JP2022095855A - Copper foil for flexible printed substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a copper foil for a flexible printed substrate having improved bendability of CCL.

SOLUTION: A copper foil for flexible printed substrate is a rolled copper foil containing 99.9 mass% or more of Cu and, as an additional element, 0.0005-0.0220 mass% of P, with the balance being inevitable impurities. A crystal orientation density in Copper orientation is less than 10, and a crystal orientation density in Brass orientation is less than 20.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2022,JPO&INPIT

Description

The present invention relates to a copper foil suitable for use in a wiring member such as a flexible printed substrate.

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 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 covering the wiring with a resin layer called a coverlay.

By the way, the copper foil which is the conductor of the FPC is required to have bending resistance which is hard to break even if it is repeatedly bent, and further high-speed transmission characteristic.
Normally, copper foil for FPC is subjected to roughening treatment to form fine metal particles called roughened particles on the surface, and various surface treatments are applied to impart heat resistance, chemical resistance, and adhesiveness. Be given. Then, CCL is formed by a method of pressure laminating this copper foil with a film-shaped insulating resin base material, a method of applying an insulating resin base material to the copper foil, and then drying or high-temperature treatment. The copper foil portion is etched to form a circuit, and the FPC is manufactured.

As a method for improving the bendability of the rolled copper foil, it is known to increase the final reduction rate to develop a cubic texture, which is the Cube orientation of the recrystallized texture, but when the final reduction rate is high, it is known. There is a problem that strain is accumulated and the softening temperature is lowered.
Therefore, as a measure to improve the bendability of rolled copper foil without increasing the final reduction ratio, a technology has been developed in which the crystal orientation density in the Copper orientation is 10 or more and the crystal orientation density in the Brass orientation is 20 or more. (Patent Document 1).

Japanese Patent No. 6442020

By the way, once a single copper foil is bent, the bent portion is work-hardened, and the copper foil is bent while avoiding the work-hardened portion at the next bending. On the other hand, when the CCL in which the copper foil is laminated with the support (resin) is bent, the bent portion is restrained by the resin, so that the same portion is bent, and the bending test is stricter than that of the copper foil alone.
When the crystal orientation density of the Copper orientation is increased as in the technique described in Patent Document 1, the Cube orientation develops in the recrystallized texture and the recrystallized grain size increases, so that the bendability of the CCL decreases. There was found.
Further, when the crystal orientation density of the Brass orientation is high, the texture consisting of the Brass orientation develops in the recrystallized texture. The Brass azimuth is an azimuth in which distortion is easily accumulated, and the strain is released at the time of bending to easily generate cracks, which is inferior in the bending property of CCL.

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 improved bendability of a CCL.

As a result of various studies by the present inventors, in order to improve the bendability of CCL, it is necessary to suppress the development of the Cube orientation after recrystallization, that is, to suppress the abundance ratio of the Copper orientation at the stage of the rolled structure. I found. In addition, it was decided to reduce the crystal orientation density of the Brass orientation, which deteriorates the bendability of the CCL.

That is, the copper foil for a flexible printed substrate of the present invention is a rolled copper foil containing 99.9% by mass or more of Cu and 0.0005 to 0.0220% by mass of P as an additive element, and the balance is unavoidable impurities. The crystal orientation density of is less than 10, and the crystal orientation density of the Brass orientation is less than 20.
The Copper orientation and the Brass orientation are defined by {112} <111> and {110} <112>, respectively.

The copper foil for a flexible printed substrate of the present invention may contain 0.0005 to 0.0220% by mass of P as an additive element in tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper in JIS-H3100 (C1020). good.
The copper foil for a flexible printed substrate of the present invention preferably has a surface roughness Sa of less than 0.2 μm.
The copper foil for a flexible printed circuit board of the present invention is preferably 12 μm or less in thickness.

According to the present invention, a copper foil for a flexible printed circuit board having improved bendability of a CCL can be obtained.

It is a figure which shows the method of determining the heat treatment temperature for final annealing of a copper strip. It is a figure which shows the bending test.

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 99.9% by mass or more of Cu and 0.0005 to 0.0220% by mass of P as an additive element, and the balance is unavoidable impurities. It is preferable that Cu is 99.96% by mass or more.
When P is contained as an additive element, the crystal orientation density of Copper orientation can be reduced to less than 10.

When the P content is less than 0.0005 mass% (5 mass ppm), the crystal orientation density in the Copper orientation becomes 10 or more, and the bendability of the CCL is lowered. If the P content exceeds 0.0220 mass% (220 mass ppm), the conductivity will decrease, making it unsuitable for flexible printed substrates.

The copper foil according to the present invention may be composed of tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1020) containing 0.0005 to 0.0220% by mass of P as an additive element. good.

<Crystal orientation density>
The crystal orientation density of the Copper orientation of the copper foil is less than 10. As described above, in order to improve the bendability of CCL, it is necessary to suppress the development of the Cube orientation after recrystallization, that is, to suppress the abundance ratio of the Copper orientation at the stage of the rolled structure.
When the crystal orientation density of the Copper orientation is 10 or more, the Cube orientation develops after recrystallization, the recrystallization grain size becomes large, and the bendability of the CCL decreases.
The crystal orientation density of the Brass orientation of the copper foil is less than 20. The Brass orientation is easy to accumulate strain, and when the crystal orientation density of the Brass orientation is 20 or more, the strain is released at the time of bending and cracks are likely to occur, and the bending property of the CCL is lowered.

The crystal orientation density was calculated using a perfect pole figure. The perfect pole figure was calculated using the incomplete pole figure. The incomplete pole figure can be obtained by measuring the X-ray pole figure of the rolled texture. Rigaku's RINT2500 (trade name) was used for the X-ray pole figure measurement, and the X-ray pole figure measurement was performed on the (111), (200), and (220) surfaces of the copper foil under the conditions of Schultz's reflection method. .. The measurement conditions are incident X-ray source: Co, tube voltage: 30 kV, tube current: 100 mA, divergence slit: 1 °, scattering slit: 0.05 mm, light receiving slit: 0.05 mm, divergence vertical limiting slit: 1.2 mm, The scanning speed is 360 ° / min and the step width is 5 °. The range of 2θ (θ is the diffraction angle) in which the diffraction intensity was measured on each surface is (111): 48.0 to 54.0 °, (200): 56.5 to 62.5 °, (220): 86. It is 0.0 to 92.0 °.

The positive point data processing software attached to RINT2500 manufactured by Rigaku was used for the data processing of the incomplete pole figure obtained by the X-ray pole figure measurement. The processing conditions are RD correction, smoothing, background processing, and random standardization. The processed data was converted into text using Rigaku's ASC conversion software.

Standard ODF Ver2.4 was used for the conversion from the incomplete pole figure converted into text data to the perfect pole figure and the extraction of the crystal orientation distribution function. Since the Copper orientation and the Brass orientation appear at multiple points in the Euler angle space, in the present invention, the Euler angles of the crystal orientation density functions f (gCopper) and f (gBrass) are gCopper = (90 °, 35 °, 45 °). ) And gBrass = (35 °, 45 °, 90 °) were adopted.

The surface roughness Sa of the copper foil is preferably less than 0.2 μm. When the surface roughness Sa is less than 0.2 μm, the transmission loss when the copper foil is made into FPC can be suppressed.
Surface roughness S _a is specified by ISO 25178.

The thickness of the copper foil is preferably 12 μm or less, which is the nominal thickness specified in JIS C6515. The thinner the thickness, the smaller the stress applied to the copper foil, which contributes to the improvement of bendability and the miniaturization, thinning, and weight reduction of portable devices.

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, and then hot rolled, and cold rolling and annealing are repeated to produce a foil. Annealing may be performed once.
Here, the final annealing is called final annealing, and cold rolling before and after final annealing is called cold rolling before final annealing and cold rolling after final annealing, respectively.

Here, the rolling reduction before final annealing is preferably 80% or more. This is to accumulate sufficient strain in the copper strip.
The reduction rate (final reduction rate) of cold rolling after final annealing is preferably 95% or more, more preferably 99% or more. When the rolling reduction is 95% or less, the strain accumulated in the copper foil is non-uniform, so that the rolled structure becomes non-uniform. Moreover, when the reduction rate is 95% or more, the growth in the Copper direction is suppressed.
The rolling reduction ratio (R) is represented by the rolling reduction ratio R = {(T ₀ −T ₁ ) / T ₀ } × 100, where the foil thickness before rolling is T ₀ and the foil thickness after rolling is T ₁ .

By controlling the heat treatment conditions of the final annealing, the Copper orientation and the Brass orientation can be suppressed.
Here, the heat treatment temperature for final annealing is set to a temperature in the “recrystallization” region. Assuming that the heat treatment temperature of the final annealing is the temperature in the "recovered" state, the Brass orientation is developed by cold rolling after the final annealing. If the heat treatment temperature of the final annealing is set to the temperature of the "grain growth" state so that the temperature is higher than that of the "recovery" state, the Copper orientation is developed by cold rolling after the final annealing. Therefore, the heat treatment temperature for final annealing is set to be in the "recrystallization" range.

The "recrystallized" region temperature can be determined by the following method.
First, the copper strips before the final annealing after cold rolling and before the final annealing are changed in 10 ° C. increments of 25, 100, 150 ° C. and 200 to 260 ° C., and the temperature is changed to 280, 300, 350, 380 and 400 ° C. The tensile strength (N / mm2) when heat-treated in a nitrogen atmosphere for 30 minutes is measured in accordance with JIS Z2241.
Next, as shown in FIG. 1, the measurement data is plotted on a graph with the heat treatment temperature on the X-axis and the tensile strength on the Y-axis. The point where the tensile strength drops sharply to 10 MPa / 10 ° C or higher between adjacent plots from the low temperature side (25 ° C.) to the high temperature side is defined as inflection point 1. The point where the tensile strength is less than 10 MPa / 10 ° C. between the adjacent plots from the inflection point 1 toward the high temperature side is defined as the inflection point 2. The point where the tensile strength drops to 5% or more between adjacent plots from the inflection point 2 toward the high temperature side is defined as the inflection point 3.
The temperature lower than the inflection point 2 is defined as the "recovery region", the temperature at which the inflection point 2 or more is less than the inflection point 3 is defined as the "recrystallization region", and the temperature higher than the inflection point 3 is defined as the "grain growth region".

In this way, it is preferable that the heat treatment temperature of the final annealing is set to be in the “recrystallization region”, and the final annealing is performed in a nitrogen atmosphere with a holding time of 1 to 30 minutes.

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.
An ingot was prepared by adding the elements shown in Table 1 to oxygen-free copper (JIS-H3100C1020). This ingot was hot-rolled and cold-rolled at around 900 ° C., and then annealed to perform surface milling to remove the oxide scale on the surface. After that, it was cold-rolled before final annealing by a multi-stage cold rolling machine until the thickness of the rolled copper strip became 2.0 mm. Then, the final annealing was performed for 30 minutes in a nitrogen atmosphere at the heat treatment temperature of the final annealing determined by the above method (the temperature of the inflection point 2 which is the lowest temperature in the recrystallization region of FIG. 1). Then, cold rolling was performed after final annealing at a rolling reduction of 99.4% to the final copper foil thickness of 12 μm.

<Evaluation of copper foil sample>
1. 1. Conductivity For each copper foil sample after the final cold rolling, the conductivity (% IACS) at 20 ° C. was measured by the 4-terminal method based on JIS H 0505.
If the conductivity is greater than 80% IACS, the conductivity is good.
2. 2. Surface roughness of copper foil Sa
The surface roughness S _a of each copper foil sample after the final cold rolling was measured according to ISO 25178.
The surface roughness Sa of the copper foils of each Example and Comparative Example was 0.1.
3. 3. Crystal orientation density (Copper orientation and Brass orientation)
It was measured as described above.

4. Bending resistance characteristics A flexible printed wiring board was prepared from each copper foil sample after the final cold rolling, and a bending test was performed to evaluate the bending resistance characteristics.
The flexible printed wiring board was manufactured as follows. Copper foil samples were laminated on both sides of a polyimide resin film (FRS-142 # SW manufactured by Kaneka Corporation; thickness 25 um), and heated at 360 ° C. for 5 minutes with a vacuum heat press to prepare a copper-clad laminate. The copper foil on one side of this copper-clad laminated plate is removed by etching on the entire surface, and the copper foil on the other side is L (line) / S (space) = 300 μm / so as to be parallel to the MD (rolling parallel direction). Eight 300 μm circuits were formed to form a flexible printed wiring board.

A bending test was performed as shown in FIG. First, as shown in (A), the flexible printed wiring board F was bent as in (B) with the copper side facing outward by a bending jig. Subsequently, the bent flexible printed wiring board F was opened using a return jig (C), and the opened bent portion was returned flat (D). (A) to (D) were made one 180 ° close contact bending, and this was repeated.

The number of bends was determined as follows. That is, a constant current is passed through the copper foil circuit of the flexible printed wiring board for each 180 ° close contact bending, the voltage value required to pass the current is measured, and the measured voltage value is used to determine the flexible printed wiring board. The resistance value of the copper foil circuit was calculated. When the calculated resistance value was 120% or more of the initial value (the resistance value before bending), it was determined that the fracture occurred.
The number of bendings was the number of 180 ° close contact bendings until immediately before breaking.
In the bending test, five flexible printed wiring boards were prepared for each copper foil sample and performed at n = 5, and the average value of the number of bendings in each test was adopted. It is good if the number of bends is 5 or more.

The results obtained are shown in Table 1.

As is clear from Table 1, each example in which the crystal orientation density in the Copper orientation is less than 10 and the crystal orientation density in the Brass orientation is less than 20 has excellent bending resistance and is more severe than the copper foil alone. The bendability of CCL has been improved.

On the other hand, in the cases of Comparative Examples 1 and 2 in which the final annealing temperature was set to the temperature in the "recovered" state, the Brass orientation was developed by the final cold rolling, the crystal orientation density of the Brass orientation became 20 or more, and the bending resistance was inferior. .. In the case of Comparative Example 1, since P was not added, the Copper orientation was easy to grow, and the crystal orientation density was 10 or more.
In the case of Comparative Example 3 in which the P content was less than 0.0005 mass% (5 mass ppm), the crystal orientation density in the Copper orientation was 10 or more, and the bending resistance was inferior.
In the case of Comparative Examples 4 and 5 in which the final annealing temperature is set to the temperature in the "grain growth" state, which is higher than the "recovery" state, the Copper orientation is developed by the final cold rolling, and the crystal orientation density becomes 10 or more, and the resistance to resistance. The bending characteristics were inferior.

Claims (4)

A rolled copper foil containing 99.9% by mass or more of Cu and 0.0005 to 0.0220% by mass of P as an additive element, and the balance is unavoidable impurities.
Copper foil for flexible printed substrates with a crystal orientation density of less than 10 in the Copper orientation and a crystal orientation density of less than 20 in the Brass orientation. The copper for a flexible printed substrate according to claim 1, wherein the tough pitch copper specified in JIS-H3100 (C1100) or the oxygen-free copper of JIS-H3100 (C1020) contains 0.0005 to 0.0220% by mass of P as an additive element. Foil. The copper foil for a flexible printed substrate according to claim 1 or 2, wherein the surface roughness Sa is less than 0.2 μm. The copper foil for a flexible printed circuit board according to any one of claims 1 to 3, wherein the thickness is 12 μm or less.

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