TWI633195B - Copper foil for flexible printed circuit board, copper-clad laminate using the same, flexible printed circuit board, and electronic device - Google Patents

Abstract

Provided is a copper foil for a flexible printed circuit board having excellent bendability and etching properties.

A copper foil for a flexible printed circuit board is a copper foil composed of 99.0% by mass or more of Cu, and the remainder is unavoidable impurities. The average crystal grain size is 0.5 to 4.0 μm, and the tensile strength is 235 to 290 MPa.

Description

Copper foil for flexible printed circuit board, copper-clad laminate using the same, flexible printed circuit board, and electronic device

The present invention relates to a preferred copper foil for 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 in bent or movable parts of electronic circuits because of their flexibility. For example, FPC is used for the movable part of HDD or DVD- and CD-ROM-related equipment, or the folding part of a folding mobile phone.

FPC is formed by etching a Copper Clad Laminate (copper-clad laminate, hereinafter referred to as CCL) formed by laminating a copper foil and a resin, and a resin layer called a cover lay is formed thereon. Covered by. At the stage before laminating the cover layer, as part of the surface modification step to improve the adhesion between the copper foil and the cover layer, the surface of the copper foil is etched. In addition, in order to reduce the thickness of the copper foil and improve the flexibility, the thickness may be reduced by etching.

In addition, along with the miniaturization, thinness, and high performance of electronic devices, it is required that FPCs be installed inside these devices at a high density. However, in order to perform high-density installations, FPCs must be folded and housed inside miniaturized devices That is, high bendability.

On the other hand, copper foils have been developed that have improved high-cycle flexibility as typified by IPC flexibility (Patent Documents 1 and 2).

[Patent Document 1] Japanese Patent Laid-Open No. 2010-100887

[Patent Document 2] Japanese Patent Laid-Open No. 2009-111203

However, as described above, in order to construct FPC with a high density, it is necessary to improve the bending property represented by the MIT folding resistance, and the conventional copper foil has a problem that the improvement of the bending property is not sufficient.

In addition, along with the miniaturization, thinness, and high performance of electronic devices, the circuit width and gap width of FPC have been reduced to about 20 to 30 μm. When etching a circuit, there is a problem that the etching factor or the linearity of the circuit is easily deteriorated. Asked to resolve this issue.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to provide a copper foil for a flexible printed circuit board having excellent bendability and etching properties, a copper-clad laminate, a flexible printed circuit board, and an electronic device using the same.

As a result of various studies conducted by the present inventors, it has been found that by refining the crystal grains of the copper foil after recrystallization, the strength can be increased and the bendability can be improved. The reason is that according to the Hall-Petch formula, the finer the crystal grains, the higher the strength and the higher the bendability. However, if the crystal grains are excessively refined, the strength will become too high, the flexural rigidity will increase, and back) becomes large and is not suitable for flexible printed circuit board applications. Therefore, the range of the crystal grain size is also specified.

In addition, by refining the crystal grain size to about 1/10 of the circuit width of about 20 to 30 μm of FPC in recent years, the etching factor or circuit linearity when forming a circuit by etching can also be improved.

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 the remainder is unavoidable impurities. The average crystal grain size is 0.5 to 4.0 μm, and the tensile strength is 235. ~ 290MPa.

The copper foil for a flexible printed circuit board of the present invention is preferably composed of fine copper conforming to JIS-H3100 (C1100) standard or oxygen-free copper conforming to JIS-H3100 (C1011).

Furthermore, it is preferable to contain one or more kinds of additive elements selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg in a total amount of 0.003 to 0.825% by mass.

The average crystal grain size after the heat treatment at 300 ° C. for 30 minutes is preferably 0.5 to 4.0 μm, and the tensile strength is 235 to 290 MPa.

It is preferable that the copper-clad laminate formed by laminating a polyimide resin film having a thickness of 25 μm on one side of the above-mentioned copper foil is bent tightly at 180 degrees with a bending radius of 0.05 mm and the above-mentioned copper foil becomes the outside, and The folded portion returned to 0 degrees, and after repeating this test 3 times, when the copper foil was observed at 200 times, no crack was visually recognized.

The copper-clad laminated system of the present invention is formed by laminating the above-mentioned copper foil for a flexible printed circuit board and a resin.

The flexible printed circuit board of the present invention is formed by forming a circuit on the copper foil using the copper-clad laminate.

The L / S of the above circuit is preferably 40/40 to 15/15 (μm / μm). In addition, the so-called L / S (line and gap) of a circuit is the width (L: line) of the wiring constituting the circuit, and the interval (S: Clearance). L uses the minimum value of L in the circuit, and S uses the minimum value of S in the circuit.

In addition, L and S only need to be 15 to 40 μm, and they do not need to be the same value. For example, values such as L / S = 20.5 / 35, 35/17 can also be taken.

The electronic device of the present invention is formed using the above-mentioned flexible printed circuit board.

According to the present invention, a copper foil for a flexible printed circuit board having excellent bendability and etching properties can be obtained.

10‧‧‧ compression test machine

10a‧‧‧mould

10b‧‧‧ Upper mold

20‧‧‧board

30‧‧‧CCL samples

30s‧‧‧Bend front

FIG. 1 is a diagram showing a bending test method of CCL.

Hereinafter, embodiments of the copper foil of the present invention will be described. In addition,% in the present invention means mass% unless otherwise specified.

<Composition>

The copper foil of the present invention is composed of 99.0% by mass or more of Cu, and the remainder is unavoidable impurities.

As described above, in the present invention, by refining the crystal grains of the copper foil after recrystallization, the strength is increased and the bendability is improved.

However, in the case of the above-mentioned pure copper-based composition, it is difficult to refine the crystal grains. Therefore, in the initial stage of cold rolling, recrystallization annealing is performed only once, and thereafter, recrystallization annealing is not performed. The processing strain causes dynamic recrystallization to occur, thereby enabling the refinement of crystal grains.

In addition, in order to increase the processing strain in cold rolling, as the workability of the final cold rolling (finish rolling performed after the final annealing in all steps of annealing and rolling repeatedly), it is preferable that η = ln (Sheet thickness before final cold rolling / sheet thickness after final cold rolling) = 3.5 ~ 7.5.

When η is less than 3.5, the accumulation of strain during processing is small, and the nucleus of recrystallized grains is reduced, so there is a tendency that the recrystallized grains become coarse. When η is larger than 7.5, excessive strain accumulates and becomes a driving force for grain growth, and the grain tends to become coarse. More preferably, η = 5.5 to 7.5.

In addition, as an additive element for miniaturizing crystal grains, if it contains 0.003 to 0.825 mass% with respect to the above-mentioned composition, one or more selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg. With the addition of elements, the miniaturization of crystal grains can be achieved more easily. These added elements increase the dislocation density during cold rolling, so that it is easier to refine the crystal grains. In addition, if recrystallization annealing is performed only once at the beginning of cold rolling, and recrystallization annealing is not performed thereafter, a large amount of processing strain is introduced by cold rolling to generate dynamic recrystallization, so that the grains can be more reliably realized. Miniaturization.

If the total content of the above-mentioned added elements is less than 0.003% by mass, it becomes difficult to refine the crystal grains, and if it exceeds 0.825% by mass, the conductivity is reduced. In addition, there is a case where the recrystallization temperature does not recrystallize when it is laminated with the resin, and the strength becomes too high, which results in poor bendability of the copper foil and CCL.

In addition, as a method for refining the crystal grains of the copper foil after recrystallization, in addition to a method of adding an additional element, a method of polymerizing rolling, a method of using a pulse current when using electrolytic copper foil for electrodeposition, Or use electrolytic copper foil to add an appropriate amount of thiourea or glue to the electrolyte.

The copper foil of the present invention can also be made of a copper foil conforming to JIS-H3100 (C1100). Composition of refined copper (TPC) or JIS-H3100 (C1011) oxygen-free copper (OFC).

Moreover, the said TPC or OFC can also be set as the composition which contains the said addition element.

<Average crystal grain size>

The average crystal grain size of the copper foil is 0.5 to 4.0 μm. If the average crystal grain size is less than 0.5 μm, the strength will be too high, the flexural rigidity will increase, and the springback will increase, which is not suitable for flexible printed circuit board applications. If the average crystal grain size exceeds 4.0 μm, it is impossible to achieve miniaturization of the crystal grains, it is difficult to increase the strength to improve the bendability, and the etching factor or the linearity of the circuit is deteriorated to cause a decrease in the etchability.

In order to avoid errors, the average crystal grain size is measured by observing the surface of the foil with a field of view of 100 μm × 100 μm for 3 or more fields. Observation of the foil surface can be performed using SIM (Scanning Ion Microscope) or SEM (Scanning Electron Microscope), and the average crystal grain size can be determined in accordance with JIS H 0501.

However, the twin crystal system is measured as individual crystal grains.

<Tensile strength (TS)>

The tensile strength of copper foil is 235 ~ 290MPa. As described above, the fineness of the crystal grains improves the tensile strength. If the tensile strength is less than 235 MPa, it will be difficult to increase the strength and improve the bendability. If the tensile strength exceeds 290 MPa, the strength becomes too high, the flexural rigidity becomes too large, and the springback becomes large, making it unsuitable for flexible printed circuit board applications.

The tensile strength is based on the tensile test according to IPC-TM650. The width of the test piece is 12.7mm, the room temperature (15 ~ 35 ° C), the tensile speed is 50.8mm / min, and the gauge length is 50mm. The tensile test was performed in a direction parallel to the rolling direction (or MD direction).

<300 ° C for 30 minutes heat treatment>

After the copper foil is heat-treated at 300 ° C. for 30 minutes, the average crystal grain size can be 0.5 to 4.0 μm, and the tensile strength can be 235 to 290 MPa.

The copper foil of the present invention is used for a flexible printed circuit board. At this time, the CCL obtained by laminating the copper foil and the resin is subjected to a heat treatment at 200 to 400 ° C to harden the resin. Therefore, recrystallization may cause crystal grains. Coarse.

Therefore, before and after lamination with the resin, the average crystal grain size and tensile strength of the copper foil change. Therefore, the copper foil for a flexible printed circuit board of claim 1 of the present application specifies a copper foil in a state where the resin has been cured and heat-treated after the copper-clad laminate is laminated with the resin.

On the other hand, the copper foil for flexible printed circuit board of claim 4 of the present application specifies the state when the above-mentioned heat treatment has been performed on the copper foil before being laminated with the resin. The heat treatment at 300 ° C for 30 minutes is a simulation of the temperature conditions at which the resin is hardened and heat-treated when the CCL is laminated.

The copper foil of this invention can be manufactured as follows, for example. First, after adding the above-mentioned additives to a copper ingot, melting and casting, hot rolling, cold rolling, and annealing are performed, and the above-mentioned final cold rolling is performed, whereby a foil can be manufactured.

<Copper-clad laminate and flexible printed circuit board>

Furthermore, the copper foil (1) of the present invention is casted with a resin precursor (for example, a polyimide precursor called varnish) and heated to polymerize it, and (2) a thermoplastic of the same type as the base film is used The adhesive film is used to laminate a base film to the copper foil of the present invention, thereby obtaining a copper-clad laminate (CCL) composed of two layers of a copper foil and a resin substrate. Further, a base film coated with an adhesive was laminated on the copper foil of the present invention, thereby obtaining a copper-clad laminate (CCL) composed of three layers of a copper foil, a resin substrate, and an adhesive layer therebetween. During the production of these CCLs, the copper foil is heat-treated and recrystallized.

To form a circuit using photolithography technology, and plating the circuit as necessary, and The coverlay film is laminated to obtain a flexible printed circuit board (flexible wiring board).

Therefore, the copper-clad laminated system of the present invention is formed by laminating a copper foil and a resin. 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 PET (polyethylene terephthalate), PI (polyimide), LCP (liquid crystal polymer), and PEN (polyethylene naphthalate). Limited to this. As the resin layer, such a resin film may be used.

As a method for laminating a resin layer and a copper foil, the surface of the copper foil may be coated with a material that becomes a resin layer and then heated to form a 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 thermally compression-bonded to the copper foil without using an adhesive. However, from the viewpoint of not applying excessive heat to the resin film, it is preferable to use an adhesive.

When a film is used as the resin layer, the film may be laminated on a copper foil via an adhesive layer. In this case, it is preferable to use an adhesive having the same composition as the film. For example, in the case where a polyimide film is used as the resin layer, it is preferable to use a polyimide-based adhesive in the adhesive layer. Here, the polyfluorene imine adhesive refers to an adhesive containing a fluorene imine bond, and also includes polyether fluorene imine and the like.

The present invention is not limited to the embodiments described above. Moreover, as long as the effect of this invention is exhibited, the copper alloy in the said embodiment may contain other components.

For example, the surface of the copper foil may be subjected to a roughening treatment, an anti-rust treatment, a heat-resistant treatment, or a surface treatment based on a combination of these.

[Example]

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these. Add the elements shown in Table 1 to electrolytic copper with a purity of 99.9% or more. Casting in the environment to obtain ingots. The oxygen content in the ingot did not reach 15 ppm. The ingot was homogenized and annealed at 900 ° C, then hot-rolled to a thickness of 30 mm, and then cold-rolled to a thickness of 14 mm. After annealing, the surface was subjected to surface shaving, and the processing degrees η shown in Table 1 Final cold rolling was performed to obtain a foil having a final thickness of 17 μm. The obtained foil was heat-treated at 300 ° C for 30 minutes to obtain a copper foil sample.

<A. Evaluation of copper foil samples>

Electrical conductivity

For each copper foil sample after the heat treatment, the conductivity (% IACS) at 25 ° C. was measured by the 4-terminal method in accordance with JIS H 0505.

When the conductivity is 75% IACS or more, the conductivity is good.

2.Particle size

The surface of each copper foil sample after the heat treatment was observed using a SEM (Scanning Electron Microscope), and the average particle diameter was determined in accordance with JIS H 0501. However, the twin crystal system is measured as individual crystal grains. The measurement area is 100 μm × 100 μm on the surface.

3. Bending property of copper foil (MIT resistance)

With respect to each of the copper foil samples after the heat treatment, the number of times of MIT folding resistance (the number of times of reciprocating bending) was measured in accordance with JIS P 8115. Here, R of the bending clip is set to 0.38, and the load is set to 500 g.

When the MIT folding resistance number is 75 or more, the bendability of the copper foil is good.

4. Tensile strength of copper foil

The tensile strength of each copper foil sample after the heat treatment was measured under the above conditions by a tensile test according to IPC-TM650.

<Evaluation of B.CCL>

5.Bending of CCL

Copper roughening was performed on one side of a copper foil sample (copper foil before heat treatment) that had not been subjected to the above heat treatment after final cold rolling. As a copper roughening plating bath, a composition of Cu: 10-25 g / L and sulfuric acid: 20-100 g / L is used, and plating is performed at a bath temperature of 20-40 ° C and a current density of 30-70 A / dm ² for 1 to 5 seconds. The copper adhesion amount was 20 g / dm ² .

A layer of polyimide film (the product name "Upilex VT", manufactured by Ube Kosan Co., Ltd., with a thickness of 25 μm) was applied to the roughened plated area of the copper foil sample, and heat-pressed (4 MPa) was subjected to a heat treatment at 300 ° C for 30 minutes to Lamination to obtain CCL samples. Regarding the size of the CCL sample used in the bending test, the rolling direction (long side direction) was 50 mm, and the width direction was 12.7 mm.

As shown in Fig. 1, for this CCL sample 30, a 0.1 mm thick plate 20 (a titanium copper plate conforming to JIS-H3130 (C1990) specifications) was sandwiched so that the copper foil surface became the outer side, and folded in half at the center of the long side direction. It is arranged between the lower mold 10a and the upper mold 10b of the compression tester 10 (product name "Autograph AGS" manufactured by Shimadzu Corporation).

In this state, the upper mold 10b is lowered and the CCL sample 30 is bent so that the half-folded portion is in close contact with the plate 20 (FIG. 1 (a)). Immediately remove the CCL sample 30 from the compression tester 10, and use a microscope (the product name "Single Trigger 3D Measurement Microscope VR-3000" manufactured by Keyence Manufacturing Co., Ltd.) for the "transverse V" -shaped bending front end portion 30s of the folded portion The presence or absence of cracks in the copper foil surface was visually checked at a magnification of 200 times. In addition, the bending front end portion 30s corresponds to a 180-degree close-contact bending with a bending radius of 0.05 mm.

The test was terminated when cracking was confirmed, and the number of times of compression in FIG. 1 (a) was set as the number of times of bending of CCL.

When no crack is confirmed, as shown in Figure 1 (b), The CCL sample 30 is placed between the lower mold 10a and the upper mold 10b of the compression tester 10 with the end portion 30s facing upward, and the upper mold 10b is lowered in this state to open the bent front end portion 30s.

Next, the bending shown in FIG. 1 (a) was performed again, and the presence or absence of cracking at the bending front end portion 30 s was similarly confirmed visually. Hereinafter, the steps of (a) to (b) of FIG. 1 are repeatedly performed in the same manner to determine the number of bending times.

If the number of bending times of the CCL is 3 or more, the bending property of the CCL is good.

6. Etching

In the copper foil portion of the above CCL sample, short strip-shaped circuits of L / S (line / gap) = 40/40 μm, 35/35 μm, 25/25 μm, 20/20 μm, and 15/15 μm were formed. For comparison, a circuit is formed in the same manner as a commercially available rolled copper foil (fine copper foil). Next, the etching factor (the ratio represented by the (etching depth / average etching width above and below) of the circuit) and the linearity of the circuit were visually determined using a microscope, and evaluated according to the following criteria. An evaluation of ○ is good.

○: Compared with commercially available rolled copper foil, the etching factor and the linearity of the circuit are good.

△: Compared with commercially available rolled copper foil, the etching factor and the linearity of the circuit are the same

×: Compared with commercially available rolled copper foil, the etching factor and the linearity of the circuit are poor.

The obtained results are shown in Table 1.

As is clear from Table 1, in the case of each example in which the average crystal grain diameter of the copper foil is 0.5 to 4.0 μm and the tensile strength is 235 to 290 MPa, the bendability and etching properties are excellent. In addition, Example 1 performed polymerization rolling in the last stroke of the final cold rolling.

On the other hand, in the case of Comparative Examples 1 and 4 in which the degree of processing η in the final cold rolling did not reach 3.5, the average crystal grain size of the copper foil exceeded 4.0 μm, and the tensile strength did not reach 235 MPa. The bendability is poor. In addition, in the case of Comparative Example 4, since the average crystal grain size of the copper foil was 4.5 μm slightly larger than 4.0 μm, the etching property was good.

In the case of Comparative Example 3 where the total content of the added elements did not reach the lower limit, the recrystallization of the recrystallized grains due to the added elements was insufficient, and the average crystal grain size of the copper foil was significantly larger than 4.0 μm and coarsened. The tensile strength does not reach 235 MPa, so the bendability and etchability of copper foil and CCL are poor. When the total content of the added elements exceeds the upper limit of Comparative Example 2, the conductivity is poor.

In the case of Comparative Example 5 in which the total content of the added elements exceeds the upper limit value, the recrystallization temperature becomes high and recrystallization does not occur during the heat treatment at 300 ° C., the conductivity decreases, and the tensile strength becomes higher than 290 MPa. Therefore, the bendability of copper foil and CCL is greatly deteriorated.

Claims (9)

A copper foil for a flexible printed circuit board. The copper foil is composed of 99.0% by mass or more of Cu and the remainder is unavoidable impurities. The average crystal grain size is 0.5 to 4.0 μm, and the tensile strength is 235 to 290 MPa. . For example, the copper foil for flexible printed circuit boards in the scope of application for patent No. 1 is composed of fine copper in accordance with JIS-H3100 (C1100) or oxygen-free copper in JIS-H3100 (C1011). For example, the copper foil for flexible printed circuit boards in the scope of patent application No. 1 or 2 further contains 0.003 to 0.825 mass% of a group selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg. One or more kinds of added elements. For example, the copper foil for a flexible printed circuit board in the first or second patent application range has an average crystal grain size of 0.5 to 4.0 μm after heat treatment at 300 ° C. for 30 minutes, and the tensile strength is 235 to 290 MPa. For example, the copper foil for flexible printed circuit board of the scope of application patent No. 1 or 2, wherein a copper-clad laminated body formed by laminating a polyimide resin film with a thickness of 25 μm on one side of the copper foil with a bending radius of 0.05 The copper foil was bent at 180 degrees so that the copper foil became the outer side, and then the bent portion was returned to 0 degrees. After repeating this test 3 times, when the copper foil was observed at 200 times, no crack was visually recognized. A copper-clad laminate is formed by laminating a copper foil for a flexible printed circuit board and a resin in any one of claims 1 to 5. A flexible printed circuit board is formed by forming a circuit on the copper foil by using a copper-clad laminate in the sixth aspect of the patent application. For example, the flexible printed circuit board of the seventh scope of the patent application, wherein the L / S of the circuit is 40/40 ~ 15/15 (μm / μm). An electronic device using a flexible printed circuit board with a scope of patent application No. 7 or 8.