JP6294376B2 - Copper foil for flexible printed circuit board, copper-clad laminate using the same, flexible printed circuit board, and electronic device - Google Patents

Description

  The present invention relates to a copper foil suitable for 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.

A flexible printed circuit board (flexible wiring board, hereinafter referred to as “FPC”) has flexibility, and is widely used in a bent portion and a movable portion of an electronic circuit. For example, FPCs are used for movable parts of disk-related devices such as HDDs, DVDs, and CD-ROMs, and for folding parts of foldable mobile phones.
The FPC is formed by etching a copper clad laminate (copper-clad laminate, hereinafter referred to as CCL) in which a copper foil and a resin are laminated, and then coating the wiring with a resin layer called a coverlay. The copper foil surface is etched as part of the surface modification step for improving the adhesion between the copper foil and the coverlay before the coverlay is laminated. Further, in order to improve the flexibility by reducing the thickness of the copper foil, thinning etching may be performed.

By the way, along with the downsizing, thinning, and high performance of electronic devices, it is required to mount FPCs in these devices at a high density. The FPC needs to be folded and accommodated, that is, high bendability is required.
On the other hand, copper foils with improved high-cycle flexibility represented by IPC flexibility have been developed (Patent Documents 1 and 2).

JP 2010-100877 A JP 2009-111203 A

However, in order to mount the FPC at a high density as described above, it is necessary to improve the bendability represented by MIT fold resistance, and the conventional copper foil cannot be said to have sufficient improvement in bendability. There's a problem.
In addition, as electronic devices become smaller, thinner, and higher in performance, the circuit width and space width of FPC are also reduced to about 20-30 μm, and etching factors and circuit linearity are likely to deteriorate when forming circuits by etching. There is a problem that this is solved, and this solution is also required.
The present invention has been made to solve the above-described problems, and provides a copper foil for a flexible printed circuit board excellent in bendability and etching property, a copper-clad laminate using the same, a flexible printed circuit board, and an electronic device. With the goal.

As a result of various studies, the present inventors have found that by refining crystal grains after recrystallization of copper foil, the strength can be increased and the bendability can be improved. This is because as the crystal grains are refined according to the Hall Petch rule, the strength increases and the bendability also increases. However, if the crystal grains are made too fine, the strength becomes too high and the bending rigidity increases, and the spring back becomes large, which is not suitable for flexible printed circuit board applications. Therefore, the range of crystal grain size was also defined.
Also, by reducing the crystal grain size to about 1/10 of the circuit width of about 20-30 μm of recent FPC, the etching factor and circuit linearity when forming a circuit by etching are also improved. Can do.

That is, the copper foil for a flexible printed circuit board of the present invention is a copper foil comprising 99.0% by mass or more of Cu and the balance unavoidable impurities, the average crystal grain size is 0.5 to 4.0 μm, and the tensile strength is 235 to 290 MPa . The number of MIT folding endurances based on JIS P 8115 (the number of reciprocal foldings, where the bending clamp R is 0.38 and the load is 500 g) is 75 to 129 .

The copper foil for a flexible printed circuit board of the present invention is preferably made of tough pitch copper standardized to JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1011).
Furthermore, it is preferable to contain 0.003-0.825 mass% in total of 1 or more types of additional elements chosen from the group of P, Ti, Sn, Ni, Be, Zn, In, and Mg.
It is preferable that the average crystal grain size when subjected to heat treatment at 300 ° C. for 30 minutes is 0.5 to 4.0 μm, the tensile strength is 235 to 290 MPa, and the MIT folding resistance is 75 to 129 .
A copper-clad laminate formed by laminating a polyimide resin film with a thickness of 25 μm on one side of the copper foil is bent 180 degrees so that the copper foil is on the outside with a bending radius of 0.05 mm, and then the bent portion is formed. It is preferable that no crack is visually recognized when the copper foil is observed at 200 times after repeating the test for returning to 0 degree three times.

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

The flexible printed board of the present invention is formed by using the copper-clad laminate and forming a circuit on the copper foil.
The L / S of the circuit is preferably 40/40 to 15/15 (μm / μm). Note that the L / S (line and space) of a circuit is the ratio of the width (L: line) of the wiring constituting the circuit and the interval (S: space) between adjacent wirings. L adopts the minimum value of L in the circuit, and S adopts the minimum value of S in the circuit.
In addition, L and S should just be 15-40 micrometers, and both do not need to be the same value. For example, values such as L / S = 20.5 / 35 and 35/17 can be taken.

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

  ADVANTAGE OF THE INVENTION According to this invention, the copper foil for flexible printed circuit boards excellent in the bendability and etching property is obtained.

It is a figure which shows the bendability test method of CCL.

  Hereinafter, embodiments of the copper foil according to the present invention will be described. In the present invention, “%” means “% by mass” unless otherwise specified.

<Composition>
The copper foil according to the present invention comprises 99.0% by mass or more of Cu and the balance of inevitable impurities.
As described above, in the present invention, by refining crystal grains after recrystallization of copper foil, the strength is increased and the bendability is improved.
However, in the case of the above pure copper-based composition, since it is difficult to refine the crystal grains, recrystallization annealing is performed only once in the initial stage of cold rolling, and thereafter recrystallization annealing is not performed. A large amount of processing strain can be introduced by hot rolling to cause dynamic recrystallization, thereby achieving refinement of crystal grains.

In order to increase the work strain in cold rolling, the degree of work in final cold rolling (finish rolling performed after the last annealing in the entire process of annealing and rolling) is η = ln (final cold rolling). (Thickness before cold rolling / thickness after final cold rolling) = 3.5 to 7.5.
When η is less than 3.5, the accumulation of strain during processing is small and the nuclei of the recrystallized grains are reduced, so that the recrystallized grains tend to be coarse. When η is greater than 7.5, excessive strain is accumulated to serve as a driving force for crystal grain growth, and the crystal grains tend to be coarse. More preferably, η = 5.5 to 7.5.

Further, as an additive element for refining crystal grains, a total of 0.003 to 0.825 mass of one or more additive elements selected from the group of P, Ti, Sn, Ni, Be, Zn, In and Mg with respect to the above composition If it is contained in%, the crystal grains can be made finer more easily. Since these additive elements increase the dislocation density during cold rolling, crystal grain refinement can be realized more easily. Also, if recrystallization annealing is performed only once in the initial stage of cold rolling and no subsequent recrystallization annealing is performed, a large amount of work strain is introduced by cold rolling to cause dynamic recrystallization. In this way, it is possible to realize further refinement of crystal grains.
When the total content of the additive elements is less than 0.003 mass%, it is difficult to refine the crystal grains, and when it exceeds 0.825 mass%, the conductivity may be lowered. In addition, when the recrystallization temperature rises and is laminated with the resin, it does not recrystallize, and the strength becomes too high, and the bendability of the copper foil and CCL may deteriorate.

  In addition, as a method of refining crystal grains after recrystallization of copper foil, in addition to a method of adding an additive element, a method of polymerization rolling, a method of using a pulse current when electrodepositing on electrolytic copper foil Alternatively, a method of adding an appropriate amount of thiourea, glue or the like to the electrolytic solution with electrolytic copper foil can be mentioned.

The copper foil according to the present invention may be composed of tough pitch copper (TPC) standardized to JIS-H3100 (C1100) or oxygen-free copper (OFC) of JIS-H3100 (C1011).
Moreover, it is good also as a composition which contains the above-mentioned additional element with respect to said TPC or OFC.

<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 becomes too high, the bending rigidity becomes large, the spring back becomes large, and it is not suitable for flexible printed circuit board applications. If the average crystal grain size exceeds 4.0 μm, it is difficult to refine the crystal grains, and it becomes difficult to increase the strength and improve the bendability. descend.
The average crystal grain size is measured by observing at least 3 fields of view on the surface of the foil in a 100 μm × 100 μm field in order to avoid errors. For observation of the foil surface, the average crystal grain size can be determined based on JIS H 0501 using a SIM (Scanning Ion Microscope) or SEM (Scanning Electron Microscope).
However, twins are measured as being considered as separate crystal grains.

<Tensile strength (TS)>
The tensile strength of the copper foil is 235 to 290 MPa. As described above, the tensile strength is improved by refining the crystal grains. If the tensile strength is less than 235 MPa, it is difficult to increase the strength and improve the bendability. If the tensile strength exceeds 290 MPa, the strength becomes too high and the bending rigidity becomes large, and the spring back becomes large, which is not suitable for flexible printed circuit board applications.
Tensile strength is determined by a tensile test according to IPC-TM650, with a test piece width of 12.7 mm, room temperature (15 to 35 ° C.), tensile speed of 50.8 mm / min, gauge length of 50 mm, and the rolling direction of copper foil (or MD direction) ) In the direction parallel to.

<Heat treatment at 300 ° C for 30 minutes>
The average crystal grain size of the copper foil after heat treatment at 300 ° C. for 30 minutes may be 0.5 to 4.0 μm and the tensile strength may be 235 to 290 MPa.
The copper foil which concerns on this invention is used for a flexible printed circuit board, In that case, since CCL which laminated | stacked copper foil and resin performs the heat processing for hardening resin at 200-400 degreeC, a crystal grain is formed by recrystallization. There is a possibility of coarsening.
Therefore, the average crystal grain size and tensile strength of the copper foil change before and after lamination with the resin. Then, the copper foil for flexible printed circuit boards concerning Claim 1 of this application has prescribed | regulated the copper foil of the state which received the hardening heat processing of resin after it became a copper clad laminated body laminated | stacked with resin.
On the other hand, the copper foil for flexible printed circuit boards according to claim 4 of the present application defines a state when the heat treatment is performed on the copper foil before being laminated with the resin. This heat treatment at 300 ° C. for 30 minutes simulates the temperature condition for curing and heat-treating the resin during CCL lamination.

  The copper foil of this invention can be manufactured as follows, for example. First, after adding the said additive to a copper ingot and melt | dissolving and casting, it hot-rolls, performs cold rolling and annealing, and can manufacture foil by performing the above-mentioned final cold rolling.

<Copper-clad laminate and flexible printed circuit board>
Also, (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) a thermoplastic adhesive of the same type as the base film is used. 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 the resin base material is obtained. Further, by laminating a base film obtained by applying an adhesive to the copper foil of the present invention, a copper clad laminate (CCL) comprising three layers of a copper foil, a resin base material, and an adhesive layer therebetween is obtained. During the production of these CCLs, the copper foil is heat-treated and recrystallized.
A circuit is formed on these using a photolithographic technique, a circuit is plated as needed, and a cover-lay film is laminated, and a flexible printed circuit board (flexible wiring board) is obtained.

Therefore, the copper clad laminate of the present invention is formed by laminating a copper foil and a resin layer. Moreover, the flexible printed circuit board of this invention forms a circuit in the copper foil of a copper clad laminated body.
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 for forming the resin layer may be applied to the surface of the copper foil and 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 thermocompression bonded to the copper foil without using the adhesive. However, it is preferable to use an adhesive from the viewpoint of not applying excessive heat to the resin film.
When a film is used as the resin layer, this film may be laminated on the copper foil via an adhesive layer. In this case, it is preferable to use an adhesive having the same component as the film. For example, when a polyimide film is used as the resin layer, it is preferable to use a polyimide-based adhesive for the adhesive layer. In addition, the polyimide adhesive here refers to the adhesive agent containing an imide bond, and polyether imide etc. are also included.

In addition, this invention is not limited to the said embodiment. Moreover, as long as there exists an effect of this invention, the copper alloy in the said embodiment may contain another component.
For example, the surface of the copper foil may be subjected to a surface treatment by roughening treatment, rust prevention treatment, heat resistance treatment, or a combination thereof.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to these. Each element shown in Table 1 was added to electrolytic copper with a purity of 99.9% or more, and cast 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 to a thickness of 30 mm, cold-rolled to 14 mm, and then annealed once, and then the surface was chamfered. Final cold rolling was performed at a working degree η shown in Fig. 1 to obtain a foil having a final thickness of 17 µm. A heat treatment at 300 ° C. for 30 minutes was added to the obtained foil to obtain a copper foil sample.

<A. Evaluation of copper foil sample>
1. Electrical conductivity The electrical conductivity (% IACS) at 25 ° C. was measured for each copper foil sample after the heat treatment by a four-terminal method based on JIS H 0505.
If 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 size was determined based on JIS H 0501. However, the twins were measured as if they were separate crystal grains. The measurement area was 100 μm × 100 μm on the surface.

3. Copper foil bendability (MIT folding resistance)
About each copper foil sample after the said heat processing, the MIT folding endurance (number of reciprocal bending) was measured based on JISP8115. However, the bending clamp R was 0.38, and the load was 500 g.
If the MIT folding endurance number is 75 times or more, the bendability of the copper foil is good.
4). Tensile strength of copper foil About each copper foil sample after the said heat processing, the tensile strength was measured on the said conditions by the tensile test based on IPC-TM650.

<B. Evaluation of CCL>
5. CCL bendability Copper roughening plating was performed on one side of a copper foil sample (copper foil before heat treatment) that was not subjected to the heat treatment after the final cold rolling. Using copper: 10-25g / L, sulfuric acid: 20-100g / L as the copper roughening plating bath, electroplating at a bath temperature of 20-40 ° C and a current density of 30-70A / dm2 for 1-5 seconds, copper The adhesion amount was 20 g / dm2.
A polyimide film (product name “Iupilex VT” manufactured by Ube Industries Co., Ltd., thickness 25 μm) is laminated on the roughened plating surface of the copper foil sample, and bonded by applying a heat treatment at 300 ° C. for 30 minutes with a heating press (4 MPa). A CCL sample was obtained. The dimensions of the CCL sample used for the bending test are 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 with a 0.1 mm thick plate 20 (titanium copper plate standardized to JIS-H3130 (C1990)) with the copper foil surface facing outward, and at the center in the longitudinal direction. Folded in half and placed 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 was lowered and the CCL sample 30 was bent so as to be in close contact with the plate 20 at the two-folded portion (FIG. 1A). Immediately remove the CCL sample 30 from the compression tester 10 and fold the “folded V-shaped” tip 30 s into a microscope (product name “One-shot 3D measurement microscope VR-3000” manufactured by Keyence Corporation). ”Was visually confirmed at 200 × magnification for cracking of the copper foil surface. Note that the bending tip 30s corresponds to 180 ° contact bending with a bending radius of 0.05 mm.
When cracking was confirmed, the test was terminated, and the number of times of compression shown in FIG.

When no crack is confirmed, the CCL sample 30 is placed between the lower mold 10a and the upper mold 10b of the compression tester 10 so that the bent tip 30s faces upward as shown in FIG. In this state, the upper die 10b was lowered and the bent tip portion 30s was opened.
And the bending of Fig.1 (a) was performed again, and the presence or absence of the crack of the bending front-end | tip part 30s was confirmed visually. Hereinafter, similarly, the steps of FIGS. 1A to 1B were repeated to determine the number of bendings.
If the number of times the CCL is bent is 3 times or more, the CCL bendability is good.

6). Etchability Strip-like circuits of L / S (line / 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 CCL sample. For comparison, a circuit was formed in the same manner as a commercially available rolled copper foil (tough pitch copper foil). Then, the etching factor (ratio represented by (etching depth / average upper and lower average etching width) of the circuit) and the linearity of the circuit were visually determined with a microscope and evaluated according to the following criteria. If evaluation is (circle), it is good.
○: Etching factor and circuit linearity are better than commercially available rolled copper foils △: Etching factor and circuit linearity are comparable to commercially available rolled copper foils ×: Etching factor compared to commercially available rolled copper foils And circuit linearity is poor

  The obtained results are shown in Table 1.

  As is clear from Table 1, in each of the examples in which the average crystal grain size of the copper foil was 0.5 to 4.0 μm and the tensile strength was 235 to 290 MPa, the bending property and the etching property were excellent. In Example 1, polymerization rolling was performed in the last one pass of the final cold rolling.

On the other hand, in Comparative Examples 1 and 4 in which the degree of work η in the final cold rolling is less than 3.5, the average crystal grain size of the copper foil exceeds 4.0 μm, the tensile strength becomes less than 235 MPa, and the copper foil and the CCL are bent. Inferior. In the case of Comparative Example 4, the average crystal grain size of the copper foil was 4.5 μm, which is slightly larger than 4.0 μm, so that the etching property was good.

In the case of Comparative Example 3 in which the total content of the additive elements is less than the lower limit value, the recrystallized grains are not sufficiently refined by the additive elements, and the average crystal grain size of the copper foil greatly exceeds 4.0 μm, The tensile strength was less than 235 MPa, and the copper foil and CCL were inferior in bending property and etching property. In the case of Comparative Example 2 in which the total content of additive elements exceeded the upper limit value, the conductivity was inferior.
In the case of Comparative Example 5 in which the total content of the additive elements exceeded the upper limit value, the recrystallization temperature was increased and the heat treatment at 300 ° C. did not recrystallize, the conductivity decreased, and the tensile strength exceeded 290 MPa. became. Therefore, the bendability of the copper foil and CCL was greatly deteriorated.

Claims (9)

99.0% by mass or more of Cu, the remaining copper foil consisting of inevitable impurities,
An average crystal grain size of 0.5 to 4.0 μm and a tensile strength of 235 to 290 MPa ,
A copper foil for a flexible printed circuit board having a MIT folding endurance number (the number of reciprocal bendings , where the bending clamp R is 0.38 and the load is 500 g) of 75 to 129 based on JIS P 8115 .   The copper foil for flexible printed circuit boards of Claim 1 which consists of tough pitch copper specified to JIS-H3100 (C1100) or oxygen-free copper of JIS-H3100 (C1011).   Furthermore, the flexible print of Claim 1 or 2 which contains 0.003-0.825 mass% of 1 or more types of additional elements chosen from the group of P, Ti, Sn, Ni, Be, Zn, In, and Mg in total. Copper foil for substrates. The average crystal grain size when subjected to heat treatment at 300 ° C for 30 minutes is 0.5 to 4.0 µm, the tensile strength is 235 to 290 MPa, and the MIT folding resistance is 75 to 129 . The copper foil for flexible printed circuit boards of one term.   A copper-clad laminate formed by laminating a polyimide resin film with a thickness of 25 μm on one side of the copper foil is bent 180 degrees so that the copper foil is on the outside with a bending radius of 0.05 mm, and then the bent portion is formed. The copper foil for flexible printed circuit boards as described in any one of Claims 1-4 in which a crack is not visually recognized when the said copper foil is observed by 200 time after repeating the test returned to 0 degree | times 3 times.   The copper clad laminated body formed by laminating | stacking the copper foil for flexible printed circuit boards as described in any one of Claims 1-5, and a resin layer.   A flexible printed circuit board obtained by forming a circuit on the copper foil using the copper-clad laminate according to claim 6.   The flexible printed circuit board according to claim 7, wherein L / S of the circuit is 40/40 to 15/15 (μm / μm).   The electronic device using the flexible printed circuit board of Claim 7 or 8.