WO2025127112A1 - Stretched liquid crystal polymer film, laminate, circuit board, and production method for stretched liquid crystal polymer film - Google Patents

Abstract

Provided is a stretched liquid crystal polymer film formed from a liquid crystal polymer. The stretched liquid crystal polymer film has an average film thickness of less than 25 μm. The Cv value of the film thickness of the stretched liquid crystal polymer film is 10% or lower, the Cv value being represented by equation (1).　(1): Cv value (%)=(standard deviation of measured film thickness)/(average film thickness)×100

Description

Stretched liquid crystal polymer film, laminate, circuit board, and method for producing stretched liquid crystal polymer film

The present invention relates to a stretched liquid crystal polymer film, a laminate, a circuit board, and a method for producing a stretched liquid crystal polymer film.

Advances in mobile communications technology have led to faster and larger-capacity communications, and the frequencies used in communications are becoming higher; in the fifth-generation mobile communications system, the Sub6 band of 3.7 GHz and 4.5 GHz and the millimeter-wave band of 28 GHz are used, and in the sixth-generation mobile communications system, frequency bands in the range of 90 GHz to 300 GHz are being considered. The higher the frequency of a signal flowing through a circuit, the greater the transmission loss, so there is a demand for circuits and circuit materials with low transmission loss.

In devices used for mobile communications, such as smartphones, lightweight and easily bendable flexible printed circuit boards (FPCs) are used to meet the demands of compactness and weight reduction. FPCs are composed of conductors, such as copper wires, through which signals pass, and insulating materials that hold them in place, and a lightweight and flexible polymer film is used as the insulating material. This polymer film is required to be heat resistant so that it can be used continuously even in high-temperature environments, and to have a low dielectric constant and dielectric tangent in order to reduce transmission loss when high-frequency signals flow (for example, non-patent document 1).

Polyimide film and liquid crystal polymer film are low dielectric constant and low dielectric loss tangent polymer films used in FPCs. While polyimide film has excellent heat resistance and flexibility, it has a high water absorption rate and a large rate of dimensional change due to moisture absorption, which results in low connection reliability in circuits with fine pitch patterns. For this reason, FPCs using liquid crystal polymer film, which has excellent heat resistance, low water absorption, and a small rate of dimensional change, are currently being developed. (For example, Patent Document 1)

Liquid crystal polymers have a tendency to have molecular orientation in the direction of flow. In the melt extrusion method, which is a common method for manufacturing films, the polymer is melted and extruded from a T-die or the like to form a film, so the liquid crystal polymer in the film manufactured by this method has molecular orientation in the longitudinal direction of the film. Therefore, in films manufactured by this method, the film properties (strength, linear expansion coefficient, etc.) differ greatly between the longitudinal direction of the film and the direction perpendicular to it, resulting in a state of high anisotropy. On the other hand, FPCs are manufactured by laminating copper foil on a polymer film, or by laminating a metal layer by copper plating on a polymer film, and then forming a wiring pattern by etching or the like. In this case, if the expansion coefficients (linear expansion coefficients) of the polymer film and the metal layer differ, problems such as peeling of the insulating film and the metal layer due to temperature changes, warping of the FPC, and misalignment of holes after drilling the FPC occur. In this way, if a liquid crystal polymer film with high anisotropy is used as an insulating material for an FPC, the linear expansion coefficient differs greatly depending on the direction, making it easy for the above problems to occur.

A method for reducing the anisotropy of polymer films such as polyethylene terephthalate (PET) films is to stretch the film in a direction perpendicular to the molecular orientation at a temperature above the glass transition temperature of the polymer film but below its melting point. However, liquid crystal polymer films extruded from a T-die have extremely low tensile strength, especially in the direction perpendicular to the molecular orientation, and therefore easily break when pulled in a direction perpendicular to the molecular orientation at a temperature below the melting point of the liquid crystal polymer.

Therefore, methods that have been proposed include a method in which a liquid crystal polymer film extruded from a T-die is heated and pressurized between a pair of endless belts of a double belt press (for example, Patent Document 2), a method in which a liquid crystal polymer film is extruded into a cylindrical shape from an inflation die and then inflated by blowing hot air into the film before it cools and hardens (for example, Patent Document 3), and a method in which porous polytetrafluoroethylene (PTFE) films are heated and bonded to both sides of a liquid crystal polymer film extruded from a T-die, and then stretched in a direction perpendicular to the longitudinal direction at a temperature above the melting point of the liquid crystal polymer film (for example, Patent Document 4).

Furthermore, as the demand for smaller, lighter devices increases, FPCs are becoming more multi-layered and lighter, and there is a growing demand for thin liquid crystal polymer films.

However, the molecular chains of liquid crystal polymers are rigid and have little flexibility, which means that they tend to be oriented in the flow direction as described above, and because the molecular chains are not entangled, their viscosity in the molten state is significantly reduced (Non-Patent Document 2). For this reason, the above-mentioned methods of heating and pressurizing between the pair of endless belts of a double belt press, extruding a liquid crystal polymer film into a cylindrical shape from an inflation die and blowing hot air into the film before it cools and hardens, and laminating porous polytetrafluoroethylene (PTFE) films to both sides of a liquid crystal polymer film extruded from a T-die while heating it, and stretching it in a direction perpendicular to the longitudinal direction at a temperature above the melting point of the liquid crystal polymer film, result in large film thickness unevenness, and holes are likely to occur when attempting to produce a particularly thin film, making it difficult to produce a thin liquid crystal polymer film.

A solution casting method has also been proposed in which a liquid crystal polymer solution is cast onto a metal plate or the like, the solvent is removed, and the liquid crystal polymer is solidified to produce a liquid crystal polymer film (for example, Non-Patent Document 2). However, in order to turn a liquid crystal polymer film into a solution, not only is it necessary to use a special solvent or a liquid crystal polymer with a special structure, but it is also necessary to remove the solvent after casting. In addition, the removed solvent must be recovered to prevent environmental pollution. For these reasons, when producing a liquid crystal polymer film using the solvent casting method, the cost is high, and the liquid crystal polymers that can be used are limited, making it difficult to produce liquid crystal polymer films suitable for various applications.

Non-Patent Document 1: Matsushita, Yukio et al., "Materials for High-Speed Transmission Boards," Journal of the Japan Institute of Electronics Packaging,
Vol. 4, No. 7, p. 551, 2001. Non-Patent Document 2: Satoshi Okamoto, "Film Formation by Solution Casting of LCP", Seisaku-Kao, Vol. 20, No. 5, p. 270, 2008.

Patent Document 1: Patent No. 5308295 Patent Document 2: Patent No. 6930046 Patent Document 3: Patent No. 6656231 Patent Document 4: Patent No. 3958629

The object of the present invention is to provide a thin, uniformly thick stretched liquid crystal polymer film that can be suitably used as an insulating material for circuit boards having a multilayer structure.

[1] According to aspect 1 of the present invention, there is provided a stretched liquid crystal polymer film made of a liquid crystal polymer, the stretched liquid crystal polymer film having an average thickness of less than 25 μm and a Cv value of the thickness represented by the following formula (1) of 10% or less.
Cv value (%) = (standard deviation of measured film thickness) / (average film thickness) × 100 (1)

[2] According to aspect 2 of the present invention, there is provided a stretched liquid crystal polymer film according to aspect 1, in which the difference between the maximum and minimum film thicknesses is 5 μm or less.

[3] According to aspect 3 of the present invention, there is provided a stretched liquid crystal polymer film according to aspect 1 or 2, in which the surface roughness Ra of at least one surface measured by a laser microscope is 0.5 μm or less.

[4] According to a fourth aspect of the present invention, there is provided a stretched liquid crystal polymer film according to any one of the first to third aspects, in which, in a pole measurement by X-ray diffraction, the film is tilted 45° (α=45° in the Schulz method) and rotated in an in-plane direction (β direction) while the diffraction intensity of the 110 plane is measured, the integrated intensities of β=45 to 135°, 135° to 225°, 225 to 315°, and 315 to 45° are calculated with β=0° being the longitudinal direction of the film, and the sum of the integrated intensity of β=45 to 135° and the integrated intensity of β=225° to 315° is defined as the longitudinal integrated intensity, and the sum of the integrated intensity of β=135 to 225° and the integrated intensity of β=315 to 45° is defined as the widthwise integrated intensity, and the degree of planar orientation represented by the following formula (2) is -0.5 or more and 0.5 or less.
Planar orientation degree=(integrated intensity in the longitudinal direction−integrated intensity in the transverse direction)/(integrated intensity in the longitudinal direction+integrated intensity in the transverse direction) (2)

[5] According to aspect 5 of the present invention, there is provided a stretched liquid crystal polymer film according to any one of aspects 1 to 4, the average thickness of which is less than 10 μm.

[6] According to aspect 6 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film, comprising a first step of bonding a support film made of a support polymer and having a surface roughness Ra of 1.5 μm or less as measured by a laser microscope to at least one side of an unstretched liquid crystal polymer film made of a liquid crystal polymer to obtain a laminated film, a second step of stretching the laminated film at least in the width direction using a tenter-type stretching device, and a third step of peeling off the stretched support film.

[7] According to aspect 7 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film, comprising a first step of extruding molten liquid crystal polymer and supporting polymer into a film using an extruder so that a layer of the supporting polymer is laminated on at least one side of the layer of the liquid crystal polymer to obtain a laminated film, a second step of stretching the laminated film at least in the width direction using a tenter-type stretching device, and a third step of peeling off the stretched layer of the supporting polymer.

[8] According to aspect 8 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to aspect 6 or 7, in which the thickness of the unstretched liquid crystal polymer film or the layer made of the liquid crystal polymer is 5 to 100 μm.

[9] According to aspect 9 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to aspect 6 or 7, wherein in the second step, the stretching speed of the laminated film is set to 500%/min to 10,000%/min, and the temperature T inside the furnace of the stretching device is set to a temperature that satisfies the following formula (3):
T ₁ <T < T ₂ (3)
(In the above formula (3), _T1 is a temperature that is the melting point of the liquid crystal polymer minus 100° C., and _T2 is a temperature that is the melting point of the liquid crystal polymer plus 30° C.)

[10] According to aspect 10 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to aspect 6, in which the stretching of the laminated film in the second step is performed under conditions in which the value _A1 represented by the following (4) is 0.20 to 5.00.
A ₁ = furnace temperature T (° C.) / stretching speed (%/min) / thickness of unstretched liquid crystal polymer film (μm) × 100 (4)

[11] According to aspect 11 of the present invention, there is provided a method for producing the stretched liquid crystal polymer film of aspect 7, wherein in the second step, the laminated film is stretched under conditions in which a value _A2 represented by the following formula (5) is 0.20 to 5.00.
_A2 = furnace temperature T (°C) / stretching speed (%/min) / thickness of liquid crystal polymer layer (μm) × 100 (5)

[12] According to aspect 12 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to aspect 6 or 10, in which the first step includes performing a surface treatment on the bonding surface of the unstretched liquid crystal polymer film and the bonding surface of the support film before bonding the support film to the unstretched liquid crystal polymer film.

[13] According to aspect 13 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to aspect 12, in which the surface treatment is one selected from the group consisting of plasma treatment, corona treatment, and chemical treatment.

[14] According to aspect 14 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to any one of aspects 6 to 13, in which the second step includes performing stretching at a temperature below the melting point of the liquid crystal polymer.

[15] According to aspect 15 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to any one of aspects 6, 10, and 12 to 14, in which in the second step, the stretching load calculated by multiplying the tensile stress of the support film at the temperature during stretching by the cross-sectional area of the support film is equal to or greater than the stretching load calculated by multiplying the tensile stress of the unstretched liquid crystal polymer film at the temperature during stretching by the cross-sectional area of the unstretched liquid crystal polymer film.

[16] According to aspect 16 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to any one of aspects 7, 11, and 14, in which in the second step, the stretching load calculated by multiplying the tensile stress of the layer made of the support polymer at the temperature during stretching by the cross-sectional area of the layer made of the support polymer is equal to or greater than the stretching load calculated by multiplying the tensile stress of the layer made of the liquid crystal polymer at the temperature during stretching by the cross-sectional area of the layer made of the liquid crystal polymer.

[17] According to aspect 17 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to any one of aspects 6 to 16, in which the support polymer is an aromatic polyether ketone or polyester.

[18] According to aspect 18 of the present invention, there is provided a method for producing a stretched liquid crystal polymer film according to aspect 17, in which the polyester is at least one polymer selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate.

[19] According to aspect 19 of the present invention, there is provided a laminate comprising a film layer containing the stretched liquid crystal polymer film of any one of aspects 1 to 5 and a metal layer.

[20] According to aspect 20 of the present invention, a circuit board is provided that includes the laminate of aspect 19.

The stretched liquid crystal polymer film of the present invention has an average thickness of less than 25 μm and a Cv value of the film thickness of 10% or less, so that it has a thin and uniform thickness. Therefore, the stretched liquid crystal polymer film of the present invention can be suitably used as an insulating material for circuit boards having a multilayer structure.

FIG. 1(a) is a diagram comparing the size of unevenness measured by a contact-type surface roughness meter and a laser microscope, and FIG. 1(b) is a diagram comparing the size of unevenness at small intervals measured by a contact-type surface roughness meter and a laser microscope.

<Liquid crystal polymer film>
The stretched liquid crystal polymer film of the present invention is a film made of a liquid crystal polymer. The liquid crystal polymer is not particularly limited, but is preferably a liquid crystal polyester that exhibits thermotropic liquid crystal properties and has a melting point of 250°C or higher, preferably 280°C to 380°C. Examples of such liquid crystal polyesters include aromatic polyesters that are synthesized from monomers such as aromatic diols, aromatic carboxylic acids, and hydroxycarboxylic acids and exhibit liquid crystallinity when melted. Specific examples include polycondensates of ethylene terephthalate and parahydroxybenzoic acid, polycondensates of phenol, phthalic acid, and parahydroxybenzoic acid, and polycondensates of 2,6-hydroxynaphthoic acid and parahydroxybenzoic acid. In particular, from the viewpoint of excellent mechanical properties, electrical properties, heat resistance, etc., an aromatic polyester-based liquid crystal polymer having 6-hydroxy-2-naphthoic acid and its derivatives as a basic structure and at least one monomer component selected from the group consisting of parahydroxybenzoic acid, terephthalic acid, isophthalic acid, 6-naphthalenedicarboxylic acid, 4,4'-biphenol, bisphenol A, hydroquinone, 4,4-dihydroxybiphenol, ethylene terephthalate, and derivatives thereof is preferred. The liquid crystal polymers may be used alone or in any combination and ratio of two or more. The content of the liquid crystal polymer is preferably 50 to 100% by mass, more preferably 60 to 100% by mass, and even more preferably 70 to 100% by mass, based on the total amount of the stretched liquid crystal polymer film.

The liquid crystal polyester can be synthesized by any known method, and is not particularly limited. For example, melt polymerization, melt acidolysis, slurry polymerization, etc. can be used. When applying these polymerization methods, acylation or acetylation may be performed according to the usual method.

The stretched liquid crystal polymer film may contain, within a range that does not excessively impair the effects of the present invention, polymers such as fluororesins, polyolefins, polycycloolefins, and the like; release improvers such as higher fatty acids having 10 to 25 carbon atoms, higher fatty acid esters, higher fatty acid amides, and higher fatty acid metal salts; colorants such as dyes, pigments, and carbon black; and additives such as organic fillers, inorganic fillers, hollow particles, antioxidants, heat stabilizers, light stabilizers, UV absorbers, flame retardants, lubricants, antistatic agents, surfactants, rust inhibitors, foaming agents, defoamers, and fluorescent agents. These polymers and additives can be included in the molten resin composition during the film formation of the liquid crystal polymer film. In addition, these polymers and additives can be used alone or in combination of two or more types. The content of the polymers and additives is not particularly limited, but from the viewpoint of moldability and thermal stability, it is preferably 0.01 to 50% by mass, more preferably 0.1 to 40% by mass, and even more preferably 0.5 to 30% by mass, based on the total amount of the liquid crystal polymer film. These polymers, additives, etc. may be added to the liquid crystal polymer in advance, or they may be added to the liquid crystal polymer when forming the stretched liquid crystal polymer film described below.

The average thickness of the stretched liquid crystal polymer film of the present invention is less than 25 μm, and preferably less than 10 μm. By having the average thickness within the above range, when the stretched liquid crystal polymer film is laminated with a conductor layer to form a circuit board, the thickness of the circuit board can be reduced, and the overall thickness of such laminated circuit boards can be reduced. Therefore, the stretched liquid crystal polymer film can be suitably used in the manufacture of FPCs having a multilayer structure.

The average thickness of a stretched liquid crystal polymer film can be determined, for example, by measuring the film thickness at 5 mm intervals along the width direction (TD) of the film using a micrometer, contact thickness gauge, etc. at positions (three or more positions) spaced 300 to 500 mm apart in the longitudinal direction (MD) and averaging these measurements. Note that in the present invention, if unevenness in film thickness occurs, this is due to the stretching in the width direction during the manufacturing process of the stretched liquid crystal polymer film. For this reason, the average value of film thickness measured along the width direction at several positions in the longitudinal direction of the film can be used as the average film thickness of the stretched liquid crystal polymer film.

In addition, in the stretched liquid crystal polymer film of the present invention, the Cv value of the film thickness is 10% or less, preferably 7% or less. The Cv value of the film thickness is expressed by the following formula (1) using the standard deviation of the film thicknesses measured at multiple locations when determining the average film thickness and the average film thickness. The Cv value is an index representing the uniformity of the film thickness of the stretched liquid crystal polymer film. When the Cv value is within the above range, the stretched liquid crystal polymer film and the conductor layer can be closely attached to each other when manufacturing a circuit board, and the stability of the circuit board is improved. Therefore, the stretched liquid crystal polymer film can be suitably used for manufacturing FPCs having a multilayer structure.
Cv value (%) = standard deviation of film thickness / average film thickness × 100 (1)

Furthermore, in the stretched liquid crystal polymer film of the present invention, the difference between the maximum and minimum film thickness is preferably 5 μm or less, and more preferably 4 μm or less. The smaller the difference between the maximum and minimum film thickness, the more uniform the film thickness of the stretched liquid crystal polymer film is.

In the stretched liquid crystal polymer film of the present invention, it is preferable that the surface roughness Ra of one or both sides measured by a laser microscope is 0.5 μm or less. Specifically, the surface roughness Ra in this invention is the arithmetic mean roughness of the film surface measured non-contact based on the confocal principle, which states that the amount of reflected light is maximized when the object is in focus.

Figure 1(a) is a diagram comparing the size of unevenness measured by a contact-type surface roughness gauge and a laser microscope, and Figure 1(b) is a diagram comparing the size of unevenness at small intervals measured by a contact-type surface roughness gauge and a laser microscope.

1(a), for irregularities (concave portions) with a spacing (W ₁ ) large enough for the stylus (N) of a contact-type surface roughness measuring instrument to penetrate, the size (depth) (D ₁ -N) of the irregularities measured by the contact-type surface roughness measuring instrument and the size (D ₁ -L) of the irregularities measured by the laser microscope (L) are roughly the same. For this reason, when irregularities with such spacing are mainly present on the surface (S ₁ ) to be measured, it is believed that there will not be a large difference in the surface roughness values measured by each method.

However, as shown in FIG. 1(b), when the surface (S ₂ ) to be measured has irregularities with a very small interval (W ₂ ), the width of the stylus of a contact-type surface roughness measuring instrument is wider than the width of the irregularities, and the stylus cannot penetrate into the interior of the irregularities. For this reason, in a contact-type surface roughness measuring instrument, the size of the irregularities with a small interval is measured smaller than the actual size (D ₂ -N) or is ignored, resulting in a smaller surface roughness value than the actual value. On the other hand, in a measurement using a laser microscope, the laser can penetrate the irregularities with a small interval, so that the exact size of the irregularities can be measured (D ₂ -L). For this reason, in the measurement of surface roughness using a laser microscope, it is possible to perform an accurate evaluation of the surface roughness that reflects the irregular shapes with a smaller interval than in the conventional contact-type surface roughness measurement. In particular, since a high-frequency signal can penetrate the irregularities with such a small interval, in order to reduce the transmission loss of high-frequency signals when a liquid crystal polymer film is used in an FPC, it is extremely important to control the surface roughness measured by a laser microscope of the liquid crystal polymer film.

By having a surface roughness Ra of 0.5 μm or less on at least one side measured by a laser microscope, when a laminate is manufactured by laminating a stretched liquid crystal polymer film and a conductor, the interface between the film and the conductor becomes smooth, and transmission loss during the flow of high-frequency signals is suppressed, making it possible to increase the speed and capacity of communication using the laminate. The surface roughness Ra can be measured along any direction on the surface of the stretched liquid crystal polymer film. That is, it is preferable that the surface roughness Ra measured along any direction is 0.5 μm or less, and it is more preferable that the average surface roughness Ra measured multiple times along any direction is 0.5 μm or less. It is also preferable that the surface roughness Ra measured along the longitudinal direction (MD) or width direction (TD) of the film is 0.5 μm or less, and it is more preferable that both the surface roughness Ra (MD) measured along the longitudinal direction and the surface roughness Ra (TD) measured along the width direction are 0.5 μm or less. In particular, it is preferable that the average surface roughness Ra (MD) obtained by measuring multiple times along the longitudinal direction is 0.5 μm or less, and the average surface roughness Ra (TD) obtained by measuring multiple times along the width direction is 0.5 μm or less. In addition, from the viewpoint of suppressing the transmission loss of the two conductors when conductors are attached to both sides of the stretched liquid crystal polymer film, it is more preferable that the surface roughness Ra of both sides of the stretched liquid crystal polymer film measured by a laser microscope is 0.5 μm or less. The surface roughness Ra of the stretched liquid crystal polymer film surface is preferably 0.4 μm or less, particularly preferably 0.2 μm or less in any direction and on any side. If Ra is 0.2 μm or less, the skin depth when a signal with a frequency of 90 GHz considered for the 6th generation mobile communication system flows on the surface of copper is below 0.22 μm, and the transmission loss is reduced.

The anisotropy of the molecular orientation of the stretched liquid crystal polymer film is preferably within a predetermined range of the degree of planar orientation as defined below. First, in a pole measurement by X-ray diffraction, the stretched liquid crystal polymer film is tilted by 45° (α=45° in the Schulz method) and rotated in the in-plane direction (β direction) while measuring the diffraction intensity of the 110 plane, and an X-ray diffraction intensity profile is prepared. In this profile, the longitudinal direction of the film is set to β=0°, and the integrated intensities of β=45 to 135°, 135° to 225°, 225° to 315°, and 315° to 45° are obtained, and the sum of the integrated intensity at β=45 to 135° and the integrated intensity at β=225° to 315° is defined as the integrated intensity in the longitudinal direction. In addition, the sum of the integrated intensity at β=135 to 225° and the integrated intensity at β=315 to 45° is defined as the integrated intensity in the width direction. In this case, the degree of planar orientation represented by the following formula (2) is preferably −0.5 or more and 0.5 or less. The degree of planar orientation is preferably −0.3 or more and 0.3 or less, and more preferably −0.2 or more and 0.2 or less.
Planar orientation degree=(integrated intensity in the longitudinal direction−integrated intensity in the transverse direction)/(integrated intensity in the longitudinal direction+integrated intensity in the transverse direction) (2)

The diffraction intensity of the 110 plane is the diffraction intensity of the crystal plane (110 plane) of the liquid crystal polymer. For example, the diffraction intensity of the 110 plane in a liquid crystal polymer obtained by polycondensation of 2,6-hydroxynaphthoic acid and parahydroxybenzoic acid in a molar ratio of 73:27 is the maximum diffraction intensity observed at 2θ=20° when X-ray diffraction is measured at a diffraction angle (2θ) range of 10° to 40°. The diffraction intensity of the (110 plane) of a liquid crystal polymer oriented in the longitudinal direction is maximum at β=90° and 270° when the longitudinal direction of the film is β=0°. Therefore, the sum of the integrated intensity at β=45-135° and the integrated intensity at β=225°-315° is the integrated intensity in the longitudinal direction, and the sum of the integrated intensity at β=135-225° and the integrated intensity at β=315-45° is the integrated intensity in the transverse direction. The integrated intensity is calculated by the area when β is plotted on the horizontal axis and the diffraction intensity on the vertical axis. If the value expressed by the above formula (2) is a positive value, it indicates that the molecular chains are oriented in the longitudinal direction, and if it is a negative value, it indicates that they are oriented in the width direction.

By setting the value of the degree of planar orientation expressed by the above formula (2) to -0.5 or more and -0.5 or less, the anisotropy of the linear expansion coefficient of the stretched liquid crystal polymer film of the present invention can be reduced, and therefore, when the stretched liquid crystal polymer film is laminated with copper to form an FPC, deformation due to the difference in linear expansion coefficient can be suppressed. In particular, this effect is more pronounced when the stretched liquid crystal polymer film is subjected to a heat treatment (described later). The value of the degree of planar orientation is preferably -0.2 to 0.2, and within this range, the linear expansion coefficient of the stretched liquid crystal polymer film can be approximately 10 to 30 ppm in both the longitudinal and transverse directions of the film. Furthermore, by setting the value of the degree of planar orientation to -0.1 to 0.1, the linear expansion coefficient becomes even closer to the linear expansion coefficient of copper, which is 18 ppm.

<Method of manufacturing stretched liquid crystal polymer film>
The method for producing the stretched liquid crystal polymer film of the present invention is described below. The stretched liquid crystal polymer film of the present invention is obtained by laminating supporting polymer films on both sides of an unstretched liquid crystal polymer film to make them adhere to each other to form a laminated film (first step), stretching this laminated film (second step), and then peeling off the supporting polymer films (third step).

The unstretched liquid crystal polymer film used in the first step can be manufactured by a known method. For example, the liquid crystal polymer can be formed into a film by a melt extrusion film-forming method using a T-die (T-die melt extrusion) to form an unstretched liquid crystal polymer film. Specifically, the liquid crystal polymer is melt-kneaded in an extruder, the molten resin is extruded through a T-die, and solidified on a metal roll to obtain an unstretched liquid crystal polymer film. The temperature of the cylinder of the extruder is preferably 230 to 360°C, more preferably 280 to 350°C. The slit interval of the T-die can be appropriately set depending on the type and composition of the liquid crystal polymer used, the performance of the desired film, etc. The slit interval of the T-die is not particularly limited, but is preferably 0.1 to 1.5 mm, more preferably 0.1 to 1.0 mm.

The thickness of the unstretched liquid crystal polymer film obtained by the above method is not particularly limited, but in order to control the average film thickness of the stretched liquid crystal polymer film within the desired range and to set the value of the planar orientation degree of the stretched liquid crystal polymer film to -0.5 or more and -0.5 or less, the thickness is preferably 5 to 100 μm, more preferably 10 to 100 μm, even more preferably 20 to 100 μm, and particularly preferably 20 to 70 μm.

The supporting polymer film is a film laminated to an unstretched liquid crystal polymer film to prevent the film from breaking when the unstretched liquid crystal polymer film is stretched. Examples of supporting polymers constituting the supporting polymer film include aromatic polyetherketones and polyesters. Specific examples of aromatic polyetherketones include polyetherketone (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetheretherketoneketone (PEEKK), etc. Specific examples of polyesters include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), etc. These polymers can be used alone or in combination of two or more. Among them, polyetheretherketone (PEEK), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT) are preferred, and polyetheretherketone (PEEK) is particularly preferred. By using a support polymer film made of these supporting polymers, the laminated film can be stretched in the second step at a temperature below the melting point of the liquid crystal polymer without breaking. Furthermore, it is preferable that these films are crystallized or stretched films, as they have high heat resistance and can be stretched at high temperatures.

The surface roughness Ra of the supporting polymer film measured by a laser microscope is preferably 1.5 μm or less, more preferably 1.0 μm or less, even more preferably 0.5 μm or less, and particularly preferably 0.2 μm or less. By setting the surface roughness of the supporting polymer film within the above range, the surface roughness Ra of the resulting stretched liquid crystal polymer film measured by a laser microscope can be controlled to 0.5 μm or less.

When laminating the unstretched liquid crystal polymer and the supporting polymer film, the supporting polymer film can be laminated to only one side or both sides of the unstretched liquid crystal polymer film, but laminating to both sides is preferable because the average film thickness and Cv value of the film thickness of the resulting stretched liquid crystal polymer film can be controlled within the desired range, the surface roughness Ra of both sides of the stretched liquid crystal polymer film can be controlled to 0.5 μm or less, and defects such as breakage of the unstretched liquid crystal polymer film can be reduced when the laminated film in which the unstretched liquid crystal polymer film and the supporting polymer film are laminated is stretched in the width direction (TD) in the second step described below.

The method for bonding the unstretched liquid crystal polymer film and the supporting polymer film is not particularly limited, but a thermal lamination method is preferred since it does not require adhesives or the like. In the thermal lamination method, the unstretched liquid crystal polymer film and the supporting polymer film are pressed together while the laminated film of the unstretched liquid crystal polymer film and the supporting polymer film is heated with a pair of heated rolls. The conditions for the thermal lamination method can be appropriately selected according to the physical properties of the liquid crystal polymer and the supporting polymer. Although not particularly limited, it is preferable to perform heating and pressing at a temperature near the melting point of the liquid crystal polymer and at a temperature near the melting point of the supporting polymer.

If the unstretched liquid crystal polymer film and the support film are difficult to adhere to each other by heat lamination alone, it is preferable to perform a surface treatment on the surface of the unstretched liquid crystal polymer film that comes into contact with the support polymer film (the lamination surface) and the surface of the support polymer film that comes into contact with the unstretched liquid crystal polymer film (the lamination surface) before laminating the unstretched liquid crystal polymer film and the support polymer film. Examples of surface treatment methods include plasma treatment in which a gas in a plasma state is irradiated onto the surface by applying electrical energy, corona treatment in which the surface is activated by discharge, activation methods in which ultraviolet rays or electron beams are irradiated onto the surface, activation methods in which a flame is applied to the surface, chemical treatment in which the surface is oxidized with potassium dichromate or the like, and primer treatment in which a primer is applied. By performing such a surface treatment before laminating the unstretched liquid crystal polymer film and the support polymer film, the adhesion between the unstretched liquid crystal polymer film and the support polymer film can be improved. The surface treatment method can be appropriately selected depending on the physical properties of the liquid crystal polymer and the supporting polymer, but from the viewpoint of increasing the adhesion between the unstretched liquid crystal polymer film and the supporting polymer film and reducing damage to the resulting stretched liquid crystal polymer film, plasma treatment, corona treatment, and chemical treatment are preferred, with plasma treatment being particularly preferred.

Another method for improving the adhesion between the unstretched liquid crystal polymer film and the supporting polymer film is to provide an easy-adhesion layer made of a polyester-based resin material on the surface of the supporting polymer film.

Next, in the second step, a tenter-type stretching device is used to stretch the laminated film in which the unstretched liquid crystal polymer film and the support polymer film are laminated in the width direction (TD). By stretching the laminated film in the width direction, the average thickness of the resulting stretched liquid crystal polymer film can be controlled within a desired range, and the anisotropy of the stretched liquid crystal polymer film can be reduced. The method for stretching the laminated film is not particularly limited, but a tenter transverse stretching method in which both ends of the laminated film are clamped with clips and heated and stretched is preferred. The stretching ratio and stretching speed are appropriately selected so that the support film can be stretched and the shape and physical properties of the film made of liquid crystal polymer after stretching are within the desired range. The stretching ratio is preferably 1.5 to 10 times, more preferably 2 to 5 times. The stretching speed is preferably 500%/min to 10,000%, more preferably 1,000 to 5,000%/min. In addition, in order to adjust the degree of surface orientation after stretching, stretching in the longitudinal direction (MD) may be added as necessary.

The laminated film is preferably stretched under conditions such that the surface temperature of the laminated film during stretching (the ultimate stretching temperature) is lower than the melting point of the liquid crystal polymer. The ultimate stretching temperature is more preferably 30 to 200°C lower than the melting point of the liquid crystal polymer, even more preferably 80 to 200°C lower than the melting point of the liquid crystal polymer, and particularly preferably 100 to 170°C lower than the melting point of the liquid crystal polymer. The ultimate stretching temperature is preferably equal to or higher than the glass transition temperature of the liquid crystal polymer. The ultimate stretching temperature of the laminated film can be adjusted by appropriately setting the stretching speed and the temperature (inner furnace temperature of the stretching device) when stretching the laminated film. By stretching the laminated film under conditions such that the ultimate stretching temperature is lower than the melting point of the liquid crystal polymer, the thickness of the resulting stretched liquid crystal polymer film can be made thin and uniform, and the smoothness of the film surface can be improved. In the present invention, "stretching is performed at a temperature below the melting point of the liquid crystal polymer" means that stretching is performed under conditions where the ultimate stretching temperature is below the melting point of the liquid crystal polymer.

In the second step, it is preferable that the temperature T inside the furnace of the stretching device is set to a temperature that satisfies the following formula (3).
T ₁ <T < T ₂ (3)
However, in the above formula (1), _T1 is a temperature of the melting point of the liquid crystal polymer minus 100°C, and _T2 is a temperature of the melting point of the liquid crystal polymer plus 30°C. That is, _T1 is a temperature 100°C lower than the melting point of the liquid crystal polymer, and _T2 is a temperature 30°C higher than the melting point of the liquid crystal polymer. When T is _T2 or more, even if the ultimate stretching temperature is within the appropriate temperature range described above, the support polymer film or support polymer layer is likely to break and cannot be stretched well. The reason for this is unclear, but it is presumed that if the furnace temperature is too high, when the liquid crystal polymer layer after stretching is thin, the temperature of the support polymer film or support polymer layer on its surface becomes high and it becomes easy to break. On the other hand, when T is T1 or _less , the ultimate stretching temperature does not reach an appropriate temperature during stretching, and the film breaks during stretching.

Furthermore, it is preferable that the stretching of the laminated film in the second step is carried out under conditions in which the value _A1 represented by the following formula (4) is 0.20 to 5.00.
A ₁ = furnace temperature T (° C.) / stretching speed (%/min) / thickness of unstretched liquid crystal polymer film (μm) × 100 (4)
_A1 is preferably 0.24 to 2.50, more preferably 0.28 to 0.48. When _A1 exceeds 5.00, the Cv value, which is an index showing the uniformity of the film thickness, becomes large, and the uniformity of the film thickness decreases. When _A1 is less than 0.20, the film becomes easily broken. By stretching the laminated film under conditions that satisfy the above formula (4), the film thickness of the obtained stretched liquid crystal polymer film can be made thin and uniform, and the smoothness of the film surface can be improved.

In order to control the average thickness of the resulting stretched liquid crystal polymer film to 25 μm or less and to control the Cv value of the thickness within a predetermined range, it is preferable to adjust the stretching ratio and stretching speed of the laminated film as follows. Specifically, when a laminated film including an unstretched liquid crystal polymer film having a thickness of 40 to 60 μm is stretched at a stretching ratio of 3 times, it is preferable to set the stretching speed to 1200 to 2800%/min. When an unstretched liquid crystal polymer film having a thickness of 40 to 60 μm is stretched at a stretching ratio of 3.5 times, it is preferable to set the stretching speed to 1600 to 3000%/min. When an unstretched liquid crystal polymer film having a thickness of 40 to 60 μm is stretched at a stretching ratio of 4 times, it is preferable to set the stretching speed to 2000 to 3500%/min.

When stretching an unstretched liquid crystal polymer film having a thickness of 15 to 35 μm at a stretch ratio of 3 times, the stretching speed is preferably set to 1200 to 4600%/min. When stretching an unstretched liquid crystal polymer film having a thickness of 15 to 35 μm at a stretch ratio of 3.5 times, the stretching speed is preferably set to 1600 to 6000%/min. When stretching an unstretched liquid crystal polymer film having a thickness of 15 to 35 μm at a stretch ratio of 4 times, the stretching speed is preferably set to 2000 to 7500%/min.

In addition, while the laminated film is being stretched, it is preferable that the sum of the stretching loads applied to the two supporting polymer films laminated on both sides of the unstretched liquid crystal polymer film is always equal to or greater than the stretching load applied to the unstretched liquid crystal polymer film. Here, the stretching load refers to the load applied to the film when it is stretched, and is the value obtained by multiplying the tensile stress of the film by the cross-sectional area of the film.

While stretching the laminated film, by ensuring that the sum of the stretching loads applied to the two supporting polymer films is equal to or greater than the stretching load applied to the unstretched liquid crystal polymer film, the unstretched liquid crystal polymer film can be stretched without breaking, even at temperatures below the melting point of the liquid crystal polymer. The reason for this is not clear, but it is thought to be because by adhering a supporting polymer film, which has a higher stretching load than the unstretched liquid crystal polymer film, to the unstretched liquid crystal polymer film, the tensile load applied to the liquid crystal polymer film is distributed, suppressing stress concentration in areas where breakage is likely to occur.

In order to ensure that the sum of the stretching loads applied to the two supporting polymer films is equal to or greater than the stretching load applied to the unstretched liquid crystal polymer film, this can be achieved by appropriately selecting the ratio of the thickness of the unstretched liquid crystal polymer film to the thickness of the supporting polymer film, the material and surface roughness of the supporting polymer film, and the temperature and stretching speed during stretching. For example, the material of the supporting polymer film can be selected depending on the temperature during stretching, which is determined by the type of liquid crystal polymer.

In addition, when using a laminated film in which a supporting polymer film is laminated to only one side of an unstretched liquid crystal polymer film, the laminated film can be stretched so that the stretching load on one supporting polymer film is equal to or greater than the stretching load on the unstretched liquid crystal polymer film.

From the viewpoint of being able to satisfactorily adjust the relationship between the stretching load applied to the support polymer film and the stretching load applied to the unstretched liquid crystal polymer film, the thickness ratio of the support polymer film and the unstretched liquid crystal polymer film, expressed as the ratio of "thickness of one support polymer film/thickness of one unstretched liquid crystal polymer film", is preferably 0.01 to 10.0, more preferably 0.1 to 5.0, and even more preferably 0.2 to 1.2.

Finally, in the third step, the supporting polymer film is peeled off from the stretched laminated film to obtain a stretched liquid crystal polymer film.

In the third step, after peeling off the support polymer film, the stretched liquid crystal polymer film may be heat-treated in the range of its melting point -50°C to its melting point. This can improve the heat resistance of the liquid crystal polymer film and reduce the linear expansion coefficient.

In the above method, a laminated film is obtained by laminating a film made of a liquid crystal polymer and a film made of a supporting polymer in the first step, but the method for obtaining a laminated film is not particularly limited to this. For example, a laminated film may be formed by melting a liquid crystal polymer in a first extruder and melting a supporting polymer in a second extruder, and extruding each polymer into a film (co-extrusion) so that a layer made of the supporting polymer is laminated on one or both sides of a layer made of liquid crystal polymer.

As a method for laminating a layer of supporting polymer on one or both sides of a layer of liquid crystal polymer, a method for forming a multilayer extrusion film from a T-die can be used. Specific examples include the feedblock method, in which molten liquid crystal polymer and supporting polymer supplied from two extruders are fed to a feedblock, merged, and then extruded from a T-die in the form of a film, and the multi-manifold method, in which molten liquid crystal polymer and supporting polymer are separately fed to a T-die, overlaid, and extruded in the form of a film. From the viewpoint of improving the smoothness of the resulting stretched liquid crystal polymer film, it is preferable to apply the multi-manifold method, taking into consideration cases in which the liquid crystal polymer and supporting polymer have different viscosities and flow characteristics when melted.

For the laminated film formed by coextrusion in the first step, the thickness of the liquid crystal polymer layer is preferably 5 to 100 μm, more preferably 10 to 100 μm, even more preferably 20 to 100 μm, and particularly preferably 20 to 70 μm. Furthermore, when forming a laminated film by coextrusion, it is preferable to use polyether ether ketone as the supporting polymer. By using the above supporting polymer, the laminated film can be stretched in the second step at a temperature below the melting point of the liquid crystal polymer without causing breakage. This can improve the smoothness of the surface of the resulting stretched liquid crystal polymer film.

Even when the laminated film is formed by coextrusion in the first step, it is preferable to set the temperature T inside the oven of the stretching device to a temperature that satisfies the above formula (3). Also, it is preferable to perform the second step under conditions where the value _A2 represented by the following formula (5) is 0.20 to 5.00.
_A2 = furnace temperature T (°C) / stretching speed (%/min) / thickness of liquid crystal polymer layer (μm) × 100 (5)
_A2 is preferably 0.24 to 2.50, more preferably 0.28 to 0.48. When _A2 exceeds 5.00, the Cv value, which is an index showing the uniformity of the film thickness, becomes large, and the uniformity of the film thickness decreases. When _A2 is less than 0.20, the film becomes easily broken. By stretching the laminated film under conditions that satisfy the above formula (5), the film thickness of the obtained stretched liquid crystal polymer film can be made thin and uniform, and the smoothness of the film surface can be improved.

If in the first step the laminated film is formed by extruding the liquid crystal polymer and the supporting polymer into a film shape, then in the second step it is preferable to stretch the laminated film so that the stretching load applied to the layer made of the supporting polymer is equal to or greater than the stretching load applied to the layer made of the liquid crystal polymer.

<Laminate>
The laminate of the present invention includes a film layer made of the above-mentioned stretched liquid crystal polymer film and a metal layer. Examples of the metal material constituting the metal layer include gold, silver, copper, iron, nickel, aluminum, and alloy metals thereof, and copper is preferably used.

The laminate can be manufactured by a known method as long as the smoothness and molecular orientation of the stretched liquid crystal polymer film can be maintained. For example, the laminate can be manufactured by vapor-depositing a metal layer on the surface of the stretched liquid crystal polymer film, or a metal layer can be formed on the surface of the stretched liquid crystal polymer film by electroless plating or electrolytic plating. The laminate can also be manufactured by superposing the stretched liquid crystal polymer film and the metal foil using a roll-to-roll method or a continuous isostatic pressing method (double belt method) and continuously thermocompressing the metal foil, etc., to manufacture the laminate. The laminate can also be manufactured by surface-activated bonding, in which the surfaces of the stretched liquid crystal polymer film and the metal foil are activated by removing oxides and dirt using a method such as sputter etching, and the stretched liquid crystal polymer film and the metal foil are brought into contact with each other and rolled to bond them.

<Circuit board>
The circuit board of the present invention comprises an insulator (or dielectric) made of the above-mentioned stretched liquid crystal polymer film and a conductor layer. The form of the circuit board is not particularly limited, and it can be used as various high-frequency circuit boards by known means. The circuit board may be equipped with a semiconductor element such as an IC chip.

A circuit pattern is formed on the conductor layer of the circuit board by a known processing method. Examples of metal materials constituting the conductor layer include gold, silver, copper, iron, nickel, and aluminum, as well as alloy metals thereof. Note that a circuit pattern may be formed on the metal layer of the laminate by a known method.

Specific examples of methods for manufacturing circuit boards with circuit patterns include conventionally known methods such as the modified semi-additive method (MSAP method), the semi-additive method (SAP method), and the subtractive method. For example, in the case of the SAP method, a circuit board can be manufactured by applying electroless copper plating as a conductor layer to an insulator made of a stretched liquid crystal polymer film, masking the non-wiring parts on the conductor layer, applying electrolytic copper plating to the unmasked parts to form an additional conductor layer, removing the mask, and removing the conductor layer hidden by the mask by etching. In addition, in the case of the MSAP method, a circuit board can be manufactured by laminating an extremely thin copper foil instead of the electroless copper plating used in the SAP method.

The circuit board of the present invention can be used for various transmission lines, such as coaxial lines, strip lines, microstrip lines, coplanar lines, and parallel lines. The circuit board of the present invention can also be used for antennas and antenna devices in which an antenna and a transmission line are integrated.

Antennas include antennas that use millimeter waves or microwaves, such as waveguide slot antennas, horn antennas, lens antennas, printed antennas, triplate antennas, microstrip antennas, and patch antennas. When the circuit board of the present invention is used as an antenna, it is preferable that the circuit board be a multilayer circuit board.

The circuit board of the present invention can also be used in sensors such as vehicle-mounted radar that have semiconductor elements.

The circuit board of the present invention is made of a stretched liquid crystal polymer film that is thin and has a uniform thickness, so it can be particularly well suited for use as a multi-layer circuit board.

Next, the present invention will be specifically explained using examples, but the present invention is not limited to these.

<Film Melting Point>
Using a differential scanning calorimeter (PerkinElmer, model: DSC8500), the liquid crystal polymer film before stretching was analyzed according to the differential scanning calorimetry method based on JIS K 7121. The endothermic peak temperature observed when the temperature of the liquid crystal polymer film before stretching was raised from 0° C. at a rate of 10° C./min was taken as the melting point.

<Film appearance>
The appearance of the stretched liquid crystal polymer film was visually observed and evaluated as follows.
◯: No unevenness in thickness or holes in the film.
×: Unevenness in thickness or holes in the film are observed.

<Average Film Thickness, Cv Value of Film Thickness, and Difference Between Maximum and Minimum Film Thickness>
The film thickness was measured at three points (400 mm between each point) in the longitudinal direction of the film at 5 mm intervals along the width direction of the film at a total of 270 to 360 points using a contact thickness gauge (manufactured by Meisan Co., Ltd., model: RC-1W). The average value of these film thicknesses was taken as the average film thickness of the stretched liquid crystal polymer film. In addition, the Cv value of the film thickness of the stretched liquid crystal polymer film was calculated from the standard deviation of the film thicknesses measured at 270 to 360 points and the average film thickness. The smaller the Cv value, the more uniform the film thickness. In addition, the difference between the maximum and minimum values of the film thickness measured at 270 to 360 points was calculated. The smaller the difference between the maximum and minimum values of the film thickness, the more uniform the film thickness.

<Film surface roughness>
The surface roughness Ra of the support polymer film and the stretched liquid crystal polymer film was determined using a laser microscope equipped with a white light interferometer (Keyence Corporation, model: VK-X3000). In a field of view of 1052 x 1404 μm, the roughness curve was measured under the conditions of a measurement reference length of 0.25 mm, an evaluation length of 1 mm, and a cutoff value λc of 0.25 mm (no cutoff value λs), and the surface roughness Ra was determined by calculating the arithmetic average roughness. The surface roughness Ra was determined for each of the front and back sides of the film in the longitudinal direction (MD) and the transverse direction (TD) of the film.

<Degree of Planar Orientation of Film>
For the stretched liquid crystal polymer film, a sample horizontal type multipurpose X-ray diffractometer (Rigaku Corporation, model: Ultima IV) was used, the diffraction angle (2θ) was fixed at 20°, and the X-ray target was Cu, the voltage was 40 kV, the current was 40 mA, the α angle was 45°, and the β angle was 0 to 360° (the longitudinal direction of the film was 0°, and the step angle was 5°). The X-ray diffraction intensity profile was prepared. The integrated intensities of β = 45 to 135°, 135° to 225°, 225 to 315°, and 315 to 45° of this profile were obtained, and the sum of the integrated intensities of β = 45 to 135° and β = 225° to 315° was defined as the integrated intensity in the longitudinal direction, and the sum of the integrated intensities of β = 135 to 225° and β = 315 to 45° was defined as the integrated intensity in the width direction. The degree of planar orientation was calculated from the following formula (6).
Planar orientation degree=(integrated intensity in the longitudinal direction−integrated intensity in the transverse direction)/(integrated intensity in the longitudinal direction+integrated intensity in the transverse direction) (6)

<Linear expansion coefficient of film>
A stretched liquid crystal polymer film (width 5 mm) was attached to a thermomechanical analyzer (manufactured by Rigaku Corporation, model: TMA8310) (chuck distance 15 mm), and a load of 10 mN was applied while heating from 30° C. to 150° C. at a rate of 5° C./min., and the dimensional change was measured to determine the change.

Example 1
A liquid crystal polymer (LAPEROS A950RX, manufactured by Polyplastics Co., Ltd.) was fed into a twin-screw extruder (screw diameter 32 mm), extruded into a film form from a T-die (lip length 350 mm, lip clearance about 1 mm, die temperature 300°C) at the tip of the extruder, and cooled to obtain an unstretched liquid crystal polymer film with a thickness of 50 μm. The melting point was evaluated by the above method.

Next, both sides of the unstretched liquid crystal polymer film and one side of a polyether ether ketone (PEEK) film (Victrex, APTIV Film 1000-025G, thickness 25 μm, surface roughness Ra = 0.14 μm (MD), 0.12 μm (TD)) serving as a supporting polymer film were subjected to direct atmospheric pressure plasma treatment in an oxygen-containing gas atmosphere at a power of 1.5 kW and a conveying speed of 1.0 m/min. Next, the plasma-treated surfaces were overlapped, and a PEEK film was thermocompressed onto both sides of the unstretched liquid crystal polymer film using a first roll heated to 305°C and a second roll heated to 120°C under conditions of a nip pressure of 0.2 MPa and a conveying speed of 0.5 m/min. After thermocompression, the unstretched liquid crystal polymer film and the PEEK film were in close contact.

The laminated film thus produced was stretched 3 times in the width direction (TD) with a tenter-type transverse stretching machine (furnace temperature 300° C.) at a stretching zone length of 1.2 m and a conveying speed of 10 m/min (stretching speed 1667%/min). The temperature of the laminated film during stretching (attained stretching temperature) was 170° C. Thereafter, the PEEK film was peeled off to obtain a stretched liquid crystal polymer film, and the appearance, average film thickness, difference between the maximum and minimum film thicknesses, Cv value of the film thickness, surface roughness Ra, and degree of planar orientation were evaluated. Furthermore, the obtained stretched liquid crystal polymer film was heat-treated at 250° C. for 24 hours in a nitrogen atmosphere to evaluate the linear expansion coefficient. The results are shown in Table 1. The value A ₁ in Table 1 is a value calculated from the above formula (4) based on the thickness of the unstretched liquid crystal polymer film, the furnace temperature, and the stretching speed.

<Examples 2 and 3>
A stretched liquid crystal polymer film was obtained and evaluated in the same manner as in Example 1, except that the stretching ratio was changed to the value shown in Table 1. The results are shown in Table 1.

Example 4
A liquid crystal polymer (LAPEROS A950RX, manufactured by Polyplastics Co., Ltd.) was fed into a twin-screw extruder (screw diameter 32 mm), extruded into a film form from a T-die (lip length 350 mm, lip clearance about 1 mm, die temperature 300°C) at the tip of the extruder, and cooled to obtain an unstretched liquid crystal polymer film with a thickness of 20 μm. The melting point was evaluated by the above method.

Next, both sides of the unstretched liquid crystal polymer film and one side of a polyether ether ketone (PEEK) film (Victrex, APTIV Film 1000-025G, thickness 25 μm, surface roughness Ra = 0.14 μm (MD), 0.12 μm (TD)) serving as a supporting polymer film were subjected to direct atmospheric pressure plasma treatment in an oxygen-containing gas atmosphere at a power of 1.5 kW and a conveying speed of 1.0 m/min. Next, the plasma-treated surfaces were overlapped, and a PEEK film was thermocompressed onto both sides of the unstretched liquid crystal polymer film using a first roll heated to 270°C and a second roll heated to 120°C under conditions of a nip pressure of 0.2 MPa and a conveying speed of 0.5 m/min. After thermocompression, the unstretched liquid crystal polymer film and the PEEK film were in close contact.

The laminated film thus produced was stretched 4 times in the transverse direction (TD) in a tenter-type transverse stretching machine (furnace temperature 300°C) with a stretching zone length of 0.6 m and a conveying speed of 10 m/min (stretching speed 5000%/min). The temperature of the laminated film during stretching (attained stretching temperature) was 170°C. The PEEK film was then peeled off to obtain a stretched liquid crystal polymer film, which was evaluated for appearance, average film thickness, difference between maximum and minimum film thickness, Cv value of film thickness, surface roughness Ra, and degree of planar orientation. The obtained stretched liquid crystal polymer film was further heat-treated at 250°C for 24 hours in a nitrogen atmosphere to evaluate the linear expansion coefficient. The results are shown in Table 1.

Example 5
A liquid crystal polymer (LAPEROS A950RX, manufactured by Polyplastics Co., Ltd.) was fed into a twin-screw extruder (screw diameter 32 mm), extruded into a film form from a T-die (lip length 350 mm, lip clearance about 1 mm, die temperature 300°C) at the tip of the extruder, and cooled to obtain an unstretched liquid crystal polymer film with a thickness of 50 μm. The melting point was evaluated by the above method.

Next, both sides of the unstretched liquid crystal polymer film and one side of a biaxially stretched polybutylene terephthalate (PBT) film (Kohjin Film & Chemicals, Bobblet, thickness 25 μm, surface roughness Ra = 0.11 μm (MD), 0.15 μm (TD)) serving as a supporting polymer film were subjected to direct atmospheric pressure plasma treatment in an oxygen-containing gas atmosphere at a power of 1.5 kW and a conveying speed of 1.0 m/min. Next, the plasma-treated surfaces were overlapped, and a PBT film was thermocompressed onto both sides of the unstretched liquid crystal polymer film using a first roll heated to 200°C and a second roll heated to 120°C under conditions of a nip pressure of 0.2 MPa and a conveying speed of 0.5 m/min. After thermocompression, the unstretched liquid crystal polymer film and the PBT film were in close contact.

The laminated film thus produced was stretched three times in the transverse direction (TD) in a tenter-type transverse stretching machine (furnace temperature 220°C) with a stretching zone length of 1.2 m and a conveying speed of 10 m/min (stretching speed 1667%/min). The temperature of the laminated film during stretching (attained stretching temperature) was 140°C. The PBT film was then peeled off to obtain a stretched liquid crystal polymer film, which was evaluated for appearance, average film thickness, difference between maximum and minimum film thickness, Cv value of film thickness, surface roughness Ra, and degree of planar orientation. The obtained stretched liquid crystal polymer film was then heat-treated at 250°C for 24 hours in a nitrogen atmosphere to evaluate the linear expansion coefficient. The results are shown in Table 1.

Example 6
A liquid crystal polymer (LAPEROS A950RX, manufactured by Polyplastics Co., Ltd.) was fed into a twin-screw extruder (screw diameter 32 mm), extruded into a film form from a T-die (lip length 350 mm, lip clearance about 1 mm, die temperature 300°C) at the tip of the extruder, and cooled to obtain an unstretched liquid crystal polymer film with a thickness of 50 μm. The melting point was evaluated by the above method.

Next, both sides of the unstretched liquid crystal polymer film were subjected to direct atmospheric pressure plasma treatment under an oxygen-containing gas atmosphere at a power of 1.5 kW and a conveying speed of 1.0 m/min. Next, the easy-adhesion layer surface of a biaxially stretched PET film with an easy-adhesion layer (manufactured by Toyobo, A4300, thickness 38 μm, surface roughness Ra = 0.10 μm (MD), 0.12 μm (TD)) was superimposed on the plasma-treated surface of the unstretched liquid crystal polymer film as a supporting polymer film, and a PET film was thermocompressed to both sides of the unstretched liquid crystal polymer film using a first roll heated to 200°C and a second roll heated to 120°C under conditions of a nip pressure of 0.2 MPa and a conveying speed of 0.5 m/min. After thermocompression, the unstretched liquid crystal polymer film and the PET film were in close contact.

The laminated film thus produced was stretched three times in the transverse direction (TD) in a tenter-type transverse stretching machine (furnace temperature 280°C) with a stretching zone length of 1.2 m and a conveying speed of 10 m/min (stretching speed 1667%/min). The temperature of the laminated film during stretching (attained stretching temperature) was 160°C. The PET film was then peeled off to obtain a stretched liquid crystal polymer film, which was evaluated for appearance, average film thickness, difference between maximum and minimum film thickness, Cv value of film thickness, surface roughness Ra, and degree of planar orientation. The obtained stretched liquid crystal polymer film was then heat-treated at 250°C for 24 hours in a nitrogen atmosphere to evaluate the linear expansion coefficient. The results are shown in Table 1.

Example 7
A liquid crystal polymer (Polyplastics Co., Ltd., LAPEROS A950RX) was supplied to a twin-screw extruder (screw diameter 26 mm) and melt-kneaded at 300 ° C. In addition, a polyether ether ketone (PEEK) polymer (Daicel-Evonik, VESTAKEEP 3300G) was supplied to a single-screw extruder (screw diameter 40 mm) as a support polymer and melt-kneaded at 380 ° C. These molten polymers were supplied to a multi-manifold T-die, and a layer of a support polymer was superimposed on both sides of a layer of a liquid crystal polymer, extruded, and cooled to produce a laminated film with a liquid crystal polymer layer of 50 μm and support polymer layers on both sides of 25 μm each, for a total of 100 μm.

The laminated film thus produced was stretched 4 times in the width direction (TD) with a tenter-type transverse stretching machine (furnace temperature 300° C.) at a stretching zone length of 1.2 m and a conveying speed of 10 m/min (stretching speed 2500%/min). The temperature of the laminated film during stretching (attained stretching temperature) was 170° C. Thereafter, the PEEK film was peeled off to obtain a stretched liquid crystal polymer film, and the appearance, average film thickness, difference between the maximum and minimum film thicknesses, Cv value of the film thickness, surface roughness Ra, and degree of planar orientation were evaluated. Furthermore, the obtained stretched liquid crystal polymer film was heat-treated at 250° C. for 24 hours in a nitrogen atmosphere to evaluate the linear expansion coefficient. The results are shown in Table 1. The value A ₂ in Table 1 is a value calculated from the above formula (5) based on the thickness of the layer made of liquid crystal polymer, the furnace temperature, and the stretching speed.

<Comparative Example 1>
A stretched liquid crystal polymer film was obtained and evaluated in the same manner as in Example 1, except that the stretching speed was changed to the value shown in Table 1. The results are shown in Table 1.

<Comparative Example 2>
A stretched liquid crystal polymer film was obtained in the same manner as in Comparative Example 1, except that a polyether ether ketone (PEEK) film (Shin-Etsu Polymer Co., Ltd., Shin-Etsu Sepla Film, thickness 25 μm, surface roughness Ra=2.52 μm (MD), 2.39 μm (TD)) was used as the supporting polymer film, and the evaluation was performed in the same manner. The results are shown in Table 1.

<Comparative Example 3>
A stretched liquid crystal polymer film was obtained in the same manner as in Comparative Example 1, except that a porous polytetrafluoroethylene (PTFE) film (manufactured by Chukoh Chemical Industry Co., Ltd., C-Poruos, thickness 100 μm, surface roughness Ra=2.15 μm (MD), 2.35 μm (TD)) was used as the supporting polymer film, and the evaluation was performed in the same manner. The results are shown in Table 1.

<Comparative Example 4>
An attempt was made to produce a stretched liquid crystal polymer film in the same manner as in Example 1, except that the oven temperature, stretch ratio, and stretching speed of the transverse stretching machine were changed to the values shown in Table 1. However, when the laminated film was stretched, the support polymer film was broken, and a stretched liquid crystal polymer film could not be obtained.

<Comparative Example 5>
An attempt was made to produce a stretched liquid crystal polymer film in the same manner as in Example 1, except that the thickness of the unstretched liquid crystal polymer film and the furnace temperature of the transverse stretching machine were changed to the values shown in Table 1. However, when the laminated film was stretched, the liquid crystal polymer film was broken, and a stretched liquid crystal polymer film could not be obtained.

As shown in Table 1, the stretched liquid crystal polymer films obtained in Examples 1 to 7 had a small average thickness and excellent thickness uniformity. In addition, the surface roughness Ra was small, so the films were highly smooth. In addition, the degree of planar orientation was small, and the linear expansion coefficients in the longitudinal and transverse directions were similar, so anisotropy was reduced.

On the other hand, the stretched liquid crystal polymer films of Comparative Examples 1 to 3, which were produced under conditions where the ultimate stretching temperature was equal to or higher than the melting point of the liquid crystal polymer, had unevenness and holes in the film and had poor film thickness uniformity. Furthermore, in Comparative Example 2, in which PEEK with a surface roughness Ra of more than 2 μm was laminated and stretched, and Comparative Example 3, in which a porous PTFE film was laminated and stretched, the surface roughness of the resulting stretched liquid crystal polymer measured with a laser microscope also exceeded 0.5 μm.

Claims (20)

A stretched liquid crystal polymer film made of a liquid crystal polymer, the stretched liquid crystal polymer film having an average thickness of less than 25 μm and a Cv value of the thickness represented by the following formula (1) of 10% or less.
Cv value (%) = (standard deviation of measured film thickness) / (average film thickness) × 100 (1) The stretched liquid crystal polymer film according to claim 1, in which the difference between the maximum and minimum film thicknesses is 5 μm or less. The stretched liquid crystal polymer film according to claim 1 or 2, in which the surface roughness Ra of at least one surface measured by a laser microscope is 0.5 μm or less. 3. The stretched liquid crystal polymer film according to claim 1,
In a pole measurement by X-ray diffraction, when the diffraction intensity of the 110 plane is measured while rotating the film in an in-plane direction (β direction) with the film tilted by 45° (α=45° in the Schulz method), the integrated intensities of β=45 to 135°, 135° to 225°, 225 to 315°, and 315 to 45° are calculated with β=0° as the longitudinal direction of the film, and the sum of the integrated intensity of β=45 to 135° and the integrated intensity of β=225° to 315° is defined as the longitudinal integrated intensity, and the sum of the integrated intensity of β=135 to 225° and the integrated intensity of β=315 to 45° is defined as the widthwise integrated intensity, the degree of planar orientation of the stretched liquid crystal polymer film is represented by the following formula (2) of -0.5 or more and 0.5 or less.
Planar orientation degree=(integrated intensity in the longitudinal direction−integrated intensity in the transverse direction)/(integrated intensity in the longitudinal direction+integrated intensity in the transverse direction) (2) The stretched liquid crystal polymer film according to claim 1 or 2, having an average thickness of less than 10 μm. A method for producing a stretched liquid crystal polymer film, comprising the steps of:
A first step of obtaining a laminated film by bonding a support film made of a support polymer and having a surface roughness Ra of 1.5 μm or less as measured by a laser microscope to at least one side of an unstretched liquid crystal polymer film made of a liquid crystal polymer;
A second step of stretching the laminated film at least in the width direction using a tenter-type stretching device;
A third step of peeling off the stretched support film. A method for producing a stretched liquid crystal polymer film, comprising the steps of:
A first step of extruding the molten liquid crystal polymer and the supporting polymer into a film shape using an extruder so that a layer of the supporting polymer is laminated on at least one side of a layer of the liquid crystal polymer to obtain a laminated film;
A second step of stretching the laminated film at least in the width direction using a tenter-type stretching device;
and a third step of peeling off the layer made of the stretched support polymer. A method for producing the stretched liquid crystal polymer film according to claim 6 or 7,
The method for producing a stretched liquid crystal polymer film, wherein the thickness of the unstretched liquid crystal polymer film or the layer made of the liquid crystal polymer is 5 to 100 μm. A method for producing the stretched liquid crystal polymer film according to claim 6 or 7,
In the second step, the stretching speed of the laminated film is set to 500%/min to 10,000%/min, and the temperature T inside the furnace of the stretching device is set to a temperature that satisfies the following formula (3):
T ₁ <T < T ₂ (3)
(In the above formula (3), _T1 is a temperature that is the melting point of the liquid crystal polymer minus 100° C., and _T2 is a temperature that is the melting point of the liquid crystal polymer plus 30° C.) A method for producing the stretched liquid crystal polymer film according to claim 6,
The method for producing a stretched liquid crystal polymer film, wherein the stretching of the laminated film in the second step is performed under conditions in which the value _A1 represented by the following (4) is 0.20 to 5.00.
A ₁ = furnace temperature T (° C.) / stretching speed (%/min) / thickness of unstretched liquid crystal polymer film (μm) × 100 (4) A method for producing the stretched liquid crystal polymer film according to claim 7,
In the second step, the laminated film is stretched under conditions in which a value _A2 represented by the following formula (5) is 0.20 to 5.00.
_A2 = furnace temperature T (°C) / stretching speed (%/min) / thickness of liquid crystal polymer layer (μm) × 100 (5) A method for producing the stretched liquid crystal polymer film according to claim 6 or 10,
The first step of the method for producing a stretched liquid crystal polymer film includes subjecting the bonding surface of the unstretched liquid crystal polymer film and the bonding surface of the support film to a surface treatment before bonding the support film to the unstretched liquid crystal polymer film. A method for producing the stretched liquid crystal polymer film according to claim 12,
The method for producing a stretched liquid crystal polymer film, wherein the surface treatment is one selected from the group consisting of a plasma treatment, a corona treatment, and a chemical treatment. A method for producing the stretched liquid crystal polymer film according to claim 6 or 7,
The second step comprises performing stretching at a temperature lower than the melting point of the liquid crystal polymer. A method for producing the stretched liquid crystal polymer film according to claim 6 or 10,
A method for producing a stretched liquid crystal polymer film, in which in the second step, a stretching load calculated by multiplying the tensile stress of the support film at the temperature during stretching by the cross-sectional area of the support film is equal to or greater than a stretching load calculated by multiplying the tensile stress of the unstretched liquid crystal polymer film at the temperature during stretching by the cross-sectional area of the unstretched liquid crystal polymer film. A method for producing the stretched liquid crystal polymer film according to claim 7 or 11,
A method for producing a stretched liquid crystal polymer film, in which in the second step, a stretching load calculated by multiplying the tensile stress of the layer made of the supporting polymer at the temperature during stretching by the cross-sectional area of the layer made of the supporting polymer is equal to or greater than a stretching load calculated by multiplying the tensile stress of the layer made of the liquid crystal polymer at the temperature during stretching by the cross-sectional area of the layer made of the liquid crystal polymer. A method for producing the stretched liquid crystal polymer film according to claim 6 or 7,
The method for producing a stretched liquid crystal polymer film, wherein the supporting polymer is an aromatic polyether ketone or a polyester. A method for producing the stretched liquid crystal polymer film according to claim 17,
The method for producing a stretched liquid crystal polymer film, wherein the polyester is at least one polymer selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate. A film layer comprising the stretched liquid crystal polymer film according to claim 1 or 2;
A laminate comprising: A circuit board comprising the laminate according to claim 19.

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