Conductive paste, substrate with conductive film, and process for producing substrate with conductive film

Abstract

The present invention aims to provide a conductive paste, a substrate with a conductive film using the aforementioned conductive paste, and a method for manufacturing the substrate with the conductive film. The conductive paste is designed to effectively remove the irradiation energy of the binder resin even when irradiated and then fired, preventing copper microparticles from easily splashing and forming a conductive film with excellent conductivity. Furthermore, the present invention provides a conductive paste, a substrate with a conductive film, and a method for manufacturing the substrate with the conductive film. The conductive paste contains copper microparticles with an average particle size of 300 nm or less, coarse copper particles with an average particle size of 3 to 11 μm, a binder resin, and a dispersion medium. The binder resin content is 0.1 to 2.0 parts by mass relative to 100 parts by mass of the total copper microparticles and coarse copper particles. The substrate with the conductive film includes a substrate and a sintered product of the aforementioned conductive paste disposed on the substrate. The method for manufacturing the substrate with the conductive film involves disposing a film containing the aforementioned conductive paste on the substrate and then subjecting the film to a sintering treatment.

Description

Technical Field

[0001] This invention relates to a conductive paste, a substrate with a conductive film, and a method for manufacturing the substrate with the conductive film. Prior Technology

[0002] Substrates with conductive films, such as polyethylene terephthalate (PET) film, polyimide (PI) film, paper, and glass, are used industrially as wiring substrates for RF tags, pressure sensors, etc. Common methods for forming wiring patterns include etching after copper is deposited on the substrate or after bonding the substrate and copper foil.

[0003] In recent years, with the development of AI and IoT technologies, the importance of sensor materials has gradually increased, leading to a demand for lower cost and mass production of wiring patterns. From the perspectives of cost, productivity, and environment, forming wiring patterns using etching processes is industrially disadvantageous. Therefore, there is a growing expectation for printed electronics as a simpler method for forming wiring patterns. In printed electronics, for example, a conductive paste pattern is printed on a substrate, and then heat-treated to form a conductive film on the substrate.

[0004] Conductive paste systems that can be used in printed electronics, for example, have been proposed as (1) and (2) below. (1) A conductive paste comprising copper microparticles with an average particle size of less than 300 nm, copper coarse particles with an average particle size of 3 to 11 μm, a binder resin, and a dispersion medium, wherein at least a portion of the aforementioned copper microparticles has a film comprising cuprous oxide and copper carbonate on its surface, the mass oxygen concentration is 0.1 to 1.2 wt% g / m2 relative to the specific surface area of ​​the aforementioned copper microparticles, the mass carbon concentration is 0.008 to 0.3 wt% g / m2 relative to the specific surface area of ​​the aforementioned copper microparticles, and the binder resin content is 2.5 to 6 wt% relative to 100 parts by mass of the aforementioned copper microparticles and the aforementioned copper coarse particles (Patent Document 1). (2) A conductive paste comprising copper fine particles with an average particle diameter of 10 to 100 nm and copper coarse particles with a volume-based cumulative 50% particle diameter (D50) of 4 to 25 μm measured by a laser diffraction particle size distribution measuring device. Among them, the tapped density of the copper coarse particles is 3.9 g / cm3 or less, and the ratio of the volume-based cumulative 90% particle diameter (D90) to the cumulative 10% particle diameter (D10) measured by the laser diffraction particle size distribution measuring device is 3.65 or more. The proportion of the mass of the copper fine particles relative to the total amount of the copper fine particles and the copper coarse particles is 20% or more (Patent Document 2). [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2020-119737 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-69012 Problems to be Solved by the Invention

[0006] In a conductive film having a wiring pattern formed using a conductive paste, further improvement in conductivity is required. In order to improve conductivity, it is effective to sufficiently remove the binder resin by adjusting the sintering treatment conditions and improve the sinterability of the copper particles. However, if the printed pattern formed from the conductive paste of (1) and (2) is sintered with irradiation energy that can sufficiently remove the binder resin, there is a problem that copper particles splash onto the substrate and the conductive film is likely to break down. Thus, if the irradiation energy is not appropriately adjusted, it is difficult to sufficiently remove the binder resin and difficult to improve the sinterability, so there is still room for improvement in the conductivity of the conductive film. Also, according to the research of the present inventors, the conductive film obtained from the conductive paste of (2) is likely to have a porous structure, so the conductivity is insufficient.

[0007] The present invention provides a conductive paste, a substrate with a conductive film using the aforementioned conductive paste, and a method for manufacturing a substrate with a conductive film, wherein even when the conductive paste is fired with irradiation energy that can sufficiently remove the binder resin, the copper fine particles are not easily splashed, and a conductive film with excellent conductivity can be formed. Means for Solving the Problems

[0008] The present invention provides the following conductive paste, a substrate with a conductive film using the aforementioned conductive paste, and a method for manufacturing a substrate with a conductive film. ​[1] A conductive paste 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. Among them, relative to a total of 100 parts by mass of the aforementioned copper fine particles and the aforementioned copper coarse particles, the content of the aforementioned binder resin is 0.1 to 2.0 parts by mass. [2] The conductive paste as described in [1], wherein the mass ratio of the aforementioned copper coarse particles to the aforementioned copper fine particles (mass of copper coarse particles / mass of copper fine particles) is 30 / 70 to 90 / 10. [3] The conductive paste as described in [1] or [2], wherein the mass ratio of the aforementioned copper coarse particles to the aforementioned copper fine particles (mass of copper coarse particles / mass of copper fine particles) is 40 / 60 to 90 / 10. [4] The conductive paste as described in any one of [1] to [3], wherein the aforementioned binder resin contains polyvinylpyrrolidone. [5] The conductive paste as described in any one of [1] to [4], wherein the aforementioned dispersion medium contains at least one or more selected from the group consisting of ethylene glycol and diethylene glycol. [6] The conductive paste as described in any one of [1] to [5], wherein at least a part of the surface of the aforementioned copper fine particles has a film containing cuprous oxide and copper carbonate, and the ratio of the mass oxygen concentration to the specific surface area of the aforementioned copper fine particles is 0.1 to 1.2 mass%·g / m2, and the ratio of the mass carbon concentration to the specific surface area of the aforementioned copper fine particles is 0.008 to 0.3 mass%·g / m2. [7] A substrate with a conductive film includes a substrate and a sintered product of the conductive paste as described in any one of [1] to [6] provided on the aforementioned substrate. [8] A method for manufacturing a substrate with a conductive film includes a step of setting a film containing the conductive paste as described in any one of [1] to [6] on a substrate, and a step of subjecting the aforementioned film to a sintering treatment. [9] The method for manufacturing a substrate with a conductive film as described in [8], wherein the aforementioned sintering treatment is photo-sintering. Advantages of the Invention

[0009] According to the present invention, there can be provided a conductive paste, a substrate with a conductive film using the aforementioned conductive paste, and a method for manufacturing a substrate with a conductive film. In the conductive paste, even when sintered with irradiation energy sufficient to completely remove the binder resin, the copper fine particles are not easily splashed, and a conductive film with excellent conductivity can be formed. Brief Description of the Drawings

[0010] Figure 1 is a plan view of the wiring pattern used in the measurement of resistivity (also known as specific resistance) in the embodiment. Implementation

[0011] In this specification, the term "to" to indicate a range of values ​​means including the values ​​recorded before and after it as the lower limit and upper limit. In this specification, the term "average particle size" refers to the average primary particle size obtained by the following measurement method.

[0012] <Conductive Paste> The conductive paste of the present invention contains copper microparticles with an average particle size of less than 300 nm, copper coarse particles with an average particle size of 3 to 11 μm, binder resin, and dispersion medium. The conductive paste of the present invention may contain any components other than copper microparticles, coarse copper particles, binder resin, and dispersion medium, provided that the effect of the present invention is not impaired. The following sections describe, in order: copper microparticles, copper coarse particles, binder resin, dispersion medium, and any other components.

[0013] (Copper microparticles) The average particle size of copper microparticles is below 300 nm. Ideally, the average particle size should be below 200 nm. Because the average particle size of copper microparticles is below 300 nm, they exhibit excellent sintering properties. Furthermore, the sintering temperature of conductive paste can be lowered. The average particle size of copper microparticles is preferably 50 nm or higher, and even more preferably 100 nm or higher. If the average particle size of copper microparticles is above the aforementioned lower limit, the amount of gas generated during the sintering of the conductive paste is relatively reduced, which can reduce cracks such as crazing when making conductive films. Based on the above, the average particle size of copper microparticles is preferably 50 to 300 nm, and even more preferably 100 to 200 nm.

[0014] Regarding the average particle size of copper microparticles, the particle size of each copper microparticle was measured for 250 (totaling 2500 particles in 10 fields of view) observed using a scanning electron microscope (SEM), and the arithmetic mean of these particles was taken as the average particle size of the copper microparticles. The selection criteria for the particles measured from the particles captured in the scanning electron microscope images (photographs) are as described in (1) to (6) below. (1) Particles that are partially outside the field of view of the photograph are not measured. (2) The particle system with a very clear outline and existing in isolation is measured. (3) Even when the particle shape deviates from the average, the particle system is measured if it is independent and can be measured as a single particle. (4) Although the particles overlap, the boundaries between them are clear and the shape of the particle as a whole can be determined. Each particle is measured as a separate particle. (5) Particles that are superimposed and whose boundaries are unclear, and whose overall shape cannot be determined, are not measured because their shape cannot be determined. (6) For particles that are not perfectly round, such as ellipses, the major axis is used as the particle size.

[0015] The copper microparticles preferably have a coating comprising cuprous oxide and copper carbonate on at least a portion of their surface. When the copper microparticles contain copper carbonate in the coating, the sintering temperature of the copper microparticles can be further reduced. It is believed that the lower the copper carbonate content in the coating, the lower the sintering temperature. The optimal ratio of mass carbon concentration to the specific surface area of ​​copper microparticles is 0.008 to 0.3 wt% g / m², with 0.008 to 0.020 wt% g / m² being even better. If the ratio of mass carbon concentration to the specific surface area of ​​copper microparticles is 0.008 to 0.3 wt% g / m², the sintering temperature of copper particles can be set lower, and copper microparticles can be sintered at a lower temperature. The ratio of mass carbon concentration to the specific surface area of ​​copper microparticles can be calculated from the separately measured specific surface area and mass carbon concentration. Specific surface area can be measured using a BET adsorption apparatus for nitrogen gas (e.g., Mounttech Co., Ltd.'s "MACSORB HM-1201"). Mass carbon concentration can be measured using a carbon-sulfur analysis apparatus (e.g., Horiba Manufacturing Co., Ltd.'s "EMIA-920V").

[0016] When copper microparticles have a film comprising cuprous oxide and copper carbonate on at least a portion of their surface, the ratio of mass oxygen concentration to the specific surface area of ​​the copper microparticles is preferably 0.1 to 1.2 mass%‧g / m2, and more preferably 0.2 to 0.5 mass%‧g / m2. If the ratio of oxygen concentration to the specific surface area of ​​copper microparticles is 0.1% by mass per m² or higher, the copper microparticles exhibit high chemical stability and are less prone to combustion or heating. If the ratio is 1.2% by mass per m² or lower, there are fewer copper oxides, making sintering of the copper microparticles easier. Consequently, the sintering temperature of the conductive paste decreases. However, the copper microparticles will inevitably be oxidized by atmospheric air, resulting in the formation of an oxide film. Therefore, the lower limit for the ratio of oxygen concentration to the specific surface area of ​​copper microparticles is 0.1% by mass per m². The ratio of mass oxygen concentration to the specific surface area of ​​copper microparticles can be determined using an oxygen-nitrogen analyzer (e.g., LECO TC600).

[0017] Copper microparticles can be manufactured using the manufacturing method described in Japanese Patent Application Publication No. 2018-127657. For example, by adjusting the carbon content of the fuel gas supplied to the burner, the ratio of mass carbon concentration to the specific surface area of ​​copper microparticles can be controlled to be 0.008 to 0.3 mass%‧g / m2.

[0018] (Coarse copper particles) Coarse copper particles are copper particles with an average particle size of 3 to 11 μm. An average particle size of 3 to 7 μm is preferred for coarse copper particles. Because the average particle size of coarse copper particles is 3 μm or larger, the shrinkage of copper microparticles during sintering can be reduced, and cracks such as crazing can be minimized when forming a conductive film. Furthermore, because the average particle size of coarse copper particles is less than 11 μm, the effect of reducing copper microparticle shrinkage can be maintained, while the conductive paste can be fully sintered. As a result, a conductive film with excellent conductivity can be formed.

[0019] Regarding the average particle size of coarse copper particles, the particle size of each of the 250 coarse copper particles (totaling 2500 particles in 10 fields of view) observed using a scanning electron microscope (SEM) was measured, and the arithmetic mean of these particles was taken as the average particle size of the coarse copper particles. The selection criteria for the particles to be measured among the particles captured by the scanning electron microscope images (photographs) are as described in (1) to (6) below. (1) Particles that are partially outside the field of view of the photograph are not measured. (2) The particle system with a very clear outline and existing in isolation is measured. (3) Even when the particle shape deviates from the average, the particle system is measured if it is independent and can be measured as a single particle. (4) Particle systems that have overlaps between particles but whose boundaries are clear and whose overall shape can be determined are measured by treating each particle as an individual particle. (5) Particles that are superimposed and whose boundaries are unclear, and whose overall shape cannot be determined, are not measured because their shape cannot be determined. (6) For particles that are not perfectly round, such as ellipses, the major axis is used as the particle size.

[0020] Copper particles with a flat, flake-like shape are preferable. Using flat, flake-like copper particles results in a lower density film after the conductive paste is applied to the substrate and dried, making it easier for gases generated during sintering to escape. Therefore, cracks such as crazing are less likely to occur when fabricating conductive films.

[0021] The ideal density of coarse copper particles is 2 to 6 g / cm3, with 4 to 6 g / cm3 being even better. If the compactness of the coarse copper particles is above 2 g / cm³, the effect of reducing the shrinkage of the copper microparticles can be maintained, and the conductive paste can be sintered more fully, resulting in better conductivity of the conductive film. If the compactness of the coarse copper particles is below 6 g / cm³, the density of the film after the conductive paste is applied to the substrate and dried is lower, and the gases generated during sintering can escape more easily. Therefore, cracks such as crazing are less likely to occur when making the conductive film. The knock density (g / cm3) of coarse copper particles can be measured using a knock density meter (e.g., SEISHIN Enterprise Co., Ltd. "KYT-4000").

[0022] (Dispersion medium) The dispersion medium can be any compound that can disperse copper microparticles or coarse copper particles; there are no particular limitations. 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). One of these dispersion media may be used alone, or two or more may be used in combination. Among these, since they have the reducing effect of copper microparticles, it is preferable, in terms of dispersion medium, to include at least one of the group consisting of ethylene glycol and diethylene glycol.

[0023] (Adhesive 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 impart adhesion to the substrate when it is made into a conductive film. Examples of adhesive 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 methacrylate, ethyl methacrylate, butyl methacrylate, benzyl methacrylate, hydroxyethyl methacrylate, dimethylaminoethyl acrylate, acrylic acid, and methacrylic acid; and nonionic surfactants such as polyvinyl alcohol and polyvinylpyrrolidone. However, adhesive resins are not limited to these examples.

[0024] Among these, polyvinylpyrrolidone (PVP) is the preferred binder resin in terms of improving the dispersibility of copper microparticles. Here, PPVP not only functions as a binder resin but also as a dispersant for both copper microparticles and coarse copper particles. Using PPVP as the binder resin improves the dispersibility of copper microparticles, eliminating the need for additional dispersants. Consequently, the number of components in the conductive paste can be reduced. Therefore, fewer components affect the sintering properties of copper microparticles and their adhesion to the substrate when fabricated as a conductive film.

[0025] (Any ingredients) For any given component, examples include dispersants. Examples of dispersants include sodium hexametaphosphate and sodium β-naphthalenesulfonic acid formaldehyde condensate. These dispersants can be used alone or in combination of two or more. The preferred dispersant is a compound that can be decomposed and removed during sintering.

[0026] (content) The content of copper microparticles is preferably 10 to 60% by mass relative to the total of copper microparticles and coarse copper particles (100% by mass), and even better is 20 to 30% by mass. Compared to the total mass of copper microparticles and coarse copper particles, if the content of copper microparticles is 10% by mass or more, the conductive paste can be fully sintered, and the conductivity of the conductive film becomes better. Compared to the total mass of copper microparticles and coarse copper particles, if the content of copper microparticles is less than 60% by mass, the shrinkage of copper microparticles during sintering can be reduced, and cracks such as crazing are less likely to occur when making conductive films.

[0027] The mass ratio of coarse copper particles to micro copper particles (mass of coarse copper particles / mass of micro copper particles) is preferably 30 / 70 to 90 / 10, and even better is 40 / 60 to 90 / 10. If the mass ratio of coarse copper particles to micro copper particles is 30 / 70 or higher, the splashing of copper particles will be reduced when a conductive film is formed. Even under harsh sintering conditions, the conductive film is less likely to break down. If the mass ratio of coarse copper particles to micro-copper particles is below 90 / 10, the effect of reducing the shrinkage of micro-copper particles can be maintained, while the conductive paste can be fully sintered, and the conductivity of the conductive film becomes better.

[0028] The content of binder resin is 0.1 to 2.0 parts by weight relative to 100 parts by weight of copper microparticles and coarse copper particles, preferably 0.1 to 0.5 parts by weight. Compared to a total of 100 parts by mass of copper microparticles and coarse copper particles, the content of binder resin is more than 0.1 parts by mass, thus achieving better dispersion of copper microparticles and adhesion to the substrate, and resulting in better conductivity when made into a conductive film. Compared to the total of 100 parts by mass of copper microparticles and coarse copper particles, the content of binder resin is less than 2.0 parts by mass. Therefore, less gas is generated from the binder resin during sintering, and the conductive film is less prone to cracks or copper particle splashing during light firing. Furthermore, the conductivity is better when it is made into a conductive film.

[0029] The content of the dispersion medium is preferably 15 to 30 parts by mass relative to 100 parts by mass of copper microparticles and coarse copper particles, and more preferably 17 to 25 parts by mass. If the content of the dispersion medium is above the aforementioned lower limit, the dispersibility of copper microparticles and coarse copper particles is excellent. If the content of the dispersion medium is below the aforementioned upper limit, a conductive film with excellent conductivity can be easily formed.

[0030] (Effects) In the conductive paste of the present invention described above, since the content of binder resin is 2.0 parts by mass or less relative to 100 parts by mass of copper microparticles and coarse copper particles, even when firing with irradiation energy sufficient to remove the binder resin, the amount of gas generated by the thermal decomposition of binder or solvent residue during firing can be reduced. Therefore, copper particles are less prone to splashing, and the conductive film is less likely to break down on the substrate. More specifically, even when the irradiation energy is high in a way that minimizes the resistance value, copper particles are less prone to splashing, and the conductive film is less likely to break down on the substrate. Therefore, the sinterability of copper microparticles is improved, and a conductive film with excellent conductivity can be formed. Furthermore, since the content of binder resin is 0.1 parts by mass or more relative to 100 parts by mass of copper microparticles and coarse copper particles, the adhesion of the conductive film to the substrate can be sufficiently maintained.

[0031] The conductive paste of the present invention, as described above, can be fired with irradiation energy that can sufficiently remove the binder resin. Specifically, even with a high irradiation energy to minimize resistance, copper particles are not prone to splashing. Furthermore, the sintering properties between copper microparticles, as well as between copper microparticles and coarse copper particles, are excellent, and a resistivity of less than 10 μΩ·cm can be achieved even without post-processing steps such as pressing. Furthermore, according to the conductive paste of the present invention, because the sintering temperature of copper microparticles is low, a conductive film can be formed on the substrate at a lower temperature than in conventional products. Therefore, the thermal load on the substrate during sintering is less than in conventional products, and the durability of the substrate with the conductive film is improved.

[0032] (Manufacturing method) The conductive paste of the present invention can be manufactured, for example, by a manufacturing method including the following steps 1 and 2. Step 1: The step of pre-mixing copper microparticles, copper coarse particles, binder resin, dispersion medium, and dispersant as needed. Step 2: Disperse the prepared mixture obtained in Step 1 using a disperser such as a 3-roll mill or bead mill.

[0033] In the preparatory mixing stage of step 1, a rotary mixer, a blender, a mortar and pestle, or other similar mixing equipment can be used. Degassing can be performed simultaneously with mixing. In the dispersion process of step 2, when it is difficult to disperse copper microparticles in the dispersion medium in one dispersion process, multiple dispersion processes may be performed.

[0034] <Substrate with conductive film> The substrate with the conductive film of the present invention comprises a substrate and a sintered product of the conductive paste of the present invention disposed on the substrate. Hereinafter, the substrate and the sintered product of the conductive paste will be described in sequence.

[0035] The substrate can be anything that can withstand sintering; there are no particular limitations. 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 as they can withstand large irradiation energies that can adequately remove the binder resin.

[0036] The sintered products of conductive paste are considered to include: fused compounds formed by sintering copper microparticles together, fused compounds formed by sintering copper coarse particles together, and fused compounds formed by sintering copper microparticles and copper coarse particles. These multiple types of fused compounds are difficult to distinguish after sintering because the shapes of copper microparticles and copper coarse particles change during sintering. The binder resin and dispersion medium are decomposed and removed during the sintering process by vaporization. Therefore, the sintered product of the conductive paste generally does not contain binder resin and dispersion medium. However, as long as it does not impair the effect of the invention, residues from binder resin and dispersion medium may be included in the sintered product of the conductive paste.

[0037] The conductive film system, which is disposed on a substrate as a sintered product, is conductive. The resistivity of the conductive film is preferably less than 15 μΩ·cm, even better is less than 10 μΩ·cm, and still even better is less than 8.0 μΩ·cm. If the resistivity is less than 15 μΩ·cm, the conductive film can be considered to have excellent conductivity. The resistivity can be measured using the M-02N digital tester from CUSTOM.

[0038] The thickness of the conductive film is preferably 5 to 30 μm, and more preferably 10 to 25 μm. If the thickness of the conductive film is 5 μm or more, the resistance of the conductive film will decrease. If the thickness of the conductive film is 30 μm or less, the adhesion of the conductive film to the substrate is excellent. The film thickness was determined by the method described in the examples.

[0039] The substrate of the conductive film of the present invention described above is a sintered product of the conductive paste of the present invention, thus the conductive film has excellent conductivity. The conductive film substrate of the present invention can be used for applications such as printed wiring boards, wireless substrates such as RF tags, pressure sensing sheets, and transparent conductive films.

[0040] (Manufacturing method of substrate with conductive film) The substrate with the conductive film of the present invention can be manufactured by depositing a film containing conductive paste on the substrate and then subjecting the aforementioned film to a sintering process. For example, a film containing conductive paste can be disposed on a substrate by coating a conductive paste onto the substrate, and then the film containing conductive paste can be sintered. There are no particular limitations on the method of applying conductive paste to the substrate. For example, various printing methods such as screen printing, inkjet printing, and gravure printing can be used. The method of applying conductive paste is not limited to these examples.

[0041] By performing a sintering process, copper microparticles are sintered together, and a conductive film with conductivity is formed on the substrate. In conventional conductive pastes, when sintering with irradiation energy sufficient to remove binder resin, for example, if the irradiation energy is high during sintering, the decomposition gases of the binder or dispersant will be released, resulting in numerous copper particle splashes, cracks, and voids. Therefore, it is difficult to achieve a high irradiation energy that minimizes resistance and to improve conductivity. Consequently, organic matter remains, and sintering is insufficient, resulting in inadequate conductivity. According to the inventors' research, the light irradiation energy of the conductive paste in Patent Document 1 can only be increased to approximately 5 J / cm². In contrast, in this invention, for example, even when applying irradiation energy of 7.65 J / cm² or higher, which facilitates sintering, the generation of decomposition gases can be suppressed. Therefore, a sintered film with high sinterability can be obtained, and a conductive film with excellent conductivity can be formed.

[0042] The sintering process is not particularly limited to any state in which copper microparticles in the conductive paste can be sintered. Examples of sintering processes include heat sintering and light sintering. Among these, light sintering is preferred because it facilitates the thorough removal of binder resin and the formation of a conductive film with superior conductivity. Specific examples include: a method of sintering a substrate containing a film of conductive paste at high temperature; a method of sintering a film containing conductive paste by irradiating it with light such as laser light; and photolithography. The specific state of the sintering process is not limited to these examples.

[0043] (Light firing) The conditions for photofiring include, for example, using a device equipped with a xenon lamp, adjusting the lamp output and irradiation time, and adjusting according to the composition of the conductive paste. By increasing the output energy and extending the irradiation time, the temperature of the sample can be increased, which makes it easier for copper microparticles to sinter together or for copper microparticles to sinter with coarse copper particles.

[0044] During photoforming, an output voltage of 350V to 450V is preferred, and 400V to 440V is even better. If the output is above the aforementioned lower limit, the binder resin can be easily and completely removed, improving the sinterability of the copper particles. As a result, a conductive film with superior conductivity can be formed. If the output is below the aforementioned upper limit, the copper particles are less prone to splashing, and the conductive film is less likely to break down. Furthermore, this is advantageous in terms of cost.

[0045] The irradiation time for photo-firing is preferably between 3000 μS and 60000 μS, and even better between 3500 μS and 10000 μS. If the irradiation time is above the aforementioned lower limit, the binder resin can be easily and completely removed, and the sinterability of the copper particles can be improved. As a result, a conductive film with better conductivity can be formed. If the irradiation time is below the aforementioned upper limit, the copper particles are less likely to splatter, and the conductive film is less likely to break down. Furthermore, industrial mass production is also improved.

[0046] The irradiation energy for photo-firing is preferably 7.65 to 16 J / cm², and even better is 8.5 to 13 J / cm². If the irradiation energy is above the aforementioned lower limit, the binder resin can be easily and completely removed, and the sinterability of the copper particles can be improved. As a result, a conductive film with better conductivity can be formed. If the irradiation energy is below the aforementioned upper limit, the copper particles are less likely to splatter, and the conductive film is less likely to break down. Furthermore, it is advantageous in terms of cost.

[0047] (Heating and firing) 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 is easier to sinter copper microparticles together or copper microparticles with coarse copper 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 even better. If the processing temperature is above the aforementioned lower limit, the binder resin can be easily and completely removed, and the sinterability of the copper particles can be easily improved. As a result, a conductive film with better conductivity can be formed. If the processing temperature is below the aforementioned upper limit, cracking of the conductive film is less likely to occur, and the substrate deformation is also less. Furthermore, it is advantageous in terms of cost.

[0049] The heating and firing time is preferably between 5 and 120 minutes, and even better between 15 and 60 minutes. If the processing time is above the aforementioned lower limit, the binder resin can be easily and completely removed, and the sinterability of the copper particles can be improved. As a result, a conductive film with better conductivity can be formed. If the processing time is below the aforementioned upper limit, the conductive film is less prone to cracking, and the substrate deformation is also less. Furthermore, industrial mass production is also improved. [Example]

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

[0051] <Materials> (Copper microparticles) Copper microparticles were manufactured using the manufacturing method described in Japanese Patent Application Publication No. 2018-127657. These copper microparticles were used in all examples. The average particle size of the copper microparticles was 110 nm, the specific surface area was 5.602 m² / g, the mass oxygen concentration was 1.1204%, and the mass carbon concentration was 0.119883%. Based on these measurements, the mass oxygen concentration compared to the surface area was 0.200 wt% g / m², and the mass carbon concentration compared to the surface area was 0.0214 wt% g / m².

[0052] (Coarse copper particles) MA-C03KP: Trade name of Mitsui Metals & Minerals Co., Ltd. (average particle size 3.8μm, knock density 5.26g / cm3). FCC-TB: Trade name of Fukuda Metal Foil Powder Industrial Co., Ltd. (average particle size 6.22μm, knock density 2.57g / cm3). MA-C03K: Trade name of Mitsui Metals & Minerals Co., Ltd. (average particle size 3.21μm, knock density 5.00g / cm3).

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

[0054] (Dispersion medium) In both cases, ethylene glycol (EG) was used as the dispersion medium.

[0055] <Example 1> 2.4g of copper microparticles, 5.6g of coarse copper particles, 0.16g of PVP, and 1.86g of EG were pre-mixed using a mixer (THINKY Co., Ltd. "AR-100") to obtain a pre-mixed paste. The obtained pre-mixed paste was then dispersed using a three-roll disperser (IMEX Co., Ltd. "BR-100V") to prepare a conductive paste. Then, a wiring pattern is formed by coating a conductive paste onto a polyimide (PI) film (thickness: 50μm, KAPTON film "200EN" manufactured by TORAY DUPONT Co., Ltd.) using screen printing. This is achieved by cutting an RF tag pattern wiring pattern with a wiring width of 1mm and a line length of 124mm in half. Following this, a photoforming apparatus (Novacentrix "PulseForge Invent") is used to sinter the conductive paste, resulting in a PI film with a conductive film forming a wiring pattern with a wiring width of 1mm and a line length of 124mm. The photoforming is performed under relatively strong irradiation conditions—an output of 350 to 450V, an irradiation time of 3000μS or more, and an irradiation energy of 7.65J / cm² or more—where copper microparticles readily sinter together and between coarse copper particles. Here, if the type of substrate or the composition of the paste is changed during photofiring, the escape mode of the thermal decomposition gases generated during photofiring or the instantaneous amount of thermal decomposition gases will change. Therefore, each substrate and paste composition has its own optimal photofiring conditions. In this embodiment, the optimization of the photofiring conditions is carried out as follows. (1) The irradiation time is fixed at 4000μS to optimize the irradiation output. (2) Optimize the irradiation time under the optimal output obtained in (1). Once the output and irradiation time are determined, the irradiation energy is automatically calculated by the simulation software within the device. Among the fired products produced based on the above research, the firing conditions with the lowest resistivity are considered the most suitable photofiring conditions.

[0056] <Examples 2 to 14, 16, 17, Comparative Examples 1 to 3> Except for changing the composition of the conductive paste as shown in Table 1 or Table 2, the conductive pastes of each example were prepared in the same manner as in Example 1, and a conductive film was formed on the PI film.

[0057] <Example 15> The conductive paste of Example 10 is used and a conductive film is formed on the substrate by heating and firing. The firing process uses a reflow oven manufactured by Uniemp. After pre-oxidation in the atmosphere at 250°C for 30 minutes, the sample is kept at 250°C with 3% H2 gas flowing for 30 minutes. After that, the gas is switched to N2 gas, cooled to room temperature, and the sample is taken out, so that a conductive film is formed on the PI film.

[0058] <Determination Method> (Specific resistance) The conductivity of the conductive film in each example was measured and evaluated using the 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 the following intervals: AB: 22mm, AC: 44mm, AD: 66mm, AE: 88mm, AF: 110mm, and AG: 124mm. The resistance values ​​were measured using a CUSTOM M-02N digital measuring instrument. Subsequently, a graph was plotted with the line length on the horizontal axis and the resistance value on the vertical axis. The slope of each graph, conforming to a linear function, was calculated and used as the surface resistance.

[0059] (Thickness of conductive film) The thickness of the conductive film in each example was measured at 5 locations using a laser microscope (KEYENCE Co., Ltd. "VK-X"), and the average value was obtained. The surface resistivity was multiplied by the average film thickness to calculate the specific resistivity.

[0060] [Table 1]

[0061] [Table 2]

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

[0063] <Results> In Examples 1 to 14, even when firing with irradiation energy sufficient to remove the binder resin, copper microparticles did not splash onto the substrate, and the conductive film did not break down. Furthermore, a conductive film with excellent conductivity can be formed. No substrate deformation or cracks in the conductive film were observed during heating and firing as in Example 15. Furthermore, a conductive film with excellent conductivity can be formed. In Examples 16 and 17, copper microparticles did not splash onto the substrate, and the conductive film did not collapse. Compared to other examples, due to the higher proportion of copper microparticles, although the sintering performance was good, some cracks were generated in the sintered film. Even so, the resistivity was 15 μΩ·cm, and the conductivity was sufficient. Even in circuits where cracks were generated, the impact of the cracks could be reduced by performing additional work (repair) to improve the adhesion of the film. In Comparative Example 1, due to the excessive content of binder resin, numerous pits or cracks, believed to be from the escape of decomposition gases, were observed in the fired film. Copper particles splattered, and the conductive film also collapsed. Furthermore, the resistance value was overloaded. In Comparative Example 2, since no adhesive resin was used, there was no adhesion between the substrate and the conductive film. The film would peel off from the substrate during firing, and a conductive film could not be obtained on the substrate. In Comparative Example 3, since copper microparticles were not used, it was difficult to sinter the coarse copper particles together, resulting in a resistivity of more than 15 μΩ‧cm and insufficient conductivity. [Potential for industrial application]

[0064] According to the present invention, a conductive paste, a substrate with a conductive film using the aforementioned conductive paste, and a method for manufacturing the substrate with the conductive film are provided. The conductive paste is fired with irradiation energy that can fully remove the binder resin, and copper microparticles are not easily splashed, and a conductive film with excellent conductivity can be formed.

[0065] 1: Wiring pattern A to G: Locations on the wiring diagram

Claims

1. A conductive paste comprising: copper microparticles with an average particle size of less than 300 nm, copper coarse particles with an average particle size of 3 to 11 μm, a binder resin, and a dispersion medium; wherein, The content of the binder resin is 0.1 to 0.5 parts by mass relative to the total of 100 parts by mass of the aforementioned copper microparticles and the aforementioned coarse copper particles.

2. The conductive paste as described in claim 1, wherein, The mass ratio of the aforementioned coarse copper particles to the aforementioned micro-copper particles (mass of coarse copper particles / mass of micro-copper particles) is 30 / 70 to 90 / 10.

3. The conductive paste as described in claim 1, wherein, The mass ratio of the aforementioned coarse copper particles to the aforementioned micro-copper particles (mass of coarse copper particles / mass of micro-copper particles) is 40 / 60 to 90 / 10.

4. The conductive paste as described in any one of claims 1 to 3, wherein, The aforementioned adhesive resin contains polyvinylpyrrolidone.

5. The conductive paste as described in any one of claims 1 to 3, wherein, The aforementioned dispersion medium includes at least one selected from the group consisting of ethylene glycol and diethylene glycol.

6. The conductive paste as described in any one of claims 1 to 3, wherein, The aforementioned copper microparticles have at least a portion of a film comprising cuprous oxide and copper carbonate on their surface, the ratio of mass oxygen concentration to the specific surface area of ​​the aforementioned copper microparticles is 0.1 to 1.2 mass%·g / m2, and the ratio of mass carbon concentration to the specific surface area of ​​the aforementioned copper microparticles is 0.008 to 0.3 mass%·g / m2.

7. A substrate with a conductive film, comprising: a substrate and a sintered product of the conductive paste described in any one of claims 1 to 6 disposed on the aforementioned substrate.

8. A method for manufacturing a substrate with a conductive film, comprising: a step of depositing a film comprising any one of claims 1 to 6 onto the substrate, and a step of subjecting the aforementioned film to a sintering treatment.

9. A method for manufacturing a substrate with a conductive film as described in claim 8, wherein, The aforementioned sintering process is light firing.

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