CN107046768B - Copper foil for flexible printed circuit boards, copper clad laminates using the same, flexible printed circuit boards, and electronic devices - Google Patents

Abstract

The technical problem of this invention is providing the copper foil for flexible printed circuit boards which is excellent in flexibility and etchability. The solution of the present invention is a copper foil for a flexible printed circuit board, which is a copper foil containing more than 99.0% by mass of Cu and the balance being unavoidable impurities, having an average crystal particle size of 0.5 to 4.0 μm, and a tensile strength of It is 235~290MPa.

Description

Copper foil for flexible printed circuit boards, copper clad laminates using it, flexible printed circuit boards and electronic device

technical field

The present invention relates to a copper foil suitable for wiring members such as a flexible printed circuit board, a copper clad laminate using the same, a flexible wiring board, and an electronic device.

Background technique

A flexible printed circuit board (flexible printed circuit board, hereinafter referred to as "FPC") is flexible, and thus is widely used in curved parts or movable parts of electronic circuits. For example, FPCs are used in movable parts of optical disk-related devices such as HDD, DVD, and CD-ROM, or curved parts of flip-type mobile phones.

The FPC is obtained by etching a copper clad laminate (Copper Clad Laminate, hereinafter referred to as CCL) formed by laminating copper foil and resin to form wiring, and covering it with a resin layer called a cover layer. In the previous stage of laminating the coating layer, the surface of the copper foil is etched as part of a surface modification step for improving the adhesion between the copper foil and the coating layer. In addition, in order to reduce the thickness of the copper foil and improve the flexibility, thickness reduction (meat reduction) etching may also be performed.

However, with the miniaturization, thinner, and higher performance of electronic devices, it is required to mount FPCs in these devices at high density. of bending.

On the other hand, copper foils having improved high-cycle buckling properties represented by IPC buckling properties have been developed (Patent Documents 1 and 2).

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2010-100887

Patent Document 2: Japanese Unexamined Patent Publication No. 2009-111203.

Contents of the invention

The technical problem to be solved by the invention

However, in order to mount FPCs at a high density as described above, it is necessary to improve the bendability typified by the MIT folding resistance, and there is a problem that the improvement of the bendability of the conventional copper foil is not sufficient.

In addition, with the miniaturization, thinning and high performance of electronic devices, the circuit width and pitch width of FPC have also been miniaturized to about 20-30 μm. When forming circuits by etching, there are problems that the etching coefficient and circuit linearity are easy to deteriorate. Solve this problem.

The present invention was made to solve the above-mentioned technical problems, and an object of the present invention is to provide a copper foil for a flexible printed circuit board excellent in flexibility and etchability, a copper-clad laminate using the same, a flexible printed circuit board, and an electronic device.

Solutions for technical problems

As a result of various studies conducted by the present inventors, it was found that by making the crystal grains after recrystallization of the copper foil finer, the strength can be increased and the bendability can be improved. This is because, according to the Hall-Petch principle, the finer the crystal grains, the higher the strength and the higher the bendability. However, if the crystal grains are made too fine, the strength becomes too high, the bending rigidity becomes too large, and the recoil becomes too large, making it unsuitable for use in flexible printed circuit boards. Therefore, the range of the crystal grain size is specified.

In addition, by reducing the crystal grain size to about 1/10 of the circuit width of about 20 to 30 μm in recent FPCs, it is also possible to improve the etching coefficient and circuit linearity when forming circuits by etching.

That is, the copper foil for a flexible printed circuit board of the present invention is a copper foil containing 99.0% by mass or more of Cu and the remainder being unavoidable impurities, has an average crystal grain size of 0.5 to 4.0 μm, and has a tensile strength of 235 to 290 MPa.

In the copper foil for flexible printed boards of the present invention, it is preferable that it is formed of tough copper prescribed in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1011).

Preferably, one or more additional elements selected from the group consisting of P, Ti, Sn, Ni, Be, Zn, In, and Mg are contained in a total of 0.003 to 0.825% by mass.

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

Preferably, after repeating the following test three times, no cracks can be visually confirmed when the above-mentioned copper foil is observed at 200 magnifications: a polyimide resin film obtained by laminating a polyimide resin film with a thickness of 25 μm on one side of the above-mentioned copper foil The copper-clad laminate was subjected to 180-degree close contact bending (contact-bending) with a bending radius of 0.05 mm and the above-mentioned copper foil on the outside, and then the bent portion was returned to 0 degree.

The copper-clad laminate of the present invention is obtained by laminating the above-mentioned copper foil for a flexible printed circuit board and a resin layer.

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

Preferably, the L/S of the above circuit is 40/40~15/15 (μm/μm). The L/S (line and space) of a circuit is the ratio of the width (L: line width) of the wiring constituting the circuit to the interval (S: spacing) between adjacent wirings. L adopts the minimum value of L in the circuit, and S adopts the minimum value of S in the circuit.

It should be noted that L and S only need to be 15 to 40 μm, and they do not have to be the same value. For example, values such as L/S=20.5/35, 35/17, etc. may be used.

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

The effect of the invention

According to this invention, the copper foil for flexible printed circuit boards excellent in flexibility and etchability can be obtained.

Description of drawings

FIG. 1 is a diagram illustrating a method of a bendability test of a CCL.

Detailed ways

Hereinafter, embodiment of the copper foil of this invention is demonstrated. It should be noted that, in the present invention, unless otherwise specified, % represents mass %.

＜Composition＞

The copper foil of this invention contains 99.0 mass % or more of Cu, and the balance is an unavoidable impurity.

As described above, in the present invention, the crystal grains after the recrystallization of the copper foil are refined to improve the strength and improve the bendability.

However, in the case of the above-mentioned pure copper-based composition, it is difficult to refine the crystal grains. Therefore, recrystallization annealing is performed only once in the initial stage of cold rolling, and recrystallization annealing is not performed thereafter, so that a large amount of processing strain can be introduced by cold rolling. , dynamic recrystallization occurs, and the miniaturization of grains is realized.

In addition, in order to increase the processing strain in cold rolling, it is preferable that η=ln( Thickness before final cold rolling/thickness after final cold rolling)=3.5~7.5.

When η is less than 3.5, there is little accumulation of strain during working, and there are fewer nuclei of recrystallized grains, so the recrystallized grains tend to become coarser. When η is greater than 7.5, excessive strain accumulation becomes a driving force for crystal grain growth, and the crystal grains tend to become coarser. More preferably η=5.5~7.5.

In addition, as an additive element for refining crystal grains, one or more additives selected from P, Ti, Sn, Ni, Be, Zn, In, and Mg are contained in a total of 0.003 to 0.825% by mass relative to the above composition. element, it is easier to realize the miniaturization of crystal grains. These added elements increase the dislocation density during cold rolling, so that crystal grains can be more easily refined. In addition, if recrystallization annealing is performed only once at the initial stage of cold rolling, and recrystallization annealing is not performed thereafter, a large amount of processing strain can be introduced by cold rolling, dynamic recrystallization can occur, and crystal grains can be more reliably refined.

If the total content of the above added elements is less than 0.003% by mass, it will be difficult to refine the crystal grains, and if it exceeds 0.825% by mass, the electrical conductivity may decrease. There are also cases in which the recrystallization temperature rises and recrystallization does not occur when laminated with a resin, the strength becomes too high, and the bendability of copper foil and CCL may deteriorate.

It should be noted that, as a method of making the crystal grains after recrystallization of the copper foil finer, in addition to the method of adding additional elements, the method of performing double-layer rolling (overlapping pressure extension), and the method of electrolytic copper foil In the case of electrocrystallization, the method of using a pulse current, or in the case of electrolytic copper foil, the method of adding an appropriate amount of thiourea or animal glue to the electrolyte.

The copper foil of the present invention may be composed of tough copper (TPC) prescribed in JIS-H3100 (C1100) or oxygen-free copper (OFC) of JIS-H3100 (C1011).

In addition, it may be a composition containing the above-mentioned additional elements with respect to the above-mentioned TPC or OFC.

＜Average crystal grain size＞

The average crystal grain size of the copper foil is 0.5 to 4.0 μm. When the average crystal grain size is less than 0.5 μm, the strength becomes too high, the bending rigidity becomes too large, and the recoil becomes too large, making it unsuitable for use in flexible printed circuit boards. When the average crystal grain size exceeds 4.0 μm, the crystal grains cannot be made finer, and it is difficult to increase the strength and improve the bendability. At the same time, the etching coefficient and the linearity of the circuit are deteriorated, and the etchability is lowered.

In order to avoid errors, the surface of the foil was observed with a field of view of 100 μm×100 μm in more than 3 fields of view to measure the average crystal grain size. In the observation of the foil surface, the average crystal particle size can be determined in accordance with JIS H 0501 using SIM (Scanning Ion Microscope) or SEM (Scanning Electron Microscope).

However, twin crystals were regarded as separate crystal grains and measured.

＜Tensile Strength (TS)＞

The tensile strength of copper foil is 235~290MPa. As described above, the tensile strength is improved by making crystal grains finer. If the tensile strength is less than 235 MPa, it will be difficult to increase the strength and improve the bendability. When the tensile strength exceeds 290 MPa, the strength becomes too high, the bending rigidity is too large, and the recoil is too large, which is not suitable for use in flexible printed circuit boards.

For the tensile strength, through the tensile test according to IPC-TM650, the width of the test piece is 12.7mm, the room temperature (15~35℃), the tensile speed is 50.8mm/min, and the gauge length is 50mm, in the rolling direction with the copper foil (or MD direction) parallel to the direction of the tensile test.

<Heat treatment at 300°C for 30 minutes>

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

The copper foil of the present invention can be used in flexible printed circuit boards. At this time, CCL, which is formed by laminating copper foil and resin, is heat-treated at 200 to 400°C to cure the resin, so crystal grains may be coarsened by recrystallization. .

Therefore, the average crystal grain size and tensile strength of the copper foil change before and after being laminated with the resin. Therefore, the copper foil for flexible printed boards according to claim 1 of the present application is defined as a copper foil in a state where a resin-laminated copper-clad laminate is formed and subjected to a curing heat treatment of the resin.

On the other hand, the copper foil for flexible printed circuit boards of Claim 4 of this application is prescribed|regulated to the state when the said heat treatment was performed to the copper foil before resin lamination. This heat treatment at 300° C. for 30 minutes is a heat treatment simulating a temperature condition of a curing heat treatment for a resin during lamination of CCLs.

The copper foil of this invention can be manufactured as follows, for example. First, a copper ingot is melted and cast by adding the above additives, followed by hot rolling, cold rolling, annealing, and the above-mentioned final cold rolling to prepare a foil.

＜Copper-clad laminates and flexible printed circuit boards＞

In addition, on the copper foil of the present invention, (1) cast a resin precursor (for example, a polyimide precursor called varnish) and heat it to polymerize, (2) use the same thermoplastic adhesive as the base film Adhesive Lamination of the base film on the copper foil of the present invention yields a copper-clad laminate (CCL) consisting of two layers of the copper foil and the resin substrate. In addition, by laminating a base film coated with an adhesive on the copper foil of the present invention, a copper clad laminate (CCL) consisting of three layers of copper foil, a resin substrate, and an adhesive layer therebetween can be obtained. When producing these CCLs, the copper foil is heat-treated to be recrystallized.

A circuit is formed on them using a photolithography technique, plating is performed on the circuit as necessary, and a coverlay film is laminated to obtain a flexible printed circuit board (flexible wiring board).

Therefore, the copper-clad laminate of the present invention is obtained by laminating copper foil and a resin layer. Moreover, the flexible printed circuit board of this invention forms a circuit on the 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). this. In addition, their resin films can be used as the resin layer.

As a lamination method of a resin layer and copper foil, the material which forms a resin layer is apply|coated on the surface of copper foil, and it heats and forms into a film. In addition, 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 bonded to the copper foil by thermocompression without using an adhesive. Among these, it is preferable to use an adhesive from the viewpoint of not applying unnecessary heat to the resin film.

When using a film as a resin layer, this film can be laminated|stacked on copper foil via an adhesive bond layer. In this case, it is preferable to use an adhesive having the same composition as that of the film. For example, when using a polyimide film as a resin layer, it is preferable to use a polyimide adhesive agent also for an adhesive bond layer. It should be noted that the polyimide adhesive mentioned here refers to an adhesive including imide bonds, and also includes polyetherimide and the like.

It should be noted that the present invention is not limited to the above-mentioned embodiments. In addition, the copper alloy of the above-mentioned embodiment may contain other components as long as the effects of the present invention are exhibited.

For example, surface treatment based on roughening treatment, antirust treatment, heat resistance treatment, or a combination thereof may be performed on the surface of the copper foil.

Example

Next, examples are given to illustrate the present invention in more detail, but the present invention is not limited to them. The elements shown in Table 1 were added to electrolytic copper with a purity of 99.9% or more, and cast in an Ar atmosphere to obtain ingots. The oxygen content in the ingot is below 15 ppm. After homogenizing annealing at 900°C, the ingot was hot-rolled to a thickness of 30 mm, then cold-rolled to a thickness of 14 mm, and then the surface was scraped after one annealing, as shown in Table 1. The final cold rolling was performed to a working degree η to obtain a foil with a final thickness of 17 μm. Heat treatment was applied to the obtained foil at 300° C. for 30 minutes to obtain a copper foil sample.

＜A. Evaluation of copper foil samples＞

1. Conductivity

The electrical conductivity (%IACS) at 25° C. was measured by a four-probe method based on JIS H 0505 for each copper foil sample after the heat treatment.

If the electrical conductivity is 75%IACS or more, the electrical 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 H0501. However, twin crystals were regarded as separate crystal grains and measured. The measurement area was set at 100 μm×100 μm on the surface.

3. Flexibility of copper foil (MIT folding resistance)

About each copper foil sample after the said heat treatment, the MIT folding endurance number (number of times of reciprocating bending) was measured based on JISP8115. Among them, the R of the bending fixture is 0.38, and the load is 500g.

The bendability of copper foil is favorable that the number of times of MIT folding endurance is 75 times or more.

4. Tensile strength of copper foil

About each copper foil sample after the said heat treatment, the tensile strength was measured on the said conditions by the tensile test based on IPC-TM650.

＜B. Evaluation of CCL＞

5. Flexibility of CCL

After the final cold rolling, rough copper plating was performed on one side of the copper foil sample (copper foil before heat treatment) that was not subjected to the above-mentioned heat treatment. As a rough copper plating bath, use Cu: 10-25g/L, sulfuric acid: 20-100g/L, and conduct electroplating for 1-5 seconds at a bath temperature of 20-40°C and a current density of ^30-70A /dm2, so that The amount of copper deposition was 20 g/dm ² .

Laminate a polyimide film (product name "UPILEX VT" manufactured by Ube Industries, Ltd., thickness 25 μm) on the roughened plated surface of the copper foil sample, and apply heat treatment at 300°C x 30 minutes with a hot press (4MPa) For bonding, a CCL sample was obtained. The dimensions of the CCL sample used in the bending test were 50 mm in the rolling direction (longitudinal direction) and 12.7 mm in the width direction.

As shown in FIG. 1 , this CCL sample 30 is sandwiched between a 0.1 mm thick plate 20 (titanium copper plate specified in JIS-H3130 (C1990)) so that the copper foil surface is on the outside, folded in half in the center of the longitudinal direction, and placed Between the lower mold 10a and the upper mold 10b of a compression testing machine 10 (product name "AUTOGRAPH AGS" manufactured by Shimadzu Corporation).

In this state, the upper mold 10b was lowered, and the CCL sample 30 was bent so that the half-folded portion was in close contact with the plate 20 ( FIG. 1( a )). Immediately take out the CCL sample 30 from the compression testing machine 10, and use a microscope (product name "One-shot 3D measuring microscope VR-3000" manufactured by KEYENCE CORPORATION) to measure the "horizontal V-shaped" curved top end of the folded part at 200 s for 30 seconds. The presence or absence of cracks on the copper foil surface was visually checked at a magnification of 2. It should be noted that 30 s of the bent tip portion corresponds to a 180-degree close-contact bend with a bend radius of 0.05 mm.

The test was terminated when the rupture was confirmed, and the number of compressions in FIG. 1( a ) was counted as the number of bending times of the CCL.

When no fracture is confirmed, as shown in FIG. 1( b ), the CCL sample 30 is placed between the lower mold 10a and the upper mold 10b of the compression testing machine 10 with the curved tip 30s facing upward. In this state The upper die 10b is lowered to open the curved tip portion 30s.

Then, the bending in FIG. 1( a ) was performed again, and similarly, the presence or absence of cracks in the bent distal end portion 30 s was confirmed visually. Thereafter, the steps of Fig. 1 (a) to (b) are repeated similarly to determine the number of times of bending.

When the number of times of bending of CCL is 3 or more, the bendability of CCL is good.

6. Etching

Striped circuits of L/S (line width/space) = 40/40 μm, 35/35 μm, 25/25 μm, 20/20 μm, and 15/15 μm were formed on the copper foil portion of the above CCL sample. For comparison, a circuit was formed in the same manner as a commercially available rolled copper foil (tough copper foil). Then, the etching coefficient (ratio represented by (etching depth/average etching width of the upper and lower sides) of the circuit) and the linearity of the circuit were visually judged with a microscope, and evaluated according to the following criteria. When the evaluation was ◯, it was good.

○: Compared with the commercially available rolled copper foil, the etch coefficient and the linearity of the circuit are good

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

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

The obtained results are shown in Table 1.

[Table 1]

As is clear from Table 1, the average crystal grain size of the copper foil in each example is 0.5 to 4.0 μm, and the tensile strength is 235 to 290 MPa, and the bendability and etching properties in this case are excellent. It should be noted that, in Example 1, double-layer rolling was performed in the last pass of the final cold rolling.

On the other hand, in Comparative Examples 1 and 4, the processing degree η of the final cold rolling was lower than 3.5. In this case, the average crystal grain size of the copper foil exceeded 4.0 μm, the tensile strength was lower than 235 MPa, and the bending of the copper foil and CCL Poor sex. In addition, in the case of the comparative example 4, since the average crystal grain diameter of copper foil was 4.5 micrometers which were slightly larger than 4.0 micrometers, etching property was favorable.

In Comparative Example 3, the total content of the added elements was lower than the lower limit. In this case, the refinement of the recrystallized grains due to the added elements was insufficient, and the average crystal grain size of the copper foil greatly exceeded 4.0 μm and was coarsened. When the strength is lower than 235MPa, the bendability and etchability of copper foil and CCL are poor. In Comparative Example 2, the total content of the added elements exceeded the upper limit, and in this case, the electrical conductivity was poor.

In Comparative Example 5, the total content of the added elements exceeded the upper limit. In this case, the recrystallization temperature became high, and recrystallization did not occur during the heat treatment at 300° C., and the electrical conductivity decreased. At the same time, the tensile strength increased to exceed 290 MPa. Therefore, the bendability of copper foil and CCL deteriorates significantly.

Claims (9)

1. A copper foil for flexible printed board, which comprises 99.0 mass% or more of Cu and the balance of unavoidable impurities,

an average crystal grain size of 0.5 ~ 4.0.0 μm and a tensile strength of 235 ~ 290MPa,

the number of reciprocating bending times, which is the number of MIT bending resistance times measured according to JIS P8115, is 75 to 129, wherein R of a bending jig is 0.38mm, and a load is 500 g.

2. The copper foil for flexible printed boards according to claim 1,

it is formed of tough pitch copper prescribed in JIS-H3100-alloy designation C1100 or oxygen-free copper prescribed in JIS-H3100-alloy designation C1011.

3. The copper foil for flexible printed boards according to claim 1 or 2, wherein,

and 1 or more additional elements selected from P, Ti, Sn, Ni, Be, Zn, In and Mg In a total amount of 0.003 ~ 0.825.825 mass%.

4. The copper foil for flexible printed boards according to claim 1 or 2, wherein,

the average crystal particle size after heat treatment at 300 ℃ for 30 minutes was 0.5 ~ 4.0.0 μm, and the tensile strength was 235 ~ 290 MPa.

5. The copper foil for flexible printed boards according to claim 1 or 2, wherein,

after repeating the following test 3 times, no cracks were visually observed when the copper foil was observed at 200 times: a copper-clad laminate obtained by laminating a polyimide resin film having a thickness of 25 μm on one surface of the copper foil was bent tightly at 180 degrees with a bending radius of 0.05mm so that the copper foil was positioned outside, and then the bent portion was returned to 0 degree.

6. A copper-clad laminate obtained by laminating the copper foil for a flexible printed board according to claim 1 ~ 5 and a resin layer.

7. A flexible printed board obtained by forming a circuit on the copper foil using the copper-clad laminate according to claim 6.

8. The flexible printed substrate of claim 7, the circuit having L and S of 15 ~ 40 μm, wherein L is the width of the wires making up the circuit and L is the minimum value of L in the circuit, S is the spacing of adjacent wires and S is the minimum value of S in the circuit.

9. An electronic device using the flexible printed substrate according to claim 7 or 8.