US3466233A - Gold plating - Google Patents

Description

United States Patent U.S. Cl. 204-43 6 Claims ABSTRACT OF THE DISCLOSURE A process and electrolyte for electrodepositing bright gold. The bath used is an aqueous solution consisting essentially of about 1 to 100 g./l. of alkali metal gold cyanide, to 200 g./l. of alkali metal or ammonium pyrophosphate, and 0.0l0.5 g./1. of potassium silver cyanide. The bath has a pH of 6.5-8.5 controlled by additions of phosphoric acid.

This application is a continuation-in-part of application Ser. No. 356,657 filed Apr. 1, 1964, now abandoned.

BACKGROUND OF THE INVENTION The electrodeposition of gold is usually carried out from electrolytes which are strongly alkaline in character and which are normally based upon aqueous solutions containing cyanide anions and the cations of gold, and an alkali metal such as potassium, e.g. an aqueous solution of potassium gold cyanide KAu(CN) containing free potassium cyanide.

It has long been known that gold can be electrodeposited from a substantially neutral or even acidic aqueous electrolyte and, more recently, gold plating baths of the non-alkaline type have become of commercial interest because of modern commercial requirements, particularly in relation to plating on printed circuits and the like. In such applications, strongly alkaline plating baths cannot be tolerated since they have a deleterious effect on the support and/ or resist used in printed circuit manufacture. In such applications, strongly alkaline cyanide solutions are harmful to the adhesives used in the manufacture of printed circuits, causing deterioration in the strength of the bond between the copper foil circuitry and the printed circuit board.

It is an object of the invention to provide an electrolyte solution suitable for use in the deposition of gold at substantially neutral pH. A further object is to provide a neutral gold electrolyte suitable for deposition of bright gold, characterized by high hardness and low stress and porosity. A further object is to provide a substantially neutral electrolyte for gold deposition which is stable over long periods of time and provides bright gold deposits free of harmful codeposits.

These and other objects of the invention are provided by electrodepositing gold from a novel electrolyte containing 1 to 100 grams of alkali metal gold cyanide per liter of solution, 5 to 200 grams of alkali metal pyrophosphate per liter of solution, the pH of the solution being adjusted to a value in the range of from 6.5 to 8.5, prefer ably around 7. Preferably, the electrolyte solution contains from 5 to 50 grams of potassium gold cyanide per liter of solution.

The electrolyte solution of the present invention should not be confused with prior art gold plating solutions containing phosphoric acid, as the free acid or in the form of salts thereof. Thus, for example, it is known to electrodeposit gold from aqueous solution of potassium phosphate, and from substantially neutral solutions 3,466,233 Patented Sept. 9, 1969 "ice containing admixtures of alkali metal primary phosphates and alkali metal secondary phosphates. (Volk, U.S. Patent No. 2,812,299.)

Unlike these prior art gold electroplating baths, all of which contain substantial quantities of free phosphate ion, the present bath employs an alkali .metal pyrophosphate, which in the case of potassium salt, for example, is a well-defined crystalline substance having the formula K P O The pyrophosphate salts provide gold electrolytes far superior to those obtained by use of phosphate salts, probably because of the complexing properties of the pyrophosphate ion, which can form complexes with metal impurities in the bath and prevent deposition thereof which detracts from the quality and appearance of the gold plate.

SUMMARY OF THE INVENTION It has further been found that the maintenance of the pH value of the bath during the electrolytic deposition of gold therefrom between about 6.5 and 8.5 is necessary, to maintenance of the quality of the pyrophosphate bath. Like other polyphosphates, alkali metal pyrophosphates gradually convert to the orthophosphate, and the rate of such conversion increases rapidly with increased acidity of the aqueous medium. For best results, and to produce a bath of long life and stability, the bath of the present invention is operated at or near a neutral pH.

The solutions prepared by dissolving alkali metal pyrophosphate and alkali gold cyanide in water generally have a pH somewhat higher than desired in the practice of this invention, e.g. such solutions are at pH -105 The pH of the solution can be adjusted before and/or during use by the addition of suitable amounts of water-soluble acid acting substances, such as phosphoric or polyphosphoric acid, alkali or ammonium salts of weak organic acids, e.g. citrates, formates, acetates and the like; acid salts of inorganic acids such as sulfamic acid, boric acid, etc.

A second variable which requires control in the utilization of the baths of this invention is the plating temperature. The rate of conversion of pyrophosphate to phosphate increases rapidly with increased temperature, and it is preferred therefore to operate the bath at room temperature or slightly above. In general, temperatures between about 20 C. and 60 C., preferably 25 to about 40 C. are employed. Higher temperatures, e.g. up to about C. may be used, but the stability and life of the bath will thereby be appreciably shortened.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Gold is provided to the bath in the form of the double salt of alkali cyanide and gold cyanide, e.g. as potassium gold cyanide KAu(CN) Other alkali metal gold cyanides such as sodium gold cyanide may be used.

While the pyrophosphate salt is preferably added as the potassium salt because of its higher solubility in water, the other alkali metal or ammonium salts of pyrophosphoric acid can be employed]. Thus, sodium pyrophosphate, ammonium pyrophosphate, and the like are effective. The alkali metal pyrophosphate salt is employed in an amount from about 5 to 200 g./l. of solution, preferably from 10 to 100 g./l.

The baths of the present invention can be employed for the deposition of pure gold, or for the deposition of gold along with one or more alloying metals for hardening and/or coloring of the gold plate. Any of the usual alloying metal salts may therefore be added in the baths prepared in accordance with the present invention. For example, water soluble complexes of cyanides of silver, copper, nickel, and/or cobalt may be added to the bath. These complexes are generally complexes of an alkali metal or ammonium, the metal to be alloyed, and the cyanide ion. Silver is particularly effective as an additive in the baths of the present invention; employed in an amount of from 0.01 to about .5 g./liter, eg as KAg(CN) and provides hard, bright deposits having relatively low stress and exceptionally smooth grain structure.

The operating conditions suitable for any particular plating application may be readily determined by one skilled in the art. Thus, the average cathode current density used will depend upon the desired surface appearance and on the shape and size of the article to be plated. It has been found preferable to employ cathode current densities below 20 amperes per square foot, preferably 3 to 8 a.s.f. under normal direct current plating conditions. The electrolyte is normally employed with a working voltage of from 1.5 to 3.0 volts. It is preferred to operate the electrolyte solution using insoluble anodes. For this purpose, pure platinum anodes may be used, but an entirely satisfactory and less expensive alternative is an anode made from platinum-coated titanium or stainless steel.

When using insoluble anodes of the above type, it is necessary to replenish the gold deposited from the electrolyte solution by addition of fresh gold compound. This may conveniently be added in the form of a liquid (aqueous) concentrate. Preferably, the pyrophosphate salt is replenished from time to time, to replace drag-out. Similar periodic addition of alloying or brightening agent is made to maintain the desired physical characteristics of the gold plate deposited.

The following examples are typical of the types of solutions from which gold electrodeposits having the properties indicated may be obtained.

EXAMPLE 1 An electrolyte solution was prepared as follows:

G. Potassium gold cyanide 93 Potassium pyrophosphate 75 Potassium silver cyanide 0.1

Water to make one liter.

The solution as made up had a pH of 10, and before use was adjusted and maintained thereafter at pH 68-75 by addition of minor amounts of phosphoric acid. The electrolyte solution was used to plate base metal cathodes With .0001 inch of gold in 8 minutes at 35 40 C. and with a current density of amperes per square foot, the electrolyte being agitated during the plating period. A mirror bright gold electroplate of 23 kt.+ having low stress, smooth grain structure and a Knoop hardness of approximately 110 was obtained.

This electrolyte can be operated at higher temperature, but as temperature goes above 60 C, loss of brightness results.

EXAMPLE 2 Gold electroplating solutions were prepared according to the following formulations in which the salts in each case were dissolved at 5060 C. in water to make one liter of solution.

FORMULA A 40 g. potassium gold cyanide 7.5 g. potassium pyrophosphate Ammonium citrate, suflicient to provide a pH of 6.87.5.

FORMULA B 20 g. potassium gold cyanide 50 g. sodium pyrophosphate Polyphosphoric acid, sufiicient to provide a pH of 6.8-7.5.

Each of the electroplating solutions prepared as above is used to electroplate gold at a temperature of about 40 C. to provide a substantially bright, hard and smooth gold electrodeposit; but not the mirror-brightness nor hardness obtained with the potassium silver cyanide additive of Example 1.

EXAMPLE 3 Into an amount of water suflicient to form a one liter solution are dissolved:

7.5 g. potassium gold cyanide 5.0 g. nickel as potassium nickel cyanide 150 g. potassium pyrophosphate Polyphospheric acid, sufficient to provide a pH of 8.5.

This solution can be utilized for deposition of gold of lesser brightness than is obtained from solutions not containing nickel or an additive other than silver. Preferred conditions for electroplating are a temperature of 40-60 C., and current density of 3-8 amperes per square foot. Thus, for operation in the neutral pH range the additive nickel is not a brightener for the subsequently obtained deposit.

Additional electroplating solutions were prepared according to the following examples:

EXAMPLE 4 E- solution as used commercially and containing 0.15 gram per liter silver. Ingredient concentrations are:

Gold (as KAu(CN) g./1 12 K P O (potassium pyrophosphate) g./l 50 Silver (as KAg(CN) g./l 0.15 Superphosphoric acid ml./l 4 pH 7 Temperature of operation C 35 Current density a.s.f. 5 Agitation Moderate to strong Time of plating minutes 10 A polished piece of brass was cleaned in the conventional manner and plated as per the above conditions. The electro-deposit of gold obtained was mirror bright, of low stress, of a highly fine grain structure and hardness greater than on the Knoop scale.

EXAMPLE 5 The conditions used were identical with conditions in Example 1 except that nickel (as potassium nickel cyanide) was used, the same metallic concentration being employed.

The electrodeposit obtained was extremely dull.

EXAMPLE 6 The conditions used were the same as in Example 1 with the exception that cobalt (as potassium cobaltic cyanide) was used, the same metallic concentration being employed.

The electrodeposit of gold obtained was dull over more than 60% of the surface of the cathode with some lustre in the low current density areas.

EXAMPLE 7 The conditions used were the same as in Example 1 with the exception that palladium (as potassium palladium cyanide) was used in the same concentration as in Example 1.

The electrodeposit of gold obtained was dull over the whole surface of the cathode.

The above experiments clearly show that, all other conditions being the same, that only in the case where silver is used (as distinguished from nickel, cobalt and palladium) is it possible to obtain mirror bright deposits of gold for operations in the neutral pH range.

From the above examples, it will be noted that nickel, cobalt and palladium are not effective brighteners since in the neutral pH range they do not co-deposit uniformly with the gold during the plating process because pyrophosphates of these metals in the neutral pH range are not stable and convert rapidly on usage of the bath to cyanide complexes thereby tying these metals so strongly that they do not co-deposit suificiently to provide any efiective improvement. This is not the case with silver when present in the bath as a potassium cyanide complex. Furthermore, the preferred solution excludes organic compounds such as tartrates since it has also been discovered that the inclusion of such organic compounds from organic decomposition products aifect the plated surface during the plating operation. One important advantage of the neutral gold plating baths in accordance with the invention is a very high plating efficiency of about 90% which is of commercial importance in the plating operation. In contrast, acid pyrophosphate, polyphosphate and phosphate baths are less desirable with only about 30% plating efliciency.

As compared with gold plating solutions without the silver additive, the gold-silver alloyed co-deposition occurs with a highly refined grain structure, an increase in plate brightness, and an increase in hardness properties to about 85 to about 150 Knoop hardness.

As compared to the deposits of Examples 5, 6 and 7, the solution of the invention provides mirror bright deposits in contrast to the dull deposits with the nickel, cobalt or palladium additives and has an electrical resistivity and contact resistance not substantially greater than pure gold in contrast to the increased electrical resistance of deposits containing to some degree the nickel, cobalt or palladium additives. Thus, for printed circuit application the gold-silver alloyed deposit is quite favorable as opposed to the unfavorable deposits containing nickel, cobalt or palladium.

What is claimed is:

1. An electrolyte aqueous solution for use in the electrodeposition of gold consisting essentially of from about 1 to about 100 grams of alkali metal gold cyanide and from about 5 to about 200 grams of a compound selected from the group consisting of alkali metal and ammonium pyrophosphate per liter of solution, and from 0.01 to about 0.5 gram of potassium silver cyanide per liter of solution, the pH of the solution being adjusted by phosphoric acid to a value in the range of from 6.5 to 8.5.

2. An electrolyte solution as claimed in claim 1 wherein the solution contains from 5 to grams of potassium gold cyanide per liter.

3. An electrolyte solution as claimed in claim 1 wherein the solution contains from 10 to 100 grams of alkali metal pyrophosphate per liter of solution.

4. A process for the electrodeposition of bright gold wherein an electrolyte solution as claimed in claim 1 is employed at temperature between about 20 C. and about C.

5. A process according to claim 4 wherein the temperature is in the range of from 25 C. to 40 C.

6. A process according to claim 4 wherein a cathode current density not exceeding 20 amperes per square foot is used.

References Cited UNITED STATES PATENTS 2,812,299 11/1957 Volk 204-43 2,967,135 1/ 1961 Ostrow et al. 204-43 FOREIGN PATENTS 1,331,064 5/1963 France. 1,331,065 5/1963 France.

354,643 7/1961 Switzerland.

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R. 204--44, 46