Flexible printed circuit board

Abstract

The invention provides a flexible printed circuit board which can obtain stable characteristic impedance even if a signal line is close to another conductor when the flexible printed circuit board is bent. A flexible printed circuit board (1) is provided with a flexible insulating film (2), a signal line (3), a ground conductor (4), and an insulating coating (6). The signal line 3 is provided on the upper surface 2a of the insulating film. The ground conductor (4) is provided on the upper surface (2a) of the insulating film (2) so as to be adjacent to the signal line (3), and the thickness of the ground conductor (4) is greater than the thickness of the signal line (3). The insulating coating part 6 coats the signal line 3 and the grounding conductor 4. The insulating coating portion (6) fills a groove portion (G1) defined by the signal line (3), the ground conductor (4), and the insulating film (2), and the upper surface of the insulating coating portion (6) covering the signal line (3) is located at a position higher than the upper surface of at least the portion of the ground conductor (4) adjacent to the signal line (3) over the width (W) of the signal line (3).

Description

Technical Field

[0001] This invention relates to a flexible printed circuit board. Background Technology

[0002] To achieve miniaturization of information processing terminals such as smartphones, the density of integrated circuits is constantly being increased, and high-speed communication technologies such as 5G, 5G private networks, and 6G have accelerated their development in recent years. In addition, foldable smartphones and rollable smartphones have been developed. In foldable and rollable phones, a flexible printed circuit board (FPC) with excellent high-frequency characteristics and bending resistance is needed to connect the circuit boards.

[0003] Known high-speed transmission lines include microstrip line structures, stripline structures, and coplanar structures. Generally, compared to FPCs with microstrip or stripline structures, FPCs with coplanar structures exhibit superior bending resistance because they do not have a solid GND on the back side of the insulating film. However, in the case of coplanar structures, there are issues with high-frequency performance degradation when the FPC bends or when signal lines are close to the ground terminal of the circuit board. [Refer to...] Figure 13 Please provide a detailed explanation.

[0004] Figure 13 A flexible printed circuit board 100 with a coplanar structure, a circuit board 200, and a circuit board 300 electrically connected to the flexible printed circuit board 100 are shown. For example, circuit board 200 is equipped with an antenna for receiving wireless signals, and circuit board 300 is equipped with a chip for processing the signals received by the antenna. Figure 13 As shown, the flexible printed circuit board 100 is bent, and the circuit board 200 is close to the flexible printed circuit board 100. In this case, parasitic capacitance is generated between the ground terminal of the circuit board 200 and the signal line of the flexible printed circuit board 100, and the characteristic impedance deviates from the design value (e.g., 50Ω), becoming unstable. As a result, signal reflection waves are generated, and high-frequency characteristics are degraded.

[0005] It should be noted that, not limited to the circuit board, if a conductor has a different potential than the signal lines of the FPC, the same problem as described above will occur when the conductor is close to the FPC.

[0006] In addition, to improve bending resistance, FPCs with hollow internal sections are known. When such an FPC is bent, the grounding terminal, which is positioned opposite the signal line, gets close to the signal line through the hollow section, which also degrades its high-frequency characteristics.

[0007] Patent Document 1 describes a printed circuit board designed to obtain a stable characteristic impedance. In this printed circuit board, a ground layer is provided on the lower surface of the substrate, signal lines are provided on the upper surface, and ground lines are provided on both sides of the signal lines. By making the top surface of the ground lines protrude beyond the top surface of the signal lines, the influence of the ground layer on impedance formation is reduced, and a stable characteristic impedance of the signal lines is easily obtained regardless of the shape of the ground layer. However, the electromagnetic coupling strength between the signal lines and the ground lines is insufficient, making it difficult to meet the high-frequency characteristics required by recent high-speed communication technologies. Furthermore, since the ground layer is formed to substantially cover the entire lower surface of the substrate, its bending resistance is poor.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 6-37412

[0011] Patent Document 2: Japanese Patent Application Publication No. 6-53712

[0012] Patent Document 3: Japanese Patent Application Publication No. 7-254661

[0013] Technical issues

[0014] The present invention aims to provide a flexible printed circuit board that can achieve stable characteristic impedance even when signal lines are close to other conductors when bent. Summary of the Invention

[0015] This invention provides a flexible printed circuit board. The flexible printed circuit board includes a flexible insulating film, signal lines, a ground conductor, and an insulating cover. The flexible insulating film has a first main surface and a second main surface. The signal lines are disposed on the first main surface of the insulating film. The ground conductor is disposed adjacent to the signal lines on the first main surface of the insulating film, and the thickness of the ground conductor is greater than the thickness of the signal lines. The insulating cover covers the signal lines and the ground conductor. The insulating cover fills a groove defined by the opposing sides of the signal lines and the ground conductor and the first main surface of the insulating film located between the signal lines and the ground conductor, and, across the width of the signal lines, the upper surface of the insulating cover covering the signal lines is located at a higher position than the upper surface of at least the portion of the ground conductor adjacent to the signal lines.

[0016] In addition, in the aforementioned flexible printed circuit board, a grounding conductor may not be provided on the second main surface of the insulating film.

[0017] In addition, in the aforementioned flexible printed circuit board, the portion of the grounding conductor adjacent to the signal line may be thicker than the signal line.

[0018] In addition, in the above-mentioned flexible printed circuit board, the flexible printed circuit board may include a second ground conductor, which is disposed on the first main surface together with the ground conductor and separated by a signal line, and the insulating covering portion may cover the second ground conductor.

[0019] In addition, in the aforementioned flexible printed circuit board, the second grounding conductor can be thicker than the signal line.

[0020] In addition, in the aforementioned flexible printed circuit board, the thickness of the second grounding conductor can be the same as that of the signal line.

[0021] In addition, in the aforementioned flexible printed circuit board, the portion of the second grounding conductor adjacent to the signal line may be thicker than the signal line.

[0022] Furthermore, in the aforementioned flexible printed circuit board, the flexible printed circuit board may also include a third ground conductor, a fourth ground conductor, a first passage, a second passage, and a second insulating covering portion. The third ground conductor is disposed on the second main surface in a manner opposite to the ground conductor, separated by an insulating film. The fourth ground conductor is disposed on the second main surface in a manner opposite to the second ground conductor, separated by an insulating film. The first passage is disposed within the insulating film, electrically connecting the ground conductor and the third ground conductor. The second passage is disposed within the insulating film, electrically connecting the second ground conductor and the fourth ground conductor. The second insulating covering portion covers the third ground conductor, the fourth ground conductor, and the second main surface located between the third ground conductor and the fourth ground conductor.

[0023] In addition, in the aforementioned flexible printed circuit board, the third ground conductor and / or the fourth ground conductor may be thicker than the signal line.

[0024] In addition, in the aforementioned flexible printed circuit board, the thickness of the third and fourth grounding conductors can be the same as that of the signal lines.

[0025] In addition, in the aforementioned flexible printed circuit board, at least one of the grounding conductor, the second grounding conductor, the third grounding conductor, and the fourth grounding conductor may have a portion closer to the signal line that is thicker than the signal line.

[0026] According to the present invention, a flexible printed circuit board is provided that can achieve a stable characteristic impedance even when the signal line is close to other conductors when bent. Attached Figure Description

[0027] Figure 1A This is a cross-sectional view of the flexible printed circuit board provided in the first embodiment.

[0028] Figure 1B This is a schematic diagram illustrating the structural features of the flexible printed circuit board provided in the first embodiment.

[0029] Figure 2 This is a process cross-sectional view used to illustrate the manufacturing method of the flexible printed circuit board provided in the first embodiment.

[0030] Figure 3 This is a graph representing an example of the simulation results of the characteristic impedance of the flexible printed circuit board provided in the first embodiment.

[0031] Figure 4 This is a cross-sectional view of the flexible printed circuit board provided in a variation of the first embodiment, Example 1.

[0032] Figure 5 This is a cross-sectional view of the flexible printed circuit board provided in Variation 2 of the first embodiment.

[0033] Figure 6 This is a cross-sectional view of the flexible printed circuit board provided in the second embodiment.

[0034] Figure 7 This is a process cross-sectional view used to illustrate the manufacturing method of the flexible printed circuit board provided in the second embodiment.

[0035] Figure 8 This is a graph representing an example of the simulation results of the characteristic impedance of the flexible printed circuit board provided in the second embodiment.

[0036] Figure 9 This is a cross-sectional view of the flexible printed circuit board provided in Variation 1 of the second embodiment.

[0037] Figure 10 This is a cross-sectional view of the flexible printed circuit board provided in Variation 2 of the second embodiment.

[0038] Figure 11 This is a cross-sectional view of the flexible printed circuit board provided in Variation 3 of the second embodiment.

[0039] Figure 12 This is a cross-sectional view of the flexible printed circuit board provided on a scale.

[0040] Figure 13 It is a diagram used to illustrate a problem in the prior art. Detailed Implementation

[0041] The embodiments of the present invention will now be described with reference to the accompanying drawings. It should be noted that the drawings are schematic, and the relationship between thickness and planar dimensions, the ratio of thicknesses of each layer, etc., may sometimes differ from reality. Furthermore, terms used in this specification to define shape, geometric conditions, physical properties, and their degrees, such as "parallel," "orthogonal," "equal," "same," etc., or values ​​of dimensions and physical properties, are not strictly limited in meaning and should be interpreted as encompassing a range of degrees to which the same functionality is expected.

[0042] (First Implementation)

[0043] Reference Figure 1A and Figure 1B The flexible printed circuit board 1 provided in the first embodiment will be described.

[0044] The flexible printed circuit board 1 includes an insulating film 2, signal lines 3, grounding conductors 4 and 5, and an insulating covering 6.

[0045] The insulating film 2 is composed of a flexible insulating film, having an upper surface 2a (first main surface) and a lower surface 2b (second main surface). The material of the insulating film 2 can be, for example, a liquid crystal polymer (LCP), but is not limited to this; it can also be a material suitable for general FPCs. Specifically, it can be an insulating material such as polyimide (PI), modified polyimide (MPI), polyethylene naphthalate (PEN), polyetheretherketone (PEEK), or fluoropolymers (PFA, PTFE, etc.).

[0046] It should be noted that, since the flexible printed circuit board 1 transmits high-frequency signals, the material of the insulating film 2 is preferably a material with low dielectric constant and low dielectric loss tangent.

[0047] Furthermore, to improve the bending resistance of the flexible printed circuit board 1, it is preferable that the insulating film 2 has a thinner thickness. 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 2a of the insulating film 2 to the upper surface of the portion of the insulating covering 6 that covers the signal line 3.

[0048] Signal line 3 is disposed on the upper surface 2a of insulating film 2, along a line perpendicular to... Figure 1A It extends in the direction of the paper (first direction). Signal line 3 is made of conductive materials such as copper foil.

[0049] Grounding conductor 4 is disposed on the upper surface 2a of insulating film 2 adjacent to signal line 3. Similarly, grounding conductor 5 is disposed on the upper surface 2a of insulating film 2 adjacent to signal line 3. Grounding conductor 4 and grounding conductor 5 are thicker than signal line 3.

[0050] Grounding conductor 4 and grounding conductor 5 are arranged apart from signal line 3. In other words, grounding conductor 5 and grounding conductor 4 are arranged on insulating film 2 apart from signal line 3. Grounding conductor 4 is an example of a grounding conductor described in the claims, and grounding conductor 5 is an example of a second grounding conductor described in the claims.

[0051] It should be noted that, as Figure 1AAs shown, in this embodiment, no grounding conductor is provided on the lower surface 2b of the insulating film 2. This improves the bending resistance of the flexible printed circuit board.

[0052] The insulating cover 6 is made of insulating material and covers the signal line 3 and the grounding conductors 4 and 5. However, there may also be parts that are not covered by the insulating cover 6, such as the parts where the terminals of the signal line 3 are located. Figure 1A As shown, the insulating covering part 6 fills the groove G between the grounding conductor 4 and the grounding conductor 5.

[0053] In this embodiment, the insulating covering 6 is composed of two layers: an adhesive layer 6a and a cover film 6b. The cover film 6b is, for example, made of polyimide. It should be noted that the insulating covering 6 may be composed of one layer (e.g., only a photoresist layer), or it may be composed of three or more layers.

[0054] like Figure 1B As shown, the insulating covering portion 6 fills the groove portion G1. The groove portion G1 is a groove-shaped portion defined by the opposing sides of the signal line 3 and the ground conductors 4 and 5 and the upper surface 2a of the insulating film 2 located between the signal line 3 and the ground conductors 4 and 5. The upper surface of the insulating covering portion 6 filling the groove portion G1 is higher than the upper surface of the ground conductors 4 and 5.

[0055] Furthermore, along the width W of the signal line 3, the upper surface of the insulating covering portion 6 of the signal line 3 is positioned higher than the upper surfaces of the grounding conductors 4 and 5. That is, along the width W of the signal line 3, there is a height difference D between the upper surface of the insulating covering portion 6 of the signal line 3 and the upper surfaces of the grounding conductors 4 and 5. Therefore, as detailed below, the electromagnetic coupling strength between the signal line 3 and the grounding conductors 4 and 5 is increased, thereby stabilizing the characteristic impedance.

[0056] <Manufacturing Method of Flexible Printed Circuit Board 1>

[0057] Reference Figure 2 An example of a method for manufacturing the flexible printed circuit board 1 provided in the first embodiment will be described.

[0058] like Figure 2 (1) As shown, an insulating film 2 and a single-sided copper-clad laminate with copper foil 7 on the upper surface of the insulating film 2 are prepared. The single-sided copper-clad laminate is, for example, a copper-clad board with a copper foil of 12 μm thickness (or 17.5 μm thickness) laminated on one side of a 25 μm thick LCP substrate. It should be noted that in this step, it is not limited to copper foil, and a single-sided metal-clad laminate with metal foil such as silver or aluminum can also be prepared.

[0059] Next, as Figure 2(2) As shown, the copper foil 7 is patterned using a known photolithography method to form signal lines 3 and grounding copper foils 7a and 7b. The wiring substrate is obtained through this step.

[0060] Next, as Figure 2 (3) As shown, copper plating is performed to form plating layers 8a and 8b on the grounding copper foils 7a and 7b. The grounding copper foil 7a and plating layer 8a become the grounding conductor 4, and the grounding copper foil 7b and plating layer 8b become the grounding conductor 5. It should be noted that in this step, after laminating a dry film on the wiring substrate and exposing and developing the dry film to expose the grounding copper foils 7a and 7b, the grounding copper foils 7a and 7b are electroplated using a button electroplating (pattern electroplating) method. Other electroplating methods, such as full-board electroplating, can also be used to perform copper plating on the entire surface of the wiring substrate.

[0061] Next, a cover film 6b having an adhesive layer 6a is laminated onto the wiring substrate, and heated and pressurized using a vacuum stamping device or a vacuum laminating device to form the insulating cover 6. The thicknesses of the adhesive layer 6a and the cover film 6b are, for example, 25 μm. It should be noted that, in this step, a photosensitive photoresist can also be coated onto the wiring substrate and cured by light irradiation to form the insulating cover 6.

[0062] Then, surface treatments such as gold plating are performed on the parts of signal lines 3, grounding conductors 4 and 5 that are not covered by the insulating cover part 6. Afterwards, through shaping and other processes, a finished product is manufactured. Figure 1A The flexible printed circuit board 1 shown.

[0063] <Effects of the First Embodiment>

[0064] Figure 3 An example of the results is shown, in which the characteristic impedance of the flexible printed circuit board 1 and the characteristic impedance of the flexible printed circuit board 100 provided in the comparison are obtained by electromagnetic field simulation when the distance between the flexible printed circuit board 1 and the circuit board (not shown) is changed (hereinafter referred to as "distance").

[0065] Figure 12 This is a cross-sectional view of a flexible printed circuit board 100 provided in proportion. The flexible printed circuit board 100 includes an insulating film 120, signal lines 130, ground conductors 140 and 150 disposed on the insulating film 120 with respect to the signal lines 130, and an insulating film 160 covering the signal lines 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 lines 130 and the ground conductors 140 and 150. Furthermore, the upper surface of the insulating film 160 is located at a lower position than the upper surfaces of the ground conductors 140 and 150.

[0066] The values ​​used for electromagnetic field simulation are as follows: The relative permittivity of insulating films 2 and 120 is 2.9, the dielectric loss tangent is 0.002, and the thickness is 25 μm. The thickness of signal lines 3 and 130 is 12 μm. The thickness of grounding conductors 4, 5, 140, and 150 is 24 μm. The relative permittivity of adhesive layer 6a is 2.3, the dielectric loss tangent is 0.0015, and the relative permittivity of cover film 6b and insulating film 160 is 2.9, the dielectric loss tangent is 0.002. The thickness of insulating film 160 is 5 μm. In the flexible printed circuit board 1, the height difference D between the upper surface of the insulating cover 6 covering signal line 3 and the upper surface of grounding conductors 4 and 5 is 41 μm.

[0067] The width of the slot G1 in the flexible printed circuit board 1 is 50 μm. In contrast, the width of the slot (the distance between the signal line 130 and the ground conductor 140 (150)) in the flexible printed circuit board 100 is 40 μm. The difference in slot width is to ensure that the characteristic impedance (characteristic impedance without the influence of the circuit board substrate) of the flexible printed circuit board 100 is 50 Ω. The total width of the flexible printed circuit boards 1 and 100 is 2000 μm.

[0068] The above values ​​are selected so that the characteristic impedance (characteristic impedance without the influence of the circuit board) of the flexible printed circuit board 1 is 50Ω. In addition, the thickness from the upper surface 2a of the insulating film 2 to the upper surface of the insulating covering portion 6 covering the signal line 3 is 65μm.

[0069] from Figure 3 The simulation results show that, compared with the comparative flexible printed circuit board 100, the characteristic impedance of flexible printed circuit board 1 is more stable with respect to distance. Compared with flexible printed circuit board 100, the characteristic impedance of flexible printed circuit board 1 changes less, achieving a characteristic impedance close to the design value of 50Ω across the entire distance range. This is because, compared with the comparative flexible printed circuit board 100, the characteristics of the insulating covering portion 6 in the flexible printed circuit board 1 provided in this embodiment result in a stronger electromagnetic coupling strength between the signal line 3 and the ground conductors 4 and 5. It should be noted that the widths of the slots used in the simulations of flexible printed circuit board 1 and flexible printed circuit board 100 are different, but since reducing the width of the slot tends to stabilize the characteristic impedance, it does not affect the verification results of the effect of this embodiment.

[0070] As described above, in the first embodiment, the insulating covering portion 6 covering 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 upper surface of the insulating covering portion 6 covering the signal line 3 is located at a higher position than the upper surface of the ground conductors 4 and 5 in the width W of the signal line 3. Therefore, since the electromagnetic coupling between the signal line 3 and the ground conductors 4 and 5 is strengthened, even when the circuit board (the conductor portion) is close to the signal line 3, the change in characteristic impedance of the flexible printed circuit board 1 is controlled to be small, and the degradation of the transmission characteristics of high-frequency signals can be suppressed.

[0071] Furthermore, according to the first embodiment, since no grounding conductor is provided on the lower surface 2b of the insulating film 2, the bending resistance of the flexible printed circuit board can be improved.

[0072] 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 covering 6 that covers the signal line 3. As a result, since the insulating film 2 is relatively thin, a flexible printed circuit board with excellent bend resistance can be provided.

[0073] It should be noted that signal line 3 is not limited to one; for example, two signal lines can be used in the case of a differential circuit. In this case, the characteristic impedance stabilization effect can also be achieved.

[0074] Alternatively, in the grounding conductor, either grounding conductor 4 or grounding conductor 5 may be placed on the insulating film 2. That is, grounding conductor 4 or grounding conductor 5 may be omitted.

[0075] Alternatively, either ground conductor 4 or ground conductor 5 may be thicker than signal line 3. For example, ground conductor 4 may be thicker than signal line 3, and the thickness of ground conductor 5 may be the same as the thickness of signal line 3.

[0076] Two variations of the first embodiment will be described below.

[0077] <Modification 1 of the first embodiment>

[0078] Figure 4 This is a cross-sectional view of the flexible printed circuit board 1A provided in Modified Example 1.

[0079] In this variation, such as Figure 4 As shown, the portions of grounding conductors 4 and 5 adjacent only to signal line 3 are thicker than signal line 3. Specifically, plating layer 8a of grounding conductor 4 is formed only on the portion adjacent to signal line 3, and plating layer 8b of grounding conductor 5 is formed only on the portion adjacent to signal line 3. The widths of plating layers 8a and 8b can, for example, be greater than or equal to the width of the groove G1.

[0080] As shown in this modified example, even if only the portions of the grounding conductors 4 and 5 adjacent to the signal line 3 are thicker than the signal line 3, the coupling strength between the signal line 3 and the grounding conductors 4 and 5 can be ensured, thus achieving characteristic impedance stabilization. Therefore, in order to obtain the characteristic impedance stabilization effect, the upper surface of the insulating covering portion 6 can be located at a position higher than the upper surface of at least the portion of the grounding conductors 4 and 5 adjacent to the signal line 3.

[0081] Furthermore, according to this modified example, since the portions of the grounding conductors 4 and 5, except for the portion adjacent to the signal line 3, are relatively thin, the bending resistance of the flexible printed circuit board can be further improved.

[0082] <Modification 2 of the first embodiment>

[0083] Figure 5 This is a cross-sectional view of the flexible printed circuit board 1B provided in Modification 2. In this modification, as... Figure 5 As shown, the thickness of the ground conductor 4 is the same as that of the signal line 3. Even in this case, since the coupling strength between the ground conductor 5 and the signal line 3 is greater than that of the comparative flexible printed circuit board 100, the characteristic impedance can be stabilized.

[0084] It should be noted that the flexible printed circuit 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. Additionally, the ground conductor 4 can also be thinner than the signal line 3.

[0085] (Second Implementation)

[0086] Next, refer to Figure 6 The flexible printed circuit board 1C provided in the second embodiment will be described below. One difference from the first embodiment is that a grounding 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.

[0087] The flexible printed circuit board 1C has an insulating film 2, signal lines 3, ground conductors 4 and 5, an insulating covering portion 6, ground conductors 14 and 15, an insulating covering portion 16, and vias 19 and 20. The insulating film 2, signal lines 3, ground conductors 4 and 5, and insulating covering portion 6 are the same as in the first embodiment, and therefore descriptions are omitted.

[0088] The grounding conductor 14 is disposed on the lower surface 2b of the insulating film 2, opposite to the grounding conductor 4, separated by the insulating film 2. For example, as Figure 6As shown, the grounding conductor 14 is positioned such that its projection on the upper surface 2a of the insulating film 2 is approximately the same as that of the grounding conductor 4. The grounding conductor 15 is positioned opposite the grounding conductor 5 on the lower surface 2b of the insulating film 2, separated by the insulating film 2. For example, as... Figure 6 As shown, the grounding conductor 15 is configured such that its projection on the upper surface 2a of the insulating film 2 is substantially the same as that of the grounding conductor 5. Furthermore, the grounding conductor 14 is an example of a third grounding conductor as described in the claims, and the grounding conductor 15 is an example of a fourth grounding conductor as described in the claims.

[0089] The insulating cover portion 16 covers the grounding conductor 14, the grounding conductor 15, and the lower surface 2b of the insulating film 2 located between the grounding conductor 14 and the grounding conductor 15. Furthermore, the insulating cover portion 16 is an example of a second insulating cover portion described in the claims.

[0090] The insulating covering portion 16 consists of two layers: an adhesive layer 16a and a cover film 16b. The cover film 16b is made of, for example, polyimide. Alternatively, the insulating covering portion 16 may consist of a single layer (e.g., only a photoresist layer) or three or more layers. The insulating covering portion 16 fills the groove between the grounding conductor 14 and the grounding conductor 15.

[0091] Passage 19 is disposed within the insulating film 2, electrically connecting grounding conductor 4 and grounding conductor 14. Similarly, passage 20 electrically connects grounding conductor 5 and grounding conductor 15.

[0092] <Manufacturing Method of Flexible Printed Circuit Board 1C>

[0093] Reference Figure 7 An example of a method for manufacturing a flexible printed circuit board 1C provided in the second embodiment will be described.

[0094] like Figure 7 (1) As shown, an insulating film 2 and a double-sided copper-clad laminate with copper foil 7 on the upper surface and copper foil 17 on the lower surface of the insulating film 2 are prepared. The double-sided copper-clad laminate is, for example, a copper-clad laminate with copper foil of 12 μm thickness (or 17.5 μm thickness) laminated on both sides of a 25 μm thick LCP substrate. It should be noted that in this step, it is not limited to copper foil, and a double-sided metal-clad laminate with metal foil such as silver or aluminum can also be prepared.

[0095] Next, as Figure 7 As shown in (2), a through hole H (e.g., φ150μm) is formed at a specified position on the double-sided copper-clad laminate using a drill bit. It should be noted that the through hole H can also be formed by a laser such as a CO2 laser.

[0096] Next, as Figure 7As shown in (3), electroplating is performed to fill the through-hole H with electroplated metal, forming passages 19 and 20 that electrically connect copper foil 7 and copper foil 17. In this step, button electroplating (pattern electroplating) or full-board electroplating can be used. Alternatively, passages 19 and 20 can also be electroplated through-holes. Furthermore, the electroplating method is not limited; passages 19 and 20 can also be formed by filling the through-hole H with conductive paste.

[0097] Next, as Figure 7 (4) As shown, copper foil 7 and copper foil 17 are patterned using a known photolithography method, forming signal lines 3 and grounding copper foils 7a and 7b on the upper surface of the insulating film 2, and grounding copper foils 17a and 17b on the lower surface of the insulating film 2. The wiring substrate is obtained through this step.

[0098] Next, as Figure 7 As shown in (5), copper plating is performed on both sides of the wiring substrate. This forms plating layers 8a and 8b on the grounding copper foils 7a and 7b, and plating layers 18a and 18b on the grounding copper foils 17a and 17b. The grounding copper foil 7a and plating layer 8a become grounding conductor 4, and the grounding copper foil 7b and plating layer 8b become grounding conductor 5. The grounding copper foil 17a and plating layer 18a become grounding conductor 14, and the grounding copper foil 17b and plating layer 18b become grounding conductor 15.

[0099] In this step, a dry film is laminated onto the wiring substrate. After exposing and developing the dry film to expose the grounding copper foils 7a and 7b (grounding copper foils 17a and 17b), the grounding copper foils 7a and 7b (grounding copper foils 17a and 17b) are electroplated using a button electroplating (pattern electroplating) method. It should be noted that other electroplating methods, such as full-board electroplating, can also be used to perform copper plating on the entire surface of the wiring substrate.

[0100] Next, a cover film 6b having an adhesive layer 6a (e.g., 25 μm thick) is laminated onto the upper surface of the wiring substrate. Then, a cover film 16b having an adhesive layer 16a (e.g., 25 μm thick) is laminated onto the lower surface of the wiring substrate. Afterwards, insulating covers 6 and 16 are formed by heating and pressurizing using a vacuum stamping apparatus or a vacuum laminating apparatus. It should be noted that in this step, a photosensitive photoresist can also be coated onto the wiring substrate and cured by light irradiation to form the insulating covers 6 and 16.

[0101] Then, surface treatments such as gold plating are performed on the parts of signal lines 3, grounding conductors 4, 5, 14, and 15 that are not covered by the insulating covering part 6. Afterwards, through shaping and other processes, a finished product is manufactured. Figure 6 The flexible printed circuit board 1C shown is shown.

[0102] <Effects of the Second Embodiment>

[0103] Figure 8 An example of the results is shown, in which the characteristic impedance of the flexible printed circuit board 1C and the comparative flexible printed circuit board 100 are determined by electromagnetic field simulation, with the distance between the flexible printed circuit board 1C and the circuit substrate (not shown) being changed.

[0104] The numerical values ​​used for electromagnetic field simulation are as follows. Descriptions of structures identical to those in the first embodiment are omitted. The thickness of grounding conductors 14 and 15 is 24 μm. The relative permittivity of adhesive layer 16a is 2.3, and the dielectric loss tangent is 0.0015. The relative permittivity of cover film 16b is 2.9, and the dielectric loss tangent is 0.002. It should be noted that the thickness from the lower surface 2b of insulating film 2 to the lower surface of insulating covering portion 16 is 55 μm.

[0105] from Figure 8 The simulation results show that, in the flexible printed circuit board 1C, the characteristic impedance is more stable with respect to distance compared to the comparative flexible printed circuit board 100. Furthermore, in the flexible printed circuit board 1C, the stability of the characteristic impedance is improved compared to the flexible printed circuit board 1 of the first embodiment.

[0106] As described above, in the second embodiment, in addition to the structure already described in the first embodiment, grounding conductors 14 and 15, insulating covering portion 16, and passages 19 and 20 are also provided. This further improves the stability of the characteristic impedance of the flexible printed circuit board 1C.

[0107] The following describes three variations of the second embodiment.

[0108] <Modification 1 of the Second Embodiment>

[0109] Figure 9 This is a cross-sectional view of the flexible printed circuit board 1D provided in Modification 1 of the second embodiment. In this modification, the structure described in Modification 1 of the first embodiment is applied.

[0110] In this modified example, similar to Modified Example 1 of the first embodiment, the upper surface of the insulating film 2 is thicker than the signal line 3 only in the portions of the grounding conductors 4 and 5 adjacent to the signal line 3. Specifically, the plating layer 8a of the grounding conductor 4 is formed only in the portion adjacent to the signal line 3, and the plating layer 8b of the grounding conductor 5 is formed only in the portion adjacent to the signal line 3. This allows for the stabilization of the characteristic impedance while improving the bending resistance of the flexible printed circuit board.

[0111] Furthermore, in this modified example, on the lower surface side of the insulating film 2, only the portions of the grounding conductors 14 and 15 closest to the signal line 3 (i.e., the opposing ends of the grounding conductors 14 and 15) are thicker than other portions. Specifically, the plating layer 18a of the grounding conductor 14 and the plating layer 18b of the grounding conductor 15 are formed only on the portions of the grounding conductors 14 and 15 that are disposed opposite to each other. The flexible printed circuit board 1C of the second embodiment has lower bending resistance than the flexible printed circuit board 1 of the first embodiment, but the reduction in bending resistance of the flexible printed circuit board can be suppressed by the flexible printed circuit board 1D.

[0112] As described above, according to this modified example, for grounding conductors 4, 5, 14, and 15, by making the portion only close to signal line 3 thicker than signal line 3, it is possible to suppress the reduction in the bending resistance of the flexible printed circuit board.

[0113] It should be noted that grounding conductors 14 and 15 can also be formed in the same shape as grounding conductors 4 and 5. That is, as shown below. Figure 9 As shown, the shapes of grounding conductors 14 and 15 can also be symmetrical about the insulating film 2 and the grounding conductors 4 and 5.

[0114] In addition, the widths of plating 18a and plating 18b are only required to be, for example, greater than or equal to the width of the groove G1.

[0115] In addition, in this modified example, for all grounding conductors 4, 5, 14, and 15, only the portion of their respective proximity signal line 3 is thickened, but it is also possible to only thicken the portion of the proximity signal line 3 of at least one of the grounding conductors 4, 5, 14, and 15.

[0116] <Modification 2 of the Second Embodiment>

[0117] Figure 10 This is a cross-sectional view of the flexible printed circuit board 1E provided in Modification 2 of the second embodiment.

[0118] In this modified example, the thickness of the grounding conductors 14 and 15 is the same as the thickness of the signal line 3. That is, the grounding conductors 14 and 15 are made of grounding copper foils 17a and 17b and do not have a plating.

[0119] In this modified example, compared with the flexible printed circuit board 1 of the first embodiment, the characteristic impedance can be stabilized, and compared with the flexible printed circuit board 1C of the second embodiment, the bending resistance can be improved.

[0120] <Modification 3 of the Second Embodiment>

[0121] Figure 11 This is a cross-sectional view of the flexible printed circuit board 1F provided in Modification 3 of the second embodiment.

[0122] In this modified example, the thickness of grounding conductors 14 and 15 is the same as the thickness of signal line 3. Additionally, grounding conductor 4 is made of grounding copper foil 7a and has the same thickness as signal line 3.

[0123] According to this variation, the characteristic impedance can be stabilized, and the reduction in bending resistance can be further suppressed.

[0124] The above describes three variations of the second embodiment. Additionally, although not shown, it is possible to leave one of the grounding conductors 14 and 15 unplated, making its thickness the same as the signal line 3, while plating the other grounding conductor to make it thicker than the signal line 3.

[0125] Based on the above description, those skilled in the art will conceive of additional effects and various modifications of the present invention, but the present invention is not limited to the various embodiments described above. Elements of different embodiments can also be appropriately combined. Various additions, modifications, and partial deletions can be made without departing from the scope defined in the claims and the conceptual idea and spirit of the invention derived from its equivalents.

[0126] Explanation of main component symbols

[0127] 1, 1A, 1B, 1C, 1D, 1E Flexible Printed Circuit Boards

[0128] 2. Insulating film

[0129] 3 signal lines

[0130] 4, 5 Grounding conductors

[0131] 6. Insulation Covering Part

[0132] 6a Adhesive layer

[0133] 6b Covering film

[0134] 7 Copper foil

[0135] 7a, 7b Grounding copper foil

[0136] 8a and 8b coatings

[0137] 14, 15 Grounding conductors

[0138] 16 Insulation Covering Section

[0139] 16a Adhesive layer

[0140] 16b covering film

[0141] 17 Copper Foil

[0142] 17a, 17b Grounding copper foil

[0143] 18a and 18b coatings

[0144] 19, 20 access

[0145] 100 Flexible Printed Circuit Board

[0146] 120 Insulating Film

[0147] 130 signal line

[0148] 140, 150 grounding conductors

[0149] 160 Insulating Film

[0150] 200 and 300 circuit boards

[0151] D Height difference

[0152] H through hole

[0153] W width

Claims

1. A flexible printed circuit board, wherein, The flexible printed circuit board includes: A flexible insulating film having a first main surface and a second main surface; Signal lines are disposed on the first main surface of the insulating film; A grounding conductor is disposed on the first main surface of the insulating film adjacent to the signal line, and the thickness of the grounding conductor is greater than the thickness of the signal line; An insulating covering portion, which covers the signal line and the grounding conductor; The insulating covering portion fills a groove defined by the opposing sides of the signal line and the ground conductor and the first main surface of the insulating film located between the signal line and the ground conductor, and the upper surface of the insulating covering portion covering the signal line is located at a higher position than the upper surface of at least the portion of the ground conductor adjacent to the signal line in the width of the signal line.

2. The flexible printed circuit board according to claim 1, wherein, No grounding conductor is provided on the second main surface of the insulating film.

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

4. The flexible printed circuit board according to claim 1, wherein, The flexible printed circuit board includes a second ground conductor, which is disposed on the first main surface together with the ground conductor and separated from the signal line, and the insulating cover covers the second ground conductor.

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

6. The flexible printed circuit board according to claim 4, wherein, The thickness of the second grounding conductor is the same as that of the signal line.

7. The flexible printed circuit board according to claim 4, wherein, In the second grounding conductor, only the portion adjacent to the signal line is thicker than the signal line.

8. The flexible printed circuit board according to claim 4, wherein, The flexible printed circuit board also includes: A third grounding conductor is disposed on the second main surface in such a way that it is positioned opposite to the grounding conductor through the insulating film; The fourth grounding conductor is disposed on the second main surface in such a manner that it is positioned opposite to the second grounding conductor through the insulating film; A first path is provided within the insulating film, electrically connecting the grounding conductor and the third grounding conductor; A second path is provided within the insulating film to electrically connect the second grounding conductor and the fourth grounding conductor; The second insulating covering covers the third grounding conductor, the fourth grounding conductor, and the second main surface located between the third grounding conductor and the fourth grounding conductor.

9. The flexible printed circuit board according to claim 8, wherein, The third grounding conductor and / or the fourth grounding conductor are thicker than the signal line.

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

11. The flexible printed circuit board according to claim 8, wherein, At least one of the grounding conductor, the second grounding conductor, the third grounding conductor, and the fourth grounding conductor, is thicker than the signal line only in the portion closest to the signal line.

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