Flexible substrate and electronic device

Abstract

To suppress a change in characteristic impedance due to deformation of a signal line and a dielectric.SOLUTION: A flexible substrate includes a signal line that transmits an electrical signal, a first dielectric layer and a second dielectric layer arranged to sandwich the signal line, and a ground layer arranged to sandwich the first dielectric layer and the second dielectric layer, and the first dielectric layer which is directed outward during bending is thicker than the second dielectric layer which is directed inward during bending.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a flexible substrate and an electronic device. [Background technology]

[0002] In order to suppress reflection and attenuation of signals transmitted at high speed within electronic devices, the characteristic impedance of the transmission path through which the signal is transmitted is controlled. The characteristic impedance is determined by the size of the signal line and dielectric that form the transmission path, among other factors.

[0003] In flexible substrates, changes in parasitic capacitance occur between when the substrate is stretched and when it is bent. Therefore, even if the size of the signal lines and dielectrics are designed so that the desired characteristic impedance is achieved when the flexible substrate is stretched, the impedance may deviate from the desired value when the flexible substrate is bent. Therefore, it has been proposed to design a flexible substrate taking into account changes in parasitic capacitance so that the desired characteristic impedance is achieved when the flexible substrate is bent when mounted in an electronic device (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 186468 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-004875 Summary of the Invention [Problem to be solved by the invention]

[0005] The stress generated in a bent flexible substrate deforms the signal lines and dielectrics included in the flexible substrate. Such deformation of the signal lines and dielectrics also changes the characteristic impedance of the flexible substrate. However, previous technologies have not taken into consideration such changes in characteristic impedance due to deformation of the signal lines and dielectrics.

[0006] An object of one aspect of the disclosed technology is to provide a flexible substrate that can suppress changes in characteristic impedance due to deformation of signal lines and dielectrics, and an electronic device that includes the flexible substrate. [Means for solving the problem]

[0007] One aspect of the disclosed technology is exemplified by the following flexible substrate: The flexible substrate includes a signal line that transmits an electrical signal, a first dielectric layer and a second dielectric layer that are arranged to sandwich the signal line, and ground layers that are arranged to sandwich the first dielectric layer and the second dielectric layer, wherein the first dielectric layer that faces outward when bent is formed thicker than the second dielectric layer that faces inward when bent. [Effects of the Invention]

[0008] According to the disclosed technology, it is possible to suppress changes in characteristic impedance due to deformation of the signal line and the dielectric. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating an example of a flexible substrate according to an embodiment. [Figure 2] FIG. 2 is a diagram illustrating a cross section taken along line AA in FIG. [Figure 3] FIG. 3 is a diagram illustrating the distortion that occurs when a plate-like member is bent. [Figure 4] FIG. 4 is a diagram illustrating distortion that occurs in a bent flexible substrate. [Figure 5] FIG. 5 is a diagram for explaining calculation of the distance from the lower surface of the second dielectric layer to the neutral axis. [Figure 6] FIG. 6 is a first diagram illustrating an example of a mobile phone terminal according to an application example. [Figure 7] FIG. 7 is a second diagram illustrating an example of a mobile phone terminal according to the application example. DETAILED DESCRIPTION OF THE INVENTION

[0010] <Embodiment> The configurations of the embodiments described below are examples, and the disclosed technology is not limited to the configurations of the embodiments. A flexible substrate according to the embodiments has, for example, the following configuration. The flexible substrate according to the present embodiment is a flexible substrate including a signal line that transmits an electrical signal, a first dielectric layer and a second dielectric layer that are arranged to sandwich the signal line, and ground layers that are arranged to sandwich the first dielectric layer and the second dielectric layer. The first dielectric layer, which faces outward when bent, is formed thicker than the second dielectric layer, which faces inward when bent.

[0011] When a flexible substrate is bent with the second dielectric layer facing inward, the first dielectric layer stretches and becomes thinner, while the second dielectric layer compresses and becomes thicker. This variation in the thickness of the first and second dielectric layers causes the characteristic impedance of the signal line to vary. In the flexible substrate, the first dielectric layer, which stretches when bent, is made thicker than the second dielectric layer, which compresses when bent, thereby suppressing the variation in the characteristic impedance of the signal line when bent compared to when it is not bent (straight).

[0012] Hereinafter, the embodiments of the flexible substrate will be further described with reference to the drawings. FIG. 1 is a diagram showing an example of a flexible substrate 1 according to an embodiment. FIG. 1 illustrates a cross-sectional view of the flexible substrate 1 perpendicular to the longitudinal direction. FIG. 2 is a diagram showing a cross-section taken along line AA in FIG. 1. The flexible substrate 1 includes a first ground layer 11, a second ground layer 12, a dielectric layer 13, and a signal line 14. Hereinafter, the longitudinal direction of the flexible substrate 1 is referred to as the Y direction, the height direction of the flexible substrate 1 as the Z direction, and the width direction of the flexible substrate 1 as the X direction. The +Z direction is referred to as the top, and the -Z direction as the bottom. The signal line 14 is an example of a "signal line." The flexible substrate 1 is an example of a "flexible substrate."

[0013] The flexible substrate 1 is a flexible substrate. The flexible substrate 1 is formed by a first ground layer 11, a dielectric layer 13, and a second ground layer 12, all of which are flexible. The flexible substrate 1 is used, for example, when connecting electronic components such as two boards in a flexible manner. In other words, the flexible substrate 1 is used when connecting electronic components such as two boards in a manner that allows them to move relative to each other.

[0014] As can be seen from FIG. 2, in the flexible substrate 1, the first ground layer 11, the first dielectric layer 131, the signal line 14, the second dielectric layer 132, and the second ground layer 12 are arranged in this order from the +Z direction to the -Z direction. The first dielectric layer 131 is an example of a "first dielectric layer." The second dielectric layer 132 is an example of a "second dielectric layer." The first ground layer 11 and the second ground layer 12 are examples of "ground layers."

[0015] The first ground layer 11 and the second ground layer 12 are grounded conductors. The first ground layer 11 is provided on the upper surface of the first dielectric layer 131. The second ground layer 12 is provided on the lower surface of the second dielectric layer 132. In other words, the first ground layer 11 and the second ground layer 12 are arranged so as to sandwich the dielectric layer 13 therebetween.

[0016] The dielectric layer 13 includes a first dielectric layer 131 and a second dielectric layer 132. The first dielectric layer 131 and the second dielectric layer 132 are arranged to sandwich the signal line 14. Here, the width W of the signal line 14, the height t of the signal line 14, the height h1 of the first dielectric layer 131, the height h2 of the second dielectric layer 132, and the relative dielectric constant ε of the dielectric layer 13 are r Using this, the characteristic impedance Z0 of the signal line 14 when the flexible substrate 1 is not bent can be expressed by the following equation (1). The state of the flexible substrate 1 when not bent is an example of the "first state."

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[0017] The flexible substrate 1 is designed so that the characteristic impedance of the signal line 14 becomes a desired characteristic impedance by adjusting the width W of the signal line 14, the thickness t of the signal line 14, the thickness h1 of the first dielectric layer 131, and the thickness h2 of the second dielectric layer 132. However, when the flexible substrate 1 is bent, the thickness t of the signal line 14, the thickness h1 of the first dielectric layer 131, and the thickness h2 of the second dielectric layer 132 fluctuate. As a result, the characteristic impedance of the signal line 14 also fluctuates.

[0018] For example, when the flexible substrate 1 is bent so that the second dielectric layer 132 is on the inside, the second dielectric layer 132 is compressed and becomes thicker, and the first dielectric layer 131 is stretched and becomes thinner. The width W of the signal line 14 in the bent flexible substrate 1, the thickness t' of the signal line 14, the thickness h1' of the first dielectric layer 131, the thickness h2' of the second dielectric layer 132, and the relative dielectric constant ε of the dielectric layer 13 are r Using this, the characteristic impedance Z0' of the signal line 14 when the flexible substrate 1 is bent can be expressed by the following equation (2).

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[0019] If the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 were made the same, the characteristic impedance of the signal line 14 would have different values ​​when the flexible substrate 1 is straight and when it is bent.

[0020] Therefore, in this embodiment, taking into consideration variations in the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 due to bending, the thickness h1 of the first dielectric layer 131, which is on the outer side when bent, is determined to be thicker than the thickness h2 of the second dielectric layer 132, which is on the inner side when bent. For example, the thickness h1 of the first dielectric layer 131 is determined so that the characteristic impedance of the signal line 14 when the flexible substrate 1 is not bent and the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent are approximately equal. An example of a method for determining the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 will be described below.

[0021] The thickness h1' of the first dielectric layer 131, the thickness h2' of the second dielectric layer 132, and the thickness t' of the signal line 14 in the flexible substrate 1 bent at a bending radius R are determined by the Poisson's ratio ν n , Poisson's ratio ν of the signal line 14 t , the radial strain of the first dielectric layer 131 due to bending Miε r1 , the radial strain ε of the second dielectric layer 132 due to bending r2 , the strain ε in the circumferential direction of the first dielectric layer 131 due to bending θ1 , the strain ε in the circumferential direction of the second dielectric layer 132 due to bending θ2 , the strain ε in the circumferential direction of the signal line 14 due to bending θt , can be calculated using the following equations (3), (4), and (5).

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[0022] When the flexible substrate 1 is bent with the second dielectric layer 132 on the inside, a tensile force acts on the first dielectric layer 131 and a compressive force acts on the second dielectric layer 132. FIG. 3 is a diagram illustrating the distortion that occurs when a plate-like member is bent. In FIG. 3, ρ is the radius of curvature when the flexible substrate 1 is bent. The neutral axis 300 is the part of the dielectric layer 13 where the tensile force and the compressive force balance each other. The distortion ε when a line segment PQ radially away from the neutral axis 300 by η becomes an arc PQ' can be expressed as in the following equation (6).

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[0023] Then, by solving the above equation (6) for ε, the following equation (7) is obtained.

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[0024] It can be seen from equation (7) that the strain ε is proportional to the distance η from the neutral axis 300 and inversely proportional to the radius of curvature ρ. Then, using equation (7), the strain in the first dielectric layer 131, the second dielectric layer 132, and the signal line 14 can be calculated. FIG. 4 is a diagram illustrating the strain that occurs in the bent flexible substrate 1. The distance from the center of curvature to the lower surface of the second dielectric layer 132 is R, and the distance from the lower surface of the second dielectric layer 132 to the neutral axis 300 is η0. Furthermore, if the reference in the radial direction (position 0) is the position of the neutral axis 300, the strain ε on the upper surface of the first dielectric layer 131 is θ1外 can be expressed by the following equation (8): θ1内 can be expressed by the following equation (9).

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[0025] The strain ε of the first dielectric layer 131 θ1 For example, ε θ1外 and ε θ1内For example, the strain ε of the first dielectric layer 131 can be calculated as the arithmetic mean of θ1 can be expressed by the following equation (10).

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[0026] Similarly, the strain ε θt外 can be expressed by the following equation (11): θt内 can be expressed by the following equation (12).

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[0027] The distortion ε of the signal line 14 θt For example, ε θt外 and ε θt内 For example, the strain ε of the signal line 14 can be calculated as the arithmetic mean of θt can be expressed by the following equation (13).

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[0028] Similarly, the strain ε θ2外 can be expressed by the following equation (14): θ2内 can be expressed by the following equation (15).

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[0029] The strain ε of the second dielectric layer 132 θ2 For example, ε θ2外 and ε θ2内 For example, the strain ε of the second dielectric layer 132 can be calculated as the arithmetic mean of θ2 can be expressed by the following equation (16).

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[0030] Here, since the sum of the tensile stress and the compressive stress in the radial direction of the flexible substrate 1 bent at the neutral axis 300 becomes 0, the bending stress from the lower surface of the second dielectric layer 132 to the neutral axis 300 5 is a diagram for explaining the calculation of the distance η0 from the lower surface of the second dielectric layer 132 to the neutral axis 300. Note that the first ground layer 11 and the second ground layer 12 are not shown in FIG. 5. Assuming an infinitesimal section with a length dA in the Y direction and a thickness dη in the Z direction, the distance between the infinitesimal section and the central axis 300 is η, and the length of the flexible substrate 1 is b. Furthermore, the dielectric constant of the dielectric layer 13 is E n , Young's modulus of the signal line 14 is E c Then, the following equation (17) holds.

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[0031] By transforming equation (17), the following equation (18) can be obtained.

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[0032] That is, the position η of the neutral axis 300 is determined by the formula (18), and the determined position η of the neutral axis 300 is substituted into the formulas (10), (13), and (16), whereby the strain ε of the first dielectric layer 131 is calculated. θ1 , distortion ε of the signal line 14 θt , the strain ε of the second dielectric layer 132 θ2 is calculated.

[0033] Calculated strain ε of the first dielectric layer 131 θ1 , distortion ε of the signal line 14 θt , the strain ε of the second dielectric layer 132 θ2are substituted into equations (3), (4), and (5), the thickness h1' of the first dielectric layer 131, the thickness h2' of the second dielectric layer 132, and the thickness t' of the signal line 14 when bent with a radius of curvature R are calculated.

[0034] By substituting the calculated thickness h1' of the first dielectric layer 131, the thickness h2' of the second dielectric layer 132, and the thickness t' of the signal line 14 into equation (2), the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent at a radius of curvature R with the second dielectric layer 132 on the inside is calculated. The state in which the flexible substrate 1 is bent at a radius of curvature R with the second dielectric layer 132 on the inside is an example of "a second state in which the flexible substrate is bent at a predetermined radius of curvature with the second dielectric layer on the inside."

[0035] Here, the Poisson's ratio ν of the dielectric layer 13 n , Poisson's ratio ν of the signal line 14 t , Young's modulus E of the dielectric layer 13 n and Young's modulus E of the signal line 14 c are values ​​determined depending on the materials used for the dielectric layer 13 and the signal line 14. Using these values, the thickness h1 and thickness h2 of the first dielectric layer 131 are determined so as to show desired characteristic impedances in equation (1) which represents the characteristic impedance of the signal line 14 when the flexible substrate 1 is in a straight state, and equation (2) which represents the characteristic impedance of the signal line 14 when the flexible substrate 1 is in a bent state. By determining the thickness h2 of the second dielectric layer 132, the signal line 14 can be made to exhibit the desired characteristic impedance whether the flexible substrate 1 is in a straight state (unbent state) or a bent state.

[0036] When the flexible substrate 1 is bent so that the second dielectric layer 132 is on the inside, the second dielectric layer 132 is compressed and thickened, and the first dielectric layer 131 is stretched and thinned, in the bent portion of the flexible substrate 1. Such a change in thickness may cause a change in the characteristic impedance of the signal line 14. In this embodiment, by forming the first dielectric layer 131 to be thicker than the second dielectric layer 132, it is possible to suppress a change in the characteristic impedance of the signal line 14 when the flexible substrate 1 is not bent (straight state) and when it is bent.

[0037] In this embodiment, the thickness of the first dielectric layer 131 and the thickness of the second dielectric layer 132 are determined so that the characteristic impedance of the signal line 14 when the flexible substrate 1 is not bent is approximately equal to the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent with the second dielectric layer 132 on the inside at a curvature radius R. By determining the thicknesses of the first dielectric layer 131 and the second dielectric layer 132 in this manner, it is possible to suppress fluctuations in the characteristic impedance of the signal line 14 when the flexible substrate 1 is not bent (straight state) and when it is bent.

[0038] <Application example> The flexible substrate 1 described above can be applied to, for example, a mobile phone terminal. Figs. 6 and 7 are diagrams showing an example of a mobile phone terminal 500 according to an application example. The mobile phone terminal 500 is a terminal in which a keyboard-side housing 510 and a display-side housing 520 are connected by a hinge 501, so that the keyboard-side housing 510 and the display-side housing 520 can be opened and closed relative to each other. In other words, the mobile phone terminal 500 is a foldable electronic device. Fig. 6 illustrates an example of the mobile phone terminal 500 in an open state. Fig. 7 illustrates an example of the mobile phone terminal 500 in a closed state. The mobile phone terminal 500 is an example of an "electronic device".

[0039] 6 and 7, a keyboard-side substrate 511 in a keyboard-side housing 510, a display-side substrate 521 in a display-side housing 520, and a hinge 501 are illustrated by dotted lines. The mobile phone terminal 500 can be opened and closed by connecting the keyboard-side housing 510 and the display-side housing 520 by the hinge 501. The hinge 501 is an example of a "joint portion."

[0040] In the mobile phone terminal 500, the keyboard side substrate 511 and the display side substrate 521 are connected by the flexible substrate 1. Here, when the mobile phone terminal 500 is closed, the flexible substrate 1 is arranged so that the flexible substrate 1 is bent with the second dielectric layer 132 facing inward.

[0041] When the mobile phone terminal 500 is in an open state, the flexible substrate 1 is in a straight, stretched state. When the mobile phone terminal 500 is in a closed state, the flexible substrate 1 is in a bent state. In this way, the keyboard-side housing 510 and the display-side housing 520 move relative to each other, causing the flexible substrate 1 to change between a straight state and a bent state. When the flexible substrate 1 changes between a straight state and a bent state depending on the mode of use, it is preferable that the flexible substrate 1 be designed so that the characteristic impedance of the signal line 14 becomes a desired characteristic impedance in either state.

[0042] In this embodiment, the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 are determined so that Z0 expressed by the formula (1) and Z0' expressed by the formula (2) are equal.

[0043] For example, assume that the Young's modulus of signal line 14 is 120 GPa, the Young's modulus of dielectric layer 13 is 3.4 GPa, the Poisson's ratio of signal line 14 is 0.3, and the Poisson's ratio of dielectric layer 13 is 0.3. Also assume that the thickness t of signal line 14 is 50 μm and the width w of signal line 14 is 102 μm. Under these conditions, consider the case where flexible substrate 1 is bent with a curvature radius of 0.3 mm.

[0044] When the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 are determined so that Z0 and Z0' are equal, the thickness h1 of the first dielectric layer 131 can be set to 200 μm, and the thickness h2 of the second dielectric layer 132 can be set to 100 μm. In this case, according to equation (1), the characteristic impedance Z0 of the signal line 14 when the flexible substrate 1 is straight is 50 Ω.

[0045] Furthermore, when the thickness h1 of the first dielectric layer 131 is 200 μm and the thickness h2 of the second dielectric layer 132 is 200 μm, according to equations (3), (4), and (5), the thickness h1' of the first dielectric layer 131 of the bent flexible substrate 1 is 183.87 μm, the thickness h2' of the second dielectric layer 132 is 102.31 μm, and the thickness of the signal line 14 is 50.29 μm. In this case, according to equation (2), the characteristic impedance Z0' of the signal line 14 when the flexible substrate 1 is bent is 50 Ω. In other words, the characteristic impedance of the signal line 14 can be made as equal as possible whether the flexible substrate 1 is straight or bent.

[0046] Suppose that the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 are both 150 μm, the thickness t of the signal line 14 is 50 μm, the width w of the signal line 14 is 135 μm, and the relative dielectric constant ε r is set to 3.4, the characteristic impedance of the signal line 14 when the flexible substrate 1 is in a straight state is 50Ω according to equation (1).

[0047] Furthermore, if the Young's modulus of the signal line 14 is 120 GPa, the Young's modulus of the dielectric layer 13 is 3.4 GPa, the Poisson's ratio of the signal line 14 is 0.3, and the Poisson's ratio of the dielectric layer 13 is 0.3, and the flexible substrate 1 is bent with a curvature radius of 0.3 mm, according to equations (3), (4), and (5), the thickness h1' of the first dielectric layer 131 fluctuates to 140.5 μm, the thickness h2' of the second dielectric layer 132 fluctuates to 159.5 μm, and the thickness of the signal line 14 fluctuates to 50 μm. As a result, according to equation (2), the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent is 50.3 Ω. In other words, when the flexible substrate 1 is bent such that the thickness h1 of the first dielectric layer 131 and the thickness h2 of the second dielectric layer 132 are equal, the characteristic impedance of the signal line 14 deviates by 0.6%.

[0048] In this embodiment, by making the thickness of the first dielectric layer 131 thicker than the second dielectric layer 132, it is possible to eliminate as much as possible the difference between the characteristic impedance of the signal line 14 when the flexible substrate 1 is straightened and the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent. Consequently, it is possible to make the characteristic impedance of the signal line 14 when the flexible substrate 1 is straightened and the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent both to be the desired characteristic impedance. Therefore, it is possible to improve the quality of the transmission signal whether the flexible substrate 1 is straight or bent.

[0049] In this embodiment, the characteristics of the signal line 14 when the flexible substrate 1 is straightened are The thickness of the first dielectric layer 131 and the thickness of the second dielectric layer 132 are determined so that the impedance Z0 is equal to the characteristic impedance Z0' of the signal line 14 when the flexible substrate 1 is bent. Therefore, according to this embodiment, the characteristic impedance of the signal line 14 when the flexible substrate 1 is straight and the characteristic impedance of the signal line 14 when the flexible substrate 1 is bent can both be made to be the desired characteristic impedance.

[0050] Furthermore, as described in the application example, by employing a flexible substrate 1 in which the thickness of the first dielectric layer 131 is greater than the thickness of the second dielectric layer 132, the characteristic impedance of the signal line 14 can be set to a desired characteristic impedance whether the mobile phone terminal 500 is in an open state or a closed state. Therefore, whether the mobile phone terminal 500 is in an open state or a closed state, the transmission quality of the signal transmission between the keyboard side substrate 511 and the display side substrate 521 can be improved.

[0051] <Modification> In the embodiment described above, the flexible substrate 1 has been described as having a three-layer structure of the first dielectric layer 131, the signal line 14, and the second dielectric layer 132, excluding the first ground layer 11 and the second ground layer 12. However, the flexible substrate 1 is not limited to a three-layer structure. The flexible substrate 1 may have, for example, a four-layer or more structure including three or more dielectric layers. [Explanation of symbols]

[0052] 1. Flexible substrate 11. First ground layer 12. Second ground layer 13. Dielectric layer 14 Signal line 131 First dielectric layer 132...Second dielectric layer 500··Mobile phone terminal 501··Hinge 510··Keyboard side housing 511··Keyboard side board 520··Display side housing 521 Display side board

Claims

1. a signal line for transmitting an electrical signal; a first dielectric layer and a second dielectric layer disposed so as to sandwich the signal line; a flexible substrate including: a first dielectric layer and a ground layer disposed so as to sandwich the second dielectric layer; the first dielectric layer, which faces outward when the substrate is bent, is formed thicker than the second dielectric layer, which faces inward when the substrate is bent; the thickness of the first dielectric layer and the thickness of the second dielectric layer are determined so that a characteristic impedance of the signal line in a first state in which the flexible substrate is not bent is equal to a characteristic impedance of the signal line in a second state in which the flexible substrate is bent at a predetermined radius of curvature with the second dielectric layer on the inside. Flexible substrate.

2. a first substrate and a second substrate arranged to be bendable via a joint portion; and the flexible substrate according to claim 1 , which connects the first substrate and the second substrate at the joint portion. electronic equipment.

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