Field of the I nvention

The present invention concerns an electroless gold plating bath, its use and a method for deposit ing a gold layer on a surface of a substrate. The present invention is further directed at a kit-of- parts suitable for providing said electroless gold plating bath. The electroless gold plating bath is particularly suitable in the manufacturing of printed circuit boards, I C substrates, semiconducting devices, interposers made of glass and the like.

Background of the I nvention

Gold layers are of paramount interest in the manufacturing of electronic components and in the semiconductor industry. They are frequently used as solderable and/or wire bondable surfaces in the manufacture of printed circuit boards, I C substrates, semiconducting devices and the like. Typ ically, they are used as a final finish before soldering and/or wire bonding. I n order to provide elec trical connections of sufficient conductivity and robustness between the copper lines and wires that are bonded thereto while providing a good strength for wire bonding, there are various layer as semblies which are used conventionally in the art. Such layer assemblies typically comprise on the copper line one or more of nickel, palladium and gold.

Among others, there are electroless nickel / electroless gold ( ENI G) , electroless nickel / electroless palladium / immersion gold ( ENEPI G) , direct immersion gold ( DI G) , electroless palladium / immer sion gold ( EPI G) and electroless palladium / autocatalytic gold ( EPAG) .

All of these finishes require some electroless plating process. Due to the minuscule sizes of electri cal components nowadays, it is not possible to use electrolytic processes which require electrical connections to the substrates. Therefore, electroless plating processes (electroless metal deposi tion) are used. Electroless plating generally describes methods without using external current sources for reduction of metal ions. Plating processes using external current sources are commonly described as electrolytic or galvanic plating methods. Non-metallic surfaces may be pretreated to make them receptive or catalytic for metal deposition. All or selected portions of a surface may suitably be pretreated. One type of electroless plating is autocatalytic plating. The main compo nents of an autocatalytic metal baths are the metal salt, a reducing agent, and, as optional ingre dients, a complexing agent, a pH adj uster, and additives, as for example stabilizing agents. Com- plexing agents (also called chelating agents in the art) are used to chelate the metal being depos ited and prevent the metal from being precipitated from solution (i.e. as the hydroxide and the like) . Chelating metal renders the metal available to the reducing agent that converts the metal ions to its metallic form.

A further form of metal deposition is immersion plating. I mmersion plating is another deposition of metal using neither external current sources nor chemical reducing agents. The mechanism relies on the substitution of metals from an underlying substrate for metal ions present in the immersion plating solution. One disadvantage of immersion plating is that the deposition of thicker layers is normally limited by the porosity of the formed layer. Thus, once a dense layer of the metal to be deposited has formed, plating ceases.

I n most cases, electroless gold plating baths use one or both types of electroless plating. Even if reducing agents have been added to the plating bath, immersion-type plating may occur albeit then in a reduced proportion. I n the context of the present invention, electroless plating is to be understood as a combined deposition of autocatalytic plating with the aid of a chemical reducing agent (referred to as“reducing agent” herein) and immersion plating.

Even though these techniques have been established some time ago, there are still many challeng es unresolved. Such challenges are for example the corrosion of nickel layers which are placed between gold and copper lines (also referred to as nickel corrosion in the art) and insufficient sta bilities of the prior art gold plating baths which is highly undesirable due to the costs of said baths. Some prior art plating baths yield satisfactory results when freshly prepared but lose their proper ties quickly when being in use. This also then calls for frequent replacements of those baths.

Another aspect of importance in the manufacturing industry is the plating rate. I t is highly desira ble to deposit gold layers with sufficient plating rates to run manufacturing processes in an eco nomical viable manner. I t is typically required that gold plating bath have plating rates of 1 00 nm/h or more, preferably of 150 nm/h or more, particularly preferably of 200 nm/h or more to comply with today’s industrial requirements.

Another desired property of the gold layer is the optical appearance that should be lemon yellow as a discoloration of the gold layer is not acceptable resulting in scrap production.

Although there are many electroless gold plating bath known in the prior art, many of those fail to deliver a long-term performance satisfying above-outlined technical requirements. This necessi tates frequent replacements of the gold plating baths which is an economic and environmental concern.

Objective of the present I nvention

I t is therefore an objective of the present invention to overcome the shortcomings of the prior art. Thus, it is an objective of the present invention to provide an electroless gold plating bath having a constant plating rate and performance for at least 5 metal turnovers (MTO), preferably for at least 10 metal turnovers.

Summary of the I nvention

The objectives are solved by the electroless gold plating bath according to the invention, compris ing

a) gold ions;

b) sulfite ions;

c) iodide ions; and d) at least one phosphonate compound according to form ula ( 1 ) wherein

each X is an alkanediyl group;

R ¹ , R ² , R ³ and each R ⁴ are independently alkanediyl groups;

M is independently hydrogen, a metal atom or a cation form ing radical ;

each n is a rational number and selected in accordance with the valency of the re spective M ; and

b is an integer ranging from 1 to 1 0.

In case b is more than 1 , X can be an independently selected alkanediyl group e.g. as a 1 ,2- ethanediyl group and a 1 ,3-propandiyl group. The occurrence and t he ratio depends on the used educts to form the compou nd according to form ula ( 1 ) .

The objectives are further solved by the use of the inventive electroless gold plating bath for de positing a gold layer on a surface of a substrate.

The obj ectives are also solved by the inventive method for depositing a gold layer on a surface of a substrate, comprising , in this order, the method steps

(i) providing the substrate with the surface;

( ii) contacting at least a portion of the surface of the substrate with the inventive electroless gold plating bath ; and

thereby depositing a gold layer onto at least one portion of t he su rface of the substrate, preferable the at least one portion of the substrate consists of nickel or phosphorus nickel.

Advantageously, t he plating rate of the inventive electroless gold plating bath is 240 nm/ h or greater.

Advantageously, t he electroless gold plating bath provides solderable gold surfaces. The gold su r- faces obtained from the inventive electroless gold plating bath are also suitable for wire bonding applications such as mou nting of copper wires, gold wires or silver wires to the formed gold layers. A further beneficial aspect of the present invention is that t he gold layers formed from the in ventive electroless gold plating bath adhere strongly to the surface of the substrate on which they have been deposited.

The electroless gold plating bath is particularly suitable for gold deposition on high-phosphorus nickel layers and m id-phosphorus nickel layers, e.g. such as those present on a copper surface such as a copper pad. High-phosphorus nickel layers ( HiP-Ni) are herein understood as surfaces comprising nickel and phosphorus wherein the phosphorus content ranges from 1 0.0 to 15.0 wt.-% with the balance being (essentially) nickel. Mid-phosphorus nickel layers (midP-Ni) are herein understood as surfaces comprising nickel and phosphorus wherein the phosphorus content ranges from 6.0 to 9.9 wt.-% with the balance being (essentially) nickel. The nickel corrosion was at an acceptable level or even reduced compared to many prior art plating baths when using the inventive electroless gold plating bath to deposit gold on such nickel comprising surfaces.

Further advantageously, the inventive electroless gold plating bath achieves its outstanding per formance without the need of toxic chemicals such as cyanide and thiourea and is thus m uch more environmentally friendly and safer to use than many electroless gold plating bath of the prior art.

Detailed Description of the I nvention

Percentages throughout this specification are weight-percentages (wt.-% ) unless stated other wise. Concentrations given in this specification refer to the volume or mass of the entire solutions / compositions unless stated otherwise. The terms“deposition” and“plating” are used interchangea bly herein. Also, “layer” and“deposit” are also used synonymously in this specification. The terms “deposition composition” and“plating bath” are synonymous in the context of the present inven tion. The terms“substitution” and“functionalization” are used interchangeably in this specification. An“electrolytic metal or metal alloy deposition composition” is herein understood as a composition suitable to be used for electrolytic deposition of a metal or a metal alloy.

The term“alkyl group” according to the present invention comprises branched or unbranched alkyl groups comprising cyclic and/or non-cyclic structural elements, wherein cyclic structural elements of the alkyl groups naturally require at least three carbon atoms. C1 -CX-alkyl group in this specifi cation and in the claims refers to alkyl groups having 1 to X carbon atoms (X being an integer) . C1 - C8-alkyl group for example includes, among others, methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, n-pentyl, iso-pentyl, sec-pentyl, tert-pentyl, neo-pentyl, hexyl, hep- tyl and octyl. Substituted alkyl groups may theoretically be obtained by replacing at least one hy drogen by a functional group. Unless stated otherwise, alkyl groups are preferably selected from substituted or unsubstituted C1 -C8-alkyl groups, more preferably from substituted or unsubstituted C1 -C4-alkyl groups because of their improved solubility in water.

The term“alkanediyl group” is the corresponding derivative of an alkyl group having two bonding sites. Sometimes alkanediyl groups are referred to as alkylene groups in the art. C1 -CX-alkanediyl group in this specification (X being an integer) and in the claims refers to alkanediyl groups having 1 to X carbon atoms, e.g. 1 to 12 or 2 to 4. The explanations and preferences outlined for alkyl groups apply to alkanediyl groups as well.

Unless stated otherwise, above-defined groups are substituted or unsubstituted. Functional groups as substituents are preferably selected from the group consisting of oxo (= 0) , hydroxyl (-OH) , amino (-NH ₂ ), carbonyl (-CHO) and carboxyl (-C0 ₂ H) to improve the solubility of the relevant com- pounds in polar solvents such as water, more preferably hydroxyl. I n one embodiment of the pre sent invention, the groups are preferably unsubstituted unless stated otherwise hereinafter. Oxo is not to be mistaken for oxy (-0-) which is usually an oxygen atom of an ether moiety (and thus placed between two carbon atoms) .

I f more than one substituent is to be selected from a certain group, each substituent is selected independently from each other unless stated otherwise herein. The embodiments described here inafter can be combined without restraints unless this is technically not feasible or specifically ex cluded. Preferred embodiments described for one aspect of the present invention are applicable mutatis mutandis to all the other aspects of the present invention unless stated otherwise herein. A metal turnover of an electroless gold plating bath is typically understood as the complete deple tion of the gold ions in the bath due to deposition and its subsequent replenishment. Thus, an electroless gold plating bath capable of plating for five metal turnovers can deposit gold in five times the amount of the initially contained gold ion concentration therein.

Preferred embodiments of the present invention which have been found to be particularly useful in fulfilling above described objectives underlying the present invention are described in the depend ent claims and hereinafter.

The inventive electroless gold plating bath comprises:

a) gold ions;

b) sulfite ions;

c) iodide ions; and

d) at least one phosphonate compound according to formula ( 1 ) .

I t is to be understood that the listed ions (of the inventive electroless gold plating bath) refer to sources which are represented by components having these sources of the listed ions.

The inventive electroless gold plating bath comprises gold ions. Gold ions usually have oxidation states selected from the group consisting of + 5, + 3, + 2, + 1 , 0, -1 , -2 and -3. I n the context of the present invention, preferred oxidation states are + 1 and + 3, more preferred is + 1 . Typically, gold ions in the plating bath are selected from the group consisting of Au ⁺ ions, Au ³⁺ ions and mixtures of the aforementioned. Preferably, Au ⁺ ions are selected for their ease of being reduced under the given conditions resulting in favorable plating rates.

Suitable sources of gold ions can be any water soluble gold salt or coordination complex suitable for liberating one of the aforementioned gold ions into the inventive electroless gold plating bath. Preferably, the source of gold ions is free of cyanide. More preferably, the source is gold ions is selected from the group consisting of trialkali gold disulfite such as tripotassium gold disulfite and trisodium gold disulfite; triammonium gold disulfite; gold thiosulfate; gold sulfate; gold chloride and gold bromide. Even more preferably, the source of gold ions is selected from the group con sisting of trialkali gold disulfite; triammonium gold disulfite; gold sulfate; gold chloride and gold bromide.

The concentration of gold ions in the inventive electroless gold plating bath preferably ranges from 0.5 to 50 mmol/L, more preferably from 1 to 30 mmol/L, even more preferably from 1 .5 to 10 mmol/ L, yet even more preferably 3.5 to 6 mmol/L.

The inventive electroless gold plating bath comprises sulfite ions (SOf-). Suitable sources of sulfite ions can be any water soluble sulfite salt or coordination complex suitable for liberating sulfite ions into the inventive electroless gold plating bath. Preferably, the source of sulfite ions is selected from the group consisting of alkali sulfite such as lithium sulfite, sodium sulfite, potassium sulfite; ammonium sulfite; and sulfurous acid. I n case, one of above-mentioned sources for gold ions is used which already contains sulfite ( e.g . tripotassium gold disulfite) , this source may also be used (additionally to one of the aforementioned or solely) to provide sulfite ions to the inventive electro less gold plating bath. Preferably, the concentration of the sulfite ions ranges from 10 to 150 mmol/L, preferably 15 to 1 00 mmol/L and most preferably from 20 to 80 mmol/L.

I n one embodiment of the invention, the amount of substance of the sulfite ions is at least as high as the amount of substance of the gold ions in the inventive electroless gold plating bath. Prefera bly, the molar ratio of gold ions to sulfite ions in the inventive electroless gold plating bath ranges from 1 /2 to 1 /30, more preferably from 1 /3 to 1 /20 and most preferably from 1 /4 to 1 / 1 6. A too low concentration of sulfite ions may lead to an unstable electroless gold plating bath and fewer metal turnover to be achieved. I t was observed that in some cases a too high concentration of sulfite ions in the electroless gold plating bath lead to lower plating rates.

The inventive electroless gold plating bath comprises iodide ions. Suitable sources of iodide ions can be any water soluble iodide salt or coordination complex suitable for liberating iodide ions in the inventive electroless gold plating bath. Preferably, the source of sulfite ions is selected from the group consisting of alkali iodide such as lithium iodide, sodium iodide, potassium iodide; ammoni um iodide; and hydrogen iodide. Further or alternatively, gold iodine compounds such as gold tri iodide may be used as source of both gold ions and iodide ions.

Preferably, the concentration of iodide ions ranges from 4 to 1 00 mmol/L, more preferably from 6 to 50 mmol/L, even more preferably from 7 to 25 mmol/L. I n one embodiment of the invention and in particular in cases the inventive electroless gold plating bath does not comprise any poly- alkylene glycols ( vide infra) , the concentration of the iodide ions preferably ranges from 4 to 12.5 mmol/L, more preferably from 6 to 1 0 mmol/L, and even more preferably from 7 to 9 mmol/L. The inventive electroless gold plating bath comprises at least one phosphonate compound accord ing to formula ( 1 ) .

Each M is a moiety suitable to bind to the phosphonate moiety in formula ( 1 ) . Each M is inde pendently hydrogen, a metal atom or a cation forming radical. M is not particularly restricted and the person skilled in the art can select the M with routine experiments. Each M has a valency. Such valencies are typically 1 , 2, 3 or 4. Exemplarily, hydrogen has a valency of 1 . Preferred metal at oms in this context are alkaline metals which also have a valency of 1 as well. Cation forming radi cals can be derived from many organic amines like triethylamine. A preferred cation forming radical is ammonium and has a valency of 1 as well.

Each n is a rational number and selected in accordance with the valency of the respective M. Typi cally, n is 2 divided by the valency of M. Above-mentioned M are used in quantities sufficient to satisfy the valency of the phosphonate groups in the phosphonate compound according to formula ( 1 ) . Since each phosphonate moiety in formula ( 1 ) has a valency of 2, if M would be sodium or hydrogen, n would be 2. I f M would be a zirconium atom (valency of 4) , n would be ½ . The bond between the M and the respective phosphonate moiety may be ionic, polar or covalent depending on the M of choice.

R ¹ , R ² , R ³ and each R ⁴ are preferably independently C1 -C6-alkanediyl groups; R ¹ , R ² , R ³ and each R ⁴ are more preferably C1 -C3-alkanediyl groups; R ¹ , R ² , R ³ and each R ⁴ are even more preferably 1 , 1 -methanediyl groups or 1 ,2-ethanediyl groups; most preferably R ¹ , R ² , R ³ and each R ⁴ are 1 , 1 - methanediyl groups. For the ease of synthesis, all of R ¹ , R ² , R ³ and each R ⁴ are preferably the same.

I nteger b preferably ranges from 1 to 6, more preferably from 1 to 3 and even more preferably from 1 to 2; most preferably, b is 1 .

Preferably, each X independently is a C1 -C12-alkanediyl group, each X is preferably a C1 -C6- alkanediyl group; each X is more preferably a C2-C4-alkanediyl group; most preferably, X is a 1 ,2- ethanediyl group. Preferably, all X (if there are more than one) are the same.

I n one embodiment of the invention, the phosphonate compound represented by formula ( 1 ) is selected from the group consisting of

salts and mixtures of the aforementioned.

Typical salts in this embodiment are alkali salts and ammonium salts.

Preferably, the concentration of the phosphonate compound according to formula ( 1 ) ranges from 20 to 120 mmol/L, preferably from 25 to 90 mmol/L and more preferably from 30 to 60 mmol/ L.

I n a preferred embodiment of the invention, the inventive electroless gold plating bath comprises at least one hydroxycarboxylic acid. I t was found by the inventors that the hydroxycarboxylic acid even further improves the stability of the electroless gold plating bath and allows for a greater metal turnover number to be reached compared to an inventive electroless gold plating bath wit h out said hydroxycarboxylic acid. Preferably, the at least one hydroxycarboxylic acid is aliphatic. Suitable hydroxycarboxylic acids are inter alia aliphatic a-hydroxycarboxylic acids and b-hydroxycarboxylic acids such as citric acid, isocitric acid, gluconic acid, glyceric acid, glycolic acid, pantoic acid, lactic acid, tartaric acid, tartronic acid, threonic acid, a-hydroxyglutaric acid, b- hydroxyglutaric acid, b-hydroxybutytic acid, and malic acid.

Preferably, the at least one hydroxycarboxylic acid is a C1 -C12-hydroxycarboxylic acid; more pref erably a C1 -C6-hydroxycarboxylic acid; even more preferably a C2-C4-hydroxycarboxylic acid. Pref- erably, the at least one hydroxycarboxylic acid comprises 1 to 6 hydroxyl groups; more preferably, the at least one hydroxycarboxylic acid comprises 1 to 4 hydroxyl groups; even more preferably, the at least one hydroxycarboxylic acid comprises 1 to 2 hydroxyl groups. Preferably, the hy droxycarboxylic acid comprises at least two carboxylic acid groups. I n one embodiment of the in vention, the hydroxycarboxylic acid is free of nitrogen. I n this embodiment, the hydroxycarboxylic acid does not comprise any amino groups and imino groups.

Preferably, the concentration of the at least one hydroxycarboxylic acid ranges from 1 00 to 300 mmol/L, more preferably from 130 to 270 mmol/L and even more preferably from 1 60 to 240 mmol/L.

The pH value of the inventive electroless gold plating bath is not particularly limited. I t was found to be preferable that the pH value range of the inventive bath is from 5.5 to 8.5. More preferably, the pH value ranges from 6.0 to 8.0 and most preferably from 6.5 to 7.5. Too low pH values may result in inferior stability of the electroless gold plating bath, possibly due to low stability of the sulfite ions at low pH values. Notably, high pH values are should be avoided if vulnerable sub strates are to be used in the plating process such as soldermasks which are frequently used in the electronics industry. The inventive electroless gold plating bath optionally comprises one or more acids, preferably a mineral acid such as phosphoric acid or sulfuric acid; bases such as sodium hydroxide or potassium hydroxide; and/or buffers, e.g. hydrogen phosphate / dihydrogen phosphate. These compounds can be used to adj ust the pH value of the inventive electroless gold plating bath. I t is advanta geous and thus preferable to continuously control and adj ust the pH value during usage of the inventive electroless gold plating bath as this also improves the plating bath lifetime.

I t was found by the inventors of the present invention that the inventive electroless gold plating bath comprising sulfite ions, iodide ions, and the at least one phosphonate compound according to formula ( 1 ) is particularly suitable to fulfill the objectives underlying the present invention and al low for very stable electroless gold plating baths which can be used for 1 0 or more metal turnovers while retaining the favorable plating results over the whole lifetime of the inventive electroless gold plating bath. This synergism of the aforementioned components in the inventive electroless gold plating bath can be further enhanced by the addition of at least one hydroxycarboxylic acid.

Optionally, the inventive electroless gold plating bath comprises a polyalkylene glycol such as PEG 400 or PEG 1500. Preferable polyalkylene glycols have molecular weight average weights ranging from 100 to 3000 g/mol, preferably from 300 to 1500 g/mol. Such polyalkylene glycols are prefer ably used in concentrations of 0.05 to 2.5 g/L.

The inventive electroless gold plating bath optionally comprises one or more components usually used in electroless gold plating baths of the prior art such as stabilizing agents and grain refiners. These components and adequate concentrations thereof are known in the art.

Preferably, the inventive electroless gold plating bath is predominantly an immersion gold plating bath. Thus, while in use the mechanism of gold depositions relies mostly on substitution processes rather than reducing processes. Preferably, the ratio of gold deposited by an immersion process in relation to the entire gold deposition is at least 50% ; more preferably at least 60% and most pref erably at least 70% . I f the inventive electroless gold plating bath has such higher ratios of immer sion deposition, it is less prone to plate-out and can be used for a greater number of metal turno vers. I n one embodiment of the invention, the ratio of gold deposited by an autocatalytic process in relation to the entire gold deposition is at least 1 0% ; preferably at least 20% . The combined deposition process was found to be advantageous in reaching the objectives underlying the pre sent invention such as high bath stability against plate-out and high metal turnovers while allowing for sufficient plating rates and desired properties of the formed gold layers. The ratio of gold de posited by an immersion process in relation to the entire gold deposition can be calculated based on the deposited mass of gold or depletion of gold ions in the bath and the amount of metal ions dissolved in the bath from the substrate to be plated. For example, if a nickel surface is used for plating, the amount of nickel ions in the inventive electroless gold plating bath after plating can be used for this purpose. I n one embodiment of the invention, the concentration of aldehyde-based reducing agents in the inventive electroless gold plating bath is 5 mmol/L or below, more preferably 1 mmol/L or below. Most preferably, the inventive electroless gold plating bath is (essentially) free of aldehyde-based reducing agents. Among others, aldehyde-based reducing agents are aliphatic and aromatic alde hydes such as formaldehyde; sugars having an aldehyde moiety such as glucose and mannose; benzaldehyde and precursors for any of the aforementioned such as urotropin and paraformalde hyde. This preference is due to the occasional occurrence of plate-outs and an occasionally ob served reduction of achievable metal turnovers.

I n one embodiment of the invention, the concentration of ascorbic acid and salts thereof in the inventive electroless gold plating bath is preferably 5 mmol/L or below, more preferably 1 mmol/L or below for the same reasons as outlined for aldehyde-based reducing agents. Most preferably, the inventive electroless gold plating bath is (essentially) free of ascorbic acid and salts thereof.

I n one embodiment of the invention, the concentration of hypophosphorous acid ( H ₃ P0 ₂ ) and salts thereof in the inventive electroless gold plating bath is preferably 5 mmol/L or below, more prefer ably 1 mmol/L or below for the same reasons as outlined for aldehyde-based reducing agents. Most preferably, the inventive electroless gold plating bath is (essentially) free of hypophosphorous acid and salts thereof.

I n one embodiment of the invention, the concentration of hydroxylamine and salts thereof in the inventive electroless gold plating bath is preferably 5 mmol/L or below, more preferably 1 mmol/L or below for the same reasons as outlined for aldehyde-based reducing agents and because of the acute toxicity of these compounds. Salts of hydroxylamine are for example hydroxylamine hydro chloride and hydroxylamine sulfate. Most preferably, the inventive electroless gold plating bath is (essentially) free of hydroxylamine and salts thereof.

I n one embodiment of the invention, the concentration of hydrazine and salts thereof (such as hydrazine sulfate, hydrazine hydrate, hydrazine carboxylate like hydrazine acetate, hydrazine hy drohalide like hydrazine hydrochloride) thereof in the inventive electroless gold plating bath is preferably 5 mmol/L or below, more preferably 1 mmol/L or below for the same reasons as out lined for aldehyde-based reducing agents. Moreover, these compounds are very toxic. Most prefer ably, the inventive electroless gold plating bath is (essentially) free of hydrazine and salts thereof.

I n one embodiment of the invention, the concentration of boron-based reducing agents in the in ventive electroless gold plating bath is preferably 5 mmol/L or below, more preferably 1 mmol/L or below for the same reasons as outlined for aldehyde-based reducing agents. Most preferably, the inventive electroless gold plating bath is (essentially) free of boron based reducing agents. Boron- based reducing agents are those boron containing compounds capable of reducing gold ions to metallic gold and include inter alia boranes (such as DMAB and morpholine borane) , borohydrides (such as NaBH ₄ ) I n one embodiment of the invention, the concentration of thiosulfate ions in the electroless gold plating bath is 1 0 mmol/L or below. More preferably, the concentration of thiosulfate ions is 1 mmol/ L or below. Particularly preferably, the electroless gold plating bath is (essentially) free of thiosulfate ions. Thiosulfate ions may in some case be detrimental to the plating performance of the inventive electroless gold plating bath and for example can reduce the plating rate (see exam ple X in Table 2) . Further, thiosulfate in electroless gold plating bath might result in nickel corro sion and/or to poor adhesion of the gold layer on the underlying substrate.

I n one embodiment of the invention, the concentration of cyanide ions in the inventive electroless gold plating bath is 1 mmol/L or less, preferably 0.1 mmol/L or less. Most preferably, the electro less gold plating bath is (essentially) free of cyanide ions for cyanide ions being highly toxic and harmful to the environment.

I n one embodiment of the invention, the concentration of thiourea, N-alkylthiourea and /y /V-dialkylthiourea in the inventive electroless gold plating bath is 1 0 mmol/ L or less, preferably 1 mmol/L or less. Most preferably, the electroless gold plating bath is free of thiourea, /V-alkylthiourea and N, L/'-dialkylthiourea for them being harmful to human and environment.

I n one embodiment of the invention, the concentration of aminopolycarboxylic acids is 1 0 mmol/L or less, preferably 1 mmol/ L or less. Aminopolycarboxylic acids comprise at least one amino group and at least two carboxylic acid groups. Examples of aminopolycarboxylic acids are ethylenedia- minetetraacetate (EDTA) , nitrilotriacetic acid (NTA) , iminodiacetic acid ( I DA) and pentetic acid. Most preferably, the electroless gold plating bath is free of aminopolycarboxylic acids. Aminopoly carboxylic acids in some cases might result in inferior plating results. Further, aminopolycarboxylic acids such as NTA and EDTA require extra waste-water treatment and are environmentally harm ful.

Surprisingly, it was found by the inventors that commonly used plating rate enhancers such as amino acids (like alanine, glycine, glutamine, lysine, valine) did not increase the plating rate of the electroless gold plating bath (of. example V in table 2 and US 5,31 8,621 ) . Thus, it is preferable that the concentration of amino acids in the inventive electroless gold plating bath is 2 g/L or below, more preferably of 0.5 g/L or below. Most preferably, the inventive electroless gold plating bath is (essentially) free of amino acids.

The inventive electroless gold plating bath is preferably an aqueous solution. The term “aqueous solution” means that the prevailing liquid medium, which is the solvent in the solution, is water. Further liquids, that are miscible with water, as for example alcohols and other polar organic liq uids, that are miscible with water, may be added. Generally, an aqueous solution comprises more than 50 percent water by weight. I t is preferred that 99 wt.-% of the solvents used in the inventive electroless gold plating bath are water for its ecological benign character. I n one embodiment of the invention, the inventive electroless gold plating bath consists of an aqueous solution containing

a) gold ions;

b) sulfite ions;

c) iodide ions;

d) at least one phosphonate compound according to formula ( 1 ) ;

e) optionally, at least one hydroxycarboxylic acid;

f) optionally, a polyalkylene glycol; and

g) optionally, one or more acids, preferably a mineral acid such as phosphoric acid or sul- furic acid; bases such as sodium hydroxide or potassium hydroxide; and/or buffers, e.g. hydrogen phosphate / dihydrogen phosphate, to adj ust the pH value of the inventive elec troless gold plating bath to above-described values.

The components are preferably used in the concentrations described hereinbefore.

The present invention further concerns a kit-of-parts suitable for providing the inventive electroless gold plating bath, comprising

I ) an aqueous solution comprising

- sulfite ions;

- iodide ions;

- at least one phosphonate compound according to formula ( 1 ) wherein

each X is independently an alkanediyl group;

R ¹ , R ² , R ³ and each R ⁴ are independently alkanediyl groups;

M is independently hydrogen, a metal atom or a cation forming radical; each n is a rational number and selected in accordance with the valency of the respective M; and

b is an integer ranging from 1 to 1 0;

- optionally, at least one hydroxycarboxylic acid ;

- optionally, a polyalkylene glycol; and

I I ) an aqueous solution of gold ions. This kit-of-parts can easily be combined to form the inventive electroless gold plating bath. Thus, by combing parts I ) and I I ) of the kit-of-parts, and optionally, further dilution with a solvent such as water, the inventive electroless gold plating bath can be provided. This kit-of-parts allows for improved storage and easier transportation as concentrations of the listed components can be much higher than in the inventive electroless gold plating bath. Thus, smaller volumes can be han dled which then, can be diluted with a suitable solvent such as water prior to any plating process.

I n step (i) of the inventive method, the substrate with the surface is provided. The surface of the substrate to be used in the present invention is preferably a metal or metal alloy surface. Gold is then deposited onto said metal or metal alloy surface in step (ii) . Preferable metal or metal alloys are selected from the group consisting of nickel, nickel alloys such as nickel phosphorous alloys, nickel boron alloys, cobalt, cobalt alloys such as cobalt phosphorous alloys, cobalt molybdenum phosphorous alloys, cobalt molybdenum boron alloys, cobalt molybdenum boron phosphorous al loys, cobalt tungsten phosphorous alloys, cobalt tungsten boron alloys, cobalt tungsten boron phosphorous alloys, palladium, palladium alloys such as palladium phosphorous alloys, palladium boron alloys, copper and copper alloys and gold or gold alloys. More preferable metal or metal alloys are selected from the group consisting of nickel phosphorus alloys, palladium and palladium phosphorus alloys. I t was found by the inventors that surfaces comprising nickel phosphorus alloys wherein the phosphorus content ranges from 1 0 to 15 wt.-% with the balance being (essentially) nickel ( HiP-Ni surfaces) are particularly suitable for the inventive electroless gold plating bath and the advantages described in this specification are particularly pronounced if such a surface is em ployed in the inventive method. The inventive electroless gold plating bath is particularly suitable for high end electronic devices due to the extreme good corrosion resistance obtained from said inventive electroless gold plating bath, particularly on HiP-Ni surfaces.

I n one embodiment of the invention, the substrate is selected from the group consisting of printed circuit boards, I C substrates, circuit carriers, interconnect devices, semiconducting wafers and glass substrates. The substrate then comprises a surface suitable for plating, preferably selected from above groups.

Optionally, the surface of the substrate is pretreated prior to step (ii) . Such pretreatment is known in the art. Typical pretreatments include one or more of cleaning steps (employing solvents and/or surfactants to remove mostly organic contaminants) , etching steps (using acids and optionally, oxidizing or reducing agents to remove oxides) and activation steps. The latter is to deposit a no ble metal catalyst on the surface or a part thereof to make it more receptive for plating. Such no ble metal catalyst is for example palladium which is either deposited as a salt before it is reduced to elementary palladium on the surface of the substrate or it is deposited in a colloidal form and - where appropriate - subjected to an acceleration step with an acid such as hydrochloric acid to remove any protective colloids such as tin colloids. Such treatments give activation layers which normally are not a discrete layers but an aggregation of island structures of palladium. However, activation layers are considered to be metal substrates in the context of the present invention.

I n step (ii) of the inventive method, at least a portion of the surface of the substrate is contacted with the inventive electroless gold plating bath. Thereby, a gold layer is deposited onto at least a portion of the surface of the substrate. This contact is preferably accomplished by dipping or im mersing the substrate (entirely or partially) into the inventive electroless gold plating bath or by spraying the inventive electroless gold plating bath onto the surface of the substrate.

The temperature of the inventive electroless gold plating bath is preferably in the range of 40 to 85 °C, more preferably from 50 to 75 °C, even more preferably from 60 to 70 °C during step (ii) . The duration of step (ii) is preferably in the range of 3 to 40 min, more preferably 5 to 30 min and most preferably 10 to 20 min. However, if thinner or thicker deposits are desired, the duration of step (ii) can be outside above-described ranges and adjusted accordingly.

I t is preferable to replenish components in the inventive electroless gold plating bath which are being consumed during plating. The replenishment can be done continuously, in certain time inter vals or whenever necessary. Such components are inter alia the gold ions and the sulfite ions.

I n one embodiment of the invention, the inventive electroless gold plating bath is used with hori zontal, vertical or spray plating equipment.

I t is an advantage of the present invention that the stability of the inventive electroless gold plat ing bath is high compared to prior art plating baths. Stability herein is to be understood as the lifetime of the bath prior to precipitation of compounds from the bath (“plate-out”) rendering it useless for plating purposes. Further advantageously, the inventive electroless gold plating bath is very little adversely affected by the presence of contaminants such as copper ions and/or nickel ions (see Example 2) . The inventive gold plating bath form homogeneous gold deposits with little gold layer thickness deviation. The coefficient of variance (standard deviation) of the gold layer thickness is below 1 0% or even below 7% . This little deviation is advantageously achievable even when forming gold deposits on various surface having different sizes at the same time.

The inventive electroless gold plating bath is useful for depositing gold on a surface of a substrate.

Thus, in one embodiment of the invention, the inventive electroless gold plating bath is used to deposit a gold layer on a surface of a substrate. The surfaces whereon gold is preferably deposited are those described above.

The present invention further concerns a gold layer deposited by the inventive method. Additional ly, the present invention is directed to a substrate comprising a gold layer deposited by the in ventive method. Preferable gold layer thicknesses to be used in the industry range from 20 to 80 nm, more preferably from 30 to 70 nm, even more preferably from 40 to 60 nm. I ndustrial Applicability

The present invention is particularly useful in the electronic industry. Applications of the inventive electroless gold plating bath include the manufacturing of printed circuit boards and I C substrates. Of particular interest are the formed gold layers as wire bondable and solderable surfaces to form interconnections in electronic devices.

Examples

The invention will now be illustrated by reference to the following non-limiting examples.

Commercial products were used as described in the technical datasheet available on the date of filing of this specification unless stated otherwise hereinafter. Cleaner Pro Select SF, etch cleaner MicroEtch C (Vertical) , Aurotech Pre Dip, Aurotech Activator 1 000 and Aurotech HP were products obtained from Atotech Deutschland GmbH. Test panels were copper-clad double-sided printed circuit boards in all plating experiments described herein. The deposit thickness was measured at 10 copper pads on each side of the test panels. The cho sen copper pads had different sizes and are used to determine the layer thickness by XRF using the XRF instrument Fischerscope X-RAY XDV-m ( Helm ut Fischer GmbH, Germany) . By assuming a layered structure of the deposit the layer thickness can be calculated from such XRF data. The plating rate was calculated by dividing the obtained layer thickness by the time necessary to obtain said layer thickness.

Example 1 (inventivet :

An electroless gold plating bath containing the following components was prepared by dissolution of the following components in water. Firstly, a suitable vessel was charged with deionized water (Dl water) . Then, the components were added in the sequence given below :

1 . EDTMP (provided as free acid) 40 mmol/L

2. tartaric acid 200 mmol/L

3. sulfite ions (provided as sodium sulfite) 50 mmol/L

4. iodide ions (provided as potassium iodide) 7.5 mmol/L

5. gold ions (provided as sodium gold sulfite) 5 mmol/l Prior to the addition of the gold ions, the pH value was adj usted to approximately pH 5. As a final step prior to any plating processes, the pH value of the electroless gold plating bath was adj usted with sulfuric acid and sodium hydroxide to 7.0.

The above mentioned test panels were subjected to the following process steps (Table 1 ) by dip- ping the substrates into the respective solutions employing the given parameters:

Table 1 : Process sequence for gold plating.

After this process sequence the thicknesses of the individual metal layers were measured. Gold thicknesses of more than 40 nm were obtained at a coefficient of variance (COV) up to 7% , re spectively. The plating rate thus was approximately 240 nm/h and, thus, exceeded today’s industry requirements. The gold layers formed on the substrate were lemon yellow and adhered well to the underlying substrate.

Further, the thus obtained substrates were submitted to solderability tests according to I PC J-STD- 003C WAM1 (published in 2014) : Edge dip test: All test boards at as received condition, 2x reflow at I PC condition (accord ing to above named standard) , and after 8 h aging at 72 °C and 85% relative humidity passed the test.

Wetting balance test: All test boards at as received condition passed the set A criteria. All test boards at 2x reflow at I PC condition, and after 8 h aging at 72 °C and 85% relative humidity passed the set B criteria

Wire bonding tests according to DVS 281 1 (published in 201 7) were passed at as received condi tion and after 3 h ageing at 120°C. No corrosion events could be found in the micro cross sectional investigations by means of FESEM (field emission secondary electron microscope) .

Plating tests in a gold plating tank (made of PVDF) showed these good results even at 10 MTO whereby the ageing were reached by continuous plating of copper clads on FR4 materials.

Example 2: Artificial longevity tests of electroless gold plating baths

Electroless gold plating baths containing the components as given in the subsequent table were prepared by dissolving the named components in water (using the order of the previous example wherever possible) . The pH of the plating baths were set to 7.0 with sulfuric acid and sodium hy droxide. I n a first run, the electroless gold plating baths were used to plate gold on test panels. The results are summarized in the following table.

I n a second run, the electroless gold plating baths were additionally contaminated with 5 mg/L Cu (provided as copper( l l ) sulfate pentahydrate) and 1 .5 g/L Ni (provided as nickel( l l ) sulfate hexa- hydrate) . These concentrations were typically found in electroless gold plating baths after 1 0 MTOs when plating on test panels such as those described in Example 1 (having a NiP-surface on copper lines) . The electroless gold plating baths were again used for plating on test panels.

Plating tests were performed as explained in example 1 .

Table 2: Overview of plating results.

¹⁾ 1 -hydroxyethane 1 , 1 -diphosphonic acid (etidronic acid) , ² * corresponds to a plating bath of 1 0

MTOs, ³⁾ complete bath plate out Comparative examples I I I to V showed plating rates which are not acceptable in the electronic industry. Comparative examples VI I and VI I I allowed for initially sound plating rates, but once they contained contaminants such as Cu ions and Ni ions, their plating rates dropped completely.

The inventive examples 1 , 1 1 and VI allowed for sufficiently high plating rates before and after addi tion of the contaminants. The gold layers were of a lemon yellow color and adhered well to the substrates. I nventive example X which additionally contained thiosulfate achieved a considerably lower plating rate both, before and after addition of the contaminants.

Other embodiments of the present invention will be apparent to those skilled in the art from a con sideration of this specification or practice of the invention disclosed herein. I t is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being defined by the following claims only.