JP2018139295A - Flexible circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flexible circuit board having superior durability against bending and folding, which is arranged so that a break of circuitry and a crack of the board can be prevented.SOLUTION: A flexible circuit board comprises: a polyimide layer having a thickness within a range of 5 to 30 μm and a tensile elasticity within a range of 4 to 10 GPa; and copper interconnections laminated on at least one face of the polyimide layer made of copper foil, and having a thickness within a range of 6 to 20 μm and a tensile elasticity within a range of 25 to 35 GPa. The flexible circuit board has a ten-point average roughness (Rz) within a range of 0.7 to 2.2 μm about the copper foil on the face on a side where the circuit board is in contact with the polyimide layer, and a creasing habit coefficient [PF] within a range of 0.96±0.025, which is calculated according to the formula (1) in a bending test with a gap 0.3 mm. [In the formula (1), |ε| denotes an absolute value of a flexural average strain value of the copper interconnections and εc denotes a tensile elastic limit strain of the copper interconnections].SELECTED DRAWING: None

Description

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

  In recent years, with the miniaturization and high functionality of electronic devices, higher performances such as electrical characteristics, mechanical characteristics, heat resistance, and the like have been demanded for FPCs, which are one of the electronic components constituting these. Many of FPCs are manufactured by forming a circuit on a copper foil of a flexible copper-clad laminate obtained by laminating a polyimide foil as an insulating layer on a copper foil as a metal layer. A copper-clad laminate using such polyimide as an insulating layer is a copper-clad laminate (“three layers”) in which polyimide and copper foil are laminated between a polyimide and copper foil via a thermosetting adhesive layer such as an epoxy resin. And a copper-clad laminate (also referred to as “two-layer CCL”) in which polyimide and copper foil are directly laminated without interposing 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 that requires high-temperature processing, such as a process of bonding an electrode on an FPC wiring to a monitor panel substrate, a rigid substrate, a semiconductor chip, or the like using solder or a heat tool. In addition, the three-layer CCL is a high-end electronic device in that the thickness of the adhesive layer is added to the two-layer CCL, the dimensional control is difficult due to the difference in thermal expansion coefficient between different materials, and from the viewpoint of dielectric characteristics. There is a problem with mounting. In view of this, two-layer CCL that does not use a thermosetting adhesive such as an epoxy resin has been put on the market especially in applications that require high heat resistance and reliability.

  By the way, with the recent diversification of models of portable terminal devices, the usage forms of FPCs used therein are also changing. Unlike usage patterns where a certain amount of bending radius is ensured, such as hinge bending portions and slide bending portions found in conventional mobile phones, it is more severe that it can be folded with a crease for storage in a thin casing. Bending resistance has been demanded. Hereinafter, in this specification, folding the FPC so that the upper surface side of the FPC is inverted by approximately 180 degrees to become the lower surface side may be referred to as “shell folding”.

  As intended for application to such applications, Patent Document 1 proposes a highly flexible flexible circuit board that exhibits high flexibility and excellent dimensional stability. However, in the invention of Patent Document 1, a metal wiring pattern is formed on a polyimide base film via an adhesive layer, and a polyimide having a relatively low elastic modulus range is used as a base substrate. Moreover, since an adhesive layer is required, the characteristics such as heat resistance of the two-layer CCL made of only polyimide cannot be fully utilized.

  Patent Document 2 proposes a polyimide metal laminate suitable for a circuit board used in a state of being folded in an electronic device. However, although the polyimide metal laminate disclosed here pays attention to the elastic modulus of the non-thermoplastic polyimide film constituting the polyimide layer, it does not pay attention to the elastic modulus of the copper foil side used together. Since the folding resistance is shown only once, it was not practical enough.

  Further, in the FPC design, from the viewpoint of impedance matching with the bonding destination substrate, the wiring can be thickened if the thickness of the polyimide layer that is the insulating layer of the flexible copper-clad laminate is thick. That is, wiring processing is easy, but on the other hand, when it is intended to be housed in a thin or narrow housing, it is difficult to fold due to the repulsive force of the substrate, and there is a problem in handling FPC. On the other hand, if the thickness of the polyimide layer is thin, it is necessary to make the wiring thinner from the viewpoint of impedance matching. That is, while the difficulty of wiring workability is increased, it is low repulsion, so that it is relatively easy to store in a thin or narrow housing, and the handling property of the FPC is good.

JP 2007-208087 A JP 2012-6200 A

  An object of the present invention is to provide a flexible copper-clad laminate that provides an FPC with excellent bending resistance that can prevent disconnection and cracking of a wiring circuit even when used in a thin or narrow electronic device casing. And

  As a result of intensive studies, the present inventors have optimized the characteristics of the copper foil and the polyimide film, and can solve the above problems by paying attention to the characteristics of the printed circuit board obtained by processing the flexible copper-clad laminate. The present inventors have found that a flexible copper-clad laminate can be provided and have completed the present invention.

That is, the flexible copper-clad laminate of the present invention is a flexible copper-clad laminate used for a flexible circuit board that is folded and stored in a casing of an electronic device,
A polyimide layer (A) having a thickness in the range of 5 to 30 μm and a tensile modulus of elasticity in the range of 4 to 10 GPa;
A copper foil (B) having a thickness of 6 to 20 μm and a tensile modulus 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 surface 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 folding coefficient [PF] calculated by the following formula (1) in a bending test with a gap of 0.3 mm of an arbitrary flexible circuit board in which copper wiring is formed by circuit processing is 0.96 ± 0.025 It is 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 Expression (1), | ε | is the absolute value of the bending average strain value of the copper wiring, and εc is the 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 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) is in direct contact with the copper foil (B).

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

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

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

  Since the flexible copper-clad laminate of the present invention can express the high bending resistance required for a wiring board, it provides a flexible circuit board material that is excellent in connection reliability in a state of being bent in an electronic device. be able to. Therefore, the flexible copper-clad laminate of the present invention is suitably used for electronic parts that require bending resistance such as a bent portion around a small liquid crystal such as a smartphone.

FIG. 1 is an explanatory 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 on a wiring circuit. It is plane explanatory drawing which shows the mode of the copper wiring of the test circuit board piece used in the Example. It is side surface explanatory drawing which shows the mode of the sample stage and test circuit board piece in a bending test (state figure which fixed the test circuit board piece on the sample stage). It is side surface explanatory drawing which shows the mode of the sample stage and test circuit board piece in a bending test (state figure before pressing the bending location of a test circuit board piece with a roller). It is side surface explanatory drawing which shows the mode of the sample stage and test circuit board piece in a bending test (state figure which pressed the bending location of the test circuit board piece with the roller). It is side surface explanatory drawing which shows the mode of the sample stage and test circuit board piece in a bending test (the state figure which opened the bending location and returned the test piece to the flat state). It is side surface explanatory drawing which shows the mode of the sample stage and test circuit board piece in a bending test (state figure which presses and equalizes the crease | fold part of a bending location with a roller). It is a section explanatory view (part) of a flexible circuit board.

  Embodiments of the present invention will be described below. The flexible copper clad laminate of this embodiment is composed of a polyimide layer (A) and a copper foil (B). The copper foil (B) is provided on one 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 processed by wiring circuit processing, such as etching a copper foil, to form a copper wiring, and is folded and housed in a casing of an electronic device.

<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. It is particularly preferred. When the thickness of the polyimide layer (A) exceeds 30 μm, when the FPC is bent, a large bending stress is applied to the copper wiring, and the bending resistance is remarkably lowered.

  Moreover, the tensile elasticity modulus of a polyimide layer (A) is in the range of 4-10 GPa, Preferably it is good in the range of 6-10 GPa. If 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 film tearing when the flexible copper clad laminate is processed into a circuit board. Sometimes. On the other hand, if the tensile modulus of the polyimide layer (A) exceeds 10 GPa, the bending resistance of the flexible copper clad laminate increases, resulting in an increase in bending stress applied to the copper wiring when the FPC is bent. The nature will decline.

  As the polyimide layer (A), a commercially available polyimide film can be used as it is. However, in order to easily control the thickness and physical properties of the insulating layer, the polyamic acid solution is directly applied on the copper foil, followed by heat treatment. It is preferable to use a so-called cast (coating) method in which drying and curing are performed. Moreover, although a polyimide layer (A) may be formed only from a single layer, when the adhesiveness etc. of a polyimide layer (A) and copper foil (B) are considered, what consists of multiple layers is preferable. When the polyimide layer (A) has a plurality of layers, it can be formed by sequentially applying another polyamic acid solution on a polyamic acid solution composed of different components. 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, the polyimide layer (A) is a low thermal expansion polyimide layer having a thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K. A laminated structure including (i) and a high thermal expansion polyimide layer (ii) having a thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more is preferable. More preferably, the polyimide layer (A) has a laminated structure having a high thermal expansion polyimide layer (ii) on at least one of the low thermal expansion polyimide layers (i), preferably on both sides thereof, and a high thermal expansion polyimide. It is preferable that the layer (ii) is in direct contact with the copper foil (B). Here, the “low thermal expansion polyimide layer (i)” is 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. Means a polyimide layer in the range of 3 × 10 ^{−6 to} 20 × 10 ⁻⁶ / K. The “high thermal expansion polyimide layer (ii)” refers to a polyimide layer having a thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more, preferably in the range of 30 × 10 ⁻⁶ to 80 × 10 ⁻⁶ / K. Of these, a polyimide layer in the range of 30 × 10 ⁻⁶ to 70 × 10 ⁻⁶ / K is particularly preferable. Such a polyimide layer can be made into a polyimide layer having a desired thermal expansion coefficient by appropriately changing the combination of raw materials used, thickness, and drying / curing conditions.

  The polyamic acid solution that gives 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 in the range of 500 cps to 35,000 cps, for example.

  Examples of the diamine used as a raw material for polyimide include 4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2,4-diaminomesitylene, and 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, 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'-diame 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.

  Examples of the acid anhydride used as a raw material for polyimide include pyromellitic dianhydride, 3,3 ′, 4,4′-benzophenone tetracarboxylic dianhydride, 2,2 ′, 3,3 ′. -Benzophenone tetracarboxylic dianhydride, 2,3,3 ', 4'-benzophenone tetracarboxylic dianhydride, naphthalene-1,2,5,6-tetracarboxylic dianhydride, naphthalene-1,2, 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 dianhydride, 4,8-dimethyl-1,2,3,5,6,7- Hexahydronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1, 4,5,8-tetracarboxylic 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 , 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 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-dicarboxyl) 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 dianhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, perylene-4,5,10,11-tetracarboxylic dianhydride, perylene-5, 6,11,12-tetracarboxylic dianhydride, phenanthrene-1,2,7,8-tetracarboxylic dianhydride, phenanthrene-1,2,6,7-tetracarboxylic dianhydride, phenanthrene-1 , 2,9,10-Tetracarboxylic dianhydride, cyclopentane-1,2,3,4-tetracarboxylic dianhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, pyrrolidine -2,3,4,5-tetracarboxylic dianhydride, thiophene-, 3,4,5-tetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 2,3,6,7 -Naphthalene tetracarboxylic dianhydride, etc. And the like.

  Each of the diamine and acid anhydride may be used alone or in combination of two or more. Examples of the solvent used for the polymerization include dimethylacetamide, N-methylpyrrolidinone, 2-butanone, diglyme, xylene and the like, and they can be used alone or in combination of two or more.

In this 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, 3,3 ′ , 4,4'-biphenyltetracarboxylic dianhydride and 2,2'-dimethyl-4,4'-diaminobiphenyl, 2-methoxy-4,4'-diaminobenzanilide as the diamine component Particularly preferred are those containing pyromellitic dianhydride and 2,2′-dimethyl-4,4′-diaminobiphenyl as the main components of the respective raw materials.

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

  Further, 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 the high thermal expansion The ratio of the thickness of the conductive 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 the copper foil is etched increases. If it exceeds 15, the high thermal expansion polyimide layer becomes thin, so that the reliability of adhesion between the polyimide film and the copper foil is lowered. In addition, 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 in calculating the folding 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 reduced during the production of a flexible copper-clad laminate, for example, in the step of forming a polyimide layer on the copper foil. The problem that wrinkles etc. generate | occur | produce on a tension laminated board arises. On the other hand, if the thickness of the copper foil (B) exceeds 20 μm, the bending stress applied to the copper wiring when the FPC is bent increases, so that the bending resistance decreases.

  Furthermore, in this Embodiment, it is preferable that 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. . When this thickness ratio is less than 0.9 or greater than 1.1, the bending resistance is lowered due to an increase in the maximum tensile strain when the plastically deformed portion is stretched during bending.

  Moreover, about the tensile elasticity modulus of copper foil (B), it exists in the range of 25-35 GPa. If the tensile elastic modulus of the copper foil (B) is less than 25 GPa, the rigidity of the copper foil itself is affected by the heating conditions of the copper foil itself in the process of forming a polyimide layer on the copper foil, for example, during the production of a flexible copper-clad laminate. Will fall. As a result, the problem that wrinkles etc. generate | occur | produce on a flexible copper clad laminated board arises. On the other hand, when the tensile modulus exceeds 35 GPa, a large bending stress is applied to the copper wiring when the FPC is bent, and the bending resistance is remarkably lowered.

  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. Within the range, preferably within the range of 0.8 to 1.6 μm. When the value of the surface roughness (Rz) of the copper foil (B) is less than 0.7 μm, it is difficult to ensure the reliability of adhesion with the polyimide film, and when it exceeds 2.2 μm, when the FPC is repeatedly bent, Roughness of the roughened particles tends to be the starting point for cracks. As a result, the bending resistance of the FPC is reduced. The surface roughness Rz is a value measured according to JIS B0601.

  The copper foil used for the flexible copper clad laminate of the present 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. From the viewpoint of ease of manufacture and price in the case of performing, it is preferable to use an electrolytic copper foil. 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 Nihon Electrolytic Co., Ltd., and HTE foil manufactured by Mitsui Metal Mining Co., Ltd. Moreover, even if it is a case where other than that including these commercial items is used, according to the heat processing conditions at the time of forming the polyimide layer (A) on copper foil mentioned above, copper foil (B) Since the tensile elastic modulus of can be changed, it is only necessary that the resultant flexible copper-clad laminate falls within these predetermined ranges in the present embodiment.

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

<FPC>
The flexible copper clad laminate of the present embodiment is mainly useful as an FPC material. That is, the FPC which is one 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 a conventional 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 the flexible copper-clad laminate is processed into a wiring circuit to produce a copper wiring. The bending coefficient [PF] calculated by the following (1) in the bending test (gap 0.3 mm) of an arbitrary flexible circuit board formed must be in the range of 0.96 ± 0.025. , Preferably in the range of 0.96 ± 0.02, more preferably in the range of 0.96 ± 0.015. This crease coefficient [PF] is a value determined by a stress-strain curve obtained from a uniaxial tensile test of the copper foil used. When the folding coefficient [PF] is out of the above range, the stress is concentrated locally (one point or two points), so that the bending resistance is lowered. On the other hand, when the crease coefficient [PF] is in the above range, the bending resistance such as goby folding is improved by the appropriate dispersion of the stress. For example, in the case where an electrolytic copper foil is used in the present invention, in order to make the folding coefficient [PF] defined in the present invention in the above range, in the stress-strain curve obtained from the uniaxial tensile test of the electrolytic copper foil used, An example of using a copper foil in which the slope of the initial straight line portion, that is, the elastic modulus is 29 GPa or less, the stress value at the portion where the curvature is maximum is 130 MPa or less, the strain is 5%, and the stress is 175 MPa or less.

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 elastic 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. Calculated by (2). In the following, with respect to the folding coefficient [PF], a circuit board provided with a copper wiring 12 in which one layer of copper foil is processed into a wiring circuit on one side of the polyimide layer 11 made of one layer of polyimide shown in FIG. 8 is used as a model. The case where the circuit board is bent so that the reference plane SP that is the lower surface of the polyimide layer 11 that is the first layer has a convex shape downward (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 transverse cross section) cut perpendicular to the longitudinal direction of the circuit board.
ε = − (yc− [NP] _Line ) / R (2)

  Here, with respect to 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 half, and yc in the equation is the polyimide layer 12 is a distance from the reference plane SP which is the lower surface of 12 to the center plane of the copper wiring 12. The symbol NP represents the neutral plane of the circuit board. Here, the distance between the neutral plane NP and the reference plane SP is defined as a neutral plane position [NP]. The neutral plane position [NP] is formed by copper wiring and copper wiring formed by wiring circuit processing of copper foil. It calculates separately with the space part formed between. The neutral plane position [NP] is calculated by the following equation (3).

Here, E _i is the tensile modulus of the material constituting the i-th layer (in the example shown in FIG. 8, the first layer is the polyimide layer 11 and the second layer is the copper wiring 12) in the circuit board. It is. This elastic modulus E _i corresponds to “relation between stress and strain in each layer” in the present embodiment. B _i is the width of the i-th layer and corresponds to the width B shown in FIG. 8 (dimension in the direction 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 The value of the line width SW of the wiring is used. h _i is the distance between the center plane of the i-th layer and the reference plane SP. The central surface of the i-th layer is a virtual surface located at the center in the thickness direction of the i-th layer. t _i is the thickness of the i-th layer. Further, the symbol “Σ _{i = 1} ⁿ ” represents the sum of i from 1 to n. The neutral plane position in the copper wiring is denoted as [NP] _Line .

Further, R in the expression (2) represents an effective radius of curvature, and the effective radius of curvature R is a distance from the bending center at the bent portion to the neutral plane NP of the copper wiring when the circuit board is bent in the 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, the crease coefficient [PF] representing the degree of crease of the entire circuit board is calculated by obtaining the neutral plane position, effective radius of curvature, and bending average strain. Further, as described above, the folding coefficient [PF] is 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 description (FIG. 8), for the sake of convenience, the model in which the circuit board has two layers has been described. However, the above description also applies to the case where the circuit board is formed of two or more layers. That is, when the number of layers of the circuit board 1 is n, n is an integer equal to or greater than 2, and the i-th (i = 1, 2,... The layer n) is called the i-th layer.

  Further, as shown in FIG. 1, the circuit board has a copper foil patterned by wiring circuit processing, and has a portion where the copper wiring 12 is present and a portion where the copper wiring 12 is not present. Here, if 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 wiring portion and the space portion have different configurations. 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 10 rows of copper wiring 12 (only four rows are shown in FIG. 1), and the space portion is the main portion other than the wiring portion. And a gap between the copper wirings 12. As described above, the folding coefficient [PF] can be calculated separately for the wiring portion and the space portion.

  Hereinafter, based on an Example, this invention is demonstrated in detail. In addition, each characteristic evaluation in the following Example was performed with the following method.

[Measurement of tensile modulus]
The value of the tensile elastic modulus was measured in an environment of a temperature of 23 ° C. and a relative humidity of 50% using a strograph R-1 manufactured by Toyo Seiki Seisakusho.

[Measurement of coefficient of thermal expansion (CTE)]
Using a thermomechanical analyzer manufactured by Seiko Instruments Inc., the temperature was raised to 250 ° C., held at that temperature for 10 minutes, cooled at a rate of 5 ° C./minute, and an average thermal expansion coefficient from 240 ° C. to 100 ° C. ( (Linear thermal expansion coefficient) was determined.

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

[Measurement of seam fold (bending test)]
A test piece (test circuit board piece) formed by etching copper foil of a flexible copper-clad laminate and forming 10 rows of copper wiring having a line width of 100 μm and a space width of 100 μm and a length of 40 mm along the longitudinal direction. It produced (FIG. 2). As shown in FIG. 2 showing only the copper wiring in the test piece, the 10 rows of copper wirings 51 in the test piece 40 are all continuously connected via the U-shaped portion 52, and resistance is provided at both ends thereof. An electrode portion (not shown) for value measurement is provided. The test piece 40 was fixed on the sample stages 20 and 21 that can be folded in half, and a resistance value measurement wiring was connected to start monitoring of the resistance value (FIG. 3). In the bending test, the copper wiring 51 of 10 rows was bent at the central portion in the longitudinal direction while controlling the gap G of the folding portion 40C to be 0.3 mm using the urethane roller 22. After moving the roller in parallel with the line and bending all 10 rows of copper wiring 51 (FIGS. 4 and 5), the bent portion is opened to return the test piece to a flat state (FIG. 6), and the creased portion Was moved again while being held down by a roller (FIG. 7), and this series of steps was counted as one folding. While constantly monitoring the resistance value of the wiring, the bending test was repeated, and when the predetermined resistance value (3000 Ω) was reached, it was judged that the wiring was broken. . This case was evaluated as “good” when the measured value of the folding angle was 50 times or more, and “bad” when it was less than 50 times.

  The method for producing the flexible copper-clad laminate described in Examples and Comparative Examples will be described below.

[Synthesis of polyamic acid solution]
(Synthesis Example 1)
Bottom resin synthesis:
A reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen was charged with N, N-dimethylacetamide, and further, 2,2-bis [4- (4-aminophenoxy) phenyl] propane (BAPP ) Was added and dissolved in the container with stirring. Next, pyromellitic dianhydride (PMDA) was added so that the total amount of monomers added was 12% by mass. Thereafter, 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 the polyimide film having a thickness of 25 μm formed from the polyamic acid a was 55 × 10 ⁻⁶ / K.

(Synthesis Example 2)
A reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen is charged with N, N-dimethylacetamide, and this reaction vessel is further filled with 2,2′-dimethyl-4,4′-diaminobiphenyl (m-TB). And 4,4′-diaminodiphenyl ether (DAPE) was added so that the molar ratio of each diamine (m-TB: DAPE) was 60:40, and dissolved in the container with stirring. Next, pyromellitic dianhydride (PMDA) was added so that the total amount of monomers added was 16% by mass. Thereafter, 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 the polyimide film having a thickness of 25 μm formed from the polyamic acid b was 22 × 10 ⁻⁶ / K.

(Synthesis Example 3)
A reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen is charged with N, N-dimethylacetamide, and this reaction vessel is further filled with 2,2′-dimethyl-4,4′-diaminobiphenyl (m-TB). Was dissolved while stirring in a container. Next, 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) were added in a total amount of 15% by mass of monomers, and the molar ratio of each acid anhydride. (BPDA: PMDA) was added so as to be 20:80. Thereafter, 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 the polyimide film having a thickness of 25 μm formed from the polyamic acid c was 22 × 10 ⁻⁶ / K.

Example 1
The thickness after curing 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 with a thickness of 12 μm is 2.5 μm. After uniformly coating, the solvent was removed by heating at 130 ° C. Next, the resin solution of the polyamic acid b prepared in Synthesis Example 2 was uniformly applied to the coated surface side so that the thickness after curing was 20.0 μm, and the solvent was removed by heating and drying at 120 ° C. Furthermore, the same polyamic acid a resin solution as that applied in the first layer is applied on the coated surface side uniformly so that the thickness after curing is 2.5 μm, and the solvent is removed by heating at 130 ° C. did. This continuous laminate was heat-treated in a continuous curing furnace set so that the temperature gradually increased from 300 ° C. to 300 ° C. over a period of about 6 minutes. A 25 μm single-sided flexible copper-clad laminate was obtained. Properties of the polyimide layer and the copper foil constituting the obtained flexible copper-clad laminate, such as tensile modulus, thickness, thickness ratio between the polyimide layer and the copper foil, folding coefficient, and bending resistance of the flexible copper-clad laminate The evaluation results of the property (number of times of folding) are shown in Table 1 (the same applies to Examples 2 and below). In addition, evaluation of the polyimide layer used what removed the copper foil by etching from the manufactured flexible copper clad laminated board.

Here, a specific calculation procedure for the calculation of the folding coefficient [PF] of the flexible copper-clad laminate manufactured in the example will be described using Example 1 as an example.
Considering a two-layer structure as shown in FIG. 8 for the wiring portion where the copper wiring 12 exists, the materials constituting the first layer and the second layer are polyimide and copper, respectively. As shown in Table 1 (Example 1), the elastic modulus of each layer is E ₁ = 4 GPa, E ₂ = 29 GPa, and the thicknesses are t ₁ = 25 μm and t ₂ = 12 μm. Further, the distance between the center 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. Further, regarding the width B, the width B ₂ of the copper wiring 12 and the width B _{2 ′} of the space portion are both 100 μm, and the width B ₁ of the polyimide immediately below the copper wiring 12 is also set to 100 μm (space portion). The width B _{1 ′} of the polyimide immediately below is also 100 μm).

When these values are substituted into 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, the neutral plane position [NP] _Line and the gap interval G = 0.3 mm are substituted into the equation (4), and the effective bending radius R = 0.123 mm is calculated. Further, since the distance yc from the reference plane SP to the center plane of the copper wiring 12 is yc = h ₂ = 31 μm, the bending average strain ε is the value of [NP] _Line , R previously determined by yc and the equation ( Substituting into 2), ε = −0.0333 is calculated. Here, a minus sign indicates a compressive strain. The tensile elastic limit strain εc of the copper wiring was determined to be εc = 0.00058 from the stress-strain curve obtained from the tensile test of the copper foil serving as the copper wiring in Example 1. By substituting this and the value of the bending average strain ε obtained previously into the equation (1), the folding coefficient [PF] is calculated as [PF] = 0.983. In this example, since the space portion is composed only of the polyimide layer, the operation for obtaining [NP] is not required, and the folding coefficient [PF] of other examples and comparative examples in Table 1 is not required. Is a value calculated by the above procedure.

(Example 2)
Thickness after curing the resin solution of polyamic acid a prepared in Synthesis Example 1 on one side (surface roughness Rz = 1.2 μm) of a long commercial electrolytic copper foil with a thickness of 12 μm is 2.0 μm. After uniformly coating, the solvent was removed by heating at 130 ° C. Next, the polyamic acid c resin solution prepared in Synthesis Example 3 was uniformly applied on the coated surface side so that the thickness after curing was 16 μm, and the solvent was removed by heating and drying at 130 ° C. Furthermore, the same polyamic acid a resin solution as that applied in the first layer is applied uniformly to the coated surface so that the thickness after curing is 2.0 μm, and the solvent is removed by heating at 130 ° C. did. This continuous laminate was heat-treated in a continuous curing furnace set so that the temperature gradually increased from 300 ° C. to 300 ° C. over a period of about 6 minutes. 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 the resin solution of the 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 with a thickness of 12 μm is 2.2 μm. After uniformly coating, the solvent was removed by heating at 130 ° C. Next, the polyamic acid c resin solution prepared in Synthesis Example 3 was uniformly applied to the coated surface side so that the thickness after curing was 7.6 μm, and the solvent was removed by heating at 130 ° C. Further, apply the same polyamic acid a resin solution as that applied in the first layer on the coated surface side so that the thickness after curing is 2.2 μm, and heat dry at 130 ° C. to remove the solvent. did. This continuous laminate was heat-treated in a continuous curing furnace set so that the temperature gradually increased from 300 ° C. to 300 ° C. over a period of about 6 minutes. 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)
The thickness 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, commercially available electrolytic copper foil with a thickness of 12 μm is 2.0 μm. After uniformly coating, the solvent was removed by heating at 130 ° C. Next, the polyamic acid c resin solution prepared in Synthesis Example 3 was uniformly applied to the coated surface side so that the thickness after curing was 5.0 μm, and the solvent was removed by heating and drying at 130 ° C. Furthermore, the same polyamic acid a resin solution as that applied in the first layer is applied uniformly to the coated surface so that the thickness after curing is 2.0 μm, and the solvent is removed by heating at 130 ° C. did. This continuous laminate was heat-treated in a continuous curing furnace set so that the temperature gradually increased from 300 ° C. to 300 ° C. over a period of about 6 minutes. 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 of a commercially available electrolytic copper foil having a thickness of 9 μm (surface roughness Rz = 1.2 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 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 of a commercially available electrolytic copper foil with a thickness of 12 μm and a thickness of 12 μm (surface roughness Rz = 1.2 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 that. Here, the polyimide layer has a thickness of 4.0 μm after curing the polyamic acid a resin solution prepared in Synthesis Example 1 on a copper foil, and the polyamic acid b resin prepared in Synthesis Example 2 thereon. The thickness after curing of the solution was 42.0 μm, and the thickness after curing of the polyamic acid a resin solution prepared in Synthesis Example 1 was set to 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 commercially available electrolytic copper foil with a thickness of 12 μm and having the characteristics shown in Table 1, 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 2 except that. Here, the thickness structure of the polyimide layer was 3.0 μm after curing the polyamic acid a resin solution prepared in Synthesis Example 1 on a copper foil, and the polyamic acid c resin prepared in Synthesis Example 3 thereon. The thickness after curing of the solution was 32.0 μm, and the thickness after curing of the polyamic acid a resin solution prepared in Synthesis Example 1 was 3.0 μm.

(Comparative Example 3)
Using one side of a commercially available electrolytic copper foil with a thickness of 12 μm and a thickness of 12 μm (surface roughness Rz = 1.8 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 2 except that. Here, the thickness structure of the polyimide layer is 2.5 μm after curing the polyamic acid a resin solution prepared in Synthesis Example 1 on a copper foil, and the polyamic acid c resin prepared in Synthesis Example 3 thereon. The thickness after curing of the solution was 20.0 μm, and the thickness after curing of the resin solution of polyamic acid a prepared in Synthesis Example 1 was 2.5 μm.

  From Table 1, the polyimide layer has a thickness of 5 to 30 μm, a tensile modulus of 4 to 10 GPa, a copper foil thickness of 6 to 20 μm, a tensile modulus of 25 to 35 GPa, and 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 crease coefficient [PF] is in the range of 0.96 ± 0.025. The flexible copper-clad laminates of Examples 1 to 8 were satisfactory in bending resistance. 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 elastic modulus of the copper foil exceeds 35 GPa, the number of helix folds was small and the bending resistance was poor.

  As mentioned above, although embodiment of this invention was described in detail for the purpose of illustration, this invention is not restrict | limited to the said embodiment.

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

Claims (5)

A flexible circuit board that is folded and stored in a casing of an electronic device,
A polyimide layer (A) having a thickness in the range of 5 to 30 μm and a tensile modulus of elasticity in the range of 4 to 10 GPa;
A copper wiring made of a copper foil (B) within a range of a thickness of 6 to 20 μm and a tensile modulus 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 surface in contact with the polyimide layer (A) is in the range of 0.7 to 2.2 μm, and the gap of the flexible circuit board is 0. A flexible circuit board having a folding coefficient [PF] calculated by the following formula (1) in a bending test at 3 mm in a range of 0.96 ± 0.025.

[In Expression (1), | ε | is the absolute value of the bending average strain value of the copper wiring, and εc is the tensile elastic limit strain of the copper wiring. ] The polyimide layer (A) is a low thermal expansion polyimide layer (i) having a thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K, and a high thermal expansion polyimide layer (ii) having a thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more. 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 polyimide layer (A) has a thickness in the range of 8 to 15 µm and a tensile modulus in the range of 6 to 10 GPa.   The thickness ratio [polyimide layer (A) / copper foil (B)] of the polyimide layer (A) and the copper foil (B) is in the range of 0.9 to 1.1. The flexible circuit board as described. The flexible circuit board according to claim 1, wherein the copper foil (B) is an electrolytic copper foil.

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