Acidic aqueous binary silver-bismuth alloy electroplating composition and method

Abstract

Aqueous acidic binary silver-bismuth alloy electroplating compositions and methods for electroplating silver-rich binary silver-bismuth deposits are disclosed. The aqueous acidic binary silver-bismuth alloy electroplating compositions include a thiol-terminated aliphatic compound having a carboxyl or sulfonic acid group, which enables deposition of a silver-rich binary silver-bismuth alloy having a matte to semi-bright, uniform, and low coefficient of friction deposit.

Description

Technical Field

[0001] The present invention relates to acidic aqueous binary silver-bismuth alloy electroplating compositions and methods. More specifically, the present invention relates to acidic aqueous binary silver-bismuth alloy electroplating compositions and methods, wherein the acidic aqueous binary silver-bismuth alloy electroplating compositions include a thiol-terminated aliphatic compound having a carboxyl group or a sulfonic acid group, which enables the electrodeposition of a silver-rich binary silver-bismuth alloy having good electrical conductivity, low electrical contact resistance, and a low coefficient of friction. Background Art

[0002] Silver and silver alloy plating baths are highly desirable for depositing silver and silver alloys on substrates in applications involving electronic components and jewelry manufacturing. Due to the excellent electrical properties of substantially pure silver, it is used as a contact finish. It has high electrical conductivity and low electrical contact resistance. However, its use as a contact finish, such as in electrical connectors, is limited due to its poor resistance to mechanical wear and the high silver-silver coefficient of friction. Poor resistance to mechanical wear causes physical damage to connectors after a relatively low number of insertion and removal cycles. The high coefficient of friction contributes to this wear problem. When connectors have a high coefficient of friction, the forces required to insert and remove the connectors are very high, which can damage the connector or limit connector design options. Silver alloy deposits (such as silver-antimony and silver-tin) result in improved wear characteristics but have unacceptably poor contact resistance, especially after heat aging. Maintaining good contact resistance over time when exposed to high heat is important, as silver alloys are commonly used in components of automotive engines and in electrical connectors exposed to high soldering temperatures.

[0003] Because many silver salts are essentially water-insoluble and water-soluble silver salts often form insoluble salts with various compounds that are often present in the plating bath, the plating industry faces many challenges in formulating silver or silver alloy plating baths that are stable for a long enough time for practical plating applications and at least solve the aforementioned problems. Many silver and silver-tin alloy plating baths contain cyanide compounds to enable practical applications. However, cyanide compounds are highly toxic. Therefore, special wastewater treatment is required. This leads to increased processing costs. In addition, because these baths can only be used in the alkaline range, the types of alloy metals are limited. Many metals are insoluble under alkaline conditions and precipitate out of solution, such as metal hydroxides. Another disadvantage of alkaline baths is their incompatibility with many photoresist materials, which are used to shield areas on the substrate from plating. Such photoresists may dissolve under alkaline conditions.

[0004] Alkaline baths can also passivate the substrate, resulting in poor adhesion between the plated metal and the substrate. This is often addressed by an additional step known as "bump" plating, which increases the number of processing steps, thereby reducing the overall efficiency of the metal plating process.

[0005] Therefore, there is a need for a silver alloy plating bath that is stable, acidic, and produces a silver alloy having high electrical conductivity, low electrical contact resistance, and a low coefficient of friction. Summary of the Invention

[0006] The present invention relates to a binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, and a thiol-terminated aliphatic compound having the following general formula:

[0007] HS-AR 1 (I)

[0008] wherein A is a substituted or unsubstituted (C1-C4)alkanediyl group and R 1 is a carboxyl group, a carboxylate group, a sulfonic acid group or a sulfonate group, and has a pH of less than 7, wherein the substituent is selected from the group consisting of: (C1-C3)alkyl, carboxyl (C1-C3)alkyl and -NH2.

[0009] The present invention also relates to a method for electroplating a binary silver-bismuth alloy on a substrate, the method comprising:

[0010] a) providing the substrate;

[0011] b) contacting the substrate with a binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, and a thiol-terminated aliphatic compound having the general formula:

[0012] HS-AR 1 (I)

[0013] wherein A is a substituted or unsubstituted (C1-C4)alkanediyl group and R 1 is carboxyl, carboxylate, sulfonic acid, or sulfonate, and has a pH of less than 7, wherein the substituent is selected from the group consisting of: (C1-C3)alkyl, carboxyl(C1-C3)alkyl, and -NH2; and

[0014] c) applying an electric current to the binary silver-bismuth alloy electroplating composition and the substrate to electroplate a silver-bismuth alloy deposit on the substrate.

[0015] The present invention further relates to an article comprising a binary silver-bismuth alloy layer adjacent to a surface of a substrate, wherein the binary silver-bismuth alloy layer comprises 90% to 99% silver and 1% to 10% bismuth and has a coefficient of friction of 1 or less.

[0016] The inclusion of a mercapto-terminated aliphatic compound having the above formula (I) in an aqueous binary silver-bismuth electroplating composition in an acidic environment enables the deposition of a silver-rich binary silver-bismuth alloy on a substrate, such that the silver-rich binary silver-bismuth alloy substantially possesses the favorable electrical properties of a silver deposit, such as good electrical conductivity and low electrical contact resistance (comparable to gold). Furthermore, the silver-rich binary silver-bismuth alloy deposit has a low coefficient of friction, such that the silver-rich binary silver-bismuth alloy deposit has good resistance to mechanical wear. The silver-rich binary silver-bismuth deposit is uniform and shiny in appearance. The binary silver-bismuth alloy electroplating composition of the present invention is stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a SEM of a binary silver-bismuth alloy at 30,000X showing finely dispersed bismuth in a silver matrix.

[0018] Figure 2 is a 2D profilometry map of the surface of a silver metal deposit, where the x- and y-axes are calibrated in micrometers (μm).

[0019] Figure 3 is a 3D profilometry map of the surface of a silver metal deposit, where the x-, y-, and z-axes are calibrated in micrometers (μm).

[0020] Figure 4 is a 3D profilometry plot of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 98% silver and 2% bismuth, with the x-, y-, and z-axes calibrated in micrometers (μm). DETAILED DESCRIPTION

[0021] As used throughout the specification, unless the context clearly indicates otherwise, the abbreviations have the following meanings: °C = degrees Celsius; ppm = parts per million; 1 ppm = 1 mg / L; g = gram; mg = milligram; L = liter; mL = milliliter; mm = millimeter; cm = centimeter; μm = micrometer; DI = deionized; A = ampere; ASD = ampere / dm 2 = plating rate; DC = direct current; v = volt, which is the SI unit of electromotive force; mΩ = milliohm = electrical resistance; cN = hundredth Newton = unit of force; N = Newton; COF = coefficient of friction; rpm = revolutions per minute; s = second; SEM = scanning electron microscopy; 2D = two-dimensional; 3D = three-dimensional; Ag = silver; Bi = bismuth; Au = gold; and Cu = copper.

[0022] The term "alkanediyl" (plural = alkanediyls) refers to a radical having the general formula C n H 2nAny of a series of divalent groups, unless otherwise specified, such alkanediyl groups include substituted alkanediyl groups. The term "alkylene" is an obsolete term or synonym for "alkanediyl". The term "aliphatic" means relating to or representing an organic compound in which carbon atoms form open chains (as in alkanes) rather than aromatic rings. The term "binary" when referring to an alloy means a metallic solid composed of a homogeneous mixture of two metals. The term "adjacent" means direct contact such that the two metal layers have a common interface. The term "contact resistance" means the electrical resistance resulting from contact between two conductive articles, the resistance being measured based on the force applied between the two articles. The term "reduction potential" means a measure of the tendency of a metal ion to acquire electrons and thereby be reduced to metal. The abbreviation "N" means Newton, which is the SI unit of force and is equal to the force that would give a mass of 1 kilogram an acceleration of 1 meter per second per second and is equal to 100,000 dynes. The term "friction coefficient" is a value showing the relationship between the friction force between two objects and the normal reaction force between the objects involved; and is represented by F f =μF n Shown, where F f is the friction force, μ is the friction coefficient, and F n is the normal force, wherein the normal force is the force applied between the two articles perpendicular to the direction of relative motion between the two articles when measuring the friction between the two articles. The term "tribology" means the science and engineering of surfaces that interact in relative motion, and includes the study and application of the principles of lubrication, friction, and wear. The term "wear resistance" means the loss of material from a surface by mechanical action. The term "aqueous" means water or water-based. Throughout this specification, the terms "composition" and "bath" are used interchangeably. Throughout this specification, the terms "deposit" and "layer" are used interchangeably. Throughout this specification, the terms "electroplating," "plating," and "deposition" are used interchangeably. The term "matte" means dull or lacking gloss. Throughout this specification, the terms "a / an" may refer to both the singular and the plural. Unless otherwise indicated, all percentage (%) values ​​and ranges indicate weight percentages. All numerical ranges are inclusive and combinable in any order, except where it is logical to constrain such numerical ranges to add up to 100%.

[0023] The present invention relates to an aqueous acidic binary silver-bismuth electroplating composition, wherein the aqueous acidic binary silver-bismuth electroplating composition comprises a silver ion source, a bismuth ion source, and a thiol-terminated aliphatic compound having the following general formula:

[0024] HS-AR 1 (I)

[0025] wherein A is a substituted or unsubstituted (C1-C4)alkanediyl group and R 1is a carboxyl group, a carboxylate group and a counter cation, a sulfonic acid group or a sulfonate group and a counter cation, and the pH is less than 7, wherein the substituent is selected from the group consisting of: (C1-C3)alkyl, carboxyl (C1-C3)alkyl and -NH2.

[0026] Such compounds of formula (I) above are complexing agents that are selective for bismuth ions. Preferably, the aqueous acidic binary silver-bismuth alloy electroplating composition of the present invention comprises a molar ratio of the thiol-terminated aliphatic compound of formula (I) to bismuth ions of at least 3:1, more preferably 3:1 to 10:1, even more preferably 3:1 to 6:1, and most preferably 3.5:1 to 4.5:1.

[0027] The matte to semi-bright and uniform silver-rich binary silver-bismuth alloy deposit has substantially good electrical properties, such as good electrical conductivity and low electrical contact resistance. The silver-rich binary silver-bismuth alloy deposit has a low coefficient of friction, which makes the silver-rich binary silver-bismuth alloy layer have good mechanical wear resistance. The acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention is stable. The aqueous binary silver-bismuth alloy electroplating composition does not contain any additional alloying metals, such as but not limited to antimony, tin, copper, nickel, cobalt, cadmium, gold, lead, indium, iron, palladium, platinum, rhodium, ruthenium, tellurium, thallium, selenium and zinc. Preferably, the acidic mercury-containing bismuth electroplating composition does not contain cyanide.

[0028] Preferably, the thiol-terminated aliphatic compound of the present invention is selected from one or more of the following:

[0029]

[0030] Thioglycolic acid;

[0031]

[0032] 2-Mercaptopropionic acid;

[0033]

[0034] 3-Mercaptopropionic acid;

[0035]

[0036] Cysteine;

[0037]

[0038] Mercaptosuccinic acid;

[0039]

[0040] 3-Mercapto-1-propanesulfonic acid;

[0041]

[0042] 2-Mercaptoethanesulfonic acid; and

[0043] The salt of the thiol-terminated aliphatic compound. More preferably, the thiol-terminated aliphatic compound of the present invention is selected from one or more of the following: 2-mercaptopropionic acid, 3-mercaptopropionic acid, cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts of the thiol-terminated aliphatic compound; even more preferably, the thiol-terminated aliphatic compound of the present invention is selected from one or more of the following: cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts of the thiol-terminated aliphatic compound; further preferably, the thiol-terminated aliphatic compound of the present invention is selected from one or more of the following: mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts of the thiol-terminated aliphatic compound; and most preferably, the thiol-terminated aliphatic compound of the present invention is selected from one or more of the following: 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts of the thiol-terminated aliphatic compound. Salts of the mercapto compounds of the present invention include, but are not limited to, alkali metal salts such as sodium salts, potassium salts, lithium salts, and cesium salts, ammonium salts, and tetraalkylammonium salts.

[0044] Examples of preferred salts include ammonium thioglycolate, sodium thioglycolate, sodium mercaptosuccinate, sodium 3-mercapto-1-propanesulfonic acid, sodium 3-mercapto-1-ethanesulfonic acid, and potassium 3-mercapto-1-ethanesulfonic acid. Mixtures of these preferred salts may also be included in the binary silver-bismuth electroplating composition of the present invention. More preferably, the salts are sodium mercaptosuccinate, sodium 3-mercapto-1-propanesulfonic acid, and sodium 3-mercapto-1-ethanesulfonic acid.

[0045] The thiol-terminated aliphatic compound of the present invention is included in an amount sufficient to enable electroplating of a silver-rich binary silver-bismuth alloy in an aqueous acidic environment. Preferably, the thiol-terminated aliphatic compound of the present invention is included in an amount of 5 g / L or greater, more preferably, the thiol compound is included in an amount of 10 g / L to 100 g / L, further preferably 15 g / L to 60 g / L, even more preferably 20 g / L to 50 g / L, and most preferably 30 g / L to 50 g / L.

[0046] The aqueous acidic silver-bismuth alloy electroplating composition of the present invention comprises a silver ion source. The silver ion source can be provided by a silver salt, such as, but not limited to, silver halide, silver gluconate, silver citrate, silver lactate, silver nitrate, silver sulfate, silver alkanesulfonate, silver alkanolsulfonate, or a mixture thereof. When a silver halide is used, the halide is preferably a chloride. Preferably, the silver salt is silver sulfate, silver alkanesulfonate, silver nitrate, or a mixture thereof, and more preferably, the silver salt is silver sulfate, silver methanesulfonate, or a mixture thereof. A mixture of silver salts may also be included in the composition. Silver salts are generally commercially available or can be prepared by methods described in the literature. Preferably, the silver salt is readily soluble in water.

[0047] The amount of silver salt included in the aqueous acidic binary silver-bismuth electroplating composition is an amount sufficient to provide the desired matte to semi-bright and uniform silver-rich binary silver-bismuth alloy deposit, preferably, wherein the silver content of the silver-rich binary silver-bismuth alloy deposit contains 90% to 99.8% silver, more preferably 90% to 99.7% silver, more preferably 93% to 99.7% silver, and most preferably 95% to 99% silver. Preferably, the silver salt is included in the composition to provide a silver ion concentration of at least 10 g / L, more preferably, the silver salt is included in the composition in an amount to provide a silver ion concentration in the range of 10 g / L to 100 g / L, further preferably, the silver salt is included in the composition in an amount to provide a silver ion concentration in the range of 20 g / L to 80 g / L, even more preferably, the silver salt is included in the composition in an amount to provide a silver ion concentration in the range of 20 g / L to 70 g / L, and most preferably, the silver salt is included in the composition in an amount to provide a silver ion concentration in the range of 20 g / L to 60 g / L.

[0048] The aqueous acidic silver-bismuth alloy electroplating composition includes a bismuth ion source providing a solution having Bi 3+ The composition may also contain a bismuth ion electroplating bath. Sources of bismuth ions include, but are not limited to, bismuth salts of alkanesulfonic acids such as bismuth methanesulfonate, bismuth ethanesulfonate, bismuth propanesulfonate, bismuth 2-propanesulfonate, and bismuth p-phenolsulfonate; bismuth salts of alkanolsulfonic acids such as bismuth hydroxymethanesulfonate, bismuth 2-hydroxyethane-1-sulfonate, and bismuth 2-hydroxybutane-1-sulfonate; and bismuth salts such as bismuth nitrate, bismuth sulfate, bismuth chloride, and bismuth oxide. The composition may also contain a mixture of bismuth salts. Preferably, the bismuth salt is water-soluble.

[0049] The amount of bismuth salt included in the aqueous acidic binary silver-bismuth electroplating composition is sufficient to provide the desired matte to semi-bright and uniform silver-rich binary silver-bismuth alloy deposit, preferably wherein the bismuth content of the silver-rich binary silver-bismuth alloy deposit contains 0.2% to 10% bismuth, more preferably 0.3% to 10% bismuth, more preferably 0.3% to 7% bismuth, and most preferably 1% to 5% bismuth. Preferably, the bismuth salt is included in the silver-bismuth composition to provide bismuth (III) ions in an amount of 50 ppm to 10 g / L, more preferably 100 ppm to 5 g / L, more preferably 200 ppm to 1 g / L, and most preferably 300 ppm to 800 ppm. Such bismuth salts are commercially available or can be prepared according to disclosures in the chemical literature. They are generally commercially available from various sources, such as Aldrich Chemical Company, Milwaukee, Wisconsin.

[0050] Preferably, in the aqueous acidic silver-bismuth alloy electroplating composition of the present invention, water contained as a solvent is at least one of deionized water and distilled water to limit incidental impurities.

[0051] Optionally, an acid may be included in the binary silver-bismuth alloy electroplating composition to help provide conductivity to the composition. Acids include, but are not limited to, organic acids such as acetic acid, citric acid, arylsulfonic acid, alkanesulfonic acid (such as methanesulfonic acid, ethanesulfonic acid and propanesulfonic acid), arylsulfonic acid (such as benzenesulfonic acid and toluenesulfonic acid); and inorganic acids such as sulfuric acid, aminosulfonic acid, hydrochloric acid, hydrobromic acid and fluoroboric acid. Water-soluble salts of the aforementioned acids may also be included in the binary silver-bismuth alloy electroplating composition of the present invention. Preferably, the acid is acetic acid, citric acid, alkanesulfonic acid, arylsulfonic acid, or a salt thereof, more preferably, the acid is acetic acid, citric acid, methanesulfonic acid, or a salt thereof. Such salts include, but are not limited to, alkali metal salts such as sodium salts, potassium salts, lithium salts and cesium salts, ammonium salts, tetraalkylammonium salts and magnesium salts. Such salts also include, but are not limited to, sodium acetate and potassium acetate, trisodium citrate, disodium hydrogen citrate, sodium dihydrogen citrate, trisodium citrate, tripotassium citrate, dipotassium citrate, dipotassium hydrogen citrate, and potassium dihydrogen citrate. Although a mixture of acids can be used, it is preferred to use a separate acid when used. Acids are typically commercially available or can be prepared by methods known in the literature. Such acids can be included in an amount that provides desired conductivity. Preferably, the acid or its salt is included in an amount of at least 5g / L, more preferably 10g / L to 250g / L, even more preferably 30g / L to 150g / L, most preferably 30g / L to 125g / L.

[0052] The pH of the aqueous acidic binary silver-bismuth alloy electroplating composition is less than 7. Preferably, the pH is 0 to 6, more preferably the pH is 0 to 5, further preferably the pH is 0 to 3, even more preferably the pH is 0 to 2.5, and most preferably the pH is 0 to 2.

[0053] Optionally, a pH adjuster may be included in the aqueous acidic binary silver-bismuth alloy composition of the present invention. Such pH adjusters include inorganic acids, organic acids, inorganic bases, or organic bases and their salts. Such acids include, but are not limited to, inorganic acids such as sulfuric acid, hydrochloric acid, sulfamic acid, boric acid, phosphoric acid, and their salts. Organic acids include, but are not limited to, acetic acid, citric acid, glycine, and ascorbic acid and their salts. Such salts include, but are not limited to, trisodium citrate. Inorganic bases such as sodium hydroxide and potassium hydroxide, as well as organic bases such as various types of amines, may be used. Preferably, the pH adjuster is selected from acetic acid, citric acid, and glycine and their salts, most preferably, acetic acid, citric acid, and their salts. The pH adjuster may be added in an amount required to maintain the desired pH range.

[0054] Optionally, but preferably, a dihydroxydisulfide compound or mixtures thereof may be included in the aqueous acidic silver-bismuth alloy electroplating composition of the present invention. Such dihydroxydisulfide compounds include, but are not limited to, 2,4-dithia-1,5-pentanediol, 2,5-dithia-1,6-hexanediol, 2,6-dithia-1,7-heptanediol, 2,7-dithia-1,8-octanediol, 2,8-dithia-1,9-nonanediol, 2,9-dithia-1,10-decanediol, 2,11-dithia-1,12-dodecanediol, 5,8-dithia-1,12-dodecanediol, 2,15-dithia-1,16-hexadecanediol, 2,21-dithia-1,22-docosanediol, 3,5-dithia-1,7-heptanediol, 3,6-dithia-1,8-octanediol, 3,8-dithia-1,10-decanediol, 3,10-dithia-1,10-decanediol, 3 ...0-dithia-1,10-decanediol, 3,10-dithia-1,10-decanediol, 3,10-dithia-1,10-decanediol, 3 Thia-1,8-dodecanediol, 3,13-dithia-1,15-pentadecanediol, 3,18-dithia-1,20-eicosanediol, 4,6-dithia-1,9-nonanediol, 4,7-dithia-1,10-decanediol, 4,11-dithia-1,14-tetradecanediol, 4,15-dithia-1,18-octadecanediol, 4,19-dithia-1,22-docosanediol, 5,7-dithia-1,11-undecanediol, 5,9-dithia-1,13-tridecanediol, 5,13-dithia-1,17-heptadecanediol, 5,17-dithia-1,21-heneicosanediol, and 1,8-dimethyl-3,6-dithia-1,8-octanediol. Preferably, the dihydroxydisulfide compound is selected from 3,6-dithia-1,8-octanediol, 3,8-dithia-1,10-decanediol, 2,4-dithia-1,5-pentanediol, 2,5-dithia-1,6-hexanediol, 2,6-dithia-1,7-heptanediol, 2,7-dithia-1,8-octanediol, more preferably 3,6-dithia-1,8-octanediol, 2,4-dithia-1,5-pentanediol, 2,5-dithia-1,6-hexanediol, 2,6-dithia-1,7-heptanediol, 2,7-dithia-1,8-octanediol, 4-dithia-1,5-pentanediol, 2,5-dithia-1,6-hexanediol, 2,6-dithia-1,7-heptanediol and 2,7-dithia-1,8-octanediol, even more preferably 3,6-dithia-1,8-octanediol, 2,6-dithia-1,7-heptanediol and 2,7-dithia-1,8-octanediol, most preferably 3,6-dithia-1,8-octanediol.

[0055] Preferably, the dihydroxy disulfide compound may be included in the aqueous acidic binary silver-bismuth alloy electroplating composition in an amount of at least 0.5 g / L, more preferably 10 g / L to 200 g / L, even more preferably 50 g / L to 150 g / L, further preferably 50 g / L to 125 g / L, and most preferably 80 g / L to 115 g / L.

[0056] Optionally, one or more surfactants may be included in the aqueous acidic silver-nickel alloy electroplating composition of the present invention. Such surfactants include, but are not limited to, ionic surfactants, such as cationic and anionic surfactants, nonionic surfactants, and amphoteric surfactants. The surfactant may be included in conventional amounts, such as 0.05 g / L to 30 g / L.

[0057] Examples of anionic surfactants are sodium di(1,3-dimethylbutyl)sulfosuccinate, sodium 2-ethylhexyl sulfate, sodium diamylsulfosuccinate, sodium lauryl sulfate, sodium lauryl ether sulfate, sodium dialkylsulfosuccinate and sodium dodecylbenzenesulfonate. Examples of cationic surfactants are quaternary ammonium salts, such as perfluorinated quaternary amines.

[0058] Other optional additives may include, but are not limited to, brighteners and biocides. Conventional brighteners and biocides well known in the art may be included in the aqueous acidic binary silver-bismuth electroplating composition. Such optional additives may be included in conventional amounts.

[0059] Preferably, the acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention consists of water, silver ions and counter anions, bismuth (III) ions and counter anions, a thiol-terminated aliphatic compound having the following general formula, optionally a dihydroxydisulfide compound, optionally an acid or a salt thereof, optionally a pH adjuster, optionally a surfactant, optionally a brightener, and optionally a biocide:

[0060] HS-AR 1 (I)

[0061] wherein A is a substituted or unsubstituted (C1-C4)alkanediyl group and R 1 is carboxyl, carboxylate, sulfonic acid or sulfonate, wherein the substituent is selected from the group consisting of (C1-C3)alkyl, carboxyl(C1-C3)alkyl and -NH2, wherein the pH is less than 7.

[0062] Further preferably, the acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention is composed of water, silver ions and counter anions, bismuth (III) ions and counter anions, a thiol-terminated aliphatic compound having the following general formula, a dihydroxydisulfide compound, optionally an acid or a salt thereof, optionally a pH adjuster, optionally a surfactant, optionally a brightener, and optionally a biocide:

[0063] HS-AR 1 (I)

[0064] wherein A is a substituted or unsubstituted (C1-C4)alkanediyl group and R 1is carboxyl, carboxylate, sulfonic acid or sulfonate, wherein the substituent is selected from the group consisting of (C1-C3)alkyl, carboxyl(C1-C3)alkyl and -NH2, wherein the pH is 0-6.

[0065] More preferably, the acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention consists of water, silver ions and counter anions, bismuth (III) ions and counter anions, a thiol-terminated aliphatic compound having the following general formula, a dihydroxydisulfide compound, an acid or a salt thereof, optionally a pH adjuster, optionally a surfactant, optionally a brightener, and optionally a biocide:

[0066] Hs-AR 1 (I)

[0067] wherein A is a substituted or unsubstituted (C1-C4)alkanediyl group and R 1 is carboxyl, carboxylate, sulfonic acid or sulfonate, wherein the substituent is selected from the group consisting of (C1-C3)alkyl, carboxyl(C1-C3)alkyl and -NH2, wherein the pH is 0-6.

[0068] Even more preferably, the acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention consists of water, silver ions and counter anions, bismuth (III) ions and counter anions, a thiol-terminated aliphatic compound, a dihydroxydisulfide compound, an acid or a salt thereof, optionally a pH adjuster, optionally a surfactant, optionally a brightener, and optionally a biocide, wherein the pH is 0-3, and the thiol-terminated aliphatic compound is selected from the group consisting of thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, salts of thiol-terminated aliphatic compounds, and mixtures thereof.

[0069] The acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention can be used to deposit binary silver-bismuth alloy layers on a variety of substrates, both conductive and semiconductive. Preferably, the substrate on which the silver-bismuth alloy layer is deposited is a copper and copper alloy substrate. Such copper alloy substrates include, but are not limited to, brass and bronze. During plating, the electroplating composition temperature can range from room temperature to 70°C, preferably from 30°C to 60°C, and more preferably from 40°C to 60°C. The silver-bismuth alloy electroplating composition is preferably under continuous stirring during electroplating.

[0070] The acidic aqueous binary silver-bismuth alloy electroplating method of the present invention comprises providing a substrate, providing the acidic mercury-containing silver-bismuth alloy electroplating composition of the present invention, and contacting the substrate with the acidic mercury-containing silver-bismuth alloy electroplating composition, such as by immersing the substrate in the composition or spraying the substrate with the composition. Current is applied using a conventional rectifier, wherein the substrate acts as a cathode and a counter electrode or anode is present. The anode can be any conventional soluble or insoluble anode used for electroplating binary silver-bismuth alloys to deposit adjacent to the surface of the substrate.

[0071] The acidic mercury-containing silver-bismuth alloy electroplating composition of the present invention enables the deposition of a matte to semi-bright, uniform silver-rich silver-bismuth alloy layer over a wide current density range. The silver-rich silver-bismuth alloy comprises 90% to 99.8% silver and 0.2% to 10% bismuth, preferably 90% to 99.7% silver and 0.3% to 10% bismuth, more preferably 93% to 99.7% silver and 0.3% to 7% bismuth, and most preferably 95% to 99% silver and 1% to 5% bismuth, excluding unavoidable impurities in the alloy.

[0072] The current density for electroplating the matte to semi-bright and uniform silver-rich silver-bismuth alloy of the present invention may be 0.1 ASD or higher. Preferably, the current density is 0.5 ASD to 70 ASD, more preferably 1 ASD to 40 ASD, more preferably 1 ASD to 30 ASD, and even more preferably 1 ASD to 15 ASD.

[0073] The thickness of the binary silver-bismuth alloy layer of the present invention can vary depending on the function of the silver-bismuth alloy layer and the type of substrate it is plated on. Preferably, the silver-bismuth alloy layer is 1 μm or greater. Further preferably, the silver-bismuth layer has a thickness ranging from 1 μm to 100 μm, more preferably from 1 μm to 50 μm, even more preferably from 1 μm to 10 μm, and most preferably from 1 μm to 5 μm.

[0074] While it is contemplated that the acidic aqueous binary silver-bismuth alloy electroplating compositions of the present invention may be used to plate a variety of substrates that may include a silver-bismuth alloy layer, preferably, the acidic aqueous silver-bismuth alloy electroplating compositions of the present invention are used to electroplate a top layer or coating on electrical connectors where substantial contact forces and wear are expected to be prevalent. Silver-rich silver-bismuth alloy deposits are highly desirable alternatives to conventional silver coatings found on conventional connectors. The silver-bismuth alloy deposits have low electrical contact resistance. Additionally, the silver-bismuth alloy deposits of the present invention have a low COF, preferably 1 or less, more preferably a COF of 0.3 or less. The COF of the silver-nickel alloy deposits of the present invention has a COF that is preferably 40% less than, more preferably 80% less than, that of a substantially pure silver deposit, thus, the binary silver-bismuth alloys of the present invention have a significant improvement in wear resistance relative to substantially pure silver. Surface wear of the metal deposits can be determined according to conventional tribological and profilometry measurements well known in the art.

[0075] The following examples are included to further illustrate the invention but are not intended to limit its scope.

[0076] Binary silver-bismuth alloy electroplating examples 1-8:

[0077] Unless otherwise specified, in all cases the electroplated substrates were 5 cm x 5 cm brass (70% copper, 30% zinc) coupons. Prior to electroplating, the coupons were TM The samples were electro-cleaned at room temperature for 30 seconds with DC at a current density of 5 ASD in GP-100 electrolytic alkaline degreaser (available from DuPont de Nemours). After electro-cleaning, the samples were rinsed with DI water, activated in 10% sulfuric acid for 30 seconds, rinsed again with DI water, and then placed in an electroplating bath. Electroplating was carried out for 6 minutes with DC at a current density of 1 ASD (the actual current applied was 0.28 A) to deposit a silver-bismuth deposit of approximately 4 μm. Electroplating was carried out in a square glass beaker using a platinum-plated titanium anode. Stirring was provided at a speed of 400 rpm by a 5 cm long TEFLON-coated stirring rod. Electroplating was carried out at a temperature of 55°C. All silver-bismuth electroplating baths were water-based. Water was added to each bath to reach the desired volume. The pH of the electroplating bath was adjusted with potassium hydroxide or methanesulfonic acid.

[0078] The thickness and elemental composition of the electroplated silver-bismuth alloy were measured using a Bowman Series P X-ray fluorimeter (XRF) available from Bowman, Schaumburg, IL. The XRF was calibrated using pure element thickness standards of silver and bismuth from Bowman, and the alloy composition and thickness were calculated by combining the pure element standards with the fundamental parameter (FP) calculations in the XRF process specification.

[0079] Example 1 (present invention)

[0080] An aqueous acidic binary silver-bismuth electroplating bath was prepared having the following composition:

[0081] Silver methanesulfonate that supplies 20g / L of silver ions

[0082] 3,6-dithia-1,8-octanediol: 102g / L

[0083] Bismuth methanesulfonate supplying 2g / L of bismuth ions

[0084] Cysteine: 9g / L

[0085] 3-Mercapto-1-propanesulfonic acid sodium salt: 2g / L

[0086] Adjust pH to 2

[0087] After the plating procedure, the electrodeposited coating is metallic and matte, with a composition of 98% silver and 2% bismuth. Figure 1 is a SEM of a binary silver-bismuth alloy at 30,000X showing finely dispersed bismuth in the silver matrix.

[0088] Example 2 (present invention)

[0089] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0090] Silver methanesulfonate that supplies 20g / L of silver ions

[0091] 3,6-dithia-1,8-octanediol: 102g / L

[0092] Bismuth methanesulfonate supplying 5g / L of bismuth ions

[0093] Cysteine: 9g / L

[0094] 2-Mercaptoethanesulfonic acid: 400ppm

[0095] Adjust pH to 2

[0096] After the plating procedure, the electrodeposited coating is metallic and semi-bright, with a composition of 95% silver and 5% bismuth.

[0097] Example 3 (present invention)

[0098] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0099] Silver methanesulfonate that supplies 20g / L of silver ions

[0100] 3,6-dithia-1,8-octanediol: 102g / L

[0101] Bismuth methanesulfonate supplying 5g / L of bismuth ions

[0102] 3-Mercapto-1-propanesulfonic acid sodium salt: 13.2g / L

[0103] Cysteine: 400ppm

[0104] Adjust pH to 2

[0105] After the plating procedure, the electrodeposited coating is metallic and semi-bright, with a composition of 96% silver and 4% bismuth.

[0106] Example 4 (present invention)

[0107] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0108] Silver methanesulfonate that supplies 20g / L of silver ions

[0109] 3,6-dithia-1,8-octanediol: 102g / L

[0110] Bismuth methanesulfonate supplying 5g / L of bismuth ions

[0111] 3-Mercapto-1-ethanesulfonic acid sodium salt: 12.2g / L

[0112] Cysteine: 400ppm

[0113] Adjust pH to 2

[0114] After the plating procedure, the electrodeposited coating is metallic and semi-bright, with a composition of 96% silver and 4% bismuth.

[0115] Example 5 (present invention)

[0116] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0117] Silver methanesulfonate that supplies 20g / L of silver ions

[0118] 3,6-dithia-1,8-octanediol: 102g / L

[0119] Bismuth methanesulfonate supplying 5g / L of bismuth ions

[0120] Mercaptosuccinic acid: 11.1 g / L

[0121] 3-Mercapto-1-ethanesulfonic acid sodium salt: 400ppm

[0122] Adjust pH to 2

[0123] After the plating procedure, the electrodeposited coating is metallic and matte, with a composition of 98% silver and 2% bismuth.

[0124] Example 6 (present invention)

[0125] An aqueous acidic binary silver-bismuth electroplating bath was prepared having the following composition:

[0126] Silver methanesulfonate that supplies 20g / L of silver ions

[0127] 3,6-dithia-1,8-octanediol: 102g / L

[0128] Bismuth methanesulfonate supplying 5g / L of bismuth ions

[0129] Mercaptosuccinic acid: 11.9 g / L

[0130] 2-Mercaptopropionic acid: 400ppm

[0131] Adjust pH to 2

[0132] After the plating procedure, the electrodeposited coating is metallic and matte, with a composition of 94% silver and 6% bismuth.

[0133] Example 7 (present invention)

[0134] An aqueous acidic binary silver-bismuth electroplating bath was prepared having the following composition:

[0135] Silver methanesulfonate that supplies 20g / L of silver ions

[0136] 3,6-dithia-1,8-octanediol: 102g / L

[0137] Bismuth methanesulfonate supplying 5g / L of bismuth ions

[0138] Thioglycolic acid: 9g / L

[0139] 2-Mercaptoethanesulfonic acid: 400ppm

[0140] Adjust pH to 2

[0141] After the plating procedure, the electrodeposited coating is metallic and semi-bright, with a composition of 95% silver and 5% bismuth.

[0142] Example 8 (comparison)

[0143] An aqueous acidic binary silver-bismuth electroplating bath was prepared having the following composition:

[0144] Silver methanesulfonate that supplies 20g / L of silver ions

[0145] Bismuth methanesulfonate supplying 10g / L of bismuth ions

[0146] Methanesulfonic acid: 150g / L

[0147] Pluronic L-44 surfactant (purchased from BASF): 10 g / L

[0148] o-Chlorobenzaldehyde: 100ppm

[0149] 3,6-dithia-1,8-octanediol: 80g / L

[0150] pH<1

[0151] After the plating procedure, the electrodeposited coating was metallic and semi-bright, with a composition of 46% silver and 54% bismuth.

[0152] Example 9 (present invention)

[0153] Contact resistance measurement

[0154] Contact resistance was evaluated using a custom-designed apparatus comprising a Starrett MTH-550 manual dynamometer stand equipped with a Starrett DFC-20 digital dynamometer. The digital dynamometer was equipped with a gold-plated copper probe having a hemispherical tip with a diameter of 2.5 mm. The contact resistance between the gold-plated probe and a flat specimen plated with the silver alloy of interest was measured using a 4-wire resistance measurement as the contact force varied. The current source was a Keithley 6220 DC current source, and the voltmeter was a Keithley 2182A nanovoltmeter. These instruments were operated in thermoelectric compensation mode to achieve maximum accuracy.

[0155] Testing was conducted using flat brass coupons plated with approximately 3 μm of binary silver-bismuth alloy from the aqueous acidic binary silver-bismuth alloy electroplating bath disclosed in Example 1 above. The applied force was measured using a Starrett DGF-20 digital force gauge and adjusted using a manual height stage. The contact resistance is reported in Table 1 below.

[0156] Table 1

[0157] Contact resistance

[0158] Force (cN) Ag(98%)-Bi(2%) / Brass(mΩ) 0 800 5 225 10 120 20 90 30 80 40 70 50 60 60 50 70 40 80 20 90 10 100 10

[0159] Example 10 (comparison)

[0160] Silver wear resistance

[0161] Tribological measurements were performed using an Anton Paar TRB3 pin-on-disc tribometer (available from Anton Paar GmbH, Graz, Austria) equipped with a linear reciprocating stage. All tests were performed using a 1 N load, a 10 mm stroke length, and a 5 mm / s sliding speed. All tests were performed "like-on-like," meaning that the flat coupons and the spherical balls were each coated with the same silver metal deposit, which was obtained from SILVER GLO® available from DuPont Nemours. TM Produced in an electrolytic silver bath. The ball used was made of C260 brass (70% copper, 30% zinc) and had a diameter of 5.55 mm and was electroplated with about 5 μm of silver. The flat specimen was also made of C260 brass and was electroplated with about 5 μm of silver. During the test, the friction coefficient was monitored using a tribometer. The wear mark depth was measured using laser profilometry. The measurement was carried out for 100 cycles, where each cycle was a reciprocating stroke of the ball on the specimen. It only takes 100 cycles to break through the silver-plated deposit. Profilometry measurements were performed using a Keyence VK-X laser scanning confocal microscope (available from Keyence Corporation of America, Elmwood Park, NJ, USA). Wear marks were measured using laser profilometry at a magnification of 200X. 3D and 2D profilometry maps were created based on these measurements using VK-X analysis software from Keyence.

[0162] Figure 2 is a 2D profilometry plot of a silver deposit showing primary surface wear of the silver from 600 μm to 800 μm along the x-axis and from +2 μm to -5 μm along the y-axis. The vertical dashed line indicates the depth of the indentation wear scar, which is 7.3 μm. Figure 3 is a 3D profilometry image of the silver deposit, which further illustrates the severe surface wear of the silver deposit after 100 cycles. Figure 2 The depth of the dent wear marks.

[0163] The coefficient of friction (COF) was determined to be about 1.6. The COF was measured directly by the tribometer described above using the software Tribometer (version 8.1.5).

[0164] Example 11 (present invention)

[0165] Wear resistance of binary silver-bismuth alloy

[0166] As in Example 10 above, tribological measurements were performed using an Anton Paar TRB3 pin-on-disc tribometer equipped with a linear reciprocating stage. All tests were performed using a 1N load, a stroke length of 10mm, and a sliding speed of 5mm / s. The flat specimens and spherical balls were each plated with the silver-bismuth alloy in Example 1 above. The balls used were made of C260 brass (70% copper, 30% zinc) and had a diameter of 5.55mm and were electroplated with approximately 5μm of silver-bismuth alloy. The flat specimens were also made of C260 brass and electroplated with approximately 2μm of alloy. During the test, the friction coefficient was monitored using a tribometer. As in Example 10, the wear mark depth was measured using a Keyence VK-X laser scanning confocal microscope using laser profilometry. The measurement was performed for 500 cycles. The wear marks were measured using laser profilometry at a magnification of 200X. 3D profilometry maps were created based on these measurements using software from Keyence Corporation.

[0167] Figure 4 3D profilometry of the silver-bismuth deposit. Even after 500 cycles, there was no indication of surface wear. The coefficient of friction was determined to be approximately 0.3, which is an 80% reduction compared to the silver in Example 10.

Claims

1. A binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, one or more hydroxydisulfide compounds, and a thiol-terminated aliphatic compound selected from one or more of the following: thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts of the thiol-terminated compounds.

2. The binary silver-bismuth alloy electroplating composition of claim 1, further comprising one or more acids or salts thereof.

3. The binary silver-bismuth alloy electroplating composition of claim 1 , further comprising one or more pH adjusters.

4. The binary silver-bismuth alloy electroplating composition according to claim 1, wherein: The binary silver-bismuth alloy electroplating composition has a pH of 0 to 6.

5. A method for electroplating a binary silver-bismuth alloy on a substrate, the method comprising: a) providing the substrate; b) contacting the substrate with a binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, one or more hydroxydisulfide compounds, and a thiol-terminated aliphatic compound selected from one or more of the group consisting of thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts of the thiol-terminated compounds; and c) applying an electric current to the binary silver-bismuth alloy electroplating composition and a substrate to electroplate a binary silver-bismuth deposit on the substrate.

6. The method according to claim 5, wherein: The binary silver-bismuth electroplating composition further comprises one or more acids and salts thereof.

7. The method according to claim 5, wherein: The binary silver-bismuth alloy electroplating composition further comprises one or more pH adjusters.

8. The method of claim 5, wherein: The binary silver-bismuth alloy electroplating composition has a pH of 0 to 6.

9. An article comprising a binary silver-bismuth alloy layer formed from the binary silver-bismuth alloy electroplating composition of claim 1 and adjacent to a surface of a substrate, wherein the binary silver-bismuth alloy layer comprises 90% to 99.8% silver and 0.2% to 10% bismuth and has a coefficient of friction of 1 or less.

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