JP6734925B2 - Silver paste for flexible substrates - Google Patents

Description

The present invention relates to a silver paste capable of forming a conductive film on a substrate having low heat resistance such as resin.
This application claims priority based on Japanese Patent Application No. 2016-182291 filed on September 16, 2016. The entire content of that application is incorporated herein by reference.

Regarding a circuit board on which electronic components are mounted, a conductive film has been printed on a polymer substrate in place of the inorganic substrate as the circuit board becomes smaller, thinner, lighter, and more sophisticated. Since the polymer substrate is inferior in heat resistance to the inorganic substrate, it is required that the conductive paste for forming the conductive film can be formed at a low temperature (for example, 400° C. or lower). Patent Documents 1 and 2 disclose, for example, a silver powder having excellent low-temperature sinterability for suitably performing film formation at low temperature, and a silver paste containing the silver powder. Patent Document 3 discloses a silver paste containing silver powder and a thermoplastic resin that cures at a low temperature.

International Publication No. 2014/084275 Japanese Patent Application Publication No. 2013-36057 International Publication No. 2013/081664

By the way, in recent years, a flexible printed circuit board (FPC) is mass-produced at high throughput and low cost by printing a conductive paste typified by a silver paste using a thin and flexible polymer film as a substrate. Is being done. As a silver paste used for this type of substrate, since the formed conductive film itself is required to have flexibility, a thermosetting resin containing a thermosetting resin that cures at a temperature lower than the heat resistant temperature of the base material as a binder is used. Mold silver paste is commonly used. However, the conductive film obtained from this thermosetting silver paste has a problem that it is easily peeled off from the substrate when the polymer film substrate is repeatedly curved. Further, since the polymer film substrate has higher heat sensitivity due to the thin layer, it is required to form the film at a temperature lower than that in the past (for example, 140° C. or lower).

The present invention has been made in view of the above points, and an object thereof is to provide a silver paste for a flexible substrate, which can form a conductive film having good adhesiveness at low temperature on a flexible substrate having low heat resistance. is there.

A conventional conductive film obtained from this type of thermosetting silver paste has a thickness of about 10 to 30 μm. According to the study of the present inventors, it was found that when a polymer film substrate including a conductive film formed of a thermosetting silver paste is repeatedly curved, the conductive film is easily peeled off due to its hardness and thickness. did. Here, this type of thermosetting silver paste uses an insulating thermosetting resin as a binder for silver particles. Therefore, in order to secure the conductivity of the formed conductive film, it is difficult to use fine silver nanoparticles having a large specific surface area because it leads to an increase in the binder, and the average particle diameter is about 2 to 5 μm or more. It was necessary to use flake-shaped silver powder. As a result, a conductive film formed of particles having such an average particle diameter is stably formed only when the thickness is approximately 10 μm or more, and the conductivity of the conductive film and the film strength, adhesiveness, etc. It was difficult to reduce the thickness of the conductive film in consideration of compatibility with the physical characteristics of.

Therefore, the present inventors have completed the present invention as a result of earnest research from the viewpoint that the use of nanometer-order silver powder is indispensable for forming a flexible conductive film on a flexible substrate. It was That is, the silver paste for a flexible substrate (hereinafter, may be simply referred to as “silver paste”, “paste”, etc.) provided by the present invention to solve the above problems is used for forming a conductive film on a flexible film substrate. It is a silver paste. The silver paste for a flexible substrate contains (A) silver powder, (B) a thermoplastic polyester resin as a binder, and (C) a solvent that dissolves the thermoplastic polyester resin. The silver powder (A) is characterized by having an average particle diameter of 40 nm or more and 100 nm or less. Further, the thermoplastic polyester resin (B) has a glass transition point of 60° C. or higher and 90° C. or lower, and is contained in a ratio of 5 parts by mass or more and 8 parts by mass or less with respect to 100 parts by mass of the silver powder. I am trying. The solvent (C) is characterized by having a boiling point of 180° C. or higher and 250° C. or lower and containing a phenyl group in its molecular structure.

By using such a silver paste for a flexible substrate, a flexible conductive film having a thickness of, for example, 3 μm or less can be favorably formed on the flexible substrate at a low temperature of 140° C. or less with good adhesion. This makes it possible to form a conductive film which is hard to peel off from the substrate and whose peeling is suppressed even when the substrate is repeatedly curved, for example. Further, for example, even when the substrate is bent at 90° or stretched (straightened out), a conductive film in which peeling is suppressed can be formed.

The average particle size of the silver powder means a cumulative 50% particle size in the number-based particle size distribution measured based on electron microscope observation. For the particle size distribution, specifically, for example, using a scanning electron microscope (SEM) or the like, the silver powder is observed at an appropriate magnification (for example, 50,000 times), and 100 or more (for example, 100 to 1000) silver particles are observed. It can be created based on the equivalent circle diameter obtained for the particles.

The glass transition point of the thermoplastic polyester resin can be measured according to the glass transition point measuring method defined in JIS K7121:1987 "Plastic transition temperature measuring method". Specifically, the glass transition point of the thermoplastic polyester resin is measured, for example, by using a differential thermal analysis (DTA) device or a differential scanning calorimetry (DSC) device to heat a measurement sample and a standard substance at a constant rate. The glass transition point of the sample can be grasped by measuring the difference in the amount of heat between the sample and the standard substance based on the change in the heat capacity of the sample. Upon heating, for example, it is preferable that the sample is heated to a temperature at least about 30° C. higher than that at the end of the glass transition, held at that temperature for about 10 minutes, and then rapidly cooled to a temperature about 50° C. lower than the glass transition point. .. When a commercially available thermoplastic polyester resin is used, the glass transition point described in the data sheet of the product can be used.

In a preferred embodiment of the silver paste for a flexible substrate disclosed herein, the silver powder (A) comprises spherical silver fine particles having an aspect ratio of 1.5 or less and non-spherical silver fine particles having an aspect ratio of more than 1.5. It is characterized by including. By this, the sintering of the silver powder is favorably promoted, and a conductive film having a lower sheet resistance can be formed.

In a preferred embodiment of the silver paste for a flexible substrate disclosed herein, a protective agent composed of an organic amine having 5 or less carbon atoms is attached to the surface of (A) silver powder. This increases the dispersion stability of the silver powder in the paste, and the silver particles are arranged at suitable positions from the storage state of the paste until the paste is applied to the substrate and baked, resulting in a dense and uniform conductivity. A film can be formed.

In another aspect, the technology disclosed herein provides an electronic device. The electronic device includes a flexible film substrate and a conductive film provided on the flexible film substrate. The conductive film is characterized by being a cured product of the silver paste for a flexible substrate described in any of the above. The silver paste described above can form a conductive film having good followability and adhesiveness with respect to bending and bending of the flexible substrate. Therefore, even when the substrate is bent or bent, an electronic element in which floating and peeling of the conductive film are suppressed can be realized.

In a preferred aspect of the electronic device disclosed herein, the average thickness of the conductive film is 0.2 μm or more and 3 μm or less. For example, the sheet resistance of the conductive film is characterized by being 100 mΩ/□ or less in terms of a value when the thickness of the conductive film is converted to 10 μm. This realizes an electronic element including a conductive film having a better balance between sheet resistance and flexibility.

In yet another aspect, the technology disclosed herein provides a method for manufacturing an electronic device. This manufacturing method comprises preparing a flexible film substrate, preparing any one of the above-described flexible substrate silver pastes, supplying the above-mentioned flexible film substrate with the above-mentioned flexible substrate silver paste, and the above-mentioned flexible substrate Drying the flexible film substrate supplied with the silver paste, and heat treating the dried flexible film substrate supplied with the dried silver paste for flexible substrate to form a conductive film. The silver paste for a flexible substrate is supplied so that the average thickness of the conductive film to be formed is 3 μm or less, and the temperature for drying is the heat contained in the silver paste for a flexible substrate. The temperature is lower than the glass transition point of the plastic polyester resin, and the temperature of the heat treatment is 20° C. or more higher than the glass transition point. Accordingly, the silver paste for a flexible substrate disclosed herein can be used to favorably manufacture an electronic element including a conductive film having good adhesiveness and conductivity on the flexible substrate.

Hereinafter, preferred embodiments of the present invention will be described. In addition, matters other than matters particularly referred to in the present specification (for example, the configuration and properties of the silver paste for a flexible substrate) and matters necessary for carrying out the present invention (for example, a base material of the paste). The detailed technique for applying the above) can be carried out based on the technical contents taught by the present specification and general technical common sense of those skilled in the art. In this specification, the notation “A to B” indicating a numerical range means A or more and B or less.

[Silver paste for flexible substrates]
The silver paste for a flexible substrate disclosed herein can essentially form a cured product by a heat treatment at a low temperature (for example, 140° C. or lower), and the cured product is a conductive film exhibiting electrical conductivity (conductivity). Is. The silver paste may be dried prior to the heat treatment. Characteristically, the conductive film itself is provided with flexibility and adhesiveness to the substrate, and is realized, for example, as having good substrate followability even on a flexible substrate. Such a silver paste for a flexible substrate contains (A) silver powder, (B) a thermoplastic polyester resin as a binder, and (C) a solvent that dissolves the thermoplastic polyester resin as main components. Hereinafter, each constituent component of the silver paste for a flexible substrate disclosed herein will be described.

(A) Silver Powder Silver powder is a material mainly for forming a film body (conductive film) having high electric conductivity (hereinafter, simply referred to as “conductivity”) such as electrodes, lead wires and conductive films in electronic devices and the like. is there. Silver (Ag) is preferable as a conductor material because it is less expensive than gold (Au), is less likely to be oxidized, and has excellent conductivity. The composition of silver powder is not particularly limited as long as it is a powder (aggregate of particles) containing silver as a main component, and silver powder having desired conductivity and other physical properties can be used. Here, the main component means that it is the maximum component among the components constituting the silver powder. Examples of silver particles constituting the silver powder include particles made of silver and silver alloys, and mixtures or composites thereof. Preferred examples of the silver alloy include silver-palladium (Ag-Pd) alloy, silver-platinum (Ag-Pt) alloy, and silver-copper (Ag-Cu) alloy. For example, core-shell particles in which the core is made of a metal other than silver, such as copper or a silver alloy, and the shell covering the core is made of silver can be used.

As the silver powder, the higher the purity (content of silver (Ag)), the higher the conductivity tends to be. Therefore, it is preferable to use silver powder having high purity. The silver powder preferably has a purity of 95% or more, more preferably 97% or more, particularly preferably 99% or more. For example, it is more preferable to use a silver powder having a purity of about 99.5% or more (for example, about 99.8% or more) because a conductive film having an extremely low resistance can be formed.

In addition, in the technique disclosed herein, average particles are used as silver powder so that sintering by heat treatment at a relatively low temperature (for example, 140° C. or lower, typically about 110° C. to 135° C.) is preferably realized. The diameter is 40 nm or more and 100 nm or less. Generally, finer particles (for example, fine particles of several nm to several tens of nm) have higher sinterability at low temperatures, and are therefore preferable in that the amount of binder used can be reduced. However, a binderless silver particle sintered body forms a dense sintered body, and bulk properties are strongly exhibited, and cracks occur when the conductive film is curved. That is, the flexibility (flexibility) cannot be exhibited. When the average particle size is smaller than 40 nm, silver particles are generated even at a temperature lower than the heat treatment (for example, a lower temperature environment such as supply of silver paste to a base material or drying; for example, about 20° C. to 100° C.). Sintering (including self-sintering) is likely to occur and stable film formation becomes impossible, which is not preferable. From this viewpoint, the average particle size of the silver powder is preferably 40 nm or more (exceeding 40 nm), more preferably 45 nm or more, and particularly preferably 50 nm or more.

On the other hand, if the average particle size of the silver particles is too large, it may be difficult to sinter at low temperature, and it may be difficult to obtain a conductive film having good conductivity. Further, even if it can be sintered at a low temperature, the minimum thickness of the conductive film that can be stably deposited with low resistance becomes large, and as a result, it becomes difficult to exhibit sufficient flexibility due to the thickness of the conductive film. From this viewpoint, the average particle size of the silver powder is preferably 100 nm or less (less than 100 nm), more preferably 95 nm or less, and particularly preferably 90 nm or less. For example, 55 nm or more and 85 nm or less is suitable.

From the viewpoint of quality stability, it is preferable that the silver powder does not contain particles having a too small particle diameter or particles having a too large particle diameter. For example, preferably, in the particle size distribution based on the number of silver powder, the minimum value (Dmin) of the particle size is preferably 10 nm or more, more preferably 20 nm or more, and particularly preferably 30 nm or more. In other words, it is preferable that substantially no ultrafine particles of less than 10 nm, preferably less than 20 nm, for example less than 30 nm are contained. Further, for example, in the number-based particle size distribution, the maximum value (Dmax) of the particle size is preferably 300 nm or less, more preferably 250 nm or less, and particularly preferably 200 nm or less. In other words, it is preferable that substantially no coarse particles exceeding 300 nm, preferably exceeding 250 nm, for example exceeding 200 nm, are contained.

The silver powder disclosed herein preferably has an appropriate spread in the particle size distribution. For example, specifically, the value (D90-D10) obtained by subtracting the cumulative 10% particle size (D10) from the cumulative 90% particle size (D90) in the number-based particle size distribution is 70 nm or more, typically 75 nm or more. It is preferably about 220 nm or less, for example, 210 nm or less. By allowing the particle size distribution to have an appropriate spread in this way, when the silver powder is sintered, the silver particles having a relatively small particle size are arranged so as to fill the gaps between the silver particles having a relatively large particle size. Can be sintered. As a result, the silver powder is sintered in a more densely packed state, and a conductive film having excellent conductivity can be realized.

Further, it is preferable that the ratio (D10/D90) of the cumulative 90% particle size (D90) and the cumulative 10% particle size (D10) is approximately 0.3 or more. It is more preferably 0.33 or more. The ratio (D10/D90) is preferably 0.6 or less, more preferably 0.55 or less.

The shape of the silver particles constituting the above silver powder is not particularly limited. For example, it may be spherical, elliptical, crushed, scaly, flat, fibrous and the like. From the viewpoint of forming a thinner and more uniform film by printing, the shape of the silver particles is preferably spherical or close to spherical. An aspect ratio when the shape of silver particles is two-dimensionally evaluated is one index that represents the sphericity of silver particles. The aspect ratio is, for example, 100 or more (for example, 100 to 1000) silver particles are observed by an electron microscope or the like, and a rectangular shape circumscribing the outer shape of the silver particles in the observed image is drawn. It can be calculated as the ratio of the length of the long side to the length (major axis/minor axis). Here, the arithmetic average value of the aspect ratio of each silver particle is adopted as the aspect ratio of the silver powder. The closer the aspect ratio is to 1, the more isotropic the silver particles are, and the three-dimensional shape of the silver particles becomes closer to a spherical shape. On the other hand, the larger the aspect ratio, the higher the anisotropy, and the shape of the silver particles becomes closer to a non-spherical shape such as a flat plate shape or a fiber shape. Here, silver particles having an aspect ratio of 1.5 or less are called “spherical particles”, and silver particles having an aspect ratio of less than 1.5 are called “non-spherical particles”.

The silver powder disclosed herein may also contain spherical silver particles and non-spherical silver particles. In a mixture of spherical silver particles and non-spherical silver particles, for example, spherical particles may enter the gaps in which the non-spherical particles are arranged, or non-spherical particles may enter in the gaps in which the spherical particles are arranged, so that the total filling A highly sintered body can be obtained. As a result, the contact area between silver particles is increased, and a conductor film having excellent conductivity can be formed.
In the silver powder disclosed herein, the proportion of spherical silver particles having an aspect ratio of 1.5 or less is preferably 60 number% or more of the total silver powder. In other words, it is preferable that the non-spherical silver particles having an aspect ratio of less than 1.5 are 40% by number or less of the silver particles constituting the silver powder. The spherical silver particles are more preferably 70% by number or more of the total silver powder, particularly preferably 80% by number or more, for example, 85% by number or more, or 90% by number or more. Since the silver powder is composed of particles having such a shape, the stability, surface smoothness, homogeneity, filling property, etc. of the silver particles from the time when the silver paste is supplied to the base material to the heat treatment are effective. Be raised to. As a result, the filling property of silver particles and the surface smoothness of the formed conductive film are improved, and a conductive film having higher conductivity can be obtained.

As described above, the silver powder disclosed herein has an average particle size on the order of nanometers and is relatively fine. Therefore, since silver powder having such a size generally easily aggregates, a protective agent that suppresses aggregation can be provided on the surface of the silver particles. Typically, the surface of silver powder (silver particles) is covered with a protective agent. Thereby, the surface stability of the silver particles can be maintained and the aggregation of the silver particles can be efficiently suppressed. As a result, the silver paste disclosed herein can suppress the aggregation of silver particles in the solvent and can be stably stored with good dispersibility for a long period of time. Further, for example, when the silver paste is supplied to the base material by various printing methods, the fluidity of the silver particles can be increased and the printability can be improved. As a result, it is possible to form a coating film that is uniform and has less unevenness.

The type of this surface protective agent is not particularly limited, but from the viewpoint that it can be burned off from the surface of silver particles by heat treatment (calcination) at low temperature for a short time, the protective agent is detached from the surface of silver particles during heat treatment. It is preferably easy. The protective agent preferably has, for example, a low sublimation point or boiling point at atmospheric pressure and a decomposition temperature and forms a relatively weak bond (for example, a coordinate bond) with silver.

Therefore, in the technique disclosed herein, the protective agent is preferably an organic amine having 5 or less carbon atoms. Specific examples of the organic amine having 5 or less carbon atoms include methylamine, ethylamine, n-propylamine, isopropylamine, butylamine, pentylamine, 2-methoxyethylamine, 2-ethoxyethylamine, 3-methoxypropylamine, 3-ethoxy. Primary aliphatic amines such as propylamine; secondary aliphatic amines such as dimethylamine, diethylamine, methylbutylamine, ethylpropylamine, ethylisopropylamine; tertiary aliphatics such as trimethylamine, dimethylethylamine, diethylmethylamine Amine; is illustrated. The carbon number of the organic amine is preferably 3 or more, and more preferably 4 or more. Further, this organic amine may contain an alkoxy group such as a methoxy group or an ethoxy group in the structure. Any one of these organic amines may be used alone, or two or more thereof may be used in combination. Thereby, the above-mentioned dispersion stability can be realized more preferably.

As will be described later, when the silver paste (coating film) supplied to the substrate and dried is subjected to heat treatment (baking) at a low temperature for a short time, in order to develop high conductivity in the conductive film after baking, It is important to keep the amount of residual protective agent and the amount of heat shrinkage of silver particles small. Then, in order to suppress the residual of the protective agent and the thermal shrinkage of the silver particles, it may be effective to reduce the ratio of the protective agent in the silver powder as much as possible. In the technique disclosed herein, by setting the average particle diameter of the silver particles within the above range, a remarkably low content ratio of the protective agent as compared with the conventional case is realized. Specifically, when the silver powder (silver particle portion) is 100 parts by mass, the ratio of the protective agent can be 1.2 parts by mass or less. In other words, 98.8 parts by mass or more of the silver powder can be composed of silver particles. The ratio of the protective agent is preferably 1.1 parts by mass or less, for example, 1 part by mass or less. As a result, it is possible to effectively suppress the residual of the protective agent and the heat shrinkage of the silver particles even if it is baked at a low temperature for a short time, and it is possible to form a coating film having excellent conductivity.

Further, it is preferable that the silver paste disclosed herein does not substantially contain a component (corrosion component) capable of generating an erosion gas during firing. That is, although it is acceptable that a corrosive component is inevitably mixed in due to, for example, a manufacturing process of silver powder or a manufacturing facility, it is preferable that such a corrosive component is not intentionally included. Examples of such a corrosive component include a halogen component such as fluorine (F) and chlorine (Cl), and a sulfur (S) component. It is preferable that these components are not contained in the silver powder itself, and it is also preferable that they are not contained in the protective agent. It is preferable that the silver paste does not substantially contain such a corrosive component, because it is possible to suppress the corrosion deterioration of the semiconductor manufacturing apparatus, the inclusion of foreign matter in the semiconductor element, and the alteration of the electrode or substrate of the semiconductor element. In addition, it is preferable that a component that may adversely affect the human body or the environment, such as a lead (Pb) component or an arsenic (As) component, is not included. For example, these corrosive components such as fluorine (F), chlorine (Cl), sulfur (S), lead (Pb), and arsenic (As) are 0.1 parts by mass when the silver powder is 100 parts by mass. It is preferably suppressed to (1000 ppm) or less. These corrosive components are preferably suppressed to 0.1 parts by mass (1000 ppm) or less in total when the silver powder is 100 parts by mass.

(B) Thermoplastic Polyester Resin Thermoplastic polyester (polyester: PEs) resin functions as a binder component in the silver paste disclosed herein. Due to the inclusion of this thermoplastic polyester resin, the silver paste disclosed herein softens the binder by heating and hardens the binder by subsequent heat dissipation (cooling), thereby supporting the bonding between silver particles and the adhesion to the substrate. To be done. It is typically considered to contribute to the bonding between the sintered silver powder and the substrate. There are thermosetting and thermoplastic polyester resins, and in the conventional silver paste of this type, thermosetting polyester resin or the like was used as a binder. On the other hand, in the technique disclosed herein, as described above, the binder function is realized by utilizing the reversible manifestation of the plasticity due to the heating of the thermoplastic polyester resin.

In addition, as described above, the behavior of the thermoplastic polyester resin as the binder is softened by the heat treatment and hardened by the subsequent cooling.
Here, it is preferable that (1) the thermoplastic polyester resin is softened before the sintering of the silver powder and cured after the sintering, from the viewpoint of enhancing the adhesion to the substrate. In other words, as the thermoplastic polyester resin, one having a relatively low appropriate glass transition point (Tg) corresponding to the heat treatment temperature for sintering the silver powder can be preferably used. Although the details are not clear, this causes most of the thermoplastic polyester resin to soften during the heat treatment to reach the interface between the silver powder and the substrate, without impeding the sintering of the silver powder and the substrate. It is considered that it contributes favorably to binding with.

Further, (2) the thermoplastic polyester resin, like the silver powder, is not preferable to undergo a large volume change due to heat treatment or temperature change. Further, since the thermoplastic polyester resin is in a cured state at room temperature, it is preferably soluble in the solvent described below. Amorphous (non-crystalline) ones can be preferably used as the thermoplastic polyester resin that preferably satisfies such requirements. An amorphous resin can be understood as a resin having a structure in which a regular arrangement of molecular chains is not seen in a cured state and the molecular chains are randomly mixed. The amorphous thermoplastic polyester resin can be understood as a compound that is soluble in a solvent and has a glass transition point but no clear crystal melting point, for example.

The glass transition point of the thermoplastic polyester resin cannot be generally determined because it depends on the heat resistant temperature of the material constituting the substrate, but for example, the heat treatment temperature for sintering the silver powder (for example, 140° C. or lower, typically Is about 110° C. to 135° C.). As a suitable example, for example, a temperature lower than the heat treatment temperature by 20° C. or more (for example, about 20° C. to 50° C.) is preferable. From this viewpoint, the glass transition point is preferably 90°C or lower, more preferably 85°C or lower, and particularly preferably 80°C or lower. On the other hand, the glass transition point of the thermoplastic polyester resin is preferably higher than the temperature at which the solvent in the silver paste almost volatilizes, for example, the drying temperature of the silver paste. For example, it is preferably higher than the drying temperature by about 20°C. From this viewpoint, the glass transition point is preferably 60° C. or higher (exceeds 60° C.), more preferably 63° C. or higher, and particularly preferably 65° C. or higher. Such a glass transition point belongs to a relatively high class in the commonly used thermoplastic polyester resins. For example, it can be said that the temperature is extremely high for binders for resin substrates such as PET.

As such a thermoplastic polyester resin, various compounds containing a polyester structure obtained by polycondensation of a polycarboxylic acid and a polyalcohol as a main component or a main monomer can be used as a repeating unit constituting the resin.
In addition, the "main component" means a monomer component corresponding to a repeating unit that is most contained on a mass basis among the repeating units that form the main skeleton of the thermoplastic polyester resin. This main component may preferably be a monomer component contained in the thermoplastic polyester resin in an amount of more than 50% by mass.

The monomer component corresponding to the polycarboxylic acid forming the polyester structure is not particularly limited. The polycarboxylic acid may be an acyclic polycarboxylic acid or a saturated or unsaturated alicyclic polycarboxylic acid. For example, aliphatic dibasic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, tartaric acid, glutamic acid, sebacic acid, dodecanedioic acid, brassic acid, dimer acid; furandicarboxylic acid, diphenyldicarboxylic acid, 1 Preferred examples include alicyclic dicarboxylic acids such as 4-cyclohexanedicarboxylic acid; dibasic acids such as aromatic dibasic acids such as phthalic acid, isophthalic acid, terephthalic acid, and naphthalenedicarboxylic acid. Of these, aromatic dibasic acids are preferred. Further, the monomer component corresponding to the polyalcohol constituting the polyester structure is not particularly limited. Examples of polyalcohols include aliphatic polyalcohols, alicyclic polyalcohols, aromatic polyalcohols, and the like. Aliphatic or alicyclic diols are preferable from the viewpoint of obtaining high adhesiveness. Specific examples of the monomer component corresponding to polyalcohol include ethylene glycol, propylene glycol, trimethylene glycol, 1,2-butanediol, 1,3-butanediol, and 1,4-cyclohexanedimethanol. It is preferably a diol. These may have an alicyclic skeleton in the side chain or may not have such an alicyclic skeleton in the side chain.

Further, the thermoplastic polyester resin may be, for example, a polymer of a monomer raw material that contains a polyester structure as a main monomer and may further contain a sub-monomer copolymerizable with the main monomer. Here, the main monomer refers to a component that accounts for more than 50% by weight of the monomer composition in the monomer raw material. The main monomer can be, for example, an ester of a polycarboxylic acid represented by the dibasic acid exemplified above and a polyol represented by diols. As another example, the main monomers are polycondensation reaction products of terephthalic acid and ethylene glycol (PET-based main monomers), polycondensation reaction products of terephthalic acid and butanediol (PBT-based main monomers), and naphthalenedicarboxylic acid. It may be a polycondensation reaction product with ethylene glycol (PEN-based main monomer) or a polycondensation reaction product with naphthalenedicarboxylic acid and butanediol (PBN-based main monomer). These main monomers may be contained alone or in combination of two or more.

As the sub-monomer, a component capable of introducing a cross-linking point into the polyester structure or enhancing the adhesive force of the polyester structure is preferable. Examples of the sub-monomer include carboxy group-containing monomers typified by monocarboxylic acids, dicarboxylic acids and their anhydrides; typified by hydroxyalkyl (meth)acrylate compounds, alcohol compounds, ether compounds, polyether compounds, etc. Hydroxyl group-containing monomers; amide group-containing monomers represented by (meth)acrylamide; isocyanate group-containing monomers represented by (meth)acryloyl isocyanate; phenyl group-containing monomers represented by styrene compounds, phenyl ether compounds, etc. Can be mentioned. These sub-monomers may be contained alone or in combination of two or more.

Further, the thermoplastic polyester resin preferably has appropriate flexibility after curing. Therefore, a component such as a cross-linking agent or a cross-linking aid may be contained for the purpose of improving flexibility and adhesiveness after curing. Examples of such a cross-linking agent and a cross-linking aid may include an isocyanate compound, a polyfunctional melamine compound, a polyfunctional epoxy compound, and a polyhydroxy compound. Specific examples include aliphatic polyisocyanates, alicyclic polyisocyanates, aromatic polyisocyanates, araliphatic polyisocyanates, and polyhydroxy compounds. The cross-linking agent and the cross-linking aid can be used alone or in combination of two or more.

In order to enhance the chemical and photochemical stability of the conductive film, the sub-monomer, the cross-linking agent, the cross-linking aid, and the like preferably have a chemical structure that does not introduce an unsaturated group into the thermoplastic polyester resin. That is, the thermoplastic polyester resin is preferably a saturated copolyester. This is particularly preferable because the adhesiveness to a PET film substrate which is widely used as a flexible film substrate is enhanced.

When the above thermoplastic polyester resin is used, the number average molecular weight (Mn) is not particularly limited, but if the number average molecular weight is less than 2000, it is difficult to exhibit the adhesiveness and/or the tackiness required as a binder. There is a case where it is not preferable. From this viewpoint, the number average molecular weight is preferably 2,000 or more, more preferably 5,000 or more, still more preferably 10,000 or more. On the other hand, if the number average molecular weight of the thermoplastic polyester resin exceeds 100,000, the solubility in a solvent may be extremely reduced and the printability may be deteriorated. From this viewpoint, the number average molecular weight may be 100,000 or less, preferably 50,000 or less, and more preferably 10,000 or more and 30,000 or less. Such a number average molecular weight belongs to a relatively high class among the commonly used amorphous thermoplastic polyester resins.

The properties such as the above-mentioned flexibility and adhesiveness and solvent solubility in the thermoplastic polyester resin can be determined by those skilled in the art having the disclosure of the present specification, depending on the film base material to be used, as a main monomer and a sub-monomer. It is possible to appropriately design and prepare by combining with and adjusting the compounding amount thereof and the glass transition point and the molecular weight.
Moreover, such a thermoplastic polyester resin can also be obtained and used as a commercially available product. Examples of such commercially available products include, for example, Elitel (registered trademark) UE3200, UE9200, UE3201, UE3203, UE3600, UE9600, UE3660, UE3690 manufactured by Unitika Ltd., and Polyester (registered trademark) manufactured by Nippon Synthetic Chemical Industry Co., Ltd. Trademark) TP236, TP220, TP235, Dynapol (registered trademark) L205, L206, L208, L952, L907 manufactured by Evonik Industries AG, and VITEL (registered trademark) 2100, 2200 manufactured by Bostig.

The thermoplastic polyester resin is used in an amount of 5 parts by mass or more based on 100 parts by mass of the silver powder in order to impart sufficient flexibility and adhesiveness to a conductive film containing a sintered body of silver powder. It is essential to be included in the ratio of. The amount of the thermoplastic polyester resin is more preferably 5.3 parts by mass or more, and particularly preferably 5.5 parts by mass or more.
On the other hand, since the thermoplastic polyester resin exhibits insulating properties, it is preferable to keep the content in the silver paste as low as possible. From this viewpoint, the content of the thermoplastic polyester resin is preferably 8 parts by mass or less, more preferably 7.8 parts by mass or less, and particularly preferably 7.5 parts by mass or less with respect to 100 parts by mass of the silver powder. ..

(C) Solvent As the solvent, various solvents capable of dissolving the above-mentioned (B) thermoplastic polyester resin can be used. It also has the function of dispersing the above-mentioned silver powder, which is the solid content of the silver paste. The solvent is not particularly limited, but for example, it is possible to suitably realize the firing of the silver paste using the above-mentioned (A) silver powder and (B) thermoplastic polyester resin in combination to obtain a conductive film having excellent conductivity. From the viewpoint that it can be produced, a solvent having a boiling point of 180° C. or higher and 250° C. or lower is preferable. Further, it is preferable that the molecular structure contains a phenyl group.

The solvent is a high-boiling solvent having a boiling point of 180° C. or higher. For example, when the silver paste is continuously supplied to an arbitrary substrate by a printing method, the solvent volatilizes and the properties of the silver paste change. It can be suppressed. Volatilization of the solvent before the silver paste is supplied to the base material increases the viscosity of the silver paste to make the printing conditions unstable, and the conductive film formed by increasing the content ratio of silver powder in the silver paste. It is not preferable because it causes variations in the film thickness. In addition, since the boiling point of the solvent is 250° C. or lower, the solvent can be quickly volatilized in a short time at a temperature sufficiently lower than the heat treatment temperature for sintering the silver powder. Further, if the boiling point of this solvent exceeds 250° C., the solvent component tends to remain in the coating film obtained by drying the silver paste, which makes it difficult to form a suitable film, which is not preferable.

In addition, it is preferable that the solvent contains a phenyl group in the molecular structure because the solubility of the thermoplastic polyester resin is increased and the paste properties suitable for printing are easily adjusted. That is, since the solvent contains a phenyl group, the affinity for the non-aqueous thermoplastic polyester resin is increased, and it is thermodynamically stable and less susceptible to oxidation/reduction. Further, due to the presence of the phenyl ring exhibiting rigidity, appropriate viscosity can be stably and suitably imparted to the silver paste. As a result, it is possible to prepare a silver paste having high workability and printing stability when the silver paste is supplied to the base material. As a result, a uniform coating film (including a conductive film) can be stably formed. The number of phenyl groups in the molecular structure may be one.

As described above, by adjusting the silver paste using an appropriate solvent, it is possible to supply the silver paste to the base material by a printing method while keeping the properties of the silver paste stable. This can be an extremely advantageous characteristic in the future manufacturing of electronic devices, for example, when the roll-to-roll process is entirely adopted. Although the boiling point and volatility of the solvent do not exactly match, in consideration of the use of the silver paste disclosed herein and the characteristics of the solvent to be used, the volatility should be understood based on the boiling point of the solvent. It can be said that it does not matter.

As such a solvent, a non-aqueous solvent satisfying the above boiling point and containing a phenyl group can be used without particular limitation. As an example of such a solvent, for example, oxyalkylene monophenyl ether represented by ethylene glycol monophenyl ether (245° C.), propylene glycol monophenyl ether (243° C.) and the like; methyl phenyl ether (154° C.), ethyl phenyl ether ( 184° C.), butyl phenyl ether (210° C.), methyl phenyl ethyl ether (184° C.) and other alkyl phenyl ethers; benzyl alcohol (205° C.), isophorone (215° C.), benzaldehyde (179° C.), benzyl acetate (212° C.) and the like benzenes; and the like. In consideration of the solubility of the binder, it is preferable that the chain length of the alkyl group in the molecular structure is short. For example, in the molecular structure of the solvent, the linear carbon number of the alkyl group is preferably 3 or less.

In a non-aqueous dispersion medium, hydrophobic interaction, interface adsorption by ions, electrostatic repulsion effect, etc. are suppressed. On the other hand, depending on the type of the protective agent used in the silver powder as the dispersoid, the use of a highly polar solvent is not preferable because it corrodes the protective agent. Therefore, it is also preferable to select a solvent having a dispersibility suitable for the surface characteristics of the silver powder as the dispersoid. Further, for example, since the size of the silver powder used here is the sub-nanometer size as described above, it is expected that the effect of improving the dispersibility by using the dispersant cannot be expected or becomes difficult. Therefore, from the viewpoint of stabilizing the dispersion of the silver powder, a solvent that can suitably disperse the silver powder without using a dispersant can be preferably used as the solvent. From this viewpoint, the use of the above-mentioned oxyalkylene monophenyl ether is particularly suitable as the solvent.

The ratio of the (C) solvent in the silver paste is not particularly limited as long as it is an amount capable of dissolving the thermoplastic polyester resin. For example, it can be appropriately adjusted according to the feeding method so that the workability and the feedability when the silver paste is fed to the base material are good. For example, when the silver paste is supplied to the base material by a printing method, as a rough guide, the proportion of the silver powder can be about 50 mass% or more of the total silver paste, and 60 mass% or more is preferable, for example, It may be 70% by mass or more. Further, the proportion of the silver powder can be 90% by mass or less of the total silver paste, preferably 85% by mass or less, and for example, it is exemplified to be adjusted to 80% by mass or less. Further, the proportion of the solvent can be about 10% by mass or more of the entire silver paste, preferably 15% by mass or more, and for example, 20% by mass or more. Further, the proportion of the solvent can be 50% by mass or less of the entire silver paste, preferably 40% by mass or less, and for example, 30% by mass or less. By increasing the proportion of the silver powder in this way, the denseness of the conductive film can be improved. As a result, it is possible to stably form a conductive film having excellent conductivity even if it is fired at a low temperature for a short time. Further, even when a relatively thin conductive film (for example, having a thickness of 3 μm or less) is formed, a uniform conductive film without unevenness can be formed.

(D) Other components The silver paste disclosed herein does not essentially need to contain components other than the above (A) silver powder, (B) thermoplastic polyester resin, and (C) solvent. However, in addition to the above-mentioned (A) silver powder, (B) thermoplastic polyester resin, and (C) solvent, the inclusion of various components is acceptable without departing from the object of the present application. As these components, an additive added for the purpose of improving the properties of the silver paste for flexible substrates, an additive added for the purpose of improving the characteristics of the conductive film as a cured product, and the like can be considered. .. Examples include surfactants, dispersants, fillers (organic fillers and inorganic fillers), viscosity modifiers, defoamers, plasticizers, stabilizers, antioxidants, preservatives and the like. One kind of these additives (compounds) may be contained alone, or two or more kinds thereof may be contained in combination. However, it is not preferable to include (A) a component that inhibits the sintering of the silver powder and (B) a binder performance of the thermoplastic polyester resin, or to include an additive in an amount that inhibits these components. From this point of view, for example, it is not preferable to include an inappropriate protective agent for silver particles or an inorganic filler. Moreover, when an additive is included, the total content of these components is preferably about 5% by mass or less, more preferably 3% by mass or less, and particularly preferably 1% by mass or less based on the entire silver paste.

The silver paste for a flexible substrate can be prepared by blending the above-mentioned constituents in a predetermined ratio and uniformly mixing and kneading. At the time of mixing, the constituent materials may be mixed at the same time. For example, (B) a thermoplastic polyester resin and (C) a solvent are mixed to prepare a vehicle, and then (A) silver powder is added to the vehicle. You may mix it. When other additives are added, the timing of addition is not particularly limited. For mixing, for example, a three roll mill can be used.

The thus prepared silver paste for a flexible substrate can be cured at a lower temperature (typically 140° C. or lower, for example, 110 to 135° C.) as compared with a conventional one. Then, the conductive paste (cured product) having a desired pattern can be formed on the substrate by supplying the flexible substrate silver paste in a desired pattern on the substrate and then curing the silver paste.

[Conductive film]
Since the conductive epoxy film uses the thermoplastic epoxy resin as a binder, the conductive film itself has flexibility. The average thickness of the conductive film is not strictly limited. However, in the case where the conductive film is formed on a flexible substrate, the thickness of the conductive film should be 3 μm or less in order to improve the adhesiveness and the substrate followability of the conductive film when the substrate is curved. Preferably. By controlling the thickness of the conductive film in this manner, excellent adhesion to the base material can be maintained not only when the substrate is repeatedly bent but also when the substrate is repeatedly bent.

In addition, the thickness of the conductive film is preferably 0.2 μm or more in order to improve the conductivity of the conductive film. Thereby, the silver powder can be laminated in the thickness direction to form a conductive film, and a good conductive path can be formed. The conductivity of such a conductive film cannot be generally stated because it depends on the shape and thickness of the conductive film, but, for example, assuming that the sheet resistance is 100 mΩ/□ or less when the thickness of the conductive film is converted to 10 μm. Obtainable. The sheet resistance can be, for example, 80 mΩ/□ or less, and preferably 60 mΩ/□ or less.

[substrate]
The material to which the silver paste for a flexible substrate disclosed herein is applied is not strictly limited. For example, when a thin substrate (film) made of polymer (plastic), paper, cloth, etc. and having flexibility is targeted, the excellent characteristics of the silver paste disclosed herein are remarkably exhibited. Is preferred. As the flexible film substrate (hereinafter sometimes simply referred to as “flexible substrate”), polyester resin such as polyethylene terephthalate (PET), polypropylene, polyolefin resin such as ethylene-propylene copolymer, polyimide resin, polychlorinated A polymer film made of a thermoplastic resin such as vinyl is preferably used. These base materials may have a single-layer form or a multi-layer form. In the case of multiple layers, film base materials of different materials may be stuck together, or film base materials of the same kind of material may be stuck together. Such a flexible substrate may constitute a flex portion of a rigid flexible substrate including a rigid portion for mounting components and the like and a flex portion for bending.

Further, “flexible” means being flexible and capable of being bent or bent. Usually, it means that it can be bent or bent with a comparatively weak force at room temperature without damaging the object itself. Flexible substrate is a term for a rigid substrate that does not flex without temperature changes or the presence of coating layers. There is no particular limitation on the amount of bending of the flexible substrate (the amount of bending that can be bent). However, if necessary, the amount of deflection when a load is applied to the tip of the cantilever substrate at room temperature is 0.001 or more (typically 0.1 or more, for example, It can be understood as a substrate on which one or more deformations can occur. The silver paste disclosed here can be applied to a flexible substrate having extremely high flexibility. For example, such a substrate may be a film base material having a bendability capable of being wound around a winding core (core) having a cross-sectional diameter of 20 mm or less. Regarding this bendability, for example, the base material may be a film which can be wound around a core having a diameter of preferably 10 mm or less, particularly preferably 6 mm or less. Further, the silver paste disclosed here can be applied to a flexible substrate having excellent flexibility. Such a substrate may be, for example, a substrate that is not damaged by repeated bending at a folding angle of 90° to 120°. The thickness of the flexible substrate is not particularly limited, but for example, a flexible substrate having a thickness of about 3 to 200 μm (for example, 5 to 100 μm, typically 10 to 50 μm) is widely adopted.

Note that the flexible substrate preferably has a predetermined rigidity (strength) because it can be repeatedly curved and bent. From this viewpoint, a polyester film can be preferably used as the flexible substrate, and a PET film substrate can be particularly preferably used. The PET film substrate is also preferable in that it is frequently used as a flexible printed circuit (FPC) substrate or a flexible cable substrate. Therefore, a method of suitably forming an electronic element by forming a conductive film on a PET film substrate using the silver paste for a flexible substrate disclosed herein will be described below.

[Method of manufacturing electronic device]
The method of manufacturing an electronic device disclosed herein essentially includes the following steps (1) to (5).
(1) Prepare a flexible substrate.
(2) Prepare the silver paste disclosed herein.
(3) A silver paste is supplied onto the flexible substrate.
(4) The flexible substrate supplied with the silver paste is dried.
(5) The flexible substrate supplied with the dried silver paste is heat-treated to form a conductive film.

It should be noted that steps (1) and (2) can be understood from the above description of the silver paste and the substrate, and therefore the description thereof will be omitted here.
In step (3), the silver paste disclosed herein is supplied onto the prepared flexible substrate. The method of supplying the silver paste is not particularly limited. For example, various printing methods such as inkjet printing, gravure printing, screen printing, flexographic printing, offset printing, spin coating, aerosol jet printing, etc. can be adopted. These printings may be performed by a step (intermittent) method or a continuous method such as roll-to-roll. The silver paste is prepared to have properties suitable for each printing method. The silver paste disclosed herein can be preferably used for the purpose of forming a conductive film having an arbitrary pattern over a relatively large area on a flexible substrate by screen printing, for example.

As described above, in order to make the conductive film after firing have sufficient flexibility, the thickness of the conductive film obtained after the heat treatment should be 3 μm or less (for example, less than 3 μm). It is preferable to control the amount of paste supplied. When the thickness of the conductive film exceeds 3 μm, it is preferable from the viewpoint of conductivity. However, for example, even when there is no problem with the slight bending of the flexible substrate, it is preferable that the conductive film is likely to be cracked or cracked when the flexible substrate is largely bent. Absent. Damage to the conductive film leads to a decrease in the conductivity of the conductive film and is a form that should be avoided. The thickness of the conductive film is more preferably 2.7 μm or less, and particularly preferably 2.5 μm or less. Further, if the thickness of the conductive film obtained after the heat treatment is less than 0.2 μm, it is difficult to achieve sufficient adhesion between the conductive film and the flexible substrate, which is not preferable. Further, it is also not preferable in that the conductivity of the conductive film may decrease. The thickness of the conductive film is preferably 0.2 μm or more, more preferably 0.5 μm or more, and particularly preferably 0.7 μm or more.

The thickness of the conductive film can be obtained as an arithmetic average value (that is, average thickness) when measuring the dimension in the direction perpendicular to the substrate surface at 10 or more points.

In subsequent steps (4) and (5), "drying" and "heat treatment" are applied to the silver paste supplied onto the flexible substrate, respectively. A conductive film is formed by this drying and heat treatment. When forming the conductive film, the components contained in the silver paste volatilize the solvent, soften the resin, and then harden, and the silver powder sinters. A conductive film is formed by the sintered silver powder and the cured resin.

Here, it is not preferable that the volatilization of the solvent, the softening and hardening of the resin, and the sintering of the silver powder proceed at the same time. In other words, the solvent is completely volatilized and the silver powder sinters in a state where only the solid content of the silver paste remains densely on the substrate, so that the silver particles bond with each other at more points of contact and the low resistance. This is preferable because it is possible to obtain a different conductive film. Further, the resin is softened and hardened in a state where the solvent is completely volatilized and only the solid content of the silver paste is left on the substrate, so that it is possible to develop an appropriate binder function with a smaller amount of resin. Is preferred. Furthermore, the resin is preferable because the resin can be bonded after the silver powder is completely sintered, and the sintered silver can be bonded to the substrate while suppressing the inhibition of the sintering of the silver powder.

The “drying” in step (4) is a step mainly performed for the purpose of volatilizing the solvent contained in the silver paste and leaving only the solid content of the silver paste on the substrate. The "heat treatment" in the step (5) is a step mainly performed for the purpose of sintering the silver powder on the substrate. Then, after the step (4), the resin is softened in the course of reaching the step (5), and the resin is cured in the course of cooling after the heating in the step (5). Therefore, in manufacturing the electronic device disclosed herein, it is important to appropriately control the temperatures of the drying in the step (4) and the heat treatment in the step (5) according to the silver paste.

The drying in the step (4) may be natural drying, or may be performed by means of blast drying, heat drying, vacuum drying, freeze drying, or the like. Heat drying is preferable because it can be dried easily in a shorter time. The heating means in the heat drying is not particularly limited, and it can be dried using various known dryers.

This drying step is performed at a temperature lower than the glass transition point (Tg) of the thermoplastic epoxy resin used in the silver paste. From the viewpoint of shortening the drying time, for example, the drying step is preferably performed by heating to a temperature lower by about 2° C. to 30° C. than the glass transition point. The drying temperature is a temperature lower than the glass transition point, and is preferably set in the range of, for example, about 60°C ± 10°C.

The heat treatment in the step (5) is performed at a temperature at which the silver powder can be sintered and higher than the glass transition point of the thermoplastic epoxy resin. In the technique disclosed herein, since the conductive film is formed by heat treatment at a lower temperature, heat treatment can be performed in a temperature range of 140° C. or lower. Further, in the technology disclosed herein, since the glass transition point of the thermoplastic epoxy resin is 60° C. or higher and 90° C. or lower, the heat treatment temperature is the thermoplastic epoxy resin contained in the silver paste. It can be set according to the glass transition point of. The heat treatment temperature is preferably set to a temperature of (glass transition point+20)° C. or higher depending on the glass transition point of the thermoplastic epoxy resin used for the silver paste. For example, the heat treatment temperature is preferably about 100°C to 135°C, more preferably 100°C to 130°C, and particularly preferably 100°C to 120°C. This heat treatment can be carried out using various known heating devices, drying devices and the like.

By this heat treatment, the silver powder, which is a solid component of the silver paste, sinters and the silver particles form good electrical contacts. Further, by cooling after this heat treatment, the thermoplastic epoxy resin is hardened to support the bonding of the sintered bodies of the silver powder to each other more reliably, and the sintered body and the PET substrate are flexible and strong. Achieve adhesion. As a result, a conductive film having high conductivity and good adhesiveness can be easily formed at low temperature even on a flexible substrate. Since this conductive film is formed by using a printing technique, it is realized as a film having a uniform thickness in an arbitrary pattern.

Therefore, the flexible substrate having such a conductive film formed thereon can maintain extremely good adhesion between the substrate and the conductive film even after being deformed. Further, the conductive film can maintain conductivity even after being deformed. Specifically, peeling or cracking of the conductive film is highly suppressed even when the substrate is repeatedly bent greatly. Further, as shown in Examples described later, even when the substrate is repeatedly bent, peeling or cracking of the conductive film is highly suppressed. Therefore, according to the technique disclosed herein, for example, a conductive film such as a movable portion wiring can be preferably formed by printing on a flexible substrate that is repeatedly bent at a hinge portion or the like. As a result, an electronic element (for example, FPC) that can be arranged in the uneven portion and the space is realized. In addition, a flexible cable that has excellent repetitive bendability and can be wired in a space-saving drive section or the like is realized.

Such a flexible substrate with a conductive film can be suitably used as an electronic element used in various fields such as electric devices, semiconductor devices, solar cells, displays, sensors and biomedical devices.

Hereinafter, some embodiments of the present invention will be described, but the present invention is not intended to be limited to those shown in the embodiments.

(Embodiment 1)
[Preparation of silver powder]
Three types of silver powders A to C having different average particle diameters were prepared. Specifically, at room temperature (25° C.), butylamine as a surface modifier and butanol as a solvent/particle size controlling agent are mixed at a predetermined molar ratio, and silver oxalate is added, followed by stirring. While heating to about 100° C., substantially spherical silver powders B and C whose surfaces were stabilized with an organic amine were obtained. The average particle size of the silver powder was controlled by adjusting the addition amount of the particle size controlling agent (molar ratio of the organic amine and the particle size controlling agent) and further classifying. Further, the silver powder A is a commercially available flake-shaped silver powder, and has a relatively large average particle diameter of 2000 nm in a plan view, and thus no surface modifier is used. The average particle diameter (D50) and shape of the thus prepared silver powders A to C were calculated by SEM observation and are shown in Table 1 below.

[Preparation of vehicle]
Then, a vehicle in which the silver powder was dispersed was prepared. Specifically, first, as a binder resin, two kinds of resins, a crystalline epoxy resin (EP) and an amorphous polyester resin (PEs) were prepared. The epoxy resin used has a softening point of 65° C. and a number average molecular weight (Mn) of 1×10 ³ , which is widely used as a binder for thermosetting conductive paste. As the polyester resin, a thermoplastic resin having a number average molecular weight (Mn) of 23×10 ³ and a glass transition point (Tg) of 65° was used. As a solvent, propylene glycol monophenyl ether having a phenyl group in its molecular structure and capable of suitably dissolving the above binder resin was prepared. The boiling point (Tb) of propylene glycol monophenyl ether is 243°C. Then, each resin and solvent were weighed in predetermined amounts in a glass bottle container, stirred by hand, and then heated in a steam oven at about 100°C for about 10 to 20 hours. During heating, manual stirring was performed as needed. This gave two different vehicles.

[Preparation of silver paste]
The prepared silver powders A to C and the vehicle were mixed at a predetermined ratio, and mixed and kneaded using a three-roll mill to prepare the silver pastes of Examples 1 to 5. In addition, as shown in Table 1, the ratio of the silver powder and the vehicle was such that the resin in the vehicle was 10 parts by mass or 6 parts by mass with respect to the mass (100 parts by mass) of the silver powder. The silver paste was adjusted by adding a solvent so that the viscosity at 25° C.-20 rpm was 50 to 150 Pa·s.

[Formation of conductive film]
The silver pastes of Examples 1 to 5 thus prepared were applied to the surface of a PET resin film substrate (thickness 100 μm) by a screen printing method. For screen printing, a calendered stainless steel mesh of #640 was used, and printing was performed in a thin layer so that the film thickness after heat treatment (baking) was about 1 μm. As the print pattern, one rectangular solid coating pattern of 3 cm×1.5 cm and a pattern for sheet resistance measurement described later are arranged side by side. The pattern for sheet resistance measurement was a linear pattern whose dimensions after firing were adjusted so that the total length was 10 cm or more and the width was 0.5 mm. The silver paste of Example 4 was printed by changing the stainless mesh so that the thickness after heat treatment was 10 μm (this is referred to as Example 4 ^* ). The printed substrate was dried in a dryer at 60° C. for 10 minutes and then heat-treated at 120° C. for 20 minutes to form the conductive films of Examples 1 to 5. The film thickness of the obtained conductive film was measured and shown in the column of "Firing thickness" in Table 1 below. Further, with respect to the obtained conductive film, the sheet resistance, adhesiveness and bending adhesiveness were evaluated by the following methods, and the results are shown in the relevant column of Table 1 below.

[Sheet resistance]
The sheet resistance of the conductive film for sheet resistance measurement formed as described above was measured. Specifically, using a digital multimeter, the resistance value of the linear conductive film was measured by a two-terminal method under the conditions of a terminal interval (conductor length) of 100 mm and a line width (conductor width) of 0.500 mm. Then, from this resistance value, the sheet resistance value was calculated based on the following formula. The converted thickness was 10 μm. The results are shown in Table 1. A film having a sheet resistance value of more than 1000 mΩ/□ is indicated as “>1000” rather than an actual measured value because it is difficult to use as a conductive film.
Sheet resistance value (mΩ/□)=resistance value (Ω)×{conductor width (mm)/conductor length (mm)}×{conductor thickness (μm)/converted thickness (μm)}

[Adhesiveness]
The conductive film formed as described above was subjected to an adhesiveness test using a double-sided tape to evaluate the adhesiveness of the conductive film to the substrate. Specifically, first, a double-sided tape (manufactured by Nichiban Co., Ltd., NW Stack NW-10 for general use, width 1 cm×length 1.5 cm) was attached on a test bench. Then, the conductive film portion formed on the PET substrate was attached to the adhesive surface on the upper side of the double-sided tape attached on the table, and it was sufficiently adhered by pushing from the back surface of the PET substrate with a finger. Then, pinch the end of the PET substrate with a finger, and in the direction along the longitudinal direction of the double-sided tape and in the direction of 120° to 150° from the initial attachment position of the PET substrate (that is, the angle made by the PET substrate is 60° to The film substrate was peeled from the double-sided tape by pulling the film substrate at an angle of 30° obliquely upward and rearward.

Observing the conductive film of the film substrate after peeling, when the area ratio of the part remaining on the film substrate after peeling is 95% or more among the parts adhered to the double-sided tape, the area ratio of the remaining part is The results are shown in Table 1 when the ratio is less than 95% and 80% or more is “Δ”, and when the area ratio of the remaining portion is less than 80% is “x”.

[Folding resistance]
With respect to the conductive film formed as described above, the durability of the conductive film when the substrate was bent was evaluated. Specifically, first, the sheet resistance (initial sheet resistance) of the conductive film formed on the PET substrate was measured as described above. Next, the PET substrate provided with the conductive film was fitted along the surface of the test stand having a right angle (90°) corner portion, and was bent at a right angle together with the PET substrate at the conductive film portion. After that, the bent PET substrate was straightly extended, and the PET substrate and the conductive film were bent at a right angle again at the same bending position. This operation was repeated until the total number of times of bending was 6 times.

The two terminals of the digital multimeter were brought into contact with the conductive film after the six-fold bending so that the bent portions were sandwiched between the terminals, and the sheet resistance of the conductive film including the bent portions was measured as described above. Then, the sheet resistance increase rate was measured from the following formula, and the sheet resistance after six-fold bending was 120% or more of the initial sheet resistance was “×”, and the case where it was less than 120% was “◯”. The folding resistance of the membrane was evaluated. The rate of increase in sheet resistance was similarly measured for a film having an initial sheet resistance of more than 1000 mΩ/□. The results are shown in Table 1.
Sheet resistance increase rate (%)={(sheet resistance after 6-fold bending)−(initial sheet resistance)}÷(initial sheet resistance)×100

(Evaluation)
As shown in Table 1, the silver paste of Example 1 is an example of a commonly used thermosetting silver paste that uses a flake type silver powder A having an average particle diameter of 2000 nm and an epoxy resin. According to the conventional general thermosetting type silver paste, a conductive film having a thickness of about 10 μm can be preferably formed. When the silver paste of Example 1 is used, it is not possible to print a thin conductive film up to 1 μm in thickness under the same printing conditions, and the thickness of the conductive film becomes thick based on the particle diameter of silver particles. I was able to confirm that it would end up. In this example, it was found that it is possible to form a conductive film having a thickness of about 2 μm, which is similar to the average particle diameter of silver particles.

In such a conductive film of Example 1, the silver particles do not sinter with each other by heat treatment at 120° C., and the film is formed by fixing the silver particles through or with a thermosetting binder. ing. Therefore, the amount of binder required for film formation is relatively large at 10 parts by mass, and it is difficult to reduce the sheet resistance even when forming a conductive film having a thickness of 10 μm, for example. In this example, a conductive film having a smaller film thickness of 5 μm was formed. Therefore, it was found that the electrical contact between silver particles was poor even in the thickness direction, and the sheet resistance was significantly increased to the extent that it was not suitable as a conductive film. In addition, the conductive film of Example 1 is weak in film strength because the silver particles are not sintered and the thickness is small relative to the size of the silver particles. Was found to peel off. Further, since the thermosetting resin is crystalline, the hardness after curing is high. Therefore, although the conductive film was relatively thin, it was confirmed that cracks and peeling from the substrate were formed in the conductive film by one bending, and the bending resistance was also poor. That is, according to the silver paste of Example 1, it was found that it was difficult to form a conductive thin film having low sheet resistance and good adhesiveness on a flexible substrate having low heat resistance.

The silver pastes of Examples 2 and 4 are low-temperature curing type silver pastes using a polyester resin having a glass transition point of 65° C., and the average particle diameter of the silver powder used is (Example 2) silver powder B: The difference is 500 nm and (Example 4) silver powder C: 70 nm. It was confirmed that the target conductive film having a thickness of 1 μm can be formed by using these silver pastes. In addition, since the glass transition point of the binder resin is 65° C., the binder resin is sufficiently softened by the heat treatment at 120° C. and then hardened, so that the silver particles are satisfactorily bonded to each other and the silver particles and the substrate, and the adhesiveness It was confirmed that a good conductive film could be formed. In addition, since the polyester resin used as the binder is non-crystalline, the conductive film has flexibility and substrate followability, and has high bending resistance, and even if the base material is repeatedly bent, the sheet resistance of the conductive film is significantly increased. It was confirmed that there was no. From this, it was found that by using a thermoplastic resin having an appropriate glass transition point as the binder resin, it is possible to form a conductive thin film having good adhesiveness to a flexible substrate having low heat resistance.

On the other hand, although not shown in Table 1, in the conductive film of Example 4 using the fine silver powder C, a part of the silver powder is sintered by heat treatment, which is higher than that of the conductive film of Example 2. It showed adhesiveness. Further, the sheet resistance of the conductive film of Example 4 was as low as 50 mΩ/□, whereas the sheet resistance of the conductive film of Example 2 using the silver powder B having an average particle diameter of 500 nm still exceeded 1000 mΩ/□. .. It can be considered that this is because the conductive film of Example 4 had a sufficiently small average particle size that silver particles sinter with each other to form a good contact in the conductive film. However, since the silver particles B used in Example 2 are not sintered by heat treatment at a low temperature, it shows that the silver particles cannot form a good contact in a film having a limited thickness of, for example, about 1 μm. From this, it was found that the average particle diameter of the silver powder is preferably about 100 μm or less.

For the silver pastes of Examples 3, 4, and 5, the silver powder C was used, and the amount of the polyester resin having a glass transition point of 65° C. was (Example 3) 0 parts by mass (not used), (Example 4) 6 parts by mass, (Example 5) 10 parts by mass was changed. It was confirmed that the target conductive film having a thickness of 1 μm can be formed by using these silver pastes. It was also found that the sheet resistance of the conductive film was extremely low at 10 mΩ/□ for the conductive film of Example 3 in which the amount of binder resin used was zero, and the sheet resistance increased as the binder resin increased. Here, although the sheet resistance of the conductive film of Example 4 with the binder resin amount of 6 parts by mass is sufficiently low as 50 mΩ/□, the conductive film of Example 5 with the binder resin amount of 10 parts by mass is similar to the conductive film of Example 1. It was found that the sheet resistance was extremely high, exceeding 1000 mΩ/□. From this, it was found that the binder resin content was preferably less than 10 parts by mass, for example 8 parts by mass or less, based on the silver powder. Further, by using a thermoplastic binder resin having an appropriate glass transition point, a film having good adhesiveness can be formed by low temperature firing while reducing the amount of resin, and low sheet resistance and adhesiveness can be obtained. It turned out to be compatible at a high level.

In the conductive film of Example 3 in which the amount of the binder resin was 0 part by mass, the silver particles were sintered by the heat treatment at 120° C., so that the conductive film having a thickness of 1 μm could be formed. However, it was found that since the conductive film of Example 3 does not contain a binder resin, the adhesiveness between the conductive film and the substrate and the bending resistance deteriorate. From this, it was found that the use of a thermoplastic binder resin having an appropriate glass transition point is essential for forming a conductive thin film on a flexible substrate (for example, 0.2 parts by mass or more). ..

As described above, it was found that by using the silver paste of Example 4, a conductive film having extremely good adhesiveness can be formed on a flexible PET substrate having low heat resistance. Therefore, the silver paste of Example 4 was used to form a conductive film of Example 4 ^* having a thickness of 10 μm as in the case of using the conventional thermosetting silver paste. As a result, like the conductive film of Example 4, the conductive film of Example 4 ^* had low sheet resistance and excellent adhesiveness even though it was fired at a low temperature. However, since the film thickness was as thick as 10 μm, the conductive film was floated or cracked by bending, and in this example, satisfactory results were not obtained regarding bending resistance. Although specific data are not shown, it was found that the silver paste of Example 4 can be particularly preferably used for forming a thin film having a thickness of less than 10 μm, for example, a film thickness of 3 μm or less in order to have flexibility. It was

(Embodiment 2)
[Preparation of silver paste]
In the same manner as in Embodiment 1 described above, a substantially spherical silver powder C having an average particle diameter of 70 nm and using butylamine as a surface modifier was prepared. In the silver powder C prepared here, almost all the particles were substantially spherical, but non-spherical silver fine particles having an aspect ratio of more than 1.5 were present at a ratio of about 10% by mass or less. It could be confirmed.
As the binder resin, as shown in Table 2, five types of non-crystalline polyester resins (PEs) A to E having different glass transition points (Tg) of 7 to 84 were prepared. As the solvent, as shown below, in addition to propylene glycol monophenyl ether, three kinds of organic solvents having different boiling points (Tb) were prepared.

(A) Propylene glycol monophenyl ether (boiling point: 243° C.)
(B) 2-(2-butoxyethoxy)ethyl acetate (boiling point: 255° C.)
(C) Ethylene glycol monophenyl ether (boiling point: 245°C)
(D) Diethylene glycol monophenyl ether (boiling point: 298°C)
Then, first, the prepared resin and solvent were mixed in a combination shown in Table 2 in a ratio of resin:solvent of 4:21 by weight ratio, and the mixture was mixed with a steam oven at about 100° C. in a steam oven at about 100° C. while appropriately stirring by hand. The vehicle was prepared by heating for about 20 hours.

The prepared silver powder and vehicle are blended at a weight ratio of silver powder:resin of 75:4 (that is, 100 parts by mass:5.3 parts by mass), and mixed and kneaded using a three-roll mill. Thus, the silver pastes of Examples 6 to 13 were prepared. The silver paste was prepared by adding a solvent so that the viscosity at 25° C.-20 rpm was 50 to 150 Pa·s.

[Formation of conductive film]
The silver pastes of Examples 6 to 13 thus prepared were applied to the surface of a PET resin film substrate (thickness 100 μm) by a screen printing method in the same manner as in the first embodiment. For the screen printing, a calendered #640 stainless mesh was used, and printing was performed in a thin layer so that the film thickness after firing was approximately 1.5 μm. The substrates after printing were dried at 60° C. for 10 minutes in a dryer and then subjected to heat treatment at 105° C., 115° C. or 125° C. for 20 minutes, thereby producing Examples 6 to 13 at different heat treatment temperatures. Was formed.
The film thickness of the obtained conductive film was measured, and the average value thereof is shown in the column of "fired thickness" in Table 2 below. The sheet resistance and adhesiveness of the obtained conductive film were evaluated in the same manner as in Embodiment 1, and the results are shown in the relevant column of Table 2 below.

(Evaluation)
Examples 6 to 10 in Table 2 are silver pastes prepared under the same conditions except that polyester resins A to E having different glass transition points were used.
The sheet resistance of the conductive film formed of these silver pastes was a small value in Examples 6 and 7 in which the glass transition point of the binder resin was lower than the drying temperature. Looking only at this point, it seems that the glass transition point of the binder resin is preferably a low value, for example, 50° C. or lower. However, the conductive films of Examples 6 and 7 having low sheet resistance have extremely low adhesion to the base material even though the amount of resin is the same. It is considered that this is because, for example, when the silver paste is dried at 60° C., the solvent volatilizes and the binder softens at the same time, and stable film formation cannot be performed.

On the other hand, for the silver pastes of Examples 8 to 10 in which the glass transition point of the binder resin is higher than the drying temperature, the drying and firing of the silver paste are performed in different steps (in two steps), and the solvent is sufficiently added. The volatilized coating body can be baked. From this, it was found that the conductive films of Examples 8 to 10 have a high sheet resistance, but have a high adhesiveness to the base material, and can achieve both low sheet resistance and base material adhesiveness. In particular, with the silver pastes of Examples 8 and 9, it was found that a conductive film having excellent sheet resistance characteristics and adhesiveness could be stably formed without being significantly affected by the firing temperature.

Regarding the conductive film of Example 10, the sample whose heat treatment temperature was as high as 125° C. showed a slight decrease in adhesion. It is considered that this is because the glass transition point of the binder resin was too close to the heat treatment temperature, so that the softening of the binder and the baking of the silver particles proceeded at the same time and the stability was lowered. From this, it can be said that the glass transition point of the binder resin tends to be sufficiently lower than the heat treatment temperature in order to form the conductive film on the film substrate having low heat resistance. For example, it is preferably lower than the heat treatment temperature by about 30° C. or more.

Examples 11 to 13 are examples in which the silver paste was prepared by changing the type of solvent.
In Example 11, 2-(2-butoxyethoxy)ethyl acetate (b) was used as the solvent. When 2-(2-butoxyethoxy)ethyl acetate was used as the solvent, although the binder resin could be dissolved to prepare the silver paste, the viscosity of the obtained silver paste was too low to print. could not. It is considered that this is because 2-(2-butoxyethoxy)ethyl acetate does not have phenyl. In addition, it is not practical to add a thickener to such a low-viscosity silver paste because the characteristics of the obtained conductive film are significantly deteriorated.

In Example 12, ethylene glycol monophenyl ether (c) was used as the solvent. In this case, the silver paste could be adjusted to have a viscosity suitable for printing. It was also found that by setting the temperature of the heat treatment in the range of 105°C to 125°C, a conductive film having excellent sheet resistance and adhesiveness can be formed. Note that by lowering the temperature of the heat treatment to 105° C. to 115° C., the adhesiveness can be further improved, and a conductive film having a better balance between sheet resistance and adhesiveness can be obtained. It was also confirmed that a conductive film having a lower sheet resistance can be formed by raising the temperature to a high temperature.

In Example 13, diethylene glycol monophenyl ether (d) was used as the solvent. When diethylene glycol monophenyl ether was used as the solvent, the binder resin could not be dissolved in the solvent, and preparation of the silver paste itself was impossible. The diethylene glycol monophenyl ether (d) is an alkylene glycol like the ethylene glycol monophenyl ether (c), but its boiling point greatly exceeds 250°C. Since the boiling point can also reflect the molecular structure and its characteristics, diethylene glycol monophenyl ether having a boiling point of higher than 250° C. is considered to be unsuitable as a solvent for the silver paste for flexible substrates disclosed herein.

Specific examples of the present invention have been described above in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

Claims (7)

A silver paste for forming a conductive film on a flexible film substrate,
(A) silver powder, (B) a thermoplastic polyester resin as a binder, and (C) a solvent that dissolves the thermoplastic polyester resin,
(A) The silver powder has an average particle size of 40 nm or more and less than 100 nm,
(B) The thermoplastic polyester resin is
Has a glass transition point of 60° C. or higher and 90° C. or lower,
It is contained in a ratio of 5 parts by mass or more and 8 parts by mass or less with respect to 100 parts by mass of the silver powder,
(C) The solvent is
A boiling point of 180° C. or higher and 250° C. or lower,
Including phenyl group in the molecular structure,
A silver paste for a flexible substrate, which is characterized in that The silver paste for a flexible substrate according to claim 1, wherein (A) the silver powder contains spherical silver fine particles having an aspect ratio of 1.5 or less and non-spherical silver fine particles having an aspect ratio of more than 1.5. (A) The silver paste for a flexible substrate according to claim 1 or 2, wherein a protective agent composed of an organic amine having 5 or less carbon atoms is attached to the surface of the silver powder. A flexible film substrate,
A conductive film provided on the flexible film substrate,
Including
The said conductive film is an electronic element which is the hardened|cured material of the silver paste for flexible substrates of any one of Claims 1-3. The electronic element according to claim 4, wherein the conductive film has an average thickness of 0.2 μm or more and 3 μm or less. The electronic element according to claim 4, wherein the sheet resistance of the conductive film is 100 mΩ/□ or less. Prepare a flexible film substrate,
Preparing the silver paste for flexible substrates according to any one of claims 1 to 3,
Supplying the flexible substrate silver paste on the flexible film substrate,
Drying the flexible film substrate to which the flexible substrate silver paste is supplied,
Heat-treating the flexible film substrate supplied with the dried silver paste for flexible substrate to form a conductive film;
Including
The flexible substrate silver paste is supplied such that the conductive film formed has an average thickness of 3 μm or less,
The temperature for the drying is a temperature lower than the glass transition point of the thermoplastic polyester resin contained in the flexible substrate silver paste,
The temperature of the heat treatment is 20° C. or more higher than the glass transition point,
Manufacturing method of electronic device.