Chemical Aspects of Printed Wiring Printed wiring, as we know it today, had its real beginning in the proximity fuse of World War 11. Since then, as a replacement for conventional point-to-point wiring, its use has grown by leaps and bounds. It is estimated that approximately 60,000,000 printed wiring boards, of average size 4 X 6 inches, will be produced in 1959. In the past symposia have dealt with printed wiring-with what might be termed the unit operations of printed wiring production (etching, plating, soldering, etc.) without emphasis on the chemical nature of the processes that occur in these operations. There have also been symposia dealing with applications of printed wiring, both general and particular, by and large for electronic engineers and production men. The chemist, as a chemist, has been on the outside, looking in. It is hoped that this symposium is a significant reversal of the trend. It is the first to deal specifically with chemical aspects of printed wiring. Its purpose is to direct attention to a previously neglected facet of the art, to provide a common meeting ground for dissemination of new knowledge and exchange of ideas by those interested in the chemistry associated with the art, and to stimulate research directed toward understanding the chemical processes involved both in producing a printed wiring board and the changes in its behavior in use. Only with understanding will come the ability to translate design ideas rapidly into reliable printed circuits.
LOUISH. SH-ARPE Bell T e l e p h o n e Laboratories, I n c . ?rIurra>- Hill, N. J.
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E. B. SAUBESTRE' Sylvania Research Laboratories, Division of Sylvania Electric Products Inc., Bayside, N. Y
Chemical Problems in Printed Wiring The most pressing problem is development of accelerated testing procedures for predicting the performance of a board
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CONKECTION with wider usage of printed wiring, many chemical problems have arisen, Xvhich can be broadly subdivided into nvo groups: those associated with producing printed circuits, and those associated with electrical performance of boards in the field. This discussion is confined to a general presentation of the many problems facing chemists in this field, and does not include chemical problems in development and manufacture of laminates. solders. and fluxes.
Production of Printed Circuits
Stop-Offs (Resists). \\'hen unwanted copper is etched away from a copper-clad Present address. Enthone, Inc., Nc\v Havrn 8, Conn.
Reprints of any one group of these articles may be purchased at $1 .OO for single copies or $0.75 each, in lots of ten or more. Address Special Issue Sales Department, American Chemical Society, 1 1 55 16th St., N.W., Washington 6, D. C.
laminate, a stop-off. or resist? must he first applied to protect the pattern. T h e techniques are conventional and include photo-resist and silk screening. Conventional resist materials d o not completely meet the detailed requirements of printed wiring. This is surprising, because these materials are satisfactory in photoengraving and other uses where even finer detail is required. T h e main differences seem to lie in the roughness. unevenness, and absorptive properties of the laminates. The most serious problem is pinholing, which often causes rejection of the printed wiring board. Pinholing involves four variables: resist composition, methods of application, development or setting, and cleaning and etching. Misuse of any of these four parameters will lead to trouble-for example. inadequate cleaning or a film of resist that is too thin will lead to pinholes. Development work has centered about the cause of pinholes when all four variables are seemingly under control. Developing conditions must be defined more carefully than in the past, and choice of etch influences spread of pinholes. Thus, \Then the resist is changed, both the etch and conditions must be re-evaluated. But because of technical and economic rea-
sons: this may not he simple. Therefore, one conclusion is inevitable: Further development work is required in the composition of resist materials. Solder resists can be used to avoid pinholing. I n this method, a reverse stopoff is applied, then solder is plated onto the laminate. If the organic stop-off contains pinholes. small spots of solder plate in unwanted areas are the only damage ; these are readily brushed away ivhen the reverse stop-off is removed. The unwanted copper is then removed by etching in a medium which \vi11 not attack solder. If the copper was properly cleaned prior to solder plating, little if any pinholing should occur in the pattern. Etching. Many materials etch copper at economically feasible rates. However, to prevent attack or contamination of plastic laminates, attack or pinholing of resists. and subsequent contamination of boards from residues difficult to rinse off, the etchants of interest are largely limited to chlorides (ferric or cupric), nitric acid, chromic-sulfuric acid, and peroxysulfates. CHLORIDES.Use of ferric and cupric chloride etches is the most satisfactory commercial method-these etches stabilize and complex the cuprous ion and VOL. 5 1 , NO. 3
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thus make copper easy to etch. The following standard electrode potentials illustrate the relative effectiveness of these etches. Ferric chloride, a t the copper surface:
tweencupric and cuprous ions in the bath is slow to be established. Beyond this point, etching rate becomes proportional to the ferric ion content. When ?S% of thecopper has been dissolved, the etching rate becomes negligibly slow. .4ged Fe(H?0jjClTi Cu C13H?O = baths tend to form insoluble ferric and Fe(H20j8++ C L I ( H ~ O ) ~ C I , - cuprous salts which interfere with difE“ = +0.474 volt fusion a t the copper surface, further slowing etching rate. The p H of an aged and, in the bulk of the solution: bath rises, precipitating ferric hydroxide; Fe(HaO),Cl-+ Cu(HrOjnC1?in a fresh bath ferric chloride hydrolyzes 3H2O = Fe(H20)8-+ extensively, lowering pH, while in an Cu(H*O)?-* 3C1aged bath neither the ferrous nor cuprous E” = +0.22 volt ions formed hydrolyze nearly as much. As the bath ages, the Cu++,’Cu+ratio deCupric chloride : creases and the free chloride content decreases, causing precipitation of inC U ( H ? O ) ~ + CU ~ 4C1- = soluble cuprous chloride. Addition of 2Cu(H?O)rC12hydrochloric acid increases etching rate E“ = f0.257 volt in aged baths by preventing such ferric These potentials show that ferric chloand cuprous precipitates. ride has a somewhat larger oxidation-reBath life may be extended in txvo other duction potential; therefore, it should ways-by oxidizing or chelating agents, operate a t a slightly higher rate in etchor by periodic regeneration of the bath. ing of copper. Also, cuprous ion is furThe latter is preferred because addition ther oxidized to cupric ion in the bulk of agents are only partly successful, and the solution. This has a twofold adpollution la\vs in most localities restrict vantage-the cuprous ion content is dumping of residues of such etchants. lowered, which increases the reaction Regeneration of ferric chloride etches inrate, and the cupric ion so formed further volves two steps: oxidation of the ferrous oxidizes copper. Thermodynamically, ion to ferric ion by use of peroxides. peiferric and cupric ions contribute equally oxysulfates, or anodic oxidation. and reto the etching action a t all stages during moval of copper, either by precipitation life of the etch. O n the other hand, the as an insoluble salt, or by cathodic recupric chloride etch involves no metal duction to copper metal. Cupric chloother than copper, simplifying regeneraride etches are easier to regenerate; election. I n some applications, rinsing of trolysis provides for simultaneous reducresidues is less troublesome in the cupric tion of copper a t the cathode, and oxidachloride etch; this does not apply to all tion of cuprous ion to cupric ion a t the conditions. anode. I n some localities, dumping of In etching with chlorides, three princicupric chloride residues is less stringently pal problems are involved: Without controlled than dumping of ferric chlovigorous agitation the rate is slow, soluride residues. tion life is relatively short, and residues SITRIC ACID. Nitric acid etching of copper offers obvious advantages over may seriously contaminate the laminates. Because vigorous agitation is needed, chloride etchants: The rate is faster, no splash etching is commonly used. It has insoluble residues form, chemical costs been said that aeration is important; are low, and there is no serious disposal however, there is no evidence for this problem. Principal disadvantages are claim. Although the method is successful formation of noxious fumes and a tendcommercially, it requires specialized ency to “run aivay.” The over-all reequipment and involves health and corroaction is: sion hazards. Suitable chemical addi2x038H(H20)+ 3Cu tives are highly desirable for avoiding 3 C u ( H ~ O j r + + 2x0 splash etching. No completely satisE o = +0.623 volt factory additives have been reported, but claims have been made for several, This reaction proceeds through a series of usually of a n oxidizing nature. intermediate steps, symbolically repreThe bath life is not long enough: .4 sented as : high percentage of the theoretical amount N(V)-.N(IV)+N(III)-N(1I) of copper may be dissolved by the etchant, but after a short period of use, the The rapid step is reduction of K (IV) to etching time increases greatly. The etchN (111); the rate-limiting step is reducing rate is a first-order reaction which is tion of N (111) to N (11). Sormally, the diffusion-controlled-the rate decreases slow step acts as a brake on the entire in proportion to the concentration of resequence, preventing “running away.” actant. For ferric chloride etches, the reHowever, N (111) can engage in two actants are ferric and cupric ions. Howother reactions: ever, when 50%of the theoretical amount 2N (111) = N (11) N ( I V ) of copper has dissolved, litfle ferric ion remains in the bath and equilibrium beN (111) N (V) = 2 N (IV)
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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In both cases, N (IV) forms, so that if a source of electrons is available (as from copper metal), the fast reduction of S (IV) to N (111) occurs, and the reaction runs away. Running away may be prevented by two methods. N (111) may be removed from the copper surface by vigorous agitation-in fact, splash etching techniques will practically stop attack on copper in nitric acid. A better method is to add to the bath, agents which either complex S (111) or destroy it. Work on such agents is currently under kvay, and when all problems have been satisfactorily answered, nitric acid etches should compete easily with chloride etches. Life of nitric acid etches is good, being limited largely by build-up of cupric ion \vhich disproportionates Lvith copper to form cuprous ion. Because no complexing agents for cuprous ion are present, insoluble cuprous oxide forms on the metal surface, slowing etching rate. Aeration accelerates this effect. Thus, either oxidizing agents or complexing agents stable in nitric acid are required. Fuming is a serious problem in concentrated solutions. Dilute solutions fume less but etch more slowly, so that a compromise concentration is used. Concentrated solutions are also troublesome, i n that they attack some grades of laminates, requiring use of more dilute but slower e tchan ts. CHROMIC-SULFURIC. Previously discussed etchants cannot be used when solder resists are present. The solder must be passivated, usually by chromates. Such coatings are removed by chloridesor nitrates, but not so readily by sulfates. Chromic-sulfuric acid etchants are used for solder-plated boards, as the combination will attack copper but not solder. Such etches are slow and have short life. These limitations have not yet been satisfactorily solved. Splash etching increases rate of etching, but presents corrosion and safety problems, and control of etching rate is not good. Oxidizing agents have been added: but often attack the coating on the solder. Addition agents may increase useful life of the bath considerably, but not the slo\v etching rate. With suitable oxidizing and other addition agents, the chromic-sulfuric etch is useful when solder, nickel, rhodium, and other metals have been plated onto copper as resists. This is its main field of application. Much further work is needed to make these etches commercially attractive for other purposes. PEROXYSULFATES. Peroxysulfates have been used for many years for stripping copper from steel. Application to printed circuitry shows promise, though problems remain. In the absence of a catalyst, such as mercury or silver salts, etching rate is slow. Catalyst control is important: A low content leads to slow etching; too high may lead to displace-
PRINTED W I R I N G ment coatings on copper. Peroxysulfates are not too stable, tending to hydrolyze to sulfuric acid and peroxide, a reaction catalyzed by some laminates. In fact, some laminates may be attacked, though most of the standard types are unaffected. Peroxysulfates, however, permit copper etching a t fairly high rates in the presence of solder, nickel? rhodium, gold. and silver resists. In other words, they combine the rapid rate of chloride etches \vith the selectivity of chromic-sulfuric etches. Other desirable features include lack of fuming, immersion is adequate (although splash etching may be used ifdesired), there is no sludge on aging, copper is recoverable by cathodic reduction, and the only waste product is essentially a dilute sulfuric acid solution. Metallizing. Cnclad laminates must be made conductive, then plated or otheririse metallized to the desired pattern. Methods include: metal spraying, application of metal powders, metallic paints, conductive inks, "electroless" or chemical reduction plating, and electroplating. Most attention lately has been on electroless plating. PREPARATION OF LAMINATES.Firsr step is deglazing the board, as with a slurry of pumice and trisodium phosphate or by abrasive blasting, as with aluminum oxide. Then a mildly alkaline cleaner is used. Yext. a sensitizer is applied to limit deposition in the plating solution to the laminate only. Because of the hydrophobic nature of the laminate, the sensitizer does not wet the board well, and absorption of the sensitizer is inadequate. Two solutions to the problem are use of a strong alkaline cleaner just prior to sensitizing, or dissolution of the sensitizer in an organic solvent \rhich \rets the laminate. Either may lead to subsequent contamination problems. Adhesion bemeen laminates and metallized films is generally poor, and common practice calls for an adhesive applied to the uncoated laminate. This causes problems in electroless plating. The uncured, or partly cured, adhesive limits use of alkaline cleaning solutions to make the boards wettable in the sensitizing solution. If the cleaning step is omitted, wetting agents are needed in the sensitizer solution, but may be inadequate. If the sensitizer is dissolved in organic solvents, the adhesive may be softened. One solution is to add the sensitizer directly to the adhesive, but this is wasteful of sensitizer. Better adhesives and better methods of sensitizing adhesive-coated boards are required. The metal being deposited determines choice of sensitizer. For silvering, acidified stannous chloride is common, with anionic surface active agents, or quinol added when used on adhesive-coated laminates. If an organic sensitizer is
used, ethanol and 2-propanol are good solvents; dimethyl- or diethyltin chloride may be used. For coppering, chlorides of gold, palladium, and platinum are common, with anionic surface active agents commonly added. Ketones are suitable organic solvents. SILVERIKG.Well-known silver mirroring techniques are suitable. Ammoniacal silver nitrate is used with formaldehyde, hydrazine, Rochelle salt, or susgar as reducing agent. Explosions nil1 occur if free alkali metal hydroxides are present. if residues are allowed to solidify, or if storage containers are not clean. Immersion silvering is possible, but spraying is far better. The sensitized board is sprayed by a two-nozzle gun, one delivers the silver solution, \vhile the other delivers the reducing solution. Proprietary solutions render boards conductive in seconds, and are satisfactorv if the boards are properly prepared. Occasionally, such films may become hydrophobic after spraying, making subsequent plating difficult. An occasional drawback of silvering is the problem of removing unwanted silver after the pattern has been defined by electroplating. More seriously, military specifications often forbid silvering because of silver migration. This problem has been exaggerated and is of no importance in the application considered here COPPERING. Interest in coppering plastics has grown recently. -4cid (sulfate) or alkaline (tartrate complex) solutions may be used, depending on whether
the reducing agent chosen is more effective in acid or alkaline solutions. Tartrates, formaldehyde, hydrazine, and sugar have been used as reducing agents, but only forma1deh)de in alkaline tartrate solutions readily lends itself to catalytic reductions. In other systems. copper tends to be deposited throughout the solution. In the preferred system, copper is itself a catalyst for the reduction; once a thin flash of copper has developed on the sensitized surface, copper build-up continues, catalyzed by the copper already present. The main problem in coppering is a tendency to reduce only to the cuprous state, forming an insoluble film of cuprous oxide on the laminate. Reduction then either ceases or forms loosely adherent, nonconductive films of copper and cuprous oxide. Complexing agents Ivhich solubilize cuprous oxide (cyanide: chloride, thiocyanate, Thiosulfate) severely repress copper plating by favoring the reaction:
Cu+' f c:u = 2 c u + The only complexing agents for cuprous ion lvhich do not favor this reaction are ammonia, aqueous solutions containing halides plus organics such as ethylene, and unsaturated organic acids such as maleic and fumaric acids. However, little work has been clone in this direction. Tendency to form cuprous oxide films on the board is a function of the reducing agent chosen, pH, and total salt concentration in the bath.
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Single etched copper wiring pattern shown here i s usdally m a d e on a glass epoxy laminate VOL. 51,
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sorb in oriented fashion: with the organuphilic end of the dipole exposed. making surfaces markedly hydrophobic. These materials are normally removed easily in the cleaning cycle prior to plating, but after aging, or if heat is applied to the surface in processing. become exceedingly difficult, if not impossible, to remove in ordinary cleaning cycles. llechaiiical cleaning with pumice and trisodium phosphate may be required. Nickel-rhodium plated switching patterns are neat without complex diagramming
Even under best conditions. heavy copper deposits darken or become spongy or blistered. Complexing agents for cupric ions help here: carbonates, EDT.4, phydroxycarbonyl compounds such as the p-diketones. chelates based on oxalates and tartrates. catechol, and organic disulfides. Under ideal conditions, 1 mil per hour of sound copper may be deposited from coppering solutions. Choice of adhesive for use in conjunction with coppering is severely limited by the high pH (10 to 11) of most coppering baths, causing many adhesives to soften or react with the barh. Curing of the adhesive is also troublesome-volatile matter evolved in curing blisters and lifts the deposit. Best solution is to cure the adhesive as soon as coverage is complete. Pressure curing is helpful here. After curing, the coppered surface may have to be cleaned before continuing with deposition. T h e copper film is thin, and the cleaner should be selected with care. Coppering baths have t ~ v olimitations : short life and slow deposition rate. Life is shortened by formation ofcopper nuclei in the bath, which act as catalytic centers, causing rapid reduction of all the copper in the bath to metal. Use of addition agents and pH control help prevent this, but only at the cost of lowered deposition rates on the laminate. The relatively slow deposition rate means that immersion processes are required-spraying is uneconomic because of the long induction time for the initiation of deposition. Much work is under way on this problem, and commercially interesting copper spraying methods should soon appear. ?VICKELISC.Electroless nickel may be deposited from acid baths, simplifying choice of adhesive compared to coppering. A conventional nickel sulfate-sodium hypophosphite-organic acid buffer type of solution is used. Sensitizers used for coppering are adequate for this application. The high bath temperature (90° C.) may cause premature curing of some adhesives, but condensation-type adhesives curing at about 130’ 6 . are sui table, Electroplating. METALSDEPOSITED. Electroplating has many applications: to build u p conductor thickness (copper) ; serve as an etching resist (solder, rhodium) ; improve corrosion resistance and
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provide good sliding. wiping: and static electrical contacts (gold. silver. nickel, and rhodium). Electroplating procedures are conventional. except that low current densities must be employed at the outset \\-bile the conductive film is thin. However: use of adhesive-bonded copper introduces new problems: Hot electrolytes (especially at pH > 9) soften the adhesive. This eliminates conventional cvanide baths. If possible. high metal content, room temperature, acid plating solutions are used to minimize adhesive attack. But dendritic or spongy deposits are normally obtained from such baths; thus, the proprietary organic addition agents used to prevent this must be carefully controlled. Copper, solder, tin. and rhodium are deposited from baths of the type recommended above. Gold and silver cannot be easily deposited from such baths even lvhen organic addition agents are used, because the high rate of crystal growth relative to the nucleation rate leads to dendrites. Thus, modified cyanide baths are used to minimize attack on adhesives. These provide for higher metal contents, lo\.ver free cyanide, room temperature operation. and the minimum practicable pH. Gold baths containing a small amount of silver are particularly useful. Alternatively, other complexing agents may be used \vhich are suitable in baths M-ith a pH below 9. LVhile considerable progress has been made, more work is needed. PREPARATION FOR PLATING. Treatment of copper clad laminates is conventional: solvent or solvent-emulsion degreasing, mild alkaline cleaning, acid dipping, and plating. Presence of cured adhesive on the copper-clad laminate calls for mechanical cleaning. This is common. as the electroformed copper commonly used today is porous, so that adhesive bleeds through the pores during the bonding of the copper foil to the laminate. Use of electroformed copper in manufacture of preclad laminates presents another serious cleaning problem: T o avoid staining and tarnishing after plating: many producers employ rinse waters containing organic addition agents such as “rinse aids” and water-displacing fluids. which coat the copper Jvith thin layers of polar organics. These materials ab-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Electrical Performance of Boards in the Field Effects of Contamination. Contamination of printed circr-it boards chanjes electrical properties: Residue of prior processing cause bridging and leaksqe between adjacent conductors; conductivit)of the laminate itself may increase (surface and bulk); corrosion may lead to mechanical failure of the hoard; and superficial contamination such as tarnishing leads to increased contact rcsistance between contact tabs on the board and external harnesses. and decreased solderability. Sources of Contamination. RALV MATERIAL.Organic decomposition of partially currd. thermoplastic laminates may occur. Cleaning and plaring procedures used in the production of plated boards are sources of contamination. The susceptibility of a board to the effecn of contamination depends on the type used: Some react Ivith or absorb contaminants and ”or absorb moisture more readily than others. PROCESSING. Etching solution residues are the principal source of contamination in producing etched boards. Other major factors include residual electrolyte from cleaning. sensitizing. chemical plating. and electroplating solutions. Secondary contamination sources include solder flux residues. metallic migration, and growth of clhiskers on cadmium-, zinc-, and tin-plated surfaces. extraneous contamination from fingerprints. spillages and the like. and decomposition of organic protective coatings. ESVIRONMENT.Contamination may arise as a result of the environment of the completed assembly. For example, oxidation and tarnishing make soldering difficult? and further metal migration may occur in humid environments. Tropical environments stimulate fungus and similar gro\vths. Corona discharges form ozone and nitrogen oxides in air. causing deterioration of electrical properties of the wiring assembly. Measuring a n d Predicting Contamination. S o one test suffices, as the effects of contamination are many, but it is customary to test for contamination by electrical leakage tests. Surface and bulk resistivit). measurements are typical direct current tests, as they measure the tendency of contaminants to permit ionic conduction. Dielectric dissipation meas-
PRINTED W I R I N G urements are typical alternating current tests, as they measure the tendency of contaminants to increase dilectric leakage. Specifications for such tests have been developed by the Electronic Industries Association, American Society for Testing Materials, Xational Electric hlanufacturers Association, and various military and industrial groups. Research institutes such as Stanford Research Institute are also very active here. Prediction of whether a given board will give rise to contamination effects in the field is complex, and no fully satisfactory methods exist. M'ork on accelerated contamination tests should continue; this is a very important aspect of printed circuitry in which the chemist may make important contributions. Most common method today is to expose the test board to a constant hig'l humidity or a prescribed humidity cycle. T h e electrical leakage is measured before and after the test. Two other tests have been studied in the past 2 years by a working group of the Electronic Industries Association. O n e method consists of extracting ionizable matter from the surface of the board by refluxing with boiling conductivity water. Conductivity of the water is measured as a function of time of refluxing, to provide a measure of the degree of contamination. T h e other method involves applying 220 volts direct current behveen adjacent conductors on the assembly a t high humidity, washing the board, and performing a microdetermination of copper. This test assumes that the presence of ionizable matter on the board \vi11 cause anodic oxidation of copper when a potential is so appiied. Unfortunately, attempts to relate the results of these t\ro tests to actual field evaluation of the boards have been largely unsuccessful. Undoubtedly, equating electrical leakage of the board after use in the field with the presence of ionizable matrer on the board after processing represents a n oversimplification. Obviousl) . there remains much lvork to be done in the field of accelerated contamination testing. Elimination of Contamination. Half the battle in eliminating contamination of printed wiring is good housekeeping and adherence to proper processing specifications, especially in rinsing and after cleaning, etching, and platins operations. CLEANINGRESIDUES. Proprietary cleaners vary in their ease of rinsing, strongly alkaline solutions containing caustic and silicates being ivorst. Mild cleaners, such as those designed for nonferrous metal cleaning, are satisfactory. Care must be taken with solvent and solvent-emulsion cleaners that the solvent has little or no interaction Xvitli either the
laminate itself or the adhesive. Conventional solvents such as tri- and perchloroethylene are such effective solvents for organic compounds that they may soften or otherwise attack adhesives, swell laminates, or remove desired markings and color codings. It is preferable to use solvents which remove contamination such as solder fluxes and fingerprints, yet do not attack adhesives or laminates. Such a solvent is a toluene-isopropyl alcohol mixture. Recent claims have been made for fluorinated organics, such as Freons and perfluorinated compounds in such applications. ETCHINGRESIDUES.Rinsing of residues is not simply a matter of being thorough; ion exchange and hydrolysis phenomena must be considered. For example, in ferric chloride etches, the ferric ion is readily taken u p by ion exchange on the surface of plastics which exhibit anionic ion exchange tendencies, probably as FeCId-. During rinsing. dilution occurs, and iron is eluted (even if the rinse water is acidified with hydrochloric acid). Because of its extensive hydrolysis, the iron tends to precipitate as ferric hydroxide during elution; ion exchange is responsible for the presence of ferric ion on the board, and hydrolysis is responsible for its precipitation and consequent contamination of the board. This problem is minimized by adding wetting agents to aid in removal of contaminants by the rinse and by using acidified rinse waters for the preliminary rinse and complexing or chelating agents, to prevent or minimize precipitation of hydrous oxides or hydroxides of the metals present in the etchant. Although ferric chloride etches present the worst problem, cupric chloride etches present problems too, owing to the insolubility of the cuprous ion. For ferric ion, suitable complexing agents include ammonia, fluorides, cyanides, thiocyanate, citrate, oxalate. EDTA, phenol, and catechol; for cuprous ion, ammonia, halides. cyanide, thiosulfate, thiourea, ethylene. maleic acid. and fumaric acid. PLATING RESIDUES. Rinsing problems resemble those discussed above: Insoluble salts of rhe metals in the plating solution may precipitate on the laminate to cause trouble. Hoirever, proper rinsing practice is normally adequate, especially after acid electrolytes. Alkaline electrolytes, especially those containing cyanide, may be a source of contamination by being absorbed by the laminate. h rather specific problem in rinsing of plating solution residues may arise in the production of "plated-throuyh holes." I n this process. the hole is sensitized, rendered conductive by chemical plating of copper or silver, then electroplated with the desired thickness of copper. T h e laminations inside the board are exposed
to the action of a variety of electrolytes, such as the cleaning, and electroplating solutions. I n each case, capillary action may cause considerable absorption of electrolyte. Such absorbed electrolyte may be very difficult to remove subsequently, and can affect bulk resistivity of the laminate. A-0 cornpletely satisfactory solution to t h i s problem exists. Wetting, complexing, and chelating agents help remove some matter, but some of the absorbed electrolyte is reduced to metallic form, and such material cannot be removed. SOLDERINGRESIDIJES.Alcohol-rosin or activated rosin fluxes are almost universally used, and are not a major source of difficulty in Contamination. I t may be necessary to remove solder fluxes to meet specifications or military requirements. I n such cases, solvents must completely remove solder fluxes without affecting either the adhesives used between the laminate and the copper, or the laminate itself. POTTING COMPOUNDS. T h e effects of contaminants are often decreased by application of a final protective coating of organic material, such as varnishes or epoxy-type coatings, to cover the entire assembly. However, under conditions of high humidity, contaminated boards deteriorate gradually in electrical properties, indicating moisture penetration of thr organic coating. Silicone and epoxy coatings are the best knobrn to date. O n e advantage of epoxy-type potting compounds is their compatibility with the commonly used epoxy-glass filled laminated board.
Conclusions Rfuch remains to be done. Thc reliability of printed circuits and components is a serious problem, especially i n such military applications as rocketry and missiles. LVhile m,my of the problems involved in reliability of printed citcuitry lie within the domain of the electrical engineer, the chemist has responsibilities as well. If any one problem might be singled out as of pressing importance today, it is the need for adequate testing procedures for predicting tvhether or not a given board \\ill meet performance specifications. Existence of such tests would make possible screening. of production. and go a long way toirard enabling the manufacturer to pinpoint sources of difficulty in the manufacturing operation. RECEIVED for revie\v April 13, 1958 ACCEPTED December 4. 1958 Divisim of Industrial and Engineerinq Chemistry, Symposium o n Chemical X s pects of Printed IVirinp. 133rd Meeting, .\CS, San Francisco. Calif., April 1958. VOL. 51, NO. 3
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