JP2021038281A - Biaxially stretched polyethylenenaphthalate film and method for producing biaxially stretched polyethylenenaphthalate film - Google Patents

Abstract

To provide a biaxially stretched polyethylenenaphthalate film that makes it possible to produce a flexible circuit board having excellent via connection reliability.SOLUTION: A biaxially stretched polyethylenenaphthalate film has polyethylenenaphthalate as the main component, has a refractive index in thickness direction of 1.499-1.512, has a dielectric loss tangent at 5.0 GHz of 0.007 or less and a relative dielectric constant at 5.0 GHz of 3.20 or less, and is used for a flexible circuit board that transmits a high frequency signal of 6.0 GHz or more.SELECTED DRAWING: None

Description

The present invention relates to a biaxially stretched polyethylene naphthalate film and a method for producing a biaxially stretched polyethylene naphthalate film.

In recent years, in the advanced information age, the transmission speed of mobile communication has been exponentially increased and increased in frequency. Millimeter-wave band radio waves are used in so-called fifth-generation mobile communications. Such a high-frequency circuit board is required to significantly reduce transmission loss in order to suppress attenuation of output signals and ensure good communication quality.

The transmission loss mainly consists of a dielectric loss caused by a dielectric and a conductor loss caused by a conductor. The dielectric loss decreases as the dielectric constant, dielectric loss tangent (tan δ), etc. of the dielectric become smaller. Is known, and the dielectric constant or dielectric loss tangent of a dielectric is reduced to reduce the dielectric loss.

Various types of low transmission loss materials have been developed. For example, a thermoplastic liquid crystal on a metal foil, which can reduce the transmission loss of an electric signal in a high frequency circuit and has high adhesion between a metal foil and a resin. A high-frequency circuit board made of a laminated body in which a polymer film is laminated has been proposed (see, for example, Patent Document 1). Further, a circuit board having high reliability, particularly excellent hygroscopicity characteristics and high frequency characteristics, has also been proposed (see, for example, Patent Document 2).

Japanese Unexamined Patent Publication No. 2011-216598 Japanese Unexamined Patent Publication No. 2002-171466

Examples of the low-loss dielectric substrate material include a liquid crystal polymer film, a fluororesin film, and a modified polyimide film. The liquid crystal polymer has an extremely small dielectric loss tangent and is excellent in transmission characteristics, but it is difficult to control orientation crystallization during film formation and uneven thickness is likely to occur. Fluororesin also has a very low dielectric constant and dielectric loss tangent and is excellent in transmission characteristics, but has a problem that it is difficult to adhere to copper foil, other materials, etc., and it is difficult to laminate with these materials. The modified polyimide has good dielectric properties in an absolutely dry state, but has a high water absorption rate and deteriorates in dielectric properties when it absorbs moisture in the atmosphere.

Circuit boards for mobile devices, which are remarkably required to be miniaturized, are becoming more densely packed and multi-layered, and the performance required for vias connecting the wiring between layers is becoming stricter. Especially in high-speed communication, the communication quality deteriorates due to the parasitic elements of the vias, so the processing accuracy of the vias, the thermal dimensional stability, and the like are very important factors. If the via length, that is, the film thickness is not uniform, it causes signal disturbance. Further, when the substrate expands and contracts due to changes in temperature, humidity, etc., not only the signal quality is adversely affected, but also a repeated stress load is applied to the plating layer on the inner surface of the via, which eventually leads to disconnection. Therefore, the plating layer on the inner surface of the via is repeatedly stressed with the passage of time, which may cause problems such as cracking and disconnection, and it is desired to improve the connection reliability of the via. There is.

The uniformity of the film thickness and the thermal dimensional stability in the thickness direction strongly depend on the molecular orientation state of the film, but the molecular orientation of the liquid crystal polymer film, the fluororesin film, the modified polyimide film, etc. is difficult to control. It is known that the film is formed with a large amount of variation in thickness. Therefore, it is preferable to apply a biaxially stretched film such as polyolefin, polyamide, polyester, which is a semi-crystalline polymer that can precisely control the molecular orientation and has excellent thickness uniformity, and among them, heat resistance, low water absorption, and processing characteristics. Polyester having excellent properties such as is preferable.

An object of the present invention is to provide a biaxially stretched polyethylene naphthalate film capable of producing a flexible circuit board having excellent via connection reliability, and a method for producing the same.

Specific means for achieving the above-mentioned problems are as follows.
<1> Polyethylene naphthalate is the main component, the refractive index in the thickness direction is 1.499 to 1.512, the dielectric loss tangent at 5.0 GHz is 0.007 or less, and the relative permittivity at 5.0 GHz is A biaxially stretched polyethylene naphthalate film for use in a flexible circuit board that transmits a high frequency signal of 3.20 or less and 6.0 GHz or more.
<2> The biaxially stretched polyethylene naphthalate film according to <1>, wherein the heat shrinkage after heat treatment at 200 ° C. for 10 minutes is 1.0% or less in both the vertical and horizontal directions.

<3> A resin layer made of the biaxially stretched polyethylene naphthalate film according to <1> or <2> and a copper foil layer made of copper foil arranged on one main surface of the resin layer are provided. Laminated body.

<4> The method for producing a biaxially stretched polyethylene naphthalate film according to <1> or <2>, wherein an unstretched polyethylene naphthalate film is prepared and the unstretched polyethylene naphthalate film is vertically formed. The step of biaxially stretching the biaxially stretched polyethylene naphthalate film independently under the condition that the stretching ratio and the lateral stretching ratio are 3.5 times or less, and heating the biaxially stretched polyethylene naphthalate film under the condition that the maximum temperature of the heat fixing zone is 250 ° C. or more. A method for producing a biaxially stretched polyethylene naphthalate film, which comprises the steps of
<5> Production of the biaxially stretched polyethylene naphthalate film according to <4>, further comprising a step of post-treating the biaxially stretched polyethylene naphthalate film at a temperature of 200 ° C. or higher after the heating step. Method.

According to the present invention, it is possible to provide a biaxially stretched polyethylene naphthalate film capable of producing a flexible circuit board having excellent via connection reliability, and a method for producing the same.

In the present disclosure, the numerical range represented by using "~" means a range including the numerical values before and after "~" as the lower limit value and the upper limit value.
In the numerical range described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range described stepwise. .. Further, in the numerical range described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

<Biaxially stretched polyethylene naphthalate film>
The biaxially stretched polyethylene naphthalate film (hereinafter, also simply referred to as “PEN film”) of the present disclosure contains polyethylene naphthalate as a main component and has a refractive index in the thickness direction of 1.499 to 1.512. It is used for a flexible circuit board that transmits a high-frequency signal of 6.0 GHz or more, having a dielectric constant tangent of 0.007 or less at 5.0 GHz and a relative permittivity of 3.20 or less at 5.0 GHz.

By using the PEN film of the present disclosure, it is possible to manufacture a flexible circuit board having excellent via connection reliability. Since the PEN film of the present disclosure has a refractive index of 1.499 or more in the thickness direction, the molecular orientation in the plane direction orthogonal to the thickness direction is not too large, and the refractive index in the thickness direction is 1.512 or less. Therefore, the flexibility and heat resistance of the PEN film can be ensured. Since the molecular orientation in the plane direction orthogonal to the thickness direction is not too large, the thermal expansion in the thickness direction of the PEN film is small and the thermal dimensional stability in the thickness direction is excellent. As a result, it is considered that the flexible circuit board manufactured by using the PEN film is less likely to be disconnected and has excellent via connection reliability. Further, the PEN film of the present disclosure has a dielectric loss tangent at 5.0 GHz of 0.007 or less and a relative permittivity of 3.20 or less at 5.0 GHz, so that transmission loss of a high frequency signal can be reduced. , 6.0 GHz or higher can be suitably used for a flexible circuit board that transmits a high frequency signal. The reason for this is considered as follows. Since the PEN film of the present disclosure has a small dielectric loss tangent and relative permittivity at 5.0 GHz, the dielectric loss tangent and relative permittivity can be reduced even at a high frequency of 6.0 GHz or higher. As a result, it is possible to suitably transmit the high frequency signal while reducing the transmission loss of the high frequency signal of 6.0 GHz or more.
The upper limit of the high frequency signal transmitted by the PEN film of the present disclosure is preferably 60 GHz or less, more preferably 40 GHz or less, and further preferably 30 GHz or less from the viewpoint of the magnitude of practical transmission loss.

Polyethylene naphthalate, which is a main component of the PEN film of the present disclosure, is obtained, for example, by polycondensation of naphthalenedicarboxylic acid and ethylene glycol. As the polyethylene naphthalate, polyethylene-2,6-naphthalene dicarboxylate is particularly preferable. Polyethylene-2,6-naphthalenedicarboxylate is obtained, for example, by polycondensation of 2,6-naphthalenedicarboxylic acid and ethylene glycol. In the manufacture of flexible printed circuit boards, they are often exposed to high temperature environments due to thermosetting, prebaking, etc. of the bonding sheets between layers. On the other hand, by using polyethylene naphthalate, which has a high glass transition temperature, as a main component, the thermal dimensional stability of the film can be improved.

In the PEN film of the present disclosure, the content of polyethylene naphthalate is preferably 50% by mass or more, more preferably 70% by mass or more, and preferably 90% by mass or more with respect to the total amount of the film. The PEN film may be substantially made of polyethylene naphthalate, may be 99% by mass or less, may be 97% by mass or less, or may be 95% by mass or less. The polyethylene naphthalate contained in the PEN film of the present disclosure may be one kind or two or more kinds.

The polyethylene naphthalate which is a main component of the PEN film of the present disclosure may be a polymer consisting of only a structural unit derived from a naphthalenedicarboxylic acid and a structural unit derived from ethylene glycol, other than naphthalenedicarboxylic acid and ethylene glycol. It may be a polymer further containing a structural unit derived from another monomer.

Constituent units derived from monomers other than naphthalenedicarboxylic acid and ethylene glycol include constituent units derived from diol components (excluding ethylene glycol) such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, adipic acid, and sebacic acid. Examples thereof include constituent units derived from dicarboxylic acid components (excluding dicarboxylic acid naphthalene acid) such as phthalic acid, isophthalic acid, terephthalic acid, and 5-sodium sulfoisophthalic acid.

The content of structural units derived from monomers other than naphthalene dicarboxylic acid and ethylene glycol in polyethylene naphthalate is preferably 10 mol% or less, more preferably 5 mol% or less, based on all the structural units.

Polyethylene naphthalate can be produced by applying a known method. For example, a naphthalenedicarboxylic acid and ethylene glycol are subjected to an esterification reaction with at least one selected from the group consisting of a diol component (excluding ethylene glycol) and a dicarboxylic acid component (excluding dicarboxylic acid naphthalene acid), if necessary. Then, a method is known in which the reaction product obtained is subjected to a polycondensation reaction to obtain a polyester. Further, it may be produced by a method in which derivatives of these raw material monomers are subjected to a transesterification reaction, and then the obtained reaction product is subjected to a polycondensation reaction to obtain a polyester.

Further, for the purpose of improving the adhesion between the film-like substance described later and the rotary cooling drum, quaternary phosphonium sulfonic acid having an ester-forming functional group may be added to naphthalene dicarboxylic acid, ethylene glycol, etc. and subjected to an esterification reaction. Good. The amount of the quaternary phosphonium sulfonic acid having an ester-forming functional group added is preferably 0.1 mmol% to 10 mmol% with respect to the bifunctional carboxylic acid component such as naphthalene dicarboxylic acid. As the quaternary sulfonic acid having an ester-forming functional group, for example, a tetrabutylphosphonium salt of 3,5-dicarboxybenzenesulfonic acid is preferable.

The PEN film of the present disclosure is mainly composed of polyethylene naphthalate, but may contain additives as long as the object of the present invention is achieved. Examples of the additive include a lubricant, an ultraviolet absorber, an antioxidant, an antistatic agent, a light stabilizer, a heat stabilizer and the like.

Examples of the lubricant include inorganic particles such as calcium carbonate, silica, talc and clay, organic particles such as silicone, thermoplastic resin and thermosetting resin, and inert particles such as pigments such as barium sulfate and titanium oxide. The lubricant may be used alone or in combination of two or more.

When the PEN film of the present disclosure contains an additive, the content of the additive is preferably 5% by mass or less, more preferably 3% by mass or less, still more preferably 1% by mass or less, based on the total amount of the PEN film.

The PEN film of the present disclosure has a refractive index in the thickness direction of 1.499 to 1.512. When the above-mentioned refractive index is 1.499 or more, an increase in the coefficient of thermal expansion in the thickness direction is suppressed, an increase in the relative permittivity and the dielectric loss tangent is suppressed, and a flexible circuit manufactured using a PEN film is used. Excellent connection reliability of vias for the board. Further, when the above-mentioned refractive index is 1.512 or less, the orientation in the plane direction of the film is excellent, and the film is excellent in flexibility and heat resistance.

The PEN film of the present disclosure preferably has a refractive index in the thickness direction of 1.501 to 1.511, more preferably 1.503 to 1.511, and 1.503 to 1.510. It is more preferable, and it is particularly preferable that it is 1.505 to 1.508.

The PEN film of the present disclosure has a dielectric loss tangent of 0.007 or less at 5.0 GHz. Dielectric loss can be suppressed when the above-mentioned dielectric loss tangent is 0.007 or less. The dielectric loss tangent at 5.0 GHz is preferably 0.006 or less. Further, the dielectric loss tangent at 5.0 GHz may be 0.001 or more, or 0.003 or more.

The PEN film of the present disclosure has a relative permittivity of 3.20 or less at 5.0 GHz. When the above-mentioned relative permittivity is 3.20 or less, the dielectric loss can be suppressed. The relative permittivity at 5.0 GHz is preferably 3.16 or less. The relative permittivity at 5.0 GHz may be 2.70 or more, or 2.90 or more.

In the PEN film of the present disclosure, the refractive index, dielectric loss tangent and relative permittivity in the thickness direction are values measured by the method described in Examples described later.

The thickness of the PEN film of the present disclosure is not particularly limited, and may be, for example, 25 μm to 250 μm or 30 μm to 200 μm. When the thickness of the PEN film is 25 μm or more, the handleability during processing tends to be excellent. Since the thickness of the PEN film is 250 μm or less, the flexibility of the flexible circuit board tends to be excellent.
In the present disclosure, the thickness of the PEN film means the center thickness.

The heat shrinkage rate of the PEN film of the present disclosure in the longitudinal direction (mechanical direction) is preferably 0.01% to 2.0% after heat treatment at 200 ° C. for 10 minutes, and heat treatment at 150 ° C. for 30 minutes. Later, it is preferably 0.01% to 1.0%. Further, the above-mentioned heat shrinkage rate in the vertical direction is preferably 1.0% or less, preferably 0.8% or less, from the viewpoint that a fine circuit can be formed after heat treatment at 200 ° C. for 10 minutes. Is more preferable.

The lateral heat shrinkage of the PEN film of the present disclosure is preferably −0.5% to 2.0% after heat treatment at 200 ° C. for 10 minutes, and after heat treatment at 150 ° C. for 30 minutes. It is preferably −0.1% to 1.0%. Further, the above-mentioned heat shrinkage rate in the lateral direction is preferably 1.0% or less, preferably 0.8% or less, from the viewpoint that a fine circuit can be formed after heat treatment at 200 ° C. for 10 minutes. Is more preferable, 0.5% or less is further preferable, and 0.3% or less is particularly preferable.

When the heat shrinkage rate in the vertical direction and the horizontal direction of the PEN film of the present disclosure is equal to or higher than the above-mentioned lower limit value, it is easy to maintain the pattern of the circuit formed by using the PEN film, and a higher definition circuit pattern is formed. And the device can be miniaturized. In addition, deformation such as wrinkles and waviness is unlikely to occur after processing the PEN film. On the other hand, when the heat shrinkage rate in the vertical direction and the horizontal direction of the PEN film of the present disclosure is equal to or higher than the above-mentioned upper limit value, the flatness of the circuit board formed by using the PEN film is excellent.

The heat shrinkage of the PEN film of the present disclosure in the vertical and horizontal directions after heat treatment at 200 ° C. for 10 minutes can be determined by the method described in Examples. The heat shrinkage rate of the PEN film of the present disclosure in the vertical and horizontal directions after heat treatment at 150 ° C. for 30 minutes is a method in which the heating temperature and heating time conditions described in Examples are replaced with the conditions of 150 ° C. and 30 minutes. Can be obtained by.

It is preferable that the difference between the heat shrinkage rate of the PEN film in the vertical direction and the horizontal direction and the difference between the heat shrinkage rate of the central portion and the heat shrinkage rate of the edge portion of the PEN film are small. The proper balance of these differences also depends on the conditions of use and is therefore adjusted appropriately according to the application.

<Manufacturing method of biaxially stretched polyethylene naphthalate film>
In the method for producing a biaxially stretched polyethylene naphthalate film of the present disclosure, the step of preparing an unstretched polyethylene naphthalate film and the longitudinal stretching ratio and the transverse stretching ratio of the unstretched polyethylene naphthalate film are independent of each other. It includes a step of biaxially stretching under the condition of 5 times or less, and a step of heating the biaxially stretched polyethylene naphthalate film under the condition that the maximum temperature of the heat fixing zone is 250 ° C. or higher.

(Preparation process)
In the manufacturing method of the present disclosure, an unstretched polyethylene naphthalate film is prepared (preparation step). In the unstretched polyethylene naphthalate film, for example, as described above, polyethylene naphthalate obtained by polycondensation of naphthalenedicarboxylic acid and ethylene glycol is supplied to an extruder, and a melt kneaded and melted by the extruder is produced. It is obtained by discharging from a T-die to form a film, and quenching the film-like material.

The intrinsic viscosity of polyethylene naphthalate used for producing an unstretched polyethylene naphthalate film is preferably 0.40 dl / g or more at 35 ° C. in o-chlorophenol, and is 0.40 dl / g to 0.90 dl. It is more preferably / g. When the above-mentioned intrinsic viscosity is 0.40 dl / g or more, cutting is unlikely to occur during film production, and the strength of the printed circuit board after molding tends to be excellent. Further, since the above-mentioned intrinsic viscosity is 0.90 dl / g or less, the productivity at the time of polymerization is excellent, the melt viscosity does not become too high, and the film tends to be easily produced.

The temperature at which polyethylene naphthalate is kneaded and melted in an extruder is preferably, for example, at least its melting point (hereinafter, also referred to as Tm) and at least (Tm + 70) ° C.

The film-like material discharged from the T-die may be rapidly cooled on the surface of the rotary cooling drum. At this time, an electrostatic charge may be applied to the film-like material for the purpose of improving the adhesion between the film-like material and the rotary cooling drum. When a static charge is applied to a film-like material, for example, a sulfon having an ester-forming functional group of 0.1 mmol% to 10 mmol% with respect to a bifunctional carboxylic acid component such as naphthalenedicarboxylic acid which is a raw material of polyethylene naphthalate. It is preferable to add quaternary phosphonium acid to produce polyethylene naphthalate. As the quaternary sulfonic acid having an ester-forming functional group, for example, a tetrabutylphosphonium salt of 3,5-dicarboxybenzenesulfonic acid is preferable. Since the melt of polyethylene naphthalate has high electrical resistance, electrostatic adhesion with the above-mentioned rotary cooling drum may be insufficient, while the above-mentioned quaternary phosphonium sulfonic acid having an ester-forming functional group is used. As a result, the adhesion between the film-like material and the rotary cooling drum can be improved.

(Stretching process)
In the production method of the present disclosure, an unstretched polyethylene naphthalate film is biaxially stretched independently under the condition that the longitudinal stretching ratio and the transverse stretching ratio are 3.5 times or less (stretching step).

The unstretched polyethylene naphthalate film is, for example, preheated on a roll heated to a temperature of 100 to 160 ° C., preferably 110 ° C. to 140 ° C., and then vertically preferably on a heated infrared heater. It is stretched at a stretching ratio of 5 times or less, more preferably 2.5 times to 3.5 times, still more preferably 2.8 times to 3.3 times.

The polyethylene naphthalate film stretched in the longitudinal direction is preferably 3.5 times or less, more preferably 2.5 times to 3.5 times, still more preferably 3. It is stretched at a stretching ratio of 0 to 3.5 times. When stretching in the lateral direction, the temperature may be raised during the stretching process. The temperature rise in the transverse stretching process may be continuous or stepwise (sequential), and it is preferable to raise the temperature stepwise. For example, the lateral stretching zone of the stenter may be divided into a plurality of zones along the film traveling direction, and the temperature may be gradually raised by flowing a heating medium having a predetermined temperature for each zone.

When the stretching ratio in the lateral direction is 2.5 times or more, the strength of the PEN film as a circuit board tends to be sufficiently obtained. When the stretching ratio in the lateral direction is 3.5 times or less, it is easy to adjust the range of the refractive index in the thickness direction of the PEN film of the present disclosure, breakage during film production is unlikely to occur, and the thickness of the PEN film is increased. Roughness is less likely to occur, and the thickness uniformity of the PEN film tends to be excellent.

The horizontal stretching ratio (horizontal stretching ratio / longitudinal stretching ratio) with respect to the vertical stretching ratio is preferably 1.06 times to 1.20 times. These stretchings may be multi-stage stretching performed by dividing into a plurality of stages, or simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed at the same time.

(Heating process)
In the manufacturing method of the present disclosure, the biaxially stretched polyethylene naphthalate film is heated under the condition that the maximum temperature of the heat fixing zone is 250 ° C. or higher (heating step).

By subjecting the biaxially stretched polyethylene naphthalate film to a heating step, a PEN film having the above-mentioned refractive index in the thickness direction and further excellent in thermal dimensional stability can be obtained.

The temperature of the heat fixing zone is preferably (Tm-100) ° C. or higher, and more preferably (Tm-70) ° C. to (Tm-10) ° C.

Further, from the viewpoint of reducing the heat shrinkage rate of the PEN film after heat treatment at 200 ° C., at least one of the vertical direction and the horizontal direction is obtained when the maximum temperature of the heat fixing zone is 250 ° C. or higher, or after heating. In addition, it is preferable to perform the heat relaxation treatment in the range of the relaxation rate of 0.1% to 15%.

In the PEN film, there is a correlation between the refractive index in the thickness direction and the molecular orientation in the plane direction, and the lower the molecular orientation in the plane direction, the higher the refractive index in the thickness direction tends to be. Tends to be excellent in thermal dimensional stability and dielectric properties.

As a means for lowering the molecular orientation in the plane direction, for example, there are two types, a method of lowering the draw ratio and a method of relaxing the molecular chain after the drawing step of once stretching at a normal ratio. Since the method of lowering the draw ratio tends to increase the thickness variation, a method of performing a relaxation treatment after the drawing step is preferable. In the PEN film, the range of thickness unevenness is less than ± 10%, more preferably less than ± 5% of the center thickness.

(Post-treatment process)
The production method of the present disclosure may further include a step of post-treating the biaxially stretched polyethylene naphthalate film at a temperature of 200 ° C. or higher after the above-mentioned heating step. This makes it possible to further reduce the relative permittivity of the PEN film. The detailed reason for the decrease in the relative permittivity of the PEN film is unknown, but it is considered that the reason is an increase in crystallinity, the influence of local orientation of molecules, and the like.

The method of post-treatment is not particularly limited, and for example, a suspension type relaxation heat treatment method is preferable. As a suspension type relaxation heat treatment method, the PEN film to be treated is hung down by its own weight through a roller installed above, heated in the middle, and then turned almost horizontally while being cooled by the lower roller. A method of winding after blocking the winding tension with a nip roller is preferable.

In the suspension type relaxation heat treatment method, the hanging distance is preferably about 2 m to 10 m. When the hanging distance is 2 m or more, the own weight can be secured, the flatness is excellent, and the heating range can be secured, so that a sufficient relaxing effect can be obtained. When the hanging distance is 10 m or less, it is suppressed that the self-weight becomes too large, and the variation in the heat shrinkage rate depending on the position of the heating region is suppressed.

The heating method of the PEN film in the post-treatment step is not particularly limited, and infrared heating is preferable because the PEN film can be heated quickly.

In the post-treatment step, it is preferable to heat the PEN film so that the temperature becomes 200 to 240 ° C. By heating the temperature of the PEN film to 200 ° C. or higher, the heat shrinkage rate of the PEN film after heat treatment at 200 ° C. can be reduced, and the temperature of the PEN film is heated to 240 ° C. or lower. By doing so, a PEN film having excellent flatness and suppressed whitening due to oligomer precipitation can be obtained.
The film temperature can be measured using a non-contact infrared thermometer (for example, a Burns type radiation thermometer).

Among the heat treatment methods, the suspension type relaxation heat treatment method is preferable because it is easy to more uniformly suppress the heat shrinkage rate in a wide range of the PEN film.

<Laminated body>
The laminate of the present disclosure includes a resin layer made of a biaxially stretched polyethylene naphthalate film and a copper foil layer made of a copper foil arranged on one main surface of the resin layer. By using the laminate of the present disclosure, it is possible to manufacture a flexible circuit board which transmits a high frequency signal of 6.0 GHz or more and has excellent via connection reliability.

The copper foil used for forming the laminate is not particularly limited, and a conventionally known copper foil may be used. Above all, it is preferable to use a copper foil having high surface smoothness from the viewpoint of reducing conductor loss. For example, the copper foil preferably has a surface roughness Rz of 2 μm or less from the viewpoint of excellent surface smoothness. For example, the copper foil described in Japanese Patent No. 3155920 and Japanese Patent Application Laid-Open No. 2019-51709 may be used. A commercially available product may be used as the copper foil, and examples thereof include "TQM4-VSP" manufactured by Mitsui Mining & Smelting Co., Ltd.
In the present disclosure, the surface roughness Rz is a parameter in the height direction called "maximum height roughness", and is the peak height of the roughness curve at the reference length specified in JIS B 0601: 2013. Is the sum of the maximum value of and the maximum value of the valley depth.

The resin layer and the copper foil layer may be in contact with each other and directly laminated, or the resin layer and the copper foil layer may be laminated via another layer such as an adhesive layer.

When an adhesive layer is provided between the resin layer and the copper foil layer, the adhesive layer may be provided by using a conventionally known adhesive. Of these, an adhesive made of a material having a small dielectric constant, dielectric loss tangent, and the like is preferable. Further, the higher the adhesion with the PEN film and the copper foil, the more preferable, and for example, the adhesion with 0.5 N / 10 mm or more is preferable. As the adhesive, for example, the adhesive described in Japanese Patent No. 6485577 may be used. A commercially available product may be used as the adhesive, and for example, "Aronmighty AF-700" manufactured by Toagosei Co., Ltd. may be used.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples. In addition, it is of course possible to carry out with appropriate modifications within the range that can be adapted to the gist of the present invention, and all of them are included in the technical scope of the present invention.

In this example, each physical property value was measured as follows.

(Refractive index)
The refractive index of the biaxially stretched polyethylene naphthalate film was measured using an Abbe refractometer equipped with a polarizing plate using a sodium D line (589 nm) as a light source.

(Heat shrinkage after treatment at 200 ° C)
A 30 cm square film with a length of 30 cm square, which was previously measured and marked with an accurate length in the vertical and horizontal directions of the biaxially stretched polyethylene naphthalate film, was placed in an oven set at 200 ° C. without load for 10 minutes. After allowing to stand, it was taken out, returned to room temperature, and the dimensional change was read.
From the length (L0) before the heat treatment and the dimensional change amount (ΔL) due to the heat treatment, the heat shrinkage rates in the vertical direction and the horizontal direction were obtained according to the following equation (1), respectively. The heat shrinkage rate in each direction was evaluated with the number of samples n = 5, and the average value was used.
Heat shrinkage rate (%) = (ΔL / L0) × 100 ... (1)

(Relative permittivity and dielectric loss tangent)
The relative permittivity and dielectric loss tangent at 5.0 GHz of the biaxially stretched polyethylene naphthalate film were measured by the cylindrical cavity resonator method.

(Cycle test)
The heat cycle test was carried out using the laminate according to the thermal shock test method of JIS C5016: 1994. The temperature was −55 ° C. to 100 ° C., the number of cycles was 100, and the via continuity after the test was evaluated.
Evaluation A was given when there was no poor continuity in the measurement of 10 samples, and evaluation B was given when there was no poor continuity in one or more samples.

[Example 1]
(Synthesis of polymer)
To a mixture of 100 parts by mass of dimethyl 2,6-naphthalenedicarboxylic acid and 60 parts by mass of ethylene glycol, 0.03 parts by mass of manganese acetate / tetrahydrate was added, and transesterification was performed while gradually raising the temperature from 150 ° C. to 240 ° C. The reaction was carried out. When the reaction temperature reached 170 ° C. on the way, 0.024 parts by mass of antimony trioxide was added, 0.15% by mass of porous silica having an average particle size of 0.3 μm was added, and then 220 ° C. was reached. At this time, 0.042 parts by mass (corresponding to 2 mmol%) of tetrabutylphosphonium salt of 3,5-dicarboxybenzenesulfonic acid was added. Subsequently, a transesterification reaction was carried out, and 0.023 parts by mass of trimethyl phosphate was added after the transesterification reaction was completed. After that, the reaction product was transferred to a polymerization reactor, the temperature was raised to 290 ° C., a polycondensation reaction was carried out under a high vacuum of 0.2 mmHg or less, and the intrinsic viscosity measured with an o-chlorophenol solution at 25 ° C. was 0. A polyethylene-2,6-naphthalenedicarboxylate polymer having 61 dl / g and a DEF copolymerization amount of 1.1 mol% was obtained.

(Manufacture of biaxially stretched polyethylene naphthalate film)
After drying this polymer at 170 ° C. for 6 hours, it was supplied to a single-screw extruder, melted at a melting temperature of 302 ° C., filtered through a non-woven fabric filter having an average opening of 24 μm made of fine stainless steel wire having a wire diameter of 13 μm, and then filtered. The film was discharged from a slit die and extruded onto a rotary cooling drum having a surface finish of 0.3S and a surface temperature of 60 ° C. to obtain an unstretched film. A wire electrode to which a direct current voltage of 6.5 kV was applied was placed close to the molten polymer in contact with the rotary cooling drum to promote adhesion to the rotary cooling drum due to static electricity. The unstretched film thus obtained was passed through a preheating roller preheated to 140 ° C. and stretched 3.0 times in the vertical direction on an infrared heater heated to a temperature of 600 ° C. or higher to obtain a vertically stretched film.

Next, this longitudinally stretched film was continuously passed through a stenter stretching machine, and stretched 3.3 times in the lateral direction in a four-step temperature raising zone of 110 ° C., 117 ° C., 130 ° C., and 145 ° C. Furthermore, the heat fixing zones set at 220 ° C. and 250 ° C. are continuously passed in this order, and while the heat fixing treatment is performed for 20 seconds in the heat fixing zone at 250 ° C., 0.7% relaxation treatment is performed in the lateral direction. , A biaxially stretched polyethylene naphthalate film having a thickness of 50 μm was obtained by passing through a cooling zone at 135 ° C. and winding with a winder.
Further, the obtained stretched film was further heat-treated at 230 ° C. for 5 minutes using a suspension type relaxation heat treatment apparatus.

[Example 2]
A biaxially stretched polyethylene naphthalate film was obtained in the same manner as in Example 1 except that the heat treatment using the above-mentioned relaxation heat treatment apparatus was not performed after the biaxial stretching.

[Comparative Example 1]
The same as in Example 2 except that the stretching ratio was 3.7 times in the vertical direction and 3.8 times in the horizontal direction, the temperature of the heat fixing zone was 224 ° C. and 244 ° C., respectively, and the relaxation rate in the heat fixing zone was 0%. A biaxially stretched polyethylene naphthalate film was obtained.

(Manufacturing of laminate)
A thermosetting adhesive sheet "Alonmighty AF-700" (manufactured by Toagosei Co., Ltd.) was temporarily bonded to both sides of the obtained biaxially stretched polyethylene naphthalate films of Examples 1 and 2 and Comparative Example 1 at 100 ° C. , A copper foil "TQ-M4-VSP" (manufactured by Mitsui Metal Mining Co., Ltd.) having a thickness of 18 μm was laminated, and then heat-pressed at 180 ° C. and 1 MPa for 30 minutes to prepare a flexible copper-clad laminate.
A via having a diameter of 0.1 mm was formed on the obtained copper-clad laminate with a drill, the surface was acid-cleaned, and then the inner surface of the via was plated with copper by electroless plating to produce a laminate.

Table 1 shows the evaluation results of each physical property of the biaxially stretched polyethylene naphthalate film and the laminate of Examples 1 and 2 and Comparative Example 1.

As shown in Table 1, in Examples 1 and 2, a biaxially stretched polyethylene naphthalate film capable of producing a flexible circuit board having better cycle test results and excellent via connection reliability than Comparative Example 1. was gotten.
As shown in Table 1, in Examples 1 and 2, the numerical value of the heat shrinkage rate after the treatment at 200 ° C. was smaller than that in Comparative Example 1. As a result, it was found that the biaxially stretched polyethylene naphthalate films obtained in Examples 1 and 2 are resistant to heat shrinkage and are suitable for producing fine or small flexible circuit boards.

Claims (5)

Polyethylene naphthalate is the main constituent,
The refractive index in the thickness direction is 1.499 to 1.512.
The dielectric loss tangent at 5.0 GHz is 0.007 or less, and the relative permittivity at 5.0 GHz is 3.20 or less.
A biaxially stretched polyethylene naphthalate film for use in a flexible circuit board that transmits high-frequency signals of 6.0 GHz or higher. The biaxially stretched polyethylene naphthalate film according to claim 1, wherein the heat shrinkage after heat treatment at 200 ° C. for 10 minutes is 1.0% or less in both the vertical and horizontal directions. The resin layer made of the biaxially stretched polyethylene naphthalate film according to claim 1 or 2.
A copper foil layer made of copper foil arranged on one main surface of the resin layer,
Laminated body comprising. The method for producing a biaxially stretched polyethylene naphthalate film according to claim 1 or 2.
The process of preparing an unstretched polyethylene naphthalate film and
A step of biaxially stretching the unstretched polyethylene naphthalate film under the condition that the longitudinal stretching ratio and the transverse stretching ratio are each independently 3.5 times or less.
A step of heating the biaxially stretched polyethylene naphthalate film under the condition that the maximum temperature of the heat fixing zone is 250 ° C. or higher, and
A method for producing a biaxially stretched polyethylene naphthalate film, which comprises. The method for producing a biaxially stretched polyethylene naphthalate film according to claim 4, further comprising a step of post-treating the biaxially stretched polyethylene naphthalate film at a temperature of 200 ° C. or higher after the heating step.