JP7688903B2 - Film for flexible printed wiring boards - Google Patents

Description

The present invention relates to a film for flexible printed wiring boards that has excellent dielectric properties and excellent interlayer adhesion, and a laminate containing the same.

Electronic devices such as information processing devices and communication devices generally incorporate circuit boards. Circuit boards usually have a substrate made of an insulating material and a layer of conductive material (hereinafter referred to as a conductor layer) formed on the substrate, and the circuit is formed by this conductor layer. Various electronic components are attached to the circuit board by processes such as soldering.

In response to the demand for smaller and thinner circuit boards used in small electronic devices such as mobile phones, development is underway for circuit boards (flexible printed wiring boards) that use flexible resin films as insulating substrates. Resin films are used in insulating base materials for conductor circuit boards, bonding sheets that join boards with conductor circuits formed on the surface of the base material, and coverlay films that are formed on the surface of circuit layers.

In recent years, as the speed of transmitted signals has increased, signals have become increasingly high frequency, and multilayer circuit boards with multiple conductor layers have come into widespread use.

Therefore, for flexible printed wiring boards, there is a demand for resin films that have a low dielectric tangent to suppress transmission loss in the high frequency range, and excellent interlayer adhesion to allow multiple conductor layers to be multi-layered.

Patent Document 1 describes a resin composition for flexible printed wiring boards that is made of a urethane resin, an epoxy resin, and a filler, and a sheet or film made from the composition. Patent Document 2 describes a flexible printed wiring board made from a liquid crystal polymer film.

International Publication No. WO 2016/129565 JP 2016-10967 A

However, the resin composition and its sheet or film described in Patent Document 1 have a high dielectric tangent and are not suitable for use in the high frequency range. In addition, the long heat curing time poses the problem of a long manufacturing process.

The flexible printed wiring board made of the liquid crystal polymer film described in Patent Document 2 requires lamination processing at high temperatures to melt the bonding sheet, which causes problems such as warping, voids, and thermal expansion in the flexible printed wiring board. In addition, although it has a relatively low dielectric tangent, it is insufficient for use in the high frequency range, and there is a demand for a material with an even lower dielectric tangent.

The object of the present invention is to provide a film for flexible printed wiring boards that has a low dielectric tangent and can be laminated at low temperatures. Another object of the present invention is to provide a laminate composed of the film for flexible printed wiring boards.

In view of the above problems, the inventors conducted intensive research and discovered that by using a liquid crystal polymer that satisfies certain conditions, a film for flexible printed wiring boards that has a low dielectric tangent and can be laminated at low temperatures can be obtained, thus completing the present invention.

［式中、
Ａｒ_１およびＡｒ_２は、それぞれ１種または２種以上の２価の芳香族基を表し、ｐ、ｑ、ｒおよびｓは、それぞれ、液晶ポリマー中での各繰返し単位の組成比（モル％）であり、以下の条件を満たす：
０．５≦ｐ／ｑ≦２．５
０．１≦ｒ≦２５、および
０．１≦ｓ≦２５］
で表される繰返し単位を含む全芳香族液晶ポリエステルである、〔１〕に記載のフィルム。
〔３〕 式（ＩＩＩ）および／または式（ＩＶ）は、Ａｒ_１およびＡｒ_２が、互いに独立して、式（１）～（４）

からなる群から選択される芳香族基である繰返し単位の１種または２種以上を含む全芳香族液晶ポリエステルである、〔２〕に記載のフィルム。
〔４〕フレキシブルプリント配線板のボンディングシートである、〔１〕～〔３〕のいずれかに記載のフィルム。
〔５〕フレキシブルプリント配線板のカバーレイである、〔１〕～〔３〕のいずれかに記載のフィルム。
〔６〕〔１〕～〔５〕のいずれかに記載のフィルムおよび金属層を含む、積層体。
〔７〕〔１〕～〔５〕のいずれかに記載のフィルムおよび樹脂層を含む、積層体。 That is, the present invention includes the following preferred embodiments.
[1] A film for flexible printed wiring boards, comprising a liquid crystal polymer having a crystal melting temperature of 260° C. or lower and a dielectric loss tangent of 0.0015 or lower at 10 GHz as measured by a cavity resonator perturbation method.
[2] The liquid crystal polymer is represented by the formulas (I) to (IV)

[In the formula,
_Ar1 and _Ar2 each represent one or more divalent aromatic groups, and p, q, r, and s each represent a composition ratio (mol %) of each repeating unit in the liquid crystal polymer, and satisfy the following condition:
0.5≦p/q≦2.5
0.1≦r≦25, and 0.1≦s≦25]
The film according to [1], which is a wholly aromatic liquid crystal polyester containing a repeating unit represented by the following formula:
[3] In the formula (III) and/or formula (IV), Ar ₁ and Ar ₂ are each independently represented by any one of the formulas (1) to (4):

The film according to [2], which is a wholly aromatic liquid crystal polyester containing one or more repeating units which are aromatic groups selected from the group consisting of:
[4] The film according to any one of [1] to [3], which is a bonding sheet for a flexible printed wiring board.
[5] The film according to any one of [1] to [3], which is a coverlay for a flexible printed wiring board.
[6] A laminate comprising the film according to any one of [1] to [5] and a metal layer.
[7] A laminate comprising the film according to any one of [1] to [5] and a resin layer.

The flexible printed wiring board film of the present invention has a low dielectric tangent, can be laminated at low temperatures, and has excellent interlayer adhesion, making it suitable for use as a flexible printed wiring board material. The flexible printed wiring board film of the present invention is particularly suitable for use as a bonding sheet or coverlay.

The liquid crystal polymer used in the flexible printed wiring board film of the present invention is a liquid crystal polyester or liquid crystal polyester amide that forms an anisotropic molten phase and is called a thermotropic liquid crystal polymer by those skilled in the art.

The anisotropic melt phase properties of the liquid crystal polymer can be confirmed by conventional polarimetry using cross polarizers, ie, by observing a sample placed on a hot stage under a nitrogen atmosphere.
Examples of the repeating units constituting the liquid crystal polymer used in the present invention include aromatic oxycarbonyl repeating units, aromatic dicarbonyl repeating units, aromatic dioxy repeating units, aliphatic dicarbonyl repeating units, aliphatic dioxy repeating units, aromatic aminooxy repeating units, aromatic diamino repeating units, aromatic aminocarbonyl repeating units, and combinations thereof.

Specific examples of monomers that provide aromatic oxycarbonyl repeating units include aromatic hydroxycarboxylic acids such as 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 2-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, 5-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4'-hydroxyphenyl-4-benzoic acid, 3'-hydroxyphenyl-4-benzoic acid, 4'-hydroxyphenyl-3-benzoic acid, and the like, as well as their alkyl, alkoxy, or halogen-substituted derivatives, and their acylated derivatives, ester derivatives, acid halides, and other ester-forming derivatives. Among these, 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid are preferred because they allow the mechanical properties, heat resistance, crystal melting temperature, and moldability of the resulting liquid crystal polymer to be easily adjusted to appropriate levels.

Specific examples of monomers that provide aromatic dicarbonyl repeating units include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, and 4,4'-dicarboxybiphenyl, as well as their alkyl, alkoxy, or halogen-substituted derivatives, and their ester-forming derivatives such as ester derivatives and acid halides. Of these, terephthalic acid and 2,6-naphthalenedicarboxylic acid are preferred because they allow the mechanical properties, heat resistance, crystal melting temperature, and moldability of the resulting liquid crystal polymer to be easily adjusted to appropriate levels.

Specific examples of monomers that give aromatic dioxy repeating units include aromatic diols such as hydroquinone, resorcinol, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 4,4'-dihydroxybiphenyl, 3,3'-dihydroxybiphenyl, 3,4'-dihydroxybiphenyl, 4,4'-dihydroxybiphenyl ether, and their alkyl, alkoxy or halogen-substituted derivatives, as well as ester-forming derivatives such as acylated derivatives of these. Among these, hydroquinone and 4,4'-dihydroxybiphenyl are preferred because they are easy to adjust to appropriate levels the reactivity during polymerization, and the mechanical properties, heat resistance, crystal melting temperature, and moldability of the resulting liquid crystal polymer.

Specific examples of monomers that provide aliphatic dicarbonyl repeating units include, for example, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, tetradecanedioic acid, fumaric acid, maleic acid, cyclohexanedicarboxylic acid, and hexahydroterephthalic acid.

Specific examples of monomers that provide aliphatic dioxy repeating units include ethylene glycol, 1,4-butanediol, 1,6-hexanediol, and acylated products thereof.

Monomers that give aromatic aminooxy repeating units, aromatic diamino repeating units, and aromatic aminocarbonyl repeating units include aromatic hydroxyamines, aromatic diamines, and aromatic aminocarboxylic acids.

The liquid crystal polymer used in the present invention may contain aromatic oxydicarbonyl repeating units and thioester bonds, provided that the object of the present invention is not impaired. Examples of monomers that give thioester bonds include mercaptoaromatic carboxylic acids, aromatic dithiols, and hydroxyaromatic thiols. The amount of these monomers used is preferably 10 mol% or less based on the total amount of the aromatic oxycarbonyl repeating units, aromatic dicarbonyl repeating units, aromatic dioxy repeating units, aromatic aminooxy repeating units, aromatic diamino repeating units, and monomers that give aromatic aminocarbonyl repeating units.

Copolymers that combine these repeating units may or may not form an anisotropic molten phase, depending on the monomer composition and composition ratio, and the sequence distribution of each repeating unit in the copolymer. However, the liquid crystal polymers used in the present invention are limited to copolymers that form an anisotropic molten phase.

The liquid crystal polymer used in the present invention may be a blend of two or more liquid crystal polymers.

The crystalline melting temperature of the liquid crystal polymer used in the present invention, measured by a differential scanning calorimeter, is 260°C or less, preferably 150 to 250°C, more preferably 170 to 240°C, and even more preferably 190 to 230°C.

The crystalline melting temperature of the liquid crystal polymer is 260°C or lower, which improves low-temperature lamination processability when manufacturing flexible printed wiring boards, making it easier to suppress the occurrence of shape defects such as warping, voids, and thermal expansion in flexible printed wiring boards.

In this specification and claims, the "crystalline melting temperature" is determined from the crystalline melting peak temperature measured by a differential scanning calorimeter (hereinafter abbreviated as DSC) at a heating rate of 20°C/min. More specifically, after observing the endothermic peak temperature (Tm1) observed when a sample of a liquid crystal polymer is measured under heating conditions of 20°C/min from room temperature, the sample is held at a temperature 20 to 50°C higher than Tm1 for 10 minutes, and then cooled to room temperature under a cooling condition of 20°C/min, and the endothermic peak is observed when measured again under heating conditions of 20°C/min, and the temperature showing the peak top is taken as the crystalline melting temperature of the liquid crystal polymer. For example, an Exstar 6000 manufactured by Seiko Instruments Inc. can be used as a measuring device.

The liquid crystal polymer used in the present invention has a dielectric loss tangent at 10 GHz measured by the cavity resonator perturbation method of 0.0015 or less, preferably 0.0013 or less, and more preferably 0.0010 or less.

With a dielectric tangent of 0.0015 or less at 10 GHz, transmission loss in the high frequency range is suppressed, making the film suitable for flexible printed wiring boards for high-speed communication.

［式中、
Ａｒ_１およびＡｒ_２は、それぞれ１種または２種以上の２価の芳香族基を表し、ｐ、ｑ、ｒおよびｓは、それぞれ、全芳香族液晶ポリエステル中での各繰返し単位の組成比（モル％）であり、以下の条件を満たすものである：
０．５≦ｐ／ｑ≦２．５
０．１≦ｒ≦２５、および
０．１≦ｓ≦２５］ As the liquid crystal polymer used in the present invention, a wholly aromatic liquid crystal polyester is preferably used, and a wholly aromatic liquid crystal polyester containing the repeating units represented by the formulae (I) to (IV) is more preferably used.

[In the formula,
_Ar1 and _Ar2 each represent one or more divalent aromatic groups, and p, q, r and s each represent a composition ratio (mol %) of each repeating unit in the wholly aromatic liquid crystal polyester, and satisfy the following condition:
0.5≦p/q≦2.5
0.1≦r≦25, and 0.1≦s≦25]

The molar ratio (p/q) of the composition ratio p (mol%) in the above formula (I) to the composition ratio q (mol%) in the above formula (II) is preferably 0.5 to 2.5, more preferably 0.6 to 1.8, and even more preferably 0.8 to 1.6.

For the fully aromatic liquid crystal polyesters preferably used above, the total composition ratio of p and q is preferably 50 to 99.8 mol%, more preferably 60 to 96 mol%, and even more preferably 70 to 90 mol%.

For the fully aromatic liquid crystal polyesters preferably used above, the composition ratio p in formula (I) and the composition ratio q in formula (II) are each preferably 20 to 60 mol%, and more preferably 30 to 55 mol%.

In the wholly aromatic liquid crystal polyester preferably used in the present invention, by containing the repeating units represented by formula (I) and formula (II) at least in the above molar ratio (p/q), and optionally in the above composition ratio of the sum of p and q and/or the individual composition ratios of p and q (mol %), a wholly aromatic liquid crystal polyester exhibiting a crystalline melting temperature of 260°C or less can be preferably obtained.

In addition, for the wholly aromatic liquid crystal polyester preferably used in the present invention, the composition ratio r according to formula (III) and the composition ratio s according to formula (IV) are each preferably 0.1 to 25 mol%, more preferably 2 to 20 mol%, and even more preferably 5 to 15 mol%. It is preferable that r and s are equimolar amounts.

In the above repeating unit, for example, Ar ₁ (or Ar ₂ ) represents two or more kinds of divalent aromatic groups means that the wholly aromatic liquid crystal polyester contains two or more kinds of repeating units represented by formula (III) (or (IV)) according to the type of divalent aromatic group. In this case, the composition ratio r according to formula (III) (or the composition ratio s according to formula (IV)) represents the composition ratio of the two or more kinds of repeating units in total.

Specific examples of monomers that give the repeating unit represented by formula (I) include, for example, 4-hydroxybenzoic acid and its acylation products, ester derivatives, acid halides, and other ester-forming derivatives.

Specific examples of monomers that provide the repeating unit represented by formula (II) include 6-hydroxy-2-naphthoic acid and its acylated products, ester derivatives, acid halides, and other ester-forming derivatives.

Specific examples of monomers that provide the repeating unit represented by formula (III) include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, and 4,4'-dicarboxybiphenyl, as well as their alkyl, alkoxy, or halogen-substituted derivatives, and their ester derivatives, acid halides, and other ester-forming derivatives.

Specific examples of monomers that provide the repeating unit represented by formula (IV) include aromatic diols such as hydroquinone, resorcinol, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 4,4'-dihydroxybiphenyl, 3,3'-dihydroxybiphenyl, 3,4'-dihydroxybiphenyl, 4,4'-dihydroxybiphenyl ether, and their alkyl, alkoxy or halogen substituted derivatives, as well as acylated derivatives of these esters.

Among the wholly aromatic liquid crystal polyesters preferably used in the present invention, those in which Ar ₁ and Ar ₂ in the repeating units represented by formulas (III) and (IV) each independently contain one or more aromatic groups selected from the group consisting of aromatic groups represented by formulas (1) to (4) are more preferably used.

Among these, the aromatic groups represented by formula (1), formula (2) and formula (4) are particularly preferred as the repeating unit represented by formula (III), that is, terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid and their ester-forming derivatives are used as monomers that give these repeating units, because the mechanical properties, heat resistance, crystalline melting temperature and moldability of the resulting wholly aromatic liquid crystal polyester are easily adjusted to appropriate levels.

As the repeating unit represented by formula (IV), it is particularly preferred to use aromatic groups represented by formulas (1) and (3), i.e., hydroquinone and 4,4'-dihydroxybiphenyl and their ester-forming derivatives as monomers that give these repeating units, because this makes it easier to adjust the reactivity during polymerization and the mechanical properties, heat resistance, crystal melting temperature, and moldability of the resulting wholly aromatic liquid crystal polyester to appropriate levels.

In the above repeating unit, for example, Ar ₁ (or Ar ₂ ) containing two or more aromatic groups means that the repeating unit represented by formula (III) (or (IV)) contains two or more types according to the type of divalent aromatic group in the wholly aromatic liquid crystal polyester. That is, the wholly aromatic liquid crystal polyester of formula (III) and/or formula (IV) preferably contains one or more types of repeating units in which Ar ₁ and Ar ₂ are each independently an aromatic group selected from the group consisting of formulas (1) to (4). In this case, the composition ratio r according to formula (III) (or the composition ratio s according to formula (IV)) represents the composition ratio of the two or more types of repeating units in total.

In the wholly aromatic liquid crystal polyester preferably used in the present invention, the total composition ratio of the repeating units [p+q+r+s] is preferably 100 mol %, but other repeating units may also be contained within a range that does not impair the object of the present invention.

Examples of monomers that provide other repeating units constituting the wholly aromatic liquid crystal polyester preferably used in the present invention include other aromatic hydroxycarboxylic acids, aromatic hydroxyamines, aromatic diamines, aromatic aminocarboxylic acids, aromatic hydroxydicarboxylic acids, aromatic mercaptocarboxylic acids, aromatic dithiols, aromatic mercaptophenols, and combinations thereof.

Specific examples of other aromatic hydroxycarboxylic acids include, for example, 3-hydroxybenzoic acid, 2-hydroxybenzoic acid, 5-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4'-hydroxyphenyl-4-benzoic acid, 3'-hydroxyphenyl-4-benzoic acid, 4'-hydroxyphenyl-3-benzoic acid, and their alkyl, alkoxy or halogen substituted derivatives, as well as their acylated products, ester derivatives, acid halides and other ester-forming derivatives.

The total composition ratio of the repeating units provided by these other monomer components is preferably 10 mol % or less of the total repeating units.

The method for producing the liquid crystal polymer used in the present invention is described below.

There are no particular limitations on the method for producing the liquid crystal polymer used in the present invention, and any known polycondensation method that forms ester bonds or amide bonds from the combination of the above-mentioned monomers, such as the molten acidolysis method or the slurry polymerization method, can be used.

The molten acidolysis process is the preferred method for producing the liquid crystal polymers used in the present invention, in which the monomers are first heated to form a molten reactant liquid, which is then reacted to obtain a molten polymer. A vacuum may be applied to facilitate removal of volatile by-products (e.g., acetic acid, water, etc.) produced during the final stage of condensation.

Slurry polymerization is a process in which the reaction is carried out in the presence of a heat exchange fluid, and the solid product is obtained in a state suspended in the heat exchange medium.

In both the molten acidolysis method and the slurry polymerization method, the polymerizable monomer components used in producing the liquid crystal polymer can be subjected to the reaction at room temperature in a modified form in which the hydroxyl group and/or amino group is acylated, i.e., as a lower acylated product. The lower acyl group preferably has 2 to 5 carbon atoms, and more preferably has 2 or 3 carbon atoms. Particularly preferred is a method in which an acetylated product of the monomer is used in the reaction.

The acylated monomer may be prepared by separately acylation and synthesis, or may be produced in the reaction system by adding an acylating agent such as acetic anhydride to the monomer during the production of the liquid crystal polymer.

In either the molten acidolysis method or the slurry polymerization method, a catalyst may be used during the reaction, if necessary.

Specific examples of the catalyst include, for example, organic tin compounds (dialkyl tin oxides such as dibutyltin oxide, diaryl tin oxides, etc.), titanium dioxide, antimony trioxide, organic titanium compounds (alkoxy titanium silicates, titanium alkoxides, etc.), alkali and alkaline earth metal salts of carboxylic acids (potassium acetate, sodium acetate, etc.), Lewis acids ( _BF3 , etc.), gaseous acid catalysts such as hydrogen halides (HCl, etc.), and the like.

The catalyst is usually used in an amount of 1 to 1000 ppm, preferably 2 to 100 ppm, based on the total amount of monomer.

The liquid crystal polymer obtained by this polycondensation reaction is withdrawn from the polymerization reaction tank in a molten state and then processed into pellets, flakes, or powder.

The liquid crystal polymer used in the present invention may contain inorganic and/or organic fillers, if necessary.

The inorganic and/or organic fillers include, for example, one or more selected from the group consisting of talc, mica, graphite, silica, wollastonite, dolomite, clay, glass flakes, glass beads, glass balloons, calcium carbonate, barium sulfate, titanium oxide, glass fiber, silica alumina fiber, alumina fiber, carbon fiber, potassium titanate fiber, aluminum borate fiber, and aramid fiber. Among these, talc, mica, and silica are preferred because of their excellent balance between physical properties and cost.

When an inorganic filler and/or an organic filler is used, the amount of the filler is preferably 0.01 to 100 parts by mass, more preferably 0.05 to 50 parts by mass, and even more preferably 0.1 to 30 parts by mass, per 100 parts by mass of the liquid crystal polymer.

The liquid crystal polymer used in the present invention may further contain other resin components and additives within the scope of the present invention. Examples of other resin components include thermoplastic resins and modified products thereof, such as polyolefins such as polypropylene and polyethylene, amorphous polyarylates, polycarbonates, polyamides, other polyesters, polyacetals, polyphenylene ethers, polysulfones, polyethersulfones, polyetherimides, and polyamideimides, and thermosetting resins such as phenolic resins, epoxy resins, and polyimide resins. Examples of additives include antioxidants, hydrochloric acid absorbers, heat stabilizers, light stabilizers, ultraviolet absorbers, lubricants, antistatic agents, flame retardants, nucleating agents, pigments, dyes, dispersants, copper inhibitors, neutralizing agents, foaming agents, plasticizers, bubble inhibitors, crosslinking agents, flow improvers such as peroxides, and antiblocking agents.

The other resin components and additives may be used alone or in combination of two or more types.

When other resin components are blended, the blending amount of the resin components is preferably 0.1 to 100 parts by mass, and more preferably 5 to 80 parts by mass, per 100 parts by mass of the liquid crystal polymer.

When an additive is added, the amount of the additive is preferably 0.001 to 5 parts by mass, and more preferably 0.01 to 3 parts by mass, per 100 parts by mass of the liquid crystal polymer.

The film for flexible printed wiring boards of the present invention can be obtained by subjecting the liquid crystal polymer obtained as described above to known molding methods such as extrusion molding, press molding, and injection molding, among which extrusion molding is preferred. Any method can be used as the extrusion molding method, but the well-known T-die method, laminate stretching method, inflation method, and the like are industrially advantageous. In particular, the inflation method and laminate stretching method apply stress not only in the mechanical axis direction of the film (or the mechanical processing direction: hereinafter abbreviated as MD direction) but also in the direction perpendicular to this (hereinafter abbreviated as TD direction), and a film can be obtained in which the molecular orientation, dielectric properties, etc. in the MD and TD directions are controlled.

In extrusion molding, it is preferable to include a stretching process to control the orientation. For example, in extrusion molding using a T-die method, the molten sheet extruded from the T-die may be stretched not only in the MD direction of the film, but also in both the TD direction and this direction simultaneously, or the molten sheet extruded from the T-die may be stretched first in the MD direction and then in the TD direction.

In addition, in extrusion molding using the inflation method, the cylindrical sheet melt-extruded from the ring die may be stretched at a predetermined draw ratio (corresponding to the stretch ratio in the MD direction: also called the draw-down ratio) and blow ratio (corresponding to the stretch ratio in the TD direction: also called the blow-up ratio).

The stretch ratio in such extrusion molding may be, for example, about 1.0 to 10 as the stretch ratio (or draw ratio) in the MD direction, preferably about 1.2 to 8, and more preferably about 1.3 to 7. Also, the stretch ratio (or blow ratio) in the TD direction may be, for example, about 1.5 to 20, preferably about 2 to 15, and more preferably about 2.5 to 14.

The ratio of the stretch ratios in the MD direction to the TD direction (TD direction/MD direction) may be, for example, 2.6 or less, preferably about 0.4 to 2.5.

After extrusion, the liquid crystal polymer film may be stretched as necessary. The stretching method itself is well known, and either biaxial stretching or uniaxial stretching may be used, but biaxial stretching is preferred because it is easier to control the degree of molecular orientation. For stretching, a well-known uniaxial stretching machine, simultaneous biaxial stretching machine, or sequential biaxial stretching machine may be used.

If necessary, known or conventional heat treatment may be performed to adjust the melting point and/or thermal expansion coefficient of the liquid crystal polymer film. Heat treatment conditions can be set appropriately depending on the purpose. For example, the melting point (Tm) of the liquid crystal polymer film may be increased by heating for several hours at a temperature equal to or higher than the melting point (Tm0) of the liquid crystal polymer -10°C (for example, about Tm0-10 to Tm0+30°C, preferably about Tm0 to Tm0+20°C).

The film for flexible printed wiring boards of the present invention may be of any thickness. When a liquid crystal polymer film is used as the electrical insulation layer, the thickness of the film is preferably within the range of 1 to 1000 μm, more preferably within the range of 5 to 500 μm, and even more preferably within the range of 10 to 300 μm. If the film thickness is too thin, the rigidity and strength of the film will be reduced, so a method of laminating films with thicknesses in the range of 10 to 300 μm to obtain any thickness may be adopted.

The flexible printed wiring board film of the present invention thus obtained has the characteristics of a low dielectric tangent, excellent gas barrier properties, and low moisture absorption. In addition, it can be laminated at low temperatures and has excellent interlayer adhesion, making it suitable for use as a circuit board material.

Specific examples of circuit board materials include insulating substrates for conductor circuit boards, bonding sheets for joining substrates with conductor circuits formed on the substrate surface, and coverlays formed on the surface of circuit layers.

When two or more unit circuit boards each having a conductor layer are stacked, the bonding sheet is laminated between the unit circuit boards. The coverlay is placed on the top and/or bottom layer of the laminate.

The film for flexible printed wiring boards of the present invention has excellent interlayer adhesion and can therefore be suitably used as a laminate with metals or resins.
Examples of metals constituting the film and laminate of the present invention include copper, gold, silver, nickel, aluminum, etc., of which copper is preferred. The metal is preferably used to constitute the laminate as a metal foil, and copper foil is particularly preferred.

In addition, examples of resin films constituting the laminate with the film of the present invention include resin films such as polyolefins such as polypropylene and polyethylene, liquid crystal polymers, amorphous polyarylates, polycarbonates, polyamides, other polyesters, polyacetals, polyphenylene ethers, polysulfones, polyethersulfones, polyetherimides, polyetheretherketones, polyamideimides, fluororesins such as polytetrafluoroethylene, phenolic resins, epoxy resins, and polyimide resins. Among these, laminates with resin films selected from the group consisting of liquid crystal polymer films, polyimide films, polytetrafluoroethylene films, and polyetheretherketones are preferred, laminates with liquid crystal polymer films or polyimide films are more preferred, and laminates with liquid crystal polymer films are even more preferred.

The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto.

The crystal melting temperature and dielectric tangent in the examples were measured using the method described below.

Measurement of Crystal Melting Temperature
The measurement was carried out using a differential scanning calorimeter (DSC) Exstar 6000 manufactured by Seiko Instruments Inc. A liquid crystal polymer sample was measured under a temperature increase condition of 20°C/min from room temperature, and the endothermic peak temperature (Tm1) was observed, and then the sample was held at a temperature 20 to 50°C higher than Tm1 for 10 minutes. The sample was then cooled to room temperature under a temperature decrease condition of 20°C/min, and the endothermic peak was observed when the sample was measured again under a temperature increase condition of 20°C/min, and the temperature showing the peak top was taken as the crystalline melting temperature of the liquid crystal polymer.

<Measurement of dielectric tangent>
A liquid crystal polymer sample was molded into a stick-shaped test piece having a length of 85 mm, a width of 1.70 mm, and a thickness of 1.70 mm using an injection molding machine (NEX-15-1E manufactured by Nissei Plastic Industrial Co., Ltd.) with a mold having a single gate at the end point in the length direction. The dielectric loss tangent at 10 GHz was measured by the cavity resonator perturbation method using the obtained stick-shaped test piece and a cavity resonator for dielectric constant measurement (manufactured by Kanto Electronics Application Development Co., Ltd.) and a network analyzer.

In the examples and comparative examples, the following abbreviations represent the following compounds.
LCP: Liquid crystal polymer POB: 4-hydroxybenzoic acid BON6: 6-hydroxy-2-naphthoic acid HQ: Hydroquinone BP: 4,4'-dihydroxybiphenyl TPA: Terephthalic acid NDA: 2,6-naphthalenedicarboxylic acid IPA: Isophthalic acid

[Synthesis Example 1 (Synthesis of LCP-1)]
In a reaction vessel equipped with a stirrer with a torque meter and a distillation tube, POB, BON6, HQ and TPA were charged in a total amount of 6.5 moles in the composition ratio shown in Table 1, and acetic anhydride was further charged in an amount of 1.03 moles relative to the amount (mol) of hydroxyl groups of all monomers, and deacetic acid polymerization was carried out under the following conditions.

Under a nitrogen gas atmosphere, the temperature was raised from room temperature to 145°C over 1 hour and maintained at that temperature for 30 minutes. Next, the temperature was raised to 330°C over 7 hours while distilling off the by-product acetic acid, and then the pressure was reduced to 10 mmHg over 80 minutes. When a predetermined torque was reached, the polymerization reaction was terminated, the contents of the reaction vessel were removed, and pellets of LCP-1 were obtained using a grinder. The amount of acetic acid distilled during polymerization was almost the theoretical value.

The crystalline melting temperature of the obtained LCP-1 measured by DSC was 218°C. In addition, using a stick-shaped test piece molded at a molding temperature of 250°C and a mold temperature of 70°C, the dielectric tangent at 10 GHz measured by the cavity resonator perturbation method was 0.0008.

[Synthesis Example 2 (Synthesis of LCP-2)]
A reaction vessel equipped with a stirrer with a torque meter and a distillation tube was charged with POB, BON6, BP, NDA and IPA in a total amount of 6.5 moles in the composition ratio shown in Table 2, and acetic anhydride was further charged in an amount of 1.03 moles relative to the amount (mol) of hydroxyl groups of all monomers, and deacetic acid polymerization was carried out under the following conditions.

Under a nitrogen gas atmosphere, the temperature was raised from room temperature to 150°C over one hour and maintained at that temperature for 30 minutes. Next, the temperature was quickly raised to 210°C while distilling off the by-product acetic acid, and maintained at that temperature for 30 minutes. The temperature was then raised to 340°C over four hours, after which the pressure was reduced to 10 mmHg over 80 minutes. The polymerization reaction was terminated when a predetermined torque was reached, the contents of the reaction vessel were removed, and pellets of LCP-2 were obtained using a grinder. The amount of acetic acid distilled during polymerization was almost the theoretical value.

The crystalline melting temperature of the obtained LCP-2 measured by DSC was 183°C. In addition, using a stick-shaped test piece molded at a molding temperature of 250°C and a mold temperature of 70°C, the dielectric loss tangent at 10 GHz measured by the cavity resonator perturbation method was 0.0006.

[Synthesis Example 3 (Synthesis of LCP-3)]
Into a reaction vessel equipped with a stirrer with a torque meter and a distillation tube, POB and BON6 were charged in a total amount of 6.5 moles in the composition ratio shown in Table 3, and acetic anhydride was further charged in an amount of 1.03 moles relative to the amount (mol) of hydroxyl groups of all monomers, and deacetic acid polymerization was carried out under the following conditions.

Under a nitrogen gas atmosphere, the temperature was raised from room temperature to 150°C over 1 hour and maintained at that temperature for 30 minutes. Next, the temperature was quickly raised to 210°C while distilling off the by-product acetic acid, and maintained at that temperature for 30 minutes. The temperature was then raised to 325°C over 5 hours, and the pressure was then reduced to 20 mmHg over 90 minutes. The polymerization reaction was terminated when a predetermined torque was reached, the contents of the reaction vessel were removed, and pellets of LCP-3 were obtained using a grinder. The amount of acetic acid distilled during polymerization was almost the theoretical value.

The crystalline melting temperature of the obtained LCP-3 measured by DSC was 280°C. In addition, using a stick-shaped test piece molded at a molding temperature of 300°C and a mold temperature of 70°C, the dielectric tangent at 10 GHz measured by the cavity resonator perturbation method was 0.0021.

[Synthesis Example 4 (Synthesis of LCP-4)]
A reaction vessel equipped with a stirrer with a torque meter and a distillation tube was charged with BON6, BP, HQ, and TPA in a total amount of 6.5 moles in the composition ratio shown in Table 4, and acetic anhydride was further charged in an amount of 1.03 moles relative to the amount (mol) of hydroxyl groups of all monomers, and deacetic acid polymerization was carried out under the following conditions.

Under a nitrogen gas atmosphere, the temperature was raised from room temperature to 150°C over 1 hour and maintained at that temperature for 60 minutes. Next, the temperature was raised to 350°C over 7 hours while distilling off the by-product acetic acid, and then the pressure was reduced to 10 mmHg over 90 minutes. When the specified torque was reached, the polymerization reaction was terminated, the contents of the reaction vessel were removed, and pellets of LCP-4 were obtained using a grinder. The amount of acetic acid distilled during polymerization was almost the theoretical value.

The crystalline melting temperature of the obtained LCP-4 measured by DSC was 338°C. In addition, using a stick-shaped test piece molded at a molding temperature of 350°C and a mold temperature of 70°C, the dielectric tangent at 10 GHz measured by the cavity resonator perturbation method was 0.0006. LCP-4 was produced in order to measure the adhesive strength with LCP-1 to LCP-3.

<Measurement of adhesive strength>
Adhesive strength with LCP-4 (Adhesive strength 1, 2)
For each of LCP-1 to LCP-4, a film-like test piece (length 150 mm x width 13 mm x thickness 0.8 mm) was prepared using an injection molding machine (UH1000-110 manufactured by Nissei Plastic Industrial Co., Ltd.) under the following molding conditions.
LCP-1 and LCP-2: Molding temperature 250°C, mold temperature 70°C
LCP-3: Molding temperature 300°C, mold temperature 70°C
LCP-4: Molding temperature 350°C, mold temperature 70°C

The film test pieces of LCP-1 to LCP-3 were placed in an oven heater at 250°C for 60 seconds so that they overlapped with the film test piece of LCP-4 with an area of 70 mm length x 13 mm width, and the film test pieces were bonded together. The adhesive strength (tensile shear strength) of each test piece was measured in accordance with JIS-K-7162 using an AUTOGRAPH AG-Xplus made by Shimadzu Corporation (adhesive strength 1). The adhesive strength was also measured in the same manner, except that the temperature of the oven heater was changed to 300°C (adhesive strength 2).

Adhesive strength with copper foil (adhesive strength 3, 4)
Using a vacuum press and a metal spacer having a hole of 160 mm in length and 50 mm in width formed in a 0.7 mm thick metal plate, the LCPs were thermally melted by leaving them at 10 torr for 2 minutes and 30 seconds under temperature conditions of 250° C. for LCP-1 and LCP-2, and 300° C. for LCP-3, and then pressed at 50 MPa for 3 minutes to produce film-like molded pieces of LCP1 to LCP-3 having a length of 160 mm, a width of 50 mm, and a thickness of 0.7 mm.

A copper foil (length 200 mm x width 70 mm x thickness 0.05 mm) was layered on a film-shaped piece of LCP integrated with a metal spacer so that the matte side of the copper foil was adhered to the LCP. The piece was then placed in a vacuum press at 250°C and left to stand for 2 minutes and 30 seconds at 10 torr, after which it was pressed at 50 MPa for 3 minutes to adhere the LCP and copper foil together, the metal spacer was removed, and a laminated film of LCP and copper foil was produced.

This laminated film was cut in half lengthwise, and a 10 mm wide cut was made on the copper foil side. Using an AUTOGRAPH AG-Xplus manufactured by Shimadzu Corporation, the peel strength was measured when peeling the copper foil from the LCP film under conditions of a peel width of 10 mm, a peel speed of 25 mm/min, and a peel angle of 90 degrees (adhesive strength 3). In addition, the peel strength was measured in the same manner, except that the temperature of the vacuum press was changed to 300°C (adhesive strength 4).

When the liquid crystal polymers of Examples 1 and 2 are used, the adhesive strength with the copper foil and the liquid crystal polymer film is high, and the dielectric tangent is 0.001 or less, indicating that the dielectric properties are excellent.

On the other hand, when the liquid crystal polymer of Comparative Example 1 was used, the adhesive strength (adhesive strength 3) with the copper foil at low temperature (250°C) was poor, and the adhesive did not adhere to the liquid crystal polymer film at either 250°C or 300°C. Furthermore, the dielectric tangent was greater than 0.002, and the dielectric properties were poor.

Claims (7)

A film for flexible printed wiring boards composed of a liquid crystal polymer with a crystal melting temperature of 260°C or less and a dielectric loss tangent of 0.0015 or less at 10 GHz measured by the cavity resonator perturbation method. The liquid crystal polymer is represented by the formulas (I) to (IV):

[In the formula,
_Ar1 and _Ar2 each represent one or more divalent aromatic groups, and p, q, r, and s each represent a composition ratio (mol %) of each repeating unit in the liquid crystal polymer, and satisfy the following condition:
0.5≦p/q≦2.5
0.1≦r≦25, and 0.1≦s≦25]
2. The film according to claim 1, which is a wholly aromatic liquid crystal polyester containing a repeating unit represented by the following formula: Formula (III) and/or Formula (IV) are each independently a group selected from the group consisting of Ar ₁ and Ar ₂ , each selected from the group consisting of formulas (1) to (4):

3. The film according to claim 2, which is a wholly aromatic liquid crystal polyester containing one or more repeating units which are aromatic groups selected from the group consisting of: The film according to any one of claims 1 to 3, which is a bonding sheet for a flexible printed wiring board. The film according to any one of claims 1 to 3, which is a coverlay for a flexible printed wiring board. A laminate comprising the film according to any one of claims 1 to 5 and a metal layer. A laminate comprising the film according to any one of claims 1 to 5 and a resin layer.