JP6961942B2 - Transparent film for flexible substrate and flexible circuit board - Google Patents

Description

The present invention relates to a transparent film for a flexible substrate. More specifically, the present invention relates to a transparent film for a flexible substrate, which has excellent heat resistance and excellent process suitability in an electronic circuit manufacturing process.

In recent years, electronic devices (particularly mobile devices) have been made lighter and have larger screens. However, since conventional devices are made of rigid housings, there is an upper limit to the size that can be carried, and there is a limit to the increase in screen size. Therefore, various flexible displays that can store the screen in a small size have been proposed.

Flexible displays, like ordinary displays, have a base material layer and a light emitting layer (LED, OLED).
), TFT, cover layer, etc. are laminated, but since it is necessary to repeatedly deform to a certain state, a transparent and repeatedly bendable base material is required. As such a base material, polyester films such as polycarbonate film, polyethylene naphthalate film and polyethylene terephthalate film, as well as light-transmitting films such as polymethylmethacrylate film have been proposed, and among them, rigid. Polyester films with a small linear expansion coefficient are often used.

On the other hand, various silver paste materials have been proposed for forming wiring on these substrates. In particular, a silver paste having excellent process suitability and capable of low-temperature firing has been proposed (Patent Document 1). It is stated that the reason is that the heat resistant temperature of the polyester film of the substrate is up to about 120 ° C., and it is a problem to lower the firing temperature. On the other hand, it is said that a circuit without bleeding can be obtained by providing a primer layer on a substrate in a recent thinned circuit (Patent Document 2).

However, even if these technologies are combined, in recent thinned circuits, the energy generated when the silver paste material is cured causes minute irregularities on the surface of the base material, which affects conductivity and durability. It has become a problem that affects.

Japanese Unexamined Patent Publication No. 2015-040319 Japanese Unexamined Patent Publication No. 2015-032734

The present invention has been made against the background of the problems of the prior art. That is, an object of the present invention is to provide a transparent film for a flexible substrate, which has excellent heat resistance and is suitable for circuit thinning.

As a result of diligent studies, the present inventors have found that the above problems can be solved by the means shown below, and have arrived at the present invention. That is, the present invention has the following configuration.
(1) The polyester film is characterized by having an undercoat layer having a surface free energy of 20 to 40 mJ / m ² , a surface roughness Ra of 10 nm or less, and a film thickness of 0.3 to 3.0 μm on at least one surface of the polyester film. Transparent film for flexible substrates.
(2) The transparent film for a flexible substrate according to (1) above, wherein the undercoat layer contains a melamine crosslinked acrylic resin.
(3) When four spherical beads having a particle size of 0.5 mm are placed on the undercoat layer in an environment of 100 ° C. and pressed against a load of 15 g, the transfer depth of the undercoat layer is 1.5 μm or less. The transparent film for a flexible substrate according to the above (1) or (2).
(4) A flexible circuit board characterized in that a circuit having a line width of 30 μm or less is formed on the transparent film for a flexible substrate according to any one of (1) to (3) above.

According to the present invention, a circuit cannot be formed due to deformation caused by deformation during curing after forming a thin wire on a circuit in the conventional technique. It has become possible to provide a transparent film for a flexible substrate suitable for the above.

Hereinafter, the present invention will be described in detail.
(Base film)
The transparent film for a flexible substrate of the present invention has a structure in which an undercoat layer is laminated on a light-transmitting base material. The transparent film for a flexible substrate of the present invention is used for applications such as forming a circuit on the coating layer with a silver paste or the like, and the coating layer is referred to as an undercoat layer.

The resin forming the light-transmitting base material is not particularly limited, and for example, polyester resin, acetate resin, polyether sulfone resin, polycarbonate resin, polyamide resin, polyimide resin, polyolefin resin, ( Meta) Acrylic resin, polyvinyl chloride resin, polyvinylidene chloride resin, polystyrene resin, polyvinyl alcohol resin, polyallylate resin, polyphenylene sulfide resin and the like can be mentioned. Among them, a polyester resin which is inexpensive and has excellent mechanical strength is preferably used, and polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate and the like are preferably used.

The thickness of the light-transmitting substrate is preferably 2 μm or more. When it is 2 μm or more, there is little possibility that the mechanical strength of the light-transmitting substrate will be insufficient, which is preferable. More preferably, it is 25 μm or more. The thickness of the light-transmitting substrate is preferably 250 μm or less. When it is 250 μm or less, the rigidity of the film is not too large, and the handleability as a flexible substrate is maintained, which is preferable. More preferably, it is 188 μm or less.

The surface of the light-transmitting substrate may be subjected to etching treatment such as sputtering, corona discharge, plasma discharge, ultraviolet irradiation, electron beam irradiation, chemical conversion, oxidation, or undercoating treatment in advance. By performing these treatments in advance, it is possible to improve the adhesion to the undercoat layer formed on the light transmissive substrate. Further, before forming the undercoat layer or the conductive layer, the surface of the light-transmitting base material may be dust-removed and cleaned by solvent cleaning, ultrasonic cleaning, or the like, if necessary.

(Undercoat layer)
As the undercoat layer, a resin obtained by cross-linking an acrylic resin with a cross-linking material having 3 or more reaction points in the molecule is preferable. In particular, a heat-crosslinking resin represented by an acrylic resin crosslinked with a melamine crosslinking agent is preferable.

Since the melamine cross-linking agent has a plurality of reaction points and has a dense cross-linked structure, it has high heat resistance and does not deform even when the circuit is formed. Further, another cross-linking agent such as polyisocyanate, melamine having a different chemical structure, epoxy, aluminum chelate, titanium chelate, and ultraviolet curable resin can be mixed and used.

As the melamine cross-linking agent, alkoxymethylol melamine or the like can be used.

As the resin of the undercoat layer, in addition to the acrylic resin, any resin having a hydroxyl group in the molecule such as an acrylic polyol, a polyester resin, an epoxy resin, and a polyether resin can be used or used together without particular limitation, but the crosslink density is controlled. It is preferable to include an acrylic resin from the viewpoints of (amount of hydroxyl group introduced) and control of glass transition temperature.

The acrylic resin is a copolymerized polymer copolymerized from two or more (meth) acrylic acid esters, and at least one kind of (meth) acrylic acid ester having a hydroxyl group must be copolymerized. Examples of the (meth) acrylic acid ester having a hydroxyl group include, but are not limited to, hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate.

Examples of the (meth) acrylic acid ester that copolymerizes with the (meth) acrylic acid ester having a hydroxyl group include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, and n-butyl acrylate. , N-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, n-hexyl acrylate, n-hexyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, stearyl methacrylate and the like. One of these may be used alone, or two or more thereof may be used in combination.

The molecular weight of the acrylic resin is preferably 5,000 to 200,000, more preferably 10,000 to 100,000. When the molecular weight is 5000 or more, the heat resistance of the coating film is not likely to decrease, which is preferable. On the other hand, when the molecular weight is 200,000 or less, the viscosity does not become too high and the coating suitability is maintained, which is preferable.

Examples of the method of laminating the undercoat layer on the surface of the base film include a gravure roll coating method, a reverse roll coating method, a knife coater method, a dip coating method, a spin coating method, and the like. The coating method of the undercoat layer suitable for laminating is not particularly limited. Further, it can be provided by an in-line coating method in which a coating layer is provided in the film manufacturing process, or an offline coating method in which a coating layer is provided after film production.

The drying temperature for forming the undercoat layer is usually preferably 60 ° C. or higher, more preferably 90 ° C. or higher. A drying temperature of 60 ° C. or higher is preferable in terms of productivity. On the other hand, the drying temperature is preferably 170 ° C. or lower, more preferably 150 ° C. or lower. When the drying temperature is 170 or less, the flatness of the film is maintained, which is preferable.

The film thickness of the undercoat layer is preferably 0.3 μm or more, and more preferably 0.5 μm or more as the film thickness after drying. A film thickness of 0.3 μm or more is preferable from the viewpoint of heat resistance. On the other hand, the film thickness after drying is preferably 3 μm or less, more preferably 2 μm or less. When the film thickness is 3 μm or less, there is no possibility of causing a problem of curling of the film, which is preferable. Here, the curl amount is preferably 10 mm or less in the evaluation by the measurement method described later.

(Surface free energy)
The surface free energy of the undercoat layer ^{is preferably 20 mJ / m 2} or more. When the surface free energy of the undercoat layer is 20 mJ / m ² or more, there is no possibility that the slurry of the circuit forming material becomes repellent, and a uniform line width can be formed, which is preferable. On the other hand, the surface free energy of the undercoat layer is preferably not more than 40 mJ / ^{m 2,} more preferably not more than 35 mJ / ^{m 2.} When the surface free energy of the undercoat layer is 40 mJ / m ² or less, there is no possibility that the slurry of the circuit forming material will get wet and spread, and a uniform line width can be formed, which is preferable.

(Surface roughness)
The surface roughness Ra of the undercoat layer is preferably 10 nm or less, more preferably 8 nm or less. When the surface roughness Ra is 10 nm or less, a uniform line width is easily formed and disconnection is less likely to occur, which is preferable. On the other hand, the surface roughness Ra of the undercoat layer may be 0.05 nm or more, and may be 0.1 nm or more in the present invention.

The transparent film for a flexible substrate of the present invention is undercoated when four spherical beads having a particle size of 0.5 mm are placed on the undercoat layer described above in an environment of 100 ° C. and pressed against a load of a 15 g glass plate. The transfer depth of the coat layer is preferably 1.5 μm or less. More preferably, it is 1.2 μm or less. When the transfer depth of the undercoat layer is 1.5 μm or less, the heat resistance when forming a circuit on the undercoat layer is sufficient, and it is resistant to heat in various processes and environments after the circuit is formed. Since it is difficult to deform, it is preferable because the line width of a thin circuit can be beautifully formed and maintained. Further, the transfer depth of the undercoat layer is preferably small, but it may be 0.1 μm or more, or 0.2 μm or more.

(Circuit forming material)
The circuit-forming material laminated on the undercoat layer of the transparent film for a flexible substrate of the present invention is not particularly limited, but is preferably a nanodisperse of a metal having excellent conductivity, and a dispersion of silver or copper. Especially preferably used. The average particle size of these fillers is preferably 100 nm or less, more preferably 30 nm or less. When the average particle size of the filler is 100 nm or less, clogging does not occur during thin circuit printing, and the amount of heat for firing is small, which is preferable.

(Printing method)
The method for printing the circuit on the undercoat layer of the transparent film for a flexible substrate of the present invention is not particularly limited, but a method such as screen printing or a metal mask can be preferably used.

(Curing method)
The method for curing the circuit after printing on the undercoat layer of the transparent film for a flexible substrate of the present invention is not particularly limited to thermosetting, ultraviolet curing, electron beam curing, etc., and can be cured by any method. can. In particular, thermosetting, ultraviolet curing and the like are preferable.

For example, the thermosetting temperature is preferably 150 ° C. or lower, more preferably 130 ° C. or lower. When the temperature is 150 ° C. or lower, there is no risk of film deformation or discoloration, and the film can be preferably used for flexible substrate applications.

The UV crosslinking of the ultraviolet curing, preferably the amount of light 2000 mJ / cm ² or less, more preferably 1000 mJ / cm ² or less. When irradiated with a light amount of 2000 mJ / cm ² or less, there is no risk of deterioration of the film substrate due to heat due to UV or UV light, which is preferable.

The circuit formed on the transparent film for a flexible substrate of the present invention preferably has a line width of 30 μm or less. Since the undercoat layer of the transparent film for a flexible substrate of the present invention has excellent properties such as appropriate surface free energy and heat resistance, even a circuit having a thin line width of 30 μm or less can be beautifully formed. Is.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the Examples. First, the measurement method and the evaluation method carried out in the present invention will be described.

(Surface free energy)
Using a contact angle meter (“FACE contact angle meter CA-X” manufactured by Kyowa Interface Science Co., Ltd.), 1.8 μl of water was dropped on the release surface under the conditions of 22 ° C. and 60% RH, and the drops were formed. The contact angle after 60 seconds was set to θw, 0.9 μl of methylene iodide was dropped on the release surface, and the contact angle after 30 seconds of drip was set to θy. From these measured values, γsh (hydrogen bonding force component term) and γsd (hydrogen dispersion force component term) were calculated according to the method described in Journal of Applied Polymer Science, vol.13, p1741-1747 (1969). The sum of each component was calculated as the surface free energy γs.

(Surface roughness Ra)
Using a laser microscope (manufactured by KEYENCE CORPORATION, product name: VK-X110), the surface roughness Ra of the surface shape of the undercoat layer was measured at a magnification of 50 times.

(Curl amount)
A 100 mm x 100 mm sample is hung in an oven at 150 ° C for 60 minutes with the four corners moving freely, cooled to room temperature, and then curled and raised when placed flat. The value was taken as the curl amount (mm).

(Initial appearance evaluation of forming circuit)
Using silver paste for circuit formation (manufactured by Toyobo Co., Ltd., product name: DX-152H-75), screen printing is performed on the undercoat layer of a transparent film for flexible substrates, and the film is baked at 130 ° C. for 30 minutes to have a line width of 20 μm. A circuit was formed. The formed circuit was visually judged, and in Table 1, good ones without cissing and bleeding were marked with ◯, and those with bleeding and bleeding were marked with x.

(Heat resistance: heat transfer depth)
In an environment of 100 ° C., four spherical zirconia beads (manufactured by Tosoh Corporation, trade name: YTZ, particle size 0.5 mm) were placed on the undercoat layer of the sample, and a 15 g weight was placed on the beads and left for 3 minutes. Then, after removing the weight and cooling the sample to room temperature, the deformed part of the film was observed using a laser microscope (manufactured by KEYENCE, trade name: VK-X110, 10 times observation), and the transfer depth of the spherical zirconia beads (transfer depth of spherical zirconia beads). μm) was measured. The average value of 4 points was used as the measured value.

(Example 1)
A mixture of a melamine cross-linking agent and a stearyl-modified acrylic resin (manufactured by Hitachi Chemical Co., Ltd., trade name: Tessfine (registered trademark) 322, solid content: 40% by mass), p-toluenesulfonic acid as an acid catalyst with respect to 100 parts by mass. (Manufactured by Hitachi Kasei Polymer Co., Ltd., trade name: dryer 900, solid content: 50% by mass) 1 part by mass is mixed, diluted with a mixed solvent of toluene and MEK (blending ratio 1: 1), and the solid content concentration is 2% by mass. The coating solution of was prepared. Next, the film thickness after drying is 0 on the opposite surface (surface roughness: 8 nm) of the biaxially stretched polyethylene terephthalate film (manufactured by Toyobo Co., Ltd., Cosmo Shine (registered trademark) A4100, film thickness 125 μm) with a single-sided easy-adhesive coat. The film was applied using a Meyer bar so as to have a thickness of .4 μm, dried in a solvent at a temperature of 160 ° C. for 30 seconds, and heat-cured to obtain an undercoat layer. Table 1 shows the characteristics of the obtained transparent film for flexible substrates.

(Example 2)
The same as in Example 1 except that the film thickness after drying was set to 2.0 μm in Example 1.

(Example 3)
In Example 1, a long-chain acrylic was added to 100 parts by mass of a mixture of a melamine cross-linking agent and a stearyl-modified acrylic resin (manufactured by Hitachi Chemical Co., Ltd., trade name: Tessfine (registered trademark) 322, solid content: 40% by mass). Modified resin (manufactured by Yushi Kogyo Co., Ltd., Piroyl (registered trademark) 1050, solid content: 10% by mass) 400 parts by mass, p-toluenesulfonic acid as an acid catalyst (manufactured by Hitachi Chemical Polymer Co., Ltd., trade name: dryer 900, solid) Minutes: 50% by mass) 1 part by mass was mixed, diluted with a mixed solvent of toluene and MEK (blending ratio 1: 1), and a coating solution having a solid content concentration of 2% by mass was prepared in the same manner as in Example 1. A transparent film for a flexible substrate was obtained.

(Example 4)
Toluene diisocyanate (solid content concentration: 35% by mass) with respect to 100 parts by mass of a copolymer (solid content concentration: 35% by mass) having a molecular weight of 30,000 obtained by copolymerizing 5 parts by mass of hydroxyethyl acrylate in MEK with respect to 95 parts by mass of stearyl acrylate. Made by Toso Co., Ltd., trade name: Coronate (registered trademark) L, solid content concentration: 75% by mass) 3.2 parts by mass, dioctyl tin as a urethanization catalyst (manufactured by Nitto Kasei Co., Ltd., trade name: Neostan (registered trademark) U- 860) 0.03 parts by mass was added and diluted with a mixed solvent of toluene and MEK (blending ratio 1: 1) to prepare a coating solution having a solid content concentration of 2% by mass. Next, a film after drying on the opposite surface (surface roughness: 8.0 nm) of the biaxially stretched polyethylene terephthalate film (manufactured by Toyobo Co., Ltd., Cosmo Shine (registered trademark) A4100, film thickness 125 μm) with a single-sided easy-adhesive coating. The film was applied using a Meyer bar so as to have a thickness of 0.5 μm, dried in a solvent at a temperature of 160 ° C. for 30 seconds, and heat-cured, and an undercoat layer was laminated. Table 1 shows the characteristics of the obtained transparent film for flexible substrates.

(Comparative Example 1)
A coated film was obtained in the same manner as in Example 1 except that the film thickness after drying was changed to 0.2 μm in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 2)
In Example 1, the film thickness after drying was changed to 0.5 μm, and the base film to which the Andocoat layer was applied was a polyester film (manufactured by Toyobo Co., Ltd., trade name: E5100, film thickness 100 μm, surface roughness: 36.0 nm). ) Was changed, and a coated film was obtained in the same manner as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 3)
A coat film was obtained in the same manner as in Example 1 except that the film thickness after drying was 3.5 μm in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 4)
In Example 1, a mixture of a melamine cross-linking agent and a stearyl-modified acrylic resin (manufactured by Hitachi Chemical Co., Ltd., trade name: Tessfine (registered trademark) 322, solid content: 40% by mass), silicone oil (manufactured by Hitachi Kasei Polymer Co., Ltd., solid content: 40% by mass), silicone oil (manufactured by Hitachi Chemical Co., Ltd.) Momentive Performance Materials Japan Co., Ltd., TSF4446, solid content: 100% by mass) 20 parts by mass, p-toluenesulfonic acid as an acid catalyst (Hitachi Kasei Polymer Co., Ltd., trade name: dryer 900, solid content: 50 mass) %) 1 part by mass was mixed and diluted with a mixed solvent of toluene and MEK (blending ratio 1: 1) to prepare a coating solution having a solid content concentration of 2% by mass. Got The evaluation results are shown in Table 1.

By using the transparent film for a flexible substrate of the present invention, it is possible to print with higher accuracy, it is possible to easily form a thin line circuit, and it is also very easy to form a flexible circuit. It greatly contributes to the world.

Claims (6)

Wherein at least one surface of the polyester film, the surface free energy of 20 mJ / ^{m 2} or more 35 mJ / ^m less than ^2, the surface roughness Ra of 10nm or less, to have an undercoat layer having a thickness of 0.3~3.0μm A transparent film for flexible substrates. The transparent film for a flexible substrate according to claim 1, wherein the undercoat layer contains a melamine-crosslinked acrylic resin. When four spherical beads having a particle size of 0.5 mm are placed on the undercoat layer in an environment of 100 ° C. and pressed against a load of 15 g, the transfer depth of the undercoat layer is 1.5 μm or less. The transparent film for a flexible substrate according to claim 1 or 2. The surface free energy of the undercoat layer is 20 mJ / ^{m 2} or more 32 mJ / ^{m 2} or less, the transparent flexible substrate according to any one of claims 1 to 3 film. The acrylic resin is a copolymerized polymer obtained by copolymerizing at least a (meth) acrylic acid ester having a hydroxyl group and one or more (meth) acrylic acid esters selected from the group consisting of stearyl acrylate and stearyl methacrylate. , The transparent film for a flexible substrate according to claim 2. A flexible circuit board characterized in that a circuit having a line width of 30 μm or less is formed on the transparent film for a flexible substrate according to any one of claims 1 to 5.