JP6856688B2 - 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 disk-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, thinning etching may be performed.

By the way, as electronic devices become smaller, thinner, and have higher performance, the flexibility of FPC is further required. Therefore, a technique for improving flexibility by regulating the average crystal grain size and the maximum crystal grain size of copper foil has been reported (Patent Document 1). Further, a technique for improving MIT flexibility by defining the relationship between the plate thickness and the elongation at break has been reported (Patent Document 2).
Further, as electronic devices become smaller, thinner, and have higher performance, miniaturization of FPC circuit width and space width (for example, about 20 to 30 μm) is also required.

Japanese Unexamined Patent Publication No. 2016-188415 Japanese Unexamined Patent Publication No. 2018-131653

However, when the FPC circuit is miniaturized, when the FPC is bent, low-strain repeated deformation is applied to the circuit (copper foil), the surface roughness becomes large, stress is concentrated in the recesses, and the flexibility is lowered. There's a problem.
That is, when the circuit width is sufficiently wide with respect to the thickness of the copper foil, the deformation in the direction parallel to the bending direction is dominant, but in the case of a fine circuit, the value of the copper foil (thickness / width) becomes large. Therefore, it is necessary to consider the deformation in the width direction perpendicular to the bending direction. In addition, it is generally considered that the vicinity of the end portion is less constrained from the surroundings and is more easily deformed than the central portion in the width direction of the circuit, but in a fine circuit, the proportion of the region that can be regarded as the vicinity of the easily deformable end portion is large. .. For the above reasons, it is considered that the flexibility becomes more severe as the circuit width becomes narrower.
The present invention has been made to solve the above problems, and is a copper foil for a flexible printed circuit board having excellent flexibility after forming a fine circuit, a copper-clad laminate using the copper foil, a flexible printed circuit board, and an electronic device. The purpose is to provide.

As a result of various studies by the present inventors, an increase in the surface roughness of the copper foil (circuit) due to repeated deformation that reduces the flexibility of the fine circuit of the FPC is related to the strain rate of the final pass in the final cold rolling. We found that, and defined the range of surface roughness that does not reduce the flexibility.

That is, the copper foil for a flexible printed circuit board of the present invention is a copper foil composed of 99.0% by mass or more of Cu and inevitable impurities in the balance, and has a width of 12.7 mm and a length of 200 mm in which a circuit is formed from the copper foil. The surface roughness Ra of the circuit surface after performing 2000 IPC sliding bends with a radius of curvature R = 2.0 using a two-layer single-sided CCL sample is 0.030 μm or more and 0.400 μm or less. However, in the two-layer single-sided CCL sample, after copper roughening plating is performed on one side of the copper foil, the copper roughening plating side of each of the two copper foils is directed to both sides of a polyimide film having a thickness of 25 μm. The layers are laminated and bonded at 4 MPa with a heating press at 300 ° C. for 30 minutes, and the copper foil on one side is completely removed by etchout to prepare a two-layer single-sided CCL. Then, a circuit extending along the rolling direction with a line width of 25 μm is etched and formed on the surface of the two-layer single-sided CCL sample on the copper foil side with eight circuits and a circuit interval of 125 um.

In the copper foil for a flexible printed circuit board of the present invention, it is preferable that the surface roughness Ra of the circuit surface before the IPC sliding bending is 0.010 μm or more and 0.200 μm or less.
The copper foil for a flexible printed circuit board of the present invention is preferably made of tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1020).
The copper foil for flexible printed circuit boards of the present invention further contains at least one or two or more selected from the group consisting of P, Ag, Si, Ge, Al, Ga, Zn, Sn and Sb as additive elements in total. It is preferably contained in an amount of 0.7% by mass 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 in the copper-clad laminate.

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 flexibility after forming a fine circuit can be obtained.

It is a figure which shows the bending test method.

Hereinafter, embodiments of the copper foil according to the present invention will be described. In the present invention,% means mass% unless otherwise specified.
First, the evaluation of flexibility after forming a fine circuit will be described.
As described above, in the case of a fine circuit, since the value of the copper foil (thickness / width) becomes large, the deformation in the width direction perpendicular to the bending direction becomes large, and the ratio of the region at the end of the circuit which is easily deformed. Becomes larger. As a result, when the circuit (copper foil) is repeatedly deformed with low strain, the surface roughness becomes large, stress is concentrated in the recesses, and the flexibility is lowered.

Therefore, a fine circuit was simulated, the surface roughness of the circuit (copper foil) before and after the predetermined bending test was measured, and the Ra after bending was defined as 0.030 μm or more and 0.400 μm or less.
By controlling the Ra after bending within the above range, the stress applied to the surface can be uniformly dispersed, and excellent flexibility is exhibited.
If Ra after bending is less than 0.030 μm, the surface is too smooth, stress is concentrated on small undulations (oil pits, etc.) existing before deformation, and the flexibility is rather lowered. When Ra after bending exceeds 0.400 μm, the surface roughness becomes large, stress is concentrated in the recesses, and the flexibility is lowered.

Further, it is preferable that Ra before bending is 0.010 μm or more and 0.200 μm or less.
If the Ra before bending is less than 0.010 μm, the Ra after bending tends to be less than 0.030 μm, and if the Ra before bending exceeds 0.200 μm, the Ra after bending tends to exceed 0.400 μm.

A simulated microcircuit is produced as follows. First, copper roughening plating is performed on one side of the copper foil after the final cold rolling, and the copper foil is laminated on both sides of the polyimide film (thickness 25 μm) on the roughened plating side of the copper foil, and bonded by a heating press (4MPa). Obtain a three-layer double-sided copper foil CCL sample. A heat treatment of 300 ° C. for 30 minutes is applied when laminating the film. Further, out of the three-layer double-sided copper foil CCL sample, the single-sided copper foil is completely removed by etchout to prepare a two-layer single-sided CCL.
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. Assuming this heat treatment, the temperature was set to 300 ° C. × 30 minutes.

A circuit extending along the rolling direction with a line width of 25 μm is etched and formed on the copper foil side surface of the two-layer single-sided CCL sample with eight circuits and a circuit interval of 125 um. The etching factor EF of the circuit shall be 4.0 or more.

The rough plating is not particularly limited as long as the copper foil and the resin can be prevented from peeling during the bending test, and the plating conditions and the like are not particularly limited. For example, the following are exemplified as those generally used for FPC applications. Plating bath composition: Cu 15 g / L, Co 8.5 g / L, Ni 8.6 g / L, plating solution pH: 2.5, plating temperature: 38 ° C, current density: 20 A / dm ² , plating time: 2.0 seconds Etching The liquid may be adjusted so that, for example, CuCl ₂ -2H ₂ O: 3 mol / L, HCl: 4 mol / L, the etching temperature is, for example, 50 ° C., and the etching time is 25 um.

Then, the CCL in which this circuit is formed is subjected to sliding bending 2000 times by the IPC (American Print Circuit Industry Association) bending test apparatus shown in FIG. 1, and then the surface roughness in the direction parallel to the circuit direction is roughened using a laser microscope. Measure Ra.
This device has a structure in which the vibration transmission member 3 is coupled to the oscillation drive body 4, and the FPC 1 is fixed to the device at a total of four points, the screw 2 portion indicated by the arrow and the tip portion of the vibration transmission member 3. To. When the vibration transmitting member 3 is driven up and down, the intermediate portion of the FPC 1 is bent like a hairpin with a predetermined radius of curvature r. In this test, bending is repeated under the following conditions.
The test conditions are as follows: test piece width: 12.7 mm, test piece length: 200 mm, test piece sampling direction: sampled so that the length direction of the test piece is parallel to the rolling direction, radius of curvature r : 2 mm, vibration stroke: 20 mm, vibration speed: 100 times / minute, bending direction: FPC1 with copper foil inside.

Surface roughness (arithmetic mean roughness) Ra is the center line average roughness calculated according to JIS B 0601-1994 from the unevenness profile of the copper foil surface.
For the measurement of the surface roughness Ra, a shape analysis laser microscope VK-X1050 manufactured by KEYENCE Corporation can be used. The measurement conditions are an objective lens: 50 times and an intermediate lens: 24 times. Ten line roughness analyzes are performed parallel to the circuit direction, and the average value is adopted as the surface roughness Ra. The interval of line analysis is adjusted to equal intervals so that the distance between the 1st and 10th lines is 80% or more of the width of the circuit surface.

<Composition>
The copper foil according to the present invention is composed of 99.0% by mass or more of Cu and unavoidable impurities in the balance.
Further, as the additive element, at least one or two or more selected from the group consisting of P, Ag, Si, Ge, Al, Ga, Zn, Sn and Sb are added to the above composition in a total of 0.7% by mass or less. When it is contained, the recrystallized grains can be made finer and the increase in surface roughness due to repeated deformation can be suppressed.
Since the additive element increases the frequency of dislocation entanglement during cold rolling, the recrystallized grains can be made finer.
If the above additive elements are contained in excess of 0.7% by mass in total, the conductivity is lowered and the copper foil for a flexible substrate may not be suitable. Therefore, the upper limit is 0.7% by mass. The lower limit of the content of the additive element is not particularly limited, but for example, it is industrially difficult to control each element to be smaller than 0.0005% by mass, so the lower limit of the content of each element is preferably 0.0005% by mass.

The copper foil according to the present invention may be composed of tough pitch copper (TPC) standardized for JIS-H3100 (C1100) or oxygen-free copper (OFC) of JIS-H3100 (C1020).
Further, the composition may be such that P is contained in the TPC or OFC.

The copper foil of the present invention can be produced, for example, as follows. First, a copper ingot is melted and cast, then hot-rolled, cold-rolled and annealed, and finally cold-rolled to produce a foil.
Here, if the strain rate of the final pass in the final cold rolling is increased, it is possible to suppress an increase in the surface roughness of the circuit (copper foil) due to repeated deformation. The reason for this is that when the strain rate is increased, a large strain is concentrated and accumulated on the rolled surface of the copper foil as compared with the inside of the copper foil. As a result, when the copper foil is recrystallized, fine crystal grains are arranged in random directions on the rolled surface, and the deformation due to repeated deformation is not concentrated locally and the surface is prevented from becoming rough and smooth. Be maintained. The strain rate of the final pass in the final cold rolling ^{is preferably 7.4 × 10 3} (1 / s) or more. However, if the strain rate is too high, the copper foil may break during rolling and the manufacturability may decrease. Therefore, 9.5 × 10 ³ (1 / s) should be set as the upper limit of the final pass strain rate.

<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 the same type of thermoplastic adhesive 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 and a heat film may be formed. 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 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 the 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. Further, electrolytic copper foil may be used.
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 the electrolytic copper to obtain the composition shown in Table 1, 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., hot-rolled, then cold-rolled and recrystallized annealed repeatedly, and finally recrystallized and finally cold-rolled to obtain rolled copper foil.
A CCL was prepared on the obtained rolled copper foil as described above.

<Surface roughness of copper foil (circuit) before and after bending test>
It was measured as described above.
<Bending fatigue life>
In the IPC sliding bending test, the sample prepared by the same method as the CCL (two-layer single-sided CCL) for which the circuit for the bending test has been prepared described above exceeds 10% from the initial electrical resistance value flowing through both ends of the circuit. The time when it became high was defined as the bending fatigue life. The measurement conditions for determining the bending fatigue life are as follows: test piece width: 12.7 mm, test piece length: 200 mm, test piece collection direction: so that the length direction of the test piece is parallel to the rolling direction. Collection, radius of curvature r: 5 mm, vibration stroke: 20 mm, vibration speed: 1500 times / minute, bending direction: FPC (two-layer single-sided CCL) 1 with copper foil inside.
When the bending fatigue life was 80,000 times or more, it was evaluated as having excellent flexibility, and when the bending fatigue life was less than 80,000 times, it was evaluated as being inferior in flexibility.

The results obtained are shown in Table 1.

As is clear from Tables 1 and 2, in each example in which the surface roughness Ra of the circuit surface after the bending test was 0.030 μm or more and 0.400 μm or less, the bending fatigue life was excellent.

In the case of Comparative Example 1 in which Ra was less than 0.030 μm, the bending fatigue life was inferior. It is considered that this is because the surface is too smooth and the stress is concentrated on the small undulations (oil pits, etc.) that existed before the deformation.
In Comparative Examples 2 to 6 in which Ra exceeded 0.400 μm, the bending fatigue life was inferior. In the case of Comparative Examples 2 to 6, it is considered that the strain rate of the final pass in the final cold rolling was lower than that of the examples, and the strain was not sufficiently accumulated on the rolled surface.

Claims (7)

A copper foil consisting of 99.0% by mass or more of Cu and unavoidable impurities in the balance.
Using a two-layer single-sided CCL sample having a width of 12.7 mm and a length of 200 mm formed from the copper foil, the surface of the circuit surface is roughened after performing 2000 IPC sliding bends with a radius of curvature R = 2.0. Copper foil for flexible printed circuits with a Ra of 0.030 μm or more and 0.400 μm or less.
However, in the two-layer single-sided CCL sample, after copper roughening plating is performed on one side of the copper foil, the copper roughening plating side of each of the two copper foils is directed to both sides of a polyimide film having a thickness of 25 μm. The layers are laminated and bonded at 4 MPa with a heating press at 300 ° C. for 30 minutes, and the copper foil on one side is completely removed by etchout to prepare a two-layer single-sided CCL. Then, a circuit extending along the rolling direction with a line width of 25 μm is etched and formed on the surface of the two-layer single-sided CCL sample on the copper foil side with eight circuits and a circuit interval of 125 um. The copper foil for a flexible printed circuit board according to claim 1, wherein the surface roughness Ra of the circuit surface before the IPC sliding bending is 0.010 μm or more and 0.200 μm or less. The copper foil for a flexible printed circuit board according to claim 1, which is made of tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1020). Further, as an additive element, at least one or two or more selected from the group consisting of P, Ag, Si, Ge, Al, Ga, Zn, Sn and Sb are contained in an amount of 0.7% by mass or less in total. Item 2. The copper foil for a flexible printed circuit board according to Item 1 or 2. A copper-clad laminate obtained by laminating the copper foil for a flexible printed circuit board according to any one of claims 1 to 4 and a resin layer. A flexible printed circuit board formed by forming a circuit on the copper foil in the copper-clad laminate according to claim 5. An electronic device using the flexible printed circuit board according to claim 6.

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