JP6793138B2 - Copper foil for flexible printed circuit boards, copper-clad laminates using it, flexible printed circuit boards, and electronic devices - Google Patents

Description

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

Since a flexible printed circuit board (flexible wiring board, hereinafter referred to as "FPC") has flexibility, it is widely used for bent parts and movable parts of electronic circuits. For example, FPCs are used for moving parts of disc-related devices such as HDDs, DVDs, and CD-ROMs, and bent parts of foldable mobile phones.
FPC is formed by etching Copper Clad Laminate (copper-clad laminate, hereinafter referred to as CCL) in which copper foil and resin are laminated to form wiring, and coating the wiring with a resin layer called a coverlay. Before laminating the coverlay, the surface of the copper foil is etched as part of the surface modification step for improving the adhesion between the copper foil and the coverlay. Further, in order to reduce the thickness of the copper foil and improve the flexibility, thinning etching may be performed.

By the way, as electronic devices become smaller, thinner, and have higher performance, miniaturization of FPC circuit width and space width (for example, about 20 to 30 μm) is required. When the FPC circuit is miniaturized, there is a problem that the etching factor and the circuit linearity tend to deteriorate when the circuit is formed by etching (Patent Documents 1 and 2).

JP-A-2017-141501 JP-A-2017-179390

However, in the conventional technique, although the average crystal grain size and the like are optimized as a measure for improving the etching property, there is room for improvement in the etching property in the formation of a fine circuit.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a copper foil for a flexible printed circuit board having excellent etching properties, a copper-clad laminate using the same, a flexible printed circuit board, and an electronic device. To do.

As a result of various studies, the present inventors have found that the etching rate in the <220> direction is high. In particular, it has been said that there is no difference in etching rate depending on the orientation in etching with a cupric chloride etchant. Therefore, we succeeded in further improving the etching property (particularly the soft etching property and the etching factor) by increasing the number of crystal grains in the <220> orientation.

That is, the copper foil for a flexible printed substrate of the present invention is a rolled copper foil composed of 99.0% by mass or more of Cu and unavoidable impurities in the balance, and is annealed at 300 ° C. × 30 min (however, the heating rate is 100 ° C./min to 300 ° C.). At ° C / min), it has recrystallized grains with an average crystal grain size of 0.5 to 4.0 μm , and the degree of aggregation indicated by the X-ray diffraction intensity I (220) / I ₀ (220) on the surface of the copper foil. Is 1.3 or more and less than 7.0, and the conductivity is 80% or more.

The copper foil for a flexible printed circuit board of the present invention is preferably made of tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1020).
The copper foil for a flexible printed circuit board of the present invention further contains at least one or a total of two or more selected as additive elements from the group consisting of P, Ag, Si, Ge, Al, Ga, Zn, Sn and Sb. It is preferably contained in an amount of 0.7% by mass or less .

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

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

The electronic device of the present invention uses the flexible printed circuit board.

According to the present invention, a copper foil for a flexible printed circuit board having excellent etching properties (particularly soft etching properties and etching factors) can be obtained.

It is a figure which shows the measuring method of the etching factor EF. It is another figure which shows the measuring method of the etching factor EF.

Hereinafter, embodiments of the copper foil according to the present invention will be described. In the present invention,% means mass% unless otherwise specified.
First, the soft etching property and the etching factor EF in the etching property will be described.
Soft etching property is an index showing the accuracy of the circuit due to etching due to the adhesion between the copper foil surface and the resist. The better the adhesion of the resist and the more the resist follows the copper foil surface, the more the etching solution penetrates between them. As a result, the problem of a part of the circuit being chipped is suppressed, a uniform circuit pattern is obtained over the entire surface of the copper foil, and the yield is improved.
The etching factor EF is an index of the cross-sectional shape of the circuit formed by etching, and the higher the EF, the sharper the cross section of the circuit formed by etching, so that the accuracy of the circuit pattern is improved when the circuit is miniaturized.

Even if the soft etching property is good, if the etching factor EF is inferior, a uniform circuit pattern is obtained on the entire surface of the copper foil and the yield is improved, but the accuracy of the circuit pattern is lowered when the circuit is miniaturized.
On the contrary, even if the etching factor EF is good, if the soft etching property is inferior, the accuracy of the circuit pattern is improved when the circuit is miniaturized, but (the etching solution easily penetrates between the copper foil surface and the resist). Therefore, a problem occurs in which a part of the circuit is chipped, and a uniform circuit pattern cannot be obtained on the entire surface of the copper foil, resulting in a decrease in yield.

<Composition>
The copper foil according to the present invention is composed of 99.0% by mass or more of Cu and unavoidable impurities in the balance.
In the examples of the present invention, by refining the crystal grain size of the copper foil before the final cold rolling, the accumulation of dislocations of the copper foil is promoted during the cold rolling, and the recrystallized grains become fine during recrystallization. Become. Further, when the strain rate is extremely high in the final pass of cold rolling, the recrystallized grains are oriented in a specific direction at the time of recrystallization, that is, the {200} plane aggregation is suppressed and the {220} plane aggregation is suppressed. Can be increased and the etchability is improved.

Further, in order to refine the crystal grains after recrystallization of the copper foil, the crystal grain size before the final cold rolling performed after the final annealing in the entire process of repeating annealing and rolling is set to 5 μm or more and 20 μm or less. preferable.
Specifically, the particle size can be controlled by adjusting the temperature of the final annealing and the workability of cold rolling before the final annealing. The final annealing temperature varies depending on the manufacturing conditions of the copper foil and is not limited, but may be, for example, 300 to 400 ° C. Further, the workability of cold rolling before final annealing is not limited, but for example, the workability η may be 1.6 to 3.0.
The workability η is represented by η = ln (A0 / A1), where the thickness of the material immediately before cold rolling before final annealing is A0 and the thickness of the material immediately after cold rolling before final annealing is A1.

When the crystal grain size before the final cold rolling is more than 20 μm, the entanglement of dislocations during processing becomes small and the strain accumulation becomes small, so the strain is not released after recrystallization and the grain refinement is insufficient. It tends to be. If the crystal grain size before the final cold rolling is smaller than 5 μm, dislocation entanglement during processing occurs in almost the entire region of the copper foil and no further entanglement is possible, and recrystallized grains are formed during recrystallization of the copper foil. The effect of miniaturization is saturated. Therefore, the lower limit of the crystal grain size before the final cold rolling was set to 5 μm.

Further, as the additive element, at least one or two or more selected from the group consisting of P, Ag, Si, Ge, Al, Ga, Zn, Sn and Sb are added to the above composition in a total of 0.7% by mass or less. When it is contained, the recrystallized grains can be made finer.
Since the additive element increases the frequency of dislocation entanglement during cold rolling, the recrystallized grains can be made finer.
If the above additive elements are contained in excess of 0.7% by mass in total, the conductivity is lowered and the copper foil for a flexible substrate may not be suitable. Therefore, the upper limit is 0.7% by mass. The lower limit of the content of the additive element is not particularly limited, but for example, it is industrially difficult to control each element to be smaller than 0.0005% by mass, so the lower limit of the content of each element is preferably 0.0005% by mass.

The copper foil according to the present invention may be composed of tough pitch copper (TPC) standardized for JIS-H3100 (C1100) or oxygen-free copper (OFC) of JIS-H3100 (C1020).
Further, the composition may be such that P is contained in the TPC or OFC.

<Average crystal grain size>
The average crystal grain size of the copper foil is 0.5-4.0 μm. If the average crystal grain size is less than 0.5 μm, the strength becomes too high, the bending rigidity becomes large, and the springback becomes large, which is not suitable for flexible printed circuit board applications. If the average crystal grain size exceeds 4.0 μm, the soft etchability deteriorates.
The average crystal grain size is measured by observing the foil surface in a field of view of 100 μm × 100 μm in three or more fields of view to avoid errors. For the observation of the foil surface, SIM (Scanning Ion Microscope) or SEM (Scanning Electron Microscope) can be used, and the average crystal grain size can be determined based on JIS-H0501. However, twins are measured as separate crystal grains.

<Aggregation>
The degree of aggregation represented by the X-ray diffraction intensity I (220) / I ₀ (220) on the surface of the copper foil is 1.3 or more and less than 7.0.
When the degree of aggregation is less than 1.3, the etching rate in the thickness direction becomes low, and the etching factor described later for the copper foil decreases. When the strain rate is 7.0 or more, the etching factor is good, but the shape of the rolled copper foil is deteriorated, which may make it difficult to use as a copper foil for FPC.

The degree of aggregation is measured as follows. First, the X-ray diffraction intensity of the {220} plane of the rolled surface of the copper foil is measured and designated as I (220).
Further, under the same conditions, the X-ray diffraction intensity of the {220} plane was measured for pure copper powder (325 mesh (JIS Z8801, purity 99.5%), used after heating at 300 ° C. for 1 hour in a hydrogen stream). Then, I ₀ (220).

Then, it is standardized as follows.
-{220} surface aggregation degree: I (220) / I ₀ (220)
The measurement conditions for X-ray diffraction are as follows.
・ Incident X-ray source: Co,
・ Acceleration voltage: 25kV,
・ Tube current: 20mA,
・ Divergence slit: 1 degree,
・ Scattering slit: 1 degree,
・ Light receiving slit: 0.3 mm,
・ Divergence vertical restriction slit: 10 mm,
・ Monochrome light receiving slit 0.8mm

<Heat treatment at 300 ° C for 30 minutes>
The copper foil according to the present invention is used for a flexible printed circuit board, and at that time, the CCL in which the copper foil and the resin are laminated is subjected to a heat treatment for curing the resin at 200 to 400 ° C. The degree of aggregation displayed by I (220) / I ₀ (220) changes.
Therefore, the average crystal grain size and the degree of aggregation change before and after laminating with the resin. Therefore, the copper foil for a flexible printed circuit board according to claim 1 of the present application defines a copper foil in a state of being heat-treated when it becomes a copper-clad laminate after being laminated with a resin. That is, since it has already been heat-treated, it is a copper foil in a state where no new heat treatment is performed.
On the other hand, the copper foil for a flexible printed circuit board according to claim 4 of the present application is in a state when the copper foil before being laminated with the resin is subjected to the above heat treatment (for example, the copper foil coil before the heat treatment is delivered to the CCL manufacturing factory. The heated state when laminated on the CCL) is specified. This heat treatment at 300 ° C. for 30 minutes imitates the temperature conditions for curing and heat-treating the resin during lamination of CCL. In order to prevent oxidation of the copper foil surface due to the heat treatment, the heat treatment atmosphere is preferably a reducing or non-oxidizing atmosphere, for example, a vacuum atmosphere, argon, nitrogen, hydrogen, carbon monoxide, etc., or these. The atmosphere may be a mixed gas. The heating rate may be between 100 and 300 ° C./min.

The copper foil of the present invention can be produced, for example, as follows. First, the copper ingot is melted and cast, then hot-rolled, cold-rolled and annealed.Preferably, recrystallization annealing is performed at the initial stage of cold rolling, and final recrystallization annealing and final cold rolling are performed. By doing so, the foil can be produced.
The total working ratio in the final cold rolling eta, before final cold rolling and the average grain size after final recrystallization annealing, and by adjusting the strain rate of the final pass of the final cold rolling, the average crystal grain size, and You can control the degree of aggregation.

When the total workability η of the final cold rolling is 6.10 or more, the degree of aggregation displayed by I (220) / I ₀ (220) can be increased more reliably.
Assuming that the average crystal grain size before the final cold rolling and after the final recrystallization annealing is 5 to 20 μm, the average crystal grain size of the product can be surely set to 0.5 to 4.0 μm.
When the strain rate of the final pass of the final cold rolling is 1000 (/ sec) or more, the degree of aggregation can be increased more reliably.

<Copper-clad laminate and flexible printed circuit board>
Further, (1) a resin precursor (for example, a polyimide precursor called varnish) is cast on the copper foil of the present invention and polymerized by applying heat, and (2) using the same type of thermoplastic adhesive as the base film. By laminating the base film on the copper foil of the present invention, a copper-clad laminate (CCL) composed of two layers of the copper foil and a resin base material can be obtained. Further, by laminating a base film coated with an adhesive on the copper foil of the present invention, a copper-clad laminate (CCL) composed of three layers of a copper foil, a resin base material, and an adhesive layer between them can be obtained. During the production of these CCLs, the copper foil is heat treated and recrystallized.
A flexible printed circuit board (flexible wiring board) can be obtained by forming a circuit on these using photolithography technology, plating the circuit as necessary, and laminating a coverlay film.

Therefore, the copper-clad laminate of the present invention is formed by laminating a copper foil and a resin layer. Further, 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, but are not limited to, PET (polyethylene terephthalate), PI (polyimide), LCP (liquid crystal polymer), and PEN (polyethylene naphthalate). Moreover, you may use these resin films as a resin layer.
As a method of laminating the resin layer and the copper foil, a material to be a resin layer may be applied to the surface of the copper foil to form a heat 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 thermocompression bonded to the copper foil without using the adhesive. However, it is preferable to use an adhesive from the viewpoint of not applying extra heat to the resin film.

When a film is used as the resin layer, this film may be laminated on the copper foil via the adhesive layer. In this case, it is preferable to use an adhesive having the same composition as the film. For example, when a polyimide film is used as the resin layer, it is preferable to use a polyimide-based adhesive as the adhesive layer. The polyimide adhesive referred to here refers to an adhesive containing an imide bond, and also includes polyetherimide and the like.

The present invention is not limited to the above embodiment. Further, the copper alloy in the above-described embodiment may contain other components as long as the effects of the present invention are exhibited. Further, electrolytic copper foil may be used.
For example, the surface of the copper foil may be roughened, rust-proofed, heat-resistant, or surface-treated by a combination thereof.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto. The elements shown in Table 1 were added to the electrolytic copper to obtain the composition shown in Table 1, and casting was performed in an Ar atmosphere to obtain an ingot. The oxygen content in the ingot was less than 15 ppm. The ingot was homogenized and annealed at 900 ° C., hot-rolled, then cold-rolled and recrystallized annealed repeatedly, and finally recrystallized and finally cold-rolled to obtain rolled copper foil.
The obtained rolled copper foil was heat-treated at 300 ° C. for 30 minutes in an argon atmosphere to obtain a copper foil sample. The copper foil after the heat treatment imitates the state of being heat-treated at the time of laminating the CCL.

<Evaluation of copper foil sample>
1. 1. Conductivity For each copper foil sample after the above heat treatment, the conductivity (% IACS) at 25 ° C. was measured by the 4-terminal method based on JIS H 0505.
If the conductivity is greater than 80% IACS, the conductivity is good.
2. 2. Aggregation degree For each copper foil sample after the above heat treatment, the aggregation degree was measured by the above-mentioned method using an X-ray diffractometer (RINT-2500: manufactured by Rigaku Denki). In addition to the degree of aggregation indicated by I (220) / I ₀ (220), the X-ray diffraction intensity of the {200} plane was measured in the same manner, and I (200) / I ₀ (200) was also determined.

3. 3. Etching factor EF
A copper foil and a resin were laminated, and then a dry film resist was laminated on the surface of the copper foil to form a strip-shaped (L / S = 25/25) circuit pattern on the resist. Etching was carried out by changing the etching time by spray etching of cupric chloride etchant.

There are various methods for measuring EF, but in the present invention, the etching factor EF is evaluated by the etching rate in the depth direction with respect to the width direction, which is the most common method for obtaining the etching factor EF. In the present invention, the measurement is performed as shown in FIGS. 1 and 2. The following equation (1) focuses only on the etching rates in the depth direction and the width direction, and does not consider the etching rates in the oblique direction.
As shown in FIG. 1, the EF is obtained by the following equation (1) from the etching rates in the width direction and the depth direction of the cross section of one circuit.
EF = etching rate in the depth direction / etching rate in the width direction (1)

However, since it is difficult to measure the etching rate itself, the width and depth of the circuit when the etching time is changed are measured. Then, as shown in FIG. 2, each data is plotted with the horizontal axis as the width of the circuit and the vertical axis as the depth of the circuit, and approximately obtained by the following equation (2). That is, the slope of the graph in FIG. 2 is obtained from a linear approximation formula by the least squares method and used as EF.
EF ≒ time change of depth / (time change of width / 2) = 2 × time change of depth / time change of width (2)
Here, the coefficient "2" in the equation (2) needs to be halved because the etching in the width direction proceeds to both the left and right sides of FIG.

Then, it was evaluated by the following indexes according to the value of EF. If the evaluation is ◎ or ○, it is good.
⊚: EF is 1.4 or more ×: EF is 1.1 or more and less than 1.4 ×: EF is less than 1.1

4. Soft etching property The surface of each copper foil sample after the above heat treatment was soft-etched under the following conditions. As an index for evaluating the soft etching property, the arithmetic mean roughness Ra based on JIS-B0601 (2001) of the copper foil surface after the soft etching was measured.
As the soft etching conditions, soft etching for imparting adhesion between the copper foil and the resist is simulated, and 420 in an aqueous solution (liquid temperature 25 ° C) with a sodium persulfate concentration of 100 g / L and a hydrogen peroxide concentration of 35 g / L. The second copper foil was to be immersed. When the arithmetic mean roughness Ra was 0.2 μm or less, the soft etching property was good (◯), and when the arithmetic average roughness Ra was more than 0.2 μm, the soft etching property was poor (x).

If the resist follows the copper foil surface well after soft etching, the adhesion is excellent, the accuracy of the circuit pattern is improved, and the soft etching property is good. When Ra exceeds 0.2 μm, it is difficult for the resist to follow the copper foil surface, and a gap is likely to be generated between the resist and the copper foil surface. Then, the etching solution invades the gap, and the accuracy at the time of forming the circuit pattern is lowered.

5. Crystal grain size For each copper foil sample after the heat treatment, the rolled surface was observed using an SEM (Scanning Electron Microscope), and the average particle size was determined based on JIS H 0501. However, twins were measured as separate crystal grains. The measurement area was 400 μm × 400 μm with a cross section parallel to the rolling direction.

The obtained results are shown in Tables 1 and 2.

As is clear from Tables 1 and 2, the grain size is 0.5 to 4.0 μm, and the degree of aggregation indicated by I (220) / I ₀ (220) is 1.3 or more and less than 7.0. In the case of each of the examples, both the etching factor and the soft etching property were excellent. As a result, a uniform circuit pattern is obtained on the entire surface of the copper foil, the yield is improved, and the accuracy of the circuit pattern is improved when the circuit is further miniaturized.

In the cases of Comparative Examples 1 to 4 in which the strain rate of the final pass of the final cold rolling was less than 1000 (s ^-1 ), the degree of aggregation was less than 1.3, the soft etching property was good, but the etching factor was lowered. Therefore, when the circuit is miniaturized, the accuracy of the circuit pattern is lowered.
In the case of Comparative Example 5 in which the average crystal grain size before the final cold rolling and after the final recrystallization annealing exceeded 20 μm, the average crystal grain size of the product exceeded 4.0 μm, and the etching factor was good, but the soft etching property was obtained. Was inferior. Therefore, a uniform circuit pattern cannot be obtained on the entire surface of the copper foil, and the yield is lowered.
In the case of Comparative Example 6 in which the total workability of the final cold rolling was less than 6.10, the degree of aggregation was less than 1.3, and the etching factor was lowered.
In the case of Comparative Example 7 in which the total content of the added elements exceeded 0.7% by mass, the conductivity was less than 80% and the conductivity was inferior.

Claims (6)

A rolled copper foil consisting of 99.0% by mass or more of Cu and unavoidable impurities in the balance.
300 ° C. × 30min annealing (however, heating rate 100 ℃ / min~300 ℃ / min) when the average crystal grain size has a recrystallized grains of 0.5~ 4.0μ m, X-ray diffraction of the copper foil surface The degree of aggregation displayed by the intensity I (220) / I ₀ (220) is 1.3 or more and less than 7.0.
Copper foil for flexible printed circuit boards with conductivity of 80% or more. The copper foil for a flexible printed circuit board according to claim 1, which is made of tough pitch copper specified in JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1020). Further, as an additive element, at least one or two or more selected from the group consisting of P, Ag, Si, Ge, Al, Ga, Zn, Sn and Sb are contained in a total of 0.7% by mass or less. Item 2. The copper foil for a flexible printed circuit board according to Item 1 or 2. A copper-clad laminate obtained by laminating the copper foil for a flexible printed circuit board according to any one of claims 1 to 3 and a resin layer. A flexible printed circuit board formed by forming a circuit on the copper foil in the copper-clad laminate according to claim 4. An electronic device using the flexible printed circuit board according to claim 5.

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