SE2250388A1 - Compositions, methods, and preparations of cyanide-free gold solutions, suitable for electroplating of gold deposits and alloys thereof - Google Patents

Abstract

The present disclosure relates to a gold electroplating composition comprising one or more sources of Au(I) or Au(III), a strong base for solution make-up, an acid for pH adjustment and for providing conductivity, and the combination of at least two different mercapto-ligands, selected from the groups of mercapto-triazoles, mercapto-tetrazoles, and mercapto-thiadiazoles. Also disclosed herein are methods for electroplating gold or a gold alloy.

Description

Compositions, methods, and preparations of cyanide-free gold solutions, suitable for electroplating of gold deposits and alloys thereof TECHNICAL FIELD The present invention relates to the environmentally friendly replacement of cyanide in solutions suitable for coating in general and electroplating in particular. More specifically, the present invention provides information for the preparation of fully stable cyanide-free gold solutions, which can be used either directly or in combination with already existing solutions suitable for plating, in particular electroplating of difficult to plate substrates.

BACKGROUND Gold electroplating has been conventionally used in jewelry but found, due to its excellent electric characteristics and corrosion resistance, a broad application range in the electronics industry over the last decades.

To date, most gold electrolytes are still based on cyanide, which is mainly owing to its high stability constant as compared to other well-known gold complexes used in electroplating, such as sulfite, thiosulfate and thiourea and its derivatives.

Over the last two decades, mercapto-triazoles and mercapto-tetrazoles have found increasing interest in stabilizing low valent metal cations for electroplating purposes. Examples include copper (Egli SE 2150946-8), copper in white bronze plating (Egli and Chen US 7'780'839, Foyet et al. US 2016/298249) and silver plating (Foyet at al. US 10'889'907). ln gold electroplating this class of ligand has not yet been used as a major complexing agent, but only as anti-immersion or anti-displacement agent to reduce unwanted immersion plating on reactive substrates (Zhang et al EP 2'309'036, Breitfelder et al. US 2017/0159195) with a concentration maximum of 1 g/L of the mercapto compound. As with other low valency cations, this limited use can be attributed to the scarce solubility of such complexes in aqueous media. However, as could be shown recently with analogous copper complexes (Egli SE 2150946-8), the low solubility can be resolved by combining different types of mercapto-ligands.

SUMMARY OF THE INVENTION A gold electroplating bath consists of one or more gold +l and/or gold +lll sources and a combination of at least two different compounds, chosen from the groups of mercapto- tetrazoles, mercapto-triazoles and mercapto-thiadiazoles, designated in the following as mercapto-ligands. At least one mercapto-ligand provides one co-ordination site to gold, whereas at least another mercapto-ligand provides two co-ordination sites to gold bearing 2 thereby the potential of cross-linking two gold cations. The one-co-ordination site mercapto-ligand and the two-co-ordination site mercapto-ligand are preferably, but not limited, applied in a 1 to 1 molar ratio. A gold electrolyte as described above can be combined with any source of the following metals: copper, silver, zinc, nickel, palladium, platinum, cobalt, rhodium, iron, ruthenium, tungsten, rhenium, osmium, tin, indium, and bismuth. ln one aspect, the present invention provides a gold electroplating composition comprising one or more sources of Au(|) or Au(|||), a strong base for solution make-up, an acid for pH adjustment and for providing conductivity, and the combination of at least two different mercapto-ligands, selected from the groups of mercapto-triazoles, mercapto-tetrazoles, and mercapto-thiadiazoles. ln one embodiment thereof, the gold concentration in the electrolyte ranges from 0.05 to 50 g/L and more preferably from 0.5 to 15 g/L. ln one embodiment, at least one of the mercapto-ligands has no additional nitrogen- or sulfur-comprising functional group suitable for co-ordination and can therefore be considered as mono-dentate or terminal ligand. ln one embodiment, at least another one of the mercapto-ligands comprises an additional nitrogen- or sulfur-comprising functional group, suitable for co-ordination and can therefore be considered as bi-dentate, not chelating but rather cross-linking ligand. ln one embodiment, the stoichiometric ratio of the cross-linking mercapto-ligand and the terminal mercapto-ligand equals or closely equals 1 to 1. ln one embodiment, the concentration of each individual mercapto-ligand is applied in any concentration that is allowed by the solubility of the individual compound, but mainly in the range of 0.1 to 500 g/L, and preferably between 10 and 150 g/L. ln one embodiment, at least one chemical with at least one thioether and/or thioether functionality is added to the formulation. ln one embodiment, the concentration of the thioether and/or thiourea compound is applied in any concentration that is allowed by the solubility of the individual compound, but preferably from 0.5 to 500 g/L and more preferably from 1 to 200 g/L. ln one embodiment, the pH ranges from 0 to 14. ln another aspect, the present invention provides to a method of electroplating gold or a gold alloy using a formulation as defined herein on a substrate. ln one embodiment of the method aspect, the substrate is a magnesium or magnesium alloy substrate. ln one embodiment of the method aspect, the substrate is a zinc or zinc alloy substrate. ln one embodiment of the method aspect, the substrate is an aluminum or aluminum alloy substrate. ln one embodiment of the method aspect, the substrate is an iron or steel substrate. ln one embodiment of the method aspect, the substrate is a copper or copper a||oy substrate. ln one embodiment of the method aspect, the substrate is a nickel or nickel a||oy su bstrate.

DETAILED DESCRIPTION OF THE INVENTION The purpose of this invention is the reliable and fully adherent application of electroplates, more specifically gold electroplates from a nontoxic electroplating bath that is substantially free of cyanide, while providing the same deposit quality, which is expected from cyanide- based electrolytes. lt has been shown in the past that mercapto-ligands, as defined above, have the potential to substitute cyanides, constituting low-valent complexes of sufficiently high stability for electroplating. The prerequisite for a successful application of metal-mercapto-ligand compounds (MMLC) includes, however, a sufficiently high metal concentration to allow economically reasonable plating rates, preferably at different pH- values. ln spite of favorable stabilities of MMLCs, their low solubility has been a limiting factor for a successful technical implementation. lt has now been observed that the combination of two different mercapto-ligands has a very favorable influence on the solubility of MMLCs, particularly if one of the mercapto- ligands has cross-linking and another mercapto-ligand has terminal binding properties to the low-valent metal center. The stoichiometry of the cross-linking to the terminal mercapto-ligand is preferably 1 to 1. The solubility of the MMLCs can further be improved by the addition of a thioether and/or a thiourea compound, without the necessity to care about an exact stoichiometric ratio in that case. There is also no need to discriminate between aurous or auric gold sources. Trivalent gold is readily reduced to the monovalent oxidation state upon the addition of the corresponding gold salts.

Sources of gold ions include both aurous and auric metal salts, such as, but not limited to, Au(l)Cl, Au(lll)Cl3, HAu(lll)Cl4, NaAu(lll)Cl4, KAu(lll)Cl4, (NH4)Au(lll)Cl4, Au(lll)Brs, HAu(lll)Br4, Au(l)l, Au2(lll)O3, Au(lll)(OH)3, NaAu(lll)(SO3)2, Na3Au(l)(S2O3)2, HAu(lll)(NO3)4, Au(lll)acetate, Au(l)thiomalate, and any method to generate in situ mono- or trivalent gold solutions from solid gold. All gold sources can be applied in their anhydrous as well their hydrated forms. Aurous or auric salts can be introduced in an amount into the electrolyte that the gold concentration ranges from 0.05 to 50 g/L, and more preferably from 0.5 to 15 g/L. 4 The core of this invention is the combined application of two different mercapto-ligands, one cross-linking and one terminal in an approximate 1 to 1 stoichiometric ratio. Each one by itself shows very high complex stability with gold (I) and in some cases even relatively good initial solubility in water. However, over time precipitation or gel formation can be observed, pointing to a dynamic mechanism, where different intermediate compounds are soluble, but one (not necessarily the most stable one) is insoluble, consuming ultimately all other species in the equilibrium. This problem is largely reduced by the combination of t\No different mercapto-ligands. lt is possible that already the combination of two different mercapto-ligands reduces the formation-probability of a low solubility product. lt has been observed, however, that this effect is particularly pronounced by the simultaneous addition of a cross-linking and a terminal mercapto-ligand. Mercapto-ligands include mercapto- triazoles, mercapto-tetrazoles and mercapto-thiadiazoles and have the general formula, Y==N where X is either S and Y is CR (thiadiazole), X is NR and Y is CR (triazole), or X is NR and Y is N (tetrazole). R can be hydrogen, a saturated or unsaturated organic side chain, but also any heteroatom or a salt forming cation.

Derivatives of mercapto-triazoles include, but are not limited to, 1H-1,2,4-triazole-3-thiol, 4-methyl-4H-1,2,4-triazole-3-thiol, 4-amino-5-hydrazino-1,2,4-triazole-3-thiol and 5-(4- pyridyl-1H-1,2,4-triazole-3-thiol. 4-methyl-4H-1,2,4-triazole-3-thiol is shown as an illustrative example of a mercapto-triazole compound: SH HsCgNÅN gN/ Derivatives of mercapto-thiadiazoles include, but are not limited to, 5-methy|-1,3,4- thiadiazole-2-thiol, 2,5-dimercapto-thiadiazole and 5-amino-1,3,4-thiadiazole-2-thiol, the latter being shown as an illustrative example of a mercapto-thiadiazole compound: SH H2N Derivatives of mercapto-tetrazoles include but are not limited to 5-mercapto-1H-tetrazole- 1-acetic acid, 5-mercapto-1H-tetrazole-1-methanesulfonic acid, 1-methyl-1H-tetrazole-5- thiol, 5-mercapto-1-phenyl-1H-tetrazole, 1-(2-dimethylaminoethyl)-5-mercapto-1H- tetrazole, 1-(2-diethylaminoethyl)-5-mercapto-1H-tetrazole and 1-(2-hydroxyethy|)-5- merca pto-1 H-tetrazole. SH t *vw N \ N H3C/ t NïN All mercapto-ligands can be used in any concentration that is allowed by the solubility of the individual compound, but mainly in the range of 0.1 to 500 g/L, and preferably between 5 and 150 g/L.

The gold or gold alloy electrolyte may further include one or more compounds out of the class of thioethers. Such compounds may extend the applicable gold concentration range and may further specify the applicable pH range of the electroplating bath. These substances may have one or more thioether functionalities, with the general formula, S R R' Where R and R' are aliphatic or arylic carbons, Which by themselves may have other functional groups. Furthermore, the thioethers may be open-chained, branched, or cyclic. Typical thioethers as described above are 2,2'-dithioethanol, 2,2'-dithioacetic acid, 3,6- dithia-1,8-octanediol or 1,4,7-trithiacyclononane. As observed With mercapto-ligands, the upper concentration limits of the thioether compounds are given by the solubility of the individual substances. Reasonable concentrations range from 0.5 to 500 g/L, and more preferably from 1 to 200 g/L.

Analogously to thioether-compounds, thiourea substances can be introduced to the electrolyte, the general formula of which are depicted below. R, R', R", R"' are hydrogen, aliphatic or arylic carbons, which by themselves may have other functional groups. Furthermore, the thioureas may be open-chained, branched, or cyclic. Typical members are thiourea, N,N'-dimethyl-thiourea, N,N'-diethyl-thiourea and N-allyl-thiourea.

S R|RN NRIIRIII As described with mercapto-ligands and thioethers, the upper concentration limits of the thioureas are given by the solubility of the individual substances. Reasonable concentrations range from 0.5 to 500 g/L, and more preferably from 1 to 100 g/L.

As well known in the art, wetting agents are added to the plating solution in a concentration from 0.1 to 20 g/L. Conveniently, the same type of wetting agents can be used that are known from gold cyanide plating, e.g., alkyl ether phosphates and sulfates, and alkoxylated amine oxides.

For the make-up of an electrolyte, it is advised to pre-dissolve the mercapto-ligands in a one-to-one stoichiometry in alkaline water having a pH-value of 12 or higher, containing any chemical with strong alkaline character, but preferably the well-known KOH or NaOH. To such a solution an appropriate amount of a water-soluble gold compound is added, either pre-dissolved in water or as a solid. Both mercapto-ligands should be in at least a fivefold excess compared to the final gold concentration for plating at pH above 12. Electroplating at neutral or acidic pH require higher mercapto-ligand excess. Too high gold concentrations result in greenish to blueish gold plates from such under-stabilized solutions.

After the full dissolution of the gold compound in the alkaline mercapto-ligand solution, the electroplating bath can be acidified by any known moderate to strong proton donator and in particular by oxo-acids, such as sulfuric acid, alkyl sulfonic acids, aryl sulfonic acids, phosphoric acid, oligomeric phosphoric acids, phosphorous acid, organo-phosphonic and -biphosphonic acids, e.g., etidronic acid, boric acid, nitric acid, amidosulfonic acid and 7 carboxylic acids, such as formic acid, acetic acid, lactic acid, succinic acid, to mention only a few typical members. The final pH of the plating solution can be anywhere between 0 and 14. lt is also in the scope of the invention that any type of pH-buffering compound, namely aromatic nitrogen compounds, e.g., imidazole, or Good's buffers to stabilize near-neutral pH-values, can be introduced into the electroplating solution.

Optionally, the bath can be combined with allyl-type brighteners such as butyne-diol or propargylic alcohol and their derivatives. Carbon-disulfide-adducts, such as trithio- carbonate, dithio-carboxylic acids, xanthogenates and carbamates can also be introduced as grain refiners. Also well-known in the art metallic grain refiners may be included into the electrolyte such as selenium, tellurium, antimony, bismuth, cobalt, thallium, indium, and others in the form of their commercially available salts. The concentration range of brighteners and grain refiners is between 0.5 to 1000 ppm, but mostly between 5 and 100 ppm.

A gold bath is preferably prepared by the method above in a tank, suitable for electroplating. Although not common in practice, pure gold anodes can be used. Preferably, however, platinized titanium anodes or mixed metal oxide anodes, well-known from gold cyanide plating, are used.

EXAMPLE 1 As indicated above, several methods for the in situ preparation of a gold solution can be used for the make-up of a gold electrolyte. ln a well-ventilated hood, for instance, 1 liter of a gold cyanide plating solution (2 g/L Au) was refluxed and concentrated to one fifth of its original volume. The concentrate was then digested by 30 milliliter nitric acid and 100 milliliter hydrochloric acid. After the evolution of nitrous oxide gases has stopped, the solution was allowed to cool and a yellow slurry with lemon-yellow needle-like crystals of presumably HAu(lll)Cl4 formed. The aqueous mother liquor has carefully been decanted and the needles have been collected without further purification.

At ambient temperature and under vigorous stirring the yellow needles have been added in several portions to an aqueous solution of 11.79 g 3-Amino-5-mercapto-1,2,4-triazole, 11.69 g 4-Methyl-1,2,4-triazole-3-thiol, and 18 g KOH. Upon every addition the solution turned shortly dark yellow and immediately changed back to the original yellow color. The pH of the clear solution was then adjusted to 11 and used without further additions for plating on a plain brass and on a bright nickel-plated brass Hull cell panel at room temperature, paddle agitation and at 1 A current for 5 minutes. The resulting panel showed an evenly bright gold deposit over the whole current density range.

EXAMPLE 2 An electrolyte of the following composition has been prepared, by mixing the mercapto- ligands and KOH first, followed by the addition of the gold solution and the wetting agent and concluded by pH adjustment: Sodium gold sulfite solution (100 g/L Au) 20 ml/I (2 g/l Au) 3-Amino-5-mercapto-1,2,4-triazole 11.7 g/l 4-Methyl-1,2,4-triazole-3-thiol 11.6 g/l KOH 18 g/l Polyoxyethylene alkyl phenyl ether phosphate 0.5 g/l Methanesulfonic acid 85% to a pH of 12 Subsequently, the solution was used in a Hull cell to plate polished brass panels and polished iron panels applying the following parameters: ambient temperature, paddle agitation, 1 A per panel and mixed metal oxide anodes. After conventional pre-treatment (2 minutes cathodic electrocleaning, 30 seconds alkaline pickling for brass, diluted hydrochloric acid to remove the protecting zinc for iron panels) the panels were plated for 5 minutes, resulting in a deep yellow bright gold plate over the whole current density range.

EXAMPLE 3 An electrolyte of the following composition has been prepared, by mixing the mercapto- ligands and KOH first, followed by the addition of the gold solution and the wetting agent and concluded by pH adjustment: Sodium gold sulfite solution (100 g/L Au) 10 ml/I (1 g/l Au) 3-Amino-5-mercapto-1,2,4-triazole 11.7 g/l 4-Methyl-1,2,4-triazole-3-thiol 11.6 g/l KOH 18 g/l Polyoxyethylene alkyl phenyl ether phosphate 0.5 g/l Methanesulfonic acid 85% to a pH of 7.5 Subsequently, the solution was used in a Hull cell to plate polished brass panels and polished iron panels applying the following parameters: ambient temperature, paddle agitation, 1 A per panel and mixed metal oxide anodes. After conventional pre-treatment 9 (2 minutes cathodic electrocleaning, 30 seconds alkaline pickling for brass, diluted hydrochloric acid to remove the protecting zinc for iron panels) the panels were plated for 5 minutes, resulting in a deep yellow bright gold plate over the whole current density range.

EXAMPLE 4 An electrolyte of the following composition has been prepared, by mixing the mercapto- ligands and KOH first, followed by the addition of the gold solution and the wetting agent and concluded by pH adjustment: Sodium gold su|fite solution (100 g/L Au) 4 ml/I (0.4 g/l Au) 3-Amino-5-mercapto-1,2,4-triazole 11.7 g/l 4-Methyl-1,2,4-triazole-3-thiol 11.6 g/l KOH 18 g/l Methanesulfonic acid 85% to a pH of 2 Subsequently, the solution was used in a Hull cell to plate polished brass panels and polished iron panels applying the following parameters: ambient temperature, paddle agitation, 1 A per panel and mixed metal oxide anodes. After conventional pre-treatment (2 minutes cathodic electrocleaning, 30 seconds alkaline pickling for brass, diluted hydrochloric acid to remove the protecting zinc for iron panels) the panels were plated for 5 minutes, resulting in a deep yellow bright gold plate over the whole current density range.

Claims (1)

1.Claims A gold electroplating composition comprising one or more sources of Au(|) or Au(|||), a strong base for solution make-up, an acid for pH adjustment and for providing conductivity, and the combination of at least two different mercapto- ligands, selected from the groups of mercapto-triazoles, mercapto-tetrazoles, and mercapto-thiadiazoles. A plating composition according to c|aim 1 in which the gold concentration in the electrolyte ranges from 0.05 to 50 g/L and more preferably from 0.5 to 15 g/L. A plating composition according to c|aim 1 or 2 characterized in that at least one of the mercapto-ligands has no additional nitrogen- or sulfur-comprising functional group, suitable for co-ordination and can therefore be considered as mono-dentate or terminal ligand and at least another one of the mercapto-ligands comprising an additional nitrogen- or sulfur-comprising functional group, suitable for co-ordination and can therefore be considered as bi-dentate, not chelating but rather cross- linking ligand. A plating composition according to any one of the preceding claims characterized in that the stoichiometric ratio of the cross-linking mercapto-ligand and the terminal mercapto-ligand equals or closely equals 1 to A plating composition according to any one of the preceding claims in which the concentration of each individual mercapto-ligand is applied in any concentration that is allowed by the solubility of the individual compound, but mainly in the range of 0.1 to 500 g/L, and preferably bet\Neen 10 and 150 g/L. A plating composition according to any one of the preceding claims characterized in that at least one chemical with at least one thioether and/or thioether functionality is added to the formulation. A plating composition according to c|aim 6 in which the concentration of the thioether and/or thiourea compound is applied in any concentration that is allowed by the solubility of the individual compound, but preferably from 0.5 to 500 g/L and more preferably from 1 to 200 g/L. A plating composition according to any one of the preceding claims characterized in that the pH ranges from 0 to A method of electroplating gold or a gold alloy using a formulation according to any one of the preceding claims on a substrate. A method of electroplating gold or a gold alloy according to c|aim 9 wherein the substrate is selected from a. a magnesium or magnesium alloy substrate; "WP-OP"a zinc or zinc alloy substrate; an aluminum or aluminum alloy substrate; an iron or steel substrate; a copper or copper alloy substrate, and a nickel or nickel alloy substrate.