Flexible printed circuit board

Abstract

In flexible printed circuit boards, when transmitting high-frequency signals in the millimeter-wave band or higher, this reduces phase fluctuations, group delay variations, and electromagnetic interference (EMI) caused by the spread of electromagnetic fields around the wiring and instability of the return current path. [Solution] A groove-shaped grounding conductor is electrically connected continuously to the pair of grounding conductor layers, and the signal line is substantially surrounded in a closed loop by the upper, lower, and side grounding conductors, thereby forming electromagnetically continuous boundary conditions around the signal line. This suppresses electromagnetic field leakage and intrusion of external electromagnetic fields, and stabilizes the return current path. As a result, phase fluctuations and group delay fluctuations are reduced, and stable signal transmission and high EMI immunity are obtained even for high-frequency signals in the millimeter wave band and above.

Description

TECHNICAL FIELD

[0001] The present invention relates to a flexible printed circuit (FPC). In particular, the present invention relates to a wiring structure of a flexible printed circuit capable of stably transmitting high-frequency signals in the millimeter-wave band and above, and further relates to a semiconductor package mounting structure that applies the flexible printed circuit to an optical transceiver or a multi-die package. BACKGROUND ART

[0002] In recent years, demand for technologies handling high-frequency signals from several tens of GHz to over 100 GHz has been increasing in data centers, communication infrastructure, in-vehicle radars, optical communication devices, and the like.

[0003] In wiring that transmits such high-frequency signals, not only reduction of transmission loss but also suppression of phase fluctuation, group delay variation, electromagnetic interference (EMI) and unnecessary radiation are important issues.

[0004] Flexible printed circuits are widely used for high-frequency signal wiring applications due to their light weight, flexibility and high degree of mounting freedom.

[0005] However, in high frequency bands, electromagnetic fields around the wiring easily spread and the return current path becomes unstable, so there is a problem that signal quality degradation and electromagnetic radiation to the outside are likely to occur.

[0006] In particular, in the frequency region of the millimeter-wave band and above, slight electromagnetic discontinuity caused by the wiring structure becomes apparent as phase fluctuation and group delay variation, and greatly affects system performance.

[0007] To address such problems, the prior art has proposed a so-called via fence structure, in which grounding wiring for guard is arranged on the sides of a signal line, and via holes are periodically formed to connect the upper and lower ground layers.

[0008] However, in via fence structures, grounding is discrete along the wiring direction, making it difficult to form electromagnetically continuous boundary conditions in the high-frequency band.

[0009] As a result, as the frequency increased, the return current path would diffuse or meander, leading to problems such as insufficient suppression of phase fluctuations and electromagnetic field leakage. [Prior art documents] [Patent Documents]

[0010] Patent Document 1: JP2012069813A (Printed Wiring Board) Patent Document 2: WO2006065539A2 (Multi-layer printed circuit board comprising a through connection for high frequency applications) [Overview of the Initiative]

[0011] The present invention was made to solve the above-mentioned problems, and aims to provide a flexible printed circuit board that can reduce phase fluctuations, group delay fluctuations, and electromagnetic interference in the transmission of high-frequency signals by stably confining the electromagnetic field and structurally defining the return current path.

[0012] The flexible printed circuit board according to the present invention is characterized in that a groove-shaped grounding conductor is formed on the side of the signal lines, extending along the wiring direction, and the groove-shaped grounding conductor is continuously connected to the upper and lower grounding conductor layers.

[0013] The groove-shaped grounding conductor forms a continuous conductor path in a planar manner along the wiring direction, and is configured such that the current path along the wiring direction is substantially uninterrupted, including the connection with the upper and lower grounding conductor layers.

[0014] As a result, the signal line is effectively surrounded in a closed loop by the grounding conductors above, below, and to the sides, and electromagnetically continuous boundary conditions are formed around the signal line.

[0015] As a result, signal quality stabilization in the high-frequency band, which could not be achieved with conventional periodic and discrete grounding structures, is realized. The present invention includes applications to signal wiring that transmits high-frequency signals in the millimeter-wave band or higher, and applications to power wiring that transmit power supply current or its high-frequency components, but these applications can be adopted independently. [Problems that the invention aims to solve]

[0016] The problem that this invention aims to solve is to effectively suppress phase fluctuations, group delay fluctuations, and electromagnetic interference that occur during the transmission of high-frequency signals in a flexible printed circuit board.

[0017] In particular, when handling high-frequency signals in the millimeter-wave band or higher, a challenge is to suppress the spread of electromagnetic fields around the wiring and the instability of the return current path.

[0018] Conventional via fence structures have a problem in that the grounding structure is prone to electromagnetic discontinuities, and the degradation of signal quality becomes significant as the frequency increases.

[0019] The problem addressed by this invention is not merely the reduction of electromagnetic interference and crosstalk, but rather the suppression of phase fluctuations and group delay variations that occur as a result of fluctuations in the return current path caused by electromagnetic discontinuities in the grounding structure formed along the wiring direction, through the wiring structure itself. [Means for solving the problem]

[0020] The flexible printed circuit board according to the present invention comprises signal lines formed within an insulating layer, a pair of grounding conductor layers arranged above and below the signal lines, and groove-shaped grounding conductors formed on the sides of the signal lines.

[0021] The grooved grounding conductor is electrically connected in a continuous manner to the pair of grounding conductor layers, thereby surrounding the signal line with a substantially closed-loop grounding structure.

[0022] With this configuration, the electromagnetic field around the signal line is stably confined, and the return current path is induced to the shortest and stable path.

[0023] The present invention is characterized in that the groove-shaped ground conductors are formed substantially continuously along the wiring direction, and are continuously electrically connected to the upper and lower ground conductor layers, thereby making the electromagnetic boundary conditions formed around the signal line continuous along the wiring direction. Thereby, the return current path corresponding to the signal line is structurally defined, and stable high-frequency transmission characteristics that are less dependent on frequency conditions and manufacturing variations can be obtained. Effects of the Invention

[0024] According to the present invention, by surrounding a signal line with a substantially closed-loop ground structure, phase fluctuation and group delay variation of high-frequency signals can be reduced compared to conventional structures.

[0025] Furthermore, since the return current path is structurally defined, stable high-frequency characteristics can be obtained even against design variations and manufacturing variations.

[0026] Furthermore, since the electromagnetic field around the signal line is stably confined, electromagnetic coupling with adjacent wiring and electromagnetic radiation to the outside are suppressed, and EMI resistance is improved.

[0027] These effects are particularly remarkable when handling high-frequency signals in the millimeter wave band or higher, and enable stable signal transmission in high-frequency bands, which has been difficult with conventional techniques.

[0028] It should be noted that the above operational effects can be obtained both when the present configuration is applied as a high-frequency signal line and when the present configuration is applied as a power supply line, and these can be grasped as independent application modes respectively.

[0029] According to the present invention, the electromagnetic field distribution around the signal line and the return current path are stabilized along the wiring direction, thereby reducing phase fluctuations and group delay variations during high-frequency signal transmission. These effects stem from the structural characteristic of continuous electromagnetic boundary conditions and manifest independently of frequency band or signal application.

[0030] In particular, in so-called multi-row differential wiring configurations, where multiple pairs of wires transmitting differential signals are arranged in parallel, the present invention's configuration, which forms a substantially closed-loop grounding structure with a groove-shaped grounding conductor and grounding conductor layer corresponding to each differential signal line pair, independently defines the return current path and electromagnetic field distribution for each pair. As a result, even when a large number of differential signal lines are arranged in close proximity, crosstalk between adjacent pairs, common-mode conversion, and phase fluctuations between channels are suppressed, and stable impedance characteristics and timing matching can be maintained even in the simultaneous transmission of multi-row differential signals.

[0031] Furthermore, since multiple differential signal lines arranged in multiple rows can be integrally formed on a single flexible wiring body, the number of alignment points during mounting is significantly reduced compared to conventional configurations where multiple coaxial cables are individually wired for each channel and each is soldered and connected with connectors. As a result, soldering alignment between modules and connection with flexible wiring body connectors becomes easier, contributing to reduced assembly man-hours and improved connection reliability, while also suppressing the volume and weight of the cable bundle. [Brief explanation of the drawing]

[0032] [Figure 1] This figure shows an overall image of an embodiment (reference numeral 101) of the flexible printed circuit board according to the present invention and a conventional structure (reference numeral 102) as a comparative example. (A) and (B) are top views, and (Aa) and (Bb) are cross-sectional views along the dashed line positions shown in the corresponding top views.

[0033] [Figure 2]This figure shows an example of the cross-sectional structure of a flexible printed circuit board according to the present invention, illustrating the positional relationship between the grooved grounding conductor, internal wiring, and grounding conductor layer. (A), (B), (C), and (D) show cross-sectional views of the wiring structure, while (Aa), (Ba), (Ca), and (Da) show the cross-sectional structure of the electrode portion in the corresponding wiring cross-section.

[0034] [Figure 3] This figure shows the manufacturing process of a flexible printed circuit board according to one embodiment of the present invention. (A), (B), (C), (D), and (E) show the cross-sectional structure during the manufacturing process, and (Aa), (Ba), (Da), and (Ea) show the top view at the corresponding process stage.

[0035] [Figure 4] This figure shows the dependence of equivalent phase fluctuations on structural parameters in the structure of the present invention and the comparative example structure.

[0036] [Figure 5] This figure shows the frequency dependence of equivalent phase fluctuations in the structure of the present invention and the comparative example structure.

[0037] [Figure 6] This is a schematic diagram showing the positional relationship between the grooved grounding conductor and the internal electrode in the present invention.

[0038] [Figure 7] This figure shows an embodiment of the electrode structure of a flexible printed circuit board according to the present invention.

[0039] [Figure 8] This figure shows the configuration of an electronic device using the flexible printed circuit board of the present invention.

[0040] [Figure 9] This figure shows the conceptual differences between the electromagnetic boundary and return current path in the structure of the present invention (reference numeral 101) and the comparative example structure (reference numeral 102).

[0041] [Figure 10] This table shows the results of determining whether or not LO (Low-Level) is achieved for the frequency conditions in the structure of the present invention and the comparative example structure.

[0042] [Figure 11] This table shows the correspondence between the parameters and symbols used in the evaluation according to the present invention.

[0043] These evaluation results are an example demonstrating that the phase behavior in the high-frequency band differs qualitatively between cases where electromagnetic boundary conditions and return current paths around signal lines are structurally defined by a continuous grooved grounding conductor and cases with discrete grounding structures. [Modes for carrying out the invention]

[0044] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

[0045] In each figure, identical or corresponding components are denoted by the same reference numeral and described accordingly.

[0046] Furthermore, the drawings are schematic diagrams intended to facilitate understanding of the present invention and do not accurately represent the actual dimensional ratios or shapes.

[0047] The following embodiments are illustrative to facilitate understanding of the present invention, and the present invention is not limited thereto.

[0048] In this specification, "signal line" refers to wiring (reference numeral 6) formed within the insulating layer, and "signal electrode" refers to an electrode (reference numeral 1) provided at an external connection or electrode section. Whenever "wiring" is used in this text, it refers to a conductor pattern including the signal line (reference numeral 6), unless otherwise specified. <Basic structure>

[0049] Figure 1 is an overall diagram showing one embodiment (reference numeral 101) of the flexible printed circuit board according to the present invention and a conventional structure (reference numeral 102) as a comparative example. The flexible printed circuit board of this embodiment has signal lines 6 for transmitting high-frequency signals in its inner layer, and ground conductor layers 5 and 5a are formed above and below the signal lines 6, respectively.

[0050] A groove-shaped grounding conductor 8 is formed on the side of the signal line 6, extending along the wiring direction of the signal line 6. The groove-shaped grounding conductor 8 is formed by removing the insulator and applying a conductive treatment and electroplating to the side surface of the groove, and is electrically connected to the upper and lower grounding conductor layers 5 and 5a.

[0051] As a result, the signal line 6 is substantially surrounded in a closed loop by the upper and lower grounding conductor layers 5, 5a and the lateral groove-shaped grounding conductor 8. This structure creates continuous electromagnetic boundary conditions around the signal line 6. <Function of a continuous grooved grounding conductor>

[0052] The groove-shaped grounding conductor 8 is formed substantially continuously along the wiring direction, ensuring equipotentiality along the wiring direction. Therefore, during high-frequency signal transmission, the electromagnetic field generated from the signal line 6 is enclosed by the upper, lower, and lateral grounding conductors, suppressing electromagnetic field leakage from the sides and ends.

[0053] Furthermore, the return current corresponding to the high-frequency current flowing through the signal line 6 flows along the shortest path along the groove-shaped grounding conductor 8 and the upper and lower grounding conductor layers 5 and 5a, thereby suppressing instability in the return current path and an increase in loop area.

[0054] Unlike conventional structures that connect grounding conductors periodically and discretely using via fences, etc. (see reference numeral 102), this structure is characterized by forming electromagnetically continuous boundary conditions.

[0055] By forming these groove-shaped grounding conductors continuously along the wiring direction, abrupt changes in electromagnetic boundary conditions along the signal line are suppressed, and the return current forms a stable closed-loop path along the wiring direction. As a result, unlike simply optimizing dimensions to improve high-frequency characteristics, the wiring structure itself restricts the degrees of freedom of the electromagnetic field.

[0056] As shown in Figure 9, in the present invention structure (reference numeral 101), a continuous groove-shaped grounding conductor 8 formed on the side of the signal line 6 creates a continuous electromagnetic boundary around the signal line 6, forming the shortest and closed-loop path for the return current corresponding to the high-frequency signal.

[0057] In contrast, in the comparative example structure (reference numeral 102) shown in Figure 9, the grounding structure is formed discretely along the wiring direction, resulting in discontinuous electromagnetic boundaries and a tendency for the return current path to diffuse or meander. <Application to differential signal lines>

[0058] In the embodiment shown in Figure 2, the signal line 6 may consist of a pair of wires that transmit differential signals. In this case, each differential signal line is surrounded by a corresponding groove-shaped grounding conductor 8 and grounding conductor layers 5, 5a.

[0059] This suppresses the conversion from differential signals to common-mode signals, which is problematic in differential signals, and reduces phase fluctuations and group delay variations in the high-frequency band. <Allowance for discontinuities in grooved grounding conductors>

[0060] The grooved grounding conductor 8 is preferably formed substantially continuously along the wiring direction, but may include short discontinuous sections due to manufacturing constraints or other reasons.

[0061] Even in this case, if the length of the discontinuity is sufficiently short compared to the effective wavelength of the high-frequency signal within the substrate, it functions as an electromagnetically continuous ground conductor and does not hinder the effects of the present invention.

[0062] For example, if the length of the discontinuity is one-tenth or less of the effective wavelength within the substrate at the operating frequency, it is preferable in that it functions as an electromagnetically continuous grounding conductor.

[0063] Furthermore, it is even more preferable when the length of the discontinuity is 1 / 20th or less of the effective wavelength, as this further suppresses the effects of the electromagnetic field discontinuity. <Distance between the side grounding conductor and the signal line>

[0064] The distance between the groove-shaped grounding conductor 8 and the signal line 6 is an important design element for achieving the effects of the present invention.

[0065] If the distance is excessively small, the electromagnetic coupling between the signal line 6 and the groove-shaped grounding conductor 8 may become excessive, potentially leading to impedance fluctuations, increased transmission loss, or degradation of signal quality.

[0066] On the other hand, if the distance is excessively large, the electromagnetic field formed around the signal line 6 cannot be sufficiently confined, and the effectiveness of electromagnetic boundary formation by the groove-shaped grounding conductor 8 tends to decrease.

[0067] Therefore, it is preferable to set the distance between the groove-shaped grounding conductor 8 and the signal line 6 within a range that balances the electromagnetic field confinement effect and signal characteristics.

[0068] For example, in applications transmitting high-frequency signals in the millimeter-wave band, setting the distance to several hundred micrometers or more is preferable because it allows for a stable confinement effect of the electromagnetic field.

[0069] Furthermore, setting this distance with a design margin is preferable because it allows for stable high-frequency characteristics even with manufacturing variations and fluctuations in the operating environment.

[0070] In particular, when the distance range is set to one in which the desired impedance characteristics and phase stability are both achieved, the electromagnetic field confinement effect and the return current path stabilization effect of the groove-shaped grounding conductor 8 are most pronounced, making it optimal in that the effects of the present invention are maximized.

[0071] The distances and ratios to effective wavelengths mentioned above are examples provided to facilitate understanding of the present invention, and do not limit the scope of the present invention to these numerical values. <Multiple wiring and multi-layer structure>

[0072] The present invention is not limited to a single signal line, but can also be applied to configurations in which multiple signal lines are arranged in parallel within the same layer.

[0073] In this case, each signal line is effectively surrounded in a closed loop by its corresponding groove-shaped grounding conductor 8, thereby suppressing electromagnetic coupling and crosstalk between adjacent signal lines.

[0074] Furthermore, the present invention is also applicable to multilayer flexible printed circuit boards.

[0075] For example, a configuration may be one in which a first signal line layer containing signal lines and a second signal line layer located on a different layer are stacked vertically.

[0076] In a flexible printed circuit board having a multilayer structure, the metal film thickness of the wiring layers or ground conductor layers arranged vertically can be made to differ from each other. For example, one metal film thickness can be formed relatively thickly to serve as a power supply wiring layer for transmitting power supply current or its high-frequency components, while the other can be used as a grounding conductor layer to form the reference potential of the signal line.

[0077] In this case, the signal lines arranged in each signal line layer are surrounded by their respective grounding conductor layers and grooved grounding conductors, thereby suppressing signal interference and phase fluctuations between layers. <Relationship with manufacturing method>

[0078] The flexible printed circuit board according to the present invention can form via holes and grooves for via formation by removing the insulator from a substrate having an insulator and metal foil, and then applying a conductive treatment and electroplating to the sides thereof to form via holes and groove-shaped ground conductors 8. The conductive treatment can be performed using wet or dry methods such as electroless copper plating, black hole, graphite treatment, organic conductive film, or sputtering.

[0079] The process of forming via holes and grooves may be carried out before lamination of the substrate, or after lamination of multiple layers.

[0080] In other words, the process of forming the via-forming holes and grooves may be performed before the lamination process, simultaneously with the lamination process, or after the lamination process, and the order of the manufacturing process can be changed as appropriate without departing from the gist of the present invention. When the positional accuracy of the vias and internal electrodes is to be increased and the internal electrodes are to be made smaller, it is preferable to form the via-forming openings 14 simultaneously with the wiring 6 before the lamination process.

[0081] Furthermore, to improve productivity, via-forming holes and grooves may be formed in the same process by direct laser processing. <Application example>

[0082] Furthermore, when forming the groove 4 for forming the groove-shaped grounding conductor 8 by laser processing or the like, an outer groove may be formed simultaneously along the outer shape of the flexible wiring body 24, and a metal film may be formed along the outer shape of the flexible wiring body 24 by applying a conductive treatment and electroplating to the side surface of the outer groove in the same process as the groove 4.

[0083] The metal film on the outer surface may be configured to be electrically connected to the grounding conductor layers 5, 5a and the grooved grounding conductor 8, thereby further enhancing the shielding effect at the periphery of the flexible printed circuit board 101.

[0084] The flexible printed circuit board according to the present invention is suitably used as wiring on the input and output sides of electrical signals in an optical transceiver or optical transceiver module.

[0085] Furthermore, the flexible printed circuit board according to the present invention can also be used in a multi-die package as a power supply wiring to supply power to multiple semiconductor dies by being directly connected to the upper surface of the interposer or the upper surface of the package substrate.

[0086] In this case as well, the continuous groove-shaped grounding conductor 8 stabilizes the return current path, suppressing the effects of electromagnetic noise caused by high-frequency components in the power supply current.

[0087] As a result, the flexible printed circuit board according to the present invention can be applied to both high-frequency signal wiring and power supply wiring including high-frequency components, and contributes to improving signal quality and electromagnetic reliability in various electronic devices that require high-density mounting.

[0088] The flexible wiring according to the present invention can be suitably used in applications such as transmitting multiple high-speed differential signals in multiple lines between a transmitting / receiving circuit and an optical element in an optical transceiver or optical transceiver module, and in applications such as densely integrating high-speed signal lines and power wiring between multiple semiconductor dies in a multi-die package. It can also be suitably used for multi-channel simultaneous transmission of millimeter-wave signals from multiple antenna elements in an automotive millimeter-wave radar system, and for integrating multiple video signals and multiple sensor signals between a head-mounted display and a host device in a VR / AR system.

[0089] When applied to an in-vehicle millimeter-wave radar system, multiple high-frequency signal lines corresponding to multiple transmitting and receiving antennas are arranged in multiple rows on the flexible wiring body of the present invention, and a substantially closed-loop grounding structure is formed for each channel. This makes it possible to suppress inter-channel phase fluctuations and group delay differences even in the millimeter-wave band. As a result, while suppressing a decrease in azimuth resolution and an increase in side lobes in beamforming processing, the connection between the antenna module and the radar signal processing circuit can be realized with a single flexible wiring body and a small number of connectors instead of multiple coaxial cables, thus contributing to space saving and improved ease of implementation in the in-vehicle environment.

[0090] When applied to a VR / AR system, multiple differential video signal lines supplied to the left and right display panels, and multiple differential sensor signal lines from inertial sensors and distance image sensors for head position detection can be arranged in multiple rows and layers on the flexible wiring body of the present invention. In this case, since each differential signal line pair is surrounded by a groove-shaped ground conductor and a ground conductor layer, crosstalk and group delay differences between the video signal and the sensor signal are suppressed, reducing the synchronization delay between the user's head movement and the displayed image. Furthermore, compared to a configuration in which multiple coaxial cables are routed to the head-mounted display, the wiring is lighter and easier to handle, which is advantageous in terms of wearing comfort and mechanical reliability.

[0091] The width of the groove 4 is preferably, for example, 10 micrometers or more and 1 millimeter or less, and particularly preferably 60 micrometers or more and 200 micrometers or less, but the present invention is not limited to these numerical ranges.

[0092] If the thickness is less than 10 micrometers, it may be difficult to stably form a plating film of sufficient thickness on the inner wall of groove 4. On the other hand, if it significantly exceeds 1 millimeter, it may be disadvantageous in terms of reduced mechanical strength and interference with high-density wiring.

[0093] The thickness of the wiring 6 is preferably, for example, 1 micrometer or more and 100 micrometers or less, and particularly preferably 10 micrometers or more and 75 micrometers or less, but is not limited to these values.

[0094] There is a risk of wire breakage at sizes less than 1 micrometer, and forming fine patterns can become difficult at sizes exceeding 100 micrometers.

[0095] The thickness of the groove side plated surface 19 is preferably, for example, 0.1 micrometers or more and 100 micrometers or less, and particularly preferably 5 micrometers or more and 30 micrometers or less, but the present invention is not limited to these values.

[0096] If the thickness is less than 0.1 micrometers, electrical conductivity may be impaired due to cracks, etc., and if it is too thick, it may be disadvantageous in terms of increasing the plating time.

[0097] The thickness of the insulator 7 is preferably, for example, 10 micrometers or more and 1 millimeter or less, and particularly preferably 50 micrometers or more and 500 micrometers or less, and can be appropriately changed depending on the application and required insulation performance.

[0098] A thickness of less than 10 micrometers is disadvantageous in terms of insulation reliability, while a thickness exceeding 1 millimeter may be disadvantageous in terms of workability and other factors.

[0099] The insulator 7 is not particularly limited, but suitable materials include epoxy resins, polyimide (PI), fluororesins containing PFA and PTFE, olefin resins containing PP and HDPE, styrene resins, LCP, and MPI. Alternatively, materials containing glass cloth impregnated into these resins (CCL) or inorganic fillers such as silica, alumina, and barium sulfate may be used. [Explanation of Symbols]

[0100] 101 … Flexible printed circuit board according to the present invention (embodiment) 102... Conventional structure as a comparative example 103… Multi-die package 1 ... Signal electrode 2 … Ground electrode (GND electrode) 3. Solder mask (insulating film) 4 … Groove 5. Ground plane (ground conductor layer) 5a ... Ground plane 6… Signal line 7. Insulator 8… Grooved grounding conductor 9. Guard grounding conductor (Guard GND) 10 ... Continuity via 11 … Internal electrode 12… Gap 13… Metal foil 14… Via formation opening 15 … Groove formation opening 16 … Via formation hole 17… Through-hole 18 … Groove side 19… Groove side plated surface 20… Interposer 21… Package Substrate 22… Implementation board 23… Semiconductor die 24. Flexible printed circuit (FPC) 25... Soldering section 26… Connector

Claims

1. A signal line formed within the insulating layer, A pair of grounding conductor layers positioned above and below the signal line, The signal line is provided with a groove-shaped grounding conductor extending along the wiring direction of the signal line, The grooved grounding conductor is electrically connected in a continuous manner to the pair of grounding conductor layers, so that the signal line is substantially surrounded in a closed loop by the pair of grounding conductor layers and the grooved grounding conductor. The groove-shaped grounding conductor is configured to include a discontinuous portion having a length sufficiently short with respect to the effective wavelength of the high-frequency signal within the substrate, and is characterized in that it functions as an electromagnetically continuous grounding conductor.

2. The flexible printed circuit board according to Claim 1, wherein the length of the discontinuity is one-tenth or less of the effective wavelength within the substrate at the operating frequency.

3. The flexible printed circuit board according to claim 1 or claim 2, wherein the groove-shaped grounding conductor is formed by applying a conductive treatment and electroplating to the side surface of a groove formed in an insulator.

4. The flexible printed circuit board according to claim 1 or 2, wherein the groove-shaped grounding conductor is formed substantially continuously along the wiring direction of the signal line.

5. The flexible printed circuit board according to claim 1 or claim 2, wherein the distance between the groove-shaped grounding conductor and the signal line is set to a range in which the electromagnetic field confinement effect and signal characteristics can be balanced.

6. The flexible printed circuit board according to claim 5, wherein the distance between the groove-shaped grounding conductor and the signal line is set to suppress degradation of signal quality due to excessive electromagnetic coupling and to effectively exert the electromagnetic field confinement effect.

7. The flexible printed circuit board according to claim 1 or claim 2, wherein the signal line is a wiring that transmits a single-wire signal.

8. The flexible printed circuit board according to claim 1 or claim 2, wherein the signal line is composed of a pair of wires that transmit differential signals.

9. The flexible printed circuit board according to claim 8, wherein each pair of wires transmitting the differential signal is substantially surrounded in a closed loop by the groove-shaped ground conductor and the ground conductor layer.

10. The flexible printed circuit board according to claim 1 or claim 2, wherein the signal line transmits high-frequency signals in the millimeter-wave band or higher.

11. The flexible printed circuit board according to claim 1 or claim 2, wherein the signal line is a power line that transmits power supply current or high-frequency components contained in the power supply current.

12. The flexible printed circuit board according to claim 1 or claim 2, wherein the groove-shaped grounding conductor structurally defines a return current path corresponding to the signal line.

13. The flexible printed circuit board according to claim 1 or claim 2, wherein a plurality of the signal lines are arranged within the same layer, and each signal line is surrounded by a corresponding groove-shaped grounding conductor.

14. The flexible printed circuit board according to claim 1 or claim 2, wherein the flexible printed circuit board has a multilayer structure, and signal lines arranged in different layers are each surrounded by the ground conductor layer and the groove-shaped ground conductor.

15. A flexible printed circuit board according to claim 1 or claim 2, A flexible printed circuit board characterized in that a metal film is formed along the side surface of the outer shape of the flexible printed circuit board, and the metal film is electrically connected to the grounding conductor layer and the groove-shaped grounding conductor.

16. A method for manufacturing a flexible printed circuit board according to Claim 1, A substrate comprising an insulator and a metal foil formed on at least one surface thereof, comprising the steps of forming via-forming holes and grooves by removing the insulator, A step of applying a conductive treatment and electroplating to the sides of the via-forming holes and grooves to form via holes and groove-shaped ground conductors, A method for manufacturing a flexible printed circuit board, comprising the step of forming the signal lines and the ground conductor layer by patterning the metal foil.

17. The method for manufacturing a flexible printed circuit board according to claim 16, wherein the via-forming holes and grooves are formed in the same process by direct laser processing.

18. A method for manufacturing a flexible printed circuit board according to claim 16 or claim 17, wherein the step of forming the via-forming holes and the grooves is performed before the substrate is laminated.

19. A method for manufacturing a flexible printed circuit board according to claim 16 or claim 17, wherein the step of forming the via-forming holes and the grooves is performed after the substrates are laminated.

20. An electronic device comprising a flexible printed circuit board according to claim 1 or claim 2.

21. The electronic device according to claim 20, wherein the electronic device is an optical transceiver or an optical transceiver module.

22. The flexible printed circuit board according to claim 1 or claim 2 is directly connected to the upper surface of the interposer or the upper surface of the package substrate in a multi-die package. A semiconductor package mounting structure used as power supply wiring to supply power to multiple semiconductor dies.

23. The semiconductor package mounting structure according to claim 22, wherein in the power supply wiring, the return current path is stabilized by a continuous groove-shaped grounding conductor, and the influence of electromagnetic noise caused by high-frequency components contained in the power supply current is suppressed.

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