US3174918A

US3174918A - Bright gold electroplating

Info

Publication number: US3174918A
Application number: US84517A
Authority: US
Inventors: Rinker Edwin Cornell; Foulke Donald Gardner
Original assignee: Sel Rex Corp
Current assignee: Occidental Chemical Corp; Sel Rex Corp
Priority date: 1961-01-24
Filing date: 1961-01-24
Publication date: 1965-03-23
Anticipated expiration: 1982-03-23
Also published as: FR1311023A; NL273924A; GB939007A; CH399865A

Description

dillfild Patented Mar. 23, 1965 3,174,918 BRIGHT QQLD ELECTRQPLATING Edwin Cornell Thinker, Morristown, and Donald Gardner l 'oulke, Watchnng, Plainlield, Nah, assignors to Sol-Rex (Jorporation, Nutiey, NJ., a corporation of New .l'ersey No Drawing. Filed lien. 24, $61, Ser. No. 34,517 2 Claims. (Cl. 294- 43) This invention relates to the electrodeposition of gold to obtain bright gold deposits and to the electrolytes for obtaining such deposits.

Among the objects of the invention are to provide a gold plating bath and process which will produce bright gold deposits.

In the past, it has been possible to obtain bright gold deposits from gold cyanide baths, particularly in the case of baths rather high in free cyanide, by the addition of silver ions as a brightening agent. The effectiveness of the silver ion as a brightener appears to be related to the complexing ability of the cyanide ion on the silver whereby a limited amount of silver is free to join the gold atoms in the crystal lattice.

This invention is based on the discovery that silver acts as a brightener, in what are essentially non-cyanide baths providing the silver ion concentration is maintained small by adding complexing or chelating agents to the bath, for example. Under these circumstances, the bath pH may be maintained over relatively wide pH values; 6 to 13 in contrast to the normal bright gold plating baths employing silver as an addition agent where the pH values in practice have been of the order of 12.

Although the baths of the present invention may be operated at a pH of 6l3, there is a definite advantage in being able to operate a bright gold bath at a pH of 6.5 to 7.0, with a low content of free cyanide, especially when plating workpieces such as printed circuit boards where the solvent action of the alkaline cyanide bath on the copper-plastic laminate is a problem.

There is a definite advantage in being able to operate a gold plating bath in the lower portion of the pH range of 6 to 13, for example, at a pH of 6.5 to 7 or even up to 9 or 10. Thus, such, baths having little or substantially no free cyanide or alkali content do not deleteriously effect the copper-plastic laminate of printed circuit boards which are immersed therein for plating.

Among other objects of the invention, therefore, is to provide a process and electrolyte capable of plating bright gold deposits without the presence of any substantial amount of alkali or alkali cyanides.

This invention provides a bath which has relatively low susceptibility to the effect or". foreign ions such as nickel, copper and other base metal ions, which ions normally have a deleterious effect on brightness effected by the silver ion by virtue of the presence of the silver complexing agent which in general will complex higher valent cations more effectively.

Among other objects of the invention, therefore, is to provide a bright gold plating solution containing silver ions which has a low susceptibility to foreign ions.

The objects of the invention are attained by providing an aqueous bath containing about 4-20 g./l. of potas sium gold cyanide, 25-300 mg./l. of silver ion and 5400 g./l. of one or more alkali metal salts. To this hath no free cyanide is added although a limited amount of free cyanide may be set free as the bath is operated.

The alkali metal gold cyanide provides the necessary gold ions for the orderly disposition of the gold, but does not supply sufiicient ions to provide the conductivity required for good commercial operation of the bath. Consequently, alkali metal such as sodium or, preferably potassium salts of organic acids, inorganic and organo-inorganic acids, (re added to provide the nectill essary conductivity. We have found that the alkali metal sulfates, phosphates, nitrates, sulfamates, and the alkali metal salts of such organic acids as citrate, tartrate, lactate, as well as the organo-inorganic acid such as naphthalene sulfonic, benzene sulfonic acids are useful in this connection.

Thus, the bath contains the following ingredients:

l Au(CN) (calculated as Au) 4 to 20 g./l. KAg(CN) (calculated as Ag) 25 to 500 m./l.

Conducting salt 5 to g./l.

Alkali hydroxide Suflicient to produce pH of 6.5l3 preferably pH=6.57.5.

lQNilICN), or K CMCNL,

(optional) Up to 15 g./l.

Chelating agent l to 50 g./l.

The chelating or complexing agent for silver must be such that the silver ion concentration, with 5 to 50 grams per liter of the agent present, will be reduced to no more than that provided by potassium or sodium silver cyanide in the presence of free potassium cyanide. Otherwise, the silver salt must be kept at low concentration and fed frequently. If silver is added as potassium silver cyanide, as silver-sulfate, nitrate or other soluble silver salt, the available silver ions in the cathode film are considerable with the result that rapid deposition of silver along with the gold occurs. This results in a large concentration gradient in the diffusion layer at the cathode and the concomitant rapid supply of silver ions to the cathode which, of course, makes for rapid silver deposition on a continuing basis unless the silver salt concentration is very low. Under such conditions, the gold deposit assumes a brownish, greenish, unattractive appearance. However, the addition of chelating or complexing compounds such as the salts of triglycollamic acid, ethylenediamine tetraacetic acid (EDTA), and amines which complex the silver ion so that supply to the cathode surface is sufiicient to provide just enough silver ions to provide a gold deposit which is bright over wide current density ranges.

Other minor brightening agents, such as potassium nickel and cobalt cyanides may be added to the aforedescribed bath to provide a small increase in brilliance, but in general, these materials are unnecessary and contribute to deposit brittleness.

The practice of this invention can best be described by citing a number of examples of baths which have been found to provide excellent bright gold deposits. In all these examples, a bright gold deposit of a thickness of more than 10 millionths of an inch was obtained. Thus, the term bright gold deposit will be understood to mean deposits which are substantially thicker than flash deposits.

Example 1 In an amount of water sufficient to provide one liter of solution was dissolved:

Silver (as KAg(Ci l) mg K PO gm 6 g[].'l 2 Gold (as KAu(CN) gm 8 The deposit upon a polished brass panel 3 /2" 7; 1" was very bright at 2 a.s.f. and 4 a.s.f. (3 and 3.9 volts), at a pH of 11.0. The efficiency Was 100 mg./ampereminute.

Example 2 in an amount of Water sufiicient to provide one liter of solution was added:

Triglycollamic acid 10 gm.

KOH To pH of 11.0. Gold (as l IAu(CN) 8 gm.

Silver (as l Ag(CN) 100 mg.

In an amount of water sufiicient to provide one liter of solution was added:

Disodium EDTA 10 gm. KOH To pH of 10.

Gold (as KAu(CN) 8 gm.

Silver (as KAg(CN) 1 mg.

Excellent bright deposits were obtained at a current density of 3 and 4 a.s.f.

Example 4 In an amount of water sufficient to provide one liter of solution was added:

Triglycollamic acid 10* gm.

Naphthalene sulfonic acid, Na salt 10 gm.

KOH To pH of 8.

Gold (as KAu(CN) 8 gm.

Silver (as KAg(CN) 100 mg.

Example 5 In an amount of water sufiicient to provide one liter of solution was added:

Triglycollamic acid gm.

Potassium hydroxide To adjust pH of 6.5. Silver nitrate 155 mg.

Gold (as KAu(CN) 8 gm.

Even though all the silver nitrate did not dissolve at once, bright deposits were obtained.

Example 6 To the bath of Example 5, absent the silver nitrate, was added 145 mg./l. of silver sulfate. Again not all the silver salt dissolved at once, but a bright deposit was obtained.

Example 7 To a liter of water were added:

Disodium EDTA g1 l0 KH PO gm 5 Gold (as KAu(CN) gm 8 Silver (as KAg(CN) mg 200 pH 7.0

Bright deposits were obtained at 3 a.s.f.

Example 8 To a liter of water were added:

Potassium dihydrogen phosphate gm 10 Diethylethanolamine gm 5 Gold (as KAu(CN) gm 8 Silver (as KAg(CN) mg 100 pH 7.0

Bright deposits were obtained at 3 a.s.f.

Example 9 To a liter of water were added:

Sodium citrate gm 10 KAI1(CN) gm 11 KAg(CN) mg 200 pH 10.5

Bright deposits were obtained from this bath at 4 a.s.f.,

4 and when 10 g./l. o1"- K NKCNM were added, the deposit was brilliant, even after 40 minutes plating time.

In addition to the examples detailed above, satisfactory deposits were obtained from baths containing the following alternative ingredients.

A. Conducting salts: potassium sulfate, potassium nitrate,

sodium glycolate B. Chelating agents: pyridine diethylethanol amine, 2,4-

lutidine, triethylene tetramine, and

C. Silver salts: silver fluoride, silver acetate.

In all cases, the silver ion was responsible for the bright deposits obtained, and the best deposits and most stable bath resulted when a compound capable of chelating silver was present in the bath.

The above baths were normally operated at room temperature, but they can be operated up to a temperature of 45 C. Above this temperature hazy deposits are apt to be obtained. The baths can be operated at temperatures down to about 15 C.

The normal current density range is 1 to 5 a.s.f. (0.1O.5 amp/43m?) depending upon the gold content, the temperature, and the rate of agitation. At higher temperatures, high gold content and very vigorous agitation, considerably higher current densities can be employed, but the control of commercial baths becomes somewhat more critical.

In general, agitation, either solution volume or mechanical agitation is useful and, actually air agitation is etleetive for this bath in contrast to high free cyanide gold baths where the ethciency drops rapidly when air agitation is used.

Obviously, current knowledge will suggest adaptation of the method of this invention and such adaptations should be comprehended within the meaning of this invention.

We claim:

1. An aqueous electrolyte for the production of bright gold deposits consisting essentially of the following ingredients:

KAu(CN) (calculated as Au) 4 to 20 g./l. KAg(CN) (calculated as Ag) 25 to 500 mg./l. Alkali metal conducting salt (other than cyanide) 5 to g./l. Alkali hydroxide to pH 6.513. Triglycollamic acid 5 to 10 g./l.

the proportion of triglycollamic acid within said limits increasing as the amount of silver is increased within said limits.

2. An aqueous electrolyte for the production of bright gold deposits consisting essentially of the following ingredients:

the proportion of disodium EDTA within said limits increasing as the amount of silver is increased within said limits.

References Cited in the file of this patent UNITED STATES PATENTS Ostrow Nov. 24, 1953 Ostrow et al Jan. 3, 1961