Copper foil, and copper-clad laminate and flexible printed wiring board each using same

Abstract

The present invention provides: a copper foil which has high resistance to folding by sliding or bending; and a copper-clad laminate and a flexible printed wiring board, each of which uses this copper foil. The present invention provides: a copper foil which has an area ratio of the Cube orientation (001)<100> of 92% or more; and a copper-clad laminate and a flexible printed wiring board, each of which uses this copper foil.

Description

Copper foil, and copper-clad laminate and flexible printed wiring board using the same

[0001] The present invention relates to a copper foil and a flexible printed circuit (FPC) using the same. In particular, the present invention relates to a copper foil suitable for use in bent portions of electric circuits, and a copper-clad laminate and a flexible printed circuit using the same.

[0002] Flexible printed wiring boards are flexible and can be bent, twisted, wound, and stacked within electronic devices to save space, and are therefore used in a variety of fields. For example, FPCs used in bending sections of wiring in mobile phones and other devices are manufactured by a method called the casting method, in which a polyimide varnish is applied to copper foil, which is then heated to dry and harden it to form a laminate, or by a method called the lamination method, in which a polyimide film pre-coated with an adhesive thermoplastic polyimide is stacked on copper foil and then pressure-bonded using a heated roll or the like. Flexible copper-clad laminates obtained by these methods are called two-layer flexible copper-clad laminates.

[0003] In addition to mobile phones, flexible printed wiring boards are also widely used for wiring in computer-related products, audio-visual products, cameras, automobiles, and the like.

[0004] The properties required for flexible printed wiring boards include good bending properties, as represented by MIT folding endurance, and high cycle bending properties, as represented by IPC bending endurance, and conventionally, copper foils and copper-resin substrate laminates (copper-clad laminates) having such properties have been developed.

[0005] For example, Patent Document 1 (JP 2010-100887 A) discloses a copper foil in which recrystallization nuclei with a preferred orientation are formed in the copper foil, thereby growing crystal grains with a preferred orientation in (200). It is disclosed that this copper foil has excellent flexibility and can ensure strength.

[0006] Patent Document 2 (JP 2009-111203 A) discloses that flexibility can be improved by increasing the number of crystal grain boundaries that intersect with a straight line connecting one surface to the other surface at the shortest distance, and that it is possible to provide a rolled copper foil for flexible printed wiring boards that can suppress crack propagation and obtain high flexibility even under severe bending conditions.

[0007] Patent Document 3 (JP 2007-207812 A) discloses that by forming a Ni—Cr alloy layer on the surface of a copper foil and then forming an oxide layer of a predetermined thickness on the surface of this alloy layer, adhesion to a resin substrate is significantly improved even when the copper layer surface is smooth and has little anchoring effect, and that it is possible to provide a copper foil for printed wiring boards that has a smooth copper layer surface and is excellent in etching properties and adhesion to a resin substrate.

[0008] JP 2010-100887 A JP 2009-111203 A JP 2007-207812 A

[0009] Recent technological trends have seen the emergence of many new types of mobile devices, including foldable, wearable, and rollable devices. Unlike conventional electronic devices in which components are placed inside a rigid housing, these devices have deformable housings, which means different characteristics are required of the flexible printed wiring boards used. For example, foldable phones require higher sliding and bending durability than conventional flexible printed wiring boards, so they can withstand repeated bending deformation at the hinge radius.

[0010] In response to such market demands, efforts have been made to control the crystal structure of copper foils used in conventional flexible printed wiring boards, as typified by the techniques disclosed in the above Patent Documents 1 to 3, but it is expected that these efforts will not be able to keep up with future technological trends.

[0011] The present invention was completed in view of the above problems, and an object of the present invention is to provide a copper foil having high sliding, bending, and folding resistance in one embodiment. An object of the present invention is to provide a copper-clad laminate and a flexible printed wiring board using such a copper foil in another embodiment.

[0012] As a result of intensive research, the present inventors have found that by increasing the area ratio of the Cube orientation {001}<100> of the copper foil to an extremely high range, the sliding bending folding endurance of the copper foil can be dramatically improved. The present invention has been completed based on the above findings, and is exemplified below.

[0013] [1] A copper foil having an area ratio of 92% or more in Cube orientation {001}<100>. [2] The copper foil according to [1], having an area ratio of 99% or more in Cube orientation {001}<100>. [3] The copper foil according to [1] or [2], having an average crystal grain size of 27 μm or more in Cube orientation {001}<100>. [4] A copper-clad laminate having the copper foil according to any one of [1] to [3] and a resin. [5] A flexible printed wiring board using the copper-clad laminate according to [4].

[0014] According to the present invention, it is possible to provide a copper foil having high sliding, bending and folding resistance, and a copper-clad laminate and a flexible printed wiring board using the same.

[0015] Next, an embodiment of the present invention will be described. It should be understood that the present invention is not limited to the following embodiment, and that appropriate design changes and improvements may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0016] (Composition of Copper Foil) The material of the copper foil of the present invention is not particularly limited, but for example, tough pitch copper specified in JIS-H3100-C1100 or oxygen-free copper specified in JIS-H3100-C1020 is preferred. Because these compositions are close to pure copper, the conductivity of the copper foil does not decrease, making it suitable for circuit formation. The oxygen concentration contained in the copper foil is usually 0.05 wt% or less in the case of tough pitch copper and usually 0.001 wt% or less in the case of oxygen-free copper.

[0017] The copper foil according to the present invention is made of industrially used copper and contains unavoidable impurities. Examples of unavoidable impurities include P, Fe, Zr, Mg, S, Ge, and Ti. If the content of these unavoidable impurities is too high, bending deformation of the copper foil tends to cause rotation of the crystal orientation, shear bands tend to occur, and cracks and breaks tend to occur when the copper foil is repeatedly bent. Therefore, it is preferable that the copper foil according to the present invention contains one or more unavoidable impurities selected from the group consisting of P, Fe, Zr, Mg, S, Ge, and Ti in total, controlled to 0.002 wt % or less.

[0018] In this specification, when the term "copper foil" is used alone, it also includes copper alloy foil, and when the term "tough pitch copper and oxygen-free copper" is used alone, it also includes copper alloy foil based on tough pitch copper and oxygen-free copper.

[0019] (Cube Orientation Area Ratio of Copper Foil) In one embodiment of the present invention, the area ratio of the Cube orientation {001}<100> is 92% or more. By increasing the area ratio of the Cube orientation {001}<100> to such an extremely high range, the sliding bending and folding endurance of the copper foil is dramatically improved.

[0020] Although the present invention is not intended to be constrained by theory, the reason why the sliding bending folding endurance of copper foil is improved by increasing the area ratio of Cube orientation {001}<100> is presumed to be as follows. That is, copper is a metal having a face-centered cubic lattice. In Cube orientation ({100}<001>) crystal grains of a face-centered cubic lattice, when stress is applied in the rolling direction (RD) or the direction perpendicular to the rolling direction (TD), which are the stress axes of the FPC circuit, eight slip systems are simultaneously activated, making it difficult for strain to accumulate. Therefore, the higher the area ratio of Cube orientation {001}<100> of copper foil, the less strain accumulates throughout the structure, resulting in high bending resistance.

[0021] From the above viewpoints, in the copper foil of another embodiment of the present invention, the area ratio of Cube orientation {001}<100> is preferably 93% or more, more preferably 94% or more, even more preferably 95% or more, even more preferably 96% or more, even more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more. There is no particular upper limit for the area ratio of Cube orientation {001}<100>, but it is typically 99.99% or less, for example 99.9% or less.

[0022] In the present invention, the area ratio of the Cube orientation {001}<100> is determined by performing a single cleanup using the grain dilation method, measuring the crystal orientation distribution using the EBSD method (Electron Backscatter Diffraction), and using the crystal orientation distribution function. Specifically, electrolytic polishing is performed using the following electrolyte and test conditions, and a thickness of about 1 μm is removed from the sample surface. After that, a square sample measuring 1 mm long x 1 mm wide is arbitrarily set so that one side of the observation field is parallel to the rolling direction. The sample is scanned in 3 μm steps to measure the crystal orientation distribution, and crystal orientation distribution function analysis is performed to determine the area ratio of the region having an orientation difference of 15° or less from the Cube orientation {001}<100>. An OIM Analysis manufactured by AMETEK can be used for the above analysis.

[0023] The details of the cleanup using the grain dilation method are as follows: Software used: TSL OIM Analysis 8 Parameter settings (Cleanup Parameters): Grain Tolerance Angle = 0.5° Minimum Grain Size = 10 Grain must contain multiple rows = yes

[0024] The specific conditions for the EBSD method are as follows:<Electrolyte composition (example)> Distilled water 250 ml Phosphoric acid 125 ml Urea 2.5 g Ethanol grade 1 125 ml 1-propanol 25 ml <Electrolytic polishing conditions> Applied voltage: 10 V Electrolysis time: 10 seconds <EBSD measurement conditions, etc.> SEM conditions Equipment: Scanning electron microscope manufactured by JEOL Ltd. (JSM-IT500HR or equivalent) Electron gun type: Field emission electron gun (Schottky type) Electron gun emitter: ZrO tungsten cathode Objective lens type: Out-lens type Focus correction: Yes (dynamic focus: 50) Beam conditions Acceleration voltage: 15 kV Working distance: 15 mm Irradiation current: 15 nA SEM probe diameter: 0.5 to 2 nm Observation magnification: 90x EBSD equipment conditions Detector: Slow scan CCD camera manufactured by TSL Solutions Co., Ltd. Data processing conditions Data collection software: OIM manufactured by TSL Solutions Co., Ltd. Data Collection Phase: Copper Number of pixels of CCD camera: 1394 x 1040 pixels Binning: 8 x 8 Exposure (exposure time): 8 milliseconds Gain: 0.9 to 0.95 Presence or absence of background processing: Yes Measurement point scanning method: hexagonal lattice Hough transformation (1) Hough Type: Classic (2) Hough Resolution: Low (3) Classic Hough Convolution Mask: 9 x 9 Min Peak Magnitude (minimum peak intensity): 5 Min Peak Distance (minimum distance between peaks): 23; Peak Symmetry: 0.75; Vertical Bias: 0 (4) General Parameters; Binned Pattern Size: 120; Theta Step Size: 1°; Rho Fraction: 90%; Max Peak Count: 8; Min Peak Count: 3.

[0025] Then, the crystal orientation distribution function is analyzed, and the area of ​​crystal grains having an orientation within 15° of the Cube orientation is divided by the measured area to obtain the area ratio. The above measurement data is collected using OIM Data Collection from TSL Solutions Co., Ltd., and the data is analyzed using OIM Analysis V8 from TSL Solutions Co., Ltd. Note that the information obtained in the orientation analysis by the EBSD method includes orientation information up to a depth of several tens of nanometers, the penetration depth of the electron beam into the sample, but since this is sufficiently small compared to the area being measured, it is reported as an area ratio.

[0026] <Data analysis conditions for OIM Analysis V8> ・New Map window Map Style ・Grayscale: Select <None>. ・Color Coded: Select Crystal Orientation. Boundaries ・Second Partition: Select <None>. ・Crystal Orientation window (screen displayed when you click Edit under Color Coded in the Map Style window) ・Representation: Select Euler Angles (Bunge). ・Enforce Orthotropic Sample Symmetry checkbox: Check the box.・Add Crystal Orientation Range window (screen displayed when you click Add in the Crystal Orientation window) ・Orientation tab ・Phase: Select Copper. ・Euler Angles (Bunge): (φ1, φ, φ2) = (0, 0, 0) ・Input values ​​for hkl: 001 ・Input values ​​for uvw: 100 ・Tolerance tab ・Input value for Minimum: 0 ・Input value for Maximum: 15

[0027] The total fraction value in the crystal orientation measurement results carried out under the above set conditions was taken as the Cube orientation area ratio.

[0028] (Average Cube grain size of copper foil) In one embodiment of the present invention, the average grain size of the Cube orientation {001}<100> is preferably 27 μm or more. In order to improve the sliding bending and folding endurance of the copper foil, it is preferable that the average grain size of the Cube orientation {001}<100> is large. This can further suppress the accumulation of strain.

[0029] In the present invention, the average grain size of the Cube orientation {001}<100> can be determined by the line segment method, where the grain boundary is defined as the area between two measurement points with an orientation difference of 0.5° or more within the square region of 1 mm length by 1 mm width measured by the EBSD method after one cleanup by the grain dilation method as described above. The line segment method means that 76 horizontal lines are drawn within the region, the average length of the line segments passing through each crystal grain is defined as the grain size of the crystal grain, and the average grain size of the crystal grains within the region is determined.

[0030] Specifically, the average crystal grain size of the copper foil can be determined by the line segment method by performing the following operations in OIM Analysis V8. <Average crystal grain size analysis conditions in OIM Analysis V8> - Select Chart Style - GS (Intercept length) in the New Chart window. After the above operations, the <Average Intercept Length> displayed in the GS (Intercept length) window was used as the average crystal grain size of the copper foil determined by the line segment method.

[0031] From the above viewpoints, in the copper foil of another embodiment of the present invention, the average crystal grain size in the Cube orientation {001}<100> is preferably 30 μm or more, more preferably 33 μm or more. There is no particular upper limit to the average crystal grain size in the Cube orientation {001}<100>, but it is typically 100 μm or less, for example 85 μm or less.

[0032] (Thickness of Copper Foil) The thickness of the copper foil of the present invention is not particularly limited, but can be, for example, 2 to 100 μm. If the copper foil is too thin, it is not preferable in terms of maintaining sliding, bending, and folding endurance, and if it is too thick, it is not preferable in terms of flexibility and miniaturization. Therefore, the thickness of the copper foil can be 5 μm or more, or 10 μm or more. In addition, the thickness of the copper foil can be 40 μm or less, or 35 μm or less.

[0033] (Method for Producing Copper Foil) The method for producing the copper foil of this embodiment is not particularly limited, but for example, an ingot is obtained by casting in a non-oxidizing atmosphere, followed by homogenization annealing at a temperature of 900° C. or higher, followed by processing such as hot rolling, appropriate facing, cold rolling, intermediate annealing, and final cold rolling, and then high-temperature annealing as described below, to obtain a copper foil of a predetermined thickness.

[0034] The composition of the rolled copper foil is measured using ICP optical emission spectroscopy as a wet analysis. Specifically, the measurement can be performed using an ICP optical emission spectroscopy analyzer (ICP-OES) SPS3100 manufactured by Hitachi High-Tech Science Corporation. In the case of ICP optical emission spectroscopy, a sample is dissolved in an aqueous nitric acid solution (volume ratio of nitric acid:water = 1:1) and then diluted.

[0035] In the prior art, measures to increase the area ratio of Cube orientation {001}<100> have included reviewing the additive elements of raw materials, the grain size during intermediate annealing, rolling conditions, and the degree of processing of copper foil. However, the limit of the area ratio of Cube orientation {001}<100> was approximately 90% in copper foil (for a copper foil with a thickness of 12 μm). In one embodiment of the present invention, high-temperature annealing (e.g., 600°C or higher) is performed in a hydrogen atmosphere on copper foil after final cold rolling (i.e., copper foil processed to a predetermined thickness), thereby achieving an area ratio of Cube orientation {001}<100> of 92% or more. Furthermore, this high-temperature annealing can achieve an average grain size of Cube orientation {001}<100> of 27 μm or more. Annealing is preferably performed in a hydrogen atmosphere from the viewpoint of increasing the vacancy density within the copper, which is the driving force behind the increase in the area ratio of Cube orientation {001}<100>.

[0036] (Copper-clad laminate and flexible printed wiring board) When the flexible printed wiring board of the present invention uses the copper-clad laminate in which copper foil and resin are laminated according to the present invention, the resin may be a film-like resin laminated on the copper foil with or without an adhesive, or the resin may be a resin that is not a film but is coated on the copper foil and then formed into a film. Examples of the resin layer include, but are not limited to, polyimide, liquid crystal polymer, and PTFE (polytetrafluoroethylene).

[0037] The present invention will be specifically described below with reference to examples, but the description here is for the purpose of illustration only and is not intended to be limiting.

[0038] First, a copper ingot was cast in a non-oxidizing atmosphere. The copper content of the copper ingot was 99.98% or more. The ingot was subjected to homogenization annealing at a temperature of 900°C or more, followed by hot rolling, facing, cold rolling, intermediate annealing, and final cold rolling to obtain copper foils having a thickness of 12 μm or 33 μm.

[0039] Thereafter, the copper foils of the Examples and Comparative Examples were annealed at the temperatures shown in Table 1 or Table 2. The Examples were annealed for 60 minutes in a hydrogen atmosphere at the temperatures shown in Table 1 or Table 2, and the temperature was raised at 400°C / 30 minutes for temperatures up to 800°C, and at 120°C / 10 minutes for temperatures from above 800°C to 1,020°C. The Comparative Examples were annealed for 30 minutes at the temperatures shown in Table 1, and then placed in a high-temperature hot air dryer that had been heated to 250°C. The copper foils of the Comparative Examples were annealed not at high temperatures, but at a temperature (250°C) that simulates normal primary recrystallization.

[0040] (Area Ratio of Cube Orientation {001}<100>) Thereafter, for each of the Examples and Comparative Examples, as described above, a square sample of 1 mm length × 1 mm width was scanned in 3 μm steps by an EBSD method using a scanning electron microscope JSM-IT500HR manufactured by JEOL Ltd., to measure the crystal orientation distribution, and a crystal orientation distribution function analysis was performed to determine the area ratio of the region having an orientation difference of 15° or less from the Cube orientation {001}<100>.

[0041] (Average crystal grain size of Cube orientation {001}<100>) Within the aforementioned square region of 1 mm length × 1 mm width, the boundary between two measurement points with an orientation difference of 0.5° or more was defined as a grain boundary, and the average grain size was determined by the line segment method (76 horizontal lines) as described above.

[0042] (Sliding Flex Test) For Examples 1 and 2 and Comparative Example 1, NIKAFLEX-CISV1215 manufactured by Nikkan Industries Co., Ltd. was used as a base film, and copper foil was annealed at the temperature shown in Table 1 below and then hot-pressed to be attached. For the hot press, the copper foil and base film were stacked and placed in a furnace, the set temperature was raised to 180°C over 30 minutes, and then pressed at the same temperature for 40 minutes under a pressure of 2.5 MPa, and then the furnace was allowed to cool to room temperature before being removed. Thereafter, a circuit was formed in a 300 μm-wide pattern, and a coverlay was further provided with NIKAFLEX-CISV1215 to produce a three-layer flexible printed wiring board. For Examples 3 and 4, NIKAFLEX-CISV1215 manufactured by Nikkan Industries Co., Ltd. was used as a base film, and copper foil was annealed at the temperature shown in Table 2 below and then hot-pressed to be attached, to produce flexible printed wiring boards on which a circuit was formed with a pattern having a width of 300 μm.

[0043] An IPC sliding flex test was performed on the obtained flexible printed wiring boards under conditions of a speed of 1500 cpm (cycles / minute), a stroke of 20 mm, and a bending radius of 0.5 mm, and the number of cycles until breakage of the copper foil was counted for Examples 1 and 2 and Comparative Example 1. For Examples 3 and 4, an IPC sliding flex test was performed under conditions of a speed of 1500 cpm (cycles / minute), a stroke of 20 mm, and a bending radius of 4.0 mm, and the number of cycles until breakage of the copper foil was counted.

[0044]

[0045]

[0046] As can be seen from Tables 1 and 2, in the examples in which high-temperature annealing was performed under the specified conditions, the area ratio of the Cube orientation {001}<100> was 92% or more, and the average crystal grain size of the Cube orientation {001}<100> was 27 μm or more. Also, in the sliding bending test, a significantly larger number of bending cycles was obtained compared to the comparative examples.

[0047] On the other hand, in Comparative Example 1, annealing was performed at a temperature assumed to be normal primary recrystallization, but the area ratio of the Cube orientation {001}<100> was outside the range of the present invention, and the results of the sliding bending test were inferior to those of the Examples.

Claims

1. A copper foil having an area ratio of 92% or more of Cube orientation {001}<100>.

2. The copper foil according to claim 1, wherein the area ratio of Cube orientation {001}<100> is 99% or more.

3. The copper foil according to claim 1 or 2, wherein the average crystal grain size in the Cube orientation {001}<100> is 27 μm or more.

4. A copper-clad laminate comprising the copper foil according to claim 1 or 2 and a resin.

5. A flexible printed wiring board using the copper clad laminate according to claim 4.