JP6624756B2 - Flexible circuit board, method of using the same, and electronic equipment - Google Patents

Description

  The present invention relates to a flexible circuit board (FPC), and more particularly, to a flexible circuit board that is used by being folded and stored in a housing of an electronic device, a method of using the same, and an electronic device.

  In recent years, electronic devices typified by mobile phones, notebook computers, digital cameras, game machines, and the like have been rapidly becoming smaller, thinner, and lighter. High-density, high-performance materials capable of storing components have been desired. With the spread of high-performance small-sized electronic devices such as smartphones, the density of parts storage has also increased with the spread of flexible circuit boards. I have. Therefore, the flexible copper-clad laminate, which is the material of the flexible circuit board, is also required to have improved bending resistance from the material side. Hereinafter, in this specification, folding the FPC so that the upper surface side is inverted by approximately 180 ° C. and becomes the lower surface side may be referred to as “folding”.

  As an application intended for such an application, Patent Document 1 discloses a technique of controlling the elastic modulus of a polyimide base film or a cover film used for a flexible copper-clad laminate to reduce the total stiffness of a flexible circuit board. By doing so, a technique of improving the bending resistance has been proposed. However, controlling only the characteristics of polyimide and the cover film is not sufficient for a severe bending mode in which the film is folded and stored in an electronic device, and a flexible circuit board with sufficient bending resistance cannot be provided. .

  Patent Document 2 discloses a copper foil for heat treatment that suppresses springback resistance by focusing on the crystal grain size of the copper foil as an approach from the copper foil side from the viewpoint of increasing the density in electronic devices. Has been proposed. This technology uses a rolled copper foil containing various suitable additives in the copper foil, and by applying a sufficient amount of heat to enlarge the crystal grains, the crystal grain size grows large, and as a result, the copper foil This is a technique for improving the spring-back resistance of the device.

  However, for small electronic devices represented by smartphones, it is required that FPCs be housed in a narrow housing at a higher density. For this reason, it is difficult to meet the demand for higher density with only the above-mentioned conventional technology.

JP 2007-208087 A JP 2010-280191 A

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a flexible circuit board having excellent bending resistance and a method of using the same, which can prevent disconnection or cracking of a wiring circuit even in a narrow housing. To provide.

  The present inventors have conducted intensive studies to solve the above problems, and as a result, the thickness of the polyimide insulating layer constituting the flexible circuit board, the thickness and cube ratio of the copper wiring constituting the circuit wiring layer, and the flexible circuit By focusing on the relationship with the equivalent bending stiffness of the entire board, it has been found that a flexible circuit board that can solve the above problem can be provided, and the present invention has been completed.

The flexible circuit board of the present invention is laminated on a polyimide insulating layer (A), a circuit wiring layer (B) provided on at least one surface of the polyimide insulating layer (A), and the circuit wiring layer (B). Coverlay (C). The flexible circuit board of the present invention has the following structures a to c:
a) the thickness of the polyimide insulating layer (A) is in the range of 23 to 27 μm;
b) The thickness of the copper wiring constituting the circuit wiring layer (B) is in the range of 10 to 14 μm, and the cube ratio of the copper wiring is 85% or more;
as well as,
c) the equivalent bending stiffness when bending the coverlay (C) inside is in the range of 0.07 to 0.10 N · m ² ;
It is characterized by having.

  The flexible circuit board of the present invention may be used by being housed in a housing of an electronic device in a folded state with the coverlay (C) inside.

  In the electronic device of the present invention, the flexible circuit board is housed in a housing in a folded state with the cover lay (C) inside.

  INDUSTRIAL APPLICABILITY The flexible circuit board of the present invention can exhibit high bending resistance required for a wiring board, and thus has excellent connection reliability in a state of being bent in an electronic device. Therefore, the flexible circuit board of the present invention can be suitably used particularly for an electronic component that requires bending resistance, such as a bent portion around a small liquid crystal such as a smartphone.

FIG. 3 is an explanatory plan view showing a state of copper wiring of a test circuit board piece used in an example. It is a side explanatory view showing a situation of a sample stage and a test circuit board piece in a bending test (state figure which fixed a test circuit board piece on a sample stage). FIG. 4 is an explanatory side view showing a state of a sample stage and a test circuit board piece in a bending test (a state diagram before a bent portion of the test circuit board piece is pressed by a roller). FIG. 4 is an explanatory side view showing a state of a sample stage and a test circuit board piece in a bending test (a state diagram in which a bent portion of the test circuit board piece is pressed by a roller). FIG. 4 is an explanatory side view showing a state of a sample stage and a test circuit board piece in a bending test (a state diagram in which a bent portion is opened and the test piece is returned to a flat state). FIG. 4 is a side view illustrating a state of a sample stage and a test circuit board piece in a bending test (a state diagram in which a fold portion of a bent portion is pressed and leveled by a roller). It is sectional explanatory drawing (part) of a flexible circuit board.

Hereinafter, embodiments of the present invention will be described.
<Flexible circuit board>
The flexible circuit board of the present embodiment is laminated on a polyimide insulating layer (A), a circuit wiring layer (B) provided on one or both surfaces of the polyimide insulating layer (A), and a circuit wiring layer (B). And a coverlay (C). This flexible circuit board, for example, by etching the copper foil layer of a flexible copper-clad laminate having a polyimide insulating layer and a copper foil layer, forming a wiring circuit, forming copper wiring, and attaching a coverlay It is produced by In the case where the circuit wiring layers (B) are provided on both sides of the polyimide insulating layer (A) in the flexible circuit board, the circuit wiring layers that become inside when bent have the configuration b described later. Just do it. In this case, the cover lay that covers the circuit wiring layer that is inside when folded is the cover lay (C) of the configuration c described later.

<Polyimide insulating layer (A)>
The polyimide insulating layer (A) has a thickness in the range of 23 to 27 μm (configuration a). If the thickness of the polyimide insulating layer (A) is less than 23 μm, the equivalent bending stiffness of the flexible circuit board will be reduced, and the resistance to seam folding will be reduced. If it exceeds 27 μm, copper will be lost when the flexible circuit board is bent. More stress is applied to the wiring, and the resistance to seam folding tends to be reduced.

  For the polyimide insulating layer (A), a commercially available polyimide film can be used as it is. However, from the viewpoint of easy control of the thickness and physical properties of the insulating layer, a polyamic acid solution is directly applied on the copper foil. A so-called cast (coating) method of drying and curing by heat treatment is preferable. The polyimide insulating layer (A) may be formed of only a single layer, but is preferably formed of a plurality of layers in consideration of the adhesiveness between the polyimide insulating layer (A) and the circuit wiring layer (B). . When the polyimide insulating layer (A) has a plurality of layers, it can be formed by sequentially applying another polyamic acid solution on a polyamic acid solution having different components. When the polyimide insulating layer (A) is composed of a plurality of layers, a polyimide precursor resin having the same configuration may be used twice or more.

The polyimide insulating layer (A) will be described in more detail. As described above, it is preferable that the polyimide insulating layer (A) has a plurality of layers. As a specific example, the polyimide insulating layer (A) has a low thermal expansion coefficient of less than 30 × 10 ⁻⁶ / K. It is preferable to have a laminated structure including the polyimide layer (i) and the polyimide layer (ii) having a high thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more. More preferably, the polyimide insulating layer (A) has a laminated structure having a high thermal expansion polyimide layer (ii) on at least one, preferably both sides, of the low thermal expansion polyimide layer (i). It is preferable that the polyimide layer (ii) is in direct contact with the circuit wiring layer (B). Here, the “polyimide layer (i) with low thermal expansion” refers to a coefficient of thermal expansion 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 “high thermal expansion polyimide layer (ii)” refers to a polyimide layer having a thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more, and preferably in a range of 30 × 10 ⁻⁶ to 80 × 10 ⁻⁶ / K. Of these, particularly preferably, it refers to a polyimide layer in the range of 30 × 10 ⁻⁶ to 70 × 10 ⁻⁶ / K. Such a polyimide layer can be a polyimide layer having a desired coefficient of thermal expansion by appropriately changing the combination of raw materials to be used, the thickness, and drying / curing conditions.

  The polyamic acid solution for providing the polyimide insulating 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 or more and 35,000 cps or less.

  Examples of the diamine used as a raw material of the polyimide include, for example, 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, 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.

  Examples of the acid anhydride used as a raw material of the polyimide include, for example, pyromellitic dianhydride, 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride, 2,2 ′, 3,3 ′ -Benzophenonetetracarboxylic dianhydride, 2,3,3 ', 4'-benzophenonetetracarboxylic 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-dicarboxy) (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 -Naphthalenetetracarboxylic dianhydride And the like.

  The diamine and the 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 one or more of them can be used in combination.

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

In order to obtain a polyimide layer (ii) having a high thermal expansion coefficient of 30 × 10 ⁻⁶ / K or more, for example, pyromellitic dianhydride, 3,3 ′, 4 , 4'-biphenyltetracarboxylic dianhydride, 3,3 ', 4,4'-benzophenone tetracarboxylic dianhydride, 3,3', 4,4'-diphenyl sulfone tetracarboxylic dianhydride, As the diamine component, 2,2′-bis [4- (4-aminophenoxy) phenyl] propane, 4,4′-diaminodiphenyl ether, and 1,3-bis (4-aminophenoxy) benzene are often used. Particularly preferably, pyromellitic dianhydride and 2,2′-bis [4- (4-aminophenoxy) phenyl] propane are used as the main components of the raw materials. The preferable glass transition temperature of the polyimide layer (ii) having high thermal expansion obtained in this manner is in the range of 300 to 400 ° C.

  When the polyimide insulating 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 polyimide layer (i) are used. The thickness ratio of the polyimide layer (ii) to the expandable polyimide layer (ii) is preferably in a range of 2 to 15. If the value of this ratio is less than 2, the low thermal expansion polyimide layer (i) with respect to the entire polyimide insulating layer (A) becomes thin, and it becomes difficult to control the dimensional characteristics of the polyimide film. When the circuit wiring layer (B) is formed, the dimensional change rate becomes large, and when it exceeds 15, the polyimide layer (ii) having high thermal expansion becomes thin, so that the polyimide insulating layer (A) and the circuit wiring layer (B) become thin. The bonding reliability is reduced.

<Circuit wiring layer (B)>
In the flexible circuit board according to the present embodiment, the circuit wiring layer (B) is made of, for example, copper wiring using copper foil as a raw material. The thickness of the copper wiring forming the circuit wiring layer (B) is in the range of 10 to 14 μm, and the cube ratio of the copper wiring is 85% or more (configuration b). If the thickness of the copper wiring constituting the circuit wiring layer (B) is less than 10 μm, the rigidity of the copper-clad laminate will decrease, and handling when manufacturing the flexible circuit board tends to deteriorate. The bending stress resistance tends to decrease due to an increase in stress applied to the copper wiring when the wire is bent. Further, when the cube ratio of the copper wiring constituting the circuit wiring layer (B) is less than 85%, the anisotropy of each crystal grain of the copper wiring becomes high, and microscopic stress concentration occurs at the time of bending, so that the copper wiring is resistant. The foldability tends to decrease.

  Here, the cube ratio is an index indicating an area ratio in which a predetermined plane of copper constituting a copper wiring is oriented in a crystal orientation <200>. The cube ratio can be confirmed by an EBSD (Electron Back Scattering Diffraction) method, as described in Examples below. The copper foil, which is a raw material of the circuit wiring layer (B), undergoes a sufficient thermal history during the production of the flexible circuit board, and as a result, the annealing proceeds. It must be within the range.

  In the flexible circuit board of the present embodiment, the copper foil used for the circuit wiring layer (B) is not particularly limited as long as it satisfies the above characteristics, and commercially available copper foil may be used. it can. Specific examples thereof include HA foil manufactured by JX Nippon Mining & Metals Co., Ltd.

<Coverlay (C)>
In the flexible circuit board of the present embodiment, it is preferable to use a coverlay (C) having a thickness of 27.5 μm and an elastic modulus in the range of 2.0 to 3.5 GPa. As such a coverlay (C), a commercial product can be used. Specific examples thereof include CEA0515 (trade name) manufactured by Arisawa Seisakusho Co., Ltd.

<Overall thickness>
The total thickness of the flexible circuit board of the present embodiment [that is, the total thickness of the polyimide insulating layer (A), the circuit wiring layer (B), and the coverlay (C)] is preferably in the range of 53 to 63 μm. . If the total thickness of the flexible circuit board is less than 53 μm, the equivalent bending stiffness of the flexible circuit board will decrease, and the resistance to seam folding will tend to decrease. If it exceeds 63, copper wiring will be required when the flexible circuit board is bent. In addition, more stress is applied, and the buckling resistance may be reduced.

<Equivalent bending rigidity of flexible circuit board>
The flexible circuit board of the present embodiment has an equivalent bending stiffness when bent with the coverlay (C) inside, for example, in the range of 0.07 to 0.10 N · m ² (configuration c). Equivalent bending stiffness is preferably in the range of 0.07~0.09N · ^{m 2.} When the equivalent bending stiffness of the flexible circuit board is out of the above range, the seam resistance decreases.

  Hereinafter, the equivalent bending rigidity of the flexible circuit board according to the present embodiment will be described. First, a method of calculating the neutral plane position of the flexible circuit board will be described in detail with reference to FIG. FIG. 7 is a cross-sectional view of a model of a laminate used for explaining a method of calculating a neutral plane position. FIG. 7 shows a model in which the stacked body has two layers for convenience, but the following description applies to a general case where the stacked body has two or more layers. Here, the number of layers of the stacked body is n (n is an integer of 2 or more). The i-th (i = 1, 2,..., N) layer counted from the bottom among the layers constituting the laminate is referred to as the i-th layer. In FIG. 7, reference symbol B represents the width of the laminate. Here, the width is a dimension in a direction parallel to the lower surface of the first layer and perpendicular to the longitudinal direction of the laminate.

  The flexible circuit board according to the present embodiment includes the above-described polyimide insulating layer (A), circuit wiring layer (B), and coverlay (C). When viewed from the (B) side, there are a portion where the copper wiring 51 exists and a portion where the copper wiring 51 does not exist. Here, the part where the copper wiring 51 exists is called a wiring part (Line), and the part where the copper wiring 51 does not exist is called a space part (Space). The configuration differs between the wiring section and the space section. Therefore, the wiring section and the space section are separately considered as necessary.

[Calculation of neutral plane position]
Here, the lower surface of the first layer is defined as a reference plane SP. Hereinafter, a case where the stacked body is bent so that the reference plane SP has a downwardly convex shape in FIG. 7 will be considered. In FIG. 7, reference numeral NP indicates a neutral surface of the laminate. Here, the distance between the neutral plane NP and the reference plane SP is defined as a neutral plane position [NP], and the neutral plane position [NP] is separately calculated for the wiring section and the space section. The neutral plane position [NP] is calculated by the following equation (1).

^{_{[NP] = Σ i = 1}} n E i B i h i t i / Σ i = 1 n E i B i t i ... (1)

Here, _Ei is the elastic modulus of the material constituting the i-th layer. The modulus of elasticity E _i corresponds to the "relationship stress and strain in the layers" in the present embodiment. _Bi is the width of the i-th layer and corresponds to the width B shown in FIG. In case of obtaining the neutral plane position of the wiring portion [NP], when a B _i using the values of the line width LW, obtaining the neutral plane position of the space portion [NP] is the line width SW as B _i Is used. h _i is the distance between the center plane and the reference plane SP of the i layer. Note that the central plane of the i-th layer is a virtual plane located at the center in the thickness direction of the i-th layer. t _i is the thickness of the i-th layer. The symbol “Σ _{i = 1} ⁿ ” represents the sum of i from 1 to n. Hereinafter, the neutral plane position of the wiring portion is referred to as [NP] _Line .

[Calculation of equivalent bending stiffness]
The equivalent bending stiffness [BR], which is the bending stiffness of the entire flexible circuit board, is calculated by the following equation (2).

[BR] = B _Line {{ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Line
+ B _Space {{ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Space (2)

Here, in Equation (2), B _Line is the sum of the line widths LW, and B _Space is the sum of the line intervals SW. Further, as shown in FIG. 7, a _i is the distance between the neutral plane NP as the upper surface of the i layer, b _i is the distance between the neutral plane NP as the lower surface of the i layer. {Σ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Line is a value of E _i (a _i ³ −b _i ³ ) / 3 in the wiring portion, where i is 1 to n. It is a sum. {Σ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Space is the value of E _i (a _i ³ −b _i ³ ) / 3 in the space part, where i is 1 to n. It is a sum. Although it is related to the expression (2), for the i-th layer, B _i (a _i ³ −b _i ³ ) / 3 is a parameter representing a geometric property of a cross section generally called a second moment of area. . The value obtained by multiplying the second moment of area of the i-th layer by the elastic modulus of the i-th layer is the bending rigidity of the i-th layer.

<Manufacture of flexible copper-clad laminates>
The flexible copper-clad laminate used for manufacturing the flexible circuit board of the present embodiment has, for example, a polyimide precursor resin solution (also referred to as a polyamic acid solution) on a surface of a copper foil as a raw material of the circuit wiring layer (B). ), Followed by a heat treatment step of drying and curing. In the heat treatment in the heat treatment step, after the solvent in the polyamic acid is dried and removed at a temperature of less than 160 ° C. from the applied polyamic acid solution, the temperature is further raised stepwise in a temperature range of 150 ° C. to 400 ° C., and the curing is performed. It is done by letting it do. 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 a separately prepared copper foil are heated at 300 to 400 ° C. There is a method of crimping.

<FPC>
The flexible circuit board of the present embodiment can be manufactured by processing a metal foil of a flexible copper-clad laminate into a pattern by a conventional method to form a wiring layer, and then attaching a coverlay.

<How to use FPC>
The flexible circuit board according to the present embodiment is particularly effective in bending applications where bending performance is strictly required in a narrow gap of, for example, 0.1 to 0.5 mm. That is, it is preferable that the FPC be bent by 180 ° by applying a load of 1 kg, and be folded and stored in the housing of the electronic device for use.

<Electronic equipment>
An electronic device according to one embodiment of the present invention has an FPC that is folded and stored in a housing of the electronic device. The electronic device is a portable information communication terminal represented by, for example, a smartphone, a tablet terminal, or the like. The electronic device includes a housing made of a material such as a metal or a synthetic resin, and the FPC of the present embodiment. The FPC is folded and stored in a housing of the electronic device. Since the electronic device uses the FPC of this embodiment, which has excellent bending resistance and high connection reliability, even if the FPC is folded and housed at a high density in the case, disconnection or cracking of the wiring circuit may occur. Less likely to occur and excellent product reliability.

  Hereinafter, the present invention will be described in more detail with reference to examples. In addition, each characteristic evaluation in the following Examples was performed by the following methods.

[Measurement of tensile modulus]
In measuring the tensile modulus, a copper foil subjected to a heat treatment equivalent to that of the flexible copper-clad laminate using a vacuum oven was used. Further, as for the polyimide layer, a polyimide film from which the copper foil was completely removed by etching the flexible copper-clad laminate was used. The tensile modulus of the material thus obtained was measured using a strograph R-1 manufactured by Toyo Seiki Seisakusho under an environment of a temperature of 23 ° C. and a relative humidity of 50%.

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

[Measurement of surface roughness (Rz)]
The surface roughness of the contact surface side of the copper foil with the polyimide insulating layer was measured using a contact surface roughness measuring device (SUREF CORDER ET3000 manufactured by Kosaka Laboratory Co., Ltd.).

[Measurement of Cube Cube Ratio]
The cube ratio is an index indicating an area ratio in which a predetermined plane of the copper foil is oriented in the crystal orientation <200>. The crystal orientation of the predetermined surface of the copper foil in each example was confirmed by an EBSD (Electron Back Scattering Diffraction) method. The EBSD method analyzes a crystal from a diffraction image called a pseudo-Kikuchi line, which is diffracted from individual crystal planes generated when a convergent electron beam is irradiated on the surface of a sample to be measured, and provides azimuth data and positional information of measurement points. Is a method of measuring the crystal orientation distribution of the object to be measured, and it is possible to analyze the crystal orientation of the texture in a micro region as compared with the X-ray diffraction method. For example, it is possible to specify the crystal orientation in each of the minute regions, connect them, and map them, and paint the ones in which the inclination (orientation difference) of the plane orientation between the mapping points is equal to or less than a certain value in the same color. An orientation mapping image can be obtained by highlighting the distribution of regions (crystal grains) having substantially the same plane orientation. In addition, it is possible to define an azimuth plane including an azimuth plane having an azimuth within a predetermined angle with respect to a specific plane azimuth, and calculate an area ratio, that is, a cube ratio, of the existence ratio of each plane azimuth.

[Measurement of folding (bending test)]
A test piece (test circuit board piece) 40 formed by etching a copper foil of a copper-clad laminate and forming ten rows of copper wirings 51 having a line width of 100 μm, a space width of 100 μm, and a length of 40 mm along the longitudinal direction thereof. (FIG. 1). As shown in FIG. 1 showing only the copper wirings 51 in the test piece 40, the copper wirings 51 in ten rows in the test piece 40 are all continuously connected via a U-shaped portion 52, and are connected to both ends thereof. Is provided with an electrode portion (not shown) for measuring a resistance value.

  The test piece 40 was fixed on the sample stages 20 and 21 which can be folded in two, connected to a wiring for measuring a resistance value, and monitoring of the resistance value was started (FIG. 2). The bending test was performed on ten rows of copper wirings 51 so that the copper wirings 51 faced inward at the center part in the longitudinal direction. At this time, while using a urethane roller 22 to control the bending point 40C so that a weight of 1 kg is applied thereto, the roller 22 is moved in parallel with the bending line, and all the ten rows of copper wiring 51 are bent. (FIGS. 3 and 4), the bent portion is opened to return the test piece 40 to a flat state (FIG. 5), and the creased portion is moved again while being held down by the roller 22 (FIG. 6). In the step (2), the number of folding times is counted as one. While the bending test is repeatedly performed in such a procedure, the resistance value of the copper wiring 51 is constantly monitored, and when the resistance reaches a predetermined resistance (3000 Ω), it is determined that the copper wiring 51 is broken. The number of times was taken as a measured value. As for the fold measurement value, 70 times or more was evaluated as “good”, and less than 70 times was evaluated as “impossible”.

  In the bending test, when the test piece 40 in a state of being bent from the beginning is used, a state where the test piece 40 is once expanded and the bending is eliminated is set to be zero, and the number of times of bending is counted in the above procedure.

Coverlay A:
Arizawa Seisakusho coverlay (trade name: CEA0515, thickness: 27.5 μm, tensile modulus: 3.5 GPa)
Coverlay B:
Coverlay (trade name; CISV1215, thickness: 27.5 μm, tensile modulus; 2.0 GPa) manufactured by Nickan Industry Co., Ltd.
Coverlay C:
Arizawa Seisakusho coverlay (trade name: CMA0515, thickness: 27.5 μm, tensile modulus: 2.6 GPa)
Coverlay D:
Arizawa Seisakusho coverlay (trade name: CEA0525, thickness: 37.5 μm, tensile modulus: 3.3 GPa)
Coverlay E:
Coverlay (trade name; CISV1225, thickness: 37.5 μm, tensile modulus; 2.0 GPa) manufactured by Nickan Industry Co., Ltd.
Coverlay F:
Coverlay manufactured by Arisawa Seisakusho Co., Ltd. (trade name: CMA0525, thickness: 37.5 μm, tensile modulus: 2.3 GPa)

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

[Synthesis of polyamic acid solution]
(Synthesis example 1)
N, N-dimethylacetamide is placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen, and furthermore, 2,2-bis [4- (4-aminophenoxy) phenyl] propane (BAPP) is added to the reaction vessel. ) Was added and dissolved in the vessel with stirring. Next, pyromellitic dianhydride (PMDA) was introduced so that the total amount of the introduced monomers became 12 wt%. Thereafter, the polymerization reaction was carried out by continuing stirring for 3 hours to obtain a resin solution of polyamic acid a. The coefficient of thermal expansion (CTE) of the polyimide film having a thickness of 25 μm formed from polyamic acid a was 55 × 10 ⁻⁶ / K.

(Synthesis example 2)
N, N-dimethylacetamide is charged into a reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen, and 2,2′-dimethyl-4,4′-diaminobiphenyl (m-TB) is further added to the reaction vessel. Was added and dissolved while stirring in a container. Next, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) were added at a total monomer input amount of 15 wt%, and a molar ratio of each acid anhydride ( (BPDA: PMDA) was set to 20:80. Thereafter, stirring was continued for 3 hours to carry out a polymerization reaction to obtain a resin solution of polyamic acid b. The coefficient of thermal expansion (CTE) of the polyimide film having a thickness of 25 μm formed from polyamic acid b was 22 × 10 ⁻⁶ / K.

(Example 1)
Resin solution of polyamic acid a prepared in Synthesis Example 1 on a commercially available long 12 μm-thick copper foil (surface roughness Rz = 0.4 μm on the coated surface) having the characteristics shown in Table 1. Was uniformly applied so that the thickness after curing became 2.5 μm, and then heated and dried at 130 ° C. to remove the solvent. Next, the resin solution of polyamic acid b prepared in Synthesis Example 2 was uniformly applied to the application surface side so that the thickness after curing became 20.0 μm, and heated and dried at 120 ° C. to remove the solvent. Further, a resin solution of the same polyamic acid a as that applied in the first layer is uniformly applied to the application surface side so that the thickness after curing becomes 2.5 μm, and is dried by heating at 130 ° C. to remove the solvent. did. The long laminate was heat-treated in a continuous curing furnace set at 130 ° C. and gradually increased in temperature to 300 ° C. over a total time of about 6 minutes to give a polyimide resin layer thickness of 25 μm. To obtain a single-sided flexible copper-clad laminate. A copper foil layer having a cube ratio of 85% or more by thermocompression bonding the single-sided flexible copper-clad laminate and a commercially available long copper foil having a thickness of 12 μm separately prepared at 300 to 400 ° C. Was obtained. After a wiring circuit was formed by etching the double-sided flexible copper-clad laminate to form a copper wiring, a coverlay A having a thickness of 27.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent flexural rigidity and the crimp resistance of the obtained flexible circuit board.

  Tensile modulus of the copper foil constituting the obtained flexible copper-clad laminate, physical property values such as the tensile modulus of the polyimide layer, tensile elasticity of the coverlay, equivalent bending rigidity of the flexible circuit board, heat resistance The evaluation results are shown in Tables 1 and 2. The evaluation of the polyimide layer used the thing which removed the copper foil from the manufactured flexible copper-clad laminate by etching.

  Here, a specific calculation procedure of the method of calculating the equivalent bending stiffness [BR] of the flexible copper-clad laminate used in the example will be described using the example 1 as an example.

Considering a two-layer structure as shown in FIG. 7 for the wiring part where the copper wiring exists, the materials constituting the first and second layers are polyimide and copper, respectively. As shown in Table 1 (Example 1), the elastic modulus of each layer is E ₁ = 7.5 GPa, E ₂ = 18 GPa, E ₃ = 3.5 GPa, and the thickness is t ₁ = 25 μm, t ₂ = 12 μm, and t _3. = 21.5 μm. The distance between the center plane and the reference plane SP in the thickness direction in each layer is h ₁ = 12.5 μm, h ₂ = 31 μm, and h ₃ = 47.8 μm, respectively. When these values are substituted into equation (1), first, the neutral plane position (the distance between the reference plane SP and the neutral plane NP) in the wiring section where the copper wiring exists is calculated as [NP] = 26.4 μm. Is done. Next, the distance between the upper surface of each layer and the neutral plane NP is a ₁ = 1.387 μm, a ₂ = 10.613 μm, a ₃ = 32.113 μm, and the distance between the lower surface of each layer and the neutral plane NP is b ₁ = 26.387 μm, b ₂ = 1.387 μm, b ₃ = 10.613, respectively. The width B is considered by focusing on the unit width of the copper wiring, B _Line = 1, B _Space = 0, and these values and the elastic moduli E ₁ , E _{2, and} E ₃ are substituted into the equation (2). The equivalent bending stiffness is calculated as [BR] = 0.089 N · m ² .

(Example 2)
In the same manner as in Example 1, a double-sided flexible copper-clad laminate was obtained. After a wiring circuit was formed by etching the double-sided flexible copper-clad laminate to form a copper wiring, a coverlay B having a thickness of 27.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 1)
It has the characteristics shown in Table 1, and is manufactured by using a copper foil having a thickness of 12 μm and laminating a commercially available copper foil having a polyimide layer thickness of 25 μm and a commercially available polyimide film with a laminating roll. After a wiring circuit was formed by etching the double-sided flexible copper-clad laminate to form a copper wiring, a coverlay A having a thickness of 27.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 2)
In the same manner as in Comparative Example 1, after a commercially available copper foil and a commercially available polyimide film are laminated with a laminating roll, a double-sided flexible copper-clad laminate manufactured by etching the wiring circuit and the like to form a copper wiring, A coverlay B having a thickness of 27.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 3)
In the same manner as in Comparative Example 1, after a commercially available copper foil and a commercially available polyimide film are laminated with a laminating roll, a double-sided flexible copper-clad laminate manufactured by etching the wiring circuit and the like to form a copper wiring, A coverlay C having a thickness of 27.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 4)
In the same manner as in Comparative Example 1, after a commercially available copper foil and a commercially available polyimide film are laminated with a laminating roll, a double-sided flexible copper-clad laminate manufactured by etching the wiring circuit and the like to form a copper wiring, A coverlay D having a thickness of 37.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 5)
In the same manner as in Comparative Example 1, after a copper foil is formed by etching a double-sided flexible copper-clad laminate manufactured by laminating a commercially available copper foil and a commercially available polyimide film with a laminating roll, for example, by etching. Then, a coverlay E having a thickness of 37.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 6)
In the same manner as in Comparative Example 1, after a commercially available copper foil and a commercially available polyimide film are laminated with a laminating roll, a double-sided flexible copper-clad laminate manufactured by etching the wiring circuit and the like to form a copper wiring, A coverlay F having a thickness of 37.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

(Comparative Example 7)
In the same manner as in Example 1, a double-sided flexible copper-clad laminate was obtained. After a wiring circuit was formed by etching the double-sided flexible copper-clad laminate to form a copper wiring, a coverlay E having a thickness of 37.5 μm was attached to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of the equivalent bending rigidity and the crimp resistance of the obtained flexible circuit board.

  From Tables 1 and 2, all of the flexible circuit boards of Examples 1 and 2 have the above-mentioned configurations a to c, and have a good value of 70 times or more in the fold measurement value, and have bending resistance. Was a satisfactory result. On the other hand, Comparative Examples 1 to 6 in which the cube ratio was 75.9% were inferior in creasing resistance. In addition, the crimp resistance of Comparative Example 7, which did not satisfy the structure c, was inferior.

  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.

20, 21: sample stage, 22: roller, 40: test piece, 40C: bent portion of test piece, 51: copper wiring, 52: U-shaped part of copper wiring

Claims (6)

A polyimide insulating layer (A),
A circuit wiring layer (B) provided on at least one surface of the polyimide insulating layer (A);
A coverlay (C) laminated on the circuit wiring layer (B);
A flexible circuit board that is used by being bent at a gap of 0.1 to 0.5 mm by "bent fold" in which the upper surface side is reversed by approximately 180 ° C. and turned to the lower surface side,
Configurations of the following a to c:
a) the thickness of the polyimide insulating layer (A) is in the range of 23 to 27 μm;
b) The thickness of the copper wiring constituting the circuit wiring layer (B) is in the range of 10 to 14 μm, and the cube ratio of the copper wiring is 85% or more;
as well as,
c) The tensile modulus of the coverlay (C) is in the range of 2.0 to 3.5 GPa, and the coverlay (C) is formed of the polyimide insulating layer (A), the circuit wiring layer (B), and the coverlay (C). The total thickness is in the range of 53 to 63 μm;
A flexible circuit board comprising: ^2. The flexible circuit board according to claim 1, wherein an equivalent bending stiffness when bending the coverlay (C) inside is in a range of 0.07 to 0.09 N · m ^2. 3. The polyimide insulating layer (A) has a coefficient of thermal expansion of 30 × 10 ^-6 / K low-expansion polyimide layer (i) having a thermal expansion coefficient of less than 30 × 10 ^-6 / K or more high thermal expansion polyimide layer (ii),
  The thickness ratio of the low thermal expansion polyimide layer (i) to the high thermal expansion polyimide layer (ii) (low thermal expansion polyimide layer (i) / high thermal expansion polyimide layer (ii)) is 2-15. The flexible circuit board according to claim 1, wherein The polyimide constituting the low thermal expansion polyimide layer (i) is a polyimide obtained by reacting an acid anhydride component and a diamine component, wherein the acid anhydride component is pyromellitic dianhydride and / or 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride, wherein the diamine component is 2,2'-dimethyl-4,4'-diaminobiphenyl and / or 2-methoxy-4,4'- 4. The flexible circuit board according to claim 3, comprising diaminobenzanilide. The flexible circuit board according to any one of claims 1 to 4, wherein the flexible circuit board is used by being housed in a housing of an electronic device in a state where the coverlay (C) is folded inside. An electronic device comprising the flexible circuit board according to any one of 1 to 4, which is housed in a housing in a folded state with the coverlay (C) inside.

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