Palladium Plating Procedure and Bath

The invention is a process for electroplating palladium and plating bath for electroplating palladium.

Palladium and palladium alloys are extensively used in a variety of industrial applications including the fabrication of jewelry, optical devices, and electronic circuits and devices. Palladium and its alloys are attractive because of chemical inertness, surface luster, high electrical conductivity, and excellent surface properties, particularly for electrical contacts. In many applications, chemical inertness is highly advantageous for long life and high reliability. This is particularly true for applications in electrical and electronic devices.

Because of chemical inertness and reasonable surface hardness, palladium is especially attractive as an electrical contact material in electrical connectors, relay contacts, switches, etc. Various palladium alloys such as palladium silver are also useful for the same applications. Indeed, because of the increasing cost of gold, palladium becomes more and more attractive economically as a contact material, surface material, and in other applications. In many applications where gold is used, it is often economically attractive to use palladium, provided an inexpensive and efficient method of plating ductile and adherent palladium is available.

Highly desirable is a process for plating palladium from an aqueous solution which is highly reliable despite the possible introduction of foreign ions into the bath which might adversely affect the plating process. This is most critical in plating various types of electrical connectors (switches, relays, etc.) and other electrical devices because of the possibility of introducing copper ions into the plating bath.

Palladium electroplating processes and baths have been described in a number of references including U.S. Pat. No. 1,970,950, issued to E. M. Wise on Aug. 21, 1934 and U.S. Pat. No. 1,993,623, issued to A. R. Raper on Mar. 5, 1935.

The invention is a process and bath composition for electroplating palladium. The process is carried out in an ammoniacal aqueous bath containing an amine palladium complex and benzotriazole or a benzotriazole derivative. It is preferred that the benzotriazole derivative comprise benzotriazole compounds with inert substituents on at least one of the 4, 5, 6 and 7 positions on the benzene ring, typically hydrocarbon substituents up to 5 carbon atoms, chlorine, bromine and nitrate groups. Typical examples are 5-methylbenzotriazole, 5-chlorobenzotriazole, 5-bromobenzotriazole and 5-nitrobenzotriazole. Most preferred are unsubstituted benzotriazole and alkane substituents with up to 5 carbon atoms. Such palladium plating processes and baths yield excellent results even where palladium is plated on copper or copper alloys or where copper is somehow exposed to the plating solution. The process and bath are ideal for use on electrical contact surfaces (switches, relays, connectors) where copper or copper alloy is extensively used.

The FIGURE shows a typical plating apparatus useful for plating palladium.

For convenience, the structural formula for benzotriazole is set forth below. ##STR1##

The invention in its broadest terms is the use of benzotriazole and derivatives of benzotriazole (collectively referred to as benzotriazole compound) to prevent interference in palladium electroplating processes by introduction of copper ions into the plating bath. Benzotriazole is useful as are many derivatives, particularly those with relatively inert (alkane, halide, etc.) substituents attached to at least one of the benzene ring (4, 5, 6 and 7) positions. More than one compound may be used. Concentration of the benzotriazole compound may vary from 0.1 mg/l to saturation. Preferred is a range from 2 mg/l to 20 mg/l. Less than 2 mg/l gives protection for an inconveniently short time; over 20 mg/l does not materially increase the protection or time of protection and is wasteful of material. The benzotriazole compound may be added to the palladium bath and the palladium plating process carried out in the presence of the benzotriazole or the benzotriazole compound applied to the surface to be plated before exposure to the plating bath.

In broad terms, the electroplating bath should contain a source of palladium in an amine complex and some free ammonia to adjust the pH and stabilize the palladium amine complex.

The concentration of palladium may vary over large limits provided plating takes place. Concentration ranges (in terms of palladium metal) between one mg/l and saturation are useful but more usually the concentration range is between 10 g/l and saturation. For certain applications, especially where rapid plating is desirable, a concentration range between 50 g/l or even 100 g/l and saturation is preferred. Economic considerations often limit the concentration rather than plating considerations. Conducting salts are often added such as ammonium chloride, ammonium phosphate, ammonium sulfamate, ammonium formate, ammonium sulfate, ammonium carbonate, etc. Aqueous ammonia is added to adjust pH and stabilize the palladium complex.

The pH may vary over large limits but is typically between 7 and 12, with 8-10 preferred. Such a pH range stabilizes the palladium--amine complex, yields high plating rates with good adhesion and minimizes attack on the substrate being plated. Stirring or agitation of the bath is usually beneficial, particularly for higher plating rates. The pH is adjusted by the addition of aqueous ammonia solution.

Some typical ammoniacal palladium baths are as follows:

Pd(NH.sub.3).sub.2 (NO.sub.2).sub.2

Benzotriazole compound

NH.sub.4 NO.sub.3 (optional)

NaNO.sub.2 (optional)

Aqueous ammonia to pH 8-10

Pd(NH.sub.3).sub.4 (NO.sub.3).sub.2

Benzotriazole compound

Conducting salts (optional)

Aqueous ammonia to pH 7-10

Pd(NH.sub.3).sub.2 Cl.sub.2

Benzotriazole compound

Ammonium salts (optional)

Aqueous ammonia to pH 8-12, 8.8 to 9.2 preferred.

Pd(NH.sub.3).sub.4 Br.sub.2

Benzotriazole compound

Aqueous ammonia to pH 9-10, 9.5 to 10 preferred.

Other palladium complexes such as the corresponding sulfate, phosphate, tartrate, citrate, oxalate and carbonate also may be useful.

The preferred plating bath contains Pd(NH.sub.3).sub.4 Cl.sub.2 as the source of palladium. Amounts of at least 10 g/l (in terms of palladium metal) are preferred with (optionally) various salts such as NH.sub.4 Cl added as conducting salts. Sufficient aqueous ammonia is added to yield a pH between 8 and 10, preferably 9.0. Higher concentrations of Pd(NH.sub.3).sub.4 Cl.sub.2 are more preferred, say greater than 20 g/l or even 100 g/l. Increased concentration of the palladium complex reduces the amount of conducting salts (i.e., NH.sub.4 Cl) that can be dissolved in the bath. Indeed, a particular advantage of the invention process is that larger concentrations of palladium species can be used with superior plating results. Where very high concentrations of palladium salt is used, the addition of conducting salt such as NH.sub.4 Cl may be omitted.

An excellent bath for the practice of the invention is 80 g/l (in terms of Pd metal) of Pd(NH.sub.3).sub.4 Cl.sub.2, 20 mg/l 6-methylbenzotriazole and sufficient aqueous ammonia to pH=9.+-.0.2.

It is contemplated that the bath may be replenished. For example, palladium may be added (either in solid or solution form) as palladium salt (PdCl.sub.2) or complex (Pd(NH.sub.3).sub.4 Cl.sub.2). Also, the benzotriazole may be added separately or replenished. Generally, ammonia water is added to stabilize the complex or adjust the pH to a desired range. The bath is usually supplied in solution form but may be supplied either in concentrated form for dilution with water or aqueous ammonia or supplied as solid for dilution to the bath composition.

The compound 2-mercaptobenzothiazole and related compounds and derivatives are also useful in preventing copper poisoning of palladium baths.

The FIGURE is a schematic view of a typical plating apparatus 10, showing plating cell 11 with working electrode 12 and counter electrode 13. Also shown is a reference electrode 14 together with a voltmeter 15 for monitoring the plating potential and a potentiostat 16. The potentiostat supplies sufficient current as measured by the ammeter 17 to maximize plating current without exceeding a predetermined plating potential. Plating solution is pumped into the plating cell 11 by means of a liquid pump 18. A reservoir 19 receives bath solution coming out of the plating cell. Typically, the potentiostat supplies sufficient voltage so that current passes from the working electrode through the plating solution and into the counter electrode. Current is controlled by the potentiostat so as to fix the working electrode potential at a preset value. Naturally, where close control of the working electrode potential is not necessary, the potentiostat can be replaced by a source of electrical energy of sufficiently high voltage and current to carry out the plating procedure.