Coating film forming method, printed wiring board manufacturing method, and coating film forming system

Abstract

A coating film forming method according to the present invention is a coating film forming method for forming a coating film on a printed wiring board, the coating film forming method involving: an uncured coating film forming step for applying, with an inkjet method, a photocurable ink to a printed wiring board having a substrate and a conductor that has a thickness of 50 μm or more and is disposed on the substrate, thereby forming an uncured coating film so as to cover the conductor; and a photocured coating film forming step for subjecting the uncured coating film to multiple exposures each curing a thickness of 6-18 μm, thereby curing the uncured coating film to form a photocured coating film.

Description

Coating film forming method, printed wiring board manufacturing method, and coating film forming system

[0001] The present invention relates to a coating film forming method, a printed wiring board manufacturing method, and a coating film forming system. More specifically, the present invention relates to a coating film forming method, a printed wiring board manufacturing method, and a coating film forming system that can suppress ink bleeding and the occurrence of voids between the coating film and the substrate.

[0002] Conventionally, photolithography and screen printing have been used to form etching resists, solder resists, markings, and the like on printed wiring boards. Examples of utilizing inkjet printing in printed wiring board manufacturing methods include applying an inkjet ink containing a photopolymerizable compound to a copper-clad laminate for printed wiring boards (see, for example, Patent Documents 1 to 3). Forming solder resist using an inkjet printer has also been proposed. Inkjet printing can significantly reduce the number of steps and labor compared to photolithography, which requires a photomask, and screen printing methods using resist inks or marking inks, which require a screen plate. Furthermore, inkjet printing can reduce consumables such as developers, various inks, and cleaning solvents, and can also reduce wastewater, which is expected to contribute to a cleaner environment.

[0003] JP 2015-89540 A JP 2022-63284 A International Publication No. 2022 / 85183

[0004] When applying a solder resist film to the wiring of a printed wiring board, the thicker the wiring, the thicker the coating film (film thickness) must be to protect the wiring. When the wiring on the board is thin, sequential exposure is used. Sequential exposure involves applying a portion of the ink, immediately followed by exposure, and then repeating this partial ink application and subsequent exposure. However, as the wiring and coating thickness increase, sequential exposure creates a problem: voids form between the coating and the substrate. Therefore, ink is first applied to cover all or most of the wiring on the printed wiring board to form a pre-cured coating film (hereinafter also referred to as a "pre-cured coating film"). The pre-cured coating film is then exposed multiple times with time intervals to form a coating film (hereinafter also referred to as a "divided exposure"). This method successfully suppressed the formation of voids up to a certain wiring thickness, but when the wiring thickness exceeded 50 μm, it became difficult to suppress the formation of voids. In this regard, further improvements were needed for the inventions described in Patent Documents 1 to 3. Furthermore, there is also a problem that bleeding occurs when the illuminance of the exposure is reduced during the divided exposure. Note that the invention described in Patent Document 2 involves multiple exposures, but also multiple ink applications to form multiple layers of ink, and by forming multiple layers of ink, it is intended to improve the adhesion of the cured product film.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a coating film forming method, a printed wiring board manufacturing method, and a coating film forming system that can suppress the occurrence of voids between the coating film and the substrate and also suppress ink bleeding during coating film formation.

[0006] The present inventors investigated the causes of the above-mentioned problems in order to solve them. As a result, the present inventors found that by exposing the photocurable ink multiple times by increasing the number of exposures at a specific low irradiation amount of light, it is possible to suppress the occurrence of voids between the coating film and the substrate. Furthermore, the present inventors found that by forming a coating film by exposing the photocurable ink multiple times at a specific low irradiation amount of light, as long as the total exposure time of the photocurable ink is not too long, it is possible to suppress bleeding of the photocurable ink, and thus arrived at the present invention. That is, the above-mentioned problems according to the present invention are solved by the following means.

[0007] 1. A method for forming a coating film on a printed wiring board, comprising: a pre-cured coating film forming step of applying a photo-curable ink to the printed wiring board, which has a substrate and a conductor having a thickness of 50 μm or more arranged on the substrate, by an inkjet method, to form a pre-cured coating film so as to cover the conductor; and a photo-cured coating film forming step of exposing the pre-cured coating film multiple times to light so as to cure it to a thickness of 6 to 18 μm per time, thereby curing the pre-cured coating film to form a photo-cured coating film.

[0008] 2. The exposure is performed with an irradiation light amount of 200 to 700 mJ / cm ２ 2. The method for forming a coating film according to claim 1, wherein the exposure is by means of a photoirradiation.

[0009] 3. The time for each exposure is 40×10 －３ ~119 x 10 －３ 3. The method for forming a coating film according to claim 1, wherein the heating time is 10 seconds.

[0010] 4. The method for forming a coating film according to claim 1 or 2, wherein the photocurable ink contains a photopolymerizable compound having a molecular weight of 200 to 600.

[0011] 5. The method for forming a coating film according to claim 1 or 2, wherein the thickness of the conductor is 60 to 150 μm.

[0012] 6. The method for forming a coating film according to claim 1 or 2, wherein the photocurable ink has a linear expansion coefficient of 20 to 250 ppm / k.

[0013] 7. The method for forming a coating film according to claim 1 or 2, wherein the coating film has a survival rate of 90% or more as a result of a test in accordance with the adhesion test of JIS K5600.

[0014] 8. The method for forming a coating film according to claim 1 or 2, wherein the width of the spread from the boundary line between the coating film and the substrate toward the substrate is 20 μm or less.

[0015] 9. A method for producing a printed wiring board, comprising the steps of: forming a printed wiring board using the coating film forming method according to claim 1 or 2.

[0016] 10. A coating film forming system for use in the coating film forming method according to claim 1 or 2, comprising: an inkjet head that applies the photocurable ink to the printed wiring board to form the pre-cured coating film; and a curing unit that cures the pre-cured coating film applied to the printed wiring board with active energy rays.

[0017] The coating film forming method of the present invention can suppress the occurrence of voids between the coating film and the substrate, thereby suppressing ink bleeding during coating film formation. Furthermore, the printed wiring board manufacturing method of the present invention can suppress the occurrence of voids between the coating film and the substrate, thereby suppressing ink bleeding during coating film formation. Furthermore, the coating film forming system of the present invention can be used in the coating film forming method of the present invention, thereby suppressing the occurrence of voids between the coating film and the substrate during coating film formation, thereby suppressing ink bleeding during coating film formation.

[0018] The mechanism by which the effects of the present invention are manifested or the mechanism of action is not clear, but is presumed to be as follows.

[0019] When a coating film is formed on a printed wiring board, a gap between the coating film and the substrate, on which a conductor having a thickness of 50 μm or more is disposed, occurs near the conductor on the printed wiring board. The gap between the coating film and the substrate is sometimes referred to as a void. Furthermore, since voids do not occur when the substrate is heated before the conductor is disposed, it is presumed that foaming from the substrate is not the cause of the voids. Furthermore, since voids occur even when the ink before coating is degassed, it is presumed that foaming from the ink is not the cause of the voids. Thus, it is presumed that foaming from the substrate or ink, which are components of the printed wiring board, is not the cause of the voids. Therefore, it is presumed that one cause of the voids is a phenomenon that prevents the coating film from adhering to the substrate surface during coating film formation. Specifically, when the pre-cured coating film is cured, the pre-cured coating film shrinks while curing to form the coating film. It is presumed that this shrinkage of the pre-cured coating film causes the coating film to peel off from the substrate surface, resulting in the formation of the voids. It is presumed that one of the causes of peeling of the pre-cured coating film from the substrate surface is the occurrence of significant shrinkage in a short period of time. In contrast, the coating film forming method of the present invention is a method in which the pre-cured coating film is exposed to light multiple times, curing the film to a thickness of 6 to 18 μm per exposure. Therefore, the coating film forming method of the present invention can reduce the exposure dose (also referred to as "irradiation dose") in each exposure, thereby reducing the amount of shrinkage in each exposure. This prevents significant shrinkage from occurring in a short period of time. It is therefore presumed that the coating film forming method of the present invention can prevent peeling of the coating film from the substrate surface.

[0020] Furthermore, to form a coating film by curing the pre-cured coating film through multiple exposures, it is necessary to reduce the exposure dose per exposure. When the pre-cured coating film is exposed multiple times over a long period of time with a small exposure dose per exposure, bleeding occurs around the periphery of the coating film. It is believed that this bleeding is due to photopolymerizable compounds (monomers) or the like that are released during the curing of the pre-cured coating film. In contrast, the coating film formation method of the present invention exposes the pre-cured coating film multiple times, curing it to a thickness of 6 to 18 μm per exposure. Therefore, the pre-cured coating film can be cured without excessively long curing times. It is believed that this allows the polymerization of photopolymerizable compounds or the like (curing of the pre-cured coating film) to proceed while suppressing bleeding. In other words, the coating film formation method of the present invention allows the polymerization of photopolymerizable compounds or the like (curing of the pre-cured coating film) to proceed at a certain rate or faster. It is believed that this prevents the photopolymerizable compounds or the like from bleeding out and suppresses bleeding.

[0021] Fig. 1 is a plan view schematically showing a part of a state in which a pre-cured coating film has been formed on a printed wiring board. Fig. 2 is a cross-sectional view taken along the line A-A in Fig. 1. Fig. 3 is a flowchart showing an example of a coating film forming method of the present embodiment. Fig. 4 is a schematic diagram showing a process of forming a coating film using the coating film forming system of the present embodiment. Fig. 5 is a schematic diagram showing a process of forming a coating film using the coating film forming system of the present embodiment.

[0022] One embodiment of the coating film forming method of the present invention has the following features. A photocurable ink is applied to a printed wiring board by an inkjet method to form a pre-cured coating film that covers the conductor. The pre-cured coating film is then exposed to light multiple times, curing the pre-cured coating film to a thickness of 6 to 18 μm per exposure. The pre-cured coating film is thereby cured to form a photocured coating film. The printed wiring board has a substrate and a conductor having a thickness of 50 μm or more disposed on the substrate. Note that the "photocurable ink" is also simply referred to as "ink." This feature is a technical feature common to or corresponding to the following embodiments.

[0023] In the coating film forming method of this embodiment, the exposure (light irradiation) per one time is performed at an irradiation light amount of 200 to 700 mJ / cm ２This allows multiple exposures to be performed, each time curing a thickness of 6 to 18 μm, making it possible to suppress the occurrence of voids and bleeding of the ink.

[0024] In the coating film forming method of this embodiment, the exposure time per exposure is 40×10 －３ ~119 x 10 －３ This allows multiple exposures to be performed, each time curing a thickness of 6 to 18 μm, making it possible to suppress the occurrence of voids and bleeding of the ink.

[0025] In the coating film forming method of this embodiment, the photocurable ink preferably contains a photopolymerizable compound having a molecular weight of 200 to 600, and more preferably contains a photopolymerizable compound having a molecular weight of 200 to 350. This provides the photocurable ink with excellent ejection stability when ejected from an inkjet head.

[0026] In the coating film forming method of this embodiment, the thickness of the conductor is preferably 60 to 150 μm. In this way, the coating film forming method of this embodiment can suppress the occurrence of voids between the coating film and the substrate even for printed wiring boards with thick conductors, and can suppress bleeding of the ink during coating film formation.

[0027] In the coating film forming method of this embodiment, the photocurable ink preferably has a linear expansion coefficient of 20 to 250 ppm / k. This reduces the amount of shrinkage that occurs when the pre-cured coating film is cured and transformed into a coating film. This makes it possible to suppress the occurrence of voids during coating film formation. Here, the linear expansion coefficient of the photocurable ink refers to the linear expansion coefficient of the photocured coating film after the pre-cured coating film has cured.

[0028] In the coating film forming method of this embodiment, it is preferable that the result of an adhesion test of the coating film after multiple exposures is a survival rate of 90% or more. The adhesion test is a test based on the adhesion test method of JIS K5600. This shows that the occurrence of voids during coating film formation can be suppressed. The adhesion test is a test based on "JIS K 5600-5-6 Part 5 - Section 6: Adhesion (Cross-Cut Method)."

[0029] In the coating film forming method of this embodiment, the width of the bleeding is preferably 20 μm or less. The bleeding width is the width of bleeding from the boundary line between the coating film and the substrate toward the substrate after multiple exposures. This demonstrates that ink bleeding during coating film formation can be suppressed.

[0030] One embodiment of the printed wiring board manufacturing method of the present invention is characterized in that a printed wiring board is produced using the coating film forming method of the present invention. This makes it possible to suppress the occurrence of voids between the coating film and the substrate. Furthermore, it is possible to suppress ink bleeding during coating film formation. In this specification, a distinction is made between a "substrate having a conductor disposed thereon" on which no coating film is formed and a "substrate having a conductor disposed thereon" on which a coating film is formed. Specifically, a "substrate having a conductor disposed thereon" on which no coating film is formed is referred to as a printed wiring board, and a "substrate having a conductor disposed thereon" on which a coating film is formed is referred to as a printed wiring board.

[0031] One embodiment of the coating film forming system of the present invention includes an inkjet head that applies a photocurable ink to a printed wiring board to form a pre-cured coating film. Furthermore, the coating film forming system of this embodiment includes a curing unit that cures the pre-cured coating film applied to the printed wiring board with active energy rays. The coating film forming system of this embodiment is used in the coating film forming method of this embodiment. This can prevent voids from forming between the coating film and the substrate. Furthermore, it can prevent ink bleeding during coating film formation.

[0032] Hereinafter, the present invention, its components, and modes and aspects for carrying out the present invention will be described in detail with reference to the drawings. In this application, the symbol "to" is used to mean that the numerical values ​​before and after it are included as lower and upper limits.

[0033] 1. Coating Film Forming Method One embodiment of the coating film forming method of the present invention will be described. As shown in Figure 3, the coating film forming method of this embodiment has a pre-curing coating film forming step S01 and a photo-cured coating film S02. The coating film forming method of this embodiment may further have a heat-cured coating film forming step S03.

[0034] As shown in FIGS. 1 to 3 , the coating film forming method of this embodiment involves applying a photocurable ink to a printed wiring board 3 to form a pre-cured coating film 4 covering the conductor. This is the pre-cured coating film forming step S01. Next, the pre-cured coating film 4 is exposed to light multiple times, curing the pre-cured coating film 4 to a thickness of 6 to 18 μm per exposure, thereby curing the pre-cured coating film 4 and forming a photocured coating film. This is the photocured coating film forming step S02. The printed wiring board 3 includes a substrate 1 and a conductor 2 having a thickness of 50 μm or more disposed on the substrate 1. The photocurable ink is applied to the printed wiring board 3 using an inkjet method. In the coating film forming method of this embodiment, the photocured coating film may be further thermally cured to form a thermocured coating film. This is the thermocured coating film forming step S03.

[0035] As described above, the coating film forming method of this embodiment involves exposing the pre-cured coating film 4 multiple times, curing the film to a thickness of 6 to 18 μm per exposure. Therefore, the coating film forming method of this embodiment can suppress the occurrence of voids between the coating film and the substrate. Furthermore, the coating film forming method of this embodiment can suppress ink bleeding during coating film formation. Here, "curing the pre-cured coating film to a thickness of 6 to 18 μm per exposure" means that the pre-cured coating film is cured to a thickness of 6 to 18 μm in the thickness direction. Furthermore, "curing" the pre-cured coating film means that the pre-cured coating film has reached a state where color transfer onto white paper is no longer visible after the pre-cured coating film is pressed against and rubbed against the white paper through exposure.

[0036] The coating film formed by the coating film forming method of this embodiment is preferably a solder resist film. In this specification, the term "coating film" refers to a film finally formed on a printed wiring board. When the photocured coating film obtained by photocuring the pre-cured coating film is the final film, this photocured coating film is the coating film formed by the coating film forming method of this embodiment. Furthermore, when the thermocured coating film obtained by further thermocuring the photocured coating film is the final film, this thermocured coating film is the coating film formed by the coating film forming method of this embodiment.

[0037] 1-1. Pre-curing coating film formation process The pre-curing coating film formation process is a process in which a photocurable ink is applied to a printed wiring board to form a pre-curing coating film that covers the conductors. Furthermore, when applying the photocurable ink to the printed wiring board, the photocurable ink is applied using an inkjet method.

[0038] 1-1-1. Inkjet Method (Ejection Method) In this embodiment, the inkjet method ejects ink from an inkjet head to apply the ink to the surface of a printed wiring board. The coating film forming method of this embodiment preferably applies a photocurable ink, such as an inkjet ink for solder resist, onto a pattern on a printed wiring board using the inkjet method. The ejection method from the inkjet head may be either an on-demand method or a continuous method. An on-demand inkjet head may be either an electro-mechanical conversion method or an electro-thermal conversion method. Examples of electro-mechanical conversion methods include single-cavity, double-cavity, bender, piston, shear mode, and shared-wall types. Examples of electro-thermal conversion methods include thermal inkjet and bubble jet ("Bubble Jet" is a registered trademark of Canon Inc.) types.

[0039] (Ejection Amount) From the viewpoint of increasing the pattern formation speed and forming a finer pattern, the amount of ink droplets ejected is preferably 3.0 pL or more and 9.0 pL or less.

[0040] (Ejection Temperature) From the viewpoint of further improving ejection properties from the inkjet head, it is preferable to set the temperature of the ink when the ink is filled into the inkjet head to 40°C or higher and 100°C or lower. It is even more preferable to set this temperature to 40°C or higher and 90°C or lower. In particular, when the ink contains a gelling agent, the ink droplets are heated and solated before being ejected from the inkjet head. Therefore, it is preferable to set the temperature of the ink when filled into the inkjet head to a gelation temperature of the ink + 10°C or higher and a gelation temperature + 30°C or lower. When the temperature of the photocurable ink in the inkjet head is a gelation temperature + 10°C or higher, degradation of ejection properties due to gelation of the ink inside the inkjet head or on the nozzle surface is unlikely to occur. On the other hand, when the temperature of the ink in the inkjet head is a gelation temperature + 30°C or lower, degradation of the components due to high temperatures is unlikely to occur.

[0041] The method for heating the ink is not particularly limited. For example, at least one of the ink supply system, such as the ink tank constituting the head carriage, the supply pipe, and the anterior ink tank immediately before the head, the piping with a filter, and the piezo head can be heated by a panel heater, a ribbon heater, or heated water.

[0042] 1-1-2. Printed Wiring Board The printed wiring board has a substrate and a conductor with a thickness of 50 μm or more arranged on the substrate. In this specification, a substrate without a coating film formed on its surface as described above will be referred to as a "printed wiring board." A substrate with a coating film formed on its surface will be referred to as a "printed wiring board."

[0043] 1-1-2 (1). Substrate In the coating film forming method of this embodiment, the printed wiring board to which the ink is applied has a conductor disposed on the surface of the board. The conductor is preferably a conductive film such as copper foil disposed on the board. Substrates include rigid boards and flexible boards. Examples of rigid boards include paper phenolic boards, paper epoxy boards, glass epoxy boards, glass polyimide boards, Teflon (registered trademark) boards, PPO boards, and composite substrate epoxy boards. Examples of flexible board materials include PET film and polyimide film.

[0044] The thickness and size of the substrate are not particularly limited.

[0045] 1-1-2(2). Conductor In the coating film formation method of this embodiment, the conductor thickness is 50 μm or more. The coating film formation method of this embodiment is more effective when the conductor thickness is 60 to 150 μm. Here, "effective" means "effective in suppressing the occurrence of voids and bleeding." When the conductor thickness is less than 50 μm, the occurrence of voids and bleeding when forming a coating film on a printed wiring board is not significant. Therefore, even without using the coating film formation method of this embodiment, a coating film can be formed with reduced occurrence of voids and bleeding. In contrast, when the conductor thickness is 50 μm or more, the occurrence of voids and bleeding becomes significant during coating film formation. In this case, the "effect of suppressing the occurrence of voids and bleeding" achieved by the coating film formation method of this embodiment is significantly achieved. Furthermore, when the conductor thickness is 60 μm or more, the occurrence of voids and bleeding becomes even more significant during coating film formation. Even with such a thick conductor, the "effect of suppressing the occurrence of voids and bleeding" achieved by the coating film formation method of this embodiment is significantly achieved.

[0046] The printed wiring board preferably has wiring formed on the surface of the board using the conductor, and a circuit pattern formed on the surface of the board using this conductor wiring. It is also preferable that the conductor material is copper foil, and the conductor wiring is copper wiring. The width of the wiring formed by the conductor is not particularly limited and is determined depending on the characteristics of the printed wiring board, etc. The width of the wiring is preferably in the range of 30 to 100 μm, and more preferably in the range of 50 to 70 μm. When the width of the wiring is in the above range, the "effect of suppressing the occurrence of voids and bleeding" achieved by the coating film forming method of this embodiment is significantly exhibited.

[0047] In the coating film forming method of this embodiment, the photocurable ink is applied to the surface of a printed wiring board to form a pre-cured coating film that covers the conductors. The photocurable ink that has formed the pre-cured coating film is then exposed to specific light multiple times to form a photocured coating film.

[0048] 1-1-3 (1). Photopolymerizable Compound The photocurable ink preferably contains a photopolymerizable compound. The photopolymerizable compound is a compound that polymerizes and crosslinks when irradiated with active energy rays. The photopolymerizable compound may be a compound that further polymerizes and crosslinks when heated after being irradiated with active energy rays. The photocurable ink may contain only one type of photopolymerizable compound, or two or more types of photopolymerizable compounds.

[0049] The molecular weight of the photopolymerizable compound is preferably 200 or more and 600 or less. When the molecular weight of the photopolymerizable compound is 200 or more, the liquid components of the photocurable ink are less likely to volatilize. This makes it possible to further prevent the photocurable ink adhering to the nozzles of the inkjet head from drying, and non-volatile components such as pigments contained in the photocurable ink from adhering to the nozzles, resulting in a decrease in ejection stability. When the molecular weight is 600 or less, it is possible to prevent the viscosity of the photocurable ink from increasing too much, thereby preventing a decrease in ejection stability. From this perspective, the molecular weight is more preferably 250 or more and 500 or less.

[0050] The polymerizable compound may be a cationically polymerizable compound or a radically polymerizable compound, but is preferably a radically polymerizable compound. The radically polymerizable compound is a compound having a radically polymerizable ethylenically unsaturated bond.

[0051] Specific examples of the photopolymerizable compound include unsaturated carboxylic acid esters. The unsaturated carboxylic acid ester is preferably a (meth)acrylate compound (hereinafter simply referred to as (meth)acrylate). In this specification, "(meth)acrylate" means acrylate or methacrylate.

[0052] Examples of (meth)acrylates include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomylstyryl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, monofunctional acrylates including t-butylcyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, and t-butylcyclohexyl (meth)acrylate, as well as triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-Nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, bisphenol A PO adduct di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, polyethylene glycol diacrylate, tripropylene glycol diacrylate, tricyclodecane dimethanol diacrylate, tricyclodecane dimethanol diacrylate, propoxylated (3) neopentyl glycol diacrylate, Examples of the (meth)acrylate include bifunctional acrylates such as pentyl glycol diacrylate, and polyfunctional acrylates including trifunctional or higher acrylates such as trimethylolpropane tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxy tri(meth)acrylate, and pentaerythritol ethoxy tetra(meth)acrylate. Examples of the (meth)acrylate also include o-phenylphenol EO acrylate.

[0053] From the viewpoint of photosensitivity, etc., the (meth)acrylate is preferably stearyl (meth)acrylate, lauryl (meth)acrylate, isostearyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, isobornyl (meth)acrylate, tetraethylene glycol di(meth)acrylate, glycerin propoxy tri(meth)acrylate, etc. The (meth)acrylate may be a modified product. Examples of modified (meth)acrylates include ethylene oxide-modified trimethylolpropane tri(meth)acrylate, ethylene oxide-modified pentaerythritol tetraacrylate, ethylene oxide-modified (meth)acrylates including 4EO-modified hexanediol diacrylate, etc., caprolactone-modified (meth)acrylates including caprolactone-modified trimethylolpropane tri(meth)acrylate, etc., caprolactam-modified (meth)acrylates including caprolactam-modified dipentaerythritol hexa(meth)acrylate, nonylphenol 2PO-modified acrylate, etc., and the like.

[0054] The (meth)acrylate may be a polymerizable oligomer. Examples of the (meth)acrylate that is a polymerizable oligomer include an epoxy (meth)acrylate oligomer, an aliphatic urethane (meth)acrylate oligomer, an aromatic urethane (meth)acrylate oligomer, a polyester (meth)acrylate oligomer, and a linear (meth)acrylic oligomer.

[0055] 1-1-3(2). Photopolymerization initiator In the coating film forming method of this embodiment, the photocurable ink preferably contains a photopolymerization initiator. The photopolymerization initiator is a compound that initiates polymerization and crosslinking of the photopolymerizable compound when irradiated with active energy rays.

[0056] The photocurable ink can be cured by radical polymerization. In the coating film forming method of this embodiment, it is preferable to use a radical polymerization initiator as the photopolymerization initiator when curing the photocurable ink. Hereinafter, the "radical polymerization initiator" may also be simply referred to as "initiator." The photocurable ink may contain only one type of photopolymerization initiator, or two or more types of photopolymerization initiators.

[0057] Examples of radical polymerization initiators include α-cleavage radical polymerization initiators and hydrogen abstraction radical polymerization initiators. α-cleavage radical polymerization initiators are also called Norrish type I polymerization initiators. Hydrogen abstraction radical polymerization initiators are also called Norrish type II polymerization initiators.

[0058] The content of the α-cleavage type radical polymerization initiator is preferably within a range of 0.3 to 6 mass % relative to the total mass of the photocurable ink, and the content of the hydrogen abstraction type radical polymerization initiator is preferably within a range of 0.5 to 10 mass % relative to the total mass of the photocurable ink.

[0059] An α-cleavage radical polymerization initiator is an initiator that cleaves after photoexcitation to directly give an initiating radical. A hydrogen abstraction radical polymerization initiator is a photopolymerization initiator that is activated by active energy rays (e.g., ultraviolet light) and generates a free radical by abstracting hydrogen from a second compound. In this case, the second compound becomes the actual initiating free radical. This second compound is called a polymerization synergist or coinitiator. In the coating film formation method of this embodiment, both an α-cleavage radical polymerization initiator and a hydrogen abstraction radical polymerization initiator can be used alone or in combination.

[0060] Examples of the α-cleavage type radical polymerization initiator include acetophenone-based initiators, benzoin-based initiators, acylphosphine oxide-based initiators, benzyl and methylphenyl glyoxyesters.

[0061] Examples of acetophenone-based initiators include diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzil dimethyl ketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexyl-phenyl ketone, 2-methyl-2-morpholino(4-thiomethylphenyl)propan-1-one, and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone.

[0062] Examples of benzoin-based initiators include benzoin, benzoin methyl ether, and benzoin isopropyl ether.

[0063] Examples of the acylphosphine oxide initiator include 2,4,6-trimethylbenzoindiphenylphosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

[0064] Examples of hydrogen abstraction type radical initiators include benzophenone-based initiators, thioxanthone-based initiators, aminobenzophenone-based initiators, 10-butyl-2-chloroacridone, 2-ethylanthraquinone, 9,10-phenanthrenequinone, and camphorquinone.

[0065] Examples of benzophenone-based initiators include benzophenone, o-benzoylmethylbenzoate-4-phenylbenzophenone, 4,4'-dichlorobenzophenone, hydroxybenzophenone, 4-benzoyl-4'-methyl-diphenyl sulfide, acrylated benzophenone, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 3,3'-dimethyl-4-methoxybenzophenone.

[0066] Examples of thioxanthone initiators include 2-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, and 2,4-dichlorothioxanthone.

[0067] Examples of aminobenzophenone initiators include Michler's ketone and 4,4'-diethylaminobenzophenone.

[0068] 1-1-3(4). Other Components [A. Polymerization Inhibitor] The photocurable ink preferably further contains a polymerization inhibitor. By containing a polymerization inhibitor, the adhesiveness between multiple curable compounds can be reduced. In this specification, the term "polymerization inhibitor" includes all compounds added to inhibit polymerization reactions during preparation of ink containing polymerizable compounds or during storage after preparation.

[0069] The photocurable ink used in the coating film forming method of this embodiment can contain various conventionally known polymerization inhibitors. From the viewpoint of effective expression, it is more preferable that the polymerization inhibitor contains at least one of an N-oxyl-based polymerization inhibitor, a phenol-based polymerization inhibitor containing an o-t-butyl group, and a polymerization inhibitor having two or more aromatic rings. Furthermore, of these, it is even more preferable to contain an N-oxyl-based polymerization inhibitor from the viewpoint of adhesion to printed wiring boards.

[0070] The content of the polymerization inhibitor is preferably within a range of 0.05 to 0.5% by mass relative to the total mass of the ink.

[0071] [A-1. N-oxyl Polymerization Inhibitors] Examples of N-oxyl polymerization inhibitors include 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, 4-oxo-2,2,6,6-tetramethyl-piperidine-N-oxyl, 4-methoxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, 4-acetoxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, Irgastab (registered trademark), and UV10 (manufactured by BASF).

[0072] [A-2. Phenol-Based Polymerization Inhibitors] Examples of phenol-based polymerization inhibitors include 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2-tert-butyl-4,6-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-t-butyl-p-cresol (butylated hydroxytoluene: BHT), 4-methoxyphenol, and 2-methoxy-4-methylphenol.

[0073] [A-3. Quinone-Based Polymerization Inhibitors] Examples of quinone-based polymerization inhibitors include hydroquinone, methoxyhydroquinone, benzoquinone, 1,4-naphthoquinone, and p-tert-butylcatechol.

[0074] [A-4. Amine-Based Polymerization Inhibitors] Examples of amine-based polymerization inhibitors include alkylated diphenylamine, N,N'-diphenyl-p-phenylenediamine, and phenothiazine.

[0075] [A-5. Other Polymerization Inhibitors] Examples of other polymerization inhibitors include copper dithiocarbamate polymerization inhibitors such as copper dimethyldithiocarbamate, copper diethyldithiocarbamate, and copper dibutyldithiocarbamate.

[0076] Only one of these may be contained, or two or more of them may be contained. Among these, N-oxyl-based and quinone-based polymerization inhibitors are preferred, and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), 2,6-di-t-butyl-p-cresol (butylated hydroxytoluene: BHT), 2,4-di-tert-butylphenol, and naphthoquinone, etc., are preferred as polymerization inhibitors having two or more aromatic rings.

[0077] [B. Gelling Agent] The photocurable ink preferably further contains a gelling agent. By containing a gelling agent, the ink on the recording medium can be gelled and temporarily fixed (pinned), and the ink can be prevented from wetting and spreading.

[0078] The gelling agent preferably crystallizes at a temperature equal to or lower than the gelling temperature of the ink. The "gelling temperature" refers to the temperature at which, when a composition that has been solated or liquefied by heating is cooled, the gelling agent undergoes a phase transition from sol to gel, causing a sudden change in the viscosity of the composition. Specifically, the solated or liquefied composition is cooled while measuring its viscosity with a viscoelasticity measuring device, and the temperature at which the viscosity suddenly increases can be taken as the gelling temperature of the composition. An example of a viscoelasticity measuring device is the MCR300 (manufactured by Anton Paar).

[0079] When the gelling agent crystallizes in the ink, a so-called house-of-cards structure is formed. The house-of-cards structure is a structure in which the photopolymerizable compound is encapsulated in a three-dimensional space formed by the gelling agent crystallized into plates. To form this house-of-cards structure, it is preferable that the photopolymerizable compound and the gelling agent dissolved in the ink are compatible with each other.

[0080] Examples of gelling agents suitable for forming house-of-card structures include aliphatic ketones, aliphatic esters, petroleum waxes, vegetable waxes, animal waxes, mineral waxes, hydrogenated castor oil, modified waxes, higher fatty acids, higher alcohols, hydroxystearic acid, fatty acid amides including N-substituted fatty acid amides and special fatty acid amides, higher amines, esters of sucrose fatty acids, synthetic waxes, dibenzylidene sorbitol, dimer acid and dimer diol.

[0081] Among these, from the viewpoint of improving pinning properties, aliphatic ketones, aliphatic esters, higher fatty acids, and higher alcohols having a hydrocarbon group with a carbon number of 9 to 25 are preferred. Only one type of gelling agent may be contained, or two or more types may be contained.

[0082] [B-1. Aliphatic Ketones] Examples of aliphatic ketones include dilignoceryl ketone, dibehenyl ketone, distearyl ketone, dieicosyl ketone, dipalmityl ketone, dilauryl ketone, dimyristyl ketone, myristyl palmityl ketone, and palmityl stearyl ketone.

[0083] [B-2. Fatty Acid Esters] Examples of fatty acid esters include fatty acid esters of monoalcohols such as behenyl behenate, eicosanoic acid icosyl, and oleyl palmitate; and fatty acid esters of polyhydric alcohols such as glycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, ethylene glycol fatty acid esters, and polyoxyethylene fatty acid esters.

[0084] Examples of commercially available products of the above aliphatic esters include the EMALEX (registered trademark) series manufactured by Nippon Emulsion Co., Ltd., and the Rikemal (registered trademark) series and Poem (registered trademark) series manufactured by Riken Vitamin Co., Ltd.

[0085] [B-3. Higher Fatty Acids] Examples of higher fatty acids include behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid.

[0086] [B-4. Higher Alcohols] Examples of higher alcohols include stearyl alcohol and behenyl alcohol.

[0087] [B-5. Suitable Gelling Agent] In the coating film forming method of the present embodiment, the gelling agent is particularly preferably an aliphatic ketone represented by the following general formula (G1) or an aliphatic ester represented by the following general formula (G2).

[0088] General formula (G1): R １ -CO-R ２

[0089] In the above general formula (G1), R １ and R ２ R each independently represents an alkyl group having 12 to 26 carbon atoms, which includes a linear portion and may include a branched portion. １ and R ２ may be the same or different.

[0090] General formula (G2): R ３ -COO-R ４

[0091] In the above general formula (G2), R ３ and R４ R each independently represents an alkyl group having 12 to 26 carbon atoms, which includes a linear portion and may include a branched portion. ３ and R ４ may be the same or different.

[0092] In general formulas (G1) and (G2), when the number of carbon atoms in the linear or branched hydrocarbon group is 12 or more, the crystallinity of the aliphatic ketone represented by general formula (G1) or the aliphatic ester represented by general formula (G2) is further increased, and more sufficient spaces are generated in the house-of-card structure, which makes it easier for the photopolymerizable compound to be sufficiently encapsulated in the spaces, improving the pinning ability of the ink.

[0093] Furthermore, since the number of carbon atoms in the linear or branched hydrocarbon group is 26 or less, the melting point of the aliphatic ketone represented by general formula (G1) or the aliphatic ester represented by general formula (G2) does not increase excessively, that is, the melting point can be set to a temperature that is easy to handle, and there is no need to excessively heat the ink when ejecting it.

[0094] Examples of aliphatic ketones represented by general formula (G1) include dilignoceryl ketone (number of carbon atoms: 23, 24), dibehenyl ketone (number of carbon atoms: 21, 22), distearyl ketone (number of carbon atoms: 17, 18), dieicosyl ketone (number of carbon atoms: 19, 20), dipalmityl ketone (number of carbon atoms: 15, 16), dimyristyl ketone (number of carbon atoms: 13, 14), dilauryl ketone (number of carbon atoms: 11, 12), lauryl myristyl ketone (number of carbon atoms: 13, 14), and methyl ketone (number of carbon atoms: 13, 14). Examples of suitable ketones include lauryl palmityl ketone (carbon number: 11, 14), lauryl palmityl ketone (carbon number: 11, 16), myristyl palmityl ketone (carbon number: 13, 16), myristyl stearyl ketone (carbon number: 13, 18), myristyl behenyl ketone (carbon number: 13, 22), palmityl stearyl ketone (carbon number: 15, 18), palmityl behenyl ketone (carbon number: 15, 22), and stearyl behenyl ketone (carbon number: 17, 22). The number of carbon atoms in parentheses indicates the number of carbon atoms in each of the two hydrocarbon groups separated by the carbonyl group.

[0095] Commercially available examples of the aliphatic ketone represented by general formula (G1) include 18-Pentatriacontanon and Hentriacontan-16-on manufactured by Alfa Aeser, and Kaowax T-1 manufactured by Kao Corporation.

[0096] Examples of the aliphatic ester represented by general formula (G2) include behenyl behenate (number of carbon atoms: 21, 22), icosanoic acid icosyl (number of carbon atoms: 19, 20), stearyl stearate (number of carbon atoms: 17, 18), palmityl stearate (number of carbon atoms: 16, 17), lauryl stearate (number of carbon atoms: 12, 17), cetyl palmitate (number of carbon atoms: 6, 15), stearyl palmitate (number of carbon atoms: 15, 18), myristoyl stearate (number of carbon atoms: 16, 17), and methyl stearate (number of carbon atoms: 16, 17). Examples of esters include myristyl myristate (carbon number: 13, 14), cetyl myristate (carbon number: 13, 16), octyldodecyl myristate (carbon number: 13, 20), stearyl oleate (carbon number: 17, 18), stearyl erucate (carbon number: 18, 21), stearyl linoleate (carbon number: 17, 18), behenyl oleate (carbon number: 18, 22), and arachidyl linoleate (carbon number: 17, 20). The number of carbon atoms in parentheses indicates the number of carbon atoms in each of the two hydrocarbon groups separated by the ester group.

[0097] Commercially available examples of the aliphatic ester represented by general formula (G2) include Unistar (registered trademark) M-2222SL and Sperm Acetate manufactured by NOF Corporation, Exepar (registered trademark) SS and Exepar (registered trademark) MY-M manufactured by Kao Corporation, EMALEX (registered trademark) CC-18 and EMALEX (registered trademark) CC-10 manufactured by Nippon Emulsion Co., Ltd., and Amleps (registered trademark) PC manufactured by Kokyu Alcohol Kogyo Co., Ltd.

[0098] The content of the gelling agent is preferably within a range of 1 to 10% by mass relative to the total mass of the photocurable ink.

[0099] [C. Other Components] [C-1. Surfactant] In the coating film forming method of this embodiment, the photocurable ink may further contain a surfactant, if necessary. Examples of the surfactant include anionic surfactants such as dialkyl sulfosuccinates, alkyl naphthalene sulfonates, and fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surfactants such as alkylamine salts and quaternary ammonium salts; and silicone-based and fluorine-based surfactants.

[0100] [C-2. Colorant] In the coating film forming method of this embodiment, the photocurable ink may further contain a colorant, if necessary. The colorant may be a pigment or a dye, but is preferably a pigment from the viewpoints of good dispersibility in the constituent components of the ink and excellent weather resistance. The pigment is not particularly limited, and examples include organic pigments or inorganic pigments with the following numbers listed in the Color Index.

[0101] In the coating film forming method of this embodiment, only one type of colorant may be contained, or two or more types may be contained, and the color may be toned to a desired color. The content of the colorant is preferably within a range of 0.1 to 20 mass % relative to the total mass of the photocurable ink, and more preferably within a range of 0.2 to 10 mass %.

[0102] (Pigment) <Red or Magenta Pigment> Examples of red or magenta pigments include Pigment Red 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149, 166, 168, 169, 170, 177, 178, 179, 184, 185, 208, 216, 226, 257, and Pigment Violet. Pigment Orange 13, 16, 20, 36, or a mixture thereof.

[0103] <Blue or cyan pigment> Examples of blue or cyan pigments include pigments selected from Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 17:1, 22, 27, 28, 29, 36 and 60, and mixtures thereof.

[0104] <Green Pigment> Examples of green pigments include pigments selected from Pigment Green 7, 26, 36, and 50, and mixtures thereof.

[0105] <Yellow Pigment> Examples of yellow pigments include pigments selected from Pigment Yellow 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180, 185, and 193, and mixtures thereof.

[0106] <Black Pigment> Examples of black pigments include pigments selected from Pigment Black 7, 28, and 26, and mixtures thereof.

[0107] <Examples of Commercially Available Pigments> Examples of commercially available pigments include Black Pigment (manufactured by Mikuni Co., Ltd.), Chromofine Yellow 2080, 5900, 5930, AF-1300, 2700L, Chromofine Orange 3700L, 6730, Chromofine Scarlet 6750, Chromofine Magenta 6880, 6886, 6891N, 6790, 6887, and Chromofine Violet. RE, Chromofine Red 6820, 6830, Chromofine Blue HS-3, 5187, 5108, 5197, 5085N, SR-5020, 5026, 5050, 4920, 4927, 4937, 4824, 4933GN-EP, 4940, 4973, 5205, 5208, 5214, 5221, 5000P, Chromofine Green 2GN, 2GO, 2G-550D, 5310, 5370, 6830, Chromofine Black A-1103, Seika Fast Yellow 10GH, A-3, 2035, 2054, 2200, 2270, 2300, 2400 (B), 2500, 2600, ZAY-260, 2700 (B), 2770, Seika Fast Red 80 40, C405(F), CA120, LR-116, 1531B, 8060R, 1547, ZAW-262, 1537B, GY, 4R-4016, 3820, 3891, ZA-215, Seikafast Carmine 6B1476T-7, 1483LT, 3840, 3870, Seikafast Bordeaux 10B-430, Seikalite Rose R40, Seikalite Violet B800, 7805, Seikafast Maroon 460N, Seikafast Orange 900, 2900, Seikalite Blue C718, A612, Cyanine Blue 4933M, 4933GN-EP, 4940, 4973 (all manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.; "Chromofine" is a registered trademark of the company); KET Yellow 401, 402, 403, 404, 405, 406, 416, 424, KET Orange 501, KET Red 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 336, 337, 338, 346, KET Blue 101, 102, 103, 104, 105, 106, 111, 118, 124, KET Green 201 (manufactured by DIC);Colortex Yellow 301, 314, 315, 316, P-624, 314, U10GN, U3GN, UNN, UA-414, U263, Finecol Yellow T-13, T-05, Pigment Yellow1705, Colortex Orange 202, Colortex Red101, 103, 115, 116, D3B, P-625, 102, H-1024, 105C, UFN, UCN, UBN, U3BN, URN, UGN, UG276, U456, U457, 105C, USN, Colortex Maroon601, Colortex Brown B610N, Colortex Violet 600, Pigment Red 122, Colortex Blue 516, 517, 518, 519, A818, P-908, 510, Colortex Green 402, 403, Colortex Black 702, U905 (all manufactured by Sanyo Dye Co., Ltd.; "Colortex" and "Finecol" are registered trademarks of the company); Lionol Yellow 1405G, Lionol Blue FG7330, FG7350, FG7400G, FG7405G, ES, ESP-S (all manufactured by Toyo Ink Co., Ltd.; "Lionol" is a registered trademark of the company), Toner Magenta E02, Permanent Rubin F6B, Toner Yellow HG, Permanent Yellow GG-02, Hostaperm Blue B2G (all manufactured by Hoechst Industries); Novoperm P-HG, Hostaperm Pink E, Hostaperm Blue B2G (all manufactured by Clariant; "Novoperm" and "Hostaperm" are registered trademarks of the company); carbon black #2600, #2400, #2350, #2200, #1000, #990, #980, #970, #960, #950, #850, MCF88, #750, #650, MA600, MA7, MA8, MA11, MA100, MA100R, MA77, #52, #50, #47, #45, #45L, #40, #33, #32, #30, #25, #20, #10, #5, #44, and CF9 (all manufactured by Mitsubishi Chemical);

[0108] <Dispersion of Pigment> The pigment can be dispersed using, for example, a ball mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, paint shaker, or the like.

[0109] The pigment is preferably dispersed so that the volume average particle size of the pigment particles is within the range of 0.08 to 0.5 μm. Furthermore, the pigment is preferably dispersed so that the maximum particle size of the pigment particles is within the range of 0.3 to 10 μm, more preferably 0.3 to 3 μm. The pigment dispersion is adjusted by selecting the pigment, dispersant, and dispersion medium, dispersing conditions, filtration conditions, etc.

[0110] <Dispersant> The photocurable ink may further contain a dispersant to improve the dispersibility of the pigment. Examples of dispersants include carboxylic acid esters having a hydroxy group, salts of long-chain polyaminoamides and high-molecular-weight acid esters, salts of high-molecular-weight polycarboxylic acids, salts of long-chain polyaminoamides and polar acid esters, high-molecular-weight unsaturated acid esters, polymer copolymers, modified polyurethanes, modified polyacrylates, polyether ester-type anionic surfactants, naphthalene sulfonic acid formalin condensate salts, aromatic sulfonic acid formalin condensate salts, polyoxyethylene alkyl phosphate esters, polyoxyethylene nonylphenyl ether, and stearylamine acetate. Examples of commercially available dispersants include the Solsperse (registered trademark) series manufactured by Avecia and the PB series manufactured by Ajinomoto Fine-Techno Co., Ltd.

[0111] <Dispersion Aid> The photocurable ink may further contain a dispersion aid, if necessary. The dispersion aid may be selected depending on the pigment. The total content of the dispersant and dispersion aid is preferably within the range of 1 to 50% by mass relative to the total mass of the pigment.

[0112] <Dispersion Medium> The photocurable ink may further contain a dispersion medium for dispersing the pigment, if necessary. The photocurable ink may contain a solvent as the dispersion medium, but in order to prevent the solvent from remaining in the formed image, it is preferable to use the above-mentioned photopolymerizable compound (particularly a photopolymerizable compound with low viscosity) as the dispersion medium.

[0113] [C-3. Other Additives] The photocurable ink may further contain a coupling agent, a solvent, etc., as needed.

[0114] (Coupling Agent) The photocurable ink may further contain various coupling agents as needed. The inclusion of a coupling agent can improve adhesion to a printed wiring board. Examples of various coupling agents include silane-based, titanium-based, and aluminum-based coupling agents.

[0115] (Solvent) The photocurable ink is preferably solvent-free from the viewpoint of rapid curing and ejection stability, but a solvent may be added to adjust the ink viscosity.

[0116] 1-1-4. Formation of Pre-Cured Coating Film In the coating film forming method of this embodiment, a photocurable ink is applied to the surface of a printed wiring board to form a pre-cured coating film that covers the conductors.

[0117] 1-1-4 (1). Application of Photocurable Ink The method for applying the photocurable ink to the surface of a printed wiring board is the inkjet method. The photocurable ink is preferably applied so as to cover the wiring along the wiring pattern of the printed wiring board. That is, the photocurable ink is applied onto the conductors that make up the wiring, and a pre-cured coating film is formed to cover the conductors. Forming a pre-cured coating film to cover the conductors means applying the photocurable ink onto the conductors so that the pre-cured coating film extends beyond the conductor surface and its outer edge is in contact with the substrate surface. In other words, the pre-cured coating film is formed on the surface of the conductor (wiring pattern), and also extends beyond the conductor surface and its outer edge is in contact with the substrate surface. This maintains insulation without exposing the conductor to the surface.

[0118] 1-1-4(2). Pre-cured coating film The pre-cured coating film is a coating film formed by applying photocurable ink to the surface of a printed wiring board to cover the conductors. The pre-cured coating film is a coating film made of photocurable ink before it is photocured.

[0119] The thickness of the pre-cured coating film is preferably 40 to 200 μm, more preferably 50 to 180 μm, and particularly preferably 60 to 160 μm. By setting the thickness of the pre-cured coating film within this range, a highly insulating coating film is formed when the coating film is formed on a printed wiring board to produce a printed wiring board. If the thickness of the pre-cured coating film is thinner than 40 μm, the insulating properties of the formed coating film may be reduced. If the thickness of the pre-cured coating film is thicker than 200 μm, the resulting coating film may be too thick, making voids more likely to occur.

[0120] The pre-cured coating film is disposed on the conductor forming the wiring pattern in a shape that conforms to the conductor, and the outer edge of the pre-cured coating film extends beyond the outer edge of the conductor and contacts the surface of the substrate. This results in a coating film with high insulating properties. The width by which the outer edge of the pre-cured coating film extends beyond the outer edge of the conductor is preferably 2 to 5 μm. By setting the width (extending length) in this range, sufficient insulating properties can be maintained.

[0121] 1-2. Photocured Coating Film Formation Step The photocured coating film formation step is a step in which the pre-cured coating film is subjected to multiple exposures, each time curing the pre-cured coating film to a thickness of 6 to 18 μm, thereby curing the pre-cured coating film to form a photocured coating film. Here, "exposure that cures a thickness of 6 to 18 μm per exposure" is sometimes referred to as "exposure under specific conditions." As such, in the photocured coating film formation step, the coating film formation method of this embodiment subjects the pre-cured coating film to multiple exposures under specific conditions, thereby curing the pre-cured coating film. Therefore, the coating film formation method of this embodiment can suppress the occurrence of voids between the coating film and the substrate, and can suppress ink bleeding during coating film formation.

[0122] 1-2-1. Exposure Method In the photocured coating film formation process, an exposure method in which "exposure under specific conditions is performed multiple times" is used. When "exposure under specific conditions" is performed on the pre-cured coating film, the amount of irradiation light in each exposure is 200 to 700 mJ / cm. ２ Therefore, it is possible to reduce the shrinkage of the coating film before curing during one exposure, and to suppress the occurrence of voids between the coating film and the substrate. Furthermore, the exposure time per one exposure is 40×10 －３ ~119 x 10 －３ The rate of curing in one exposure is preferably determined by the illuminance of the light (W / cm ２ In order to keep the exposure time within the above range, it is necessary to perform exposure at a specific low illuminance. The illuminance of the light is 4.0 to 6.0 W / cm ２ is preferably 4.5 to 5.5 W / cm ２ More preferably, it is 5.0 W / cm ２ It is most preferable that the pre-cured coating film shrinks when exposed to light, becoming a photo-cured coating film. If large shrinkage occurs in a short period of time, it is thought that voids are more likely to occur between the formed coating film and the substrate. The coating film forming method of this embodiment forms a photo-cured coating film by curing the pre-cured coating film while repeating small shrinkages multiple times. In this way, the coating film forming method of this embodiment can prevent large shrinkage from occurring in a short period of time and suppress the occurrence of voids between the formed coating film and the substrate. In this specification, the term "illuminance" of light refers to "radiant illuminance (W / cm ２ The illuminance of the light is the illuminance on the surface of the coating film before it is cured.

[0123] As described above, the coating film forming method of this embodiment performs "exposure under specific conditions" on the pre-cured coating film. Therefore, bleeding of the ink (photocurable ink) during coating film formation can be suppressed. When exposing the pre-cured coating film, using light with low illuminance results in a single exposure that is too small, resulting in bleeding around the periphery of the formed coating film. Reducing the illuminance of light in a single exposure increases the time required for curing. This is thought to result in uncured photopolymerizable compounds and the like being unable to remain within the coating film and instead flowing to the periphery. In contrast, the coating film forming method of this embodiment performs "exposure of the pre-cured coating film to a thickness of 6 to 18 μm per exposure" multiple times. It is preferable to avoid excessively long exposure times. This allows the exposure and curing of the pre-cured coating film to proceed at a rate that does not cause bleeding of the photopolymerizable compounds and the like. This is thought to prevent bleeding of the photopolymerizable compounds and the like and suppress bleeding during coating film formation. As described above, if the curing of the pre-cured coating film proceeds too quickly, bleeding will not occur, but voids will form between the coating film and the substrate. On the other hand, if the curing of the pre-cured coating film proceeds too slowly, voids will not form between the coating film and the substrate, but bleeding will occur. To achieve both of these contradictory properties, the inventors discovered an exposure method in which "the pre-cured coating film is exposed to light multiple times to cure a thickness of 6 to 18 μm per exposure." The thickness of the pre-cured coating film cured in one exposure is a value obtained by measuring the film thickness of the photocured coating film after the entire pre-cured coating film has been cured, and dividing the obtained film thickness by the number of exposures.

[0124] In the coating film forming method of this embodiment, if the thickness of exposure per one time is less than 6 μm, the number of exposures increases, and the total time for curing (exposing) the pre-cured coating film becomes too long, which is undesirable, causing bleeding. Hereinafter, the thickness of exposure per one time may be referred to as the "exposure thickness." If the exposure thickness exceeds 18 μm, when an attempt is made to increase the illuminance of light and expose and cure the pre-cured coating film for a predetermined time to prevent bleeding, the amount of shrinkage per exposure becomes large, which is undesirable, causing voids between the coating film and the substrate. Furthermore, when an attempt is made to reduce the illuminance of light and cure the pre-cured coating film to prevent voids from forming between the coating film and the substrate, the curing takes too long, causing bleeding, which is undesirable. Furthermore, if the exposure time per one time is 40×10 －３ If the exposure time is shorter than 119 × 10 seconds, it is necessary to increase the illuminance of the light to cure the predetermined thick pre-cured coating film. This is undesirable because it causes gaps between the coating film and the substrate. －３ If the exposure time is longer than 10 seconds, it is necessary to reduce the illuminance of the light in order to cure a predetermined thick pre-cured coating film, which is undesirable as it causes bleeding.

[0125] The exposure of the pre-cured coating film disposed on the surface of the printed wiring board involves irradiating the pre-cured coating film with active energy rays, thereby curing the pre-cured coating film. The active energy rays can be selected from, for example, electron beams, ultraviolet rays, α rays, γ rays, X-rays, etc., but ultraviolet rays or electron beams are preferred. The ultraviolet rays are preferably light having a peak wavelength of 360 nm or more and 410 nm or less. The ultraviolet rays are preferably emitted from an LED light source. LEDs emit less radiant heat than conventional light sources (e.g., metal halide lamps), so using LEDs makes it less likely for the ink to melt when irradiated with active rays, making it less likely for uneven gloss to occur.

[0126] 1-2-2. Coating Film The coating film formed by the coating film forming method of this embodiment preferably has a thickness of 40 to 200 μm, more preferably 50 to 180 μm, and particularly preferably 60 to 160 μm. The thickness of the coating film is the height in the vertical direction from the substrate surface. By having the coating film thickness within this range, the insulation properties of a printed wiring board having a thick conductor disposed thereon can be improved. If the thickness is thinner than 40 μm, it may be difficult to maintain insulation properties. If the thickness is thicker than 200 μm, voids may be more likely to occur. Furthermore, the thickness of the coating film is preferably 20 to 50 μm thicker than the thickness of the conductor, more preferably 25 to 45 μm thicker, and particularly preferably 30 to 40 μm thicker. By having the coating film thickness within this range, the insulation properties of a printed wiring board having a thick conductor disposed thereon can be improved. If the thickness is thinner than 20 μm, it may be difficult to maintain insulation properties. If the thickness is thicker than 50 μm, voids may be more likely to occur.

[0127] 1-3. Heated Coating Film Forming Step The coating film forming method of this embodiment may include a heated coating film forming step after the photocured coating film forming step. The heated coating film forming step is a step of further heating the photocured coating film to form a heated coating film. By further heating the photocured coating film, the photocured coating film can be further cured. The heated coating film is a photocured coating film further cured to improve its curability. As described above, when a photocured coating film is the coating film that is finally obtained, the photocured coating film is the coating film obtained by the coating film forming method of this embodiment. Furthermore, when a heated coating film is the coating film that is finally obtained, the heated coating film is the coating film obtained by the coating film forming method of this embodiment. This can improve the adhesion of the coating film to the conductor and the substrate, and further improve the heat resistance of the coating film.

[0128] 1-3-1. Heating Method The method for heating the photocured coating film is not particularly limited, but for example, heating using an oven is possible. The heating temperature for the photocured coating film is not particularly limited as long as the coating film can be thermally cured, but is preferably 140°C or higher and 180°C or lower. The heating time for the photocured coating film is preferably 30 minutes or higher and 120 minutes or lower, and more preferably 30 minutes or higher and 60 minutes or lower.

[0129] 2. Printed Wiring Board Manufacturing Method The printed wiring board manufacturing method of the present invention is a method for producing a printed wiring board using the coating film forming method of the present invention. The resulting printed wiring board has a structure comprising a printed wiring board having a substrate and a conductor with a thickness of 50 μm or more arranged on the substrate, and a coating film disposed so as to cover the conductor.

[0130] The embodiment of the method for manufacturing a printed wiring board uses the embodiment of the method for forming a coating film to produce a printed wiring board, and therefore the resulting printed wiring board has reduced voids between the coating film and the substrate and reduced ink bleeding.

[0131] 3. Coating Film Forming System The coating film forming system of this embodiment will be described with reference to FIGS. 4A and 4B. FIGS. 4A and 4B are schematic diagrams illustrating the process of forming a coating film (photocured coating film 13) using the coating film forming system of this embodiment. FIG. 4A is a schematic diagram illustrating the state in which a photocurable ink is applied to a printed wiring board 3 by an inkjet head 11. FIG. 4B is a schematic diagram illustrating the state in which a pre-cured coating film is cured by a curing unit 3 to form a photocured coating film 13. The coating film forming system of this embodiment has an inkjet head 11 and a curing unit 12, and is used in the coating film forming method of this embodiment. The coating film forming system is shown separately in FIGS. 4A and 4B. The inkjet head 11 applies photocurable ink to the printed wiring board 3 to form a pre-cured coating film 4. The curing unit 12 cures the pre-cured coating film 4 applied to the printed wiring board 3 with active energy rays (light) to form a photocured coating film 13. That is, the active energy rays used for curing are irradiated from the curing section 12 onto the coating film before curing.

[0132] The inkjet head 11 is not particularly limited, but a piezoelectric inkjet head is preferred. An example of a piezoelectric inkjet head is the KM1800iSHC-C (resolution 600 dpi) manufactured by Konica Minolta. Furthermore, the inkjet head is preferably formed by arranging two independently driven piezoelectric inkjet heads with their nozzles staggered, so that the nozzle row has a resolution of 1200 dpi. This inkjet head is preferably configured to be filled with photocurable ink from an ink tank storing the photocurable ink via an ink flow path.

[0133] As described above, the curing unit 12 cures the pre-cured coating film applied to the printed wiring board with active energy rays (light). The curing unit 12 that irradiates active energy rays preferably includes an LED light source, a metal halide lamp, or the like, and more preferably includes an LED light source. The LED light source preferably irradiates ultraviolet light.

[0134] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these. In the following examples, unless otherwise specified, operations were carried out at room temperature (25°C). Furthermore, unless otherwise specified, "%" and "parts" mean "% by mass" and "parts by mass", respectively.

[0135] <Preparation of Pigment Dispersion A> Pigment Dispersion A is a cyan pigment dispersion. Dispersant 1 and Dispersant 2 shown below and a dispersion medium were placed in a stainless steel beaker, heated on a hot plate at 65°C for 1 hour while stirring and dissolving, and then cooled to room temperature. The pigment shown below was added to the mixture, and the mixture was placed in a glass bottle together with 200 g of zirconia beads with a diameter of 0.5 mm and sealed. The mixture was dispersed in a paint shaker until the desired particle size was reached, after which the zirconia beads were removed. Note that, in this specification, the term "pigment dispersion" refers to the entire dispersion in which the pigment and dispersant are dispersed in the dispersion medium.

[0136] Dispersant (PX4701 (manufactured by BASF)): 7.0 parts by mass Dispersion medium (dipropylene glycol diacrylate (containing 0.2% UV-10)): 70 parts by mass Pigment (Pigment Blue 15:4 (manufactured by Dainichiseika Chemicals, CFB6332JC (Chromofine (registered trademark) Blue 6332JC))): 23 parts by mass

[0137] <Preparation of Pigment Dispersion B> Pigment Dispersion B is a yellow pigment dispersion liquid. It was prepared in the same manner as Pigment Dispersion A, except that the dispersant, dispersion medium, and pigment in Pigment Dispersion A were changed as shown below.

[0138] Dispersant 1 (PX4701 (manufactured by BASF)): 6.0 parts by mass Dispersant 2 (Solsperse 22000 (manufactured by Lubrizol Japan)): 0.3 parts by mass Dispersion medium (dipropylene glycol diacrylate (containing 0.2% UV-10)): 61.5 parts by mass Pigment (Pigment Yellow 147 (manufactured by BASF, Oracet Y 140)): 10.2 parts by mass

[0139] The photopolymerizable compounds and photopolymerization initiators used in the preparation of the photocurable inks are as follows:

[0140] <Photopolymerizable Compounds> Photopolymerizable compound A: dipropylene glycol di(meth)acrylate, product name (manufacturer) = M222 (manufactured by Miwon), molecular weight 242 Photopolymerizable compound B: tricyclodecane dimethanol diacrylate, product name (manufacturer) = M262 (manufactured by Miwon), molecular weight 304 Photopolymerizable compound C: isobornyl (meth)acrylate, product name (manufacturer) = IBXA (manufactured by Miwon), molecular weight 208

[0141] <Photopolymerization initiators> Photopolymerization initiator A: Product name (manufacturer) = Omnirad 379 EG (manufactured by IGM) Photopolymerization initiator B: Product name (manufacturer) = Omnirad TPO H (manufactured by IGM) Photopolymerization initiator C: 2-isopropylthioxanthone, product name (manufacturer) = ITX (manufactured by Sartomer)

[0142] <Preparation of Photocurable Ink> The following components were mixed and filtered through a 3 μm Teflon (registered trademark) membrane filter manufactured by ADVATEC to prepare a photocurable ink. The resulting photocurable ink was evaluated for ejection stability using the following method. The composition of the photocurable ink and the results of the ejection stability evaluation are shown in Table I.

[0143] (Photopolymerizable compounds) Photopolymerizable compound A: 36.7 parts by mass Photopolymerizable compound B: 21.0 parts by mass Photopolymerizable compound C: 33.0 parts by mass (Pigment dispersions) Pigment dispersion A: 0.2 parts by mass Pigment dispersion B: 0.4 parts by mass

[0144] (Photopolymerization initiators) Photopolymerization initiator A: 3.6 parts by mass Photopolymerization initiator B: 4.8 parts by mass Photopolymerization initiator C: 0.3 parts by mass

[0145] Example 1 Formation of a Coating Film by Inkjet Method (Pre-Cured Coating Film Formation Step) The prepared photocurable ink was loaded into an inkjet recording device having an inkjet head equipped with a piezoelectric inkjet nozzle. Using this device, the photocurable ink was applied to a printed wiring board by inkjet printing to form a pre-cured coating film covering the conductor. The printed wiring board had a substrate and a conductor disposed on the substrate. The conductor in the printed wiring board was copper foil disposed on the substrate to form a linear pattern. The conductor had a thickness of 70 μm. The printed wiring board had a thickness of 1.6 mm and a size of 150 mm × 95 mm.

[0146] The photocurable ink supply system consisted of an ink tank, an ink flow path, a sub-ink tank immediately preceding the inkjet recording head, piping with a metal filter, and a piezo head. The ink was heated to 80°C from the ink tank to the head. A heater was also built into the piezo head, and the ink temperature inside the recording head was heated to 80°C. The piezo head had a nozzle diameter of 22 μm and a nozzle resolution of 360 dpi, arranged in a staggered pattern to form a nozzle array of 720 dpi. Using this inkjet device, a voltage was applied to form dots with a droplet volume of 6.0 pL, and the photocurable ink was applied.

[0147] (Photocured Coating Film Formation Step) The pre-cured coating film was exposed to active energy rays six times, each time curing a thickness of 10.0 μm, to form a photocured coating film. The amount of active energy rays irradiated per exposure was 300 mJ / cm. ２ The total light intensity is 1800 mJ / cm ２ The irradiance of the active energy rays was 5.0 W / cm ２ Irradiation with active energy rays was performed using an LED lamp (Fire Jet™ FJ100, manufactured by Phoseon Technology, wavelength 395 nm).

[0148] The device that cures the pre-cured coating film applied to the wiring board with active energy rays is the curing unit in the coating film forming system of the present invention. The curing unit is equipped with the LED lamp and irradiates active energy rays. The coating film forming system of the present invention comprises the inkjet head and the curing unit.

[0149] (Thermosetting coating film (coating film) forming step) After forming the photocured coating film, the photocured coating film was heated to form a thermosetting coating film. Specifically, the printed wiring board provided with the photocured coating film was placed in an oven set at 150°C for 60 minutes to obtain a printed wiring board. The coating film had a thickness of 60 μm. In this example, the thermocured coating film is the coating film obtained by the coating film forming method of this example. The coating film thickness was measured using a contact film thickness meter (MH-15M).

[0150] The amount of bleeding was measured for the printed wiring board using the method described below. The results are shown in Table II. In Tables II to IV, "Working" means "Example", for example, "Working 1" means "Example 1". Furthermore, "Comparative" means "Comparative Example", for example, "Comparative 1" means "Comparative Example 1".

[0151] Furthermore, a printed wiring board was prepared in the same manner as in Example 1, except that the pre-cured coating film was applied to the entire printed wiring board. The printed wiring board obtained in this manner is also called a "solid pattern." This was used as the sample of Example 1 and an adhesion test was conducted. The results are shown in Table II.

[0152] Examples 2 to 11, Comparative Examples 1 to 4 Printed wiring boards were produced in the same manner as in Example 1, except that the ingredients and photocuring conditions were changed as shown in Tables II to IV.

[0153] [Evaluation] <Jet Stability> Photocurable ink was continuously ejected (driven) using a piezo head. The conditions for continuous ejection (driving) were a droplet volume of 3.5 pl, a droplet speed of 7 m / sec, an ejection frequency of 40 kHz, and a printing rate of 100%. The number of nozzles that were not ejecting (missing nozzles) was counted 1 minute, 5 minutes, and 10 minutes after the start of continuous ejection (driving). The evaluation criteria for this evaluation test were as follows, with A and B being pass and C being fail. A: The number of missing nozzles was less than 5 in all counts after 1 minute, 5 minutes, and 10 minutes. B: The number of missing nozzles was 5 or more but less than 20 in all counts after 1 minute, 5 minutes, and 10 minutes. C: The number of missing nozzles was 20 or more in all counts after 1 minute, 5 minutes, and 10 minutes.

[0154] <Linear expansion coefficient> The linear expansion coefficient was measured by a compression load method using a thermomechanical analyzer. Measurement samples were obtained by cutting the coating film using a cutting machine. The measurement samples had vertical lengths, horizontal lengths, and thicknesses of 5 mm, 10 mm, and 10 mm. The thermomechanical analyzer used was a TMA 4000SE (manufactured by NETZSCH Japan). The measurement conditions were a nitrogen atmosphere, a compression method measurement mode, a temperature range from room temperature to 180°C, a heating rate of 5°C / min, and a load of 5 gf.

[0155] <Amount of Bleeding> The obtained printed wiring board was observed at 500x magnification using a microscope (VHX-5000 manufactured by KEYENCE Corporation). The amount of bleed-out of the monomer component of the photopolymerizable compound not containing a dye component from the boundary line of the outer edge of the coating film (hereinafter also referred to as "amount of bleed-out") was measured. The evaluation criteria for this evaluation test are as follows, with A to C being pass and D being fail. A: Amount of bleed-out is 20 μm or less B: Amount of bleed-out is more than 20 μm but not more than 30 μm C: Amount of bleed-out is more than 30 μm but not more than 40 μm D: Amount of bleed-out is more than 40 μm

[0156] <Adhesion> A test was conducted on a solid pattern in accordance with the adhesion test of JIS K5600 (JIS K 5600-5-6 Part 5 - Section 6: Adhesion (Cross-Cut Method)). Specifically, grid-like cuts were made in the coating film, adhesive tape was applied, and the film was peeled off to observe the state of peeling. The residual adhesion rate was determined using the method described below and evaluated according to the following criteria. Here, the residual adhesion rate is calculated by taking the number of squares made by the cuts as the denominator and the number of squares remaining on the printed wiring board after the tape was peeled off as the numerator. A to C are pass, and D is fail. (Criteria) A: residual adhesion rate 100% B: residual adhesion rate 80% or more but less than 100% C: residual adhesion rate 60% or more but less than 80% D: residual adhesion rate less than 60%

[0157]

[0158]

[0159]

[0160]

[0161] From the above results, it can be seen that the coating film obtained by the coating film forming method of the Example has less bleeding and higher adhesion than the coating film obtained by the coating film forming method of the Comparative Example.

[0162] It is possible to provide a coating film forming method, a printed wiring board manufacturing method, and a coating film forming system that can suppress ink bleeding and also suppress the occurrence of voids between the coating film and the substrate.

[0163] 1: Substrate 2: Conductor 3: Printed wiring board 4: Pre-cured coating film 11: Inkjet head 12: Curing section 13: Photo-cured coating film S01: Pre-cured coating film forming step S02: Photo-cured coating film forming step S03: Thermo-cured coating film forming step

Claims

1. A method for forming a coating film on a printed wiring board, comprising: a pre-cured coating film forming step of applying a photo-curable ink to the printed wiring board, which has a substrate and a conductor of 50 μm or more in thickness, by an inkjet method, to form a pre-cured coating film so as to cover the conductor; and a photo-cured coating film forming step of exposing the pre-cured coating film multiple times to light so as to cure it to a thickness of 6 to 18 μm per time, thereby curing the pre-cured coating film to form a photo-cured coating film.

2. The exposure is performed with an irradiation light amount of 200 to 700 mJ / cm ２ 2. The method for forming a coating film according to claim 1, wherein the exposure is by means of a photoirradiation.

3. The exposure time is 40 x 10 －３ ~119 x 10 －３ 3. The method for forming a coating film according to claim 1, wherein the heating time is 10 seconds.

4. The method for forming a coating film according to claim 1 or 2, wherein the photocurable ink contains a photopolymerizable compound having a molecular weight of 200 to 600.

5. The method for forming a coating film according to claim 1 or 2, wherein the thickness of the conductor is 60 to 150 μm.

6. The method for forming a coating film according to claim 1 or 2, wherein the photocurable ink has a linear expansion coefficient of 20 to 250 ppm / k.

7. A method for forming a coating film according to claim 1 or claim 2, wherein the coating film has a survival rate of 90% or more as a result of a test performed in accordance with the adhesion test of JIS K5600.

8. A method for forming a coating film according to claim 1 or 2, wherein the width of the spread from the boundary line between the coating film and the substrate toward the substrate is 20 μm or less.

9. A method for producing a printed wiring board, which comprises using the coating film forming method according to claim 1 or 2.

10. A coating film forming system for use in the coating film forming method described in claim 1 or claim 2, comprising: an inkjet head that applies the photocurable ink to the printed wiring board to form the pre-cured coating film; and a curing unit that cures the pre-cured coating film applied to the printed wiring board with active energy rays.

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