CN111669898A

CN111669898A - Rolled copper foil for flexible printed circuit board, flexible copper-clad laminate, and flexible printed circuit board

Info

Publication number: CN111669898A
Application number: CN202010147685.8A
Authority: CN
Inventors: 工藤雄大
Original assignee: Jks Metal Co ltd
Current assignee: Jks Metal Co ltd; JX Nippon Mining and Metals Corp
Priority date: 2019-03-05
Filing date: 2020-03-05
Publication date: 2020-09-15
Also published as: JP2020143321A; KR20200106834A; TW202039891A; JP6944963B2; TWI717998B

Abstract

The invention provides a rolled copper foil for a flexible printed circuit board, a flexible copper-clad laminate and a flexible printed circuit board, wherein the rolled copper foil has excellent bendability, especially folding performance, stably regardless of the heating condition during the production of the flexible copper-clad laminate. The solution of the problem is a rolled copper foil for a flexible printed board, which comprises 99.0 mass% or more of Cu and the balance of unavoidable impurities, and is heated from 25 ℃ to 350 ℃ in 5 seconds or more and then held at 350 ℃ for 30 minutes in a heating modeA or heating mode B which takes 1 second to reach 350 ℃ from 25 ℃ and heats the steel sheet to the atmosphere, and the intensity (I) of the (200) surface obtained by X-ray diffraction of the rolled surface is compared with the intensity (I) of the (200) surface obtained by X-ray diffraction of the fine copper powder₀) Is I/I₀≥45。

Description

Rolled copper foil for flexible printed circuit board, flexible copper-clad laminate, and flexible printed circuit board

Technical Field

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 require bendability.

Background

A Flexible Printed Circuit board (FPC) has a Circuit formed on a Flexible Copper Clad Laminate (FCCL). FCCL is formed by laminating a resin on one surface or both surfaces of a copper foil, but polyimide is often used as the resin. As the FCCL, there are three-layer FCCL and two-layer FCCL from the viewpoint of its structure.

The three-layer FCCL has a structure in which a resin film such as polyimide is bonded to a copper foil as a conductive material 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 is directly bonded to a copper foil as a conductive material. 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).

Copper foil for FPC is required to have high flexibility. As a method for imparting flexibility to a copper foil, a technique of increasing the degree of orientation of crystal orientation of the (200) plane of the copper foil (patent document 1), a technique of increasing the proportion of crystal grains penetrating the thickness direction of the copper foil (patent document 2), and a technique of reducing the surface roughness Ry (maximum height) corresponding to the depth of an oil pit (oil pit) of the copper foil to 2.0 μm or less (patent document 3) are known.

The FPC used for the bending portion uses a two-layer FCCL manufactured by a method of: a method called a tape casting method in which a varnish of polyimide is applied to a copper foil and then cured by heating and drying to form a laminate; a method called a lamination method in which a polyimide film coated with a thermoplastic polyimide having an adhesive force is previously laminated on a copper foil and pressure-bonded with a heating roller or the like.

For example, a flexible copper-clad laminate having high flexibility obtained by a casting method is known (patent document 4). The copper foil is recrystallized by the heat treatment in the FCCL production process.

However, in order to accommodate the FPC in a small space of a housing of a mobile phone, a tablet PC, or the like, the FPC may be folded and then bent, or may be continuously and repeatedly bent with a small radius of curvature such as a read/write cable (read/write cable) of a hard disk drive, and thus, more severe bending property is required.

Here, folding is a method of folding by cutting a fold line to store in a thin case, and folding by inverting 180 degrees of the upper surface side of the FPC to the lower surface side is referred to as "folding".

In order to cope with severe bending such as folding, the technique described in patent document 1 described above is designed to add a small amount of Ag, Sn, or the like to a copper foil, thereby softening the copper foil by annealing during heat treatment for FCCL production and developing a cubic structure having uniform crystal orientation in a specific direction (200 planes).

Thus, in the case where stress at the time of bending is applied to the copper foil, dislocations generated within the crystal and their movement do not accumulate at the grain boundaries but move in the surface direction, thereby suppressing the occurrence and progression of cracks at the grain boundaries to cause destruction, thereby exhibiting excellent bending characteristics.

One important point of the FPC for realizing high bendability is to recrystallize the metal structure of the copper foil to a preferable state for bendability in the heat treatment in the production of the FCCL. The most preferable metal structure for bendability is a structure in which the cubic orientation is very developed and the grain boundaries are few, in other words, the crystal grains are large. Here, the degree of development of the cubic orientation can be determined by the X-ray diffraction intensity ratio I/I of 200 planes₀(I: diffraction intensity of 200-face of copper foil, I)₀: diffraction intensity of 200 plane of copper powder), and the larger the value, the more developed the cubic orientation.

When a two-layer FCCL is produced by a casting method, in the course of increasing the temperature stepwise at the time of lamination (at the time of applying a resin material to a copper foil), nucleation of recrystallization and growth of recrystallized grains occur in the copper foil. When the copper foil is heated to 200 ℃ for 4 seconds or more by the tape casting method, is held at 200 ℃ for 30 minutes, and is cooled to room temperature, the X-ray diffraction intensity ratio I/I of the 200 plane measured at room temperature is measured₀A value of 40 or more can provide high bendability.

On the other hand, in the case of producing a two-layer FCCL by a lamination method, since the polyimide film coated with an adhesive and dried is pressure-bonded to the copper foil by a heating roller and there is no need to evaporate a solvent or the like, the temperature can be raised all at once to a temperature at which the polyimide undergoes a curing reaction. However, when the temperature is rapidly raised, nuclei oriented in multiple directions are generated and grow, and development of cubic orientation is suppressed. Therefore, the flexibility tends to be reduced in the lamination method as compared with the casting method in which heating is performed relatively slowly in the lamination (patent document 5).

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 3009383

Patent document 2: japanese patent laid-open publication No. 2006-117977

Patent document 3: japanese patent laid-open No. 2001 and 058203

Patent document 4: japanese laid-open patent publication No. 2006 and 237048

Patent document 5: japanese laid-open patent publication No. 2009-292090

Non-patent document

Non-patent document 1: FUJIKURA technology report, FUJIKURA, No.109 pp.31-35 (2005).

Disclosure of Invention

Problems to be solved by the invention

As described above, as a method for producing the two-layer FCCL, there are a casting method and a lamination method, which have different heating conditions, and a copper foil for FPC, which can stably obtain bendability regardless of the heating conditions, is required.

In particular, a copper foil for FPC having more severe bendability, i.e., excellent folding property, is desired.

Accordingly, an object of the present invention is to provide a rolled copper foil for a flexible printed circuit board, a flexible copper-clad laminate, and a flexible printed circuit board, which can stably obtain bendability, particularly excellent foldability, regardless of the heating conditions used in the production of the flexible copper-clad laminate.

Means for solving the problems

The present inventors have conducted various studies and, as a result, have found that: manufacturing of two-layer FCCL by simulationIf the strength of the (200) plane is I/I₀The copper foil of not less than 45 can stably obtain the bendability regardless of the heating condition in the production of the two-layer FCCL.

In order to achieve the above object, a rolled copper foil for a flexible printed board of the present invention contains 99.0 mass% or more of Cu, and the balance is made of inevitable impurities, and the intensity (I) of the (200) plane obtained by X-ray diffraction of the rolled plane is compared with the intensity (I) of the (200) plane obtained by X-ray diffraction of fine powder copper after heating in a heating mode a in which heating from 25 ℃ to 350 ℃ takes 5 seconds or more and holding at 350 ℃ for 30 minutes or heating in a heating mode B in which heating from 25 ℃ to 350 ℃ takes 1 second and then heating in the atmosphere₀) Is I/I₀≥45。

The rolled copper foil for a flexible printed board of the present invention may contain 280 to 360ppm by mass of Ag relative to the oxygen-free copper specified in JIS-H0500 (C1011).

The flexible copper-clad laminate of the present invention is obtained 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.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, the flexibility can be stably obtained regardless of the heating conditions used in the production of the flexible copper-clad laminate, and in particular, a rolled copper foil for a flexible printed circuit board, a flexible copper-clad laminate, and a flexible printed circuit board, which have excellent foldability, can be obtained.

Drawings

Fig. 1 is a diagram schematically showing the appearance of the FPC of the embodiment.

Fig. 2 is a diagram schematically showing the steps of the folding test.

Fig. 3 is a graph showing the relationship between the Ag concentration and the final cold rolling degree (true strain) η in examples and comparative examples.

FIG. 4 is a graph showing I/I of annealed copper foils corresponding to a casting method and a laminating method of examples and comparative examples₀The figure (a).

Fig. 5 is a graph showing the number of breaks based on the folding test of the FPCs of the examples and comparative examples.

Detailed Description

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

(composition)

The composition of the rolled copper foil for a flexible printed board contains 99.0 mass% or more of Cu, and the balance is made up of unavoidable impurities.

Particularly preferably, the composition contains 280 to 360 mass ppm of Ag relative to oxygen-free copper specified in JIS-H0500 (C1011).

When the Ag content is less than 280ppm by mass, the amount of strain introduced into the material by rolling is small, the growth of the cube texture becomes insufficient, and I/I described later may not be realized₀Not less than 45. In particular, in the case of a lamination method corresponding to a copper foil that is rapidly heated at the time of lamination, the cubic aggregate structure is further difficult to grow.

When the content of Ag exceeds 360 mass ppm, the recrystallization temperature of the copper foil becomes high, and recrystallization does not occur sufficiently even when heating is performed in the production of the two-layer FCCL, and a large amount of unrecrystallized grains remain in the copper foil, resulting in a significant deterioration in the folding property of the obtained FPC.

More preferably, the copper alloy contains 290 to 340 ppm by mass of Ag relative to the 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 the folding property and the fine circuit forming property, the thickness is preferably small, and it is preferably 6 to 35 μm, more preferably 9 to 18 μm.

[ collective organization ]

In the rolled copper foil for a flexible printed board according to the embodiment of the present invention, the heating pattern a is a heating pattern which takes 5 seconds or more to heat from room temperature (25 ℃) to 350 ℃ and then holds at 350 ℃ for 30 minutes or takes 1 second to heat from room temperature (25 ℃) to 350 ℃After heating to 350 ℃ in heating mode B in the atmosphere, the intensity (I) of the (200) plane obtained by X-ray diffraction of the rolled plane was compared with the intensity (I) of the (200) plane obtained by X-ray diffraction of fine copper powder (325 mesh, heated at 300 ℃ for 1 hour in a hydrogen gas stream and reused)₀) Is I/I₀≥45。

Heating at 350 ℃ for 30 minutes simulates the heating conditions for producing a two-layer FCCL by the tape casting method, and means that the copper foil is gradually heated from room temperature (25 ℃) to 350 ℃.

The heating condition for producing a two-layer FCCL by the lamination method was simulated by heating at 350 ℃ for 1 second, and the heating condition was expressed by rapidly heating to the maximum temperature (350 ℃) by the lamination method (from room temperature (25 ℃) to 350 ℃ in 1 second).

The strengths (I) and (I)₀) The measurement was carried out at normal temperature (25 ℃). In addition, the copper foil heated to the maximum temperature of 350 ℃ by the heating pattern A, B is naturally cooled to room temperature, and it is considered that: the cooling rate at this time is not particularly specified, and the texture of the copper foil assembly is not affected.

As described above, by specifying to I/I₀Not less than 45, the cube orientation with excellent bendability is very developed, the bendability can be stably obtained regardless of the heating condition in the production of the flexible copper-clad laminate, and the copper foil with particularly excellent foldability is formed.

I/I₀For example, 100.

(production)

The rolled copper foil for a flexible printed circuit board according to the embodiment of the present invention can be generally produced by repeating hot rolling, cold rolling, and annealing in this order.

The rolling degree in the final cold rolling is set to 92.0 to 99.8% (the true strain eta is 2.53 to 6.21).

Fig. 3 shows the relationship between the Ag concentration and the true strain η in examples and comparative examples to be described later.

As shown in FIG. 3, the higher the Ag concentration in the rolled copper foil, the higher the rolling pressure in the final cold rolling step is, the lower the rolling pressure isWhen the degree of elongation (true strain) η is determined, it is difficult to introduce strain that is a driving force for recrystallization, and it is difficult to realize I/I₀On the other hand, when η is excessively increased, a large amount of shear bands inhibiting the growth of cubic aggregates are introduced into the rolled copper foil, and it is still difficult to realize I/I₀A tendency of 45 or more.

Therefore, in order to perform final cold rolling in a region between two rightward-rising straight lines B-C, A-D experimentally found for distinguishing the example and the comparative example of FIG. 3, the Ag concentration in the rolled copper foil is denoted by C_Ag(mass ppm) was 0.04 × C_Ag-9.3）≤η≤（0.04C_Ag-7.3）。

Line A-D was defined as η ═ 0.04 × C_Ag-9.3), straight line B-C is η ═ 0.04 × C_Ag-7.3). The line A-B represents C, which is the lower limit of the concentration of Ag in the rolled copper foil_AgThe straight line C-D represents C, which is the upper limit of the concentration of Ag in the rolled copper foil, 280ppm by mass _Ag360 mass ppm.

The true strain η is defined by the following equation.

η ═ ln { (i.e., the sectional area of the material before the final cold rolling)/(the sectional area of the material immediately after the final cold rolling) }.

Examples

Hereinafter, examples of the present invention will be described, but these are provided for the purpose of sufficiently understanding the present invention, and are not intended to limit the present invention.

[ production of rolled copper foil ]

A copper alloy having a composition shown in Table 1 was cast into an ingot as a raw material, hot-rolled at 800 ℃ or higher until the thickness became 10mm, the surface scale was subjected to end face cutting, then cold rolling and annealing were repeated, and finally, finish rolling was performed to a thickness of 0.009 to 0.018 mm. The oxygen-free copper described in Table 1 was standardized in JIS-H0500 (C1011).

The rolling degree in the final cold rolling was set to 85 to 99.9% (1.9 to 6.6 in terms of true strain η), and the rolling degree in the final cold rolling of examples (true strain η) was set toThe Ag concentration of the sample was adjusted to the above-mentioned value (0.04 × C) in the range of 280 to 360ppm as shown in FIG. 3_Ag-9.3）≤η≤（0.04C_Ag-7.3).

For each rolled copper foil sample thus obtained, I/I was performed₀And evaluation of folding endurance.

(1) Cube collection organization (I/I)₀）

After the copper foil samples were heated in the heating modes A and B, the integrated value (I) of the (200) surface intensity was obtained by X-ray diffraction at 25 ℃ on the rolled surface. This value was divided by the previously determined integral value (I) of the (200) surface intensity of copper micropowder (325 mesh, heated at 300 ℃ for 1 hour in a hydrogen gas stream and then used)₀) Thereby calculating I/I₀The value of (c).

(2) Folding endurance

After the copper foil samples were heated and recrystallized in the above heating modes a and B, a 2 μm thermoplastic polyimide adhesive was applied to one surface (surface to be bonded to the copper foil) of the polyimide film, and then dried, thereby forming a 27 μm thick resin layer. A copper foil was laminated on the adhesive surface of the resin layer, and vacuum heat pressing was performed to produce FCCL. Thereafter, a circuit is formed by etching, thereby producing an FPC shown in fig. 1.

As shown in fig. 2, folding and bending of the FPC were repeatedly performed under a load of 100N while confirming conduction of the FPC with a tester, and folding resistance of the FPC was examined.

Specifically, the FPC gently bent into a ring shape was placed on a stainless steel stage as shown in fig. 2 (1), and a indenter made of the same stainless steel was lowered at a speed of 6mm/min to fold the FPC under a load of 100N as shown in fig. 2 (2). After the application of the load of 100N for 5 seconds, the press head was raised at a rate of 1,000mm/min as in FIG. 2 (3), and the folded FPC was unfolded. Thereafter, a load of 100N was applied to the FPC for 5 seconds as in FIG. 2 (4), the FPC was bent back, and the indenter was again raised at a speed of 1,000mm/min as in FIG. 2 (5), thereby gently bending the FPC into a ring shape.

Fig. 2 (1) - (5) were taken as 1 cycle, and it was examined that the FPC circuit broke and no longer conducted at the next cycle (that is, the FPC circuit broke).

The number of folds until rupture was 7 or less, and was judged as poor (x), 8 or more and 14 or less, and was judged as normal (Δ), and 15 or more, and was judged as good (o). If the evaluation is Δ or o, there is no practical problem.

The results are shown in Table 1. The overall judgment is as follows. When judged as ∈ x, ≈ Δ and Δ, FCCL produced by either the casting method or the lamination method showed high folding endurance.

Very good: judgment of folding test after annealing corresponding to the casting method and after annealing corresponding to the lamination method was O

O: in the judgment of the folding test after annealing corresponding to the casting method and the folding test after annealing corresponding to the lamination method, one is O and the other is Delta

And (delta): the judgment of the folding test after annealing corresponding to the casting method and after annealing corresponding to the lamination method was Δ

X: at least one of judgment of folding test after annealing corresponding to the casting method and judgment of folding test after annealing corresponding to the lamination method was x.

[ Table 1]

As is clear from table 1: in each example, the copper foil satisfied I/I after any annealing equivalent to the casting method and equivalent to the lamination method₀Not less than 45. Therefore, FPCs made using any annealed copper foil equivalent to the casting method and equivalent to the lamination method also exhibit high folding endurance.

In comparative examples 1, 4, and 5, the lower side of the degree of working η with respect to the line a-D in fig. 3 means that the degree of working is insufficient with respect to the Ag concentration. Therefore, the amount of accumulated strain that becomes a driving force for recrystallization is small, and the recrystallization temperature of the copper foil becomes high. As a result, the copper foil is not sufficiently recrystallized in at least one of annealing corresponding to the casting method and annealing corresponding to lamination, and folding endurance is poor. Further, it is considered that unrecrystallized grains which are likely to accumulate strain and may become crack starting points remain.

In the case of comparative example 2 in which the Ag concentration was less than 280ppm, the amount of strain introduced into the material by rolling was small, and the growth of the cube texture was insufficient in annealing corresponding to lamination, and I/I was not satisfied₀Not less than 45. Therefore, the folding endurance is poor.

In comparative example 2, the cube texture grew sufficiently to satisfy the I/I in the annealing corresponding to the casting₀The reason for ≧ 45 is that, in the case of the casting method, the copper foil is slowly heated at the time of lamination, and therefore, the cubic collective structure is easily grown.

In comparative example 3 in which the Ag concentration exceeded 360ppm, the recrystallization temperature became high, and therefore, the copper foil did not recrystallize sufficiently in at least one of the annealing equivalent to the casting method and the annealing equivalent to lamination, and the folding endurance was poor. Further, it is considered that unrecrystallized grains which are likely to accumulate strain and may become crack starting points remain.

In comparative examples 6 and 7, the fact that the degree of working η was located on the upper side of the line B-C in FIG. 3 means that the degree of working η was too high with respect to the Ag concentration, and therefore, I/I was achieved in at least one of the annealing processes corresponding to the casting process and the annealing processes corresponding to the lamination process₀<45, folding endurance was poor.

This is considered to be because: when the degree of working η is too high, a large amount of shear band is introduced into the copper foil, and as a result, the development of a cubic aggregate structure is inhibited, and crystal grains having another orientation grow. In other words, it is considered that, when the cubic microstructure grows, the grains of the cubic microstructure grow while being packed with the surrounding grains having other orientations, but if the shear band is present, the growth of the cubic microstructure is inhibited, and the grains having other orientations remain and grow.

Claims

1. A rolled copper foil for a flexible printed board, which contains 99.0 mass% or more of Cu, and the balance being made up of unavoidable impurities,

heating the rolled surface in a heating mode A of heating from 25 ℃ to 350 ℃ for 5 seconds or more and holding the rolled surface at 350 ℃ for 30 minutes or in a heating mode B of heating from 25 ℃ to 350 ℃ for 1 second, and then subjecting the rolled surface to X-ray diffraction to obtain an intensity (I) of the (200) surface relative to an intensity (I) of the (200) surface obtained by X-ray diffraction of fine copper powder₀) Is I/I₀≥45。

2. The rolled copper foil for flexible printed boards according to claim 1, wherein the Ag is contained in an amount of 280 to 360ppm by mass based on the oxygen-free copper specified in JIS-H0500 (C1011).

3. A flexible copper-clad laminate obtained by laminating the rolled copper foil for a flexible printed board according to claim 1 or 2 and a resin.

4. A flexible printed circuit board comprising the flexible copper-clad laminate according to claim 3.