Printed circuit board

Abstract

To provide a printed circuit board capable of improving the transmission characteristics of high-frequency signals in signal patterns. [Solution] The printed circuit board 100 is bendable along the bending direction in a plan view and includes a bendable portion 10 having a first dielectric layer 20, a signal pattern 30, a second dielectric layer 50, a cloth, and a ground pattern. The first dielectric layer 20 has a first main surface 20a. The signal pattern 30 is arranged on the first main surface 20a. The second dielectric layer 50 has a second main surface 50a and a third main surface 50b which is the opposite surface of the second main surface 50a, and is arranged so that the second main surface 50a faces the first main surface 20a. The cloth is made by weaving together a plurality of first reinforcing fibers extending along a first direction in a plan view and a plurality of second reinforcing fibers extending along a second direction which is inclined with respect to the first direction in a plan view.

Description

Technical Field

[0001] This disclosure relates to a printed wiring board.

Background Art

[0002] Japanese Unexamined Patent Application Publication No. 2011-54919 (Patent Document 1) describes a printed wiring board. The printed wiring board described in Patent Document 1 has a first dielectric layer, a signal pattern disposed on the first dielectric layer, an adhesive layer disposed on the first dielectric layer so as to cover the signal pattern, a second dielectric layer disposed on the adhesive layer, and a ground pattern disposed on the second dielectric layer.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When the printed wiring board described in Patent Document 1 is bent such that the second dielectric layer is located on the inner side of the bend rather than the first dielectric layer, a compressive stress acts on the second dielectric layer. If the second dielectric layer buckles due to this compressive stress, the distance between the signal pattern and the ground pattern, and thus the impedance of the signal pattern, changes, and the transmission characteristics of high-frequency signals in the signal pattern deteriorate. This disclosure provides a printed wiring board capable of improving the transmission characteristics of high-frequency signals in a signal pattern.

Means for Solving the Problems

[0005] The printed circuit board of this disclosure includes a bendable portion. The bendable portion is bendable along the bending direction in a plan view and includes a first dielectric layer, a signal pattern, a second dielectric layer, a cloth, and a ground pattern. The first dielectric layer has a first main surface. The signal pattern is arranged on the first main surface. The second dielectric layer has a second main surface and a third main surface which is the opposite surface of the second main surface, and is arranged so that the second main surface faces the first main surface. The cloth is made by weaving together a plurality of first reinforcing fibers extending along a first direction in a plan view and a plurality of second reinforcing fibers extending along a second direction which is inclined with respect to the first direction in a plan view. The cloth is arranged within the second dielectric layer. The ground pattern is arranged on the third main surface. The first direction and the second direction, each of them, are inclined with respect to the bending direction. [Effects of the Invention]

[0006] According to the printed circuit board of this disclosure, the transmission characteristics of high-frequency signals in the signal pattern can be improved. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a schematic side view of the printed circuit board 100. [Figure 2] Figure 2 is a plan view of the printed circuit board 100. [Figure 3] Figure 3 is a cross-sectional view taken along line III-III in Figure 2. [Figure 4] Figure 4 is a schematic diagram showing the orientation of reinforcing fibers 51a and 51b in the cloth 51 arranged within the dielectric layer 50 at the bent portion 10 of the printed circuit board 100. [Figure 5] Figure 5 is a cross-sectional view of the printed circuit board 100 according to Modification 1. [Figure 6] Figure 6 is a cross-sectional view of the printed circuit board 100 according to modified example 2. [Figure 7] Figure 7 is a diagram illustrating the manufacturing process of the printed circuit board 100. [Figure 8]Figure 8 is a first cross-sectional view illustrating the preparation process S1. [Figure 9] Figure 9 is a second explanatory diagram illustrating the preparation process S1. [Figure 10] Figure 10 is a cross-sectional view illustrating the patterning process S2. [Figure 11] Figure 11 is a schematic diagram showing the orientation of reinforcing fibers 51a and 51b in the cloth 51 arranged within the dielectric layer 50 at the bent portion 10 of the printed circuit board 200. [Figure 12] Figure 12 shows the load-strain curves when the angle between the longitudinal direction and the first direction DR1 of each sample is varied. [Figure 13] Figure 13 is a graph showing the relationship between the angle between the longitudinal direction and the first direction DR1 and the fracture strain in each sample. [Modes for carrying out the invention]

[0008] [Description of Embodiments in this Disclosure] First, embodiments of this disclosure will be listed and described.

[0009] (1) The printed circuit board according to the embodiment includes a bendable portion. The bendable portion is bendable along the bending direction in a plan view and has a first dielectric layer, a signal pattern, a second dielectric layer, a cloth, and a ground pattern. The first dielectric layer has a first main surface. The signal pattern is arranged on the first main surface. The second dielectric layer has a second main surface and a third main surface which is the opposite surface of the second main surface, and is arranged so that the second main surface faces the first main surface. The cloth is made by weaving together a plurality of first reinforcing fibers extending along a first direction in a plan view and a plurality of second reinforcing fibers extending along a second direction which is inclined with respect to the first direction in a plan view. The cloth is arranged within the second dielectric layer. The ground pattern is arranged on the third main surface. Each of the first and second directions is inclined with respect to the bending direction. According to the printed circuit board of (1) above, the transmission characteristics of high-frequency signals in the signal pattern can be improved.

[0010] (2) In the printed wiring board of (1) above, the first direction may be perpendicular to the second direction. (3) In the printed wiring board of (2) above, the angle formed by the first direction and the bending direction in a plan view may be 30° or more.

[0011] (4) In the printed wiring board of (2) or (3) above, the angle formed by the first direction and the bending direction in a plan view may be 40° or more.

[0012] (5) In the printed wiring board of (1) to (4) above, the plurality of first reinforcing fibers and the plurality of second reinforcing fibers may be formed of glass.

[0013] (6) In the printed wiring board of (1) to (5) above, the second dielectric layer may be formed of a fluororesin.

[0014] (7) The printed wiring board of (1) to (6) above may further include an adhesive layer disposed between the first main surface and the second main surface so as to cover the signal pattern.

[0015] (8) The printed wiring board of (1) to (7) above may further include a second cross disposed in the second dielectric layer. The second cross may be formed by knitting a plurality of third reinforcing fibers extending along a third direction in a plan view and a plurality of fourth reinforcing fibers extending along a fourth direction inclined with respect to the third direction in a plan view in a cloth shape. Each of the third direction and the fourth direction may be inclined with respect to the bending direction.

[0016] (9) In the printed wiring board of (1) to (8) above, the bent portion may be bendable such that the second dielectric layer is compressed.

[0017] [Details of Embodiments of the Present Disclosure] Next, the details of the embodiments of this disclosure will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions will not be repeated. The printed circuit board according to the embodiment will be referred to as printed circuit board 100.

[0018] (Configuration of printed circuit board 100) The configuration of the printed circuit board 100 is described below.

[0019] As shown in Figure 1, the printed circuit board 100 has a bent portion 10. The bent portion 10 is bendable along the bending direction BD in a plan view.

[0020] As shown in Figures 2 and 3, the printed circuit board 100 has a dielectric layer 20, a signal pattern 30, a ground pattern 31, an adhesive layer 40, a dielectric layer 50, and a ground pattern 60 at the bent portion 10.

[0021] The dielectric layer 20 has a main surface 20a and a main surface 20b. The main surface 20b is the opposite surface to the main surface 20a. The main surfaces 20a and 20b are end faces in the thickness direction of the dielectric layer 20. The dielectric layer 20 is made of a dielectric material. The dielectric layer 20 is made of, for example, a fluororesin.

[0022] The signal pattern 30 is arranged on the dielectric layer 20 (main surface 20a). In a plan view, the signal pattern 30 extends, for example, along the bending direction BD. A high-frequency signal, for example, flows through the signal pattern 30. The signal pattern 30 is formed of a conductive material. The signal pattern 30 is formed of, for example, copper or a copper alloy.

[0023] The ground pattern 31 is located on the dielectric layer 20 (main surface 20b). The ground pattern 31 is at ground potential. The ground pattern 31 is made of a conductive material. For example, the ground pattern 31 is made of copper or a copper alloy.

[0024] The adhesive layer 40 is positioned on the dielectric layer 20 (main surface 20a) so as to cover the signal pattern 30. The adhesive layer 40 is formed of an adhesive.

[0025] The dielectric layer 50 has a main surface 50a and a main surface 50b. The main surface 50b is the opposite surface to the main surface 50a. The main surfaces 50a and 50b are end faces in the thickness direction of the dielectric layer 50. The dielectric layer 50 is placed on the adhesive layer 40 such that the main surface 50a faces the adhesive layer 40. The bent portion 10 is bent such that the dielectric layer 50 is located inside the bend relative to the dielectric layer 20.

[0026] The ground pattern 60 is located on the dielectric layer 50 (main surface 50b). The ground pattern 60 is at ground potential. The ground pattern 60 is made of a conductive material. For example, the ground pattern 60 is made of copper or a copper alloy.

[0027] A cloth 51 is arranged within the dielectric layer 50. As shown in Figure 4, the cloth 51 has a plurality of reinforcing fibers 51a extending along a first direction DR1 in a plan view, and a plurality of reinforcing fibers 51b extending along a second direction DR2 which is inclined with respect to the first direction DR1 in a plan view. The cloth 51 is formed by weaving the plurality of reinforcing fibers 51a and the plurality of reinforcing fibers 51b into a cloth-like structure. The reinforcing fibers 51a and the reinforcing fibers 51b are made of, for example, glass (glass fibers). That is, the cloth 51 is, for example, a glass cloth.

[0028] In a plan view, the angle between the first direction DR1 and the second direction DR2 is, for example, 90°. However, if the first direction DR1 is inclined relative to the second direction DR2 in a plan view, the angle between the first direction DR1 and the second direction DR2 in a plan view does not have to be 90°.

[0029] In a plan view, the first direction DR1 and the second direction DR2 are inclined with respect to the bending direction BD. In a plan view, the angle between the first direction DR1 and the bending direction BD is, for example, 15° or more. In a plan view, the angle between the first direction DR1 and the bending direction BD may be 30° or more, or 40° or more. If the angle between the first direction DR1 and the second direction DR2 is 90° in a plan view, then ideally the angle between the first direction DR1 and the second direction DR2 in a plan view is 45°. Note that the angle between the first direction DR1 and the bending direction BD in a plan view is the smaller of the two angles that the first direction DR1 and the bending direction BD make in a plan view.

[0030] Although not shown in the figures, crosses may also be arranged within the dielectric layer 20. The reinforcing fibers forming the crosses arranged within the dielectric layer 20 may be oriented in the same way as the reinforcing fibers 51a and 51b. That is, the direction of extension of one of the reinforcing fibers forming the crosses arranged within the dielectric layer 20 and the direction of extension of the other reinforcing fiber forming the crosses located within the dielectric layer 20 are inclined with respect to the bending direction BD. The direction of extension of the one reinforcing fiber may or may not be parallel to the first direction DR1. The direction of extension of the other reinforcing fiber may or may not be parallel to the second direction DR2.

[0031] <Example 1> As shown in Figure 5, the printed circuit board 100 may further have coverlays 70 and 71. Coverlay 70 has an adhesive layer 70a and a film 70b. The adhesive layer 70a is placed on the ground pattern 60. The film 70b is placed on the adhesive layer 70a. Coverlay 71 has an adhesive layer 71a and a film 71b. The adhesive layer 71a is placed on the ground pattern 31. The film 71b is placed on the adhesive layer 71a.

[0032] <Modification 2> As shown in Figure 6, the printed circuit board 100 may further include an adhesive layer 80, a dielectric layer 81, a signal pattern 82, an adhesive layer 90, a dielectric layer 91, and a ground pattern 92.

[0033] The adhesive layer 80 is located on the ground pattern 31. The adhesive layer 80 is made of an adhesive. The dielectric layer 81 has a main surface 81a and a main surface 81b opposite to the main surface 81a, and is located on the adhesive layer 80 such that the main surface 81a faces the adhesive layer 80. The dielectric layer 81 is made of a dielectric material, such as fluororesin. The signal pattern 82 is located on the main surface 81b. The signal pattern 82 is made of a conductive material, such as copper or a copper alloy.

[0034] The adhesive layer 90 is positioned on the main surface 81b so as to cover the signal pattern 82. The adhesive layer 90 is made of an adhesive. The dielectric layer 91 has a main surface 91a and a main surface 91b opposite to the main surface 91a, and is positioned on the adhesive layer 90 so as to face the adhesive layer 90. The dielectric layer 91 is made of a dielectric material, such as fluororesin. The ground pattern 92 is positioned on the main surface 91b. The ground pattern 92 is made of a conductive material, such as copper or a copper alloy. In this way, multiple signal patterns may be arranged in the inner layers of the printed circuit board 100.

[0035] (Manufacturing method for printed circuit board 100) The manufacturing method for the printed circuit board 100 is described below.

[0036] As shown in Figure 7, the manufacturing method of the printed circuit board 100 includes a preparation step S1, a patterning step S2, and a dielectric layer attachment step S3. The patterning step S2 is performed after the preparation step S1, and the dielectric layer attachment step S3 is performed after the patterning step S2.

[0037] As shown in Figure 8, in preparation step S1, the dielectric layer 20 is prepared. In the dielectric layer 20 prepared in preparation step S1, a copper layer 32 is placed on the main surface 20a and a ground pattern 31 is placed on the main surface 20b. As shown in Figure 9, in preparation step S1, the dielectric layer 50 is also prepared. In the dielectric layer 50 prepared in preparation step S1, an adhesive layer 40 is placed on the main surface 50a and a ground pattern 60 is prepared on the main surface 50b. At this stage, the adhesive layer 40 is not yet cured.

[0038] As shown in Figure 10, in patterning step S2, the copper layer 32 is patterned to form the signal pattern 30. In patterning step S2, firstly, a resist pattern is formed on the copper layer 32. The resist pattern is formed, for example, by attaching a dry film resist to the copper layer 32 and then exposing and developing the dry film resist. Secondly, the copper layer 32 exposed from the openings of the resist pattern is removed by etching. As a result, the copper layer 32 is patterned and the signal pattern 30 is formed.

[0039] In dielectric layer attachment step S3, the dielectric layer 50 is attached. In dielectric layer attachment step S3, firstly, the dielectric layer 50 is positioned so that the adhesive layer 40 covers the signal pattern 30. Secondly, the dielectric layer 50 is hot-pressed against the dielectric layer 20. That is, while heated, the dielectric layer 50 is pressed toward the dielectric layer 20. As a result, the adhesive layer 40 hardens, and the dielectric layer 50 is attached to the dielectric layer 20 by the adhesive layer 40. Thus, the structure of the printed circuit board 100 shown in Figures 2 and 3 is formed.

[0040] (Effect of printed circuit board 100) The effects of printed circuit board 100 will be explained below in comparison with the printed circuit board of the comparative example. The printed circuit board of the comparative example will be referred to as printed circuit board 200.

[0041] Because fluororesin has a low dielectric constant, it is suitable as a material for the dielectric layer 50 from the viewpoint of improving the transmission characteristics of high-frequency signals flowing through the signal pattern 30. However, fluororesin has a high coefficient of thermal expansion. Therefore, in order to suppress the thermal expansion of the dielectric layer 50, it is conceivable to place cloth 51 within the dielectric layer 50.

[0042] As shown in Figure 11, in the bent portion 10 of the printed circuit board 200, in a plan view, the first direction DR1 is parallel to the bending direction BD, and the second direction DR2 is perpendicular to the bending direction BD. The bent portion 10 is bent such that the dielectric layer 50 is located inside the bend of the dielectric layer 20. Therefore, in the printed circuit board 200, compressive stress acts on the cloth 51 arranged within the dielectric layer 50 as the bent portion 10 is bent. Since the first direction DR1 is parallel to the bending direction BD in the bent portion 10 of the printed circuit board 200, the reinforcing fibers 51a buckle due to the above compressive stress, and consequently the ground pattern 60 arranged on the dielectric layer 50 and the main surface 50b also buckle. As a result, in the printed circuit board 200, the distance between the ground pattern 60 and the signal pattern 30 changes, and consequently the impedance of the signal pattern 30 changes.

[0043] On the other hand, in the bent portion 10 of the printed circuit board 100, both the first direction DR1 and the second direction DR2 are inclined with respect to the bending direction BD in a plan view. Therefore, the compressive stress does not concentrate on either the reinforcing fiber 51a or the reinforcing fiber 51b, and buckling of the reinforcing fiber 51a and the reinforcing fiber 51b is unlikely to occur. As a result, with the printed circuit board 100, the distance between the ground pattern 60 and the signal pattern 30 and the impedance of the signal pattern 30 do not change easily, and the signal transmission characteristics of the signal pattern 30 are improved.

[0044] To investigate the effect of the angle between the first direction DR1 and the bending direction BD on the load-strain curve of the dielectric layer 50 in a plan view, samples 1 to 4 were prepared as samples of the dielectric layer 50. Each sample was rectangular in shape, measuring 10 mm × 50 mm in a plan view. The direction in which the length of each sample is 50 mm is called the longitudinal direction.

[0045] In Sample 1, the angle between the first direction DR1 and the longitudinal direction in a plan view was set to 0° (the first direction DR1 is parallel to the bending direction BD). In Sample 2, the angle between the first direction DR1 and the longitudinal direction in a plan view was set to 15°. In Sample 3, the angle between the first direction DR1 and the longitudinal direction in a plan view was set to 30°. In Sample 4, the angle between the first direction DR1 and the longitudinal direction in a plan view was set to 45°. In Samples 1 through 4, the angle between the first direction DR1 and the second direction DR2 in a plan view was set to 90°.

[0046] Tensile tests were performed on samples 1 through 4. In the tensile tests, each sample was pulled along its longitudinal direction, and load-strain curves were obtained. As shown in Figures 12 and 13, the fracture strain of each sample increased as the angle between the longitudinal direction and the first direction DR1 increased in the plan view.

[0047] When the bending portion 10 bends, tensile stress is applied to the dielectric layer 20. From the results in Figures 12 and 13, it was found that stress is unlikely to concentrate on one side of the cross-shaped reinforcing fibers arranged within the dielectric layer 20, making it unlikely for the reinforcing fibers to break. Furthermore, when the bending portion 10 bends, compressive stress is applied to the dielectric layer 50, attempting to cause it to buckle. However, similar to the case where tensile stress is applied, stress is unlikely to concentrate on one side of the cross-shaped reinforcing fibers arranged within the dielectric layer 50, making it unlikely for the reinforcing fibers to buckle. Thus, it was found that the inclination of the first direction DR1 and the second direction DR2 with respect to the bending direction BD in a plan view makes it unlikely for the dielectric layer 50 to buckle, and consequently, the impedance of the signal pattern 30 to change, improving the signal transmission characteristics of the signal pattern 30.

[0048] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the embodiments described above, and all modifications within the meaning and scope of the claims are intended to be included. [Explanation of symbols]

[0049] 10. Bending section 20 Dielectric layer 20a,20b main surface 30 signal patterns 31 Ground Patterns 32 copper layer 40 Adhesive layer 50 Dielectric layer 50a,50b main surface 51 Cross 51a, 51b Reinforced fibers 60 Ground Patterns 70 Coverlays 71a Adhesive layer 71b film 71 Coverlay 71a Adhesive layer 71b film 80 Adhesive layer 81 Dielectric layer 81a, 81b Main surface 82 signal patterns 90 Adhesive layer 91 Dielectric layer 91a,91b Main surface 92 Ground Patterns 100,200 Printed circuit boards BD bending direction DR1 1st direction DR2 2nd direction S1 Preparation process S2 Patterning Process S3 Dielectric layer attachment process

Claims

1. Equipped with a bend, The bent portion is bendable along the bending direction in a plan view and has a first dielectric layer, a signal pattern, a second dielectric layer, a first cross, and a ground pattern. The first dielectric layer has a first main surface, The signal pattern is arranged on the first main surface, The second dielectric layer has a second main surface and a third main surface which is the opposite surface of the second main surface, and is arranged such that the second main surface faces the first main surface. The first cloth is formed by weaving together a plurality of first reinforcing fibers extending along a first direction in a plan view and a plurality of second reinforcing fibers extending along a second direction inclined with respect to the first direction in a plan view, The first cloth is placed within the second dielectric layer, The ground pattern is arranged on the third main surface, A printed circuit board in which each of the first and second directions is inclined with respect to the bending direction.

2. The printed circuit board according to claim 1, wherein the first direction is perpendicular to the second direction.

3. The printed circuit board according to claim 2, wherein the angle between the first direction and the bending direction in a plan view is 30° or more.

4. The printed circuit board according to claim 2, wherein the angle between the first direction and the bending direction in a plan view is 40° or more.

5. The printed circuit board according to claim 1, wherein the plurality of first reinforcing fibers and the plurality of second reinforcing fibers are formed of glass.

6. The printed circuit board according to claim 1, wherein the second dielectric layer is made of fluororesin.

7. The printed circuit board according to claim 1, further comprising an adhesive layer disposed between the first main surface and the second main surface so as to cover the signal pattern.

8. The first dielectric layer further comprises a second cloth disposed within the first dielectric layer, The second cloth is formed by weaving together a plurality of third reinforcing fibers extending along a third direction in a plan view and a plurality of fourth reinforcing fibers extending along a fourth direction that is inclined with respect to the third direction in a plan view, The printed circuit board according to claim 1, wherein each of the third and fourth directions is inclined with respect to the bending direction.

9. The printed circuit board according to any one of claims 1 to 8, wherein the bent portion is bendable so as to compress the second dielectric layer.

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