TWI637071B - Copper foil for flexible printed circuit boards, copper-clad laminates using the same, flexible printed circuit boards, and electronic devices - Google Patents

Abstract

The present invention provides a copper foil for a flexible printed circuit board having excellent etching properties.

The copper foil for a flexible printed circuit board of the present invention is composed of 99.0% by mass or more of Cu, and the remainder is unavoidable impurities. The average crystal grain size is 0.6 to 4.3 μm, and the tensile strength in the MD direction is 230 to 287 MPa. The surface after immersion in the MD direction and the CD direction for 420 seconds after being immersed in an aqueous solution (liquid temperature 25 ° C.) of 100 g / L of sodium persulfate concentration and 35 g / L of hydrogen peroxide concentration is based on the skewness Rsk of JIS B 0601-2001. For 16 measurements, the absolute value of each measurement was averaged to obtain a value of 0.05 or less.

Description

Copper foil for flexible printed circuit boards, copper-clad laminates using the same, flexible printed circuit boards, and electronic devices

The present invention relates to a copper foil suitable for a wiring member such as a flexible printed circuit board, a copper clad laminate using the same, a flexible wiring board, and an electronic device.

A flexible printed circuit board (flexible wiring board, hereinafter referred to as "FPC") has flexibility, and is therefore widely used in a bent portion or a movable portion of an electronic circuit. For example, FPC is used for moving parts of HDD or DVD- and CD-ROM-related equipment, or bending parts of folding mobile phones.

FPC is formed by etching a copper clad laminate (copper clad laminate) (hereinafter referred to as CCL) laminated with a copper foil and a resin, and covering it with a resin layer called a cover layer. At the stage before laminating the cover layer, the surface of the copper foil is etched as part of a surface modification step to improve the adhesion between the copper foil and the cover layer. In order to reduce the thickness of the copper foil and improve the flexibility, soft etching may be performed.

The surface of the copper foil used for FPC to form a photoresist for etching to form a circuit is soft-etched in order to impart adhesion to the photoresist. Soft etching is a surface treatment that removes the oxide film on the surface of the copper foil and flattens the surface. However, when soft etching is performed, An abnormality called a dish-down, in which a recessed portion is formed on the surface of a rolled copper foil. This dish-shaped depression is caused by the difference in etching speed in the thickness direction of the rolled copper foil and the surface becomes uneven, thereby reducing the photoresist adhesiveness. The reason why the etching speed is different is that the etching speed is different depending on the crystal orientation of the surface of the rolled copper foil.

Therefore, a rolled copper foil has been developed that reduces the ratio of the surface whose etching speed is slower (200) than other crystal surfaces, and improves the soft etchability (Patent Document 1).

[Patent Document 1] Japanese Patent Laid-Open No. 2014-77182

However, along with the miniaturization, thinness, and high performance of electronic devices, it is necessary to mount FPCs inside these devices at high density. However, in order to perform high-density mounting, it is necessary to further refine the circuit and bend the FPC with a small thickness Folded and housed inside a miniaturized machine. In addition, in order to miniaturize the circuit, the adhesion between the photoresist for etching the circuit and the copper foil is further required. That is, if the adhesion between the copper foil and the photoresist is low, the etchant will penetrate between the copper foil and the photoresist, and it is difficult to form fine wiring.

However, in the case of the conventional copper foil, it is difficult to say that the planarization of the surface after soft etching is sufficient, and it is difficult to miniaturize the circuit.

In addition, along with the miniaturization, thinness, and high performance of electronic devices, there is a problem that the circuit width and gap width of the FPC are also reduced to about 20 to 30 μm, and the etching factor or the linearity of the circuit is easily deteriorated when the circuit is formed by etching. Also need to solve this problem.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to provide an excellent etching property. Copper foil for flexible printed boards, copper-clad laminates using the same, flexible printed boards, and electronic devices.

The present inventors have conducted various studies, and as a result, have found that by miniaturizing the grains of the copper foil and specifying the skewness Rsk of the copper foil after etching, the etching properties can be improved. However, if the crystal grains are made too fine, the strength becomes too high, the flexural rigidity becomes large, and the springback becomes large, making it unsuitable for flexible printed circuit board applications. Therefore, the range of crystal grain size and tensile strength is specified.

In addition, it is possible to refine the crystal grain size to about 1/10 of the circuit width of about 20 to 30 μm of FPC in recent years, thereby improving the etching factor or circuit linearity when forming a circuit by etching.

That is, the copper foil for a flexible printed circuit board of the present invention is composed of 99.0% by mass or more of Cu, and the remainder is unavoidable impurities. The average crystal grain size is 0.6 to 4.3 μm, and the tensile strength in the MD direction is 230 to 287 MPa, in the MD direction and the CD direction, respectively, immersed in an aqueous solution of sodium persulfate concentration of 100 g / L and hydrogen peroxide concentration of 35 g / L (liquid temperature 25 ° C) for 420 seconds based on the skewness of JIS B 0601-2001 Rsk performs 16 measurements and averages the absolute value of each measurement to obtain a value of 0.05 or less.

The copper foil for a flexible printed circuit board of the present invention is preferably composed of fine copper conforming to JIS-H3100 (C1100) standard or oxygen-free copper conforming to JIS-H3100 (C1011).

The copper foil for a flexible printed circuit board of the present invention preferably further contains one or more additional elements selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg in a total amount of 0.003 to 0.825% by mass. to make.

Preferably, the above-mentioned copper foil is a rolled copper foil, and is subjected to a heat treatment at 300 ° C for 30 minutes. The average crystal grain size after the treatment is 0.6 to 4.3 μm, the tensile strength is 230 to 287 MPa, and the skewness Rsk after the heat treatment is 0.05 or less.

The copper clad laminate of the present invention is formed by laminating the copper foil for a flexible printed circuit board and a resin layer.

The flexible printed circuit board of the present invention is formed by forming a circuit on the copper foil using the copper clad laminate.

The L / S of the above circuit is preferably 35/35 to 10/10 (μm / μm). In addition, the L / S (line and gap) of a circuit is a ratio of the width (L: line) of the wiring constituting the circuit to the interval (S: gap) of adjacent wiring. L uses the minimum value of L in the circuit, and S uses the minimum value of S in the circuit.

In addition, L and S only need to be 10 to 35 μm, and they do not need to be the same value. For example, values such as L / S = 20.5 / 35, 35/17 can also be taken.

The electronic device of the present invention is formed using the flexible printed circuit board.

According to the present invention, a copper foil for a flexible printed circuit board having excellent etching properties can be obtained.

Hereinafter, embodiments of the copper foil of the present invention will be described. In the present invention, unless otherwise specified,% means% by mass.

<Composition>

The copper foil of the present invention is composed of 99.0% by mass or more of Cu, and the remainder is unavoidable impurities.

As described above, in the present invention, by refining the crystal grains of the copper foil after recrystallization, the strength is improved and the etchability is improved.

However, in the case of the pure copper-based composition described above, it is difficult to refine the crystal grains. Therefore, in the initial stage of cold rolling, recrystallization annealing is performed only once, and then recrystallization annealing is not performed, so that a large amount of cold rolling is performed. Introduce processing strain to generate dynamic recrystallization, so that the crystal grains can be refined.

In order to increase the processing strain during cold rolling, as the workability in the final cold rolling (overall the steps of annealing and rolling repeatedly, and the final rolling after the final annealing), it is preferable to set η = ln (sheet thickness before final cold rolling / sheet thickness after final cold rolling) = 7.51 ~ 8.00.

When η is less than 7.51, the processing strain does not accumulate uniformly, that is, the strain accumulates locally, so the etching rate is different from other parts where the strain accumulates. Therefore, the absolute value of Rsk after soft etching becomes large, and the etchability deteriorates. When η is larger than 8.00, strain is excessively accumulated to become a driving force for grain growth, and the grain tends to become coarse. Further, it is preferably set to η = 7.75 to 8.00.

In addition, as an additional element for miniaturizing crystal grains, if the total content of the above-mentioned composition is 0.003 to 0.825 mass%, one or more members selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg Adding an element makes it easier to miniaturize the crystal grains. These additional elements increase the dislocation density during cold rolling, so that it is easier to refine the crystal grains. In addition, if recrystallization annealing is performed only once in the initial stage of cold rolling, and then recrystallization annealing is not performed, a large amount of processing strain is introduced by cold rolling to generate dynamic recrystallization, so that the crystal grains can be more reliably realized. Miniaturization.

When the total content of the above-mentioned added elements is less than 0.003% by mass, it becomes difficult to refine the crystal grains, and when it exceeds 0.825% by mass, the conductivity is reduced. In addition, there are cases where the recrystallization temperature increases without recrystallization when laminated with the resin, the strength becomes too high, and the bendability of the copper foil and CCL is deteriorated.

In addition, as a method for refining the crystal grains of the copper foil after recrystallization, in addition to a method of adding an element, a method of performing polymerization and rolling, and a method of using a pulse current when performing electrodeposition by using electrolytic copper foil may also be cited. Or, by adding an appropriate amount of thiourea or animal glue to the electrolytic solution through electrolytic copper foil.

The copper foil of the present invention may be composed of fine copper (TPC) conforming to JIS-H3100 (C1100) or oxygen-free copper (OFC) conforming to JIS-H3100 (C1011).

Moreover, it can also be set as the composition which made the said TPC or OFC contain the said addition element.

<Average crystal grain size>

The average crystal grain size of the copper foil is 0.6 to 4.3 μm. If the average crystal grain size is less than 0.6 μm, the strength becomes too high, the flexural rigidity becomes large, and the springback becomes large, making it unsuitable for flexible printed circuit board applications. If the average crystal grain size exceeds 4.3 μm, it is impossible to achieve miniaturization of the crystal grains, it is difficult to increase the strength to improve the bendability, and the soft etchability, etching factor, or circuit linearity is deteriorated and the etchability is lowered.

Regarding the measurement of the average crystal grain size, in order to avoid errors, the surface of the foil was observed with a field of view of 100 μm × 100 μm for 3 or more fields. For the observation of the foil surface, an average crystal grain size can be obtained based on JIS H 0501 using a scanning ion microscope (SIM) or a scanning electron microscope (SEM).

However, the twin crystal system is measured as individual crystal grains.

<Tensile strength (TS)>

The tensile strength of copper foil is 230 ~ 287MPa. As described above, the fineness of the crystal grains improves the tensile strength. If the tensile strength is less than 230 MPa, it becomes difficult to improve the strength. When the tensile strength exceeds 287 MPa, the strength becomes too high, the flexural rigidity becomes large, and the springback becomes large, making it unsuitable for flexible printed circuit board applications.

The tensile strength is based on the tensile test according to IPC-TM650, with a test piece width of 12.7mm, room temperature (15 ~ 35 ° C), a tensile speed of 50.8mm / min, and a gauge length of 50mm. It is obtained by performing a tensile test in the rolling direction (or MD direction).

<Skew Rsk>

As an index for evaluating soft etchability, the copper foil surface after etching is specified based on the skewness Rsk of JIS B 0601-2001. As the etching conditions, the soft etching to simulate the adhesion between the copper foil and the photoresist was simulated, and the copper foil was immersed in an aqueous solution (liquid temperature 25 ° C) 420 of 100 g / L sodium sulfate concentration and 35 g / L hydrogen peroxide concentration. second.

Skewness Rsk is a mean of the Z (x) cube at the base length without the dimensionization by the cube of the root mean square height Rq.

The root-mean-square height Rq is an index indicating the degree of unevenness in the surface roughness measurement using a non-contact roughness meter in accordance with JIS B 0601 (2001), and is expressed by the following formula (A), which is the surface roughness The height of the unevenness (the height of the mountain) in the Z axis direction is the root mean square of the height Z (x) of the mountain with the reference length lr.

Root mean square height Rq of the height of the mountain of reference length lr:

The skewness Rsk is expressed by the following formula (B) using the root mean square height Rq.

The skewness Rsk of the copper foil surface is an index showing the symmetry of the roughness of the copper foil surface when the average surface of the uneven surface of the copper foil surface is used as the center. Therefore, the closer the absolute value of Rsk is to 0, the more symmetrical the hills and valleys of unevenness, and the higher the peel strength (peel strength (adhesion strength) according to IPC-TM-650) becomes, the better it adheres to the resin, so soft etching The better the sex. In addition, it can be said that if Rsk <0, the height distribution is on the upper side relative to the average surface, and if Rsk> 0, the height distribution is on the lower side relative to the average surface. When the upward deflection is large, the surface of the copper foil will become convex, so the diffuse reflection inside the copper foil will become larger. When a photoresist is attached to the copper foil and exposed and etched away, the circuit The accuracy of linearity or etching factor deteriorates. When the downward deflection is large, the surface of the copper foil will become concave. If light is irradiated from the light source, the diffuse reflection on the surface of the copper foil will increase. After the photoresist is attached to the copper foil and exposed, In the case of etching removal, the linearity of the circuit or the accuracy of the etching factor is deteriorated. In addition, the closer the absolute value of Rsk is to 0, the more symmetrical the hills and valleys of the concave and convex, so that the electromagnetic force lines will not be disturbed in the height direction, so the better the high-frequency transmission characteristics.

In this regard, the copper foil of the present invention measures the skewness Rsk 16 times in the MD direction and the CD direction, respectively, and uses the value obtained by averaging the absolute values of the measured values as Rsk.

Regarding MD (Machine Direction) direction, when rolling copper foil, it is rolled. In the parallel direction, in the case of electrolytic copper foil, it is the direction of travel of the strip during manufacture. The CD (Cross Machine Direction) direction is a right-angle direction in the case of rolling a copper foil, and a direction perpendicular to the direction of travel in the case of an electrolytic copper foil.

The actual copper foil is cut in the MD and CD directions for CCL, so Rsk in the MD and CD directions is measured.

By setting the absolute value of Rsk on the surface of the copper foil to 0.05 or less, the peel strength is improved, and its adhesion to the resin is excellent, and the linearity of the circuit or the accuracy of the etching factor after the copper foil is etched and removed using a photoresist As a result, the soft etchability is improved.

If the absolute value of Rsk exceeds 0.050, the adhesion with the resin will be improved, but the unevenness on the surface will become significant, and the accuracy of the linearity of the circuit after the copper foil is etched and removed by the photoresist will be reduced, resulting in poor soft etching.

The lower limit of the absolute value of Rsk is not particularly limited, and is usually 0.001. It is industrially difficult to set the absolute value of Rsk to less than 0.001.

<300 ° C for 30 minutes heat treatment>

After the copper foil is heat-treated at 300 ° C. for 30 minutes, the average crystal grain size can be 0.6 to 4.3 μm, the tensile strength in the MD direction can be 230 to 287 MPa, and the skewness Rsk after the heat treatment can be 0.05 or less.

The copper foil of the present invention is used for a flexible printed circuit board. At this time, the CCL obtained by laminating the copper foil and the resin is subjected to a heat treatment for hardening the resin at 200 to 400 ° C. Therefore, there is a problem that the grains are coarsened due to recrystallization possibility.

The CCL obtained by laminating the copper foil and the resin is subjected to a heat treatment for curing the resin at 200 to 400 ° C. That is, the actual soft etching is performed on the copper foil subjected to the heat treatment.

Therefore, before and after lamination with the resin, the average crystal grain size and tensile strength of the copper foil changed. Therefore, the copper foil for a flexible printed circuit board of claim 1 of the present case specifies a copper foil that has been cured and heat-treated with a resin after being laminated with a resin to a copper-clad laminate.

On the other hand, the copper foil for flexible printed circuit board of claim 4 of the present case specifies the state when the above-mentioned heat treatment has been performed on the copper foil before being laminated with the resin. The heat treatment at 300 ° C. for 30 minutes simulates the temperature conditions for curing heat treatment of the resin when the CCL is laminated.

In addition, the environment for the heat treatment is not particularly limited, and it may be in the atmosphere or an inert gas environment such as Ar or nitrogen.

The copper foil of this invention can be manufactured as follows, for example. First, a foil can be manufactured by adding the above-mentioned additives to a copper ingot, melting and casting, and then performing hot rolling, cold rolling, and annealing, and performing the final cold rolling described above.

<Copper-clad laminate and flexible printed circuit board>

In addition, for the copper foil of the present invention, (1) a resin precursor (for example, a polyimide precursor called varnish) is cast and heated to polymerize it, and (2) the same type as the base film is used A thermoplastic adhesive is used to laminate a base film to the copper foil of the present invention, thereby obtaining a copper clad laminate (CCL) composed of two layers of a copper foil and a resin substrate. Further, a base film coated with an adhesive was laminated on the copper foil of the present invention, thereby obtaining a copper clad laminate (CCL) composed of three layers of a copper foil, a resin substrate, and an adhesive layer therebetween. When these CCLs are produced, the copper foil is recrystallized by heat treatment.

A circuit is formed by using the photolithography technique, and the circuit is plated as necessary, and a cover film is laminated to obtain a flexible printed circuit board (flexible wiring board).

Therefore, the copper clad laminate of the present invention is formed by laminating a copper foil and a resin layer. Again, this The flexible printed circuit board of the invention is formed by forming a circuit on a copper foil of a copper clad laminate.

Examples of the resin layer include, but are not limited to, PET (polyethylene terephthalate), PI (polyimide), LCP (liquid crystal polymer), and PEN (polyethylene naphthalate). . As the resin layer, such a resin film may be used.

As a method of laminating a resin layer and a copper foil, the surface of the copper foil can be coated with a material that becomes a resin layer and heated to form a film. Further, a resin film may be used as the resin layer, and the following adhesive may be used between the resin film and the copper foil, or the resin film may be thermally compression-bonded to the copper foil without using an adhesive. However, it is preferable to use an adhesive in terms of not adding unnecessary heat to the resin film.

When a film is used as the resin layer, the film may be laminated on a copper foil via an adhesive layer. In this case, it is preferable to use an adhesive having the same composition as the film. For example, when a polyimide film is used as the resin layer, it is preferable to use a polyimide-based adhesive for the adhesive layer. In addition, the polyfluorene imine adhesive mentioned here refers to the adhesive containing a fluorene imine bond, and also includes polyether fluorene imine and the like.

The present invention is not limited to the embodiments described above. Moreover, as long as the effect of this invention is acquired, the copper alloy in the said embodiment may contain other components.

For example, the surface of the copper foil may be subjected to a surface treatment of roughening treatment, rust prevention treatment, heat treatment, or a combination thereof.

[Example]

Next, the present invention will be described in more detail with examples, but the present invention is not limited to these examples. Elements shown in Table 1 were added to electrolytic copper having a purity of 99.9% or more, and casting was performed in an Ar environment to obtain an ingot. The oxygen content in the ingot did not reach 15 ppm. This ingot was homogenized and annealed at 900 ° C, and then hot-rolled and cold-rolled to a thickness of After 31 to 51 mm, annealing was performed once, and then the surface was surface-cut, and finally cold-rolled at the processing degree η shown in Table 1, to obtain a copper foil sample with a final thickness of 17 μm.

<A. Evaluation of copper foil samples>

Electrical conductivity

For each of the above copper foil samples, heat treatment was performed at 300 ° C for 30 minutes in the air (simulating the temperature conditions for the resin to harden and heat-treat when CCL is laminated), and then based on JIS H 0505, the 25 ° C Conductivity (% IACS) was measured.

When the conductivity is 75% IACS or more, the conductivity is good.

2.Particle size

The surface of each copper foil sample after the heat treatment was observed using a SEM (Scanning Electron Microscope), and the average particle diameter was determined based on JIS H 0501. However, the twin crystal system is measured as individual crystal grains. The measurement area was set to 100 μm × 100 μm on the surface.

3. Tensile strength of copper foil

For each copper foil sample after the above heat treatment, the tensile strength was measured under the above conditions by a tensile test according to IPC-TM650.

<Evaluation of B.CCL>

4.CCL (copper clad laminate) production

After the final cold rolling, copper roughening plating was performed on one side of a copper foil sample (copper foil before heat treatment) that had not been subjected to the above-mentioned heat treatment. As a copper roughening plating bath, a composition of Cu: 10-25 g / L and sulfuric acid: 20-100 g / L is used, and plating is performed for 1-5 seconds at a bath temperature of 20-40 ° C and a current density of 30-70A / dm ² , So that the copper adhesion amount is 20g / dm ² .

The roughened plated surfaces of the copper foil samples were laminated on each side of the polyimide film (product name "Upilex VT" manufactured by Ube Kosan Co., Ltd., thickness 25 μm) with adhesive on both sides, and pressed by heating. (4 MPa) A heat treatment was performed at 300 ° C. for 30 minutes for bonding, and CCL samples were obtained in which copper foil was laminated on both sides of the polyimide film.

5. Skew Rsk

The CCL was immersed in an aqueous solution (liquid temperature 25 ° C.) at a concentration of 100 g / L of sodium persulfate and 35 g / L of hydrogen peroxide for 420 seconds to perform soft etching. The measurement locations were changed in the rolling parallel direction and the rolling right angle direction. The soft-etched copper foil surface was measured 16 times (total 32 times) based on the skewness Rsk of JIS B 0601-2001, and the measured values were obtained. The absolute value of is the average value.

6. Etching

A short strip-shaped circuit with L / S (line / gap) = 35/35 μm, 35/35 μm, 25/25 μm, 20/20 μm, and 10/10 μm was formed on the copper foil portion of the CCL sample. For comparison, a circuit was formed in the same manner as a commercially available rolled copper foil (fine copper foil, thickness: 17 μm). Then, the etching factor (the ratio shown by the (etching depth / average etching width above and below) of the circuit) and the linearity of the circuit were visually determined using a microscope, and evaluated based on the following criteria. An evaluation of ○ is good.

○: Compared with commercially available rolled copper foil, the etching factor and the linearity of the circuit are good

△: Compared with commercially available rolled copper foil, the etching factor and the linearity of the circuit are the same

×: Compared with commercially available rolled copper foil, the etching factor and the linearity of the circuit are poor.

7. High-frequency transmission characteristics

A microstrip line with an impedance of 50Ω and a length of 100mm was etched on the copper foil portion of one side of the above-mentioned CCL as an example. In addition, the copper foil on the opposite side of CCL is not etched and becomes GND.

As a comparative example, CCL was produced in the same manner from a commercially available rolled copper foil (fine copper foil, thickness: 17 μm), and the microstrip line was formed on a copper foil portion on one side of the CCL.

Then, using a network analyzer, S21 was measured as the S-parameter (Scattering Parameter) of the microstrip line at 60 GHz. S21 uses signal A which is incident on port 1 and signal B which is transmitted to port 2, which is expressed by the following formula (C).

S ₂₁ [dB] = 10log ₁₀ (B / A) (C)

It is shown that the smaller the absolute value of S21 (S21 must be negative), the smaller the transmission loss, and the better the transmission characteristics. Therefore, the transmission characteristics (transmission loss) of the circuit were evaluated based on the following criteria. When the evaluation is ○, the transmission characteristics are excellent.

○: {(absolute value of S21 of commercially available rolled copper foil)-(absolute value of S21 of the embodiment) ≧ 5dB / mm or more

△: 5dB / mm> {(absolute value of S21 of commercially available rolled copper foil)-(absolute value of S21 of the embodiment)}>-5dB / mm

×: -5dB / mm ≧ {(absolute value of S21 of commercially available rolled copper foil)-(absolute value of S21 of the example)}

The obtained results are shown in Table 1.

As is clear from Table 1, when the average crystal grain diameter of the copper foil is 0.6 to 4.3 μm, the tensile strength is 230 to 287 Mpa, and the absolute value of the skewness Rsk is less than or equal to 0.05, each case includes The soft etchability is excellent in etching properties, and the high-frequency transmission characteristics are also excellent. In addition, in Example 1, polymerization and rolling were performed on the last stroke of the final cold rolling.

On the other hand, in the case of Comparative Examples 1 and 3 in which the final cold-rolled workability η did not reach 7.51, the average crystal grain diameter of the copper foil exceeded 4.3 μm and coarsened, and the tensile strength became less than 230 MPa, and the skewness The absolute value of Rsk becomes greater than 0.05. As a result, etchability including soft etchability is poor, and high-frequency transmission characteristics are also poor.

Moreover, in the case of Comparative Example 4 in which the final cold-rolled workability η is less than 7.51 but is 3.5 or more, the skewness Rsk is greater than 0.05, the etchability including soft etchability is poor, and the high-frequency transmission characteristics are also poor.

However, in the case of Comparative Example 4, the average crystal grain size of the copper foil was 4.3 μm or less, and the tensile strength was also 230 MPa or more. The reason is considered as follows. That is, in the case of Comparative Examples 1 and 3 in which η is less than 3.5, the accumulation of strain during final cold rolling is small, and the cores of the recrystal grains are reduced, so that the recrystal grains become coarse. On the other hand, in the case of Comparative Example 4 in which η is 3.5 or more, although the strain accumulates moderately during the final cold rolling process and the recrystal grains become fine, the strain locally exists, so Rsk becomes large. In addition, it is considered that if η is 7.51 or more, the accumulated strain amount further increases and the strain exists uniformly, so that Rsk becomes small.

In the case of Comparative Example 5 where the final cold rolling process degree η is greater than 8.00, the average crystal grain size of the copper foil is also larger than 4.3 μm and coarsened, and the tensile strength does not reach 230 MPa. The absolute deviation of the skewness Rsk The value also becomes greater than 0.05. As a result, etchability including soft etchability is poor, and high-frequency transmission characteristics are also poor.

When the total content of the added elements exceeds the upper limit of Comparative Example 2, the conductivity is poor.

Claims (9)

A copper foil for a flexible printed circuit board is composed of 99.0% by mass of Cu, and the remaining portion is unavoidable impurities. The average crystal grain size is 0.6 to 4.3 μm, and the tensile strength in the MD direction is 230 to 287 MPa. The MD and CD directions were immersed in an aqueous solution (liquid temperature 25 ° C) at a concentration of 100 g / L of sodium persulfate and 35 g / L of hydrogen peroxide for 420 seconds. For each measurement, the absolute value of each measurement value is averaged to obtain a value of 0.05 or less. For example, the copper foil for flexible printed circuit boards in the scope of application for patent No. 1 is composed of fine copper in accordance with JIS-H3100 (C1100) or oxygen-free copper in JIS-H3100 (C1011). For example, the copper foil for a flexible printed circuit board in the scope of patent application No. 1 or 2 further contains 0.003 to 0.825 mass% of a group selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg. One or more types of added elements. For example, the copper foil for flexible printed substrates according to any one of claims 1 to 2, wherein the copper foil is a rolled copper foil, and the average crystal grain size is 0.6 after heat treatment at 300 ° C for 30 minutes. ~ 4.3 μm, the tensile strength is 230 to 287 MPa, and the skewness Rsk after the heat treatment is 0.05 or less. For example, the copper foil for flexible printed substrates in the scope of patent application No. 3, wherein the copper foil is a rolled copper foil, and the average crystal grain size is 0.6 to 4.3 μm after heat treatment at 300 ° C for 30 minutes. The tensile strength is 230 to 287 MPa, and the skewness Rsk after the heat treatment is 0.05 or less. A copper clad laminate is formed by laminating a copper foil and a resin layer for a flexible printed circuit board according to any one of claims 1 to 5. A flexible printed circuit board is formed by using a copper-clad laminate with the scope of patent application No. 6 to form a circuit on the copper foil. For example, the flexible printed circuit board of the seventh scope of the patent application, in which the L / S of the circuit is 35/35 ~ 10/10 (μm / μm). An electronic device using a flexible printed circuit board having the scope of patent application No. 7 or 8.