JP5732406B2 - Flexible circuit board and bent portion structure of flexible circuit board - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a flexible circuit board that is used with a bent portion in any one of them, and a bent portion structure of the flexible circuit board. More specifically, the invention has durability against bending and is flexible. The present invention relates to an excellent flexible circuit board and a bent portion structure of the flexible circuit board.

  Since a flexible circuit board (flexible printed circuit board) having a resin layer and a wiring made of metal foil can be used by being bent, a movable part in a hard disk, a hinge part of a mobile phone, Widely used in various electronic and electrical devices such as sliding slides, printer heads, optical pickups, and notebook PCs. In recent years, especially with the downsizing, thinning, and high functionality of these devices, the flexible circuit board can be folded and stored in a limited space or used in various movements of electronic devices. Corresponding flexibility is required. Therefore, it is necessary to improve the mechanical characteristics such as the strength of the flexible circuit board so that it can cope with the bending with a smaller radius of curvature at the bent portion and the operation in which the bending is frequently repeated. It has become.

  In general, it is the wiring rather than the resin layer that causes failure due to repeated bending and bending with a small radius of curvature. As a result, it cannot be used as a circuit board. Therefore, for example, in order to reduce the bending stress on the wiring in the hinge portion, the flexible circuit board (see Patent Document 1) wired so as to be inclined with respect to the rotation axis, or the rotation direction of the hinge portion. A method has been proposed in which a spiral portion formed by spiraling one or more turns is formed and the change in the diameter of the spiral portion due to the opening / closing operation is reduced to reduce damage by increasing the number of turns (see Patent Document 2). . However, any of these methods restricts the design of the flexible circuit board.

On the other hand, the strength (I) of the (200) plane determined by X-ray diffraction (X-ray diffraction in the thickness direction of the copper foil) of the rolled copper foil was determined by X-ray diffraction of fine powder copper (200 ) It has been reported that when I / I ₀ > 20 with respect to the surface strength (I ₀ ), the film has excellent flexibility (see Patent Documents 3 and 4). That is, the flexibility of the copper foil improves as the cube orientation, which is the recrystallized texture of copper, develops. Therefore, a flexible circuit board in which the degree of development of the cube texture is defined by the parameter (I / I ₀ ) A copper foil suitable as a wiring material is known. In addition, a rolled copper alloy foil containing elements such as Fe, Ni, Al, Ag, etc. in a concentration in the range of solid solution in copper, and obtained by annealing and recrystallization under predetermined conditions is shear along the sliding surface. It has been reported that the deformation is facilitated and the flexibility is excellent (see Patent Document 5).

  In addition, a copper foil containing impurities such as oxygen and silver may be used for a flexible circuit board that requires high bending properties, and the purity is about 99% to 99.9% by mass. Copper foil. In the present invention, purity is expressed as mass concentration unless otherwise specified. Further, at the test level, there are examples in which tough pitch copper having a purity of about 99.5% and oxygen-free copper containing no oxide, which are widely used as cable conductors, are used (see Patent Documents 3 and 4). Impurities of tough pitch copper include several hundred ppm of oxygen (mostly included as copper oxide), silver, iron, sulfur, phosphorus, and the like. Oxygen-free copper is usually copper having a purity of about 99.96 to 99.995%, and is oxygen that has been greatly reduced in oxygen to 10 ppm or less. In Patent Documents 3 and 4 described above, it is reported that the bending fatigue characteristics of a copper foil made of oxygen-free copper is superior to that of a tough pitch copper foil and depends on the presence or absence of copper oxide. In addition, when further improving the purity of these copper, it is necessary to remove impurities, such as silver, phosphorus, and sulfur.

JP 2002-171033 A JP 2002-300147 A JP 2001-58203 A Japanese Patent No. 3009383 JP 2007-107036 A

  Under such circumstances, the present inventors have no restrictions on the design of the flexible circuit board, and the flexible circuit board has durability against repeated bending and bending with a small radius of curvature. As a result of diligent studies to obtain a flexible circuit that is highly oriented and has excellent bending durability and flexibility by using a metal foil having a face-centered cubic crystal structure with a large elongation at break. The inventors found that a substrate was obtained and completed the present invention.

  Accordingly, an object of the present invention is to provide a flexible circuit board having excellent durability, and in particular, repeated bending with a small radius of curvature, such as a hinge part or a slide sliding part of a mobile phone or a small electronic device. It is to provide a flexible circuit board that exhibits durability even under severe use conditions such as, and has excellent flexibility.

  In addition, another object of the present invention is to provide durability and resistance to harsh conditions such as hinge portions or slide sliding portions of mobile phones, small electronic devices, etc., particularly in repeated bending portions having a small curvature radius. An object of the present invention is to provide an excellent flexible circuit board bent portion structure.

The present invention is summarized as follows as a result of intensive studies to solve the above-described problems of the prior art.
(1) A flexible circuit board including a resin layer and a wiring formed from a metal foil, and having a bent portion at least at one place of the wiring,
The resin layer is made of polyimide having a thickness of 10 to 30 μm,
The metal foil is composed of a rolled copper foil having a face-centered cubic structure and a purity of 99.9% by mass or more, and the basic crystal axis <100> of the unit cell of the face-centered cubic structure is the thickness direction of the rolled copper foil and the foil. With respect to two orthogonal axes with respect to a certain direction existing in the plane, each of the preferential orientation regions within an orientation difference of 10 ° occupies 50% or more by area ratio, and from the normal direction of the foil surface of the rolled copper foil The crystal grain size when viewed is 25 μm or more,
Further, the elongation at break of the rolled copper foil in the normal direction relative to the cross section P of the wiring cut in the thickness direction of the rolled copper foil from the ridgeline at the bent portion is 3.5% or more and 20% or less. Circuit board.
(2) rolled copper foil, is 99.999% by mass or more (1) a flexible circuit board according to claim.
( 3 ) The flexible circuit board according to (1) or (2) , wherein the thickness of the rolled copper foil is 5 μm or more and 18 μm or less.
( 4 ) Any plane in which the cross section P of the wiring is included in the range of (20 1 0) to (1 20 0) in the rotation direction from (100) to (110) with [001] as the zone axis The flexible circuit board according to any one of (1) to ( 3 ), wherein
( 5 ) The cross section P of the wiring is any surface on the line segment connected by the point representing (20 1 0) and the point representing (110) in the stereo triangle of the (100) standard projection view. The flexible circuit board according to item ( 4 ).
( 6 ) The flexible circuit board according to any one of (1) to ( 5 ), wherein wiring is formed along a direction orthogonal to the ridgeline in the bent portion.
( 7 ) Any of the items (1) to ( 6 ) used so as to form a bent portion with any repetitive action selected from the group consisting of sliding bending, bending bending, hinge bending and sliding bending. A flexible circuit board according to claim 1.
( 8 ) An electronic device on which the flexible circuit board according to any one of (1) to ( 7 ) is mounted.
( 9 ) A bent portion structure of a flexible circuit board including a resin layer and a wiring formed from a metal foil, and having a bent portion in at least one place of the wiring,
The resin layer is made of polyimide having a thickness of 10 to 30 μm,
The metal foil is composed of a rolled copper foil having a face-centered cubic structure and a purity of 99.9% by mass or more, and the basic crystal axis <100> of the unit cell of the face-centered cubic structure is the thickness direction of the rolled copper foil and the foil. With respect to two orthogonal axes with respect to a certain direction existing in the plane, each of the preferential orientation regions within an orientation difference of 10 ° occupies 50% or more by area ratio, and from the normal direction of the foil surface of the rolled copper foil The crystal grain size when viewed is 25 μm or more,
Further, the elongation at break of the rolled copper foil in the normal direction relative to the cross section P of the wiring cut in the thickness direction of the rolled copper foil from the ridgeline at the bent portion is 3.5% or more and 20% or less. Circuit board bend structure.

  According to the present invention, the metal foil constituting the wiring at the bent portion when the flexible circuit board is bent hardly causes metal fatigue and has excellent durability against stress and strain. Therefore, it is possible to provide a flexible circuit board having excellent flexibility with sufficient strength to withstand repeated bending and bending with a small radius of curvature without any restrictions on the design of the flexible circuit board. This makes it possible to realize highly durable electronic devices such as thin mobile phones, thin displays, hard disks, printers, and DVD devices.

FIG. 1 is a diagram illustrating a relationship between a crystal zone axis in a cubic crystal structure and a plane obtained by rotating around the crystal zone axis. FIG. 2 is a stereo triangle of (100) standard projection. FIG. 3 is an explanatory cross-sectional view showing a state in which the flexible circuit board is bent. FIG. 4 is an explanatory plan view showing the relationship between the wiring in the flexible circuit board and the crystal axis of the metal foil, (a) and (b) show the flexible circuit board according to the present invention, and (c) ) And (d) show a prior art flexible circuit board. FIG. 5 is a perspective explanatory view of a single-sided copper-clad laminate. FIG. 6 is an explanatory plan view showing a state in which a test flexible circuit board is obtained from a single-sided copper-clad laminate in the embodiment of the present invention. FIG. 7 is an explanatory diagram of an MIT flex test apparatus. FIG. 8A is an explanatory view of an IPC bending test apparatus, and FIG. 8B is an X-X ′ sectional view of a test flexible circuit board used in the IPC bending test.

  The wiring provided in the flexible circuit board of the present invention is formed of a metal foil made of a metal having a face-centered cubic crystal structure. As the metal having a face-centered cubic crystal structure, for example, copper, aluminum, nickel, silver, rhodium, palladium, platinum, gold and the like are known. Copper, aluminum, and nickel are preferable because of their utility as copper, and copper foils that are mainly used as wiring for flexible circuit boards are the most common.

  The present invention provides a flexible circuit board excellent in bending durability and flexibility, and in particular, provides a flexible circuit board having excellent fatigue characteristics in a high strain region where the radius of curvature is 2 mm or less. Is. In order to achieve this object, in the present invention, one of i) the metal foil is highly oriented and ii) the elongation at break in the principal stress direction of the metal foil is large at the bent portion. However, it does not become a flexible circuit board that is resistant to fatigue failure during high bending as in the present invention. That is, by satisfying both i) and ii) at the same time, a flexible circuit board that is resistant to fatigue failure during high bending can be obtained. Specifically, i) the basic crystal axis <100> of the unit cell of the face-centered cubic structure is respectively in relation to two orthogonal axes of the thickness direction of the metal foil and a certain direction existing in the foil surface. The preferred orientation region within an orientation difference of 10 ° or more occupies 50% or more in area ratio, and ii) the elongation at break of the metal foil in the normal direction relative to the cross section P of the wiring cut in the thickness direction of the metal foil from the ridgeline at the bent portion Needs to be 3.5% or more and 20% or less.

  When the metal foil is a polycrystalline body as found in general electrolytic foils and rolled foils, a high breaking elongation can be obtained, but a flexible circuit having high fatigue characteristics against the high strain fatigue required in the present invention. It is not a substrate. On the other hand, when the texture is developed and the elongation at break is small even when the degree of orientation is large, a flexible circuit board having the characteristics required by the present invention cannot be obtained.

  In the present invention, on the condition that the texture is developed and the metal foil has a large degree of orientation, the breaking elongation of the metal foil in the flexible circuit board, which requires particularly high bending characteristics, is an important factor. This is the first time revealed.

  The metal foil may be either a rolled foil or an electrolytic foil, but is preferably a rolled foil in order to obtain high orientation. In the case of face-centered cubic metal, a metal foil having a highly oriented cubic texture with <100> main orientations in the rolling direction and the normal direction of the foil surface is produced by devising rolling conditions and heat treatment conditions. can do.

  The feature of the mechanical characteristics of the metal foil having a strong cubic orientation is not limited to the use of the flexible circuit board, and is that the elongation at break is anisotropic. The elongation at break takes a very small value when pulled in the <100> direction. In general, the greater the degree of orientation and the smaller the thickness of the metal foil, the smaller the elongation at break when a tensile test is performed in the <100> direction. The basic crystal axis <100> of the unit cell of the face-centered cubic structure is the thickness direction of the metal foil (the normal direction of the foil surface) and one direction existing in the foil surface (one of which is the rolling direction). In the case of a general rolled copper foil having a surface area ratio of 95% or more and a thickness of 18 μm or less with respect to two orthogonal axes, each of which has an orientation difference of 10 ° or less (hereinafter referred to as “conventional rolling” for convenience) It is referred to as “copper foil”), and the elongation at break in the principal stress direction at the bent portion does not reach 3.5%. The elongation at break here refers to a specimen having any width within a range of 5 to 15 mm and having a width sufficiently larger than the thickness of the metal foil. This refers to the elongation to break when a tensile test is performed at a strain rate of% / min. In the present invention, the elongation after breaking of the metal foil is determined by the measurement method shown in the following examples, and the value after the flexible circuit board is obtained by laminating with the resin layer.

  In the case of a rolled copper foil, the recrystallized texture has a <100> orientation in the rolling direction, that is, the longitudinal direction of the metal foil. In a normal flexible circuit board, when the board is extracted, the longitudinal direction of the circuit and the longitudinal direction of the copper foil coincide with each other from the viewpoint of increasing the yield. Therefore, in the normal usage mode in which the longitudinal direction of the circuit is bent, the main stress direction coincides with the <100> direction, so that the conventional rolled copper foil cannot obtain high fatigue characteristics against repeated bending.

  As a method for improving the fatigue characteristics of the flexible circuit board used in such an orientation relationship, the present invention aims to increase the purity of the metal foil used. In a flexible circuit board used for high bending applications known so far, a copper foil intentionally or inevitably contains impurities such as oxygen and silver is used. This is for the purpose of facilitating shear deformation along the slip surface and suppressing an increase in electrical resistance, as described in Patent Document 5, for example. However, these impurity elements reduce the stacking fault energy. The present inventors paid attention to this point. That is, when the stacking fault energy is decreased, dislocations are easily expanded and cross slipping is unlikely to occur, and particularly when stretched in the <100> direction, elongation is difficult to occur.

  Therefore, in the present invention, a metal foil (preferably a copper foil) having a predetermined preferred orientation as described below and preferably having a purity of 99.999% or more is used, whereby the <100> direction is used. The elongation at break can be increased to 3.5% or more, and as a result, the fatigue characteristics are increased when repeated strain is applied in a high strain region. Although the higher purity of the metal foil is desirable, it is most preferable to use 99.999% to 99.9999% from the viewpoint of manufacturing cost. Further, even if the copper foil has a purity lower than 99.999%, an oxygen-free copper foil having a low oxygen concentration has a face-centered cubic shape depending on rolling and heat treatment conditions in a narrow condition, as shown in the following examples. 98% or more of a region in which one of the basic crystal axes <100> of the structure, for example, the [001] axis is within 10 ° in orientation difference with respect to the thickness direction of the metal foil (the normal direction of the foil surface); When it is 8% or less, there is a region where the elongation at break is 3.5% or more, and the bending fatigue resistance is improved. Although it is not clear at this time about this reason, in the oxygen-free copper foil in which a predetermined texture is obtained by heat treatment, a recrystallized residual structure with respect to the <212> orientation with respect to the rolling direction dispersed at an appropriate size and volume ratio It is assumed that the elongation at break in the <100> direction is increased by the presence of.

  In the flexible circuit board of the present invention, the three-dimensional crystal orientation of the metal foil is defined with respect to the sample coordinate system of the metal foil constituting the circuit, and the degree of texture integration is in the following range. That is, a region in which one of the basic crystal axes <100> of the face-centered cubic structure, for example, the [001] axis is within 10 ° in orientation difference with respect to the thickness direction of the metal foil (the normal direction of the foil surface) is an area. In the foil plane, which has a preferential orientation that accounts for 50% or more, preferably 75% or more, more preferably 98% or more, and is in a direction parallel to the surface of the metal foil, another basic crystal An axis having a preferential orientation such that a region within 10 ° from the [100] axis occupies an azimuth difference accounts for 50% or more, desirably 85% or more, and more desirably 99% or more is used. In the present invention, it is sufficient that at least the bent portion has the above-mentioned texture integration degree, but preferably all the metal foils laminated on the resin layer have the above-mentioned integration degree. A so-called single crystal-like metal foil is preferable without being restricted in wiring design. Since the crystal orientation at the center of the preferential orientation is called the main orientation of the texture, the metal foil used in the present invention has a main orientation of <100> in the thickness direction of the metal foil, It can be said that the foil plane has a main orientation of <100>.

  There are several indices that indicate the priority of texture orientation, that is, the degree of orientation or accumulation, and it is objective to use X-ray diffraction intensity and statistical data of local three-dimensional orientation data obtained by electron diffraction. An index based on simple data can be used.

For example, when the metal foil is a copper foil, the intensity (I) from (002) perpendicular to the above-mentioned zone axis determined by X-ray diffraction (here, according to a general notation method in X-ray diffraction, (200) plane The wiring having a predetermined pattern is formed from a copper foil having I / I ₀ ≧ 25 with respect to the strength (I ₀ ) of the (200) plane obtained by X-ray diffraction of fine powder copper. The I / I ₀ is preferably in the range of 33 to 150, more preferably in the range of 50 to 150. Here, the parameter I / I ₀ represents the degree of orientation of the zone axes of (100) and (110), that is, the common axis [001], and is an objective index indicating the degree of development of the cube texture. is there. When the metal foil is a rolled copper foil, it is strongly processed at a rolling rate of a certain level or higher, and then recrystallized by applying heat, so that the rolled foil surface is rolled into the (001) main orientation and foil plane. A recrystallized cube orientation with the direction as the (100) principal orientation develops. The bending fatigue life of the copper foil is improved as the cubic orientation, which is the recrystallized texture of copper, develops. In the flexible circuit board of the present invention, if I / I ₀ is less than 25, it is not possible to sufficiently improve the bending fatigue life of the wiring. If I / I ₀ is 33 or more, the bending fatigue life is remarkably improved. Become. The X-ray diffraction in the thickness direction of the copper foil confirms the orientation on the surface of the copper foil (rolled surface in the case of a rolled copper foil), and the strength (I) of the (200) plane is X-ray. The integrated intensity value of the (200) plane obtained by diffraction is shown. The intensity (I ₀₎ shows a fine powder of copper (manufactured by Kanto Chemical Co., Inc. copper powder reagent I grade, 325 mesh) in an intensity integral value of the (200) plane.

In order to increase the I / I ₀ to 25 or more, a recrystallized texture of the copper foil may be obtained. There is no particular limitation on this means, but the intermediate annealing conditions and the cold rolling process rate are targeted. By optimizing according to the type of metal foil and the impurity concentration, a rolled copper foil having a large texture of crystal grains and I / I ₀ ≧ 25 can be obtained. In addition, for example, after a resin layer and a rolled copper foil are laminated to obtain a copper-clad laminate, the copper foil is subjected to a heating condition such that a temperature of 300 to 360 ° C. is loaded for an integrated time of 5 minutes or more. A recrystallized texture of copper foil may be obtained.

Further, in order to define the texture with a three-dimensional degree of integration, the texture can be specified using the area ratio of the preferentially oriented region that falls within 10 ° with respect to the main orientation of the texture. That is, as to what crystal orientation the predetermined surface of the metal foil has, for example, X-ray diffraction method such as EBSP (Electron Back Scattering Pattern) method, ECP (Electron Channeling Pattern) method or X-ray method such as micro Laue method It can be confirmed by a line diffraction method or the like. In particular, the EBSP method analyzes a crystal from a diffraction image called a pseudo Kikuchi line that is diffracted from each crystal plane generated when a focused electron beam is irradiated on the surface of a sample to be measured. This is a method of measuring the crystal orientation distribution of the measurement object from the position information of the above, and it is possible to analyze the crystal orientation of the texture in the microscopic region 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. It is also possible to define the orientation including the orientation plane having an orientation within a predetermined angle with respect to a specific plane orientation, and to extract the existence ratio of each plane orientation by the area ratio. In the EBSP method, in order to obtain an area ratio of a region within a specific angle from a specific orientation, in order to obtain an area ratio at least in a region larger than the circuit bent region in the flexible circuit board of the present invention. It is necessary to scan the electron beam finely so as to obtain a sufficient number of points to obtain the average information. However, in the metal foil targeted by the present invention, in view of the size of the target circuit, 0. In an area of 005 mm ² or more, 1000 points or more may be measured in order to obtain an average area ratio.

  By the way, the difference in structure between the present invention and the inventions described in Patent Documents 3 and 4 is that the orientation definition of the inventions of these Patent Documents is only the normal direction of the foil measured by X-ray. Thus, the present invention is defined in three dimensions. In order to obtain high fatigue characteristics with respect to bending, the main strain, the main stress direction, that is, the <100> integration degree in the foil surface, particularly when bent is important. In the present invention, the size of recrystallized grains, that is, crystal grains having a cubic orientation, is preferably 25 μm or more on average.

  In the present invention, particularly when high flexibility is required, the metal foil forming the flexible circuit board may be a rolled copper foil having a thickness of 5 to 18 μm, preferably a thickness of 9 to 12 μm. It is preferable to use a rolled copper foil. When the rolled copper foil is thicker than 18 μm, it becomes difficult to obtain a flexible circuit board having excellent fatigue characteristics in a high strain region where the radius of curvature is 2 mm or less. On the other hand, when the thickness is less than 5 μm, handling in laminating the metal foil and the resin layer is difficult, and it is difficult to form a homogeneous flexible circuit board.

  Apart from the first measure for improving the fatigue characteristics of the flexible circuit board described above, the present invention provides a second method for improving the breaking elongation of a face-centered cubic metal foil close to a highly oriented single crystal. As a measure of the above, there is a case where the wiring configuration of the flexible circuit board is devised so that the <100> direction having a small elongation at break does not become the principal stress direction.

  As described in the first policy, by devising the rolling and recrystallization conditions, a metal foil having a highly oriented cubic texture having a <100> main orientation in both the rolling direction and the foil surface normal direction is produced. can do. In addition, the direction of cutting the circuit as wiring is shifted by a predetermined angle from the rolling direction, that is, the <100> direction, and the circuit is pulled diagonally, so that flexibility is large in elongation at break in the main stress direction when bent. A circuit board can be obtained. By such a method, the breaking elongation of the metal foil in the normal direction (main stress direction in the bent portion) with respect to the cross section P of the wiring cut from the ridge line in the bent portion in the thickness direction of the metal foil is 3.5% or more. In order to achieve this, the cross section P needs to have a main orientation on any plane included in the range of (20 1 0) to (1 20 0) with [001] as the zone axis . Here, the relationship between the zone axis and the plane orientation is shown in FIG. (20 1 0) and (1 20 0) have a relationship in which [001] is a common axis, that is, a zone axis, from (100) to (110) with [001] as the axis [from (100) (010)] in the plane of rotation. That is, when this is shown on the reverse pole figure with respect to the normal direction of the cross section P, each plane of (001), (20 1 0), (110) is as shown in FIG. Due to symmetry, (1 20 0) is represented at the same position as (20 1 0) on the inverse pole figure. The metal of the metal foil in the present invention has a face-centered cubic structure. The crystal axes of the unit cell are [100], [010], and [001]. In the present invention, <100> preferred orientation in the thickness direction of the metal foil (the direction perpendicular to the surface of the metal foil). In this case, this axis is expressed as [001], that is, the foil plane orientation is (001), but it is equivalent even if these axes are interchanged due to the symmetry of the face-centered cubic structure, and these are of course included in the present invention. It is.

  And, the main orientation in the foil plane is in the main strain direction of the bent portion, that is, in the normal direction of the cross section of the wiring cut from the ridge line in the bent portion in the thickness direction (with respect to the perpendicular to the wiring cross section P), It is necessary to have an angle of 2.9 ° to 87.1 ° [(20 1 0) to (1 20 0)], and preferably 5.7 ° to 84.3 ° ((10 1 0) to (1 100)], more preferably 11.4 ° to 78.6 ° [(510) to (150)], and more preferably 26.6 ° to 63.4 ° [(210) to (120)], most preferably 30 ° or 60 ° [(40 23 0) or (23 40 0)]. Here, the inside of [] represents the surface orientation of the cross section P corresponding to each angle. From the symmetry of the crystal, it can also be described that the normal to the wiring cross section P has an angle of 2.9 to 45 ° with the basic crystal axis <100> in the metal foil plane.

  Here, the cross section P of the wiring when it is cut in the thickness direction from the ridgeline at the bent portion is, for example, as shown in FIG. 3, when the flexible circuit board is bent in a U shape, a ridgeline L is formed on the outside thereof. However, it refers to the wiring portion of the cross section obtained when the ridge line L is cut in the thickness direction d of the flexible circuit board. The ridge line L is an apex formed when the cross section of the flexible circuit board is viewed along the bending direction (thick arrow in FIG. 3) in a state where the flexible circuit board is bent. It is a connected line. In addition, the case where the ridgeline L moves a flexible circuit board, such as sliding bending mentioned later, for example, is also included. 3 shows a state in which the resin layer 1 is on the outside and the wiring 2 is bent inward (the side on which a circle having a radius of curvature is inscribed is inward), but the wiring 2 is bent outward. Of course, it may be.

  In various applications, when subjected to a forced displacement of a certain curvature, the metal foil is primarily subjected to tensile or compressive stress. Which part of the flexible circuit board that is bent is subjected to tension or compression depends on the configuration of the metal foil and the resin, but is bent from the neutral axis (or neutral surface) of tension and compression. It is common that the outermost part, which is the outermost part, is severe due to the destruction of the metal, and the tensile stress in the normal direction of the cross section of the wiring when cut in the thickness direction from the ridgeline at the bent part becomes the main stress. That is, the main stress direction of the wiring in the bent portion is the direction indicated by the arrow 21 in FIG. 3, and is typically the normal direction to the wiring cross section P cut from the ridge line of the bent portion in the thickness direction of the metal foil. And the direction perpendicular to the [001] axis oriented in the thickness direction of the metal foil.

  When considering the mechanical characteristics of the metal foil in the flexible circuit board, the stress-strain characteristic when the metal foil is simply pulled in the main stress direction indicated by the arrow 21 in FIG. 3 is an important characteristic. Here, as shown in the examples of FIGS. 4C and 4D, the metal foil having a face-centered cubic crystal structure is bent so that a ridge line perpendicular to the [100] axis is formed. In this case, the cross section of the wiring cut in the thickness direction of the flexible circuit board from the ridge line at the bent portion becomes the (100) plane, but the cross section P of the wiring when cut in the thickness direction from the ridge line at the bent portion is As shown in FIG. 1, it is included in the range (20 1 0) to (1 20 0) in the rotation direction from (100) to (010) (double arrow in the figure) with [001] as the zone axis. If any of the surfaces has a main orientation, the elongation at break can be improved. In FIG. 1, the range from (20 1 0) to (1 20 0) is shown, but the face-centered cubic crystal structure has a plane equivalent to the plane included in this range. Therefore, an equivalent surface having a sign different from that of a surface whose wiring cross section is in the range of (20 1 0) to (1 20 0) is included in the present invention.

  In the second policy, the cross-section P of the wiring when cut in the thickness direction from the ridgeline at the bent portion is preferentially oriented with a main orientation in a specific orientation between (20 1 0) and (1 20 0). Therefore, the reason why the elongation at break is improved is that when a tensile stress is applied in the normal direction of the cross section P, that is, the principal stress direction, the metal having a face-centered cubic structure has eight {111 }, Since the main slip surface having the largest Schmid factor is four, the shear slip is good and local work hardening is less likely to occur. In normal rolled copper foil, the longitudinal direction of the metal foil corresponds to the rolling direction, and as shown in FIGS. 4C and 4D, it is normal to form a circuit along the main direction <100>. is there. For example, the Example of patent document 5 is equivalent to the form of FIG.4 (d). Thus, when the cross-sectional orientation of the wiring when cutting in the thickness direction from the ridgeline at the bent portion is (100), the eight Schmitt factors of the eight slip planes become equivalent when bent, and the eight slip systems become simultaneous. Dislocations tend to accumulate locally. Due to such a difference from the prior art, the bending resistance of the flexible circuit board adopting the second measure is superior to that of a normal usage mode in which the flexible circuit board is bent in the longitudinal direction of the circuit.

  Regarding the cross section P in the flexible circuit board, the most desirable orientation is 30 ° or 60 ° with respect to the main strain direction of the bent portion, that is, the normal direction of the cross section of the wiring when cut from the ridge line in the bent portion in the thickness direction. This is because the direction of stress coincides with the stable orientation of tension. When the above mechanism is considered, at least the cross section P of the wiring when cut in the thickness direction from the ridge line at the bent portion is between (20 10) and (1 200) with [001] as the crystal axis. It suffices to have the preferential orientation with the main orientation in the specific orientation.

  That is, the second policy in the present invention is that the metal foil has a face-centered cubic structure, the thickness direction of the metal foil has a main orientation of <100>, and the foil surface of the metal foil has <100>. And the normal direction of the cross section P of the wiring when cut in the thickness direction from the ridgeline at the bent portion is the specific direction between (20 1 0) and (1 200) A wiring that has a preferential orientation is provided. At this time, the normal direction of the cross section P is preferably preferentially oriented with a main orientation in a specific orientation between (10 1 0) and (1 100), and more preferably (510 ) To (110) having a main orientation in a specific orientation, and more preferably having a main orientation in a specific orientation between (210) and (110). Most preferably, the preferred orientation is centered in the vicinity of (40 230). In the case of a metal foil whose foil surface is preferentially oriented with (001) as the main orientation, for example, [001] and [100] in the foil surface are equivalent, and the bent portion of the flexible circuit board in the present invention The main direction of the cross section P of the wiring when cut in the thickness direction from the ridge line in FIG. 5 can also be described as a specific direction between (120) and (110), preferably (120) to (110) It can also be described that it is preferable to preferentially have a main orientation in a specific direction between them, and most preferably to have a main orientation in the vicinity of (23400).

  In addition, the thickness direction of the metal foil has a main orientation of <100>, the foil surface of the metal foil has a main orientation of <100>, and the wiring when cut from the ridgeline at the bent portion in the thickness direction That the cross section P of the image has a main orientation in a specific orientation between (20 1 0) and (1 20 0) is reversed on the stereo triangle of the (100) standard projection shown in FIG. When the pole is displayed, the cross-sectional orientation of the wiring when cut in the thickness direction from the ridgeline at the bent portion is either on the line segment connected by the point representing (20 10) and the point representing (110) It can also be said that Furthermore, the flexible circuit board in the second measure is formed by forming a wiring from a 3 (2) axis-oriented material in which the thickness direction of the metal foil is the [001] axis, and cutting the ridge line at the bent portion in the thickness direction. It can also be said that the cross-sectional normal of the wiring at that time has an angle in the range of 2.9 ° to 87.1 ° with the [100] axis in the foil plane.

  And according to such a second measure, the breaking elongation of the metal foil in the principal stress direction at the bent portion can be 3.5% or more, and the repeated strain such that the curvature radius is 2 mm or less, Alternatively, metal fatigue is less likely to occur due to stress, and a flexible circuit board with high flexibility can be obtained. Further, in the present invention, a flexible circuit board excellent in metal fatigue characteristics and bendability can be obtained more reliably by combining the first policy and the second policy described above, and the main stress direction can be obtained. The breaking elongation of the metal foil can be 3.5% or more, preferably 4% or more, more preferably 9% or more. As for the upper limit of the elongation at break, the basic crystal axis <100> of the unit cell of the face-centered cubic structure is in the thickness direction of the metal foil (the normal direction of the foil surface) and in one direction (in the direction of the foil surface) Rolling that can be taken within the scope of the present invention, in which the preferential orientation region with an orientation difference of 10 ° or less is 50% in area ratio and the thickness is 18 μm with respect to two orthogonal axes (one is the rolling direction) The upper limit of the foil can be defined as 20% or less, but the basic crystal axis <100> of the copper unit cell is two orthogonal directions of the thickness direction of the copper foil and one direction existing in the foil plane. In the case where the preferential orientation regions each having an orientation difference within 10 ° with respect to the axis occupy 95% or more in area ratio and the thickness is 12 μm or less, the upper limit of elongation at break is 15% or less. .

  With respect to the resin layer of the flexible circuit board in the present invention, the type of resin forming the resin layer is not particularly limited, and examples thereof include those used in ordinary flexible circuit boards, such as polyimide and polyamide. , Polyester, liquid crystal polymer, polyphenylene sulfide, polyether ether ketone and the like. Of these, polyimide and liquid crystal polymer are preferred because they exhibit good flexibility when used as a circuit board and are excellent in heat resistance.

  The thickness of the resin layer can be appropriately set according to the use, shape, etc. of the flexible circuit board, but is preferably in the range of 5 to 75 μm and in the range of 9 to 50 μm from the viewpoint of flexibility. Is more preferable, and the range of 10 to 30 μm is most preferable. If the thickness of the resin layer is less than 5 μm, the insulation reliability may decrease. On the other hand, if it exceeds 75 μm, the thickness of the entire circuit board may be too thick when mounted on a small device, etc. A decline in sex is also possible.

  In addition, when a flexible circuit board is applied to a slide sliding portion of a mobile phone, a cover material made of a coverlay film or the like may be used on a wiring formed from a metal foil. In that case, it is preferable to design the structure of the cover material and the resin layer in consideration of the balance of stress applied to the wiring. According to the knowledge of the present inventors, for example, a polyimide having a tensile elastic modulus at 25 ° C. of 4 to 6 GPa and a thickness of 14 to 17 μm is used as a resin layer, and a thermosetting resin having a thickness of 8 to 17 μm is used. And an adhesive layer and a polyimide layer having a thickness of 7 to 13 μm, and a cover lay film having a tensile elastic modulus of 2 to 4 GPa of the adhesive layer and the entire polyimide layer as a cover material, A polyimide layer having a tensile modulus of elasticity at 25 ° C. of 6 to 8 GPa and a thickness of 12 to 15 μm is used as a resin layer, an adhesive layer made of a thermosetting resin having a thickness of 8 to 17 μm, and a polyimide layer having a thickness of 7 to 13 μm. And a cover lay film having a tensile elastic modulus of 2 to 4 GPa of the adhesive layer and the entire polyimide layer is used as a cover material.

  Regarding the means for laminating the resin layer and the metal foil, for example, when the resin layer is made of polyimide, the metal foil may be thermally laminated by applying or interposing a thermoplastic polyimide to the polyimide film (so-called laminating). Law). Examples of the polyimide film used in the laminating method include “Kapton” (Toray DuPont Co., Ltd.), “Apical” (Kanebuchi Chemical Industry Co., Ltd.), “Upilex” (Ube Industries Co., Ltd.), and the like. When the polyimide film and the metal foil are heat-bonded, a thermoplastic polyimide resin exhibiting thermoplasticity is preferably interposed. In addition, from the viewpoint of easy control of the thickness and bending characteristics of the resin layer, a polyimide precursor solution (also referred to as a polyamic acid solution) may be applied to the metal foil, and then dried and cured to obtain a laminate (so-called “so-called”). Cast method).

  The resin layer may be formed by laminating a plurality of resins. For example, two or more kinds of polyimides having different linear expansion coefficients may be laminated, and in that case, heat resistance and flexibility are ensured. In view of the above, it is desirable that substantially all of the resin layer is formed of polyimide without using an epoxy resin or the like as an adhesive. Including the case of being composed of a single polyimide and the case of being composed of a plurality of polyimides, the tensile elastic modulus of the resin layer is preferably 4 to 10 GPa, and preferably 5 to 8 GPa.

  In the flexible circuit board of the present invention, the linear expansion coefficient of the resin layer is preferably in the range of 10 to 30 ppm / ° C. When the resin layer is made of a plurality of resins, the linear expansion coefficient of the entire resin layer may be in this range. In order to satisfy such conditions, for example, a low linear expansion polyimide layer having a linear expansion coefficient of 25 ppm / ° C. or less, preferably 5 to 20 ppm / ° C., and a linear expansion coefficient of 26 ppm / ° C. or more, preferably 30 to It is a resin layer composed of a high linear expansion polyimide layer of 80 ppm / ° C., and can be adjusted to 10 to 30 ppm / ° C. by adjusting the thickness ratio thereof. A preferred ratio of the thickness of the low linear expansion polyimide layer to the high linear expansion polyimide layer is in the range of 70:30 to 95: 5. The low linear expansion polyimide layer is the main resin layer of the resin layer, and the high linear expansion polyimide layer is preferably provided so as to be in contact with the metal foil. In addition, the linear expansion coefficient is obtained by using polyimide with the imidization reaction sufficiently completed as a sample, raising the temperature to 250 ° C. using a thermomechanical analyzer (TMA), cooling at a rate of 10 ° C./min, and 240 to 100 ° C. It can obtain | require from the average linear expansion coefficient in the range.

  Moreover, the flexible circuit board in this invention is equipped with the wiring formed from the resin layer and metal foil, and has a bending part in any one, and is used. In other words, it is widely used in various electronic and electrical devices such as movable parts in hard disks, hinges and slides of mobile phones, printer heads, optical pickups, movable parts of notebook PCs, etc., and the circuit board itself Is bent, twisted, or deformed according to the operation of the mounted device, and a bent portion is formed in either of them. In particular, since the flexible circuit board of the present invention has a bent portion structure with excellent bending durability, it is frequently bent with repeated operations such as sliding bending, bending bending, hinge bending, and sliding bending. In addition, in order to cope with the downsizing of devices to be mounted, the radius of curvature is 0.38 to 2.0 mm in bending behavior, 1.25 to 2.0 mm in sliding bending, and 3. 0 to 5.0 mm, suitable for severe use conditions such as 0.3 to 2.0 mm in slide bending, and in slide applications where bending performance is severe with a narrow gap of 0.3 to 1 mm Especially effective.

  Regarding the method for producing a flexible circuit board according to the present invention, as one of them, i) a rolled metal exhibiting a cubic texture in which the [001] axis is finally oriented in the normal direction to the foil surface (perpendicular to the surface of the metal foil) Obtain a composite in which the foil and the resin layer are bonded to each other with the foil surface of the metal foil, and design the principal stress direction of the bending, that is, the normal direction of the cross section of the wiring when cutting from the ridgeline at the bent portion in the thickness direction. The wiring is formed so that the ridge line of the bent portion is formed at an angle of 2.9 ° to 87.1 ° with respect to the [100] main direction in the metal foil plane, or ii) the metal constituting the wiring The foil may have a purity of 99.999% or higher, or iii) the methods i) and ii) may be adopted at the same time.

  At this time, the metal foil does not necessarily have to exhibit a cubic texture from the beginning, and the cubic texture may be formed by heat treatment. For example, a manufacturing process of a flexible circuit board, specifically a resin A cubic texture may be formed by heat treatment in the layer formation process. That is, by heat treatment, one of the basic crystal axes <100> of the unit cell is given priority in the thickness direction of the metal foil so that the area within 10 ° from the <100> axis occupies an area ratio of 50% or more. In addition to the orientation, another one of the basic crystal axes <100> is preferentially oriented in the horizontal direction with respect to the surface of the metal foil so that the area within 10 ° from the <100> axis occupies an area ratio of 50% or more. You can make it. The recrystallized texture of the rolled copper foil usually has a rolling plane orientation of {100} and a rolling direction of <100>. Therefore, the (001) main direction is formed as the rolling surface direction. In addition, when using a metal foil with a purity of 99.999% or more, the elongation at break can be secured to 3.5% or more regardless of the orientation of the circuit formed and wired, and the flexible application range is wide. Circuit board can be formed.

  More specifically, in the case of adopting the second measure, as shown in FIG. 3, for example, when a flexible circuit board is bent in a U shape, an outer circle (an inscribed circle having a radius of curvature) is formed. A ridge line L is formed on the opposite side of the ridge line L, and the ridge line L is α = 2.9 to 87.1 (°) from a state orthogonal to the [100] axis of the metal foil forming the wiring. What is necessary is just to make it have inclination in a range. An example of such a state is shown in FIGS. Incidentally, (c) and (d) of FIG. 4 are states (α = 0) in which the ridge line is orthogonal to the [100] axis. Here, when α is less than 2.9 °, a clear effect on flexibility is not confirmed. When α = 11.4 to 78.6 (°), the bending durability of the bent portion structure is further improved. In the present invention, when α = 2.9 °, the cross section P of the wiring when cut from the ridge line in the thickness d direction corresponds to the (20 10) plane, and when α = 45, the cross section P corresponds to the (110) plane, and when α = 87.1, the cross section P corresponds to the (1 200) plane. In addition, in the face-centered cubic structure, [100] and [010] are equivalent, and therefore, the angle α formed between the orthogonal axis in the foil plane of [100] and the ridge line as shown in FIGS. 4 (a) and 4 (b). Is the same as the angle range formed by [100] and the cross-section P normal line, and the angle range formed by [100] and the ridge line.

  In addition, the width, shape, pattern, etc. of the wiring are not particularly limited, and may be appropriately designed according to the use of the flexible circuit board, the mounted electronic device, etc. Even if the second measure is adopted, it is necessary to dare to wire in an oblique direction with respect to the rotation axis of the hinge portion, for example, in order to reduce the bending stress on the wiring. Therefore, wiring along the direction orthogonal to the ridgeline in the bent portion, that is, wiring with the minimum necessary minimum distance is possible. FIGS. 4A and 4B show a flexible circuit board used for, for example, a hinge part of a cellular phone, which is an example having a resin layer 1, a wiring 2 formed from a metal foil, and a connector terminal 3. is there. 4A and 4B show the position of the ridge line L in the bent portion near the center, and this ridge line L is (with respect to the [100] axis direction of the metal foil forming the wiring 2 ( 90 + α) °. Here, FIG. 4A is an example in which the wiring is formed obliquely in the vicinity of the ridge line L in the middle of the connector terminals 3 at both ends. However, as shown in FIG. Wiring is also possible. In addition to the case where the position of the ridge line L in the bent portion is fixed as in a folding mobile phone, the slide sliding bend in which the ridge line L in the bent portion moves as in a slide type mobile phone (see FIG. 4 (b) may be the direction of the thick arrow). In addition, although the flexible circuit board in this invention is equipped with the wiring which consists of metal foil in at least one surface of a resin layer, you may make it equip both surfaces of a resin layer with metal foil as needed.

  As described above, the metal foil constituting the wiring in the bent portion when the flexible circuit board is bent is highly oriented and has a large elongation at break in the main stress and main strain directions. By configuring the metal foil, even when repeated bending with a small bending radius and high bending is performed, local stress concentration caused by crystal anisotropy hardly occurs and dislocation accumulation hardly occurs. Due to these effects, metal fatigue is less likely to occur, it has excellent durability against stress and strain, and there is no restriction on the design of the flexible circuit board, and even against repeated bending and bending with a small radius of curvature. It is possible to provide a flexible circuit board that has sufficient strength and can be bent.

  Hereinafter, based on an Example and a comparative example, the present invention is explained more concretely. In addition, the kind of copper foil used by the Example etc. and the synthesis | combination of a polyamic-acid solution are as follows.

[Copper foil A]
Commercial rolled copper foil, purity 99.9%, thickness 9 μm.
[Copper foil B]
Commercially available electrolytic copper foil, purity 99.9%, thickness 9 μm.
[Copper foil C]
Oxygen-free copper foil, purity 99.99%, thickness 9 μm, process condition A.
Impurities (mass ppm) Oxygen: 2, Silver: 18, Phosphorus: 2.1, Sulfur: 4, Iron: 1.5
[Copper foil D]
Purified copper foil, purity 99.999%, thickness 9 μm, process condition A.
Impurities (mass ppm) Oxygen: 2, Silver: 5, Phosphorus: 0.01, Sulfur: 0.01, Iron 0.002
[Copper foil E]
Purified copper foil, purity 99.9999%, thickness 9 μm, process condition A.
Impurities (mass ppm) Oxygen: <1, Silver: 0.18, Phosphorus: <0.005, Sulfur: <0.005, Iron: 0.002
[Copper foil F]
Purified copper foil, purity 99.9999%, thickness 9 μm, process condition B.
Impurities (mass ppm) Oxygen: <1, Silver: 0.18, Phosphorus: <0.005, Sulfur: <0.005, Iron: 0.002
[Copper foil G]
Purified copper foil, purity 99.9999%, thickness 9 μm, process condition C.
Impurities (mass ppm) Oxygen: <1, Silver: 0.18, Phosphorus: <0.005, Sulfur: <0.005, Iron: 0.002
[Copper foil H]
Commercial rolled copper foil, purity 99.9%, thickness 12 μm.

[Manufacturing method of copper foil]
Copper foil A and copper foil H are commercially available rolled copper foils, and copper foil B is a commercially available electrolytic copper foil produced in a copper sulfate bath. These are all copper foils that are commercially available as high-bending products, and the purity is 99.9%, which is high as a commercial product. Copper foil C to copper foil G are processed by the present inventors, and a copper material having a predetermined purity is cast and solidified in a graphite mold and rolled to a predetermined thickness. The thickness of the casting ingot is 10 mm, and after dropping to 1 mm by cold rolling, for copper foil C, copper foil D, and copper foil E, after carrying out intermediate annealing at 300 ° C. for 30 minutes, up to 9 μm Cold rolling was performed (process condition A). Moreover, the copper foil F was cold-rolled to 9 micrometers (process condition B), without performing intermediate annealing. Further, the copper foil G was subjected to an intermediate annealing temperature of 800 ° C. and cold-rolled to 9 μm (process condition C).

[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 introducing nitrogen. 2,2-bis [4- (4-aminophenoxy) phenyl] propane (BAPP) was dissolved in the reaction vessel with stirring. Next, pyromellitic dianhydride (PMDA) was added. The total amount of monomers charged was 15 wt%. Thereafter, stirring was continued for 3 hours to obtain a resin solution of polyamic acid a. The solution viscosity of this polyamic acid a resin solution was 3,000 cps.

(Synthesis Example 2)
N, N-dimethylacetamide was placed in a reaction vessel equipped with a thermocouple and a stirrer and capable of introducing nitrogen. 2,2′-Dimethyl-4,4′-diaminobiphenyl (m-TB) was charged into the reaction vessel. Next, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) were added. The total amount of monomers charged was 15 wt%, and the molar ratio of each acid anhydride (BPDA: PMDA) was 20:80. Thereafter, stirring was continued for 3 hours to obtain a resin solution of polyamic acid b. The solution viscosity of this polyamic acid b resin solution was 20,000 cps.

[Example 1]
The polyamic acid solution a prepared above is applied to seven types of copper foils from copper foil A to copper foil G, dried (after curing, a 2 μm-thick thermoplastic polyimide film is formed), and then polyamic acid b is applied thereon. And dried (forms a low thermal expansion coefficient polyimide with a thickness of 9 μm after curing), and further coated with polyamic acid a and dried (after curing, forms a thermoplastic polyimide with a thickness of 2 μm), 300- A polyimide layer having a three-layer structure was formed through a heating condition in which a temperature of 360 ° C. was loaded for 5 minutes or more in the integration time. Next, the copper foil was cut out to a rectangular size of 250 mm in length along the rolling direction (MD direction) and 150 mm in the direction perpendicular to the rolling direction (TD direction), as shown in FIG. A single-sided copper clad laminate 4 having a 13 μm thick polyimide layer (resin layer) 1 and a 9 μm thick copper foil 2 was obtained. At that time, the tensile elastic modulus of the entire resin layer was 7.5 GPa.

  After the single-sided copper clad laminate 4 obtained above is subjected to mechanical and chemical polishing using colloidal silica on the rolled surface 2a of each copper foil 2 from the copper foil A to the copper foil G. The azimuth analysis was performed with an EBSP apparatus. The apparatuses used were FE-SEM (S-4100) manufactured by Hitachi, Ltd., EBSP apparatus manufactured by TSL, and software (OIM Analysis 5.2). The measurement area was an area of approximately 800 μm × 1600 μm, and the measurement acceleration voltage was 20 kV and the measurement step interval was 4 μm. The evaluation of the orientation was shown by the ratio of the measurement points where <100> is within 10 ° with respect to the thickness direction of the foil and the rolling direction of the foil to the total measurement points. The number of measurements was made on five different samples of each variety and rounded to the nearest whole number. Further, using the obtained data, the crystal grain size was evaluated by setting the crystal grain boundaries to those having an orientation difference of 15 ° or more between adjacent crystal grains, and the average grain size was determined for the polycrystal. The results are shown in Table 1.

  It was found that all the rolled copper foils excluding the copper foil B had a cubic texture, and both the copper foil surface orientation and the rolling direction had a main orientation of {001} <100>. This is because the rolled copper foil was recrystallized by heat at the time of curing of the polyimide, and a recrystallized texture was formed. However, the degree varied depending on the type, and the orientation of the copper foils A, C, D, and E with respect to the cube orientation was extremely high. The degree of orientation of the cube orientation was highly dependent on the copper foil processing method regardless of the purity of the copper foil having a purity of 99.9% or more. These copper foils were composed of grains having a cubic orientation in a field of 800 × 1600 μm, and crystal grains having different orientations of 5 μm or less were dispersed in an island shape inside. Since the area ratio of the island structure is as small as 2% or less, the recrystallized grains having a cubic orientation are integrated with the same orientation, and the size of the recrystallized grains is the same as the foil thickness in the thickness direction. 9 μm, and 800 μm or more in the foil surface. In addition, the recrystallized grains having a cubic orientation of the copper foil F and the copper foil G are not high in area ratio, and therefore exist independently of each other, and the average grain sizes in the foil surface are 25 μm and 20 μm, respectively. It was. On the other hand, the electrolytic copper foil B was a polycrystal having an average particle diameter of 1 μm, and almost no orientation was observed.

  Next, a predetermined mask is put on the copper foil 2 side of the single-sided copper clad laminate 4 obtained above, and etching is performed using an iron chloride / copper chloride solution, as shown in FIG. The angle between the wiring direction H and the MD direction is 0 °), and the wiring direction H (H direction) of the linear wiring 2 having a line width (l) of 150 μm is parallel to the MD direction (<100> axis). The wiring pattern was formed so that the space width (s) was 250 μm. And in order to serve as a sample for bending resistance test to be described later, in accordance with JIS 6471, for the test having a width of 150 mm in the longitudinal direction along the wiring direction H of the circuit board and a width of 15 mm in the direction orthogonal to the wiring direction H. A flexible circuit board 5 was obtained.

  Using the test flexible circuit board obtained above, an MIT flex test was performed according to JIS C5016. A schematic diagram of the test is shown in FIG. The equipment is manufactured by Toyo Seiki Seisakusho (STROGRAPH-R1), one end of the test flexible circuit board 5 in the longitudinal direction is fixed to the holding jig of the bending test apparatus, and the other end is fixed with a weight. The wire 2 on the circuit board 5 is cut off from conduction while being rotated to 135 ± 5 degrees alternately left and right at a vibration speed of 150 times / min. Was determined as the number of flexing.

  Under this test condition, the main stress applied to the copper circuit is obtained because the ridge line formed in the bent portion is bent so as to be orthogonal to the wiring direction H of the wiring 2 of the test flexible circuit board 5. The main strain is a tensile stress or tensile strain parallel to the rolling direction. When the circuit was observed from the thickness direction of the copper foil after the bending test, it was confirmed that a crack occurred in the vicinity of the ridgeline of the bent portion, almost perpendicular to the rolling direction, and a broken line. The results of the bending life are shown in Table 1. The flex life in Table 1 is an average of the results of the test flexible circuit boards prepared for each type of copper foil.

  From the results shown in Table 1, the bending fatigue life depends on the degree of integration of the cube texture, but the copper foil C, the copper foil D, and the copper foil E, which are produced by the same processing method and have substantially the same degree of orientation. It has been found that the bending fatigue life varies greatly.

  Next, in order to investigate the governing factor of the bending life, a tensile test was performed in parallel with the main stress of bending, the main strain direction, that is, the rolling direction. In order to investigate the characteristics of the copper foil alone, the resin layer was dissolved from the single-sided copper clad laminate 4 before etching, and a tensile test was conducted on the copper foil alone. In the process of dissolving the polyimide, it was confirmed that there was no change in the structure of the copper foil.

  The tensile test uses a sample cut to a length of 150 mm in the rolling direction (MD direction) of the copper foil and a width of 10 mm in the direction perpendicular to the rolling direction in the foil plane, and the distance between the gauge points is 100 mm in the length direction. Tensile speed 10 mm / min. Measured with For the measurement, seven samples were prepared for each type of copper foil, and the breaking stress (breaking strength) obtained by measuring these samples and the average value of breaking elongation are shown in Table 1.

  As a result, it was found that in the copper foil having a developed texture, the elongation at break is correlated with the bending fatigue life, not at the breaking strength. Moreover, although copper foil B is large in both strength and elongation at break, this reflects that it is a fine polycrystalline body of crystal grains. However, since the copper foil B did not develop a texture, the fatigue life was inferior. Further, when the copper foil C with a purity of 99.99% and the copper foil D with a purity of 99.999%, which have the same degree of cube texture accumulation, were compared, the fatigue characteristics of the copper foil D with respect to bending were greatly improved. . The oxygen concentration of these two copper foils is the same, and the amount of copper oxide inside is also small and equal, so it is not due to the difference due to copper oxide but due to the difference in elongation at break due to different purity. .

  As described above, from the results shown in Example 1, in order to obtain better characteristics than a general high bending copper foil, the basic crystal axis <100> exists in the thickness direction of the metal foil and in the foil plane. The main orientation is such that the preferential orientation region with an azimuth difference of 10 ° or less occupies 50% or more with respect to two orthogonal axes with a certain direction, and the ridge line at the bent portion It was found that the breaking elongation of the metal foil in the normal direction relative to the cross section P of the wiring cut in the thickness direction of the metal foil was required to be 3.5% or more. Also, the purity is extremely high at 99.999% or more, and by developing the cube orientation, the elongation at break is improved, and the fatigue life against repeated bending in which principal stress and principal strain are applied in that direction. It has been found to be a long flexible circuit board.

[Example 2]
Next, for a single-sided copper-clad laminate using copper foil A and copper foil E produced by the same method as in Example 1, as shown in FIG. 6, the wiring of linear wiring 2 having a line width (l) of 150 μm The wiring pattern was formed such that the direction H (H direction) had an angle of 30 ° and 45 ° with respect to the MD direction ([100] axis) and the space width (s) was 250 μm. And in order to serve as a sample for bending resistance test to be described later, in accordance with JIS 6471, for the test having a width of 150 mm in the longitudinal direction along the wiring direction H of the circuit board and a width of 15 mm in the direction orthogonal to the wiring direction H. A flexible circuit board 5 was obtained. FIG. 6 shows an example in which the angle formed by the wiring direction H of the test flexible circuit board 5 and the MD direction is cut out at an angle of 45 °.

  For the test flexible circuit board 5 obtained above, a bending fatigue test was repeatedly performed under the same conditions as in Example 1. Also, the resin layer is melted from the single-sided copper clad laminate 4 before etching so that the angle formed by the wiring direction H and the MD direction of the flexible circuit board 5 for testing is the same, and the longitudinal direction is rolled. A tensile test was performed in the same manner as in Example 1 using a sample of 150 mm × 10 mm cut so as to have an angle of 30 ° and 45 ° with respect to the direction. That is, the principal stress and principal strain direction in the fatigue test of the copper foil coincide with the tensile direction of the tensile test, and both the copper foil A and the copper foil E have a highly developed cubic texture. In the tensile test, the same crystal orientation is subjected to principal strain and principal stress. Table 2 shows the results of the fatigue test and the tensile test.

  From the test results shown in Table 2, high fatigue characteristics can be obtained by removing the principal stress and principal strain directions from the <100> orientation. In these orientations, the elongation at break is significantly larger than that in the <100> orientation. In particular, in the case of 30 °, the fatigue life is increased along with the elongation at break.

  From the results of Example 2 above, when the copper foil was highly oriented between the fatigue life of the flexible circuit board against repeated bending of high strain and the breaking elongation of the copper foil constituting the wiring, It was found that there was a high correlation. As seen in Example 1, the polycrystalline body provides higher strength and ductility, but is not effective in high bending applications. Therefore, the relationship between the fatigue life and the elongation at break under conditions that have a highly integrated texture is that the slip system plays an important role, not just copper, but the same slip system. Even a face-centered cubic orientation metal holds, and if the metals have different stacking fault energies, it is expected that the elongation at break can be increased and the fatigue life can be expected to be increased.

[Example 3]
The polyamic acid solution a prepared by the same method as in Synthesis Example 1 is applied to a rolled copper foil H having a purity of 99.9 mass% and a thickness of 12 μm and dried (after curing, a 2 μm-thick thermoplastic polyimide is formed) ), And polyamic acid b is applied and dried (forms a low thermal expansion coefficient polyimide having a film thickness of 8 μm after curing), and further polyamic acid a is applied and dried (film thickness after curing). 2 μm thermoplastic polyimide is formed), and as shown in the following conditions a to d, the polyimide layer (resin layer) is subjected to a heating condition such that a temperature of a maximum temperature of 180 to 240 ° C. is added for an integrated time of 10 minutes. Formed.

  Next, a polyimide layer (resin) having a thickness of 250 μm along the rolling direction (MD direction) of the copper foil and a rectangular size of 150 mm width in a direction orthogonal to the rolling direction (TD direction) and having a thickness of 12 μm. A single-sided copper-clad laminate 4 according to Example 3 having a layer 1 and a copper foil 2 having a thickness of 12 μm was obtained.

  A predetermined mask is placed on the copper foil side of the single-sided copper-clad laminate 4 obtained above, etching is performed using an iron chloride / copper chloride solution, and a straight line having a line width of 150 μm and a space width of 250 μm is based on the IPC standard. A low-speed IPC test wiring 2 having a shape-like wiring was formed. In this manufacturing process, the maximum temperature of the heating conditions in forming the polyimide layer is set to four levels of 180 ° C. (condition a), 200 ° C. (condition b), 220 ° C. (condition c), and 240 ° C. (condition d). Moreover, the wiring direction (H direction) of the linear wiring 2 is 0 °, 2 °, 2.9 °, 5.7 °, 9.5 °, 11.4 ° with respect to the rolling direction (MD direction). 14 °, 18.4 °, 25 °, 26.6 °, 30 °, 40 °, 45 °, 55 °, 60 °, 63.4 °, 78.6 °, 80 °, 82.9 °, Wiring patterns were formed so as to have angles of 22 levels of 87.1 °, 88 °, and 90 °, respectively.

  Next, as shown in FIG. 8B, a cover material 7 (CVK-0515KA manufactured by Arisawa Manufacturing Co., Ltd., thickness: 12.5 μm) was laminated on each wiring pattern side surface using an epoxy adhesive. The thickness of the adhesive layer 6 made of an adhesive was 15 μm in a portion without a copper foil circuit, and 6 μm in a portion where a copper foil circuit was present. And it cut out so that it might become 15 cm in a length direction along a wiring direction (H direction), and 8 mm in the direction orthogonal to a wiring direction, and the flexible circuit board for a test for setting it as an IPC test sample was obtained.

  On the other hand, in order to investigate the characteristics of the copper foil alone, a tensile test was performed as follows. The resin layer is dissolved from the single-sided copper-clad laminate 4 before etching so that the angle relationship between the wiring direction H and the MD direction of the test flexible circuit board 5 is the same 22 level, and the copper foil is dissolved. A rectangular sample having a length of 150 mm and a width of 10 mm, which was cut out so that the longitudinal direction had the above-described 22 level angle with respect to the rolling direction, was prepared. At this time, it was confirmed that there was no change in the structure of the copper foil in the process of dissolving the polyimide. In the tensile test, the distance between the gauge points in the length direction is 100 mm, and the tensile speed is 10 mm / min. Measured with

  In addition, as a sample for performing a structural analysis by EBSP, a single-sided copper-clad laminate produced under the heat treatment conditions of conditions a to d is 0 °, 2.9 °, 30 °, 63.4 with respect to the rolling direction. A total of 20 samples without wiring patterns cut out at 5 angles of 7 ° and 78.6 ° were produced. In order to align the thermal history with the IPC test sample, a simulated heat treatment was applied under the same conditions as the circuit formation etching, and a cover material was further laminated under the same conditions. However, these influences on the copper foil structure are minor, and it has been found later that the copper foil structure is determined by the heat treatment conditions of conditions a to d at the time of polyimide formation.

  Then, 20 copper foils H having the four-level heat treatment conditions and the five-level angle conditions prepared for the EBSP measurement as described above are polished in the substrate thickness direction. The foil surface of the copper foil H was exposed. Furthermore, it finished-polished using colloidal silica, and the structure of the copper foil H was evaluated by EBSP. The measurement area was 0.8 mm × 1.6 mm, and the measurement interval was 4 μm. That is, the number of measurement points in one area is 80000 points. As a result, all the samples heat-treated under the heat treatment conditions from condition a to condition d have a cubic texture, and have a copper foil plane orientation and a main orientation of {001} <100> in the rolling direction. I understood. Based on the obtained results, the number of points where the unit cell axis <001> is within 10 ° is counted in the thickness direction and the rolling direction of the copper foil, and the ratio to the total number of points is calculated. The average value was obtained. The results are shown in Table 3. The variation between samples under the same heating condition is 1% or less, and it can be said that the same heat treatment condition has the integration degree shown in Table 3 over the entire surface of the copper foil. It was found that the higher the maximum heat treatment temperature and the greater the thermal history, the more recrystallization progressed and the higher the degree of integration of the cubic recrystallization texture. Further, as a result of orientation analysis in the foil plane, the cutting direction of the sample cut at five angles of 0 °, 2.9 °, 30 °, 63.4 °, and 78.6 ° with respect to the rolling direction. The main orientations of [100], [20 1 0], [40 23 0], [120], and [150] were almost as specified. On the other hand, using the obtained EBSP data, the crystal grain size was evaluated when viewed from the normal direction of the foil surface, in which the crystal grain boundary was analyzed when the difference in orientation between adjacent crystal grains was 15 ° or more. The average particle size was determined for the polycrystal. The results are shown in Table 3.

  As schematically shown in FIG. 8, the IPC test is a test simulating slide bending, which is one of the bending forms used for mobile phones and the like. The IPC test is a test in which a bent portion is provided with a determined gap length 8 as shown in FIG. 8, one side is fixed by a fixing portion 9, and the slide operating portion 10 on the opposite side is repeatedly reciprocated as shown in the drawing. . Therefore, the substrate is repeatedly bent in a region corresponding to the stroke amount of the reciprocating portion. In this example, the test was conducted by repeatedly sliding the polyimide layer (resin layer) 1 with the cap length being 1 mm, that is, the bending radius being 0.5 mm and the stroke being 38 mm. During the test, the electrical resistance of the circuit of the test flexible circuit board was measured, and the progress of fatigue cracks in the copper foil circuit was monitored by increasing the electrical resistance. In this example, the circuit breakage life was defined as the number of strokes at which the electrical resistance of the circuit reached twice the initial value.

  The test was conducted on a total of 88 levels in which wiring patterns having 22 levels of angles were formed under the four heat treatment conditions of the above conditions a to d. At each test level, four test pieces were measured, and the average number of strokes at which the circuit was broken was obtained. Regarding the copper foil after the circuit breaking life, when the cross section of the copper foil cut in the thickness direction so as to be orthogonal to the sliding direction is observed with a scanning electron microscope, there is a difference in degree, but the resin layer side and the cover material side It was observed that cracks occurred on the surfaces of the copper foils, and that many cracks were introduced on the surface of the copper foil on the resin layer side, which is outside the bent portion.

  Table 4 shows the average value of circuit break life at each level and the elongation at break in the tensile test. In the angle column of Table 4, the surface index is also shown only in the case of the low index direction of the cross section P of the wiring when the circuit is cut in the length direction (wiring direction), that is, the ridgeline at the bent portion in the thickness direction. .

  The fracture life (fatigue life) in the IPC test is the angle between the circuit length direction (H direction) and the rolling direction (MD direction), that is, the normal direction of the cross section of the wiring when cutting from the ridge line at the bent portion in the thickness direction. And [100] were found to depend greatly on the angle. This orientation dependency is manifested under conditions b, c, and d. The higher the degree of integration of the cube orientation, the greater the fatigue life against repeated bending and the greater the orientation dependency. Regarding this orientation dependency, the <001> main direction is such that the area where the copper [001] is within an orientation difference of 10 ° occupies an area ratio of 50% or more in the evaluation by the EBSP method with respect to the thickness direction of the metal foil. [100] The orientation is preferentially oriented in the thickness direction of the metal foil, and the region within 10 ° of the orientation difference from the [100] axis of copper occupies an area ratio of 50% or more as evaluated by the EBSP method. It was confirmed that the main azimuth was expressed when preferentially oriented in the metal foil plane. In particular, when the thickness direction and the rolling direction show the area ratios of 75% or more and 85% or more, respectively, and the condition c has a high degree of integration of cube orientation, the fatigue life is large and the orientation dependency is also high. The effect becomes large, and in the condition d in which the thickness direction and the rolling direction show an area ratio of 98% or more and 99% or more, respectively, and the accumulation degree of the cube orientation is extremely high, the fatigue life is further increased and the orientation dependency is increased. It turns out that the effect is great.

  When the results of conditions b, c, and d are examined in detail, the normal direction of the wiring cross section P when cutting in the thickness direction from the ridgeline at the bent portion, that is, the main stress direction is the <100> main orientation of the copper foil. The fatigue life of the circuit against bending is higher when it deviates from the above. In the IPC test of this example, the effect was observed with respect to the main strain direction of the bent portion, that is, with respect to the cross-sectional normal direction of the wiring when cutting from the ridge line in the bent portion in the thickness direction. It was a case having an angle of 9 ° to 87.1 °. When this is expressed in terms of a plane index, the cross section P of the wiring when it is cut in the thickness direction from the ridgeline at the bent portion passes from (20 1 0) to (110) with [001] as the crystal zone axis, and (1 20 0 ). In particular, the effect is great at 11.4 ° to 78.6 ° with respect to the main strain direction of the bent portion, that is, with respect to the normal direction of the cross section of the wiring when cut from the ridge line in the bent portion in the thickness direction. It was the case with an angle. When this is expressed in terms of a plane index, the cross section P of the wiring when it is cut in the thickness direction from the ridgeline at the bent portion ranges from (510) to (110) to (150) with [001] as the zone axis. It is. The bending characteristic further has an angle of 26.6 ° to 63.4 ° with respect to the main strain direction of the bent portion, that is, with respect to the cross-sectional normal direction of the wiring when cut from the ridge line in the bent portion in the thickness direction. In the case of 30 ° and 60 °, the best results were obtained. When this is expressed in terms of a plane index, the cross section P is in the range from (210) to (110) to (120) with [001] as the zone axis, and the most excellent are (40 230) and ( 23 40 0) When in the vicinity.

  When these results are compared with the elongation at break, the basic crystal axis <100> of the unit cell having a face-centered cubic structure has two orthogonal axes of the thickness direction of the metal foil and a certain direction existing in the foil surface. On the other hand, when the main orientation is such that the preferential orientation regions having an orientation difference of 10 ° or less each occupy 50% or more in area ratio, the wiring cut from the ridge line in the bent portion in the thickness direction of the metal foil It was found that when the breaking elongation of the metal foil in the normal direction relative to the cross section P is 3.5% or more, the metal foil has good bending fatigue characteristics with respect to bending that generates principal stress and principal strain in the direction. On the other hand, when the area ratio of the <100> preferentially oriented region was 49% or less, even if the elongation at break in that direction showed a value of 3.5% or more, good bending fatigue characteristics were not obtained.

[Example 4]
A heat treatment (preliminary heat treatment) for 30 minutes was applied to copper foil C having a purity of 99.99% at a five-level temperature of 180 ° C. to 400 ° C. in an Ar stream, and the polyamic acid solution a was applied in the same manner as in Example 1. , Dried (after forming a 2 μm-thick thermoplastic polyimide film after curing), coated with polyamic acid b and dried (after curing, formed a 9 μm-thick low thermal expansion polyimide), and further The polyamic acid a is applied and dried (forms a thermoplastic polyimide having a film thickness of 2 μm after curing), and the three-layer structure is subjected to heating conditions such that a temperature of 300 to 360 ° C. is loaded for 5 minutes or more in an integrated time. A polyimide layer was formed. Next, the copper foil was cut out to a rectangular size of 250 mm in length along the rolling direction (MD direction) and 150 mm in the direction perpendicular to the rolling direction (TD direction), as shown in FIG. A single-sided copper clad laminate 4 having a 13 μm thick polyimide layer (resin layer) 1 and a 9 μm thick copper foil 2 was obtained. At that time, the tensile elastic modulus of the entire resin layer was 7.5 GPa.

  About the single-sided copper clad laminated board 4 obtained above, colloidal silica was used with respect to the rolling surface 2a of the copper foil 2, and after performing mechanical and chemical polishing, orientation analysis was performed with an EBSP apparatus. The apparatuses used were FE-SEM (S-4100) manufactured by Hitachi, Ltd., EBSP apparatus manufactured by TSL, and software (OIM Analysis 5.2). The measurement area was an area of approximately 800 μm × 1600 μm, and the measurement acceleration voltage was 20 kV and the measurement step interval was 4 μm. The evaluation of the orientation was shown by the ratio of the measurement points where <100> is within 10 ° with respect to the thickness direction of the foil and the rolling direction of the foil to the total measurement points. The number of measurements was performed on five different samples of each variety, and the percentages after the second decimal place were rounded off. Further, using the obtained data, the crystal grain size was evaluated by setting the crystal grain boundaries to those having an orientation difference of 15 ° or more between adjacent crystal grains, and the average grain size was determined for the polycrystal. The results are shown in Table 5.

  It was found that each of the copper foils C had a cubic texture, and both the copper foil surface orientation and the rolling direction had a main orientation of {001} <100>. This is because the rolled copper foil was recrystallized by pre-heat treatment and heat at the time of curing of the polyimide, and a recrystallized texture was formed. Here, the higher the preliminary heat treatment temperature, the greater the degree of orientation of {001} <100>. In addition, the orientation other than the <100> orientation was confirmed by the EBSP apparatus in the same manner as described above. As a result, the recrystallized residual orientation having an orientation of <212> with respect to the rolling direction and an equivalent circle diameter of 5 μm or less Was dispersed. However, such an island-like structure was hardly observed in the copper foil subjected to the preliminary heat treatment at 400 ° C. In addition, since the area ratio of the confirmed island-like structure was as small as 2% or less, the recrystallized grains having the cubic orientation were integrated with the same orientation. Moreover, the size of the recrystallized grains was 9 μm, which was the same as the foil thickness in the thickness direction, and was 800 μm or more in the foil surface.

  Next, a predetermined mask is put on the copper foil 2 side of the single-sided copper clad laminate 4 obtained above, and etching is performed using an iron chloride / copper chloride solution, as shown in FIG. The angle between the wiring direction H and the MD direction is 0 °), and the wiring direction H (H direction) of the linear wiring 2 having a line width (l) of 150 μm is parallel to the MD direction (<100> axis). The wiring pattern was formed so that the space width (s) was 250 μm. And in order to serve as a sample for bending resistance test to be described later, in accordance with JIS 6471, for the test having a width of 150 mm in the longitudinal direction along the wiring direction H of the circuit board and a width of 15 mm in the direction orthogonal to the wiring direction H. A flexible circuit board 5 was obtained.

  Using the test flexible circuit board obtained above, an MIT flex test was performed according to JIS C5016. A schematic diagram of the test is shown in FIG. The equipment is manufactured by Toyo Seiki Seisakusho (STROGRAPH-R1), one end of the test flexible circuit board 5 in the longitudinal direction is fixed to the holding jig of the bending test apparatus, and the other end is fixed with a weight. The wire 2 on the circuit board 5 is cut off from conduction while being rotated to 135 ± 5 degrees alternately left and right at a vibration speed of 150 times / min. Was determined as the number of flexing.

  Under this test condition, the main stress applied to the copper circuit is obtained because the ridge line formed in the bent portion is bent so as to be orthogonal to the wiring direction H of the wiring 2 of the test flexible circuit board 5. The main strain is a tensile stress or tensile strain parallel to the rolling direction. When the circuit was observed from the thickness direction of the copper foil after the bending test, it was confirmed that a crack occurred in the vicinity of the ridgeline of the bent portion, almost perpendicular to the rolling direction, and a broken line. The results of the bending life are shown in Table 5. The flex life in Table 5 is an average of the results of five test flexible circuit boards prepared for each copper foil pre-heat treatment temperature. From the results shown in Table 5, it was found that the bending fatigue life becomes particularly large when the degree of cube texture accumulation is 98.0% or more and 99.8%.

  Next, in order to investigate the governing factor of the bending life, a tensile test was performed in parallel with the main stress of bending, the main strain direction, that is, the rolling direction. In order to investigate the characteristics of the copper foil alone according to the preliminary heat treatment temperature, the resin layer was dissolved from the single-sided copper clad laminate 4 before etching, and a tensile test was conducted on the copper foil alone. In the process of dissolving the polyimide, it was confirmed that there was no change in the structure of the copper foil.

  The tensile test uses a sample cut to a length of 150 mm in the rolling direction (MD direction) of the copper foil and a width of 10 mm in the vertical direction in the foil plane, the distance between the gauge points is 100 mm, and the tensile speed is 10 mm / min in the length direction. . Measured with For the measurement, seven samples were prepared for each preliminary heat treatment temperature of the copper foil, and the breaking stress (breaking strength) and the average value of breaking elongation obtained by measuring these were shown in Table 5.

  Contrary to the results so far, the elongation at break increased as the degree of integration increased in a region where the <100> degree of integration (%) was 98.0% or more and 99.8% or less. On the other hand, the elongation at break was small in the copper foil from which the island-like structure disappeared. This is presumed to be related to the slip surface. From the above, it was confirmed that the elongation at break and the bending fatigue life have a strong correlation. That is, a texture with a <100> accumulation degree (%) of 98.0% or more and 99.8% or less is highly developed, and the flexural fatigue life is increased when the elongation at break is 3.5% or more. I understand.

  On the other hand, when a copper foil was produced under the same conditions using tough pitch copper containing 0.035% by mass of oxygen under the same conditions and under the same conditions, the test was performed under the same conditions. <100> Degree of integration (%) However, even at 98.0% or more, the breaking elongation decreased as the degree of integration increased, and a copper foil of 3.5% or more could not be obtained, and a fatigue life of 1000 times or more could not be obtained.

  The flexible circuit board according to the present invention can be widely used in various electronic and electrical devices. The circuit board itself is bent, twisted, or deformed according to the operation of the mounted device. , Suitable for use with a bent portion in either. In particular, since the flexible circuit board of the present invention has a bent portion structure with excellent bending durability, it is frequently bent with repeated operations such as sliding bending, bending bending, hinge bending, and sliding bending. Alternatively, it is suitable for the case where a bent portion is required in which the radius of curvature is required to be extremely small in order to cope with downsizing of the equipment to be mounted. Therefore, it can be suitably used for various electronic devices such as thin mobile phones, thin displays, hard disks, printers, and DVD devices that require durability.

1: Resin layer 2: Wiring (metal foil)
2a: Rolled surface
2b: Side surface 3: Connector terminal 4: Single-sided copper clad laminate 5: Test flexible circuit board 6: Adhesive layer 7: Cover material 8: Gap length 9: Fixing part 10: Slide operating part 21: Method of section P Line direction L: Ridge line P: Cross section of wiring when cut in the thickness direction from the ridge line at the bent portion

Claims (9)

A flexible circuit board comprising a resin layer and a wiring formed from a metal foil, and having a bent portion in at least one part of the wiring,
The resin layer is made of polyimide having a thickness of 10 to 30 μm,
The metal foil is composed of a rolled copper foil having a face-centered cubic structure and a purity of 99.9% by mass or more, and the basic crystal axis <100> of the unit cell of the face-centered cubic structure is the thickness direction of the rolled copper foil and the foil. With respect to two orthogonal axes with respect to a certain direction existing in the plane, each of the preferential orientation regions within an orientation difference of 10 ° occupies 50% or more by area ratio, and from the normal direction of the foil surface of the rolled copper foil The crystal grain size when viewed is 25 μm or more,
Further, the elongation at break of the rolled copper foil in the normal direction relative to the cross section P of the wiring cut in the thickness direction of the rolled copper foil from the ridgeline at the bent portion is 3.5% or more and 20% or less. Circuit board. Flexible circuit board according to claim 1 rolled copper foil, it is 99.999% by mass or more. The flexible circuit board according to claim 1 or 2 , wherein the rolled copper foil has a thickness of 5 µm or more and 18 µm or less. The cross section P of the wiring has a main orientation on any plane included in the range of (20 1 0) to (1 20 0) in the rotation direction from (100) to (110) with [001] as the crystal zone axis The flexible circuit board according to any one of claims 1 to 3 . Section P of wiring, (100) according to claim 4 which is one of surfaces that are on knotted line segment in the stereo triangle of a standard projection diagram and points representing the (20 1 0) and a point representing the (110) A flexible circuit board according to claim 1. Flexible circuit board according to any one of claims 1 to 5, arranged along the direction orthogonal to the ridge line in the bend portion is formed. The flexibility according to any one of claims 1 to 6 , which is used so as to form a bent portion with any repetitive motion selected from the group consisting of sliding bending, bending bending, hinge bending, and sliding bending. Circuit board. Electronic device equipped with flexible circuit board according to any one of claims 1-7. A flexible circuit board bent portion structure comprising a resin layer and a wiring formed from a metal foil, and having a bent portion in at least one portion of the wiring;
The resin layer is made of polyimide having a thickness of 10 to 30 μm,
The metal foil is composed of a rolled copper foil having a face-centered cubic structure and a purity of 99.9% by mass or more, and the basic crystal axis <100> of the unit cell of the face-centered cubic structure is the thickness direction of the rolled copper foil and the foil. With respect to two orthogonal axes with respect to a certain direction existing in the plane, each of the preferential orientation regions within an orientation difference of 10 ° occupies 50% or more by area ratio, and from the normal direction of the foil surface of the rolled copper foil The crystal grain size when viewed is 25 μm or more,
Further, the elongation at break of the rolled copper foil in the normal direction relative to the cross section P of the wiring cut in the thickness direction of the rolled copper foil from the ridgeline at the bent portion is 3.5% or more and 20% or less. Circuit board bend structure.

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