JP7601633B2 - Manufacturing method for flexible substrate - Google Patents

Description

The present invention relates to a method for manufacturing a flexible substrate, which is a precursor to a flexible wiring substrate.

Flexible wiring boards are flexible and allow for three-dimensional wiring and movable wiring within electronic devices, so they are used in a variety of electronic devices, including mobile terminals, LCD televisions, notebook computers, and digital cameras. Flexible wiring boards are produced by forming a wiring pattern on a flexible substrate (precursor) that has a conductive layer formed on a base material. Flexible substrates are produced by a variety of known methods, and various improvement techniques have been proposed to increase connection reliability.

For example, Patent Document 1 discloses a bronze plating solution containing a cupric salt, a reducing agent, at least two types of complexing agents, and a stabilizer, and a method for manufacturing a flexible substrate using the plating solution.

Patent Document 2 discloses an electroless copper plating bath containing a specific phosphine compound as a stabilizer, and a method for manufacturing a flexible substrate using the plating bath.

Patent document 3 discloses a flexible substrate in which an electroless copper plating layer is formed on a polyimide substrate having a specified surface roughness.

China Patent Publication No. 108559980 JP 2005-290415 A JP 2012-15448 A

Electrolytic copper foil and rolled copper foil are used as conductive layers, but rolled copper foil is generally used for flexible wiring boards used in moving parts such as hinges of electronic devices, which require even higher bending and folding resistance. Electrolytic copper foil has a columnar crystal structure developed in the thickness direction, so when it is folded, cracks propagate along the grain boundaries, causing early breakage, whereas rolled copper foil has an isotropic crystal structure, so cracks do not propagate easily and it has high bending resistance reliability.
In recent years, flexible substrates have come into use in which a conductive layer is formed on a resin substrate by combining rolled copper foil, electroless copper plating, and electrolytic copper plating. However, the following plating film defects have occurred due to conventional electroless copper plating processes using electroless copper plating solutions.
First, i) when a resin substrate is used, even if electroless copper plating is performed in a conventional electroless copper plating solution, plating does not deposit on the resin substrate portion where rolled copper foil is not formed, and an electroless copper plating film cannot be formed on the resin substrate in some portions. Therefore, even if electrolytic copper plating is performed after electroless copper plating, an electrolytic copper plating film cannot be formed on the non-deposited portion, and the non-deposited portion may cause wiring pattern defects or decrease connection reliability.
In addition, ii) the electroless copper plating film formed on the resin substrate may have blisters, i.e., the electroless copper plating film may swell to form gaps between the resin substrate and the plating film, which may cause the plating film to peel off from the resin substrate or reduce connection reliability.

The present invention was made in consideration of the above problems, and its purpose is to provide a manufacturing method for flexible substrates that can solve the above problems i and ii.

[1] A method for producing a flexible substrate in which an electroless copper plating film and an electrolytic copper plating film are laminated on a resin substrate and a rolled copper foil formed on a part of the resin substrate, comprising:
The manufacturing method includes:
A step of forming the electroless copper plating film on the resin substrate and the rolled copper foil by performing an electroless copper plating process;
a step of forming the electrolytic copper plating film on the electroless copper plating film by performing an electrolytic copper plating process;
The electroless copper plating process uses an electroless copper plating solution containing a copper compound, a reducing agent, a tertiary alkanolamine, a nickel compound, and a nitrogen-containing aromatic compound.

[2] The concentration of the tertiary alkanolamine in the electroless copper plating solution is 0.001 g/L to 1000 g/L;
The concentration of the nickel compound is 0.0005 g/L to 5 g/L in terms of nickel concentration,
The method for producing a flexible substrate according to the above-mentioned [1], wherein the concentration of the nitrogen-containing aromatic compound is 0.00001 g/L to 2 g/L.

[3] The method for producing a flexible substrate according to the above [1] or [2], wherein the tertiary alkanolamine is at least one selected from the group consisting of triethanolamine and triisopropanolamine.

[4] The method for producing a flexible substrate according to any one of [1] to [3] above, wherein the nitrogen-containing aromatic compound is at least one selected from the group consisting of bipyridines and phenanthrolines.

According to the present invention, it is possible to form a good electroless copper plating film in which non-plated areas and blisters are suppressed.
Furthermore, according to the present invention, a good electrolytic copper plating film in which pit-like defects are suppressed can be formed.
Therefore, according to the manufacturing method of the present invention, it is possible to provide a flexible substrate having a good electroless copper plating film and a good electrolytic copper plating film.

FIG. 1 is a schematic diagram illustrating a manufacturing process for a flexible substrate according to the present invention. FIG. 2 is a schematic diagram for explaining the process for producing a flexible wiring board using the flexible substrate of the present invention.

The flexible substrate of the present invention can be manufactured by sequentially performing electroless copper plating and electrolytic copper plating on the rolled copper foil surface of the resin substrate to form an electroless copper plating film and an electrolytic copper plating film. The present invention is characterized in that an electroless copper plating bath having a predetermined composition is used as a means for solving the above problems.

The electroless copper plating solution of the present invention contains a tertiary alkanolamine, a nickel compound, a nitrogen-containing aromatic compound, a copper compound, and a reducing agent. The electroless copper plating solution of the present invention has a synergistic effect of the tertiary alkanolamine, the nickel compound, and the nitrogen-containing aromatic compound,
1) Suppression of non-deposition of electroless copper plating film on the substrate surface,
2) suppression of blistering of the electroless copper plating film formed on the surface of the substrate; and 3) a electroless copper plating film that does not have the same crystal orientation as the rolled copper foil can be formed on the surface of the rolled copper foil.
In particular, the above 3) is a finding obtained by the present inventors as a result of repeated investigations into the above conventional problems i) and ii). That is, it was found that if an electroless copper plating film formed on a rolled copper foil has the same crystal orientation as the rolled copper foil, the electrolytic copper plating film also has the same crystal orientation, and that such an electrolytic copper plating film has no surface gloss and pit-like defects occur on the surface of the electrolytic copper plating film. Such an electrolytic copper plating film causes pattern defects and circuit defects such as reduced connection reliability. In order to avoid such problems, it is effective to form an electroless copper plating film as described in the above 3).
By using the electroless copper plating solution of the present invention, the above-mentioned effects 1 to 3 can be obtained for a substrate. Furthermore, an electroless copper plating film formed by subjecting an electroless copper plating process to an electroless copper plating film formed using the electroless copper plating solution of the present invention has suppressed pit-like defects.

First, the electroless copper plating solution of the present invention will be described.
The electroless copper plating solution of the present invention contains a copper compound, a reducing agent, a tertiary alkanolamine, a nickel compound, and a nitrogen-containing aromatic compound.

Tertiary alkanolamines not only act as complexing agents, but are also particularly effective in the above-mentioned 3) formation on the surface of a rolled copper foil of an electroless copper plating film that does not have the same crystal orientation as the rolled copper foil, and contribute to suppressing pit-like defects in the electrolytic copper plating film formed on the surface of the electroless copper plating film.

The tertiary alkanolamine is a compound having three or more linear or branched alkanol groups. The tertiary alkanolamine is preferably a compound having an alkanol group having 1 to 18 carbon atoms, and more preferably trimethanolamine, triethanolamine, tripropanolamine, triisopropanolamine, tributanolamine, tripentanolamine, trihexanolamine, triheptanolamine, trioctanolamine, trinonanolamine, tridecanolamine, tridodecanolamine, tritetradecanolamine, trihexadecanolamine, or trioctadecanolamine. The tertiary alkanolamine is more preferably triethanolamine or triisopropanolamine.
The tertiary alkanolamines can be used alone or in combination of two or more kinds in any desired ratio.

If the concentration of the tertiary alkanolamine is too low, the effect of adding the tertiary alkanolamine may not be sufficient, whereas if the concentration of the tertiary alkanolamine is too high, the electroless copper plating film may not be uniformly formed (hereinafter, sometimes referred to as film uniformity), for example, due to the occurrence of areas on the substrate or rolled copper foil that are not covered with the electroless copper plating film.
The concentration of tertiary alkanolamine in the electroless copper plating solution is preferably 0.001 g/L or more, more preferably 0.005 g/L or more, even more preferably 0.01 g/L or more, still more preferably 0.5 g/L or more, and is preferably 1000 g/L or less, more preferably 600 g/L or less, and even more preferably 200 g/L or less.

Nickel Compounds Nickel compounds particularly contribute to the above-mentioned 2) prevention of blistering of the electroless copper plating film formed on the surface of the substrate.
The nickel compound is preferably a water-soluble nickel compound, more preferably nickel sulfate, nickel nitrate, nickel chloride, nickel acetate, nickel citrate, nickel tartrate, nickel gluconate, or the like.
The nickel compounds can be used alone or in combination of two or more kinds in any ratio.

If the concentration of the nickel compound is too low, the effect of adding the nickel compound may not be sufficient, whereas if the concentration of the nickel compound is too high, the uniformity of the coating may not be obtained.
The nickel compound concentration in the electroless copper plating solution is preferably 0.0005 g/L or more, more preferably 0.005 g/L or more, and even more preferably 0.05 g/L or more, and is preferably 5 g/L or less, more preferably 2.5 g/L or less, and even more preferably 0.5 g/L or less, in terms of nickel concentration.

Nitrogen-Containing Aromatic Compound Nitrogen-containing aromatic compounds particularly contribute to the above-mentioned 1) suppression of non-deposition of the electroless copper plating film on the surface of the substrate.
The nitrogen-containing aromatic compound is not particularly limited, but is preferably pyrrolidines, imidazoles, triazoles, tetrazoles, benzimidazoles, benzotriazoles, piperidines, morpholines, piperazines, pyridines, bipyridines, phenanthrolines, oxazoles, benzoxazoles, pyrimidines, quinolines, and isoquinolines, and more preferably bipyridines and phenanthrolines.
The nitrogen-containing aromatic compounds can be used alone or in combination of two or more kinds in any desired ratio.

More specifically, 4,4'-dimethyl-2,2'-dipyridyl, 2,2'-biquinolyl, 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine, 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p'-disulfonic acid disodium salt, 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine, 2,2'-bis(5,6-dimethyl-1,2,4-triazine), 6,7- It is preferable that the compound is at least one selected from the group consisting of dimethyl-2,3-di(2-pyridyl)-quinoxaline, 2,3-bis(2-pyridyl)-6,7-dimethylquinoxaline, 2,2'-bipyridyl, 6,7-diethyl-2,3-di(2-pyridyl)-quinoxaline, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, and 2,9-diphenyl-1,10-phenanthroline.

If the concentration of the nitrogen-containing aromatic compound is too low, the effect of adding the compound may not be sufficient, whereas if the concentration of the nitrogen-containing aromatic compound is too high, the coating may not be uniform.
The concentration of the nitrogen-containing aromatic compound in the electroless copper plating solution is preferably 0.00001 g/L or more, more preferably 0.0001 g/L or more, even more preferably 0.001 g/L or more, and is preferably 2 g/L or less, more preferably 1 g/L or less, even more preferably 0.1 g/L or less, and still more preferably 0.05 g/L or less.

Copper Compounds Copper compounds are essential compounds for the formation of electroless copper plating films.
The copper compound is a water-soluble copper salt, and preferred examples thereof include copper sulfate, copper nitrate, copper chloride, copper acetate, copper citrate, copper tartrate, and copper gluconate.
The copper compounds can be used alone or in combination of two or more kinds in any desired ratio.

If the concentration of the copper compound is too low, the deposition rate will be slow and the plating time will be long, while if the concentration of the copper compound is too high, the plating solution will become unstable.
The concentration of the copper compound in the electroless copper plating solution is preferably 0.05 g/L or more, more preferably 0.5 g/L or more, and is preferably 30 g/L or less, more preferably 10 g/L or less, in terms of copper concentration.

The reducing agent may be any known reducing agent for copper ions in the electroless copper plating solution, and is preferably formaldehyde, paraformaldehyde, glyoxylic acid, glyoxylates, aminoboranes such as dimethylamine borane, alkali metal borohydrides, hydrazine, polysaccharides, sugars such as glucose, hypophosphorous acid, hypophosphites, glycolic acid, glycolate salts, formic acid, formates, etc.
The reducing agent can be used alone or in combination of two or more kinds in any ratio.

The reducing agent may be present in an amount necessary for the reduction of copper ions. The reducing agent concentration in the electroless copper plating solution is preferably 0.01 g/L or more, more preferably 0.5 g/L or more, and is preferably 100 g/L or less, more preferably 80 g/L or less, and even more preferably 50 g/L or less.

The electroless copper plating solution of the present invention may further contain optional additives such as complexing agents, surfactants, pH adjusters, and stabilizers as necessary.

Complexing Agent In the present invention, the tertiary alkanolamine also acts as a complexing agent. Therefore, other complexing agents may not be added, but may be added optionally. As the other complexing agent, known complexing agents used in electroless copper plating solutions can be used. The complexing agent is preferably monocarboxylic acid such as acetic acid and formic acid, and their ammonium salts, potassium salts, sodium salts, etc.; dicarboxylic acid such as malonic acid, succinic acid, adipic acid, maleic acid, fumaric acid, and their ammonium salts, potassium salts, sodium salts; hydroxycarboxylic acid such as malic acid, lactic acid, glycolic acid, gluconic acid, citric acid, and their ammonium salts, potassium salts, sodium salts, etc.; ethylenediaminediacetic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and their ammonium salts, potassium salts, sodium salts, etc.; aminopolycarboxylic acid such as ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and their ammonium salts, potassium salts, sodium salts, etc.
The complexing agent can be used alone or in combination of two or more kinds in any ratio.

The concentration of other complexing agents in the electroless copper plating solution may be at a level that does not impair the effects of the present invention, and is preferably 0.0001 g/L or more, more preferably 0.01 g/L or more, and is preferably 1000 g/L or less, more preferably 500 g/L or less.

In the present invention, a known surfactant used in an electroless copper plating solution can be used. The surfactant can be any of an ionic surfactant, a nonionic surfactant, and an amphoteric surfactant, and can be appropriately selected and used.
The surfactants can be used alone or in combination of two or more in any desired ratio.

The concentration of the surfactant in the electroless copper plating solution is not particularly limited, but is preferably 0.000001 g/L or more, more preferably 0.00001 g/L or more, and is preferably 5 g/L or less, more preferably 2 g/L or less.

pH Adjusting Agent In the present invention, it is preferable to add a pH adjusting agent as necessary to appropriately adjust the pH of the electroless copper plating solution to be within the following range.
Any known pH adjuster used in electroless copper plating solutions can be used, and the pH adjuster is preferably a basic compound, an inorganic acid, or an organic acid, and more preferably an inorganic acid such as sulfuric acid, hydrochloric acid, or phosphoric acid, or sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, rubidium hydroxide, ammonium hydroxide, tetramethylammonium hydroxide, or butylammonium hydroxide.
The pH adjusters can be used alone or in combination of two or more in any desired ratio.

pH of electroless copper plating solution
The pH of the electroless copper plating solution is preferably in the range from acidic to alkaline, more preferably in the range from neutral to alkaline. The pH of the electroless copper plating solution is preferably 2 or more, more preferably 7 or more, and is preferably 13 or less.

The manufacturing method for the flexible substrate of the present invention is described below.

Resin Substrate The resin substrate 1 is not particularly limited as long as it is a resin substrate having the required flexibility, and various known resin substrates can be used (FIG. 1(a)). For example, a substrate made of a resin having excellent heat resistance, such as polyimide or polyethylene terephthalate (PET), is preferable. Polyimide-based substrates and PET-based substrates have dimensional stability, little thermal shrinkage, and excellent flexibility.

Rolled copper foil A rolled copper foil 2 is laminated on at least one surface of a resin substrate 1 (FIG. 1(b)). The rolled copper foil may be laminated on both sides of the resin substrate. The rolled copper foil may be laminated on at least a part of the surface of the resin substrate. Various known methods can be adopted as the raw material and manufacturing method of the rolled copper foil, and the method of laminating the rolled copper foil to the resin substrate. In addition, in the present invention, a resin substrate having a commercially available known rolled copper foil can be used. The size and wiring pattern of the rolled copper foil can be appropriately selected according to the required properties.
The flexible substrate of the present invention has an electroless copper plating film formed on both the surface of the rolled copper foil and the surface of the substrate. In the present invention, it is preferable to use a flexible substrate having a line and space pattern formed with rolled copper foil. Specific examples include flexible substrates used for flexible displays and FPC connectors.

Before electroless copper plating, the resin substrate having the rolled copper foil may be subjected to a suitable pretreatment, if necessary. Examples of the pretreatment include a degreasing step, a conditioning step, an acid washing step, a sensitization step (a catalyst application step), and an adhesion promotion step.

In the present invention, electroless copper plating is performed to form an electroless copper plating film on the rolled copper foil side of the resin substrate. The resin substrate is immersed in an electroless copper plating solution to form an electroless copper plating film 3 on the substrate surface 1 and on the rolled copper foil 2 surface (FIG. 1(c)).
The electroless copper plating process uses the electroless copper plating solution of the present invention. In the present invention, the processing conditions are not limited except for the use of the electroless copper plating solution, and known electroless copper plating processing conditions can be adopted.

The thickness of the electroless copper plating film can be appropriately changed depending on the required properties. The electroless copper plating film is preferably thick enough not to dissolve during the copper electroplating process.
The thickness of the electroless copper plating film is preferably 0.01 μm or more, more preferably 0.1 μm or more, and is preferably 5 μm or less, more preferably 2 μm or less.

Pretreatment Step Before carrying out the electrolytic copper plating process, the surface of the electroless copper plating film may be subjected to a suitable pretreatment step, if necessary. Examples of the pretreatment step include various known steps such as a degreasing step, a conditioning step, and an acid washing step.

Electrolytic Copper Plating Treatment In the present invention, electrolytic copper plating treatment is carried out to form an electrolytic copper plating film 4 on the surface of the electroless copper plating film 3 (FIG. 1(d)).
The composition of the electrolytic copper plating solution used in the electrolytic copper plating process is not particularly limited. For example, a known electrolytic copper plating solution such as the electrolytic copper plating solution used in the examples can be used. The electrolytic copper plating process conditions are also not limited, and known electrolytic copper plating process conditions can be used.

The thickness of the electrolytic copper plating film can be appropriately changed depending on the required properties. The thickness of the electrolytic copper plating film may be appropriately adjusted to adjust the thickness of the conductive layer.
The thickness of the electrolytic copper plating film is preferably 0.1 μm or more, more preferably 1 μm or more, and is preferably 100 μm or less, more preferably 50 μm or less.

The copper electroplating process produces a multilayer flexible substrate in which an electroless copper plating film and an electrolytic copper plating film are laminated in that order on a resin substrate and on a rolled copper foil.

A flexible wiring board can be obtained by forming a wiring pattern on the flexible board of the present invention (FIG. 2(a)) by a known method.
For example, a resist layer 5 is provided on an electrolytic copper plating film 4, and the resist layer 5 is patterned into a desired circuit pattern (FIG. 2(b)). The exposed electrolytic copper plating film 4 and its underlying electroless copper plating film 3 are then etched away (FIG. 2(c)). The resist layer 5 is then peeled off to obtain a flexible wiring board (FIG. 2(d)).
By using the flexible substrate of the present invention, a flexible wiring substrate free from defects such as chipping or breaking of wiring can be obtained.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to the following examples, and it is of course possible to carry out the invention with appropriate modifications within the scope of the purpose described above and below, and all such modifications are included in the technical scope of the present invention. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

Experiment 1
A polyimide film (Kapton 200EN, manufactured by DuPont-Toray Co., Ltd.: size 50 mm x 50 mm) was used as the resin substrate. A high-performance rolled copper foil HA-V2 copper foil with a line and pattern (manufactured by JX Metals Corporation: each line width 1 mm x line length 50 mm, space width 1 mm) was provided on the surface of this polyimide film.
This polyimide film was pretreated according to the process shown in Table 1, and then electroless copper plating was performed using electroless copper plating solutions having compositions shown in Tables 2 to 7 to prepare samples in which electroless copper plating films were formed on the resin substrate surface and the rolled copper foil surface.
The electroless copper plating films of the samples were evaluated for the following two points.
(1) The presence or absence of undeposited portions of the electroless copper plating film was visually confirmed.
When there was a portion where the polyimide film was exposed, i.e., when there was a portion where the electroless copper plating film was not deposited, it was rated as "poor", and when there was no undeposited portion, it was rated as "good".
(2) The presence or absence of blisters on the electroless copper plating film was visually confirmed.
When the electroless copper plating film on the polyimide film had one or more blisters, it was rated as "present", and when there were no blisters, it was rated as "absent".

Experiment 2
After electroless copper plating was performed in the same manner as in Experiment 1, the electroless copper-plated film was subjected to the pickling cleaner treatment and pickling treatment shown in Table 1, and then an electrolytic copper plating treatment was performed to prepare each sample in which an electrolytic copper-plated film was formed on the electroless copper-plated film.
The electrolytic copper plating films of the samples were evaluated as follows.
(3) The presence or absence of gloss on the surface of the electrolytic copper plating film of the sample was evaluated using a scanning electron microscope. If there was gloss, it was rated as "present," and if there was no gloss, it was rated as "absent."

As can be seen from Tables 2 to 7, each of the samples of Examples 1 to 62 prepared using the electroless copper plating solution of the present invention was able to form a good electroless copper plating film on the resin substrate without (1) undeposited areas or (2) blisters. In addition, the electroless copper plating film formed on the electroless copper plating film had (3) a gloss, and the flexible substrate of the present invention had no defects such as pits. Therefore, flexible wiring substrates with high reliability can be manufactured using the flexible substrate obtained by the manufacturing method of the present invention.

Comparative examples 1 and 2 are examples in which an electroless copper plating solution that does not contain nitrogen-containing aromatic compounds was used, and there were areas on the resin substrate where the electroless copper plating film had not been deposited. Therefore, it was not possible to form an electrolytic copper plating film on the undeposited areas.

Comparative examples 3 and 4 are examples in which an electroless copper plating solution that does not contain nickel compounds was used, and blisters were formed in the electroless copper plating film on the resin substrate. As a result, the electroless copper plating film in the blistered areas had poor adhesion to the substrate, and the electrolytic copper plating film formed in the blistered areas had reduced smoothness.

Comparative examples 5 and 6 are examples in which an electroless copper plating solution that does not contain nickel compounds or nitrogen-containing aromatic compounds was used, and there were undeposited areas and blisters in the electroless copper plating film on the resin substrate. As a result, the same problems as those in comparative examples 1 to 4 occurred.

Comparative Example 7 is an example in which an electroless copper plating solution containing no tertiary alkanolamines, nickel compounds, or nitrogen-containing aromatic compounds was used, and there were undeposited areas and blisters in the electroless copper plating film on the resin substrate, and the electrolytic copper plating film formed had no surface gloss. Therefore, in addition to the same problems as in Comparative Examples 1 to 4, pit-like defects occurred in the electrolytic copper plating film.

The flexible boards of Comparative Examples 1 to 7 have defects in the electroless copper plating film and/or the electrolytic copper plating film, which causes defects in the wiring patterns, etc. Therefore, it is not possible to manufacture a reliable flexible wiring board.

REFERENCE SIGNS LIST 1 resin substrate 2 rolled copper foil 3 electroless copper plating film 4 electrolytic copper plating film 5 resist layer

Claims (4)

A method for producing a flexible substrate in which an electroless copper plating film and an electrolytic copper plating film are laminated on a resin substrate and a rolled copper foil formed on a part of the resin substrate, the method comprising the steps of:
The manufacturing method includes:
A step of forming the electroless copper plating film on the resin substrate and the rolled copper foil by performing an electroless copper plating process;
a step of forming the electrolytic copper plating film on the electroless copper plating film by performing an electrolytic copper plating process;
The electroless copper plating process uses an electroless copper plating solution containing a copper compound, a reducing agent, a tertiary alkanolamine, a nickel compound, and a nitrogen-containing aromatic compound. The concentration of the tertiary alkanolamine in the electroless copper plating solution is 0.001 g/L to 1000 g/L.
The concentration of the nickel compound is 0.0005 g/L to 5 g/L in terms of nickel concentration,
The method for manufacturing a flexible substrate according to claim 1, wherein the concentration of the nitrogen-containing aromatic compound is 0.00001 g/L to 2 g/L. The method for manufacturing a flexible substrate according to claim 1 or 2, wherein the tertiary alkanolamine is at least one selected from the group consisting of triethanolamine and triisopropanolamine. The method for manufacturing a flexible substrate according to any one of claims 1 to 3, wherein the nitrogen-containing aromatic compound is at least one selected from the group consisting of bipyridines and phenanthrolines.

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