Electroconductive film, electroconductive base material, electronic device, RF tag, electromagnetic wave-shielding film, and planar heating body

Abstract

To form a conductive film having good conductivity by using dendritic copper particles.SOLUTION: An electroconductive film is formed by sintering a plurality of copper particles, wherein the plurality of copper particles include dendritic copper particles, and the arithmetic average height Sa as defined by ISO 25178 on at least one surface of the electroconductive film is 0.1-0.9 μm, and the maximum height Sz as defined by ISO 25178 is 1.5-11.0 μm.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a conductive film, a conductive substrate, an electronic device, an RF tag, an electromagnetic wave shielding film, and a sheet heating element, and more specifically to a conductive film formed by sintering copper particles. [Background technology]

[0002] A technique for obtaining conductive patterns on a substrate using a conductive film formed from a conductive composition containing conductive particles is known. This technique is thought to be applicable to printed electronics, which has been actively developed in recent years. Printed electronics is a technology for forming electronic circuits, sensors, elements, etc. on a substrate such as a film using printing technology.

[0003] Patent Document 1 discloses a copper powder characterized by being composed of flat copper particles each having a dendritic shape with a linearly growing main trunk and multiple branches branching off from the main trunk, the particles being 1.0 μm or more in size and 0.2 to 0.5 μm in cross-sectional thickness, and the particles being aggregated together so that the thickness of the branches is 10 μm or less. Patent Document 1 further discloses the use of this copper powder to prepare a conductive paste, and the use of this conductive paste to form a conductive film.

[0004] Patent Document 2 discloses copper powder characterized by an angle of repose of more than 50 degrees and a grind gauge value (μm) of 40 to 4. Patent Document 2 further discloses that a conductive paste is prepared using this copper powder, and that a conductive film is formed using this conductive paste. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-8333 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-136818 Summary of the Invention [Problem to be solved by the invention]

[0006] Dendritic copper particles are known as conductive particles, and are described in, for example, Patent Document 1 mentioned above. According to the findings of the present inventors, there is room for improvement in the conductive film formed using dendritic copper particles, for example, in terms of conductivity.

[0007] The present invention has been made in view of the above circumstances, and one of the objects of the present invention is to form a conductive film with good conductivity by using dendritic copper particles. [Means for solving the problem]

[0008] The present inventors have completed the invention provided below and solved the above problems.

[0009] 1. A conductive film formed by sintering a plurality of copper particles, the plurality of copper particles include dendritic copper particles, The conductive film has an arithmetic mean height Sa of 0.1 to 0.9 μm, as defined in ISO 25178, on at least one surface thereof, and a maximum height Sz of 1.5 to 11.0 μm, as defined in ISO 25178. 2. 1. The conductive film according to claim 1, The conductive film has a root mean square height Sq, as defined in ISO 25178, of 0.1 to 1.0 μm on the one surface. 3. 1. The conductive film according to 1. or 2., The conductive film has a maximum peak height Sp, as defined in ISO 25178, of 0.9 to 5.0 μm on the one surface. 4. The conductive film according to any one of 1. to 3., The conductive film has a core level difference Sk on the one surface, as defined by ISO 25178, of 0.1 to 2.5 μm. 5. The conductive film according to any one of 1. to 4., The void volume Vvc of the core portion on the one surface as defined in ISO 25178 is 0.1 to 1.3 ml / m 2 A conductive film. 6. The conductive film according to any one of 1. to 5., The core material volume Vmc on the one surface as defined in ISO 25178 is 0.05 to 1.0 ml / m 2 A conductive film. 7. The conductive film according to any one of 1. to 6., The conductive film has a surface resistivity of 5 to 100 mΩ / □ on one surface. 8. The conductive film according to any one of 1. to 5., A conductive film having a thickness of 2 to 100 μm. 9. The conductive film according to any one of 1. to 8., The particle diameter D at which the cumulative frequency reaches 50% in a volume-based cumulative particle diameter distribution curve obtained when the particle diameters of the plurality of copper particles are measured by a laser diffraction scattering method. 50 The conductive film has a thickness of 0.5 to 100 μm. 10. A substrate; the conductive film according to any one of 1. to 9. provided on at least one surface of the substrate; A conductive substrate comprising: 11. 10. The conductive substrate according to claim 10, A conductive substrate, wherein the substrate is flexible. 12. 10. An electronic device comprising the conductive substrate according to 11. 13. An RF tag comprising the conductive substrate according to 10. or 11. 14. 10. An electromagnetic wave shielding film comprising the conductive substrate according to 11. 15. A sheet heating element comprising the conductive substrate according to 10. or 11. [Effects of the Invention]

[0010] According to the present invention, a conductive film with good conductivity can be formed using dendritic copper particles. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram illustrating a conductive substrate. [Figure 2] 1 is an electron microscope image of dendritic copper particles used in the examples. [Figure 3] 1 is an electron microscope image of dendritic copper particles used in the examples. [Figure 4] 1 is an SEM image of a cross section of the conductive substrate obtained in Example 1. [Figure 5] 1 is an SEM image of a cross section of the conductive substrate obtained in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, similar components are denoted by similar reference numerals and descriptions thereof will be omitted where appropriate. The drawings are for illustrative purposes only, and the shapes and dimensional ratios of the components in the drawings do not necessarily correspond to the actual products.

[0013] In this specification, unless otherwise specified, the expression "X to Y" in the description of a numerical range means at least X and at most Y. For example, "1 to 5% by mass" means "at least 1% by mass and at most 5% by mass."

[0014] In this specification, the term "(meth)acrylic" represents a concept that encompasses both acrylic and methacrylic. The same applies to similar terms such as "(meth)acrylate." In this specification, the term "electronic device" is used to encompass elements, devices, final products, etc. to which electronic engineering technology is applied, such as semiconductor chips, semiconductor elements, printed wiring boards, electric circuit display devices, information and communication terminals, light-emitting diodes, physical batteries, and chemical batteries.

[0015] <Conductive film> The conductive film of this embodiment is formed by sintering copper particles, including dendritic copper particles. At least one surface of the conductive film of this embodiment has an arithmetic mean height Sa defined by ISO 25178 of 0.1 to 0.9 μm, and a maximum height Sz defined by ISO 25178 of 1.5 to 11.0 μm. Sa is preferably 0.2 to 0.8 μm, more preferably 0.3 to 0.7 μm, and even more preferably 0.4 to 0.7 μm. Sz is preferably 2.0 to 10.0 μm, more preferably 3.0 to 9.5 μm, and even more preferably 4.0 to 9.0 μm.

[0016] The conductive film described above has good conductivity. Although the reasons for this are not entirely clear, the inventors believe that indicators such as Sa and Sz may be correlated with the density of copper particles or their sintered products in the conductive film, the porosity of the conductive film, etc. Specifically, as shown in the examples below, applying an appropriate temperature and pressure to a film formed from a conductive composition containing dendritic copper particles can be interpreted as the results of sufficient compression and sintering of the copper particles, which are reflected in the values ​​of Sa and Sz.

[0017] It is believed that dendritic copper particles are prone to voids between particles due to their shape. For this reason, conventionally, conductive films formed using dendritic copper particles have tended to have low conductivity. However, by employing appropriate manufacturing methods and conditions, for example, a conductive film with Sa and Sz in the above-mentioned ranges can be formed, making it possible to achieve good conductivity for the conductive film even when dendritic copper particles are used.

[0018] As already briefly explained, the conductive film of this embodiment can be obtained by using dendritic copper particles and employing appropriate manufacturing methods and manufacturing conditions. Details of the manufacturing methods and manufacturing conditions will be described later, but to briefly explain here, the conductive film of this embodiment can be produced by applying appropriate heat and pressure to a film formed using a conductive composition containing dendritic copper particles. Depending on the heating and pressure conditions, the conductive film of this embodiment may not be obtained. Just to be clear, the conductive film of this embodiment is not to be interpreted as being limited by the manufacturing method or manufacturing conditions.

[0019] The conductive film of this embodiment will be further described.

[0020] (Various indicators of surface roughness) In the conductive film of this embodiment, conductivity can be further improved by ensuring that other surface roughness-related indices, in addition to Sa and Sz, are within predetermined numerical ranges. Specifically, these indices are as follows. These indices are also likely correlated with the density of copper particles or sintered products thereof in the conductive film, the porosity in the conductive film, etc.

[0021] The root mean square height Sq defined by ISO 25178 on at least one surface of the conductive film is, for example, 0.1 to 1.0 μm, preferably 0.2 to 1.0 μm, more preferably 0.3 to 1.0 μm, and even more preferably 0.5 to 0.9 μm. The maximum peak height Sp, as defined by ISO 25178, on at least one surface of the conductive film is, for example, 0.9 to 5.0 μm, preferably 1.5 to 5.0 μm, and more preferably 2.0 to 5.0 μm. The level difference Sk of the core portion defined by ISO 25178 on at least one surface of the conductive film is, for example, 0.1 to 2.5 μm, preferably 1.0 to 2.5 μm, and more preferably 1.0 to 2.0 μm. The void volume Vvc of the core portion defined by ISO 25178 on at least one surface of the conductive film is, for example, 0.1 to 1.3 ml / m 2 , preferably 0.3 to 1.2 ml / m 2 , more preferably 0.5 to 1.1 ml / m 2 is. The core material volume Vmc of at least one surface of the conductive film as defined in ISO 25178 is, for example, 0.05 to 1.0 ml / m 2 , preferably 0.3 to 0.9 ml / m 2 , more preferably 0.4 to 0.8 ml / m 2 is.

[0022] (Indicator of conductivity) The conductive film of this embodiment has good conductivity. This can be quantitatively expressed, for example, by surface resistivity. Specifically, the surface resistivity of at least one surface of the conductive film is preferably 5 to 100 mΩ / □.

[0023] (Thickness) Although the thickness of the conductive film of this embodiment is not particularly limited, considering application to printed electronics, the thickness of the conductive film is, for example, 2 to 100 μm, preferably 5 to 50 μm, and more preferably 10 to 30 μm. If the thickness of the conductive film is not uniform, it is preferable that the thickness at least at the points where Sa and Sz are measured is within the above ranges. A conductive film that is appropriately thick facilitates current flow and improves conductivity, while a conductive film that is not too thick can sometimes prevent peeling when the conductive film is provided on the surface of a substrate, as described below.

[0024] (About copper particles) As already explained, the copper particles (i.e., copper powder) used to form the conductive film include dendritic copper particles.

[0025] The term "dendritic copper particles" is commonly used to describe the shape of copper particles. "Dendrite" generally refers to a shape having a main trunk extending in one direction and at least one branch branching from the main trunk. Copper particles produced by electrolysis are usually dendritic due to the crystallization mechanism of copper. In the technical field of copper particles, dendritic copper particles produced by electrolysis are usually referred to as "dendritic copper particles." The meaning of the term "dendritic copper particles" in this specification is the same as the ordinary meaning of the term "dendritic copper particles" in the technical field of copper particles.

[0026] The proportion of dendritic copper particles in the total copper particles used to form the conductive film is preferably 50% by mass or more, more preferably 75% by mass or more, and even more preferably 90% by mass or more. Of course, all of the copper particles (100% by mass) may be dendritic copper particles. Because dendritic copper particles are relatively inexpensive, a high proportion of dendritic copper particles in a plurality of copper particles (copper powder) leads to reduced manufacturing costs for the conductive film. Furthermore, in this embodiment, even if the proportion of dendritic copper particles in a plurality of copper particles (copper powder) is high, the conductivity of the conductive film is good as long as Sa and Sz are within the above-mentioned numerical ranges when the conductive film is formed.

[0027] By using multiple copper particles (copper powder) with an appropriate particle size distribution to form the conductive film, the conductivity of the final conductive film can be further improved. The conductivity of a conductive film is thought to be correlated with the number of grain boundaries in the film and the porosity of the film. It is thought that the fewer the number of grain boundaries in the film, the higher the conductivity. It is also thought that the lower the porosity of the film, the higher the conductivity. It is thought that the balance between these two can be optimized by using multiple copper particles (copper powder) with an appropriate particle size distribution. Specifically, the particle diameter D at which the cumulative frequency reaches 50% is determined in the volume-based cumulative particle diameter distribution curve obtained when the particle diameters of multiple copper particles (copper powder) used to form the conductive film are measured by the laser diffraction scattering method. 50 is preferably 0.5 to 100 μm, more preferably 1 to 50 μm, further preferably 3 to 30 μm, and particularly preferably 4 to 20 μm.

[0028] D 50 Regarding D, if the copper particles are measured at the supplier, the value, i.e., the catalog value, can be used. If the catalog value is unknown, D can be determined by measuring the copper particles using the laser diffraction scattering method. 50 All we need to do is find the answer.

[0029] The copper particles (copper powder) generally contain copper as a main component. "Containing copper as a main component" means that the ratio of copper element to all constituent elements in the copper particles (copper powder) is preferably 50 mol% or more, more preferably 75 mol% or more, even more preferably 90 mol% or more, and particularly preferably 95 mol% or more. This ratio may be 100 mol%. Just to be clear, the copper particles (copper powder) may contain elements other than copper, as long as the desired conductivity is obtained. Examples of elements other than copper include gold, silver, aluminum, platinum, palladium, iridium, tungsten, nickel, tantalum, lead, and zinc. Of course, the copper particles (copper powder) do not have to be substantially free of elements other than copper. "Substantially free" means that the copper particles (copper powder) may unavoidably contain elements other than copper as impurities, but do not intentionally or artificially contain elements other than copper. The plurality of copper particles (copper powder) may or may not be surface-treated.

[0030] Copper particles, particularly dendritic copper particles (copper powder), are commercially available from, for example, DOWA Electronics Co., Ltd., Fukuda Metal Foil and Powder Co., Ltd., and Mitsui Mining & Smelting Co., Ltd. Two or more different copper powders may be mixed together for the purpose of adjusting or optimizing the particle size distribution or for other purposes.

[0031] <Conductive base material> The conductive substrate 10 of this embodiment is, for example, as shown in FIG. A substrate 1, The conductive film (conductive film 3) described above is provided on at least one surface of the substrate 1; It can be equipped with:

[0032] The specific embodiment of the conductive film 3 has already been described. Usually, the surface of the conductive film 3 opposite to the substrate 1 satisfies the above-mentioned numerical ranges for Sa, Sz, and the like.

[0033] The substrate 1 is usually in the form of a film, sheet or plate, and from the viewpoint of industrial productivity, the shape of the substrate 1 is preferably any of these. The substrate 1 is preferably flexible. A flexible printed circuit (FPC) can be manufactured by using a flexible substrate 1. Of course, the substrate 1 may also be a rigid substrate that does not have flexibility.

[0034] Considering cost and end use, the substrate 1 is preferably at least one selected from the group consisting of polyesters such as PET (polyethylene terephthalate) and PEN (polyethylene naphthalate), polyolefins such as polyethylene and polypropylene, polycarbonate, polyimide, and paper. Here, the paper may be coated paper (paper whose surface is coated with a coating agent) or ordinary uncoated paper. Furthermore, the substrate 1 is not limited to PET and other materials, and general resin films can also be used. Furthermore, the substrate 1 may be transparent or opaque. Examples of opaque resin films include foamed resin films such as foamed PET film and foamed resin sheets. The heating temperature described below may not necessarily be so high. Therefore, a substrate 1 having low heat resistance, such as polyester, polyolefin, polycarbonate, or paper, can also be suitably used as the substrate. Furthermore, when a substrate 1 having high heat resistance, such as polyimide, is used, the resistivity of the finally obtained conductive pattern can be further reduced by heating at a high temperature.

[0035] The thickness of the substrate 1 is not particularly limited and can be appropriately set depending on the final use (electronic device, RF tag, electromagnetic wave shielding film, planar heating element, etc.) described below and various other circumstances. The thickness of the substrate 1 is typically 10 to 250 μm, preferably 30 to 100 μm. However, from the viewpoint of suppressing the occurrence of curling, it is preferable that the substrate 1 is somewhat thick. Specifically, the thickness of the substrate 1 is preferably 100 to 250 μm, more preferably 100 to 150 μm. The substrate 1 may have a single layer structure or a laminate structure of two or more layers.

[0036] There may or may not be any layer between the substrate 1 and the conductive film 3. For example, to improve the adhesion of the conductive film 3 (to prevent peeling), a cured product of a curable resin material (such as a cured product of an adhesive) may be present between the substrate 1 and the conductive film 3. The substrate 1 may or may not be subjected to some surface treatment (for example, treatment to improve the adhesion of the conductive film).

[0037] <Method of manufacturing conductive film / conductive substrate> As already mentioned briefly, the conductive film of this embodiment can be obtained by using dendritic copper particles and employing appropriate manufacturing methods and manufacturing conditions. Simply put, the conductive film of this embodiment can be produced by applying appropriate heat and pressure to a film formed using a conductive composition containing dendritic copper particles. Depending on the heating and pressure conditions, the conductive film of this embodiment may not be obtained.

[0038] An example of a manufacturing method for manufacturing a conductive substrate by providing a conductive film on one surface of a substrate will be described below.

[0039] (Preparation of Conductive Composition) First, a conductive composition containing dendritic copper particles is prepared. The conductive composition can be prepared by uniformly mixing copper particles such as dendritic copper particles (more precisely, copper powder composed of multiple copper particles), a solvent, and, if necessary, a resin component.

[0040] Specific embodiments of copper particles such as dendritic copper particles have already been described. Two or more copper particles may be used for the purpose of optimizing particle size distribution. Of course, only one copper particle (dendritic copper particle) may also be used. To further enhance the conductivity of the conductive film, the proportion of conductive particles in the conductive composition is preferably high. Specifically, the proportion of conductive particles in the total nonvolatile components of the conductive composition is preferably 90% by mass or more, more preferably 92% by mass or more, and even more preferably 94% by mass or more.

[0041] The type of solvent is not particularly limited. The solvent is typically an organic solvent. Any solvent may be used as long as it does not substantially alter the components in the conductive composition. When a solvent is used, only one solvent may be used, or two or more solvents may be used. The amount of the solvent used may be adjusted appropriately depending on the method of applying the conductive composition, etc. The amount of the solvent used is, for example, 3 to 30 mass %, and preferably 5 to 25 mass %, of the total amount of the conductive composition.

[0042] The resin component may or may not be used as appropriate, depending on the adhesiveness to the substrate, coatability, printability, and the like. When a resin component is used, the type thereof is not particularly limited, and specific examples thereof include polyvinylpyrrolidone, polyester, epoxy resin, (meth)acrylic resin, polyvinyl acetal, cellulose resin (e.g., ethyl cellulose), and phenol resin. From the viewpoint of particularly enhancing the conductivity of the conductive film, the amount of the resin component in the total nonvolatile components of the conductive composition is, for example, 10% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, and particularly preferably 1% by mass or less. The lower limit of the amount of the resin component may be 0. However, when it is intended to actively obtain effects such as improved adhesion by using the resin component, the amount of the resin component in the total nonvolatile components of the conductive composition is preferably 1% by mass or more, and more preferably 2% by mass or more. That is, from the viewpoint of balancing various performances, the amount of the resin component is preferably 1 to 10% by mass, more preferably 2 to 5% by mass, of the total nonvolatile components of the conductive composition.

[0043] The conductive composition may or may not contain various additive components found in conventional ink compositions and conductive pastes.

[0044] In order to mix the components uniformly, it is preferable to use, for example, a planetary mixer.

[0045] (film formation) A film of the conductive composition is formed on the substrate by an appropriate means such as coating, printing, or transfer. Specific embodiments of the substrate are as described above.

[0046] The conductive composition may be formed into a film over the entire surface of the substrate, or may be formed into a film only over a portion of the surface of the substrate. In the former case, application can be performed using a device such as a blade coater, air knife coater, doctor coater, roll coater, bar coater (rod coater), or curtain coater. In the latter case, various printing methods can be applied, such as screen printing, gravure printing, letterpress printing, lithographic printing (offset printing), inkjet printing, and transfer printing. In short, any method can be used as long as it is possible to form a film of the conductive composition having the desired pattern shape. By appropriately designing the shape of the pattern formed by coating, printing, etc., it is possible to produce a conductive substrate having a pattern structure, such as a conductive film (circuit pattern) that can function as a circuit or a mesh pattern that has electromagnetic wave shielding ability. When the conductive composition is formed into a film on only a part of one surface of the substrate, it is preferable that the shape of the film (pattern) be appropriately designed depending on the application of the finally obtained conductive substrate. To prevent the formation of a film of the conductive composition in areas other than the desired locations, a process may be carried out in which, for example, a film with holes cut out therein is placed on the substrate 1, the conductive composition is applied or printed on top of it, and then the film is removed.

[0047] From the viewpoint of finally obtaining a conductive film having sufficient conductivity and from the viewpoint of ease of film formation, the thickness of the layer provided here (dry thickness when the conductive composition contains a solvent) is preferably 5 to 100 μm, more preferably 10 to 50 μm.

[0048] When the conductive composition contains a solvent, it is preferable to carry out a heat treatment to dry the solvent. The conditions for the heat treatment are not particularly limited as long as the solvent is sufficiently dried, but from the viewpoint of sufficiently drying the solvent and preventing the conductive particles from being altered by excessive heating, the heat treatment temperature is preferably 50 to 150°C, more preferably 80 to 120°C. The heat treatment time is preferably 1 to 60 minutes, more preferably 3 to 30 minutes. The heat treatment for drying the solvent can be carried out by applying hot air to the formed film, although other methods of heat treatment may also be used.

[0049] (heat and pressure) The laminate of the substrate and the layer formed of the conductive composition obtained as described above is heated and pressurized. Heating and pressurization are preferably performed simultaneously. This sinters the copper particles (including dendritic copper particles) in the layer formed of the conductive composition, connecting the particles together. This allows the production of a conductive film with good conductivity, and a conductive substrate comprising the conductive film and the substrate.

[0050] Heating and pressure can be performed using, for example, a flat press equipped with a heating mechanism. That is, a laminate of the substrate and the layer formed of the conductive composition is sandwiched between two (a pair of) flat plates and pressed while being heated, thereby sintering the copper particles in the layer formed of the conductive composition. By appropriately setting the heating and pressing conditions, a conductive film can be obtained in which indices such as Sa and Sz are appropriately controlled. Specifically, when a flat press is used, the heating temperature is preferably 105 to 120°C, more preferably 105 to 115°C, the pressing pressure is preferably 5 to 80 MPa, more preferably 5 to 75 MPa, and the heating and pressing time is preferably 3 to 60 seconds, more preferably 3 to 40 seconds.

[0051] When applying heat and pressure using a flat press, it is preferable to interpose a resin film between the pressing surface (flat surface) of the device and the layer made of the conductive composition. The presence of this resin film may contribute to the appropriate control of Sa and Sz because it distributes and uniforms the pressure from the pressing surface. After heating and pressure are completed, the resin film used is usually peeled off. The resin film that can be used here is not particularly limited. For example, the same resin film as the substrate can be used. From the viewpoint of durability, the resin film is preferably a polyimide film. This resin film may be disposable or may be reused if possible. From the viewpoint of dispersing and equalizing pressure, it is also possible to use glass cloth, rubber-based sheets, carbon-based sheets, etc. instead of resin films. Examples of materials for rubber-based sheets include silicone rubber and fluororubber. Examples of materials for carbon-based sheets include carbon fiber, graphite, and graphene.

[0052] In addition to a flat press, heating and pressurization can also be performed using a roll press or other device. However, when heating and pressurization are performed using a roll press, the heating and pressurization time per unit area tends to be shorter than when heating and pressurization is performed using a flat press, so care must be taken to appropriately control the heating, pressurization, conveying speed (roll rotation speed), etc.

[0053] (Oxide film removal process) In this embodiment, it is preferable to perform an oxide film removal step in which a component X capable of removing the oxide film on the surface of the copper particles is penetrated into the layer formed by the conductive composition before the above-mentioned heating and pressurization. The oxide film removal step is thought to facilitate the sintering of the copper particles by heating and pressurization, which is likely to change the Sa and Sz of the finally obtained conductive film. Furthermore, since the sintering of the copper particles is facilitated, the conductivity of the finally obtained conductive film tends to be further increased.

[0054] It is preferable that component X penetrates into the layer formed by the conductive composition. This tends to further increase the conductivity of the finally obtained conductive film. Therefore, in the oxide film removal step, the layer formed by the conductive composition with which component X is in contact may be pressed to promote penetration of component X into the layer. In this case, the oxide film removal step and the above-mentioned heating and pressurization may be performed simultaneously. Of course, the oxide film removal step may be carried out separately from the heating and pressurizing.

[0055] The oxide film removal step can be carried out by contacting, and preferably allowing penetration of, a liquid containing component X capable of removing the oxide film on the surface of copper particles with the layer formed from the conductive composition by any method such as dropping, spraying, immersion, etc. Alternatively, the liquid containing component X may be contacted with the layer formed from the conductive composition by an inkjet method or a dispenser method. The liquid containing component X is preferably water in which component X is dissolved or dispersed. The use of water is preferable from the viewpoints of reducing the environmental load and process safety (non-flammability). Of course, an organic solvent in which component X is dissolved or dispersed can also be used. Instead of using a liquid containing component X, the oxide film removal step may be carried out by bringing component X in a gaseous state into contact with the layer provided by the conductive composition. It is also conceivable to bring a sheet containing component X into contact with a layer formed of a conductive composition, preferably by applying pressure, thereby causing component X to penetrate into the layer formed of the conductive composition. Specific examples of the sheet here include paper or nonwoven fabric containing component X, and resin sheets having component X coated or printed on their surfaces. Additionally, the method for bringing component X into contact with and permeating the layer formed from the conductive composition is not particularly limited.

[0056] Component X is not particularly limited as long as it is capable of removing the oxide film on the surface of the copper particles. In this specification, "removal" of an oxide film includes not only the removal of the oxide itself present on the surface of the copper particles, but also the case where the oxide undergoes a chemical change such as reduction, causing the oxide to return to a non-oxide state.

[0057] According to the findings of the present inventors, it is preferable that the acidic acid contains at least one selected from the group consisting of organic acids, phosphorus oxoacids, and hydrazine or derivatives thereof.

[0058] Examples of organic acids include carboxylic acids such as citric acid, formic acid, acetic acid, malonic acid, malic acid, tartaric acid, ascorbic acid, succinic acid, fumaric acid, and propionic acid. Specific examples of phosphorus oxoacids include phosphinic acid, phosphonic acid, phosphorous acid, phosphoric acid, diphosphoric acid, triphosphoric acid, and metatriphosphoric acid. Of these, phosphinic acid is particularly preferred. Examples of hydrazine or its derivatives include hydrazine itself; hydrazine salts such as hydrazine monohydrochloride, hydrazine dihydrochloride, hydrazine monohydrobromide, and hydrazine sulfate; and other compounds having an -NH-NH2 structure.

[0059] Additionally, from the viewpoint of removing an oxide film, a compound having a small pKa in water can be used as component X. Specifically, a compound having a pKa in water of -5.0 to 5.0 is preferred as component X, and a compound having a pKa of -4.0 to 4.5 is more preferred as component X. Incidentally, when component X is a polybasic acid, it is preferred that the smallest pKa among the multiple pKas is within the above range. Considering only the small pKa and the resulting ability to remove oxide films, it is conceivable to use an inorganic acid such as hydrochloric acid, nitric acid, or sulfuric acid as component X. However, considering the inconvenience that may occur if the acid remains in the layer formed by the conductive composition, an organic acid is preferred as component X. The pKa value here can be the value at room temperature (e.g., 25°C).

[0060] Any compound capable of returning a copper oxide film to a non-oxidized state by a reduction reaction can also be used as component X. For example, a compound having an aldehyde group can be used as component X because it may be able to reduce copper oxide.

[0061] Compounds that have a small pKa in water and can return a copper oxide film to a non-oxidized state through a reduction reaction are also preferably used as component X. Formic acid is an example of such a compound. Formic acid has the advantage that it is easily volatile and therefore does not easily remain in the layer formed from the conductive composition.

[0062] In addition to the above, examples of component X include pyrogallol, phenidone, hydroquinone, and orthoaminophenol, which are known to function as reducing agents in the field of silver halide photography. When a conductive film having a lower resistivity is obtained by infiltrating a certain compound A into a layer formed from a conductive composition compared to when the compound A is not infiltrated, the compound A can be used as component X.

[0063] When a liquid in which component X is dissolved or dispersed is brought into contact with a layer formed from a conductive composition, the concentration of component X in the liquid may be adjusted appropriately from the viewpoints of allowing a sufficient amount of component X to penetrate into the layer formed from the conductive composition and reducing the amount of residual component X to suppress corrosion or deterioration of the conductive film. The concentration of component X in the liquid is, for example, 0.05 to 50 mol / L, preferably 0.1 to 40 mol / L, more preferably 0.1 to 30 mol / L, even more preferably 0.1 to 10 mol / L, and particularly preferably 0.15 to 5.0 mol / L. Of course, a liquid containing component X at a concentration lower than the concentrations shown here may be used, or a liquid containing component X at a concentration higher than the concentrations shown here (for example, saturation concentration) may be used.

[0064] <Applications of conductive substrates> The conductive substrate of this embodiment can be used in a variety of applications.

[0065] (Electronic Devices) As an example, it is possible to manufacture an electronic device including the conductive substrate of this embodiment. By appropriately designing the pattern shape of the conductive layer, it is possible to manufacture a substrate having a conductive layer (circuit pattern) that can function as a circuit. Then, by combining this substrate with other electronic elements, it is possible to manufacture an electronic device.

[0066] Some examples of electronic devices are listed below, but it should be noted that the electronic devices are of course not limited to these. Sensors: For example, the conductive substrate of this embodiment can be applied to conductive members / circuits in sensors such as pressure sensors and vital sensors. Solar cells: For example, the conductive substrate of this embodiment can be applied to the current collecting wiring of solar cells. Membrane switch: A membrane switch is a thin sheet-like switch made by printing circuits and contacts on a film and then laminating it. The matters described in this specification can be applied to forming the circuits and contacts of this switch. Touch sensors and touch panels: For example, the matters described herein can be applied to forming lead wiring in touch sensors and touch panels. It is also conceivable that the matters described herein can be applied to forming transparent electrodes in touch sensors and touch panels. Flexible substrate: Conventionally, circuits are formed by first coating the entire surface of a flexible film with a metal film and then removing unnecessary parts of the metal film using chemicals. Instead of this conventional method, it is conceivable to form circuits as described herein.

[0067] A particularly preferred example of the electronic device is an RF tag, in which the conductive circuit portion, such as the antenna portion, of the RF tag is a conductive film formed by sintering a plurality of copper particles, including dendritic copper particles, and the Sa and Sz of the surface thereof are preferably within the above-mentioned numerical ranges. For the specific structure of the RF tag, reference can be made to, for example, Japanese Patent Application Laid-Open No. 2003-332714 and Japanese Patent Application Laid-Open No. 2020-46834.

[0068] (Electromagnetic wave shielding film) Another application apart from electronic devices is electromagnetic wave shielding films. When forming a film on a substrate by coating or printing a conductive composition, the pattern of the film can be made into a pattern specific to electromagnetic wave shielding films (such as a mesh pattern), allowing the production of electromagnetic wave shielding films.

[0069] (sheet heating element) Another application is a sheet heating element. A sheet heating element is an element that generates heat by passing electric current through electrical wiring provided on a substrate. A specific example of a sheet heating element is a sheet heating element for anti-fogging or cold protection, such as on the rear window of a passenger car. When forming a film on a substrate by coating or printing a conductive composition, the pattern shape of the film can be made into a pattern specific to the sheet heating element, thereby manufacturing the sheet heating element.

[0070] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0071] The embodiments of the present invention will be described in detail based on Examples, Comparative Examples and Reference Examples. However, it should be noted that the present invention is not limited to the Examples. In the following, exponential notation may be indicated by the symbol "E." For example, 1.3E-06 is 1.3 x 10 -6 means.

[0072] <Preparation of Conductive Composition> ·Conductive composition A 75 parts by mass of conductive particles, 4 parts by mass of commercially available polyester resin soluble in organic solvents, and 21 parts by mass of organic solvent were mixed using a planetary mixer. This resulted in a uniform paste-like conductive composition. The conductive particles were dendritic copper particles (D) obtained from Fukuda Metal Foil and Powder Co., Ltd. 50 :5μm) was used. ·Conductive composition B Dendritic copper particles (D) obtained from Fukuda Metal Foil and Powder Co., Ltd. were used as conductive particles. 50 A uniform paste-like conductive composition was obtained in the same manner as in conductive composition A, except that a powder of 1000 sintered particles (0.7 μm) was used. ·Conductive composition C As conductive particles, dendritic copper particles (electrolytic copper powder, D) manufactured by Mitsui Mining & Smelting Co., Ltd. were used. 50A uniform paste-like conductive composition was obtained in the same manner as in conductive composition A, except that a powder of 1000 sintered particles (0.7 μm) was used. ·Conductive composition D As conductive particles, dendritic copper particles (electrolytic copper powder, D) manufactured by Mitsui Mining & Smelting Co., Ltd. were used. 50 A uniform paste-like conductive composition was obtained in the same manner as in conductive composition A, except that a powder of 1000 sintered particles (0.5 μm) was used.

[0073] ·Conductive composition E (reference) The conductive particles used were not dendritic copper particles but atomized copper powder (D) manufactured by Fukuda Metal Foil and Powder Co., Ltd. 50 A uniform paste-like conductive composition was obtained in the same manner as in conductive composition A, except that a powder of 1000 sintered particles (0.5 μm) was used. ·Conductive composition F (reference) The conductive particles used were not dendritic copper particles but wet reduced copper powder (D) manufactured by Mitsui Mining & Smelting Co., Ltd. 50 A uniform paste-like conductive composition was obtained in the same manner as in conductive composition A, except that a powder of 1000 sintered particles (0.5 μm) was used.

[0074] For reference, the dendritic copper particles (D) used in the preparation of the conductive composition A 50 Electron microscope images of the copper particles (diameter: 5 μm) are shown in Figures 2 and 3. These images show that the shape of the copper particles is "dendritic."

[0075] <Formation of a conductive film on a substrate (manufacturing a conductive substrate)> (film formation) Using the above conductive composition, a coating film (solid film) measuring 15 mm x 5 mm was formed on a polyimide film. Specifically, 3M Scotch tape was first applied to a polyimide film (substrate) to create a 15mm x 5mm "cutout." A conductive composition was then applied to the cutout using a squeegee, filling the cutout with the conductive composition. The Scotch tape was then removed. The thickness of the undried film at this point was approximately 40μm. The polyimide film with the above-described film formed thereon was placed in a hot air circulating atmospheric oven and heated at 100° C. for 15 minutes, thereby volatilizing the solvent. In this manner, a laminate having a conductive particle-containing layer provided on a polyimide film was obtained.

[0076] (pressure and heat) 2 μL of an oxide film removing solution (formic acid solution) was dropped onto the conductive particle-containing layer of the laminate to wet the layer, and then a polyimide film (different from the substrate) was placed on top to obtain a laminate of polyimide film (substrate)-conductive particle-containing layer-polyimide film. This stack was placed on a metal plate (30 mm × 20 mm) attached to the lower plate of a flat press (HC300-15K, manufactured by AS ONE Corporation). Pressing was then performed under the conditions of pressure, temperature, and other factors as shown in Table 1. (supplement) The press temperature was adjusted by accurately measuring the temperature using a thermocouple installed on the plate (pressing surface), rather than using the value displayed on the machine. The pressure was calculated as follows: First, the area of ​​the metal plate placed on the flat press (lower plate) is 30 × 20 mm = 600 mm 2 The pressure was calculated as F. 600mm 2 Since the force F was applied to the area, F÷600mm 2 The pressure was calculated by the following calculation.

[0077] After the pressure application, the polyimide film (different from the substrate) was peeled off, and a conductive substrate was produced in which a conductive film was formed on the polyimide film (substrate).

[0078] <Measurement of various indexes related to surface roughness> The conductive film on the conductive substrate was measured for each surface roughness index using a laser microscope (Keyence VK-X3000, 50x objective lens). The measurement method was performed in accordance with ISO 25178. The measurement was performed three times, and the arithmetic mean value of the three values ​​obtained for each index was used as the value for each index. As a side note, when measuring Vvc and Vmc, it is necessary to specify the areal material ratio that separates the core from the protruding peaks and the areal material ratio that separates the core from the protruding valleys. In this study, the default values ​​for the device were used.

[0079] <Evaluation: Surface resistivity and specific resistance> The surface resistance of the obtained conductive film on the conductive substrate was measured using a four-terminal resistance measuring device. The film thickness was measured with a film thickness meter, and the resistivity was calculated from the measured surface resistance and film thickness. The smaller the resistivity value, the better.

[0080] The conductive composition and manufacturing conditions (heating and pressure conditions) are summarized in Table 1, and the results of various measurements and evaluations are summarized in Table 2. In Table 2, "outside the measurement range" for resistivity means 1.4 Ω·cm or more, and "outside the measurement range" for surface resistivity means 1 kΩ / □ or more.

[0081] [Table 1]

[0082] [Table 2]

[0083] As shown in Table 2, the conductive films of Examples 1 to 5, which were formed using a conductive composition containing dendritic copper particles and had Sa of 0.1 to 0.9 μm and Sz of 1.5 to 11.0 μm, exhibited better conductivity than the conductive film of Comparative Example 1, which had Sa greater than 0.9 μm and Sz greater than 11.0 μm.

[0084] In Reference Examples 1 and 2, in which copper particles other than dendritic copper particles were used as raw materials, the specific resistance and surface resistance values ​​were comparable to those of Examples 1 to 5. However, the atomized copper powder used in Reference Example 1 and the wet-reduced copper powder used in Reference Example 2 are more expensive than dendritic copper particles (electrolytic copper powder). In other words, Examples 1 to 5 demonstrate that by using dendritic copper particles, conductive films with good conductivity could be obtained at low cost.

[0085] <Reference image> Figure 4 shows an SEM image of a cross section of the conductive substrate obtained in Example 1, and Figure 5 shows an SEM image of a cross section of the conductive substrate obtained in Example 3. In these cross-sectional images, the interfaces between particles that are in contact at least partially are unclear, suggesting that sintering between the particles is progressing. [Explanation of symbols]

[0086] 1 Base material 3 Conductive film 10 Conductive base material

Claims

1. A conductive film formed by sintering a plurality of copper particles, the plurality of copper particles include dendritic copper particles, The conductive film has an arithmetic mean height Sa of 0.1 to 0.9 μm, as defined in ISO 25178, on at least one surface thereof, and a maximum height Sz of 1.5 to 11.0 μm, as defined in ISO 25178.

2. The conductive film according to claim 1 , The conductive film has a root mean square height Sq of 0.1 to 1.0 μm as defined in ISO 25178 on the one surface.

3. The conductive film according to claim 1 or 2, a conductive film, wherein the one surface has a maximum peak height Sp, as defined in ISO 25178, of 0.9 to 5.0 μm.

4. The conductive film according to claim 1 or 2, The conductive film, wherein a level difference Sk of a core portion on the one surface, as defined in ISO 25178, is 0.1 to 2.5 μm.

5. The conductive film according to claim 1 or 2, The void volume Vvc of the core portion on the one surface as defined in ISO 25178 is 0.1 to 1.3 ml / m ２ A conductive film.

6. The conductive film according to claim 1 or 2, The core material volume Vmc of the one surface as defined in ISO 25178 is 0.05 to 1.0 ml / m ２ A conductive film.

7. The conductive film according to claim 1 or 2, The conductive film has a surface resistivity of 5 to 100 mΩ / □ on one surface.

8. The conductive film according to claim 1 or 2, A conductive film having a thickness of 2 to 100 μm.

9. The conductive film according to claim 1 or 2, The particle diameter D at which the cumulative frequency reaches 50% in a volume-based cumulative particle diameter distribution curve obtained when the particle diameters of the plurality of copper particles are measured by a laser diffraction scattering method. ５０ The conductive film has a thickness of 0.5 to 100 μm.

10. A substrate; The conductive film according to claim 1 or 2 provided on at least one surface of the substrate; A conductive substrate comprising:

11. The conductive substrate according to claim 10, A conductive substrate, wherein the substrate is flexible.

12. An electronic device comprising the conductive substrate of claim 10.

13. An RF tag comprising the conductive substrate according to claim 10.

14. An electromagnetic wave shielding film comprising the conductive substrate according to claim 10.

15. A sheet heating element comprising the conductive substrate according to claim 10.

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