Polyimide films, metal-clad laminates and flexible circuit boards

Abstract

To provide an FPC having excellent continuous bending resistance by suppressing the deformation of a polyimide film at the bending portion tip in the bending cycle in a hinge portion of a foldable device.SOLUTION: A polyimide film is used as an insulating resin layer of a flexible circuit board in which an operation of reversing and folding a second non-bent portion by 180 degrees with respect to a first non-bent portion is repeated so as to have a shape including the first non-bent portion and the second non-bent portion whose shape has not changed compared to the flat state, and a bent portion that is curved and deformed so as to be located between the first non-bent portion and the second non-bent portion, and the slope of a plastic deformation region of a stress-strain curve is within a range of 1.0 MPa / % or more and less than 5.0 MPa / %.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a polyimide film, a metal-clad laminate, and a flexible circuit board, and more particularly to a flexible circuit board used, for example, in the hinge portion of a foldable device, and a polyimide film and a metal-clad laminate used therein. [Background technology]

[0002] In recent years, flexible printed circuits (FPCs) have been widely used in electronic devices such as mobile phones, smartphones, laptop computers, hard disk drives, optical pickup devices, and printers. Because FPCs allow for three-dimensional, high-density mounting even in limited spaces, their applications are expanding to include wiring for moving parts in electronic devices such as HDDs, DVDs, mobile phones, and smartphones, as well as cables, connectors, and other components. Such FPCs require high flex resistance.

[0003] Patent Document 1 proposes an FPC that is suitable for use in a manner that ensures a certain bending radius, such as the sliding bending parts found in conventional mobile phones, and Patent Document 2 proposes a flexible copper-clad laminate that is suitable for use in a manner that allows it to be folded with creases for storage in a thin housing.

[0004] Furthermore, portable electronic devices such as smartphones that use touch panels are used in a wide range of fields, and among these, electronic devices that have a hinge portion formed within a display area such as a display and are flexible enough to be foldable (so-called foldable devices) have begun to be considered (for example, Patent Documents 3 and 4).

[0005] For FPCs that are actually incorporated into electronic devices and bent repeatedly, the shape of the FPC in the bent state, particularly the shape of the bent part, varies depending on the electronic device, so an FPC that meets such needs is required. To meet such needs, the inventors have proposed an FPC bending resistance test device that can control the shape of the bent part, which was not possible with conventional test devices, to a desired shape (Patent Application No. 2017-249096). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. WO2012 / 020677 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-80021 [Patent Document 3] Japanese Patent Application Laid-Open No. 2109-12098 [Patent Document 4] Japanese Patent Application Publication No. 2019-61194 Summary of the Invention [Problem to be solved by the invention]

[0007] The difference between the FPCs used in foldable devices and those used in conventional smartphones is that they are subjected to harsh conditions that cross the bending axis of the hinge, and FPCs used in this manner are required to have excellent "resistance to continuous bending." Therefore, an object of the present invention is to provide an FPC that has excellent resistance to repeated bending by suppressing deformation at the tip of the bent portion of the FPC during bending cycles in the hinge portion of a foldable device. [Means for solving the problem]

[0008] As a result of extensive research, the inventors discovered that an FPC that can solve the above-mentioned problems can be provided by focusing on the relationship between the slope of the plastic deformation region in the stress-strain curve of the polyimide film used in the insulating resin layer of the FPC and the tendency for stress to concentrate at the tip of the bent portion under the bending conditions expected in foldable devices, and thus completed the present invention.

[0009] That is, the polyimide film of the present invention is a polyimide film used as an insulating resin layer of a flexible circuit board in which the second non-bending portion is repeatedly bent 180 degrees relative to the first non-bending portion so as to form a shape including a first non-bending portion and a second non-bending portion whose shape has not changed compared to when the flexible circuit board is flat, and a bent portion that is located between the first non-bending portion and the second non-bending portion and has been curved and deformed. The polyimide film of the present invention has a stress-strain curve with a slope of 0.01 mm in the plastic deformation region. 100MPa or more but less than 500MPa The range is characterized in that:

[0010] In the polyimide film of the present invention, when an axial direction parallel to the thickness direction of the first non-bending portion and the second non-bending portion is defined as a Y-axis direction and an axial direction perpendicular to the Y-axis direction and parallel to the longitudinal direction of the flexible circuit board is defined as an X-axis direction, in two-dimensional coordinate axes in the X-axis direction and the Y-axis direction, the maximum length of the deformation region in the X-axis direction in the bent portion may be in the range of 1.0 mm or more and 10.0 mm or less, and the maximum length of the Y-axis direction may be in the range of 1.0 mm or more and 6.0 mm or less.

[0011] The metal-clad laminate of the present invention comprises an insulating resin layer made of any of the above polyimide films, and a metal layer laminated on at least one surface of the insulating resin layer.

[0012] The flexible circuit board of the present invention comprises an insulating resin layer made of any of the above polyimide films, and a wiring layer formed on at least one surface of the insulating resin layer.

[0013] The flexible circuit board of the present invention may further include a coverlay for protecting the wiring layer.

[0014] The flexible circuit board of the present invention may be used in a manner in which it is repeatedly bent by 180 degrees so that the wiring layer faces inward. [Effects of the Invention]

[0015] An FPC using the polyimide film of the present invention as an insulating resin layer exhibits high resistance to repeated bending, and therefore can be suitably used for electronic components that require resistance to repeated bending, such as hinge portions of foldable devices. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a side view illustrating a mode of use of the FPC. [Figure 2] 2 is a side view illustrating a state in which the FPC of FIG. 1 is bent. FIG. [Figure 3] 1 is a diagram showing a stress-strain curve in a tensile test of a polyimide film. [Figure 4] FIG. 10 is a cross-sectional view of a laminate model used to explain a method for calculating a neutral plane position. [Figure 5] FIG. 1 is a plan view illustrating the copper wiring of a test piece used in a continuous bending test. [Figure 6] FIG. 1 is a diagram illustrating a continuous bending test, showing a state in which a test piece is set. [Figure 7] FIG. 1 is a diagram for explaining a continuous bending test, illustrating a state in which a test piece is bent. DETAILED DESCRIPTION OF THE INVENTION

[0017] Next, an embodiment of the present invention will be described with reference to the drawings as needed.

[0018] [Polyimide film] First, a description will be given of an embodiment of an FPC in which the polyimide film of this embodiment can be used as an insulating resin layer. FIGS. 1 and 2 are side views of an FPC 100 in the form of a long, thin film. Layer structures such as an insulating resin layer and a circuit wiring layer are not shown in FIGS. 1 and 2. The FPC 100 has a first surface 101 on which a circuit wiring layer is formed, and a second surface 102 opposite the first surface 101. A circuit wiring layer may also be formed on the second surface 102. A coverlay may be formed on the first surface 101 and the second surface 102, laminated on the circuit wiring layer.

[0019] During use, FPC 100 is repeatedly bent so that first surface 101, on which a circuit wiring layer is formed, faces inward. In the bent state, as shown in Fig. 2, first surfaces 101 face each other, and the FPC 100 takes on a shape including first non-bent portion 110 and second non-bent portion 120, whose shapes have not changed compared to the FPC 100 in its entirely flat, unfolded state (Fig. 1), and bent portion 130, which is located between first non-bent portion 110 and second non-bent portion 120 and has been curved. In other words, in the bent state shown in Fig. 2, second non-bent portion 120 is bent 180 degrees relative to first non-bent portion 110, with bent portion 130 as the boundary. For example, when applied to the hinge portion of a foldable device, as the display area is repeatedly unfolded and folded, FPC100 will repeat unfolding and folding operations between the unfolded state of Figure 1 and the folded state of Figure 2 (it may also be held in an intermediate position between these).

[0020] 1 and 2, the axial direction parallel to the thickness direction of the first non-bending portion 110 and the second non-bending portion 120 of the FPC 100 is defined as the Y-axis direction, and the axial direction perpendicular to the Y-axis direction and parallel to the longitudinal direction of the FPC 100 is defined as the X-axis direction. In this case, the usage mode of the FPC 100 is defined as the maximum length L in the X-axis direction of the deformation region in the bending portion 130 in the two-dimensional coordinate axes in the X-axis direction and the Y-axis direction. X is in the range of 1.0 mm or more and 10.0 mm or less, and the maximum length L in the Y-axis direction YIn this case, the distance (gap) G between the outer surface of the first non-bending portion 110 (i.e., the second surface 102) and the outer surface of the second non-bending portion 120 (i.e., the second surface 102) in FIG. 2 is, for example, in the range of 1.0 to 3.0 mm. In particular, the FPC 100 of this embodiment has a maximum length L of the deformation region in the X-axis direction. X is 1.0 to 3.0 mm, and the maximum length in the Y-axis direction L Y The FPC 100 exhibits excellent resistance to repeated bending even in a relatively narrow installation space where the gap G is limited to a range of 1.0 to 2.0 mm and the FPC 100 has a width of 1.0 to 3.0 mm. A typical example of such a usage mode is a usage method in which the FPC 100 crosses the hinge part of a foldable device. The "deformation region" refers to the region that deforms when FPC 100 is bent to the bent state shown in FIG. 2 compared to the unfolded state shown in FIG. 1, and more specifically, can be defined as the region between inflection point P1, which is the boundary between the first non-bent portion 110 and the bent portion 130, and inflection point P2, which is the boundary between the second non-bent portion 120 and the bent portion 130. The Y-axis direction may also be defined as an axial direction parallel to a straight line passing through inflection point P1, which is the boundary between the first non-bending portion 110 and the bending portion 130, and inflection point P2, which is the boundary between the second non-bending portion 120 and the bending portion 130.

[0021] As described above, the polyimide film of the present embodiment can be preferably applied as an insulating resin layer of an FPC used in the hinge portion of a foldable device, for example. The polyimide film of the present embodiment has a slope of the plastic deformation region of the stress-strain curve (described later). 100MPa or more but less than 500MPa and preferably 200MPa or more but less than 500MPa more preferably in the range 200MPa or more but less than 300MPa By having the slope of the plastic deformation region within the above range, it is possible to maintain a relatively large rigidity even after plastic deformation, and therefore when used as an insulating resin layer of an FPC 100 used in a foldable device, breakage during continuous bending can be prevented. The reasons for this are as follows. The FPC 100 used in foldable devices differs from that used in conventional smartphones in that it is subjected to harsh usage that crosses the bending axis of the hinge. Therefore, the FPC 100 used in foldable devices is required to have excellent "resistance to continuous bending." Furthermore, because the hinge portion of the foldable device has a narrow gap, it is expected that the insulating resin layer used in the FPC 100 will move in the plastic deformation region of the stress-strain curve. Under such bending conditions, if the rigidity of the insulating resin layer in the plastic deformation region is low, the shape of the tip of the bending portion 130 (i.e., the end of the curved portion on the opposite side of the X-axis direction from the inflection points P1 and P2 in Figure 2) will become acute, causing large localized stresses to act, increasing the risk of disconnection. In contrast, the slope of the plastic deformation region of the polyimide film, which is the insulating resin layer of FPC100, is 100MPa or more but less than 500MPa By keeping the thickness within this range, plastic deformation is unlikely to occur, and the bent shape is maintained in an arc shape, which is thought to enable stress to be dispersed during bending, leading to a long bending life. More specifically, when bent, the bent portion 130 in FIG. 2 can be curved and deformed into a shape that bulges in the thickness direction (Y-axis direction) of the first non-bent portion 110 and the second non-bent portion 120. In other words, the inclination of the plastic deformation region is 100MPa or more but less than 500MPa By being within the range, the maximum length L of the bending portion 130 in the Y-axis direction Y becomes larger than the gap G (L Y >G) Since the curved shape is maintained, stress concentration at the tip during bending is avoided, improving the resistance to repeated bending.

[0022] <Calculation method for the slope of the plastic deformation region> Next, the plastic deformation region and its slope will be described with reference to Figure 3. Figure 3 shows a stress-strain curve in a tensile test of a polyimide film according to this embodiment, with the vertical axis representing stress (MPa) and the horizontal axis representing strain (elongation; %). E1 on the horizontal axis represents the strain at the yield point when the stress is S1, E2 represents the strain (=8%) when the stress is S2, and E3 represents the strain at the break point when the stress is S3. The "plastic deformation region" refers to the strain region (E1 to E3) from the yield point to the break point in the stress-strain curve of a polyimide film in a tensile test. The strain at the yield point in a tensile test of a polyimide film varies depending on the material, but it never exceeds 8%, and since the stress-strain curve is nearly linear in the plastic deformation region, in the present invention, the slope between the strain "8% (E2)" and "breaking strain (E3)" in the stress-strain curve in Figure 3 is defined as the "slope of the plastic deformation region." The "slope of the plastic deformation region" can be calculated using the following formula (a): The term "resistance to plastic deformation" refers to the large increase in stress in the plastic deformation region. In other words, the term "resistance to plastic deformation" can be expressed as the magnitude of the gradient in the plastic deformation region.

[0023] Slope of plastic deformation region = (S3 - S2) / (E3 - E2) (a) S2: Stress at 8% strain S3: Breaking stress E2: 8% strain E3: Breaking strain

[0024] As the polyimide film of this embodiment, a commercially available polyimide film can be used as is, but because it is easy to control the thickness and physical properties, a method called a casting (coating) method is preferred, in which a polyamic acid solution is applied to a substrate such as copper foil, followed by drying and curing by heat treatment. A polyimide film having multiple layers can be formed by sequentially coating a polyamic acid solution consisting of one component on another polyamic acid solution consisting of a different component. In the case of the casting method, the polyimide film is obtained by peeling it off from the substrate or by etching the substrate.

[0025] The tensile modulus, thickness, laminate structure, raw materials, thermal expansion coefficient, etc. of the polyimide film of this embodiment are similar to those of the polyimide insulating layer (A) in the FPC described later, so please refer to the description of the polyimide insulating layer (A) and the description will be omitted.

[0026] [FPC] Next, an FPC according to one embodiment of the present invention, in which the above-mentioned polyimide film is used as an insulating resin layer, will be described. Since the FPC of this embodiment has the same configuration as the FPC 100 shown in Figures 1 and 2, the same components will be denoted by the same reference numerals. The FPC 100 comprises a polyimide insulating layer (A) as an insulating resin layer, and a circuit wiring layer (B) provided on one or both sides of the polyimide insulating layer (A). The FPC 100 may further comprise a coverlay (C) laminated on the circuit wiring layer (B).

[0027] <Polyimide insulating layer (A)> The polyimide insulating layer (A) has a stress-strain curve in which the slope of the plastic deformation region in a tensile test is 100MPa or more but less than 500MPa , preferably 200MPa or more but less than 500MPa , more preferably 200MPa or more but less than 300MPa The slope of the plastic deformation region is within the range 100MPa or more but less than 500MPaIn this case, when the FPC 100 laminated with the coverlay (C) is bent, compressive stress is mainly applied to the circuit wiring layer (B), so that necking (partial constriction) of the wiring tends to be less likely to occur. As a result, the tip shape of the bent portion 130 of the FPC 100 is easily maintained during the bending cycle, and stress is less likely to concentrate at the tip. On the other hand, the inclination of the plastic deformation region Less than 100 MPa In this case, tensile stress is mainly applied to the circuit wiring layer (B), which tends to cause necking of the wiring. As a result, the tip shape of the bent portion 130 of the FPC 100 is likely to deform into an acute angle during the bending cycle, and stress tends to concentrate at the tip.

[0028] The tensile modulus of the polyimide insulating layer (A) is preferably, for example, in the range of 4 to 10 GPa, and more preferably in the range of 6 to 8 GPa. If the tensile modulus is below the lower limit, the tip of the bent portion 130 of the FPC 100 is likely to deform at an acute angle, which may cause stress concentration at the tip of the bent portion 130 and reduce the resistance to repeated bending. If the tensile modulus exceeds the upper limit, a large stress is applied to the circuit wiring layer (B) when the FPC 100 is bent, which may reduce the resistance to repeated bending.

[0029] The thickness of the polyimide insulating layer (A) is within the range of 12 to 35 μm, preferably within the range of 17 to 32 μm, and more preferably within the range of 23 to 27 μm. If the thickness of the polyimide insulating layer (A) is less than 12 μm, the tip of the bent portion 130 of the FPC 100 is likely to deform at an acute angle, which may cause stress concentration at the tip of the bent portion 130 and reduce the resistance to repeated bending. If the thickness of the polyimide insulating layer (A) is more than 35 μm, a large stress is applied to the circuit wiring layer (B) when the FPC 100 is bent, which may reduce the resistance to repeated bending.

[0030] For the polyimide insulating layer (A), commercially available polyimide films can be used as they are. However, because of the ease of controlling the thickness and physical properties of the insulating layer, a so-called casting (coating) method is preferred, in which a polyamic acid solution is directly applied to a copper foil and then dried and cured by heat treatment. The polyimide insulating layer (A) may be formed from a single layer, but considering the adhesiveness between the polyimide insulating layer (A) and the circuit wiring layer (B), a polyimide insulating layer consisting of multiple layers is preferred. A polyimide insulating layer (A) consisting of multiple layers can be formed by sequentially applying a polyamic acid solution consisting of one component onto another polyamic acid solution consisting of a different component. When the polyimide insulating layer (A) consists of multiple layers, a polyamic acid solution of the same component may be used more than once.

[0031] The polyimide insulating layer (A) will be explained in more detail. As mentioned above, it is preferable that the polyimide insulating layer (A) be made of a plurality of layers. As a specific example, it is preferable that the polyimide insulating layer (A) has a laminated structure including a low-thermal expansion polyimide layer (i) and a high-thermal expansion polyimide layer (ii). More preferably, the polyimide insulating layer (A) has a laminated structure having a high-thermal expansion polyimide layer (ii) on at least one side of the low-thermal expansion polyimide layer (i), preferably on both sides thereof, so that the high-thermal expansion polyimide layer (ii) is in direct contact with the circuit wiring layer (B). Here, the "low-thermal expansion polyimide layer (i)" refers to a polyimide layer having a thermal expansion coefficient of 30×10 -6 / K, preferably less than 1 × 10 -6 ~25×10 -6 / K, particularly preferably 3×10 -6 ~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 30 × 10 -6 ~80×10 -6 / K, particularly preferably 30×10 -6 ~70×10 -6This refers to a polyimide layer having a desired thermal expansion coefficient by appropriately changing the combination of raw materials used, thickness, and drying and curing conditions.

[0032] 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. In this case, the viscosity of the polyamic acid solution obtained by polymerization is preferably in the range of, for example, 500 cps to 35,000 cps.

[0033] Diamines used as raw materials for polyimides 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'-diaminodiphenyl 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-xylylenediamine, p-xylylenediamine, 2,6-diaminopyridine, 2,5-diaminopyridine, 2,5-diamino-1,3,4-oxadiazole, piperazine, 2,2'-dimethyl-4,4'-diaminobiphenyl, 2-methoxy-4,4'-diaminobenzanilide, 3,7-diaminodibenzofuran, 1,Examples include 5-diaminofluorene, dibenzo-p-dioxin-2,7-diamine, and 4,4'-diaminobenzyl.

[0034] Examples of acid anhydrides used as raw materials for polyimides include pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,3,3',4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, naphthalene-1,2,5,6-tetracarboxylic dianhydride, naphthalene-1,2,4,5-tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, and naphthalene-1,2,6 ,7-tetracarboxylic acid dianhydride, 4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylic acid dianhydride, 4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-2,3,6,7-tetracarboxylic acid dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid 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-dicarboxyphenyl)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- Examples include 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, and 2,3,6,7-naphthalenetetracarboxylic dianhydride.

[0035] The diamines and acid anhydrides may be used singly or in combination of two or more thereof. Solvents used in the polymerization include dimethylacetamide, N-methylpyrrolidinone, 2-butanone, diglyme, xylene, etc., and these may be used singly or in combination of two or more thereof.

[0036] In this embodiment, the thermal expansion coefficient is 30×10 -6 To obtain a polyimide layer (i) having a low thermal expansion coefficient of less than 1 / K, for example, it is preferable to use pyromellitic dianhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride as the raw material acid anhydride components, and 2,2'-dimethyl-4,4'-diaminobiphenyl and 2-methoxy-4,4'-diaminobenzanilide as the diamine components, and it is particularly preferable to use a polyimide layer (i) containing pyromellitic dianhydride and 2,2'-dimethyl-4,4'-diaminobiphenyl as the main raw material components.

[0037] Also, the thermal expansion coefficient is 30 x 10 -6 To obtain a polyimide layer (ii) with a high thermal expansion of 1 / K or more, for example, the acid anhydride components of the raw materials include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, and 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, and the diamine components include 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenyl ether, and 1,3-bis(4-aminophenoxy)benzene. The preferred raw material components are pyromellitic dianhydride and 2,2'-bis[4-(4-aminophenoxy)phenyl]propane. The preferred glass transition temperature of the resulting high thermal expansion polyimide layer (ii) is in the range of 300 to 400°C.

[0038] Furthermore, when the polyimide insulating layer (A) has a laminate structure of a low-thermal expansion polyimide layer (i) and a 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 preferably in the range of 2 to 15, more preferably in the range of 8 to 11. If this ratio value is less than 2, the low-thermal expansion polyimide layer (i) becomes thin relative to the entire polyimide insulating layer (A), making it difficult to control the dimensional characteristics of the polyimide insulating layer (A), and increasing the dimensional change rate when the copper foil is etched to form the circuit wiring layer (B). If it exceeds 15, the high-thermal expansion polyimide layer (ii) becomes thin, reducing the adhesive reliability between the polyimide insulating layer (A) and the circuit wiring layer (B).

[0039] <Circuit wiring layer (B)> In the FPC 100 of this embodiment, the circuit wiring layer (B) is composed of copper wiring made of, for example, copper foil. The copper foil used for the circuit wiring layer (B) is not particularly limited, and either a commercially available rolled copper foil or an electrolytic copper foil may be used.

[0040] The thickness of the copper wiring constituting the circuit wiring layer (B) is preferably within the range of, for example, 10 to 14 μm. 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 decreases, and handling during production of the FPC 100 tends to deteriorate. If the thickness exceeds 14 μm, the stress applied to the copper wiring when the FPC 100 is bent increases, and the resistance to repeated bending tends to deteriorate.

[0041] <Coverlay (C)> In the FPC 100 of this embodiment, the coverlay (C) preferably has a thickness in the range of 35 to 40 μm and a tensile modulus in the range of 2.0 to 3.5 GPa. Commercially available products can be used as such coverlay (C). Specific examples include CEA0525 (product name) manufactured by Arisawa Manufacturing Co., Ltd.

[0042] The FPC 100 of this embodiment preferably has an overall thickness (i.e., the total thickness of the polyimide insulating layer (A), the circuit wiring layer (B), and, if an optional coverlay (C) is laminated, the thickness of the coverlay (C) [where this refers to the thickness after wiring filling (see Table 1 below)]) within a range of 63 to 73 μm. In this case, the thickness ratio of the polyimide insulating layer (A) to the circuit wiring layer (B) (polyimide insulating layer (A) / circuit wiring layer (B)) is preferably within a range of 1.5 to 2.5. Within this thickness ratio range, the physical properties of the polyimide insulating layer (A) dominate during bending, resulting in good bending resistance. If this ratio is less than 1.5, the tip of the bent portion 130 of the FPC 100 is likely to deform at an acute angle, which may result in stress concentration at the tip of the bent portion 130 and reduce bending resistance. If the value of this ratio exceeds 2.5, a larger stress is applied to the circuit wiring layer (B) when the FPC 100 is bent, and this may reduce its resistance to repeated bending. The thickness ratio of the polyimide insulating layer (A) to the circuit wiring layer (B) is the neutral plane position [NP] in the wiring portion described next. Line It is preferable to decide taking into consideration the following. [NP]Line The closer to the center of the circuit wiring layer (B), the smaller the stress applied to the circuit wiring layer (B) when bent, and the tendency is for the resistance to repeated bending to be improved.

[0043] <Laminate model for calculating neutral plane position> A method for calculating the neutral plane position of the FPC 100 will be described in detail with reference to FIG. 4. FIG. 4 is a cross-sectional view of a model of a laminate used to explain the method for calculating the neutral plane position. For convenience, FIG. 4 shows a model of a laminate with two layers, but the following description applies to laminates with two or more layers in general. Here, the number of layers of the laminate is n (n is an integer equal to or greater than 2). Furthermore, of the layers constituting this laminate, the i-th layer (i=1, 2, . . . , n), counting from the bottom, is referred to as the i-th layer. In FIG. 4, the symbol B represents the width of the laminate. Note that the width here refers to the dimension parallel to the bottom surface of the first layer and perpendicular to the longitudinal direction of the laminate. The FPC 100 in this embodiment is composed of a polyimide insulating layer (A), a circuit wiring layer (B), and a coverlay (C). When viewed from the circuit wiring layer (B) side without the coverlay (C), there are areas where copper wiring exists and areas where copper wiring does not exist. Here, the areas where copper wiring exists are called wiring areas (Line), and the areas where copper wiring does not exist are called space areas (Space). The wiring areas and space areas have different configurations. Therefore, the wiring areas and space areas are considered separately as necessary.

[0044] <Calculation of neutral plane position> Here, the bottom surface of the first layer is the reference plane SP. Below, we consider the case where the laminate is bent so that the reference plane SP has a downward convex shape in Figure 4. In Figure 4, 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 the neutral plane position [NP], and this neutral plane position [NP] is calculated separately for the wiring section and the space section. The neutral plane position [NP] is calculated using the following formula (1):

[0045] [NP]=Σ i=1 n E i B i hi 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 the relationship between stress and strain in each layer. B i is the width of the i-th layer and corresponds to the width B shown in FIG. 4. When obtaining the neutral plane position [NP] of the wiring portion, the value of the line width (line width) is used as B i , and when obtaining the neutral plane position [NP] of the space portion, the value of the space width (space width) is used as B i . h i is the distance between the center plane of the i-th layer and the reference plane SP. Note that the center 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. Also, the symbol "Σ i=1 n " represents the sum from i = 1 to n. Hereinafter, the neutral plane position of the wiring portion is denoted as [NP] Line .

[0046] [NP] Line The closer [NP] is to the center plane of the circuit wiring layer (B), the smaller the stress applied to the circuit wiring layer (B) during bending, and the tendency for the continuous foldability to improve. For example, when the thickness of the polyimide insulating layer (A) is within the range of 23 to 27 μm, the thickness of the circuit wiring layer (B) is within the range of 10 to 14 μm, and the thickness of the coverlay (C) is within the range of 35 to 40 μm, and the total thickness of these is within the range of 63 to 73 μm, [NP] Line is preferably within the range of 28 to 32 μm.

[0047] <Manufacture of FPC> The FPC 100 is manufactured, for example, by processing the copper foil layer of a flexible copper-clad laminate provided with a polyimide insulating layer and a copper foil layer into a pattern by etching or the like to form a wiring layer, and attaching a coverlay thereon as necessary.

[0048] [Metal-clad laminate] The metal-clad laminate used to manufacture the FPC 100 of this embodiment includes a polyimide layer that becomes the polyimide insulating layer (A) when processed into the FPC 100, and a metal layer that becomes the circuit wiring layer (B). The configuration of the polyimide layer is the same as the polyimide insulating layer (A) in the FPC 100. The configuration of the metal layer is the same as the circuit wiring layer (B) in the FPC 100, except that it is not circuit-processed.

[0049] A flexible copper-clad laminate, which is a preferred embodiment of a metal-clad laminate, can be produced, for example, by applying a polyimide precursor resin solution (also referred to as a polyamic acid solution) to the surface of copper foil, which is the 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 involves heating the applied polyamic acid solution at a temperature below 160°C to dry and remove the solvent in the polyamic acid, and then gradually increasing the temperature in the range of 150°C to 400°C to cure it. To convert the single-sided flexible copper-clad laminate obtained in this way into a double-sided copper-clad laminate, one method is to thermocompress the single-sided flexible copper-clad laminate and separately prepared copper foil at 300 to 400°C. [Example]

[0050] The features of the present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. In the following examples, various measurements and evaluations are as follows, unless otherwise specified.

[0051] [Create stress-strain curve] To create the stress-strain curve for polyimide, we used a polyimide film prepared by etching a flexible copper-clad laminate to completely remove the copper foil. The material thus obtained was subjected to a tensile test at a temperature of 23°C and a relative humidity of 50% using a Strograph R-1 manufactured by Toyo Seiki Seisaku-sho, Ltd., to create a stress-strain curve, and the slope of the plastic deformation region was calculated based on the above formula (a).

[0052] [Continuous bending test] The test was performed using a similar bending resistance test device to that described in Japanese Patent Application No. 2017-249096. First, the copper foil of a copper-clad laminate was etched to form 16 rows of copper wiring 201 along its longitudinal direction, as shown in FIG. 5, with a line width of 100 μm, a space width of 100 μm, and a length of 110 mm, to produce a test specimen (test circuit board specimen) 200. FIG. 5 shows only the copper wiring 201 in the test specimen 200, and only 9 of the 16 rows are shown. All 16 rows of copper wiring 201 in the test specimen 200 were continuously connected via U-shaped portions 202, and electrodes (not shown) for measuring resistance were provided at both ends.

[0053] A 37.5 μm thick coverlay was crimped onto the test piece 200, excluding the electrode portion, and then the test piece was fixed on two foldable sample stages 220 and 230, as shown in FIG. 6 . Next, a resistance measurement wire (not shown) was connected, and resistance monitoring was initiated. The bending test was performed by bending the 16 rows of copper wiring 201 so that the copper wiring 201 faced each other with the inside facing each other, forming a first non-bent portion, a second non-bent portion, and a bent portion, as shown in FIG. 7 . For ease of explanation, in FIG. 7 , the first non-bent portion is designated by the symbol 110, the second non-bent portion by the symbol 120, and the bent portion by the symbol 130, as in FIG. 2 .

[0054] When the test piece 200 is bent, the lengths in the X-axis direction and the Y-axis direction of the deformation region of the bent portion 130 can be set arbitrarily by the distance S between the sample stages 220 and 230 when unbent and the gap G between the sample stages 220 and 230 when bent 180 degrees from the unbent state. In this test, the lengths in the X-axis direction and the Y-axis direction were changed in three stages as shown in Table 2 (described later). The bending test was performed up to a maximum of 200,000 bending cycles, and the point at which the resistance value rose by 10% or more from the value before the bending test was considered to be a failure, and the number of bending cycles up to that point was taken as the measurement value. Also, if the device had not failed after 200,000 bending cycles, the measurement value was taken as 200,000 cycles.

[0055] [Measurement of tensile modulus] The tensile modulus was measured using a Strograph R-1 manufactured by Toyo Seiki Seisakusho Co., Ltd. under an environment of a temperature of 23°C and a relative humidity of 50%. Test piece size: length 160mm x width 12.7mm Grip distance: 101.6 mm Tensile speed: 10 mm / min (when measuring copper foil), 50 mm / min (when measuring polyimide film)

[0056] [Measurement of coefficient of thermal expansion (CTE)] Using a thermomechanical analyzer manufactured by Seiko Instruments, the sample was heated to 250°C, held at that temperature for 10 minutes, and then cooled at a rate of 5°C / min to determine the average thermal expansion coefficient (linear thermal expansion coefficient) from 240°C to 100°C.

[0057] [Synthesis of polyamic acid solution] (Synthesis Example 1) N,N-dimethylacetamide was placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of nitrogen gas introduction. 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) was then added to the reaction vessel and dissolved while stirring. Next, pyromellitic dianhydride (PMDA) was added so that the total amount of monomers added was 12 wt%. Stirring was continued for 3 hours to carry out the polymerization reaction, yielding a resin solution of polyamic acid a. The coefficient of thermal expansion (CTE) of a 25 μm-thick polyimide film formed from polyamic acid a was 55 × 10 -6 / K.

[0058] (Synthesis Example 2) N,N-dimethylacetamide was placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of nitrogen gas introduction. 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB) was then added to the reaction vessel and dissolved while stirring. Next, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) were added so that the total amount of monomers was 15 wt% and the molar ratio of the respective acid anhydrides (BPDA:PMDA) was 20:80. Stirring was continued for 3 hours to carry out the polymerization reaction, yielding a resin solution of polyamic acid b. The coefficient of thermal expansion (CTE) of a 25 μm-thick polyimide film formed from polyamic acid b was 22 × 10 -6 / K.

[0059] [Medium plane position] Table 1 shows the neutral plane positions ([NP] Line The neutral plane position was calculated by substituting the thickness, tensile modulus, and distance between the center plane and the reference plane of each of the polyimide layer, copper foil layer, and coverlay layer listed in Table 1 into the above formula (1).

[0060] [Example 1] On copper foil 1 (rolled copper foil, long, thickness: 12 μm), the resin solution of polyamic acid a prepared in Synthesis Example 1 was uniformly applied to a cured thickness of 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 coated surface to a cured thickness of 20.0 μm, and then heated and dried at 120 ° C to remove the solvent. Furthermore, the resin solution of polyamic acid a was uniformly applied to the coated surface to a cured thickness of 2.5 μm, and then heated and dried at 130 ° C to remove the solvent. This long laminate 1 was heat-treated stepwise from 130 ° C to 360 ° C to obtain a single-sided flexible copper-clad laminate 1 (polyimide layer thickness: 25 μm, polyimide layer tensile modulus: 7.5 GPa). Copper foil 1 was thermocompressed onto the polyimide layer of single-sided flexible copper-clad laminate 1 at 300-400°C to obtain double-sided flexible copper-clad laminate 1. The copper foil layer on the coated side of double-sided flexible copper-clad laminate 1 was etched away, and the copper foil layer (tensile modulus: 19 GPa) on the other bonding side was processed to form a copper wiring circuit. Coverlay A (thickness: 37.5 μm, tensile modulus: 3.3 GPa) was then attached to obtain FPC 1. The thickness, tensile modulus, distance between the center plane and the reference plane, and center plane position of each of the polyimide layer, copper foil layer, and coverlay layer of the obtained FPC 1 are shown in Table 1. The slope of the plastic deformation region in the stress-strain curve of the polyimide layer and the evaluation results of continuous bending resistance are shown in Table 2.

[0061] [Example 2] Single-sided flexible copper-clad laminate 2, double-sided flexible copper-clad laminate 2 (tensile modulus of the copper foil layer on the bonding surface: 29 GPa), and FPC 2 were prepared in the same manner as in Example 1, except that copper foil 2 (electrolytic copper foil, long, thickness: 12 μm) was used instead of copper foil 1 in Example 1. The thickness, tensile modulus, distance between the center plane and the reference plane, and center plane position of each of the polyimide layer, copper foil layer, and coverlay layer of the obtained FPC 2 are shown in Table 1, and the slope of the plastic deformation region in the stress-strain curve of the polyimide layer and the evaluation results of continuous bending resistance are shown in Table 2.

[0062] [Example 3] The copper foil layer on one side of a commercially available double-sided flexible copper-clad laminate 3 (copper foil thickness: 12 μm, polyimide layer thickness: 25 μm, polyimide layer tensile modulus: 4.5 GPa) was etched away, and the copper foil layer on the other side (tensile modulus: 29 GPa) was processed to form copper wiring, followed by application of coverlay A to obtain FPC 3. The thickness, tensile modulus, distance between the center plane and the reference plane, and center plane position of each of the polyimide layer, copper foil layer, and coverlay layers of the obtained FPC 3 are shown in Table 1, and the slope of the plastic deformation region in the stress-strain curve of the polyimide layer and the evaluation results of continuous bending resistance are shown in Table 2.

[0063] Comparative Example 1 The copper foil layer on one side of a commercially available double-sided flexible copper-clad laminate 4 (copper foil thickness: 12 μm, polyimide layer thickness: 25 μm, polyimide layer tensile modulus: 5.0 GPa) was etched away, and the copper foil layer on the other side (tensile modulus: 19 GPa) was processed to form copper wiring, followed by application of coverlay A to obtain FPC 4. The thickness, tensile modulus, distance between the center plane and the reference plane, and center plane position of each polyimide layer, copper foil layer, and coverlay layer of the obtained FPC 4 are shown in Table 1, and the slope of the plastic deformation region in the stress-strain curve of the polyimide layer and the evaluation results of continuous bending resistance are shown in Table 2.

[0064] Comparative Example 2 The copper foil layer on one side of a commercially available double-sided flexible copper-clad laminate 5 (copper foil thickness: 12 μm, polyimide layer thickness: 25 μm, polyimide layer tensile modulus: 5.0 GPa) was etched away, and the copper foil layer on the other side (tensile modulus: 23 GPa) was processed to form copper wiring, followed by application of coverlay A to obtain FPC 5. The thickness, tensile modulus, distance between the center plane and the reference plane, and center plane position of each of the polyimide layer, copper foil layer, and coverlay layers of the obtained FPC 5 are shown in Table 1, and the slope of the plastic deformation region in the stress-strain curve of the polyimide layer and the evaluation results of continuous bending resistance are shown in Table 2.

[0065] The above results are shown in Tables 1 and 2. In Table 2, the unit of "Slope of stress-strain curve (polyimide layer)" is 10 2 MPa The "failure rate" refers to the number of failures per test run (5 times).

[0066] [Table 1]

[0067] [Table 2]

[0068] Although the embodiments of the present invention have been described in detail above for the purpose of illustration, the present invention is not limited to the above-described embodiments and various modifications are possible. [Explanation of symbols]

[0069] 100...FPC, 101...first surface, 102...second surface, 110...first non-bending portion, 120...second non-bending portion, 130...bending portion, 200...test piece, 201...copper wiring, 202...U-shaped portion, 220, 230...sample stage, P1, P2...inflection point

Claims

1. an insulating resin layer; a wiring layer laminated on at least one surface of the insulating resin layer; a coverlay for protecting the wiring layer; A flexible circuit board comprising: the second non-bending portion is repeatedly bent 180 degrees relative to the first non-bending portion so as to have a shape including a first non-bending portion and a second non-bending portion whose shape has not changed compared to the flat state, and a bent portion that is positioned between the first non-bending portion and the second non-bending portion and has been curved and deformed, the wiring layer has a tensile modulus of elasticity in the range of 19 GPa to 29 GPa in a state of a metal layer without circuit processing; When an axial direction parallel to a thickness direction of the first non-bending portion and the second non-bending portion is defined as a Y-axis direction, and an axial direction perpendicular to the Y-axis direction and parallel to a longitudinal direction of the flexible circuit board is defined as an X-axis direction, in the insulating resin layer, a maximum length of the deformation region in the X-axis direction in the bent portion is within a range of 1.0 mm or more and 10.0 mm or less, and a maximum length of the deformation region in the Y-axis direction is within a range of 1.0 mm or more and 6.0 mm or less on two-dimensional coordinate axes in the X-axis direction and the Y-axis direction, the insulating resin layer has a tensile modulus of elasticity in the range of 6 GPa to 8 GPa; The thickness of the coverlay is in the range of 35 to 40 μm, A flexible circuit board in which the gradient of the plastic deformation region of the stress-strain curve of the insulating resin layer is within a range of 100 MPa or more and less than 500 MPa.

2. 2. The flexible circuit board according to claim 1, which is used by repeatedly bending the flexible circuit board by 180 degrees so that the wiring layer is on the inside.

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