Metal wiring and conductive sheet excellent in bending resistance and metal paste for forming the same

Abstract

The present invention relates to a metal wiring formed on a substrate having flexibility, which is composed of a sintered body of silver particles. The sintered body constituting the metal wiring is characterized in that the volume resistivity is 20 μΩ·cm or less, and the hardness is 0.38 GPa or less and the Young's modulus is 7.0 GPa or less. A conductive sheet provided with the metal wiring can be manufactured by applying and firing a metal paste on a substrate, the metal paste containing silver particles having a prescribed particle diameter and particle diameter distribution as a solid component, and further containing ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less as a regulator. The metal wiring of the present invention can suppress changes in electrical characteristics even when repeatedly subjected to bending deformation, and is excellent in bending resistance.

Description

Technical Field

[0001] This invention relates to metallic wiring for use in wiring materials constituting mobile devices such as tablet computers and smartphones, wearable devices, and displays. More specifically, it relates to metallic wiring and conductive sheets with excellent bending resistance that do not break even when repeatedly bent and maintain their electrical properties, said metallic wiring being formed on a flexible substrate. Background Technology

[0002] In recent years, printed electronics (PCB) technology has attracted attention as a highly efficient design and manufacturing process for circuit boards of various electrical / electronic devices. PCB technology is a process based on printing wiring on a circuit board by coating a metal paste (metal ink). Besides its high efficiency, PCB technology is particularly effective in manufacturing circuit boards for touch panels / displays in mobile devices and wearable devices, where miniaturization and lightweighting are required. By using organic materials, such as lightweight and flexible PET film, as substrates, not only can devices be made lightweight and flexible, but continuous roll-to-roll manufacturing of circuit boards is also possible, thus offering advantages in terms of productivity.

[0003] Metal paste used in printed electronics is a dispersion obtained by dispersing conductive metal particles such as silver in a solvent. The metal paste is then coated onto a substrate, followed by heating and sintering to sinter the metal particles, thereby forming conductive wiring / electrodes. When using organic substrates, the heating and sintering temperature cannot be set above the heat resistance temperature of the organic substrate; therefore, it must be carried out at a low temperature below 200°C.

[0004] The applicant of this application, taking into account the advantages of the aforementioned printed electronics technology and the resulting metal wiring, has made many discoveries regarding various metal pastes and metal wirings, as well as their manufacturing methods (Patent Documents 1 to 5). Among these prior art applications proposed by the applicant, a basic technique is the metal paste / metal ink obtained by dispersing metal particles such as silver, protected by a protective agent such as an amine, in a suitable solvent and optional additives. Furthermore, these prior art documents also clearly define features that satisfy various requirements for the properties and manufacturing efficiency of metal wirings and conductive sheets. Specifically, the metal paste proposed by the applicant can be sintered at a lower temperature and can produce low-resistance metal wirings (Patent Documents 1 and 2). By imparting low-temperature sintering properties to the metal paste, there is no need to worry about thermal damage to the substrate; therefore, resin materials and the like can be used on the substrate, expanding the range of substrate material choices. In addition, according to the metal wiring formation technique proposed by the applicant, extremely fine metal wirings can be formed with highly precise patterns (Patent Document 3). Furthermore, the applicant has also disclosed a method for preventing the visualization of metal wirings caused by light reflection by suppressing the metallic luster of the metal particles (Patent Document 4).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Specification of Japanese Patent No. 5795096

[0008] Patent Document 2: Specification of Japanese Patent No. 6491753

[0009] Patent Document 3: Description of Japanese Patent No. 5916159

[0010] Patent Document 4: Specification of Japanese Patent No. 6496775

[0011] Patent Document 5: Specification of Japanese Patent No. 6496784 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, in recent years, there has been development of miniaturized smartphones and other devices with foldable touchscreens. Foldable displays have also been developed for PCs, tablets, and other devices. Furthermore, for wearable devices, considering comfort and style, curved designs are required to adapt to body movements. Therefore, the conductive sheets used in these various devices are subjected to repeated bending deformation. Moreover, the bending direction is not necessarily fixed in one direction; there are also cases where bidirectional bending deformation occurs.

[0014] For conductive sheets like these, there is a concern that repeated bending and deformation could cause changes in the electrical properties (resistance) of the metal wiring, potentially leading to wire breakage in the worst-case scenario. This is especially true for the touch panels / displays of foldable smartphones, where the radius of curvature (R) is quite small, suggesting significant changes in the electrical properties of the metal wiring. Therefore, for conductive sheets used in such applications, the metal wiring must exhibit durability with minimal changes in electrical properties even when subjected to repeated bending and deformation (it should be noted that in this invention, this durability against repeated bending and deformation is referred to as bending resistance).

[0015] Regarding the bending resistance of metal wiring, the inventors have confirmed instances where conventional metal pastes and the resulting metal wiring exhibit difficulty in achieving the required durability. In this regard, the applicant of this application discloses a conductive sheet as described in Patent Document 5, which possesses bending resistance. This prior art ensures durability by forming a layer of metal wiring for the two systems required for device control on each side of the substrate, thereby increasing the radius of curvature of each metal wiring subjected to bending deformation.

[0016] However, the conductive sheet proposed by the applicant in this application is a conductive sheet with improved bending resistance from a structural perspective, not a conductive sheet aimed at improving the properties of the metal wiring itself. Therefore, it may not be suitable for all device structures. Furthermore, even with this conductive sheet, it is unclear whether it can withstand the harsh repeated bending deformations experienced by foldable smartphones. To cope with harsh repeated bending deformations independently of the conductive sheet structure, it is preferable to impart bending resistance to the metal wiring itself.

[0017] The present invention is made based on the above background and relates to metal wiring formed on a conductive sheet on which a flexible substrate is applied, providing metal wiring with excellent bending resistance that can suppress changes in electrical properties even when repeatedly subjected to bending deformation with a small radius of curvature. Furthermore, a conductive sheet having this metal wiring is provided.

[0018] Methods for solving problems

[0019] The inventors sought to optimize the structure of metal wiring obtained from metal paste containing metal particles in the prior art. Conventional metal wiring obtained from metal powder is not entirely without durability against bending deformation. This can be inferred from the application of metal paste in the bend-resistant conductive sheet proposed by the applicant (Patent Document 5).

[0020] The inventors used silver-containing powder as the metal powder and conducted a rigorous study on the composition of the silver-containing metal paste from multiple aspects. Then, the bending resistance of the metal wiring manufactured from these metal pastes was investigated, and a correlation was found between the specified physical properties of the silver-containing metal wiring and its bending resistance. It was found that, particularly for silver-containing metal wiring, metal wiring with strictly specified hardness and Young's modulus exhibited excellent bending resistance, leading to the present invention.

[0021] That is, the present invention is a metal wiring, which is a metal wiring composed of a sintered body of silver particles formed on a flexible substrate, characterized in that the volume resistivity of the sintered body is 20 μΩ·cm or less, the hardness is 0.38 GPa or less, and the Young's modulus is 7.0 GPa or less. Hereinafter, the structure of the metal wiring of the present invention and the conductive sheet having the metal wiring will be described in detail.

[0022] (I) Metal wiring of the present invention

[0023] As described above, the metal wiring of the present invention is composed of a sintered body of silver particles. Silver was chosen as the constituent metal of the metal wiring because of its suitability as a wiring material, considering its electrical conductivity (low resistivity) and chemical stability. Furthermore, the use of silver particles is considered one of the main reasons why the sintered body of the metal wiring exhibits the hardness and Young's modulus described later. In the present invention, sintering refers to a state in which adjacent silver particles are bonded together, that is, a state in which the sintered body is bonded with a force greater than that required to prevent it from collapsing under its own weight. This is not limited to the state in which plastic deformation and necking occur between the constituent particles, as in "sintering" formed by powder metallurgy, resulting in a strong bond between the particles; it should be interpreted broadly. To improve bending resistance, the metal wiring of the present invention, composed of a sintered body of silver particles, is characterized in terms of hardness and Young's modulus, and has a preferred range in terms of the particle size and particle size distribution of the silver particles. These components will be described below.

[0024] (I-1) Hardness and Young's modulus of sintered body

[0025] Regarding the metal wiring of the present invention, the hardness and Young's modulus of the sintered body constituting the metal wiring are specified because it has been confirmed that sintered bodies exceeding the upper limit of these physical property values ​​have poor bending resistance, and that electrical properties (resistance) increase with the number of bending cycles during repeated bending deformation. Furthermore, when defining the structure of the metal wiring of the present invention, which is composed of a sintered body of silver particles, the optimal method is to determine the hardness and Young's modulus of the sintered body. When directly determining the structure of the sintered body of silver particles, the constituent elements used for this purpose involve many aspects. That is, in addition to the particle size, particle size distribution, and interparticle bonding strength of the silver particles, many constituent elements such as the content (residual amount) of organic matter from additives (such as ethyl cellulose) contained in the metal paste described later can be inferred. It is inferred that these constituent elements participate in each other and in a complex manner in the metal wiring of the present invention. Although it is difficult to determine each individual constituent element, the hardness and Young's modulus of the sintered body are manifested due to the participation of these elements. In the present invention, the structure of the metal wiring is determined by specifying the hardness and Young's modulus of the sintered body.

[0026] As described above, the metal wiring of the present invention requires a hardness of 0.38 GPa or less and a Young's modulus of 7.0 GPa or less. Metal wiring with values ​​higher than these has insufficient bending resistance. Both hardness and Young's modulus need to be below the aforementioned values; metal wiring with either value greater than these values ​​has poor bending resistance. Furthermore, in order to have durability against bending deformation at even smaller radii of curvature, a hardness of 0.30 GPa or less and a Young's modulus of 6.0 GPa or less are preferred.

[0027] It should be noted that regarding the lower limits of hardness and Young's modulus, a hardness of 0.18 GPa or higher and a Young's modulus of 5.2 GPa or higher are preferred. Metal wiring with hardness and Young's modulus below these values ​​is insufficient in strength, becomes brittle, and may break under slight pressure, failing to maintain its shape stably. Such wiring may cause resistance variations during use, and may also lead to problems in other situations, such as during the conductive sheet manufacturing process. In the conductive sheet manufacturing process, a winding process is sometimes added. During this winding, the pressure applied to the sheet surface may cause wiring breakage and an increase in resistance.

[0028] Regarding the determination of the hardness and Young's modulus of the metal wiring of the present invention, when the wiring width and thickness are large, they can be determined by combining general hardness testers (Vickers hardness testers, micro Vickers hardness testers) and tensile tests. However, the metal wiring of the present invention is intended for application to micro-wires. As a technique for determining the hardness and Young's modulus of such narrow and extremely thin metal wiring, nanoindentation is known and is effectively applied in the present invention. Nanoindentation is a method for determining hardness and Young's modulus based on the load and indentation depth when an indenter of a measuring device called a nanoindenter is indented into the test object. The load generated by the indenter of the nanoindenter is precisely controlled by electromagnetic control, and the amount of movement of the indenter is also precisely measured electrically. By measuring with a nanoindenter, the contact stiffness and contact depth of the test object can be determined, thereby allowing the calculation of hardness and Young's modulus. The method for determining physical properties using nanoindentation has been standardized by the International Organization for Standardization (ISO) for micro-indentation testing (ISO 14577).

[0029] (I-2) Composition of silver particles constituting the sintered body

[0030] (i) Average particle size of silver particles

[0031] The metal wiring of the present invention is a sintered body of silver particles. Here, the average particle size of the silver particles constituting the sintered body is preferably 100 nm or more and 200 nm or less. According to the inventors, the average particle size of the silver particles is related to the hardness and Young's modulus of the metal wiring as its sintered body, and can affect the bending resistance. In particular, when the average particle size is less than 100 nm, the sintered body of fine silver particles tends to increase the hardness and Young's modulus, resulting in insufficient bending resistance of the metal wiring. Furthermore, regarding silver particles with an average particle size greater than 200 nm, due to poor low-temperature sintering performance, the resistivity after sintering becomes higher, potentially exceeding 20 μΩ·cm. To improve bending resistance, the average particle size of the silver particles is preferably 120 nm or more and 180 nm or less, more preferably 140 nm or more and 180 nm or less.

[0032] (ii) Particle size distribution of silver particles (standard deviation)

[0033] Furthermore, regarding the particle size distribution of the silver particles constituting the metal wiring, a sintered body with a relatively large deviation of silver particles is preferred in this invention. Specifically, the standard deviation of the silver particle size is preferably 40 nm or more and 120 nm or less. When the silver particle size is uniform, i.e., when the standard deviation is small, an excessively dense sintered body is formed. Such a sintered body increases hardness and Young's modulus, but decreases bending resistance. On the other hand, if the deviation of the silver particle size is too large, i.e., when the standard deviation is too large, the hardness and Young's modulus may be insufficient.

[0034] (iii) Purity of silver particles

[0035] It should be noted that the purity of the silver particles constituting the metal wiring of the present invention is preferably 97% by weight or more. This is because when the purity of the silver particles is less than 97% by weight, it may lead to an increase in sintering temperature, an increase in resistivity, and a decrease in bending resistance.

[0036] (I-3) Composition of sintered body

[0037] (i) Volume resistivity of sintered body

[0038] The metal wiring of the present invention is composed of a sintered body of silver particles, thereby achieving a volume resistivity of 20 μΩ·m or less (at 20°C). This volume resistivity differs from that exhibited by fine wires / films composed of dense (bulk) silver that is not a sintered body. For bulk silver, the volume resistivity is approximately 1.6 μΩ·cm. It should be noted that a lower volume resistivity of the metal wiring is preferred, but the lower limit of the volume resistivity of the metal wiring of the present invention is 2.0 μΩ·cm or more.

[0039] (ii) Thickness of sintered body

[0040] In this invention, the thickness of the sintered body of silver particles forming the metal wiring is preferably set to 1 μm or more and 20 μm or less. Excessively thick metal wiring tends to have poor bending resistance, and the resistance value changes more significantly due to repeated bending deformation. Furthermore, for metal wiring composed of a sintered body of silver particles, if the thickness is too small, the uniformity of the resistance value may decrease. Taking these factors into consideration, the thickness of the metal wiring is preferably within the above-mentioned range. More preferably, the thickness of the metal wiring is set to 10 μm or less.

[0041] (iii) Other components of the sintered body

[0042] The sintered body of the silver particles constituting the metal wiring of the present invention is basically composed of silver (silver powder). However, since it is a sintered body, it has a small amount of porosity. In addition, sometimes organic matter such as ethyl cellulose is contained in the pores of the silver powder. Such organic matter comes from the components of the metal paste, which is a precursor to the sintered body, i.e., modifiers and additives. However, such organic matter is present in extremely small amounts and is difficult to quantify using conventional analytical methods. The inventors believe that the presence or absence of these pores and organic matter, as well as their state of existence, along with the particle size of the silver particles, affect the bending resistance of the sintered body (metal wiring). However, it is difficult to determine the content of organic matter. In the present invention, it is believed that these various factors have a complex influence on bending resistance, and hardness and Young's modulus are used as means to determine the metal wiring.

[0043] (II) The conductive sheet of the present invention

[0044] The conductive sheet of the present invention having the above-described metal wiring will be described. The conductive sheet of the present invention is constructed by forming the above-described metal wiring of the present invention on at least one surface of a flexible substrate.

[0045] (II-1)Substrate

[0046] The material, shape, and size of the substrate used in the conductive sheet of this invention are not particularly limited. However, considering the problem to be solved by this invention, it is made of a flexible material that can be bent and deformed. Examples of substrate materials include metals, resins, and plastics. It should be noted that although the flexibility of the substrate does not need to be specified, since this invention takes into account the application of conductive sheets used in foldable devices, a substrate with a flexible material and size that can be bent to a radius of curvature of 1 mm or less, more preferably 0.5 mm or less, is preferred.

[0047] It should be noted that, in this invention, the radius of curvature of the conductive sheet generated by bending deformation refers to half the value of the interval (gap) generated on the inner side of the bend. For example, as... Figure 1 As shown, when bending deformation is performed with the surface containing the metal wiring as the inner side, the distance between the metal wirings is defined as half the distance between each other as the radius of curvature R. When bending deformation is performed with the surface containing the metal wiring as the outer side, the distance between the inner substrates is defined as half the distance between each other as the radius of curvature R.

[0048] Furthermore, since the present invention can handle bending deformation with a small radius of curvature R, it is also durable for bending deformation with a large radius of curvature R. In addition, the substrate must be flexible, but does not need to be subjected to frequent bending deformation. Therefore, there is no upper limit to the radius of curvature R of the conductive sheet, and it can include cases where the curvature of the conductive sheet is 0.

[0049] Furthermore, if the application is considered in display devices such as monitors and touch panels, the substrate is preferably made of a transparent material. Considering the above aspects, specific materials that can be used as substrates include polyethylene terephthalate (PET), polyimide (PI), and polyamide.

[0050] It should be noted that the substrate does not necessarily have to be a single-layer structure. For example, a multi-layer substrate with a primer layer made of resin or the like formed on a substrate of the aforementioned material can also be considered the substrate of this invention. This primer layer is formed to improve the adhesion between the metal wiring and the substrate, and is made of resin materials such as epoxy resin. The primer layer is related to the adhesion of the metal wiring, but it does not affect the bending resistance of the metal wiring itself. Since the metal wiring of this invention has its own bending resistance, it is not necessary to form a primer layer on the substrate.

[0051] (II-2) Metal wiring on the substrate

[0052] In the conductive sheet of the present invention, a metal electrode is formed on at least one of the surface or back surface of the substrate. There are no limitations on the formation pattern of the metal electrode, and it can be freely set according to the design of the circuit board, such as parallel wiring patterns or interlaced grid-like wiring patterns. The configuration of the metal wiring formed on the substrate (average particle size of silver particles, etc.) is as described above.

[0053] There are no particular restrictions on the size of the metal wiring. However, regarding the thickness of the metal wiring, as mentioned above, from the viewpoint of bending resistance, it is preferable to set it to 1 μm or more and 20 μm or less.

[0054] (II-3) Other structures of conductive sheets

[0055] The conductive sheet of the present invention has the aforementioned substrate and metal wiring as its basic structure. However, other structures may also include a coating layer that covers the substrate together with the metal wiring. The coating layer is formed to prevent displacement of the metal wiring, provide moisture / oxidation protection, and prevent peeling. Furthermore, the coating layer may also be formed as a surface layer of the conductive sheet to prevent scratches, etc. Examples of materials for such coating layers include, for example, resins used to prevent displacement such as fluorinated resins, acrylic resins, epoxy resins, alkyd resins, vinyl resins, phenolic resins, and silicone resins. Examples of materials for coating layers intended as surface coatings include, for example, fluorinated resins, acrylic resins, epoxy resins, alkyd resins, vinyl resins, phenolic resins, and silicone resins. The coating layer can be formed as a single layer or a combination of multiple layers can be used. The thickness of the coating layer can be adjusted according to its application and the materials used, and there are no particular limitations.

[0056] One of the particularly preferred applications of the conductive sheet of the present invention described above is as a component of display devices such as displays and touch panels. For example, by attaching a flexible protective glass or protective film to the conductive sheet of the present invention and connecting a control connector, displays and the like can be manufactured.

[0057] (III) Metal paste for forming metal wiring according to the present invention

[0058] As described above, the metal wiring of the present invention is composed of a sintered body of silver particles, which is manufactured by using a metal paste containing silver particles as a precursor. In the present invention, the relationship between the composition of the aforementioned metal paste and the physical properties (hardness, Young's modulus) of the sintered body obtained from the metal paste was studied, and a suitable metal paste for manufacturing metal wiring with bend resistance was discovered.

[0059] The metal paste of the present invention comprises a solid component consisting of silver particles, a solvent, a modifier, and optional organic additives. The solid component consists of silver particles with an average particle size of 100 nm to 200 nm and a standard deviation of 40 nm to 120 nm. The silver particles constituting the solid component are bonded with at least one amine compound having 4 to 8 carbon atoms as a protective agent. The modifier is ethyl cellulose with a number average molecular weight of 10,000 to 90,000. As described above, the metal paste of the present invention comprises a solid component consisting of silver particles, a solvent, a modifier, and optional organic additives. The components will be described below.

[0060] (III-1) Silver particles in metallic paste (solid component)

[0061] In the metal paste of the present invention, the average particle size of the silver particles, which are the solid component, is set to be 100 nm or more and 200 nm or less. This is to ensure that the hardness and Young's modulus, which affect the bending resistance of the sintered body (metal wiring), are within an appropriate range. When the average particle size is less than 100 nm, the sintered body of fine silver particles forms a denser sintered body than necessary, tending to increase the hardness and Young's modulus. On the other hand, with respect to silver particles with an average particle size greater than 200 nm, there is a problem with the increased resistivity after sintering due to poor low-temperature sintering performance. Moreover, a more preferred average particle size of silver particles is 120 nm or more and 180 nm or less, and more preferably 140 nm or more and 180 nm or less.

[0062] Furthermore, the standard deviation of the silver particle size distribution in the metal paste is set to be 40 nm or more and 120 nm or less. When using silver particles with uniform particle size and small standard deviation, the silver particles aggregate without gaps during the paste coating stage, forming a dense sintered body. This dense sintered body increases hardness and Young's modulus, but decreases flexural strength. In this respect, conventional metal pastes tend to use silver particles with uniform particle size and sharp peaks in their particle size distribution. In this invention, from the viewpoint of adjusting the hardness and Young's modulus to improve the flexural strength of the sintered body, a metal paste with a larger deviation of silver particles, opposite to conventional ones, is used. However, if the deviation of the silver particle size is too large, it is possible that the paste contains silver particles with undesirable particle sizes. For example, when the average particle size is 180 nm, if the standard deviation is too large, the content of particles larger than 200 nm increases, which may result in a decrease in low-temperature sintering performance. Based on the above reasons, in this invention, the standard deviation of the silver particle size is set to be above 40 nm and below 120 nm.

[0063] As a method for manufacturing silver particles having the average particle size and particle size distribution described above, a thermal decomposition method using a thermally decomposable silver compound as a raw material is preferred. The thermal decomposition method is as follows: Silver compounds such as silver oxalate (Ag₂C₂O₄), silver carbonate (Ag₂CO₃), and silver oxide (Ag₂O), which can precipitate silver through heating and decomposition, are used as raw materials. An organic compound such as an amine is mixed with this raw material to form a silver-amine complex. This complex is then heated and decomposed to precipitate silver particles. A metal paste can be manufactured by recovering the silver particles precipitated in this manner and adding them to a solvent. The silver particle protectant (amine compound) described later is added during the formation stage of the silver-amine complex.

[0064] In the thermal decomposition method, the average particle size of silver particles can be adjusted by regulating various manufacturing conditions such as the heating temperature of the amine complex, the heating rate, and the water content of the reaction system. Furthermore, the metal paste of the present invention requires increasing the deviation (standard deviation) of the particle size distribution of silver particles. As a method for imparting such a deviation to the silver particles in the metal paste, a method can be listed as follows: controlling the cooling rate after heating and synthesizing the silver particles during the silver particle manufacturing process.

[0065] The reaction in which silver particles precipitate from the amine complex is carried out at a specified high temperature (e.g., above 100°C). By continuously heating at this temperature, the silver particles can grow to the target particle size. During this heating stage, the average particle size increases due to the sintering of all particles in the reaction system, but it is not easy to cause an increase in deviation. Then, heating is stopped when the silver particles reach the target average particle size, but because there is residual heat in the reaction system after heating is stopped, the sintering of silver particles continues due to this heat. At this time, if the reaction system is rapidly cooled after heating is stopped, the sintering of silver particles caused by residual heat can be uniformly suppressed, resulting in silver particles with uniform particle size. Therefore, in order to cause deviation in the particle size of silver particles, it is necessary to partially allow the sintering of silver particles to occur during the cooling process of the reaction system after heating is stopped. Specifically, by controlling the cooling rate in the intermediate temperature region (60°C to 90°C) during the cooling of the reaction system, the particle size of silver particles can be deviated. As for the cooling rate, it is preferable to set it to 1°C / min or less between the high temperature region where the reaction stops and the reaction system reaches 60°C. When cooling is performed at a rate higher than this, as mentioned above, sintering is suppressed and particle size deviation is not sufficient.

[0066] Regarding the silver particles described above, their content in the metal paste, relative to the overall mass of the paste, is preferably set to 60% by mass or more and 75% by mass or less. Increasing the silver particle content in the metal paste allows for the efficient production of sintered bodies with thickness. If the silver powder content is less than 60% by mass, the production efficiency of the sintered body is poor. However, when the silver particle content is greater than 75% by mass, the hardness and Young's modulus increase, resulting in a sintered body with poor flexural strength.

[0067] Furthermore, in the metal paste of the present invention, silver particles are dispersed in the solvent in a state bound to amine compounds that serve as protective agents, having an average carbon number of 4 to 8. A protective agent is a compound bound to a portion or the entire surface of the metal particles; it is an additive used to inhibit the aggregation of metal particles in a dispersion such as the metal paste. In the present invention, one or more amine compounds are bound to the silver particles as protective agents. In this case, the protective agent bound to the silver particles is an amine compound with an overall average carbon number of 4 to 8. The overall average carbon number refers to the number of carbon atoms calculated by distributing these carbon atoms according to the amount (mole fraction) of one or more amine compounds contained in the metal paste. This is a value obtained by averaging these carbon atoms assuming that the amine compounds contained in the metal paste are uniformly bound to the silver particles.

[0068] To achieve sintering of silver particles at a relatively low temperature (e.g., below 150°C), the protective agent used in this invention is limited to an amine compound. In many cases, flexible resins or plastics are used as substrates in this invention. High temperatures are required for the sintering of silver particles, which may cause thermal damage to the substrate. Amine compounds volatilize at lower temperatures, promoting the sintering of silver particles together. Furthermore, the average carbon number of the amine compound used as the protective agent is set to 4-8 because amines with an average carbon number less than 4 lack protective properties and are difficult to stably maintain the silver particles. On the other hand, amines with an average carbon number greater than 8 require a high sintering temperature to form wiring with a specified low resistance value. It should be noted that as long as the average carbon number of the protective molecule is 8 or less, a larger molecular weight amine with 9 or more carbon atoms can also be used. For example, an amine with 4 carbon atoms can be mixed with an amine with 12 carbon atoms to use as a protective agent. In this case, an average carbon number of 8 or less per molecule is sufficient.

[0069] As amine compounds, (mono)amines having one amino group or diamines having two amino groups can be used. Furthermore, the number of hydrocarbon groups bonded to the amino group is preferably one or two, and they are preferably primary amines (RNH2) or secondary amines (R2NH). Moreover, when using a diamine as a protecting agent, it is preferable that at least one amino group is a primary or secondary amine. The hydrocarbon group bonded to the amino group can be a chain hydrocarbon group with a straight-chain or branched structure, or it can be a cyclic hydrocarbon group. Additionally, oxygen may be included in a portion of the hydrocarbon group.

[0070] Preferred specific examples of the protective agents used in this invention include amine compounds such as butylamine (4 carbon atoms), 1,4-diaminobutane (4 carbon atoms), 3-methoxypropylamine (4 carbon atoms), pentylamine (5 carbon atoms), 2,2-dimethylpropylamine (5 carbon atoms), 3-ethoxypropylamine (5 carbon atoms), N,N-dimethyl-1,3-diaminopropane (5 carbon atoms), hexylamine (6 carbon atoms), heptylamine (7 carbon atoms), benzylamine (7 carbon atoms), N,N-diethyl-1,3-diaminopropane (7 carbon atoms), octylamine (8 carbon atoms), 2-ethylhexylamine (8 carbon atoms), nonylamine (9 carbon atoms), decylamine (10 carbon atoms), and dodecylamine (12 carbon atoms).

[0071] The amount of the protective agent (amine compound) in the metal paste of the present invention is preferably 80 ppm or more and 27,000 ppm or less based on the weight of the metal paste. When the amount of the protective agent is less than 80 ppm, the protective effect on silver particles is insufficient, and the dispersibility of silver particles in the metal paste is reduced. When the amount of the protective agent is greater than 27,000 ppm, it may remain on the sintered body, which may affect the hardness and Young's modulus of the sintered body.

[0072] (III-2) Regulator

[0073] The metal paste of this invention comprises silver particles as the aforementioned solid component, and ethyl cellulose as a modifier with a number average molecular weight of 10,000 or more and 90,000 or less. Ethyl cellulose having this number average molecular weight range is an essential component when forming the metal wiring (sintered body) of this invention.

[0074] Regarding this point, the particle size and particle size distribution of the silver particles mentioned above are also factors affecting the physical properties of the sintered body. However, even if the silver particles in the metal paste are appropriate, a metal paste without ethyl cellulose of the aforementioned number-average molecular weight range cannot form a metal wire with bend resistance. The inventors believe that the modifier plays a role in the sintering process of the silver particles. It is believed that by dispersing an appropriate number-average molecular weight modifier together with silver particles of a specified particle size / particle size distribution, the movement and filling state of the silver particles when the metal paste is applied to the substrate becomes appropriate, thus resulting in a sintered body suitable for bend resistance.

[0075] The number-average molecular weight of ethyl cellulose, an essential component of the metal paste, is kept below 90,000 because adding high-molecular-weight ethyl cellulose with a number-average molecular weight greater than 90,000 as a modifier increases the paste viscosity, making it unsuitable for printing. On the other hand, excessively low number-average molecular weight ethyl cellulose makes it difficult to ensure the compactness of the sintered body, hindering the production of low-resistance metal wiring. From this perspective, the lower limit of the number-average molecular weight of ethyl cellulose is set above 10,000.

[0076] The metal paste preferably contains ethyl cellulose with a number average molecular weight of 10,000 or more but less than 60,000. This is because such low molecular weight ethyl cellulose is believed to further improve the flexural strength of the sintered body. Furthermore, even when a metal paste containing low molecular weight ethyl cellulose also contains high molecular weight ethyl cellulose with a number average molecular weight greater than 60,000 but less than 90,000, good flexural strength is still desirable, making this a preferred approach. High molecular weight ethyl cellulose is considered to have a good effect on adjusting the printability of the metal paste. In processes that use metal paste to form metal wiring, most involve coating the metal paste onto a substrate using methods such as screen printing. By adding high molecular weight ethyl cellulose in addition to low molecular weight ethyl cellulose, printability can be improved, thereby suppressing issues such as the paste not adhering to the applicator and poor transfer to the substrate during metal paste coating. However, as mentioned above, excessively high molecular weight ethyl cellulose may cause a decrease in printability due to increased paste viscosity; therefore, even when containing low molecular weight ethyl cellulose, ethyl cellulose with a molecular weight greater than 90,000 should not be added.

[0077] The content of ethyl cellulose, which is a modifier with a number average molecular weight of 10,000 or more and 90,000 or less, is preferably 0.50% by mass or more and 2.6% by mass relative to the total mass of the paste. When it is less than 0.50% by mass, it is difficult to achieve the desired effect of adding ethyl cellulose. Furthermore, since excessive mixing of ethyl cellulose reduces the resistivity of the sintered body, 2.6% by mass is set as an upper limit. As described above, both low molecular weight ethyl cellulose (number average molecular weight of 10,000 or more and 60,000 or less) and high molecular weight ethyl cellulose (number average molecular weight greater than 60,000 and 90,000 or less) can be added simultaneously as modifiers for the metal paste. Even in this case, the total content of each ethyl cellulose is preferably within the above range. It should be noted that when manufacturing a metal paste containing two or more types of ethyl cellulose, each ethyl cellulose can be added to the solvent sequentially, or the two or more ethyl celluloses can be mixed beforehand and then added to the solvent.

[0078] Furthermore, in a metal paste containing both low molecular weight ethyl cellulose (number average molecular weight of 10,000 or more and 60,000 or less) and high molecular weight ethyl cellulose (number average molecular weight of 60,000 or more and 90,000 or less), the content of high molecular weight ethyl cellulose relative to the mass of the metal paste is set as C. H (mass%), the content of low molecular weight ethyl cellulose is set as C. L When (mass%), it is preferable to use C H Compared to C L The proportion (C) H / C L Set it to 5 or higher. Regarding C... H Compared to C L The proportion (C) H / C L There is no specific upper limit for the content of ethyl cellulose. This is because even using only high molecular weight ethyl cellulose can impart a certain degree of bending resistance to the sintered body. However, in order to form metal wiring capable of withstanding bending deformation with extremely low radii of curvature, it is preferable to use C... H Compared to C L The ratio is set to be below 9.

[0079] (III-3) Solvent

[0080] The metal paste of the present invention is formed by mixing and dispersing the aforementioned silver particles and modifier in a solvent. Preferably, the solvent is an organic solvent with 8 to 16 carbon atoms and an OH group in its structure, having a boiling point of 280°C or lower. When sintering the silver particles coated with the metal paste, the solvent must be evaporated and removed. To perform sintering of the silver particles and removal of the solvent at a lower temperature, a solvent with a boiling point of 280°C or lower is preferred. Preferred specific examples of the solvent include: terpineol (C10, boiling point 219°C), dihydroterpineol (C10, boiling point 220°C), dodecyl alcohol ester (C12, boiling point 260°C), 2,4-dimethyl-1,5-pentanediol (C9, boiling point 150°C), and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (C16, boiling point 280°C). Multiple solvents can be used in combination or individually.

[0081] Regarding the mixing ratio of solvent and other components (silver particles (protectant), regulator) in the overall metal paste, the solvent content is preferably set to 19% by mass or more and less than 40% by mass relative to the overall paste. When the solvent content is less than 19% by mass, the viscosity is too high and difficult to print or coat; when the solvent content is greater than 40% by mass, the viscosity is too low and difficult to screen print.

[0082] (III-4) Optional organic additives

[0083] In addition to the essential components described above, the metal paste of the present invention may also contain organic compounds as optional additives. Specifically, a primer or similar material for improving adhesion to the printing substrate may be added. The amount of these optional organic additives added is preferably set to 1% by weight or less relative to the total weight of the paste. It should be noted that the metal paste of the present invention does not contain glass frit commonly used in metal pastes for forming so-called electrode films (thick films). Thick film forming metal pastes contain metal particles and are therefore similar to the metal paste of the present invention. However, the metal paste of the present invention is used to form fine metal wiring and is significantly different from thick film forming metal pastes. Furthermore, the metal wiring formed by the metal paste of the present invention naturally does not contain glass frit components.

[0084] (IV) Method for manufacturing the metal wiring and conductive sheet of the present invention

[0085] The metal wiring of the present invention is manufactured by applying the aforementioned metal paste onto a substrate and sintering silver particles. Furthermore, the conductive sheet of the present invention is manufactured by forming metal wiring on a substrate in such a manner.

[0086] Methods for applying metal paste to a substrate include screen printing, dip coating, spin coating, and roll coating. Screen printing is preferred for forming patterned, fine metal wiring. In screen printing, by appropriately using a screen mask, metal wiring of the desired thickness and pattern can be formed.

[0087] After the metal paste is applied, a firing process (heat treatment) is performed to sinter the silver particles and form a sintered body. The purpose of the firing process is also to volatilize and remove the solvent, modifier, and protective agent (amine compound) from the metal paste. This firing process is preferably performed at a temperature above 50°C and below 200°C. Above 50°C, the volatilization of solvents, etc., takes a long time, and the sintering of silver particles is difficult. On the other hand, below 200°C, substrate deformation and thermal damage may occur. The firing time is preferably 3 minutes or more and 120 minutes or less. It should be noted that the firing process can be performed in an atmospheric atmosphere, or in a vacuum atmosphere, a reduced pressure atmosphere, or an inert gas atmosphere. Furthermore, to remove the solvent and protective agent before and after the firing process, a cleaning treatment using a poor solvent can be performed. As the solvent for the cleaning treatment, highly polar organic solvents such as water or alcohols are preferred.

[0088] Metal wiring is formed by coating the metal paste described above and sintering and bonding the sintered metal particles. A conductive sheet is then manufactured by forming this metal wiring. The aforementioned coating layer or similar material can be appropriately formed on the manufactured conductive sheet.

[0089] Invention Effects

[0090] As explained above, the metal wiring of the present invention, composed of a sintered body of silver particles, exhibits excellent bending resistance and is not prone to changes in electrical properties even when subjected to repeated bending deformation. In particular, it can maintain good resistance values ​​even under repeated bending deformation with extremely small radii of curvature of less than 1 mm and further less than 0.5 mm. Attached Figure Description

[0091] Figure 1 A diagram illustrating the radius of curvature (R) when a conductive sheet is bent and deformed.

[0092] Figure 2 A graph showing the results of repeated bending tests on the metal wiring manufactured in the first embodiment (relationship between the number of bends and the resistance value).

[0093] Figure 3 SEM images showing the cross-sectional structure of the metal wiring of No.1 to No.6 in the second embodiment. Detailed Implementation

[0094] First Implementation MethodHereinafter, a preferred embodiment of the present invention will be described. In this embodiment, silver particles having a suitable average particle size and particle size distribution are manufactured, and a metal paste is prepared by dispersing the silver particles and low molecular weight ethyl cellulose in a solvent. Then, metal wiring is formed on a resin substrate using the metal paste, and the bending resistance is evaluated.

[0095] [The manufacture of silver particles]

[0096] As the silver compound used as a raw material, 102.2 g of silver carbonate (silver content 80.0 g) was used. To prepare this silver compound, 37.3 g of water (36.4 wt% relative to 100 parts by mass of silver carbonate) was added to form a wetted silver compound. Then, 3-methoxypropylamine, an amine compound used as a protecting agent (at a molar ratio of 6 times the mass of silver in the silver compound), was added to the silver compound to prepare a silver-amine complex. The silver compound and the amine were mixed at room temperature to appropriately reduce the surface area of ​​the uncomplexed portion of the silver compound.

[0097] For the aforementioned silver-amine complex, water was added as needed, taking into account the moisture content. Then, the moisture content of the reaction system was checked before heating. For reaction systems where the moisture content was confirmed, heating was initiated at room temperature to decompose the silver-amine complex and precipitate silver particles. The heating temperature at this point was assumed to be 110°C to 130°C, which is the decomposition temperature of the complex, and this was taken as the arrival temperature. Furthermore, the heating rate was set to 10°C / minute. During the heating process, the generation of carbon dioxide was confirmed from near the decomposition temperature. Heating continued until the generation of carbon dioxide ceased.

[0098] After the heating process, while the reaction solution is allowed to cool to room temperature by stopping heating, it is simultaneously kept at the original temperature and allowed to return to room temperature at a cooling rate of approximately 0.4°C / min. After the silver particles precipitate, methanol is added to the reaction solution for washing, followed by centrifugation. This washing and centrifugation process is repeated twice.

[0099] [Making of Metal Paste]

[0100] Texanol ester, used as a solvent, was compounded into the silver particles prepared above, and low molecular weight ethyl cellulose was further added to produce a metallic paste (silver paste). Commercially available ethyl cellulose (manufactured by Dow Chemical Company, ETHOCEL (registered trademark) STD7 (number average molecular weight 17347)) was used as the low molecular weight ethyl cellulose. The silver particle content was set at 70% by mass relative to the total paste, and the low molecular weight ethyl cellulose content was set at 1.95% by mass relative to the total paste.

[0101] For the metal paste manufactured in this embodiment, the average particle size and particle size distribution of silver particles were measured. In this measurement, the metal paste was appropriately sampled and observed using SEM. For the obtained SEM images, the particle size of each of 500 silver particles was measured using the biaxial averaging method, and the average value (median diameter) and standard deviation were calculated. In this embodiment, the average particle size of the silver particles in the metal paste was 120 nm, and the standard deviation was 71.3 nm.

[0102] [Manufacturing of conductive sheets]

[0103] The conductive sheet is manufactured using the prepared metal paste. A transparent resin substrate made of polyethylene terephthalate (PET) (size: 150mm × 150mm, thickness 38μm) is used as the substrate. The prepared metal paste is then screen-printed onto the substrate using an SUS screen mask. After leveling for 10 minutes, the substrate is fired at 120°C for 1 hour to produce a conductive sheet with metal wiring consisting of a sintered body of silver particles. In this embodiment, multiple screen masks are used to form parallel metal wiring (60mm in length) with linewidths of 0.1mm, 0.2mm, and 0.5mm on the substrate. It should be noted that the thickness of the metal wiring is approximately 4.1μm.

[0104] [Repeated bending test]

[0105] The conductive sheets (0.1 mm, 0.2 mm, and 0.5 mm in line width) manufactured in this embodiment were subjected to repeated bending tests to study the bending resistance of the metal wiring. During the repeated bending test, for any wire of the manufactured conductive sheet, a terminal was connected at a position 10 mm from one end and another 10 mm from the opposite end. In the repeated bending test, the resistance value of the metal wiring before the test was first measured using a digital tester.

[0106] Then, the radius of curvature for the bending deformation is set to 1.0 mm, and bending deformation is applied along the center line of the substrate of the conductive sheet. In this embodiment, as... Figure 1 As shown, bending deformation was applied with the metal wiring facing inwards. Regarding the number of bends, the number of bends was counted as one when the film reached a V-shaped state, and 100,000 bending deformations were applied. In this repeated bending test, the resistance value of the metal wiring was measured using a digital tester every 20,000 bends. For this repeated bending test, in... Figure 2 The graph shown in the image represents the change in resistance value based on the number of bends.

[0107] Depend on Figure 2It can be seen that the metal wiring manufactured in this embodiment exhibits almost no change in resistance even after 100,000 repeated bending deformations (with a radius of curvature of 1.0 mm), demonstrating excellent bending resistance. Specifically, compared to the initial resistance value (R0) during the test... i The maximum resistance value (R) observed during 100,000 deformation cycles. max The ratio is approximately 1.4 times. Moreover, it has been confirmed that this excellent bending resistance is maintained even when the line width of the metal wiring is set to an extremely fine 0.1 mm.

[0108] Second Implementation Method In this embodiment, a metal paste is manufactured by producing six types of silver particles (lots a to f; the silver particles of the first embodiment are lot c) with an average particle size of 80 nm to 180 nm and two types of silver particles (lots g and h) with a sharp particle size distribution and a small standard deviation. In the manufacture of these silver particles, the average particle size (lots a to f) is adjusted by changing the amount of water added in the manufacturing process of the first embodiment. Furthermore, the standard deviation of the particle size (lots g and h) is adjusted by rapidly cooling the reaction tank with cold water during the cooling process after the heating step in the manufacturing process of the first embodiment. The average particle size and standard deviation of the particle size of the silver particles manufactured in this embodiment are shown in Table 1 below.

[0109] [Table 1]

[0110] Ag particles batch number Average particle size (nm) Standard deviation (nm) a 80 44.5 b 100 41.9 C 120 71.3 d 140 78.3 e 160 77.4 f 180 107.1 g 80 34.5 h 120 32.1

[0111] Then, the above eight types of silver particles are dispersed together with the regulator in a solvent to produce a metal paste. In this embodiment, when producing the metal paste, metal pastes with different mixing amounts of silver particles and different types of regulators (one or a combination of low molecular weight ethyl cellulose with a number average molecular weight of 10,000 or more and 60,000 or less, and high molecular weight ethyl cellulose with a number average molecular weight of more than 60,000 and less than 90,000) are produced.

[0112] Then, metal wiring is formed using the various metal pastes produced, conductive sheets are manufactured, and their bending resistance is evaluated. The method for manufacturing the metal wiring is the same as in the first embodiment. However, regarding the line width, the most stringent condition for evaluating bending resistance is met, i.e., a line width of 0.1 mm. The bending resistance evaluation test is the same as in the first embodiment. In addition, in some embodiments, the firing temperature after coating the metal paste is set to 150°C to manufacture the conductive sheets.

[0113] To evaluate the test results, calculate the maximum resistance value (R) observed during the test. max ) relative to the resistance value at the beginning of the test (R) i The proportion of ) (R)max / R i ), R max / R i Metal wiring with a strength of 1.5 or less is judged as "◎ (Excellent)" and R is set accordingly. max / R i Metal wiring with a strength of 1.5 or higher but less than 2.0 is judged as "0 (Good)" and R is set accordingly. max / R i Metal wiring with a strength of 2.0 or higher but lower than 5.0 is judged as "△ (passing)" and R is set accordingly. max / R i Metal wiring with a strength of 5.0 or higher is classified as "× (defective)". It should be noted that metal wiring that breaks during 100,000 bends is also classified as "×". The evaluation results of the bending resistance of various metal wirings manufactured in this embodiment are shown in Table 2.

[0114] [Table 2]

[0115]

[0116] *1: The number-average molecular weight of ethyl cellulose is as follows.

[0117] STD200: Number-average molecular weight 80733 STD100: Number-average molecular weight 63420

[0118] STD50: Number-average molecular weight 56489 STD7: Number-average molecular weight 17347

[0119] Benchmark: A mixture of STD100 and STD7 (C) H / C L =6.8)

[0120] MIX A: A mixture of STD100 and STD50 (C H / C L =1.0)

[0121] MIX B: A mixture of STD100 and STD20 (number average molecular weight 38984) (C) H / C L =9.0)

[0122] *2: The amount of ethyl cellulose added is expressed as a relative amount when the amount of No.1 added is set to 1.

[0123] The addition amount of ethyl cellulose (baseline) in No. 1 is STD100: 1.70 wt% + STD7: 0.25 wt%.

[0124] *3: Test No. 10 is the result of the first implementation method (Ag particles are batch number c).

[0125] *4: The resistance value could not be measured because the metal wiring broke during a single bend.

[0126] As confirmed by Table 2, the sintered body (metal wire) of the metal paste obtained by using appropriate average particle size and standard deviation of silver particles and adding ethyl cellulose with a number average molecular weight of 10,000 to 90,000 as a modifier exhibits good bending resistance. Moreover, it can be said that by keeping both the hardness and Young's modulus of the metal wire below constant values, the resistivity is maintained despite repeated bending deformation.

[0127] Upon detailed examination of the various components of the metal paste, it can be concluded that the metal wiring obtained from the No. 1 metal paste, with an average silver particle size of less than 100 nm (80 nm), has excessively high hardness and poor bending resistance. Furthermore, for metal pastes with small standard deviations and uniform silver particle size, metal wiring with higher hardness and Young's modulus but lower bending resistance is obtained (No. 13, No. 14). Even adjusting the average silver particle size does not improve this tendency.

[0128] Regarding ethyl cellulose as a modifier, particularly good bending resistance was observed in metal wires obtained from metal pastes containing low molecular weight (number average molecular weight of 60,000 or less) ethyl cellulose (No. 10: First Embodiment). However, even metal pastes containing only high molecular weight (number average molecular weight greater than 60,000) ethyl cellulose can be considered to have an improved bending resistance effect on metal wires by making the average particle size and standard deviation of the silver particles appropriate (No. 7, No. 8). In addition, metal wires formed from metal pastes containing both low molecular weight and high molecular weight ethyl cellulose also showed good bending resistance (No. 2 to No. 6, No. 9, No. 12).

[0129] Furthermore, the metal wiring obtained from the metal paste (No. 15) without ethyl cellulose as a modifier resulted in low hardness and particularly poor bending resistance. Regarding the conductive sheet of No. 15, wire breakage due to a single bend was confirmed. In contrast, for other conductive sheets (No. 1, No. 13, No. 14) that received a bending test rating of "× (poor)," although the resistance value of the wiring increased after bending, no wire breakage occurred. Considering these factors, from the viewpoint of preventing wiring breakage caused by bending, it can be said that ethyl cellulose in the metal paste is an essential component. Moreover, it can be said that in order to suppress changes in resistance value caused by bending, it is necessary to add ethyl cellulose to the metal paste and adjust the particle size and particle size distribution of the silver particles.

[0130] Furthermore, regarding the content of ethyl cellulose, when the addition amount of No.1 and others (1.70% by mass + 0.25% by mass) is taken as the standard addition amount, it can be said that based on the results of adding 0.3 times that amount (No.16), it is preferable to be 0.50% by mass or more, and more preferably 0.50% by mass or more (No.17 to No.19).

[0131] It should be noted that even when increasing the silver particle content of the metal paste to form metal wiring with a diameter of 10 μm or larger, bending resistance can still be ensured (No. 20 to No. 22). In addition, if the metal paste is appropriate, the bending resistance of the metal wiring is good even when the firing temperature is set to 150°C (No. 23).

[0132] [Cross-sectional structure of metallic wiring]

[0133] Figure 3 This is a photograph showing the cross-sectional microstructure of the metallic wiring of batches No.1 to No.6 (silver particles from batches a to f), as observed by SEM. Figure 3 It was found that the metal wiring obtained using silver particles with an average particle size of 80 nm formed a dense sintered body with very few pores. Although the porosity increased with the increase of the average particle size of the silver particles, no significant difference was found in the average particle size and the amount (area) of pores when the average particle size was above 140 nm (No. 4). The cross-sectional microstructure of wiring other than those No. 1 to No. 6 was also observed, and the same porosity was observed in the cross-sectional microstructure of the metal wiring with good results in repeated bending tests (evaluated as ○ or ◎).

[0134] Third Implementation Method In this embodiment, for conductive sheets No. 4 to No. 6, No. 8, No. 10, No. 11, and No. 21, which showed good results in the repeated bending test (R = 1.0 mm) in the second embodiment, the test conditions for the repeated bending test were made more stringent, and the radius of curvature R was set to 0.5 mm. The configuration of each conductive sheet (metal wiring) and other test conditions are the same as in the second embodiment. The results are shown in Table 3.

[0135] [Table 3]

[0136]

[0137] *1: The number-average molecular weight of ethyl cellulose is as follows.

[0138] STD100: Number-average molecular weight 63420 STD50: Number-average molecular weight 56489

[0139] STD7: Number-average molecular weight 17347

[0140] Benchmark: A mixture of STD100 and STD7 (C) H / C L =6.8)

[0141] *2: The amount of ethyl cellulose added is expressed as a relative amount when the amount of No.1 added is set to 1.

[0142] The addition amount of ethyl cellulose (baseline) in No. 1 is STD100: 1.70 wt% + STD7: 0.25 wt%.

[0143] Table 3 shows that metal wirings No. 24, No. 25, and No. 28 (No. 4, No. 5, and No. 10) exhibit good bending resistance with minimal change, even under extremely stringent conditions (R = 0.5 mm) where the radius of curvature for bending deformation is set to 0.5 mm. Furthermore, referring to No. 27 (No. 8), it is evident that in metal wirings formed from metal pastes with only high molecular weight ethyl cellulose added as a modifier, the change in resistance increases with stricter bending test conditions. However, even with the inclusion of high molecular weight ethyl cellulose, harsh bending deformation can be addressed by also including low molecular weight ethyl cellulose (No. 24, No. 25 (No. 4, No. 5)). It should be noted that when the silver particle content of the metal paste is high (75% by mass), the bending resistance tends to decrease when the radius of curvature for bending deformation is extremely small (No. 30 (No. 21)). While metal pastes with a high silver particle content can be considered suitable for forming thick wiring, the degree of bending and deformation of the conductive sheet used should be taken into account.

[0144] Industrial availability

[0145] As explained above, the metal wiring of the present invention, composed of a sintered body of silver particles, exhibits excellent bending resistance and is not prone to changes in electrical properties even under repeated bending deformation. Regarding this bending deformation, it can withstand even extremely severe deformations with a radius of curvature of 0.5 mm. The conductive sheet of the present invention, with its extremely fine metal wiring and excellent bending resistance, can be used as a component of flexible displays and wearable devices.

Claims

1. A metal wiring, comprising a sintered body of silver particles formed on a flexible substrate, characterized in that, The silver particles have an average particle size of 100 nm or more and 200 nm or less, and the standard deviation of the particle size is 40 nm or more and 120 nm or less. The sintered body has a volume resistivity of less than 20 μΩ·cm, a hardness of less than 0.38 GPa, and a Young's modulus of less than 7.0 GPa.

2. The metal wiring as described in claim 1, wherein, The thickness of the sintered body is more than 1 μm and less than 20 μm.

3. A conductive sheet comprising a flexible substrate and metal wiring formed on at least one surface of the substrate, wherein, As the metal wiring, the metal wiring of claim 1 or claim 2 is formed.

4. A metal paste for forming the metal wiring of claim 1 or claim 2, wherein, It contains a solid component consisting of silver particles, a solvent, a modifier, and optional organic additives. It contains a solid component consisting of silver particles, a solvent, and optional organic additives. The solid component consists of silver particles with an average particle size of 100 nm to 200 nm and a standard deviation of 40 nm to 120 nm. The silver particles constituting the solid component are bonded to at least one amine compound having 4 or more but less than 8 carbon atoms as a protective agent. The regulator is ethyl cellulose with a number average molecular weight of 10,000 or more and 90,000 or less.

5. The metal paste as described in claim 4, wherein, The regulator is ethyl cellulose with a number average molecular weight of 10,000 or more and 60,000 or less.

6. The metal paste as described in claim 5, wherein, It also contains ethyl cellulose with a number average molecular weight greater than 60,000 and less than 90,000 as the modifier.

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