JP2019194360A - Copper foil for flexible printed wiring board, and copper clad laminate, flexible printed wiring board and electronic device using the same - Google Patents

Abstract

To provide a copper foil for flexible printed wiring board excellent in etching property.SOLUTION: There is provided a copper foil for flexible printed wiring board consisting of Ag of 0.001 to 0.05 mass% and one or more addition element selected from a group of P, Ti, Sn, Ni, Be, Zn, In and Mg of total 0.003 to 0.825 mass% based on a tough pitch copper specified in JIS-H3100 (C1100) and oxygen free copper of JIS-H3100 (C1011) and having average crystal particle diameter of 0.6 to 4.3 μm, tensile strength in a MD direction of 230 to 287 MPa and a value by measuring skewness Rsk based on JIS B 0601-2001 of a surface in the MD direction and a CD direction 16 times respectively after impregnating the copper foil in a solution with sodium persulfate concentration of 100 g/L and hydrogen peroxide concentration of 35 g/L (liquid temperature of 25°C) for 420 sec. and averaging absolute values of each time measurement values of 0.05 or less.SELECTED DRAWING: None

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.

A flexible printed circuit board (flexible wiring board, hereinafter referred to as “FPC”) has flexibility, and is widely used in a bent portion and a movable portion of an electronic circuit. For example, FPCs are used for movable parts of disk-related devices such as HDDs, DVDs, and CD-ROMs, and for folding parts of foldable mobile phones.
The FPC is formed by etching a copper clad laminate (copper-clad laminate, hereinafter referred to as CCL) in which a copper foil and a resin are laminated, and then coating the wiring with a resin layer called a coverlay. The copper foil surface is etched as part of the surface modification step for improving the adhesion between the copper foil and the coverlay before the coverlay is laminated. In addition, soft etching may be performed in order to improve the flexibility by reducing the thickness of the copper foil.

Of the copper foil used for FPC, soft etching is performed on a surface on which a resist for etching a circuit is formed in order to provide 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 a recess is formed on the surface of the rolled copper foil. This dishdown is caused by the etching rate being different in the thickness direction of the rolled copper foil, resulting in irregularities on the surface and lowering the resist adhesion. The difference in etching rate is said to be caused by the difference in etching rate depending on the crystal orientation of the surface of the rolled copper foil.
Then, the rolled copper foil which reduced the ratio of the (200) plane whose etching rate is slow compared with another crystal plane, and improved soft etching property is developed (patent document 1).

JP 2014-77182 A

  By the way, along with the downsizing, thinning, and high performance of electronic devices, it is required to mount FPCs in these devices at a high density. To achieve high density mounting, the circuit is made finer. At the same time, it is necessary to fold and store the FPC having a small thickness inside the downsized device. And in order to miniaturize a circuit, the adhesiveness of the resist and copper foil which etch-form a circuit is requested | required further. That is, if the adhesion between the copper foil and the resist is low, the etchant enters between the copper foil and the photoresist, and it becomes difficult to form fine wiring.

However, in the case of the conventional copper foil, it cannot be said that the surface after soft etching is sufficiently flattened, and it is difficult to miniaturize the circuit.
In addition, as electronic devices become smaller, thinner, and higher in performance, the circuit width and space width of FPC are also reduced to about 20-30 μm, and etching factors and circuit linearity are likely to deteriorate when forming circuits by etching. There is a problem that this is solved, and this solution is also required.
The present invention has been made to solve the above-mentioned problems, and aims to provide a copper foil for a flexible printed circuit board excellent in etching property, 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 refining the crystal grains of the copper foil 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 increases, and the spring back becomes large, which is not suitable for flexible printed circuit board applications. Therefore, the range of crystal grain size and tensile strength was defined.
Also, by reducing the crystal grain size to about 1/10 of the circuit width of about 20-30 μm of recent FPC, the etching factor and circuit linearity when forming a circuit by etching are also improved. Can do.

  That is, the copper foil for a flexible printed circuit board of the present invention is based on JIS-H3100 (C1100) tough pitch copper or JIS-H3100 (C1011) oxygen-free copper. , Sn, Ni, Be, Zn, In and Mg are contained in a total amount of one or more additive elements selected from the group of 0.003 to 0.825 mass%, the average crystal grain size is 0.6 to 4.3 μm, and MD direction Skewness Rsk based on JIS B 0601-2001 on the surface after immersion for 420 seconds in an aqueous solution (liquid temperature 25 ° C.) having a tensile strength of 230 to 287 MPa and a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L Is measured 16 times in the MD direction and CD direction, respectively, and the average value of the measured values of each time is 0.05 or less.

  The copper foil is a rolled copper foil, the average 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 flexible printed circuit board copper foil and a resin layer.

The flexible printed board of the present invention is formed by using the copper-clad laminate and forming a circuit on the copper foil.
The L / S of the circuit is preferably 35/35 to 10/10 (μm / μm). Note that the L / S (line and space) of a circuit is the ratio of the width (L: line) of the wiring constituting the circuit and the interval (S: space) between adjacent wirings. L adopts the minimum value of L in the circuit, and S adopts the minimum value of S in the circuit.
In addition, L and S should just be 10-35 micrometers, and both do not need 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.

  ADVANTAGE OF THE INVENTION According to this invention, the copper foil for flexible printed circuit boards excellent in etching property is obtained.

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

<Composition>
The copper foil according to the present invention is based on JIS-H3100 (C1100) tough pitch copper or JIS-H3100 (C1011) oxygen-free copper. Ag is 0.001 to 0.05 mass%, and P, Ti, Sn, Ni, One or more additive elements selected from the group of Be, Zn, In and Mg are contained in a total amount of 0.003 to 0.825 mass%.
As described above, in the present invention, by refining crystal grains after recrystallization of the copper foil, the strength is enhanced and the etching property is improved.
However, in the case of the pure copper-based composition described above, since it is difficult to refine crystal grains, recrystallization annealing is performed only once in the initial stage of cold rolling, and thereafter recrystallization annealing is not performed. A large amount of processing strain can be introduced by hot rolling to cause dynamic recrystallization, thereby achieving refinement of crystal grains.

In order to increase the work strain in cold rolling, the degree of work in final cold rolling (finish rolling performed after the last annealing in the entire process of annealing and rolling) is η = ln (final cold rolling). (Thickness before cold rolling / thickness after final cold rolling) = 7.51 to 8.00.
When η is less than 7.51, the processing strain is not accumulated uniformly, that is, the strain is accumulated locally, so that the etching rate is different between the accumulated portion and other portions. For this reason, the absolute value of Rsk after soft etching increases, and the etching property deteriorates. When η is greater than 8.00, excessive strain is accumulated to serve as 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 additive elements are contained as additive elements for refining crystal grains, the dislocation density is increased during cold rolling, and the crystal grains can be reliably refined.
Among these, Ag makes the recrystallized grain size less sensitive to recrystallization annealing conditions. That is, as will be described later, heat treatment for curing the resin is performed at the time of CCL lamination. Actually, the temperature and time of the heat treatment vary, and the rate of temperature rise varies depending on the manufacturing apparatus, manufacturer, and the like. For this reason, there exists a possibility that the particle size of the recrystallized grain of copper foil may become large depending on heat processing. Thus, by containing Ag, the crystal grains can be stably refined even if the heat treatment conditions during CCL lamination change.

If the Ag content is less than 0.001% by mass, it is difficult to refine 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 becomes too high and the bending properties of the copper foil and CCL may deteriorate. .
When the total content of the additive elements is less than 0.003 mass%, it is difficult to refine the crystal grains, and when it exceeds 0.825 mass%, the conductivity may be lowered. In addition, when the recrystallization temperature rises and is laminated with the resin, it does not recrystallize, and the strength becomes too high, and the bendability of the copper foil and CCL may deteriorate.

<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 bending rigidity becomes large, the spring back becomes large, and it is not suitable for flexible printed circuit board applications. If the average crystal grain size exceeds 4.3 μm, crystal grain refinement will not be realized, and it will be difficult to increase strength and improve bendability, and soft etchability, etching factor and circuit linearity will deteriorate. As a result, the etching property decreases.
The average crystal grain size is measured by observing at least 3 fields of view on the surface of the foil in a 100 μm × 100 μm field in order to avoid errors. For observation of the foil surface, the average crystal grain size can be determined based on JIS H 0501 using a SIM (Scanning Ion Microscope) or SEM (Scanning Electron Microscope).
However, twins are measured as being considered as separate crystal grains.

<Tensile strength (TS)>
The tensile strength of the copper foil is 230 to 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 is difficult to increase the strength. If the tensile strength exceeds 287 MPa, the strength becomes too high and the bending rigidity becomes large, and the spring back becomes large, which is not suitable for flexible printed circuit board applications.
Tensile strength is determined by a tensile test according to 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 the rolling direction of copper foil (or MD direction) ) In the direction parallel to.

<Skness Rsk>
As an index for evaluating the soft etching property, the skewness Rsk based on JIS B 0601-2001 on the surface of the copper foil after etching is defined. Etching conditions are simulated for soft etching to provide adhesion between copper foil and resist, and are placed in an aqueous solution with a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L (liquid temperature 25 ° C) 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。

The skewness Rsk represents the Z (x) cube average at the reference length made dimensionless 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 surface roughness measurement with a non-contact type roughness meter in accordance with JIS B 0601 (2001), and is expressed by the following equation (A). The height of the unevenness (mountain) in the Z-axis direction is the root mean square of the height Z (x) of the mountain at the reference length lr.
The root mean square height Rq of the height of the mountain at the reference length lr:

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

  The skewness Rsk on the surface of the copper foil is an index indicating the symmetry of the unevenness on the surface of the copper foil when the average surface of the uneven surface on the surface of the copper foil is the center. Accordingly, the closer the absolute value of Rsk is to 0, the more symmetrical the ridges and valleys are, and the higher the peel strength (peel strength (adhesion strength) according to IPC-TM-650), the better the adhesion to 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 copper foil surface has a convex shape, so the irregular reflection inside the copper foil increases, and if the resist is attached to the copper foil and then exposed and etched away, circuit linearity and The accuracy of the etching factor is deteriorated. When the bias to the lower side is large, the copper foil surface has a concave shape, and when light is irradiated from the light source, irregular reflection on the copper foil surface increases, and after applying the resist to the copper foil, it is exposed to light and etched away In this case, the accuracy of the circuit linearity and the etching factor deteriorates. Further, the closer the absolute value of Rsk is to 0, the more symmetrical the peaks and valleys of the unevenness are, so that the disturbance of electromagnetic field lines does not occur in the height direction, and the high frequency transmission characteristics are better.

For this reason, the copper foil of the present invention measures the skewness Rsk 16 times in the MD direction and the CD direction, respectively, and adopts the value obtained by averaging the absolute values of the measured values for each time as Rsk.
The MD (Machine 1 Direction) direction is a rolling parallel direction in the case of a rolled copper foil, and is a flow direction of a strip during production in the case of an electrolytic copper foil. The CD (Cross Machine Direction) direction is a direction perpendicular to rolling in the case of rolled copper foil, and is a direction perpendicular to the flow direction in 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 defining the absolute value of Rsk on the copper foil surface to be 0.05 or less, the peel strength is high and the adhesion to the resin is excellent, and the linearity and etching factor of the circuit after the copper foil is removed by etching with the resist Since the accuracy is increased, the soft etching property is improved.
When the absolute value of Rsk exceeds 0.050, the adhesion to the resin is improved, but the surface irregularities become prominent and the accuracy of the linearity of the circuit after etching away the copper foil with the resist decreases, and the soft etching property 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 copper foil may have an average crystal grain size of 0.6 to 4.3 μm after heat treatment at 300 ° C. for 30 minutes, a tensile strength in the MD direction of 230 to 287 MPa, and a skewness Rsk after the heat treatment of 0.05 or less.
The copper foil which concerns on this invention is used for a flexible printed circuit board, In that case, since CCL which laminated | stacked copper foil and resin performs the heat processing for hardening resin at 200-400 degreeC, a crystal grain is formed by recrystallization. There is a possibility of coarsening.
Moreover, CCL which laminated | stacked copper foil and resin performs the heat processing for hardening resin at 200-400 degreeC. That is, the actual soft etching is performed on the copper foil subjected to the heat treatment.

Therefore, the average crystal grain size and tensile strength of the copper foil change before and after lamination with the resin. Then, the copper foil for flexible printed circuit boards concerning Claim 1 of this application has prescribed | regulated the copper foil of the state which received the hardening heat processing of resin after it became a copper clad laminated body laminated | stacked with resin. +
On the other hand, the copper foil for flexible printed circuit boards according to claim 4 of the present application defines a state when the heat treatment is performed on the copper foil before being laminated with the resin. This heat treatment at 300 ° C. for 30 minutes simulates the temperature condition for curing and heat-treating the resin during CCL lamination.
The atmosphere for the heat treatment is not particularly limited, and may be in the air or an inert gas atmosphere such as Ar or nitrogen.

  The copper foil of this invention can be manufactured as follows, for example. First, after adding the said additive to a copper ingot and melt | dissolving and casting, it hot-rolls, performs cold rolling and annealing, and can manufacture foil by performing the above-mentioned final cold rolling.

<Copper-clad laminate and flexible printed circuit board>
Also, (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) a thermoplastic adhesive of the same type as the base film is used. 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 the resin base material is obtained. Further, by laminating a base film obtained by applying an adhesive to the copper foil of the present invention, a copper clad laminate (CCL) comprising three layers of a copper foil, a resin base material, and an adhesive layer therebetween is obtained. During the production of these CCLs, the copper foil is heat-treated and recrystallized.
A circuit is formed on these using a photolithographic technique, a circuit is plated as needed, and a cover-lay film is laminated, and a flexible printed circuit board (flexible wiring board) is obtained.

Therefore, the copper clad laminate of the present invention is formed by laminating a copper foil and a resin layer. Moreover, the flexible printed circuit board of this invention forms a circuit in the copper foil of a copper clad laminated body.
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 for forming the resin layer may be applied to the surface of the copper foil 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 thermocompression bonded to the copper foil without using the adhesive. However, it is preferable to use an adhesive from the viewpoint of not applying excessive 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 component 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 polyimide adhesive here refers to the adhesive agent containing an imide bond, and polyether imide etc. are also included.

In addition, this invention is not limited to the said embodiment. Moreover, as long as there exists an effect of this invention, the copper alloy in the said embodiment may contain another component.
For example, the surface of the copper foil may be subjected to a surface treatment by roughening treatment, rust prevention treatment, heat resistance treatment, or a combination thereof.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to these. Each element shown in Table 1 was added to electrolytic copper with a purity of 99.9% or more, and cast in an Ar atmosphere to obtain an ingot. The oxygen content in the ingot was less than 15 ppm. The ingot is homogenized and annealed at 900 ° C., hot rolled and cold rolled to a thickness of 31 to 51 mm, and then subjected to one annealing, and then the surface is chamfered. A final cold rolling was performed at a degree η to obtain a copper foil sample having a final thickness of 17 μm.

<A. Evaluation of copper foil sample>
1. Conductivity Each of the copper foil samples described above was subjected to a heat treatment at 300 ° C. for 30 minutes in the atmosphere (simulating the temperature condition for curing the resin during CCL lamination), and then in accordance with JIS H 0505 by a four-terminal method. The electrical conductivity (% IACS) at 25 ° C. 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, the twins were measured as if they were separate crystal grains. The measurement area was 100 μm × 100 μm on the surface.

3. Tensile strength of copper foil About each copper foil sample after the said heat processing, the tensile strength was measured on the said conditions by the tensile test based on IPC-TM650.

<B. Evaluation of CCL>
4). Preparation of CCL (copper-clad laminate) Copper roughening plating was performed on one side of a copper foil sample (copper foil before heat treatment) that was not subjected to the heat treatment after the final cold rolling. Using copper: 10-25g / L, sulfuric acid: 20-100g / L as the 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 adhesion amount was 20 g / dm2.
The roughened plating surface of the copper foil sample was laminated on each adhesive surface of a polyimide film with a double-sided adhesive (product name “Upilex VT” manufactured by Ube Industries, Ltd., thickness 25 μm), and heated with a heating press (4 MPa). A CCL sample in which a copper foil was laminated on each side of the polyimide film was obtained by applying heat treatment at 30 ° C. for 30 minutes and bonding.

5). Skness Rsk
Soft etching was performed by immersing the CCL in an aqueous solution having a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L (liquid temperature: 25 ° C.) for 420 seconds. The skewness Rsk based on IS B 0601-2001 on 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 perpendicular direction of rolling, and the absolute value of each measurement value was measured. The average value was obtained.

6). Etchability Strip-like circuits of L / S (line / space) = 35/35 μm, 35/35 μm, 25/25 μm, 20/20 μm, and 10/10 μm were 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 (tough pitch copper foil, 17 μm thickness). Then, the etching factor (ratio represented by (etching depth / average upper and lower average etching width) of the circuit) and the linearity of the circuit were visually determined with a microscope and evaluated according to the following criteria. If evaluation is (circle), it is good.
○: Etching factor and circuit linearity are better than commercially available rolled copper foils △: Etching factor and circuit linearity are comparable to commercially available rolled copper foils ×: Etching factor compared to commercially available rolled copper foils And circuit linearity is poor

7). High-frequency transmission characteristics A microstrip line having an impedance of 50Ω and a length of 100 mm was formed by etching on the copper foil portion on one side of the above-mentioned CCL. Note that the copper foil on the opposite side of the CCL is not etched and becomes GND.
As a comparative example, a CCL was similarly produced from a commercially available rolled copper foil (tough pitch copper foil, 17 μm thickness), and the microstrip line was formed on the copper foil portion on one side of the CCL.
Then, using a network analyzer, S21 which is an S parameter (Scattering Parameter) of the microstrip line was measured at 60 GHz. S21 is expressed 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 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)}>-5 dB / mm
×: −5 dB / mm ≧ {(Absolute value of S21 of commercially available rolled copper foil) − (Absolute value of S21 of Example)}

  The obtained results 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 the skewness Rsk is 0.05 or less, soft etching properties are included. Excellent etching properties and excellent high-frequency transmission characteristics.

  On the other hand, in the case of Comparative Example 1 containing Ag but no additive element, and in Comparative Example 6 in which the total content of additive elements is less than the lower limit, the average crystal grain size of the copper foil is significantly increased to 4.3 μm. The coarseness was exceeded, the tensile strength was less than 230 MPa, and the absolute value of skewness Rsk was larger than 0.05. As a result, the etching property including soft etching property was inferior, and the high frequency transmission property was also inferior.

In the case of Comparative Examples 3, 4, and 7 in which the degree of work η in the final cold rolling is less than 7.51, the average crystal grain size of the copper foil becomes coarser than 4.3 μm, the tensile strength becomes less than 230 MPa, and the skewness Rsk The absolute value was greater than 0.05. As a result, the etching property including soft etching property was inferior, and the high frequency transmission property was also inferior.
Further, in the case of Comparative Example 4 in which the degree of work η in the final cold rolling is less than 7.51, but in the case of Comparative Example 4 which is 3.5 or more, the skewness Rsk is larger than 0.05, the etching property including the soft etching property is inferior, and the high frequency transmission property is also achieved. 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 is considered as follows. That is, in Comparative Examples 3 and 7 in which η is less than 3.5, the accumulation of strain during the final cold rolling process is small, and the nuclei of the recrystallized grains are reduced, so that the recrystallized grains are coarse. On the other hand, in the case of Comparative Example 4 in which η is 3.5 or more, although the strain was moderately accumulated during the final cold rolling process and the recrystallized grains became fine, the Rsk increased because the strain was locally present. . When η is 7.51 or more, the accumulated strain amount is further increased, and the strain exists uniformly, so that Rsk is considered to be small.

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

Claims (6)

For tough pitch copper standardized to JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1011),
0.001 to 0.05 mass% of Ag and 0.003 to 0.825 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 immersion for 420 seconds in an aqueous solution with a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L (liquid temperature 25 ° C.) The copper foil for flexible printed circuit boards which measured 16 times and averaged the absolute value of the measured value of each time is 0.05 or less. The copper foil is a rolled copper foil,
2. 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.   The copper clad laminated body formed by laminating | stacking the copper foil for flexible printed circuit boards of Claim 1 or 2, and a resin layer.   The flexible printed board formed by forming a circuit in the said copper foil using the copper clad laminated body of Claim 3.   The flexible printed circuit board according to claim 4, wherein L / S of the circuit is 35/35 to 10/10 (μm / μm).   The electronic device using the flexible printed circuit board of Claim 4 or 5.