Acidic aqueous binary silver-bismuth alloy electroplating composition and method

Abstract

Aqueous acidic binary silver-bismuth alloy electroplating compositions and methods enabling electroplating of silver-rich binary silver-bismuth deposits are disclosed. The aqueous acidic binary silver-bismuth alloy electroplating compositions include a 5-membered heterocyclic nitrogen compound having mercapto functionality, which enables deposition of a silver-rich binary silver-bismuth alloy. The silver-rich silver-bismuth deposit is matte to semi-bright, uniform, and has a low coefficient of friction.

Description

Technical Field

[0001] The present invention relates to an acidic aqueous binary silver-bismuth alloy electroplating composition and method. More specifically, the present invention relates to an acidic aqueous binary silver-bismuth alloy electroplating composition and method, wherein the acidic aqueous binary silver-bismuth alloy electroplating composition comprises a 5-membered aromatic heterocyclic nitrogen compound having mercapto functionality, which enables the electrodeposition of a silver-rich binary silver-bismuth alloy having low electrical contact resistance and good wear resistance. 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 its excellent electrical properties, particularly low electrical contact resistance, substantially pure silver is used as a contact finish. However, its use as a contact finish, such as an electrical connector, is limited (due to its poor resistance to mechanical wear and the high silver-silver friction coefficient). The poor resistance to mechanical wear causes the connector to be physically damaged after a relatively low number of insertion-deinsertion cycles of the connector. The high friction coefficient contributes to this wear problem. When a connector has a high friction coefficient, the force required to plug and unplug the connector is very high and this may 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 exposure to high temperatures for extended periods of time.

[0003] Because many silver salts are substantially water-insoluble and water-soluble silver salts often form insoluble salts with various compounds commonly present in plating baths, 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 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 rising treatment costs. In addition, because these baths can only be used in the alkaline range, the types of alloy metals are limited because most metals are insoluble in alkaline environments. In addition, exposing the substrate to an alkaline bath can cause passivation of the substrate surface, making the surface difficult to plate.

[0004] Therefore, there is a need for a silver alloy plating bath that is stable, acidic, and produces a deposit having low electrical contact resistance and a low coefficient of friction to provide improved wear resistance. Summary of the invention

[0005] The present invention relates to a binary silver-bismuth alloy electroplating composition, which comprises a silver ion source, a bismuth ion source, and a 5-membered aromatic heterocyclic nitrogen compound having the following general formula:

[0006]

[0007] wherein Q1-Q4 can be substituted or unsubstituted nitrogen or substituted or unsubstituted carbon, with the proviso that at least two of Q1-Q4 are nitrogen, wherein substituents include, but are not limited to, hydrogen, (C1-C4)alkyl, amino, aminoalkyl, carboxyl, carboxyl(C1-C4)alkyl or alkyl sulfonate, and the pH is less than 7.

[0008] The present invention also relates to a method for electroplating a binary silver-bismuth alloy on a substrate, the method comprising:

[0009] a) providing the substrate;

[0010] b) contacting the substrate with a binary silver-bismuth alloy electroplating composition, the binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, and a 5-membered aromatic heterocyclic nitrogen compound having the following general formula:

[0011]

[0012] wherein Q1-Q4 may be substituted or unsubstituted nitrogen or substituted or unsubstituted carbon, with the proviso that at least two of Q1-Q4 are nitrogen, wherein substituents include, but are not limited to, hydrogen, (C1-C4)alkyl, amino, aminoalkyl, carboxyl, carboxyl(C1-C4)alkyl, or alkyl sulfonate, and the pH is less than 7; and

[0013] 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.

[0014] 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.

[0015] The inclusion of a 5-membered aromatic heterocyclic nitrogen compound having the above formula (I) in an aqueous binary silver-bismuth alloy electroplating composition in an acidic environment enables the deposition of a silver-rich binary silver-bismuth alloy on a substrate, so that the silver-rich binary silver-bismuth alloy substantially has the good electrical properties of a silver deposit, particularly low electrical contact resistance. In addition, the silver-rich binary silver-bismuth alloy deposit has good mechanical wear resistance. The silver-rich binary silver-bismuth deposit is uniform in appearance and matte to semi-bright. The binary silver-bismuth alloy electroplating composition of the present invention is stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1is a scanning electron microscope image of a binary silver-bismuth alloy at 10,000X showing discrete sectors of bismuth in the silver matrix.

[0017] Figure 2 is a 2D profilometry map of the surface of a silver metal deposit, where the x-axis and y-axis are calibrated in micrometers (μm).

[0018] Figure 3 is a 3D profilometry map of the surface of a silver metal deposit, where the x-axis, y-axis, and z-axis are calibrated in micrometers (μm).

[0019] Figure 4 is a 2D profilometry plot of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 95% silver and 5% bismuth, and the x-, y-, and z-axes are calibrated in micrometers (μm).

[0020] Figure 5 is a 3D profilometry map of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 95% silver and 5% bismuth, and the x-axis, y-axis, and z-axis are calibrated in micrometers (μm).

[0021] Figure 6 is a 2D profilometry plot of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 97% silver and 3% bismuth, and the x-, y-, and z-axes are calibrated in micrometers (μm).

[0022] Figure 7 is a 3D profilometry plot of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 97% silver and 3% bismuth, with the x-axis, y-axis, and z-axis calibrated in micrometers (μm).

[0023] Figure 8 is a 2D profilometry plot of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 97% silver and 3% bismuth, and the x-, y-, and z-axes are calibrated in micrometers (μm).

[0024] Fig. 9 is a 3D profilometry plot of the surface of a silver-bismuth alloy deposit of the present invention, wherein the alloy is composed of 97% silver and 3% bismuth, with the x-axis, y-axis, and z-axis calibrated in micrometers (μm). DETAILED DESCRIPTION

[0025] As used throughout the specification, the abbreviations have the following meanings, unless the context clearly indicates otherwise: °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 speed; DC = direct current; v = volt, which is the SI unit of electromotive force; mΩ = milliohm = contact resistance; cN = hundredth of a newton = unit of force; N = Newton; COF = coefficient of friction; rpm = revolutions per minute; s = seconds; 2D = two-dimensional; 3D = three-dimensional; Ag = silver; Bi = bismuth; Au = gold; Cu = copper; Ag + Molecular weight = 107.868 g / mole; and 3,6-dithia-1,8-octanediol molecular weight = 182.3 g / mole.

[0026] The term "alkyl" refers to a group having the general formula C n H 2n+1 An organic functional group in which C is carbon, H is hydrogen and n is an integer. The term "aliphatic" means relating to or representing an organic compound in which the carbon atoms form open chains (as in alkanes) rather than aromatic rings. The term "binary" when referring to an alloy means a metallic solid consisting of a homogeneous mixture of two metals. The "----" dashed line in a chemical structure means an optional covalent bond. The term "adjacent" means direct contact so that the two metal layers have a common interface. The term "contact resistance" means resistance as a function of applied force. The term "Newton" is the SI unit of force, and it is equal to the force that gives a mass of 1 kilogram an acceleration of 1 meter / second / second, and is equal to 100,000 dynes. The term "coefficient of friction" 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 It is shown that F f is the friction force, μ is the friction coefficient, and F nIt is a normal force. The term "normal force" means a force applied perpendicular to the contact plane between two objects. The term "discrete" means individually separated and different. The term "segment" means an area or part that is different from other areas. The term "matrix" means the surrounding metal or most of the metal, i.e., more than 50% of the metal alloy. The term "prerequisite" means a condition or restriction. The term "tribology" means the science and engineering of surfaces that interact in relative motion, and includes the study and application of lubrication, friction and wear principles. The term "cold welding" means a solid-state welding process, in which a connection occurs at the interface of the two parts to be welded without melting or heating, and there is no molten liquid or molten phase at the joint. The term "aqueous" means water or water-based. Throughout the specification, the terms "composition" and "bath" are used interchangeably. Throughout the specification, the terms "deposit" and "layer" are used interchangeably. Throughout the specification, the terms "electroplating", "plating" and "deposition" are used interchangeably. The term "matte" means dull or without luster. Throughout the specification, the term "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 that such numerical ranges be limited to add up to 100%.

[0027] The present invention relates to an aqueous acidic binary silver-bismuth alloy electroplating composition, wherein the aqueous acidic binary silver-bismuth alloy electroplating composition comprises a silver ion source, a bismuth ion source, and a 5-membered aromatic heterocyclic nitrogen compound having the following general formula:

[0028]

[0029] Wherein Q1-Q4 can be substituted or unsubstituted nitrogen or substituted or unsubstituted carbon, provided that at least three of Q1-Q4 are nitrogen, wherein the substituents include, but are not limited to, hydrogen, (C1-C4) alkyl, amino, aminoalkyl, carboxyl, carboxyl (C1-C4) alkyl or alkyl sulfonate, and the pH is less than 7. Preferably, the substituent is hydrogen, (C1-C2) alkyl, primary amino such as NH2, secondary amino such as NH-NH2, or amino (C1-C4) alkyl, further preferably, the substituent is hydrogen, methyl or NH2, more preferably, the ring nitrogen is substituted by hydrogen or methyl or the ring nitrogen is substituted by NH2, or the ring carbon is substituted by NH2, and most preferably, the ring nitrogen is substituted by hydrogen or methyl.

[0030] Matt to semi-bright and uniform silver-rich binary silver-bismuth alloy deposits have substantially good electrical properties, such as low electrical contact resistance. The silver-rich binary silver-bismuth alloy deposit has a low friction coefficient so that the silver-rich binary silver-bismuth alloy layer has 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 silver-bismuth alloy electroplating composition does not contain cyanide.

[0031] Preferred examples of the 5-membered aromatic heterocyclic nitrogen compounds of the present invention are the following:

[0032]

[0033] 3-Mercapto-1,2,4-triazole;

[0034]

[0035] 3-amino-1,2,4-triazole-5-thiol;

[0036]

[0037] 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole;

[0038]

[0039] 3-Mercapto-4-methyl-4H-1,2,4-triazole;

[0040]

[0041] 1H-imidazole-2-thiol; and

[0042] A salt of a 5-membered aromatic heterocyclic nitrogen compound having thiol functionality. More preferably, the 5-membered aromatic heterocyclic nitrogen compound having thiol functionality of the present invention is selected from one or more of the following: 3-mercapto-1,2,4-triazole, 3-amino-1,2,4-triazole-5-thiol, 3-amino-5-mercapto-1,2,4-triazole, 3-mercapto-4-methyl-4H-1,2,4-triazole, and a salt of a 5-membered aromatic heterocyclic nitrogen compound having thiol functionality; even more preferably, the 5-membered aromatic heterocyclic nitrogen compound having thiol functionality of the present invention is selected from one or more of the following: 3-mercapto-1,2,4-triazole, 3-amino-1,2,4-triazole-5-thiol, Thiol, 3-amino-5-mercapto-1,2,4-triazole, 3-mercapto-4-methyl-4H-1,2,4-triazole, and salts of 5-membered aromatic heterocyclic nitrogen compounds with thiol functionality of the present invention; further preferably, the 5-membered aromatic heterocyclic nitrogen compound with thiol functionality of the present invention is selected from one or more of the following: 3-mercapto-1,2,4-triazole, 3-mercapto-4-methyl-4H-1,2,4-triazole, and salts of 5-membered aromatic heterocyclic nitrogen compounds with thiol functionality of the present invention. Salts of 5-membered aromatic heterocyclic nitrogen compounds with thiol functionality of the present invention include, but are not limited to, alkali metal salts, such as sodium salts, potassium salts and lithium salts, ammonium salts, tetraalkylammonium salts, magnesium salts, silver salts and bismuth salts.

[0043] The 5-membered aromatic heterocyclic nitrogen compound with thiol functionality of the present invention and its salt are contained in an amount sufficient to enable electroplating of a silver-rich binary silver-bismuth alloy in an aqueous acidic environment. Preferably, the 5-membered aromatic heterocyclic nitrogen compound with thiol functionality of the present invention is contained in an amount of 50 ppm or more, more preferably in an amount of 100 ppm to 100 g / L, further preferably in an amount of 100 ppm to 20 g / L, even more preferably in an amount of 100 ppm to 10 g / L, and most preferably in an amount of 100 ppm to 5 g / L.

[0044] 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 alkane sulfonate, silver alkane sulfonate or a mixture thereof. When a silver halide is used, preferably the halide is a chloride. Preferably, the silver salt is silver sulfate, silver alkane sulfonate, silver nitrate or a mixture thereof, more preferably, the silver salt is silver sulfate, silver methane sulfonate or a mixture thereof. A mixture of silver salts can 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 soluble in water.

[0045] The amount of silver salt included in the aqueous acidic binary silver-bismuth alloy electroplating composition is an amount sufficient to provide a 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, further preferably 90% to 99.7% silver, most preferably 95% to 99.7% silver, the remainder of the alloy metal being bismuth, excluding unavoidable impurities. 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 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 of 20 g / L to 80 g / L, and most preferably, the silver salt is included in the composition in an amount to provide a silver ion concentration of 20 g / L to 60 g / L.

[0046] The aqueous acidic silver-bismuth alloy electroplating composition includes a bismuth ion source providing a Bi 3+ The composition may also contain a mixture of bismuth salts. The bismuth ion source includes, but is 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 and bismuth chloride. The composition may also contain a mixture of bismuth salts. Preferably, the bismuth salt is water-soluble.

[0047] The amount of bismuth salt included in the aqueous acidic binary silver-bismuth alloy 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, further preferably 0.3% to 10% bismuth, and most preferably 0.3% 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, further preferably 100 ppm to 5 g / L, and most preferably 200 ppm to 5 g / L. 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.

[0048] 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.

[0049] 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 or propanesulfonic acid), arylsulfonic acid (such as benzenesulfonic acid or toluenesulfonic acid); and inorganic acids such as sulfuric acid, aminosulfonic acid, hydrochloric acid, hydrobromic acid or 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, and more preferably, the acid is acetic acid, citric acid, methanesulfonic acid, or a salt thereof. Such salts include, but are not limited to, sodium salts, potassium salts, silver salts, bismuth salts, magnesium salts, tetraalkylammonium salts, and ammonium salts. Although a mixture of acids may be used, it is preferred to use a single acid when used. Acids are generally commercially available or may be prepared by methods known in the literature. Such acids may be included in an amount that provides the desired conductivity. Preferably, the acid or salt thereof is included in an amount of at least 5 g / L, more preferably from 10 g / L to 250 g / L, even more preferably from 30 g / L to 150 g / L, most preferably from 30 g / l to 125 g / L.

[0050] 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.

[0051] Optionally, a pH regulator may be included in the aqueous acidic binary silver-bismuth alloy electroplating composition of the present invention. Such pH regulators 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, aminosulfonic acid, boric acid, phosphoric acid or their salts. Organic acids include, but are not limited to, acetic acid, citric acid, aminoacetic acid and ascorbic acid or their salts. Such salts include, but are not limited to, trisodium citrate. Inorganic bases such as sodium hydroxide and potassium hydroxide can be used, as well as organic bases such as various types of amines. Preferably, the pH regulator is selected from acetic acid, citric acid, aminoacetic acid or its salt, most preferably, acetic acid, citric acid or its salt. The pH regulator can be added in the amount required to maintain the desired pH range.

[0052] Optionally, but preferably, a thioether is included in the aqueous acidic binary silver-bismuth alloy electroplating composition of the present invention. Such thioethers include, but are not limited to, 2,2'-thiodiethanol, 4,4'-thiobiphenol and 4,4'-thiobis(2-methyl-6-tert-butylphenol), dihydroxybissulfide compounds or mixtures thereof. Such dihydroxy disulfide 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-docosahexanediol, 3,5-dithia-1,7-heptanediol, 3,6-dithia-1,8-octanediol, 3,8-dithia-1,10-decanediol, 3,10-dithia-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-eicosanediol, 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 or 2,7-dithia-1,8-octanediol, even more preferably 3,6-dithia-1,8-octanediol, 2,6-dithia-1,7-heptanediol or 2,7-dithia-1,8-octanediol. Dithia-1,8-octanediol, most preferably 3,6-dithia-1,8-octanediol.

[0053] Preferably, the sulfide compound may be included in the aqueous acidic binary silver-bismuth alloy electroplating composition in a molar ratio of at least 1 mole of sulfide to 1 mole of silver, more preferably, 1 to 5 moles of sulfide to 1 mole of silver, even more preferably, 2 to 4 moles of sulfide to 1 mole of silver, and most preferably, 2.5 to 4 moles of sulfide to 1 mole of silver.

[0054] Optionally, but preferably, the aqueous acidic binary silver-bismuth alloy electroplating composition of the present invention may contain an aliphatic thiol-terminated compound. Such aliphatic thiol-terminated compounds include, but are not limited to, mercaptoacetic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, and salts thereof. Salts of the aliphatic thiol-terminated compounds of the present invention include, but are not limited to, alkali metal salts such as sodium and potassium salts, or ammonium salts.

[0055] Preferably, the aliphatic thiol-terminated aliphatic compound of the present invention is contained in an amount of 1 g / L or more, more preferably, the aliphatic thiol-terminated compound is contained in an amount of 1 g / L to 100 g / L, most preferably 1 g / L to 60 g / L.

[0056] Optionally, one or more surfactants may be included in the aqueous acidic silver-bismuth 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] Additional optional additives may be included in the silver-bismuth alloy electroplating composition of the present invention, such as biocides. Conventional biocides well known in the art may be included in the silver-bismuth alloy electroplating composition of the present invention. The biocides are included in conventional amounts well known to those of ordinary skill in the art. Preferably, compounds that act as levelers are not included in the silver-bismuth alloy electroplating composition of the present invention.

[0059] 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 5-membered aromatic heterocyclic nitrogen compound having the following general formula, optionally a thioether, optionally an aliphatic thiol-terminated compound or a salt thereof, optionally an acid or a salt thereof, optionally a pH adjuster, optionally a biocide, and optionally a surfactant:

[0060]

[0061] wherein Q1-Q4 can be substituted or unsubstituted nitrogen or substituted or unsubstituted carbon, with the proviso that at least two of Q1-Q4 are nitrogen, wherein substituents include, but are not limited to, hydrogen, (C1-C4)alkyl, amino, aminoalkyl, carboxyl, carboxyl(C1-C4)alkyl or alkyl sulfonate, 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 5-membered aromatic heterocyclic nitrogen compound having the following general formula, a thioether, optionally an aliphatic thiol-terminated compound or a salt thereof, optionally an acid or a salt thereof, optionally a pH adjuster, optionally a biocide, and optionally a surfactant:

[0063]

[0064] wherein Q1-Q4 can be substituted or unsubstituted nitrogen or substituted or unsubstituted carbon, with the proviso that at least two of Q1-Q4 are nitrogen, wherein substituents include, but are not limited to, hydrogen, (C1-C4)alkyl, amino, aminoalkyl, carboxyl, carboxyl(C1-C4)alkyl or alkyl sulfonate, wherein the pH is 0-6.

[0065] More 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 5-membered aromatic heterocyclic nitrogen compound having the following general formula, a dihydroxydisulfide compound, an aliphatic thiol-terminated compound or a salt thereof, an acid or a salt thereof, optionally a pH adjuster, optionally a biocide, and optionally a surfactant:

[0066]

[0067] wherein Q1-Q4 can be substituted or unsubstituted nitrogen or substituted or unsubstituted carbon, with the proviso that at least two of Q1-Q4 are nitrogen, wherein substituents include, but are not limited to, hydrogen, (C1-C4)alkyl, amino, aminoalkyl, carboxyl, carboxyl(C1-C4)alkyl or alkyl sulfonate, wherein the pH is 0-6.

[0068] Even more 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 compound selected from the group consisting of: 3-mercapto-1,2,4-triazole, 3-amino-1,2,4-triazole-5-thiol, 3-amino-5-mercapto-1,2,4-triazole, triazole, 3-mercapto-4-methyl-4H-1,2,4-triazole, 1H-imidazole-2-thiol, salts of the 5-membered aromatic heterocyclic nitrogen compounds having thiol functionality, and mixtures thereof, dihydroxydisulfide compounds, aliphatic thiol-terminated compounds 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, and salts thereof, acids or salts thereof, optionally pH adjusters, optionally biocides, and optionally surfactants, wherein the pH is 0-3.

[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 various substrates (both conductive and semiconducting substrates). Preferably, the substrate on which the silver-bismuth alloy layer is deposited is a nickel, nickel alloy, copper and copper alloy substrate. Such nickel alloy substrates include, but are not limited to, nickel phosphorus and nickel boron. Such copper alloy substrates include, but are not limited to, brass and bronze.

[0070] During plating, the electroplating composition temperature may be from room temperature to 70° C., preferably from 30° C. to 60° C., more preferably from 40° C. to 60° C. During electroplating, the binary silver-bismuth alloy electroplating composition is preferably under continuous stirring.

[0071] The acidic aqueous binary silver-bismuth alloy electroplating method of the present invention comprises providing a substrate, providing an acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention, and contacting the substrate with the acidic aqueous binary silver-bismuth alloy electroplating composition, such as by immersing the substrate in the composition or spraying the substrate with the composition, and applying an electric current with a conventional rectifier, wherein the substrate acts as a cathode and there is a counter electrode or anode. The anode can be any conventional soluble or insoluble anode used for electroplating binary silver-bismuth alloys. The binary silver-bismuth alloy is deposited adjacent to the surface of the substrate, wherein the surface of the substrate is nickel, a nickel alloy, copper or a copper alloy.

[0072] The acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention enables the deposition of a matte to semi-bright and uniform silver-rich silver-bismuth alloy layer over a wide current density range. The silver-rich binary 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, and most preferably 95% to 99.7% silver and 0.3% to 5% bismuth, excluding unavoidable impurities in the alloy.

[0073] The current density for electroplating the matte to semi-bright and uniform silver-rich silver-bismuth alloy of the present invention can be 0.1ASD or higher. Preferably, the current density is 0.5ASD to 70ASD, further preferably 1ASD to 40ASD, more preferably 1ASD to 30ASD, even more preferably 1ASD to 15ASD.

[0074] 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 binary silver-bismuth alloy layer is 1 μm or greater. Further preferably, the binary silver-bismuth alloy layer has a thickness range of 1 μm to 100 μm, more preferably 1 μm to 50 μm, even more preferably 1 μm to 10 μm, and most preferably 1 μm to 5 μm.

[0075] Although it is contemplated that the acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention can be used to plate a variety of substrates that may include a silver-bismuth alloy layer, preferably, the acidic aqueous binary silver-bismuth alloy electroplating composition of the present invention is used to electroplate a top layer or coating on an electrical connector where substantial contact forces and wear are expected to be prevalent. Silver-rich binary 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. In addition, the silver-bismuth alloy deposits of the present invention have a low COF, preferably 1 or less, more preferably a COF of 0.6 or less. The COF of the silver-bismuth alloy deposits of the present invention has a COF of 40% or less of the COF of a substantially pure silver deposit, and therefore, the binary silver-bismuth alloy of the present invention has a significant improvement in wear resistance relative to substantially pure silver. The surface wear of the metal deposit can be determined according to conventional tribology and profilometry measurements well known in the art.

[0076] The following examples are included to further illustrate the invention but are not intended to limit its scope.

[0077] Binary silver-bismuth alloy electroplating examples 1-22:

[0078] Electroplating process

[0079] Unless otherwise specified, in all cases, the electroplating substrate is a 5cm x 5cm brass (70% copper, 30% zinc) sample. Before electroplating, the sample was electro-cleaned at 80°C for 30 seconds with DC at 5 ASD current density in RONACLEANT MGP-300LF electrolytic cleaner (alkaline degreaser) (available from DuPont). After electro-cleaning, the sample was rinsed with DI water, activated in 10% sulfuric acid for 30 seconds, rinsed with DI water again, and then placed in an electroplating bath. Electroplating was carried out for 6 minutes with DC at a current density of 1ASD (the actual current applied was 0.56A). Electroplating was carried out in a square glass beaker using a platinum-plated titanium anode. Stirring was provided at a speed of 400rpm by a 5cm long TEFLONTM coated stirring rod. Electroplating was carried out at a temperature of 55°C. The binary silver-bismuth alloy deposit was about 6 μm thick. All binary silver-bismuth alloy electroplating baths were water-based. Water was added to each bath to bring it to the desired volume. The pH of the plating baths was adjusted with potassium hydroxide or methanesulfonic acid.

[0080] The thickness and elemental composition of the electroplated binary silver-bismuth alloys 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 (available from Hitachi (Chiyoda, Tokyo, Japan), Fischer GmbH (Sindelfingen, Germany), and Veeco (Plainview, New York, USA), and the software package included with the Bowman Series P X-ray fluorimeter was used to calculate the alloy composition and thickness by combining the pure element standards with fundamental parameter (FP) calculations.

[0081] Contact resistance measurement

[0082] Contact resistance was evaluated using a custom designed apparatus comprising a Starrett MTH-550 manual dynamometer stand (Athol, MA, USA) equipped with a Starrett DFC-20 digital dynamometer. The digital dynamometer was equipped with a gold-plated copper probe with a hemispherical tip of 2.5 mm in diameter. 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 (running at 10 mA), and the voltmeter was a Keithley 2182A nanovoltmeter (Cleveland, Ohio, USA). These instruments were run in thermoelectric compensation mode to achieve maximum accuracy.

[0083] Wear resistance measurement

[0084] Tribological measurements were performed using an Anton Paar TRB3 pin-on-disk tribometer (available from Anton Paar GmbH, Graz, Austria) equipped with a linear reciprocating stage. All tests were performed using a 1 N load, a stroke length of 10 mm, and a sliding speed of 5 mm / s. All tests were performed "like-on-like", meaning that the flat specimens and the spherical ball were each coated with the same silver (SILVER GLOSS®). TM 3K bright silver plating bath, a cyanide-based product available from DuPont) or silver-bismuth alloy metal deposits. The balls used were made of C260 brass (70% copper, 30% zinc) and had a diameter of 5.55 mm and were electroplated with about 5 μm of silver. Flat specimens were also made of C260 brass and electroplated with about 2 μm of silver or silver-bismuth alloy. During the test, the friction coefficient was monitored using a tribometer. The wear scar depth was measured using laser profilometry. Measurements were made for 100 or 500 cycles as specified in each case.

[0085] One cycle is one round trip of the tribometer. The sample is moved in one direction, then the direction is reversed and the tribometer is moved back to its original position, always under a load of 1N and in contact with the spherical ball. This is one cycle.

[0086] 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 plots were created from these measurements using VK-X MultiAnalyzer software from Keyence.

[0087] Example 1 (the present invention)

[0088] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0089] Silver methanesulfonate that supplies 20g / L of silver ions

[0090] 3,6-dithia-1,8-octanediol: 102g / L

[0091] Bismuth methanesulfonate supplying 0.8g / L of bismuth ions

[0092] Cysteine: 1.4 g / L

[0093] 3-Mercapto-1,2,4-triazole: 1.5 g / L

[0094] Adjust pH to 1.6

[0095] After plating, the electrodeposited coating appeared semi-bright, with a composition of 98.9% silver and 1.1% bismuth.

[0096] Example 2 (the present invention)

[0097] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0098] Silver methanesulfonate that supplies 20g / L of silver ions

[0099] 3,6-dithia-1,8-octanediol: 102g / L

[0100] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0101] Cysteine: 9g / L

[0102] 2-Mercapto-1,2,4-triazole: 400ppm

[0103] Adjust pH to 2

[0104] After plating, the electrodeposited coating appeared metallic and semi-bright, with a composition of 95% silver and 5% bismuth. Figure 1is a scanning electron microscope image of a binary silver-bismuth alloy at 10,000X magnification showing discrete areas of bismuth (indicated by bright spots) in the silver matrix.

[0105] Example 3 (the present invention)

[0106] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0107] Silver methanesulfonate that supplies 20g / L of silver ions

[0108] 3,6-dithia-1,8-octanediol: 102g / L

[0109] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0110] 3-Mercapto-1-propanesulfonic acid sodium salt: 13.2g / L

[0111] 3-Mercapto-1,2,4-triazole: 400ppm

[0112] Adjust pH to 2

[0113] After plating, the electrodeposited coating appeared metallic and semi-bright, with a composition of 96% silver and 4% bismuth.

[0114] Example 4 (the present invention)

[0115] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0116] Silver methanesulfonate that supplies 20g / L of silver ions

[0117] 3,6-dithia-1,8-octanediol: 102g / L

[0118] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0119] 3-Mercapto-1-ethanesulfonic acid sodium salt: 12.2g / L

[0120] 3-Mercapto-1,2,4-triazole: 400ppm

[0121] Adjust pH to 2

[0122] After the plating procedure, the electrodeposited coating appeared metallic and semi-bright, with a composition of 96% silver and 4% bismuth.

[0123] Example 5 (the present invention)

[0124] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0125] Silver methanesulfonate that supplies 20g / L of silver ions

[0126] 3,6-dithia-1,8-octanediol: 102g / L

[0127] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0128] Mercaptosuccinic acid: 11.1 g / L

[0129] 3-Mercapto-1,2,4-triazole: 400ppm

[0130] Adjust pH to 2

[0131] After plating, the electrodeposited coating appeared metallic and matte, with a composition of 98% silver and 2% bismuth.

[0132] Example 6 (present invention)

[0133] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0134] Silver methanesulfonate that supplies 20g / L of silver ions

[0135] 3,6-dithia-1,8-octanediol: 102g / L

[0136] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0137] Cysteine: 9g / L

[0138] 3-Mercapto-4-methyl-4H-1,2,4-triazole: 400ppm

[0139] Adjust pH to 2

[0140] After plating, the electrodeposited coating appeared metallic and semi-bright, with a composition of 95% silver and 5% bismuth.

[0141] Example 7 (present invention)

[0142] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0143] Silver methanesulfonate that supplies 20g / L of silver ions

[0144] 3,6-dithia-1,8-octanediol: 34g / L

[0145] Mercaptosuccinic acid: 35g / L

[0146] Bismuth methanesulfonate supplying 2g / L of bismuth ions

[0147] 3-Mercapto-1,2,4-triazole: 400ppm

[0148] Adjust pH to 2

[0149] After plating, the electrodeposited coating appeared metallic and semi-bright, with a composition of 97% silver and 3% bismuth.

[0150] Example 8 (present invention)

[0151] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0152] Silver methanesulfonate that supplies 20g / L of silver ions

[0153] 3,6-dithia-1,8-octanediol: 34g / L

[0154] Mercaptosuccinic acid: 35g / L

[0155] Bismuth methanesulfonate supplying 2g / L of bismuth ions

[0156] 3-Mercapto-4-methyl-4H-1,2,4-triazole: 400ppm

[0157] Adjust pH to 2

[0158] After plating, the electrodeposited coating appeared metallic and semi-bright, with a composition of 97% silver and 3% bismuth.

[0159] Example 9 (present invention)

[0160] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0161] Silver methanesulfonate that supplies 20g / L of silver ions

[0162] 3,6-dithia-1,8-octanediol: 34g / L

[0163] Mercaptosuccinic acid: 35g / L

[0164] Bismuth methanesulfonate supplying 2g / L of bismuth ions

[0165] 1H-imidazole-2-thiol: 400ppm

[0166] Adjust pH to 2

[0167] After plating, the electrodeposited coating appeared metallic and semi-bright, with a composition of 97% silver and 3% bismuth.

[0168] Example 10 (Comparison)

[0169] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0170] Silver methanesulfonate that supplies 20g / L of silver ions

[0171] Bismuth methanesulfonate supplying 10g / L of bismuth ion

[0172] Methanesulfonic acid: 150g / L

[0173] PLURONIC TM L-44 surfactant (purchased from BASF): 10 g / L

[0174] o-Chlorobenzaldehyde: 100ppm

[0175] 3,6-dithia-1,8-octanediol: 80g / L

[0176] pH < 1

[0177] After the plating procedure, the electrodeposited coating was metallic and semi-bright, with a composition of 46% silver and 54% bismuth.

[0178] Example 11 (Comparison)

[0179] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0180] Silver methanesulfonate that supplies 20g / L of silver ions

[0181] 3,6-dithia-1,8-octanediol: 102g / L

[0182] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0183] Cysteine: 9g / L

[0184] 4-Amino-1,2,4-triazole: 400ppm

[0185] Adjust pH to 2

[0186] After plating, the electrodeposited coating appeared metallic and matte, with a composition of 97% silver and 3% bismuth.

[0187] Example 12 (Comparison)

[0188] An aqueous acidic binary silver-bismuth alloy electroplating bath was prepared having the following composition:

[0189] Silver methanesulfonate that supplies 20g / L of silver ions

[0190] 3,6-dithia-1,8-octanediol: 102g / L

[0191] Bismuth methanesulfonate supplying 5g / L of bismuth ion

[0192] Cysteine: 9g / L

[0193] 1,2,4-Triazole: 400ppm

[0194] Adjust pH to 2

[0195] After plating, the electrodeposited coating appears metallic and matte, with a composition of 97% silver and 3% bismuth.

[0196] Example 13 (Comparison)

[0197] An aqueous alkaline cyanide-based silver electroplating bath was prepared having the following composition:

[0198] Potassium silver cyanide that supplies 40g / L of silver ions

[0199] Potassium cyanide: 120g / L

[0200] Potassium carbonate: 15g / L

[0201] Potassium selenocyanate: 7mg / L

[0202] After plating, the electrodeposited coating appears metallic and shiny, with a composition of 100% silver.

[0203] Example 14 (Comparison)

[0204] An aqueous alkaline cyanide based binary silver-antimony electroplating bath was prepared having the following composition:

[0205] Potassium silver cyanide that supplies 40g / L of silver ions

[0206] Potassium cyanide: 120g / L

[0207] Potassium carbonate: 15g / L

[0208] Potassium selenocyanate: 6mg / L

[0209] Potassium sodium tartrate: 10g / L

[0210] Tripotassium citrate: 19g / L

[0211] Potassium antimony tartrate: 7.8g / L

[0212] After plating, the electrodeposited coating appeared metallic and bright, with a composition of 98% silver and 2% antimony.

[0213] Example 15

[0214] Contact resistance measurement

[0215] Flat nickel coupons plated with about 3 μm of silver from Comparative Example 13, silver-antimony alloys (98% silver, 2% antimony) from Comparative Example 14, and binary silver-bismuth alloys from aqueous acidic binary silver-bismuth alloy electroplating baths disclosed in Examples 2, 8, and 9 (invention) above were tested. The coupons were heated in an oven at 150° C. for 100 hours prior to measurement. The contact resistances of the coupons are in Table 1 below.

[0216] Table 1

[0217]

[0218] Example 16 (Comparison)

[0219] Wear resistance measurements were made on silver metal deposits electroplated using the bath described in Example 13 (Comparative). The test was conducted for 100 cycles. 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. Figure 3 is a 3D profilometry plot of the silver deposit, which further illustrates the significant surface wear of the silver deposit after 100 cycles. Inspection of the ball indicated material accumulation, and significant prow formation was observed at the end of the wear track, indicating cold welding during the wear process. The coefficient of friction throughout the experiment was determined to be approximately 1.6. The coefficient of friction is measured by a sensor on the tribometer, which measures the friction force, and then divides the friction force by the reported down force to obtain the coefficient of friction.

[0220] Example 17 (Comparison)

[0221] Wear resistance measurements were performed on a silver-bismuth alloy deposit plated using the bath described in Example 10 (Comparative). The test was performed for 500 cycles. The coefficient of friction was determined to be about 0.3 throughout the experiment. However, the wear marks observed were essentially the same as those observed for Example 16 above, showing significant wear.

[0222] Example 18 (Comparison)

[0223] Wear resistance measurements were performed on a silver-bismuth alloy deposit plated using the bath described in Example 11 (Comparative). The test was conducted for 500 cycles. The coefficient of friction was determined to be about 0.7 throughout the experiment. However, the wear marks observed were essentially the same as those observed for silver metal in Example 16 above, showing significant wear.

[0224] Example 19 (Comparison)

[0225] Wear resistance measurements were performed on a silver-bismuth alloy deposit plated using the bath described in Example 12 (Comparative). The test was performed for 500 cycles. The coefficient of friction was determined to be about 0.7 throughout the experiment. However, the wear marks observed were essentially the same as those observed in Example 16 above, indicating severe wear.

[0226] Example 20 (present invention)

[0227] Wear resistance measurements were performed on silver-bismuth alloy deposits electroplated using the bath described in Example 2 (invention). The test was performed for 500 cycles. Figure 4 is a 2D profilometry image of a silver-bismuth alloy deposit that shows no observable wear of the silver-bismuth alloy along the width of the sample. Figure 5 3D profilometry plot of the silver-bismuth alloy deposit, which further shows no observable surface wear of the silver-bismuth alloy deposit after 500 cycles. Inspection of the spherical ball shows no material accumulation, and no observable prow formation at the end of the wear scar, indicating that no cold welding occurred during the wear process. The coefficient of friction throughout the experiment was determined to be about 0.6. These results represent a significant improvement over the comparative example.

[0228] Example 21 (present invention)

[0229] Wear resistance measurements were performed on silver-bismuth alloy deposits electroplated using the bath described in Example 8 (invention). The test was performed for 500 cycles. Figure 6 is a 2D profilometry plot of the silver deposit, which shows less wear of the silver-bismuth alloy along the width of the sample, although significantly less than the comparative example. Figure 7 3D profilometry plot of a silver-bismuth alloy deposit, further showing less wear of the silver-bismuth deposit after 500 cycles. Inspection of the spherical ball showed no material buildup, and no protrusion formation was observed at the end of the wear scar, indicating that no cold welding occurred during the wear process. The coefficient of friction was determined to be about 0.9 throughout the experiment. These results represent a significant improvement over the comparative example.

[0230] Example 22 (present invention)

[0231] Wear resistance measurements were performed on silver-bismuth alloy deposits electroplated using the bath described in Example 10 (invention). The test was performed for 500 cycles. Figure 8 is a 2D profilometry plot of the silver deposit showing minimal wear of the silver-bismuth alloy along the width of the sample. Fig. 93D profilometry plot of a silver-bismuth alloy deposit, further showing minimal surface wear of the silver-bismuth deposit after 500 cycles. Inspection of the spherical ball showed no material accumulation, and no protrusion formation was observed at the end of the wear scar, indicating that no cold welding occurred during the wear process. The coefficient of friction was determined to be about 0.5 throughout the experiment. These results represent a significant improvement over the comparative example.

Claims

1. A binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, and a 5-membered aromatic heterocyclic nitrogen compound selected from the group consisting of: The salt of the 5-membered aromatic heterocyclic nitrogen compound, and a mixture thereof, and a thioether or a mixture thereof, wherein: Thioethers are hydroxy disulfide compounds.

2. The binary silver-bismuth alloy electroplating composition of claim 1, further comprising an aliphatic thiol-terminated compound or a mixture thereof.

3. The binary silver-bismuth alloy electroplating composition of claim 2, wherein: The aliphatic thiol-terminated compound is selected from the group consisting of mercaptoacetic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, cysteine, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptoethanesulfonic acid, salts of the aliphatic thiol-terminated compound, and mixtures thereof.

4. 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, the binary silver-bismuth alloy electroplating composition comprising a silver ion source, a bismuth ion source, and a 5-membered aromatic heterocyclic nitrogen compound selected from the group consisting of: The salt of the 5-membered aromatic heterocyclic nitrogen compound, and a mixture thereof, and a thioether or a mixture thereof, wherein the thioether is a hydroxydisulfide compound; 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.

5. The method of claim 4, wherein: The binary silver-bismuth alloy electroplating composition further comprises an aliphatic thiol-terminated compound or a mixture thereof.

6. An article comprising a binary silver-bismuth alloy layer adjacent to a substrate surface, 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, and wherein, The binary silver-bismuth alloy layer is prepared by the binary silver-bismuth alloy electroplating composition as claimed in claim 1 or by the method for electroplating a binary silver-bismuth alloy on a substrate as claimed in claim 4.

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