The present invention relates to a method for production of a noble metal salt preparation, the noble metal salt preparation comprising at least one noble metal sulfonate and thiourea and the use for surface coating by electroplating or electroless plating of a noble metal or metal alloy.

In the field of electronics and electrical engineering coatings with noble metals, especially gold, are widespread because of their high electrical conductivity, the high corrosion resistance, the low contact resistance and good solderability.

Commonly, coatings with noble metals are produced by electroplating or electroless plating from noble metal cyanide complexes. The alkaline, acidic or neutral plating solutions comprise as source of a noble metal for example potassium dicyanoaurate, potassium tetracyanoaurate or alkaline sulfites.

Disadvantages of the common plating solutions are the use of toxic cyanides and thus the high requirements for plant safety, employment protection and storage.

WO 2014/054429 A1 or US 2015/0137356 A1 , respectively, disclose a non-cyan metal plating bath comprising alkaline gold sulfites or ammonium gold sulfites and a conductive salt including sulfite and sulfate. The main disadvantage of the disclosed plating solution is the poor stability.

EP 0 61 1 840 A1 discloses a cyanide-free electroplating solution for depositing monovalent copper, silver, gold and other metals complexed by a thiosulfate ion, and a stabilizer of an organic sulfinate compound in an amount sufficient to stabilize the thiosulfate ion when the solution is operated at an acidic pH of less than 7. The main disadvantage of the disclosed plating solution is the limitation of the current density and the formation of toxic sulphur dioxide at the presence of sulfites.

JP 3365866 B2 discloses a non-cyan noble metal plating bath comprising a water-soluble noble metal salt and a nonionic surfactant. The acidic noble metal plating bath comprises an alkane sulfonic acid or the like with a noble metal selected from gold or silver. Nonionic surfactants are preferred Pururafakku LF401 (manufactured by BASF), Tetronic TR-702 (manufactured by Asahi Denka Kogyo Co., Ltd.), Nymeen L-207 (manufactured by NOF Corporation) or Liponox NC- 100 (manufactured by Lion Corporation). US 6,251 ,249 B1 discloses iodide-free formulations comprising organosulfur compounds and/or carboxylic acids and a source of a soluble precious metal ion; and procedures for the deposition of precious metals onto solid substrates. The source of a soluble precious metal ion is selected from precious metal alkanesulfonate, precious metal alkanesulfonamide or precious metal alkanesulfonimide, preferred silver methanesulfonate, silver methanesulfonamide or silver dimethanesulfonimide. The organosulfur compound is selected from alkyl mercaptan, aryl mercaptan, heterocyclic mercaptan, dialkyl sulfide, diaryl sulfide, aryl alkyl sulfide, organic disulfide, organic polysulfide, organic xanthate, organic thiocyanate, or thiourea and the carboxylic acid is selected from alkanecarboxylic acid, aromatic carboxylic acid, oarmino acid, amino acid, dicarboxylic acid or polycarboxylic acid. The substrate for electroplating are selected from brass, bronze, silver, gold, palladium, copper, copper alloys, nickel, nickel alloys, iron, iron alloys, tin, tin alloys, zinc, zinc alloys, aluminum or organic based plastics. Precious metals are selected from silver, gold, platinum, palladium, iridium, rhodium, osmium and ruthenium, preferred silver, palladium and gold. The main disadvantage of this formulation is the use of gold (III) complexes produced from tetrachloroaurate (III) complexes which comprise chloride ions.

DE 10 2009 024 396 A1 discloses cyan-free, neutral or alkaline, aqueous electrolytes for the electroplating of gold or gold alloys comprising an anionic gold thiolate complex. Furthermore, DE 10 2009 024 396 A1 describes the addition of complexing agents, brighteners, surfactants and/or conducting salts.

DE 10 2103 215476 B3 discloses cyan-free, acidic and aqueous electrolytes for the electroplating of silver-palladium alloys comprising a silver compound, a palladium compound, a tellurium or selenium compound, urea and/or at least one amino acids, and a sulfonic acid. Furthermore, DE 10 2103 215 476 B3 discloses a method for electroplating.

CN 105316718 A discloses an electroplating liquid with a good dispersion capacity and covering capacity and an electroplating method for cyanide-free gold electroplating with sulfite. The electroplating liquid comprises gold chloride, a sulfite as coordination agent and an alkali metal mercaptopropionic sulfonate as auxiliary coordination agent.

US 2016/0230287 A1 discloses a reductive electroless gold plating solution, comprising a water- soluble gold compound, citric acid or a citrate salt, ethylenediamintetratacetic acid (EDTA) or an ethylenediamine tetraacetate salt, hexamethylenetetramine, and a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups; and a method for electroless gold plating using the plating solution. Disadvantages of the disclosed plating solutions and methods for electroplating is the use of toxic substances or persistent substances. Thus, problems occur in the degradation of the plating solution after electroplating.

Alternatively, DE 199 28 047 A1 discloses low-pollutant to pollutant-free and thus, environmentally compatible, aqueous solutions for electroplating of noble metals and noble metal alloys. The plating solutions comprise a gold and/or silver complex with amino acid derivatives and/or at least one water soluble sulfonic acid and/or at least one water soluble nitro compound. Disadvantageously, the plating solution comprise gold (III). The main disadvantage of the use of gold (III) comprising plating solutions is the considerably higher current demand in comparison to gold (I).

The object of the present invention is to provide an alternative to cyanide based noble metal plating solutions and to provide noble metal plating solutions which are biologicaldegradable after metal plating.

The object has been solved by a method for production of a noble metal salt preparation by membrane electrolysis comprising the steps a) Provision of an electrolytic cell comprising an anode comprising a noble metal and a cathode, wherein the anodic region and the cathodic region are separated by a membrane, b) Provision of at least one sulfonate solution in the anodic region,

c) Anodic oxidation of the noble metal and formation of a noble metal salt preparation comprising a noble metal sulfonate by passing a current through the electrolytic cell, wherein thiourea is added in the anodic region after step b) or after step c),

wherein the concentration of thiourea is 0.005 g/l to 200 g/l, preferred 10 g/l to 100 g/l, more preferred 35 g/l to 55 g/l.

According to the the invention, the term "noble metals" refers to metals that are resistant to corrosion and oxidation in moist air. Preferably, the term "noble metals" relates to elements of the groups 8 to 10 according to the lUPAC nomenclature of inorganic chemistry of 1989 with exception of the iron metals (Fe, Co and Ni). However, in an embodiment of the invention the term "noble metal" includes indium (In). Preferably the at least one noble metal is selected from gold (Au), platinum (Pt), palladium (Pd), rhodium ( h), iridium (Ir), ruthenium (Ru) or indium (In). More preferred the at least one noble metal is gold (Au).

Preferably the membrane is a cation-exchange membrane or an anion-exchange membrane, more preferred the membrane is a cation-exchange membrane, preferable with a sulfonated tetrafluoroethylene based fluoropolymer-copolymer (like Nafion™).

Sulfonate according to the invention is an alkyl sulfonate or an alkali or alkaline earth salt thereof, a sulfonamide or a sulfonimide. In an embodiment, the sulfonate is an alkyl sulfonate, preferably a C1 - to C10-alkyl sulfonate. As used herein, the term "alky sulfonate" refers to a linear, cyclic or aromatic organosulfonate. The term C1- to C10-alkyl refers to alkyl groups with 1 to 10 carbon atoms.

In preferred embodiments, the sulfonate is a methanesulfonate, ethanesulfonate, propanesulfonate, benzenesulfonate or an alkali or alkaline earth salt thereof.

Preferably the concentration of the at least one sulfonate solution is 0.1 % (w/w) to20% (w/w). In an embodiment, the sulfonate solution is an aqueous solution.

Useful molar ratios of noble metal to thiourea lie between 10.000:1 to 1 :10, preferred 100:1 to 1 :5, more preferred 10:1 to 1 :2.

The pH value of the at least one sulfonate solution is in the range between pH 1 and pH 8, preferably between pH 3 and pH 8.

In an embodiment, the anodic oxidation according to step c) is carried out with a current between 0.1 A and 500 A, preferred a current between 0.5 A and 50 A.

In an embodiment, the anodic oxidation according to step c) is carried out with a voltage between 0.1 V and 10 V.

In a further embodiment, the at least one sulfonate solution further comprises at least one further complexing agent, preferably selected from chelating agents or organosulfur compounds, more preferably selected from methylglycinediacetic acid or ethylenediamine-N,N'-disuccinic acid (EDDS) or an alkali or alkaline earth salt of these acids (e. g. Trilon™ M), methionine or cysteine. The term complexing agent refers to a compound which forms complexes with metal ions. The term chelating agent refers to a compound which forms chelates with metal ions. The term chelates refers to complexes with the presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single metal ion.

Advantageously, the at least one chelating agent or organosulfur compound stabilizes the noble metal salt preparation. Further advantageously, the at least one further complexing agent is biodegradable. As used herein, the term "biodegradable" refers to compounds which break down under composting conditions, by bacteria, fungi or other biological means.

In a further embodiment, the method for production of a noble metal salt preparation comprises at least one further step selected from precipitation, flocculation, complexation, oxidation and/or reduction.

In a further embodiment, the method for production of a noble metal salt preparation further comprises a step after step c), wherein the further step is mixing at least two noble metal salt preparations.

In a further embodiment, the method for production of a noble metal salt preparation further comprises a step after step c), wherein the further step is solvation or dilution of the noble metal salt preparation in an aqueous, organic or ionic solution.

The present invention further comprises a noble metal salt preparation comprising at least one noble metal sulfonate and thiourea,

wherein the molar ratio of noble metal to thiourea is 10.000:1 to 1 :10, preferred 100:1 to 1 :5, more preferred 10:1 to 1 :2.

The at least one noble metal sulfonate is preferably selected from gold (Au), platinum (Pt), palladium (Pd), rhodium ( h), iridium (Ir), ruthenium (Ru) or indium (In) sulfonates. Preferably the at least one noble metal salt is a gold (Au) salt, more preferably a gold (I) (Au (I)) salt.

The at least one noble metal sulfonate is preferably an alkyl sulfonate, more preferred a C1 - to C10-alkyl sulfonate. In preferred embodiments, the at least one noble metal sulfonate is a methanesulfonate, ethanesulfonate, propanesulfonate, benzenesulfonate or an alkali or alkaline earth salt thereof. In an embodiment, the noble metal salt preparation is an aqueous, organic or ionic solution with a concentration of the noble metal of 0.001 mol to 5 mol, preferably 0.01 mol to 0.5 mol.

The concentration of the noble metal sulfonate in the preparation is 0.1 % (w/w) to 20% (w/w).

In a further embodiment, the noble metal salt preparation is an aqueous solution with a pH value between pH 1 and pH 8, preferably between pH 3 and pH 8.

In a further embodiment, the noble metal salt preparation comprises at least one further complexing agent, preferably selected from chelating agents or organosulfur compounds, more preferably selected from methylglycinediacetic acid or ethylenediamine-N,N'-disuccinic acid (EDDS) or an alkali or alkaline earth salt of these acids (e. g. Trilon™ M), methionine or cysteine.

In an embodiment, the stability of an aqueous solution of the noble metal salt preparation is at least some month, preferred at least some years.

Advantageously, the noble metal salt preparation according to the invention does not comprise cyanide. Further advantageously, the noble metal salt preparation according to the invention does not comprise halogen ions and/or resistant complexing agents. As used herein, the term "resistant" refers to non-biodegradable compounds.

In an embodiment, the noble metal salt preparation according to the invention further comprises carboxylic acids. As used herein, the term "carboxylic acids" refers to an organic compound with at least one carboxyl group and 1 to 20 carbon atoms. Advantageously, the carboxylic acid is a complexing agent and adjusts the pH value.

In an embodiment, carboxylic acids are selected from formic acid, acetic acid, succinic acid, citric acid or salts thereof. In a further embodiment salts of the carboxylic acids are selected from sodium acetate, sodium succinate or sodium citrate.

In a further embodiment, the noble metal salt preparation according to the invention further comprises an aldehyde, an alcohol, a ketone, an ether and/or an ester. Advantageously, the aldehyde, alcohol, ketone, ether and/or ester are complexing agents and surfactants (surface active agent). As used herein, the term "surface active substance" refers to a compound, which lowers the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. In a further embodiment, the noble metal salt preparation according to the invention further comprises a surfactant. In a further embodiment, the surfactant is selected from cationic, anionic, nonionic and betaine tensides, preferably the surfactant is sodium dodecyl sulfate (SDS).

In a further embodiment, the noble metal salt preparation according to the invention further comprises an amine, preferred a primary amine. Advantageously, the amine is a complexing agent and a buffer. In an embodiment, the amine is selected from amino acids, ethylenediamine and ethylamine.

Advantageously, the noble metal salt preparation according to the invention is stable at temperatures between 0°C and 100°C.

The present invention further comprises a plating solution comprising the noble metal salt preparation according to the invention.

Another object of the invention is the use of a noble metal salt preparation according to the invention for surface coating by electroplating or electroless plating of a noble metal or metal alloy. The term electroplating refers to a process that uses electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode.

Advantageously, the surface coating with the noble metal salt preparation according to the invention noble metals are metals is resistant to corrosion and oxidation in moist air.

Further advantageously, the noble metal salt preparation according to the invention is biodegradable according to OECD criteria after surface coating of the noble metal (OECD 1992).

The present invention further comprises a method for electroplating of a noble metal or metal alloy comprising the steps a) Provision of an electroplating bath comprising an anode, a cathode and a solution of at least one noble metal salt preparation according to the invention,

b) Applying a current to the electroplating bath, whereby the at least one noble metal forms on the cathode, and

c) Removing the cathode from the electroplating bath. In an embodiment, the anode is an inert or soluble anode, preferred a mixed oxide anode or a noble metal anode.

In an embodiment, the anode is selected from ruthenium ( u), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), gold (Au) or indium (In).

In a further embodiment, the solution of at least one noble metal salt preparation has a metal contend of 0.001 mol to 0.25 mol, preferred 0.01 mol to 0.1 mol.

In a further embodiment, the method for electroplating of a noble metal or metal alloy is carried out at a pH value between pH 1 and pH 8, preferred at a pH value between pH 2 and pH 6.

In a further embodiment, the method for electroplating of a noble metal or metal alloy is carried out at a temperature between 0 °C and 100 °C.

In an embodiment, the method for electroplating of a noble metal or metal alloy is carried out with a current density between 0,1 A/dm ² and 20 A/dm ² .

In a further embodiment, the method for electroplating is carried out by pulse-plating,

In an embodiment, the method for electroplating of a metal alloy is carried out with a solution of at least one noble metal salt preparation according to the invention further comprising at least one noble metal or other metals. The at least one noble metal is selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt) or gold (Au). In an embodiment, the at least one other metal is selected from nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), cadmium (Cd) or indium (In).

The present invention further comprises a method for electroless plating of a noble metal comprising the steps a) Provision of a plating bath comprising a solution of a noble metal salt preparation according to the invention,

b) Contacting of a solid substrate with the plating bath, whereby the noble metal forms on the solid substrate, and

c) Removing the solid substrate from the plating bath. In a further embodiment, the solution of a noble metal salt preparation has a metal contend of 0.001 mol to 0.25 mol, preferred 0.01 mol to 0.1 mol.

In a further embodiment, the method for electroless plating of a noble metal is carried outat a pH value between pH 1 and pH 8, preferred at a pH value between pH 2 and pH 6.

In a further embodiment, the method for electroless plating of a noble metal is carried out at a temperature between 0 °C and 100 °C, preferred at a temperature between 60 °C and 80 °C.

In an embodiment, the thickness of the layer of the at least one noble metal is 10 nm to 100 μηη, preferred 100 nm to 10 μηη.

Advantageously, the layer of the at least one noble metal acts as soldering aid or anticorrosive coating.

In a further embodiment, the recently described embodiments can be combined.

The present invention will now be further explained by the following non-limiting figures and examples.

Fig. 1 shows a schema of the electrolytic cell comprising an anode comprising a noble metal and a cathode, wherein the anodic region and the cathodic region are separated by a membrane

Production of an Au (I) salt solution

For the production of Au (I) salt solutions the electrolytic cell of Fig. 1 was used. The electrolytic cell comprises an anode comprising gold with a high surface, which is produced according to known procedures, in particular by the Wohlwill process (Gmelin 1974), and a cathode, wherein the anodic region and the cathodic region are separated by a membrane. In the anodic region, the electrolyte is an alkyl sulfonic acid and thiourea. In the cathodic region, the electrolyte is an alkyl sulfonic acid.

The anodic oxidation is carried out with direct current of 0.5 A, 1 A or 10 A. The membrane electrolysis is carried out at 25°C. Alternatively, the membrane electrolysis can be carried out at temperatures between 20°C and 80°C.

In the anodic region, the Au (I) salt solution is received and filtrated. Example 1 : Preparation of Au(l) methanesulfonate

The catholyte is mixed of 70% methane sulfonic acid and distilled water to a solution of 5% methane sulfonic acid. The anolyte comprises an aqueous solution of methane sulfonic acid (5%) with 20 g/l thiourea. Platinated titanium was used as cathode and the anode was pure gold from the Wohlwill process. The membrane between cathode and anode was a cation selective membrane from DuPont (Nafion).

The anodic oxidation is carried out with direct current of 1 .0 A for two hours. Afterwards the anolyte contained 13.9 g gold as gold (I).

Example 2 Preparation Au(l) methanesulfonate

The catholyte is mixed of 70% methane sulfonic acid and distilled water to a solution of 20% methane sulfonic acid. The anolyte comprises an aqueous solution of methane sulfonic acid (20%) with 40 g/l thiourea. Platinated titanium was used as cathode and the anode was pure gold from the Wohlwill process. The membrane between cathode and anode was a cation selective membrane from DuPont (Nafion).

The anodic oxidation is carried out with direct current of 1 .0 Aforfive hours. Afterwards the anolyte contained 33.1 g gold as gold (I).

Example 3 Preparation Au(l) methanesulfonate

The catholyte and anolyte are 100% methane sulfonic acid. Platinated titanium was used as cathode and the anode was pure gold from the Wohlwill process The membrane between cathode and anode was a cation selective membrane from DuPont (Nafion).

The anodic oxidation is carried out with direct current of 0.5 A for four hours. Afterwards the anolyte contained 13.4 g gold as gold (I) and was diluted with distilled water with solved thiourea in a concentration of 20 g/l.

Example 4 Electroplating

The Au(l) methanesulfonate solution of example 1 was used for electroplating a nickel plated messing plate in a regular Hull Cell. The Au (I) plating solution comprised 2g/l Au (I), 5 g/l thiourea, 5 g/l sodium methanesulfonate and 5 ml of a 0,005 % solution of sodium dodecyl sulfate. After plating with 0,5 A/dm ² for 5 min the cathodic nickel/messing plate was yellow by the gold plating. Example 5 Electroless plating

The Au(l) methanesulfonate solution of example 2 was used for electroless plating a nickel plated printed circuit board in a regular beaker. The Au (I) plating solution comprised 2g/l Au (I), 5 g/l thiourea, 2 g/l Trilon M and 5 ml 0,005 % solution of sodium dodecyl sulfate. After heating to 62 °C and plating for 5 min the printed circuit board was yellow by the gold plating.

Cited non-patent literature

OECD (1992) OECD Guideline for testing of chemicals, Section 3-Degradation and Accumulation, Test No. 301 : Ready Biodegradability, DOI: 10.1787/9789264070349-en.

Gmelin L (1974) Gmelin Handbuch der anorganischen Chemie: Gold. Lieferung 2, Springer- Verlag Berlin Heidelberg GmbH, 8 ^th Edition, ISBN 3-540-93265-8.

Reference signs

Cathode

Anode

Membrane

Catholyte

Anolyte