JP5026133B2 - Flexible printed circuit board - Google Patents

Description

  The present invention relates to a flexible printed board used mainly for signal transmission in an optical apparatus such as a camera. This flexible printed circuit board is provided with an antireflection layer (film) for suppressing obstacles such as flare and ghost.

In general, in an optical apparatus such as a still camera or a video camera, there is a problem of a failure caused by reflection of incident light that has passed through a lens. Specifically, when reflected light from the inner wall or reflected light from a flexible printed circuit board used for signal transmission around the lens barrel is incident on the light receiving element or the like, flare or Disturbances such as ghosts occur.
Known means that can reduce the reflected light in order to solve the problems caused by the reflected light include (1) a method of attaching an antireflection film to a flexible printed circuit board via a thermosetting adhesive, and (2) A method of applying matte ink to a flexible printed circuit board is widely known. It is also widely known that this type of matte ink is blended with carbon black as an antireflection agent (see, for example, Patent Documents 1 to 3).

  On the other hand, if we look at elements other than the antireflection function, the trend toward miniaturization of optical devices such as cameras will not stop, and flexible printed circuit boards are inevitably required to be smaller and thinner. Yes. In other words, in order to cope with the miniaturization and thinning of optical equipment, it is not sufficient to simply pursue the miniaturization and thinning of a flexible printed circuit board. It is necessary to sufficiently satisfy the properties and flexibility.

  In addition, with the improvement in performance of optical equipment, the influence of dust attached to components such as flexible printed circuit boards has become ignorable, and this is one of the important issues that should be solved. Cleaning with a solvent is effective for removing dust. Therefore, the flexible printed circuit board is required to have solvent resistance sufficient to withstand the solvent.

  In addition, since there is concern about the influence of lead on the human body and the environment in recent years, there are many cases where component mounting is performed using a solder paste that does not use lead. As a result of not containing the lead component, the melting point of the solder paste is increased, so that the flexible printed circuit board is also required to have heat resistance at a higher temperature than before.

JP-A-10-275964 Japanese Patent Laid-Open No. 02-050826 JP 07-31904 A

  However, in the above-described method (1) of applying the antireflection film, an adhesive layer (generally, a thermosetting adhesive layer such as epoxy or acrylic) is inevitably formed. The thickness will increase. Such an increase in thickness leads to a decrease in flexibility and an increase in repulsive force during bending. Further, there is a disadvantage that the thermosetting adhesive that connects the antireflection film and the substrate does not have sufficient heat resistance to withstand the use of lead-free solder paste.

  Further, in the above-described method (2) method of applying matte ink, the main component of general matte ink is epoxy, acrylic, urethane, etc., so that heat resistance tends to be insufficient. In addition, the resistance to organic solvents such as alcohols and ketones used in dust cleaning and substrate cleaning is insufficient.

In addition, the carbon black blended in this kind of matte ink has a function of attenuating light energy, which is a kind of electromagnetic wave, but carbon black is a kind of conductive material. When ink for reducing reflected light to be applied is applied to a printed circuit board, an undercoat with an insulating paint is required. Carbon black lacks adhesiveness with the binder resin and is difficult to be blended in large amounts in inks like ordinary fillers (fillers) such as calcium carbonate, magnesium hydroxide and kaolin clay. The coating amount must be increased to increase the thickness of the coating film.
However, when an undercoat is applied with an insulating paint or the thickness of the coating film that reduces reflected light is increased, the flexibility of the flexible printed circuit board is impaired, and the coating film is cracked and easily peeled off.

Silicone resin (refractive index 1.40 to 1.45) having a refractive index of 1.5 or less, fluororesin (refractive index 1.35 to 1.40), cationic acrylic resin (refractive index 1.48), polyamide Low refractive index resins such as resins and polyurethane resins are considered to be suitable as an ink binder for reducing reflected light because of little reflected light on the resin surface.
However, silicone resin and fluororesin have poor adhesion to the base substrate and are not suitable for flexible printed boards that are constantly bent. In addition, acrylic resin, polyamide resin, polyurethane resin, and the like have good adhesiveness but lack heat resistance, and are not suitable for a flexible printed circuit board of an optical device in a heated state. There is also a disadvantage in that the coating film made of such ink does not have sufficient heat resistance to withstand the use of lead-free solder paste.

  The present invention has been made in order to solve the above-described problems, and has a base substrate made of a matte ink coating film that exhibits an excellent anti-reflective ability as well as an anti-reflective ability. It is an object of the present invention to obtain a novel flexible printed circuit board that does not impair its original flexibility and has both excellent solvent resistance and heat resistance.

  As described above, in recent years, in order to cope with lead-free solder, the requirement for heat resistance has become more severe from 220 ° C. to 250 ° C. Further, matte ink used for flexible printed boards as well as heat resistance is required to satisfy adhesion to the board, flexibility, and folding resistance. However, conventional matte inks using epoxy, acrylic and urethane resins as binders cannot satisfy these requirements. As the resin having heat resistance at reflow temperature, (1) polyamide imide, (2) polyimide, and (3) polyamide imide with a specific amount of epoxy can be used, and these are considered as binders for matte ink. As a result, it was found that polyamideimide and polyimide are preferable, and polyamideimide which is the same resin as our flexible printed circuit board is most preferable as a binder from the viewpoint of reliability such as flexibility. In addition, silica particles have been used for matte ink, but as a result of studying the surface characteristics and processing method of silica, the hydrophobic index of the silica can be selected to determine the refractive index of the resin and the necessity of carbon black. Regardless, we have found conditions that allow optimal control of glossiness (evaluation characteristics of antireflection properties), flexibility, and folding resistance, and the addition of carbon black also has the effect of reducing reflection and is non-conductive. The present inventors have obtained knowledge that carbon black is suitable and have completed the present invention.

That is, the flexible printed circuit board according to the present invention has a film thickness of 5 to 20 μm by matte ink mainly composed of silica fine particles having an average particle diameter of 2 to 8 μm, an aromatic polyamideimide resin and an organic solvent subjected to a hydrophobic treatment. The matte ink contains 5 to 8 parts by weight of silica fine particles per 100 parts by weight of the aromatic polyamideimide resin, and the matte ink has a viscosity of The first characteristic is that the surface glossiness is 2.0% or less, which is adjusted to 2000 poise or less and is measured by a measuring method specified in JIS-Z-8741.

A second aspect of the flexible printed circuit board according to the present invention, in addition to the first feature, the shape of the silica fine particles is in terms Ru spherical der.

The third feature of the flexible printed circuit board according to the present invention is that, in addition to any of the first and second features, the organic solvent is N · N · dimethylformamide, N · N · dimethylacetamide, dimethyl sulfoxide, N -It contains any one kind of polar organic solvent selected from methyl 2, pyrrolidone and hexamethylphosphoramide, and is characterized in that non-conductive carbon black is blended in matte ink.

  According to the present invention, the particle size (2 to 8 μm) of the silica fine particles 12 is large for the film thickness (5 to 20 μm) of the matte ink 14, and fine irregularities corresponding to the particle size of the silica fine particles are formed. Reflected light on the coating surface is suppressed (see FIG. 1). Thus, the surface glossiness of the flexible printed circuit board 11 is 2.0 or less, and it is possible to effectively prevent the illusion (flare or ghost) of the flexible printed circuit board from appearing in the image to be properly projected.

  As the solvent for the aromatic polyamideimide resin 13, any one of polar organic solvents such as N · N · dimethylformamide, N · N · dimethylacetamide, dimethyl sulfoxide, N · methyl · 2 · pyrrolidone, and hexamethylphosphoramide is used. In some cases, a polyamide resin ink having thixotropic characteristics suitable for a coverlay ink can be obtained, so that an ultrathin coating film having excellent adhesion without bleeding can be formed on the surface of the flexible printed board 11.

The matte ink 14 formed on the surface of the flexible printed board 11 has a very thin film thickness (5 to 20 μm), is highly flexible, and does not crack in the coating film (antireflection layer). In the present invention, since the coating film of the matte ink 14 is made extremely thin, it is well adapted to the base substrate 15 and the printed circuit 16 and does not peel off. That is, it is possible to obtain the flexible printed circuit board 11 in which the coating film made of the matte ink 14 is formed on the base substrate 15 and the printed circuit 16 in an integral manner.
In addition, the aromatic polyamideimide resin is excellent in adhesiveness as compared with a fluororesin and a silicone resin, and is excellent in heat resistance as compared with a polyamide resin, an acrylic resin, and a polyurethane resin. In addition, the aromatic polyamideimide resin has solvent solubility (solvent resistance) that is difficult to dissolve in a general organic solvent such as alcohol and methyl ethyl ketone but dissolves in a polar solvent.
For this reason, according to the present invention, it is possible to obtain a flexible printed circuit board 11 that can be cleaned and wiped using a general organic solvent, has high solvent resistance, and withstands the use of lead-free solder paste, and has high durability.

  According to the present invention, a matte coating film having a surface glossiness of 2.0 or less is formed on the surface of the flexible printed circuit board 11 by the silica fine particles 12 and the aromatic polyamideimide resin 13, and the use of carbon black for the coating film is as follows. It is not always necessary. However, by adding carbon black, the reflectance and gloss can be further reduced.

  FIG. 1 shows an embodiment of a flexible printed circuit board according to the present invention. This flexible printed circuit board 11 is formed by forming a coating film having a function of reducing reflected light by printing matte ink 14 as shown below on a base substrate 15 having a printed circuit 16. is there. The coating film made of the matte ink 14 can be formed on one side as well as on both sides of the base substrate 15 as in the illustrated example. The part where the coating film is formed is not limited to the entire surface, but may be a part.

  This matte ink 14 is composed mainly of an aromatic polyamideimide resin 13, silica fine particles 12, and an organic solvent, and has a thickness of 5 to 20 μm. Specifically, this matte ink 14 is prepared by blending and dissolving an aromatic polyamideimide resin 13 in a polar organic solvent, blending silica fine particles 12 and passing through a stirrer, a ball mill, a three roll mill or the like. It is.

  The aromatic polyamideimide resin 13 in the present invention is represented by Chemical Formula 1 or Chemical Formula 2. In Chemical Formula 1 or Chemical Formula 2, n represents an integer of 2 or more. In Chemical Formula 2, X represents an oxygen atom, a sulfur atom, a sulfonyl group, a carboxylic group or a methyl group, and n represents an integer of 2 or more.

  The aromatic polyamideimide resin 13 can be obtained, for example, by a reaction between an aromatic diamine and trimellitic anhydride chloride or a reaction between an aromatic isocyanate and bisimide dicarboxylic acid.

  There are various types of silica fine particles, such as spherical ones and scale-like ones, which are added during the preparation of the ink of the present invention, and finally the silica fine particles 12 constituting the coating film are glossy. From the viewpoint of degree and folding resistance, the shape is preferably spherical (see Table 11). In view of the flexibility and matting effect of the coating film by the ink of the present invention, in consideration of the affinity with the spherical shape, in particular, the aromatic polyamideimide resin, by silicon oil or silane coupling treatment The use of hydrophobized silica or alumina coated silica treated with basic aluminum chloride is recommended (see Table 4).

  The average particle diameter of the silica fine particles 12 is preferably in the range of 2 to 8 μm (see FIGS. 2 to 5). This is because the glossiness increases when the average particle size of the silica fine particles is less than 2 μm, and the folding resistance becomes poor when the average particle size exceeds 8 μm.

  The blending amount of the silica fine particles 12 is set in consideration of the viscosity of the matte ink, the surface roughness of the coating film, the glossiness, and the folding resistance, which vary depending on the particle size. According to the knowledge of the present inventors, when silica fine particles 12 having an average particle diameter of 2 to 8 μm are used, 5 to 8 parts by weight with respect to 100 parts by weight of aromatic polyamideimide resin. The silica fine particles 12 are preferably blended, and 5 to 7 parts by weight is optimal (see Table 10 and FIGS. 6 to 7).

  For the base substrate 15, a polyamideimide film, a polyester film, a polycarbonate film, a polyimide film, or the like is used. A printed circuit 16 is engraved by bonding a metal foil to the surface of the base substrate 15 and performing an etching process. Alternatively, the matte ink 14 is applied to the surface of the base substrate 15 on the back side.

  Carbon black can be added to the matte ink 14 according to the present invention. Such carbon black has an effect of reducing the total reflectance by partially absorbing the remaining light incident on the antireflection film other than the light reflected on the surface among the light hitting the antireflection film ( (See Table 5). The amount of carbon black added is preferably in the range of 0.5 to 5% by weight.

The flexible printed circuit board is required to be insulative, and the JPCA standard is defined for the insulative property. However, carbon black usually has electrical conductivity. Therefore, in the present invention, the above-mentioned JPCA standard can be satisfied by using non-conductive carbon black (see Table 6, Table 7, and Table 8). Specific examples of such non-conductive carbon black include MA78 manufactured by Mitsubishi Chemical Corporation. However, the present invention is not limited to this, and what is necessary is just non-conductive carbon black.

The matte ink 14 used in the present invention can be used for coating electronic parts mounted on optical equipment and inner wall surfaces of optical equipment in addition to the flexible printed circuit board 11. For example, the antireflection film according to the present invention can be formed on the coverlay. Forming the antireflection film according to the present invention on various cover lay films made of Kapton, polyimide, polyimide amide, etc. can cause obstacles such as flare and ghost on the electronic parts mounted on the optical equipment and the inner wall surface of the optical equipment. It has a favorable effect to prevent. Therefore, of course, implementation of this invention to a coverlay film is not prevented at all.
In that case, a silicone resin having a refractive index of 1.5 or less can be used in place of the aromatic polyamideimide resin in a portion where flexibility and heat resistance are not required, and titanium oxide is also used as a matting agent. It can also be used in combination with silica fine particles.

Next, although an example and a comparative example explain the present invention concretely, the present invention is not limited to these examples.
The evaluation methods and conditions of flexible printed circuit boards performed in the following examples and comparative examples are as follows.
<Glossiness>
In accordance with JIS-Z-8741, the glossiness for incident light of 60 degrees is measured.
<Solder heat resistance>
When the test piece of the flexible printed circuit board is immersed in a solder bath at 270 ° C. for 30 seconds, the coating of the matte ink is visually checked for swelling and peeling.
<Solvent resistance>
Prepare five types of acetone, xylene, 2-propanol (IPA), ethanol, and water as solvents used for general cleaning, impregnate them into a nonwoven fabric, wipe off the matte ink film, Visually check the change in appearance of the coating.

<Preparation of base substrate>
As the base substrate of the flexible printed circuit board, one having excellent flexibility and heat resistance and having no adhesive applied on the front and back sides thereof is used.
<Preparation of flexible printed circuit board>
Screened with an organic polyamide-imide resin (Sokuseal (registered trademark) made by Nippon Kogyo Kogyo Co., Ltd.) on the copper foil surface of Espanex M Series (manufactured by Nippon Steel Chemical Co., Ltd.), a two-layer copper clad laminate. The coating thickness is set to 20 μm using a printing apparatus, preliminarily dried at 100 ° C. for 8 minutes, and then subjected to final heat drying at 300 ° C. for 50 minutes in a far-infrared furnace to produce a flexible printed circuit board.

[Example 1]
100 parts by weight of a polar organic solvent-soluble aromatic polyamideimide resin synthesized from 4,4-diaminodiphenyl ether and trimellitic anhydride chloride was dissolved in 450 parts by weight of N-methylpyrrolidone.
Subsequently, 7 parts by weight of hydrophobic spherical silica having an average particle diameter of 5 μm (SS-70, manufactured by Tosoh Silica Co., Ltd.) and 1 part by weight of carbon black having an average particle diameter of 20 nm (manufactured by Mitsubishi Chemical Industries, Ltd.) were added, and a stirrer was added. And uniformly dispersed to prepare a matte ink.
A matte ink coating having a thickness of 10 μm is applied to the previously prepared flexible printed circuit board by a screen printing apparatus, preliminarily dried at 100 ° C. for 8 minutes, and then finally heated and dried at 300 ° C. for 50 minutes in a far-infrared furnace. Formed.

[Example 2 ]
A flexible printed circuit board was prepared in the same manner as in Example 1 except that hydrophobic spherical silica (manufactured by Tosoh Silica Co., Ltd.) having an average particle diameter of 5 μm was used as the silica fine powder, and the addition amount was 5 parts by weight.

[Example 3 ]
A flexible printed circuit board was produced in the same manner as in Example 1 except that carbon black was not used.

[Comparative Example 1]
A commercially available matte ink (manufactured by Seiko Advance Co., Ltd.) was applied to the previously prepared flexible printed circuit board with a screen printing apparatus and dried at 80 ° C. for 60 minutes to form a matte ink coating film having a thickness of 10 μm.

[Comparative Example 2]
A commercially available matte ink (manufactured by Toyo Ink Co., Ltd.) was applied to the flexible printed board prepared in advance by a screen printing apparatus and dried at 60 ° C. for 60 minutes to form a matte ink coating film having a thickness of 10 μm.

Table 1 shows the physical properties of the flexible printed boards obtained in Examples 1 to 3 and Comparative Examples 1 and 2. As Table 1 shows, the flexible printed circuit boards of Comparative Examples 1 and 2 all showed high reflectivity, and were not satisfactory in heat resistance and solvent resistance. On the other hand, in the flexible printed circuit boards of Examples 1 to 3 of the present invention, the gloss value is low (the gloss value is 2.0% or less), and the heat resistance and the solvent resistance are also excellent. there were. Further, from Example 3 , it was confirmed that the flexible printed circuit board according to the present invention can achieve a low reflectance even without the addition of carbon black to the matte ink.

[Experimental example]
Below, the basis regarding the best mode for carrying out the present invention will be described in detail based on experimental results. Specifically, the effect on the surface roughness, glossiness, and folding resistance of the antireflection film due to factors such as the particle size of the silica, the shape of the silica, and the chemical affinity of the silica surface. The evaluation test is shown.

  In the following experiment, an ink sample was prepared by kneading various types of silica according to the purpose of the experiment with polyamide imide resin ink (Socsea (registered trademark), solvent N methylpyrrolidone) manufactured by Nippon Kogyo Paper Industries Co., Ltd. A film having a thickness of 50 μm was prepared through printing and a subsequent drying step, and the glossiness, folding resistance, and surface roughness of the film prepared under various conditions were evaluated. About folding resistance, it tested by the MIT test method (R = 0.38mm, weight 500g) which is one of the indexes of film folding resistance. For ink, viscosity was used as an index for evaluating ink characteristics.

<ink>
Disperse uniformly by adding various amounts of silica and carbon black having an average particle size of 20 nm (manufactured by Mitsubishi Chemical Corporation) to 100 parts by weight of sock seal powder and adding 450 parts by weight of N-methylpyrrolidone according to the experimental conditions. The desired ink was prepared using a stirrer while paying attention to the aggregation / sedimentation of the solid particles so that the obtained ink was obtained. Since the coating composition after coating and drying is composed of sock seal, silica, and carbon black, for the convenience of the experiment, the silica addition amount, the carbon black addition amount, and the solvent use amount are weight percent with respect to 100 parts by weight of the sock seal resin powder. Displayed.

  Table 4 shows the experimental results regarding the chemical affinity of the silica surface. In Table 4, sample 1 is a film prepared using general silica particles (G) that are not particularly surface-treated. On the other hand, Sample 2 is prepared using silica (D) whose surface is hydrophobized with a silane coupling agent. Tables 2 and 3 below collectively show the types of silica used in the following experimental examples.

  As will be described in detail later, when the ink viscosity in screen printing exceeds 2000 poise (hereinafter referred to as P), printing is impossible. Comparing sample 1 and sample 2, it can be seen that when silica (D) whose surface is subjected to hydrophobic treatment as in sample 2 is used, the ink viscosity is 2000 P or less and the ink viscosity is improved. And as a result of using the ink which concerns on this sample 2, the favorable coating film was able to be formed by screen printing.

  In addition, as shown in Table 4, the film of Sample 2 formed using this ink has a folding resistance of more than 1000 times and a significant improvement of 10 times or more compared to 100 times or less when there is no hydrophobic treatment. I was able to achieve it. This is considered to be because the chemical affinity of the silica interface to the sock seal resin was improved by subjecting the silica to a hydrophobization treatment, and as a result, the fracture strength at this interface was increased. Based on the above findings, silica subjected to a hydrophobic treatment with a silane coupling agent was used in the subsequent experiments. In addition, the surface glossiness of the coating film was 2.0% or less which is the target in both cases where the silica is not hydrophobic and hydrophobic.

<Influence of carbon addition>
Table 5 shows the results of studies on the effects of carbon addition. Specifically, Table 5 shows differences in physical properties (ink viscosity, surface roughness, glossiness, folding resistance (number of times)) when carbon black is not added and when carbon black is added (black). Here, the amount of silica added was 7 wt% of the weight of the sock seal powder (hereinafter, except for special cases, the wt% of silica is the weight percent with respect to the sock seal). In each of the following experiments, MA78 manufactured by Mitsubishi Chemical Corporation having non-conductivity was used as carbon black.

  As can be seen from Table 5, although the viscosity increases slightly and the folding resistance tends to decrease with the addition of carbon black, the effect of adding carbon black is ignored if it is 1 wt% of the weight of the sock seal resin powder. I understand that I can do it. In the following experiment, carbon black was added.

<Insulation properties of flexible printed circuit boards>
A flexible printed circuit board is required to have a certain level of insulation. However, carbon black usually has electrical conductivity. Therefore, after preparing inks having the conditions shown in Table 6 according to the ink preparation method shown in the paragraph No. 0045, a sample having a coating thickness of 10 μm formed on both surfaces of the flexible printed board was prepared. Evaluated. As the carbon black here, MA78 manufactured by Mitsubishi Chemical Corporation having non-conductivity was used.

Tables 7 and 8 show the measurement results of the insulation resistance value between lines in the environmental load test for each of the high temperature storage and the low temperature storage. The measured value after various environmental load tests of the sample to which no carbon black was added was 1E + 13Ω or more. This evaluation result sufficiently satisfied level 3 (5.0E + 8Ω) of the JPCA standard.
On the other hand, the resistance value before the test was 1E + 13 or more in both the case of adding 1 part by weight and the case of adding 5 parts by weight of the flexible printed circuit board having the black antireflection film made of black ink using carbon black. Similarly to the case where carbon black was not used, the resistance values after the environmental load tests for both high temperature and low temperature exposure were all 1E + 13 or more. These evaluation results sufficiently satisfied level 3 (5.0E + 8Ω) of the JPCA standard. From the above, it was confirmed that there was no practical problem if non-conductive carbon black was used.

<Silica particle size and physical properties>
Ink was prepared by changing the particle size of silica in the range of 1.6 μm to 10 μm under the condition of silica addition amount of 7 wt% (sock seal weight ratio). Table 9 shows the ink composition and the measurement results of the film characteristics.

<Silica particle size and surface roughness>
FIG. 2 shows the relationship between the particle diameter of silica and the surface roughness at a silica addition amount of 7 wt% (soccer ratio). From this figure and Table 9, it can be seen that Ra (surface roughness) increases almost linearly as the particle size of silica increases. Therefore, it can be seen that the surface roughness can be adjusted by the particle size of the silica to be added when the silica particle size is in the range of 1 to 10 μm.

<Silica particle size and gloss>
FIG. 3 shows the relationship between the average particle diameter of silica and the glossiness at a silica addition amount of 7 wt% (sock seal ratio). From this figure and Table 9, it can be seen that there is a certain regularity between the glossiness and the average particle diameter of silica. That is, as the average particle diameter of the silica is reduced, the glossiness increases, and when the particle diameter is 1.6 μm, it exceeds 2%. Therefore, the preferable numerical value range of the surface glossiness is 2.0% or less. In consideration of the setting, the lower limit of the particle size of the silica particles is suitably 2.0 μm. From the above, from the viewpoint of glossiness, the preferred average particle diameter of silica in the present invention is 2.0 μm or more.

<Surface roughness and gloss>
FIG. 4 shows the relationship between the surface roughness of the film and the glossiness at a silica addition amount of 7 wt% (sock seal ratio). It can be seen that when the surface roughness is below 0.5 μm, the glossiness increases rapidly and the reflected light increases. Therefore, from the viewpoint of glossiness and surface roughness, it can be seen that at least the surface roughness may be 0.5 to 5 μm in order to stably adjust the glossiness to 1.5% or less.

  The effect of silica particle size on folding resistance was investigated. FIG. 5 shows the average particle diameter of silica at a silica addition amount of 7 wt% (sock seal ratio) and the results of the MIT test. From this figure and Table 7, it is clear that when the folding resistance is evaluated by the MIT test, the folding resistance is almost lost when the average particle diameter of silica is 10 μm. Therefore, even if it is the silica which processed the silane coupling agent, what can be used as a flexible printed circuit board will not be obtained if the average particle diameter is 10 micrometers or more. Therefore, from the viewpoint of folding resistance, the upper limit of the particle size of the silica particles used for forming the antireflection film is 8 μm.

<Silica addition amount>
In order to confirm the relationship between the silica addition amount, the ink properties, and the film properties after coating and drying, a sample was prepared with the silica addition amount of 3 to 7 wt% under the condition that the silica particle size was 5 μm. Table 10 shows the measurement results of the ink composition and the properties of the prepared film.

  FIG. 6 shows the relationship between the amount of silica added and the viscosity (p; poise) of the ink. From the figure and Table 10, it can be seen that the ink viscosity increases as the silica addition amount increases. In the screen printing, the viscosity of the ink is an important factor for forming a coating film. Generally, if it exceeds 1500 p, printing becomes difficult, and if it exceeds 2000 p, it may be impossible. In FIG. 6, when the relationship between the silica addition amount and the viscosity is extrapolated, if the silica addition amount exceeds 10%, the ink viscosity becomes 2000 (p) or more, and printing by screen printing becomes difficult. From the above, from the viewpoint of the viscosity of the ink and the amount of silica added, the amount of silica added in the ink preparation is 8% or less, preferably 6% or less.

  FIG. 7 shows the relationship between the amount of silica added and the surface roughness when silica having an average particle size of 5 μm is used. FIG. 8 shows the relationship between the amount of silica added and the glossiness when silica having an average particle diameter of 5 μm is used. From these, it can be seen that when silica is not added, the surface is smooth, and therefore the glossiness is large. As the amount of silica added increases, the surface roughness increases and, at the same time, the glossiness decreases. In order to make the glossiness 2% or less, the amount of silica added is preferably 5 wt% or more.

  FIG. 9 shows the relationship between the amount of silica added and the number of bendings by the MIT method. From the figure, it can be seen that the folding resistance index (times) of the MIT test decreases (deteriorates) as the silica addition amount increases. When the amount of silica added exceeds 10 wt%, the MIT test is estimated to be 200 times, which is a characteristic that is not practical as the folding resistance of the flexible printed circuit board. As described above, the upper limit of the amount of silica added is 8 wt%, preferably 7 wt% or less, from the results of the MIT test.

  The influence of the shape of silica is described. An ink and a film thereof were prepared using silica having a scaly shape under the same conditions as in the above experimental example, and the glossiness was compared. The results are shown in Table 11. In the case of silica having the same average particle diameter and scale-like shape, the glossiness was 7.0% even when the same addition amount of 5 wt% was obtained, which was larger than in the case of the spherical shape. In the case of a scale-like form, since it is oriented in a planar shape, surface irregularities are reduced even with the same addition amount. For this reason, the amount of reflected light is larger than when spherical. Accordingly, the shape of the particles to be added is preferably a shape having no anisotropy (spherical).

  As described above, according to the present invention, it has excellent heat resistance and solvent resistance, which cannot be achieved by a conventional method, and the antireflection layer has a thickness of 20 μm or less and low reflection. It is possible to obtain a flexible printed circuit board capable of realizing the rate and excellent in flexibility. By using the flexible printed board having the antireflection layer of the present invention, the reliability of the electronic component with respect to the solvent and heat is improved. The flexible printed circuit board according to the present invention can greatly contribute to miniaturization and high resolution of a digital camera, a mobile phone with a camera, and the like.

It is sectional drawing which shows typically the flexible printed circuit board which concerns on this invention. It is a figure which shows the relationship between the average particle diameter of silica, and surface roughness. It is a figure which shows the relationship between the average particle diameter of a silica, and film glossiness. It is a figure which shows the relationship between surface roughness and film glossiness. It is a figure which shows the relationship between the average particle diameter of a silica, and a MIT test (folding resistance). It is a figure which shows the relationship between a silica addition amount and a viscosity. It is a figure which shows the relationship between the amount of silica addition, and surface roughness. It is a figure which shows the relationship between a silica addition amount and glossiness. It is a figure which shows the relationship between the addition amount of a silica, and a MIT test (folding resistance).

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Flexible printed circuit board 12 Silica microparticle 13 Aromatic polyamide-imide resin 14 Matte ink 15 Base board 16 Printed circuit

Claims (3)

The surface is covered with a coating film having a film thickness of 5 to 20 μm made of matte ink mainly composed of silica fine particles having an average particle diameter of 2 to 8 μm subjected to a hydrophobization treatment , an aromatic polyamideimide resin and an organic solvent,
The matte ink contains 5 to 8 parts by weight of silica fine particles per 100 parts by weight of the aromatic polyamideimide resin.
The viscosity of the matte ink is adjusted to 2000 poise or less,
The flexible printed circuit board whose surface glossiness measured by the measuring method prescribed | regulated to JIS-Z-8741 is 2.0% or less. The flexible printed circuit board according to claim 1, wherein the shape of the silica fine particles Ru spherical der. The organic solvent includes any one type of polar organic solvent selected from N · N · dimethylformamide, N · N · dimethylacetamide, dimethyl sulfoxide, N · methyl · 2 · pyrrolidone, hexamethylphosphoramide,
The flexible printed circuit board according to claim 1, wherein non-conductive carbon black is blended in the matte ink.

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