conductive adhesive

Abstract

The objective of this invention is to provide a conductive paste that can produce fine conductive patterns with excellent conductivity even at low temperatures below 100°C. This invention is a conductive paste containing conductive particles (a), resin (b), and imidazolium salt (c), wherein the imidazolium salt (c) has a melting point of 30°C to 100°C, and the content of the imidazolium salt (c) is 0.05 parts by mass to 3.0 parts by mass relative to 100 parts by mass of the conductive particles (a).

Description

conductive paste This invention relates to a conductive paste. In recent years, touch panels have been widely used as input devices. A touch panel consists of a display unit such as a liquid crystal panel or an organic electroluminescence (EL) panel, and a touch sensor that detects information input to a specific location. Touch panels are broadly categorized based on the method of input location detection, including resistive film, capacitive, optical, electromagnetic induction, and ultrasonic methods. Among these, capacitive touch panels are widely used due to their optical brightness, superior design, simple structure, and excellent functionality. Capacitive touch sensors have a second electrode orthogonal to the first electrode, separated by an insulating layer. A voltage is applied to the electrode on the touch panel surface, and the contact position, obtained by detecting changes in electrostatic capacitance when a conductive object such as a finger touches the sensor, is output as a signal. For the wiring electrodes used in capacitive touch sensors, transparent wiring electrodes such as indium tin oxide (ITO) are generally used to avoid making the wiring electrodes less visible. However, in recent years, due to increased sensitivity and larger screen sizes, the use of opaque wiring electrodes made of metallic materials has been expanding. Furthermore, to improve the precision, thinness, and visibility of touch sensors, it is required to directly form opaque wiring electrodes on the display portion, such as a liquid crystal panel or an organic EL panel. Therefore, in addition to forming fine patterns, it is also necessary to form conductive patterns at low temperatures. Therefore, as a technique for forming fine conductive patterns that exhibit conductivity under low-temperature curing conditions, a conductive paste containing conductive fillers, zwitterionic compounds, and thermosetting compounds has been proposed (for example, see Patent Document 1). Additionally, a conductive paste containing conductive fillers and quaternary ammonium salt compounds has also been proposed (for example, see Patent Document 2). [Prior Art Documents] [Patent Documents] Patent Document 1: International Publication No. 2014 / 208445 Patent Document 2: International Publication No. 2019 / 073926 [Problem to be solved by the invention] According to the technology of Patent Document 1 and Patent Document 2, although conductivity can be exhibited at lower temperatures than before, in recent years there has been a demand for conductivity at even lower temperatures, and conductivity is still insufficient at low temperatures below 100°C. The objective of this invention is to provide a conductive paste capable of producing fine conductive patterns that exhibit excellent conductivity even at low temperatures below 100°C. [Means for Solving the Objective] The present invention and its preferred embodiments include the following structures. [1] A conductive paste comprising conductive particles (a), a resin (b), and an imidazolium salt (c), wherein the imidazolium salt (c) has a melting point of 30°C to 100°C and the content of the imidazolium salt (c) is 0.05 parts by mass to 3.0 parts by mass relative to 100 parts by mass of the conductive particles (a). [2] The conductive paste of [1] wherein the imidazolium salt (c) has an alkyl chain at the nitrogen atom at the 1 or 3 position of the imidazolium ring. [3] The conductive paste of [2] wherein the alkyl chain has 2 to 5 carbon atoms. [4] The conductive paste of any one of [1] to [3] wherein the anion of the imidazolium salt (c) has a molecular weight of 80 or less. [5] The conductive paste of any one of [1] to [4] wherein the resin (b) comprises a resin having a carboxyl group, and the conductive paste further comprises a photopolymerization initiator (d). [6] The conductive paste of any one of [1] to [5], wherein the resin (b) contains a reactive monomer having unsaturated double bonds, and the conductive paste further contains a photopolymerization initiator (d). [7] The conductive paste of any one of [1] to [6], wherein the volume average particle size of the conductive particles (a) is 0.1 μm to 2.0 μm. [8] The conductive paste of any one of [1] to [7], wherein the conductive particles (a) constitute 60% to 85% by mass of the total solid components. [9] The conductive paste of any one of [1] to [8], wherein the conductive particles (a) are particles of a metal selected from the group consisting of gold, silver, and copper. [Effects of the Invention] The present invention enables the formation of fine conductive patterns with excellent conductivity at low temperatures below 100°C. The conductive paste of the present invention contains conductive particles (a), resin (b), and imidazolium salt (c). [Conductive Particle (a)] The conductive particle (a) is a particle that has electrical conductivity, such as particles containing silver, gold, copper, platinum, lead, tin, nickel, aluminum, tungsten, molybdenum, chromium, titanium, or indium, or alloys of these metals. Among these, particles of gold, silver, or copper with high conductivity are preferred, and particles of silver with high stability and price advantage are even more preferred. The conductive particle (a) may have a layered structure with two or more layers. For example, it may have a core-shell structure with a silver-containing shell on the surface of a copper-containing core. Furthermore, the surface of the conductive particle (a) may be coated with organic components or inorganic oxides. The organic components function as dispersants or conductive aids for small-particle-size conductive particles. Examples of organic components include fatty acids, amines, thiols, and cyanides. The volume average particle size of the conductive particles (a) is preferably 0.1 μm to 2.0 μm. By making the volume average particle size of the conductive particles (a) 0.1 μm or more, more preferably 0.3 μm or more, the contact probability between the conductive particles (a) is increased, the resistivity of the formed conductive pattern is reduced, and the exposure light during exposure can smoothly pass through the coating film of the conductive paste of the present invention, thus making the formation of fine patterns easier. On the other hand, by making the volume average particle size of the conductive particles (a) 2.0 μm or less, more preferably 1.0 μm or less, the surface smoothness and dimensional accuracy of the formed conductive pattern are improved. The volume average particle size of conductive particles (a) can be determined as follows: the conductive paste is diluted and centrifuged using a solvent soluble in resin components such as tetrahydrofuran (THF) to precipitate and recover the solid components other than the resin components. The recovered solid components are observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to observe the conductive particles (a). 100 primary particles of conductive particles (a) are randomly extracted and images are obtained. The diameter of each primary particle is calculated by converting it to a circle using image analysis, and the average diameter obtained by weighting by volume is calculated. The proportion of conductive particles (a) in the total solid components is preferably 60% to 85% by mass. By making the proportion of conductive particles (a) 60% by mass or more, more preferably 70% by mass or more, the contact probability between conductive particles (a) is increased, and the resistivity of the formed conductive pattern is reduced. On the other hand, by making the proportion of conductive particles (a) 85% by mass or less, more preferably 80% by mass or less, the light used for exposure during exposure can smoothly pass through the coating film of the conductive paste of the present invention, thus making it easier to form fine patterns. The term "total solid components" here refers to all components of the conductive paste excluding the solvent. The proportion of conductive particles (a) in the total solid components of the conductive paste of the present invention can be determined as follows: First, the conductive paste is heated at 60°C to 120°C to evaporate the solvent and recover all solid components. Second, the total solid components are heated at 400°C to 600°C using thermogravimetric-differential thermal analysis (TG-DTA) to burn the resin components, thereby determining the proportion of inorganic solid components in the total solid components. Third, the remaining inorganic solid components are dissolved in nitric acid or the like, and inductively coupled plasma (ICP) luminescence spectrophotometry is performed to determine the proportion of conductive particles (a) in the inorganic solid components. [Resin (b)] In this invention, resin (b) includes monomers and oligomers in addition to polymers. Examples of resins (b) include: acrylic resins, polyester resins, phenolic resins, epoxy resins, acrylate resins, polyether acrylate resins, phenoxy resins, polycarbonate resins, polyimide resins, polyamide resins, and polyamide-imide resins. It may also contain two or more of the aforementioned resins. In the case where the conductive paste of the present invention further contains a photopolymerization initiator (d), that is, when the pattern is formed using the conductive paste of the present invention by photolithography, it is preferable that the resin (b) has carboxyl groups. Examples of resins containing carboxyl groups include: acrylic copolymers, carboxylic acid modified epoxy resins, carboxylic acid modified phenolic resins, polyamides, carboxylic acid modified silicone polymers, etc. Two or more of the above resins may also be contained. Among these, acrylic copolymers or carboxylic acid modified epoxy resins with high ultraviolet light transmittance are preferred. As an acrylic copolymer, it is preferred to be a copolymer of an acrylic monomer and an unsaturated acid or its anhydride. Examples of acrylic monomers include methyl acrylate and ethyl acrylate. Acrylate (hereinafter referred to as "EA"), 2-ethylhexyl acrylate, n-butyl acrylate, isobutyl acrylate, isopropyl acrylate, glycidyl acrylate, butoxytriethylene glycol acrylate, dicyclopentyl acrylate, dicyclopentenyl acrylate, 2-hydroxyethyl acrylate, isobornyl acrylate, 2-hydroxypropyl acrylate, isodecanyl acrylate, isoodecyl acrylate, isooctyl acrylate, lauryl acrylate, 2-methoxyethyl acrylate, methoxyethylene glycol acrylate, methoxydiethylene glycol acrylate, octafluoropentyl acrylate, phenoxyethyl acrylate, octadecyl acrylate, trifluoroethyl acrylate, aminoethyl acrylate, phenyl acrylate, phenoxyethyl acrylate, 1-naphthyl acrylate, 2-naphthyl acrylate, thiophenol acrylate, benzyl mercaptan acrylate, allylated cyclohexyl diacrylate, methoxylated cyclohexyl diacrylate, 1,4-butanediol diacrylate, 1,3-butanediol diacrylate, ethylene glycol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate Ester, polyethylene glycol diacrylate, neopentyl glycol diacrylate, propylene glycol diacrylate, polypropylene glycol diacrylate, triglyceride diacrylate, trimethylolpropane triacrylate, di-trimethylolpropane tetraacrylate, dipentaerythritol monohydroxy pentaacrylate, dipentaerythritol hexaacrylate, acrylamide, N-methoxymethacrylamide, N-ethoxymethacrylamide, N-n-butoxymethacrylamide, N-isobutoxymethacrylamide, methacrylphenol, methyl Compounds including acrylamide phenol, γ-acryloxypropyltrimethoxysilane, N-(2-hydroxyphenyl)acrylamide, N-(3-hydroxyphenyl)acrylamide, N-(4-hydroxyphenyl)acrylamide, o-hydroxyphenyl acrylate, m-hydroxyphenyl acrylate, p-hydroxyphenyl acrylate, 2-(2-hydroxyphenyl)ethyl acrylate, 2-(3-hydroxyphenyl)ethyl acrylate, 2-(4-hydroxyphenyl)ethyl acrylate, etc., or compounds in which the acrylate groups are replaced with methacrylate groups. Among these, monomers selected from ethyl acrylate, 2-hydroxyethyl acrylate, and isobornyl acrylate are particularly preferred. Two or more of these monomers may also be used. Examples of unsaturated acids or their anhydrides include: acrylic acid (hereinafter referred to as "AA"), methacrylic acid, itaconic acid, butenoic acid, maleic acid, fumaric acid, vinyl acetate, or their anhydrides. Two or more of these may also be used. The acid value of the acrylic copolymer can be adjusted by the copolymerization ratio of the unsaturated acids. Other monomers with unsaturated double bonds include, for example, o-hydroxystyrene, m-hydroxystyrene, and p-hydroxystyrene. Two or more of these may be used. As a carboxylic acid-modified epoxy resin, it is preferably a reaction product of an epoxy compound and an unsaturated acid or anhydride. Here, carboxylic acid-modified epoxy resin refers to one obtained by modifying the epoxy groups of an epoxy compound using carboxylic acid or carboxylic anhydride, and does not contain epoxy groups. Examples of epoxy compounds include glycidyl ethers, glycidyl amines, and epoxy resins. More specifically, examples of glycidyl ethers include: methyl glycidyl ether, ethyl glycidyl ether, butyl glycidyl ether, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A diglycidyl ether, hydrogenated bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol S diglycidyl ether, bisphenol fluorene diglycidyl ether, biphenol diglycidyl ether, tetramethylbiphenol glycidyl ether, trimethylolpropane triglycidyl ether, and 3',4'-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylic acid ester, etc. Examples of glycidylamines include, for instance, tert-butylglycidylamine. Examples of epoxy resins include: bisphenol A type epoxy resin, bisphenol F type epoxy resin, biphenyl type epoxy resin, phenolic varnish type epoxy resin, hydrogenated bisphenol A type epoxy resin, etc. Two or more of these epoxy compounds may also be used. Examples of unsaturated acids or anhydrides that react with epoxy compounds include, for example, substances previously exemplified as raw materials for acrylic copolymers. In the case where the conductive paste of the present invention further contains a photopolymerization initiator (d), that is, when the pattern is formed using the conductive paste of the present invention by photolithography, the resin (b) is also preferably having unsaturated double bonds. By having unsaturated double bonds in the resin (b), the crosslinking density of the exposed portion can be increased during exposure, the development margin can be expanded, and a finer pattern can be formed. Unsaturated double bonds can be introduced by reacting a compound having unsaturated double bonds, such as glycidyl (meth)acrylate, with the acrylic copolymer or carboxylic acid-modified epoxy resin. In the case where the conductive paste of the present invention further contains a photopolymerization initiator (d), that is, when the pattern of the conductive paste of the present invention is formed by photolithography, the resin (b) is preferably a reactive monomer having unsaturated double bonds. Examples of reactive monomers having unsaturated double bonds include, for example, acrylic monomers previously exemplified as raw materials for acrylic copolymers, and styrene (hereinafter referred to as (St)). Two or more of these may also be included. The content of reactive monomers with unsaturated double bonds in resin (b) is preferably 1% to 50% by mass. By making the content of reactive monomers with unsaturated double bonds 1% or more by mass, fine patterns can be formed. On the other hand, by making it 50% or less by mass, hardening shrinkage can be moderately suppressed and conductivity can be further improved. As resin (b), a resin having phenolic hydroxyl groups can be used more preferably. By giving resin (b) phenolic hydroxyl groups, hydrogen bonds can be formed with polar groups such as hydroxyl or amine groups on the surface of the substrate, thereby improving the adhesion between the pattern and the substrate. The acid value of resin (b) is preferably 50 mgKOH / g to 250 mgKOH / g. If the acid value is 50 mgKOH / g or higher, more preferably 60 mgKOH / g or higher, the solubility in the developer increases, which can suppress the formation of developing residue. On the other hand, if the acid value is 250 mgKOH / g or lower, more preferably 200 mgKOH / g or lower, excessive dissolution in the developer can be suppressed, and the reduction of pattern film can be suppressed. The acid value of resin (b) can be determined according to Japanese Industrial Standards (JIS) K 0070 (1992). If we consider the content of conductive particles (a) in the case that the content of the imidazolium salt (c) described later is trace, then the content of resin (b) in the total solid components of the conductive paste is preferably 15% to 40% by mass. [Imidazolium Salt (c)] The conductive paste of the present invention contains imidazolium salt (c). By adding imidazolium salt (c), the diffusion of metal atoms from conductive particles (a) is significantly promoted, thereby effectively sintering the conductive particles (a) together, and maintaining conductivity even under low temperature and short-time thermosetting. The melting point of imidazolium salt (c) must be above 30°C and below 100°C. Imidazolium salts with melting points below 30°C will promote sintering even at room temperature. Therefore, sintering before pattern formation makes pattern processing difficult and compromises the storage stability of the paste. Furthermore, by setting the melting point below 100°C, electrical conductivity can be achieved even at low temperatures below 100°C. The melting point of imidazolium salt (c) can be determined by differential scanning calorimetry (DSC). Examples of imidazolium salts (c) include: 1-butyl-2,3-dimethylimidazolium chloride, 1-(2-hydroxyethyl)-3-methylimidazolium chloride, 1-methyl-3-propylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-benzyl-3-methylimidazolium chloride, 1-dodecyl-3-methyl-1H-imidazol-3-onium chloride, and 1-ethyl-3-methylimidazolium bromide. 1-Dodecyl-3-methylimidazolium bromide, 1-butyl-2,3-dimethylimidazolium hexafluorophosphate, 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium methane sulfonate, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-benzyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium nitrate, 1-ethyl-3-methylimidazolium iodide, etc. It may also contain two or more of these. The imidazolium salt (c) preferably has an alkyl chain on the nitrogen atom at the 1 or 3 position of the imidazolium ring. The presence of an alkyl chain on the nitrogen atom at the 1 or 3 position facilitates the ionization of anions and cations. Furthermore, since the alkyl chain does not readily cause steric hindrance, the diffusion of cations within the coating film becomes easier, effectively allowing metal atoms to diffuse from the conductive particles (a). The alkyl chain preferably has 2 to 5 carbon atoms. By having the alkyl chain have 2 to 5 carbon atoms, the diffusion of cations within the coating film becomes easier. The imidazolium salt (c) is preferably anionic with a molecular weight of 80 or less. By making the molecular weight 80 or less, and more preferably 70 or less, the diffusion of the anion within the coating film becomes easier. The content of imidazolium salt (c) is 0.05 to 3.0 parts by mass relative to 100 parts by mass of conductive particles (a). When the content of imidazolium salt (c) is 0.05 parts by mass or more, preferably 0.1 parts by mass or more, it easily promotes atomic diffusion of conductive particles (a), and conductivity can be improved by low temperature and short-time thermosetting. On the other hand, when the content of imidazolium salt (c) is 3.0 parts by mass or less, sintering before pattern processing can be suppressed, and high-precision wiring can be formed. [Photopolymerization initiator (d)] When patterning the conductive paste of the present invention using photolithography, the conductive paste of the present invention preferably contains a photopolymerization initiator (d). The photopolymerization initiator absorbs short-wavelength light such as ultraviolet light and undergoes decomposition or hydrogen abstraction reaction to generate free radicals. Examples of photopolymerization initiators include: benzophenone derivatives, acetophenone derivatives, thioxanone derivatives, benzyl derivatives, benzoin derivatives, oxime compounds, α-hydroxyketone compounds, α-aminoalkylphenyl ketone compounds, phosphine oxide compounds, anthrone compounds, and anthraquinone compounds. Examples of benzophenone derivatives include: benzophenone, methyl phthalate, 4,4'-bis(dimethylamino)benzophenone, 4,4'-bis(diethylamino)benzophenone, 4,4'-dichlorobenzophenone, fluorenone, 4-benzoyl-4'-methyldiphenyl ketone, etc. Examples of acetophenone derivatives include p-tert-butyldichloroacetophenone, 4-azidobenzylideneacetophenone, and 2,2'-diethoxyacetophenone. Examples of thioxanthone derivatives include: thioxanthone, 2-methylthioxanthone, 2-chlorothioxanthone, 2-isopropylthioxanthone, diethylthioxanthone, etc. Examples of benzyl derivatives include: benzyl, benzyl dimethyl ketal, benzyl-β-methoxyethyl acetal, etc. Examples of benzoin derivatives include: benzoin, benzoin methyl ether, and benzoin butyl ether. Examples of oxime compounds include: 1,2-octanedione-1-[4-(phenylthio)-2-(O-benzoyl oxime)], acetone-1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazole-3-yl]-1-(O-acetylated oxime), 1-phenyl-1,2-butanedione-2-(O-methoxycarbonyl)oxime, 1-phenyl-propanedione-2-(O-ethoxycarbonyl)oxime, 1-phenyl-propanedione-2-(O-benzoyl)oxime, 1,3-diphenyl-triketone-2-(O-ethoxycarbonyl)oxime, and 1-phenyl-3-ethoxy-triketone-2-(O-benzoyl)oxime. Examples of α-hydroxy ketone compounds include 2-hydroxy-2-methyl-1-phenyl-propane-1-one and 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one. Examples of α-aminoalkylphenyl ketone compounds include: 2-methyl-(4-methylthiophenyl)-2-morpholinylpropane-1-one, 2-benzyl-2-dimethylamino-1-(4-morpholinylphenyl)-butane-1-one, 2-dimethylamino-2-(4-methylbenzyl)-1-(4-morpholin-4-yl-phenyl)butane-1-one, etc. Examples of phosphine oxide compounds include 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide. Examples of anthrone compounds include: anthrone, benzoanthrone, dibenzoanthrone, methyleneanthrone, etc. Examples of anthraquinone compounds include: anthraquinone, 2-tert-butylanthraquinone, 2-pentylanthraquinone, β-chloroanthraquinone, etc. It may also contain two or more of these compounds. Among these, oxime compounds with high photosensitivity are preferred. The content of photopolymerization initiator (d) is preferably 1 to 30 parts by mass relative to 100 parts by mass of resin (b). If the content of photopolymerization initiator is 1 part by mass or more, the curing density of the exposed area increases, which can improve the residual film yield after development. On the other hand, if the content of photopolymerization initiator is 30 parts by mass or less, excessive light absorption caused by the photopolymerization initiator in the upper part of the pattern can be suppressed. As a result, the pattern can be easily formed into a conical shape, which can improve the adhesion to the substrate. In addition to the aforementioned components, the conductive paste of the present invention may also contain additives such as solvents, plasticizers, leveling agents, surfactants, silane coupling agents, defoamers, and pigments. Specific examples of plasticizers include: dibutyl phthalate, dioctyl phthalate, polyethylene glycol, and glycerin. Specific examples of leveling agents include: special vinyl polymers and special acrylic polymers. Examples of silane coupling agents include: methyltrimethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, hexamethyldisilazane, 3-methacryloxypropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, vinyltrimethoxysilane, and 3-glycidoxypropylmethyldiethoxysilane. Examples of solvents include: N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, 2-dimethylaminoethanol (hereinafter referred to as dimethylaminoethanol, DMEA), dimethylimidazolium, dimethyl sulfoxide, γ-butyrolactone, ethyl lactate, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, ethylene glycol mono-n-propyl ether, diacetone alcohol, tetrahydrofurfuryl alcohol, propylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether, diethylene glycol, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, etc. It may also contain two or more of these. The solvent preferably has a boiling point above 150°C. If the boiling point is above 150°C, the evaporation of the solvent is suppressed, which can inhibit the thickening of the paste. [Preparation] The conductive paste of the present invention can be prepared, for example, using a disperser or mixer such as a three-roll mill, ball mill, or planetary ball mill. [Formation of conductive patterns] A method for forming conductive patterns using the conductive paste of the present invention includes, for example, a coating step, in which the conductive paste of the present invention is coated onto a substrate to obtain a coating film; a photolithography step, in which the coating film is exposed and developed to obtain a pattern; and a curing step, in which the pattern is heated at 60°C to 250°C to obtain a conductive pattern. The coating step is the step of coating the conductive paste of the present invention onto the substrate to obtain a coating film. Examples of substrates include: polyethylene terephthalate (PET) film, polyimide film, polyester film, aromatic polyimide film, epoxy resin substrate, polyetherimide resin substrate, polyetherketone resin substrate, polyurethane resin substrate, glass substrate, silicon wafer, alumina substrate, aluminum nitride substrate, or silicon carbide substrate. Examples of coating methods used in the coating step include: rotary coating using a rotator, spray coating, roller coating, screen printing, or coating using a doctor blade coater, die coater, calender coater, meniscus coater, or bar coater. In cases where the conductive paste of the present invention contains a solvent, the resulting coated film can also be dried to remove the solvent. Methods for drying the coated film include, for example, heating drying using an oven, a heating plate, or infrared irradiation, or vacuum drying. The heating drying temperature is typically 50°C to 80°C, and the heating drying time is typically 1 minute to several hours. The thickness of the coated film obtained in the coating step can be appropriately determined by the coating method, the concentration or viscosity of all solid components of the conductive paste, etc., and preferably the thickness of the coated film after drying is 0.1 μm to 50 μm. The photolithography step is a step of exposing and developing the coated film obtained in the coating step to obtain a pattern. As the light source for exposing the coated film, it is preferably an i-ray (365 nm), h-ray (405 nm), or g-ray (436 nm) from a mercury lamp. After exposure, the unexposed areas are removed using a developer, thereby obtaining the desired pattern. Examples of developers used in alkaline development include: tetramethylammonium hydroxide, diethanolamine, diethylaminoethanol, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, triethylamine, diethylamine, methylamine, dimethylaminoethyl acetate, dimethylaminoethanol, dimethylaminoethyl methacrylate, cyclohexylamine, ethylenediamine, or hexamethylenediamine aqueous solutions. Polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, or γ-butyrolactone, alcohols such as methanol, ethanol, or isopropanol, esters such as ethyl lactate or propylene glycol monomethyl ether acetate, ketones such as cyclopentanone, cyclohexanone, isobutyl ketone, or methyl isobutyl ketone, or surfactants, can also be added to these aqueous solutions. Examples of developing solutions used in organic development include polar solvents such as N-methyl-2-pyrrolidone, N-acetyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylmethamide, dimethyl sulfoxide, or hexamethylphosphonic triamine, or mixtures of these polar solvents with methanol, ethanol, isopropanol, xylene, water, methylcarbitol, or ethylcarbitol. Examples of development methods include: spraying developer onto the surface of the coated film while the substrate is stationary or rotating; immersing the substrate in developer; or applying ultrasound while immersing the substrate in developer. The pattern obtained in the developing step can also be cleaned using a cleaning solution. Examples of such cleaning solutions include water or aqueous solutions containing alcohols such as ethanol or isopropanol, or esters such as ethyl lactate or propylene glycol monomethyl ether acetate. The curing step is a step of heating the pattern obtained in the photolithography step at 60℃~250℃ to obtain a conductive pattern. Methods of curing include, for example, drying by heating with an oven, an inert oven or a heating plate, drying by heating with electromagnetic waves from an infrared heater or the like, or vacuum drying. The curing temperature is preferably 60°C to 250°C. By setting the curing temperature to 60°C or higher, more preferably 80°C or higher, sufficient sintering of the metal particles can be achieved, significantly reducing the resistivity of the obtained conductive pattern. On the other hand, by setting the curing temperature to 250°C or lower, more preferably 140°C or lower, and even more preferably 100°C or lower, conductive patterns can be stably formed on substrates with low heat resistance. [Example] The present invention will be described in detail below with examples and comparative examples. However, the present invention is not limited to these examples. The materials used in each embodiment and comparative example are as follows. [Conductive particles (a)] Silver particles with volume average particle sizes of 0.5 μm, 0.1 μm, and 2.0 μm. [Resin (b)] (B-1) 150 g of 2-dimethylaminoethanol (DMEA) was added to a reaction vessel under nitrogen atmosphere, and the temperature was raised to 80°C using an oil bath. A mixture comprising 20 g of ethyl acrylate (EA), 40 g of 2-ethylhexyl methacrylate (2-EHMA), 20 g of styrene (St), 15 g of acrylic acid (AA), 0.8 g of 2,2'-azobisisobutyronitrile (2,2'-azobisisobutyronitrile), and 10 g of DMEA was added dropwise over 1 hour. After the addition was complete, polymerization was carried out for another 6 hours. Then, 1 g of hydroquinone monomethyl ether was added to stop the polymerization. Subsequently, a mixture comprising 5 g of GMA, 1 g of triethylbenzylammonium chloride, and 10 g of DMEA was added dropwise over 0.5 hours. After the addition was complete, an addition reaction was carried out for another 2 hours. The obtained reaction solution was purified with methanol to remove unreacted impurities, followed by vacuum drying for 24 hours to obtain B-1, an acrylic copolymer with carboxyl groups formed by adding 5 parts by mass of GMA to an acrylic copolymer of EA / 2-EHMA / St / AA (copolymer ratio of 20 / 40 / 20 / 15 by mass). The acid value of the obtained B-1 was 103 mgKOH / g. (B-2) As a reactive monomer with unsaturated double bonds, an acrylic monomer (light acrylate BP-4EA manufactured by Kyoei Chemical Co., Ltd.) is used. It is referred to as B-2. [Imidazolium Salts (c)] IS-1: 1-Methyl-3-propylimidazolium chloride (melting point: 64℃) IS-2: 1-Butyl-3-methylimidazolium chloride (melting point: 70℃) IS-3: 1-(2-hydroxyethyl)-3-methylimidazolium chloride (melting point: 83℃) IS-4: 1-Benzyl-3-methylimidazolium chloride (melting point: 80℃) IS-5: 1-Dodecyl-3-methyl-1H-imidazol-3-onium chloride (melting point: 48℃) IS-6: 1-Ethyl-3-methylimidazolium bromide (melting point: 74℃) IS-7: 1-Butyl-2,3-dimethylimidazolium tetrafluoroborate (melting point: 34℃) IS-8: 1-Ethyl-3-methylimidazolium hexafluorophosphate (melting point: 61℃) IS-9: 1-Butyl-2,3-dimethylimidazolium chloride (melting point: 99℃) IS-10: 1-Butyl-3-methylimidazolium trifluoromethane sulfonate (melting point: 16℃) IS-11: 1,3-dimethylimidazolium chloride (melting point: 125℃) IS-12: 1-Ethyl-3-methylimidazolium nitrate (melting point: 41℃) [Alternatives to imidazolium salt (c)] AS-1: Tetramethylammonium chloride (melting point: 425℃) AS-2: Tetrabutylammonium chloride (melting point: 70℃). [Photopolymerization Initiator (d)] · IRGACURE (registered trademark) OXE01 (trade name, manufactured by BASF Japan, oxime compound) (hereinafter referred to as OXE01). [Solvent] ·DMEA (manufactured by Tokyo Chemical Industry Co., Ltd.). [Evaluation and Measurement Methods] (1) Pattern Formation: The conductive paste obtained in each example was coated onto a 50 μm thick PET film by screen printing to achieve a dry coating film thickness of 2.0 μm. The obtained coating film was dried in a drying oven at 60°C for 20 minutes. A group of straight lines arranged with a certain line and gap (hereinafter referred to as "L / S"), i.e., a light-transmitting pattern, was used as a unit. Three photomasks with different L / S values ​​were used as intervening units to expose and develop the dried coating film, thereby obtaining three patterns with different L / S values. Regarding exposure, an exposure device (PEM-6M; manufactured by United Optical Co., Ltd.) was used with an exposure dose of 300 mJ / cm. 2 (Converted to wavelength 365 nm) Full-line exposure is performed, and development involves exposing the substrate to 0.20% by mass Na. 2CO After immersing in the solution for 30 seconds, a cleaning process using ultrapure water is carried out. Subsequently, the obtained patterns were thermocured at 80°C and 100°C for 1 hour to obtain three conductive patterns with different L / S values. Furthermore, the L / S values ​​of each unit in the photomask were set to 15 / 15, 10 / 10, and 7 / 7 (representing linewidth (μm) / interval (μm), respectively). The obtained conductive patterns were observed using an optical microscope. The conductive pattern with the smallest L / S value, exhibiting no residue between patterns and no pattern peeling, was identified. The case with an L / S value of 7 / 7 was classified as A, 10 / 10 as B, 15 / 15 as C, and the case with an L / S value of 15 / 15, which could not form a conductive pattern, was classified as D. Following the order A > B > C > D, the pattern formation performance was excellent, indicating the ability to form fine patterns. A through C were deemed acceptable. (2) The resistivity value is applied to a 50 μm thick PET film. The conductive paste obtained in each embodiment is applied by screen printing to make the dried coating film thickness 1.5 μm. The obtained coating film is dried in a drying oven at 80°C for 15 minutes. A photomask having 100 light-transmitting patterns 100 as shown in Figure 1 is used to expose and develop the dried coating film to obtain the pattern. The film thickness was measured using a stylus-type step gauge (Surfcom 1400 manufactured by Tokyo Seimitsu Co., Ltd.). More specifically, the film thickness was measured at 10 randomly selected locations under the conditions of a measurement length of 1 mm and a scanning speed of 0.3 mm / sec, and the average value of these film thicknesses was calculated to determine the film thickness. Subsequently, the obtained pattern was thermocured at 80°C and 100°C for 1 hour to obtain a conductive pattern for resistivity measurement. The obtained conductive pattern has a linewidth of 0.10 mm and a line length of 80 mm. Furthermore, the exposure and development conditions were set to be the same as those used in the pattern formation evaluation method. In addition, the line width can be calculated by observing the line width at 10 randomly selected locations using an optical microscope, analyzing the image data, and calculating the average value of these line widths. The resistance value was measured by connecting the ends of the conductive pattern obtained by the ohmmeter (RM3544; manufactured by HIOKI) to the ohmmeter. The ohmmeter value was calculated according to the following formula: Specific resistance value = Resistance value × Film thickness × Line width / Line length ... The smaller the specific resistance value, the better the conductivity. [Example 1] In a 100 mL clean bottle, 10.00 g of B-1 and 2.00 g of B-2 as resin (b), 0.255 g of IS-1 as imidazolium salt (c), 0.40 g of OXE01 as photopolymerization initiator, and 6.00 g of DMEA as solvent were added. The mixture was stirred using a rotary mixer (Thinky ARE-310 "Defoaming Stirrer" manufactured by Thinky Co., Ltd.) to obtain 18.66 g of resin solution (67.8% by mass of total solids). 10.00 g of the resin solution and 27.10 g of Ag particles (volume average particle size: 0.5 μm) as conductive particles (a) were mixed and kneaded using a three-roll mill (EXAKT M-50 manufactured by EXAKT Corporation) to obtain 37.10 g of conductive paste 1. [Examples 2 to 10] Except that IS-2 to IS-9 and IS-12 were used instead of IS-1 as imidazolium salt (c) in each example according to the correspondence shown in Table 1, conductive paste 2 to conductive paste 10 were obtained by operating in the same manner as in Example 1. [Example 11] The conductive paste 11 was obtained by operating in the same manner as in Example 1, except that the amount of imidazole salt (c) IS-1 added to the resin solution was set to 0.025 g instead of 0.255 g, and the amount of resin solution mixed with conductive particles (a) was set to 10.03 g instead of 10.00 g. [Example 12] The conductive paste 12 was obtained by operating in the same manner as in Example 1, except that the amount of imidazole salt (c) IS-1 added to the resin solution was set to 0.515 g instead of 0.255 g, and the amount of resin solution mixed with conductive particles (a) was set to 9.95 g instead of 10.00 g. [Example 13] The conductive paste 13 was obtained by operating in the same manner as in Example 1, except that the amount of imidazole salt (c) IS-1 added to the resin solution was set to 1.69 g instead of 0.255 g, and the amount of resin solution mixed with conductive particles (a) was set to 9.66 g instead of 10.00 g. [Examples 14 and 15] Conductive paste 14 and conductive paste 15 were obtained in the same manner as in Example 1, except that Ag particles with a volume average particle size of 0.1 μm and 2.0 μm were used instead of Ag particles with a volume average particle size of 0.5 μm as conductive particles (a). [Comparative Example 1] The conductive paste 16 was obtained by operating in the same manner as in Example 1, except that the amount of imidazole salt (c) IS-1 added to the resin solution was set to 0.015 g instead of 0.255 g, and the amount of resin solution mixed with conductive particles (a) was set to 10.05 g instead of 10.00 g. [Comparative Example 2] The conductive paste 17 was obtained by operating in the same manner as in Example 1, except that the amount of imidazole salt (c) IS-1 added to the resin solution was set to 2.36 g instead of 0.255 g, and the amount of resin solution mixed with conductive particles (a) was set to 9.55 g instead of 10.00 g. [Comparative Examples 3 to 6] Conductive pastes 18 to 21 were obtained by operating in the same manner as in Example 1, except that AS-1, AS-2, IS-10, and IS-11 were used instead of IS-1 as imidazolium salt (c) or its substitutes. Conductive patterns were manufactured by forming predetermined patterns using conductive pastes 18 to 21 obtained in each embodiment and comparative example, followed by thermosetting. The pattern formation properties and conductivity were then evaluated. Comparative Examples 2 and 5 exhibited poor pattern formation properties and could not be processed. The evaluation results are shown in Table 2. [Table 1] [Table 2] 100: Translucent Pattern Figure 1 is a schematic diagram of the light transmission pattern of the photomask used for conductivity evaluation in the embodiment.

Claims

1. A conductive paste comprising conductive particles (a), resin (b), and imidazolium salt (c), wherein the imidazolium salt (c) has a melting point of 30°C to 100°C and the content of the imidazolium salt (c) relative to 100 parts by mass of the conductive particles (a) is 0.05 parts by mass to 3.0 parts by mass, and the resin (b) comprises a resin having a carboxyl group and a reactive monomer having an unsaturated double bond.

2. The conductive paste as claimed in claim 1, wherein the imidazole onium salt (c) has an alkyl chain on the nitrogen atom at the 1 or 3 position of the imidazole ring.

3. The conductive paste as claimed in claim 2, wherein the alkyl chain has 2 to 5 carbon atoms.

4. The conductive paste as claimed in claim 1 or claim 2, wherein the anion of the imidazolium salt (c) has a molecular weight of 80 or less.

5. The conductive paste as claimed in claim 1 or claim 2, wherein the conductive paste further contains a photopolymerization initiator (d).

6. The conductive paste as claimed in claim 1 or claim 2, wherein the volume average particle size of the conductive particles (a) is 0.1 μm to 2.0 μm.

7. The conductive paste as claimed in claim 1 or claim 2, wherein the conductive particles (a) constitute 60% to 85% by mass of the total solid components.

8. The conductive paste as claimed in claim 1 or claim 2, wherein the conductive particles (a) are particles of a metal selected from the group consisting of gold, silver and copper.

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