GB2065175A - Method and Composition for Producing Palladium Electrodeposition - Google Patents

Abstract

A new aqueous electrolytic bath suitable for the electrodeposition of ductile, non-porous palladium coatings and a method for its use are disclosed. Prior palladium plating methods are slow and uneven in plating characteristics and do not produce pore-free coatings unless complex baths are used which cannot be readily regenerated. The prior art baths also produce environmental problems. The bath contains a palladosamine salt (chloride or bromide) as well as a dual electrolyte system composed of sulfamic acid and ammonium chloride. The method entails use of the bath under certain temperature and pH conditions. The new electrolytic bath is readily formulated and does not require the addition of additives. Significant amounts of ammonia and other undesirable fumes are not released from the bath during plating. Coatings produced from the bath are bright, highly adherent, ductile and generally pore- and stress-free. <IMAGE>

Description

SPECIFICATION Method and Composition for Producing Palladium Electrode position Technical Field The present invention relates to improvements in and relating to the electrodeposition of palladium onto base metal substrates, and in preferred embodiments to the electrodeposition of palladium onto electrical contact surfaces.

Background of Prior Art Electrical contact surfaces must exhibit stable, low electrical resistance. This characteristic is generally obtained by either forming contacts out of materials exhibiting the requisite stability and low resistance or by forming the contacts out of a base metal and then plating them with such a stable, conductive material.

Gold, as well as some of the other noble metals, satisfies electrical contact surface requirements for stability and low resistance. Gold coatings are generally resistant to oxidation, sulfide formation and other chemical reactions which might increase their electrical resistance. However, since gold is a precious, expensive element, it is desirable to find alternate plating materials.

It has, for example, been suggested that the relatively inexpensive noble metal palladium be electrodeposited upon a base metal substrate in order to achieve these desirable characteristics. It has been recognized, however, that the palladium coatings produced by the various electrodepositing means developed in the art suffer a number of significant drawbacks. In many applications, including particularly the coating of electrical contacts, it is essential that the palladium layer be pore-free.

Porosity and pitting in this layer permit foreign matter to penetrate through to the supporting surface and spread out onto the contact itself. This results in tarnishing and direct-couple corrosion between the palladium and the supporting base metal, both of which ultimately impair the electrical conductivity and usefulness of the electrical contact. One significant drawback of prior methods for electroplating palladium is that they have failed to provide simple, direct means for producing pore-free palladium coatings.

Moreover, prior art proposals for electrodepositing palladium on base metal substrates have been less than fully satisfactory for other reasons. Some of the electrodeposition methods are slow acting and produce uneven, stressed coatings with high hydrogen embrittlement characteristics which tend to crack and peel, especially when applied to surfaces in high-use applications such as electrical contact surfaces. Other prior art methods are simply undesirable because they are inherently inefficient. Some of these other baths are difficult and expensive to regenerate once contaminated, some exhibit poor throwing power, and some are so highly sensitive that they require a very narrow current density range and/or a very narrow palladium ion concentration.

Finally, prior palladium electro-plating techniques have produced unpleasant, harmful vapors, requiring that the electro-deposition be carried out under vacuum hoods. This venting requirement is not only cumbersome, but adds to the expense of the overall electro-deposition method. Furthermore, the presence of a significant volume of an unpleasant, hazardous vapor creates serious environmental, health and safety problems.

A typical prior art plating solution releasing significant amounts of unpleasant, harmful ammonia vapor is that described in U.S. Patent No. 3,920,526. The plating solution described in this patent includes palladosamine chloride, ammonium chloride and aqueous ammonia. Another prior plating solution is described in U.S. Patent No. 3,925,170 which discloses a solution containing a soluble palladium compound, and a soluble electrolyte, such as ammonium sulfate. When used in electroplating, the above solutions release significant quantities of ammonia gas, causing the solutions to go acidic which interfers with plating and, therefore, must be controlled by continual addition of an appropriate base such as ammonium hydroxide. Once contaminated, these solutions cannot be readily regenerated.

The aqueous electrolytic bath of the present invention, when used in conventional electroplating operations releases little or no ammonia gas or other undesirable vapors, and, therefore, does not require venting during use. Palladium coatings electrodeposited from this solution exhibit various chemical and physical properties which are desirable in a wide range of applications, including particularly coating of electrical contacts. Thus, the coatings are generally pore-free, and exhibit outstanding ductility, stress and adherence characteristics. Throwing power from this electrolytic bath is excellent, and plating may be carried out over a wide range of current densities and palladium ion concentrations. Finally, these baths, when contaminated in the course of regular use, can be readily and inexpensively regenerated.

A further important benefit derived from the present invention is that the various additives often required with prior palladium plating dolutions are not needed. These additives have, in the past, been used to control pitting, porosity, stress, adherence and other coating characteristics. Unfortunately, in addition to the expense inherent in the use of such additives, it has been found that they often plate out with the palladium, impairing its conductivity or resulting in a trade-off of enhancement of one characteristic, such as ductility, in return for an impairment of some other characteristic, such as adherence. Thus, the expense and the undesirable trade-offs which result from the use of plating solution additives are eliminated in the practice of the present invention.

Brief Summary of the Invention it is, therefore, the object of the present invention to provide an aqueous electrolytic bath suitable for quickly and evenly depositing palladium coatings exhibiting such highly desirable properties as brightness, low electrical resistance, good ductility, low stress, lack of pitting and porosity, minimal hydrogen embrittlement and good adhesion, which will not release significant amounts of ammonia gas or other undesirable fumes during plating. Other features and advantages of the present invention will become apparent upon examination of the specification and claims. While the invention is described below in connection with preferred or illustrative embodiments, these embodiments are not intended to be exhaustive or limiting.Rather, the invention is intended to cover any alternatives, modifications and equivalents that may be included within its spirit and scope, as defined by the appended claims.

The present aqueous electrolytic bath includes the components: palladosamine chloride (Pd(NH3)2CIz), sulfamic acid (HSO3NH2), and ammonium chloride (NH4CI). Although less preferred due to economic considerations, palladosamine bromide (Pd(NH ,)2Br2) may be used in lieu of the palladosamine chloride.

Detailed Description of Invention The concentrations of the various components which make up the present aqueous electrolytic bath may be varied over relatively wide ranges. The following concentration ranges, however, are preferred: Palladosamine chloride/bromide 5-50 gpl(grams per liter of H20) Sulfamic acid 10-150 gpl Ammonium chloride 10-150 gpl In addition, the bath is preferably operated in the pH range of about 6.5 to 10.0, and most preferably in the pH range 8.0-9.0.

The above palladosamine chloride/bromide level is preferred because at salt levels below the preferred limit of 5 grams per liter plating will become spotty due to the shortage of palladium ion. Any plating which does occur will be dark and porous. On the other hand, when the preferred upper limit of 50 grams per liter of the palladosamine salt is exceeded, excessive and uneconomic drag-out losses will occur. Furthermore, dark burned deposits will appear and throwing power of the solution will be impaired. The electroplating process as well as the palladium coating itself will be similarly adversely affected when ammonium chloride and sulfamic acid levels are outside of these ranges.

In a more preferred embodiment of the present invention, the palladosamine salt will be present in an amount of about 30 4-0 grams per liter. The palladium ion levels provided by this salt concentration will insure good current density levels and hence ideal coating characteristics as well as minimal drag-out losses and excellent throwing power. It is further preferred that the concentration of both the ammonium chloride and the sulfamic acid lie in the range of about 40-60 grams per liter to optimize plating and film characteristics.

The sulfamic acid and ammonium chloride components of the present invention comprise a new and unique dual electrolyte system exhibiting outstanding compatibility with palladosamine salts.

Although the precise chemical and physical mechanisms by which electrodeposition from aqueous electrolytic baths proceeds is not well understood, this dual electrolyte system, when combined with the palladosamine salts, results in an electroplating bath with outstanding conductivity and throwing power capable of quickly and efficiently electrodepositing palladium films with outstanding physical, chemical and electrical properties.

The present aqueous electrolytic bath may be made up in the following manner: 1) Palladosamine chloride in powder form is dissolved in an aqueous solution of ammonium hydroxide (NH4OH:H20 at about 1:1, by weight).

2) Aqueous ammonium chloride is added to the solution of step 1.

3) Aqueous sulfamic acid is added to the solution of step 2.

4) Sufficient ammonium hydroxide orsulfamic acid is added to bring the pH into the desired range of 6.5-10.0, or the more preferred pH range of about 8.0-9.0.

The present aqueous electrolytic bath is suitable for use in conjunction with conventional systems for plating of palladium coatings onto base metal workpieces. The workpiece substrate may be any conductive material or metal which does not adversely influence the electro-deposition bath, including such base metals as nickel, copper, beryllium-copper alloys, brass, phosphor-bronze, stainless steel, mild steel, silver and the like. In general, prior to plating the workpiece must be cleaned and activated in an acid dip which removes surface oxidation, to provide a good surface for palladium adherence. In addition, the surface may be preplated with a strike of palladium or of other metals to further enhance adherence and to prevent or minimize corrosion due to direct-coupling between the palladium overlayer and the base metal.The strike layer, which is a very thin metal film also protects the base metal from attack in the concentrated electro-plating bath, which would result in bath contamination.

A desirable palladium strike may easily be obtained by subjecting the workpiece to electro-deposition using the present bath components at a low concentration.

Once cleaning, activating and, where desirable, pre-plating have been completed, the workpiece is immersed in the present electrolytic bath. The bath should be maintained under agitation at a temperature of from about 60--1 F, and preferably at a temperature of about 65-1 050F in order to obtain the best balance of coating characteristics, plating rate and bath evaporation.

An electrical potential is then applied across the immersed workpiece, as cathode, and a nonreactive anode such as platinum-plated titanium or tantalum of any appropriate configuration present in the bath. The potential is applied either continuously or in pulses for a total period of time sufficient to produce a coating of the desired thickness. Generally, coatings of from 1 microinches to as high as 200 microinches or greater can readily be obtained. Thinner coatings which tend to be porous may also be obtained. Finally, it may be desirable to apply the palladium in the form of several separate coatings to obtain optimal crystal orientation.The coating time will, of course, be related to the current density which should be in the range of from 1-100 amperes per square foot and will preferably be in the range of 5-50 amperes per square foot for rack plating, 3-1 5 amperes per square foot for barrel plating, and 5-100, 300 or more amperes per square foot for selective plating. Once the desired coating thickness is obtained, the workpiece is removed from the bath and rinsed.

As plating proceeds, the pH generally slowly rises. Acid addition is therefore required to maintain the pH in the desired range. This is a most important characteristic of the present invention, since the evolution of harmful ammonia fumes experienced in many prior art palladium plating systems is for the most part, eliminated. Of course, palladium is also lost from the bath as plating proceeds. Additional palladosamine salt must, therefore, be introduced into the bath during plating in amounts sufficient to maintain the salt concentration within the desired range.

Electrolytic baths used in electro-deposition of palladium and other metal coatings tend to become contaminated during use over a period of time. Contaminants come from the surfaces being plated, as well as from replenishment materials, containers, and other sources. The presence of contaminants in these baths, even in minor amounts, is undesirable because the contaminants tend to plate out along with the primary plating metal to produce contaminated coatings. It is therefore very important to be able to simply and cheaply regenerate, or purify contaminated electrolytic plating baths.

Prior to the advent of the present invention, it has generally been both difficult and expensive to regenerate electrolytic plating baths. This has been due, in part, to the difficulty of recovering plating metal compounds in a pure state. It has therefore often been the case that these prior baths had to be subjected to lengthy and complex purification procedures or discarded.

In contrast to prior practice, the electrolytic bath of the present invention may be regenerated in a most straightforard and economical manner. Since plating additives of the nature commonly employed in prior baths are not required, these additives will not be lost upon in the course of regeneration of the bath. It has been found, for example, that contaminated aqueous electrolytic baths initially prepared in accordance with the present invention and containing pailadosamine salts, sulfamic acid, ammonium chloride and a minor amount of various contaminants may be regenerated as follows: 1. Adjust the pH of the contaminated bath to about 3.0 to 5.0 to precipitate the palladosamine salt and to dissolve contaminants which might include, for example, copper, zinc and tin, and compounds derived therefrom.Since the bath will generally be at a pH in excess of 6.0, prior to regeneration, it has been found that either sulfamic acid or hydrochloric acid may be used to reduce the pH to the desired level. Sulfamic acid is preferred in this application, since the use of hydrochloric acid will introduce excess chloride into the bath which may affect palladium coating stress characteristics.

2. Filter the pH adjusted bath to separate the precipitated salt from the filtrate containing the dissolved contaminants. Rinse the remaining precipitated salt.

3. The precipitated palladium salt, which is by far the most expensive component of the bath, may then be redissolved in an alkaline medium such as an aqueous ammonium hydroxide solution containing ammonium hydroxide and water in a ratio of about 1:1. Sulfamic acid and ammonium chloride may then be added to form a regenerated electrolytic plating bath.

Having thus generally described the invention, the following specific examples are given to illustrate preferred embodiments and advantages thereof.

Example 1 The plating efficiency of an electrolytic bath prepared according to the teaching of the present invention was compared to the plating efficiency of a commercially available palladium bath and to the copper plating efficiency of a copper sulfate bath. Brass hull panels suspended in plating cells were plated in each case by applying 1.0 amperes to the plating cell for an effective current density of about 1 5 amperes per square foot (still plating).

An electrolytic bath examplary of the present invention (Bath A) was used, consisting of: 18 gpl (grams per liter of H20) palladium as Pd(NH3)2CI2 50 gpl ammonium chloride 50 gpl sulfamic acid pH was adjusted to 8.5 with ammonium hydroxide or sulfamic acid (temperature was maintained at about 740F) The commercial bath used was Palladure 100 (Bath B), which is sold by Lea Ronal. Since 100% efficiency is expected for copper sulfate under proper plating conditions, a copper sulfate standard (Bath C) was used to confirm the accuracy of the test results.

The results obtained are listed below: Bath A Run 1(5 minutes) Run 2(4.5 Minutes) Actual Pd plating: 1.9956 g/amp-hr. 1.963 g/amp-hr.

Predicted Pd plating: 1.990 g/amp-hr. 1.990 g/amp-hr.

Efficiency: 100.0% 98.6% Bath B Run 1(5 minutes) Run 2r4.5 minutes) Actual Pd plating: 1.4316 g/amp-hr. 1.430 g/amp-hr.

Predicted Pd plating: 1.990 g/amp-hr. 1.990 g/amp-hr.

Efficiency: 71.9% 71.8% Bath C (Standard) Run 1(5 minutes) Run 2(4.5 minutes) Actual Cu plating: 1.1976 g/amp-hr. 1.192 g/amp-hr.

Predicted Cu plating: 1.186 g/amp-hr. 1.186 g/amp-hr.

Efficiency: 101 .0%* 100.5%* The above test results indicate that outstanding plating efficiency can be obtained with the present bath. This efficiency is commensurate with that obtained from an "ideal" copper sulfate plating solution and far exceeds that exhibited by Palladure 100, a typical commercially available plating solution.

Example 2 Change of pH upon plating from electrolytic baths prepared according to the teaching of the present invention was examined in tests carried out as follows. Four 275 ml test baths were prepared according to the teaching of the present invention with bath components in the following proportions: 18 gpl (grams per liter of H2O) palladium as Pd(NH3)2CI2 50 gpl ammonium chloride 50 gpl sulfamic acid Starting pH's of the test baths were set respectively at the following pH's: 7.0, 8.0, 9.0, and 10.0.

Plating onto brass bull panels from each bath was carried out in hull cells maintained at about 740F. Plating current was applied to each cell for five minutes at one amp (still plating), and pH of each solution was measured after plating.

The results obtained appear in Figure 1. An examination of these results shows that when plating was carried out at pH 7.0 or pH 8.0, the pH rose somewhat, requiring addition of acid. At pH 9.0, no change in pH was detected. At pH 10.0, a slight decrease in pH was observed, which would require a small addition of base. Plating at pH's above 10.0 produced generally unsatisfactory coatings.

Example 3 The nature of a palladium coating produced from a bath prepared according to the teaching of the present invention was examined as follows. A brass hull cell panel was suspended in a 2000 ml beaker holding 1 500 ml of the Bath A solution described in Example 1. The panel was plated for 60 amperes minutes (still plating), whereupon additional palladium (as Pd(NH3)2C12) was added to bring the palladium concentration back to 1 8 grams per liter. Suifamic acid was also added at this point to reestablish the pH at 8.5. Plating was then continued for an additional four hours at one amp current, which corresponds to 1 5 amps per square foot.

The plated panel was then removed and inspected. Contrary to expectations for most commercially available electroplating baths, the edges of the panel contained very little roughness or treeing.

*Within experimental error, 100.0% efficiency.

The panel was weighed both before and after plating, and it was determined that 10 grams of palladium were deposited. The panel curved out slightly indicating a moderate tensile stress due to the heavy planting.

The final pH of the plating bath was pH 8.9. In view of the heavy plating carried out from this bath, this very modest 0.4 pH rise illustrates a highly desirable characteristic of the bath.

If the panel were cut into strips and the strip edges polished, close examination of the edges would show that the palladium deposit was even across the plate. A knoop hardness test would show the coating to be sufficiently hard for use on electrical contacts. If the plate were etched and examined microscopically, the grain structure would be fine and relatively free of inclusions.

In this particular case, a portion of the above panel was immersed in a 25% nitric acid solution to dissolve the brass panel. The remaining palladium sheet was from 0.003-0.005 inches thick and very flexible. A portion of this sheet was analyzed and found to contain: copper 0.09% zinc trace tin trace silver 0,03% palladium remainder The latter analysis shows that the palladium coating was very pure, which is a highly desirable characteristic in many applications, including especially where the palladium is coated onto conducting portions of electrical contacts.

Thus, it can be seen from the foregoing detailed specification and examples that a new electroplating bath and a method for its use in the electrodeposition of palladium is disclosed. The bath itself, which is readily formulated, does not require the use of additives to produce outstanding palladium coatings. It is generally trouble-free since it is relatively insensitive to minor variations in operating conditions such as Pd concentration, pH and temperature. Ammonia gas and other undesirable fumes are not released from the bath in significant amounts during plating. The bath exhibits good conductivity, efficiency and throwing power. Finally, the coatings produced from the bath are bright, highly adherent, ductile, and generally pore- and stress-free.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art with various changes in form and details may be made without departing from the spirit and scope of the invention.

Claims (11)

Claims

1. An aqueous electrolytic bath suitable for the electrodeposition of ductile, non-porous palladium coatings comprising: about 5 to 50 grams per liter of a palladosamine salt chosen from the group consisting of palladosamine chloride and palladosamine bromide; about 10 to 1 50 grams per liter of sulfamic acid; and about 10 to 1 50 grams per liter of ammonium chloride; said bath having a pH of about 6.5 to 10.0.

2. The electrolytic bath as claimed in Claim 1 wherein said palladosamine salt is present in an amount of about 30 to 40 grams per liter, said sulfamic acid is present in an amount of about 40 to 60 grams per liter and said ammonium chloride is present in an amount of about 40 to 60 grams per liter.

3. The electrolytic bath as claimed in Claim 1 wherein said pH is about 8.0 to 9.0.

4. The electrolytic bath as claimed in Claim 1 including sufficient base or acid to obtain a pH of about 8.0 or 9.0.

5. The electrolytic bath as claimed in Claim 4 wherein said base is ammonium hydroxide.

6. The electrolytic bath as claimed in Claim 4 wherein the acid is sulfamic acid.

7. A method of electroplating ductile, non-porous palladium coatings upon a workpiece having a conductive surface comprising the steps of: immersing said workpiece in an aqueous electrolytic bath which includes about 5 to 50 grams per liter of a palladosamine salt chosen from the group consisting of palladosamine chloride and palladosamine bromide, about 10 to 1 50 grams per liter of sulfamic acid, and about 10 to 1 50 grams per liter of ammonium chloride, said bath having a pH of about 6.5 to 10.0; maintaining the temperature of said bath at about 60 to 1300 Farenheit; applying a potential across said workpiece and an anode inert to said bath; and, removing said workpiece from said bath.

8. The method as claimed in Claim 7 wherein sulfamic acid is added to said bath to maintain said pH at about 8.0 to 9.0.

9. The method as claimed in Claim 7 wherein said bath is maintained under agitation and said potential provides a still plating current density of from 1 to 100 amperes per square foot.

10. A method of regenerating a contaminated aqueous electrolytic bath having a pH in excess of pH 6.0 and including sulfamic acid, ammonium chloride, a palladosamine salt chosen from the group consisting of palladosamine chloride and palladosamine bromide, and at least a minor amount of contaminants soluble in an acidic medium at about pH 3.0-5.0, comprising of steps of: adjusting said contaminated bath to a pH of about 3.0 to 5.0 with an acid chosen from the group consisting of sulfamic acid and hydrochloric acid to precipitate the palladosamine salt and to dissolve the contaminates: filtering said adjusted bath to separate said precipitated salt: redissolving said precipitated salt in an alkaline medium; and, adding sulfamic acid and ammonium chloride to form a regenerated electrolytic bath.

11. The method as claimed in Claim 10 wherein said acid is sulfamic acid and said alkaline medium is an aqueous ammonium hydroxide solution.