Plating bath composition for plating precious metals and method for depositing a precious metal layer

Abstract

A water-based precious metal plating bath comprising (a) at least one source of precious 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 reducing agent for precious metal ions; (f) optionally at least one wetting agent; and (g) optionally at least one accelerator, and a method for depositing precious metals. The water-based precious metal plating bath is suitable for providing a soft precious metal layer useful for wire bonding and soldering applications required for electronic components.

Description

Technical Field

[0001] The present invention relates to an aqueous noble metal plating bath composition for plating a noble metal layer on a substrate and a method for depositing a noble metal. The plating bath is particularly suitable for the manufacture of printed circuit boards, IC substrates, semiconductor devices, glass interposers, and the like.

Background Art

[0002] Noble metal layers are of utmost importance in the manufacture of electronic components and in the semiconductor industry. Gold layers are often used as solderable surfaces and / or wire bondable surfaces in the manufacture of printed circuit boards, IC substrates, semiconductor devices, and the like. Typically, they are used as the final finish before soldering and wire bonding. There are various layer assemblies conventionally used in the art to provide a sufficiently conductive and robust electrical connection between a copper wire and a wire bonded thereto while providing sufficient strength for wire bonding. In particular, there 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 techniques have been established quite some time ago, many unsolved problems still remain. Such problems are the corrosion of the nickel layer (nickel corrosion) disposed between the gold wire and the copper wire and the insufficient stability of the gold plating bath, which is highly undesirable due to the cost of the bath. Also, it is highly desirable to deposit the gold layer at a sufficient plating rate in order to economically carry out the manufacturing process. Another desirable property of the gold layer is an optical appearance that should be lemon yellow when discoloration of the gold layer is not tolerated.

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

[0004] Due to the extremely small size of recent electrical components, it is impossible to use electrolytic methods that require electrical connection to the substrate. Therefore, electroless metal deposition methods (electroless plating) are used. Electroless plating generally describes a method that does not use an external current source for the reduction of metal ions. Plating methods that use an external current source are generally described as electrolytic plating methods or galvanic plating methods. Non-metallic surfaces can be pretreated to make them sensitive or catalytic to metal deposition. All or selected parts of the surface can be appropriately pretreated.

[0005] In principle, two types of electroless precious metal plating baths can be identified: autocatalytic precious metal plating baths and replacement-type precious metal plating baths. Mixed forms of autocatalytic plating baths, replacement-type plating baths, and electrolytic plating baths are also known. In the context of the present invention, the foregoing terms apply. However, in the literature, the identification is often not very clear. For example, the more general term "electroless" can be used instead of the more detailed term "autocatalytic". This also applies, with the necessary modifications, to the replacement of the term "replacement-type" with the term "electroless" in the literature.

[0006] Typically, the main components of an autocatalytic metal bath are metal salts, reducing agents, and, as optional components, complexing agents, pH adjusters, and additives, such as stabilizers. Complexing agents (also referred to as chelating agents in the art) are used to chelate the deposited metal and prevent the metal from precipitating from the solution (i.e., as hydroxides, etc.). Chelated metals make the metal available to reducing agents that convert metal ions to the metallic form.

[0007] A further form of metal deposition is replacement plating. Replacement plating is another form of metal deposition that uses neither an external current source nor a chemical reducing agent. The mechanism relies on replacing the metal from the underlying substrate with metal ions present in the replacement plating solution. This is a particular drawback of replacement plating because the deposition of thicker layers is usually limited by the porosity of the layer.

[0008] In most cases, electroless gold plating baths use one or both types of electroless plating. Even if a reducing agent is added to the plating bath, displacement plating can still occur, albeit at a significantly reduced rate.

[0009] EP 343 816 A1 discloses an aqueous liquor for use as an electroless gold deposition bath containing a source of gold and a reducing agent. The liquor also contains a variety of compounds, namely mixtures of alkali metals or ammonium ferrocyanide and mixtures of alkali metals or ammonium ferricyanide; 1-H-tetrazole; redox mediator; and reducing stabilizers selected from mixtures thereof.

[0010] WO2017 / 050662 discloses ethylenediamine derivatives as electroless plating baths and plating bath enhancers. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] EP 343 816 A1 [Patent Document 2] WO2017 / 050662 [Overview of the project] [Problems that the invention aims to solve]

[0012] An object of the present invention is to provide an aqueous precious metal plating bath composition capable of depositing a precious metal layer at a sufficient plating rate, and a method for the purposes of the above. Another object of the present invention is to provide an aqueous precious metal plating bath that has sufficient stability and can be used for a long period of time.

[0013] Another objective is to improve the corrosion resistance of plating baths, given the current state of technology in this field.

[0014] It is common to use cyanide-containing compounds as a source of gold ions. However, it is common to use additional cyanide-containing compounds, such as potassium cyanide, to stabilize the plating bath. These baths have two drawbacks: the toxicity of these additional cyanide-containing compounds poses problems in the process itself and in wastewater treatment, requiring the establishment of appropriate safety measures. In addition, the addition of additional cyanide-containing compounds is not easy, and insufficient or excessive addition leads to problems such as reduced plating. Therefore, another objective is to provide a plating bath with a reduced cyanide content. Yet another objective is to ensure that the formed precious metal layer does not show discoloration.

[0015] Hydrazine is a well-known reducing agent. However, its high toxicity is also well known. Therefore, a further objective of the present invention was to reduce the toxicity of aqueous precious metal plating baths by providing an aqueous precious metal plating bath that does not contain hydrazine and / or any hydrazine derivatives. [Means for solving the problem]

[0016] These objectives are, (a) A source of at least one precious metal ion; (b) at least one source of hexacyanoferrate(II) ions and / or hexacyanoferrate(III) ions; (c) Optionally, at least one halide ion source; (d) at least one complexing agent; (e) At least one reducing agent for precious metal ions; (f) at least one humectant of their choice; and (g) Optionally, at least one accelerator This is solved by an aqueous precious metal plating bath according to the present invention, which includes [the specified component].

[0017] These objectives can be further addressed by the method of depositing a gold layer from the above-described plating bath, and by using the above-described plating enhancing compounds in a gold plating bath comprising at least one gold ion source and at least one reducing agent for gold ions. [Brief explanation of the drawing]

[0018] [Figure 1] This graph shows the stability of various plating baths (see Example 1). [Figure 2] This graph shows the stability of the plating bath and the ENEPIG plating rate in Example 2. [Figure 3] This graph shows the stability of the plating bath and the ENEPIG plating rate in Example 3. [Figure 4] This graph shows the stability of the plating bath and the ENEPIG plating rate in Example 4. [Figure 5] This graph shows the stability of the plating bath and the ENEPIG plating rate in Example 5. [Modes for carrying out the invention]

[0019] Plating bath The aqueous precious metal plating bath according to the present invention is synonymously called an aqueous solution. The term "aqueous solution" means that the main liquid medium, the solvent in the solution, is water. Further liquids that are compatible with water, such as alcohols and other polar organic liquids that are compatible with water, may be added. As a rule, an aqueous solution contains more than 50 percent by mass of water.

[0020] Preferably, the plating bath of the present invention is an electroless precious metal plating bath. The electroless precious metal plating bath includes, but is not limited to, various types such as displacement-type precious metal plating baths, autocatalytic precious metal plating baths, and precious metal plating baths using a mixture of autocatalytic and displacement-type 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.

[0021] A displacement-type precious metal plating bath typically contains at least the following components: at least one chelating agent; at least one gold source; and at least one pH adjuster. In addition, the displacement-type precious metal plating bath may contain, but is not limited to, additives such as at least one grain refiner, at least one wetting agent, and / or at least one accelerator.

[0022] A self-catalytic precious metal plating bath typically contains 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. In addition, a self-catalytic precious metal plating bath may contain, but is not limited to, at least one grain refining agent, at least one wetting agent, and / or at least one accelerator.

[0023] The plating bath according to the present invention can be prepared by dissolving all components in an aqueous liquid medium, preferably water.

[0024] (a) Source of precious metal ions The aqueous precious metal plating bath according to the present invention comprises at least one precious metal ion source.

[0025] In the context of the present invention, the term "precious metal" refers to a metal selected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. These are elements of groups 8 to 11 of the fifth and sixth periods. Preferably, the precious metal is selected from the group consisting of Au and Ag. More preferably, the precious metal is Au. The concentration of precious metal ions in the aqueous precious metal plating 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. When two or more precious metal ions are contained in the aqueous precious metal plating bath according to the present invention, the concentration is based on the total amount of precious metal ions.

[0026] Gold ions are Au + Au 3+It may be either or both oxidation states. The gold ion source may be any water-soluble gold salt having the above oxidation states. Preferably, the gold ion source is selected from the group consisting of alkali gold(I) cyanides such as gold cyanide, ammonium gold cyanide, potassium gold(I) cyanide, sodium gold(I) cyanide, trisodium gold bisulfite, tripotassium gold bisulfite, and triammonium gold bisulfite, gold thiosulfate, gold thiocyanide, gold sulfate, gold chloride, and gold bromide. Preferably, the gold ion source is alkali gold(I) cyanide, which can be added to the aqueous plating bath in the form of a solution containing this salt. The concentration of gold ions in the aqueous gold plating 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.

[0027] Silver ions are Ag + The silver ion source is in an oxidized state. The silver ion source may be any water-soluble silver salt having the above-described oxidized state. Preferably, the silver ion source is at least one metal cyanosilate M[Ag(CN)2] (wherein M is selected from the group consisting of alkali metals and ammonium). Preferably, the silver ion source is potassium cyanosilate K[Ag(CN)2], which can be added to the aqueous plating bath in the form of a solution containing this salt. The concentration of silver ions in the aqueous silver plating 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.

[0028] (b) Sources of hexacyanoferrate(II) and / or hexacyanoferrate(III) ions Cyanide ions are known to form stable complexes with iron(II) and / or iron(III) ions. These complexes are known as hexacyanoferrate(II) and hexacyanoferrate(III), respectively. These are anions with four negative charges (in the case of hexacyanoferrate(II)) and three negative charges (in the case of hexacyanoferrate(III)). Typically, they are neutralized by suitable cations. The source of hexacyanoferrate(II) and / or hexacyanoferrate(III) ions may be any of their water-soluble forms. Preferably, the source of the hexacyanoferrate(II) ion and / or hexacyanoferrate(III) ion is selected from the group consisting of alkali hexacyanoferrate(II) salts such as ammonium hexacyanoferrate(II), potassium hexacyanoferrate(II), and sodium hexacyanoferrate(II); alkali hexacyanoferrate(III) salts such as ammonium hexacyanoferrate(III), potassium hexacyanoferrate(III), and sodium hexacyanoferrate(III); and mixtures thereof. Preferably, the source of the hexacyanoferrate(II) ion and / or hexacyanoferrate(III) ion is selected from potassium hexacyanoferrate(II), potassium hexacyanoferrate(III), and mixtures thereof, and can be added to the aqueous plating bath in the form of a solution containing this salt. In the aqueous precious metal plating bath according to the present invention, the concentration of hexacyanoferrate(II) ions and / or hexacyanoferrate(III) ions 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. When both hexacyanoferrate(II) ions and hexacyanoferrate(III) ions are contained in the aqueous precious metal plating bath according to the present invention, the concentration is based on the total amount of hexacyanoferrate(II) ions and hexacyanoferrate(III) ions.

[0029] (c) Halide sources The halide source may be any water-soluble halide. Preferably, the halide is a bromide and / or iodide. Preferably, the halide source is selected from the group consisting of alkaline bromides such as ammonium bromide, potassium bromide, and sodium bromide; alkaline iodides such as ammonium iodide, potassium iodide, and sodium iodide; and mixtures thereof. Preferably, the halide source may be added to the aqueous plating bath in the form of a solution containing the salt. The halide concentration in the electroless aqueous gold plating 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 two or more halide ions are contained in the aqueous precious metal plating bath according to the present invention, the concentration is based on the total amount of halide ions.

[0030] If silver ions are present as noble metal ions (a) in the noble metal plating bath, the halide ion concentration is zero, meaning that halide ions are not present in the noble metal plating bath.

[0031] (d) at least one complexing agent The aqueous precious metal plating 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 plating 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 as a complexing agent for metal ions that dissolve from the substrate during plating, such as nickel ions or copper ions. Preferred carboxylic acids are, for example, oxalic acid or a salt thereof. 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). A preferred phosphonic acid is 1-hydroxyethane-(1,1-diphosphonic acid) (HEDP). Preferred aminophosphonic acids are nitrilotri(methylphosphonic acid) (generally abbreviated as ATMP), diethylenetriaminepentakis(methylphosphonic acid) (generally abbreviated as DTPMP), and ethylenediaminetetra(methylenephosphonic acid) (generally abbreviated as EDTMP). In all cases, sodium, potassium, and ammonium salts of the above compounds are also preferred. The concentration of at least one optional complexing agent is preferably in the range of 0.25 to 250 mmol / L, more preferably 1.0 to 150 mmol / L.

[0032] More preferably, the aqueous precious metal plating bath according to the present invention comprises two different complexing agents and / or salts thereof, for example, an aminocarboxylic acid or a salt thereof and a phosphonic acid or a salt thereof.

[0033] (e) Reducing agents for precious metal ions The aqueous precious metal plating bath further comprises at least one reducing agent for precious metal ions.

[0034] The reducing agent for precious metal ions is preferably an aliphatic aldehyde, such as formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, α-methylvaleraldehyde, β-methylvaleraldehyde, γ-methylvaleraldehyde, etc.; an aliphatic dialdehyde, such as glyoxal, succinidaldehyde, etc.; an aliphatic unsaturated aldehyde, such as crotonaldehyde, etc.; an aromatic aldehyde, such as benzaldehyde, o-nitrobenzaldehyde, m-nitrobenzaldehyde, p-nitrobenzaldehyde, o-tolualdehyde, m-tolualdehyde. The following are selected from the group consisting of p-tolualdehyde, o-hydroxybenzaldehyde, m-hydroxybenzaldehyde, p-hydroxybenzaldehyde, phenylacetaldehyde, etc.; sugars having an aldehyde group (-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 sources. The term "glyoxylic acid source" includes glyoxylic acid and all compounds that can be converted to glyoxylic acid in aqueous solution. In aqueous solution, aldehyde-containing acids are in equilibrium with their hydrates. A suitable source of glyoxylic acid is dihaloacetic acid, for example, dichloroacetic acid, which hydrolyzes to glyoxylic acid hydrate in an aqueous medium. Another source of glyoxylic acid is a bisulfite adduct, which is a hydrolyzable ester or other acid derivative. The bisulfite adduct may be added to the electroless aqueous gold plating bath according to the present invention or may be formed in situ. The bisulfite adduct can be produced from glyoxylate and any of bisulfite, sulfite, or metabisulfite. Formaldehyde, a source of glyoxylic acid, and glyoxylic acid are preferred, with formaldehyde being the most preferred.

[0035] Hydrazine is a known reducing agent. However, its high toxicity is also well known. Therefore, the object of the present invention was to provide an aqueous precious metal plating bath that does not contain hydrazine and / or any hydrazine derivatives.

[0036] The aqueous precious metal plating bath is characterized by not containing hydrazine and / or any hydrazine derivative.

[0037] The concentration of at least one reducing agent for the precious metal ions 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.

[0038] (f) Wetting agents; A wetting agent is a surfactant molecule used to reduce the surface tension of water. Although many wetting agents and methods of application are known to those skilled in the art, a preferred wetting agent is ethylene glycol. The concentration of at least one wetting agent for noble metal ions 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.

[0039] (g) Promoter The aqueous precious metal plating bath according to the present invention optionally contains an accelerator selected from the group consisting of thallium ions, arsenic ions, selenium ions, and lead ions. Such crystal modifiers are preferably added to the electroless aqueous precious metal plating 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 the above ions may be their water-soluble salts, such as nitrates, sulfates, and halides, respectively.

[0040] In addition to these listed components, the precious metal plating bath of the present invention may include further components, for example, one or more stabilizers and / or one or more plating enhancers.

[0041] Optional additional ingredients - stabilizers The electroless aqueous gold plating bath according to the present invention optionally comprises a cyanide ion source, hydantoin, and at least one stabilizer selected from the group consisting of alkyl hydantoin and dialkyl hydantoin, wherein the alkyl residue in the context of the present invention includes C1-C8 alkyl, preferably methyl, and may be a cyclic and / or alicyclic, branched or unbranched sulfur compound, such as 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, mercaptoacetic acid, 3-(2-benzothiazolylthio)-1-propanesulfonic acid, mercaptosuccinic acid, thiosulfuric acid, thioglycol, thiourea, thiomalic acid, etc., and an aromatic nitrogen compound, such as benzotriazole, 1,2,4-aminotriazole, etc. A suitable cyanide ion source may be any soluble cyanide salt, such as an alkali cyanide such as sodium cyanide or potassium cyanide.

[0042] Due to toxicity and problematic input levels, an objective of the present invention was to minimize or even eliminate the use of stabilizers selected from cyanide ions. Therefore, in a preferred embodiment of the present invention, the plating solution of the present invention does not contain stabilizers selected from cyanide ions. Such cyanide ion stabilizers contain free cyanide ions. It is evident that hexacyanoferrate compounds do not fit this definition, as their cyanide ligands form highly stable complexes with the iron core.

[0043] However, for example, if HSE regulations are not very strict, the plating bath of the present invention may contain at least one stabilizer selected from cyanide ions.

[0044] The concentration of the optional stabilizer can be selected according to its chemical structure and can be determined by routine experiments using any known method in the art. The concentration of the optional stabilizer is preferably in the range of 0.0001 to 200 mmol / L, more preferably 0.001 to 125 mmol / L, and most preferably 0.01 to 75 mmol / L. Such stabilizers are conventionally added to electroless gold plating baths to improve their lifespan and prevent plate-out.

[0045] In one preferred embodiment, two or more stabilizers are used. More preferably, a cyanide ion source at a concentration of 0.0003 to 5 mmol / L, and one or more hydantoins and their alkyl derivatives 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 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.

[0046] Optional additional components - plating enhancers The plating bath of the present invention may further include, as a plating bath enhancer, at least one ethylenediamine derivative of formula (I) as disclosed in WO2017 / 050662. The ethylenediamine derivative of formula (I) is referred to herein as a plating enhancer compound.

[0047] Equation (I)

[0048] [ka]

[0049] The plating-enhancing compound contains 2 to 12 carbon atoms and is selected from the group consisting of branched alkyl, unbranched alkyl, cycloalkyl, or a combination thereof, including residue R 1 and residue R 2 It has individual residues R 1 and residue R 2 They are either the same or different.

[0050] The amine moiety of the plating enhancing compound of formula (I) is a secondary amine moiety.

[0051] In a preferred embodiment of the present invention, the residue R of the plating enhancing compound of formula (I) 1 and the residue R 2 contain from 2 to 8 carbon atoms, more preferably from 2 to 6 carbon atoms, and even more preferably from 2 to 4 carbon atoms.

[0052] In another preferred embodiment of the present invention, the residue R in formula (I) 1 and the residue R 2 are the same. In yet another preferred embodiment of the present invention, the alkyl residue R in formula (I) 1 and the alkyl residue R 2 do not contain a terminal hydroxy moiety (-OH). In yet another preferred embodiment of the present invention, the residue R in formula (I) 1 and the residue R 2 do not contain a terminal primary amino moiety. In another more preferred embodiment of the present invention, the residue R <​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​2 -Dipentylethane-1,2-diamine, N 1 ,N 2 -di-iso-pentylethane-1,2-diamine, N 1 ,N 2 -di-sec-pentylethane-1,2-diamine, N 1 ,N 2 -di-tert-pentylethane-1,2-diamine, N 1 ,N 2 -di-neo-pentylethane-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 ,N 2 -di-(1,1-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -di-(1,2-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -di-(1,3-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -di-(2,2-dimethylbutyl)ethane-1,2-diamine, N 1 ,N 2 -di-(2,3-dimethylbutyl)ethane-1,2-diamine, and N 1 ,N 2 It is particularly preferable to select a plating-enhancing compound from the group consisting of -di-(3,3-dimethylbutyl)ethane-1,2-diamine.

[0054] R 1 and R 2 It is most preferably a branched alkyl residue having 3 to 6 carbon atoms.

[0055] In the aqueous precious metal plating bath according to the present invention, the concentration of at least one plating-enhancing compound of formula (I) 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. When two or more plating-enhancing compounds are contained in the aqueous precious metal plating bath according to the present invention, the concentration is based on the total amount of the plating-enhancing compounds.

[0056] In another preferred embodiment of the present invention, the electroless aqueous precious metal plating bath according to the present invention does not contain a second intentionally added source of reducing metal ions (neglecting trace impurities commonly present in the technical raw materials), and a pure precious metal deposit can be formed. The pure precious metal deposit is soft, malleable, and particularly suitable for wire bonding and soldering. Trace impurities are understood to be less than 1% by mass of compounds present in the technical raw materials.

[0057] 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, and even more preferably 8 to 9. The target pH value is adjusted, for example, by using an acid such as phosphoric acid or a base such as sodium hydroxide or potassium hydroxide. Continuously controlling and adjusting the pH value during plating is also advantageous and therefore preferable because it improves the lifespan of the plating bath.

[0058] method A method for depositing a precious metal layer on a substrate, (i) The process of preparing the substrate; (ii) A step of bringing at least a portion of the surface of the substrate into contact with the aqueous precious metal plating bath according to the present invention; This includes in this order, This deposits a precious metal layer onto at least a portion of the substrate surface. method.

[0059] This contact process is preferably carried out by immersing the substrate or at least a portion of the surface of the substrate in the plating bath, or by spraying the plating bath onto the substrate or at least a portion of the surface of the substrate.

[0060] At least a portion of the surface of the substrate is preferably made of a metal or metal alloy, and a precious metal is deposited on at least a portion of the surface of the substrate which is made of a metal or metal alloy selected from the group consisting of nickel, nickel alloys, e.g., nickel-phosphorus alloy, nickel-boron alloy, cobalt, cobalt alloys, e.g., cobalt-phosphorus alloy, cobalt-molybdenum-phosphorus alloy, cobalt-molybdenum-boron alloy, cobalt-molybdenum-boron-phosphorus alloy, cobalt-tungsten-phosphorus alloy, cobalt-tungsten-boron alloy, cobalt-tungsten-boron-phosphorus alloy, palladium, palladium alloys, e.g., palladium-phosphorus alloy, palladium-boron alloy, copper and copper alloys, and precious metals or precious metal alloys. The aqueous precious metal plating bath according to the present invention can be used to deposit a precious metal layer on a precious metal substrate, and can be used, for example, to thicken an existing precious metal layer obtained from a substitutional precious metal plating bath.

[0061] As is well known in the art, the substrate may be pre-treated before plating. Such pre-treatment includes a cleaning step with a solvent and / or surfactant to substantially remove organic contaminants, an etching step with an acid and optionally an oxidizing or reducing agent to remove oxides, and an activation step. The activation step involves depositing a noble metal on the surface or part thereof to make the substrate more easily plated. Such a noble metal may be palladium, which can be deposited as a salt and then reduced to elemental palladium on the surface. Alternatively, the noble metal may be deposited in colloidal form and, if necessary, subjected to an accelerating step with an acid such as hydrochloric acid to remove any protective colloids, such as tin colloids. Such an activated layer is usually an aggregate of island-like structures of palladium, rather than individual layers. However, the activated layer is considered as a metal substrate in the context of the present invention.

[0062] Preferably, the plating bath in the method of the present invention is an electroless precious metal plating bath. The electroless precious metal plating bath includes, but is not limited to, various types such as displacement precious metal plating baths, autocatalytic precious metal plating baths, and precious metal plating baths using a mixture of autocatalytic plating baths, displacement plating baths, and electrolytic plating baths. In a more preferred embodiment, the precious metal plating bath in the method of the present invention is an autocatalytic precious metal plating bath.

[0063] The temperature of the aqueous precious metal plating bath according to the present invention is preferably in the range of 30 to 95°C, more preferably in the range of 70 to 90°C, even more preferably in the range of 75 to 85°C, and even more preferably in the range of 77 to 84°C during plating. The plating time is preferably in the range of 1 to 60 minutes, more preferably in the range of 5 to 30 minutes. However, if a thinner or thicker deposit is desired, the plating time may be outside the above range and can be adjusted as appropriate.

[0064] It is preferable to continuously or at regular intervals replenish the components used in the plating process. These components include, among others, a source of noble metal ions, a reducing agent for the noble metal ions, at least one stabilizer, and a plating-enhancing compound. If necessary, the pH value may also be adjusted continuously or at regular intervals.

[0065] The aqueous precious metal plating bath according to the present invention can be used in conjunction with horizontal plating apparatus, vertical plating apparatus, and spray plating apparatus.

[0066] One advantage of the present invention is that the stability of the aqueous precious metal plating bath according to the present invention is improved compared to precious metal plating baths known in the art (see Examples 1 to 5). Stability as used herein is understood as the lifespan of the bath before the precipitation of compounds from the bath ("plate-out") that renders the bath unusable for plating applications.

[0067] Another advantage is that the aqueous precious metal plating bath according to the present invention allows for a sufficient plating rate (the thickness of the plated metal layer over time) (see Examples 2 to 4). Most plating baths known in the art, while somewhat stable, do not provide a sufficient plating rate.

[0068] Therefore, the unique feature of the aqueous precious metal plating bath according to the present invention is that it provides a highly stable precious metal plating bath with a sufficient plating speed, thus enabling a more economically feasible precious metal plating method.

[0069] The aqueous precious metal plating bath according to the present invention forms a uniform precious metal deposit with virtually no variation in layer thickness. The standard deviation of the precious metal layer thickness is less than 10%, or even less than 8%. This small deviation is conveniently achievable even when plating on various substrates has a variety of sizes.

[0070] The following non-limiting embodiments further demonstrate the present invention. [Examples]

[0071] Basic Procedure Pro Select S8, MicroEtch C, Aurotech® Predip, Aurotech® Activator 1000, Aurotech® CNN mod, and PD-Tech® PC1 are products available from Atotech Deutschland GmbH. The gold ion source was K[Au(CN)2] in all cases. The silver ion source was K[Ag(CN)2].

[0072] 0.25-49mm on each side 2Printed circuit board test boards with numerous copper pads of various sizes in the range were used as substrates in all experiments. They were cleaned and etched before activation with palladium. Nickel was then deposited on the copper surface for ENIG, or on the nickel / palladium surface for ENEPIG, and then a gold layer was plated thereon. Individual pads had the following area: 1:0.25mm² 2 , 2:1mm 2 , 3:4mm 2 , 4:9mm 2 , 5:25mm 2 , 6:49mm 2 It possessed.

[0073] Determination of metal deposit thickness and plating rate The deposit thickness was measured on one side of the test board using six pads. Selected copper pads of various sizes were used to determine the layer thickness by XRF using an XRF instrument, Fischerscope XDV-SDD (Helmut Fischer GmbH, Germany). By assuming the layer structure of the deposit, the layer thickness could be calculated from this XRF data. The plating rate was calculated by dividing the obtained layer thickness by the time required to obtain the aforementioned layer thickness.

[0074] The uniformity of the layer thickness, expressed as the coefficient of variation (COV), was determined as the standard deviation from the average thickness value. In the case of gold, this term is abbreviated as Au COV.

[0075] (Example 1) The gold plating bath containing the following components was prepared by dissolving all components in water.

[0076] [Table 1]

[0077] The stability of the bath was examined at 82°C. The results are shown in Figure 1. The test was stopped after 56.5 hours.

[0078] (Example 2) The gold plating bath containing the following components was prepared by dissolving all components in water.

[0079] [Table 2]

[0080] The substrates were subjected to the following process steps (Table A) by immersing them in each solution using given parameters.

[0081] [Table 3]

[0082] After this process sequence, the thickness of each metal layer was measured. The plating rate was calculated as described above.

[0083] The stability of the bath and the ENEPIG plating rate were investigated. The results are shown in Figure 2.

[0084] (Example 3) The gold plating bath containing the following components was prepared by dissolving all components in water.

[0085] [Table 4]

[0086] The substrates were subjected to the process steps described in Table A of Example 2 by immersing them in each solution.

[0087] After this process sequence, the thickness of each metal layer was measured. The plating rate was calculated as described above.

[0088] Bath stability and ENEPIG plating rate were investigated. The results are shown in Figure 3. The test was stopped after 174 hours.

[0089] (Example 4) The gold plating bath containing the following components was prepared by dissolving all components in water.

[0090] [Table 5]

[0091] The substrates were subjected to the process steps described in Table A of Example 2 by immersing them in each solution.

[0092] After this process sequence, the thickness of each metal layer was measured. The plating rate was calculated as described above.

[0093] Cu 2+ / Ni 2+ / Fe 2+ Bath containing ions and Cu 2+ / Ni 2+ / Fe 2+ The ENEPIG plating rate and Au COV were investigated in a bath without ions. The results are shown in Figure 4.

[0094] (Example 5) The gold plating bath containing the following components was prepared by dissolving all components in water.

[0095] [Table 6]

[0096] The substrates were subjected to the process steps described in Table A of Example 2 by immersing them in each solution.

[0097] After this process sequence, the thickness of each metal layer was measured. The plating rate was calculated as described above.

[0098] The ENEPIG plating rate, Au COV, and stability of baths containing glycol derivatives and baths without glycol derivatives were investigated. All conditions were varied over time until the total Au metal turnover reached 2 g / L without Au plate-out. The results are shown in Figure 5.

[0099] Other embodiments of the present invention will become apparent to those skilled in the art from the discussion herein or from the practice of the invention disclosed herein. This specification and the examples are for illustrative purposes only, and the true scope of the invention is intended to be defined solely by the following claims.

Claims

1. (a) A source of at least one precious metal ion; (b) at least one source of hexacyanoferrate(II) ions and / or hexacyanoferrate(III) ions; (c) Optionally, at least one halogenated source; (d) at least one complexing agent; (e) At least one reducing agent for precious metal ions; (f) at least one humectant of their choice; and (g) Optionally, at least one accelerator A water-based precious metal plating bath containing [a specific component].

2. The aqueous precious metal plating bath according to claim 1, characterized in that at least one reducing agent for precious metal ions 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, hydroxylaminsulfate, and mixtures thereof, and preferably selected from formaldehyde precursors, formaldehyde / amine mixtures, DMAB, and hydroxylaminsulfate.

3. The aqueous precious metal plating bath according to claim 1 or 2, characterized in that it does not contain hydrazine and / or any hydrazine derivative.

4. The aqueous precious metal plating bath according to any one of claims 1 to 3, characterized in that the pH of the aqueous precious metal plating bath is in the range of 5 to 10, preferably 7 to 9, and more preferably 8 to 9.

5. An aqueous precious metal plating bath according to any one of claims 1 to 4, characterized in that 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 plating bath according to any one of claims 1 to 5, characterized in that the aqueous precious metal plating bath further comprises at least one complexing agent selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, phosphonic acids, or salts thereof.

7. A method for depositing a precious metal layer on a substrate, (i) Process of preparing the substrate (ii) A step of bringing at least a portion of the surface of the substrate into contact with the aqueous precious metal plating bath according to any one of claims 1 to 6. It includes them in this order, This deposits a precious metal layer onto at least a portion of the substrate surface. A method for depositing a precious metal layer onto a substrate.

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

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