US20220389588A1 - Electroless nickel alloy plating baths, a method for deposition of nickel alloys, nickel alloy deposits and uses of such formed nickel alloy deposits

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US20220389588A1

Publication number: US20220389588A1
Application number: US17/775,902
Authority: US
Inventors: Sebastian Lang
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2019-11-20
Filing date: 2020-11-19
Publication date: 2022-12-08
Also published as: KR20220103131A; WO2021099475A1; CA3158576A1; CN114729455A; JP7244709B2; EP4061981A1; MX2022006118A; JP2022548419A

The present invention relates to an electroless nickel alloy plating bath comprising nickel ions; further reducible metal ions selected from the group consisting of molybdenum ions, rhenium ions, tungsten ions, copper ions, oxo-ions thereof, and mixtures thereof; at least one reducing agent suitable to reduce the nickel ions and the further reducible metal ions to their respective metallic state; complexing agents CA1, CA2, CA3, and CA4, wherein CA1, CA2, CA3, and CA4 are all different from each other, wherein each of CA1 and CA2 is independently selected from the group consisting of compounds having at least two carboxylic acid moieties, the respective salts thereof as well as mixtures of the aforementioned; wherein CA3 is selected from the group consisting of aliphatic compounds having exactly one carboxylic acid moiety, the respective salts thereof as well as mixtures of the aforementioned; and wherein CA4 is selected from the group consisting of aromatic compounds having at least one carboxylic acid moiety, the respective salts thereof as well as mixtures of the aforementioned.

FIELD OF THE INVENTION

The present invention concerns electroless nickel alloy plating baths, a method for deposition of nickel alloys, nickel alloy deposits and uses of such nickel alloy deposits. The nickel coatings obtained by the invention show a high uniformity and a high hardness, good wear resistance and an improved corrosion resistance. Such coatings are suitable as a functional coating in aerospace, automobile, electrical and chemical industries. The metal layers deposited from such plating baths are also useful as barrier and cap layers in semiconducting devices, printed circuit boards, IC substrates and the like.

BACKGROUND OF THE INVENTION

Electroless nickel coatings are functional coatings that are applied to provide corrosion resistance, wear resistance, hardness, lubricity, solderability and bondability, uniformity of deposit, and non-magnetic properties (in the case of high-phosphorus nickel alloys), to provide a non-porous barrier layer or otherwise enhance the performance or useful life of a particular component. The hardness and corrosion resistance of electroless nickel are key factors in many successful applications. Electroless nickel coatings are used for a variety of applications including electrical connectors, microwave housings, valves and pump bodies, printer shafts, computer components, among others. Electroless nickel may be used to coat components made of various materials including but not limited to, steel, stainless steel, aluminum, copper, brass, magnesium and any of a number of non-conductive materials.
Electroless nickel plating deposits a nickel alloy onto a substrate that is capable of catalyzing the deposition of the alloy from a process solution containing nickel ions and a suitable chemical reducing agent capable of reducing nickel ions in solution to metallic nickel. Various additives are also used in the electroless nickel plating bath to stabilize the bath and further control the rate of nickel deposition on the substrate being plated. Reducing agents include, for example, borohydride (which produces a nickel boron alloy) and hypophosphite ions (which produces a nickel phosphorus alloy). In contrast with electroplating, electroless nickel does not require rectifiers, electrical current or anodes. The deposition process is autocatalytic, meaning that once a primary layer of nickel has formed on the substrate, that layer and each subsequent layer becomes the catalyst that causes the plating reaction to continue.
In electroless nickel plating baths employing hypophosphite ions as the reducing agent, the nickel deposit comprises an alloy of nickel and phosphorus with a phosphorus content of from about 2% to more than 12%. These alloys have unique properties in terms of corrosion resistance and (after heat treatment) hardness and wear resistance.
Compositions for electroless nickel (EN) plating solutions are known in the art. For example, U.S. Pat. No. 2,658,841 teaches the use of soluble organic acid salts as buffers for electroless nickel plating solutions. U.S. Pat. No. 2,658,842 teaches the use of short chain, dicarboxylic acids as exaltants to EN baths. U.S. Pat. No. 2,762,723 teaches the use of sulfide and sulfur bearing additives to an electroless nickel plating bath for improved bath stability.
U.S. Pat. No. 2,847,327 has introduced other means of stabilizing an electroless nickel plating solution. These include the use of higher purity starting materials; more effective stabilizers from the class of heavy metals such as Pb, Sb, Bi, Cu and Se; inorganic compounds such as iodates, and thio compounds; organic compounds such as unsaturated alkenes and alkynes and others.
WO 2015/187402 discloses electroless nickel alloy plating baths comprising one or more dicarboxylic acids and one or more alpha hydroxyl carboxylic acid.
WO2018/220220 discloses electroless nickel alloy plating baths for the preparation of data storage devices, wherein all complexing agents comprise at least two carboxylic acid moieties.
CN1 09112509A discloses a high corrosion resistance of a chemical nickel plating solution and its preparation method. Such chemical nickel plating solution comprise nickel sulfate, sodium hypophosphite, sodium citrate, lactic acid, propionic acid, and acetic acid as well as particular wetting agents and stabilizers.

OBJECTIVE OF THE PRESENT INVENTION

It is therefore an objective of the present invention to provide an electroless nickel alloy plating bath which improve the corrosion protection of metallic substrates which are treated by means of said plating bath.
It is a further objective of the present invention to provide an electroless nickel alloy plating bath which is stable and does not show any plate-out.
It is yet a further objective of the present invention to provide an electroless nickel alloy plating bath having a sufficient plating rate.
It is still another objective of the present invention to provide an electroless nickel alloy plating bath which produces nickel alloy deposits well-adhering to the underlying substrate surface, for example when plated on metal surfaces such as aluminum or aluminum alloys.

SUMMARY OF THE INVENTION

Above-captioned objectives are solved by the electroless nickel alloy plating bath according to the invention, its use, the method for depositing a nickel alloy on at least one surface of a substrate and the nickel alloy deposits obtained from the electroless nickel alloy plating bath according to the invention and their use.
The electroless nickel alloy plating bath according to the invention comprises

- a) nickel ions;
- b) further reducible metal ions selected from the group consisting of molybdenum ions, rhenium ions, tungsten ions, copper ions, oxo-ions thereof, and mixtures thereof;
- c) at least one reducing agent suitable to reduce the nickel ions and the further reducible metal ions to their respective metallic state; preferably selected from the group consisting of hypophosphorous acid, hypophosphite salts and mixtures of the aforementioned; and
- d) complexing agents CA1, CA2, CA3, and, optionally (preferably not optionally but rather compulsively, mandatory, necessarily; i.e. without optionally), CA4, wherein CA1, CA2, CA3, and CA4 are all different from each other;
  - wherein each of CA1 and CA2 is independently selected from the group consisting of compounds having at least two carboxylic acid moieties, the respective salts thereof as well as mixtures of the aforementioned;
  - wherein CA3 is selected from the group consisting of aliphatic compounds having exactly one carboxylic acid moiety, the respective salts thereof as well as mixtures of the aforementioned; and
  - wherein CA4 is selected from the group consisting of aromatic compounds having at least one carboxylic acid moiety, the respective salts thereof as well as mixtures of the aforementioned.

The two complexing agents CA1 and CA2 are different compounds having at least two carboxylic acid moieties, the respective salts thereof as well as mixtures of the aforementioned.
The electroless nickel alloy plating bath according to the invention is used to deposit a nickel alloy onto at least one surface of at least one substrate.
A method for depositing a nickel alloy on at least one surface of a substrate comprising in this order the following method steps

- A) providing the substrate comprising the at least one surface;
- B) contacting the at least one surface of the substrate with an electroless nickel alloy plating bath according to the present invention (preferably a plating bath as described as being preferred throughout the present text) and thereby depositing a nickel alloy on the at least one surface of the substrate.

A nickel alloy deposit is obtainable by deposition from an electroless nickel alloy plating bath according to the present invention (preferably a plating bath as described as being preferred throughout the present text).
The electroless nickel alloy plating bath according to the invention and the methods of the present invention are suitable to provide nickel alloy deposits having an attractive bright or semi-bright appearance. Further, the nickel alloy deposits adhere well to the underlying substrate surface.
The electroless nickel alloy plating bath according to the invention is stable and does not show plate-out during plating for a sufficient period of time to be used economically.

DETAILED DESCRIPTION OF THE INVENTION

Percentages throughout this specification are weight-percentages (wt.-%) unless stated otherwise. Concentrations given in this specification refer to the volume or mass (preferably volume) of the entire solutions unless stated otherwise. The terms “deposition” and “plating” are used interchangeably herein. Also, “layer” and “deposit” are also used synonymously in this specification. Plate-out means the undesired decomposition of a plating bath. It is understood that preferred embodiments of the present invention described in this specification can be combined unless this is technically not feasible or specifically excluded.
A carboxylic acid moiety in the context of the present invention is a —C(═O)OH group. It preferably includes salts thereof if not explicitly stated. The term “CX—CY-compound” according to the present invention refers to compounds having X to Y carbon atoms (including the carbon atoms of any carboxylic acid moieties); wherein X and Y refer to natural numbers, and X is smaller than Y.
Nickel alloys of the present invention comprise the elements nickel and one or more of molybdenum, tungsten, copper, and rhenium, preferably in combination with phosphorus and/or boron, more preferably with phosphorus. Even more preferred alloys comprise nickel, molybdenum and phosphorus. Most preferred alloys comprise nickel, molybdenum, copper, and phosphorus as they provide an improved corrosion protection of metallic substrates.
Electroless Nickel Alloy Plating Bath According to the Invention
The inventive electroless nickel alloy plating bath comprises nickel ions. The nickel ions can be provided by any water-soluble salt or any water-soluble nickel complex. Preferably, nickel ions are provided by any one of nickel sulfate, nickel chloride, nickel acetate, nickel methyl sulfonate, nickel sulfamate and mixtures thereof.
The concentration of the nickel ions in the electroless nickel alloy plating bath may vary widely and preferably ranges from 0.01 mol/L to 1.0 mol/L, more preferably from 0.03 mol/L to 0.8 mol/L, even more preferably from 0.04 mol/L to 0.5 mol/L, yet even more preferably from 0.05 mol/L to 0.3 mol/L, most preferably from 0.05 mol/L to 0.1 mol/L.
The inventive electroless nickel alloy plating bath comprises further reducible metal ions (other than nickel ions). The further reducible metal ions are selected from the group consisting of molybdenum ions, rhenium ions, tungsten ions, copper ions, and mixtures thereof. The further reducible metal ions can be provided by any water-soluble salt or any water-soluble complex of such further reducible metals. Preferably, molybdenum ions are provided by any one of molybdic acid, alkaline molybdate (e.g. Na₂MoO₄), ammonium molybdate, and mixtures thereof. Preferably, tungsten ions are provided by any one of tungstic acid, alkaline tungstate (e.g. Na₂WO₄), ammonium tungstate, and mixtures thereof. Preferably, rhenium ions are provided by any one of perrhenic acid, alkaline perrhenate (e.g. NaReO₄), ammonium perrhenate, and mixtures thereof. Preferably, copper ions are provided by any one of copper iodide, copper iodate, copper chloride and/or copper sulfate. Molybdenum ions are preferred as ecologically uncritical stabilizer and alloying component. Copper ions are preferred because they improve leveling and brightness and enhance process stability and corrosion resistance of the deposit. In one embodiment of the invention, the inventive electroless nickel alloy plating bath preferably comprises only molybdenum ions as the further reducible metal ions. In another preferred embodiment of the invention, the inventive electroless nickel alloy plating bath preferably comprises a mixture of molybdenum ions and copper ions as the further reducible metal ions. More preferably, the inventive electroless nickel alloy plating bath does neither comprise any tungsten ions nor any rhenium ions, most preferably if copper ions and/or molybdenum ions are present.
Thus, preferred is an electroless nickel alloy plating bath of the present invention, wherein the further reducible metal ions are molybdenum ions, copper ions, or a mixture thereof; preferably, the further reducible metal ions are molybdenum ions or a mixture of molybdenum ions and copper ions.
Insoluble components of molybdenum, rhenium and tungsten (such as molybdenum disulfide) are detrimental to the nickel alloy deposit properties and are thus preferably not used in the electroless nickel alloy plating bath according to the invention.
The total concentration of the further reducible metal ions in the electroless nickel alloy plating bath may vary and preferably ranges from 5*10⁻⁵to 1*10⁻²mol/L, based on the total volume of the electroless nickel alloy plating bath, more preferably from 1*10⁻⁴to 5*10⁻³mol/L, and even more preferably from 2.5*10⁻⁴to 2.5*10⁻³mol/L. If more than one type of further reducible metal ions is contained in the electroless nickel alloy plating bath, the overall concentration of all further reducible metal ions used is preferably in above-defined ranges (i.e. the total concentrations thereof).
In those cases where the inventive electroless nickel alloy plating bath comprises a mixture of molybdenum ions and copper ions as further reducible metal ions, the total concentration of molybdenum ions and copper ions in the electroless nickel alloy plating bath may vary and preferably ranges from 1*10⁻⁵(preferably from 5*10⁻⁵) to 1*10⁻²mol/L, based on the total volume of the electroless nickel alloy plating bath, more preferably from 1*10⁻⁵(preferably from 5*10⁻⁵) to 5*10⁻³mol/L, and even more preferably from 1*10⁻⁵(preferably from 5*10⁻⁵) to 5*10⁻⁴mol/L.
Preferably, the inventive electroless nickel alloy plating bath comprises more molybdenum ions than copper ions, based on a molar concentration. More preferred, the total concentration of molybdenum ions and copper ions in the electroless nickel alloy plating bath may vary and preferably ranges from 1*10⁻⁵(preferably from 5*10⁻⁵) to 1*10⁻²mol/L, based on the total volume of the electroless nickel alloy plating bath, more preferably from 1*10⁻⁵(preferably from 5*10⁻⁵) to 5*10⁻³mol/L, and even more preferably from 1*10⁻⁵(preferably from 5*10⁻⁵) to 5*10⁻⁴mol/L, and the molar ratio of molybdenum ions to copper ions is in the range of from 1:1 to 30:1, preferably from 5:1 to 20:1, more preferably from 10:1 to 15:1.
Concentrations outside above ranges may in some cases be applicable depending on the desired further metal content in the nickel alloy deposit to be formed.
The electroless nickel alloy plating bath of the present invention further comprises at least one reducing agent. The at least one reducing agent is preferably a chemical reducing agent. The at least one reducing agent is suitable to reduce the nickel ions and the further reducible metal ions to their respective metallic state. Preferably, the at least one reducing agent is selected from the group consisting of

- hypophosphite compounds, such as hypophosphorous acid and hypophosphite salts, such as alkaline hypophosphites, like sodium or potassium hypophosphite, ammonium hypophosphite, nickel hypophosphite and so forth;
- boron based reducing agents, such as aminoboranes, like dimethyl aminoboranes (DMAB) and morpholine borane, and borohydrides, like sodium borohydride;
- hydrazine; and
- hydrazine derivatives (such as hydrazine sulfate, hydrazine hydrochloride, hydrazine hydrate and other such components useable as reducing agent in the context of the present invention).

In case a hypophosphite compound is used as the reducing agent, a nickel alloy deposit comprising phosphorus is obtained. Such reducing agents provide the source of phosphorous in the deposited nickel alloy.
A borane-based reducing agent leads to a nickel alloy deposit comprising boron and a mixture of hypophosphite compounds and borane-based reducing agents leads to a nickel alloy deposit comprising phosphorus and boron.
A nitrogen-based reducing agent such as hydrazine and hydrazine derivatives provide neither phosphorus nor boron to be incorporated into the nickel alloy.
The at least one reducing agent is more preferably selected from the group consisting of hypophosphorous acid, hypophosphite salts and mixtures of the aforementioned. These reducing agents are preferred as the phosphorus being built into the inventive nickel alloys inter alia improves the magnetic properties of such nickel alloy deposits significantly and gives rise to (heat-resistant) paramagnetic nickel alloys. The incorporation of high phosphorous amounts (e.g. 10 wt.-% or more) also improves the protection against corrosion. Even more preferably, the at least one reducing agent is selected to be a hypophosphite salt for such salts being cost effective and easy to use. The concentration of the at least one reducing agent is generally in molar excess of the amount sufficient to reduce the nickel ions and the further reducible metal ions in the electroless nickel alloy plating bath. The concentration of the reducing agent preferably ranges from 0.01 mol/L to 3.0 mol/L, more preferably from 0.1 mol/L to 1 mol/L.
The electroless nickel alloy plating bath comprises (different) complexing agents CA1 and CA2, wherein each of CA1 and CA2 is independently selected from the group consisting of compounds having at least two carboxylic acid moieties, the respective salts thereof (e.g. carboxylates) as well as mixtures of the aforementioned. Preferably, said compounds having at least two carboxylic acid moieties are aliphatic compounds, preferably C2-C12-aliphatic compounds, more preferably C2-C8-aliphatic compounds. Aliphatic compounds include acyclic or cyclic, saturated or unsaturated carbon compounds excluding aromatic compounds; the aliphatic compounds preferably are non-cyclic.
Functionalizations may theoretically be obtained by replacing at least one hydrogen by a functional group. Such optional (but in some cases preferred) functional groups are preferably selected from hydroxy (—OH), amino (—NH₂), halides, olefinic double bonds (—C≡C—) and triple bonds (—C≡C—). The latter two naturally require two hydrogen atoms of adjacent carbon atoms to be theoretically replaced. More preferably, the optional functional group selected from the group consisting of hydroxy and double bonds. Even more preferably, the optional functional group is a hydroxy group.
More preferably, the electroless nickel alloy plating bath comprises complexing agents CA1 and CA2 that are independently selected from the group consisting of unfunctionalized and functionalized aliphatic dicarboxylic acids, unfunctionalized and functionalized aliphatic tricarboxylic acids, unfunctionalized and functionalized aliphatic tetracarboxylic acids, unfunctionalized and functionalized aliphatic pentacarboxylic acids, unfunctionalized and functionalized aliphatic hexacarboxylic acids, the respective salts thereof as well as mixtures of the aforementioned.
Even more preferably, the complexing agents CA1 and CA2 are independently selected from the group consisting of unfunctionalized and functionalized aliphatic dicarboxylic acids, unfunctionalized and functionalized aliphatic tricarboxylic acids, unfunctionalized and functionalized aliphatic tetracarboxylic acids, the respective salts thereof as well as mixtures of the aforementioned.
Preferably, the complexing agent CA1 is an unfunctionalized aliphatic C2-C12-dicarboxylic acid and/or salts thereof. Even more preferably, it is an unfunctionalized aliphatic C3-C6-dicarboxylic acid and/or salts thereof. Yet even more preferably, the complexing agent CA1 is selected from the group consisting of malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, glutaconic acid, itaconic acid, salts thereof, and mixtures of the aforementioned. Most preferably, the complexing agent CA1 is malonic acid and/or salts thereof.
Preferably, the complexing agent CA2 is a functionalized or unfunctionalized (preferably functionalized) aliphatic C3-C12-dicarboxylic acid and/or salts thereof, preferably a functionalized or unfunctionalized (preferably functionalized) aliphatic C4-C6-dicarboxylic acid and/or salts thereof. More preferably, the complexing agent CA2 comprises at least one hydroxy group (and is thus hydroxy-functionalized). Even more preferably, the complexing agent CA2 is a hydroxy-functionalized aliphatic C4-C6-dicarboxylic acid and/or salts thereof. Yet even more preferably, the complexing agent CA2 is selected from the group consisting of malic acid, tartaric acid, 1-hydroxyglutaric acid, 2-hydroxyglutaric acid, 1-hydroxyadipic acid, 2-hydroxyadipic acid, 3-hydroxyadipic acid, salts thereof, and mixtures of the aforementioned. Most preferably, the complexing agent CA2 is malic acid and/or salts thereof.
Preferably, the complexing agent CA3 is a functionalized or unfunctionalized aliphatic C1-C5-monocarboxylic acid and/or salts thereof, preferably a functionalized or unfunctionalized aliphatic C2-C4-monocarboxylic acid and/or salts thereof. More preferably, the complexing agent CA3 is propionic acid and/or salts thereof. Most preferably, the complexing agent CA3 is propionic acid. It shall be expressly noted that within the context of the present invention, it is possible to combine two or more of the aforementioned preferred embodiments regarding CA1, CA2, and CA3.
Preferably, the complexing agent CA4 is a functionalized (in some cases preferred) or unfunctionalized (in some cases preferred) aromatic carboxylic acid and/or salts thereof. A preferred functionalization is a hydroxyl-functionalization. “Aromatic carboxylic acids” are compounds that comprise both at least one COOH group (and its related salts) and at least one aromatic ring. The aromatic ring might comprise heteroatoms, preferably the aromatic ring does not contain heteroatoms. Preferably, at least one COOH group is directly bonded to an aromatic ring such as in e.g. benzoic acid, salicylic acid. More preferably, CA4 is benzoic acid and/or salts thereof. Even more preferably CA4 is sodium benzoate.
In a very preferred embodiment of the present invention, the complexing agents CA1, CA2, CA3 and C4 are separately selected from each other, representing each a different complexing agent according to the aforementioned preferred embodiments regarding CA1, CA2, and CA3.
Particularly preferred is an electroless nickel alloy plating bath according to the present invention, wherein complexing agent CA1 is selected from the group consisting of malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, glutaconic acid, itaconic acid, salts thereof, and mixtures of the aforementioned (particularly, the complexing agent CA1 is malonic acid and/or salts thereof); complexing agent CA2 is selected from the group consisting of malic acid, tartaric acid, 1-hydroxyglutaric acid, 2-hydroxyglutaric acid, 1-hydroxyadipic acid, 2-hydroxyadipic acid, 3-hydroxyadipic acid, salts thereof, and mixtures of the aforementioned (particularly, the complexing agent CA2 is malic acid and/or salts thereof); and complexing agent CA3 is selected from the group consisting of aliphatic monocarboxylic acids, salts thereof, and mixtures of the aforementioned. Most preferably, the aforementioned is combined with complexing agent CA4 being benzoic acid and/or salts thereof.
Preferably, the concentration of the complexing agent CA1 ranges from 50 to 300 mmol/L, more preferably from 50 to 250 mmol/L, and even more preferably from 50 to 200 mmol/L. Preferably, the concentration of the complexing agent CA2 ranges from 50 to 200 mmol/L, more preferably from 60 to 180 mmol/L, and even more preferably from 70 to 150 mmol/L. Preferably, the concentration of the complexing agent CA3 ranges from 40 to 1000 mmol/L, more preferably from 40 to 800 mmol/L, and even more preferably from 40 to 300 mmol/L. Preferably, the concentration of the complexing agent CA4 ranges from 0 (preferably more than 0) to 200 mmol/L, more preferably from 7 to 150 mmol/L, even more preferably from 15 to 75 mmol/L, and yet even more preferably from 30 to 75 mmol/L. Preferably, the total concentration of the complexing agents CA1, CA2, CA3 and CA4 is at least 0.15 mol/L, more preferably at least 0.20 mol/L, and even more preferably at least 0.22 mol/L.
Preferably, the ratio of the total molar concentration of complexing agents CA1, CA2, CA3 and, optionally, CA4 (most preferably of CA1, CA2, CA3, and CA4 all together) to the molar concentration of nickel ions to ranges from 4/1 to 8/1.
The electroless nickel alloy plating bath according to the invention preferably is 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 liquids, that are miscible with water, may be added. It is preferred that at least 95 wt.-%, more preferably 99 wt.-%, of all solvents used is water because of its ecologically benign character. The electroless nickel alloy plating bath according to the invention is preferably prepared by dissolving all components in aqueous liquid medium, preferably in water.
The electroless nickel alloy plating bath according to the invention may be acidic, neutral or alkaline. An acidic or an alkaline pH adjustor such as a (mineral) acid or a base may be selected from a wide range of materials such as ammonia, ammonium hydroxide, sodium hydroxide, hydrochloric acid, sulfuric acid and the like. The pH of the electroless nickel alloy plating bath according to the invention is preferably from about 2 to 12. In one embodiment, the electroless nickel alloy plating bath according to the invention has preferably a neutral or acidic pH value (referred to hereinafter as “acidic electroless nickel alloy plating bath”). This is particularly useful if molybdenum ions and/or rhenium ions are selected as further reducible metal ions, particularly, if molybdenum is selected as the sole further reducible metal ions. More preferably, the acidic electroless nickel alloy plating bath according to the invention has a pH value ranging from 3.5 to 7, even more preferably from 3.5 to 5.0, yet even more preferably from 4.0 to 4.8, most preferably from 4.2 to 4.7. This allows high stability of said bath and optimal results in terms of paramagnetic properties (including their maintenance at elevated temperatures) of the nickel alloy deposits formed from such bath. If tungsten ions are selected as the sole further reducible metal ions, it is preferred that the pH ranges from 8 to 10.
Other additives may be preferably included in the electroless nickel alloy plating bath according to the invention such as pH buffers, wetting agents, accelerators, brighteners, additional stabilizing agents which are known in the art, plating rate modifiers such as those disclosed in European patent application EP 3 034 650 A1.
It is further preferred that the inventive electroless nickel alloy plating bath is free of thiourea which is conventionally used as stabilizing agent for electroless nickel plating baths because of its toxicity and ecological concerns.
Method According to the Invention
In step A) of the method according to the present invention, the substrate comprising the at least one surface is provided.
Various kinds of substrates can be nickel alloy plated with the electroless nickel alloy plating bath according to the present invention and the method according to the present invention.
Preferably, metallic substrates are used in the method according to the present invention. The metallic substrates to be nickel alloy plated are preferably selected from the group consisting of copper, zinc, silver, palladium, iron, iridium, tin, aluminum, nickel, alloys thereof, and mixtures of the aforementioned. Preferably, the substrate provided in step A) is made entirely of metal (preferably of at least one of the aforementioned preferred metals) or it preferably comprises at least one surface made of metal, preferably at least one of the aforementioned preferred metals. Such a surface made of metal may also be one or more palladium activation layers typically employed to render electrically non-conductive surfaces such as glass, plastic or ceramics receptive for nickel alloy plating.
The substrate is optionally pretreated prior to step B). Such pretreatments are known in the art. Typical pretreatments include etching, cleaning, zincation and activation steps. Useful pretreatments may improve the plating result by removing undesired dirt or oxides from the surface of the substrate. Activation of a surface is usually understood as deposition of a thin and possibly discontinuous layer of for example palladium on an otherwise electrically non-conductive surface to render said surface suitable for subsequent metal plating, preferably nickel alloy plating. Pretreatments may vary widely depending on the substrate provided. Some guidance can exemplarily be found in WO 2015/161959 A1 (page 13, line 11 to page 15, line 30).
In step B) of the method according to the present invention, the at least one surface of the substrate is contacted with the electroless nickel alloy plating bath according to the present invention.
The substrate to be nickel alloy plated may be plated to the desired thickness and deposit quantity by contacting the substrate with the electroless nickel alloy plating bath according to the invention.
The inventive electroless nickel alloy plating is preferably maintained over a temperature range from 20° C. to 100° C., preferably from 70° C. to 95° C., more preferably from 80° C. to 90° C., even more preferably from 83° C. to 87° C. during deposition, i.e. most preferably during step B).
The substrate may be contacted with the inventive electroless nickel alloy plating bath for any period of time sufficient to plate the desired thickness of the nickel alloy deposit. The thickness of the nickel alloy deposit depends inter alia on the desired use of said deposit or the product containing said deposit. As a non-limiting example, a contact duration ranging from 30 to 180 min, preferably from 40 to 90 min, and more preferably from 50 to 70 min is often considered sufficient.
A thickness of the nickel alloy deposit of up to 100 μm, or higher may be formed. Preferably, the thickness of the nickel alloy deposits varies, preferably ranging from 1 to 60 μm. The thickness depends on the technical application and can be higher or lower for some applications. For example, if the nickel alloy deposit is to provide a corrosion resistant coating, a thickness ranging from 30 to 60 μm is typically desired, while for electronic applications a thickness of ranging from 1 to 15 μm is preferably applied. In the technical area of rigid memory disks, the thickness of the nickel alloy deposit preferably ranges from 5 to 20 μm, more preferably from 7 to 16 μm. In the technical area of semi-conductors, the thickness of the nickel or nickel-phosphorus deposits preferably ranges from 1 to 5 μm. Thicknesses of nickel alloy deposits may be measured with x-ray fluorescence (XRF) which is known in the art.
Various means of contacting the substrate with the inventive electroless nickel alloy plating bath are known in the art. For example, the substrate can be entirely or partially immersed (which is preferred) into the inventive electroless nickel alloy plating bath, the inventive electroless nickel alloy plating bath can be sprayed or wiped thereon.
Optionally, the inventive electroless nickel alloy plating bath is agitated before and/or during plating. Agitation may be accomplished for example by mechanical movement of the inventive electroless nickel alloy plating bath like shaking, stirring or continuously pumping of the liquids or intrinsically by ultrasonic treatment, by elevated temperatures or by gas feeds (such as purging the aqueous plating bath with an inert gas or air).
Industry standards require that the plating rate is preferably at least 2.5 μm/h. This allows a sufficiently economic processes. More preferably, the plating rate in combination with the present invention ranges from 8.0 to 12.0 μm/h. Such plating rates allow not only for sufficiently economic processes while improving the desired paramagnetic properties to be maintained at elevated temperatures, but further improves the plating speed without sacrificing quality during plating.
The method according to the present invention optionally comprises rinsing steps, preferably with water, and/or drying steps. The parameters above for the electroless nickel alloy plating bath according to the present invention and the method of the present invention are only provided to give general guidance for practicing the invention.
The inventive electroless nickel alloy plating bath of the present invention can be used to deposit a nickel alloy on a surface of a substrate.
Nickel Alloy Deposits According to the Invention and their Use
The present invention further concerns a nickel alloy deposit obtainable by deposition from the electroless nickel alloy plating bath according to the present invention. Surprisingly, although such deposits seem to be identical in terms of elemental composition (i.e. the contents of nickel, further reducible metal ions and optionally phosphorus and/or boron), the nickel alloy deposits formed from the electroless nickel alloy plating bath according to the present invention show superior corrosion resistance.
The content (preferably the total content) of the further reducible metal in the nickel alloy deposit preferably ranges from 0.5 to 5.0 wt.-%, more preferably from 0.8 to 4.0 wt.-%, even more preferably from 1.0 to 3.0 wt.-%, and yet even more preferably from 1.2 to 2.5 wt.-%.
Preferably, the content of phosphorus and/or boron (more preferably of phosphorus alone) in the nickel alloy deposit is 10 wt.-% or more, more preferably is ranging from 10.0 to 16.0 wt.-%, preferably from 10.5 to 15.0 wt.-% and more preferably from 11.0 to 14.5 wt.-%. Other preferred ranges are from 10 wt.-% to 15 wt.-%, more preferably from 10.5 wt.-% to 13 wt.-%, most preferably from 10.8 wt.-% to 12.5 wt.-%.
The remainder in the nickel alloy deposit which is neither further reducible metal nor phosphorus and/or boron is usually mainly nickel (usually 98 wt.-%, preferably 99 wt.-%, more preferably ≥99.9 wt.-% of said remainder) disregarding trace impurities commonly present in technical raw materials and co-deposited other materials such as those derived from e.g. organic impurities and stabilizing agents. Most preferably, the nickel alloy deposits according to the present invention (essentially) consist of nickel, molybdenum and phosphorus or (essentially) consist of nickel, molybdenum, copper and phosphorus (disregarding trace impurities commonly present in technical raw materials and involuntarily co-deposited other materials such as those derived from e.g. organic impurities and optional stabilizing agents).
The present invention further relates to the use of a nickel alloy deposit obtainable by deposition from the electroless nickel alloy plating bath according to the present invention and the method of the present invention. Such a nickel alloy deposit is preferably used to protect a work piece against ambient acidic corrosive conditions. ‘Ambient’ means that the environment of the work piece is such that the work piece might experience acidic corrosive conditions, most preferably under outdoor, natural conditions.
Details and preferred embodiments described for one aspect of the present invention apply mutatis mutandis to the other aspects. To avoid unnecessary repetition, they are not described time and again.
The invention will now be illustrated by reference to the following non-limiting examples.

Examples

Commercial products were used as described in the technical datasheet available on the date of filing of this specification unless stated otherwise hereinafter. UniClean® 155 (soak cleaner) and Nonacid® 701 (electrocleaner) are products of Atotech Deutschland GmbH.
Steel panels (Q panel type QD) of 0.7 dm²area were used in all plating experiments described hereinafter. The panels were pretreated as described hereinafter before nickel alloy plated.


1) Uniclean ® 155	10 min	70° C.	soak cleaner
2) HCL (17%)	60 s	RT	etching step
3) Nonacid ® 701	60 s	40° C.	electrocleaner
4) Sulfuric acid (10%)	60 s	RT	activation

Determination of Thickness of the Metal or Metal Alloy Deposits and Plating Rate
Phosphorus content and deposit thickness were measured at 5 points of each substrate by XRF using the XRF instrument Fischerscope XDV-SDD (Helmut 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.
pH Value Measurement
pH values were measured with a pH meter (WTW, Typ pH 3110, electrode: SenTix®41, gel electrode with temperature sensor, reference electrolyte: 3 mol/I KCl) at 25° C. The measurement was continued until the pH value became constant, but in any case at least for 2 min. The pH meter was calibrated with buffer solution standards for pH values at 2, 4, and 7 supplied by Fluka and Certipur prior to use.
Composition of the Deposited Nickel Alloys
X-ray photo electron spectroscopy (VersaProbe XPS, Physical Electronics GmbH) was used to measure the composition of the deposited nickel alloys.

Working Examples

Several nickel alloy plating baths, inventive and for comparison reasons, were prepared (each 1 L) containing the following components:


nickel ions	6 g/L (102.2 mmol/L, provided as nickel sulfate),
sodium	33 g/L (0.375 mol/L),
hypophosphite
molybdenum	24 mg/L (2.50*10^-4mol/L, provided as sodium
ions	molybdate),
copper ions	2.0 mg/L (3.15*10⁻⁵mol/L, provided as copper(II)
	sulfate pentahydrate),
complexing	in varying concentrations (in g/L) as given in
agents	Table 1,
pH	adjusted to 4.6 with ammonia solution
	(25 wt.-% in water)

The temperature of the nickel alloy plating baths was set to 86° C. and substrates were immersed into the baths for 60 min.

TABLE 1

Examples 1−16.

#

		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16

Malonic	[g/L]	5.3	5.3	5.3	5.3	5.3	0	2	10	20	5.3	5.3	5.3	5.3	5.3	5.3	5.3
acid
Malic acid	[g/L]	17	17	17	17	17	17	17	17	17	10	25	17	17	17	17	17
Propionic	[g/L]	0	1	10	25	50	3.6	3.6	3.6	3.6	3.6	3.6	3.6	3.6	3.6	3.6	3.6
Acid
Sodium	[g/L]	3	3	3	3	3	3	3	3	3	3	3	0	1	3	10	20
benzoate
Plating	[μm/h]	8.6	9.2	10.2	9.1	8.8	8.5	9.1	8.9	7.7	9.7	7.0	8.9	9.2	9.3	9.3	10.6
rate
Phosphorus	[wt.-%]	11.1	10.9	11.2	11.0	11.8	11.1	10.7	10.9	12.1	10.0	11.6	11.5	11.2	10.9	11.0	10.1
content
Stress	[N/mm²]	−17	−20	−14	−17	−18	−8	−19	−20	−20	skip	−20	−19	−18	−18	−15	−6

First trials with inventive electroless nickel alloy plating baths (Examples 2 to 5, 7 to 11, and 13 to 16) showed a sufficient plating rate and phosphorous content, even with varying amounts of CA1, CA2, CA3, and CA4.
For the sake of further testing and describing the advantages of the present invention, Example 14 was selected as a representative for the present invention.
Corrosion Test 1 (Time)
Corrosion test 1 is performed according to the standard AASS-DIN EN ISO 9227.
Material: Q panel type QD RA<0.5 μm; Deposit Thickness≈10 μm
The area of defects as red rust is determined and compared to the total area (“relative area of defects”) and rated according to the following scheme:
Rating 10 is assigned if no defects are detectable, i.e. 0% relative area of defects. Accordingly, the following ratings are assigned (corresponding relative area of defects in brackets): 9 (>0%-0.1%); 8 (>0.1%-0.25%); 7 (>0.25%-0.5%); 6 (>0.5%-1.0%); 5 (>1.0%-2.5%), 4 (>2.5%-5.0%); 3 (>5.0%-10%), 2 (>10%-25%); 1 (>25%-50%); 0 (>50%).

TABLE 2

Rating in Corrosion test 1 after certain amounts of time

	0 h	24 h	48 h	72 h	96 h

Example 12	10	8	7	6	5
(comparative)
Example 14	10	10	10	9	8
(inventive)

As shown in Table 2, Example 14 (according to the invention) shows a significantly improved corrosion resistance compared to Example 12 (comparative). In Example 12 no aromatic carboxylic acid is utilized. Thus, an aromatic carboxylic acid clearly has a beneficial effect on corrosion resistance.
Corrosion Test 2 (MTO)
Additional tests were carried out with another comparative Example, based on a commercially available product (Nichem® HP 1151, product of Atotech); in the following referred to as Example 17. This comparison allowed an in depth corrosion performance study.
Example 17 is based on the general formulation as outlined above but without utilizing copper and molybdenum ions. Furthermore, Example 17 contains three carboxylic acids corresponding to CA1, CA2, and CA3, but without an aromatic carboxylic acid (i.e. without CA4).
Corrosion test 2 is also performed according to the standard AASS-DIN EN ISO 9227.

TABLE 3

Corrosion test 2 time (in hours) until >1% red rust
on surface is determined

MTO*	0	2	4	6

Example 14	312 h	312 h	224 h	36 h
(inventive)
Example 17	204 h	156 h	100 h	12 h
(comparative)

*denotes Metal Turn Over

Table 3 shows that also for Corrosion test 2 a significant improvement of corrosion resistance is observed. Further experiments indicate that the improved effect is attributed to the presence of CA4 (data not shown)
Intrinsic Stress Test
Bent Strip
Stress in the coating is measured by using a stress strip with two fingers, which are lacquered on one side. The test strips are made from chemically etched beryllium-copper alloy and have spring like properties. Prior to stress measurement, the plating rate of the bath is determined with ±10% precision to determine the plating time for the stress measurement. After surface cleaning by a suitable pretreatment, the stress strip fingers are electrically contacted to the test panel and both are immersed very carefully in the process bath. Plating duration is calculated to yield a coating thickness of 10 μm, based on the known plating rate. After plating, the test strip will be rinsed with DI-water and dried carefully using paper tissue. Since stress strips fingers are mechanically fragile and tend to deform very quickly, they need to be evaluated immediately. Oxide formation on the freshly plated surface will change the beam deflection with time. The plated stress strip finger will be mounted on the testing stand, which measures the distance that the test strip fingers have spread after plating. The distance U determines the distance in increments between the stress strip fingers and is included in the formula which calculates the deposit stress in N/mm².
$Stress [N / {mm}^{2}] = \frac{58.4 \times U}{T} \times K$
T is the deposit thickness in μm (micrometer) and K is the strip calibration constant provided by the supplier. Each lot of test strips manufactured will respond with slight differences when used for deposit stress test. This degree of differences will be determined by the supplier when each lot of test strips is calibrated. The value for K will be supplied with each lot of test strips provided by Specialty Testing & Development Co. Stress is determined to be of compressive or tensile nature. If the test strip legs are spread outward on the side that has been plated, the deposit stress is tensile in nature. If the test strip legs are spread inward on the side that has been plated, the deposit stress is compressive in nature. Compressive stress is by convention given in negative numbers, thus the result of above equation is multiplied by −1 in the case of compressive stress.
Intrinsic stress tests were performed over bath life by linking stress values to the concentration of orthophosphite (in the following abbreviated as OP). OP is a suitable indicator representing bath age because orthophosphite is the reaction product of hypophosphite oxidation and accumulating overtime.

TABLE 4

Intrinsic Stress

Example 14	OP*	0	24.4	50.0	67.0	91.0	116.0	142.0
(inventive)	[g/L]
	Stress	−8.1	−14.5	−17.9	−15.6	−13.4	−8.4	20
	[N/mm²]
Example 17	OP [g/L]	0	21	42	59	82	105	119
(comparative)	Stress	−13.1	−24.4	−30.4	−39.5	−29.8	−12.7	92
	[N/mm²]

Table 4 shows that initially (i.e. OP=0) stress values are very similar/close to each other. Furthermore, the absolute values are in agreement with what is shown also in Table 1 regarding stress values, which are based on OP=0.
However, upon aging, Table 4 shows that on long-term bath utilization the presence of an aromatic carboxylic acid improves the compressive stress situation. This means that the presence of an aromatic carboxylic acid creates an improved tolerance over increasing concentrations of OP and thereby maintaining the internal stress closer to an optimal level (see Example 14: “20” vs. Example 17: “92”). Thus, a more optimal compressive stress level is maintained up to higher bath age as shown for Example 14, in particular if OP reaches a concentration above 100 g/L.
Bath Stability
Beaker Stability test was carried out as described below:
This method is a plating test for determination of chemical stability of an electroless nickel process bath. It applies to autocatalytic deposition of nickel phosphorous coatings. First a high shaped 1 liter beaker is cleaned and stripped with a 1:1 HNO₃-stripper solution for 1 hour to avoid any residues on the bottom and walls on the beaker that might affect the test negatively. Afterwards 1 liter of EN bath solution (test solution) is taken and processed therein as described hereinafter. A stirrer is utilized and set to 250 rpm. The test starts when operation temperature is reached. First the EN test solution is kept for one hour without plating. Afterwards the beaker is checked for any nickel deposition on the bottom and walls of the beaker. If nothing is visible the beaker test can be continued and one pretreated 1 dm²mild steel panel is plated in the EN bath solution. The panel has to be plated for one hour without any replenishment. When this is finished one cycle is achieved. Thus, one cycle corresponds to one hour without plating+one hour with plating. Replenishment is done after the plating period. If no sedimentation or nickel deposition on the bottom or walls of the beaker occurred the test can be continued with one hour without plating and one hour with plating for the next cycle. The beaker test is finished when wild plating or plating out is visible at the bottom or walls of the beaker. The number of achieved cycles is noted. With the number of achieved cycles the evaluation of the stability is possible.
For the following test procedure, said one liter sample is taken from a process bath at different ages of the respective process bath. Samples taken at 0 MTO correspond to a freshly prepared process bath, wherein higher MTOs correspond to increasing bath ages.

TABLE 5

Beaker Stability Test
(Number of cycles)

	MTO*	0	2	4	6

Table 5 shows that samples taken from a process according to Example 14 have a higher stability (5 cycles each) compared to samples taken from a process according to Example 17. Although Example 17 shows a relatively constant number of cycles (3 to 4 each), the absolute stability is improved in the presence of an aromatic carboxylic compound by at least one cycle.
Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It 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.

Example 14

(inventive)

Example 17

(comparative)

*denotes Metal Turn Over

1. An electroless nickel alloy plating bath comprising

a) nickel ions;

b) further reducible metal ions selected from the group consisting of molybdenum ions, rhenium ions, tungsten ions, copper ions, oxo-ions thereof, and mixtures thereof;

c) at least one reducing agent suitable to reduce the nickel ions and the further reducible metal ions to their respective metallic state;

d) complexing agents CA1, CA2, CA3, and, CA4, wherein CA1, CA2, CA3, and CA4 are all different from each other;

wherein each of CA1 and CA2 is independently selected from the group consisting of compounds having at least two carboxylic acid moieties, the respective salts thereof as well as mixtures of the aforementioned;

wherein CA3 is selected from the group consisting of aliphatic compounds having exactly one carboxylic acid moiety, the respective salts thereof as well as mixtures of the aforementioned; and

wherein CA4 is selected from the group consisting of aromatic compounds having at least one carboxylic acid moiety, the respective salts thereof as well as mixtures of the aforementioned.

2. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agents CA1 and CA2 are independently selected from the group consisting of unfunctionalized and functionalized aliphatic dicarboxylic acids, unfunctionalized and functionalized aliphatic tricarboxylic acids, unfunctionalized and functionalized aliphatic tetracarboxylic acids, unfunctionalized and functionalized aliphatic pentacarboxylic acids, unfunctionalized and functionalized aliphatic hexacarboxylic acids, the respective salts thereof as well as mixtures of the aforementioned.

3. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA1 is an unfunctionalized aliphatic C2-C12-dicarboxylic acid and/or salts thereof.

4. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA1 is selected from the group consisting of malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, glutaconic acid, itaconic acid, salts thereof, and mixtures of the aforementioned.

5. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA2 is a functionalized or unfunctionalized aliphatic C3-C12-dicarboxylic acid and/or salts thereof.

6. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA2 is selected from the group consisting of malic acid, tartaric acid, 1-hydroxyglutaric acid, 2-hydroxyglutaric acid, 1-hydroxyadipic acid, 2-hydroxyadipic acid, 3-hydroxyadipic acid, salts thereof, and mixtures of the aforementioned.

7. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA3 is a functionalized or unfunctionalized aliphatic C1-C5-monocarboxylic acid and/or salts thereof.

8. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA4 is a functionalized or unfunctionalized aromatic carboxylic acid and/or salts thereof.

9. The electroless nickel alloy plating bath according to claim 1 characterized in that it has a pH value ranging from 3.5 to 5.0.

10. The electroless nickel alloy plating bath according to claim 1 characterized in that the further reducible metal ions are molybdenum ions, copper ions, or a mixture thereof.

11. The electroless nickel alloy plating bath according to claim 1 characterized in that the further reducible metal ions have a total concentration ranging from 1*10⁻⁴to 5*10⁻³mol/L, based on the total volume of the electroless nickel alloy plating bath.

12. (canceled)

13. A method for depositing nickel alloys on at least one surface of a substrate comprising in this order the following method steps

A) providing the substrate comprising the at least one surface;

B) contacting the at least one surface of the substrate with an electroless nickel alloy plating bath according to claim 1 and thereby depositing a nickel alloy on the at least one surface of the substrate.

14. A nickel alloy deposit obtainable by deposition from the electroless nickel alloy plating bath of claim 1.

15. (canceled)

16. The electroless nickel alloy plating bath according to claim 1 wherein the at least one reducing agent is selected from the group consisting of hypophosphorous acid, hypophosphite salts and mixtures of the aforementioned.

17. The electroless nickel alloy plating bath according to claim 1 characterized in that the bath is free of thiourea.

18. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA3 is propionic acid and/or salts thereof.

19. The electroless nickel alloy plating bath according to claim 1 characterized in that the complexing agent CA4 is benzoic acid and/or salts thereof.

20. The electroless nickel alloy plating bath according to claim 1 characterized in that:

CA1 is malonic acid and/or salts thereof,

CA2 is malic acid and/or salts thereof,

CA3 is propionic acid and/or salts thereof, and

CA4 is benzoic acid and/or salts thereof.

21. The method of claim 13 characterized in that the nickel alloy has a content of phosphorous ranging from 10.0 to 16.0 wt.-%.