Conductive paste, substrate with conductive film, and method for manufacturing the substrate with conductive film

Abstract

The purpose of the present invention is to provide: a conductive paste that makes it possible to form a conductive film that has excellent conductivity and does not readily release fine copper particles, even when sintered by radiation with radiant energy that can sufficiently remove a binder resin; a conductive film–coated substrate that uses the conductive paste; and a production method for the conductive film–coated substrate. The present invention provides: a conductive paste that contains fine copper particles that have an average particle size of no more than 300 nm, coarse copper particles that have an average particle size of 3–11 μm, a binder resin, and a dispersion medium, there being 0.1–2.0 parts by mass of the binder resin per 100 total parts by mass of the fine copper particles and the coarse copper particles; a conductive film–coated substrate that comprises a substrate and a sintered product of the conductive paste that is provided on the substrate; and a production method for the conductive film–coated substrate that involves providing a film that includes the conductive paste on the substrate and then sintering the film.

Description

[Technical Field]

[0001] The present invention relates to a conductive paste, a substrate with a conductive film, and a method for manufacturing a substrate with a conductive film. [Background technology]

[0002] Conductive film-coated substrates, such as polyethylene terephthalate (PET) film, polyimide (PI) film, paper, and glass, with conductive wiring patterns formed on them, are industrially used as wiring boards for RF tags, pressure sensors, and the like. A common method for forming the wiring patterns is to deposit copper onto the substrate, or to laminate the substrate with copper foil, and then form the wiring pattern by etching or the like.

[0003] In recent years, with the development of AI and IoT technologies, the importance of sensor materials has increased, and there is a demand for lower costs and mass production of wiring patterns. Forming wiring patterns by etching processes is industrially disadvantageous in terms of cost, productivity, and environmental impact. Therefore, there is growing expectation for printed electronics as a simpler method for forming wiring patterns. In printed electronics, for example, a conductive paste is printed onto a substrate in a pattern, and then a conductive film is formed on the substrate by heat treatment.

[0004] Examples of conductive pastes that can be used in printed electronics include (1) and (2) below. (1) A mixture containing copper fine particles with an average particle diameter of 300 nm or less, coarse copper particles with an average particle diameter of 3 to 11 μm, a binder resin, and a dispersion medium, wherein the copper fine particles have a coating containing cuprous oxide and copper carbonate on at least a portion of their surface, and the ratio of mass oxygen concentration to the specific surface area of ​​the copper fine particles is 0.1 to 1.2 mass%·g / m² 2 The ratio of mass carbon concentration to specific surface area of ​​the copper fine particles is 0.008 to 0.3 mass%·g / m². 2A conductive paste (Patent Document 1) wherein the binder resin content is 2.5 to 6 parts by mass per 100 parts by mass of the total of the copper fine particles and the copper coarse particles. (2) Copper nanoparticles with an average particle size of 10 to 100 nm and the cumulative 50% particle size (D) based on volume measured by a laser diffraction particle size distribution analyzer. 50 ) contains copper coarse particles measuring 4-25 μm, with a tap density of copper coarse particles of 3.9 g / cm³. 3 The following are volume-based cumulative 10% particle size (D) measured by a laser diffraction particle size distribution analyzer. 10 ) for cumulative 90% particle size (D 90 A conductive paste (Patent Document 2) characterized in that the ratio of copper particles to copper coarse particles is 3.65 or higher, and the ratio of the mass of copper fine particles to the total amount of copper fine particles to copper coarse particles is 20% or higher. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-119737 [Patent Document 2] Japanese Patent Publication No. 2017-69012 [Overview of the project] [Problems that the invention aims to solve]

[0006] Conductive films, which feature wiring patterns formed using conductive paste, require further improvements in conductivity. To improve conductivity, it is effective to adjust the sintering conditions to sufficiently remove the binder resin and enhance the sinterability of the copper particles. However, when the printed patterns formed with the conductive pastes of (1) and (2) are sintered with an irradiation energy sufficient to remove the binder resin, there is a problem that copper particles scatter onto the substrate and the conductive film tends to collapse. Thus, if the irradiation energy is not properly adjusted, it is difficult to remove the binder resin sufficiently and difficult to improve the sinterability, so there is room for improvement in the conductivity of the conductive film. Furthermore, according to the inventors' research, the conductive film obtained from the conductive paste of (2) tends to have a porous structure, resulting in insufficient conductivity.

[0007] The present invention provides a conductive paste that can form a conductive film with excellent conductivity, in which copper fine particles are less likely to scatter even when fired with irradiation energy that can sufficiently remove the binder resin, a substrate with a conductive film using the conductive paste, and a method for manufacturing the substrate with a conductive film. [Means for solving the problem]

[0008] The present invention provides the following conductive paste, a substrate with a conductive film using the conductive paste, and a method for manufacturing the substrate with a conductive film. [1] A conductive paste comprising copper fine particles having an average particle diameter of 300 nm or less, copper coarse particles having an average particle diameter of 3 to 11 μm, a binder resin, and a dispersion medium, wherein the content of the binder resin is 0.1 to 2.0 parts by mass per 100 parts by mass of the total of the copper fine particles and copper coarse particles. [2] Mass ratio of the copper coarse particles to the copper fine particles (copper coarse Particle mass / copper slight A conductive paste [1] in which the particle mass is 30 / 70 to 90 / 10. [3] Mass ratio of the copper coarse particles to the copper fine particles (copper coarse Particle mass / copper slight A conductive paste of [1] or [2], where the particle mass is 40 / 60 to 90 / 10. [4] A conductive paste of any of [1] to [3] wherein the binder resin contains polyvinylpyrrolidone. [5] The conductive paste according to any one of [1] to [4], wherein the dispersion medium contains at least one selected from the group consisting of ethylene glycol and diethylene glycol. [6] The copper fine particles have a film containing cuprous oxide and copper carbonate on at least a part of the surface, and the ratio of the mass oxygen concentration to the specific surface area of the copper fine particles is 0.1 to 1.2 mass%·g / m 2 and the ratio of the mass carbon concentration to the specific surface area of the copper fine particles is 0.008 to 0.3 mass%·g / m 2 The conductive paste according to any one of [1] to [5]. [7] A substrate with a conductive film, comprising a substrate and a sintered product of the conductive paste according to any one of [1] to [6] provided on the substrate. [8] A method for manufacturing a substrate with a conductive film, comprising a step of providing a film containing the conductive paste according to any one of [1] to [6] on a substrate and a step of subjecting the film to a sintering treatment. [9] The method for manufacturing a substrate with a conductive film according to [8], wherein the sintering treatment is light firing. [Advantages of the Invention]

[0009] According to the present invention, there are provided a conductive paste in which copper fine particles are hardly scattered even when fired with irradiation energy capable of sufficiently removing a binder resin and a conductive film excellent in conductivity can be formed, a substrate with a conductive film using the conductive paste, and a method for manufacturing a substrate with a conductive film. [Brief Description of the Drawings]

[0010] [Figure 1] It is a plan view of a wiring pattern used for measuring the specific resistance of an example. [Modes for Carrying Out the Invention]

[0011] In this specification, "~" indicating a numerical range means including the numerical values described before and after it as a lower limit value and an upper limit value. In this specification, the average particle diameter means the average primary particle diameter obtained by the following measurement method.

[0012] <Conductive paste> The conductive paste of the present invention contains copper fine particles with an average particle diameter of 300 nm or less, copper coarse particles with an average particle diameter of 3 to 11 μm, a binder resin, and a dispersion medium. The conductive paste of the present invention may further contain any components other than copper fine particles, copper coarse particles, binder resin, and dispersion medium, as long as they do not impair the effects of the present invention. The following describes copper nanoparticles, copper coarse particles, binder resin, dispersion medium, and optional components in order.

[0013] (copper fine particles) The average particle size of the copper nanoparticles is 300 nm or less. Preferably, the average particle size of the copper nanoparticles is 200 nm or less. Because the average particle size of the copper nanoparticles is 300 nm or less, the copper nanoparticles exhibit excellent sinterability. Furthermore, the sintering temperature of the conductive paste can be lowered. The average particle size of the copper nanoparticles is preferably 50 nm or larger, and more preferably 100 nm or larger. When the lower limit of the copper nanoparticle size is greater than or equal to the upper limit, the amount of gas generated during the sintering of the conductive paste is relatively reduced, which can reduce cracks and other fissures when it is formed into a conductive film. Therefore, the average particle size of the copper nanoparticles is preferably 50 to 300 nm, and more preferably 100 to 200 nm.

[0014] The average particle size of copper nanoparticles was determined by measuring the particle size of each of the 250 copper nanoparticles (total of 2500 particles across 10 fields of view) observed using a scanning electron microscope (SEM), and taking the arithmetic mean of these measurements. The selection criteria for the particles to be measured from the particles visible in the scanning electron microscope image (photograph) are as follows (1) to (6). (1) Particles whose portion extends outside the field of view of the photograph will not be measured. (2) Particles with clear outlines that exist in isolation should be measured. (3) Even if a particle deviates from the average particle shape, if it is independent and can be measured as a single particle, it will be measured. (4) Particles that overlap with each other but have a clear boundary between the two and whose overall shape can be determined are measured with each particle as an individual particle. (5) Overlapping particles with unclear boundaries and an undeterminable overall shape are not measured as those whose particle shape cannot be determined. (6) For particles that are not perfect circles such as ellipses, the major axis is taken as the particle diameter.

[0015] The copper fine particles preferably have a film containing cuprous oxide and copper carbonate on at least a part of the surface. When the copper fine particles contain copper carbonate in the film, the sintering temperature of the copper fine particles can be further lowered. It is considered that the lower the content of copper carbonate in this film, the lower the sintering temperature. The ratio of the mass carbon concentration to the specific surface area of the copper fine particles is preferably 0.008 to 0.3 mass%·g / m 2 and more preferably 0.008 to 0.020 mass%·g / m. 2 When the ratio of the mass carbon concentration to the specific surface area of the copper fine particles is 0.008 to 0.3 mass%·g / m 2 the sintering temperature of the copper particles can be set even lower, and the copper fine particles can be sintered at a lower temperature. The ratio of the mass carbon concentration to the specific surface area of the copper fine particles can be calculated from the measured specific surface area and mass carbon concentration respectively. The specific surface area can be measured using a BET adsorption apparatus for nitrogen gas (for example, "MACSORB HM-1201" manufactured by Mountech Co., Ltd.). The mass carbon concentration can be measured using a carbon sulfur analyzer (for example, "EMIA-920V" manufactured by Horiba, Ltd.).

[0016] When the copper fine particles have a film containing cuprous oxide and copper carbonate on at least a part of the surface, the ratio of the mass oxygen concentration to the specific surface area of the copper fine particles is preferably 0.1 to 1.2 mass%·g / m 2 and more preferably 0.2 to 0.5 mass%·g / m. 2 This is more preferable. When the ratio of the mass oxygen concentration to the specific surface area of the copper fine particles is 0.1 mass%·g / m 2The above conditions result in high chemical stability of the copper nanoparticles, making combustion, heat generation, and other phenomena less likely to occur. The ratio of mass oxygen concentration to specific surface area of ​​the copper nanoparticles is 1.2 mass%·g / m². 2 The following conditions result in less copper oxide and easier sintering of copper nanoparticles. As a result, the sintering temperature of the conductive paste decreases. Here, since the surface of copper nanoparticles is oxidized by air in the atmosphere, and an oxide film is inevitably formed, the lower limit of the ratio of mass oxygen concentration to the specific surface area of ​​copper nanoparticles is 0.1%·g / m². 2 That is the case. The ratio of mass oxygen concentration to specific surface area of ​​copper nanoparticles can be measured using an oxygen-nitrogen analyzer (for example, LECO's "TC600").

[0017] Copper nanoparticles can be produced by the manufacturing method described in Japanese Patent Publication No. 2018-127657. For example, by adjusting the amount of carbon in the fuel gas supplied to the burner, the ratio of mass carbon concentration to the specific surface area of ​​copper particles can be changed to 0.008 to 0.3 mass%·g / m². 2 It can be controlled.

[0018] (Coarse copper particles) Coarse copper particles are copper particles with an average particle diameter of 3 to 11 μm. Preferably, the average particle diameter of the coarse copper particles is 3 to 7 μm. Because the average particle size of the coarse copper particles is 3 μm or larger, the shrinkage of the copper fine particles during sintering is reduced, thereby reducing cracks and other fissures when a conductive film is formed. Furthermore, because the average particle size of the coarse copper particles is 11 μm or smaller, the conductive paste can be sufficiently sintered while maintaining the effect of reducing the shrinkage of the copper fine particles. As a result, a conductive film with excellent conductivity can be formed.

[0019] The average particle size of coarse copper particles was determined by measuring the particle size of each of the 250 copper nanoparticles (total of 2500 particles across 10 fields of view) observed using a scanning electron microscope (SEM), and taking the arithmetic mean of these measurements. The selection criteria for the particles to be measured from the particles visible in the scanning electron microscope image (photograph) are as follows (1) to (6). (1) Particles whose portion extends outside the field of view of the photograph will not be measured. (2) Particles with clear outlines that exist in isolation should be measured. (3) Even if a particle deviates from the average particle shape, if it is independent and can be measured as a single particle, it will be measured. (4) If there is overlap between particles, but the boundary between them is clear and the overall shape of the particles can be determined, each particle will be measured as a single particle. (5) Particles that overlap and whose boundaries are unclear, and whose overall shape cannot be determined, will not be measured as their shape cannot be determined. (6) For particles that are not perfectly round, such as ellipses, the major axis shall be defined as the particle diameter.

[0020] The shape of the copper coarse particles is preferably flattened into flakes. When flattened copper coarse particles are used, the density of the film after the conductive paste is applied to the substrate and dried becomes lower, making it easier for gases generated during sintering to escape. Therefore, cracks and other fissures are less likely to occur when it is formed into a conductive film.

[0021] The tap density of coarse copper particles is 2-6 g / cm³. 3 Preferably, 4-6 g / cm³ 3 This is preferable. The tap density of coarse copper particles is 2 g / cm³. 3 With the above conditions, the conductive paste can be sintered more thoroughly while maintaining the effect of reducing the shrinkage of copper nanoparticles, and the conductivity of the conductive film is further improved. The tap density of coarse copper particles is 6 g / cm³. 3 If the following conditions are met, the density of the film after applying the conductive paste to the substrate and drying will be lower, allowing gases generated during sintering to escape more easily. As a result, cracks and other fissures are less likely to occur when the conductive film is formed. Tap density of coarse copper particles (g / cm³) 3 ) can be measured using a tap density meter (for example, "KYT-4000" manufactured by Seishin Corporation).

[0022] (dispersion medium) The dispersion medium is not particularly limited as long as it is a compound capable of dispersing copper fine particles or coarse copper particles. Examples include water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (IPA), and terpineol; polyols such as ethylene glycol, diethylene glycol, and triethylene glycol; and polar media such as N,N-dimethylformamide (DMF) and N-methylpyrrolidone (NMP). These dispersion media may be used individually or in combination of two or more. Among these, a dispersion medium containing at least one selected from the group consisting of ethylene glycol and diethylene glycol is preferred due to the reducing effect of copper fine particles.

[0023] (Binder resin) The binder resin is not particularly limited as long as it is a compound that can impart appropriate viscosity to the conductive paste and provide adhesion to the substrate when formed as a conductive film. Examples of binder resins include cellulose derivatives such as carboxycellulose, ethylcellulose, cellulose ether, carboxyethylcellulose, aminoethylcellulose, oxyethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, benzylcellulose, and trimethylcellulose; acrylic polymers such as copolymers of acrylic monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, benzyl (meth)acrylate, hydroxyethyl (meth)acrylate, dimethylaminoethyl acrylate, acrylic acid, and methacrylic acid; and nonionic surfactants such as polyvinyl alcohol and polyvinylpyrrolidone. However, binder resins are not limited to these examples.

[0024] Among these, polyvinylpyrrolidone is preferred as the binder resin because it improves the dispersibility of copper nanoparticles. Here, in addition to its function as a binder resin, polyvinylpyrrolidone can also function as a dispersant for copper nanoparticles and coarse copper particles. When polyvinylpyrrolidone is used as the binder resin, the dispersibility of copper nanoparticles is improved, and the need for a separate dispersant is eliminated. As a result, the number of components in the conductive paste can be reduced. Therefore, there are fewer components that can affect two properties of copper nanoparticles: sinterability and adhesion to the substrate when formed as a conductive film.

[0025] (optional ingredient) Optional components include, for example, dispersants. Examples of dispersants include sodium hexametaphosphate and sodium β-naphthalene sulfonic acid formalin condensate. These dispersants may be used individually or in combination of two or more. As a dispersant, a compound that can be decomposed and removed during sintering is preferred.

[0026] (Content) The copper fine particle content is preferably 10 to 60% by mass, and more preferably 20 to 30% by mass, relative to 100% by mass of the total of copper fine particles and coarse copper particles. When the copper fine particle content is 10% by mass or more relative to the total mass of copper fine particles and coarse copper particles, the conductive paste can be sufficiently sintered, and the conductivity of the conductive film is further improved. If the copper nanoparticle content is 60% by mass or less relative to the total mass of copper nanoparticles and coarse copper particles, the shrinkage of the copper nanoparticles during sintering is further reduced, making it less likely for cracks to occur when it is used as a conductive film.

[0027] Mass ratio of coarse copper particles to fine copper particles (copper coarse Particle mass / copper slight The particle mass is preferably 30 / 70 to 90 / 10, and more preferably 40 / 60 to 90 / 10. When the mass ratio of coarse copper particles to fine copper particles is 30 / 70 or higher, the scattering of copper particles during the formation of the conductive film is reduced, and the conductive film is less likely to collapse even when the sintering conditions are made more stringent. When the mass ratio of coarse copper particles to fine copper particles is 90 / 10 or less, the conductive paste can be sufficiently sintered while maintaining the effect of reducing the shrinkage of the fine copper particles, resulting in even better conductivity of the conductive film.

[0028] The binder resin content is 0.1 to 2.0 parts by mass, more preferably 0.1 to 0.5 parts by mass, per 100 parts by mass of the total of copper fine particles and copper coarse particles. Since the binder resin content is 0.1 parts by mass or more per 100 parts by mass of the total of copper fine particles and coarse copper particles, good dispersibility of the copper fine particles and adhesion to the substrate are obtained, resulting in improved conductivity when used as a conductive film. Since the binder resin content is 2.0 parts by mass or less per 100 parts by mass of total copper fine particles and coarse copper particles, the amount of gas derived from the binder resin generated during sintering is reduced, making it less likely for cracks to occur in the conductive film and for copper particles to scatter during light firing, resulting in improved conductivity when used as a conductive film.

[0029] The content of the dispersion medium is preferably 15 to 30 parts by mass, and more preferably 17 to 25 parts by mass, per 100 parts by mass of the total of fine particles and coarse copper particles. When the content of the dispersion medium is above the lower limit, the dispersibility of the fine particles and coarse copper particles is excellent. When the content of the dispersion medium is below the upper limit, it is easy to form a conductive film with excellent conductivity.

[0030] (Effects and Benefits) In the conductive paste of the present invention described above, the binder resin content is 2.0 parts by mass or less per 100 parts by mass of total copper fine particles and coarse copper particles. Therefore, even when firing with irradiation energy sufficient to remove the binder resin, the amount of gas generated by thermal decomposition of the binder and solvent residue during firing is reduced. As a result, copper particles are less likely to scatter, and the conductive film is less likely to collapse on the substrate. More specifically, even when the irradiation energy is increased to achieve the lowest resistance value, copper particles are less likely to scatter, and the conductive film is less likely to collapse on the substrate. Consequently, the sinterability of the copper fine particles is improved, and a conductive film with excellent conductivity can be formed. Furthermore, since the binder resin content is 0.1 parts by mass or more per 100 parts by mass of total copper fine particles and coarse copper particles, the adhesion of the conductive film to the substrate is also sufficiently maintained.

[0031] As described above, the conductive paste of the present invention is such that copper particles are less likely to scatter even when fired with an irradiation energy sufficient to remove the binder resin, specifically when the irradiation energy is increased to achieve the lowest resistance value. Furthermore, it exhibits excellent sinterability between copper fine particles and between copper fine particles and coarse copper particles, and can achieve a resistivity of 10 μΩ·cm or less without performing post-processing such as pressing. Furthermore, because the conductive paste of the present invention has a low sintering temperature for copper nanoparticles, a conductive film can be formed on the substrate at a lower temperature than conventional products. As a result, the thermal load on the substrate during sintering is reduced compared to conventional products, improving the durability of the substrate with the conductive film.

[0032] (Manufacturing method) The conductive paste of the present invention can be manufactured, for example, by a manufacturing method comprising the following steps 1 and 2. Step 1: A step of pre-mixing copper fine particles, copper coarse particles, binder resin, dispersion medium, and dispersant as needed. Step 2: The preliminary kneaded paste obtained in Step 1 is dispersed using a disperser such as a three-roll mill or a bead mill.

[0033] In the preliminary mixing of step 1, a mixing machine such as a self-rotating mixer, a mixer, or a mortar and pestle can be used. Mixing may also be performed while degassing. In the dispersion process of step 2, if it is difficult to disperse the copper fine particles in the dispersion medium in a single dispersion process, multiple dispersion processes may be performed.

[0034] <Substrate with conductive film> The conductive film-coated substrate of the present invention comprises a substrate and a sintered product of the conductive paste of the present invention provided on the substrate. The substrate and the sintered product of the conductive paste will be described in order below.

[0035] The substrate is not particularly limited as long as it can withstand the sintering process. Examples include glass substrates; resin substrates containing resins such as polyamide, polyimide, polyethylene, epoxy resin, phenolic resin, polyester resin, polyethylene terephthalate (PET), and polyethylene naphthalate (PEN); paper substrates; and glass substrates such as glass substrates. Among these, polyimide, glass substrates, and paper substrates are preferred because they can withstand high irradiation energy that is sufficient to remove the binder resin.

[0036] The sintered conductive paste is thought to contain fused materials formed by the sintering of copper fine particles, fused materials formed by the sintering of copper coarse particles, and fused materials formed by the sintering of copper fine particles and copper coarse particles. Because the shapes of the copper fine particles and copper coarse particles change during sintering, it can be difficult to distinguish between these multiple types of fused materials after sintering. The binder resin and dispersion medium are vaporized and decomposed during the sintering process. Therefore, the binder resin and dispersion medium are not usually present in the sintered conductive paste. However, residues derived from the binder resin and dispersion medium may be present in the sintered conductive paste, as long as the effects of the invention are not impaired.

[0037] The conductive film, which is a sintered product applied to the substrate, is electrically conductive. The resistivity of the conductive film is preferably less than 15 μΩ·cm, more preferably less than 10 μΩ·cm, and even more preferably less than 8.0 μΩ·cm. A resistivity of less than 15 μΩ·cm indicates that the conductive film has excellent conductivity. The resistivity can be measured using CUSTOM's M-02N digital tester.

[0038] The thickness of the conductive film is preferably 5 to 30 μm, and more preferably 10 to 25 μm. A thickness of 5 μm or more results in a lower resistance. A thickness of 30 μm or less provides excellent adhesion of the conductive film to the substrate. The film thickness is determined by the method described in the examples.

[0039] As described above, the conductive film-coated substrate of the present invention comprises a sintered product of the conductive paste of the present invention, and therefore exhibits excellent conductivity of the conductive film. The conductive film-coated substrate of the present invention can be applied to applications such as printed circuit boards, wireless substrates such as RF tags, pressure-sensitive sensor sheets, and transparent conductive films.

[0040] (Method for manufacturing a substrate with a conductive film) The conductive film-coated substrate of the present invention can be manufactured by providing a film containing a conductive paste on a substrate, and then subjecting the film to a sintering treatment. For example, it can be manufactured by applying a conductive paste to a substrate to form a film containing the conductive paste on the substrate, and then subjecting the film containing the conductive paste to a sintering process. The method of applying the conductive paste to the substrate is not particularly limited. For example, various printing methods such as screen printing, inkjet printing, and gravure printing can be used. The method of applying the conductive paste is not limited to these examples.

[0041] Through a sintering process, copper nanoparticles are sintered together, and a conductive film possessing electrical conductivity is formed on the substrate. In conventional conductive pastes, when fired with irradiation energy sufficient to remove the binder resin, for example, increasing the irradiation energy during the sintering process releases decomposition gases from the binder and dispersant, resulting in the scattering of copper particles, cracking, and the generation of numerous voids. This makes it difficult to increase the irradiation energy to achieve the lowest resistance value and to increase conductivity. As a result, organic matter remains, and the sinterability is insufficient, leading to insufficient conductivity. According to the inventor's research, the light irradiation energy for the conductive paste in Patent Document 1 is 5 J / cm². 2 It can only be raised to a certain extent. In contrast, in the present invention, for example, 7.65 J / cm² is used to facilitate sintering. 2 Even with the above irradiation energy, the amount of decomposition gas generated can be suppressed. As a result, a highly sinterable sintered film can be obtained, and a conductive film with excellent conductivity can be formed.

[0042] The sintering process is not particularly limited as long as it can sinter the copper fine particles in the conductive paste. Examples of sintering processes include heat firing and light firing. Among these, light firing is preferred because it allows for sufficient removal of the binder resin and facilitates the formation of a conductive film with even better conductivity. Specific examples include, for example, a method of sintering a substrate on which a film containing conductive paste is provided by firing at a high temperature; a method of sintering a film containing conductive paste by irradiating it with light such as a laser; and photolithography. The specific embodiments of the sintering process are not limited to these examples.

[0043] (Light firing) The conditions for light firing can be adjusted according to the composition of the conductive paste by, for example, using a device equipped with a xenon lamp, and by adjusting the lamp output and irradiation time. By increasing the output energy and extending the irradiation time, the sample temperature can be raised, making it easier to sinter copper nanoparticles with each other or copper nanoparticles with coarse copper particles.

[0044] The output during light firing is preferably 350V to 450V, and more preferably 400V to 440V. When the output is above the lower limit, the binder resin is easily removed sufficiently, and the sinterability of the copper particles is easily improved. As a result, a conductive film with even better conductivity can be formed. When the output is below the upper limit, the copper particles are less likely to scatter, and the conductive film is less likely to collapse. This is also advantageous in terms of cost.

[0045] The irradiation time for light firing is preferably, for example, 3000 μS to 60000 μS, and more preferably 3500 μS to 10000 μS. If the irradiation time is above the lower limit, the binder resin is easily removed sufficiently, and the sinterability of the copper particles is easily improved. As a result, a conductive film with even better conductivity can be formed. If the irradiation time is below the upper limit, the copper particles are less likely to scatter, and the conductive film is less likely to collapse. Furthermore, industrial mass production capabilities are also improved.

[0046] The irradiation energy for light firing is, for example, 7.65 to 16 J / cm². 2 Preferably, 8.5~13 J / cm 2 This is more preferable. When the irradiation energy is above the lower limit, the binder resin is easily removed sufficiently, and the sinterability of the copper particles is easily improved. As a result, a conductive film with even better conductivity can be formed. When the irradiation energy is below the upper limit, the copper particles are less likely to scatter, and the conductive film is less likely to collapse. It is also advantageous in terms of cost.

[0047] (Heating and baking) The heating and firing conditions can also be adjusted according to the composition of the conductive paste. By increasing the processing temperature and extending the processing time, it becomes easier to sinter copper fine particles with each other, or copper fine particles with copper coarse particles.

[0048] The processing temperature during heating and firing can be set according to the heat resistance of the substrate. For example, 200 to 400°C is preferred, and 250 to 300°C is more preferred. If the processing temperature is above the lower limit, the binder resin can be easily removed sufficiently, and the sinterability of the copper particles can be easily improved. As a result, a conductive film with even better conductivity can be formed. If the processing temperature is below the upper limit, cracks are less likely to occur in the conductive film, and substrate deformation is also reduced. Furthermore, it is advantageous in terms of cost.

[0049] The heating and firing process time is preferably 5 to 120 minutes, and more preferably 15 to 60 minutes. If the processing time is above the lower limit, the binder resin can be easily removed sufficiently, and the sinterability of the copper particles can be improved. As a result, a conductive film with even better conductivity can be formed. If the processing time is below the upper limit, cracks are less likely to occur in the conductive film, and substrate deformation is also reduced. Furthermore, industrial mass production capabilities are improved. [Examples]

[0050] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following description.

[0051] <Material> (copper fine particles) Copper nanoparticles were manufactured using the manufacturing method described in Japanese Patent Publication No. 2018-127657. These copper nanoparticles were used in all examples. The average particle size of the copper nanoparticles was 110 nm, and the specific surface area was 5.602 m². 2 The mass oxygen concentration was 1.1204% and the mass carbon concentration was 0.119883%. Based on these measurement results, the mass oxygen concentration relative to the specific surface area was calculated to be 0.200 mass%·g / m². 2 The carbon-mass concentration relative to the specific surface area is 0.0214 mass%·g / m³. 2 That was the case.

[0052] (Coarse copper particles) MA-C03KP: Product name manufactured by Mitsui Mining & Smelting Co., Ltd. (average particle size 3.8 μm, tap density 5.26 g / cm³) 3 ). FCC-TB: Product name manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd. (average particle size 6.22 μm, tap density 2.57 g / cm³) 3 ). MA-C03K: Product name manufactured by Mitsui Mining & Smelting Co., Ltd. (average particle size 3.21 μm, tap density 5.00 g / cm³) 3 ).

[0053] (Binder resin) In all examples, polyvinylpyrrolidone (PVP, "K-85N" manufactured by Nippon Shokubai Co., Ltd.) was used as the binder resin.

[0054] (dispersion medium) In all examples, ethylene glycol (EG) was used as the dispersion medium.

[0055] <Example 1> 2.4g of copper fine particles, 5.6g of copper coarse particles, 0.16g of PVP, and 1.86g of EG were pre-mixed using a mixing machine (AR-100, manufactured by Thinky Co., Ltd.) to obtain a pre-mixed paste. The obtained pre-mixed paste was subjected to dispersion treatment using a three-roll disperser (BR-100V, manufactured by AIMEX Co., Ltd.) to prepare a conductive paste. Next, a conductive paste was applied to a polyimide (PI) film (thickness: 50 μm, Kapton film "200EN" manufactured by Toray DuPont Co., Ltd.) by screen printing to form a wiring pattern. The wiring pattern had a width of 1 mm, and the line length was obtained by cutting a 124 mm RF tag pattern wiring in half. Subsequently, the conductive paste was sintered using a photo-sintering apparatus ("PulseForge Invent" manufactured by Novacentrix), obtaining a PI film with a conductive film forming a wiring pattern with a width of 1 mm and a line length of 124 mm. The photo-sintering was performed under relatively strong irradiation conditions that facilitate sintering of copper nanoparticles with each other and copper nanoparticles with copper coarse particles: output of 350-450 V, irradiation time of 3000 μS or more, and irradiation energy of 7.65 J / cm². 2 The procedure was carried out within the scope described above, and a conductive film was formed on the PI film. Here, when the type of substrate or paste composition is changed during light firing, the way in which the pyrolysis gases released and the instantaneous amount of pyrolysis gases generated during light firing change, so there are optimal light firing conditions for each substrate and paste composition. In this example, the optimization of the light firing conditions was carried out as follows. (1) Fix the irradiation time at 4000 μS and optimize the irradiation output. (2) Optimize the irradiation time under the optimal output obtained in (1). The irradiation energy is automatically calculated by the simulation software within the device once the output and irradiation time are determined. Among the fired products produced in the above study, the firing conditions that resulted in the lowest resistance value were defined as the optimal light firing conditions.

[0056] <Examples 2-14, 16, 17, Comparative Examples 1-3> The conductive paste for each example was prepared in the same manner as in Example 1, except that the composition of the conductive paste was changed as shown in Table 1 or Table 2, and a conductive film was formed on the PI film. Examples 1 and 12 are for reference only.

[0057] <Example 15> A conductive film was formed on the substrate by heating and firing using a conductive paste with the composition of Example 10. For firing, a Unitemp reflow oven was used. The sample was pre-oxidized in air at 250°C for 30 minutes, then 3% H2 gas was flowed through it for 30 minutes while maintaining the temperature at 250°C. After that, the gas was switched to N2, and the sample was cooled to room temperature before being removed and the conductive film was formed on the PI film.

[0058] <Measurement method> (specific resistance) The conductivity of the conductive film in each example was evaluated by measuring the resistance using wiring pattern 1 shown in Figure 1. Within a 124mm length of wiring pattern 1, point A was fixed, and the resistance values ​​were measured at 22mm between A and B, 44mm between A and C, 66mm between A and D, 88mm between A and E, 110mm between A and B, and 124mm between A and G. A CUSTOM M-02N digital multimeter was used to measure the resistance. Subsequently, the line length was plotted on the horizontal axis and the resistance value on the vertical axis. The slope of the linear function fitting each plot was determined, and this slope was defined as the surface resistance.

[0059] (Thickness of conductive film) The thickness of the conductive film in each example was measured at five locations using a laser microscope (Keyence Corporation's "VK-X"), and the average value was calculated. The resistivity was calculated by multiplying the surface resistance by the average film thickness.

[0060] [Table 1]

[0061] [Table 2]

[0062] In Tables 1 and 2, "Cu concentration (%)" represents the "ratio of the total amount of copper fine particles and copper coarse particles to 100 parts by mass of conductive paste," and was calculated using the following formula. (Cu concentration) (%) = (Mass of copper nanoparticles + Mass of copper coarse particles) × 100 / (Mass of copper nanoparticles + Mass of copper coarse particles + Mass of solvent + Mass of binder + Mass of dispersant)

[0063] <Result> In Examples 1-14, even when fired with irradiation energy sufficient to remove the binder resin, copper fine particles did not scatter onto the substrate, and the conductive film did not collapse. Furthermore, a conductive film with excellent conductivity was formed. As shown in Example 15, no deformation of the substrate or cracking of the conductive film was observed even when heat firing was performed. Furthermore, a conductive film with excellent conductivity was formed. In Examples 16 and 17, copper nanoparticles did not scatter onto the substrate, and the conductive film did not collapse. Because the proportion of copper nanoparticles was higher compared to other examples, sinterability was good, but some cracking occurred in the fired film. Nevertheless, the resistivity was 15 μΩ·cm, indicating sufficient conductivity. Even in circuits where cracking occurred, the impact of the cracks can be reduced by performing additional processing (rework) to improve film adhesion. In Comparative Example 1, the binder resin content was too high, resulting in numerous craters and cracks in the fired film, presumably due to the escape of decomposition gases. Copper particles were scattered, and the conductive film was also disintegrated. Furthermore, the resistance value was OVERLOAD. In Comparative Example 2, since no binder resin was used, there was no adhesion between the substrate and the conductive film, the fired film peeled off the substrate, and a conductive film could not be obtained on the substrate. In Comparative Example 3, since copper fine particles were not used, sintering between the coarse copper particles was difficult, resulting in a resistivity of 15 μΩ·cm or higher and insufficient conductivity. [Industrial applicability]

[0064] The present invention provides a conductive paste that can form a conductive film with excellent conductivity, even when fired with irradiation energy sufficient to remove the binder resin, in which copper fine particles are less likely to scatter; a substrate with a conductive film using the conductive paste; and a method for manufacturing the substrate with a conductive film. [Explanation of symbols]

[0065] 1. Wiring Pattern Points A-G on the wiring pattern

Claims

1. Copper nanoparticles with an average particle diameter of 300 nm or less, Coarse copper particles with an average particle size of 3 to 11 μm, Binder resin and It contains a dispersion medium, The content of the binder resin is 0.1 to 0.5 parts by mass per 100 parts by mass of the total of the copper fine particles and the copper coarse particles. A conductive paste in which the mass ratio of the coarse copper particles to the fine copper particles (mass of coarse copper particles / mass of fine copper particles) is 30 / 70 to 90 / 10.

2. The conductive paste according to claim 1, wherein the mass ratio of the coarse copper particles to the fine copper particles (mass of coarse copper particles / mass of fine copper particles) is 40 / 60 to 90 / 10.

3. The conductive paste according to claim 1, wherein the binder resin contains polyvinylpyrrolidone.

4. The conductive paste according to claim 1, wherein the dispersion medium comprises at least one selected from the group consisting of ethylene glycol and diethylene glycol.

5. The copper fine particles have a coating containing cuprous oxide and copper carbonate on at least a portion of their surface. The ratio of mass oxygen concentration to specific surface area of ​​the copper nanoparticles is 0.1 to 1.2 mass%·g / m². ２ And, The ratio of mass carbon concentration to specific surface area of ​​the copper nanoparticles is 0.008 to 0.3 mass%·g / m². ２ The conductive paste according to claim 1.

6. Substrate and A substrate with a conductive film, comprising a sintered product of a conductive paste according to any one of claims 1 to 5 provided on the substrate.

7. A method for manufacturing a substrate with a conductive film, comprising the steps of: providing a film containing a conductive paste according to any one of claims 1 to 5 on a substrate; and subjecting the film to a sintering treatment.

8. The method for manufacturing a conductive film-coated substrate according to claim 7, wherein the sintering process is photocatalysis.

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