Wiring board and electronic apparatus

Abstract

To provide a wiring board that can realize more superior high-speed transmission characteristics.SOLUTION: A wiring board has: a first conductor layer; a second conductor layer formed opposite the first conductor layer; a plurality of differential signal wires formed in the first conductor layer and each including a pair of signal wires; a plurality of first ground wires formed in the second conductor layer, and arranged along pairs of signal wires without overlapping in plan view in the opposition direction in which the first conductor layer and second conductor layer face each other; and a plurality of second ground wires formed in the second conductor layers at intervals to cross the first ground wires, and connecting two mutually adjacent first ground wires.SELECTED DRAWING: Figure 3A

Description

[Technical field]

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

[0002] Patent Document 1 describes a wiring board for differential signals having a first signal line and a second signal line, which are differential lines arranged parallel to each other on one side of an insulating layer, and a wiring pattern that serves as a ground layer and has a plurality of openings arranged on the other side of the insulating layer. In the wiring board described in Patent Document 1, the plurality of openings are arranged so as to be line-symmetrical with respect to a reference line that is set between the two signal lines and parallel to the direction in which the two signal lines extend, and that is set so as to be equally distant from the two signal lines. In addition, the tangents to the outer edges of the respective openings are formed so as to intersect with the reference line at less than 90°. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2012-227211 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the wiring board described in Patent Document 1, the ground wiring crosses between two signal wirings constituting the differential wiring in a plan view in the direction in which the layer including the signal wiring and the ground layer face each other, so that the area in which the differential wiring faces the ground layer becomes large. Therefore, in the wiring board described in Patent Document 1, a large impedance mismatch occurs in the differential wiring, particularly in high-speed signal transmission, and transmission loss can become large, making it difficult to achieve excellent high-speed transmission characteristics.

[0005] An object of the present invention is to provide a wiring board capable of achieving superior high-speed transmission characteristics. [Means for solving the problem]

[0006] According to one aspect of the present invention, there is provided a wiring board comprising: a first conductor layer; a second conductor layer formed opposite the first conductor layer; a plurality of differential signal wirings formed in the first conductor layer, each including a pair of signal wirings; a plurality of first ground wirings formed in the second conductor layer and arranged along but not overlapping the pair of signal wirings in a planar view seen in a direction in which the first conductor layer and the second conductor layer face each other; and a plurality of second ground wirings formed in the second conductor layer at intervals so as to intersect with the first ground wirings and connect two adjacent first ground wirings to each other. Effect of the Invention

[0007] According to the present invention, it is possible to realize superior high-speed transmission characteristics. [Brief description of the drawings]

[0008] [Figure 1] 1 is a schematic diagram illustrating an imaging device that is an example of an electronic device according to a first embodiment of the present invention. [Diagram 2] 1 is a schematic diagram showing an imaging unit according to a first embodiment of the present invention. [Figure 3A] 1 is a plan view showing a flexible printed wiring board according to a first embodiment of the present invention. [Figure 3B] 1 is a cross-sectional view showing a flexible printed wiring board according to a first embodiment of the present invention. [Figure 3C] 1 is a cross-sectional view showing a flexible printed wiring board according to a first embodiment of the present invention. [Figure 4A] FIG. 4 is a plan view showing a modified example of the flexible printed wiring board according to the first embodiment of the present invention. [Figure 4B] FIG. 4 is a plan view showing a modified example of the flexible printed wiring board according to the first embodiment of the present invention. [Figure 4C]FIG. 4 is a plan view showing a modified example of the flexible printed wiring board according to the first embodiment of the present invention. [Figure 5A] 1 is a graph showing a comparison of time domain reflectance characteristics of an example and a comparative example. [Figure 5B] 1 is a graph showing a comparison of signal transmission characteristics between an example and a comparative example. [Figure 6A] FIG. 4 is a plan view showing a flexible printed wiring board according to a second embodiment of the present invention. [Figure 6B] FIG. 4 is a cross-sectional view showing a flexible printed wiring board according to a second embodiment of the present invention. [Figure 6C] FIG. 4 is a cross-sectional view showing a flexible printed wiring board according to a second embodiment of the present invention. [Figure 7A] FIG. 11 is a plan view showing a modified example of the flexible printed wiring board according to the second embodiment of the present invention. [Figure 7B] FIG. 11 is a plan view showing a modified example of the flexible printed wiring board according to the second embodiment of the present invention. [Figure 7C] FIG. 11 is a plan view showing a modified example of the flexible printed wiring board according to the second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [First embodiment] An electronic device and a wiring board according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 5B.

[0010] First, an imaging device as an example of an electronic device according to the present embodiment will be described with reference to Fig. 1 and Fig. 2. Fig. 1 is an explanatory diagram of a digital camera 600 which is an imaging device as an example of an electronic device according to the present embodiment. Fig. 2 is an explanatory diagram of an imaging unit 200 according to the present embodiment.

[0011] 1, digital camera 600, which is an imaging device, is a lens-interchangeable digital camera and includes a camera body 601. A lens unit (lens barrel) 602 including a lens is detachable from camera body 601. Camera body 601 includes a housing 611, and imaging unit 200 and wireless communication unit 250 housed inside housing 611.

[0012] The imaging unit 200 includes a printed circuit board 201, a printed circuit board 202, and one flexible printed wiring board 100 which is a wiring board that electrically connects the printed circuit board 201 and the printed circuit board 202. The flexible printed wiring board 100 enables the wiring structure to be made lighter than a coaxial cable.

[0013] The printed circuit board 201 includes a printed wiring board 210 and a semiconductor device (first semiconductor device) 211 mounted on the printed wiring board 210. The printed circuit board 202 includes a printed wiring board 220 and a semiconductor device (second semiconductor device) 221 mounted on the printed wiring board 220.

[0014] The first semiconductor device 211 is an image sensor serving as an imaging element. The image sensor is, for example, a CMOS (Complementary Metal Oxide Semiconductor) image sensor or a CCD (Charge Coupled Device) image sensor. The image sensor has a function of converting light incident via the lens unit 602 into an electrical signal. The second semiconductor device 221 is a digital signal processor serving as a processing circuit. The digital signal processor has a function of acquiring an electrical signal indicating image data from the image sensor, performing a process of correcting the acquired electrical signal, and generating corrected image data.

[0015] The wireless communication unit 250 is a modularized wireless communication module that performs wireless communication in the GHz band. The wireless communication unit 250 has a printed wiring board 251 provided with an antenna (not shown) and a wireless communication IC 252 mounted on the printed wiring board 251. The antenna is provided in the same plane as the wireless communication IC 252 and is disposed in a position close to the housing 611 so as to facilitate communication with the outside. The wireless communication IC 252 transmits and receives image data by performing wireless communication with an external device (such as a PC or a wireless router) via the antenna. That is, the wireless communication IC 252 modulates a digital signal indicating image data and transmits it from the antenna as radio waves of a communication frequency of a wireless standard. The wireless communication IC 252 also demodulates the radio waves received by the antenna into a digital signal indicating image data. The wireless communication IC 252 performs wireless communication with an external device in compliance with a standard such as WiFi (registered trademark) or Bluetooth (registered trademark).

[0016] 2, a connector 212 is mounted on the printed wiring board 210. The connector 212 is electrically connected to the first semiconductor device 211 by a conductor formed on the printed wiring board 210. A connector 222 is mounted on the printed wiring board 220. The connector 222 is electrically connected to the second semiconductor device 221 by a conductor formed on the printed wiring board 220. In other words, the first semiconductor device 211 and the second semiconductor device 221 are electrically connected by the printed wiring board 210, the flexible printed wiring board 100, and the printed wiring board 220, and can communicate with each other.

[0017] Next, the flexible printed wiring board 100 according to the present embodiment will be described with reference to Fig. 3A to Fig. 3C. Fig. 3A is a plan view of the flexible printed wiring board 100 according to the present embodiment as viewed from above. Fig. 3B is a cross-sectional view showing a cross section of the flexible printed wiring board 100 taken along line A1-A2 in Fig. 3A. Fig. 3C is a cross-sectional view showing a cross section of the flexible printed wiring board 100 taken along line B1-B2 in Fig. 3A.

[0018] 3B and 3C, the flexible printed wiring board 100 has a base layer 123, a conductor layer 122, a conductor layer 124, a cover layer 121, and a cover layer 125, and has a layer structure including two conductor layers 122 and 124. Note that the layer structure of the flexible printed wiring board 100 is not limited to the structure shown in Fig. 3B and 3C, and may include other insulating layers, conductor layers, etc.

[0019] The base layer 123 has a sheet-like shape having a first surface and a second surface opposite to the first surface. Of the two surfaces of the base layer 123, the conductor layer 122 is formed on the first surface, and the conductor layer 124 is formed on the second surface. The conductor layer 122 and the conductor layer 124 face each other via the base layer 123. On the first surface of the base layer 123 on which the conductor layer 122 is formed, a cover layer 121 is formed on the outside of the conductor layer 122 so as to cover the conductor layer 122. On the second surface of the base layer 123 on which the conductor layer 124 is formed, a cover layer 125 is formed on the outside of the conductor layer 124 so as to cover the conductor layer 124. The conductor layer 122 is a signal wiring layer. The conductor layer 124 is a ground layer.

[0020] A plurality of differential signal wirings 110 are formed on the conductor layer 122. The differential signal wiring 110 is composed of a pair of wirings, a signal wiring 110A and a signal wiring 110B. A plurality of ground wirings 102 and a plurality of ground wirings 103 are formed on the conductor layer 124. The signal wirings 110A and the signal wirings 110B are formed parallel to each other along the longitudinal direction of the flexible printed wiring board 100. As a result, the differential signal wirings 110 are configured to transmit electric signals along the longitudinal direction of the flexible printed wiring board 100. Specifically, the differential signal wirings 110 transmit data signals, which are digital signals representing image data, for example. In addition to the differential signal wirings 110, wirings for transmitting single-ended signals, such as control signals and response signals, may be formed on the conductor layer 122.

[0021] The material of the base layer 123, which is the substrate, is, for example, a resin. Specific examples of the resin include polyimide-based resins such as polyimide, polyamide, and polyamideimide, thermosetting resins such as epoxy, and thermoplastic resins such as liquid crystal polymers. Among these, polyimide or liquid crystal polymer is preferable. Polyimide has excellent heat resistance and mechanical properties, and is easily available commercially. Liquid crystal polymers are suitable for high-speed signal transmission applications because of their low relative dielectric constant, and have low moisture absorption and excellent dimensional stability. The thickness of the base layer 123 is not particularly limited, but is preferably in the range of 10 μm to 100 μm. If the thickness is less than 10 μm, the distance between the differential signal wiring 110 and the ground wirings 102 and 103 may become short, and the value of the characteristic impedance may become small. On the other hand, if the thickness exceeds 100 μm, the rigidity of the resin may become high, and the flexibility may become insufficient. More preferably, the thickness is in the range of 12 μm to 75 μm.

[0022] In this embodiment, the flexible printed wiring board 100 is described as an example of the wiring board, but the wiring board may be a rigid printed wiring board. In the case of a rigid printed wiring board, for example, a fiber base material can be used as the material of the base layer 123. Specific examples of the fiber base material include glass fiber base materials such as woven glass cloth and nonwoven glass cloth, and inorganic fiber base materials such as woven cloth or nonwoven cloth containing inorganic compounds other than glass. In addition, organic fiber base materials composed of organic fibers such as aromatic polyamide, polyamide, aromatic polyester, polyester, polyimide, and fluororesin can be used. Among these, glass fiber base materials are preferable from the viewpoint of excellent strength and low water absorption.

[0023] The method of forming the conductor layers 122 and 124 is not particularly limited, and they can be formed by known methods such as laminating metal foils, metal plating, and inkjet processes. When using copper foil as the metal foil, a film laminated with an adhesive or the like can be used to form a required transmission line pattern by a photolithography and etching process. When using an inkjet process, a polymer ink containing conductive metal particles can be drawn into a required pattern, and the pattern can be baked at a temperature equal to or lower than the glass transition point (Tg) of the base layer 123 to form the pattern. The thickness of the conductor layer 122 (the thickness of the differential signal wiring 110) is not particularly limited, and is, for example, in the range of 0.1 μm to 25 μm. The thickness of the conductor layer 124 (the thickness of the ground wirings 102 and 103) is also the same as the thickness of the conductor layer 122.

[0024] In addition, in the housing 611 of the digital camera 600, which is inside the housing of the electronic device, the distance between the wireless communication unit 250 and the conductor layer 124 is preferably shorter than the distance between the wireless communication unit 250 and the conductor layer 122. In other words, the conductor layer 124 on which the ground wirings 102 and 103 are formed is preferably disposed at a position closer to the wireless communication unit 250 than the conductor layer 122 on which the differential signal wiring 110 is formed. By locating the conductor layer 124 closer to the wireless communication unit 250 than the conductor layer 122, it is possible to suppress the superposition of radiation noise generated from the conductor layer 122 on which the differential signal wiring 110 is formed on the radio waves of the external communication performed by the wireless communication unit 250.

[0025] The cover layers 121 and 125 serve as protective layers for suppressing current flow to other components in the electronic device. Therefore, the cover layers 121 and 125 are made of resin, which is insulating and flexible, such as a cover film and a coating layer of insulating resin. As the cover film, so-called engineering plastics can be used. As the insulating resin, any resin may be used as long as it has insulating properties, and examples thereof include thermosetting resin and ultraviolet curing resin. The thickness of the cover layers 121 and 125 is not particularly limited, but is preferably in the range of 5 μm to 50 μm, and particularly preferably in the range of 10 μm to 35 μm. If the thickness is less than 5 μm, the strength may be insufficient. On the other hand, if the thickness exceeds 50 μm, the sliding property and bending property may be reduced.

[0026] FIG. 3A shows the arrangement of the differential signal wiring 110 and the ground wiring 102, 103 in a plan view of the flexible printed wiring board 100 seen from the cover layer 121 side, i.e., in a plan view seen in the opposing direction in which the conductor layer 122 and the conductor layer 124 face each other.

[0027] 3A, the multiple differential signal wirings 110 are arranged parallel to each other along the longitudinal direction of the flexible printed wiring board 100. A pair of signal wirings 110A and 110B constituting each differential signal wiring 110 are arranged parallel to each other along the longitudinal direction of the flexible printed wiring board 100. The signal wirings 110A and 110B have the same width.

[0028] The ground wirings 102 are formed on the conductor layer 124 so as to be arranged along the longitudinal direction of the flexible printed wiring board 100. Here, the ground wirings 102 are arranged parallel to each other along the longitudinal direction of the flexible printed wiring board 100. The ground wirings 102 are also arranged along the signal wirings 110A and 110B. Here, the signal wirings 110A and 110B and the ground wirings 102 are arranged parallel to each other. The pair of signal wirings 110A and 110B constituting the differential signal wiring 110 are arranged so as to be located between two adjacent ground wirings 102. For this reason, the pair of signal wirings 110A and 110B do not overlap with the ground wirings 102 projected onto the conductor layer 122. Thus, the ground wirings 102 are arranged so as to be parallel to and not overlap with the pair of signal wirings 110A and 110B in a plan view seen in the opposing direction in which the conductor layer 122 and the conductor layer 124 face each other.

[0029] Two adjacent ground wirings 102 are connected by a plurality of ground wirings 103. The plurality of ground wirings 103 are formed on the conductor layer 124 so as to be arranged along a direction intersecting the ground wiring 102, for example, a direction perpendicular to the ground wiring 102. Each ground wiring 103 intersecting with the ground wiring 102 is formed in a straight line across the plurality of ground wirings 102 to connect the ground wirings 102. The plurality of ground wirings 103 are arranged at intervals in a direction along the ground wiring 102. The plurality of ground wirings 103 are formed at intervals so as to intersect with the ground wiring 102, and connect two adjacent ground wirings 102.

[0030] 3A illustrates an example in which the flexible printed wiring board 100 has a linear planar shape, but is not limited thereto. The flexible printed wiring board 100 may have a curved or bent planar shape. In this case, the pair of signal wirings 110A, 110B and the ground wiring 102 constituting the differential signal wiring 110 may be formed to be curved or bent in response to the curvature or bending of the flexible printed wiring board 100 while maintaining the above-mentioned arrangement.

[0031] The portion where the ground wiring 103 is not formed is a cross section taken along line A1-A2 as shown in Fig. 3B. In the portion where the ground wiring 103 is not formed, neither the ground wiring 102 nor the ground wiring 103 is disposed at a position facing the signal wirings 110A and 110B of the differential signal wiring 110. On the other hand, the portion where the ground wiring 103 is formed is a cross section taken along line B1-B2 as shown in Fig. 3C. In the portion where the ground wiring 103 is formed, the ground wiring 103 is disposed so as to face the signal wirings 110A and 110B of the differential signal wiring 110.

[0032] Thus, in this embodiment, the multiple ground wirings 102 are formed so as not to overlap with the signal wirings 110A and 110B projected onto the conductor layer 124 in the opposing direction in which the conductor layer 122 and the conductor layer 124 face each other. The opposing direction in which the conductor layer 122 and the conductor layer 124 face each other is the same as the lamination direction in which the conductor layer 122, the base layer 123, and the conductor layer 124 are laminated. The multiple ground wirings 102 are connected by multiple ground wirings 103 arranged at predetermined intervals in a direction along the ground wiring 102.

[0033] Data communication is performed between two semiconductor devices mounted on an electronic device using digital signals. Electrical signals are transmitted through signal wiring. From the viewpoint of easiness of wiring signal wiring in an electronic device and weight reduction, a flexible printed wiring board, which is flexible and lightweight, is connected to a printed wiring board through a connector. In addition, since the transmission speed of digital signals in electronic devices has been increasing in recent years, impedance matching of the transmission path is also required to ensure signal quality. In order to achieve impedance matching, for example, one layer of a two-layer flexible printed wiring board is used as a signal layer and the other layer is used as a ground layer, and the conductor of the ground layer is formed in a mesh shape.

[0034] Due to the increase in the volume of image data processed by electronic devices, electrical signals of the order of Gbps (Giga Bits Per Second) are transmitted per lane. In addition, differential signals are mainly used in high-speed data communication. When the transmission speed exceeds 1 Gbps, the ground becomes a factor of impedance mismatch even in a mesh shape. In particular, if the ground wiring crosses between a pair of signal wirings constituting a differential signal wiring when the ground wiring is projected onto the signal wiring layer, the area where the differential signal wiring faces the ground wiring becomes large, and as a result, a large impedance mismatch occurs in the differential signal wiring. In addition, when the transmission speed exceeds 1 Gbps, the transmission loss also affects the signal quality. In particular, if the ground wiring crosses between two signal wirings constituting a differential signal wiring when the ground wiring is projected onto the signal wiring layer, the area where the differential signal wiring faces the ground becomes large, and the transmission loss of the differential signal wiring becomes large.

[0035] On the other hand, in this embodiment, the ground wirings 102 and 103 are arranged as described above. Therefore, when the ground wirings 102 and 103 are projected onto the conductor layer 122 in the opposing direction in which the conductor layer 122 and the conductor layer 124 are opposed to each other, only the ground wiring 103 arranged at a distance is located between the pair of signal wirings 110A and 110B. In a plan view seen in the opposing direction, the multiple ground wirings 102 are arranged parallel to the pair of signal wirings 110A and 110B so as not to overlap. That is, in this embodiment, when the ground wirings 102 and 103 are projected onto the conductor layer 122, which is a signal wiring layer, the ground wirings 102 and 103 do not cross between the pair of signal wirings 110A and 110B constituting the differential signal wiring 110. Therefore, the area in which the differential signal wiring 110 faces the ground wirings 102 and 103 is reduced. As a result, in this embodiment, it is possible to reduce or prevent impedance mismatch in the differential signal wiring 110 while reducing the transmission loss of the differential signal wiring 110. The arrangement of the ground wirings 102 and 103 does not impair the flexibility of the flexible printed wiring board 100.

[0036] Thus, according to the present embodiment, it is possible to realize excellent high-speed transmission characteristics for the flexible printed wiring board 100 while ensuring the flexibility of the flexible printed wiring board 100. According to the present embodiment, it is possible to ensure signal quality even in high-speed transmission on the order of Gbps.

[0037] In order to reduce transmission loss while reducing or preventing impedance mismatch, it is preferable that the ground wirings 102 and 103 have the following relationship. Here, as shown in FIG. 3A, the width of the ground wiring 102 is W1, and the interval between the two adjacent ground wirings 102 is D1. The width of the ground wiring 103 is W2, and the interval between the two adjacent ground wirings 103 is D2. The width of the signal wirings 110A and 110B is W3, and the interval between the pair of signal wirings 110A and 110B is D3. The interval between the two adjacent differential signal wirings 110 is D4. The interval between the ground wiring 102 and the signal wiring 110A or 110B, which are adjacent to each other, is D5. The pitch of the ground wiring 103 is P. The widths W1 to W3, the intervals D1 to D5, and the pitch P are measured in a plan view in the direction in which the conductor layer 122 and the conductor layer 124 face each other.

[0038] First, it is preferable that the distance D2 between two adjacent ground wirings 103 is 40 times or more the width W2 of the ground wirings 103. By arranging the adjacent ground wirings 103 with such a distance D2 therebetween, the area where the differential signal wiring 110 faces the ground wirings 103 can be made smaller.

[0039] However, the ground wirings 103 are preferably arranged at a pitch P of 10 mm or less. By arranging the ground wirings 103 at such a pitch P, it is possible to reduce the potential difference between the two ground wirings 102 connected by the ground wiring 103 or prevent the occurrence of a potential difference.

[0040] Furthermore, it is preferable that the distance D4 between two adjacent differential signal wirings 110 is wider than the distance D3 between the pair of signal wirings 110A and 110B. By arranging the two adjacent differential signal wirings 110 with such a distance D4 between them, it is possible to more reliably electrically isolate the two adjacent differential signal wirings 110.

[0041] Moreover, it is preferable that the distance D5 between the ground wiring 102 and the signal wiring 110A or the signal wiring 110B adjacent to each other is wider than the distance D3 between the pair of signal wirings 110A, 110B. By arranging the signal wirings 110A, 110B in this manner, it is possible to improve the transmission characteristics of the differential signal wiring 110 formed by the signal wirings 110A, 110B.

[0042] Moreover, it is preferable that the width W1 of the ground wiring 102 is wider than the width W3 of the signal wirings 110A and 110B. When the ground wiring 102 has such a width W1, the ground wiring 102 can function sufficiently as a ground.

[0043] In this embodiment, the ground wiring 102 and the ground wiring 103 are perpendicular to each other, but the manner in which the ground wiring 102 and the ground wiring 103 intersect is not limited to this. Figures 4A, 4B, and 4C are plan views showing the ground wirings 102 and 103 according to modified examples of this embodiment.

[0044] As shown in Fig. 4A, the angle α at which the ground wiring 102 and the ground wiring 103 intersect may be greater than 0 degrees and less than 90 degrees. In this case, each ground wiring 103 intersecting at the angle α is formed in a straight line across the multiple ground wirings 102 to connect the ground wirings 102. Fig. 4A illustrates a case where the angle α is 45 degrees.

[0045] Also, as shown in FIG. 4B and FIG. 4C, the ground wiring 103 may cross one of two adjacent ground wirings 102 from one side at an angle α, and the ground wiring 103 may cross the other of the two adjacent ground wirings 102 from the opposite side at an angle β. In this case, the angles α and β are greater than 0 degrees and less than 90 degrees, and may be the same or different from each other. FIG. 4B and FIG. 4C illustrate a case where the angles α and β are both 45 degrees. FIG. 4B illustrates a case where a plurality of ground wirings 103 connecting two adjacent ground wirings 102 cross the ground wiring 102 at the same angle α or angle β. FIG. 4C illustrates a case where a plurality of ground wirings 103 connecting two adjacent ground wirings 102 cross the ground wiring 102 alternately at angles α and β.

[0046] In addition, in the present embodiment, the flexible printed wiring board 100 having two layers, the conductor layers 122 and 124, has been described as an example, but the layer configuration of the flexible printed wiring board 100 is not limited to this. For example, the flexible printed wiring board may have a layer configuration in which a ground layer is formed by printing a conductive material on a flexible printed wiring board having one signal wiring layer.

[0047] Next, for the flexible printed wiring board 100 according to this embodiment, examples 1 and 2 and a comparative example are used to show impedance characteristics and loss characteristics by showing specific values ​​of layer thickness, wiring width, etc.

[0048] Example 1 In Example 1, the cover layer 121 had a thickness of 30.5 μm. The conductor layer 122 had a thickness of 20 μm. The base layer 123 had a thickness of 12.5 μm. The conductor layer 124 had a thickness of 20 μm. The cover layer 125 had a thickness of 30.5 μm. The cover layers 121 and 125 had a relative dielectric constant of 3.3 and a dielectric loss tangent of 0.015. The base layer 123 had a relative dielectric constant of 3.6 and a dielectric loss tangent of 0.006.

[0049] Moreover, the width W3 of the signal wirings 110A and 110B was 60 μm. The distance D3 between the signal wirings 110A and 110B was 50 μm. The width W1 of the ground wiring 102 was 120 μm. The width W2 of the ground wiring 103 was 250 μm. The distance D5 between the signal wiring 110A and the ground wiring 102 on the left side of the signal wiring 110A was 120 μm. The distance D5 between the signal wiring 110B and the ground wiring 102 on the right side of the signal wiring 110B was also 120 μm. The distance D1 between the ground wirings 103 was 410 μm. The pitch P of the arrangement of the ground wiring 103 in the longitudinal direction of the flexible printed wiring board 100 was 10 mm. The wiring length of the flexible printed wiring board 100 was 50 mm. In the above numerical example, the differential impedance of the cross section along the A1-A2 line was 99.4 Ω.

[0050] Example 2 The following describes the modifications from Example 1. In Example 2, the width W2 of the ground wiring 103 was 120 μm. The pitch P of the arrangement of the ground wiring 103 in the longitudinal direction of the flexible printed wiring board 100 was 5 mm. Other points were the same as in Example 1.

[0051] (Transmission characteristics comparison) As a comparative example, a structure in which the ground wiring 103 was removed from Example 1 was used, and the transmission characteristics (TDR characteristics, signal transmission characteristics) were compared with Examples 1 and 2. Specifically, simulations were performed on the time domain reflectometry (TDR) characteristics and signal transmission characteristics as the transmission characteristics, and the results were compared. The simulations were performed using HyperLynx by Siemens and HSPICE by Synopsys.

[0052] Fig. 5A is a graph showing the simulation results of the TDR characteristics. In Fig. 5A, the vertical axis is differential impedance [Ω] and the horizontal axis is time [ns]. The long dashed line shows the TDR characteristics of Example 1, the solid line shows the TDR characteristics of Example 2, and the short dashed line shows the TDR characteristics of the comparative example.

[0053] In the comparative example, there is no ground wiring 103, and the cross section along the A1-A2 line shown in FIG. 3B continues. Therefore, the comparative example shows a uniform TDR characteristic. In the comparative example, the differential impedance is determined by the differential signal wiring 110 and the ground wiring 102 shown in the cross section along the A1-A2 line.

[0054] On the other hand, in Examples 1 and 2, due to the presence of the ground wiring 103, the differential impedance decreased at the portion where the differential signal wiring 110 crossed the ground wiring 103. However, compared to the comparative example, the amount of variation in Example 1 was 10% or less, and the amount of variation in Example 2 was 5% or less. Note that if the width W2 of the ground wiring 103 is narrower than in Examples 1 and 2, the distance over which the differential signal wiring 110 crosses the ground wiring 103 is shortened, and the decrease in differential impedance is further suppressed.

[0055] Fig. 5B is a graph showing the simulation results of the signal transmission characteristics. In Fig. 5B, the vertical axis represents the signal transmission amount [dB] and the horizontal axis represents the frequency [GHz]. The long dashed line represents the signal transmission characteristics of Example 1, the solid line represents the signal transmission characteristics of Example 2, and the short dashed line represents the signal transmission characteristics of the comparative example.

[0056] In the comparative example, resonance occurred in the signal transmission characteristics approximately every 2.2 GHz, and frequencies at which the signal transmission characteristics significantly decreased occurred at frequencies that were integer multiples of 2.2 GHz. As a result, in the comparative example, transmission loss increased at a transmission speed that matched the resonance frequency, for example, 4.4 Gbps (resonance frequency 2.2 GHz), and signal quality deteriorated.

[0057] On the other hand, in Example 1, the initial resonance frequency was about 11 GHz. Therefore, in Example 1, no significant transmission loss occurred up to about 20 Gbps (fundamental frequency 10 GHz). In Example 2, the initial resonance frequency was about 22 GHz. Therefore, in Example 2, no significant transmission loss occurred up to about 40 Gbps (fundamental frequency 20 GHz). Note that by further narrowing the pitch P of the ground wiring 103, it is possible to shift the resonance frequency further to the high frequency side.

[0058] In the comparative example having only thin and independent ground wiring 102 like the cross section along line A1-A2, the impedance of the ground wiring 102 becomes high, and therefore resonance occurs at a lower frequency than in Examples 1 and 2. On the other hand, in Examples 1 and 2 in which the ground wiring 103 is formed in addition to the ground wiring 102, the areas of the ground wirings 102 and 103 become large by connecting the ground wirings 102 to each other via the ground wiring 103. Therefore, in Examples 1 and 2, the impedance of the ground wirings 102 and 103 becomes low and the resonance frequency becomes high.

[0059] [Second embodiment] A flexible printed wiring board according to a second embodiment of the present invention will be described with reference to Figures 6A to 7C. Note that components similar to those in the first embodiment are given the same reference numerals and descriptions thereof will be omitted or simplified.

[0060] Fig. 6A is a plan view of the flexible printed wiring board 100 according to this embodiment as viewed from above. Fig. 6B is a cross-sectional view showing a cross section of the flexible printed wiring board 100 taken along line A1-A2 in Fig. 6A. Fig. 6C is a cross-sectional view showing a cross section taken along line B1-B2 in Fig. 6A. In this embodiment, the layer configuration and the structure of the signal wiring are the same as in the first embodiment, so differences from the first embodiment will be described.

[0061] FIG. 6A shows the arrangement of the differential signal wiring 110 and the ground wiring 102, 103 in a plan view of the flexible printed wiring board 100 seen from the cover layer 121 side, i.e., in a plan view seen in the direction in which the conductor layer 122 and the conductor layer 124 face each other.

[0062] 6A, similarly to the first embodiment, the multiple differential signal wirings 110 and the multiple ground wirings 102 are wired so as to be parallel to each other along the longitudinal direction of the flexible printed wiring board 100. Also in this embodiment, a pair of signal wirings 110A and 110B is disposed so as to be located between two adjacent ground wirings 102, and therefore does not overlap with the ground wirings 102 projected onto the conductor layer 122.

[0063] Two adjacent ground wirings 102 are connected to each other by a plurality of ground wirings 103. Two adjacent ground wirings 102 are connected to each other by a plurality of ground wirings 103 that are respectively arranged along a direction intersecting the ground wirings 102, for example, a direction perpendicular to the ground wirings 102.

[0064] In this embodiment, the positions of the ground wiring 103 connecting the ground wiring 102 are different for each differential signal wiring 110 to which the ground wiring 103 faces in the opposing direction in which the conductor layer 122 and the conductor layer 124 face each other. That is, unlike the ground wiring 103 arranged in a straight line in the first embodiment, the ground wiring 103 is arranged so as to be shifted from each other by, for example, a half pitch (1 / 2P) for each differential signal wiring 110 in the longitudinal direction of the flexible printed wiring board 100.

[0065] For example, as shown in FIG. 6A, two adjacent ground wirings 102 on the left side of the drawing are connected by ground wirings 103a, 103c, and 103e as the ground wiring 103. Also, two adjacent ground wirings 102 in the center of the drawing are connected by ground wirings 103b and 103d as the ground wiring 103. The ground wirings 103a, 103c, and 103e and the ground wirings 103b and 103d are arranged to be shifted by half a pitch from each other in the longitudinal direction of the flexible printed wiring board 100. Note that the amount of shift of the ground wirings 103 for each differential signal wiring 110 is not limited to half a pitch, and can be set to an appropriate value.

[0066] As in the first embodiment, the portion where the ground wiring 103 is not formed is a cross section taken along line A1-A2 as shown in Fig. 6B, and the ground wirings 102 and 103 are not arranged at positions facing the signal wirings 110A and 110B of the differential signal wiring 110. On the other hand, in this embodiment, as shown in Fig. 6C, the cross section taken along line B1-B2 includes a portion where the ground wiring 103 is arranged facing the signal wirings 110A and 110B of the differential signal wiring 110, and a portion where the ground wiring 103 is not arranged.

[0067] As in this embodiment, the multiple ground wirings 103 do not necessarily need to be formed in a straight line with the multiple differential signal wirings 110, but may be arranged with a shift for each differential signal wiring 110.

[0068] In the above, the ground wiring 102 and the ground wiring 103 are perpendicular to each other, but the manner in which the ground wiring 102 and the ground wiring 103 intersect is not limited to this. Figures 7A, 7B, and 7C are plan views showing ground wirings 102 and 103 according to modified examples of this embodiment.

[0069] As shown in Fig. 7A, the angle α at which the ground wiring 102 and the ground wiring 103 intersect may be greater than 0 degrees and less than 90 degrees. In this case, the ground wirings 103 intersecting at the angle α are arranged to be shifted for each differential signal wiring 110. Fig. 7A illustrates a case where the angle α is 45 degrees.

[0070] Also, as shown in FIG. 7B and FIG. 7C, the ground wiring 103 may cross one of two adjacent ground wirings 102 from one side at an angle α, and the ground wiring 103 may cross the other of the two adjacent ground wirings 102 from the opposite side at an angle β. In this case, the angles α and β are greater than 0 degrees and less than 90 degrees, and may be the same or different from each other. FIG. 7A and FIG. 7C illustrate a case where the angles α and β are both 45 degrees. FIG. 7B illustrates a case where a plurality of ground wirings 103 connecting two adjacent ground wirings 102 cross the ground wiring 102 at the same angle α or angle β. FIG. 7C illustrates a case where a plurality of ground wirings 103 connecting two adjacent ground wirings 102 cross the ground wiring 102 alternately at angles α and β.

[0071] Although a two-layer flexible printed wiring board has been shown as an embodiment, a configuration in which a conductive material is printed on the ground layer of a flexible printed wiring board with one signal wiring layer may also be used. Also, in this embodiment, the flexible printed wiring board 100 with two layers of conductor layers 122 and 124 has been described as an example, but the layer configuration of the flexible printed wiring board 100 is not limited to this. For example, a layer configuration in which a conductive material is printed on a flexible printed wiring board with one signal wiring layer to form a ground layer may also be used.

[0072] Next, the flexible printed wiring board 100 according to this embodiment will be described using Example 3.

[0073] Example 3 Example 3 was similar to Example 1, except that the ground wiring 103 was arranged so as to be shifted for each differential signal wiring 110. In Example 3, the pitch P of the arrangement of the ground wirings 103a, 103c, and 103e in the longitudinal direction of the flexible printed wiring board 100 was 10 mm. The pitch P of the arrangement of the ground wirings 103b and 103d in the longitudinal direction of the flexible printed wiring board 100 was also 10 mm. On the other hand, the pitch of the arrangement of the ground wirings 103a and 103b in the longitudinal direction of the flexible printed wiring board 100 was 5 mm, which was half the pitch P.

[0074] The ground wiring 103 in Example 3 is arranged so as to be shifted by half a pitch for each differential signal wiring 110 with respect to the ground wiring 103 in Example 1. Therefore, in Example 3, there are places where the impedance of the ground wiring 102 drops every 5 mm. However, in Example 3, since the ground wiring 103 is not arranged in a straight line as in Example 1, the resonance frequency is not twice that of Example 1, but the initial resonance frequency is 11.3 GHz, which is higher than that of Example 1.

[0075] The present invention is not limited to the above-described embodiments, and many modifications are possible within the technical concept of the present invention. Furthermore, the effects described in the embodiments are merely a list of the most preferable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiments.

[0076] In the above first and second embodiments, the wiring board of the present invention is applied to an imaging device such as a digital camera as an example of an electronic device, but the present invention is not limited thereto. For example, the wiring board of the present invention can be applied to electronic devices capable of mounting an electronic unit, such as mobile communication devices, wearable devices, and image forming devices. Examples of mobile communication devices include smartphones, tablet PCs, and game consoles. Examples of image forming devices include printers, copiers, facsimiles, and multifunction devices having these functions. In addition, when a flexible printed wiring board is used as the wiring board, its flexibility can be utilized to suitably use it in a camera shake correction mechanism.

[0077] The disclosure of this embodiment includes the following configuration. (Configuration 1) A first conductor layer; a second conductor layer formed so as to face the first conductor layer; A plurality of differential signal wirings formed on the first conductor layer, each including a pair of signal wirings; a plurality of first ground wirings formed in the second conductor layer and arranged along the pair of signal wirings so as not to overlap each other in a plan view seen in a direction in which the first conductor layer and the second conductor layer face each other; a plurality of second ground wirings that are formed in the second conductor layer at intervals so as to intersect with the first ground wirings and connect two adjacent first ground wirings; A wiring board comprising: (Configuration 2) 2. The wiring board according to claim 1, wherein the pair of signal wirings is disposed between two of the first ground wirings that are adjacent to each other in the plan view. (Configuration 3) 3. The wiring board according to configuration 1 or 2, wherein the interval is 40 times or more the width of the second ground wiring. (Configuration 4) 4. The wiring board according to claim 1, wherein the second ground wires are arranged at a pitch of 10 mm or less. (Configuration 5) 5. The wiring board according to claim 1, wherein the second ground wirings are arranged so as to be shifted by half a pitch for each of the differential signal wirings. (Configuration 6) 6. The wiring board according to any one of configurations 1 to 5, wherein a distance between the adjacent differential signal lines is wider than a distance between the pair of signal lines. (Configuration 7) 7. The wiring board according to any one of configurations 1 to 6, wherein a distance between the signal wiring and the first ground wiring is wider than a distance between the pair of signal wirings. (Configuration 8) 8. The wiring board according to claim 1, wherein the width of the first ground wiring is wider than the width of the signal wiring. (Configuration 9) a substrate having a first surface and a second surface opposite the first surface; the first conductor layer is formed on the first surface; 9. The wiring board according to any one of configurations 1 to 8, wherein the second conductor layer is formed on the second surface. (Configuration 10) 10. The wiring board according to claim 9, wherein the base material is a resin. (Configuration 11) The wiring board according to any one of configurations 1 to 10, and a housing in which the wiring board is housed. (Configuration 12) the housing further accommodates a wireless communication unit capable of wirelessly communicating with an external device; The electronic device according to configuration 11, wherein within the housing, the second conductor layer of the wiring board is positioned closer to the wireless communication unit than the first conductor layer of the wiring board. [Explanation of symbols]

[0078] 100: Flexible printed wiring board 110: Differential signal wiring 110A, 110B: Signal wiring 102, 103: Ground wiring 121, 125: Cover layer 123: base layer, 122: Conductor layer (signal wiring layer) 124: Conductor layer (ground layer) 200: Imaging unit 600: Digital camera 611: Cabinet

Claims

1. A wiring board which is a flexible printed circuit board, comprising: a first conductor layer; a second conductor layer formed to face the first conductor layer; a plurality of differential signal wirings formed on the first conductor layer, each including a pair of signal wirings; a first ground wiring formed on the second conductor layer; a second ground wiring formed on the second conductor layer; and the plurality of differential signal wirings include a first differential signal wiring and a second differential signal wiring that extend in a first direction and are arranged side by side in a second direction intersecting the first direction; the first ground wiring includes a first wiring, a second wiring, and a third wiring that extend in the first direction and are arranged side by side in the second direction; the second ground wiring includes a fourth wiring and a fifth wiring that face the first differential signal wiring in a third direction and are spaced apart from each other in the first direction in a plan view seen in the third direction, and a sixth wiring and a seventh wiring that face the second differential signal wiring in the third direction and are spaced apart from each other in the first direction in a plan view seen in the third direction; the first differential signal wiring is arranged between the first wiring and the second wiring in a plan view seen in the third direction, the second differential signal wiring is arranged between the second wiring and the third wiring in a plan view seen in the third direction, and the second wiring is arranged between the first differential signal wiring and the second differential signal wiring in a plan view seen in the third direction; the wiring board, wherein the fourth wiring and the fifth wiring, and the sixth wiring and the seventh wiring are electrically connected to each other via the second wiring.

2. The wiring board according to claim 1, wherein a distance between the first differential signal wiring and the second wiring is wider than a distance between the pair of signal wirings of the first differential signal wiring, and a distance between the second differential signal wiring and the second wiring is wider than a distance between the pair of signal wirings of the second differential signal wiring.

3. The wiring board according to claim 1 or 2, wherein a distance between the fourth wiring and the fifth wiring in the first direction is 40 times or more the width of the fourth wiring.

4. The interval between the fourth wiring and the fifth wiring in the first direction is 10 mm or less, and the interval between the sixth wiring and the seventh wiring in the first direction is 10 mm or less. The wiring board according to claim 1 or 2, characterized in that.

5. The fourth wiring is located on a straight line along the second direction, the second ground wiring is not located at a position overlapping the second differential signal wiring on the straight line, and the straight line is located between the sixth wiring and the seventh wiring. The wiring board according to claim 1 or 2, characterized in that.

6. The interval between the first differential signal wiring and the second differential signal wiring is wider than the interval between the pair of signal wirings of the first differential signal wiring and the interval between the pair of signal wirings of the second differential signal wiring. The wiring board according to claim 1 or 2, characterized in that.

7. The interval between the first wiring and the second wiring is wider than the interval between the first differential signal wiring and the second differential signal wiring. The wiring board according to claim 1 or 2, characterized in that.

8. The width of the first wiring is wider than the width of one of the pair of signal wirings of the first differential signal wiring. The wiring board according to claim 1 or 2, characterized in that.

9. It has a base material having a first surface and a second surface facing the first surface. The first conductor layer is formed on the first surface. The second conductor layer is formed on the second surface. The wiring board according to claim 1 or 2, characterized in that.

10. The thickness of the base material is 10 μm or more and 100 μm or less. The wiring board according to claim 9, characterized in that.

11. An electronic device having the wiring board according to claim 1 or 2, and a housing in which the wiring board is housed. An electronic device having a housing in which the wiring board is housed.

12. The housing further houses a wireless communication unit capable of wireless communication with an external device. In the housing, the second conductor layer of the wiring board is arranged closer to the wireless communication unit than the first conductor layer of the wiring board. The electronic device according to claim 11.

13. A unit including a first circuit board including a first semiconductor device, a second circuit board including a second semiconductor device, and the wiring board according to claim 1 or 2 for electrically connecting the first circuit board and the second circuit board. The unit according to claim 13, wherein the first semiconductor device is an image sensor and the second semiconductor device is a digital signal processor. The unit according to claim 13, and a housing in which the unit is housed, and a wireless communication unit housed in the housing, an electronic device comprising the same. The unit according to claim 14, and a housing in which the unit is housed, and is provided with an electronic device in which a lens unit is detachable.

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