Method for producing electroconductive particles

Abstract

Provided is electroconductive particles with excellent properties in terms of connection resistance and connection reliability by forming an even noble metal layer on the surface of a nickel coating after surface treatment, without leaving impurities on the nickel coating. This method for producing electroconductive particles includes: an acid treatment step in which nickel-coated particles each comprising a core particle and a nickel or nickel alloy coating formed on the surface thereof are brought into contact with an aqueous solution of an organic acid; and a plating process step in which the acid-treated nickel-coated particles are plated by electroless noble-metal plating. Formic acid, acetic acid, or the like can be used as the organic acid.

Description

Method for manufacturing conductive particles

[0001] The present invention relates to a method for manufacturing conductive particles.

[0002] As conductive particles used as a conductive material for anisotropic conductive materials such as anisotropic conductive films and anisotropic conductive pastes, generally, conductive particles having a conductive film made of metal formed on the surface of core material particles are known. With the miniaturization and refinement of recent electronic devices, electrical connection between electrode terminals with a small pitch is achieved by the conductive layer of conductive particles between electrodes and wirings.

[0003] Among the above conductive particles, conductive particles with a nickel coating on the surface of the core material particles are such that the nickel film is easily oxidized, and for this reason, the electrical resistance may increase over time. Further, usually, in order to further enhance conductivity, gold or palladium, which are noble metals, is further formed as a film on the nickel film (see, for example, Patent Document 1).

[0004] When forming a noble metal film on a nickel film, which is a base metal, there was a problem with the deposition property of the noble metal depending on the state of the nickel film surface. That is, unevenness occurred in the noble metal film depending on the degree of degreasing or etching of the nickel film surface, which affected the connection resistance and connection reliability. To solve this problem, for example, in Patent Document 2, it is described that a palladium film with uniform thickness can be formed by immersing the workpiece in a treatment solution composed of a complexing agent, a reducing agent, and a pH adjuster and then performing electroless palladium plating. Also, in Patent Document 3, it is described that when a metal material is treated with an aqueous solution containing a reducing agent such as hypophosphite or formate as an active ingredient and then electroless palladium plating is performed, a palladium film with good deposition property and excellent covering property can be formed. Further, in Patent Document 4, although it is not a nickel film, it is described that impurities on the particle surface can be removed by treating nickel particles themselves with hydrochloric acid, and stable conductivity can be imparted by performing gold plating thereafter.

[0005] JP-A-2011-159471 JP-A-2009-155668 JP-A-2011-225927 JP-A-2002-173702

[0006] While the surface treatment of the nickel film using the technology described above improves the deposition of precious metals and yields conductive particles with excellent connection resistance and reliability, the inventors' investigation revealed that there was still room for further improvement. Specifically, although the nickel oxide film is removed by the surface treatment of the nickel film, depending on the type of surface treatment agent, impurities may remain on the surface of the nickel film after treatment. This can cause corrosion of the nickel film, resulting in uneven plating of the precious metals and a decrease in connection reliability.

[0007] Therefore, the object of the present invention is to provide conductive particles with excellent connection resistance and connection reliability by forming a uniform noble metal layer on the surface of a nickel film without leaving any impurities on the nickel film after surface treatment.

[0008] As a result of diligent research to solve the above problems, the inventors of the present invention have found that treating the surface of the nickel film with an organic acid not only removes the nickel oxide film but also makes the nickel film less susceptible to corrosion, resulting in uniform plating of the precious metal, which in turn lowers connection resistance and improves connection reliability, thus completing the present invention.

[0009] In other words, the present invention provides a method for manufacturing conductive particles comprising an acid treatment step of contacting nickel-coated particles, which have a nickel or nickel alloy film formed on the surface of core material particles, with an organic acid aqueous solution, and a plating step of electroless precious metal plating the acid-treated nickel-coated particles.

[0010] According to the present invention, a uniform noble metal layer can be formed on the surface of a nickel film without leaving any impurities on the nickel film after surface treatment. Therefore, it is possible to provide a method for manufacturing conductive particles that have low connection resistance and excellent conductivity reliability.

[0011] This is a scanning electron microscope (SEM) image of the conductive particles obtained in Example 1.

[0012] The following describes preferred embodiments of the method for producing conductive particles of the present invention. The production method of the present invention is characterized by comprising an acid treatment step of contacting nickel-coated particles, on which a nickel or nickel alloy film is formed on the surface of core material particles, with an organic acid aqueous solution, and a plating step of electroless precious metal plating the acid-treated nickel-coated particles.

[0013] The core material particles can be inorganic or organic, as long as they are in particulate form, without any particular limitations. Examples of inorganic core material particles include metal particles such as gold, silver, copper, nickel, palladium, and solder; alloys; glass; ceramics; silica; metal or nonmetal oxides (including hydrated ones); metal silicates including aluminosilicates; metal carbides; metal nitrides; metal carbonates; metal sulfates; metal phosphates; metal sulfides; metal salts; metal halides; and carbon. Examples of organic core material particles include natural fibers; natural resins; thermoplastic resins such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polybutene, polyamide, polyacrylic acid esters, polyacrylonitrile, polyacetal, ionomer, and polyester; and thermosetting resins such as alkyd resins, phenolic resins, urea resins, benzoguanamine resins, melamine resins, xylene resins, silicone resins, epoxy resins, and diallyl phthalate resins. These may be used individually or in combination of two or more types.

[0014] The core material particles may be composed of a material consisting of both inorganic and organic materials, instead of the material consisting of either inorganic or organic materials as described above. When the core material particles are composed of a material consisting of both inorganic and organic materials, examples of the forms in which the inorganic and organic materials exist in the core material particles include a core-shell type configuration, such as a configuration comprising a core made of inorganic material and a shell made of inorganic material covering the surface of the core, or a core made of organic material and a shell made of inorganic material covering the surface of the core. In addition to these, examples of a blend type configuration include one in which inorganic and organic materials are mixed or randomly fused within a single core material particle.

[0015] The core material particles are preferably composed of organic materials or materials consisting of both inorganic and organic materials, and more preferably composed of materials consisting of both inorganic and organic materials. The inorganic materials are preferably glass, ceramics, silica, metal or nonmetal oxides (including hydrated materials), metal silicates including aluminosilicates, metal carbides, metal nitrides, metal carbonates, metal sulfates, metal phosphates, metal sulfides, metal acid salts, metal halides, and carbon. The organic materials are preferably natural fibers, natural resins, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polybutene, polyamide, polyacrylic acid esters, polyacrylonitrile, polyacetal, ionomers, polyester, and other thermoplastic resins. By using a core material made of such materials, the dispersion stability between particles can be improved, and appropriate elasticity can be exhibited during the electrical connection of electronic circuits to improve conductivity.

[0016] When organic materials are used as core material particles, it is preferable that they have no glass transition temperature or that their glass transition temperature is above 100°C, as this makes it easier to maintain the shape of the core material particles and to maintain their shape during the process of forming a metal film. The glass transition temperature can be determined, for example, by differential scanning calorimetry (DSC) as the intersection of the tangent line between the original baseline and the inflection point in the baseline shift portion of the DSC curve obtained by DSC.

[0017] When an organic material is used as the core particle, if the organic material is a highly crosslinked resin, even if the glass transition temperature is measured up to 200°C using the method described above, almost no baseline shift is observed. In this specification, such particles are also referred to as particles without a glass transition temperature, and such core particles may be used in the present invention. A specific example of a core particle material without a glass transition temperature can be obtained by copolymerizing the monomer constituting the organic material exemplified above with a crosslinkable monomer. Crosslinkable monomers include tetramethylene di(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, ethylene oxide di(meth)acrylate, tetraethylene oxide (meth)acrylate, 1,6-hexane di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane di(meth)acrylate, tetramethylolmethane tri(meth)acrylate, Examples include polyfunctional (meth)acrylates such as tetramethylolmethanetetra(meth)acrylate, tetramethylolpropanetetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, glycerol di(meth)acrylate, and glycerol tridi(meth)acrylate; polyfunctional vinyl monomers such as divinylbenzene and divinyltoluene; silane-containing monomers such as vinyltrimethoxysilane, trimethoxysilylstyrene, and γ-(meth)acryloxypropyltrimethoxysilane; and monomers such as triallyl isocyanurate, diallyl phthalate, diallylacrylamide, and diallyl ether. In particular, core material particles made of such rigid organic materials are frequently used in the COG (Chip on Glass) field.

[0018] There are no particular restrictions on the shape of the core material particles. Generally, the core material particles are spherical. However, the core material particles may have shapes other than spherical, for example, fibrous, hollow, plate-like, or needle-like, and may have numerous protrusions on their surface or be irregular in shape. In the present invention, spherical core material particles are preferred because they have excellent filling properties and are easy to coat with metal.

[0019] The present invention provides a method for producing conductive particles, which uses nickel-coated particles in which a nickel or nickel alloy film is formed on the surface of a core material particle. The thickness of the nickel or nickel alloy layer of the nickel-coated particle is preferably 0.1 nm to 2000 nm, and more preferably 1 nm to 1500 nm. By having a thickness within this range, conductive particles with excellent electrical properties can be obtained. In this invention, the thickness can be measured by cutting the particle to be measured in half and observing the cross-section of the cut surface with a scanning electron microscope (SEM).

[0020] The average particle diameter of the nickel-coated particles is preferably 0.1 μm to 50 μm, more preferably 1 μm to 30 μm. By having the average particle diameter of the nickel-coated particles within this range, conductive particles with excellent electrical properties can be obtained. In this invention, the average particle diameter of the nickel-coated particles is a value measured by SEM observation.

[0021] The core material particles preferably have a surface that can capture precious metal ions, or are surface-modified to have such a surface. The precious metal ions are preferably palladium or silver ions. Having the ability to capture precious metal ions means being able to capture precious metal ions as chelates or salts. For example, if amino groups, imino groups, amide groups, imide groups, cyano groups, hydroxyl groups, nitrile groups, carboxyl groups, etc. are present on the surface of the core material particles, the surface of the core material particles will have the ability to capture precious metal ions. When surface modification to have the ability to capture precious metal ions, for example, the method described in Japanese Patent Publication No. 61-64882 can be used.

[0022] Using such core material particles, precious metals are supported on their surface. Specifically, the core material particles are dispersed in a dilute acidic aqueous solution of a precious metal salt such as palladium chloride or silver nitrate. This causes the precious metal ions to be trapped on the surface of the particles. The concentration of the precious metal salt is determined by the surface area of ​​the particle (1 m²). ２ 1 x 10 －７ ~1 x 10 －２ A molar range is sufficient. The core material particles from which the precious metal ions have been captured are separated from the system and washed with water. Subsequently, the core material particles are suspended in water, and a reducing agent is added to perform the reduction treatment of the precious metal ions. Examples of reducing agents include sodium hypophosphite, sodium borohydride, potassium borohydride, dimethylamine borane, hydrazine, and formalin.

[0023] Before trapping precious metal ions on the surface of the core material particles, a sensitivity treatment may be performed to adsorb tin ions onto the particle surface. To adsorb tin ions onto the particle surface, for example, the surface-modified core material particles can be placed in an aqueous solution of stannous chloride and stirred for a predetermined time.

[0024] The core material particles, which have been pre-treated in this manner, are then subjected to a coating process in which they are coated with nickel or a nickel alloy. This coating process involves the following first and second steps.

[0025] The first step involves mixing core material particles with an electroless nickel plating bath containing nickel salt, a reducing agent, and a complexing agent to prepare a slurry containing primary plated particles on which an initial nickel thin film layer has been formed on the surface of the core material particles. In the first step, depending on the concentration of nickel salt in the plating bath, self-decomposition of the plating bath may occur simultaneously with the formation of the initial nickel thin film layer on the core material particles. Since this self-decomposition occurs near the core material particles, the self-decomposed products are captured on the surface of the core material particles when the initial nickel thin film layer is formed, generating nuclei for micro-protrusions, and simultaneously forming the initial nickel thin film layer. Protrusions grow from these generated nuclei. On the other hand, if self-decomposition of the plating bath does not occur, nuclei for micro-protrusions are not generated, resulting in the formation of a smooth initial nickel thin film layer.

[0026] In the first step, an aqueous slurry is prepared by thoroughly dispersing the aforementioned primary plating particles in water, preferably in a concentration of 0.1 to 500 g / L, and more preferably in a concentration of 1 to 300 g / L. The dispersion operation can usually be carried out by stirring, high-speed stirring, or by using a shear dispersion device such as a colloid mill or homogenizer. Ultrasound may also be used in combination with the dispersion operation. If necessary, a dispersant such as a surfactant may be added during the dispersion operation.

[0027] Examples of the dispersant include nonionic surfactants, amphoteric surfactants, and / or water-soluble polymers. As nonionic surfactants, polyoxyalkylene ether-based surfactants such as polyethylene glycol, polyoxyethylene alkyl ether, and polyoxyethylene alkylphenyl ether can be used. As amphoteric surfactants, betaine-based surfactants such as alkyldimethyl acetate betaine, alkyldimethylcarboxymethyl acetate betaine, and alkyldimethylamino acetate betaine can be used. As water-soluble polymers, polyvinyl alcohol, polyvinylpyrrolidinone, and hydroxyethylcellulose can be used. These dispersants can be used individually or in combination of two or more. The amount of dispersant used is preferably 0.5 to 30 g / L relative to the volume of the electroless nickel plating bath, depending on its type. In particular, an amount of dispersant used in the range of 1 to 10 g / L relative to the volume of the electroless nickel plating bath is preferable from the viewpoint of further improving the adhesion of the nickel or nickel alloy layer.

[0028] As the nickel salt, for example, nickel chloride, nickel sulfate, or nickel acetate can be used, and its concentration is preferably in the range of 0.01 to 50 g / L. As the reducing agent, for example, the same as that used for the reduction of precious metal ions as described above can be used, and it is selected based on the constituent material of the target undercoat. When a phosphorus compound, for example sodium hypophosphite, is used as the reducing agent, its concentration is preferably in the range of 0.1 to 50 g / L.

[0029] As the complexing agent, for example, compounds that have a complexing effect on nickel ions are used, such as citric acid, hydroxyacetic acid, tartaric acid, malic acid, lactic acid, gluconic acid or carboxylates such as alkali metal salts or ammonium salts thereof, amino acids such as glycine, amines such as ethylenediamine and alkylamine, and other ammonium compounds, EDTA, pyrophosphate or their salts. These can be used individually or in combination of two or more. The concentration is preferably in the range of 1 to 100 g / L, and more preferably in the range of 5 to 50 g / L. The preferred pH of the electroless nickel plating bath at this stage is in the range of 3 to 14. The electroless nickel plating reaction starts quickly when the aqueous slurry of core material particles is added and is accompanied by the generation of hydrogen gas. The first step is terminated when the generation of hydrogen gas is no longer observed. In this way, a slurry containing primary plating particles on which an initial thin nickel film layer has been formed on the surface of the core material particles can be obtained.

[0030] Next, in the second step, following the first step, either (i) a first aqueous solution containing one of nickel salt, a reducing agent, and an alkali, and a second aqueous solution containing the remaining two, or (ii) a first aqueous solution containing nickel salt, a second aqueous solution containing a reducing agent, and a third aqueous solution containing an alkali, are added to the solution from the first step simultaneously and over time to perform electroless nickel plating. Adding these solutions allows the plating reaction to resume. After the addition of the electroless nickel plating solution is complete, stirring is continued while maintaining the solution temperature for a while until no hydrogen gas generation is observed, in order to complete the reaction.

[0031] In case (i) above, it is preferable to use a first aqueous solution containing a nickel salt and a second aqueous solution containing a reducing agent and an alkali, but the combination is not limited to this. In this case, the first aqueous solution does not contain a reducing agent and an alkali, and the second aqueous solution does not contain a nickel salt. The nickel salt and reducing agent can be those mentioned above. As the alkali, for example, alkali metal hydroxides such as sodium hydroxide or potassium hydroxide can be used. The same applies to case (ii) above.

[0032] In the case of (ii) above, the first to third aqueous solutions each contain a nickel salt, a reducing agent, and an alkali, and each aqueous solution does not contain any other two components.

[0033] In either case (i) or (ii), the concentration of nickel salt in the aqueous solution is preferably 10 to 1000 g / L, and more preferably 50 to 500 g / L. The concentration of the reducing agent in the aqueous solution containing the reducing agent is preferably 100 to 1000 g / L, and more preferably 100 to 800 g / L, when a phosphorus compound is used as the reducing agent. When a boron compound is used as the reducing agent, it is preferably 5 to 200 g / L, and more preferably 10 to 100 g / L. When hydrazine or a derivative thereof is used as the reducing agent, it is preferably 5 to 200 g / L, and more preferably 10 to 100 g / L. The concentration of alkali in the aqueous solution containing the alkali is preferably 5 to 500 g / L, and more preferably 10 to 200 g / L.

[0034] The second step is performed immediately after the completion of the first step, but alternatively, the first and second steps may be performed intermittently. In this case, after the completion of the first step, the core material particles and the plating solution are separated by a method such as filtration, the core material particles are dispersed in water to prepare an aqueous slurry, an aqueous solution in which a complexing agent is dissolved preferably in a concentration range of 1 to 100 g / L, more preferably in a concentration range of 5 to 50 g / L is added, and a dispersant is dissolved preferably in a range of 0.5 to 30 g / L, more preferably in a concentration range of 1 to 10 g / L to prepare an aqueous slurry, and the second step is performed in which the above aqueous solutions are added to the aqueous slurry. Nickel-coated particles used in the manufacturing method of the present invention are obtained in this way.

[0035] The acid treatment step in the present invention is a step of subjecting the nickel-coated particles to an acid treatment in which they are brought into contact with an aqueous solution of organic acid. In this acid treatment step, oxide films and other impurities present on the surface of the nickel-coated particles can be removed by the organic acid. The reason why an organic acid is used for the acid treatment is that if an inorganic acid such as hydrochloric acid or sulfuric acid is used, elements that affect electrical properties, such as chlorine and sulfur, may remain on the surface of the nickel-coated particles after the acid treatment. Furthermore, such residual elements can cause corrosion of the nickel coating, making it impossible to uniformly plate the precious metal in the plating process described later. By using an organic acid for the acid treatment, not only can oxide films and impurities present on the surface of the nickel-coated particles be removed, but the particle surface can be brought to a state close to that of nickel metal or nickel alloy only, and then subjected to the plating process.

[0036] The aforementioned organic acid is not particularly limited and includes, for example, formic acid, acetic acid, oxalic acid, malonic acid, succinic acid, fumaric acid, maleic acid, glutaric acid, propionic acid, tartaric acid, malic acid, citric acid, myristic acid, stearic acid, palmitic acid, and mixtures of two or more of these. Among these, formic acid, acetic acid, oxalic acid, succinic acid, tartaric acid, and citric acid are preferred from the viewpoint of high effectiveness in removing oxide films and impurities, and formic acid or acetic acid are particularly preferred.

[0037] Methods for acid treatment involving contact between nickel-coated particles and organic acids include adding nickel-coated particles to an aqueous solution of organic acid and mixing, adding an aqueous solution of organic acid to a dispersion of nickel-coated particles in a solvent and mixing, and simultaneously adding an aqueous solution of organic acid and a dispersion of nickel-coated particles in a solvent to a reaction vessel and mixing. Of these, from the viewpoint of ease of operation and successful acid treatment of nickel-coated particles, the method of adding nickel-coated particles to an aqueous solution of organic acid and mixing is preferred.

[0038] The concentration of the organic acid aqueous solution is 0.1% by mass or more and 60% by mass or less, preferably 1% by mass or more and 50% by mass or less. If this concentration is less than 0.1% by mass, the oxide film and impurities on the surface of the nickel-coated particles cannot be sufficiently removed. On the other hand, if it exceeds 60% by mass, the nickel metal or nickel alloy of the nickel-coated particles will be excessively damaged, making it impossible to uniformly plate the precious metal in the subsequent plating process. Furthermore, regarding the amount of nickel-coated particles added to the organic acid aqueous solution, from the viewpoint of effectively treating the nickel-coated particles with acid and having excellent productivity, it is preferable that the concentration in the organic acid aqueous solution be 1% by mass or more and 70% by mass or less, particularly 10% by mass or more and 50% by mass or less.

[0039] When nickel-coated particles are brought into contact with an organic acid for acid treatment, the temperature is preferably 5°C to 40°C, and more preferably 10°C to 30°C, from the viewpoint of obtaining a better acid treatment effect. Furthermore, the duration of the acid treatment is preferably 100 minutes or less, and more preferably 1 minute to 80 minutes, from the viewpoint of balancing the degree of acid treatment with productivity.

[0040] Nickel-coated particles that have undergone an acid treatment process are subjected to a plating process in which electroless precious metal plating is performed by one of the following methods (D1) to (D3): (D1) A method of reducing electroless precious metal plating using a pure precious metal plating bath. (D2) A method of reducing electroless precious metal plating using hypophosphorous acid or its salt as a reducing agent. (D3) A method of displacement electroless precious metal plating.

[0041] Examples of precious metals used in the electroless precious metal plating include gold, silver, platinum, rhodium, ruthenium, palladium, and other metals, or alloys containing these. Of these, gold, platinum, and palladium are preferred as the precious metals used in the present invention from the viewpoint of particularly excellent connection reliability and electrical conductivity.

[0042] The method for performing the reductive electroless noble metal plating treatment of (D1) and (D2) is to deposit a noble metal on the nickel film by the action of a reducing agent on the noble metal ions in the plating solution. In the plating method of (D1), by using, for example, a formic acid compound as the reducing agent, an electroless noble metal plating film with a purity of 99% by mass or more can be obtained. On the other hand, in the reductive electroless noble metal plating method of (D2), by using hypophosphorous acid or its salt as the reducing agent, a plating film composed of a noble metal-phosphorus alloy can be obtained.

[0043] The method for performing the reductive electroless noble metal plating treatment in the pure noble metal plating bath of (D1) will be described below. The pure noble metal plating bath is composed of an aqueous solution containing a noble metal compound, a reducing agent, and a complexing agent as essential components. In the present invention, a dispersant and the nickel-coated particles are added to the pure noble metal plating bath to perform the reductive electroless noble metal plating treatment.

[0044] When using a gold compound as the noble metal compound, the gold compound is not particularly limited as long as it is soluble in the plating solution and an aqueous solution with a predetermined concentration can be obtained. For example, water-soluble gold compounds such as potassium gold cyanide, sodium gold cyanide, ammonium gold cyanide, chloroaurate, gold sulfite, and gold thiosulfate can be mentioned. These can be used alone or in combination of two or more. The content of the gold compound in the pure gold plating bath is preferably 0.1 to 30 g / L, more preferably 0.3 to 10 g / L in terms of gold.

[0045] When using a palladium compound as the noble metal compound, the palladium compound is not particularly limited as long as it is soluble in the plating solution and an aqueous solution of a predetermined concentration can be obtained. For example, water-soluble palladium compounds such as palladium sulfate, palladium chloride, palladium nitrate, palladium acetate, dichlorodiethylenediamine palladium, and tetraamine palladium dichloride can be mentioned. These can be used alone or in combination of two or more. Also, as the palladium compound, a palladium solution in which palladium is dissolved can be used. As the palladium solution, for example, a dichlorodiethylenediamine palladium solution or a tetraammine palladium dichloride solution can be used. The content of the palladium compound in the pure palladium plating bath is preferably 0.1 to 30 g / L, more preferably 0.3 to 10 g / L in terms of palladium.

[0046] Formic acid or its salt is used as the reducing agent. The content of the reducing agent in the pure noble metal plating bath is preferably 0.1 to 100 g / L, more preferably 1 to 50 g / L. Note that the formic acid added as the reducing agent to the plating solution and the organic acid such as formic acid used in the acid treatment step have different purposes of use.

[0047] As the complexing agent, for example, amines such as ethylenediamine and diethylenetriamine; aminopolycarboxylic acids such as ethylenediaminediacetic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid, and their sodium salts, potassium salts, ammonium salts, etc.; amino acids such as glycine, alanine, iminodiacetic acid, nitrilotriacetic acid, L-glutamic acid, L-glutamic acid diacetate, L-aspartic acid, and taurine, and their sodium salts, potassium salts, ammonium salts, etc.; aminotrimethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetramethylenephosphonic acid, and their ammonium salts, potassium salts, sodium salts, etc. can be blended. The complexing agent can be used alone or in combination of two or more. The content of the complexing agent in the pure noble metal plating bath is preferably about 0.5 to 100 g / L, and more preferably about 5 to 50 g / L.

[0048] Examples of dispersants include nonionic surfactants, amphoteric surfactants, and water-soluble polymers. Nonionic surfactants include polyoxyalkylene ether-based surfactants such as polyethylene glycol, polyoxyethylene alkyl ether, and polyoxyethylene alkylphenyl ether. Amphoteric surfactants include betaine-based surfactants such as alkyldimethyl acetate betaine, alkyldimethylcarboxymethyl acetate betaine, and alkyldimethylamino acetate betaine. Water-soluble polymers include polyvinyl alcohol, polyvinylpyrrolidinone, and hydroxyethylcellulose. The amount of dispersant used depends on its type, but is generally 0.5 to 30 g / L relative to the volume of the liquid (pure precious metal plating bath). In particular, an amount of dispersant in the range of 1 to 10 g / L relative to the volume of the liquid (pure precious metal plating bath) is preferable from the viewpoint of improving the adhesion of the precious metal film and further enhancing the effect of suppressing the formation of aggregated particles.

[0049] The pH of the electroless precious metal plating reaction is preferably 3 to 10, particularly 4 to 7, as this allows for a stable deposition rate. For pH adjustment, for example, sulfuric acid, hydrochloric acid, sodium hydroxide, or aqueous ammonia can be used. The reaction temperature is preferably 20 to 90°C, particularly 40 to 80°C, from the viewpoint of obtaining a smooth and dense film.

[0050] Furthermore, the pure precious metal plating bath may contain commonly used additives such as stabilizers. As a pure precious metal plating bath containing such additives, commercially available products from companies such as Kojima Chemical Co., Ltd., Chuo Chemical Industry Co., Ltd., and Nippon Kanizen Co., Ltd. may be used.

[0051] Next, a reduction-type electroless precious metal plating method using hypophosphorous acid or a salt thereof as the reducing agent for (D2) will be described. A precious metal-phosphorus plating bath is used as the plating bath for (D2). This precious metal-phosphorus plating bath consists of an aqueous solution containing a precious metal compound, a reducing agent of hypophosphorous acid or a salt thereof, and a complexing agent as essential components. In the present invention, the precious metal-phosphorus plating bath, a dispersant, and the nickel-coated particles are mixed to perform the reduction-type electroless precious metal plating treatment.

[0052] The same precious metal compounds, reducing agents, complexing agents, and dispersants as those used in the (D1) reduction-type electroless precious metal plating method described above can be used in similar amounts.

[0053] In the reduction-type precious metal plating method of (D2), the phosphorus content in the precious metal film is adjusted by adjusting the amount of reducing agent relative to the precious metal ions to a specific range. Generally, the phosphorus content in the precious metal film increases as the amount of hypophosphorous acid or its salt, which is the reducing agent, relative to the precious metal ions increases. In the reduction-type electroless precious metal plating of (D2), it is preferable to adjust the phosphorus content in the precious metal film to 3% by mass or less, particularly 0.5% to 3% by mass, by setting the molar ratio of the reducing agent to the precious metal ions to 0.1 to 100 and performing the reduction-type electroless precious metal plating treatment. If the amount of reducing agent added to the precious metal ions is less than 0.1 in molar ratio, the deposition rate decreases and it becomes impractical. On the other hand, if it is greater than 100, the phosphorus content in the precious metal film increases, making it impossible to obtain the excellent conductivity characteristics of the precious metal layer. In particular, in the present invention, it is preferable that the molar ratio of the reducing agent to the precious metal ions is in the range of 1 to 30.

[0054] The pH of the electroless precious metal-phosphorus plating reaction is preferably 5 to 10, more preferably 5.5 to 9. Setting the pH within this range increases the stability of the plating solution and makes it less likely for defects such as cracks to occur in the precious metal film. For pH adjustment, for example, sulfuric acid, hydrochloric acid, sodium hydroxide, ammonia water, etc., can be used. A reaction temperature of 25 to 80°C, particularly 40 to 70°C, is preferable from the viewpoint of obtaining a smooth and dense film.

[0055] Furthermore, the precious metal-phosphorus plating bath may contain commonly used additives such as stabilizers. As a precious metal-phosphorus plating bath containing such additives, commercially available products sold by companies such as Kojima Chemical Co., Ltd. and Ishihara Pharmaceutical Co., Ltd. may be used.

[0056] Next, the substitution type electroless precious metal plating method (D3) will be described. In the substitution type electroless precious metal plating method (D3), a precious metal plating film is formed on nickel-coated particles by substitution type electroless precious metal plating through a substitution reaction between precious metal ions and nickel ions.

[0057] The displacement-type electroless precious metal plating bath consists of an aqueous solution containing a precious metal compound and a complexing agent as essential components. In the present invention, the displacement-type electroless precious metal plating treatment is performed by adding a dispersant and the nickel-coated particles to the displacement-type electroless precious metal plating bath.

[0058] The noble metal compound, complexing agent, and dispersant can be the same as those used in the reduction-type electroless noble metal plating method described above (D1), and can be added in similar amounts.

[0059] When performing the substitutional electroless precious metal plating reaction at a pH of 3 to 10, particularly 4 to 8, a smooth and dense film is easily obtained, which is preferable. For pH adjustment, for example, sulfuric acid, hydrochloric acid, sodium hydroxide, or aqueous ammonia can be used. A reaction temperature of 25 to 80°C, particularly 30 to 70°C, is preferable because the reaction rate becomes practically fast and a dense precious metal film is easily obtained.

[0060] After the plating reactions described in (D1) to (D3) above are completed, the conductive powder consisting of the conductive particles of the present invention is obtained by filtration separation and drying using a conventional method. If necessary, the conductive powder may also be subjected to a crushing process using a ball mill. By subjecting it to this crushing process, monodisperse conductive particles can be obtained, and the ratio of primary conductive particles to the weight of the conductive powder can be preferably 85% by mass or more, more preferably 90% by mass or more.

[0061] The conductive particles of the present invention obtained in this manner are suitably used, for example, in anisotropic conductive films (ACF), heat-seal connectors (HSC), and conductive materials for connecting electrodes of liquid crystal display panels to circuit boards of driving LSI chips. In particular, the conductive powder of the present invention is suitably used as a conductive filler in conductive adhesives.

[0062] The conductive adhesive described above is preferably used as an anisotropic conductive adhesive that is placed between two substrates on which conductive substrates are formed, and adheres the conductive substrates to them by heating and pressing to establish electrical conductivity. This anisotropic conductive adhesive contains the conductive particles and adhesive resin of the present invention. The adhesive resin can be any resin that is insulating and is used as an adhesive resin, without any particular limitations. It may be either a thermoplastic resin or a thermosetting resin, and it is preferable that it exhibits adhesive properties upon heating. Examples of such adhesive resins include thermoplastic types, thermosetting types, and UV-curing types. There are also so-called semi-thermosetting types that exhibit properties intermediate between thermoplastic and thermosetting types, and composite types of thermosetting and UV-curing types. These adhesive resins can be appropriately selected according to the surface characteristics and usage of the circuit board or other object to be bonded. In particular, adhesive resins composed of thermosetting resins are preferred because they have excellent material strength after bonding.

[0063] Specifically, adhesive resins include those prepared as a main component from one or more selected materials from ethylene-vinyl acetate copolymer, carboxyl-modified ethylene-vinyl acetate copolymer, ethylene-isobutyl acrylate copolymer, polyamide, polyimide, polyester, polyvinyl ether, polyvinyl butyral, polyurethane, SBS block copolymer, carboxyl-modified SBS copolymer, SIS copolymer, SEBS copolymer, maleic acid-modified SEBS copolymer, polybutadiene rubber, chloroprene rubber, carboxyl-modified chloroprene rubber, styrene-butadiene rubber, isobutylene-isoprene copolymer, acrylonitrile-butadiene rubber (hereinafter referred to as NBR), carboxyl-modified NBR, amine-modified NBR, epoxy resin, epoxy ester resin, acrylic resin, phenolic resin, or silicone resin. Among these, styrene-butadiene rubber and SEBS are preferred as thermoplastic resins due to their excellent reworkability. Epoxy resins are preferred as thermosetting resins. Of these, epoxy resin is the most preferred due to its advantages of high adhesive strength, excellent heat resistance and electrical insulation, as well as low melt viscosity and the ability to connect at low pressure.

[0064] As the epoxy resin mentioned above, any polyvalent epoxy resin having two or more epoxy groups in one molecule can be used, and any commonly used epoxy resin can be used. Specific examples include novolac resins such as phenol novolac and cresol novolac; polyvalent phenols such as bisphenol A, bisphenol F, bisphenol AD, resorcinol, and bishydroxydiphenyl ether; polyhydric alcohols such as ethylene glycol, neopentyl glycol, glycerin, trimethylolpropane, and polypropylene glycol; polyamino compounds such as ethylenediamine, triethylenetetramine, and aniline; and polyvalent carboxyl compounds such as adipic acid, phthalic acid, and isophthalic acid, which are reacted with epichlorohydrin or 2-methylepichlorohydrin to obtain glycidyl-type epoxy resins. Other examples include aliphatic and alicyclic epoxy resins such as dicyclopentadiene epoxy and butadiene dimer epoxy. These can be used individually or in combination of two or more.

[0065] Furthermore, from the viewpoint of preventing ion migration, it is preferable to use high-purity adhesive resins with reduced impurity ions (such as Na and Cl) and hydrolyzable chlorine.

[0066] The amount of conductive particles used in the anisotropic conductive adhesive of the present invention is typically 0.1 to 30 parts by weight, preferably 0.5 to 25 parts by weight, and more preferably 1 to 20 parts by weight, per 100 parts by weight of the adhesive resin component. By keeping the amount of conductive particles within this range, it is possible to suppress increases in connection resistance and melt viscosity, improve connection reliability, and ensure sufficient anisotropy of the connection.

[0067] In addition to the conductive particles and adhesive resin described above, the anisotropic conductive adhesive may contain additives known in the art, and the amount of such additives may also be within the range known in the art. Examples of other additives include tackifiers, reaction aids, epoxy resin curing agents, metal oxides, photoinitiators, sensitizers, curing agents, vulcanizing agents, degradation inhibitors, heat-resistant additives, thermal conductivity enhancers, softeners, colorants, various coupling agents, or metal deactivators.

[0068] Examples of tackifiers include rosin, rosin derivatives, terpene resins, terpene phenol resins, petroleum resins, coumarone-indene resins, styrene resins, isoprene resins, alkylphenol resins, and xylene resins. Examples of reactive additives, i.e., crosslinking agents, include polyols, isocyanates, melamine resins, urea resins, utropines, amines, acid anhydrides, and peroxides. As for epoxy resin curing agents, any agent having two or more active hydrogen atoms in one molecule can be used without particular limitations. Specific examples include polyamino compounds such as diethylenetriamine, triethylenetetramine, metaphenylenediamine, dicyandiamide, and polyamidoamine; organic acid anhydrides such as phthalic anhydride, methylnadic anhydride, hexahydrophthalic anhydride, and pyromellitic anhydride; and novolac resins such as phenol novolac and cresol novolac. These can be used individually or in combination of two or more. Latent curing agents may also be used depending on the application and requirements. Examples of usable latent curing agents include imidazole-based, hydrazide-based, boron trifluoride-amine complexes, sulfonium salts, amineimides, polyamine salts, dicyandiamides, and modified versions thereof. These can be used individually or as a mixture of two or more.

[0069] The anisotropic conductive adhesive described above is typically manufactured using a manufacturing apparatus widely used by those skilled in the art. It involves blending the conductive particles and adhesive resin of the present invention with a curing agent and various additives as needed, and, if the adhesive resin is a thermosetting resin, mixing them in an organic solvent. If the adhesive resin is a thermoplastic resin, it is manufactured by melt-kneading at a temperature above the softening point of the adhesive resin, specifically preferably about 50 to 130°C, and more preferably about 60 to 110°C. The anisotropic conductive adhesive thus obtained may be applied by coating or applied in the form of a film.

[0070] The present invention will be further described below with reference to examples. However, the scope of the present invention is not limited to these examples. The characteristics in the examples were measured by the following methods: (1) Average particle diameter: 200 particles were arbitrarily extracted from a scanning electron microscope (SEM) image of the object to be measured, and the particle diameter was measured at a magnification of 10,000x. The arithmetic mean was taken as the average particle diameter. (2) Thickness of the conductive layer: The conductive particles were cut in half, and the cross-section of the cut surface was observed and measured with a scanning electron microscope (SEM).

[0071] [Example 1] (1) Pretreatment Spherical styrene-acrylate-silica composite resin particles with an average particle size of 3.0 μm were used as base particles. 9 g of these particles were added to 200 mL of a conditioner aqueous solution ("Cleaner Conditioner 231" manufactured by Rohm & Haas Electronic Materials) while stirring. The concentration of the conditioner aqueous solution was 40 mL / L. Subsequently, the base particles were surface modified and dispersed by stirring for 30 minutes while applying ultrasound at a liquid temperature of 60°C. This aqueous solution was filtered, and the base particles, which had been repulped and washed once, were made into a 200 mL slurry. 0.1 g of stannous chloride was added to this slurry. The mixture was stirred at room temperature for 5 minutes to perform a sensitivity treatment in which tin ions were adsorbed onto the surface of the base particles. Subsequently, this aqueous solution was filtered, and the base particles, which had been repulped and washed once, were made into a 200 mL slurry and maintained at 60°C. 1.5 mL of a 0.11 mol / L palladium chloride aqueous solution was added to this slurry. The mixture was stirred at 60°C for 5 minutes to perform an activation treatment in which palladium ions were captured on the surface of the substrate particles. Subsequently, this aqueous solution was filtered, and the substrate particles, which had been repulped and washed once with hot water, were made into a 100 mL slurry. 10 mL of a 0.5 g / L dimethylamine borane aqueous solution was added, and the mixture was stirred for 2 minutes under ultrasonic conditions to obtain a slurry of pre-treated substrate particles. (2) Preparation of Ni plating bath 3 L of an electroless nickel-phosphorus plating bath was prepared, consisting of an aqueous solution of 5 g / L sodium tartrate, 2 g / L nickel sulfate hexahydrate, 10 g / L trisodium citrate, 0.1 g / L sodium hypophosphite, and 2 g / L polyethylene glycol, and the temperature was raised to 70°C. (3) Electroless Ni Plating Treatment The slurry of the pre-treated substrate particles was added to the electroless nickel-phosphorus plating bath and stirred for 5 minutes until it was confirmed that hydrogen foaming had stopped. 420 mL of 224 g / L nickel sulfate aqueous solution and a mixed aqueous solution containing 210 g / L sodium hypophosphite and 80 g / L sodium hydroxide were added to the slurry continuously using a metering pump at an addition rate of 2.5 mL / min each, and electroless nickel plating was started. After adding the entire amounts of the nickel sulfate aqueous solution and the mixed aqueous solution of sodium hypophosphite and sodium hydroxide, stirring was continued for 5 minutes while maintaining a temperature of 70°C.Next, the liquid was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a nickel-phosphorus alloy coating with a thickness of 110 nm. (4) Acid treatment The conductive particles having the nickel-phosphorus alloy coating were added to a 10% by mass aqueous solution of formic acid to make a 100 g / L slurry, and the conductive particles were dispersed by stirring for 10 minutes while applying ultrasonic waves to obtain a slurry of formic acid-treated conductive particles. This aqueous solution was filtered, repulped and washed twice to make another 100 g / L slurry. (5) Precious metal plating treatment The slurry of formic acid-treated conductive particles was added to an electroless palladium plating bath (manufactured by Kojima Chemical Co., Ltd., product name "Palette") at a temperature of 60°C, and electroless palladium plating was performed by continuing to stir for 10 minutes while maintaining the temperature of 60°C. The pH of the plating bath was 6.0. Next, the liquid was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a palladium coating with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.20 μm, and the thickness of the conductive layer was 120 nm.

[0072] [Example 2] The same procedure as in Example 1 was followed up to "(3) Electroless Ni Plating Treatment" to obtain conductive particles having a nickel-phosphorus alloy film with a thickness of 110 nm. (4) Acid Treatment The conductive particles having the nickel-phosphorus alloy film were added to a 50% by mass formic acid aqueous solution to make a 100 g / L slurry, and the slurry was stirred for 10 minutes while applying ultrasonic waves to disperse the conductive particles, obtaining a slurry of formic acid-treated conductive particles. This aqueous solution was filtered and repulped twice to make another 100 g / L slurry. (5) Precious Metal Plating Treatment This slurry of formic acid-treated conductive particles was added to an electroless palladium plating bath (manufactured by Kojima Chemical Co., Ltd., product name "Palette") with a temperature of 60°C, and electroless palladium plating was performed by continuing to stir for 10 minutes while maintaining the temperature of 60°C. The pH of the plating bath was 6.0. Next, the liquid was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a palladium coating with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.16 μm, and the thickness of the conductive layer was 118 nm.

[0073] [Example 3] The same procedure as in Example 1 was followed up to "(4) Acid Treatment" to obtain a 100 g / L slurry of formic acid-treated conductive particles. (5) Precious Metal Plating Treatment The slurry of formic acid-treated conductive particles was subjected to electroless gold plating in an electroless gold plating bath at a temperature of 70°C. The temperature was maintained at 70°C and the mixture was stirred continuously for 10 minutes. The composition of the electroless gold plating solution was 10 g / L of disodium dihydrogen dihydrogen ethylenediaminetetraacetate dihydrate, 10 g / L of disodium hydrogen citrate 1.5 hydrate, and 3 g / L of potassium gold cyanide, and the pH was adjusted to 6.0. Next, the solution was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a gold film with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.21 μm, and the thickness of the conductive layer was 121 nm.

[0074] [Example 4] The same procedure as in Example 1 was followed up to "(3) Electroless Ni Plating Treatment" to obtain conductive particles having a nickel-phosphorus alloy film with a thickness of 110 nm. (4) Acid Treatment The conductive particles having the nickel-phosphorus alloy film were added to a 10% by mass aqueous solution of acetic acid to make a 100 g / L slurry, and the slurry was stirred for 10 minutes while applying ultrasonic waves to disperse the conductive particles, obtaining a slurry of conductive particles treated with acetic acid. This aqueous solution was filtered and repulped twice to make a 100 g / L slurry again. (5) Precious Metal Plating Treatment The slurry of conductive particles treated with acetic acid was added to an electroless palladium plating bath (manufactured by Kojima Chemical Co., Ltd., product name "Palette") at a temperature of 60°C, and electroless palladium plating was performed by continuing to stir for 10 minutes while maintaining the temperature of 60°C. The pH of the plating bath was 6.0. Next, the liquid was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a palladium film with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.22 μm, and the thickness of the conductive layer was 120 nm.

[0075] [Example 5] The same procedure as in Example 4 was followed up to "(4) Acid Treatment" to obtain a 100 g / L slurry of acetic acid-treated conductive particles. (5) Precious Metal Plating Treatment The slurry of acetic acid-treated conductive particles was subjected to electroless gold plating in an electroless gold plating bath at a temperature of 70°C. The temperature was maintained at 70°C and the mixture was stirred continuously for 10 minutes. The composition of the electroless gold plating solution was 10 g / L of disodium dihydrogen dihydrate ethylenediaminetetraacetic acid, 10 g / L of disodium hydrogen citrate 1.5 hydrate, and 3 g / L of potassium gold cyanide, and the pH was adjusted to 6.0. Next, the solution was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a gold film with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.21 μm, and the thickness of the conductive layer was 120 nm.

[0076] [Example 6] The same procedure as in Example 1 was followed up to "(3) Electroless Ni Plating Treatment" to obtain conductive particles having a nickel-phosphorus alloy film with a thickness of 110 nm. (4) Acid Treatment The conductive particles having the nickel-phosphorus alloy film were added to a 10% succinic acid aqueous solution to make a 100 g / L slurry, and the slurry was stirred for 10 minutes while applying ultrasonic waves to disperse the conductive particles, obtaining a slurry of succinic acid-treated conductive particles. This aqueous solution was filtered and repulped twice to make another 100 g / L slurry. (5) Precious Metal Plating Treatment The slurry of succinic acid-treated conductive particles was added to an electroless palladium plating bath (manufactured by Kojima Chemical Co., Ltd., product name "Palette") at a temperature of 60°C, and electroless palladium plating was performed by continuing to stir for 10 minutes while maintaining the temperature of 60°C. The pH of the plating bath was 6.0. Next, the liquid was filtered, the filtrate was washed three times, and then dried in a vacuum dryer at 110°C to obtain conductive particles having a palladium coating with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.21 μm, and the thickness of the conductive layer was 121 nm.

[0077] [Comparative Example 1] In Example 1, after "(3) Electroless Ni plating," "(5) Electroless Pd plating" was performed without performing "(4) Acid treatment," and conductive particles having a palladium film with a thickness of 10 nm were obtained. The average particle size of the obtained conductive particles was 3.22 μm, and the thickness of the conductive layer was 120 nm.

[0078] [Comparative Example 2] The same procedure as in Example 1 was performed except that a 70% by mass aqueous solution of formic acid was used in the "(4) Acid Treatment" step of Example 1, to obtain conductive particles having a palladium film with a thickness of 10 nm. The average particle diameter of the obtained conductive particles was 3.10 μm, and the thickness of the conductive layer was 115 nm.

[0079] [Comparative Example 3] The same procedure as in Example 1 was performed except that a 10% by mass aqueous hydrochloric acid solution was used in the "(4) Acid Treatment" step of Example 1, to obtain conductive particles having a palladium film with a thickness of 10 nm. The average particle diameter of the obtained conductive particles was 3.13 μm, and the thickness of the conductive layer was 115 nm.

[0080] [Comparative Example 4] The same procedure as in Example 1 was performed except that a 5% by mass aqueous sulfuric acid solution was used in the "(4) Acid Treatment" step of Example 1, to obtain conductive particles having a palladium film with a thickness of 10 nm. The average particle size of the obtained conductive particles was 3.10 μm, and the thickness of the conductive layer was 114 nm.

[0081] (Evaluation of connection resistance and connection reliability) The connection resistance and connection reliability were evaluated using the conductive particles of the examples and comparative examples by the following method. 1.0 g of the conductive particles obtained in the examples and comparative examples were placed in a vertically positioned resin cylinder with an inner diameter of 10 mm, and the electrical resistance between the upper and lower electrodes was measured under a load of 2 kN at room temperature (25°C, 50% RH) to determine the initial volume resistivity. The lower the initial volume resistivity, the more effectively the oxide film formed on the electrodes can be removed, and the lower the connection resistance of the conductive particles can be evaluated. Furthermore, the resistance value after holding for 144 hours under conditions of 85°C and 85% RH was also measured. The smaller the difference from the connection resistance value at room temperature, the better the connection reliability of the conductive particles can be evaluated.

[0082]

[0083] These results indicate that the conductive particles of the present invention have low connection resistance. Furthermore, the small difference between the initial volume resistivity and the resistance value after 144 hours at 85°C and 85% RH demonstrates excellent connection reliability.

Claims

1. A method for manufacturing conductive particles, comprising: an acid treatment step of contacting nickel-coated particles, which have a nickel or nickel alloy film formed on the surface of core material particles, with an organic acid aqueous solution; and a plating step of electroless precious metal plating the acid-treated nickel-coated particles.

2. The method for producing the product according to claim 1, wherein the organic acid is formic acid, acetic acid, oxalic acid, succinic acid, tartaric acid, or citric acid.

3. The manufacturing method according to claim 1, wherein the precious metal used in the electroless precious metal plating is gold, platinum, or palladium.

4. The manufacturing method according to claim 1, wherein the pH of the electroless precious metal plating bath is 3.0 or more and 7.0 or less.

5. The manufacturing method according to claim 1, wherein the nickel-coated particles are obtained by coating core material particles with nickel or a nickel alloy by electroless nickel plating.

6. The manufacturing method according to claim 1, wherein the concentration of the aqueous solution of the organic acid is 0.1% by mass or more and 60% by mass or less.

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