Electroplating bath composition for electroplating noble metal and method for depositing noble metal layer

Abstract

The present invention provides an aqueous precious metal electroplating bath and a method of depositing a precious metal, the aqueous precious metal electroplating bath comprising: (a) at least one source of precious metal ions; (b) at least one source of hexacyanoferrum (II) acid ions and / or hexacyanoferrum (III) acid ions; (c) optionally, at least one halide source; (d) at least one complexing agent; (e) at least one noble metal ion reducing agent; (f) optionally, at least one humectant; and (g) optionally, at least one accelerator. The aqueous noble metal electroplating baths are suitable for providing soft noble metal layers for wire bonding and soldering applications required for electronic components.

Description

Technical Field

[0001] The present invention relates to aqueous precious metal electroplating bath compositions for electroplating precious metal layers onto substrates and a method for depositing precious metals. The electroplating baths are particularly suitable for use in the manufacture of printed circuit boards, IC substrates, semiconductor devices, interposers made of glass, and the like. Background Art

[0002] Noble metal layers are of great significance in the manufacture of electronic components and the semiconductor industry. Gold layers are often used as surfaces that can be soldered and / or wire-bonded in the manufacture of printed circuit boards, IC substrates, semiconductor devices, and the like. Typically, the gold layer is used as the final finish before soldering and wire bonding. In order to provide an electrical connection with sufficient conductivity and robustness between the copper wire and the wires to which it is bonded, while providing good strength for wire bonding, there are conventional multi-layer assemblies used in the art. Among them are electroless nickel electroless gold plating (ENIG), electroless nickel electroless palladium immersion gold plating (ENEPIG), direct immersion gold plating (DIG), electroless palladium immersion gold plating (EPIG), and electroless palladium autocatalytic gold plating (EPAG). Although these technologies have been established for some time, there are still many unresolved challenges. These challenges are the corrosion (nickel corrosion) of the nickel layer placed between the gold wire and the copper wire and the insufficient stability of the gold plating bath, which is extremely undesirable due to the cost of the bath. In addition, it is very desirable to deposit the gold layer at a sufficient plating rate so as to operate the manufacturing process economically. Another desired property of the gold layer is the optical appearance, which should be lemon yellow, as discoloration of the gold layer is unacceptable.

[0003] Silver is a cost-effective alternative to gold in many applications.

[0004] Due to the extremely small size of today's electronic components, it is impossible to use an electrolytic process that needs to be electrically connected to the substrate. Therefore, an electroless metal deposition process (electroless plating) is used. Electroless plating generally describes a method that does not use an external current source to reduce metal ions. Electroplating methods using an external current source are generally described as electrolytic or current plating methods. Non-metallic surfaces can be pretreated so that they can accept or catalyze metal deposition. All or selected portions of the surface can be suitably pretreated.

[0005] In principle, electroless precious metal plating baths can be divided into two types: autocatalytic precious metal plating baths and immersion precious metal plating baths. Hybrid forms of autocatalytic and immersion plating baths and electrolytic plating baths are also known. In the context of the present invention, the aforementioned terminology applies. However, in the literature, the distinction is often less clear; for example, the more general term 'electroless' may be used instead of the more specific term 'autocatalytic'. Mutual modifications are also applicable in the literature to the term 'electroless' instead of 'immersion'.

[0006] Typically, the main components of an autocatalytic metal bath are a metal salt, a reducing agent, and optional complexing agents, pH adjusters, and additives (e.g., stabilizers). Complexing agents (also known in the art as chelating agents) serve to chelate the deposited metal and prevent the metal from precipitating out of solution (i.e., in the form of hydroxides and the like). Chelating the metal makes it available to the reducing agent, which converts the metal ion into a metallic form.

[0007] Another form of metal deposition is immersion plating. Immersion plating is a different type of metal deposition that uses neither an external current source nor a chemical reducing agent. The mechanism relies on the replacement of metal ions present in the immersion plating solution by metal from the underlying substrate. This is a significant disadvantage of immersion plating, as the deposition of thicker layers is often limited by the layer's porosity.

[0008] In most cases, electroless gold plating baths use one or two types of electroless plating. Even if a reducing agent has been added to the plating bath, immersion plating can still occur, although at a significantly reduced rate.

[0009] EP 0 343 816 A1 discloses an aqueous solution for use as an electroless gold deposition bath, comprising a gold source and a reducing agent. The solution further comprises a reducing stabilizer selected from a group consisting of a mixture of alkali metal or ammonium ferrocyanide and alkali metal or ammonium ferrocyanide; 1-H-tetrazole; a redox mediator; and mixtures thereof.

[0010] WO 2017 / 050662 discloses electroless plating baths and ethylenediamine derivatives as plating bath enhancers.

[0011] Objectives of the invention

[0012] The present invention aims to provide an aqueous precious metal electroplating bath composition and a method for depositing precious metal layers at a sufficient plating rate. Another object of the present invention is to provide an aqueous precious metal electroplating bath having sufficient stability and long-term usability.

[0013] Another aim is to improve the corrosion performance of current state-of-the-art electroplating baths.

[0014] Cyanide compounds are usually used as a source of gold ions. However, in order to stabilize the electroplating bath, additional cyanide compounds (e.g. potassium cyanide) are usually used. These baths have two disadvantages: the toxicity of these additional cyanide compounds can lead to problems in the process itself, necessitating the establishment of appropriate safety measures, and problems with wastewater disposal. In addition, the dosage of the additional cyanide compounds is not easy, and underdosing and overdosing can occur, leading to problems such as reduced coating. Therefore, a further object is to provide an electroplating bath with a reduced cyanide content. Another object is that the resulting noble metal layer does not discolor.

[0015] Hydrazine is a known reducing agent. However, it is also known for its high toxicity. Therefore, another object of the present invention is to reduce the toxicity of aqueous precious metal electroplating baths by providing an aqueous precious metal electroplating bath that does not include hydrazine and / or any hydrazine derivatives. Summary of the Invention

[0016] These objects are solved by an aqueous precious metal electroplating bath according to the invention, comprising

[0017] (a) at least one source of noble metal ions;

[0018] (b) at least one source of hexacyanoferrate(II) ions and / or hexacyanoferrate(III) ions;

[0019] (c) optionally, at least one halide ion source;

[0020] (d) at least one complexing agent;

[0021] (e) at least one noble metal ion reducing agent;

[0022] (f) optionally, at least one wetting agent; and

[0023] (g) Optionally, at least one accelerator.

[0024] These objects are solved by a method for depositing a gold layer from the above-described electroplating bath and by using the above-described electroplating enhancer compound in a gold electroplating bath comprising at least one gold ion source and at least one gold ion reducing agent. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Demonstrated stability in various electroplating baths (see Example 1).

[0026] Figures 2 to 5 The stability of the plating baths according to Examples 2 to 5 and the ENEPIG plating rates are demonstrated. DETAILED DESCRIPTION

[0027] electroplating bath

[0028] The aqueous precious metal electroplating bath according to the present invention is synonymously referred to as an aqueous solution. The term "aqueous solution" means that the primary liquid medium, serving as the solvent in the solution, is water. Other water-miscible liquids, such as water-miscible alcohols and other polar organic liquids, may be added. In principle, the aqueous solution comprises more than 50% water by weight.

[0029] Preferably, the plating bath of the present invention is an electroless precious metal plating bath. Electroless precious metal plating baths include various types, such as, but not limited to, immersion precious metal plating baths, autocatalytic precious metal plating baths, and precious metal plating baths that utilize a mixture of autocatalytic and immersion plating baths and electrolytic plating baths. In a more preferred embodiment, the precious metal plating bath of the present invention is an autocatalytic precious metal plating bath.

[0030] An immersion precious metal electroplating bath typically includes at least the following components: at least one chelating agent; at least one gold source; and at least one pH adjuster. Additionally, the immersion precious metal electroplating bath may include additives such as, but not limited to, at least one grain refiner, at least one wetting agent, and / or at least one accelerator.

[0031] An autocatalytic precious metal electroplating bath typically includes at least the following components: at least one chelating agent; at least one gold source; at least one pH adjuster; at least one reducing agent; and at least one stabilizer. Additionally, the autocatalytic precious metal electroplating bath may include additives such as, but not limited to, at least one grain refiner, at least one wetting agent, and / or at least one accelerator.

[0032] The electroplating bath according to the present invention can be prepared by dissolving all components in an aqueous liquid medium, preferably in water.

[0033] (a) Noble metal ion source

[0034] The aqueous precious metal electroplating bath according to the present invention comprises at least one source of precious metal ions.

[0035] In the context of the present invention, the term 'noble metal' shall describe a metal selected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt, Au. These are elements from Groups 8 to 11 of Periods 5 and 6 of the Periodic Table of the Elements. Preferably, the noble metal is selected from the group consisting of Au and Ag. More preferably, the noble metal is Au. The concentration of the noble metal ions in the aqueous noble metal electroplating bath according to the present invention is preferably in the range of 0.5 to 50 mmol / L, more preferably 1.0 to 30 mmol / L, and most preferably 2.0 to 10 mmol / L. If the aqueous noble metal electroplating bath according to the present invention contains two or more noble metal ions, the concentration is based on the total amount of the noble metal ions.

[0036] Gold ions can be in Au + 、Au 3+or two oxidation states. The gold ion source can be any water-soluble gold salt having the aforementioned oxidation states. Preferably, the gold ion source is selected from the group consisting of gold cyanide, ammonium gold cyanide, gold(I) alkali metal cyanide (including potassium gold(I) cyanide and sodium gold(I) cyanide), trisodium gold disulfite, tripotassium gold disulfite and triammonium gold disulfite, gold thiosulfate, gold thiocyanide, gold sulfate, gold chloride and gold bromide. Preferably, the gold ion source is gold(I) alkali metal cyanide and can be added to the aqueous electroplating bath in the form of a solution containing such a salt. The concentration of gold ions in the aqueous gold electroplating bath according to the present invention is preferably in the range of 0.5 to 50 mmol / L, more preferably 1.0 to 30 mmol / L, and most preferably 2.0 to 10 mmol / L.

[0037] Silver ions in Ag + oxidation state. The silver ion source can be any water-soluble silver salt having the aforementioned oxidation state. Preferably, the silver ion source is at least one metal silver cyanide salt M[Ag(CN)2], wherein M is selected from the group consisting of alkali metals and ammonium. Preferably, the silver ion source is potassium silver cyanide salt K[Ag(CN)2] and can be added to the aqueous electroplating bath in the form of a solution containing this salt. The concentration of silver ions in the aqueous silver electroplating bath according to the present invention is preferably in the range of 0.5 to 50 mmol / L, more preferably 1.5 to 30 mmol / L, and most preferably 4.0 to 20 mmol / L.

[0038] (b) a source of hexacyanoferrate (II) ions and / or hexacyanoferrate (III) ions

[0039] It is known that cyanide ions form stable complexes with iron (II) and / or iron (III) ions. The corresponding complexes are called hexacyanoferrate (II) and hexacyanoferrate (III), respectively. These ions are anions with four (in the case of hexacyanoferrate (II)) and three (in the case of hexacyanoferrate (III)) negative charges. Typically, these ions are neutralized by appropriate cations. The source of hexacyanoferrate (II) and / or hexacyanoferrate (III) ions can be any water-soluble substance thereof. Preferably, the source of hexacyanoferrate (II) and / or hexacyanoferrate (III) ions is selected from the group consisting of ammonium hexacyanoferrate (II), alkali metal hexacyanoferrate (II) ions (including potassium hexacyanoferrate (II) and sodium hexacyanoferrate (II)); ammonium hexacyanoferrate (III), alkali metal hexacyanoferrate (III) salts (including potassium hexacyanoferrate (III) and sodium hexacyanoferrate (III)); and mixtures thereof. Preferably, the source of hexacyanoferrate (II) and / or hexacyanoferrate (III) ions is selected from potassium hexacyanoferrate (II), potassium hexacyanoferrate (III) and mixtures thereof, and can be added to the aqueous electroplating bath in the form of a solution containing such salts. The concentration of hexacyanoferrate (II) ions and / or hexacyanoferrate (III) ions in the aqueous precious metal electroplating bath according to the present invention is preferably in the range of 0.1 to 50 mmol / L, more preferably 0.3 to 30 mmol / L and most preferably 0.6 to 10 mmol / L. If the aqueous precious metal electroplating bath according to the present invention contains both hexacyanoferrate (II) ions and hexacyanoferrate (III) groups, the concentration is based on the total amount of hexacyanoferrate (II) ions and hexacyanoferrate (III) groups.

[0040] (c) Halide source

[0041] The halide source may be any water-soluble halide. Preferably, the halide is bromide and / or iodide. Preferably, the halide source is selected from the group consisting of: ammonium bromide, alkali metal bromides (including potassium bromide, sodium bromide); ammonium iodide, alkali metal iodides (including potassium iodide and sodium iodide); and mixtures thereof. Preferably, the halide source can be added to the aqueous electroplating bath in the form of a solution containing such salts. The concentration of the halide in the electroless aqueous gold electroplating bath according to the present invention is preferably in the range of 0.1 to 50 mmol / L, more preferably 0.3 to 30 mmol / L and most preferably 1.0 to 10 mmol / L. If the aqueous precious metal electroplating bath according to the present invention contains two or more halide ions, the concentration is based on the total amount of halide ions.

[0042] If silver ions are present as the noble metal ions (a) in the noble metal electroplating bath, the concentration of halide ions is zero, ie, no halide ions should be present in the noble metal electroplating bath.

[0043] (d) at least one complexing agent

[0044] The aqueous precious metal electroplating bath according to the present invention optionally further comprises at least one complexing agent. The optional at least one complexing agent present in the aqueous precious metal electroplating bath according to the present invention is preferably selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, diaminecarboxylic acids, phosphonic acids, aminophosphonic acids, or salts thereof. The optional at least one complexing agent acts as a complexing agent for precious metal ions and metal ions (e.g., nickel ions or copper ions) dissolved from the substrate during electroplating. Preferred carboxylic acids are, for example, oxalic acid or its salts. Preferred hydroxycarboxylic acids are, for example, tartaric acid, citric acid, lactic acid, malic acid, gluconic acid, and salts thereof. Preferred aminocarboxylic acids are, for example, glycine, cysteine, methionine, and salts thereof. Preferred diaminecarboxylic acids are cyclohexanediaminetetracarboxylic acid (CDTA) and ethylenediaminetetracarboxylic acid (EDTA). Preferred phosphonic acids are 1-hydroxyethane-(1,1-diphosphonic acid) (HEDP). Preferred aminophosphonic acids are nitrogen tris (methylphosphonic acid) (commonly abbreviated as ATMP), diethylenetriamine penta (methylphosphonic acid) (commonly abbreviated as DTPMP) and ethylenediamine tetra (methylenephosphonic acid) (commonly abbreviated as EDTMP). In all cases, the sodium, potassium and ammonium salts of the compounds are also suitable. The concentration of the optional at least one complexing agent is preferably in the range of 0.25 to 250 mmol / L, more preferably 1.0 to 150 mmol / L.

[0045] More preferably, the aqueous precious metal electroplating bath according to the present invention comprises two different complexing agents and / or salts thereof, such as an aminocarboxylic acid or a salt thereof and a phosphonic acid or a salt thereof.

[0046] (e) Noble metal ion reducing agent

[0047] The aqueous precious metal electroplating bath further includes at least one precious metal ion reducing agent.

[0048] The noble metal ion reducing agent is preferably selected from the group consisting of: aliphatic aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, α-methylvaleraldehyde, β-methylvaleraldehyde, γ-methylvaleraldehyde, etc.; aliphatic dialdehydes such as glyoxal, succinaldehyde, etc.; aliphatic unsaturated aldehydes such as crotonaldehyde, etc.; aromatic aldehydes such as benzaldehyde, o-nitrobenzaldehyde, m-nitrobenzaldehyde, p-nitrobenzaldehyde, o-methylbenzaldehyde, m-methylbenzaldehyde, p-methylbenzaldehyde, etc. , o-hydroxybenzaldehyde, m-hydroxybenzaldehyde, p-hydroxybenzaldehyde, phenylacetaldehyde, etc.; sugars with aldehyde groups (-CHO), such as glucose, galactose, mannose, ribose, maltose, lactose, etc.; and formaldehyde precursors, such as urotropin, 1,3-bis(hydroxymethyl)-5,5-dimethylimidazolidine-2,4-dione (DMDM-hydantoin), paraformaldehyde, glyoxylic acid, glyoxylic acid sources, and glycolic acid. The term "glyoxylic acid source" encompasses glyoxylic acid and all compounds that can be converted to glyoxylic acid in aqueous solution. In aqueous solution, the acid containing the aldehyde is in equilibrium with its hydrate. Suitable glyoxylic acid sources are dihaloacetic acids (e.g., dichloroacetic acid), which will hydrolyze to a hydrate of glyoxylic acid in an aqueous medium. Alternative glyoxylic acid sources are bisulfite adducts, such as hydrolyzable esters or other acid derivatives. Bisulfite adducts can be added to the electroless aqueous gold plating bath according to the present invention or formed in situ. Bisulfite adducts can be made from glyoxylate and bisulfite, sulfite or metabisulfite. Formaldehyde, a glyoxylic acid source and glyoxylic acid are preferred, with formaldehyde being most preferred.

[0049] Hydrazine is a known reducing agent. However, it is also known for its high toxicity. Therefore, it is an object of the present invention to provide an aqueous precious metal electroplating bath that does not contain hydrazine and / or any hydrazine derivatives.

[0050] The aqueous precious metal electroplating bath is characterized by the absence of hydrazine and / or any hydrazine derivatives.

[0051] The concentration of the at least one noble metal ion reducing agent is preferably in the range of 0.1 to 500 mmol / L, more preferably 1 to 300 mmol / L, even more preferably 2 to 200 mmol / L and most preferably 5 to 100 mmol / L.

[0052] (f) Wetting agents;

[0053] Wetting agents are surface-active molecules used to reduce the surface tension of water. While various wetting agents and their application methods are known to those skilled in the art, the preferred wetting agent is ethylene glycol. The concentration of the at least one noble metal ion wetting agent is preferably in the range of 0.02 to 9.0 mol / L, more preferably 0.2 to 5.4 mol / L, and even more preferably 1.0 to 3.6 mol / L.

[0054] (g) Accelerator

[0055] The aqueous precious metal electroplating bath according to the present invention optionally includes an accelerator selected from the group consisting of thallium ions, arsenic ions, selenium ions, and lead ions. This crystal modifier is preferably added to the electroless aqueous precious metal electroplating bath according to the present invention in a concentration range of 0.00005 to 0.5 mmol / L, more preferably 0.0001 to 0.25 mmol / L, and most preferably 0.00015 to 0.1 mmol / L. Useful sources of such ions may be their water-soluble salts, such as the corresponding nitrates, sulfates, and halides.

[0056] In addition to these listed components, the precious metal electroplating baths of the present invention may include other components such as, but not limited to, one or more stabilizers and / or one or more plating enhancers.

[0057] Optional other ingredients - stabilizers

[0058] The electroless aqueous gold plating bath according to the present invention optionally includes at least one stabilizer selected from the group consisting of: a cyanide ion source; hydantoin and its alkyl derivatives, such as alkylhydantoin and dialkylhydantoin, wherein the alkyl residue in this context includes a C1 to C8 alkyl group which may be cyclic and / or alicyclic, branched or unbranched, preferably a methyl group; sulfur compounds, such as 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, mercaptoacetic acid, 3-(2-benzothiazolylthio)-1-propanesulfonic acid, mercaptosuccinic acid, thiosulfuric acid, thioethylene glycol, thiourea, thiomalic acid, and the like; and aromatic nitrogen compounds, such as benzotriazole, 1,2,4-aminotriazole, and the like. Suitable cyanide ion sources may be any soluble cyanide salt, such as alkali metal cyanides, including sodium cyanide and potassium cyanide.

[0059] Due to toxicity and problematic dosing, one goal of the present invention is to minimize or even eliminate the use of stabilizers selected from the group consisting of cyanide ions. Therefore, in a preferred embodiment of the present invention, the electroplating solution of the present invention does not include stabilizers selected from the group consisting of cyanide ions. These cyanide stabilizers contain cyanide ions in free form. Although obvious, hexacyanoferrate compounds are said to not fall within this definition because the cyanide ligands form highly stable complexes with the iron core.

[0060] However, for example when HSE regulations are less stringent, the electroplating bath of the present invention may comprise at least one stabilizer selected from cyanide ions.

[0061] The concentration of the optional stabilizer can be selected based on its chemical structure and can be determined by one skilled in the art in routine experimentation. The concentration of the optional stabilizer is preferably in the range of 0.0001 to 200 mmol / L, more preferably in the range of 0.001 to 125 mmol / L and most preferably in the range of 0.01 to 75 mmol / L. These stabilizers are conventionally added to electroless gold plating baths to improve their service life and prevent precipitation.

[0062] In a preferred embodiment, two or more stabilizers are used. More preferably, a cyanide ion source is selected at a concentration of 0.0003 to 5 mmol / L, and one or more of hydantoin and its alkyl derivatives are selected at a concentration of 1 to 200 mmol / L, more preferably 10 to 125 mmol / L, and most preferably 20 to 75 mmol / L, and / or a sulfur compound is selected at a concentration of 0.001 to 50 mmol / L, more preferably 0.001 to 30 mmol / L, and most preferably 0.001 to 20 mmol / L.

[0063] Optional other components - electroplating enhancers

[0064] The electroplating bath of the present invention may further comprise at least one ethylenediamine derivative of formula (I) disclosed in WO 2017 / 050662 as an electroplating bath enhancer. The ethylenediamine derivative of formula (I) will be referred to herein as an electroplating enhancer compound.

[0065] Electroplating enhancer compound of formula (I)

[0066]

[0067] With residue R 1 and R 2 , said residue comprises 2 to 12 carbon atoms and is selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl or a combination thereof, wherein the individual residue R 1 and R 2 Same or different.

[0068] The amine moiety in the plating enhancer compound of formula (I) is a secondary amine moiety.

[0069] In a preferred embodiment of the present invention, the residue R of the electroplating enhancer compound of formula (I) 1 and R 2 It comprises 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, even more preferably 2 to 4 carbon atoms.

[0070] In another preferred embodiment of the present invention, the residue R in formula (I) 1 and R 2 In another preferred embodiment of the present invention, the alkyl residue R in formula (I)1 and R 2 In another preferred embodiment of the present invention, the residue R in formula (I) is 1 and R 2 In another more preferred embodiment of the present invention, the residue R 1 and R 2 It does not contain any other amino moieties and / or any hydroxyl moieties. Even more preferably, the alkyl residue contains no substituents and consists only of carbon and hydrogen atoms.

[0071] Particularly preferably, the plating enhancer compound is selected from the group consisting of: N 1 ,N 2 -Diethylethane-1,2-diamine, N 1 ,N 2 -Dipropylethane-1,2-diamine, N 1 ,N 2 -Di-isopropylethane-1,2-diamine, N 1 ,N 2 -Dibutylethane-1,2-diamine, N 1 ,N 2 -Di-isobutylethane-1,2-diamine, N 1 ,N 2 -Di-tert-butylethane-1,2-diamine, N 1 ,N 2 -Dipentylethane-1,2-diamine, N 1 ,N 2 -Di-isopentylethane-1,2-diamine, N 1 ,N 2 -Di-sec-pentylethane-1,2-diamine, N 1 ,N 2 -Di-tert-amylethane-1,2-diamine, N 1 ,N 2 -Di-neopentylethane-1,2-diamine, N 1 ,N 2 -Dihexylethane-1,2-diamine, N 1 ,N 2 -Di-(1-methylpentyl)ethane-1,2-diamine, N 1 ,N 2 -Di-(2-methylpentyl)ethane-1,2-diamine, N 1 ,N 2 -Di-(3-methylpentyl)ethane-1,2-diamine, N 1 ,N 2 -Di-(4-methylpentyl)ethane-1,2-diamine, N 1 ,N2 -bis-(1,1-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -bis-(1,2-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -bis-(1,3-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -bis-(2,2-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -bis-(2,3-dimethylbutyl)ethane-1,2-diamine and N 1 ,N 2 -bis-(3,3-dimethylbutyl)ethane-1,2-diamine.

[0072] Most preferably, R 1 and R 2 is a branched alkyl residue having 3 to 6 carbon atoms.

[0073] The concentration of the at least one plating enhancer compound of formula (I) in the aqueous precious metal electroplating bath according to the present invention is preferably in the range of 0.5 to 1000 mmol / L, more preferably 1 to 200 mmol / L, even more preferably 5 to 75 mmol / L and most preferably 10 to 50 mmol / L. If the aqueous precious metal electroplating bath according to the present invention contains more than one plating enhancer compound, the concentration is based on the total amount of all plating enhancer compounds.

[0074] In another preferred embodiment of the present invention, the electroless aqueous precious metal plating bath according to the present invention contains no intentionally added source of a second reducible metal ion (not counting trace impurities typically present in technical raw materials), thereby allowing the formation of pure precious metal deposits. Pure precious metal deposits are soft, ductile, and particularly suitable for wire bonding and soldering. Trace impurities are understood to be compounds present in the technical raw materials at 1 wt% or less.

[0075] The pH of the electroless aqueous precious metal plating bath according to the present invention is preferably in the range of 5 to 10, more preferably 7 to 9, even more preferably 8 to 9. The target pH value is adjusted by using, for example, an acid (e.g., phosphoric acid) or a base (e.g., sodium hydroxide or potassium hydroxide). Continuous control and adjustment of the pH value during electroplating is advantageous and therefore preferred because it also improves the service life of the electroplating bath.

[0076] method

[0077] The method for depositing a noble metal layer onto a substrate comprises the following steps in sequence:

[0078] (i) providing a substrate;

[0079] (ii) contacting at least a portion of the surface of the substrate with an aqueous precious metal electroplating bath according to the present invention as described above;

[0080] And thereby a noble metal layer is deposited onto at least a portion of the surface of the substrate.

[0081] Such contacting is preferably achieved by immersing the substrate or at least a portion of the substrate surface in the electroplating bath, or by spraying the electroplating bath onto the substrate or at least a portion of the substrate surface.

[0082] At least a portion of the substrate surface is preferably composed of a metal or metal alloy, and the precious metal is deposited onto at least a portion of the substrate surface composed of a metal or metal alloy, the metal or metal alloy being selected from the group consisting of nickel, nickel alloys (e.g., nickel-phosphorus alloys, nickel-boron alloys), cobalt, cobalt alloys (e.g., cobalt-phosphorus alloys, cobalt-molybdenum-phosphorus alloys, cobalt-molybdenum-boron alloys, cobalt-molybdenum-boron-phosphorus alloys, cobalt-tungsten-phosphorus alloys, cobalt-tungsten-boron alloys, cobalt-tungsten-boron-phosphorus alloys), palladium, palladium alloys (e.g., palladium-phosphorus alloys, palladium-boron alloys), copper and copper alloys, and precious metals or precious metal alloys. The aqueous precious metal electroplating bath according to the present invention can be used to deposit a precious metal layer on a precious metal substrate and can be used to thicken an existing precious metal layer, for example, obtained from an immersion precious metal electroplating bath.

[0083] As is known in the art, the substrate can be pretreated before electroplating. This pretreatment includes a cleaning step with a solvent and / or surfactant to remove most organic contaminants; an etching step with an acid and optionally an oxidizing or reducing agent to remove oxides; and an activation step. The latter involves depositing a noble metal on the surface or a portion thereof to make it more receptive to electroplating. This noble metal can be palladium, which can be deposited on the surface in the form of a salt before being reduced to elemental palladium. Or it can be deposited in colloidal form and, where appropriate, an acceleration step with an acid (e.g., hydrochloric acid) to remove any protective colloid (e.g., tin colloid). Such activation layers are generally not discrete layers, but rather aggregates of palladium island structures. However, the activation layer is considered to be a metal substrate in the context of the present invention.

[0084] Preferably, the plating bath of the present invention is an electroless precious metal plating bath. Electroless precious metal plating baths include various types, such as, but not limited to, immersion precious metal plating baths, autocatalytic precious metal plating baths, and precious metal plating baths that utilize a mixture of autocatalytic and immersion plating baths and electrolytic plating baths. In a more preferred embodiment, the precious metal plating bath of the present invention is an autocatalytic precious metal plating bath.

[0085] During electroplating, the temperature of the aqueous precious metal electroplating bath according to the present invention is preferably in the range of 30 to 95° C., more preferably 70 to 90° C., even more preferably 75 to 85° C., even more preferably 77 to 84° C. The electroplating time is preferably in the range of 1 to 60 minutes, more preferably 5 to 30 minutes. However, if a thinner or thicker deposit is desired, the electroplating time may be outside the above range and adjusted accordingly.

[0086] Preferably, the components used during electroplating are replenished continuously or at certain time intervals. These components are in particular a source of noble metal ions, a noble metal ion reducing agent, at least one stabilizer, and a plating enhancer compound. If necessary, the pH value can also be adjusted continuously or at intervals.

[0087] The aqueous precious metal electroplating bath according to the present invention can be used with horizontal, vertical and spray electroplating equipment.

[0088] An advantage of the present invention is that the stability of the aqueous precious metal electroplating baths according to the present invention is improved compared to precious metal electroplating baths known in the art (see Examples 1 to 5). Stability is understood in this context as the useful life of the bath before compounds precipitate ("bleed out") from the bath, rendering it unusable for electroplating purposes.

[0089] Another advantage is that the aqueous precious metal electroplating baths according to the present invention allow for sufficient plating rates (deposition thickness of the electroplated metal layer over time) (see Examples 2 to 4). Most slightly stable electroplating baths known in the art do not allow for sufficient plating rates.

[0090] Thus, a unique feature of the aqueous precious metal electroplating bath according to the present invention is the provision of a highly stable precious metal electroplating bath with sufficient plating rates and thereby allowing for a more economically viable precious metal electroplating process.

[0091] The aqueous precious metal electroplating bath according to the present invention forms a uniform precious metal deposit with minimal variations in layer thickness. The standard deviation of the precious metal layer thickness is less than 10% or even less than 8%. This low deviation is advantageously achieved even when electroplating onto a variety of substrates with different dimensions.

[0092] The following non-limiting examples further illustrate the present invention.

[0093] Examples

[0094] General Procedures

[0095] Pro Select S8, MicroEtch C, Pre Dip, Activator 1000, CNN mod and PC 1 was purchased from Atotech Deutschland GmbH, Germany. In all cases, the gold ion source was K[Au(CN)2]. The silver ion source was K[Ag(CN)2].

[0096] In all experiments, a printed circuit test board was used as the substrate. On one side of the printed circuit test board, there were multiple layers with a diameter ranging from 0.25 to 49 mm. 2 Copper pads of different sizes are available in a range of sizes. The substrate is cleaned and etched, then activated with palladium. Subsequently, nickel (for ENIG) or nickel / palladium (for ENEPIG) is deposited on the copper surface, and a gold layer is electroplated on top. The individual pads have the following areas: 1: 0.25 mm 2 , 2:1mm 2 , 3:4mm 2 , 4:9mm 2 , 5:25mm 2 , 6:49mm 2 .

[0097] Determine the thickness of metal deposits and plating rates

[0098] The deposit thickness was measured at six pads on one side of the test plate. The selected copper pads had different sizes and were used to determine the layer thickness by XRF using an XRF instrument, Fischerscope XDV-SDD (Helmut Fischer GmbH, Germany). By assuming a layered structure of the deposit, the layer thickness can be calculated from the XRF data. The plating rate is calculated by dividing the obtained layer thickness by the time required to obtain the layer thickness.

[0099] The uniformity of the layer thickness, expressed as the coefficient of variation (COV), is determined as the standard deviation from the mean thickness value. In the case of gold, this term is abbreviated to Au COV.

[0100] Example 1:

[0101] Prepare a gold electroplating bath containing the following components by dissolving all components in water:

[0102] Table 1:

[0103]

[0104] The stability of the bath solution has been tested at 82°C. The results are presented in Figure 1 The test was stopped after 56.5 hours.

[0105] Example 2:

[0106] Prepare a gold electroplating bath containing the following components by dissolving all components in water:

[0107] Table 2:

[0108]

[0109]

[0110] By immersing the substrate in the corresponding solution with given parameters, the substrate is subjected to the following process steps (Table A): Table A: Gold plating process flow

[0111]

[0112] After the process flow, the thickness of the individual metal layers was measured. The plating rate was calculated as described above. The bath stability and ENEPIG plating rate were tested. The results are presented in Figure 2 middle.

[0113] Example 3:

[0114] Prepare a gold electroplating bath containing the following components by dissolving all components in water:

[0115] Table 3:

[0116]

[0117]

[0118] The substrates were subjected to the process steps described in Table A of Example 2 by immersing the substrates in the corresponding solutions:

[0119] After the process flow, the thickness of the individual metal layers was measured and the plating rate was calculated as described above.

[0120] The bath stability and ENEPIG plating rate were tested. The results are presented in Figure 3 The test was stopped after 174 hours.

[0121] Example 4:

[0122] Prepare a gold electroplating bath containing the following components by dissolving all components in water:

[0123] Table 4:

[0124]

[0125] The substrates were subjected to the process steps described in Table A of Example 2 by immersing the substrates in the corresponding solutions:

[0126] After the process flow, the thickness of the individual metal layers was measured and the plating rate was calculated as described above.

[0127] Detected with and without Cu 2+ / Ni 2+ / Fe 2+ ion bath ENEPIG plating rate and Au COV. The results are presented in Figure 4 middle.

[0128] Example 5:

[0129] Prepare a gold electroplating bath containing the following components by dissolving all components in water:

[0130] Table 5:

[0131]

[0132] The substrates were subjected to the process steps described in Table A of Example 2 by immersing the substrates in the corresponding solutions:

[0133] After the process flow, the thickness of the individual metal layers was measured and the plating rate was calculated as described above.

[0134] Baths with and without diol derivatives were tested for ENEPIG plating rate, Au COV, and stability. All conditions were aged until the total Au metal conversion was 2 g / L with no Au precipitation. The results are presented in Figure 5 middle.

[0135] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered exemplary only, with the true scope of the invention being defined solely by the following claims.

Claims

1. An aqueous precious metal electroplating bath comprising (a) at least one source of noble metal ions; (b) at least one source of hexacyanoferrate(II) ions and / or hexacyanoferrate(III) ions; (c) optionally, at least one halide source; (d) at least one complexing agent; (e) at least one noble metal ion reducing agent; (f) optionally, at least one wetting agent; and (g) Optionally, at least one accelerator.

2. The aqueous precious metal electroplating bath according to claim 1 , wherein the at least one precious metal ion reducing agent is selected from the group consisting of aliphatic aldehydes, aliphatic dialdehydes, aliphatic unsaturated aldehydes, aromatic aldehydes, sugars having an aldehyde group, formaldehyde precursors, amines, DMAB, hydroxylamine sulfate, and mixtures thereof, preferably selected from the group consisting of formaldehyde precursors, formaldehyde / amine mixtures, DMAB, and hydroxylamine sulfate.

3. An aqueous precious metal electroplating bath according to any one of the preceding claims, wherein hydrazine and / or any hydrazine derivatives are absent.

4. The aqueous precious metal electroplating bath according to any one of the preceding claims, wherein the pH of the aqueous precious metal electroplating bath is in the range of 5 to 10, preferably 7 to 9, more preferably 8 to 9.

5. An aqueous precious metal electroplating bath according to any one of the preceding claims, wherein the concentration of precious metal ions is in the range of 0.5 to 50 mmol / L, more preferably 1.0 to 30 mmol / L and most preferably 2.0 to 10 mmol / L.

6. The aqueous precious metal electroplating bath of any one of the preceding claims, wherein the aqueous precious metal electroplating bath further comprises at least one complexing agent selected from the group consisting of a carboxylic acid, a hydroxycarboxylic acid, an aminocarboxylic acid, a phosphonic acid, or a salt thereof.

7. A method for depositing a noble metal layer onto a substrate, comprising the following steps in sequence: (i) providing a substrate; (ii) contacting at least a portion of the surface of the substrate with the aqueous precious metal electroplating bath according to claims 1 to 6. And thereby a noble metal layer is deposited onto the at least a portion of the surface of the substrate.

8. A method for depositing a precious metal layer onto a substrate according to claim 7, wherein at least a portion of the surface consists of a metal or a metal alloy, and the precious metal is subsequently deposited onto at least a portion of the surface consisting of a metal or a metal alloy, the metal or metal alloy being selected from the group consisting of nickel, nickel alloys (e.g., nickel-phosphorus alloys, nickel-boron alloys), cobalt, cobalt alloys (e.g., cobalt-phosphorus alloys, cobalt-molybdenum-phosphorus alloys, cobalt-molybdenum-boron alloys, cobalt-molybdenum-boron-phosphorus alloys, cobalt-tungsten-phosphorus alloys, cobalt-tungsten-boron alloys, cobalt-tungsten-boron-phosphorus alloys), palladium, palladium alloys (e.g., palladium-phosphorus alloys, palladium-boron alloys), copper and copper alloys, and gold or gold alloys.

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