Flexible Multilayer Circuit Board

Abstract

A flexible multilayer circuit board (1) comprises: an insulation layer (2); a first conductor layer (10) disposed on one side of the insulation layer (2) in the thickness direction thereof; a second conductor layer (20) disposed on the other side of the insulation layer (2) in the thickness direction thereof; and a wiring part (30) embedded in the insulation layer (2). The first conductor layer (10) has a first ground conductor section (11), and the second conductor layer (20) has a second ground conductor section (21). The flexible multilayer circuit board (1) is provided with a bend section (F) at an intermediate section in the longitudinal direction orthogonal to the thickness direction. Outside of the bend section (F), the first ground conductor section (11) and the second ground conductor section (21) have a first thickness (T1). In the bend section (F), at least one of the first ground conductor section (11) and the second ground conductor section (21) has a second thickness (T2). The second thickness (T2) is thinner than the first thickness (T1).

Description

[Technical Field]

[0001] The present invention relates to a flexible multilayer circuit board. [Background technology]

[0002] In recent years, flexible multilayer circuit boards have been used in electronic devices such as mobile phones, smartphones, tablet devices, digital cameras, etc., as they become smaller, lighter, and more functional. As such flexible multilayer circuit boards, flexible multilayer circuit boards that can be bent in the thickness direction have been proposed (see, for example, Patent Document 1 below).

[0003] The flexible multilayer circuit board of Patent Document 1 has, in the longitudinal direction perpendicular to the thickness direction, a wide portion that is wide in the short-side direction of the conductor and a narrow portion that is narrow in the short-side direction of the conductor, so that it can be bent in the thickness direction at the narrow portion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-131375 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, flexible multilayer circuit boards are required to have even better flexibility.

[0006] The flexible multilayer circuit board of Patent Document 1 achieves a certain degree of flexibility by changing the width of the conductor in the short direction, but it is designed to be used in a nearly straight line, and has the disadvantage that the stress when bending cannot be sufficiently reduced.

[0007] The present invention provides a flexible multilayer circuit board that is highly flexible. [Means for solving the problem]

[0008] The present invention [1] provides a flexible multilayer circuit board comprising an insulating layer, a first conductor layer disposed on one side of the insulating layer in a thickness direction, a second conductor layer disposed on the other side of the insulating layer in the thickness direction, and a wiring portion embedded in the insulating layer, wherein the first conductor layer has a first ground conductor portion, and the second conductor layer has a second ground conductor portion, the flexible multilayer circuit board has a bent portion in a longitudinal intermediate portion perpendicular to the thickness direction, the first ground conductor portion and the second ground conductor portion having a first thickness except at the bent portion, and at least one of the first ground conductor portion and the second ground conductor portion having a second thickness at the bent portion, the second thickness being thinner than the first thickness.

[0009] In such a flexible multilayer circuit board, at least one of the first ground conductor and the second ground conductor has a second thickness that is thinner than the first thickness at the bent portion, thereby providing excellent flexibility in the thickness direction at the bent portion.

[0010] The present invention [2] includes the flexible multilayer circuit board according to [1], wherein the first ground conductor and the second ground conductor each have the second thickness at the bent portion.

[0011] In such a flexible multilayer circuit board, the first ground conductor and the second ground conductor each have the second thickness at the bent portion, which provides excellent flexibility in both thickness directions at the bent portion.

[0012] The present invention [3] includes the flexible multilayer circuit board according to [2], wherein at least one of the first ground conductor and the second ground conductor further has the first thickness in the bent portion.

[0013] In such a flexible multilayer circuit board, at least one of the first ground conductor and the second ground conductor further has a first thickness at the bending portion, which ensures appropriate rigidity at the bending portion and allows bending at a larger angle in the thickness direction.

[0014] The present invention [4] includes the flexible multilayer circuit board according to [3], wherein, in the bent portion, each of the first ground conductor and the second ground conductor further has the first thickness.

[0015] In such a flexible multilayer circuit board, the first and second ground conductors each have a first thickness at the bending portion, which ensures appropriate rigidity at the bending portion and allows bending at a larger angle to both sides in the thickness direction.

[0016] The present invention [5] includes the flexible multilayer circuit board according to [4], wherein, in the bent portion, the first ground conductor portion and the second ground conductor portion each have the first thickness and the second thickness alternately along the longitudinal direction.

[0017] In such a flexible multilayer circuit board, the first and second ground conductors each have a first thickness and a second thickness alternately along the longitudinal direction at the bent portion, which provides even better flexibility in both thickness directions at the bent portion.

[0018] The present invention [6] includes the flexible multilayer circuit board according to [5], wherein the first thickness of the first ground conductor is located on the opposite side of the insulating layer to the first thickness of the second ground conductor in the thickness direction, and the second thickness of the first ground conductor is located on the opposite side of the insulating layer to the second thickness of the second ground conductor in the thickness direction.

[0019] In such a flexible multilayer circuit board, the first thickness of the first ground conductor overlaps with the first thickness of the second ground conductor, and the second thickness of the first ground conductor overlaps with the second thickness of the second ground conductor in the thickness direction, thereby providing even better flexibility in both directions in the thickness direction at the bendable portion.

[0020] The present invention [7] includes a flexible multilayer circuit board according to any one of [1] to [6], wherein the insulating layer comprises a bonding layer, a first porous resin layer arranged on one side of the bonding layer in the thickness direction, and a second porous resin layer arranged on the other side of the bonding layer in the thickness direction, and the wiring portion is embedded in the bonding layer.

[0021] In such a flexible multilayer circuit board, the insulating layer includes, in order toward the other side in the thickness direction, a bonding layer, a first porous resin layer, a first base resin layer, a second porous resin layer, and a second base resin layer, which provides excellent adhesion between the first conductor layer and the insulating layer and between the insulating layer and the second conductor layer, and also reduces the dielectric constant. [Effects of the Invention]

[0022] The flexible multilayer circuit board of the present invention has a bent portion in a longitudinally intermediate portion perpendicular to the thickness direction, and at least one of the first ground conductor and the second ground conductor has a second thickness that is thinner than the first thickness at the bent portion, thereby providing excellent flexibility. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 shows a longitudinal cross-sectional view of one embodiment of a flexible multilayer circuit board of the present invention. [Figure 2] FIG. 2 shows a cross-sectional view of the flexible multilayer circuit board shown in FIG. 1 taken along line AA' in the width direction. [Figure 3] FIG. 3 shows a cross-sectional view of the flexible multilayer circuit board shown in FIG. 1 taken along line BB' in the width direction. [Figure 4]FIG. 4 shows a cross-sectional view taken along the line CC' in the width direction of the flexible multilayer circuit board shown in FIG. [Figure 5] Figures 5A to 5D show a method for manufacturing the flexible multilayer circuit board shown in Figure 1. Figure 5A shows a step of preparing a first porous laminate, Figure 5B shows a step of forming a first via, Figure 5C shows a step of patterning a third conductor layer, and Figure 5D shows a step of preparing a second porous laminate. [Figure 6] Figures 6A to 6D, following Figure 5D, show the method for manufacturing the flexible multilayer circuit board shown in Figure 1. Figure 6A shows the step of bonding the second porous laminate to the first porous laminate, Figure 6B shows the step of forming a second via, Figure 6C shows the step of forming a plating layer, and Figure 6D shows the step of forming a ground conductor portion and a terminal portion. [Figure 7] Figures 7A to 7C, following on from Figure 6D, show the method for manufacturing the flexible multilayer circuit board shown in Figure 1. Figure 7A shows the step of forming the second thickness, Figure 7B shows the step of bonding the cover insulating layer, and Figure 7C shows the step of bonding the reinforcing base material. [Figure 8] FIG. 8 shows a longitudinal cross-sectional view of a first modified example of the flexible multilayer circuit board shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0024] 1. Flexible multilayer circuit board A flexible multilayer circuit board 1 according to one embodiment of the present invention will be described with reference to FIGS.

[0025] 1, the vertical direction of the paper surface indicates the thickness direction of the flexible multilayer circuit board 1. The horizontal direction of the paper surface indicates the longitudinal direction of the flexible multilayer circuit board 1. The depth direction of the paper surface indicates the width direction (short side direction) of the flexible multilayer circuit board 1.

[0026] 2 to 4, the vertical direction of the paper indicates the thickness direction of the flexible multilayer circuit board 1. The horizontal direction of the paper indicates the width direction (short side direction) of the flexible multilayer circuit board 1. The depth direction of the paper indicates the longitudinal direction of the flexible multilayer circuit board 1.

[0027] The flexible multilayer circuit board 1 has a thickness. The flexible multilayer circuit board 1 extends in a planar direction perpendicular to the thickness direction. The flexible multilayer circuit board 1 has a generally flat plate shape that is long in the longitudinal direction. The longitudinal direction is perpendicular to both the thickness direction and the width direction.

[0028] 1, the flexible multilayer circuit board 1 includes an insulating layer 2, a first conductor layer 10 disposed on one thickness-wise side of the insulating layer 2, a second conductor layer 20 disposed on the other thickness-wise side of the insulating layer 2, and a wiring portion 30 embedded in the insulating layer 2. Specifically, the flexible multilayer circuit board 1 includes the insulating layer 2, the first conductor layer 10 disposed on one thickness-wise surface of the insulating layer 2, the second conductor layer 20 disposed on the other thickness-wise surface of the insulating layer 2, and the wiring portion 30 embedded in the insulating layer 2. The flexible multilayer circuit board 1 also includes a bent portion F in a longitudinally intermediate portion.

[0029] The first conductor layer 10 has a first ground conductor 11 and, if necessary, a first terminal 12. Preferably, the first conductor layer 10 has the first ground conductor 11 and the first terminal 12. The second conductor layer 20 has a second ground conductor 21 and, if necessary, a second terminal 22. Preferably, the second conductor layer 20 has the second ground conductor 21 and the second terminal 22. The first terminal 12 and the second terminal 22 may be collectively referred to as the terminals.

[0030] The first ground conductor portion 11 and the second ground conductor portion 21 have a first thickness T1 in areas other than the bent portion F. At least one of the first ground conductor portion 11 and the second ground conductor portion 21 has a second thickness T2 in the bent portion F. The second thickness T2 is thinner than the first thickness T1.

[0031] The flexible multilayer circuit board 1 may include a first via connection portion 51, a second via connection portion 52, a cover insulating layer 60, and a reinforcing base material 70, as required.

[0032] 2 to 4 show a portion of each cross-sectional view of the flexible multilayer circuit board 1. Specifically, the flexible multilayer circuit board 1 has a plurality of structures shown in Figs. 2 to 4 arranged in the width direction.

[0033] <Insulating layer> The insulating layer 2 includes a bonding layer 3, a first porous resin layer 4a disposed on one thickness-wise side of the bonding layer 3, and a second porous resin layer 4b disposed on the other thickness-wise side of the bonding layer 3. The insulating layer 2 may further include a first base resin layer 5a disposed on one thickness-wise side of the first porous resin layer 4a and a second base resin layer 5b disposed on the other thickness-wise side of the second porous resin layer 4b. Specifically, as shown in FIG. 1 , the insulating layer 2 includes the bonding layer 3, the first porous resin layer 4a disposed on one thickness-wise surface of the bonding layer 3, the first base resin layer 5a disposed on one thickness-wise surface of the first porous resin layer 4a, the second porous resin layer 4b disposed on the other thickness-wise surface of the bonding layer 3, and the second base resin layer 5b disposed on the other thickness-wise surface of the second porous resin layer 4b.

[0034] Furthermore, the insulating layer 2 has the wiring portion 30 buried therein, as will be described in detail later.

[0035] As shown in FIGS. 2 and 4, the insulating layer 2 has a plurality of first through holes 41 that penetrate between the first ground conductor part 11 and the second ground conductor part 21 in the thickness direction.

[0036] The insulating layer 2 also has second through holes 42 that penetrate between the terminal portions and the wiring portions 30 in the thickness direction at both ends in the longitudinal direction.

[0037] The insulating layer 2 does not have the first through-hole 41 or the second through-hole 42 at the bent portion F.

[0038] [Bonding layer] The bonding layer 3 has a thickness. The bonding layer 3 has a substantially flat plate shape. The bonding layer 3 is an adhesive layer that bonds between layers, specifically, it bonds the first porous resin layer 4a and the second porous resin layer 4b.

[0039] 1, the bonding layer 3 is disposed on the other thickness-wise side of the first porous resin layer 4a and on one thickness-wise side of the second porous resin layer 4b. That is, the bonding layer 3 is disposed between the first porous resin layer 4a and the second porous resin layer 4b. More specifically, the bonding layer 3 is in contact with the other thickness-wise surface of the first porous resin layer 4a and in contact with one thickness-wise surface of the second porous resin layer 4b.

[0040] Furthermore, the bonding layer 3 buries the wiring portion 30, which will be described in detail later.

[0041] The material (or raw material) of the bonding layer 3 is not particularly limited as long as it is used as an insulating material in a wired circuit board. A preferred material for the bonding layer 3 is a low-dielectric material. A preferred material for the bonding layer 3 is a resin. Examples of the resin include polycarbonate resin, polyimide resin, fluorinated polyimide resin, epoxy resin, phenol resin, urea resin, melamine resin, diallyl phthalate resin, silicone resin, thermosetting urethane resin, fluororesin, polyolefin resin (e.g., cycloolefin polymer), and liquid crystal polymer. A preferred material is polyimide resin.

[0042] The bonding layer 3 is preferably a non-porous, solid, dense film in order to improve adhesion between layers, that is, the bonding layer 3 is preferably a non-porous polyimide resin layer.

[0043] The porosity of the bonding layer 3 is, for example, 0.5% or less, preferably 0.1% or less, and more preferably 0%.

[0044] The dielectric constant of the bonding layer 3 at a frequency of 100 GHz is, for example, 1.0 to 3.5, preferably 1.0 to 3.2, more preferably 1.0 to 3.0, still more preferably 1.0 to 2.7, and particularly preferably 1.0 to 2.5.

[0045] The dielectric constant of the bonding layer 3 at a frequency of 100 GHz is, for example, more than 1.0 and, for example, 3.5 or less, preferably 3.2 or less, more preferably 3.0 or less, even more preferably 2.7 or less, and particularly preferably 2.5 or less.

[0046] The dielectric constant of the bonding layer 3 is actually measured by a resonator method using a frequency of 100 GHz.

[0047] The dielectric loss tangent of the bonding layer 3 at a frequency of 100 GHz is in the range of, for example, 0 to 0.005, preferably 0 to 0.004, more preferably 0 to 0.003, and still more preferably 0 to 0.002.

[0048] The dielectric loss tangent of the bonding layer 3 at a frequency of 100 GHz is, for example, more than 0 and, for example, 0.005 or less, preferably 0.004 or less, more preferably 0.003 or less, even more preferably 0.002 or less, and particularly preferably less than 0.002.

[0049] The dielectric loss tangent of the bonding layer 3 is actually measured by a resonator method using a frequency of 100 GHz.

[0050] The thickness of the bonding layer 3 is, for example, 1 μm to 300 μm, preferably 10 μm to 200 μm, more preferably 20 μm to 150 μm, still more preferably 30 μm to 100 μm, and particularly preferably 40 μm to 80 μm.

[0051] The thickness of the bonding layer 3 is, for example, 1 μm or more, preferably 10 μm or more, more preferably 20 μm or more, even more preferably 30 μm or more, particularly preferably 40 μm or more, and, for example, 300 μm or less, preferably 200 μm or less, more preferably 150 μm or less, even more preferably 100 μm or less, particularly preferably 80 μm or less.

[0052] The thickness of the bonding layer 3 is the total thickness including the thickness of the embedded wiring portion 30. The thickness of the bonding layer 3 on one thickness side of the wiring portion 30 and the thickness of the bonding layer 3 on the other thickness side of the wiring portion 30 are not particularly limited and may be the same or different. Considering the manufacturing method, the thickness of the bonding layer 3 on one thickness side of the wiring portion 30 is preferably different from the thickness of the bonding layer 3 on the other thickness side of the wiring portion 30. The thickness of the bonding layer 3 on one thickness side of the wiring portion 30 is the distance from one thickness side of the wiring portion 30 to one thickness side of the bonding layer 3, and the thickness of the bonding layer 3 on the other thickness side of the wiring portion 30 is the distance from the other thickness side of the wiring portion 30 to the other thickness side of the bonding layer 3.

[0053] The thickness of the bonding layer 3 on one side in the thickness direction of the wiring portion 30 is, for example, 1 μm to 100 μm, preferably 3 μm to 70 μm, more preferably 5 μm to 50 μm, even more preferably 7 μm to 30 μm, and particularly preferably 8 μm to 20 μm.

[0054] The thickness of the bonding layer 3 on the other side in the thickness direction of the wiring portion 30 is, for example, 5 μm to 200 μm, preferably 10 μm to 100 μm, more preferably 15 μm to 70 μm, even more preferably 18 μm to 50 μm, and particularly preferably 20 μm to 40 μm.

[0055] The ratio of the thickness of the bonding layer 3 on one side of the wiring portion 30 in the thickness direction to the thickness of the bonding layer 3 on the other side of the wiring portion 30 in the thickness direction is, for example, 1.0 to 10, preferably 1.3 to 8.0, more preferably 1.5 to 6.0, even more preferably 1.8 to 4.0, and particularly preferably 2.0 to 3.0.

[0056] That is, the thickness of the bonding layer 3 on one side of the wiring portion 30 in the thickness direction is preferably thinner than the thickness of the bonding layer 3 on the other side of the wiring portion 30 in the thickness direction.

[0057] [First porous resin layer] The first porous resin layer 4a has a certain thickness. The first porous resin layer 4a has a substantially flat plate shape. The first porous resin layer 4a is a layer that reduces the dielectric constant of the flexible multilayer circuit board 1.

[0058] 1, the first porous resin layer 4a is disposed on one thickness-wise side of the bonding layer 3 and on the other thickness-wise side of the first base resin layer 5a. That is, the first porous resin layer 4a is disposed between the bonding layer 3 and the first base resin layer 5a. More specifically, the first porous resin layer 4a is in contact with one thickness-wise surface of the bonding layer 3 and the other thickness-wise surface of the first base resin layer 5a.

[0059] The thickness of the first porous resin layer 4a is, for example, 1 μm to 200 μm, preferably 5 μm to 100 μm, more preferably 10 μm to 70 μm, still more preferably 15 μm to 50 μm, and particularly preferably 20 μm to 40 μm.

[0060] The thickness of the first porous resin layer 4a is, for example, 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, even more preferably 15 μm or more, particularly preferably 20 μm or more, and for example, 200 μm or less, preferably 100 μm or less, more preferably 70 μm or less, even more preferably 50 μm or less, particularly preferably 40 μm or less.

[0061] [Second porous resin layer] The second porous resin layer 4b has a certain thickness. The second porous resin layer 4b has a substantially flat plate shape. The second porous resin layer 4b is a layer that reduces the dielectric constant of the flexible multilayer circuit board 1.

[0062] 1, the second porous resin layer 4b is disposed on the other thickness-wise side of the bonding layer 3 and on one thickness-wise side of the second base resin layer 5b. That is, the second porous resin layer 4b is disposed between the bonding layer 3 and the second base resin layer 5b. More specifically, the second porous resin layer 4b is in contact with the other thickness-wise surface of the bonding layer 3 and with one thickness-wise surface of the second base resin layer 5b.

[0063] The thickness of the second porous resin layer 4b is adjusted to be within the same range as the thickness of the first porous resin layer 4a.

[0064] [Materials and properties of the porous resin layer] In the following description, the first porous resin layer 4a and the second porous resin layer 4b will be collectively referred to as the porous resin layer 4. The materials and physical properties of the first porous resin layer 4a and the second porous resin layer 4b described below may be the same or different. Preferably, they are the same.

[0065] Examples of materials (or raw materials) for the porous resin layer 4 include resins. There are no particular limitations on the resins, as long as they are used as insulating materials in wiring circuit boards. Examples of resins include polycarbonate resins, polyimide resins, fluorinated polyimide resins, epoxy resins, phenolic resins, urea resins, melamine resins, diallyl phthalate resins, silicone resins, thermosetting urethane resins, fluororesins, and liquid crystal polymers, preferably polyimide resins and liquid crystal polymers, and more preferably polyimide resins.

[0066] The porous resin layer 4 is porous to reduce the dielectric constant. The porous resin layer 4 has, for example, closed cells and / or open cells. That is, the porous resin layer 4 is preferably a porous polyimide resin layer.

[0067] The porosity of the porous resin layer 4 is, for example, 50% to 100%, preferably 60% to 99%, more preferably 70% to 99%, and still more preferably 80% to 99%.

[0068] The porosity of the porous resin layer 4 is, for example, 50% or more, preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more, and for example, less than 100%, preferably 99% or less.

[0069] If the porosity of the porous resin layer 4 is equal to or greater than the above lower limit, the dielectric constant can be reduced. If the porosity of the porous resin layer 4 is equal to or less than the above upper limit, the strength of the porous resin layer 4 can be ensured.

[0070] When the material of the porous resin layer 4 is a polyimide resin, that is, when the porous resin layer 4 is a porous polyimide resin layer, the porosity of the porous resin layer 4 can be calculated based on the following formula. Dielectric constant of porous polyimide resin layer = Dielectric constant of air × Porosity + Dielectric constant of polyimide × (1 - Porosity)

[0071] Here, the dielectric constant of air is 1 and the dielectric constant of polyimide resin is 3.5, so by applying the above formula, the following can be expressed. Dielectric constant of porous polyimide resin layer = porosity + 3.5 (1 - porosity) Porosity (%) = [(3.5 - dielectric constant of porous polyimide resin layer) / 2.5] × 100

[0072] The dielectric constant of the porous resin layer 4 at a frequency of 100 GHz is, for example, more than 1.0 and, for example, 3.0 or less, preferably 2.5 or less, more preferably 2.2 or less, and even more preferably 2.0 or less.

[0073] The dielectric constant of the porous resin layer 4 is actually measured by a resonator method using a frequency of 100 GHz.

[0074] The dielectric loss tangent of the porous resin layer 4 at a frequency of 100 GHz is, for example, more than 0 and, for example, 0.0050 or less, preferably 0.0040 or less, more preferably 0.0030 or less, even more preferably 0.0025 or less, and particularly preferably 0.002 or less.

[0075] The dielectric loss tangent of the porous resin layer 4 is actually measured by a resonator method using a frequency of 100 GHz.

[0076] As described above, the porous resin layer 4 has a low dielectric constant at a frequency of 100 GHz, and furthermore, a low dielectric loss tangent at a frequency of 100 GHz. Therefore, transmission loss can be suppressed in a high frequency range such as a frequency of 100 GHz. More specifically, if the dielectric constant of the porous resin layer 4 at a frequency of 100 GHz is equal to or less than the upper limit value, and furthermore, the dielectric loss tangent of the porous resin layer 4 at a frequency of 100 GHz is equal to or less than the upper limit value, transmission loss can be suppressed in a high frequency range such as a frequency of 100 GHz.

[0077] The total thickness of the porous resin layer 4 (total thickness of the first porous resin layer 4a and the second porous resin layer 4b) is, for example, 2 μm to 1000 μm, preferably 10 μm to 600 μm, more preferably 20 μm to 300 μm, even more preferably 30 μm to 100 μm, and particularly preferably 40 μm to 80 μm.

[0078] The total thickness of the porous resin layer 4 (total thickness of the first porous resin layer 4a and the second porous resin layer 4b) is, for example, 2 μm or more, preferably 10 μm or more, more preferably 20 μm or more, even more preferably 30 μm or more, particularly preferably 40 μm or more, and for example, 1000 μm or less, preferably 600 μm or less, more preferably 300 μm or less, even more preferably 100 μm or less, particularly preferably 80 μm or less.

[0079] [First base resin layer] The first base resin layer 5a has a certain thickness. The first base resin layer 5a has a generally flat plate shape. The first base resin layer 5a is a layer that improves the adhesion between the first porous resin layer 4a and the first conductor layer 10.

[0080] 1, the first base resin layer 5a is disposed on one thickness-wise side of the first porous resin layer 4a and on the other thickness-wise side of the first conductor layer 10. In other words, the first base resin layer 5a is disposed between the first porous resin layer 4a and the first conductor layer 10. More specifically, the first base resin layer 5a is in contact with one thickness-wise surface of the first porous resin layer 4a and in contact with the other thickness-wise surface of the first conductor layer 10. The first base resin layer 5a is disposed as needed.

[0081] The thickness of the first undercoat resin layer 5a is, for example, 0.1 μm to 50 μm, preferably 0.5 μm to 20 μm, more preferably 1.0 μm to 10 μm, still more preferably 1.5 μm to 7.0 μm, and particularly preferably 2.0 μm to 5.0 μm.

[0082] The thickness of the first base resin layer 5a is, for example, 0.1 μm or more, preferably 0.5 μm or more, more preferably 1.0 μm or more, even more preferably 1.5 μm or more, particularly preferably 2.0 μm or more, and for example, 50 μm or less, preferably 20 μm or less, more preferably 10 μm or less, even more preferably 7.0 μm or less, particularly preferably 5.0 μm or less.

[0083] The ratio of the thickness of the first base resin layer 5a to the thickness of the first porous resin layer 4a is in the range of, for example, 0.01 to 0.5, preferably 0.05 to 0.3, or more preferably 0.08 to 0.2.

[0084] The ratio of the thickness of the first base resin layer 5a to the thickness of the first porous resin layer 4a is, for example, 0.01 or more, preferably 0.05 or more, more preferably 0.08 or more, and for example, 0.5 or less, preferably 0.3 or less, more preferably 0.2 or less.

[0085] In other words, the first base resin layer 5a is thinner than the first porous resin layer 4a.

[0086] When the ratio of the thickness of the first base resin layer 5a to the thickness of the first porous resin layer 4a is equal to or greater than the above-mentioned lower limit, it is possible to ensure sufficient adhesion between the first porous resin layer 4a and the first conductor layer 10. When the ratio of the thickness of the first base resin layer 5a to the thickness of the first porous resin layer 4a is equal to or less than the above-mentioned upper limit, it is possible to reduce the total thickness of the flexible multilayer circuit board 1 while lowering the dielectric constant.

[0087] [Second base resin layer] The second base resin layer 5b has a certain thickness. The second base resin layer 5b has a substantially flat plate shape. The second base resin layer 5b is a layer that improves the adhesion between the second porous resin layer 4b and the second conductor layer 20.

[0088] 1, the second base resin layer 5b is disposed on the other thickness-wise side of the second porous resin layer 4b and on one thickness-wise side of the second conductor layer 20. In other words, the second base resin layer 5b is disposed between the second porous resin layer 4b and the second conductor layer 20. More specifically, the second base resin layer 5b is in contact with the other thickness-wise surface of the second porous resin layer 4b and is in contact with one thickness-wise surface of the second conductor layer 20. The second base resin layer 5b is disposed as needed.

[0089] The thickness of the second undercoat resin layer 5b is, for example, 0.1 μm to 50 μm, preferably 0.5 μm to 20 μm, more preferably 1.0 μm to 10 μm, still more preferably 1.5 μm to 7.0 μm, and particularly preferably 2.0 μm to 5.0 μm.

[0090] The thickness of the second base resin layer 5b is, for example, 0.1 μm or more, preferably 0.5 μm or more, more preferably 1.0 μm or more, even more preferably 1.5 μm or more, particularly preferably 2.0 μm or more, and for example, 50 μm or less, preferably 20 μm or less, more preferably 10 μm or less, even more preferably 7.0 μm or less, particularly preferably 5.0 μm or less.

[0091] The ratio of the thickness of the second base resin layer 5b to the thickness of the second porous resin layer 4b is in the range of, for example, 0.01 to 0.5, preferably 0.05 to 0.3, or more preferably 0.08 to 0.2.

[0092] The ratio of the thickness of the second base resin layer 5b to the thickness of the second porous resin layer 4b is, for example, 0.01 or more, preferably 0.05 or more, more preferably 0.08 or more, and for example, 0.5 or less, preferably 0.3 or less, more preferably 0.2 or less.

[0093] In other words, the second base resin layer 5b is thinner than the second porous resin layer 4b.

[0094] When the ratio of the thickness of the second base resin layer 5b to the thickness of the second porous resin layer 4b is equal to or greater than the above-mentioned lower limit, it is possible to ensure sufficient adhesion between the second porous resin layer 4b and the second conductor layer 20. When the ratio of the thickness of the second base resin layer 5b to the thickness of the second porous resin layer 4b is equal to or less than the above-mentioned upper limit, it is possible to reduce the total thickness of the flexible multilayer circuit board 1 while lowering the dielectric constant.

[0095] [Materials and properties of the base resin layer] In the following description, the first resin base layer 5a and the second resin base layer 5b are collectively referred to as the resin base layer 5. The materials and physical properties of the first resin base layer 5a and the second resin base layer 5b described below may be the same or different. Preferably, they are the same.

[0096] The material (or raw material) of the resin base layer 5 may be a resin. The resin is not particularly limited as long as it is used as an insulating material in a wiring circuit board. Examples of the resin include polycarbonate resin, polyimide resin, fluorinated polyimide resin, epoxy resin, phenol resin, urea resin, melamine resin, diallyl phthalate resin, silicone resin, thermosetting urethane resin, fluororesin, and liquid crystal polymer, preferably polyimide resin and liquid crystal polymer, more preferably polyimide resin.

[0097] The base resin layer 5 is preferably a non-porous, solid, dense film in order to improve adhesion between layers, that is, the base resin layer 5 is preferably a non-porous polyimide resin layer.

[0098] The porosity of the base resin layer 5 is, for example, 0.5% or less, preferably 0.1% or less, and more preferably 0%.

[0099] The dielectric constant of the base resin layer 5 at a frequency of 100 GHz is in the range of, for example, 1.0 to 4.5, preferably 1.0 to 4.0, more preferably 1.0 to 3.5, and still more preferably 1.0 to 3.3.

[0100] The dielectric constant of the base resin layer 5 at a frequency of 100 GHz is, for example, more than 1.0 and, for example, 4.5 or less, preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.3 or less.

[0101] The dielectric constant of the base resin layer 5 is actually measured by a resonator method using a frequency of 100 GHz.

[0102] The dielectric loss tangent of the resin base layer 5 at a frequency of 100 GHz is, for example, 0 to 0.0050, preferably 0 to 0.0040, more preferably 0 to 0.0030, still more preferably 0 to 0.0025, and particularly preferably 0 to 0.0020.

[0103] The dielectric loss tangent of the base resin layer 5 at a frequency of 100 GHz is, for example, more than 0 and, for example, 0.0050 or less, preferably 0.0040 or less, more preferably 0.0030 or less, even more preferably 0.0025 or less, and particularly preferably 0.002 or less.

[0104] The dielectric loss tangent of the base resin layer 5 is actually measured by a resonator method using a frequency of 100 GHz.

[0105] The total thickness of the base resin layer 5 (total thickness of the first base resin layer 5a and the second base resin layer 5b) is, for example, 0.2 μm to 100 μm, preferably 1.0 μm to 40 μm, more preferably 2.0 μm to 20 μm, even more preferably 3.0 μm to 14 μm, and particularly preferably 4.0 μm to 10 μm.

[0106] The total thickness of the base resin layer 5 (total thickness of the first base resin layer 5a and the second base resin layer 5b) is, for example, 0.2 μm or more, preferably 1.0 μm or more, more preferably 2.0 μm or more, even more preferably 3.0 μm or more, particularly preferably 4.0 μm or more, and for example, 100 μm or less, preferably 40 μm or less, more preferably 20 μm or less, even more preferably 14 μm or less, particularly preferably 10 μm or less.

[0107] <First conductor layer> The first conductor layer 10 has a thickness. The first conductor layer 10 extends in the longitudinal direction.

[0108] 1, the first conductor layer 10 is disposed on one side in the thickness direction of the first base resin layer 5a. Specifically, the first conductor layer 10 is disposed on one surface in the thickness direction of the first base resin layer 5a. In other words, the first conductor layer 10 is in contact with one surface in the thickness direction of the first base resin layer 5a.

[0109] The first conductor layer 10 is disposed opposite the second conductor layer 20 so as to overlap the wiring portion 30 when projected in the thickness direction. In other words, the first conductor layer 10 and the second conductor layer 20 are disposed opposite each other so as to overlap the wiring portion 30 when projected in the thickness direction.

[0110] The first conductor layer 10 has a first ground conductor portion 11 and, if necessary, a first terminal portion 12. Preferably, the first conductor layer 10 has the first ground conductor portion 11 and the first terminal portion 12. Specifically, as shown in FIG. 1 , the first conductor layer 10 has the first ground conductor portion 11 and further has the first terminal portion 12 at one end in the longitudinal direction.

[0111] The material of the first conductor layer 10 is not particularly limited as long as it is a conductor material commonly used in wired circuit boards. Examples of materials for the first conductor layer 10 include copper, iron, silver, gold, aluminum, nickel, and alloys thereof (e.g., stainless steel and bronze). Copper is preferred.

[0112] [First ground conductor] The first ground conductor 11 earths a weak current that may affect the first terminal 12. The weak current includes, for example, a current of less than 1A.

[0113] 1, the first ground conductor 11 is disposed on one thickness-wise side of the first base resin layer 5a. Specifically, the first ground conductor 11 is disposed on one thickness-wise surface of the first base resin layer 5a. In other words, the first ground conductor 11 is in contact with one thickness-wise surface of the first base resin layer 5a.

[0114] The first ground conductor 11 extends across the entire width of the flexible multilayer circuit board 1.

[0115] 4, at one longitudinal end, the center of the width of the first ground conductor part 11 is cut out toward the other longitudinal end so that the first terminal part 12 can be disposed therein. Specifically, although not shown, at one longitudinal end, the first ground conductor part 11 has a plurality of the above-mentioned cutouts spaced apart in the width direction.

[0116] Although not shown, the first ground conductor portion 11 is connected to a ground member.

[0117] [1st terminal part] The first terminal 12 receives and transmits signals via a signal terminal (not shown). The signals include, for example, differential signals. The signals include, for example, a small current of less than 1 A.

[0118] 1, the first terminal 12 is disposed on one side in the thickness direction of the first base resin layer 5a. Specifically, the first terminal 12 is disposed on one surface in the thickness direction of the first base resin layer 5a. In other words, the first terminal 12 is in contact with one surface in the thickness direction of the first base resin layer 5a.

[0119] The first terminal 12 is disposed, for example, at one end in the longitudinal direction. However, the first terminal 12 does not necessarily have to be disposed at the other end in the longitudinal direction where the second terminal 22 is disposed.

[0120] As shown in FIG. 4, the first terminal portion 12 is disposed opposite the wiring portion 30.

[0121] The first terminal portion 12 is disposed in a notched portion of the first ground conductor portion 11 in the width direction. That is, the first terminal portion 12 is disposed between the two first ground conductor portions 11 in the width direction with a gap therebetween.

[0122] Although not shown, a plurality of first terminal portions 12 are arranged at one longitudinal end at intervals in the width direction. Specifically, the first ground conductor portions 11 and the first terminal portions 12 are arranged alternately at intervals in the width direction at one longitudinal end.

[0123] The thickness of the first terminal portion 12 may be the same as or different from a first thickness T1 of the first ground conductor portion 11, which will be described later. Preferably, they are the same.

[0124] <Second conductor layer> The second conductor layer 20 has a thickness and extends in the longitudinal direction.

[0125] 1, the second conductor layer 20 is disposed on the other thickness-wise side of the second base resin layer 5b. Specifically, the second conductor layer 20 is disposed on the other thickness-wise surface of the second base resin layer 5b. In other words, the second conductor layer 20 is in contact with the other thickness-wise surface of the second base resin layer 5b.

[0126] The second conductor layer 20 is disposed opposite the first conductor layer 10 so as to overlap the wiring portion 30 when projected in the thickness direction. In other words, the first conductor layer 10 and the second conductor layer 20 are disposed opposite each other so as to overlap the wiring portion 30 when projected in the thickness direction.

[0127] The second conductor layer 20 has a second ground conductor portion 21 and, if necessary, a second terminal portion 22. Preferably, the second conductor layer 20 has both the second ground conductor portion 21 and the second terminal portion 22. Specifically, as shown in FIG. 1 , the second conductor layer 20 has the second ground conductor portion 21 and further has the second terminal portion 22 at the other end in the longitudinal direction.

[0128] The material of the second conductor layer 20 may be the same as the material of the first conductor layer 10 described above.

[0129] [Second ground conductor] The second ground conductor 21 earths a weak current that may affect the second terminal 22. The weak current includes, for example, a current of less than 1A.

[0130] 1, the second ground conductor 21 is disposed on the other thickness-wise side of the second base resin layer 5b. Specifically, the second ground conductor 21 is disposed on the other thickness-wise surface of the second base resin layer 5b. In other words, the second ground conductor 21 is in contact with the other thickness-wise surface of the second base resin layer 5b.

[0131] The second ground conductor 21 extends across the entire width of the flexible multilayer circuit board 1.

[0132] Although not shown, at the other longitudinal end, the second ground conductor 21 has a notch cut out in the center in the width direction toward the one longitudinal end to accommodate the second terminal 22. Specifically, although not shown, at the other longitudinal end, the second ground conductor 21 has a plurality of the above-mentioned notches spaced apart in the width direction.

[0133] Although not shown, the second ground conductor portion 21 is connected to a ground member.

[0134] [Second terminal part] The second terminal 22 receives and transmits signals via a signal terminal (not shown). The signals include, for example, differential signals. The signals include, for example, a small current of less than 1 A.

[0135] 1, the second terminal 22 is disposed on the other thickness-wise side of the second base resin layer 5b. Specifically, the second terminal 22 is disposed on the other thickness-wise surface of the second base resin layer 5b. In other words, the second terminal 22 is in contact with the other thickness-wise surface of the second base resin layer 5b.

[0136] The second terminal 22 is disposed, for example, at the other end in the longitudinal direction. Note that the second terminal 22 does not necessarily have to be disposed at the one end in the longitudinal direction where the first terminal 12 is disposed.

[0137] Although not shown, the second terminal portion 22 is disposed opposite the wiring portion 30 .

[0138] The second terminal portion 22 is disposed in a notched portion of the second ground conductor portion 21 in the width direction. That is, the second terminal portion 22 is disposed between the two second ground conductor portions 21 in the width direction with a gap therebetween.

[0139] Although not shown, a plurality of second terminal portions 22 are arranged at the other longitudinal end at intervals in the width direction. Specifically, the second ground conductor portions 21 and the second terminal portions 22 are arranged alternately at intervals in the width direction at the other longitudinal end.

[0140] The thickness of the second terminal portion 22 may be the same as or different from a first thickness T1 of the second ground conductor portion 21, which will be described later. Preferably, they are the same.

[0141] [Configuration of the folding part] 1, the flexible multilayer circuit board 1 has a bent portion F in the middle of its lengthwise direction, which is perpendicular to the thickness direction. The bent portion F is a portion where at least one of the first ground conductor 11 and the second ground conductor 21 has a different thickness from the remaining portions of the first ground conductor 11 and the second ground conductor 21 other than the bent portion F, in order to make it easier to bend the flexible multilayer circuit board 1.

[0142] In areas other than the bent portions F, the first ground conductor portion 11 and the second ground conductor portion 21 have a first thickness T1.

[0143] At the bent portion F, at least one of the first ground conductor portion 11 and the second ground conductor portion 21 has a second thickness T2.

[0144] The first thickness T1 and the second thickness T2 of the first ground conductor portion 11 are the thickness-wise distances between one thickness-wise surface of the first base resin layer 5a and one thickness-wise surface of the first ground conductor portion 11, and the first thickness T1 and the second thickness T2 of the second ground conductor portion 21 are the thickness-wise distances between the other thickness-wise surface of the second base resin layer 5b and the other thickness-wise surface of the second ground conductor portion 21.

[0145] One longitudinal end of the bent portion F is the side surface of one longitudinal end of the portion of the first ground conductor portion 11 and the second ground conductor portion 21 that has the second thickness T2 and is closest to one end in the longitudinal direction. The other longitudinal end of the bent portion F is the side surface of the other longitudinal end of the portion of the first ground conductor portion 11 and the second ground conductor portion 21 that has the second thickness T2 and is closest to the other end in the longitudinal direction.

[0146] If at least one of the first ground conductor portion 11 and the second ground conductor portion 21 has the second thickness T2, the flexibility in the thickness direction at the bent portion F can be improved.

[0147] At the bent portion F, each of the first ground conductor portion 11 and the second ground conductor portion 21 preferably has a second thickness T2.

[0148] When the first ground conductor 11 and the second ground conductor 21 each have the second thickness T2, the bending portion F has excellent flexibility in both sides in the thickness direction.

[0149] In addition, at the bent portion F, it is more preferable that each of the first ground conductor portion 11 and the second ground conductor portion 21 has the second thickness T2, and at least one of the first ground conductor portion 11 and the second ground conductor portion 21 also has the first thickness T1. It is even more preferable that each of the first ground conductor portion 11 and the second ground conductor portion 21 has the first thickness T1 and the second thickness T2.

[0150] If the first ground conductor part 11 and the second ground conductor part 21 have the first thickness T1 and the second thickness T2, respectively, appropriate rigidity can be ensured at the bending part F, and the first ground conductor part 11 and the second ground conductor part 21 can be bent at a larger angle to both sides in the thickness direction.

[0151] Specifically, as shown in FIG. 1, at the bent portion F, each of the first ground conductor portion 11 and the second ground conductor portion 21 has a first thickness T1 and a second thickness T2 alternately arranged along the longitudinal direction.

[0152] If the first ground conductor 11 and the second ground conductor 21 each have the first thickness T1 and the second thickness T2 alternating along the longitudinal direction, the bending portion F has even better flexibility in both thickness directions.

[0153] When projected in the thickness direction, the first thickness T1 of the first ground conductor portion 11 overlaps with the first thickness T1 of the second ground conductor portion 21, and the second thickness T2 of the first ground conductor portion 11 overlaps with the second thickness T2 of the second ground conductor portion 21.

[0154] In other words, the first thickness T1 of the first ground conductor portion 11 is located on the opposite side of the insulating layer 2 in the thickness direction from the first thickness T1 of the second ground conductor portion 21, and the second thickness T2 of the first ground conductor portion 11 is located on the opposite side of the insulating layer 2 in the thickness direction from the second thickness T2 of the second ground conductor portion 21.

[0155] If the first thickness T1 of the first ground conductor portion 11 is located on the opposite side of the insulating layer 2 in the thickness direction from the first thickness T1 of the second ground conductor portion 21, and the second thickness T2 of the first ground conductor portion 11 is located on the opposite side of the insulating layer 2 in the thickness direction from the second thickness T2 of the second ground conductor portion 21, then the rigidity of the flexible multilayer circuit board 1 can be reduced at the bending portion F, and the flexibility in both directions in the thickness direction can be further improved.

[0156] The first thickness T1 is, for example, 1 μm to 300 μm, preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, even more preferably 15 μm to 70 μm, particularly preferably 20 μm to 50 μm, and most preferably 25 μm to 35 μm.

[0157] The first thickness T1 is, for example, 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, even more preferably 15 μm or more, particularly preferably 20 μm or more, and most preferably 25 μm or more, and for example, 300 μm or less, preferably 200 μm or less, more preferably 100 μm or less, even more preferably 70 μm or less, particularly preferably 50 μm or less, and most preferably 35 μm or less.

[0158] The second thickness T2 is, for example, 0.5 μm to 100 μm, preferably 1 μm to 50 μm, more preferably 3 μm to 30 μm, still more preferably 5 μm to 20 μm, and particularly preferably 7 μm to 15 μm.

[0159] The second thickness T2 is, for example, 0.5 μm or more, preferably 1 μm or more, more preferably 3 μm or more, even more preferably 5 μm or more, particularly preferably 7 μm or more, and for example, 100 μm or less, preferably 50 μm or less, more preferably 30 μm or less, even more preferably 20 μm or less, particularly preferably 15 μm or less.

[0160] The ratio (T1 / T2) of the first thickness T1 to the second thickness T2 is, for example, 1.0 to 10, preferably 1.2 to 8.0, more preferably 1.5 to 6.0, still more preferably 1.8 to 5.0, and particularly preferably 2.0 to 4.0.

[0161] The ratio of T1 to the second thickness T2 (T1 / T2) is, for example, more than 1.0, preferably 1.2 or more, more preferably 1.5 or more, even more preferably 1.8 or more, particularly preferably 2.0 or more, and is, for example, 10 or less, preferably 8.0 or less, more preferably 6.0 or less, even more preferably 5.0 or less, particularly preferably 4.0 or less.

[0162] The difference (T1-T2) between the first thickness T1 and the second thickness T2 is, for example, 1 μm to 100 μm, preferably 5 μm to 70 μm, more preferably 10 μm to 50 μm, still more preferably 13 μm to 40 μm, and particularly preferably 15 μm to 30 μm.

[0163] The difference (T1-T2) between the first thickness T1 and the second thickness T2 is, for example, 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, even more preferably 13 μm or more, particularly preferably 15 μm or more, and for example, 100 μm or less, preferably 70 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less, particularly preferably 30 μm or less.

[0164] In other words, the second thickness T2 is thinner than the first thickness T1.

[0165] If the second thickness T2 is thinner than the first thickness T1, the flexibility in the thickness direction is excellent. Specifically, if the ratio of the first thickness T1 to the second thickness T2 (T1 / T2) is equal to or greater than the lower limit (exceeds), the flexibility in the thickness direction is excellent. Furthermore, if the difference between the first thickness T1 and the second thickness T2 (T1-T2) is equal to or greater than the lower limit, the flexibility in the thickness direction is excellent.

[0166] The first thickness T1 of the first ground conductor portion 11 and the second thickness T2 of the second ground conductor portion 21 may be the same or different as long as they are within the above-mentioned range. Preferably, they are the same. The second thickness T2 of the first ground conductor portion 11 and the second thickness T2 of the second ground conductor portion 21 may be the same or different as long as they are within the above-mentioned range. Preferably, they are the same.

[0167] When the first ground conductor part 11 and the second ground conductor part 21 have the first thickness T1 and the second thickness T2 alternately along the longitudinal direction, the respective pitches (the distance between adjacent portions having the first thickness T1 in the longitudinal direction and the distance between adjacent portions having the second thickness T2 in the longitudinal direction) are, for example, 1 μm to 200 μm, preferably 10 μm to 100 μm, more preferably 20 μm to 80 μm, and even more preferably 30 μm to 60 μm.

[0168] The pitches may be uniform or irregular.

[0169] The proportion of the bent portion F in the entire length of the flexible multilayer circuit board 1 in the longitudinal direction is, for example, 5% to 30%.

[0170] <Wiring section> The wiring section 30 is electrically connected to the terminal section and transmits signals from the terminal section.

[0171] The wiring portion 30 has a thickness and extends in the longitudinal direction.

[0172] Although not shown, a plurality of wiring portions 30 are arranged at intervals in the width direction.

[0173] The wiring portion 30 is embedded in the insulating layer 2 (specifically, the bonding layer 3). Specifically, both surfaces in the thickness direction and the side surfaces of the wiring portion 30 (in other words, the entire outer peripheral surface) are in contact with the bonding layer 3.

[0174] The wiring portion 30 may be made of the same material as the first conductor layer 10.

[0175] The thickness of the wiring portion 30 is, for example, 1 μm to 100 μm, preferably 5 μm to 70 μm, more preferably 10 μm to 50 μm, still more preferably 13 μm to 30 μm, and particularly preferably 15 μm to 20 μm.

[0176] The thickness of the wiring portion 30 is, for example, 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, even more preferably 13 μm or more, particularly preferably 15 μm or more, and, for example, 100 μm or less, preferably 70 μm or less, more preferably 50 μm or less, even more preferably 30 μm or less, particularly preferably 20 μm or less.

[0177] <First via connection part> The first via connection portion 51 is filled into the plurality of first through holes 41. That is, a plurality of first via connection portions 51 are provided.

[0178] The first via connection portions 51 are in contact with the first ground conductor portion 11 and the second ground conductor portion 21 so as to electrically connect the first ground conductor portion 11 and the second ground conductor portion 21. Specifically, one thickness-wise end of each of the first via connection portions 51 is in contact with the other thickness-wise surface of the first ground conductor portion 11, and the other thickness-wise end of each of the first via connection portions 51 is in contact with the one thickness-wise surface of the second ground conductor portion 21. Furthermore, the peripheral side surfaces of each of the first via connection portions 51 are in contact with the insulating layer 2. As a result, the first ground conductor portion 11 and the second ground conductor portion 21 are electrically connected via the first via connection portions 51.

[0179] As shown in FIGS. 2 and 4, the first via connection portions 51 are arranged in pairs, two of which are arranged side by side in the width direction, with the wiring portion located therebetween.

[0180] That is, as shown in FIG. 2, the first ground conductor portion 11, the second ground conductor portion 21, and the first via connection portion 51 form a ground path having a substantially square shape.

[0181] The first conductor layer 10 and the second conductor layer 20 are arranged opposite each other so as to overlap with the wiring portion 30 when projected in the thickness direction, and the pair of first via connection portions 51 are arranged so that the wiring portion 30 is located between them, thereby suppressing the occurrence of dielectric loss in the wiring portion 30.

[0182] 4, the first ground conductor 11, the second ground conductor 21, and the first via connection 51 form a substantially U-shaped ground path at one longitudinal end. When viewed in a cross section from the width direction, the ground path opens toward one side in the thickness direction. On the other hand, although not shown, the first ground conductor 11, the second ground conductor 21, and the first via connection 51 form a substantially inverted U-shaped ground path at the other longitudinal end. In this case, when viewed in a cross section from the width direction, the ground path opens toward the other side in the thickness direction.

[0183] Although not shown, a plurality of first via connection portions 51 are arranged at intervals in the width direction. Specifically, the plurality of wiring portions 30 and the plurality of first via connection portions 51 are arranged alternately at intervals in the width direction.

[0184] Although not shown, a plurality of pairs of first via connection portions 51 are arranged at intervals in the longitudinal direction.

[0185] As shown in FIG. 3, the first via connection portion 51 is not arranged at the bent portion F.

[0186] At the bent portion F, the first via connection portion 51 is not arranged, so that the bent portion F can be easily bent in the thickness direction.

[0187] The first via connection portion 51 is formed by filling the first through hole 41 with a material for the first via connection portion 51 .

[0188] The material of the first via connection portion 51 may be the same as the material of the first conductor layer 10.

[0189] <Second via connection> The second via connection portion 52 is filled in the second through hole 42 .

[0190] The second via connection portion 52 contacts the terminal portion and the wiring portion 30 so as to electrically connect the terminal portion and the wiring portion 30. Specifically, as shown in FIG. 1 , the second via connection portion 52 contacts the first terminal portion 12 and the wiring portion 30 at one longitudinal end, and contacts the second terminal portion 22 and the wiring portion 30 at the other longitudinal end. More specifically, at one longitudinal end, one thickness direction end of the second via connection portion 52 contacts the other thickness direction surface of the first terminal portion 12, and the other thickness direction end of the second via connection portion 52 contacts one thickness direction surface of the wiring portion 30. Furthermore, at the other longitudinal end, the other thickness direction end of the second via connection portion 52 contacts one thickness direction surface of the second terminal portion 22, and the one thickness direction end of the second via connection portion 52 contacts the other thickness direction surface of the wiring portion 30. Furthermore, the peripheral side surface of the second via connection portion 52 is in contact with the insulating layer 2. As a result, the terminal portion and the wiring portion 30 are electrically connected via the second via connection portion 52.

[0191] The second via connection portions 52 are disposed at both ends in the longitudinal direction, and the second via connection portions 52 are not disposed at any other ends in the longitudinal direction.

[0192] As shown in FIG. 4, one second via connection portion 52 is disposed between a pair of first via connection portions 51 in the width direction.

[0193] That is, the terminal portion, the wiring portion 30, and the second via connection portion 52 form a substantially I-shaped signal path at each of both ends in the longitudinal direction.

[0194] Although not shown, a plurality of second via connection portions 52 are arranged at intervals in the width direction at both ends in the longitudinal direction. Specifically, one second via connection portion 52 is arranged corresponding to one wiring portion 30 at each end in the longitudinal direction.

[0195] The second via connection portion 52 is formed by filling the second through hole 42 with the material of the second via connection portion 52 .

[0196] The second via connection portion 52 may be made of the same material as the first conductor layer 10.

[0197] <Cover insulating layer> The cover insulating layer 60 is a layer for protecting the surface of the flexible multilayer circuit board 1.

[0198] 1, the cover insulating layer 60 is disposed on one side in the thickness direction of the first conductor layer 10 and on the other side in the thickness direction of the second conductor layer 20. That is, the cover insulating layer has a first cover insulating layer 61 disposed on one side in the thickness direction of the first conductor layer 10 and a second cover insulating layer 62 disposed on the other side in the thickness direction of the second conductor layer 20.

[0199] The first insulating cover layer 61 contacts one surface of the first conductor layer 10 in the thickness direction. The second insulating cover layer 62 contacts the other surface of the second conductor layer 20 in the thickness direction.

[0200] At least a part of the terminal portion is exposed from the insulating cover layer 60. That is, the insulating cover layer 60 covers at least the first conductor layer 10 and the second conductor layer 20 other than the terminal portion.

[0201] The cover insulating layer 60 may be made of the same resin as the material of the first porous resin layer 4a, for example.

[0202] The thickness of the insulating cover layer 60 is, for example, 1 μm to 100 μm, preferably 3 μm to 70 μm, more preferably 5 μm to 50 μm, still more preferably 7 μm to 40 μm, and particularly preferably 9 μm to 30 μm.

[0203] The thickness of the cover insulating layer 60 is, for example, 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, even more preferably 7 μm or more, particularly preferably 9 μm or more, and for example, 100 μm or less, preferably 70 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less, particularly preferably 30 μm or less.

[0204] <Reinforcing substrate> The reinforcing base material 70 is a base material that reinforces the flexible multilayer circuit board 1. The reinforcing base material 70 has a substantially flat plate shape. The reinforcing base material 70 is disposed on each of both ends in the longitudinal direction.

[0205] By providing the reinforcing base material 70, damage to the flexible multilayer circuit board 1 at the terminal portion can be suppressed.

[0206] The reinforcing substrate 70 is disposed at each of both longitudinal ends on one thickness-wise side of the first conductor layer 10 that does not have a terminal portion, or on the other thickness-wise side of the second conductor layer 20 that does not have a terminal portion. Specifically, as shown in Fig. 1 , the reinforcing substrate 70 has a first reinforcing substrate 71 that is disposed at the other longitudinal end on one thickness-wise side of the first insulating cover layer 61, and a second reinforcing substrate 72 that is disposed at one longitudinal end on the other thickness-wise side of the second insulating cover layer 62.

[0207] The other longitudinal end of the first reinforcing base material 71 contacts one thickness direction surface of the first insulating cover layer 61. The other longitudinal end of the second reinforcing base material 72 contacts the other thickness direction surface of the second insulating cover layer 62.

[0208] The material of the reinforcing substrate 70 is not particularly limited. Examples of the material of the reinforcing substrate 70 include metals and hard resins. Metals are preferred. Examples of metals include stainless steel, copper, iron, and aluminum.

[0209] The thickness of the reinforcing base material 70 is not particularly limited.

[0210] <Adhesive layer> Although not shown, the flexible multilayer circuit board 1 may have adhesive layers between the above-mentioned layers. Specifically, adhesive layers may be provided between the first conductor layer 10 and the first insulating cover layer 61, between the second conductor layer 20 and the second insulating cover layer 62, between the first insulating cover layer 61 and the first reinforcing substrate 71, and between the second insulating cover layer 62 and the second reinforcing substrate 72.

[0211] The material (or raw material) of the adhesive layer is not particularly limited as long as it is a material that is commonly used for adhesive layers in wired circuit boards. Examples of the material (or raw material) of the adhesive layer include resins.

[0212] The thickness of the adhesive layer is not particularly limited.

[0213] 2. Manufacturing method for flexible multilayer circuit boards A method for manufacturing flexible multilayer circuit board 1 will be described with reference to FIGS. 5A to 7C.

[0214] (First porous laminate preparation step) As shown in FIG. 5A, first, a first porous laminate 100a is prepared.

[0215] The first porous laminate 100a includes a conductor layer on both sides in the thickness direction. Specifically, the first porous laminate 100a includes a first base conductor layer 101a, a first insulating layer 102a, and a third conductor layer 106, arranged in this order toward the other side in the thickness direction. The first insulating layer 102a includes a first porous resin layer 104a and a first bonding layer 103a, arranged in this order toward the other side in the thickness direction, and optionally further includes a first base resin layer 105a arranged on one side in the thickness direction of the first porous resin layer 104a.

[0216] The first underlying conductor layer 101a and the third conductor layer 106 in the first porous laminate 100a have not yet been patterned, and are not the first conductor layer 10 and the wiring portion 30 in the flexible multilayer circuit board 1 shown in Figures 1 to 4.

[0217] A method for preparing the first porous laminate 100a is described in, for example, Japanese Patent Application Laid-Open No. 2019-123851.

[0218] (Second through hole formation process) 5B, a second through hole 142 is then formed. Specifically, the second through hole 142 is formed to penetrate the first underlying conductor layer 101a and the first insulating layer 102a. The second through hole 142 exposes one surface of the third conductor layer 106 in the thickness direction.

[0219] The second through-holes 142 can be formed, for example, by drilling. Examples of drilling methods include laser processing, drilling, and blasting. Laser processing is preferred.

[0220] (Third conductor layer patterning process) 5C, the third conductor layer 106 is then patterned to form a wiring portion 130 and an interlayer connection portion 131. The second through-hole 142 is formed on one thickness-wise side of the wiring portion 130. That is, one thickness-wise surface of the wiring portion 130 is exposed. Examples of patterning the third conductor layer 106 include wet etching and dry etching, and preferably wet etching.

[0221] (Second porous laminate preparation step) As shown in FIG. 5D, the second porous laminate 100b is prepared on the other side in the thickness direction of the wiring portion 130 and the interlayer connection portion 131 of the first porous laminate 100a.

[0222] The second porous laminate 100b has a conductor layer only on the other surface in the thickness direction. Specifically, the second porous laminate 100b has a second base conductor layer 101b and a second insulating layer 102b, arranged in this order toward one side in the thickness direction. The second insulating layer 102b has a second porous resin layer 104b and a second bonding layer 103b, arranged in this order toward one side in the thickness direction, and optionally further has a second base resin layer 105b arranged on the other side in the thickness direction of the second porous resin layer 104b.

[0223] The method for preparing the second porous laminate 100b is described in, for example, Japanese Patent Application Laid-Open No. 2019-123851.

[0224] (Second porous laminate bonding step) As shown in FIG. 6A, one thickness-wise surface of the second bonding layer 103b of the second porous laminate 100b is bonded to the other thickness-wise side of the wiring portion 130 and the interlayer connection portion 131 of the first porous laminate 100a. At this time, a press (not shown) capable of pressing in the thickness direction is used. The pressing pressure is not limited and is, for example, 0.5 MPa to 10 MPa, preferably 3 MPa to 10 MPa. The pressing time is, for example, 1 minute to 120 minutes, preferably 10 minutes to 120 minutes. The pressing may be a heat press. The pressing temperature is, for example, 80°C to 300°C, preferably 120°C to 300°C.

[0225] That is, second bonding layer 103b contacts the other thickness-wise surface of first bonding layer 103a while covering the other thickness-wise surface and the entire outer peripheral surface of wiring portion 130 and interlayer connection portion 131. Specifically, first bonding layer 103a and second bonding layer 103b deform and enter between adjacent wiring portion 130 and interlayer connection portion 131. As a result, first bonding layer 103a and second bonding layer 103b may be integrated to form bonding layer 103. That is, the interface between first bonding layer 103a and second bonding layer 103b does not need to be observed.

[0226] The wiring portion 130 and the interlayer connection portion 131 are embedded in the first bonding layer 103a of the first porous laminate 100a and the second bonding layer 103b of the second porous laminate 100b.

[0227] As a result, a porous laminate is obtained which includes the second porous laminate 100b and the first porous laminate 100a in this order towards one side in the thickness direction.

[0228] The porous laminate comprises, in order toward one side in the thickness direction, a second underlying conductor layer 101b, a second underlying resin layer 105b, a second porous resin layer 104b, a bonding layer 103, a first porous resin layer 104a, a first underlying resin layer 105a, and a first underlying conductor layer 101a, and the bonding layer 103 has a wiring portion 130 and an interlayer connection portion 131 embedded therein.

[0229] (First through hole formation process) 6B, first through holes 141 are formed. Specifically, a first through hole 141 that penetrates the first underlying conductor layer 101a and the first insulating layer 102a is formed on one side of the interlayer connection portion 131 in the thickness direction, and a first through hole 141 that penetrates the second underlying conductor layer 101b and the second insulating layer 102b is formed on the other side of the interlayer connection portion 131 in the thickness direction.

[0230] The first through-hole 141 may be formed by the same method as the above-mentioned method for forming the second through-hole 142.

[0231] (Plating layer formation process) 6C , a first plating layer 107a is formed on the inner circumferential surfaces of the first through holes 141 and 142 and one thickness-wise surface of the first underlying conductor layer 101a, and a second plating layer 107b is formed on the inner circumferential surface of the first through holes 141 and the other thickness-wise surface of the second underlying conductor layer 101b. The combination of the first underlying conductor layer 101a and the first plating layer 107a formed on one thickness-wise surface of the first insulating layer 102a corresponds to the first conductor layer 110. The combination of the second underlying conductor layer 101b and the second plating layer 107b formed on one thickness-wise surface of the second insulating layer 102b corresponds to the second conductor layer 120.

[0232] That is, the thickness of the first conductor layer 110 is the total thickness of the first underlying conductor layer 101a and the first plating layer 107a. Similarly, the thickness of the second conductor layer 120 is the total thickness of the second underlying conductor layer 101b and the second plating layer 107b.

[0233] Furthermore, first plating layer 107a formed on the inner circumferential surface of second through hole 142 on one thickness direction side of wiring portion 130 corresponds to second via connection portion 152. Furthermore, first plating layer 107a formed on the inner circumferential surface of first through hole 141 on one thickness direction side of interlayer connection portion 131, interlayer connection portion 131, and second plating layer 107b formed on the inner circumferential surface of first through hole 141 on the other thickness direction side of interlayer connection portion 131 collectively correspond to first via connection portion 151.

[0234] (Ground conductor and terminal formation process) 6D, the first conductor layer 110 and the second conductor layer 120 are patterned. Examples of methods for patterning the first conductor layer 110 and the second conductor layer 120 include wet etching and dry etching, and preferably wet etching.

[0235] The first conductor layer 110 is patterned to form a first ground conductor portion 111 and a first terminal portion 112. The second conductor layer 120 is patterned to form a second ground conductor portion 121 and a second terminal portion (not shown).

[0236] In other words, the total thickness of the first underlying conductor layer 101a and the first plating layer 107a is the first thickness T1 of the first ground conductor portion 111, which is also the thickness of the first terminal portion 112. Similarly, the total thickness of the second underlying conductor layer 101b and the second plating layer 107b is the first thickness T1 of the second ground conductor portion 121, which is also the thickness of the second terminal portion.

[0237] (Second thickness T2 formation process) 7A , at least one of the first ground conductor portion 111 and the second ground conductor portion 121 is patterned at the bent portion F. The first ground conductor portion 111 and the second ground conductor portion 121 can be patterned by, for example, wet etching or dry etching, preferably wet etching. Note that the etching of the first ground conductor portion 111 and the second ground conductor portion 121 is light etching (soft etching), and only a portion of the first ground conductor portion 111 and the second ground conductor portion 121 is removed in the thickness direction, rather than the entire portion.

[0238] By etching, at least one of the first ground conductor part 111 and the second ground conductor part 121 has a second thickness T2 at the bent portion F. Note that the second thickness T2 is the total thickness of the first underlying conductor layer 101a and the first plating layer 107a or the total thickness of the second underlying conductor layer 101b and the second plating layer 107b after etching. When the first plating layer 107a or the second plating layer 107b is completely removed by etching, the second thickness T2 is the thickness of the first underlying conductor layer 101a or the second underlying conductor layer 101b after etching.

[0239] (Cover insulating layer lamination process) As shown in FIG. 7B, the cover insulating layer 160 is attached to one surface of the first conductor layer 110 in the thickness direction and the other surface of the second conductor layer 120 in the thickness direction.

[0240] Specifically, the first insulating cover layer 161 is attached to one surface in the thickness direction of the first conductor layer 110. The second insulating cover layer 162 is attached to the other surface in the thickness direction of the second conductor layer 120.

[0241] The above-mentioned press machine is used for lamination, and the pressing conditions are the same as those described above.

[0242] When the cover insulating layer 160 is bonded as described above, the first cover insulating layer 161 is inserted between the adjacent first ground conductor 111 and first terminal 112. The first cover insulating layer 161 is in contact with one thickness-wise surface of the first base resin layer 105a that is exposed from the first ground conductor 111 and the first terminal 112. The second cover insulating layer 162 is inserted between the adjacent second ground conductor 121 and the second terminal 122. The second cover insulating layer 162 is in contact with the other thickness-wise surface of the second base resin layer 105b that is exposed from the second ground conductor 121 and the second terminal 122.

[0243] (Reinforcing substrate bonding process) 7C, when providing a reinforcing substrate 170 on the flexible multilayer circuit board 1, the reinforcing substrate 170 is bonded to the insulating cover layer 160. The above-mentioned press machine is used to bond the reinforcing substrate 170. The pressing conditions are the same as those described above.

[0244] In this way, the flexible multilayer circuit board 1 is manufactured.

[0245] (Action and effect) The flexible multilayer circuit board 1 of the present invention has a bent portion F in the longitudinal middle portion, and at least one of the first ground conductor portion 11 and the second ground conductor portion 21 has a second thickness T2 that is thinner than the first thickness T1 at the bent portion F. This provides excellent flexibility.

[0246] 3. Variations In the following modifications, the same components and steps as those in the above-described embodiment are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. Furthermore, each modification can achieve the same effects as those in the above-described embodiment unless otherwise specified. Furthermore, the embodiment and modifications can be combined as appropriate.

[0247] (First Modification of Flexible Multilayer Circuit Board) 1, the first ground conductor 11 and the second ground conductor 21 each have a first thickness T1 and a second thickness T2 alternately along the longitudinal direction at the bent portion F, but this is not limited to this. At least one of the first ground conductor 11 and the second ground conductor 21 may have the second thickness T2 at the bent portion F. Specifically, as shown in FIG. 8, only the first ground conductor 11 may have the second thickness T2 at the bent portion F.

[0248] That is, in the flexible multilayer circuit board 1 of the first modified example, the bent portion F coincides with the portion of the first ground conductor portion 11 that has the second thickness T2 in the longitudinal direction.

[0249] Furthermore, in the flexible multilayer circuit board 1 of the first modified example, the second ground conductor 21 has a uniform thickness at the bent portion F and other than the bent portion F. Specifically, the second ground conductor 21 has only the first thickness T1 at the bent portion F and other than the bent portion F. Note that, at the bent portion F, the second ground conductor 21 may have the same second thickness T2 as the first ground conductor 11.

[0250] (Second Modification of Flexible Multilayer Circuit Board) 1 includes only one bent portion F, but is not limited to this. In other words, although not shown, the flexible multilayer circuit board 1 may include multiple bent portions F. The number of bent portions F is selected appropriately depending on the electronic device to which the flexible multilayer circuit board 1 is applied.

[0251] When multiple bent portions F are provided, the proportion of each bent portion F in the total length of the flexible multilayer circuit board 1 in the longitudinal direction is, for example, 5% to 30%. The proportion of all bent portions F in the total length of the flexible multilayer circuit board 1 in the longitudinal direction is, for example, 5% to 30%.

[0252] (Third Modification of Flexible Multilayer Circuit Board) 1, at one end in the longitudinal direction, the first conductor layer 10 has a first terminal portion 12 and the second conductor layer 20 has a second terminal portion 22, and at the other end in the longitudinal direction, the first conductor layer 10 does not have the first terminal portion 12 and the second conductor layer 20 has the second terminal portion 22. However, the flexible multilayer circuit board 1 is not limited to this.

[0253] That is, it is sufficient that either the first conductor layer 10 or the second conductor layer 20 has a terminal portion at each of both longitudinal ends. Specifically, although not shown, the first conductor layer 10 may have the first terminal portion 12 and the second conductor layer 20 may not have the second terminal portion 22 at each of both longitudinal ends. Alternatively, the first conductor layer 10 may not have the first terminal portion 12 and the second conductor layer 20 may have the second terminal portion 22 at each of both longitudinal ends. Furthermore, it is also possible that the first conductor layer 10 does not have the first terminal portion 12 and the second conductor layer 20 has the second terminal portion 22 at one longitudinal end, and the first conductor layer 10 has the first terminal portion 12 and the second conductor layer 20 does not have the second terminal portion 22 at the other longitudinal end.

[0254] As described above, in the flexible multilayer circuit board 1, the positions of the terminal portions can be selected according to the electronic device to which it is applied.

[0255] In addition, since the second via connection portion 52 electrically connects the terminal portion and the wiring portion 30, its arrangement is changed depending on the arrangement of the first terminal portion 12 and the second terminal portion 22. Furthermore, the arrangement of the reinforcing base material 70 is also changed depending on the arrangement of the first terminal portion 12 and the second terminal portion 22.

[0256] (First modified example of the manufacturing method of the flexible multilayer circuit board) In the manufacturing method of the flexible multilayer circuit board 1 shown in FIGS. 5A to 7C, at least one of the first ground conductor portion 111 and the second ground conductor portion 121 is lightly etched (soft etched) to form the second thickness T2 (subtractive method), but the method is not limited to this.

[0257] Specifically, although not shown, the first thickness T1 and the second thickness T2 may be formed by forming an additional plating layer on at least one of the first ground conductor part 111 and the second ground conductor part 121 in addition to the portion corresponding to the second thickness T2 (additive method).

[0258] (Second modified example of the manufacturing method of the flexible multilayer circuit board) In the method for manufacturing flexible multilayer circuit board 1 shown in FIGS. 5A to 7C, first through hole 141 and second through hole 142 are formed in separate steps, but the present invention is not limited to this.

[0259] Specifically, although not shown, the second through hole forming process may not be performed, and the first through hole 141 and the second through hole 142 may be formed together during the first through hole forming process, or a portion of the first through hole 141 (the first through hole 141 on the first porous laminate 100a side) may be formed during the second through hole forming process.

[0260] The above invention is provided as an exemplary embodiment of the present invention, but this is merely an example and should not be interpreted as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included in the scope of the following claims. [Industrial Applicability]

[0261] The flexible multilayer circuit board 1 of the present invention is suitable for use in electronic devices such as mobile phones, smartphones, tablet terminals, and digital cameras. [Explanation of symbols]

[0262] 1. Flexible multilayer circuit board 2. Insulation layer 10 First conductor layer 11 First ground conductor 20 Second conductor layer 21 Second ground conductor 30 Wiring section F bending part T1 First thickness T2 Second thickness

Claims

1. an insulating layer; a first conductor layer disposed on one side of the insulating layer in a thickness direction; a second conductor layer disposed on the other side of the insulating layer in the thickness direction; a wiring portion embedded in the insulating layer; A flexible multilayer circuit board comprising: the first conductor layer has a first ground conductor portion, the second conductor layer has a second ground conductor portion, The flexible multilayer circuit board has a longitudinal middle portion perpendicular to the thickness direction, A bending portion is provided, the first ground conductor and the second ground conductor have a first thickness in a portion other than the bent portion; At least one of the first ground conductor and the second ground conductor in the bent portion has a second thickness; The second thickness is less than the first thickness.

2. The flexible multilayer circuit board according to claim 1 , wherein the first ground conductor and the second ground conductor each have the second thickness at the bent portion.

3. The flexible multilayer circuit board according to claim 2 , wherein at least one of the first ground conductor and the second ground conductor further has the first thickness in the bent portion.

4. The flexible multilayer circuit board according to claim 3 , wherein the first ground conductor and the second ground conductor each further have the first thickness at the bent portion.

5. 5 . The flexible multilayer circuit board according to claim 4 , wherein the first thickness and the second thickness of each of the first and second ground conductors alternate along the longitudinal direction of the bent portion.

6. The first thickness of the first ground conductor portion with respect to the insulating layer in a thickness direction is the second ground conductor portion is located on an opposite side of the first thickness direction; The flexible multilayer circuit board according to claim 5 , wherein the second thickness of the first ground conductor is located on an opposite side of the insulating layer in a thickness direction to the second thickness of the second ground conductor.

7. The insulating layer is a bonding layer; a first porous resin layer disposed on one side of the bonding layer in a thickness direction; a second porous resin layer disposed on the other side of the bonding layer in the thickness direction; Equipped with 7. The flexible multilayer circuit board according to claim 1, wherein the wiring portion is embedded in the bonding layer.

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