US3441428A - Low temperature electroless plating - Google Patents

Description

United States Patent Oflice Patented Apr. 29, 1969 3,441,428 LOW TEMPERATURE ELECTROLESS PLATING George C. Reinhard, 16682 Robert Lane, Huntington Beach, Calif. 92647, and Michael W. OMara, 11209 Stamy Road, Whittier, Calif. 90604 No Drawing. Filed Sept. 13, 1965, Ser. No. 486,602 Int. Cl. C23c 3/00; B44d 1/20 US. Cl. 11747 7 Claims ABSTRACT OF THE DISCLOSURE A process of plating metals on the cleaned surface of an article of non-conductive material wherein said article is immersed in a solution consisting essentially of nickel and ammonium cations and hypophosphite anions and substantially devoid of chloride and sulfate ions. The solution is maintained in the temperature range of 70 F.-'l40 F., and replenished from time to time with additional amounts of hypophosphite, ammonia and nickel.

This invention relates to electroless plating of metals on physical surfaces and more particularly to the electroless plating of the metals such as nickel and cobalt, by means of chemical reduction of the metal ions to elemental metal, using such as hypophosphite as the reducing anion.

Because the reaction is catalyzed by certain metals it is termed one of catalytic plating. And because the metals deposited are also catalytic to the reaction, it is more specifically termed autocatalytic.

Some of the first patents relating to this are those issued to A. Brenner and Grace Riddell (US. 2,532,283 and 2,532,284), since which numerous other patents have issued and contributions have been made so that today the art is highly developed.

Despite the highly developed state of the art, increasing demands are being made to decrease the limiting conditions of the process, thus stimulating attempts to further improve it in some particular so as to provide greater efiiciency, more rapid plating rates at a given temperature, plating ability of suflicient rate at low temperatures, greater ductility, better adhesion.

Originally, techniques of plating of metals which in themselves catalyze or can be made receptive to a catalyst have been developed whereby even non-catalytic nonmetals, such as glass, ceramics, organics, etc., both natural and synthetic, have been made similarly receptive to ultimate plating by catalytic reduction by the means of preparing them with a film of a catalytic metal such as palladium.

Processing of organic base materials, such as plastics, resins, polymeric materials in general, nylon, etc., make it increasingly pressing that further improvements and developments of techniques, beyond those heretofore developed, be effected. Already, even with imperfectly developed techniques and inefficient methods, plastic items of various shapes and utility, produced by developed techniques of injection molding, vacuum molding, casting, etc., have been plated with metals so as to make them simulate in appearance, utility, service, decoration, beauty, etc., all kinds of metals such as gold, silver, copper, brass, chromium, etc., and because of the inherent nature of plastics and their relative cheapness plus economy of fabrication, present advantages over the base metals they are made to simulate.

The acrylonitrile-butadiene-styrene copolymer type of plastic hereafter referred to simply as ABS, as is common in the trade, is particularly emphasized because it is a strong, dense, high-impact type of polymer, capable of being molded and shaped in large volume by practical molding techniques and is making a strong impact on the commercial market in replacing metals and other plastics for myriads of uses and applications.

A demand for such articles including materials such as ABS plastics already exists but is prevented from realizing full attainment, or stimulus toward full attainment, by reason of the lack of perfection of efiicient, dependable, rapid, economical plating processes.

In contrast to the plating of metals and heat resistive nonmetals, the plastic, polymeric materials of an organic and semi-organic nature (polymeric hydrocarbons, fluorocarbons, silicones) are generally as a class, more sensitive to the effect of heat so as to present greatly limiting conditions for their processing which make it impossible or impractical to utilize many of the solutions, processes and techniques to which metals and heat insensitive nonmetals are adaptable. The degrading eflfect of heat is particularly a case in point. At temperatures far below those at which metals manifest no deleterious effect, plastics and resins do manifest effects such as softening and distortion. Not only is this manifested at elevated temperatures or under dry heat conditions but rather at temperatures, even under wet conditions, of hot to boiling water. Thus it has been established as a general condition, though not always firmly applicable, that plating processing be conducted at no higher temperature than 140 F. and, of course, as close to an ambient room temperature of approximately F. as possible.

Generally, it is the object of this invention to provide such improvements of the prior art as to realize these conditions in a practical, simple and effective manner.

More particularly, it is the object of this invention to provide means of deposition from certain nickel-hypophosphite, catalytic plating baths, electric conductive deposits of metal-phosphorous, such as characterizes deposition from such hypophosphite reduced reactions, and deposit the metal-phosphorous at higher rates, at all temperatures, and particularly, rapid plating at low temperatures. This rapid plating will occur at temperatures ranging from at least a minimum of ambient room temperature of about 70 F. to at least the maximum limit stated as established for plastics in general and for acrylonitrile-butadienestyrene plastic, commonly called ABS, which temperature is usually considered to be F.

It is also the object of this invention to provide improved stability of the plating solution whereby it can be used for long periods with only the necessity of replenishing reactants as they are consumed, and an object of this invention is to provide, within specified limits, an automatic pH control whereby it is not necessary to make frequent additions of alkali to maintain pH at the desired degree as is normally necessary to neutralize the acid of reaction that is characteristically formed in the hypophosphite reactions.

The prior art shows how or in what manner the pretreatment, conditioning and preparation of the article to be plated should be practiced. Whatever method is used that results in making the plastic or nonmetallic body catalytic initially, in order to promote deposition of metal from metal-hypophosphite, catalytic reduction baths, is satisfactory.

A thorough study of the art shows clearly and distinctly that the use of common cheap salts of nickel, and the chloride of sulphate, has been generally specified for bath makeup to provide the source of nickel required for plating.

The original work of Brenner and Riddell shows that the addition of specified anions, other than S0 and Cl, for certain purposes has been utilized and generally adhered to, indicating their essentiality. Thus, for instance, alkali salts of weak organic acids provide a buffering action for the maintenance of pH within certain limits on the acid side of the pH range. This is to prevent the rapid reduction of pH below which the plating rate is seriously reduced or prevented. Other organic salts were of a type that formed complexes with the nickel, selected from a group of reactants such as hydroxyacetic, lactic, citric, and tartaric acid. This is done so that, when addition of alkali becomes necessary to raise the pH, as recommended, with the local overneutralization that results when this is done, insoluble basic salt or hydroxide, deemed fatal to the stability of the bath, is not formed. These salts, being of weak acids in themselves, thus provide buffering property additionally to the complexing property.

The work of Gutzeit et al. determined the function of other organic anions to be exalting to the reaction, meaning thereby that they contributed to increased speed of reaction and hence, plating rate.

Other art contributions relate to chemical additions, usually cations, in trace amounts, that result in stabilization, and recourse to nickel carbonate additions to acidic baths to provide regeneration of consumed nickel at the same time increasing the pH as required, without the introduction of additional extraneous ions into the bath.

Nowhere does it appear that emphasis is placed upon the adverse effect of the inorganic ions, particularly the S and Cl, nor the emphatic exclusion of these for best results.

Pretreatment to make nonmetals and noncatalytic materials catalytic to the reaction of the applied solution, at least initially and until autocatalysis sets in, is necessary.

It is accomplished by depositing by pretreatment a thin, frequently imperceptible, invisible, adherent film of elemental palladium which, being a catalyst, initiates the reaction of reduction which, after nickel or cobalt is deposited, proceeds thereafter autocatalytically.

Such pretreatment or preparation generally requires that the basic material be cleaned and conditioned to make the palladium more readily and more completely received by and attached to the basic material for, depending upon the thoroughness of this, rests the complete success or failure in part, of all the ensuing operations of chemical and electrodeposition of metal which follow.

Pretreatment is specific to each material processed. In the case of organic, polymeric materials including the ABS type plastic, it is generally processed to remove soil, oil, grease and foreign matter using for this purpose any of the detergent alkali cleaners. It is then followed b a mild acid rinse to neutralize the detergent alkali. Following this the material is chemically delustered or etched by wet oxidation in a bath of chromic-acid composition, usually at 140 F. maximum after which, following a thorough rinse, it is deemed satisfactorily prepared to be subjected to the treatment necessary thereafter to make it susceptible to deposition of an adherent deposit of palladium. Processes utilizing proprietary and non-proprietary compositions generally consist of adsorbing to the cleaned, etched or abraded work, a film of a strong reducing agent that is capable of reducing soluble palladium salts to the elemental state. Thus when immersed in the palladium salt solution, elemental palladium, in a thin, often invisible and even monomolecular film, is deposited.

As stated originally by Brenner and Riddell, ammonium ions stimulate reaction especially in the higher alkaline pH ranges, while certain other cations, such as barium and magnesium, influenced, for better or worse, the rate of reaction.

The necessity of having weak acid salt buffers and/or complexing agents present when baths are operated in the acid' range and subjected to alkali additions to adjust pH periodically is not imperative or essential, and at best only slightly desirable when the bath is operated in an ammoniacal medium, constantly in the alkaline pH range and is subject to pH adjustment with ammonia, or rather the addition of an excess of ammonia as taught herein.

pH condition for long periods thus reducing the risk of accidental reduction of pH below that specified for proper operation. In the subject invention it, therefore, is unnecessary to select the anion with regard for any of these functions and it can be selected primarily with regard for its economics provided the inorganic anions, especially the S0 and Cl anions are avoided.

Illustrative of plating baths that conform to the requirements so disclosed and taught hereby, but not limited to them, are:

EXAMPLE NO. 1

A solution of nickel ammonium hypophosphite-Trace to 1 oz. (preferred) of Ni/gal. at pH 8 plus, made with ammonium hydroxide preferred in excess.

EXAMPLE NO. 2

A solution of nickel ammonium glycolate.Trace to 1 oz. (preferred) of Ni/gal. at pH 8 plus, made with ammonium hydroxide preferred in excess.

In Example No. 1 there is introduced no extraneous anions of any kind. There is introduced as the only anion an amount of hypophosphite combined in the compound with nickel, in an amount sufficient to reduce no more than 50% of the nickel on a theoretical basis and only about one-third the amount necessary to reduce all the nickel from a practical standpoint.

This is an ideal ratio of hypophosphite to nickel with which to operate the bath initially because, being so extremely activated, or exalted, it is advisable that the concentration of hypophosphite be not augmented initially, with further additions. Since activity of these solutions in alkaline media is increased and stimulated by the absolute concentration of hypophosphite, or the ratio of hypophosphite to nickel, too high a concentration of hypophosphite contributes excessively to activity and in turn makes it so reactive as to impair stability.

When, however,,in use, the hypophosphite is consumed at a greater rate than the nickel, the ratio relationship that develops is one that makes the solution more stable. As the hypophosphite is consumed it, of course, requires replenishment so as to effect efiicient consumption of nickel. Such additions are made from time to time in the form of an alkali or ammonium salt and in an amount that will bring the ratio of hypophosphite to nickel up to the original ratio but does not exceed this original ratio and thereby contribute unduly to the instability of the solution.

In the second of the illustrations, Example No. 2, since no hypophosphite exists in the nickel compound utilized, it must be provided supplemental thereto, at the outset. Such is preferred to be initially in the same mole ratio of 1 mol. of nickel to 1 mol. of hypophosphite for the same reason of stability of solution. Also for the same reasons as detailed, the replenishment should be effected in the same general manner and of the same kind and type as stated above for Example No. 1.

Using a bath of either type illustrated and using in the case of Example No. 2 ammonium hypophosphite as the supplement to effect reduction, in the amount stated, plating occurs quickly at ambient room temperature of approximately 70. F. as is evidenced by the evolution of gas formed by reaction on the surface of the catalyzing article, and the visual change in appearance (noticeable when plating light colored plastic bodies) that is observed as the article rapidly becoming darkened and opaque on the surface, and gradually assuming a metallic lustre and appearance.

At elevated temperatures the rate of reaction and deposition, characteristic of all solutions in this class, increases rapidly. At 100 F.110 F. it is such as to be of the order of /2 mil./hr. and the most practical, optimum rate for most purposes.

Beyond 140 F., the average limit stipulated for plastics in general, it is so rapid as to be considered impractical for plastic fabrication preparatory to electroplating. Beyond this at higher temperatures and approaching those normally utilized (of 200 plus deg. F.) the rate is so rapid that because of the corresponding instability that accompanies such rapid rate, use, except for special applications, is not advised without reducing the rate of deposition by either decreasing the ratio of hypophosphite to nickel or by addition of retardants such as those specifically avoided herein and which characterize other, slower baths of the past, many identical in all but this respect.

In previous attempts to improve the process as first disclosed by Brenner and Riddell, additives, such as the exaltants of Gutzeit et al. have been resorted to. However, the object of an improvement in performance of the solution has been attained by us, not by recourse to additives, or by making the bath more involved, but rather by simplification to basic ions essential to the reaction or formed in reaction, and particularly by eliminating from the bath those that are particularly retardant to the reaction (i.e., the S0 and Cl), commonly employed heretofore.

By the use of an excess of ammonia, and proportional to the amount of the excess used which can be varied within wide limits, the high pH of the solution is automatically maintained over long periods of time. The acid of reaction, characteristic of these nickel-hypophosphite plating solutions, is immediately neutralized as it is formed Without effecting a reduction of the pH until the excess of ammonia has been consumed. Thereafter, the change in pH is not abrupt but is gradual as the complexed'nickel compound acts as a buffer changing gradually in pH only as combined ammonia in the complex is removed by the acid of reaction, as the nickel compound in turn is converted from highly complexed to lesser complexed compound. This results at the same time in a gradual change in the color of the solution, from the dark blue of maximum complexed nickel through the variations of greenish blue, bluish green and finally the green of the lesser complexed nickel compound. Thus the color change is a visual indication of the change in pH which the operator can constantly view and observe without special tests and ultimately develops a proficiency of control that is reliable and dependable. When so controlled by visual appearance, our preference is that it be operated at the dark blue color of maximum complexed nickel. As a green cast develops, preparations should be made to add additional, excess ammonia.

In the past, the normal objection to the operation of ammoniacal, alkaline bath, even when no great excess of ammonia was utilized, and especially when excess ammonia was present, was one of economic loss of ammonia to waste in the atmosphere coupled with the unhealthy, irritating, uncomfortable and undesirable atmosphere that excessive volatilization of ammonia produces.

This is understandable because the solutions suggested heretofore were so inactive and slow in plating rate, that, at ambient room temperature they did not plate at all or Were so slow as to be impractical, and did not develop a practical rate of deposition until at least a temperature of 160 F. was attained, and usually required a temperature of between 190 F. and 210 F. for attainment of practical plating rates. At this temperature the ammonia was so volatile as to be rapidly discharged from solution to the surrounding atmosphere, the ammonia even being discharged from the double salt complex of nickel with which it had been combined. Thus the decrease in pH was more rapid than would have been caused only by the acid formed by the reaction, and an economic waste of ammonia resulted from this wanton discharge to the atmosphere and the atmosphere was contaminated.

Indeed, in these respects, when operated at the higher temperatures, the solutions of our examples are no better.

In the operation of these electroless, hypophosphite reducing baths, the phosphite anion is formed as the product of reaction and increases in concentration proportionally to the amount of nickel plated. In the baths according to this disclosure, this byproduct, accumulating as it does, does not impair the solution or retard its performance appreciably up to a replenishment of additional nickel deposited by plating, of at least three times the original of 1 oz./ gal. nickel concentration. The slight reduction in plating rate that results is readily compensated for by increase in temperature, for which ample factor is provided, or by increasing the absolute concentration of hypophosphite.

This flexibility is attributable, in our opinion, to the specific avoidance of the reaction retarding extraneous, inorganic anions, especially S0 and Cl anions, and the preponderance of ammonium cations in the ammoniacal medium, all characterizing the features of this invention.

It is apropos to mention that the improved results described herein, when using Example No. 2, result even when the glycolic acid utilized for the preparation of the nickel ammonium hydroxyacetate is the 70% commercial technical acid, and is not ascribed to purity of the acid which Brenner noted was a determinant of governing activity of his solutions but which, at best, as reported was not as active as these of our discovery.

In the preparation of ammonium double salt complexes it is usual to prepare them from the normal salt, made by reacting stoichiometric proportions of nickel and anion, either by ion exchange or by reacting the acid with a suitable basic compound of nickel such as the hydroxide, carbonate, etc., by adding to the said normal salt an amount of ammonium hydroxide to form the complex of the degree and type desired.

We have ascertained however that the double ammonium nickel complex of the kind and type desired can be directly prepared by adding to an ammoniacal solution of the normal ammonium salt, an amount of a nickel base such as nickel carbonate, equivalent to the amount of normal ammonium salt. Unexpectedly reaction takes place quite rapidly and proceeds to completion. The reaction is facilitated by even moderate heat and, when nickel carbonate is the base compound, rapid discharge of carbon dioxide accompanies the reaction. In the case of the examples illustrated, the normal ammonium salts utilized are the hypophosphite and hydroxyacetate, respectively.

We have found that the above reaction can be applied to the actual replenishment of the bath with additional nickel ions and constitutes an economical method for accomplishing this, at the same time excluding the undesirable anions or adding to those already present. When replenishment is desired, since the solution is already composed of normal ammonium salt in ammoniacal medium, by simply adding, as a slurry in water or by sifting or other suitable means, the predetermined amount of nickel base, preferably the carbonate, with agitation of the bath and at a slightly elevated temperature, the nickel enters into perfect solution rapidly and Without difficulty. This regeneration is preferably done at a time when no actual plating is taking place.

This result is quite unexpected for While it is known that insoluble nickel bases and basic compounds, do react with acid directly to form soluble salts, they generally remain inert and inactive and insoluble in alkaline media. As a consequence this departure for preparing the double ammonium, complex nickel compounds specified, both for original bath and as replenishment of nickel, is deemed to be a further and desirable contribution to, an advancement of, the art.

We need not confine ourselves to the compounds of our illustrations since other organic acids are adaptable. Hydroxyacetate has been selected as typical and is preferred for the reason that it is currently economically advantageous especially in the 70% commercial technical form that has been found suitable. It is not in any case utilized here for the reason it is normally utilized (the reason based on its known character of forming complexes of nickel in the pH range below pH 8). We do not operate in this range because the plating rate at the low temperature is inadequate and too slow.

In the plating of a non-conductive article that is to be overplated subsequently by electrolytic methods, it is necessary to deposit on the surface of the nonconductor an amount of conductive metal of suificient thickness to permit satisfactory electroplating thereon. When the deposit is adequate this can be done without ofiering too great a resistance to the electric current thus preventing burning while at the same time maintaining a satisfactory plating rate electrolytically.

In actual practice, over a period of six months, it has been possible, using a solution of the type illustrated herein, to maintain a steady production of properly plated articles, by depositing from the bath of our illustration, a thickness of conductive metal-phosphorous composition of the order of .00002" (20 millionths inch). This has been possible of accomplishment in an immersion time of only minutes at 110 F. in our catalytic solution of improved activity.

Rejects have been abnormally low (1%) compared to other baths (10% is considered good) and production rate has been excellently maintained.

Furthermore, solutions have been used daily in heavy production and have remained stable for long periods and have been subjected to repeated replenishment and regeneration equivalent to three times the original nickel concentration.

Solutions have been used for plating to virtual (95%) exhaustion of the nickel therefrom, visually evidenced by the low color intensity of the solution as imparted by the intensely colored nickel complex, and which, incidentally, is a convenient manner of estimating the amount of nickel present.

Such plating to exhaustion makes for improved economy when bath replacement is contemplated by making possible virtual complete removal of expensive nickel before discarding the bath.

We claim:

1. In the art of electroless plating for plating metals such as nickel on the cleaned surface of an article of substantially non-conductive material that has been pretreated, the process comprising the steps of preparing a solution consisting essentially of nickel and ammonium cations and hypophosphite anions,

and substantially devoid of Cl and S0 anions; maintaining the said solution in the temperature range of 70 F.140 F.; adding to said solution an amount of ammonium hydroxide in excess of that necessary to develop a dark blue color characteristic and indicative of maximum ammonium complexed nickel;

immersing said article in the solution for a period of time suflicient to deposit a thickness of conductive metal-phosphorous composition on the surface thereof;

adding to said solution, from time to time as the hypophosphite is consumed in plating, additional hypophosphite necessary to continue the plating of metal from the solution;

adding to said solution from time to time, as the excess ammonia is neutralized by the acid formed in reaction and as the pH is thereby reduced, which reduction in pH is visually indicated by the progressive change in color from dark blue to green, a further excess of ammonia to continue the maintenance of the solution with such excess of ammonia;

adding to said solution from time to time, as nickel is removed by deposition, indicated by the diminishing color intensity of the solution, replenishment nickel selected from the group consisting of nickel ammonium salt of an organic acid, nickel ammonium hypophosphite, nickel hydroxide, and nickel carbonate.

2. The process of claim 1, wherein said nickel, ammonium, and hypophosphite ions are provided by nickel ammonium hypophosphite.

3. The process of claim 1, wherein said nickel and ammonium ions are provided by a double nickel ammonium salt of an organic acid and the hypophosphite ions are separately provided.

4.. The process of claim 3, wherein said double ammonium salt is nickel ammonium glycolate.

5. The process of claim 3, wherein said hypophosphite ions are provided by ammonium hypophosphite.

6. The process of claim 1 further including the step of pretreating said article to be plated with a catalytic coating.

7. The process of claim 6 wherein said catalytic coating is palladium.

References Cited UNITED STATES PATENTS 1,207,218 12/19'16 Roux 1061 X 2,532,283 12/1950 Brenner et al 117130 X 2,871,142 1/1959 Hays 117-130 3,024,134 3/1962 Nixon 117130 3,211,578 10/1965 Gutzeit 117130 3,288,639 11/1966 Smith 1'17217 OTHER REFERENCES Saubestre: Metal Finishing, September 1962, p. 59.

RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R.