JP6827022B2 - 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.

Flexible printed circuit boards (flexible wiring boards, hereinafter referred to as "FPCs") are widely used as electronic devices become smaller, thinner, and have higher performance.
FPC forms wiring by etching Copper Clad Laminate (copper-clad laminate, hereinafter referred to as CCL) in which copper foil and resin (polyimide, liquid crystal polymer, etc.) are laminated, and a resin layer called coverlay is formed on the wiring. It is covered with.

By the way, when the copper foil and the resin are laminated by thermocompression bonding or a casting method, the copper foil is heated to recrystallize and soften. Then, when the difference in the tensile strength of the copper foil before and after laminating becomes large, there is a problem that wrinkles are generated in the copper foil in the laminating line where laminating is performed.
Therefore, a technique has been developed in which the copper foil after the final cold rolling is subjected to heat treatment at 70 to 95 ° C. to remove strain, reducing thermal shrinkage during thermocompression bonding and improving dimensional stability (patented). Document 1).

JP-A-2017-179393

However, in the case of the technique described in Patent Document 1, although the dimensional stability after lamination is improved, the reduction of wrinkles in the copper foil is insufficient.
The present invention has been made to solve the above problems, and is a copper foil for a flexible printed circuit board that suppresses wrinkles of the copper foil when laminating a copper foil and a resin, and a copper-clad laminate using the same. The purpose is to provide flexible printed circuit boards and electronic devices.

As a result of various studies, the present inventors have found that wrinkles in the copper foil when laminating the copper foil and the resin can be suppressed by reducing the reduction rate of the tensile strength before and after the predetermined heat treatment to 10% or less. I found it.

That is, the copper foil for flexible printed circuit boards of the present invention is a copper foil containing Cu in an amount of 99.9% by mass or more and P in an amount of 0.0005% by mass or more and 0.0220% by mass or less, and the balance is unavoidable impurities. The rate of decrease in tensile strength before and after heat treatment at 400 ° C. for 1 second is 10% or less, the conductivity after the heat treatment is 80% IACS or more, and the tensile strength before the heat treatment is 180 MPa or more and less than 320 MPa.

The copper foil for a flexible printed circuit board of the present invention may be made of tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1020).
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, it is possible to obtain a copper foil for a flexible printed circuit board in which wrinkles of the copper foil are suppressed when the copper foil and the resin are laminated.

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 Cu in an amount of 99.9% by mass or more and P in an amount of 0.0005% by mass or more and 0.0220% by mass or less, and the balance is unavoidable impurities. It is preferable that Cu is 99.96% by mass or more .

If the P content exceeds 0.0220 mass% (220 mass ppm), the conductivity will decrease, making it unsuitable for flexible printed circuit boards.

The copper foil according to the present invention is composed of tough pitch copper (TPC) standardized for JIS-H3100 (C1100) or oxygen-free copper (OFC) of JIS-H3100 (C1020), and P is 0.0005% by mass or more as an additive element. The composition may contain 0.0220% by mass or less.

<Thickness of copper foil>
The thickness of the copper foil shall be 0.018 mm or less.
If the thickness of the copper foil is larger than 0.018 mm, it is difficult to miniaturize the circuit by etching, which is not suitable for FPC. The lower limit of the thickness of the copper foil is not limited, but for manufacturing purposes, for example, 0.003 mm is exemplified.

<Reduction rate of tensile strength before and after heat treatment>
The rate of decrease in the tensile strength of the copper foil before and after heat treatment at 400 ° C. for 1 second is 10% or less.
When the copper foil and the resin are laminated by thermocompression bonding or a casting method, the copper foil is heated to recrystallize and soften. If the difference in tensile strength between the copper foils before and after laminating becomes large, wrinkles occur in the copper foils during laminating.
Therefore, if the rate of decrease in the tensile strength of the copper foil before and after the heat treatment is controlled to 10% or less, the difference in the tensile strength of the copper foil before and after laminating becomes small, and wrinkles of the copper foil during laminating can be suppressed.
The reason why the heat treatment conditions were set at 400 ° C for 1 second is that the harshest conditions among the laminations generally performed by the following roll-to-roll method are assumed, and the desired performance can be satisfied under these conditions. For example, it can be considered that heating at the time of general CCL lamination is sufficient.
In order to prevent oxidation of the copper foil surface due to heat treatment, the heat treatment atmosphere is preferably a reducing or non-oxidizing atmosphere, for example, a vacuum atmosphere, argon, nitrogen, hydrogen, carbon monoxide, etc., or these. The atmosphere may be a mixed gas. The rate of temperature rise until reaching 400 ° C. may be between 100 and 300 ° C./min.

<Conductivity and tensile strength>
The conductivity after the heat treatment is 80% IACS or higher. If the conductivity is less than 80% IACS, it is not suitable for flexible printed circuit boards.
The tensile strength before the heat treatment is 180 MPa or more and less than 320 MPa, preferably 210 or more and less than 320 MPa.
If the tensile strength before the heat treatment is less than 180 MPa, the copper foil is likely to break when the copper foil and the resin are laminated.
When the tensile strength before the heat treatment is 320 MPa or more, the tensile strength is significantly reduced after the heat treatment, and the rate of decrease in the tensile strength exceeds 10%.

The copper foil of the present invention can be produced, for example, as follows. First, P can be added to a copper ingot, melted and cast, then hot-rolled, cold-rolled and annealed to produce a foil.
Here, for example, by controlling the conditions of (1) the workability η of the final cold rolling and (2) the softening treatment after the final cold rolling, the reduction rate of the tensile strength of the copper foil before and after the heat treatment is ensured. Can be less than 10%.

By controlling the workability η of the final cold rolling, strain can be sufficiently accumulated, the decrease in tensile strength after the heat treatment is suppressed, and the rate of decrease in tensile strength is reduced to 10% or less. Can be done.
The workability η of the final cold rolling varies depending on the manufacturing conditions of the copper foil and is not limited, but for example, η may be 6.0 or more.
The workability η is represented by η = ln (A0 / A1), where the thickness of the material immediately before cold rolling before final annealing is A0 and the thickness of the material immediately after cold rolling before final annealing is A1.
If the workability η of the final cold rolling is too low, it is difficult to sufficiently introduce strain during the final cold rolling, the tensile strength after the heat treatment is greatly reduced, and the rate of decrease in the tensile strength is 10%. Exceed. The upper limit of the degree of processing η is not particularly limited, but is practically about 7.45.

By performing the softening treatment after the final cold rolling, the copper foil is preheated and recrystallized and softened. As a result, the degree of softening of the copper foil when laminating the copper foil and the resin is reduced, and the reduction rate of the tensile strength can be reduced to 10% or less.
The temperature of the softening treatment is preferably 300 ° C. or higher, more preferably 300 ° C. to 400 ° C. The softening treatment time may be several tens of seconds to several minutes, and is preferably 1 to 2 minutes, for example.
If the temperature of the softening treatment is too low, the copper foil is not sufficiently softened and it becomes difficult to reduce the rate of decrease in tensile strength to 10% or less. If the temperature of the softening treatment is too high, the copper foil will be softened too much and its strength will decrease.
As a method of performing the softening treatment, a dedicated heat treatment line may be provided. For example, when a surface treatment such as a roughening treatment is applied to the surface of a copper foil, a pretreatment is a dipping step in a degreasing solution and a subsequent drying step. There is. Therefore, if the drying temperature in this drying step is set to 300 ° C. or higher, the softening treatment can be combined with the drying treatment after degreasing.

<Copper-clad laminate and flexible printed circuit board>
Further, for example, (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 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 from 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 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.
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. P was added to 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., then hot-rolled and cold-rolled, and then intermediate-annealed.
Then, the oxide scale generated on the surface was removed, and the final cold rolling was performed at the workability η shown in Table 1 to obtain a foil having a desired final thickness. The obtained foil was passed through a degreasing liquid tank for degreasing, and then dried at the temperatures shown in Table 1 for roughening treatment and rust prevention treatment. The time for passing the foil through the degreasing drying tank was 1 minute.

<Evaluation of copper foil sample>
1. 1. Conductivity For each copper foil sample after passing through the degreasing drying tank, the conductivity (% IACS) at 25 ° C. was measured by the 4-terminal method based on JIS H 0505.
If the conductivity is 80% IACS or more, the conductivity is good.
2. 2. Tensile strength For each copper foil sample after passing through the degreasing drying tank, the tensile strength of the copper foil before and after heat treatment (400 ° C x 1 second) was measured by a tensile test based on IPC-TM650 to obtain a test piece width of 12.7 mm. The measurement was carried out by a tensile test at room temperature (15 to 35 ° C.), a tensile speed of 50.8 mm / min, and a gauge length of 50 mm in a direction parallel to the rolling direction (or MD direction) of the copper foil.

3. 3. Presence or absence of wrinkles during laminating Using a roll-to-roll type laminating device (a roll-shaped copper foil and a roll-shaped resin film are laminated and then wound back on a roll), the wrinkles of the copper-clad laminate after laminating are removed. The presence or absence of occurrence was judged.
To check for wrinkles, use a shape analysis laser microscope (manufactured by KEYENCE, product name: VK-X1000) to determine that wrinkles have occurred when the difference in unevenness on the surface of the copper-clad laminate on the copper foil side is 3 μm or more. did.
The unevenness difference is the value (Sz-Sa) obtained by subtracting the arithmetic mean height Sa specified in ISO-25178 from the maximum height Sz specified in ISO-25178.

The results obtained are shown in Table 1.

As is clear from Table 1, in each example in which a predetermined amount of P is contained and the rate of decrease in tensile strength before and after heat treatment is 10% or less, the copper foil is formed when laminated on a copper-clad laminate. No wrinkles occurred .

On the other hand, in the cases of Comparative Examples 1 to 4, 6 and 8 in which the reduction rate of the tensile strength before and after the heat treatment exceeded 10%, wrinkles were generated in the copper foil when laminated on the copper-clad laminate.
In Comparative Examples 1 to 3, since the drying temperature in the degreasing drying tank after the final cold rolling was less than 300 ° C., the tensile strength before the heat treatment was 320 MPa or more, and the difference from the tensile strength after the heat treatment was large. It became too much, and the rate of decrease in tensile strength exceeded 10%.

The ratio Comparative Examples 6 does not contain P, because the drying temperature in the degreasing drying tank after the final cold rolling is lower than 300 ° C., the tensile strength before heat treatment becomes more 320 MPa, the tensile strength after heat treatment The difference became too large, and the rate of decrease in tensile strength exceeded 10%. Here, in Comparative Example 6, the foil passing time of the degreasing drying tank was set to 24 hours, and the softening treatment was performed at a low temperature for a long time under the same conditions as in Patent Document 1.
In Comparative Example 8, since the workability η of the final cold rolling is less than 6.0, strain is not sufficiently accumulated, the degree of decrease in tensile strength before and after the heat treatment is large, and the rate of decrease in tensile strength is high. It exceeded 10%.

Claims (5)

A copper foil containing 99.9% by mass or more of Cu, 0.0005% by mass or more and 0.0220% by mass or less of P, and the balance being unavoidable impurities.
Thickness 0.018 mm or less,
The rate of decrease in tensile strength before and after heat treatment at 400 ° C for 1 second is 10% or less.
Conductivity after the heat treatment is 80% IACS or more,
A copper foil for a flexible printed circuit board having a tensile strength of 180 MPa or more and less than 320 MPa before the heat treatment. The flexible printed substrate according to claim 1, wherein P is contained in 0.0005% by mass or more and 0.0220% by mass or less of P in tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper specified in JIS-H3100 (C1020). For copper foil. 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 in the copper-clad laminate according to claim 3. An electronic device using the flexible printed circuit board according to claim 4.