JP2009111203A - Rolled copper foil, and flexible printed wiring board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolled copper foil for flexible printed wiring boards which suppresses crack growth even under its strict bending conditions to obtain its high bendable quality. <P>SOLUTION: The rolled copper for flexible printed wiring boards has conductivity not smaller than 80% IACS after heat-treating at 300°C for 30 minutes. Also, it has a crystal grain boundary in the cross section viewed in a direction parallel to its rolling direction. Further, it satisfies such a relation of N/t≥1 that N is the number of grain boundaries each of which intersects the straight line connecting one grain surface with the other grain surface in a shortest distance, at an angle not smaller than 45°when it has a foil thickness of t(μm). Moreover, when its grain size is D and its thickness is t0 after final annealing, it is manufactured by final rolling with workability satisfying the relation of 0.13×D+0.75≤ln(t0/t)≤0.085×D+2.95, after final annealing. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a rolled copper foil used for a flexible printed circuit board (hereinafter referred to as FPC) that is repeatedly bent, and a flexible printed wiring board, and in particular, high flexibility is required like the FPC of a sliding portion of a sliding mobile phone. The present invention relates to FPC copper foil and flexible printed wiring boards.

FPC is a flexible printed circuit board that can be bent, twisted, wound and stacked, and can be mounted in a narrow space, so it can be used for wiring in mobile phones, computer-related products, audio / visual products, cameras, automobiles, etc. Is used.
FPC is generally manufactured by laminating and heating a copper foil that becomes a circuit on a base film, and as the copper foil, rolled copper foil of tough pitch copper or oxygen-free copper having superior flexibility than electrolytic copper foil is used. The In order to improve flexibility, the rolled copper foil is used for FPC in an annealed state.

In particular, for example, in the case of an FPC that connects movable parts of a mobile phone main body (main operation unit) and a display unit (liquid crystal or the like), higher flexibility is required. In addition, as mobile phones and the like are becoming thinner, the bending radius of FPC tends to be reduced accordingly, and the demand for bending resistance is becoming stricter.
Therefore, a copper alloy for FPC has been reported which has the property of adding Sn less than 0.05 mass% to oxygen-free copper and softening at the curing temperature of the resin (see Patent Document 1).
In addition, the ratio of crystal grains penetrating in the thickness direction of the copper foil annealed after the final rolling was increased (the crystal grain size was increased), and the deformation due to bending was changed to single crystal deformation to improve the flexibility. A rolled copper foil has been reported (see Patent Document 2).
Furthermore, as an FPC that suppresses the penetration of cracks to the back surface of copper foil and obtains high bending resistance, when the crystal grain boundary is traced from the front surface to the back surface with an average crystal grain size of 10 μm or less, 4 or more branch points Has been reported (see Patent Document 3).

JP 2005-313380 A JP 2006-117977 A JP 2007-189261 A

However, in the case of the technique described in Patent Document 2 described above, if the bending radius is small and bending is performed under severe conditions, once cracking occurs, the back surface of the copper foil is often penetrated in a short time, and flexibility is deteriorated. Also, in the case of the technique described in Patent Document 3, if the bending radius is reduced, the progress of cracks cannot be suppressed and the flexibility is lowered.
The present invention has been made to solve the above problems, and provides a rolled copper foil for a flexible printed wiring board that can suppress the progress of cracks even under severe bending conditions and can obtain high flexibility. With the goal.

As a result of various studies, the present inventors have found that the flexibility can be improved by increasing the number of crystal grain boundaries intersecting with a straight line connected from one surface to the other surface at the shortest distance. In this way, although the resistance increase is constant at the initial stage of bending, the increase in resistance gradually becomes gentle, so the number of bendings is considered to be significantly greater than that of the conventional copper foil.
That is, the rolled copper foil of the present invention is a copper foil for flexible printed wiring boards, and after heat treatment at 300 ° C. for 30 minutes, the conductivity is 80% IACS or more, and there are crystal grain boundaries in the cross section in the rolling parallel direction. When the thickness of the foil is t (μm), the number N of grain boundaries intersecting at a 45 ° or more angle with the straight line connected from one surface to the other surface at the shortest distance is N / t ≧ It is characterized by satisfying 1.

When the maximum height of the contour curve defined by JIS-B0601 is Rz, it is preferable that (t−2 × Rz) /t≧0.8 is satisfied.
This is because even if the unevenness state (Rz) on the copper foil surface is the same, the flexibility becomes worse when the thickness is reduced, and bending is reduced when the ratio of the unevenness on the copper foil surface is reduced with respect to the copper foil thickness. This is because the property is improved.

It is preferable that a total of 1000 to 3000 ppm of one or more elements selected from the group consisting of Ti, Zr, Mg, Cr, Ca, Sn, In, and Ag is included.
When the average grain size after final annealing is D and the thickness after final annealing is t0, after the final annealing at a working degree satisfying 0.13 × D + 0.75 ≦ ln (t0 / t) ≦ 0.085 × D + 2.95 It is preferably manufactured by final rolling.
From the viewpoint of reducing the strain applied to the copper foil when bent, t is preferably 20 μm or less.

  The flexible printed wiring board of the present invention is formed by laminating a resin layer and a rolled copper foil, the conductivity of the rolled copper foil is 80% IACS or more, and there are crystal grain boundaries in the cross section in the rolling parallel direction. The number N of crystal grain boundaries that intersect at an angle of 45 degrees or more with the straight line connected from one surface to the other surface at the shortest distance satisfies N / t ≧ 1, where t is the thickness of (μm) It is characterized by that.

  According to the present invention, it is possible to obtain a rolled copper foil for a flexible printed wiring board that can suppress the progress of cracks even under severe bending conditions and can obtain high flexibility.

Hereinafter, the rolled copper foil for flexible printed wiring boards which concerns on embodiment of this invention is demonstrated. In the present invention, “%” means “% by mass” unless otherwise specified.
Further, since the copper alloy foil according to the embodiment of the present invention is required to have conductivity, the conductivity is 80% IACS or more.

[Number of grain boundaries N]
The rolled copper foil according to the embodiment of the present invention has a grain boundary in the cross section in the rolling parallel direction after heat treatment at 300 ° C. for 30 minutes, and the thickness of the foil is t (μm). The number N of grain boundaries that intersect the straight line connected to the other surface at the shortest distance at an angle of 45 ° or more satisfies N / t ≧ 1.
The heat treatment at 300 ° C. for 30 minutes imitates the heat treatment in producing a flexible printed wiring board by heating the rolled copper foil together with the base film.
A crack generated by bending is likely to proceed along the crystal grain boundary, but is difficult to proceed in a direction crossing the crystal grain boundary. For example, if the crystal grain boundaries are arranged in a complicated path (for example, zigzag), the progress of the crack becomes zigzag, and the progress of the crack (fatigue) can be delayed to improve the flexibility. As a result of the study by the present inventors, as the number N of crystal grain boundaries intersecting at a 45 ° angle or more with a straight line connected from one surface to the other surface at the shortest distance, the more severe the bending condition However, it has been found that it exhibits high flexibility. That is, as in the prior art, the number of intersections of crystal grain boundaries is not sufficient, and the number of crystal grain boundaries in the foil thickness direction is important.

Here, the straight line connected from one surface to the other surface at the shortest distance is usually a straight line parallel to the thickness direction of the foil (and perpendicular to the foil surface). In addition, the reason why the crystal grain boundary needs to intersect this straight line at an angle of 45 degrees or more is that when the crystal grain boundary intersects at an angle of less than 45 degrees, the crystal grain boundary is along the plate thickness direction, so that the crack is in the plate thickness direction. It is because it penetrates the front and back along and becomes easy to fracture.
Further, as the thickness of the foil increases, the number N of crystal grain boundaries needs to increase. For example, when the thickness of the foil is 10 μm, the number of crystal grain boundaries may be 10 or more, but when the thickness of the foil is 15 μm, the number of crystal grain boundaries needs to be 15 or more.

FIG. 1 shows a structure photograph of a rolled copper foil according to an example of the present invention described later. A crystal grain boundary that intersects with a vertical line (straight line from one surface to the other surface at the shortest distance) at an angle of 45 degrees or more is indicated by a point on this line. The example of FIG. 1 satisfied N / t ≧ 1, and the evaluation of flexibility was excellent.
On the other hand, FIGS. 3-5 shows the structure | tissue photograph of the rolled copper foil which concerns on the comparative example mentioned later. The examples of FIGS. 3 to 5 all had N / t <1, and the evaluation of flexibility was inferior.

[Foil surface irregularities (Rz)]
The rolled copper foil according to the embodiment of the present invention preferably satisfies (t−2 × Rz) /t≧0.8, where Rz is the maximum height of the contour curve defined by JIS-B0601.
When the bending condition is loose, the unevenness on the foil surface does not have a great influence on the flexibility. However, when the strain applied to the foil surface is large, that is, when the bending radius is very small, the surface unevenness becomes a stress concentration point, causing breakage and lowering the flexibility. Further, even when the unevenness (Rz) on the foil surface is the same, the flexibility decreases as the plate thickness decreases. This is because the ratio of the unevenness to the foil thickness increases as the foil becomes thinner.
And (t-2 × Rz) /t≧0.8, that is, the actual thickness (t-2 × Rz) obtained by subtracting the surface unevenness from the foil thickness t is set to 80% or more (the unevenness of the copper foil surface It has been found that the flexibility is improved when the ratio is reduced.

[composition]
The rolled copper foil according to the embodiment of the present invention preferably contains a total of 1000 to 3000 ppm of one or more elements selected from the group of Ti, Zr, Mg, Cr, Sn, In, and Ag.
As described above, in order to satisfy N / t ≧ 1, that is, to increase the number N of crystal grain boundaries, it is effective to reduce the crystal grains by adding an additive element. As such an element, an element effective for improving heat resistance is added. This is because the element that improves the heat resistance can suppress the coarsening of crystal grains due to recrystallization of the copper foil in the FPC processing process (film bonding or heating process after application of the film-forming solution). is there. However, if the additive element is contained in a large amount, the electrical conductivity is lowered, which is not preferable as a copper foil for FPC.

Therefore, an element that has little influence on conductivity and is effective in improving heat resistance even in a small amount is selected.
Fig. 6 shows that Cu, Sn, Mg, Ag, In, Fe, Cr, Zn, Ti, Be, Cd, and Zr, which are often used as additive elements for copper foil, are added in predetermined amounts to copper having a purity of 99.96% or more. The semi-softening temperature is shown when the ingot obtained is melted and hot rolled to a thickness of 10 mm, and then cold rolling and annealing are repeated to a thickness of 0.1 mm.
The semi-softening temperature is the Vickers hardness before annealing when the sample is annealed and the Vickers hardness is measured, and when fully softened (even if the annealing temperature is further increased after 30-minute annealing, the strength (Vickers hardness ) Shows an annealing temperature indicating intermediate Vickers hardness when it is considered that the state in which no change occurs) is completely softened.

  FIG. 6 shows that the semi-softening temperature varies depending on the additive element. Since the semi-softening temperature of pure copper is 160 ° C., the amount (ΔT) of the increase in the semi-softening temperature of each element from 160 ° C. was obtained and compared with Table 1 based on this temperature.

Sn is known to be effective as an element for improving heat resistance. In the present invention, Sn is used as a reference when selecting an additive element. Therefore, from Table 1, when ΔTSn of Sn is 1, the ratio of ΔT (ΔT / ΔTSn) of each element (at the time of adding 0.05%) was obtained. This ratio indicates the degree (effectiveness) that each element improves the heat resistance with respect to Sn. From this ratio, elements equivalent to Sn (the above ratio is 0.7 or more) are Mg, Ag, In, Cr, Ti, Cd, and Zr. However, Cd is not suitable as an additive element.
When the total addition amount of these elements is less than 1000 ppm (0.1% by mass), it is not effective for improving the heat resistance, and when the total addition amount exceeds 3000 ppm, the conductivity tends to decrease.
It should be noted that the electrical conductivity of the alloy can be calculated by the Matthiessen equation. If Δρi of the above elements is substituted into this equation, the electrical conductivity becomes less than 80 IACS when the total addition amount exceeds 3000 ppm. Δρi is based on literature values (author: Yotaro Murakami and Kiyoshi Kamei, “Asakura Metal Engineering Series Nonferrous Metallology”, first edition, first edition, Asakura Shoten, published in April 1978, p13).

In the rolled copper foil according to the embodiment of the present invention, when the average crystal grain size after final annealing is D and the thickness after final annealing is t0, 0.13 × D + 0.75 ≦ ln (t0 / t) ≦ 0.085 × D + 2 It is preferably manufactured by final rolling after the final annealing at a working degree satisfying .95.
As described above, in order to satisfy N / t ≧ 1, that is, to increase the number N of crystal grain boundaries, it is effective to increase the rolling degree and reduce the crystal grains. However, if the degree of rolling is increased too much, a shear band is generated, and cracks progress in the shear band, so that the flexibility is lowered.

As a result of examining the optimum rolling conditions from the above, the present inventors have determined that the average grain size D after the final annealing and the workability from the final annealing to the final rolling (ln (t0 / t)) It was found experimentally that there is a certain relationship between. That is, it has been found that when the crystal grain size D after the final annealing is reduced, the crystal grain size after the final rolling tends to be reduced even if the final rolling degree is small. Here, the reason for the logarithm of the degree of work is that the degree of work is high in the manufacture of the foil (usually about 90% or more), and the change in the degree of work is small. Because.
Therefore, when the foil is thin, the final rolling degree is inevitably increased. Therefore, if the crystal grain size D after the final annealing is increased to suppress the occurrence of the shear band shape, the flexibility is improved. On the other hand, when the thickness of the foil is thick (approximately 20 μm in thickness), the crystal grain size D after the final annealing is reduced, and the foil may be finished before reaching the degree of processing that causes shear band deformation. In this way, it is possible to produce a copper foil having excellent flexibility even under severe bending conditions.

  In addition, the rolled copper foil according to the embodiment of the present invention repeats cold rolling and continuous annealing after hot rolling, the cold rolling after performing the final annealing is the final rolling, and the finished product thickness by the final rolling. To do. The cold rolling degree before final annealing is preferably 80% or more, but when the crystal grain size D after final annealing is 10 μm or more, the cold rolling degree before final annealing is 85%. It is more preferable to exceed.

FIG. 7 shows a plot of the crystal grain size D and the degree of processing (ln (t0 / t)) of samples of Examples and Comparative Examples described later. In FIG. 7, when a straight line separating the example (symbol ◯) and the comparative example (symbol ×) is obtained by the least square method, a relational expression of ln (t0 / t) = 0.13 × D + 0.75 is obtained. . That is, if it is the range of 0.13 * D + 0.75 <= ln (t0 / t), it will become an Example of this invention. In all of the examples, N / t ≧ 1, and in the comparative example, N / t <1.
Similarly, FIG. 8 shows a graph plotting the crystal grain size D and the degree of processing (ln (t0 / t)) of Examples described later. In FIG. 8, when a straight line separating the optimum embodiment (symbol ◯) and the normal embodiment (symbol ×) is obtained by the least square method, the relational expression ln (t0 / t) = 0.085 × D + 2.95 is obtained. became. In other words, the range of ln (t0 / t) ≦ 0.085 × D + 2.95 is the optimum embodiment of the present invention. Note that all of the optimal examples are (t−2 × Rz) /t≧0.8, and the normal example is (t−2 × Rz) / t <0.8.
From the above, it is preferable to manufacture with a degree of processing satisfying 0.13 × D + 0.75 ≦ ln (t0 / t) ≦ 0.085 × D + 2.95.

The flexible printed wiring board of the present invention is formed by laminating a resin layer and a rolled copper foil, the conductivity of the rolled copper foil is 80% IACS or more, and there are crystal grain boundaries in the cross section in the rolling parallel direction. The number N of crystal grain boundaries that intersect at an angle of 45 degrees or more with the straight line connected from one surface to the other surface at the shortest distance satisfies N / t ≧ 1, where t is the thickness of (μm) .
As described above, the rolled copper foil of the present invention defines the characteristics when imitating the heat treatment when producing a flexible printed wiring board (at 300 ° C. for 30 minutes), but the flexible printed wiring board of the present invention is The characteristics of the rolled copper foil after the heat treatment when manufacturing the flexible printed wiring board are defined, and the contents of the regulation are the same as those of the rolled copper foil of the present invention.
In addition, if the flexible printed wiring board of this invention has laminated | stacked the resin layer and the rolled copper foil, it uses a film-like thing beforehand as a resin layer, and uses a rolled copper foil without using an adhesive agent. It may be laminated, or a resin layer may be formed by coating a resin material on a rolled copper foil instead of a film. Polyimide can be suitably used as the resin layer.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to these.

1. Preparation of sample Alloy copper composition by melting electrolytic copper in an electric furnace to melt the base oxygen-free copper and adding a predetermined amount of element shots according to the amount of molten metal supplied to the tundish The copper alloy ingot was manufactured by continuously casting from the nozzle at the bottom of the tundish. This ingot was hot-rolled, the oxide scale was removed thereafter, cold rolling and continuous annealing were repeated, and finished by cold rolling to a thickness at which final annealing was performed. Here, the cold rolling degree before final annealing was set to 80% or more. In the final annealing, the furnace temperature was adjusted so that the material temperature was 500 ° C. or higher, and the crystal grain size was adjusted by changing the plate passing speed according to the thickness. Thereafter, final cold rolling was performed to finish a rolled copper foil having a predetermined thickness. In the final cold rolling, the surface roughness (Rz) of the copper foil was adjusted according to the rolling speed, the amount of reduction per pass, and the surface roughness of the work roll used.

After degreasing after rolling, a polyimide precursor mainly composed of polyamic acid is applied to one side of the copper foil, dried and cured, processed into a copper-clad laminate of 25 μm thick polyimide and copper foil, and further by photolithography A predetermined circuit was formed to obtain an FPC test piece (a length of 12.7 mm and a circuit width of 1 mm).
In addition, the ratio (subscript of composition) of the additional element of Table 2 is mass%.

2. 2. Evaluation of sample 2-1. Electrical conductivity The electrical conductivity at 25 ° C. of the sample after FPC processing of each copper foil was measured by the 4-terminal method.
2-2. Number of grain boundaries N
For each specimen after FPC processing of each copper foil, take a cross-sectional structure image in the rolling parallel direction, draw a straight line parallel to the plate thickness direction of this image, and the grain boundary intersecting this straight line at an angle of 45 degrees or more Numbers were counted visually. The measurement of the sample was performed at three locations on the rolling parallel cross section of the circuit (copper foil) portion cut by FIB (Focused Ion Beam). The thickness t of the sample was calculated from the mass per unit size and the density.
2-3. Copper foil surface roughness (Rz)
The maximum height Rz of the contour curve of each copper foil was measured according to JIS-B0601.
2-4. Crystal grain size D
The crystal grain size D of each copper foil after final annealing was obtained by taking a reflection electron image of a cross section in the rolling parallel direction and obtaining the average crystal grain size by a cutting method specified in JIS. .

3. Evaluation of Flexibility (Bending Life) For each FPC test piece, the bending radius was adjusted so that the strain applied to the copper foil surface was 0.006 to 0.010, and the flexibility was evaluated by the following sliding flex test. Here, the strain applied to the copper foil is maximum on either the front or back foil surface, and varies depending on the FPC configuration, but the thickness of the copper foil, resin, adhesive (may not be used) and the respective materials The Young's modulus and the bending radius can be calculated.
In the sliding bending test (ICP), each FPC test piece is bent in a U shape in the longitudinal direction, one end is fixed to the movable plate, the other end is fixed to the fixed plate, and the movable plate is fixed to the long side direction of each FPC test piece. And reciprocally vibrated. Test conditions are U-shaped radius of curvature 0.6mm to 1.1mm for 9μm foil, 0.7mm to 1.2mm for 12μm foil, 1.0mm to 1.7mm for 18μm foil, vibration stroke 20mm, vibration The frequency was 1200 times / min. In addition, the electrical resistance of the sample during the bending test was measured, and the number of bendings where the resistance increased by 10% from the initial resistance was set as the end point.
The sliding bending test apparatus was the same as that described in FIG. 1 of JP-A-2001-323354.

  The obtained results are shown in Table 2.

As is apparent from Table 2, in each of Examples 1 to 11, high flexibility was obtained even under high bending conditions (stricter bending conditions) with strains of 0.009 and 0.010.
On the other hand, in the case of Comparative Examples 12 to 17 where N / t <1, the bending property was inferior when the bending conditions were as high as 0.009 and 0.010.
Reference Example 18 is a well-known brass (7/3 brass) with N / t ≧ 1 and excellent bendability even under high bending conditions of 0.009 and 0.010, but the conductivity is as low as 27% IACS. Not suitable for FPC. However, the structure of the brass is the same as the scope of the present invention, and the material whose composition is not known and has the structure within the specified range of the present invention is included in the present invention.
In addition, FIGS. 1-5 shows the cross-sectional structure | tissue photograph of the copper foil of Example 2, Reference Example 18, Comparative Example 12, Comparative Example 16, Comparative Example 17, respectively, and the up-down direction of each figure is a plate | board thickness direction. .

Example 1 had a higher degree of final rolling than Example 2, but N / t ≧ 1 was satisfied by increasing the crystal grain size D after the final annealing, and high flexibility could be obtained.
Precipitates did not precipitate and the strength decreased.
In the case of Examples 8 to 11, compared to Examples 1 to 7, the bending property was inferior under the highest bending condition of 0.010. In the case of Examples 9 to 11, the final rolling work degree is larger than those in Examples 1 to 7, the shear band deformation easily occurs and Rz becomes high. In the case of Example 8, in the final cold rolling. This is because Rz increased due to rolling with a work roll having a large surface roughness in the final pass. That is, in Examples 8 to 11, since (t−2 × Rz) / t <0.8, the flexibility was slightly lowered. In Examples 9 to 11, ln (t0 / t)> 0.085 × D + 2.95, and the degree of processing was too large as compared with the other examples.
However, in the case of Examples 8 to 11, the bending property with a strain of 0.009 is excellent in the bending property, and there is no problem in practical use.

It is a figure which shows the cross-sectional structure | tissue photograph of the rolled copper foil which concerns on Example 2 of this invention. It is a figure which shows the cross-sectional structure | tissue photograph of the copper foil of a reference example. It is a figure which shows the cross-sectional structure | tissue photograph of the copper foil of the comparative example 12. It is a figure which shows the cross-sectional structure | tissue photograph of the copper foil of the comparative example 16. It is a figure which shows the cross-sectional structure | tissue photograph of the copper foil of the comparative example 17. FIG. It is a figure which shows the semi-softening temperature of copper which added the additive element. FIG. 4 is a diagram showing the relationship between the crystal grain size D of a copper foil sample and the degree of processing (ln (t0 / t)). FIG. 6 is another diagram showing the relationship between the crystal grain size D and the degree of processing (ln (t0 / t)) of a copper foil sample.

Claims (6)

Copper foil for flexible printed circuit board, after heat treatment at 300 ° C for 30 minutes, conductivity is 80% IACS or more, there is a grain boundary in the cross section in the rolling parallel direction, and the thickness of the foil is t ( μm), the number N of crystal grain boundaries that intersect at a 45 ° angle or more with the straight line connecting from one surface to the other surface satisfies N / t ≧ 1 Rolled copper foil. The rolled copper foil according to claim 1, wherein (t−2 × Rz) /t≧0.8 is satisfied, where Rz is the maximum height of the contour curve defined by JIS-B0601. The rolled copper foil according to claim 1 or 2, comprising a total of 1000 to 3000 ppm of one or more elements selected from the group consisting of Ti, Zr, Mg, Cr, Sn, In, and Ag. When the average grain size after final annealing is D and the thickness after final annealing is t0, after the final annealing at a working degree satisfying 0.13 × D + 0.75 ≦ ln (t0 / t) ≦ 0.085 × D + 2.95 The rolled copper foil according to any one of claims 1 to 3, wherein the rolled copper foil is manufactured by final rolling. The rolled copper foil according to any one of claims 1 to 4, wherein t is 20 µm or less. A flexible printed wiring board in which a resin layer and a rolled copper foil are laminated, wherein the rolled copper foil has an electrical conductivity of 80% IACS or more, a grain boundary exists in a cross section in the rolling parallel direction, and the thickness of the foil T (μm), the number N of crystal grain boundaries that intersect at a 45 ° angle or more with the straight line connected from one surface to the other surface satisfies N / t ≧ 1 A flexible printed wiring board.