate silver migration on XXXP. Solder coating of silver parts has been used commercially to prevent or inhibit silver migration on susceptible material. Pure silver electrodes Ivere therefore solder-coated by dipping. .4 greatly decreased volume of silver migration resulted on XXXP; ho\vever. the tendency of the migrated silver to cause short circuits was hardly affected. In one sample. short circuiting occurred even though two migrated silver threads were the only prominent sign of migration (Figure 16). Consequently. little positive protection is offered by solder coating. Treatment of Insulator Surface. An
INDUSTRIAL AND ENGINEERING CHEMISTRY
approach to inhibiting silver migration arises from the hypothesis that the electrochemical process by jvhich silver migration occurs requires the presence of water. It seemed likely that reducing adsorption of water (on surfaces and in interstices) would reduce the intensity of silver migration in normally susceptible materials. T h e inhibitory effect of the Dow-Corning silicone product F1141 on silver migration was studied ivjth X X X P and melamine-glass laminates. Treated samples were visually estimated to have undergone 73% less silver migration than untreated ones. Other silicone products \vere tested; none \vas as effective as F4141. Alloying the Silver. I n furrher studies on alloys, the silver-copper alloy known as coin silver, containin% lo$& copper, was tested on -XY?iXP and on glass-bonded mica under the usual conditions of volrage, time. temperature. and humidity. O n X X X P . where pure silver causes moderate migration, coin silver produced very slight migration (7)-about 0.1 to l%, of that obtained with pure silver. A similar large reduction in migration \vas observed on thr more susceptible base material, glassbonded mica. An explanation (suggested by David Tabor, Cambridge University) for the inhibition of silver migrarion by 1 0y0 copper is that the copper concentration at the surface increases very quick1:- in the early part of the silver migration because of depletion of surface silver. An invisible copper sheath gradually develops and keeps the silver out of contact with the dielectric. This protective action may be increased by formation of copper oxide a i the surface. This mechanism was investigated by tkvo techniques. T h e results of both investigations supported the proposed mechanism. The first technique emp1o:-ed an ion scattering analyzer, or ”proton bombarder” (6). This instrument, developed at Stanford Research Institute. is a special facility for surface analysis. I n its operation, a beam of protons of uniform energy is directed against the sample. in high vacuum. Some protons collide with the atoms of the sample and are elasticallv scattered. From the veloc-
PRINTED W I R I N G ities of the scattered protons the masses of the elements in the samples may- be determined. Further, the number of scattered protons of a given velocity is related to the quantity of the particular element in the sample. Thus. in certain circumstances. a qualitative and quantitative analysis of surface elements may be obtained. Concentration gradients with depth, in the surface region. may be deduced in many cases from the velocity distribution of scattered protons. T h e samples analyzed by this technique included unused coin silver electrodes and used coin silver anodes and cathodes (Figure 1'). T h e unused coin silver yields the same curl'e as a used cathode. From tile deepest laver detectable u p to about 0.1 micron from the surface. the silver-copper ratio is very close to the nominal ratio in the volume--90 to 10. T h e copper concentration seems to decrease to about one half the nominal value u p to 0.02 micron from the surface, \vhich is the limit of resolution of the instrument; the silver concentration remains in the 70Tc range. T h e reduction in copper concentration Ivithin 0.1 micron of the surface may be normal for this alloy. T h e remainder of the atoms required to total 100% in the 0.1-micron layer may be oxygen in the form of copper oxide. T h e silver and copper concentrations in the coin silver anode are complementar>-. T h e silver concentration rises gradually from 90 in the volume to 95% at the layer 0.1 micron deep. T h e reason for this is not known. I t then falls off sharply to less than 80% as the surface is approached; the limit of instrument resolution prevented following the decline further. T h e copper concentration declines from the volume to the 0.1micron region. and increases sharply from 0.1 to 0.02. micron, the limit ofresolution. The inference is that a profound change has taken place a t the surface of the anode. T h e falling off of silver concentration as the surface is approached. from 0.1 micron, may correspond to an equally rapid build-up of
Table II.
Figure 16. Solder-coating of silver foil electrodes does not prevent short circuiting
copper in accord Lrith the proposed mechanism. The second technique for studying possible copper enrichment a t the surface of a used coin silver anode used a sensitized zinc ferrocyanide paper developed for detecting iron and copFer on the surface of etched printed circuits. T h e impregnated gelatin-coated paper changes from white to red-broivn in the presence of as little as 0.001 y of copper per square millimeter. Only weak tests for copper resulted from unused electrodes and used cathodes. A very stroEg copper test was obtained the first time with used anodes. but subsequent analyses on the same anodes gave weak tests. suggesting that the copper-rich layer was removed by reaction in the first test. Because of the promising results obtained \rith coin silver, other silver-copper alloys were tested: eutectic alloy 72:28 and alloys containing 2. 4. 6: 8. and 10% copper. O n XXXP. the 2, 4, 6: and 8% copper compositions gave silver migration virtually the same as that given by pure silver. T h e eutectic silver-copper alloy produced the same vast
High Temperature Stimulates Silver Migration a t High Relative Humidity
Temp., C. 30
+ +40
Base Material
XXXP
Relative Humidity, %" 97 (S) . ,
Melamine-glass
XXXP
(s)
Silver Migration Barelv detectable Slight
Moderate Heavy None detectable None detectable 85 XXXP 95 (S) Very extensive Melamine-glass Very extensive Epoxy-glass Very slight Teflon-glass Barely detectable a I. Ice used as source of constant relative humidity. S. Saturated K&Od used as source of constant relative humidity (8). Value for 85' C . run extrapolated.
+
Melamine-glass Epoxy-glass Teflon-glass
96
reduction as the coin silver. T h e absolute amount of migration from the 10 and 2857 alloys was so sliyht as to make it difficult to compare them. Eutectic silver-copper alloy \vas also tested on glass-bonded mica and conipared with the 10% copper alloy. Because glass-bonded mica permitted more migration than XXXP. it \vas possible to distinguish the effect of the additional copper : even greater reduction of silver migration than \rith the 10% copper alloy. Tests were also carried out \vith other alloys of silver, prepared by the insritute's Metallurgy Department : 5 5 gold. 10yclead, 5 and 15% tin, 55; platinum, 1Oyo antimony. and 10:; cadmium. Using XXXP, gold, 5 timony. and cadmium Lvere j t fect on silver migration. Lead, 15% tin, and platinum caused a slight reduction, considerably less than that given by the 10yGcopper alloy. O n glassbonded mica. only 1OyG cadmium gave significant reduction in silver migration; in fact. cadmium w i t s almost as effective as 10% copper.
Prediction Because the end result of silver migration in electronic equipment can be dielectric failure: designers of electronic equipment commonly avoid the use of silver lvhere long-term reliability is required. T h e use of silver even in equipment of short life, such as guided missiles where total operation time including testing may be no more than 6 or 8 hours, may present an uncertainty to a design engineer. I t would be advantageous to be able to predict, a t least qualitatively. the extent of migration under specified conditions. A number of experiments in this laboratory and others have indicated the parameters affecting silver migration. Converting these scattered and disconnected data into rules regarding the use of silver in a particular design and environmental situation is not a straiShtforward matter. The important parameters affecting silver migration are relative humidity, temperature. voltage. time, and base material. A scheme was developed to relate these parameters by means of a nomograph. Construction of the nomograph required accumulation of new data. T e m p e r a t u r e and R e l a t i v e Humidi t y P a r a m e t e r s . O n e set of separate, exploratory experiments delineated the effect of temperature on silver migration at high relative humidity (Table 11). T h e standard 65-hour time period and a potential of 400 volts direct current were employed. Silver migration depends greatly on temperature, a t relative humidities beVOL. 51, NO. 3
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L
A Figure 18. Nomograph predicts silver migration on XXXP ?j-AhiC
S.?CI:E
MICRONS
Figure 17. Copper builds up on surface of anodes of coin silver. reduces silver migrafion
A
tween 95 and 1007,. With XXXP and melamine-glass, tested a t -5j0, - 5 O . a n d 10’ C.: it is detectable only above 10’ C., and it increases rapidly as the temperature goes up. Epoxy-glass a n d Teflon-glass laminates, normally much less susceptible, support migration only a t 85’ C.? the highest temperature used. Development of Nomograph. I n a series of experiments: the parameters of relative humidity, temperature, voltage, a n d time Tvere varied systematically (for the base marerial XXXP) and the resnlts were plotted in the form of a nomograph. T h e experiments measured the time required for the first appearance of silver migration on XXXP samples held a t various combinations of temperature and relative humidity with the voltage held constant (Experiment A ) , a n d a t various combinations of voltage a n d relative humidity ivith the temperature held constant (Experiment B). For tIvo reasons the parameters were related through “the first appearance of silvcr migration” rather than by measurement of the extent of silver migration in a fixi=d time period. T h e need for measuring the amount of migrated silver is removed and a safety factor is built into the results if the danger point is taken as that point a t irhich silver just starts to migrate. T h e results of these experiments are presented in Figure 18. T h e d a t a obtained from Experiment A (left) yield a family of curves. one for each temperature. shoiving the effect of relative humidity on the time for appearance of minimal silver migration (appearance time). T h e left-hand ordinate (not sholvn) gave the appearance time only for in Lvhich a constant voltage (400 volts) \vas employed. These experiments were performed \vith onlp one base material? XXXP. They must be repeated with each base material for ~ v h i c ha “prediction“ nomograph is desired. I n Experiment B the temperature was held constant a t 40’ C. and the effect of
304
This greatly
various voltages a t various relative humidities on appearance time isas measured. These data \\ere plotted on the right side of the graph by placing points a t the intersections of horizontal lines from the various humidity points on the 40’ C . curve and vertical lines from the appropriate appearance time on the abscissa of the right side. This gives a family of curves. one for each test voltage (10. 90:and 400 volts). T h e determination of appearance time for Experiment B was altered to permit detection of silver migration heaveen the silver foil and the XXXP before the migrated silver emerged from the interface. Although this increased the safety factor a n d shortened the time for Experiment B, it changed a condition ivhich would otherivise make the A 40” C . experiment a duplicate of the B 400-volt experiment. This change should not: hoivever. alter the relationship of A to B. X given ordinate value in A represents the same amount of silver migration for a number of temperature and relative humidity conditions. Extensions of these ordinates into B as described relates those already-related variables i v i t h voltage and the neiver appearance time. T h e use of this nomograph is based on the assumption that the influence of teniperature and humidity on appearance time a t 400 volts is rhe same a t the other test voltages. T o illustrate the use of the nomograph \vith a simple example. assume that the silver migration possibility on XXXP is in question in an instrument \vhich is to operate a t 40” C:,. 757; rrlative humidity, a t 30 volts direct current. ’The temperature and Iiumidity are located on A as point P. From this point a horizon~al is dralvn intersectinq the 90-volr curve on B, a t point Q. This point corresponds to a n appearance time of 420 hours (17.5 days) before silver migration would be noticeable. I t is not a l ~ v a y spossible to know with certainty the temperature and humidity conditions under Xvhich a n instrument may operate. I n this case approxima-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
tions may be made of the u‘orst possible conditions (with respect to silver migrdtion) and appearance time estimated if the voltage is known. T h e temperature a n d voltage ranges studied tvere necessarily rather limited ; hokvever. extrapolation and interpolation may be used for values lokver and higher than those given. These two points map be illustrated w i t h an estimate of the \vorst teniperaturehumidity conditions during an eastern summer. Assume 30’ C.. 95 humidity, and 200 volts. T h e temperature-humidity point is represented by point R, and its extension into B by point S, giving 130 hours ( i . 4 days) as an estimated appearance time. Loosely interpreted. one ivould estimate that sonmvhat less than a \veek of exposure to the stated temperature. humidity. and voltage ivould cause appearance of silver migration. Lower temperatures and humidities during this hypothetical summer Lvould also permit migration. but to a lesser degree. T h e uncertainty in any ansiver ohtained from the use of the nomograph is believed to be large, perhaps +.50%. Xevertheless, even this estimate is useful. T h e nomograph a t least serves as a model experiment for future efforts to relate more extensively and ivith greater significance the parameters affecting silver migration. Extension to other susceptible base materials. such as glassbonded mica^ steatite: and nylon: would further broaden the coverase of silver migration data. literature Cited
(I) Chaikin, S. W.,Church, F. k i . ,
hfcClelland, C. IV., U . S. Patent pendIng. ( 2 ) Greenidge, R . hf. C., ”Proceedings of 1953 Electronic Components Symposium,” Pasadena, Calif., April 29, 1953. i(3) Kahn, L., “Proceedings of Symposium on Printed Circuits” (Radio-Electronic Television Manufacturers Association ). .Jan. 20, 1955, p. 53. (4) Kohman, G. T., Hermance, H. \V.> Downes, G. H., Bdl Sysiern 7’rch. J. 34, 1115-47 (19551. I 51 Paine, B. B., R.E.T..M.A. Electronic Ajjlzcations R d i a b i l d y Rer,.: S o . 3, 5-6 (1954). (61 Rubin, S., Passell, .r. O . , Bailey, I>. E.. Anal C h m . 29, 736 (19j71. ( 7 1 Short, (1. .i.: TPlp-Tech and Eleclronnic Ind. 15, 64: 65, 110-12 (19561. (8) M’exler, A , , Hasrgaiva, S., J . R‘rsearch .Tail. Bur. Standards 53, 19 (19541. (9) Williams, J. C.. Hermann, D. B., IRE Trans. Re/iabi/ity and Quality Control. 11-20 (February 19561. (101 Yost, D. E.: ”Proceedings of Symposium on Printed Circuits” (RadioElectronic Television Manufacturers Association‘), Jan. 20, 1955, p. 53. RECEIVED for review hfay 8, 19.58 ,-\CCEPTED December 1, 1958 Division of Industrial and Engineering Chemistry, Symposium on Chemical Xspects of Printed M’irinp. 133rd Ilfeeting, ACS, San Francisco, Calif., April 1958. LVork supported by the U. S.Army, Signal R and D Laboratory, Fort hIonmouth, N. J., under Contract No. DA-36-039SC-64454.
