JP6781562B2 - Copper foil for flexible printed circuit boards, copper-clad laminates using it, flexible printed circuit boards, and electronic devices - Google Patents

Description

The present invention relates to a copper foil suitable for use in 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.

Since a flexible printed circuit board (flexible wiring board, hereinafter referred to as "FPC") has flexibility, it is widely used for bent parts and movable parts of electronic circuits. For example, FPCs are used for moving parts of disc-related devices such as HDDs, DVDs, and CD-ROMs, and bent parts of foldable mobile phones.
FPC is formed by etching Copper Clad Laminate (copper-clad laminate, hereinafter referred to as CCL) in which a copper foil and a resin are laminated to form wiring, and coating the wiring with a resin layer called a coverlay. Before laminating the coverlay, the surface of the copper foil is etched as part of the surface modification step for improving the adhesion between the copper foil and the coverlay. Further, in order to reduce the thickness of the copper foil and improve the flexibility, soft etching may be performed.

Of the copper foil used for FPC, the surface on which the resist for etching and forming the circuit is formed is soft-etched in order to impart adhesion to the resist. Soft etching is a surface treatment that removes the oxide film on the surface of the copper foil and flattens the surface. However, during soft etching, a defect called dishdown occurs in which recesses are formed on the surface of the rolled copper foil. In this dish down, the surface becomes uneven due to the difference in etching rate in the thickness direction of the rolled copper foil, and the resist adhesion is lowered. It is said that the difference in etching rate is caused by the difference in etching rate depending on the crystal orientation of the surface of the rolled copper foil.
Therefore, a rolled copper foil has been developed in which the proportion of (200) planes having a slow etching rate as compared with other crystal planes is reduced and the soft etching property is improved (Patent Document 1).

Japanese Unexamined Patent Publication No. 2014-77182

By the way, as electronic devices become smaller, thinner, and have higher performance, it is required to mount FPCs at high density inside these devices. However, in order to perform high-density mounting, the circuit is made finer. At the same time, it is necessary to bend and accommodate a small-thickness FPC inside the miniaturized device. Then, in order to miniaturize the circuit, the adhesion between the resist forming the circuit by etching and the copper foil is further required. That is, if the adhesion between the copper foil and the resist is low, the etching solution penetrates between the copper foil and the photoresist, making it difficult to form fine wiring.

However, in the case of the conventional copper foil, the surface flattening after soft etching is not sufficient, and it is difficult to miniaturize the circuit.
In addition, as electronic devices become smaller, thinner, and have higher performance, the circuit width and space width of FPCs have become finer to about 20 to 30 μm, and the etching factor and circuit linearity tend to deteriorate when forming circuits by etching. There is a problem of becoming, and this solution is also required.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a copper foil for a flexible printed circuit board having excellent etching properties, a copper-clad laminate using the same, a flexible printed circuit board, and an electronic device. To do.

As a result of various studies, the present inventors have found that the etching property can be improved by making the crystal grains of the copper foil finer and defining the skewness Rsk of the copper foil after etching. However, if the crystal grains are made too fine, the strength becomes too high and the bending rigidity becomes large, and the springback becomes large, which is not suitable for flexible printed circuit board applications. Therefore, the range of grain size and tensile strength is defined.
Further, by reducing the crystal grain size to about 1/10 of the circuit width of about 20 to 30 μm of recent FPCs, the etching factor and circuit linearity when forming a circuit by etching can be improved. Can be done.

That is, the copper foil for a flexible printed substrate of the present invention has an Ag content of 0.001 to 0.05% by mass and P, Ti with respect to the tough pitch copper specified in JIS-H3100 (C1100) or the oxygen-free copper of JIS-H3100 (C1011). , Sn, Ni, Be, Zn, In and Mg, containing one or more additive elements in total of 0.003 to 0.825% by mass, with an average crystal particle size of 0.6 to 4.3 μm and in the MD direction. Skewness Rsk based on JIS B 0601-2001 on the surface after being immersed in an aqueous solution (liquid temperature 25 ° C) with a tensile strength of 230 to 287 MPa, a sodium persulfate concentration of 100 g / L, and a hydrogen peroxide concentration of 35 g / L for 420 seconds. Is measured 16 times in each of the MD direction and the CD direction, and the average value of the absolute values of the measured values of each time is 0.05 or less.

The copper foil is a rolled copper foil, the average crystal grain size after heat treatment at 300 ° C. for 30 minutes 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. The following is preferable.

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 uses the copper-clad laminate to form a circuit on the copper foil.
The L / S of the circuit is preferably 35/35 to 10/10 (μm / μm). The L / S (line and space) of the circuit is the ratio of the width (L: line) of the wiring constituting the circuit and the interval (S: space) of the adjacent wiring. L adopts the minimum value of L in the circuit, and S adopts the minimum value of S in the circuit.
L and S may be 10 to 35 μm, and they do not have to be the same value. For example, values such as L / S = 20.5 / 35 and 35/17 can be taken.

The electronic device of the present invention uses 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 according to the present invention will be described. In the present invention,% means mass% unless otherwise specified.

<Composition>
The copper foil according to the present invention contains 0.001 to 0.05% by mass of Ag and P, Ti, Sn, Ni, with respect to the tough pitch copper specified in JIS-H3100 (C1100) or the oxygen-free copper of JIS-H3100 (C1011). It contains 0.003 to 0.825% by mass of one or more additive elements selected from the group of Be, Zn, In and Mg in total.
As described above, in the present invention, the crystal grains after recrystallization of the copper foil are made finer to increase the strength and the etchability.
However, in the case of the above-mentioned pure copper-based composition, it is difficult to refine the crystal grains. Therefore, the recrystallization annealing is performed only once at the initial stage of cold rolling, and the recrystallization annealing is not performed thereafter. A large amount of machining strain can be introduced by inter-rolling to cause dynamic recrystallization, and grain refinement can be realized.

To increase the machining strain in cold rolling, the degree of machining in final cold rolling (finish rolling performed after the final annealing in the entire process of repeating annealing and rolling) is η = ln (final cold). Plate thickness before inter-rolling / plate thickness after final cold rolling) = 7.51 to 8.00.
When η is less than 7.51, the machining strain is not uniformly accumulated, that is, the strain is locally accumulated, so that the etching rate differs between the strain-accumulated portion and the other portion. Therefore, the absolute value of Rsk after soft etching becomes large, and the etching property deteriorates. When η is larger than 8.00, strain is excessively accumulated and becomes a driving force for crystal grain growth, and the crystal grains tend to be coarse. It is more preferable that η = 7.75 to 8.00.

Further, when Ag and the above-mentioned additive element are contained as the additive element for refining the crystal grains, the dislocation density can be increased during cold rolling, and the refining of the crystal grains can be surely realized.
Of these, Ag reduces the sensitivity of the recrystallization particle size to the recrystallization annealing conditions. That is, as will be described later, heat treatment is performed to cure the resin during CCL lamination, but the temperature and time of the heat treatment actually fluctuate, and the rate of temperature rise also differs depending on the manufacturing apparatus, the manufacturer, and the like. Therefore, the grain size of the recrystallized grains of the copper foil may increase depending on the heat treatment. Therefore, by containing Ag, the crystal grains can be stably refined even if the heat treatment conditions at the time of CCL lamination change.

If the Ag content is less than 0.001% by mass, it becomes difficult to refine the crystal grains. Further, if the Ag content exceeds 0.05% by mass, the recrystallization temperature rises and does not recrystallize when laminated with the resin, and the strength may become too high to deteriorate the bendability of the copper foil and CCL. ..
If the total content of the added elements is less than 0.003% by mass, it becomes difficult to refine the crystal grains, and if it exceeds 0.825% by mass, the conductivity may decrease. Further, when the recrystallization temperature rises and the resin is laminated, the recrystallization may not occur, and the strength may become too high to deteriorate the bendability of the copper foil and CCL.

<Average 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 bending rigidity becomes large, and the springback becomes large, which is not suitable for flexible printed circuit board applications. If the average crystal grain size exceeds 4.3 μm, the crystal grains cannot be refined, it becomes difficult to increase the strength and improve the bendability, and the soft etching property, etching factor, and circuit linearity deteriorate. The etchability is reduced.
The average crystal grain size is measured by observing the foil surface in a field of view of 100 μm × 100 μm in three or more fields of view in order to avoid errors. For the observation of the foil surface, a SIM (Scanning Ion Microscope) or an SEM (Scanning Electron Microscope) can be used, and the average crystal grain size can be determined based on JIS H 0501.
However, twins are measured as separate crystal grains.

<Tensile strength (TS)>
The tensile strength of the copper foil is 230-287 MPa. As described above, the tensile strength is improved by refining the crystal grains. If the tensile strength is less than 230 MPa, it becomes difficult to increase the strength. If the tensile strength exceeds 287 MPa, the strength becomes too high and the flexural rigidity becomes large, and the springback becomes large, which is not suitable for flexible printed circuit board applications.
Tensile strength was measured by a tensile test based on IPC-TM650, with a test piece width of 12.7 mm, room temperature (15 to 35 ° C), tensile speed of 50.8 mm / min, gauge length of 50 mm, and copper foil rolling direction (or MD direction). ), And a tensile test was performed in the direction parallel to).

<Skewness Rsk>
As an index for evaluating the soft etching property, the skewness Rsk based on JIS B 0601-2001 of the copper foil surface after etching is defined. As the etching conditions, soft etching for imparting adhesion between the copper foil and the resist was simulated, and the solution was placed in an aqueous solution (liquid temperature 25 ° C) with a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L for 420 seconds. The copper foil shall be immersed.

スキューネスRskは、二乗平均平方根高さＲｑを用いて、以下の（Ｂ）式で示される。

Skewness Rsk represents the Z (x) cubed average at a reference length made non-dimensional 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 by a non-contact roughness meter conforming to JIS B 0601 (2001), and is represented by the following equation (A), and the surface roughness is expressed by the following equation (A). It is the height of the unevenness (mountain) in the Z-axis direction, and is the root mean square of the height Z (x) of the mountain at the reference length ll.
Root mean square height Rq of mountain height at reference length rl:

Skewness Rsk is expressed by the following equation (B) using the root mean square height Rq.

Skewness Rsk on the surface of the copper foil is an index showing the symmetry of the unevenness of the copper foil surface when the average surface of the uneven surface of the copper foil surface is the center. Therefore, the closer the absolute value of Rsk is to 0, the more symmetrical the peaks and valleys of the unevenness, and the higher the peel strength (peeling strength (adhesive strength) based on IPC-TM-650), and the better the adhesion with the resin. Therefore, it has excellent soft etching properties. Further, if Rsk <0, the height distribution is biased upward with respect to the average plane, and if Rsk> 0, the height distribution is biased downward with respect to the average plane. When the bias to the upper side is large, the surface of the copper foil is convex, so the diffused reflection inside the copper foil becomes large, and when the resist is attached to the copper foil and then exposed and removed by etching, the circuit linearity and The accuracy of the etching factor deteriorates. When the downward bias is large, the copper foil surface has a concave shape, and when light is applied from a light source, diffused reflection on the copper foil surface increases, and after the resist is attached to the copper foil, it is exposed and etched. If this happens, the circuit linearity and the accuracy of the etching factor deteriorate. Further, the closer the absolute value of Rsk is to 0, the more symmetrical the peaks and valleys of the unevenness, so that the disturbance of the electromagnetic force line does not occur in the height direction, so that the high frequency transmission characteristic is good.

Therefore, in the copper foil of the present invention, the skewness Rsk is measured 16 times in each of the MD direction and the CD direction, and the value obtained by averaging the absolute values of the measured values of each time is adopted as the Rsk.
The MD (Machine1 Direction) direction is the rolling parallel direction in the case of rolled copper foil, and the flow direction of the strip at the time of manufacture in the case of electrolytic copper foil. The CD (Cross Machine Direction) direction is the direction perpendicular to rolling in the case of rolled copper foil, and the direction perpendicular to the flow direction in the case of electrolytic copper foil.
Since the actual copper foil is cut out in the MD direction and the CD direction and used for CCL, the Rsk in the MD direction and the CD direction is measured.

By specifying the absolute value of Rsk on the copper foil surface to 0.05 or less, the peel strength becomes high and the adhesion with the resin is excellent, and the linearity and etching factor of the circuit after etching and removing the copper foil with a resist Since the accuracy is high, the soft etching property is improved.
If the absolute value of Rsk exceeds 0.050, the adhesion to the resin will improve, but the surface irregularities will become noticeable, and the accuracy of the linearity of the circuit after etching and removing the copper foil with the resist will decrease, resulting in soft etching properties. Is inferior.
The lower limit of the absolute value of Rsk is not particularly limited, but is usually 0.001. It is industrially difficult to make the absolute value of Rsk less than 0.001.

<Heat treatment at 300 ° C for 30 minutes>
The average crystal grain size of the copper foil after heat treatment at 300 ° C. for 30 minutes may be 0.6 to 4.3 μm, the tensile strength in the MD direction may be 230 to 287 MPa, and the skewness Rsk after the heat treatment may be 0.05 or less.
The copper foil according to the present invention is used for a flexible printed circuit board, and at that time, the CCL in which the copper foil and the resin are laminated is subjected to a heat treatment for curing the resin at 200 to 400 ° C., so that crystal grains are formed by recrystallization. It may be oversized.
Further, the CCL in which the copper foil and the resin are laminated 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 this heat treatment.

Therefore, the average crystal grain size and tensile strength of the copper foil change before and after laminating with the resin. Therefore, the copper foil for a flexible printed substrate according to claim 1 of the present application defines a copper foil in a state of being subjected to a curing heat treatment of the resin after being formed into a copper-clad laminate after being laminated with the resin. +
On the other hand, the copper foil for a flexible printed circuit board according to claim 2 of the present application defines a state when the copper foil before being laminated with a resin is subjected to the above heat treatment. This heat treatment at 300 ° C. for 30 minutes imitates the temperature conditions for curing and heat-treating the resin during lamination of CCL.
The atmosphere of the heat treatment is not particularly limited, and may be an atmosphere or an atmosphere of an inert gas such as Ar or nitrogen.

The copper foil of the present invention can be produced, for example, as follows. First, the above additive is added to a copper ingot, melted and cast, then hot rolled, cold rolled and annealed, and the final cold rolling described above is carried out to produce a foil.

<Copper-clad laminate and flexible printed circuit board>
Further, (1) a resin precursor (for example, a polyimide precursor called varnish) is cast on the copper foil of the present invention and polymerized by applying heat, and (2) using a thermoplastic adhesive of the same type as the base film. By laminating the base film on the copper foil of the present invention, a copper-clad laminate (CCL) composed of two layers of the copper foil and a resin base material can be obtained. Further, by laminating a base film coated with an adhesive on the copper foil of the present invention, a copper-clad laminate (CCL) composed of three layers of a copper foil, a resin base material, and an adhesive layer between them can be obtained. During the production of these CCLs, the copper foil is heat treated and recrystallized.
A flexible printed circuit board (flexible wiring board) can be obtained by forming a circuit on these using photolithography technology, plating the circuit as necessary, and laminating a coverlay film.

Therefore, the copper-clad laminate of the present invention is formed by laminating a copper foil and a resin layer. Further, the flexible printed circuit board of the present 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). Moreover, you may use these resin films as a resin layer.
As a method of laminating the resin layer and the copper foil, a material to be a resin layer may be applied to the surface of the copper foil to form a heat 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 heat-bonded to the copper foil without using the adhesive. However, it is preferable to use an adhesive from the viewpoint of not applying extra heat to the resin film.
When a film is used as the resin layer, this film may be laminated on the 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 as the adhesive layer. The polyimide adhesive referred to here refers to an adhesive containing an imide bond, and also includes polyetherimide and the like.

The present invention is not limited to the above embodiment. Further, the copper alloy in the above-described embodiment may contain other components as long as the effects of the present invention are exhibited.
For example, the surface of the copper foil may be roughened, rust-proofed, heat-resistant, or surface-treated by a combination thereof.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto. The 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 atmosphere to obtain an ingot. The oxygen content in the ingot was less than 15 ppm. This ingot was homogenized and annealed at 900 ° C., then hot-rolled and cold-rolled to a thickness of 31 to 51 mm, and after one annealing, the surface was surface-cut and processed as shown in Table 1. The final cold rolling was performed at η degrees to obtain a copper foil sample having a final thickness of 17 μm.

<A. Evaluation of copper foil sample ＞
1. 1. Conductivity Each of the above copper foil samples is heat-treated at 300 ° C for 30 minutes in the atmosphere (simulating the temperature conditions for curing and heat-treating the resin during CCL lamination), and then by the 4-terminal method based on JIS H 0505. , 25 ° C conductivity (% IACS) was measured.
If 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 size was determined based on JIS H 0501. However, twins were measured as separate crystal grains. The measurement area was 100 μm × 100 μm on the surface.

3. 3. Tensile Strength of Copper Foil The tensile strength of each copper foil sample after the above heat treatment was measured under the above conditions by a tensile test based on IPC-TM650.

<B. Evaluation of CCL>
4. Preparation of CCL (Copper Clad Laminated Plate) Copper rough plating was performed on one side of a copper foil sample (copper foil before heat treatment) that was not subjected to the above heat treatment after the final cold rolling. Using a composition of Cu: 10-25g / L, sulfuric acid: 20-100g / L as a copper roughening plating bath, electroplating at a bath temperature of 20-40 ° C and a current density of 30-70A / dm2 for 1-5 seconds, copper The amount of adhesion was 20 g / dm2.
The roughened plated surface of the copper foil sample is laminated on each adhesive surface of a polyimide film with double-sided adhesive (product name "UPIREX VT" manufactured by Ube Kosan Co., Ltd., thickness 25 μm), and 300 by heating press (4MPa). After heat treatment at ° C. for 30 minutes, they were bonded together to obtain a CCL sample in which copper foils were laminated on both sides of the polyimide film.

5. Skunes Rsk
The CCL was immersed in an aqueous solution (liquid temperature 25 ° C.) having a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L for 420 seconds for soft etching. The skewness Rsk based on IS B 0601-2001 of the copper foil surface after soft etching was measured 16 times (32 times in total) at different measurement locations in the rolling parallel direction and the rolling perpendicular direction, respectively, and the absolute value of each measurement value was measured. The value obtained by averaging the values was calculated.

6. Etching property A strip-shaped circuit of L / S (line / space) = 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 above CCL sample. For comparison, a circuit was formed in the same manner as a commercially available rolled copper foil (tough pitch copper foil, 17 μm thickness). Then, the etching factor (ratio expressed by (etching depth / average etching width above and below) of the circuit) and the linearity of the circuit were visually judged with a microscope and evaluated according to the following criteria. It is good if the evaluation is ○.
◯: Good etching factor and circuit linearity compared to commercially available rolled copper foil Δ: Equivalent etching factor and circuit linearity compared to commercially available rolled copper foil ×: Etching factor compared to commercially available rolled copper foil And the linearity of the circuit is inferior

7. High-frequency transmission characteristics A microstrip line with an impedance of 50 Ω and a length of 100 mm was formed by etching on the copper foil portion on one side of the CCL described above, and used as an example. The copper foil on the opposite side of the CCL is not etched and becomes GND.
As a comparative example, a CCL was similarly prepared from a commercially available rolled copper foil (tough pitch copper foil, 17 μm thickness), and the microstrip line was formed on a copper foil portion on one side of the CCL.
Then, using a network analyzer, S21, which is the S parameter (Scattering Parameter) of the microstrip line, was measured at 60 GHz. S21 is represented by the following equation (C) using the signal A incident on the port 1 and the signal B transmitted to the port 2.

S21の絶対値が小さいほど（S21は必ずマイナスになる）、伝送損失が小さく伝送特性に優れていることを示す。従って、以下の基準で回路の伝送特性（伝送損失）を評価した。評価が○であれば伝送特性が優れている。
○：｛（市販の圧延銅箔のS21の絶対値）−（実施例のS21の絶対値）｝≧5dB/mm以上
△：5dB/mm＞｛（市販の圧延銅箔のS21の絶対値）−（実施例のS21の絶対値）｝＞-5dB/mm
×：-5dB/mm≧｛（市販の圧延銅箔のS21の絶対値）−（実施例のS21の絶対値）｝

The smaller the absolute value of S21 (S21 is always negative), the smaller the transmission loss and the better the transmission characteristics. Therefore, the transmission characteristics (transmission loss) of the circuit were evaluated according to the following criteria. If the evaluation is ○, the transmission characteristics are excellent.
◯: {(Absolute value of S21 of commercially available rolled copper foil) − (Absolute value of S21 of the example)} ≧ 5 dB / mm or more Δ: 5 dB / mm ＞ {(Absolute value of S21 of commercially available rolled copper foil) -(Absolute value of S21 in the example)}> -5dB / mm
×: -5 dB / mm ≧ {(absolute value of S21 of commercially available rolled copper foil)-(absolute value of S21 in Examples)}

The results obtained are shown in Table 1.

As is clear from Table 1, in the case of each example in which the average crystal grain size of the copper foil is 0.6 to 4.3 μm, the tensile strength is 230 to 287 Mpa, and the absolute value of skewness Rsk is 0.05 or less, soft etching property is included. It had excellent etching properties and high-frequency transmission characteristics.

On the other hand, in the case of Comparative Example 1 containing Ag but not containing the additive element, and in the case of Comparative Example 6 in which the total content of the additive elements is less than the lower limit, the average crystal grain size of the copper foil is significantly 4.3 μm. It became coarser than that, the tensile strength became less than 230 MPa, and the absolute value of skewness Rsk became larger than 0.05. As a result, the etching property including the soft etching property was inferior, and the high frequency transmission characteristic was also inferior.

In the case of Comparative Examples 3, 4 and 7 in which the workability η in the final cold rolling was less than 7.51, the average crystal grain size of the copper foil was coarsened by more than 4.3 μm, the tensile strength was less than 230 MPa, and the skewness Rsk The absolute value was greater than 0.05. As a result, the etching property including the soft etching property was inferior, and the high frequency transmission characteristic was also inferior.
Further, in the case of Comparative Example 4 in which the workability η in the final cold rolling is less than 7.51 but is 3.5 or more, the skewness Rsk is larger than 0.05, the etching property including soft etching property is inferior, and the high frequency transmission characteristic is also improved. inferior.
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 230 MPa or more. The reason for this is considered as follows. That is, in the cases of Comparative Examples 3 and 7 in which η was less than 3.5, the accumulation of strain during the final cold rolling process was small, and the number of recrystallized grains was reduced, so that the recrystallized grains became coarse. On the other hand, in the case of Comparative Example 4 in which η was 3.5 or more, strain was appropriately accumulated during the final cold rolling process, and the recrystallized grains became fine, but the strain was locally present and the Rsk became large. .. When η is 7.51 or more, the amount of accumulated strain is further increased, and the strain is uniformly present, so that Rsk is considered to be small.

Also, in the case of Comparative Example 5 in which the workability η in the final cold rolling is larger than 8.00, the average crystal grain size of the copper foil is coarsened by more than 4.3 μm, the tensile strength is less than 230 MPa, and the absolute value of skewness Rsk. Was greater than 0.05. As a result, the etching property including the soft etching property was inferior, and the high frequency transmission characteristic was also inferior.
In Comparative Example 2 in which the total content of the added elements exceeded the upper limit, the conductivity was inferior.

Claims (6)

For tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper specified in JIS-H3100 (C1011)
It contains 0.001 to 0.05% by mass of Ag and 0.003 to 0.825% by mass in total of one or more additive elements selected from the group of P, Ti, Sn, Ni, Be, Zn, In and Mg.
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 skewness Rsk based on JIS B 0601-2001 on the surface after being immersed in an aqueous solution (liquid temperature 25 ° C.) having a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L for 420 seconds was applied in the MD direction and the CD direction, respectively. A copper foil for a flexible printed substrate, which is measured 16 times and the average value of the absolute values measured each time is 0.05 or less. The copper foil is a rolled copper foil.
The flexible according to claim 1, wherein the average crystal grain size after heat treatment at 300 ° C. for 30 minutes 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. Copper foil for printed circuit boards. A copper-clad laminate obtained by laminating the copper foil for a flexible printed circuit board according to claim 1 or 2 and a resin layer. A flexible printed circuit board formed by forming a circuit on the copper foil using the copper-clad laminate according to claim 3. The flexible printed circuit board according to claim 4, wherein the L / S of the circuit is 35/35 to 10/10 (μm / μm). An electronic device using the flexible printed circuit board according to claim 4 or 5.