JP6944963B2 - Rolled copper foil for flexible printed circuit boards, flexible copper-clad laminates and flexible printed circuit boards - Google Patents

Description

The present invention relates to a rolled copper foil for a flexible printed circuit board, a flexible copper-clad laminate, and a flexible printed circuit board, which are required to have flexibility.

A flexible printed circuit board (FPC: Flexible Printed Circuit) is a circuit formed on a flexible copper-clad laminate (FCCL: Flexible Copper Clad Laminate). The FCCL is formed by laminating a resin on one side or both sides of a copper foil, and polyimide is often used for this resin. FCCL includes three-layer FCCL and two-layer FCCL due to its structure.
The three-layer FCCL has a structure in which a resin film such as polyimide and a copper foil as a conductive material are bonded together with an adhesive such as an epoxy resin or an acrylic resin. On the other hand, the two-layer FCCL has a structure in which a resin such as polyimide and a copper foil as a conductive material are directly bonded. The two-layer FCCL is superior to the three-layer FCCL in heat resistance, dimensional stability, bending resistance, and the like (Non-Patent Document 1).

The copper foil used for FPC is required to have high flexibility. As a method for imparting flexibility to the copper foil, a technique for increasing the degree of orientation of the crystal orientation of the (200) plane of the copper foil (Patent Document 1), and a large proportion of crystal grains penetrating in the plate thickness direction of the copper foil. (Patent Document 2), and a technique for reducing the surface roughness Ry (maximum height) corresponding to the depth of the oil pit of the copper foil to 2.0 μm or less (Patent Document 3) are known.
The FPC used for the bent part is a method called a casting method in which a polyimide varnish is applied to a copper foil and then dried and cured by applying heat to form a laminated plate, or a polyimide coated with a thermoplastic polyimide having adhesive strength in advance. A two-layer FCCL manufactured by a method called a laminating method in which a film and a copper foil are laminated and pressure-bonded through a heating roll or the like is used.
For example, a flexible copper-clad laminate obtained by a casting method with high flexibility is known (Patent Document 4). The copper foil is recrystallized by the heat treatment in this FCCL manufacturing process.

By the way, in order to store an FPC in a narrow space of a housing such as a mobile phone, a smartphone, or a tablet PC, it is bent on a goblet or continuously and repeatedly bent with a small radius of curvature like a read / write cable of a hard disk drive. In some cases, tighter flexibility is required.
Here, the goby fold refers to a mode in which a crease is made and bent so as to be stored in a thin housing, and the folding so that the upper surface side of the FPC is inverted 180 degrees and becomes the lower surface side is referred to as "goby fold". ..

Then, in order to cope with severe bending such as folds, in the technique described in Patent Document 1, by adding a small amount of Ag, Sn, etc. to the copper foil, softening due to annealing of the copper foil during the heat treatment of FCCL production is performed. As it progresses, it develops a cubic texture in which the crystal orientations are aligned in a specific direction (200 planes).
As a result, when stress is applied to the copper foil during bending, the transitions and their movements that occur in the crystal move toward the surface without accumulating at the grain boundaries, causing cracks at the grain boundaries and cracking at the grain boundaries. It suppresses breakage due to evolution and exhibits excellent bending characteristics.
One of the important points for realizing highly flexible FPC is to recrystallize the metal structure of the copper foil in a state preferable for flexibility during the heat treatment during the production of FCCL. The most preferable metal structure for flexibility is a structure in which the cubic orientation is very developed and the grain boundaries are small, in other words, the crystal grains are large. Here, the degree of development of the cube orientation is the magnitude of the X-ray diffraction intensity ratio I / I ₀ (I: diffraction intensity of 200 surfaces of copper foil, I ₀ : diffraction intensity of 200 surfaces of copper powder) of 200 surfaces. It can be expressed, and the larger this value is, the more the cube orientation is developed.

When two-layer FCCL is manufactured by the casting method, recrystallized nucleation and recrystallized grains are formed in the copper foil in the process of gradually increasing the temperature during lamination (when the resin material is applied to the copper foil). Growth occurs. Then, when the copper foil was heated by the casting method for 4 seconds or more until it reached 200 ° C., held at 200 ° C. for 30 minutes, and then cooled to room temperature, the X-ray diffraction intensity ratio of 200 surfaces measured at room temperature was obtained. When I / I ₀ is 40 or more, high flexibility can be obtained.
On the other hand, when the two-layer FCCL is manufactured by the laminating method, the polyimide film that has already been coated with the adhesive and dried is pressure-bonded to the copper foil with a heating roll, but the polyimide is cured because it is not necessary to evaporate the solvent or the like. It is possible to raise the temperature at once to the temperature at which the reaction occurs. However, when the temperature rises at a high rate, nuclei in multiple directions are generated and grown, and the development of cubic orientation is suppressed. Therefore, the flexibility tends to decrease in the case of the laminating method as compared with the casting method in which heating is performed relatively slowly during laminating (Patent Document 5).

Japanese Patent No. 309383 JP-A-2006-117977 Japanese Unexamined Patent Publication No. 2001-058203 Japanese Unexamined Patent Publication No. 2006-237048 JP-A-2009-292090

Fujikura Giho Co., Ltd. Fujikura, No. 109 pp. 31-35 (2005)

As described above, the two-layer FCCL manufacturing method includes a casting method and a laminating method, which have different heating conditions, but a copper foil for FPC that can stably obtain flexibility regardless of the heating conditions is required. ing.
In particular, there is a demand for a copper foil for FPC, which has more severe flexibility and excellent goby foldability.
Therefore, according to the present invention, a rolled copper foil for a flexible printed circuit board and a flexible copper-clad laminate, which can stably obtain flexibility regardless of the heating conditions at the time of manufacturing the flexible copper-clad laminate and are particularly excellent in foldability. And to provide a flexible printed circuit board.

As a result of various studies by the present inventors, if the copper foil has a strength of the surface (200) of I / I ₀ ≧ 45 by heat treatment when imitating the production of the two-layer FCCL, the production of the two-layer FCCL is performed. It was found that stable flexibility can be obtained regardless of the heating conditions at the time.

In order to achieve the above object, the rolled copper foil for a flexible printed substrate of the present invention contains 300 to 355 mass ppm of Ag with respect to the oxygen-free copper specified in JIS-H0500 (C1011), and the balance is unavoidable impurities. it was heated over a period of 5 seconds to reach 350 ° C. from 25 ° C., the heating pattern a for holding an additional 30 minutes at each 350 ° C., or after the air heated by the heating pattern B to reach per second from 25 ° C. to 350 ° C. The strength (I) of the (200) plane obtained by X-ray diffraction of the rolled surface is I _{0 according} to the heating pattern A with respect to the strength (I 0) of the (200) plane obtained by X-ray diffraction of fine powdered copper. / I ₀ ≥ 50.8, and I / I ₀ ≥ 46.0 according to the heating pattern B.

The flexible copper-clad laminate of the present invention is formed by laminating the rolled copper foil for a flexible printed circuit board and a resin.

The flexible printed circuit board of the present invention has the flexible copper-clad laminate.

According to the present invention, a rolled copper foil for a flexible printed circuit board and a flexible copper-clad laminate, which can stably obtain flexibility regardless of the heating conditions at the time of manufacturing the flexible copper-clad laminate and are particularly excellent in foldability. And a flexible printed circuit board is obtained.

It is a figure which shows typically the appearance of the FPC of an Example. It is a figure which shows typically the procedure of the goby folding test. It is a figure which showed the relationship between the Ag concentration of an Example and a comparative example, and the work degree (true strain) η of the final cold rolling. It is a figure which showed _{the I / I 0} of the copper foil after annealing corresponding to the casting method and the laminating method of an Example and a comparative example. It is a figure which showed the number of breaks by the goby folding test of FPC of an Example and a comparative example.

Hereinafter, the rolled copper foil for a flexible printed circuit board according to the embodiment of the present invention will be described. In the present invention,% means mass% unless otherwise specified.

(composition)
The composition of the rolled copper foil for a flexible printed circuit board contains 99.0% by mass or more of Cu, and the balance consists of unavoidable impurities.
In particular, a composition containing 280 to 360 mass ppm of Ag is preferable with respect to the oxygen-free copper specified in JIS-H0500 (C1011).
If the Ag content is less than 280 mass ppm, the amount of strain introduced into the material by rolling is small, the growth of the cubic texture is insufficient, and I / I ₀ ≧ 45, which will be described later, cannot be realized. be. In particular, in the case of the laminating method in which the copper foil is rapidly heated during laminating, the cubic texture becomes more difficult to grow.
When the Ag content exceeds 360 mass ppm, the recrystallization temperature of the copper foil becomes high, recrystallization does not occur sufficiently even when heated during the production of the two-layer FCCL, and unrecrystallized grains are formed in the copper foil. A large amount remains, and the obtained FPC has significantly inferior foldability.
More preferably, it contains 290 to 340 mass ppm of Ag with respect to oxygen-free copper.

The thickness of the rolled copper foil is not particularly limited and may be appropriately selected according to the required characteristics, and may be, for example, 1 to 100 μm. In particular, in order to improve goby folding property and fine circuit formability, the thickness is preferably thin, preferably 6 to 35 μm, and more preferably 9 to 18 μm.

[Aggregate organization]
In the rolled copper foil for a flexible printed substrate according to the embodiment of the present invention, heating pattern A or normal temperature, which is heated from room temperature (25 ° C.) to 350 ° C. for 5 seconds or more and further held at 350 ° C. for 30 minutes. After heating in the atmosphere with a heating pattern B that reaches 350 ° C. from (25 ° C.) over 1 second, the strength (I) of the (200) surface determined by X-ray diffraction of the rolled surface is fine powder copper (325 mesh, hydrogen stream). I / I ₀ ≥ 45 with _{respect to the intensity (I 0} ) of the (200) plane determined by X-ray diffraction (used after heating at 300 ° C. for 1 hour).
Heating at 350 ° C. for 30 minutes mimics the heating conditions during the production of the two-layer FCCL by the casting method, and represents that the copper foil is slowly heated from room temperature (25 ° C.) to 350 ° C.
Further, heating at 350 ° C. for 1 second imitates the heating conditions at the time of manufacturing the two-layer FCCL by the laminating method, and rapid heating up to the maximum temperature (350 ° C.) by the laminating method (normal temperature (25 in 1 second)). ℃) to 350 ℃).
The intensities (I) and (I ₀ ) are measured at room temperature (25 ° C.). Further, the copper foil after being heated to a maximum temperature of 350 ° C. in the above heating patterns A and B is cooled to room temperature by natural cooling, but the cooling rate at this time is not particularly specified, but the texture of the copper foil is not particularly specified. Is not considered to have any effect.

As described above, _{by defining I / I 0} ≥ 45, the cube orientation with excellent flexibility is greatly developed, and the flexibility is stable regardless of the heating conditions during the production of the flexible copper-clad laminate. Is obtained, and the copper foil is particularly excellent in goby foldability.
The upper limit of I / I ₀ is, for example, 100.

(Manufacturing)
The rolled copper foil for a flexible printed substrate according to the embodiment of the present invention can usually be produced by repeatedly performing hot rolling, cold rolling and annealing of an ingot in this order.
The degree of rolling in the final cold rolling may be 92.0 to 99.8% (true strain η is 2.53 to 6.21).
Here, FIG. 3 shows the relationship between the Ag concentration and the true strain η of Examples and Comparative Examples described later.
As shown in FIG. 3, the higher the Ag concentration in the rolled copper foil, the more difficult it is to introduce the strain that is the driving force for recrystallization unless the rolling processability (true strain) η in the final cold rolling is increased. It tends to be difficult to realize I / I _{0 ≧ 45.} On the other hand, if η is made too high, many shear bands that hinder the growth of the cubic texture are introduced into the rolled copper foil, and it tends to be difficult to realize _{I / I 0 ≧ 45.}

Therefore, in the rolled copper foil, the final cold rolling is performed in the region between the two upward-sloping straight lines BC and AD obtained experimentally to distinguish between the example and the comparative example of FIG. When the concentration of _Ag is C Ag (mass ppm), (0.04 × C _Ag -9.3) ≤ η ≤ ( _{0.04 C Ag} -7.3).
The straight line AD is represented by η = (0.04 × C _Ag -9.3), and the straight line BC is represented by η = (0.04 × C _Ag -7.3). The straight line AB represents the lower limit of the concentration of Ag in the rolled copper foil, C _Ag = 280 mass ppm, and the straight line CD represents the upper limit of the concentration of Ag in the rolled copper foil, C _Ag = 360 mass. Represents ppm.

The true strain η is defined by the following equation.
η = ln {(cross-sectional area of the material immediately before the final cold rolling) / (cross-sectional area of the material immediately after the final cold rolling)}

Examples of the present invention will be described below, but these are provided for a better understanding of the present invention, and are not intended to limit the present invention.

[Manufacturing of rolled copper foil]
An ingot was cast using a copper alloy having the composition shown in Table 1 as a raw material, hot-rolled at 800 ° C. or higher to a thickness of 10 mm, the oxide scale on the surface was chamfered, and then cold rolling and annealing were repeated. Finally, it was finally cold-rolled to a thickness of 0.009 to 0.018 mm. The oxygen-free copper shown in Table 1 is specified in JIS-H0500 (C1011).
The rolling workability in the final cold rolling is 85 to 99.9% (1.9 to 6.6 in true strain η), and the rolling workability (true strain η) in the final cold rolling of the example is a sample. As shown in FIG. 3, the above-mentioned range of (0.04 × C _Ag -9.3) ≤ η ≤ ( _{0.04 C Ag} -7.3) was adjusted within the range of Ag concentration (280 to 360 ppm).

Each rolled copper foil sample thus obtained was _{evaluated for I / I 0} and goby fold resistance.
(1) Cube assembly (I / I ₀ )
After heating the copper foil sample in the above heating patterns A and B, respectively, the (200) integrated value (I) of the surface strength obtained by X-ray diffraction of the rolled surface at 25 ° C. was obtained. _{This value is divided by the integral value (I 0} ) of (200) surface strength of fine powdered copper (325 mesh, used after heating at 300 ° C. for 1 hour in a hydrogen stream) measured in advance, and I / I. A value of _{0 was calculated.}

(2) Folding resistance Copper foil samples are recrystallized by heating them in the above heating patterns A and B, respectively, and then 2 μm of thermoplastic polyimide adhesive is applied to one side of the polyimide film (the side that adheres to the copper foil). After the work, it was dried to form a resin layer having a thickness of 27 μm. A copper foil was laminated on the adhesive surface of this resin layer and vacuum heat pressed to prepare FCCL. Then, a circuit was formed by etching to produce the FPC shown in FIG.

As shown in FIG. 2, the FPC was repeatedly folded and bent back at a load of 100 N while confirming the continuity of the FPC with a tester, and the fold resistance of the FPC was investigated.
Specifically, an FPC loosely bent in a loop shape is placed on a stainless steel stage as shown in FIG. 2 (1), and a stainless steel pusher is also lowered at a speed of 6 mm / min to be lowered in FIG. 2 (2). ), Fold the FPC with a load of 100N. After continuing to apply a load of 100 N for 5 seconds, the pusher is raised at a speed of 1,000 mm / min as shown in FIG. 2 (3) to spread the goby-folded FPC. Then, as shown in FIG. 2 (4), a load of 100 N is applied to the FPC for 5 seconds to bend the FPC back, and as shown in FIG. 2 (5), the pusher is raised again at a speed of 1,000 mm / min to loop the FPC. Bend loosely.

With FIGS. 2 (1) to 2 (5) as one cycle, it was investigated at what cycle the FPC circuit was broken and continuity could not be obtained (= the FPC circuit was broken).
It was judged that the number of goby foldings leading to breakage was 7 or less as bad (x), 8 or more and 14 or less as normal (Δ), and 15 or more as good (◯). If the evaluation is △ or ○, there is no problem in practical use.

The results obtained are shown in Table 1. The overall judgment was as follows. If the overall judgment is ⊚, ◯, or Δ, high fold resistance is exhibited regardless of whether FCCL is manufactured by the casting method or the laminating method.
◎: Both the judgment of the goby folding test after annealing equivalent to the casting method and after annealing equivalent to the laminating method is ○
◯: In the judgment of the goby folding test after annealing equivalent to the casting method and after annealing equivalent to the laminating method, one is ○ and the other is △.
△: Both the judgment of the goby folding test after annealing equivalent to the casting method and after annealing equivalent to the laminating method is △
×: At least one of the judgments of the goby folding test after annealing equivalent to the casting method and after annealing equivalent to the laminating method is ×

As is clear from Table 1, in the case of each example, the copper foil _{satisfied I / I 0} ≧ 45 after annealing corresponding to both the casting method and the laminating method. Therefore, the FPC produced by using the annealed copper foil equivalent to the casting method or the laminating method also showed high goby folding resistance.

In the cases of Comparative Examples 1, 4 and 5, the degree of processing η is lower than the straight line AD in FIG. 3, which means that the degree of processing is insufficient with respect to the Ag concentration. Therefore, the amount of accumulated strain, which is the driving force for recrystallization, is small, and the recrystallization temperature of the copper foil is high. As a result, the copper foil was not sufficiently recrystallized by annealing at least one of the casting method and the laminating method, and the goby folding resistance was inferior. Further, it is considered that unrecrystallized grains that easily accumulate strain and can be the starting point of cracks remain.

In the case of Comparative Example 2 in which the Ag concentration is less than 280 ppm, the amount of strain introduced into the material by rolling is reduced, and annealing equivalent to laminating results in insufficient growth of the cubic texture _{, satisfying I / I 0} ≧ 45. I didn't. Therefore, the goby folding resistance was inferior.
In Comparative Example 2, the reason why the cubic texture grew sufficiently in the annealing equivalent to casting _{and satisfied I / I 0} ≧ 45 is that the casting method heats the copper foil more slowly at the time of laminating. This is because the cubic texture is easy to grow.

In the case of Comparative Example 3 in which the Ag concentration exceeded 360 ppm, the recrystallization temperature was high, so that the copper foil was not sufficiently recrystallized by annealing at least one of the casting method and the laminate equivalent, and the goby folding resistance was inferior. .. Further, it is considered that unrecrystallized grains that easily accumulate strain and can be the starting point of cracks remain.

In the case of Comparative Examples 6 and 7, the degree of processing η is on the upper side of the straight line BC in FIG. 3, which means that the degree of processing η is too high with respect to the Ag concentration. _{Therefore, I / I 0} <45 by annealing at least one of the casting method and the laminating method, and the goby folding resistance was inferior.
It is considered that this is because the degree of processing η is too high and many shear bands are introduced into the copper foil, and as a result, the development of the cubic texture is hindered and crystal grains having other orientations grow. That is, when the cubic texture grows, the crystal grains of the cube texture grow while swallowing the surrounding crystal grains having other orientations, but the presence of the shear zone inhibits the growth of the cube texture and other orientations. It is considered that the crystal grains with the above remain and grow.

Claims (3)

It contains 300 to 355 mass ppm of Ag with respect to oxygen-free copper specified in JIS-H0500 (C1011), and the balance consists of unavoidable impurities.
Was heated over a period of 5 seconds to reach 350 ° C. from 25 ° C., after further heating pattern A for holding at each 350 ° C. 30 minutes, or air heated by the heating pattern B to reach per second from 25 ° C. to 350 ° C., rolling The intensity (I) of the (200) surface determined by X-ray diffraction of the surface is I / I according to the heating pattern A with _{respect to the intensity (I 0} ) of the (200) surface determined by X-ray diffraction of fine powder copper. _A rolled copper foil for a flexible printed substrate having 0 ≧ 50.8 and I / I ₀ ≧ 46.0 according to the heating pattern B. A flexible copper-clad laminate obtained by laminating the rolled copper foil for a flexible printed circuit board according to claim 1 and a resin. A flexible printed circuit board having the flexible copper-clad laminate according to claim 2.

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