JP2016146419A - Flexible circuit board and electronic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flexible circuit board excellent in bending resistance, and capable of preventing open circuit or cracking of a wiring circuit even in a narrow housing, and to provide an electronic apparatus.SOLUTION: A flexible circuit board includes a polyimide insulation layer (A), a circuit wiring layer (B) provided on at least one side of the polyimide insulation layer (A), and a coverlay (C) laminated on the circuit wiring layer (B). a) The thickness of the polyimide insulation layer (A) is in a range of 10-14 μm, b) the thickness of the copper wiring 51 constituting the circuit wiring layer (B) is in a range of 10-14 μm, and the cube ratio of the copper wiring 51 is 85% or more, and c) the equivalent bending rigidity when bending with the coverlay (C) on the inside is in a range of 0.03-0.04 N m.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a flexible circuit board (FPC), and more particularly, to a flexible circuit board and an electronic device that are folded and housed in a casing of the electronic device.

  In recent years, electronic devices represented by mobile phones, notebook computers, digital cameras, game machines, and the like have been rapidly reduced in size, thickness, and weight. High-density, high-performance materials that can accommodate components are desired. Also in flexible circuit boards, with the spread of high-performance small electronic devices such as smartphones, the density of parts storage has increased, so it has become necessary to store flexible circuit boards in a narrower housing than ever before. Yes. Therefore, also in the flexible copper clad laminated board which is a material of a flexible circuit board, the improvement of the bending resistance from the material surface is calculated | required. Hereinafter, in this specification, folding the FPC so that the upper surface side of the FPC is inverted by approximately 180 ° C. to become the lower surface side may be referred to as “shell folding”.

  As intended for application to such applications, Patent Document 1 reduces the total stiffness of flexible circuit boards by controlling the elastic modulus of the polyimide base film and cover film used in flexible copper clad laminates. Thus, a technique for improving the bending resistance has been proposed. However, the control of the characteristics of the polyimide and the cover film alone is not sufficient for the strict bending mode in which it is folded and stored in an electronic device, and a flexible circuit board having sufficient bending resistance cannot be provided. .

  Moreover, in patent document 2, from the viewpoint of high density in an electronic device, as an approach from the copper foil side, focusing on the crystal grain size of the copper foil, the copper foil for heat treatment with suppressed springback resistance Has been proposed. This technology uses a rolled copper foil with various appropriate additives in the copper foil to increase the crystal grain size by applying a sufficient amount of heat to enlarge the crystal grains. As a result, the copper foil It is a technology that tries to improve the spring back resistance.

  However, small electronic devices typified by smartphones are required to store the FPC in a narrow casing at a higher density. For this reason, it is difficult to meet the demand for higher density with the conventional technology alone.

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 and an electronic device having excellent bending resistance that can prevent disconnection and cracking of a wiring circuit even in a narrow housing. It is to be.

  As a result of intensive studies to solve the above problems, the present inventors have found that 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 The inventors have found that a flexible circuit board capable of solving the above problems can be provided by paying attention to the relationship with the total equivalent bending rigidity of the board, and the present invention has been completed.

The flexible circuit board of the present invention is laminated on the polyimide insulating layer (A), the circuit wiring layer (B) provided on at least one surface of the polyimide insulating layer (A), and the circuit wiring layer (B). Coverlay (C). And the flexible circuit board of this invention is the structure of the following ac:
a) The polyimide insulating layer (A) has a thickness in the range of 10 to 14 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) Equivalent bending rigidity when the coverlay (C) is folded inward is in the range of 0.03 to 0.04 N · m ² ;
It is characterized by having.

  The flexible circuit board of the present invention is used by being housed in a casing of an electronic device in a state of being folded with the cover lay (C) inside.

  The electronic device of the present invention is one in which the flexible circuit board is stored in a housing in a folded state with the cover lay (C) inside.

  Since the flexible circuit board of the present invention can exhibit the high bending resistance required for a wiring board, it is excellent in connection reliability in a state where it is bent in an electronic device. Therefore, the flexible circuit board of the present invention can be suitably used particularly for electronic components that require bending resistance such as a bent portion around a small liquid crystal such as a smartphone.

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.
<Flexible circuit board>
The flexible circuit board of this embodiment was laminated on the polyimide insulating layer (A), the circuit wiring layer (B) provided on one or both sides of the polyimide insulating layer (A), and the circuit wiring layer (B). A coverlay (C). For example, this flexible circuit board forms a copper wiring by etching a copper foil layer of a flexible copper clad laminate having a polyimide insulating layer and a copper foil layer to form a copper wiring, and affixes a coverlay. It is produced by. In addition, in the flexible circuit board, when the circuit wiring layer (B) is provided on both surfaces of the polyimide insulating layer (A), the circuit wiring layer that becomes the inner side when bent is provided with the configuration b described later. Just do it. In this case, the coverlay that covers the circuit wiring layer that is inside when bent corresponds to the coverlay (C) of the configuration c described later.

<Polyimide insulation layer (A)>
The polyimide insulating layer (A) has a thickness in the range of 10 to 14 μm (Configuration a). If the thickness of the polyimide insulating layer (A) is less than 10 μm, the equivalent bending rigidity of the flexible circuit board will be reduced, and the folding resistance will be reduced. If the thickness exceeds 14 μm, copper will be lost when the flexible circuit board is bent. Stress is further applied to the wiring, and the resistance to bending tends to decrease.

  The polyimide insulating layer (A) can be a commercially available polyimide film as it is, but from the ease of controlling the thickness and physical properties of the insulating layer, after applying the polyamic acid solution directly on the copper foil, A so-called cast (coating) method in which drying and curing are performed by heat treatment is preferable. In addition, the polyimide insulating layer (A) may be formed of only a single layer, but in consideration of the adhesiveness between the polyimide insulating layer (A) and the circuit wiring layer (B), a layer composed of a plurality of layers is preferable. . 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 composed of different components. When the polyimide insulating layer (A) is composed of a plurality of layers, the 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, the polyimide insulating layer (A) is preferably 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. A laminated structure including the polyimide layer (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 insulating 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 both sides thereof, and has a high thermal expansion. The polyimide layer (ii) is preferably in direct contact with the circuit wiring layer (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 provides 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 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, for example, pyromellitic dianhydride as a raw acid anhydride component, 3 ', 4,4'-biphenyltetracarboxylic dianhydride and 2,2'-dimethyl-4,4'-diaminobiphenyl and 2-methoxy-4,4'-diaminobenzanilide as diamine components It is preferable to use pyromellitic dianhydride and 2,2′-dimethyl-4,4′-diaminobiphenyl as main components of the 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, for example, pyromellitic dianhydride, 3,3 ′, 4 as an acid anhydride component of a raw material , 4′-biphenyltetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride, 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic dianhydride, As the diamine component, 2,2′-bis [4- (4-aminophenoxy) phenyl] propane, 4,4′-diaminodiphenyl ether, 1,3-bis (4-aminophenoxy) benzene is preferably used, Particularly preferably, 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.

  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 are used. The ratio of the thickness of the expandable 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 (i) with respect to the entire polyimide insulating layer (A) becomes thin, so it becomes difficult to control the dimensional characteristics of the polyimide film, and the copper foil is etched. When the circuit wiring layer (B) is formed, the rate of dimensional change increases. When the circuit wiring layer (B) exceeds 15, the polyimide layer (ii) having a high thermal expansion becomes thin. Therefore, the polyimide insulating layer (A) and the circuit wiring layer (B) The adhesion reliability of the is reduced.

<Circuit wiring layer (B)>
In the flexible circuit board of the present embodiment, the circuit wiring layer (B) is constituted by, for example, copper wiring using copper foil as a raw material. 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 (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 is lowered, and the handling during the production of the flexible circuit board tends to deteriorate. If the thickness exceeds 14 μm, the flexible circuit board As the stress applied to the copper wiring increases when the wire is bent, the folding resistance tends to decrease. Further, when the cube ratio of the copper wiring constituting the circuit wiring layer (B) is less than 85%, the anisotropy for each crystal grain of the copper wiring is increased, and microscopic stress concentration is generated at the time of bending. The foldability tends to decrease.

  Here, the cube ratio is an index representing an area ratio in which a predetermined plane of copper constituting the copper wiring is oriented in the crystal orientation <200>. The cube ratio can be confirmed by an EBSD (Electron Back Scattering Diffraction) method, as shown in the examples described later. The copper foil as the raw material for the circuit wiring layer (B) undergoes annealing through a sufficient thermal history during the manufacture of the flexible circuit board, and as a result, the cube ratio is as described above when it is processed into the circuit wiring layer (B). 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 a commercially available copper foil may be used. it can. Specific examples thereof include HA foil manufactured by JX Nippon Mining & Metals.

<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 item can be used. Specific examples include CEA0515 (trade name) manufactured by Arisawa Manufacturing Co., Ltd.

<Overall thickness>
The flexible circuit board of the present embodiment preferably has a total thickness [that is, the total thickness of the polyimide insulating layer (A), the circuit wiring layer (B), and the coverlay (C)] in the range of 41 to 50 μm. . If the total thickness of the flexible circuit board is less than 41 μm, the equivalent bending rigidity of the flexible circuit board tends to decrease, and the resistance to fold-down tends to decrease. If the thickness exceeds 50 μm, the copper wiring is bent when the flexible circuit board is bent. In addition, more stress is applied, and the folding resistance may be reduced.

<Equivalent bending rigidity of flexible circuit board>
In the flexible circuit board according to the present embodiment, the equivalent bending rigidity when folded with the coverlay (C) inside is within a range of, for example, 0.03 to 0.05 N · m ² (Configuration c). The equivalent bending rigidity is preferably within a range of 0.03 to 0.04 N · m ² . When the equivalent bending rigidity of the flexible circuit board is out of the above range, the folding resistance is lowered.

  Hereinafter, the equivalent bending rigidity of the flexible circuit board of the present embodiment will be described. First, a method for 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 the calculation method of the neutral plane position. Although FIG. 7 shows a model in which the laminated body has two layers for convenience, the following description applies to the case where the laminated 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). In addition, the i-th (i = 1, 2,..., N) layer counted from the bottom among the layers constituting this laminate is referred to as the i-th layer. In FIG. 7, the symbol B represents the width of the laminate. The width here 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 in the present embodiment is composed of the polyimide insulating layer (A), the circuit wiring layer (B), and the coverlay (C), but the circuit wiring layer is the state excluding the 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, a portion where the copper wiring 51 exists is called a wiring portion (Line), and a portion where the copper wiring 51 does not exist is called a space portion (Space). The wiring part and the space part have different configurations. Therefore, the wiring portion and the space portion are considered separately as necessary.

[Calculation of neutral plane position]
Here, the lower surface of the first layer is defined as a reference surface SP. Hereinafter, a case where the laminate is bent so that the reference plane SP has a convex shape on the lower side in FIG. 7 will be considered. In FIG. 7, the symbol NP represents the neutral plane 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 calculated separately for the wiring portion and the space portion. 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, E _i is the elastic modulus of the material constituting the i-th layer. 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. When obtaining the neutral plane position [NP] of the wiring portion, the value of the line width LW is used as B _i , and when obtaining the neutral plane position [NP] of the space portion, the line width SW as B _i. The value of 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. Hereinafter, the neutral plane position of the wiring portion is referred to as [NP] _Line .

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

[BR] = B _Line {Σ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Line
+ B _Space {Σ _{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 widths SW. Further, as shown in FIG. 7, a _i is the distance between the upper surface of the i-th layer and the neutral plane NP, and b _i is the distance between the lower surface of the i-th layer and the neutral plane NP. {Σ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Line is the value of E _i (a _i ³ −b _i ³ ) / 3 in the wiring section, where i is 1 to n It is the sum. {Σ _{i = 1} ⁿ E _i (a _i ³ −b _i ³ ) / 3} _Space is a value of E _i (a _i ³ −b _i ³ ) / 3 in the space part, where i is 1 to n It is the sum. Although related to the equation (2), for the i-th layer, B _i (a _i ³ −b _i ³ ) / 3 is a parameter representing the geometric characteristic of the cross section, generally called a cross-section second moment. . A value obtained by multiplying the sectional moment of the i-th layer by the elastic modulus E _i 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 is, for example, a polyimide precursor resin solution (also called a polyamic acid solution) on the surface of a copper foil that is a raw material for the circuit wiring layer (B). ), Followed by a heat treatment step of drying and curing. The heat treatment in the heat treatment step is to remove the solvent in the polyamic acid by drying and removing the coated polyamic acid solution at a temperature of less than 160 ° C., and then gradually increase the temperature in a temperature range of 150 ° C. to 400 ° C. to cure. It is done by letting. A single-sided flexible circuit board was obtained by the above method. 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 were heated at 300 to 400 ° C. The method of crimping is mentioned.

<FPC>
The flexible circuit board of this 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 that require severe bending performance with 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 weight of 1 kg, and folded and stored in the casing of the electronic device.

<Electronic equipment>
An electronic device according to an embodiment of the present invention includes an FPC that is folded and stored in a housing of the electronic device. An electronic device is a portable information communication terminal represented by a smart phone, a tablet terminal, etc., for example. The electronic device includes a housing made of a material such as metal or synthetic resin, and the FPC of this embodiment. The FPC is stored in a folded state in the casing 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 in the housing and stored at high density, the wiring circuit is not broken or cracked. It does not occur easily and has excellent product reliability.

  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]
In measuring the tensile elastic modulus, a copper foil subjected to a heat treatment equivalent to the treatment process of the flexible copper-clad laminate using a vacuum oven was used. Moreover, regarding the polyimide layer, the polyimide film which etched the flexible copper clad laminated board and removed copper foil completely was used. The material obtained in this manner was measured for tensile modulus 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)]
Using a contact-type surface roughness measuring machine (SUREF CORDER ET3000 manufactured by Kosaka Laboratory Ltd.), the surface roughness of the contact surface side of the copper foil with the polyimide insulating layer was measured.

[Measurement of copper foil cube ratio]
The cube ratio is an index representing an area ratio at 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. In the EBSD method, a crystal is analyzed from a diffraction image called a pseudo Kikuchi line that is diffracted from an individual crystal plane generated when a focused electron beam is irradiated on a sample surface to be measured, and orientation data and position information of a measurement point are obtained. From this, the crystal orientation distribution of the measurement object is measured, and the crystal orientation of the texture in the microscopic region can be analyzed as compared with the X-ray diffraction method. For example, it is possible to specify the crystal orientation in each minute region, connect them and map them, and separate the ones whose tilt angle (azimuth difference) between each mapping point is below a certain value with 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 calculate the area ratio, that is, the cube ratio, by defining the existence ratio of each surface orientation, including an orientation surface having an orientation within a predetermined angle with respect to a specific surface orientation.

[Measurement of seam fold (bending test)]
A test piece (test circuit board piece) 40 in which a copper foil of a copper-clad laminate was etched and 10 rows of copper wirings 51 having a line width of 100 μm and a space width of 100 μm and a length of 40 mm were formed along the longitudinal direction. Was produced (FIG. 1). As shown in FIG. 1 showing only the copper wiring 51 in the test piece 40, the 10 rows of copper wirings 51 in the test piece 40 are all continuously connected via the U-shaped portion 52, and are connected to both ends thereof. Is provided with an electrode portion (not shown) for measuring the resistance value.

  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. 2). The bending test was performed with respect to ten rows of copper wirings 51 by bending the copper wirings 51 so that they face each other at the center in the longitudinal direction. At this time, using the urethane roller 22, the roller 22 is moved in parallel with the bent line while controlling the bent portion 40C so as to be loaded with 1 kg, and all the 10 rows of copper wirings 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 by the roller 22 (FIG. 6). In this process, the number of times of folding is counted as one. While the bending test is repeated in such a procedure, the resistance value of the copper wiring 51 is constantly monitored, and when the predetermined resistance (3000 Ω) is reached, it is determined that the copper wiring 51 is broken, and the bending is repeated until that time. The number of times was taken as the measured value of the folded fold. As for the measured value of the helical fold, 30 times or more were evaluated as “good” and less than 30 were evaluated as “impossible”.

  In the bending test, when using the test piece 40 that has been bent from the beginning, the state once unfolded to cancel the bending is regarded as zero folding, and the number of folding is counted according to the above procedure.

Coverlay A means a coverlay (trade name: CEA0515, thickness: 27.5 μm, tensile elastic modulus: 3.5 GPa) manufactured by Arisawa Manufacturing Co., Ltd.
Coverlay B means a coverlay (trade name: CMA0515, thickness: 27.5 μm, tensile elastic modulus: 2.6 GPa) manufactured by Arisawa Manufacturing Co., Ltd.
Coverlay C means a coverlay (trade name; CEA0525, thickness: 37.5 μm, tensile elastic modulus: 3.3 GPa) manufactured by Arisawa Manufacturing Co., Ltd.
The coverlay D means a coverlay (trade name: CMA0525, thickness: 37.5 μm, tensile elastic modulus: 2.3 GPa) manufactured by Arisawa Manufacturing Co., Ltd.

  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)
A reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen is charged with N, N-dimethylacetamide, and 2,2-bis [4- (4-aminophenoxy) phenyl] propane (BAPP) is added to the reaction vessel. ) 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 wt%. 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 2,2′-dimethyl-4,4′-diaminobiphenyl (m-TB) is added to the reaction vessel. Was dissolved while stirring in a container. Next, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) were added in a total monomer amount of 15 wt%, and the molar ratio of each acid anhydride ( (BPDA: PMDA) was charged 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 thermal expansion coefficient (CTE) of the 25 μm thick polyimide film formed from the 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 copper foil having a characteristic shown in Table 1 and having a thickness of 12 μm (coating surface roughness Rz = 0.4 μm) Was applied uniformly so that the thickness after curing was 2.2 μm, and then dried by heating at 130 ° C. to remove the solvent. 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 7.6 μm, and the solvent was removed by heating and drying at 120 ° 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 long laminate is heat-treated in a continuous curing furnace set so that the temperature gradually increases from 300 ° C. to 300 ° C. over a period of about 6 minutes. A polyimide resin layer having a thickness of 12 μm and having a copper foil layer of 85% or more was obtained. A copper circuit was formed by etching this single-sided flexible copper-clad laminate to form a copper wiring, and then 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 folding resistance of the obtained flexible circuit board.

  Properties of physical properties such as tensile modulus of copper foil, tensile modulus of polyimide layer, tensile modulus of coverlay, equivalent flexural rigidity of flexible circuit board, folding resistance The evaluation results are shown in Tables 1 and 2. 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 method of the equivalent bending stiffness [BR] of the flexible copper-clad laminate used in the example will be described by way of example 1.

Considering a two-layer structure as shown in FIG. 7 for the wiring portion where the copper wiring 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 = 6.8 GPa, E ₂ = 18 GPa, E ₃ = 3.5 GPa, and the thicknesses are t ₁ = 12 μm, t ₂ = 12 μm, t _3. = 21.5 µm. Further, the distance between the center plane in the thickness direction and the reference plane SP in each layer is h ₁ = 6 μm, h ₂ = 18 μm, and h ₃ = 34.8 μm, respectively. When these values are substituted into the equation (1), first, the neutral plane position (distance between the reference plane SP and the neutral plane NP) in the wiring portion where the copper wiring exists is calculated as [NP] = 18.8 μm. Is done. Next, the distance between the upper surface of each layer and the neutral plane NP is a ₁ = 6.8 μm, a ₂ = 5.2 μm, a ₃ = 26.7 μm, and the distance between the lower surface of each layer and the neutral plane NP is b _1. = 18.8 μm, b ₂ = 6.8 μm, and b ₃ = 5.2. The width B is considered by paying attention to the unit width of the copper wiring. When B _Line = 1 and B _Space = 0, and substituting these values and elastic moduli E ₁ , E _{2, and} E ₃ into the equation (2), The equivalent bending stiffness is calculated as [BR] = 0.039 N · m ² .

(Example 2)
In the same manner as in Example 1, a single-sided flexible copper-clad laminate was obtained. A copper circuit was formed by etching this single-sided flexible copper-clad laminate to form a copper wiring, and then 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 equivalent bending rigidity and resistance to folding of the obtained flexible circuit board.

(Comparative Example 1)
The copper foil having the characteristics shown in Table 1 and having a thickness of 12 μm was used, and a commercially available copper foil having a polyimide layer thickness of 12 μm and a commercially available polyimide film were bonded to each other with a laminate roll. A wiring circuit was formed by etching the double-sided flexible copper-clad laminate to form a copper wiring, and then 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 equivalent bending rigidity and resistance to folding of the obtained flexible circuit board.

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

(Comparative Example 3)
In the same manner as in Comparative Example 1, after forming a copper wiring by processing a wiring circuit such as etching a double-sided flexible copper-clad laminate produced by laminating a commercially available copper foil and a commercially available polyimide film with a laminate roll, A coverlay C having a thickness of 37.5 μm was pasted to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of equivalent bending rigidity and resistance to folding of the obtained flexible circuit board.

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

(Comparative Example 5)
In the same manner as in Comparative Example 1, after forming a copper wiring by processing a wiring circuit such as etching a double-sided flexible copper-clad laminate produced by laminating a commercially available copper foil and a commercially available polyimide film with a laminate roll, A coverlay C having a thickness of 37.5 μm was pasted to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of equivalent bending rigidity and resistance to folding of the obtained flexible circuit board.

(Comparative Example 6)
In the same manner as in Comparative Example 1, after forming a copper wiring by processing a wiring circuit such as etching a double-sided flexible copper-clad laminate produced by laminating a commercially available copper foil and a commercially available polyimide film with a laminate roll, A coverlay D having a thickness of 37.5 μm was pasted to obtain a flexible circuit board. Tables 1 and 2 show the evaluation results of equivalent bending rigidity and resistance to folding of the obtained flexible circuit board.

  From Table 1 and Table 2, the flexible circuit boards of Examples 1 and 2 each have the above-described configurations a to c, and the measured values of the helix fold are as good as 30 times or more. Was a satisfactory result. On the other hand, Comparative Examples 1 to 4 having a cube ratio of 75.9% were inferior in the folding resistance. Moreover, the folding resistance of Comparative Examples 5-6 which did not satisfy the configuration c was also inferior.

  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.

DESCRIPTION OF SYMBOLS 1 ... Housing, 3 ... Display, 5 ... External connection terminal, 10 ... Flexible circuit board (FPC), 20, 21 ... Sample stage, 22 ... Roller, 40 ... Test piece, 40C ... Bending part of test piece, 51 ... Copper wiring, 52 ... U-shaped part of copper wiring

Claims (3)

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 comprising:
The following configurations a to c:
a) The polyimide insulating layer (A) has a thickness in the range of 10 to 14 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) Equivalent bending rigidity when the coverlay (C) is folded inward is in the range of 0.03 to 0.04 N · m ² ;
A flexible circuit board comprising:   2. The flexible circuit board according to claim 1, wherein the flexible circuit board is used by being housed in a casing of an electronic device in a state of being folded with the coverlay (C) inside. 3.   The electronic device which accommodated the flexible circuit board of Claim 1 in the housing | casing in the state folded with the said coverlay (C) inside.

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