Flexible printed wiring board

Abstract

[Problem] To provide a flexible printed wiring board capable of obtaining a stable characteristic impedance even when a signal line approaches another conductor when bending. [Solution] This flexible printed wiring board 1 comprises: a flexible insulation film 2; a signal line 3 that is disposed on the upper surface 2a of the insulation film 2; a ground conductor 4 that is disposed on the upper surface 2a of the insulation film 2 so as to be adjacent to the signal line 3 and that has a thickness greater than the thickness of the signal line 3; and an insulation coating part 6 that covers the signal line 3 and the ground conductor 4. The insulation coating part 6 fills a groove G1 defined by the signal line 3, the ground conductor 4, and the insulation film 2, and the upper surface of the insulation coating part 6 covering the signal line 3, over the width W of the signal line 3, is located above the upper surface of at least the portion of the ground conductor 4 that is adjacent to the signal line 3.

Description

flexible printed wiring board

[0001] The present invention relates to a flexible printed wiring board.

[0002] In information processing terminals such as smartphones, the density of integrated circuits is increasing in order to reduce their size, and in recent years, the evolution of high-speed communication technologies such as 5G, local 5G, and 6G has accelerated. Furthermore, foldable smartphones (foldable phones) and rollable smartphones (rollable phones) have been developed. Foldable phones and rollable phones require flexible printed circuit boards (FPCs) with excellent high-frequency characteristics and bending resistance to connect circuit boards.

[0003] Known high-speed transmission lines include microstripline structures, stripline structures, and coplanar structures. Generally, compared to FPCs with microstripline or stripline structures, FPCs with coplanar structures have superior flexibility because they do not have a solid GND on the back surface of the insulating film. However, the coplanar structure presents a problem in that high-frequency characteristics deteriorate when the FPC is bent and the signal line approaches the ground of the circuit board. This will be explained in detail with reference to FIG. 13 .

[0004] FIG. 13 shows a flexible printed wiring board 100 having a coplanar structure, and circuit boards 200 and 300 electrically connected by the flexible printed wiring board 100. For example, the circuit board 200 is a circuit board equipped with an antenna for receiving wireless signals, and the circuit board 300 is a circuit board equipped with a chip for processing the signals received by the antenna. Consider a situation in which the flexible printed wiring board 100 is bent and the circuit board 200 approaches the flexible printed wiring board 100, as shown in FIG. 13 . In this situation, parasitic capacitance occurs between the ground of the circuit board 200 and the signal line of the flexible printed wiring board 100, causing the characteristic impedance to deviate from the design value (e.g., 50 Ω) and become unstable. As a result, a reflected wave of the signal is generated, degrading the high-frequency characteristics.

[0005] The same problem as above occurs not only in the case of a circuit board, but also when a conductor having a different potential from the signal lines of the FPC comes close to the FPC.

[0006] In addition, FPCs with hollow spaces inside are known to enhance bending resistance, but even with such FPCs, when bent, the ground, which faces the signal lines across the hollow space, comes close to the signal lines, causing degradation of high-frequency characteristics.

[0007] Patent Document 1 describes a printed wiring board designed to achieve a stable characteristic impedance. This printed wiring board has a ground layer on the underside of the board, a signal line on the top side, and ground lines on both sides of the signal line. By projecting the top surface of the ground line beyond the top surface of the signal line, the contribution of the ground layer to impedance formation is reduced, making it easier to achieve a stable characteristic impedance for the signal line regardless of the shape of the ground layer. However, the electromagnetic coupling strength between the signal line and the ground line is insufficient, making it difficult to achieve the high-frequency characteristics required for modern high-speed communication technology. Furthermore, because the ground layer is formed to cover almost the entire bottom surface of the board, it has poor bending resistance.

[0008] JP-A-6-37412 JP-A-6-53712 JP-A-7-254661

[0009] The problem to be solved by the present invention is to provide a flexible printed wiring board that can obtain a stable characteristic impedance even when a signal line comes close to another conductor when bent.

[0010] A flexible printed wiring board according to the present invention comprises: a flexible insulating film having a first main surface and a second main surface; a signal line arranged on the first main surface of the insulating film; a ground conductor arranged adjacent to the signal line on the first main surface of the insulating film and having a thickness greater than that of the signal line; and an insulating coating portion covering the signal line and the ground conductor, wherein the insulating coating portion fills a groove defined by opposing side surfaces of the signal line and the ground conductor and the first main surface of the insulating film between the signal line and the ground conductor, and the upper surface of the insulating coating portion covering the signal line across the width of the signal line is positioned higher than the upper surface of at least a portion of the ground conductor adjacent to the signal line.

[0011] In the flexible printed wiring board, no ground conductor may be provided on the second main surface of the insulating film.

[0012] In the flexible printed wiring board, only a portion of the ground conductor adjacent to the signal line may be thicker than the signal line.

[0013] The flexible printed wiring board may further include a second ground conductor disposed on the first principal surface so as to sandwich the signal line between the second ground conductor and the ground conductor, and the insulating coating may cover the second ground conductor.

[0014] In the flexible printed wiring board, the second ground conductor may be thicker than the signal line.

[0015] In the flexible printed wiring board, the second ground conductor may have the same thickness as the signal line.

[0016] In the flexible printed wiring board, only a portion of the second ground conductor adjacent to the signal line may be thicker than the signal line.

[0017] The flexible printed wiring board may further include: a third ground conductor arranged on the second principal surface to face the ground conductor with the insulating film interposed therebetween; a fourth ground conductor arranged on the second principal surface to face the second ground conductor with the insulating film interposed therebetween; a first via provided in the insulating film and electrically connecting the ground conductor and the third ground conductor; a second via provided in the insulating film and electrically connecting the second ground conductor and the fourth ground conductor; and a second insulating coating that covers the third ground conductor, a fourth ground conductor, and a portion of the second principal surface between the third ground conductor and the fourth ground conductor.

[0018] In the flexible printed wiring board, the third ground conductor and / or the fourth ground conductor may be thicker than the signal line.

[0019] In the flexible printed wiring board, the third and fourth ground conductors may have the same thickness as the signal line.

[0020] In the flexible printed wiring board, at least one of the ground conductor, the second ground conductor, the third ground conductor, and the fourth ground conductor may be thicker than the signal line only in a portion thereof that is close to the signal line.

[0021] According to the present invention, it is possible to provide a flexible printed wiring board that can obtain a stable characteristic impedance even when a signal line comes close to another conductor when bent.

[0022] FIG. 1 is a cross-sectional view of a flexible printed wiring board according to a first embodiment. FIG. 2 is a diagram for explaining structural features of the flexible printed wiring board according to the first embodiment. FIG. 3 is a process cross-sectional view for explaining a manufacturing method of a flexible printed wiring board according to the first embodiment. FIG. 4 is a graph showing an example of a simulation result of characteristic impedance of the flexible printed wiring board according to the first embodiment. FIG. 5 is a cross-sectional view of a flexible printed wiring board according to a first modification of the first embodiment. FIG. 6 is a cross-sectional view of a flexible printed wiring board according to a second embodiment. FIG. 7 is a process cross-sectional view for explaining a manufacturing method of a flexible printed wiring board according to the second embodiment. FIG. 8 is a graph showing an example of a simulation result of characteristic impedance of the flexible printed wiring board according to the second embodiment. FIG. 9 is a cross-sectional view of a flexible printed wiring board according to a first modification of the second embodiment. FIG. 10 is a cross-sectional view of a flexible printed wiring board according to a second modification of the second embodiment.

[0023] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each layer, and the like may differ from the actual ones. Furthermore, terms used in this specification that specify shapes, geometric conditions, physical characteristics, and their degrees, such as "parallel," "orthogonal," "equal," and "same," as well as dimensions, physical characteristic values, and the like, are not limited to their strict meanings but are interpreted to include a range within which similar functions can be expected.

[0024] First Embodiment A flexible printed wiring board 1 according to a first embodiment will be described with reference to FIGS. 1A and 1B.

[0025] The flexible printed wiring board 1 includes an insulating film 2 , a signal line 3 , ground conductors 4 and 5 , and an insulating coating 6 .

[0026] The insulating film 2 is made of a flexible insulating film and has an upper surface 2a (first main surface) and a lower surface 2b (second main surface). The material of the insulating film 2 is, for example, liquid crystal polymer (LCP), but is not particularly limited and may be a material used in general FPCs. Specifically, insulating materials such as polyimide (PI), modified polyimide (MPI), polyethylene naphthalate (PEN), polyether ether ketone (PEEK), fluororesin (PFA, PTFE, etc.), etc. may be used.

[0027] Since the flexible printed wiring board 1 transmits high frequency signals, it is desirable that the material of the insulating film 2 has low dielectric constant and low dielectric loss tangent characteristics.

[0028] Furthermore, it is desirable that the thickness of the insulating film 2 be thin in order to improve the bending resistance of the flexible printed wiring board 1. In this embodiment, the thickness of the insulating film 2 is 25 μm, which is less than half the thickness (65 μm) from the upper surface 2 a of the insulating film 2 to the upper surface of the portion of the insulating coating 6 that covers the signal line 3.

[0029] The signal line 3 is disposed on the upper surface 2a of the insulating film 2 and extends in a direction perpendicular to the plane of the paper in Fig. 1A (first direction). The signal line 3 is made of a conductive material such as copper foil.

[0030] The ground conductor 4 is disposed on the upper surface 2a of the insulating film 2 so as to be adjacent to the signal line 3. Similarly, the ground conductor 5 is disposed on the upper surface 2a of the insulating film 2 so as to be adjacent to the signal line 3. The ground conductors 4 and 5 are thicker than the signal line 3.

[0031] The ground conductor 4 and the ground conductor 5 are arranged to sandwich the signal line 3. In other words, the ground conductor 5 is arranged on the insulating film 2 to sandwich the signal line 3 with the ground conductor 4. The ground conductor 4 is an example of a ground conductor in the claims, and the ground conductor 5 is an example of a second ground conductor in the claims.

[0032] 1A, in this embodiment, no ground conductor is provided on the lower surface 2b of the insulating film 2. This can improve the bending resistance of the flexible printed wiring board.

[0033] The insulating coating 6 is made of an insulating material and covers the signal line 3 and the ground conductors 4 and 5. However, some portions, such as the portion where the terminal of the signal line 3 is provided, may not be covered by the insulating coating 6. As shown in FIG. 1A , the insulating coating 6 fills the groove G between the ground conductor 4 and the ground conductor 5.

[0034] In this embodiment, the insulating coating 6 is composed of two layers: an adhesive layer 6a and a cover film 6b. The cover film 6b is made of, for example, polyimide. The insulating coating 6 may be composed of a single layer (for example, only a photoresist layer) or three or more layers.

[0035] 1B , the insulating coating 6 fills the groove G1. The groove G1 is a groove-shaped portion defined by the opposing side surfaces of the signal line 3 and the ground conductor 4 and the upper surface 2 a of the insulating film 2 between the signal line 3 and the ground conductor 4. The upper surface of the insulating coating 6 filling the groove G1 is located higher than the upper surfaces of the ground conductors 4 and 5.

[0036] The top surface of the insulating coating 6 that covers the signal wire 3 is located higher than the top surfaces of the ground conductors 4 and 5 across the width W of the signal wire 3. That is, for the width W of the signal wire 3, there is a difference D between the top surface of the insulating coating 6 that covers the signal wire 3 and the top surfaces of the ground conductors 4 and 5. This increases the electromagnetic coupling strength between the signal wire 3 and the ground conductors 4 and 5, as will be described in detail later, and stabilizes the characteristic impedance.

[0037] <Method for Manufacturing Flexible Printed Wiring Board 1> An example of a method for manufacturing the flexible printed wiring board 1 according to the first embodiment will be described with reference to FIG.

[0038] 2(1), an insulating film 2 and a single-sided copper-clad laminate having copper foil 7 provided on the upper surface of the insulating film 2 are prepared. The single-sided copper-clad laminate is, for example, a copper-clad board having 12 μm (or 17.5 μm) copper foil laminated on one side of a 25 μm thick LCP base material. Note that in this step, a single-sided metal foil-clad laminate having metal foil such as silver or aluminum may be prepared instead of copper foil.

[0039] 2B, the copper foil 7 is patterned by a known photoetching method to form the signal line 3 and the ground copper foils 7a and 7b, thereby obtaining a wiring substrate.

[0040] 2(3), copper plating is performed to form plating layers 8a and 8b on the ground copper foils 7a and 7b. The ground copper foil 7a and plating layer 8a form the ground conductor 4, and the ground copper foil 7b and plating layer 8b form the ground conductor 5. In this step, a dry film is laminated on the wiring substrate, and the dry film is exposed and developed to expose the ground copper foils 7a and 7b. After that, button plating (pattern plating) is used, in which the ground copper foils 7a and 7b are plated. Alternatively, panel plating, in which copper plating is performed on the entire surface of the wiring substrate, may be used.

[0041] Next, a cover film 6b having an adhesive layer 6a is laminated onto the wiring substrate, and heated and pressurized using a vacuum press or vacuum laminator to form the insulating coating 6. The adhesive layer 6a and the cover film 6b each have a thickness of, for example, 25 μm. In this step, the insulating coating 6 may be formed by applying a photosensitive photoresist to the wiring substrate and curing it by light irradiation.

[0042] Thereafter, surface treatment such as gold plating is performed on the portions not covered with the insulating coating 6, such as the signal line 3 and the terminals of the ground conductors 4 and 5. Thereafter, the flexible printed wiring board 1 shown in FIG. 1A is manufactured by performing external processing and the like.

[0043] <Action and effect of the first embodiment> FIG. 3 shows an example of the results of electromagnetic field simulation of the characteristic impedance of the flexible printed wiring board 1 and the flexible printed wiring board 100 according to the comparative example when the distance (hereinafter also simply referred to as "distance") between the flexible printed wiring board 1 and the circuit board (not shown) is changed.

[0044] 12 shows a cross-sectional view of a flexible printed wiring board 100 according to a comparative example. The flexible printed wiring board 100 includes an insulating film 120, a signal line 130, ground conductors 140 and 150 disposed on the insulating film 120 so as to sandwich the signal line 130, and an insulating film 160 that covers the signal line 130 and the ground conductors 140 and 150. The insulating film 160 has a uniform thickness and does not fill the groove between the signal line 130 and the ground conductor 140 (ground conductor 150). The top surface of the insulating film 160 is located below the top surfaces of the ground conductors 140 and 150.

[0045] The values ​​used in the electromagnetic field simulation are as follows: The insulating films 2 and 120 had a relative permittivity of 2.9, a dielectric dissipation factor of 0.002, and a thickness of 25 μm. The signal lines 3 and 130 had a thickness of 12 μm. The ground conductors 4, 5, 140, and 150 had a thickness of 24 μm. The adhesive layer 6 a had a relative permittivity of 2.3 and a dielectric dissipation factor of 0.0015, and the cover film 6 b and insulating film 160 had a relative permittivity of 2.9 and a dielectric dissipation factor of 0.002. The insulating film 160 had a thickness of 5 μm. In the flexible printed wiring board 1, the difference D between the top surface of the insulating coating 6 covering the signal lines 3 and the top surfaces of the ground conductors 4 and 5 was 41 μm.

[0046] The width of the groove G1 of the flexible printed wiring board 1 was set to 50 μm. In contrast, the width of the groove of the flexible printed wiring board 100 (the distance between the signal line 130 and the ground conductor 140 (150)) was set to 40 μm. The groove widths were different so that the characteristic impedance of the flexible printed wiring board 100 (the characteristic impedance when there is no influence of the circuit board) was set to 50 Ω. The overall width of the flexible printed wiring boards 1 and 100 was set to 2000 μm.

[0047] The above values ​​were selected so that the characteristic impedance of the flexible printed wiring board 1 (characteristic impedance when there is no influence of the circuit board) would be 50 Ω. The thickness from the upper surface 2 a of the insulating film 2 to the upper surface of the insulating coating 6 that covers the signal line 3 is 65 μm.

[0048] 3 , the flexible printed wiring board 1 has a more stable characteristic impedance with respect to changes in distance than the flexible printed wiring board 100 of the comparative example. The change in characteristic impedance is smaller in the flexible printed wiring board 1 than in the flexible printed wiring board 100, and a characteristic impedance close to the design value of 50Ω is obtained over the entire range of distance. This is because the flexible printed wiring board 1 of the present embodiment has a stronger electromagnetic coupling strength between the signal line 3 and the ground conductors 4 and 5 due to the characteristics of the insulating coating portion 6 than the flexible printed wiring board 100 of the comparative example. Note that the widths of the grooves used in the simulation are different between the flexible printed wiring board 1 and the flexible printed wiring board 100. However, because a smaller groove width tends to result in more stable characteristic impedance, this does not affect the verification results of the effects of the present embodiment.

[0049] As described above, in the first embodiment, the insulating coating 6 that coats the signal line 3 and the ground conductors 4 and 5 fills the groove G1 between the signal line 3 and the ground conductor 4 (and / or between the signal line 3 and the ground conductor 5), and the top surface of the insulating coating 6 that coats the signal line 3 across the width W of the signal line 3 is located higher than the top surfaces of the ground conductors 4 and 5. This strengthens the electromagnetic coupling between the signal line 3 and the ground conductors 4 and 5, so that even when (the conductor portion of) the circuit board is close to the signal line 3, the change in the characteristic impedance of the flexible printed wiring board 1 is kept relatively small, and degradation of the transmission characteristics of high-frequency signals can be suppressed.

[0050] Furthermore, according to the first embodiment, since no ground conductor is provided on the lower surface 2b of the insulating film 2, the bending resistance of the flexible printed wiring board can be improved.

[0051] Furthermore, according to the first embodiment, the thickness of the insulating film 2 is thinner than the thickness from the upper surface 2a of the insulating film 2 to the upper surface of the portion of the insulating coating 6 that covers the signal line 3. Because the insulating film 2 is thus relatively thin, it is possible to provide a flexible printed wiring board that has excellent bending resistance.

[0052] The number of signal lines 3 is not limited to one, and for example, in the case of a differential line, two signal lines are provided. In this case as well, the effect of stabilizing the characteristic impedance can be obtained.

[0053] Alternatively, only one of the ground conductors, 4 and 5, may be provided on the insulating film 2. In other words, the ground conductor 4 or the ground conductor 5 may be omitted.

[0054] Alternatively, only one of the ground conductors 4 and 5 may be thicker than the signal line 3. For example, the ground conductor 4 may be thicker than the signal line 3, and the ground conductor 5 may be the same thickness as the signal line 3.

[0055] Two modifications of the first embodiment will be described below.

[0056] First Modification of First Embodiment FIG. 4 shows a cross-sectional view of a flexible printed wiring board 1A according to a first modification.

[0057] 4 , in this modification, only the portions of the ground conductors 4 and 5 adjacent to the signal line 3 are thicker than the signal line 3. Specifically, the plating layer 8a of the ground conductor 4 is formed only in the portion adjacent to the signal line 3, and the plating layer 8b of the ground conductor 5 is formed only in the portion adjacent to the signal line 3. The widths of the plating layers 8a and 8b may be, for example, equal to or greater than the width of the groove G1.

[0058] Even if the ground conductors 4, 5 are formed thicker than the signal line 3 only in the portions adjacent to the signal line 3 as in this modification, the characteristic impedance can be stabilized because the coupling strength between the signal line 3 and the ground conductors 4, 5 is ensured. Therefore, to obtain the effect of stabilizing the characteristic impedance, it is sufficient that the top surface of the insulating coating 6 is positioned higher than the top surfaces of at least the portions of the ground conductors 4, 5 adjacent to the signal line 3.

[0059] Furthermore, according to this modification, the ground conductors 4 and 5 are thin except for the portions adjacent to the signal line 3, which further improves the bending resistance of the flexible printed wiring board.

[0060] <Modification 2 of First Embodiment> Fig. 5 shows a cross-sectional view of a flexible printed wiring board 1B according to Modification 2. In this modification, as shown in Fig. 5, the thickness of the ground conductor 4 is the same as that of the signal line 3. Even in this case, the coupling strength between the ground conductor 5 and the signal line 3 is greater than that of the flexible printed wiring board 100 of the comparative example, and therefore, the characteristic impedance can be stabilized.

[0061] The flexible printed wiring board 1B is manufactured by forming a plating layer only on the ground copper foil 7b, without forming a plating layer on the ground copper foil 7a. The ground conductor 4 may be thinner than the signal line 3.

[0062] Second Embodiment Next, a flexible printed wiring board 1C according to a second embodiment will be described with reference to Fig. 6. One of the differences from the first embodiment is that a ground conductor is also provided on the lower surface 2b of the insulating film 2. The second embodiment will be described below, focusing on the differences.

[0063] The flexible printed wiring board 1C includes an insulating film 2, a signal line 3, ground conductors 4 and 5, an insulating coating 6, ground conductors 14 and 15, an insulating coating 16, and vias 19 and 20. The insulating film 2, the signal line 3, the ground conductors 4 and 5, and the insulating coating 6 are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0064] The ground conductor 14 is disposed on the lower surface 2b of the insulating film 2 so as to face the ground conductor 4 with the insulating film 2 interposed therebetween. For example, as shown in Fig. 6 , the ground conductor 14 is disposed so that its projection onto the upper surface 2a of the insulating film 2 substantially coincides with the ground conductor 4. The ground conductor 15 is disposed on the lower surface 2b of the insulating film 2 so as to face the ground conductor 15 with the insulating film 2 interposed therebetween. For example, as shown in Fig. 6 , the ground conductor 15 is disposed so that its projection onto the upper surface 2a of the insulating film 2 substantially coincides with the ground conductor 5. Note that the ground conductor 14 is an example of a third ground conductor in the claims, and the ground conductor 15 is an example of a fourth ground conductor in the claims.

[0065] The insulating coating portion 16 covers the ground conductor 14, the ground conductor 15, and the lower surface 2b of the insulating film 2 between the ground conductors 14 and 15. The insulating coating portion 16 is an example of a second insulating coating portion in the claims.

[0066] The insulating coating 16 is composed of two layers: an adhesive layer 16a and a cover film 16b. The cover film 16b is made of, for example, polyimide. The insulating coating 16 may be composed of a single layer (for example, only a photoresist layer) or three or more layers. The insulating coating 16 fills the groove between the ground conductor 14 and the ground conductor 15.

[0067] The via 19 is provided in the insulating film 2 and electrically connects the ground conductor 4 to the ground conductor 14. Similarly, the via 20 electrically connects the ground conductor 5 to the ground conductor 15.

[0068] <Method for Manufacturing Flexible Printed Wiring Board 1C> An example of a method for manufacturing the flexible printed wiring board 1C according to the second embodiment will be described with reference to FIG.

[0069] 7(1), an insulating film 2 and a double-sided copper-clad laminate are prepared, which has copper foil 7 on the upper surface of the insulating film 2 and copper foil 17 on the lower surface of the insulating film 2. The double-sided copper-clad laminate is, for example, a copper-clad board in which 12 μm thick (or 17.5 μm thick) copper foil is laminated on both sides of a 25 μm thick LCP base material. Note that in this step, a double-sided metal foil-clad laminate may be prepared in which metal foil such as silver or aluminum is provided, rather than copper foil.

[0070] Next, as shown in Fig. 7(2), through holes H (for example, φ150 µm) are formed at predetermined positions in the double-sided copper clad laminate using a drill. ２ It may be formed by a laser such as a laser.

[0071] Next, as shown in FIG. 7(3), a plating process is performed to fill the through holes H with a plating metal, thereby forming vias 19 and 20 that electrically connect the copper foil 7 and the copper foil 17. In this step, button plating (pattern plating) or panel plating may be used. The vias 19 and 20 may be plated through holes. Furthermore, the vias 19 and 20 may be formed by filling the through holes H with a conductive paste, without being limited to plating.

[0072] 7(4), the copper foils 7 and 17 are patterned by a known photoetching method to form the signal lines 3 and ground copper foils 7a and 7b on the upper surface of the insulating film 2, and ground copper foils 17a and 17b on the lower surface of the insulating film 2. A wiring substrate is obtained by this step.

[0073] Next, as shown in Fig. 7(5), copper plating is applied to both surfaces of the wiring substrate. As a result, plating layers 8a and 8b are formed on the ground copper foils 7a and 7b, and plating layers 18a and 18b are formed on the ground copper foils 17a and 17b. The ground copper foil 7a and plating layer 8a form the ground conductor 4, and the ground copper foil 7b and plating layer 8b form the ground conductor 5. The ground copper foil 17a and plating layer 18a form the ground conductor 14, and the ground copper foil 17b and plating layer 18b form the ground conductor 15.

[0074] In this step, a dry film is laminated on the wiring substrate, and the dry film is exposed and developed to expose the ground copper foils 7 a, 7 b (ground copper foils 17 a, 17 b). After that, the ground copper foils 7 a, 7 b (ground copper foils 17 a, 17 b) are plated by button plating (pattern plating). Alternatively, a panel plating method may be used to plate the entire surface of the wiring substrate with copper.

[0075] Next, a cover film 6b (e.g., 25 μm thick) having an adhesive layer 6a is laminated onto the upper surface of the wiring substrate. Furthermore, a cover film 16b (e.g., 25 μm thick) having an adhesive layer 16a is laminated onto the lower surface of the wiring substrate. After that, the insulating coatings 6, 16 are formed by applying heat and pressure using a vacuum press or vacuum laminator. In this step, the insulating coatings 6, 16 may also be formed by applying a photosensitive photoresist to the wiring substrate and curing it with light irradiation.

[0076] Thereafter, surface treatment such as gold plating is performed on the portions not covered with the insulating coating portion 6, such as the signal line 3 and the terminals of the ground conductors 4, 5, 14, and 15. Thereafter, by performing external processing and the like, the flexible printed wiring board 1C shown in FIG.

[0077] <Action and effect of the second embodiment> Figure 8 shows an example of the results of electromagnetic field simulation of the characteristic impedance of flexible printed wiring board 1C and a comparative example flexible printed wiring board 100 when the distance between flexible printed wiring board 1C and a circuit board (not shown) is changed.

[0078] The values ​​used in the electromagnetic field simulation are as follows. The same configurations as in the first embodiment are omitted. The thickness of the ground conductors 14 and 15 was set to 24 μm. The relative dielectric constant of the adhesive layer 16a was set to 2.3 and the dielectric dissipation factor was set to 0.0015. The relative dielectric constant of the cover film 16b was set to 2.9 and the dielectric dissipation factor was set to 0.002. The thickness from the lower surface 2b of the insulating film 2 to the lower surface of the insulating coating 16 was 55 μm.

[0079] 8 shows that the characteristic impedance of the flexible printed wiring board 1C is more stable with respect to changes in distance than that of the comparative example flexible printed wiring board 100. Furthermore, the stability of the characteristic impedance of the flexible printed wiring board 1C is improved compared to that of the flexible printed wiring board 1 of the first embodiment.

[0080] As described above, in the second embodiment, in addition to the configuration described in the first embodiment, the ground conductors 14 and 15, the insulating coating 16, and the vias 19 and 20 are provided, which can further improve the stability of the characteristic impedance of the flexible printed wiring board 1C.

[0081] Three modifications of the second embodiment will be described below.

[0082] 9 shows a cross-sectional view of a flexible printed wiring board 1D according to Modification 1 of the second embodiment. This modification employs the structure described in Modification 1 of the first embodiment.

[0083] In this modification, as in the first modification of the first embodiment, only the portions of the ground conductors 4 and 5 adjacent to the signal line 3 on the upper surface of the insulating film 2 are thicker than the signal line 3. Specifically, the plating layer 8a of the ground conductor 4 is formed only in the portion adjacent to the signal line 3, and the plating layer 8b of the ground conductor 5 is formed only in the portion adjacent to the signal line 3. This makes it possible to stabilize the characteristic impedance and improve the bending resistance of the flexible printed wiring board.

[0084] Furthermore, in this modification, only the portions of the ground conductors 14, 15 that are close to the signal line 3 (i.e., the ends where the ground conductors 14, 15 face each other) are thicker than the other portions on the underside of the insulating film 2. Specifically, the plating layer 18a of the ground conductor 14 and the plating layer 18b of the ground conductor 15 are formed only in the portions where the ground conductors 14 and 15 face each other. While the flexible printed wiring board 1C according to the second embodiment has lower bending resistance than the flexible printed wiring board 1 according to the first embodiment, the flexible printed wiring board 1D can suppress the decrease in bending resistance of the flexible printed wiring board.

[0085] As described above, according to this modification, by making the ground conductors 4, 5, 14, and 15 thicker than the signal line 3 only in the portions close to the signal line 3, it is possible to prevent a decrease in the bending resistance of the flexible printed wiring board.

[0086] The ground conductors 14 and 15 may have the same shape as the ground conductors 4 and 5. That is, as shown in Fig. 9 , the ground conductors 14 and 15 may have a shape symmetrical to that of the ground conductors 4 and 5 with respect to the insulating film 2.

[0087] Furthermore, the width of the plating layers 18a and 18b may be, for example, equal to or greater than the width of the groove portion G1.

[0088] In addition, in this modified example, the ground conductors 4, 5, 14, and 15 are all configured so that only the portions thereof close to the signal line 3 are thickened. However, it is also possible to thicken only the portions thereof close to the signal line 3 for at least one of the ground conductors 4, 5, 14, and 15.

[0089] <Modification 2 of Second Embodiment> FIG. 10 shows a cross-sectional view of a flexible printed wiring board 1E according to Modification 2 of the second embodiment.

[0090] In this modification, the thickness of the ground conductors 14 and 15 is the same as the thickness of the signal line 3. That is, the ground conductors 14 and 15 are made of ground copper foils 17a and 17b and do not have a plating layer.

[0091] According to this modification, it is possible to stabilize the characteristic impedance more than the flexible printed wiring board 1 of the first embodiment, and to improve the bending resistance more than the flexible printed wiring board 1C of the second embodiment.

[0092] <Third Modification of Second Embodiment> FIG. 11 shows a cross-sectional view of a flexible printed wiring board 1F according to a third modification of the second embodiment.

[0093] In this modification, the thickness of the ground conductors 14 and 15 is the same as the thickness of the signal line 3. The ground conductor 4 is made of a ground copper foil 7a and has the same thickness as the signal line 3.

[0094] According to this modification, it is possible to stabilize the characteristic impedance and further suppress the deterioration of bending resistance.

[0095] The above describes three modified examples of the second embodiment. Alternatively, although not shown, one of the ground conductors 14 and 15 may be made the same thickness as the signal line 3 without forming a plating layer, and the other ground conductor may be made thicker than the signal line 3 by forming a plating layer.

[0096] Based on the above description, a person skilled in the art may conceive additional effects and various modifications of the present invention, but the aspects of the present invention are not limited to the individual embodiments described above. Elements from different embodiments may be combined as appropriate. Various additions, modifications, and partial deletions are possible within the scope of the conceptual idea and spirit of the present invention, which is derived from the content defined in the claims and their equivalents.

[0097] REFERENCE SIGNS LIST 1, 1A, 1B, 1C, 1D, 1E Flexible printed wiring board 2 Insulating film 3 Signal line 4, 5 Ground conductor 6 Insulating coating portion 6a Adhesive layer 6b Cover film 7 Copper foil 7a, 7b Ground copper foil 8a, 8b Plating layer 14, 15 Ground conductor 16 Insulating coating portion 16a Adhesive layer 16b Cover film 17 Copper foil 17a, 17b Ground copper foil 18a, 18b Plating layer 19, 20 Via 100 Flexible printed wiring board 120 Insulating film 130 Signal line 140, 150 Ground conductor 160 Insulating film 200, 300 Circuit board D Difference H Through hole W Width

Claims

1. A flexible printed wiring board comprising: a flexible insulating film having a first main surface and a second main surface; a signal line arranged on the first main surface of the insulating film; a ground conductor arranged adjacent to the signal line on the first main surface of the insulating film and having a thickness greater than that of the signal line; and an insulating coating that covers the signal line and the ground conductor, wherein the insulating coating fills a groove defined by the opposing side surfaces of the signal line and the ground conductor and the first main surface of the insulating film between the signal line and the ground conductor, and wherein the top surface of the insulating coating that covers the signal line across the width of the signal line is positioned higher than the top surface of at least a portion of the ground conductor adjacent to the signal line.

2. The flexible printed wiring board according to claim 1, wherein no ground conductor is provided on the second main surface of the insulating film.

3. The flexible printed wiring board according to claim 1, wherein only a portion of the ground conductor adjacent to the signal line is thicker than the signal line.

4. The flexible printed wiring board according to claim 1, further comprising a second ground conductor disposed on the first principal surface so as to sandwich the signal line together with the ground conductor, and the insulating coating portion coating the second ground conductor.

5. The flexible printed wiring board according to claim 4, wherein the second ground conductor is thicker than the signal line.

6. The flexible printed wiring board according to claim 4, wherein the second ground conductor has the same thickness as the signal line.

7. The flexible printed wiring board according to claim 4, wherein only a portion of said second ground conductor adjacent to said signal line is thicker than said signal line.

8. The flexible printed wiring board according to claim 4, further comprising: a third ground conductor arranged on the second principal surface to face the ground conductor with the insulating film sandwiched therebetween; a fourth ground conductor arranged on the second principal surface to face the second ground conductor with the insulating film sandwiched therebetween; a first via provided in the insulating film and electrically connecting the ground conductor and the third ground conductor; a second via provided in the insulating film and electrically connecting the second ground conductor and the fourth ground conductor; and a second insulating coating portion covering the third ground conductor, the fourth ground conductor, and a portion of the second principal surface between the third ground conductor and the fourth ground conductor.

9. The flexible printed wiring board according to claim 8, wherein the third ground conductor and / or the fourth ground conductor is thicker than the signal line.

10. The flexible printed wiring board according to claim 8, wherein the thickness of the third and fourth ground conductors is the same as that of the signal line.

11. The flexible printed wiring board according to claim 8, wherein at least one of the ground conductor, the second ground conductor, the third ground conductor, and the fourth ground conductor is thicker than the signal line only in a portion close to the signal line.

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