Method for preventing discoloration and corrosion

Abstract

To provide a method of preventing discoloration and corrosion.SOLUTION: A method of preventing discoloration of silver or a silver alloy and corrosion of gold or a gold alloy is performed by depositing a thin coating of bismuth on the silver, silver alloy, gold, or gold alloy. The thin bismuth coating does not spoil the electric performance of the silver, silver alloy, gold, or gold alloy even after thermal aging.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for preventing discoloration of silver and corrosion of gold. More specifically, the present invention relates to a method for preventing discoloration of silver and corrosion of gold by depositing a bismuth layer on the surface of silver or gold to a thickness sufficient to prevent discoloration of silver and corrosion of gold and to maintain good electrical performance even after thermal aging.

Background Art

[0002] Silver is used as a metal finish for applications in the electronics industry. Connector and lead frame components may include a silver finish due to its excellent electrical properties. There is also a financial incentive for using silver, as it is much less expensive than gold. A major drawback of silver is its tendency to discolor, resulting in the formation of a layer on the surface that impairs the appearance. This is not visually acceptable and, since it is insulating, the electrical performance of silver when applied as a finish for electrical components is impaired. The main product of silver discoloration is silver sulfide, which is formed via the half-reactions 8Ag + 4HS - ⇔ 4Ag2S + 2H2 + 4e - and O2 + 2H2O + 4e - ⇔ 4OH - in the presence of sulfides such as hydrogen sulfide present in the atmosphere. In dry air, discoloration does not occur. When water is present (relative humidity 5 - 50% or more), oxygen functions as a cathode species and consumes electrons as shown in the equations. As the concentration of silver sulfide increases, discoloration increases. The discoloration rate gradually decreases with an increase in the thickness of the discolored layer, but this reaction proceeds even on a significantly discolored surface. Silver sulfide does not form a protective layer for preventing surface corrosion due to its rough structure.

[0003] Therefore, for silver deposition, it is necessary to use a post-treatment for preventing discoloration of the silver surface. Conventionally, the organic anti-discoloration treatment for silver consisted of fatty acid thiols. These molecules form a dense self-assembled monolayer due to the high enthalpy of the silver-sulfur bond and the van der Waals interaction of the hydrocarbon tails of the long-chain fatty acid thiol molecules. The resulting hydrophobicity of the monolayer prevents the interaction between water and the silver surface, thus preventing the discoloration of silver. However, this method has the disadvantages that it takes a long process time for monolayer formation and that a flammable organic solvent is required as the working solution to dissolve the long-chain thiol. Another major drawback of using organic molecules for the post-treatment against discoloration is their thermal instability. Organic molecules evaporate or decompose when heated to temperatures above 100°C. The aliphatic carbon-hydrogen bonds of the long hydrocarbon chains also oxidize and thus decompose under a high-temperature oxygen-containing atmosphere, rendering them ineffective as a post-treatment for silver.

[0004] As an alternative to the organic post-treatment, metal or inorganic treatments have also been disclosed. Unlike typical organic post-treatments, metal coatings are non-volatile even at high temperatures. Metal oxides of zinc, titanium, or aluminum have been used for preventing discoloration. Chromium(VI) has also been a conventionally used coating component, but has not been widely used due to its toxicity. In addition, noble metals can also protect the silver surface. Such thin coatings are typically electroplated as an inert top coat to prevent silver from interacting with sulfur and moisture, and thus discoloration of silver is not observed, as disclosed in (Patent Document 1), (Patent Document 2), (Patent Document 3), and (Patent Document 4). These thin coatings of these metals can also retain the shiny appearance of silver. The main drawback of these treatments is the cost associated with noble metal coatings. In addition, intermetallic compounds can be formed by heating. In this way, the thermal instability is not related to the evaporation of the post-treatment, but rather to the diffusion into silver and its resulting impairment of electrical performance (i.e., an increase in contact resistance).

[0005] Hard silver or gold alloys of cobalt and nickel are widely used as contact materials for electrical connectors for high-reliability applications. Connectors with a final layer of hard gold are often electroplated onto a nickel substrate, such as nickel plating on copper. Generally, by using selective plating techniques such as spot plating to limit the plating area of gold and other precious metals, such as palladium and palladium-nickel alloys, the material cost of the connector is significantly reduced. Hard gold does not discolor like silver, but hard gold often has a thin porous surface through which the underlying nickel can corrode, potentially degrading the performance of the electrical connector.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] Therefore, there is a need for a method to prevent silver discoloration and pore corrosion of the underlying nickel by a gold or silver alloy topcoat.

Means for Solving the Problems

[0008] In a method of electroplating bismuth, providing a substrate comprising silver, a silver alloy, gold, or a gold alloy, and providing a bismuth electroplating comprising a bismuth ion source, an acid, a salt of the acid, or a combination thereof. Bath Providing a step of electroplating the substrate with bismuth BathA step of contacting; and a bismuth electroplating Bath A step of applying an electric current to a substrate; and a step of electroplating bismuth on silver, a silver alloy, gold, or hard gold of the substrate to a thickness greater than 0 nm and less than or equal to 20 nm. A method including these steps.

[0009] Bismuth electroplating comprising a bismuth ion source, an acid, a salt of the acid, or a combination thereof, water, optionally a surfactant, optionally a brightener, optionally an antibacterial agent, and optionally an antifoaming agent. Bath 。

[0010] An article including a layer of silver, a silver alloy, gold, or a gold alloy and having a bismuth layer with a thickness of 20 nm or less adjacent to a monolayer of silver, a silver alloy, or hard gold.

[0011] The bismuth layer on silver, a silver alloy, gold, or a gold alloy prevents discoloration of silver and corrosion of gold and provides a low contact resistance that can achieve good electrical performance even after heat aging.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0013] The following abbreviations have the following meanings, unless otherwise clearly interpreted in context: °C = degrees Celsius, g = gram, mL = milliliter, L = liter, A = ampere, dm = decimeter, ASD = ampere / dm 2 , mΩ = milliohm, nm = nanometer, μm = micrometer, cm = centimeter, eN = centinewton, sec = second, DI = deionized, DC = direct current, XRF = X-ray fluorescence, bismuth ion = bismuth(III) = Bi 3+ , wt% = weight percent, ASTM = American Society for Testing and Materials, NA = not available or not applicable.

[0014] All percentages and ratios are by weight, unless otherwise specified. All ranges are inclusive and can be combined in any order, except when it is inevitable that the sum of such numerical ranges must be 100%.

[0015] Throughout this specification, the terms "plating" and "electroplating" are used interchangeably. The indefinite articles (a, an) are taken to include both singular and plural. The term "adjacent" means adjacent or joined so as to have a common interface. The term "contact resistance" means the contribution that can be made to the overall resistance of a system by the contact interfaces of electrical leads and connections. The term "additional vertical force" means the force applied to an object by a person or another object, i.e., gravity or weight. The term "centinewton" is a unit of measurement of force. The term "ohm" is the SI coherent unit of electrical resistance. The term "monolayer" means a layer one molecule thick.

[0016] The bismuth electroplating of the present inventionBath comprises water, a bismuth(III) ion source, an acid, optionally a brightener, and optionally a surfactant (preferably consisting of these), and the plating Bath does not contain alloy metals, and therefore, the deposit plated from the plating of the present invention Bath is substantially 100% bismuth.

[0017] The bismuth source provides bismuth(III) (Bi 3+ ) ions and a corresponding neutralizing action. Preferably, the bismuth(II) ion source is water-soluble. Examples of the bismuth(III) ion source include bismuth salts of alkane sulfonic acids such as bismuth methanesulfonate, bismuth ethanesulfonate, bismuth propanesulfonate, 2-bismuth propane sulfonate, and bismuth p-phenol sulfonate; bismuth salts of alkanol sulfonic 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, but are not limited thereto. Mixtures of bismuth(III) ion sources can also be included in the bismuth electroplating of the present invention Bath . More preferably, the bismuth(III) ion source is selected from the group consisting of bismuth methanesulfonate, bismuth ethanesulfonate, bismuth propanesulfonate, and mixtures thereof. Most preferably, the bismuth(III) ion source is bismuth methanesulfonate.

[0018] Preferably, the bismuth salt is for plating BathAmong them, bismuth(III) ions are included in an amount of 1 to 200 g / L, more preferably 1 to 150 g / L, even more preferably 1 to 100 g / L, far more preferably 1 to 50 g / L, still more preferably 1 to 25 g / L, and most preferably 1 to 10 g / L. Such bismuth salts may be commercially available or produced according to the disclosure of chemical literature. These are generally commercially available from various vendors including Aldrich Chemical Company in Milwaukee, Wisconsin.

[0019] Bismuth plating Bath The acids included therein are organic, inorganic, or a mixture thereof. Salts of organic and inorganic acids can also be included in the bismuth electroplating of the present invention Bath can be included. A mixture of acids and salts can also be included in the bismuth electroplating of the present invention Bath can be included. Preferably, the organic acids and their salts are used in the bismuth electroplating of the present invention BathIt is included. Preferably, the organic acid includes, but is not limited to, alkane sulfonic acid, alkanol sulfonic acid, and aromatic sulfonic acid. The alkane sulfonic acid includes, but is not limited to, methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, 1-propanesulfonic acid, 2-propanesulfonic acid, 1-butanestrfonic acid, 2-butanestrfonic acid, pentanesulfonic acid, hexanesulfonic acid, decanesulfonic acid, and dodecanesulfonic acid. The alkanol sulfonic acid includes, but is not limited to, 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybutane-1-sulfonic acid, 2-hydroxyhexane-1-sulfonic acid, 2-hydroxydecane-1-sulfonic acid, 2-hydroxy-dodecane-1-sulfonic acid, 2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid, 2-hydroxybutane-1-sulfonic acid, and 2-hydroxypentane-1-sulfonic acid. The aromatic sulfonic acid includes, but is not limited to, benzenesulfonic acid, alkylbenzenesulfonic acid, phenolsulfonic acid, cresolsulfonic acid, sulfosalicylic acid, nitrobenzenesulfonic acid, sulfobenzoic acid, and diphenylamine-4-sulfonic acid. Preferably, the organic acid is alkane sulfonic acid. More preferably, the alkane sulfonic acid is selected from the group consisting of methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, its salts, and mixtures thereof. Most preferably, the alkane sulfonic acid is methanesulfonic acid or its salt.

[0020] Preferably, the organic acid is water-soluble. Preferably, the organic acid and its salt are in the plating Bath in an amount of 1 to 1000 g / L, more preferably 5 to 500 g / L, even more preferably 10 to 250 g / L, far more preferably 10 to 100 g / L, and most preferably 10 to 60 g / L. The acids as described above may be commercially available or may be prepared according to the disclosure in chemical literature. These are generally commercially available from various suppliers including Aldrich Chemical Company in Milwaukee, Wisconsin.

[0021] Inorganic acids include, but are not limited to, sulfuric acid, nitric acid, hydrochloric acid, sulfamic acid, and their salts. Preferably, the inorganic acid is sulfuric acid and its salts. Preferably, the inorganic acid and its salts can be included in the electroplating Bath in an amount of 10 to 200 g / L, more preferably 20 to 100 g / L, even more preferably 30 to 70 g / L.

[0022] The pH of the bismuth electroplating of the present invention Bath is 7 or less, preferably less than 7, more preferably in the range of 0 to 6, even more preferably 0 to 2, and most preferably less than 0 to 2.

[0023] Optionally, but preferably, the bismuth electroplating of the present invention Bath contains a surfactant. Preferably, the surfactant is polyoxymethylene ArielEthers, such as commercially available products available from ADEKA Corporation, ADEKA (trademark) TOL PC-8, amine oxides, such as commercially available products available from Evonik Operation GmbH, TOMAMINE (trademark) AO-455, branched alcohol alkoxylate nonionic surfactants, such as commercially available products, TERGITOL (trademark) CA, polyether polyols, such as commercially available products, TERGITOL (trademark) L-64, secondary alcohol ethoxylates, such as commercially available products, TERGITOL (trademark) 15-S-7, nonionic low-foaming surfactants, such as poly(oxy-1,2-ethanediyl), alpha-(phenylmethyl)-omega-(1,1,3,3-tetramethylbutyl)phenoxy, polyethylene glycol octylphenyl ether, and decanoic acid, included in a mixture, TRITON (trademark) CF-87 (all available from The Dow Chemical Company, Midland, Michigan), mixtures of organic and inorganic components, such as Wetting Agent W containing sodium dodecylbenzenesulfonate, and Wetting Agent NAW-4 containing 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-2H-isothiazole-3-one in a mixture (also available from DOW Chemical Company). Preferably, the surfactant is a nonionic surfactant.

[0024] The surfactant can be included in conventional amounts in the bismuth electroplating. Bath Preferably, the surfactant is included in an amount of 0.1 to 2 g / L, more preferably 0.5 to 2 g / L, and even more preferably 0.5 to 1 g / L.

[0025] Optionally, an antifoaming agent can be added to the bismuth plating. BathIt can be included. Conventional antifoaming agents can be used and included in conventional amounts. The antifoaming agent is preferably included in an amount of 10 to 100 mg / L. An example of a preferred commercially available antifoaming agent is FOAM BAN (registered trademark) MS-293 antifoaming agent available from Inwoo Corporation of Gobiz Korea, which contains a mixture of 5-decyne 4,7-diol, 2,4,7,9-tetramethyl (less than 2.5 wt%) and ethylene glycol (less than 2.5 wt%).

[0026] Optionally, the bismuth electroplating of the present invention Bath can contain a brightening agent. Conventional brightening agents can be included in the bismuth electroplating Bath The brightening agent is preferably 5-sulfosalicylic acid, cysteine, 1,6-hexanediol, thiodiethanol, 4,5-dihydroxy-1,3-benzenedisulfonic acid, 2,2-bis(hydroxymethyl)propionic acid, Taurine, thiodiglycolic acid, Those salts, and mixtures thereof Mixture selected from the group consisting of. More preferably, the brightening agent is selected from the group consisting of 5-sulfosalicylic acid, 4,5-dihydroxyl-1,3-benzenedisulfonic acid, thiodiglycolic acid, salts thereof, and mixtures thereof.

[0027] The brightening agent can be included in a conventional amount. Preferably, the brightening agent is included in the bismuth electroplating Bath in an amount of 0.5 to 20 molar equivalents of bismuth(III) ions in the bismuth electroplating Bath More preferably, the brightening agent is included in an amount of 0.5 to 15 molar equivalents of bismuth(III) ions in the bismuth electroplating, and even more preferably, the brightening agent is included in an amount of 0.5 to 10 molar equivalents of bismuth(III) ions in the bismuth electroplating. Bath Bath

[0028] Bath Bath Optionally, the bismuth electroplating Bath contains one or more antibacterial agents. Electroplating BathConventional antibacterial agents typically included therein may be used. Such antibacterial agents are well known in the art. They are used in conventional amounts.

[0029] Bismuth can be electroplated onto the surfaces of silver, silver alloys, hard gold, and soft gold from the electroplating bath of the present invention at a current density of 0.1 ASD or more. Preferably, bismuth can be electroplated at a current density of 0.1 to 5 ASD, more preferably 0.1 to 3 ASD, and most preferably 0.1 to 1 ASD. Bath From the electroplating bath of the present invention, bismuth can be electroplated onto the surfaces of silver, silver alloys, hard gold, and soft gold at a current density of 0.1 ASD or more. Preferably, bismuth can be electroplated at a current density of 0.1 to 5 ASD, more preferably 0.1 to 3 ASD, and most preferably 0.1 to 1 ASD.

[0030] Preferably, the bismuth electroplating is carried out at a temperature of room temperature to 60°C, more preferably room temperature to 50°C, even more preferably 30 to 50°C, and most preferably 35 to 45°C. Bath at a temperature of room temperature to 60°C, more preferably room temperature to 50°C, even more preferably 30 to 50°C, and most preferably 35 to 45°C.

[0031] The bismuth layer adjacent to silver, silver alloy, hard gold, and soft gold is in the range of more than 0 nm to 20 nm, or the bismuth is 20 nm, more preferably more than 1 nm to 20 nm, more preferably more than 1 nm to 10 nm, still more preferably more than 1 nm to 7 nm, and most preferably, the thickness of the bismuth layer is 1 to 5 nm. In addition to the bismuth metal deposited adjacent to silver, silver alloy, hard gold, and soft gold, the deposit may contain bismuth(III).

[0032] The bismuth layer adjacent to silver or silver alloy prevents discoloration of the surface of the silver or silver alloy, and the bismuth layer adjacent to hard gold or soft gold prevents corrosion of the gold. As a result, silver, silver alloy, hard gold, and soft gold can maintain a low contact resistance even under an additional vertical force of 100 cN or the like, and good conductivity can be obtained. Furthermore, the bismuth layer of the present invention prevents discoloration of silver and silver alloys, as shown by a conventional accelerated sulfurization test in which the substrate is immersed in an aqueous solution of 2 wt% potassium polysulfide. The bismuth layer prevents discoloration even after heat aging, as shown by a conventional heat aging test. The bismuth layer also prevents corrosion of hard gold even after heat aging, as shown by a conventional nitric acid vapor (NAV) and sulfur dioxide vapor test.

[0033] Preferably, the silver is substantially about 98 to 99.9 wt% silver, and the silver can be deposited on the substrate or the surface of the article by conventional methods known in the art. Preferably, the silver is silver electroplating Bath deposited by electroplating from.

[0034] Silver plating Bath contains silver ions, which can be provided by silver salts, which are silver cyanide, silver oxide, silver hydantoin, silver succinate, silver halide, silver gluconate, silver citrate, silver lactate, silver nitrate, silver sulfate, silver alkanesulfonate, silver alkanol sulfonate, or combinations thereof, but are not limited thereto. When silver halide is used, preferably the halide is chloride. Mixtures 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, included in conventional amounts in water-soluble silver electroplating compositions, and well known among those skilled in the art. Silver plating Bath can contain conventional additives such as electrolytes, complexing agents, buffers, and brighteners. Such additives are included in conventional amounts and are well known among those skilled in the art. Commercially available silver electroplating Examples of Bath is SILVERON™ GT-101 Bright Silver, or SILVERGLO™ 3K Bright Silver (both available from Dupont Electronic & Industrial, Marlboro, Massachusetts).

[0035] Preferably, the current density for electroplating the silver layer can range from 0.1 ASD to 50 ASD, or for example from 1 ASD to 5 ASD. Preferably, the silver plating Bath temperature can be from room temperature to 50 °C. Preferably, the silver layer is from 0.1 μm to 20 μm.

[0036] Silver alloys include, but are not limited to, silver-tin, silver-indium, silver-nickel, and silver-gold. Preferably, the silver alloy is a silver-tin alloy. Preferably, the silver content of the silver-tin alloy is about 70-95 wt%, and the balance is tin and trace impurities.

[0037] The silver-tin alloy can be deposited on a substrate by conventional methods known in the art. Preferably, the silver-tin alloy is electroplated from a silver-tin electroplating bath. Bath Such electroplating baths contain one or more silver ion sources of silver ions. The ion sources include, but are not limited to, silver salts such as silver halides, silver gluconate, silver citrate, silver lactate, silver nitrate, silver sulfate, silver alkanesulfonate, silver alkanol sulfonate, etc. The silver salts can generally be commercially available or prepared by methods described in the literature. Bath Preferably, the silver salt in the plating bath can range from 1 g / L to 100 g / L. 。 Preferably, the tin ion source includes salts such as, but not limited to, tin halides, tin sulfide, tin alkanesulfonate, tin alkanol sulfonate, and acids. The tin salts can generally be commercially available or prepared by methods known in the literature. Preferably, the tin salt can range from 0.1 g / L to 80 g / L. The silver / tin alloy electroplating bath Bath can also include one or more conventional plating additives, which are included in conventional amounts known in the art. Preferably, the current density for electroplating the silver-tin layer can range from 0.1 ASD to 50 ASD, for example, 1 ASD to 5 ASD. Preferably, the temperature of the silver-tin plating

[0038] can range from room temperature to 50 °C. Examples of commercially available hard gold alloy electroplating baths Bath include SILVERON™ GT-820 Silver-Tin (available from Dupont Electronic & Industrial, Marlborough, Massachusetts). Preferably, the silver-tin layer is from 0.1 μm to 20 μm. Bath can also include one or more conventional plating additives, which are included in conventional amounts known in the art. Preferably, the current density for electroplating the silver-tin layer can range from 0.1 ASD to 50 ASD, for example, 1 ASD to 5 ASD. Preferably, the temperature of the silver-tin plating Bath can range from room temperature to 50 °C. Examples of commercially available hard gold alloy electroplating baths Bath include SILVERON™ GT-820 Silver-Tin (available from Dupont Electronic & Industrial, Marlborough, Massachusetts). Preferably, the silver-tin layer is from 0.1 μm to 20 μm.

[0039] The hard gold is a gold-cobalt or gold-nickel alloy. The gold-cobalt alloy preferably has a gold content of about 98 to 99.95 wt% and a cobalt content of about 0.01 to 2 wt%. The gold-nickel alloy preferably has a gold content of about 98 to 99.95 wt% and a nickel content of about 0.01 to 2 wt%. Most preferably, the hard gold alloy is composed of 0.1 wt% to 0.4 wt% of cobalt and the balance of gold.

[0040] The hard gold can be deposited on a substrate by conventional methods known in the art. Preferably, the hard gold is electroplated on the substrate using a gold-cobalt alloy electroplating Bath bath. Gold ion sources for electroplating Bath include, but are not limited to, potassium gold cyanide, sodium dicyanoaurate(I), ammonium dicyanoaurate(I), and other dicyanoaurate(I) salts, potassium tetracyanoaurate(III), sodium tetracyanoaurate(III), ammonium tetracyanoaurate(III), and other tetracyanoaurate(III) salts, aurous cyanide, auric cyanide, aurous dichloride salts, tetrachloroauric acid(III), sodium tetrachloroaurate(III), and other tetrachloroauric acid(III) compounds, ammonium aurous sulfide, potassium aurous sulfide, sodium aurous sulfide, and other aurous sulfite, gold oxide, gold hydroxide, and other alkali metal salts, nitrosulphito gold complexes. Preferably, the gold ion source is included in a conventional amount such as 3 g / L to 8 g / L.

[0041] The gold alloy electroplating Bath bath can also include conventional additives, which are, for example, surfactants, brighteners, leveling agents, complexing agents, chelating agents, buffers, organic acids and inorganic acids, biocides, but are not limited thereto. Such additives are included in conventional amounts and are well known in the art. Commercially available hard gold alloy electroplating BathAn example is RONOVEL™ CM Cobalt-Alloyed Electrolytic Gold (available from Dupont Electronic & Industrial, Marlborough, Massachusetts).

[0042] Hard gold alloy electroplating can be carried out at a current density of preferably 0.1 ASD to 10 ASD, more preferably 0.5 ASD to 3 ASD, and a temperature of 30°C to 60°C. Hard gold alloy electroplating Bath can have a pH in the range of 4 to 8.

[0043] Preferably, soft gold or gold is about 98 to 99.9 wt% gold, and the balance is unavoidable impurities. Soft gold or gold can be deposited on a substrate using conventional methods known in the art. Preferably, soft gold or gold is electroplated onto the substrate. The gold ion source is the same as that described above for hard gold. The gold ion source can be included in the plating Bath in a conventional amount. Plating for soft gold and gold Bath can also include surfactants, brighteners, leveling agents, complexing agents, chelating agents, buffers, organic and inorganic acids, and biocides. Such additives can be included in conventional amounts and are well known among those skilled in the art. Commercially available soft gold plating Bath is AURONAL™ BGA LF Gold Electroplating Bath (available from Dupont Electronic & Industrial, Marlborough, Massachusetts).

[0044] A substrate containing silver, a silver alloy, hard gold, or soft gold is treated with the bismuth electroplating of the present invention Bath and any suitable method known in the art, for example, by immersing the substrate Bath in it, or by plating BathIt can be brought into contact by spraying onto a substrate. An insoluble electrode, for example, an insoluble platinum-plated titanium electrode, can serve as the anode. The bismuth plating is carried out according to the parameters described above for depositing a bismuth layer adjacent to silver, silver alloy, hard gold, or soft gold.

[0045] The bismuth electroplating of the present invention Bath can be used for plating bismuth onto any suitable substrate containing a silver, silver alloy, hard gold, or soft gold layer. Preferably, the method of the present invention is used for depositing bismuth adjacent to silver, silver alloy, or hard gold of a lead frame or similar electronic component. Such electronic components preferably include a brass base of a copper-zinc alloy and, optionally, a nickel barrier layer and a surface layer of silver or silver alloy. The nickel barrier layer, silver layer, and silver alloy layer are deposited using conventional plating compositions and methods well known in the art, such as electroplating.

[0046] The nickel barrier layer can be deposited by conventional methods known in the art. Preferably, the nickel barrier layer is deposited by electroplating from nickel plating Bath solutions. Sources of nickel ions for nickel electroplating Bath include, but are not limited to, nickel sulfate or its hydrated form, nickel sulfamate or its hydrated form, nickel chloride hexahydrate, nickel methanesulfonate, or nickel acetate or its hydrated form. One or more sources of nickel ions are included in conventional amounts in aqueous nickel electroplating compositions and are well known among those skilled in the art. Nickel plating Bath can include conventional additives, such as, but not limited to, surfactants, brighteners, leveling agents, complexing agents, chelating agents, buffers, and biocides. Such additives are included in conventional amounts and are well known among those skilled in the art. Commercially available nickel electroplating BathExamples include NIKAL™ PC-3 Bright Nickel and NIKAL™ SC Nickel (both available from Dupont Electronic & Industrial, Marlborough, MA).

[0047] Preferably, the current density for electroplating the nickel layer is 0.5 ASD to 20 ASD, or for example 1 ASD to 10 ASD. Preferably, the nickel plating Bath is electroplated at a temperature from room temperature to 60 °C.

[0048] The article of the present invention, as shown in FIG. 1, includes a brass base 10. The brass base preferably includes a copper-zinc alloy. Adjacent to the base 10, there is an optional nickel barrier layer 12. Preferably, the thickness of the nickel barrier is 0 to 2 μm. The silver layer 14 adjacent to the nickel barrier layer preferably has a thickness of 0.5 to 7 μm. The bismuth layer 16 adjacent to the silver layer 14 preferably has a thickness of 1 to 20 nm.

[0049] FIG. 2 shows an article of the present invention that does not include a nickel barrier layer. The brass base 20 preferably includes a copper-zinc alloy. Adjacent to the brass base 20, there is a silver layer 22, and adjacent to the silver layer 22, there is a bismuth layer 24. The thickness of the metal layers is substantially in the same thickness range as in FIG. 1.

[0050] FIG. 3 shows an article of the present invention that includes a brass base 30, preferably a copper-zinc alloy. Adjacent to the brass base 30, there is a nickel barrier layer 32. Adjacent to the nickel barrier layer 32, there is a silver-tin alloy layer 34. The bismuth layer 36 is adjacent to the silver-tin alloy layer.

[0051] FIG. 4 shows an article of the present invention that includes a brass base 40, preferably a copper-zinc alloy. Adjacent to the brass base 40, there is a nickel barrier layer 42. Adjacent to the nickel barrier layer 42, there is a hard gold layer 44. The bismuth layer 46 is adjacent to the hard gold layer 44.

[0052] Another article of the present invention is shown in FIG. 5. The base includes brass of a copper-iron-phosphite-zinc alloy (copper-C194) 50. There is a silver layer 52 adjacent to the brass base, and a bismuth layer 54 adjacent to the silver layer 52.

[0053] The following examples are included for the purpose of illustration of the present invention and are not intended to limit the scope of the present invention.

Example

[0054] Example 1 A nickel electroplating Bath , a 1-μm nickel barrier layer was electroplated from NIKAL (trademark) PC-3 Bright Nickel or NIKAL (trademark) SC Nickel on a plurality of brass substrates of a 3 cm × 4 cm copper-zinc alloy. The current density of the electroplating of the nickel layer was 4 ASD. The nickel plating Bath was at 50°C.

[0055] A 2-μm silver layer was electroplated on the nickel layer by Bath , electroplating from SILVERON (trademark) GT-101 Bright Silver or SILVERGLO (trademark) 3K Bright Silver electroplating Bath . The current density for electroplating the silver layer was 2 ASD at 50°C.

[0056] The thicknesses of the nickel and silver layers were measured by XRF using a BOWMAN (registered trademark) P-Series fluorescence analyzer. The contact resistance of the substrate was measured with an additional vertical force of 0 to 100 cN. The control of the additional force was performed using a Starrett DFC-20 force gauge. The resistance was measured using a Keithley 2010 Multimeter with a gold reference probe contact.

[0057] Subsequently, the substrate was immersed in an aqueous 2 wt% potassium polysulfide sulfidation test solution at room temperature for 5 minutes. The substrate was taken out of the sulfidation test solution, rinsed with DI water, and air-dried. The silver turned dark blue. The contact resistance was measured with an applied perpendicular load of 0 - 100 cN. The contact resistance of the discolored silver had a significantly higher value than that of the non-discolored silver. At an applied force of 100 cN, the contact resistance of the discolored silver was approximately 8 mΩ. In contrast, the contact resistance of the non-discolored silver was only 1.5 mΩ (Table 1).

[0058]

Table 1

[0059] Example 2 On a 3 cm × 4 cm brass copper-zinc alloy substrate, nickel was electroplated to a thickness of 1 μm and then silver was electroplated to a thickness of 2 μm as described in Example 1. The thickness of the silver layer was measured by XRF using a BOWMAN (registered trademark) P-Series fluorescence analyzer. The contact resistance of the substrate was measured with an applied perpendicular force of 0 - 100 cN as described in Example 1. The results are shown in Table 3 later.

[0060] Aqueous bismuth electroplating Bath was prepared as shown in the following table.

[0061]

Table 2

[0062] Electroplating Bath was heated to 40°C. An insoluble platinum-plated titanium anode was immersed in the electroplating Bath and connected to a DC power supply. The silver-plated substrate was immersed in the bismuth electroplating Bath and functioned as the cathode. A current density of 0.2 ASD was applied for 5 seconds to electroplate a 10 nm bismuth layer on the silver. The current supply was cut off, the substrate was taken out, washed with DI water, and air-dried. The contact resistance was measured immediately.

[0063] Next, the bismuth-plated substrate was heated in a conventional laboratory oven at 125 °C for 18 hours, and the contact resistance was measured. Then, an accelerated sulfurization test was performed on the substrate, and the contact resistance was measured again. The appearance of the substrate maintained its luster and gray color. The contact resistance was 1.5 mΩ at an additional vertical force of 100 cN, which was equivalent to that of freshly plated pure silver (Table 3).

[0064]

Table 3

[0065] Example 3 A 2.5 cm × 3.8 cm brass (copper-zinc alloy) substrate was electroplated with nickel to a thickness of 1 μm and then with silver to a thickness of 2 μm as described in Example 1. The thickness of the silver layer was measured by XRF using a BOWMAN (registered trademark) P-Series fluorescence analyzer. The contact resistance of the substrate was measured with an additional vertical force of 0 - 100 cN as described in Example 1, and the data is shown in Table 5 below.

[0066] Thereafter, the silver-plated substrate was heated in a conventional laboratory oven at 270 °C for 10 minutes. The substrate was cooled to room temperature. Next, the substrate was immersed in an accelerated sulfurization test aqueous solution of 2 wt% potassium polysulfide at room temperature for 5 minutes. The substrate was taken out of the sulfurization test solution, rinsed with DI water, and air-dried. The contact resistance was measured with an additional vertical force of 0 - 100 cN, and this is shown in Table 5 below.

[0067] Aqueous bismuth electroplating Bath was prepared as shown in the table below.

[0068]

Table 4

[0069] A 10 nm bismuth layer was plated on a 2.5 cm × 3.8 cm brass (copper-zinc alloy) substrate with a 1 μm nickel layer and a 2 μm silver top layer. The plating Bath was heated to 40 °C. A platinum-plated titanium anode was used for the plating BathIt was immersed therein and connected to a DC power supply as the anode. The silver-plated substrate was plated Bath It was immersed therein and connected to a cathode wire. A current density of 0.2 ASD was applied for 5 seconds. The potential was turned off, the substrate was taken out, washed with DI water, and dried. Then, the substrate was heat-treated in a conventional laboratory oven at 270 °C for 10 minutes. After the substrate was cooled to room temperature, an accelerated sulfidation test was performed and the contact resistance was measured. The data are shown in Table 5 below. The appearance of the bismuth-treated substrate maintained a gloss and a gray color.

[0070] A comparison with octadecanethiol as a post-treatment was also carried out. After plating a 1-μm nickel layer and a 2-μm silver top layer on a 2.5 cm × 3.8 cm brass copper-zinc alloy substrate, it was immersed in a 30 °C solution containing 0.1 M octadecanethiol and emulsified with a nonionic surfactant, TRITON™ X-114 (40 g / L) for 30 seconds. The substrate was heated in a conventional laboratory oven at 270 °C for 10 minutes, then an accelerated sulfidation test was performed, rinsed with DI water, and air-dried. The contact resistance was measured with an applied vertical force of 0 to 100 cN, which is shown in Table 5 below.

[0071] When the substrate was heated under the same conditions without post-treatment and with octadecanethiol post-treatment, it turned purple after the sulfidation test. In the bismuth-containing post-treatment, a shiny appearance was maintained, and at an applied vertical force of 100 cN, the low contact resistance after heating and sulfidation was as low as 1 mΩ.

[0072]

Table 5

[0073] Example 4 On a 2.5 cm × 3.8 cm brass copper-zinc alloy substrate, nickel was electroplated to a thickness of 1 μm and then silver to a thickness of 2 μm as described in Example 1. The thickness of the silver layer was measured by XRF using a BOWMAN® P-Series fluorescence analyzer. The contact resistance of the substrate was measured with an applied vertical force of 0 to 100 cN as described in Example 1 and reported in Table 7 below.

[0074] Thereafter, the silver-plated substrate was heat-treated in air at 150 °C for 1000 hours. The substrate was cooled to room temperature. Next, the substrate was immersed in an accelerated sulfidation test aqueous solution of 2 wt% potassium polysulfide at room temperature for 5 minutes. The substrate was taken out of the sulfidation test solution, rinsed with DI water, and air-dried. The appearance of silver changed to purple. The contact resistance was measured with an applied vertical force of 0 to 100 cN, which is shown in Table 7 later.

[0075] Aqueous bismuth electroplating Bath was prepared as shown in the following table.

[0076]

Table 6

[0077] Plating Bath was heated to 40 °C. A platinum-plated titanium anode was immersed in the plating Bath and connected to a DC power supply as the anode. A brass substrate of a copper-zinc alloy having a nickel layer with a thickness of 1 μm and a silver layer with a thickness of 2 μm was immersed in the plating Bath and connected to a cathode wire. A current density of 0.3 ASD was applied for 5 seconds to deposit a bismuth layer with a thickness of 10 nm on silver. The current was turned off, the substrate was taken out, washed with DI water, and dried. The substrate was heated in air at 150 °C for 1000 hours, and then an accelerated sulfidation test was performed and the contact resistance was measured. The appearance of the substrate maintained a glossy and gray color, and the contact resistance measured with an applied vertical force of 100 cN was 1.5 mΩ, which was equivalent to that of freshly plated 99.9 wt% silver as shown in Figure 7 later.

[0078]

Table 7

[0079] Example 5 A 2.5 cm × 3.8 cm brass copper-zinc alloy substrate was electroplated with 1 μm of nickel and then 2 μm of silver as described in Example 1. The contact resistance of the substrate was measured with an additional vertical force of 0 to 100 cN as described in Example 1 and is shown in Table 9 below.

[0080] Aqueous bismuth electroplating Bath was prepared as shown in the table below.

[0081]

Table 8

[0082] Plating Bath was heated to 40 °C. An insoluble platinum-plated titanium anode was immersed in the plating Bath and connected to a DC power supply as the anode. The silver-plated brass substrate was immersed in the plating Bath and connected to the cathode wire. A current density of 0.2 ASD was applied for 5 seconds to deposit a bismuth layer up to 10 nm thick on the silver. The power supply of the current was turned off, the substrate was taken out, washed with DI water, and dried. The contact resistance was measured immediately. Then, the bismuth-plated substrate was heated in a conventional laboratory oven at 180 °C for 48 hours, and the contact resistance was measured. Next, an accelerated sulfidation test was performed on the substrate, and the contact resistance was measured again. The appearance of the bismuth layer on the substrate retained a shiny and gray color. The contact resistance was 1.2 mΩ when measured with an additional vertical force of 100 cN, which was equivalent to that of the silver plating.

[0083]

Table 9

[0084] Example 6 A 2.5 cm × 3.8 cm brass copper-zinc alloy substrate was plated with NIKAL (trademark) SC Nicel electroplating Bath (Dupont Electronic and Industrial, Marlborough, Massachusetts) to 1 μm of nickel and then SILVERON (trademark) GT-820 Silver-Tin electroplatingBath Using Bath , an Ag—Sn alloy (80 wt% Ag, 20 wt% Sn) was electroplated to a thickness of 2 μm. A current density of 2 ASD was applied at 50° C. for 2 minutes.

[0085] The thickness of the Ag—Sn layer was measured by XRF using a BOWMAN (registered trademark) P-Series fluorescence analyzer. The contact resistance of the substrate was measured with an additional perpendicular force of 0 to 100 cN as described in Example 1, and is shown in Table 11.

[0086] The Ag—Sn plated substrate was then heat-treated in air at 180° C. for 48 hours. The substrate was cooled to room temperature and immersed in an accelerated sulfidation test solution at room temperature for 5 minutes. The substrate was removed from the sulfidation test solution, rinsed with DI water, and air dried. The appearance of the Ag—Sn was dull gray. The contact resistance was measured with an additional perpendicular force of 0 to 100 cN.

[0087] Aqueous bismuth electroplating Bath was prepared as shown in the table below.

[0088]

Table 10

[0089] Plating Bath was heated to 40° C. A platinum-plated titanium anode was immersed in the plating Bath and connected to a DC power supply. Nickel was electroplated to 1 μm on a 2.5×3.8 cm brass substrate using NIKAL (trademark) SC Nickel electroplating Bath at 50° C. by applying a current density of 4 ASD for 2 minutes, and then SILVERON (trademark) GT-820 Silver-Tin electroplating Bath was electroplated by applying a current density of 2 ASD for 2 minutes, Bath from BathAt 50 °C, an Ag–Sn alloy (80 wt% Ag, 20 wt% Sn) was electroplated to a thickness of 2 μm. Next, the substrate was immersed and connected to the cathode wire. A current of 0.4 ASD was applied for 5 s to deposit 10 nm of bismuth on the Ag–Sn alloy layer. The power was turned off, the substrate was taken out, washed with DI water, and dried. The substrate was heated in a conventional laboratory oven at 180 °C for 48 h, and then an accelerated sulfidation test was performed and the contact resistance was measured. The appearance of the substrate maintained a glossy and gray color, and the contact resistance was <10 mΩ when measured with an additional perpendicular force of 100 cN.

[0090] Example 7 Six 2.5 cm × 5 cm brass (copper–zinc alloy) test pieces were electroplated with a 1-μm-thick nickel layer using NIKAL™ SC Nickel electroplating Bath The nickel plating was performed at 4 ASD for 2 min at 55 °C. A hard gold alloy was plated on top of the nickel to a thickness of 0.5 μm using RONOVEL™ CM Cobalt-Alloyed Electrolytic Gold. The hard gold alloy was plated Bath at a plating temperature of 50 °C for 4 min at 1 ASD. The thicknesses of the nickel and hard gold layers were measured by XRF using a BOWMAN® P-Series fluorescence analyzer.

[0091] Four of the plated substrates were post-treated in PORE BLOCKER™ 200 discoloration inhibitor (available from Dupont Electronic & Industrial, Marlborough, Massachusetts). The substrates were immersed in the discoloration inhibitor at room temperature for 5 min, taken out, rinsed with DI water, and air-dried at room temperature.

[0092] Aqueous bismuth electroplating Bath was prepared as shown in the figure below.

[0093] [Table 12]

[0094] PlatingBath was heated to 40 °C. A platinum-plated titanium anode was immersed in the plating Bath and connected to a DC power supply. Two of the post-treated hard gold substrates were immersed in the plating Bath and electrically connected to the cathode wire. A current density of 0.2 ASD was applied for 5 seconds to deposit a 10-nm-thick bismuth layer on the hard gold alloy. The current was turned off, the substrate was taken out, washed with DI water, and air-dried at room temperature.

[0095] The corrosion tests conducted on the plated substrates were the nitric acid vapor (NAV) test according to ASTM B735 and the sulfur dioxide vapor test according to ASTM B799. The thermal stability of the anti-discoloration post-treatment was evaluated by visually comparing the corrosion of the substrates heated at 180 °C for 48 hours or 125 °C for 18 hours before the corrosion test with that of the non-heated substrates.

[0096] In the substrates without any anti-discoloration post-treatment, the nickel underlayer of the substrates on which the hard gold alloy was electroplated corroded in both tests. In the substrates coated with PORE BLOCKER (trademark) 200 Anti-tarnish Formation, no corrosion of the nickel underlayer was observed in the non-heated substrates. However, when the substrates subjected to the PORE BLOCKLER (trademark) 200 anti-discoloration post-treatment were heated at 125 °C for 18 hours before the corrosion test, substantial underlayer corrosion was observed. The substrates subjected to the bismuth plating post-treatment did not corrode under the conditions of ASTM B735 or ASTM B799, whether heat-treated or not. These results are summarized in Table 13.

[0097]

Table 13

[0098] Example 8 A 2.5 cm × 3.8 cm brass copper-zinc alloy substrate was electroplated with a 0.5-μm nickel barrier layer using NIKAL (trademark) SC Nickel electroplating Bath using a current density of 4 ASD for 1 minute, and the plating BathThe temperature was 50 °C. An upper layer of 0.4 μm of soft gold (99.9% gold) was plated on nickel using AURONAL (trademark) BGA LF gold electroplating Bath The electroplating was carried out at a current density of 1 ASD for 4 minutes at 50 °C. The pH of the electroplating Bath was 5.5 during plating.

[0099] The contact resistance of the substrate was measured with an additional vertical force of 0 to 100 cN as described in Example 1. The contact resistance was measured again at room temperature after plating and after heat treatment in air at 180 °C for 24 hours.

[0100] Aqueous bismuth electroplating Bath was prepared as shown in the following table.

[0101]

Table 14

[0102] Bath was heated to 40 °C. A platinum-plated titanium anode was immersed in the plating Bath and connected to a DC power supply. A current of 0.2 ASD was applied for 5 seconds to deposit a bismuth layer on a brass copper zinc alloy substrate measuring 2.5 cm × 3.8 cm with a 0.5 μm nickel barrier layer and a 0.4 μm thick gold layer. The current was switched off, the substrate was removed, washed with DI water and dried. The contact resistance was measured immediately and then heated in air at 180 °C for 24 hours. Visually evaluating the appearance of the substrate, gloss and gold color were observed. There was no change in the contact resistance. The contact resistance remained at approximately 2 mΩ with an additional force of 100 cN, which is shown in Table 15 below.

[0103]

Table 15

Explanation of Symbols

[0104] 10 Base 12 Nickel barrier layer 14 Silver layer 16 Bismuth layer 20 Brass base 22 Silver layer 24 Bismuth layer 30 Brass base 32 Nickel barrier layer 34 Silver-tin alloy layer 36 Bismuth layer 40 Brass base 42 Nickel barrier layer 44 Hard gold layer 46 Bismuth layer 50 Copper-iron-zinc phosphite alloy 52 Silver layer 54 Bismuth layer

Claims

1. A method for electroplating bismuth on a substrate of silver, silver alloy, gold, or gold alloy, comprising: providing a substrate comprising silver, silver alloy, gold, or gold alloy; providing a bismuth electroplating bath comprising a bismuth ion source, an acid, a salt of an acid, or a combination thereof; contacting the substrate with the bismuth electroplating bath; applying a current to the bismuth electroplating bath and the substrate; electroplating bismuth on the silver, silver alloy, gold, or gold alloy of the substrate to a thickness greater than 0 nm and less than or equal to 20 nm; and a method comprising the above steps.

2. The method according to claim 1, wherein the thickness is in the range of greater than 1 nm and less than or equal to 10 nm.

3. The method according to claim 2, wherein the thickness is in the range of greater than 1 nm and less than or equal to 7 nm.

4. The method according to claim 1, wherein the current density is 0.1 ASD or more.

5. The method according to claim 4, wherein the current density is in the range of 0.1 to 5 ASD.

6. A bismuth electroplating bath comprising a bismuth ion source, an organic acid selected from the group consisting of alkanesulfonic acid, alkanol sulfonic acid, and aromatic sulfonic acid, a salt of the organic acid, or a combination thereof, water, a brightener, optionally a surfactant, optionally an antibacterial agent, and optionally an antifoaming agent, wherein the brightener is selected from the group consisting of cysteine, 1,6 - hexanediol, thiodiethanol, 4,5 - dihydroxy - 1,3 - benzenedisulfonic acid, 2,2 - bis(hydroxymethyl)propionic acid, taurine, thioglycolic acid, salts thereof, and mixtures thereof, and the aromatic sulfonic acid is selected from the group consisting of benzenesulfonic acid, alkylbenzenesulfonic acid, phenolsulfonic acid, cresolsulfonic acid, nitrobenzenesulfonic acid, sulfobenzoic acid, and diphenylamine - 4 - sulfonic acid.

7. The bismuth electroplating bath according to claim 6, comprising a surfactant selected from the group consisting of secondary alcohol ethoxylate, polyether polyol, polyoxyethylene allyl ether, amine oxide, non - ionic low - foaming surfactant, and mixtures thereof.

8. An article comprising a layer of silver, silver alloy, gold, hard gold, or a combination thereof, having a bismuth layer greater than 1 nm and less than or equal to 20 nm adjacent to the silver, silver alloy, gold, or hard gold.

9. Further including a brass base and a nickel barrier layer, wherein the silver, silver alloy, gold, or hard gold layer is adjacent to the nickel barrier layer, and the nickel barrier layer is adjacent to the brass base, the article according to claim 8.

10. The article according to claim 8, wherein the bismuth layer ranges from more than 1 nm to 10 nm.

11. The article according to claim 10, wherein the bismuth layer ranges from more than 1 nm to 7 nm.

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