JP6534471B2 - Flexible circuit board - Google Patents

Description

  The present invention relates to a flexible circuit board (FPC) used by being folded and stored in a housing of an electronic device.

  In recent years, along with the miniaturization and high functionalization of electronic devices, even in the FPC which is one of the electronic components constituting them, higher performance products such as electrical characteristics, mechanical characteristics, heat resistance, etc. are required. Most of FPCs are manufactured by forming a circuit on the copper foil of a flexible copper-clad laminate in which polyimide, which is an insulating layer, is laminated on copper foil, which is a metal layer. A copper-clad laminate in which such a polyimide is used as an insulating layer is a copper-clad laminate in which polyimide and copper foil are laminated between polyimide and copper foil via a thermosetting adhesive layer such as an epoxy resin ("three-layer And CCL, and copper-clad laminates (also called “two-layer CCL”) in which polyimide and copper foil are directly laminated without intervention of a thermosetting adhesive layer.

  The three-layer CCL has a problem in heat resistance because an epoxy resin or the like is used for the adhesive layer. Specifically, a problem is likely to occur in a process requiring high temperature processing, such as a process of bonding electrodes on a wiring of an FPC to a monitor panel substrate, a rigid substrate, a semiconductor chip or the like using solder or a heat tool. In the three-layer CCL, the thickness of the adhesive layer is added to the two-layer CCL, the dimensional control by the difference in thermal expansion coefficient between different materials is difficult, and from the viewpoint of dielectric characteristics, high-end electronic device There is a problem with loading on the. Therefore, in applications where heat resistance and reliability are particularly required, a two-layer CCL that does not use a thermosetting adhesive or the like such as an epoxy resin has been marketed.

  By the way, with the recent diversification of models of portable terminal devices, the usage pattern of the FPC used therein is also changing. Unlike usage where a certain amount of bending radius is secured such as a hinge bending portion or a slide bending portion found in a conventional mobile phone, it is more severe that a crease is made to be folded to be stored in a thin case. Bending resistance is becoming required. Hereinafter, in the present specification, folding the upper surface side of the FPC so that the upper surface side thereof is inverted by approximately 180 degrees so as to be the lower surface side may be referred to as “zero folding”.

  As intended for application to such applications, Patent Document 1 proposes a highly flexible flexible circuit board which exhibits high flexibility and is excellent in dimensional stability. However, in the invention of Patent Document 1, a metal wiring pattern is formed on a polyimide base film through an adhesive layer, and polyimide having a relatively low elastic modulus range is used as a base material. In addition, since the adhesive layer is required, it is impossible to fully utilize the characteristics such as the heat resistance of the two-layer CCL only with the polyimide.

  Moreover, in patent document 2, the polyimide metal laminated body suitable for the circuit board used in the state bend | folded in an electronic device is proposed. However, although the polyimide metal laminate disclosed herein focuses on the modulus of elasticity of the non-thermoplastic polyimide film constituting the polyimide layer, it does not focus on the modulus of elasticity on the copper foil side used together, Since the folding resistance was also shown only once, it was practically insufficient.

  Further, in the design of the FPC, the wiring can be thickened if the thickness of the polyimide layer which is the insulating layer of the flexible copper-clad laminate is large from the viewpoint of impedance matching with the bonding destination substrate. That is, although wiring processing is easy, on the other hand, when it is going to store in a thin or narrow case, the repulsive force of a substrate influences and it becomes difficult to fold, and there is a problem in handling of FPC. On the other hand, if the thickness of the polyimide layer is thin, it is also necessary to make the wiring thinner from the viewpoint of impedance matching. That is, while the degree of difficulty in wiring processability increases, the low repulsion makes it relatively easy to store in a thin or narrow case, and the handling of the FPC is good.

Japanese Patent Application Publication No. 2007-208807 JP, 2012-6200, A

  An object of the present invention is to provide a flexible copper-clad laminate which gives an FPC having excellent bending resistance, which can prevent disconnection or breakage of a wiring circuit even when used in a thin or narrow housing of an electronic device. I assume.

  As a result of intensive investigations, the present inventors can solve the above problems by optimizing the characteristics of copper foil and polyimide film and focusing on the characteristics of a printed circuit board obtained by processing a flexible copper-clad laminate into a printed circuit. It has been found that a flexible copper-clad laminate can be provided, and the present invention has been completed.

That is, the flexible copper-clad laminate of the present invention is a flexible copper-clad laminate used for a flexible circuit board which is folded and stored in a housing of an electronic device,
A polyimide layer (A) in the range of 5 to 30 μm in thickness and in the range of tensile modulus of 4 to 10 GPa,
The copper foil (B) has a thickness in the range of 6 to 20 μm and a tensile modulus of elasticity in the range of 25 to 35 GPa laminated on at least one surface of the polyimide layer (A),
The ten-point average roughness (Rz) of the copper foil (B) on the side in contact with the polyimide layer (A) is in the range of 0.7 to 2.2 μm, and the copper foil (B) is wired The bending coefficient [PF] calculated by the following equation (1) is 0.96 ± 0.025 in a bending test at a gap of 0.3 mm of an arbitrary flexible circuit board on which circuit processing is performed to form a copper wiring It is characterized by being in the range.

［式（１）において、|ε|は銅配線の屈曲平均ひずみ値の絶対値であり、εｃは銅配線の引張弾性限界ひずみである。］

[In formula (1), | ε | is an absolute value of the bending average strain value of the copper wiring, and εc is a tensile elastic limit strain of the copper wiring. ]

In the flexible copper-clad laminate of the present invention, the polyimide layer (A) has a low thermal expansion polyimide layer (i) having a thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K and a thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more It is preferable that the high thermal expansion polyimide layer (ii) includes the high thermal expansion polyimide layer (ii), and the high thermal expansion polyimide layer (ii) is in direct contact with the copper foil (B).

  In the flexible copper-clad laminate of the present invention, the thickness of the polyimide layer (A) is preferably in the range of 8 to 15 μm, and the tensile elastic modulus is preferably in the range of 6 to 10 GPa.

  In the flexible copper-clad laminate of the present invention, the thickness ratio of the polyimide layer (A) to the copper foil (B) [polyimide layer (A) / copper foil (B)] is in the range of 0.9 to 1.1. Preferably within.

  In the flexible copper-clad laminate of the present invention, the copper foil (B) is preferably an electrodeposited copper foil.

  The flexible copper-clad laminate of the present invention can exhibit high bending resistance required for a wiring substrate, and thus provides a flexible circuit substrate material having excellent connection reliability in a bent state in an electronic device. be able to. Therefore, the flexible copper-clad laminate of the present invention is suitably used particularly for electronic parts that are required to have bending resistance such as bending parts around small liquid crystals such as smartphones.

FIG. 1 is a perspective view showing a main part of a flexible circuit board obtained by processing a copper foil of a flexible copper-clad laminate of the present invention in a printed circuit. It is plane explanatory drawing which shows the appearance of the copper wiring of the test circuit board piece used in the Example. It is side explanatory drawing which shows the mode of the sample stage in a bending test, and a test circuit board piece (The state figure which fixed the test circuit board piece on the sample stage). It is side explanatory drawing which shows the mode of the sample stage in a bending test, and a test circuit board piece (The state figure of the front side which presses down the bending location of a test circuit board piece with a roller). It is side explanatory drawing which shows the mode of the sample stage in a bending test, and a test circuit board piece (The state figure which pressed down the bending location of a test circuit board piece with a roller). It is side explanatory drawing which shows the mode of the sample stage in a bending test, and a test circuit board piece (The state figure which open | released the bending location and returned the test piece to the flat state). It is side explanatory drawing which shows the mode of the sample stage in a bending test, and a test circuit board piece (The state figure which hold | suppresses the crease | fold part of a bending location with a roller, and is equalized). It is a section explanatory drawing (a part) of a flexible circuit board.

  Hereinafter, embodiments of the present invention will be described. The flexible copper-clad laminate of this embodiment comprises a polyimide layer (A) and a copper foil (B). The copper foil (B) is provided on one side or both sides of the polyimide layer (A), and an electrolytic copper foil is preferable. This flexible copper-clad laminate is used for an FPC that is folded and housed in a housing of an electronic device by forming a copper wiring by processing a wiring circuit by etching a copper foil or the like.

<Polyimide layer>
In the flexible copper-clad laminate of this embodiment, the thickness of the polyimide layer (A) is in the range of 5 to 30 μm, preferably in the range of 8 to 15 μm, and in the range of 9 to 12 μm. Is particularly preferred. When the thickness of the polyimide layer (A) exceeds 30 μm, a large bending stress is applied to the copper wiring when the FPC is bent, and the bending resistance thereof is significantly reduced.

  The tensile modulus of the polyimide layer (A) is in the range of 4 to 10 GPa, preferably in the range of 6 to 10 GPa. When the tensile modulus of the polyimide layer (A) is less than 4 GPa, the strength of the polyimide itself is reduced, which causes handling problems such as tearing of the film when processing the flexible copper clad laminate into a circuit board Sometimes. On the other hand, when the tensile modulus of the polyimide layer (A) exceeds 10 GPa, the rigidity against bending of the flexible copper-clad laminate increases, so that the bending stress applied to the copper wiring when bending the FPC increases and the bending resistance The sex is reduced.

  Although it is possible to use a commercially available polyimide film as it is for the polyimide layer (A), a polyamide acid solution is directly coated on a copper foil and then heat treated because of easy control of the thickness and physical properties of the insulating layer. It is preferable to use a so-called cast (coating) method of drying and curing according to. The polyimide layer (A) may be formed of only a single layer, but in consideration of the adhesion between the polyimide layer (A) and the copper foil (B), a plurality of layers is preferable. When making a polyimide layer (A) into multiple layers, another polyamic-acid solution can be sequentially apply | coated and formed on the polyamic-acid solution which consists of a different structural component. When a polyimide layer (A) consists of multiple layers, you may use the polyimide precursor resin of the same structure twice or more.

The polyimide layer (A) will be described in more detail. As described above, the polyimide layer (A) is preferably a plurality of layers. As a specific example thereof, a low thermal expansion polyimide layer having a thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K is used. It is preferable to set it as the laminated structure containing (i) and the highly thermally expandable polyimide layer (ii) whose thermal expansion coefficient is 30 * 10 < ^-6 > / K or more. More preferably, the polyimide layer (A) is a laminated structure having a high thermal expansion polyimide layer (ii) on at least one side, preferably both sides of the low thermal expansion polyimide layer (i), which is a high thermal expansion polyimide It is preferred that the layer (ii) be in direct contact with the copper foil (B). Here, the “low thermal expansion polyimide layer (i)” means a thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K, preferably in the range of 1 × 10 ^{−6 to} 25 × 10 ⁻⁶ / K, particularly preferably Refers to a polyimide layer in the range of 3 × 10 ^{−6 to} 20 × 10 ⁻⁶ / K. The term "high thermal expansion polyimide layer (ii)" refers to a polyimide layer having a thermal expansion coefficient of 30 x 10 ^-6 / K or more, preferably in the range of 30 x 10 ^-6 to 80 x 10 ^-6 / K. The polyimide layer is particularly preferably in the range of 30 × 10 ⁻⁶ to 70 × 10 ⁻⁶ / K. Such a polyimide layer can be made into a polyimide layer which has a desired thermal expansion coefficient by changing suitably the combination of the raw material to be used, thickness, and a drying and curing conditions.

  The polyamic acid solution giving the polyimide layer (A) can be produced by polymerizing a known diamine and an acid anhydride in the presence of a solvent. At this time, the viscosity of the resin to be polymerized is preferably, for example, in the range of 500 cps to 35,000 cps.

  Examples of diamine used as a raw material of polyimide include 4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2,4-diaminomesitylene, 4,4'-methylenedi-o- Toluidine, 4,4'-methylenedi-2,6-xylidine, 4,4'-methylene-2,6-diethylaniline, 2,4-toluenediamine, m-phenylenediamine, p-phenylenediamine, 4,4 ' -Diaminodiphenylpropane, 3,3'-diaminodiphenylpropane, 4,4'-diaminodiphenylethane, 3,3'-diaminodiphenylethane, 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 2-Bis [4- (4-aminophenoxy) phenyl] propane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl Sulfone, 4,4'-diami Diphenyl ether, 3,3-diaminodiphenyl ether, 1,3-bis (3-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1,4-bis (4-aminophenoxy) benzene, benzidine, 3,3'-Diaminobiphenyl, 3,3'-dimethyl-4,4'-diaminobiphenyl, 3,3'-dimethoxybenzidine, 4,4'-diamino-p-terphenyl, 3,3'-diamino- p-terphenyl, bis (p-aminocyclohexyl) methane, bis (p-β-amino-t-butylphenyl) ether, bis (p-β-methyl-δ-aminopentyl) benzene, p-bis (2-) Methyl-4-aminopentyl) benzene, p-bis (1,1-dimethyl-5-aminopentyl) benzene, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene, 2,4-bis (β-amino- t-Butyl) toluene, 2,4-diaminotoluene, m-xylene-2,5-diamine, p-xylene-2,5-diamine, m- Silylenediamine, p-xylylenediamine, 2,6-diaminopyridine, 2,5-diaminopyridine, 2,5-diamino-1,3,4-oxadiazole, piperazine, 2,2'-dimethyl-4, 4'-diaminobiphenyl, 3,7-diaminodibenzofuran, 1,5-diaminofluorene, dibenzo-p-dioxin-2,7-diamine, 4,4'-diaminobenzyl and the like.

  Moreover, as an acid anhydride used as a raw material of a polyimide, for example, pyromellitic dianhydride, 3,3 ', 4,4'-benzophenonetetracarboxylic acid dianhydride, 2,2', 3,3 ' -Benzophenonetetracarboxylic acid dianhydride, 2,3,3 ', 4'-benzophenonetetracarboxylic acid dianhydride, naphthalene-1,2,5,6-tetracarboxylic acid dianhydride, naphthalene-1,2,5 4,5-tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, naphthalene-1,2,6,7-tetracarboxylic dianhydride, 4,8-dimethyl -1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylic acid dianhydride, 4,8-dimethyl-1,2,3,5,6,7- Hexahydronaphthalene-2,3,6,7-tetracarboxylic acid dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,7-dichloronaphthalene-1, 4,5,8-Tetracarboxylic acid dianhydride, 2,3,6,7-tetrachloro Phthalene-1,4,5,8-tetracarboxylic dianhydride, 1,4,5,8-tetrachloronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 3,3 ', 4 2,4'-biphenyltetracarboxylic dianhydride, 2,2 ', 3,3'-biphenyltetracarboxylic dianhydride, 2,3,3', 4'-biphenyltetracarboxylic dianhydride, 3, 3 '', 4,4 ''-p-terphenyltetracarboxylic dianhydride, 2,2 '', 3,3 ''-p-terphenyltetracarboxylic dianhydride, 2,3,3 ' ', 4' '-p-terphenyltetracarboxylic acid dianhydride, 2, 2-bis (2,3-dicarboxyphenyl) -propane dianhydride, 2, 2- bis (3,4- dicarboxyphenyl) ) -Propane dianhydride, bis (2,3-dicarboxyphenyl) ether dianhydride, bis (2,3-dicarboxyphenyl) methane dianhydride, bis (3.4-dicarboxyphenyl) methane dianhydride, Bis (2,3-dicarboxyphenyl) sulfone dianhydride, bis (3,4-dicarboxylate) Phenyl) sulfone dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, perylene-2,3 , 8,9-tetracarboxylic acid dianhydride, perylene-3,4,9,10-tetracarboxylic acid dianhydride, perylene-4,5,10,11-tetracarboxylic acid dianhydride, perylene-5, 6,11,12-tetracarboxylic acid dianhydride, phenanthrene-1,2,7,8-tetracarboxylic acid dianhydride, phenanthrene-1,2,6,7-tetracarboxylic acid dianhydride, phenanthrene-1 2,9,10-tetracarboxylic acid dianhydride, cyclopentane-1,2,3,4-tetracarboxylic acid dianhydride, pyrazine-2,3,5,6-tetracarboxylic acid dianhydride, pyrrolidine -2,3,4,5-tetracarboxylic acid dianhydride, thiophene-, 3,4,5-tetracarboxylic acid dianhydride, 4,4'-oxydiphthalic acid dianhydride, 2,3,6,7 -Naphthalenetetracarboxylic acid dianhydride etc And the like.

  The diamine and the acid anhydride may be used alone or in combination of two or more. Further, examples of the solvent used for the polymerization include dimethylacetamide, N-methyl pyrrolidinone, 2-butanone, diglyme, xylene and the like, and one or more kinds can be used in combination.

In the present embodiment, in order to obtain a low thermal expansion polyimide layer (i) having a thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K, pyromellitic dianhydride as a raw acid anhydride component, 3,3 ′. Use 4,2,4'-biphenyltetracarboxylic acid dianhydride, and use 2,2'-dimethyl-4,4'-diaminobiphenyl and 2-methoxy-4,4'-diaminobenzanilide as the diamine component It is particularly preferable to use pyromellitic dianhydride and 2,2'-dimethyl-4,4'-diaminobiphenyl as the main components of the starting materials.

Moreover, in order to obtain a highly thermally expandable polyimide layer (ii) having a thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more, pyromellitic dianhydride as an acid anhydride component of the raw material, 3,3 ′, 4,4 '-Biphenyltetracarboxylic acid dianhydride, 3,3', 4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3 ', 4,4'-diphenyl sulfone tetracarboxylic acid dianhydride, diamine component Preferably, 2,2′-bis [4- (4-aminophenoxy) phenyl] propane, 4,4′-diaminodiphenyl ether, 1,3-bis (4-aminophenoxy) benzene is used, particularly preferably It is preferable that pyromellitic dianhydride and 2,2'-bis [4- (4-aminophenoxy) phenyl] propane be the main component of each raw material. In addition, the preferable glass transition temperature of the highly thermally expandable polyimide layer (ii) obtained in this way exists in the range of 300-400 degreeC.

  When the polyimide layer (A) has a laminated structure of a low thermal expansion polyimide layer (i) and a high thermal expansion polyimide layer (ii), preferably, the low thermal expansion polyimide layer (i) and high thermal expansion The thickness ratio to the polyimide layer (ii) (low thermal expansion polyimide layer (i) / high thermal expansion polyimide layer (ii)) is preferably in the range of 2 to 15. If the value of this ratio is less than 2, the low thermal expansion polyimide layer with respect to the entire polyimide layer becomes thin, so it becomes difficult to control the dimensional characteristics of the polyimide film, and the dimensional change rate when etching the copper foil becomes large. If it exceeds 15, the high thermal expansion polyimide layer becomes thin, and the adhesion reliability between the polyimide film and the copper foil is lowered. Even when the polyimide layer (A) is composed of a plurality of layers, the thickness and elastic modulus of the entire polyimide layer (A) can be used to calculate the above-mentioned fracture coefficient [PF].

<Copper foil>
In the flexible copper clad laminate of the present embodiment, the thickness of the copper foil (B) is in the range of 6 to 20 μm, and preferably in the range of 8 to 15 μm. If the thickness of the copper foil (B) is less than 6 μm, the rigidity of the copper foil itself is lowered in the process of forming a polyimide layer on the copper foil, for example, in the production of the flexible copper clad laminate. There is a problem that wrinkles and the like occur on the tension laminated plate. Moreover, when the thickness of copper foil (B) exceeds 20 micrometers, bending resistance added to the copper wiring at the time of bending | flexing FPC will become large, and bending resistance will fall.

  Furthermore, in the present embodiment, the thickness ratio of the polyimide layer (A) to the copper foil (B) [polyimide layer (A) / copper foil (B)] is preferably in the range of 0.9 to 1.1. . When the thickness ratio is less than 0.9 or greater than 1.1, the maximum tensile strain at the time when the portion plastically deformed during bending is elongated becomes large, and the bending resistance is lowered.

  Moreover, about the tensile elasticity modulus of copper foil (B), it exists in the range of 25-35 GPa. If the tensile modulus of the copper foil (B) does not reach 25 GPa, for example, the heating conditions of the copper foil itself may affect the process of forming the polyimide layer on the copper foil during the production of the flexible copper clad laminate, Will decrease. As a result, the problem that a wrinkle etc. generate | occur | produce on a flexible copper clad laminated board arises. On the other hand, when the tensile elastic modulus exceeds 35 GPa, a large bending stress is applied to the copper wiring when the FPC is bent, and the bending resistance thereof is significantly reduced.

  The surface of the copper foil (B) may be roughened, and the surface roughness (ten-point average roughness; Rz) of the copper foil surface in contact with the polyimide layer (A) is 0.7 to 2.2 μm. It is in a range, and a range of 0.8 to 1.6 μm is preferable. If the surface roughness (Rz) value of the copper foil (B) is less than 0.7 μm, it will be difficult to secure the adhesion reliability with the polyimide film, and if it exceeds 2.2 μm, the FPC will be bent repeatedly. The unevenness of the roughening particles is likely to be the starting point of the crack generation. As a result, the bending resistance of the FPC is reduced. In addition, surface roughness Rz is a value measured according to the prescription | regulation of JISB0601.

  The copper foil used for the flexible copper-clad laminate of this embodiment is not particularly limited as long as it satisfies the above characteristics, and may be an electrolytic copper foil or a rolled copper foil, but a thin copper foil is used. It is preferable to use an electrolytic copper foil from the viewpoint of the easiness of manufacture in the case of, and the price. Commercially available products can be used as the electrolytic copper foil, and specific examples thereof include WS foil manufactured by Furukawa Electric Co., Ltd., HL foil manufactured by Nippon Electrolytic Co., Ltd., HTE foil manufactured by Mitsui Mining & Smelting Co., Ltd., and the like. In addition, even if other products including these commercial products are used, the copper foil (B) can be used under the heat treatment conditions for forming the polyimide layer (A) on the copper foil described above. In the present embodiment, the resulting flexible copper-clad laminate should fall within these predetermined ranges.

  The flexible copper-clad laminate of this embodiment can be produced, for example, by applying a polyimide precursor resin solution (also referred to as a polyamic acid solution) on the surface of a copper foil, and then drying and curing it. it can. The heat treatment conditions in the heat treatment step are such that after the solvent in the polyamic acid solution is dried and removed at a temperature of less than 160 ° C., the temperature of the coated polyamic acid solution is raised stepwise in a temperature range of 130 ° C. to 400 ° C. And curing. In order to make the single-sided flexible copper-clad laminate thus obtained into a double-sided copper-clad laminate, the single-sided flexible copper-clad laminate and the copper foil prepared separately from this are within the range of 300 to 400 ° C. A method of thermocompression bonding at a temperature of

<FPC>
The flexible copper-clad laminate of this embodiment is mainly useful as an FPC material. That is, the FPC according to an embodiment of the present invention can be manufactured by processing the copper foil of the flexible copper-clad laminate of the present embodiment into a pattern by an ordinary method to form a wiring layer.

  The flexible copper-clad laminate of the present invention is composed of the polyimide layer (A) and the copper foil (B). The copper foil (B) of this flexible copper-clad laminate is subjected to wiring circuit processing for copper wiring In a bending test (a gap of 0.3 mm) of any formed flexible circuit board, it is necessary that the tortuosity factor [PF] calculated by the following (1) is in the range of 0.96 ± 0.025. It is preferably in the range of 0.96 ± 0.02, and more preferably in the range of 0.96 ± 0.015. The fracture modulus [PF] is a value determined by the stress-strain curve obtained from the uniaxial tension test of the copper foil used. When the tortuosity factor [PF] is out of the above range, the stress is locally concentrated (one point or two points), whereby the bending resistance is lowered. On the other hand, if the toe coefficient [PF] is in the above range, the stress is appropriately dispersed to improve the bending resistance such as goby folding. For example, when the electrolytic copper foil is used in the present invention, the stress-strain curve obtained from the uniaxial tensile test of the electrolytic copper foil used is to set the fracture modulus [PF] defined in the present invention to the above range. An embodiment using a copper foil in which the initial inclination of the linear portion, that is, the elastic modulus is 29 GPa or less, the stress value of the portion where the curvature is maximum is 130 MPa or less, and the strain is 5% and the stress is 175 MPa or less is exemplified.

式（１）において、｜ε｜は銅配線の屈曲平均ひずみ値の絶対値であり、εｃは銅配線の引張弾性限界ひずみである。

In equation (1), | ε | is the absolute value of the bending average strain value of the copper wiring, and εc is the tensile elasticity limit strain of the copper wiring.

As described above, the bending coefficient [PF] is represented by the absolute value | ε | of the bending average strain value ε of the copper wiring and the tensile elastic limit strain εc of the copper wiring, and the bending average strain value ε is Calculated by (2). Hereinafter, with respect to the bending coefficient [PF], a circuit board provided with a copper wiring 12 obtained by processing one layer of copper foil on one side of the polyimide layer 11 made of one layer of polyimide shown in FIG. The case where the circuit board is bent so that the reference surface SP which is the lower surface of the polyimide layer 11 which is the first layer is convex downward (the outer surface of the bent portion) will be described. The circuit board shown in FIG. 8 shows a portion where copper wiring exists in a cross section (that is, a cross section) cut perpendicularly to the longitudinal direction of the circuit board.
ε = − (yc− [NP] _Line ) / R (2)

  Here, in the equation (2), the bending average strain ε is the bending average strain in the longitudinal direction generated in the copper wiring by pure bending when the longitudinal direction of the circuit board is folded in two, and yc in the equation is a polyimide layer It is the distance from the reference plane SP which is the lower surface of 12 to the center plane of the copper wiring 12. Further, the code NP represents the neutral plane of the circuit board. Here, the distance between the neutral plane NP and the reference plane SP is taken as a neutral plane position [NP], and for this neutral plane position [NP], copper wiring and copper wiring formed by wiring circuit processing of copper foil Calculate separately with the space part formed between. The neutral plane position [NP] is calculated by the following equation (3).

Here, E _i is (in the example shown in FIG. 8, the first layer is a polyimide layer 11, second layer copper wiring is 12) the i-th layer in the circuit board tensile modulus of the material constituting the It is. The modulus of elasticity E _i corresponds to the "relationship stress and strain in the layers" in the present embodiment. B _i is the width of the ith layer, which corresponds to the width B shown in FIG. 8 (the dimension parallel to the lower surface of the first layer and perpendicular to the longitudinal direction of the circuit board).

In case of obtaining the neutral plane position of the copper wiring [NP], when a B _i using the values of the line width LW of the copper wiring, obtaining the neutral plane position of the space portion [NP], the copper as B _i Use the value of the line width SW of the wiring. h _i is the distance between the central plane of the ith layer and the reference plane SP. The center plane of the ith layer is an imaginary plane located at the center in the thickness direction of the ith layer. t _i is the thickness of the i-th layer. Also, the symbol “Σ _{i = 1} ⁿ ” represents the total sum of i from 1 to n. In addition, the neutral plane position in copper wiring is described as [NP] _Line .

Further, R in the equation (2) represents an effective radius of curvature, and the effective radius of curvature R is a distance from a bending center at a bending portion to the neutral plane NP of the copper wiring when the circuit board is bent in a bending test. is there. That is, the effective curvature radius R is calculated by the following equation (4) from the gap interval G and the neutral plane position [NP] _{Line of the} copper wiring.
R = G / 2- [NP] _Line (4)

  As described above, by obtaining the position of the neutral surface, the effective radius of curvature, and the bending average strain, a bending coefficient [PF] representing the degree of bending of the entire circuit board is calculated. Further, as described above, the fracture coefficient [PF] is determined by the thickness of each layer constituting the circuit board, the elastic modulus of each layer constituting the circuit board, the gap interval G in the bending test, and the copper wiring 12 It can be calculated using each information such as the line width LW.

  In the above (FIG. 8), for convenience, the model in which the circuit board has two layers is described and described, but the above description also applies to the case where the circuit board is formed of two or more layers. That is, assuming that the number of layers of circuit board 1 is n, n is an integer of 2 or more, and it is i-th (i = 1, 2,. The layer n) is called the i-th layer.

  Further, as shown in FIG. 1, in the circuit board, copper foil is patterned by wiring circuit processing, and there are a portion where the copper wiring 12 exists and a portion where the copper wiring 12 does not exist. Here, when a portion where the copper wiring 12 exists is called a wiring portion and a portion where the copper wiring 12 does not exist is called a space portion, the configuration differs between the wiring portion and the space portion. For example, in the case of the circuit board 1 shown in FIG. 1, the wiring portion on the polyimide layer 11 is composed of ten rows (only four rows are shown in FIG. 1) of copper wiring 12 and the space portion is other than the wiring portion. And the gaps between the copper wires 12. From the above, the calculation of the fold coefficient [PF] can be performed by dividing the wiring portion and the space portion.

  Hereinafter, the present invention will be described in more detail based on examples. In addition, each characteristic evaluation in the following Example was performed by the following method.

[Measurement of tensile modulus]
The value of the tensile modulus of elasticity was measured under an environment of a temperature of 23 ° C. and a relative humidity of 50% using a Strograph R-1 manufactured by Toyo Seiki Seisaku-sho, Ltd.

[Measurement of coefficient of thermal expansion (CTE)]
Using a thermomechanical analyzer manufactured by Seiko Instruments Inc., raise the temperature to 250 ° C., hold at that temperature for 10 minutes, cool at a rate of 5 ° C./min, and average thermal expansion coefficient from 240 ° C. to 100 ° C. The linear thermal expansion coefficient was determined.

[Measurement of surface roughness (Rz)]
The surface roughness of the copper foil in contact with the polyimide layer was measured using a contact-type surface roughness measuring machine (SE1700 manufactured by Kosaka Laboratory Ltd.).

[Haze-fold measurement (bending test)]
A test piece (test circuit board piece) was obtained by etching a copper foil of a flexible copper clad laminate and forming 10 rows of copper wiring with a line width of 100 μm and a space width of 100 μm and a length of 40 mm along its longitudinal direction. It produced (FIG. 2). As shown in FIG. 2 representing only the copper wiring in the test piece, the ten rows of copper wiring 51 in the test piece 40 are all continuously connected via the U-shaped portion 52, and resistance is applied to both ends thereof An electrode portion (not shown) for value measurement is provided. The test piece 40 was fixed on the sample stage 20 and 21 which can be folded in half, and a wire for resistance measurement was connected to start monitoring of the resistance (FIG. 3). In the bending test, the roller G made of urethane was used while controlling the gap G at the bending portion 40C to be 0.3 mm at a center portion in the longitudinal direction with respect to 10 rows of copper wires 51. After moving the roller parallel to the line and bending all 10 rows of copper wiring 51 (Figs. 4 and 5), open the bent part and return the test piece to a flat state (Fig. 6), the creased part Was again moved while being held down by a roller (FIG. 7), and this series of steps was performed to count it once with the number of folds. The bending test is repeated while monitoring the resistance value of the wiring at all times, and it is determined that the wiring is broken when the predetermined resistance value (3000 Ω) is reached, and the number of times of bending repeated until that time is taken as the mean bending value . This case was evaluated as “good” when the measurement value was 50 times or more and “poor” when the measurement value was less than 50 times.

  The manufacturing method of the flexible copper clad laminated board as described in an Example and a comparative example is shown next.

[Synthesis of polyamic acid solution]
Synthesis Example 1
Bottom resin synthesis:
N, N-dimethylacetamide is placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen, and further, 2,2-bis [4- (4-aminophenoxy) phenyl] propane (BAPP) is placed in the reaction vessel. ) Was added and dissolved in the vessel with stirring. Next, pyromellitic dianhydride (PMDA) was charged such that the total amount of charged monomers was 12% by mass. Then, stirring was continued for 3 hours to carry out a polymerization reaction to obtain a resin solution of polyamic acid a. The thermal expansion coefficient (CTE) of a 25 μm-thick polyimide film formed of polyamic acid a was 55 × 10 ⁻⁶ / K.

(Composition example 2)
N, N-dimethylacetamide is placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen, and further, 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB) is placed in the reaction vessel. And 4,4'-diaminodiphenyl ether (DAPE) were added at a molar ratio of each diamine (m-TB: DAPE) of 60:40 and dissolved in a container with stirring. Next, pyromellitic dianhydride (PMDA) was charged such that the total amount of charged monomers was 16% by mass. Then, stirring was continued for 3 hours to carry out a polymerization reaction to obtain a resin solution of polyamic acid b. The thermal expansion coefficient (CTE) of a 25 μm-thick polyimide film formed of polyamic acid b was 22 × 10 ⁻⁶ / K.

(Composition example 3)
N, N-dimethylacetamide is placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen, and further, 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB) is placed in the reaction vessel. Was dissolved in the vessel with stirring. Next, 15% by mass of the total amount of monomers charged with 3,3 ', 4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) and pyromellitic dianhydride (PMDA), the molar ratio of each acid anhydride (BPDA: PMDA) was introduced so as to be 20:80. Then, stirring was continued for 3 hours to carry out a polymerization reaction to obtain a resin solution of polyamic acid c. The thermal expansion coefficient (CTE) of a 25 μm thick polyimide film formed of polyamic acid c was 22 × 10 ⁻⁶ / K.

Example 1
The thickness after curing of the resin solution of polyamic acid a prepared in Synthesis Example 1 on one side (surface roughness Rz = 1.2 μm) of a long, commercially available electrolytic copper foil having a thickness of 12 μm is 2.5 μm. The solution was uniformly coated and dried by heating at 130.degree. C. to remove the solvent. Next, the resin solution of polyamic acid b prepared in Synthesis Example 2 was uniformly coated on the coated surface side to a cured thickness of 20.0 μm, and dried by heating at 120 ° C. to remove the solvent. Furthermore, a resin solution of the same polyamic acid a as that applied in the first layer is uniformly applied on the coated surface side to a cured thickness of 2.5 μm and dried by heating at 130 ° C. to remove the solvent. did. This long laminate is heat-treated for a total of about 6 minutes in a continuous curing furnace set so that the temperature rises stepwise from 130 ° C. to 300 ° C., and the thickness of the polyimide layer is A 25 μm single-sided flexible copper-clad laminate was obtained. Physical properties such as tensile elastic modulus of polyimide layer and copper foil constituting the obtained flexible copper-clad laminate, thickness, thickness ratio of polyimide layer to copper foil, bending modulus, and bending resistance of flexible copper-clad laminate Table 1 shows the evaluation results of the properties (the number of times of warping) (the same applies to Example 2 and thereafter). In addition, evaluation of the polyimide layer used what etched-removed the copper foil from the manufactured flexible copper clad laminated board.

Here, a specific calculation procedure will be described by taking Example 1 as an example for calculation of the bending coefficient [PF] of the flexible copper clad laminate manufactured in the example.
Considering a two-layer structure as shown in FIG. 8 for the wiring portion in which the copper wiring 12 exists, the materials constituting the first layer and the second layer are respectively polyimide and copper. As shown in Table 1 (Example 1), the elastic modulus of each layer is E ₁ = 4 GPa, E ₂ = 29 GPa, and the thickness is t ₁ = 25 μm, t ₂ = 12 μm. The distance between the central plane and the reference plane SP in the thickness direction in each layer is h ₁ = 12.5 μm and h ₂ = 31 μm respectively. Furthermore, the width B, and the width B _{2 'are} both 100 [mu] m width _{B 2} and the space portion of the copper wiring 12, and the width _{B 1} of the polyimide just under the copper wiring 12 is also present was 100 [mu] m (the space portion The width B _{1 ′} of the polyimide immediately below is also 100 μm).

Substituting these values into the equation (3), first, the neutral plane position in the wiring portion where the copper wiring 12 exists is calculated as [NP] _Line = 26.9 μm. Next, this neutral plane position [NP] _Line and the gap interval G = 0.3 mm are substituted in the equation (4) to calculate the effective bending radius R = 0.123 mm. Furthermore, since the distance yc from the reference plane SP to the central plane of the copper wiring 12 is yc = h ₂ = 31 μm, the bending average strain ε is the value of this yc and the previously obtained [NP] _Line and R It substitutes to 2) and it calculates with (epsilon) =-0.0333. Here, a minus sign indicates that it is a compression distortion. From the stress-strain curve obtained from the tensile test of the copper foil in Example 1, the tensile elastic limit strain εc of the copper wiring was determined to be εc = 0.00058. Substituting this and the value of the bending average strain ε previously obtained into the equation (1), the bending coefficient [PF] is calculated as [PF] = 0.983. In the present embodiment, since the space portion is formed only of the polyimide layer, the operation for obtaining [NP] is not necessary, and the fracture coefficients [PF] of other embodiments and comparative examples in Table 1 are obtained. Is the value calculated by the above procedure.

(Example 2)
A thickness of 2.0 μm after curing of the resin solution of polyamic acid a prepared in Synthesis Example 1 on one side (surface roughness Rz = 1.2 μm) of a long, commercially available electrolytic copper foil having a thickness of 12 μm The solution was uniformly coated and dried by heating at 130.degree. C. to remove the solvent. Next, the resin solution of the polyamic acid c prepared in Synthesis Example 3 was uniformly coated on the coated surface side so that the thickness after curing was 16 μm, and dried by heating at 130 ° C. to remove the solvent. Furthermore, a resin solution of the same polyamic acid a as that applied in the first layer is uniformly applied on the coated surface side to a cured thickness of 2.0 μm and dried by heating at 130 ° C. to remove the solvent. did. This long laminate is heat-treated for a total of about 6 minutes in a continuous curing furnace set so that the temperature rises stepwise from 130 ° C. to 300 ° C., and the thickness of the polyimide layer is A 20 μm single-sided flexible copper-clad laminate was obtained. Table 1 shows the evaluation results of the bending resistance of the obtained single-sided flexible copper-clad laminate.

(Example 3)
The thickness after curing of the resin solution of polyamic acid a prepared in Synthesis Example 1 on one side (surface roughness Rz = 1.2 μm) of a long, commercially available electrolytic copper foil having a thickness of 12 μm is 2.2 μm. The solution was uniformly coated and dried by heating at 130.degree. C. to remove the solvent. Next, the resin solution of polyamic acid c prepared in Synthesis Example 3 was uniformly coated on the coated surface side to a cured thickness of 7.6 μm, and dried by heating at 130 ° C. to remove the solvent. Furthermore, a resin solution of the same polyamic acid a as that applied in the first layer is uniformly applied on the coated surface side to a cured thickness of 2.2 μm, and dried by heating at 130 ° C. to remove the solvent. did. This long laminate is heat-treated for a total of about 6 minutes in a continuous curing furnace set so that the temperature rises stepwise from 130 ° C. to 300 ° C., and the thickness of the polyimide layer is A 12 μm single-sided flexible copper-clad laminate was obtained. Table 1 shows the evaluation results of the bending resistance of the obtained single-sided flexible copper-clad laminate.

(Example 4)
A thickness of 2.0 μm after curing the resin solution of polyamic acid a prepared in Synthesis Example 1 on one side (surface roughness Rz = 1.20 μm) of a long 12 μm thick commercially available electrolytic copper foil The solution was uniformly coated and dried by heating at 130.degree. C. to remove the solvent. Next, the resin solution of polyamic acid c prepared in Synthesis Example 3 was uniformly coated on the coated surface side to a cured thickness of 5.0 μm, and dried by heating at 130 ° C. to remove the solvent. Furthermore, a resin solution of the same polyamic acid a as that applied in the first layer is uniformly applied on the coated surface side to a cured thickness of 2.0 μm and dried by heating at 130 ° C. to remove the solvent. did. This long laminate is heat-treated for a total of about 6 minutes in a continuous curing furnace set so that the temperature rises stepwise from 130 ° C. to 300 ° C., and the thickness of the polyimide layer is A 9 μm single-sided flexible copper-clad laminate was obtained. Table 1 shows the evaluation results of the bending resistance of the obtained single-sided flexible copper-clad laminate.

(Example 5)
A flexible copper-clad laminate was obtained in the same manner as in Example 4, except that one side (surface roughness Rz = 1.2 μm) of a long commercially available electrolytic copper foil having a thickness of 9 μm was used. The evaluation results of the bending resistance of the obtained flexible copper clad laminate are shown in Table 1.

(Example 6)
A flexible copper-clad laminate was obtained in the same manner as in Example 3, except that one side (surface roughness Rz = 1.9 μm) of a long, commercially available electrolytic copper foil having a thickness of 12 μm was used. The evaluation results of the bending resistance of the obtained flexible copper clad laminate are shown in Table 1.

(Example 7)
A flexible copper-clad laminate was obtained in the same manner as in Example 3, except that one side (surface roughness Rz = 1.2 μm) of a long, commercially available electrolytic copper foil having a thickness of 9 μm was used. The evaluation results of the bending resistance of the obtained flexible copper clad laminate are shown in Table 1.

(Example 8)
A flexible copper-clad laminate was obtained in the same manner as in Example 3, except that one side (surface roughness Rz = 2.2 μm) of a long, commercially available electrolytic copper foil having a thickness of 12 μm was used. The evaluation results of the bending resistance of the obtained flexible copper clad laminate are shown in Table 1.

(Comparative example 1)
Using one side (surface roughness Rz = 1.2 μm) of a long strip commercially available electrolytic copper foil having the characteristics shown in Table 1 and a thickness of 12 μm, the thickness configuration of the polyimide layer was changed as follows A flexible copper-clad laminate was obtained in the same manner as in Example 1 except for the above. Here, the thickness configuration of the polyimide layer is that the resin solution of the polyamic acid a prepared in Synthesis Example 1 on a copper foil has a thickness of 4.0 μm after curing and the resin of the polyamic acid b prepared in Synthesis Example 2 thereon. The cured solution thickness was 42.0 μm, and the resin solution of the polyamic acid a prepared in Synthesis Example 1 was further cured to a thickness of 4.0 μm. Table 1 shows the evaluation results of the bending resistance of the obtained flexible copper clad laminate.

(Comparative example 2)
Using one side (surface roughness Rz = 2.0 μm) of a long strip commercially available electrolytic copper foil having the characteristics shown in Table 1 and a thickness of 12 μm, the thickness configuration of the polyimide layer is changed as follows A flexible copper-clad laminate was obtained in the same manner as in Example 2 except for the above. Here, the thickness configuration of the polyimide layer is 3.0 μm after curing of the resin solution of the polyamic acid a prepared in Synthesis Example 1 on a copper foil, and the resin of the polyamic acid c prepared in Synthesis Example 3 thereon. The thickness after curing of the solution was 32.0 μm, and further, the resin solution of the polyamic acid a prepared in Synthesis Example 1 was 3.0 μm after curing.

(Comparative example 3)
Using one side (surface roughness Rz = 1.8 μm) of a long strip commercially available electrolytic copper foil having the characteristics shown in Table 1 and a thickness of 12 μm, the thickness configuration of the polyimide layer is changed as follows: A flexible copper-clad laminate was obtained in the same manner as in Example 2 except for the above. Here, the thickness configuration of the polyimide layer is such that the resin solution of polyamic acid a prepared in Synthesis Example 1 on a copper foil has a thickness of 2.5 μm after curing and the resin of polyamic acid c prepared in Synthesis Example 3 thereon. The cured solution thickness was 20.0 μm, and the resin solution of the polyamic acid a prepared in Synthesis Example 1 was further cured to a thickness of 2.5 μm.

  From Table 1, the thickness of the polyimide layer is 5 to 30 μm, the tensile elastic modulus is 4 to 10 GPa, the thickness of the copper foil is in the range of 6 to 20 μm, the tensile elastic modulus is in the range of 25 to 35 GPa, The ten-point average roughness (Rz) of the copper foil on the surface in contact with the polyimide layer is in the range of 0.7 to 2.2 μm, and the bending coefficient [PF] is in the range of 0.96 ± 0.025. The flexible copper-clad laminates of Examples 1 to 8 of the present invention show that the bending resistance is satisfactory. On the other hand, in Comparative Examples 1 and 2 in which the thickness of the polyimide layer exceeds 30 μm and Comparative Example 3 in which the tensile modulus of the copper foil exceeds 35 GPa, the number of folds is small and bending resistance is poor.

  Although the embodiments of the present invention have been described in detail for the purpose of illustration, the present invention is not limited to the above embodiments.

1: Circuit board
11: Polyimide layer
12, 51: Copper wiring
20, 21: sample stage
22: Roller
40: Test piece
40C: Bending point of test piece
52: U-shaped part of copper wiring

Claims (5)

A flexible circuit board which is folded so as to be folded and folded by being folded so that the upper surface side is turned 180 degrees and the lower surface side is folded in a housing of the electronic device,
A polyimide layer (A) in the range of 5 to 30 μm in thickness and in the range of tensile modulus of 4 to 10 GPa,
And copper wiring composed of copper foil (B) within a thickness of 6 to 20 μm and having a tensile modulus of elasticity of 25 to 35 GPa laminated on at least one surface of the polyimide layer (A),
The ten-point average roughness (Rz) of the copper foil (B) on the side in contact with the polyimide layer (A) is in the range of 0.7 to 2.2 μm, and the gap 0. A flexible circuit board characterized in that a bending modulus [PF] calculated by the following formula (1) in a bending test at 3 mm is in a range of 0.96 ± 0.025.

[In formula (1), | ε | is an absolute value of the bending average strain value of the copper wiring, and εc is a tensile elastic limit strain of the copper wiring. ] Polyimide layer (A), low thermal expansion polyimide layer is less than the thermal expansion coefficient of 30 × ^{10 -6} / K and (i), the thermal expansion coefficient of 30 × ^{10 -6} / K or more high thermal expansion of the polyimide layer (ii The flexible circuit board according to claim 1, wherein the high thermal expansion polyimide layer (ii) is in direct contact with the copper foil (B).   The flexible circuit board according to claim 1 or 2, wherein the thickness of the polyimide layer (A) is in the range of 8 to 15 μm, and the tensile elastic modulus is in the range of 6 to 10 GPa.   The thickness ratio [polyimide layer (A) / copper foil (B)] of a polyimide layer (A) and copper foil (B) exists in the range of 0.9-1.1 in any one of Claims 1-3. The flexible circuit board of description.   A copper foil (B) is an electrolytic copper foil, The flexible circuit board in any one of Claims 1-4.

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