Flexible multilayer circuit board

Abstract

This flexible multilayer circuit board includes: an insulation layer; a first conductor layer disposed on one side in a thickness direction of the insulation layer; a second conductor layer disposed on the other side in the thickness direction of the insulation layer; and a wiring part. The insulation layer has a porous insulation layer and a non-porous insulation layer. The wiring part is embedded in the porous insulation layer.

Description

Flexible Multilayer Circuit Board

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

[0002] In recent years, development of wireless communication standards for the so-called "fifth generation (5G)" has been progressing. Wireless communication standards for the "fifth generation (5G)" enable the transmission of large volumes of data at high speeds. Wireless communication standards for the "fifth generation (5G)" use high frequencies, including millimeter waves. Substrates for high-frequency antennas that emit such millimeter waves require substrates with low dielectric constants (low-dielectric substrates). Furthermore, flexible printed circuit boards (FPCs) are in demand for high-speed transmission FPCs that transmit data at high speeds, and low-dielectric substrates are also required for the substrates for these high-speed transmission FPCs.

[0003] As a flexible circuit board, for example, a wired circuit board has been proposed which includes a porous insulating layer and a conductor layer arranged in that order toward one side in the thickness direction, and the conductor layer has a first wiring portion and a second wiring portion that is thicker than the first wiring portion (see Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2022-182956

[0005] A flexible multilayer circuit board may be manufactured by bonding two or more layered precursors. An example of this is described below. FIG. 10 is a schematic diagram of an example of a conventional flexible multilayer circuit board. The flexible multilayer circuit board 60 shown in FIG. 10 includes a porous insulating layer 52 having a hole P, a first conductor layer 51 disposed on one thickness-wise side of the porous insulating layer 52, a second conductor layer 53 disposed on the other thickness-wise side of the porous insulating layer 52, and a wiring portion 54 embedded in the porous insulating layer 52. The flexible multilayer circuit board 60 shown in FIG. 10 is manufactured by the method shown in FIGS. 11A to 11F. This method is described below. First, a laminate of a wiring portion precursor 54A and a porous insulating layer first precursor 52A is prepared ( FIG. 11A ). The porous insulating layer first precursor 52A has a hole P. Next, the wiring portion precursor 54A in the prepared laminate is selectively etched to form the wiring portion 54, thereby obtaining a circuit board first precursor 70A ( FIG. 11B ). Next, a second circuit board precursor 70B is prepared, including a second conductor layer 53 and a second porous insulating layer precursor 52B disposed on one thickness-wise side of the second conductor layer 53 ( FIG. 11C ). The second porous insulating layer precursor 52B has holes P. Next, the first circuit board precursor 70A and the second circuit board precursor 70B are aligned so that the wiring portion 54 faces the second porous insulating layer precursor 52B ( FIG. 11D ). The first circuit board precursor 70A and the second circuit board precursor 70B are then bonded together. This integrates the first porous insulating layer precursor 52A and the second porous insulating layer precursor 52B to form the porous insulating layer 52 ( FIG. 11E ). Next, a first conductor layer 51 is bonded to one thickness-wise side of the porous insulating layer 52 ( FIG. 11F ). This results in the flexible multilayer circuit board 60 shown in FIG. 10 . When the first circuit board precursor 70A and the second circuit board precursor 70B are bonded together, the resulting porous insulating layer 52 is deformed by pressure. The deformation of the porous insulating layer 52 changes the insulating properties (for example, dielectric properties) of the porous insulating layer 52, which may result in the electrical properties of the wiring portion 54 deviating from the desired electrical properties.

[0006] An object of the present invention is to provide a flexible multilayer circuit board capable of suppressing fluctuations in the electrical characteristics of the wiring portion.

[0007] The present inventors have conducted extensive research to solve the above problems, and as a result have found that the above problems can be solved, and have completed the present invention having the following gist. That is, the present invention includes the following.

[0008] [1] A flexible multilayer circuit board comprising an insulating layer, a first conductor layer disposed on one side of the insulating layer in the thickness direction, a second conductor layer disposed on the other side of the insulating layer in the thickness direction, and a wiring portion, wherein the insulating layer has a porous insulating layer and a non-porous insulating layer, and the wiring portion is embedded in the porous insulating layer. [2] The flexible multilayer circuit board according to [1], wherein the insulating layer further has a second non-porous insulating layer, the non-porous insulating layer being disposed on one side of the porous insulating layer in the thickness direction, and the second porous insulating layer being disposed on the other side of the porous insulating layer in the thickness direction. [3] The flexible multilayer circuit board according to [1] or [2], wherein the porous insulating layer contains a liquid crystal polymer, a cycloolefin polymer, or a polyimide.

[0009] According to the present invention, it is possible to provide a flexible multilayer circuit board capable of suppressing fluctuations in the electrical characteristics of the wiring portion.

[0010] FIG. 1 is a schematic diagram of one embodiment of a flexible multilayer circuit board. FIG. 2A is a schematic diagram (part 1) for explaining one embodiment of a method for manufacturing a flexible multilayer circuit board. FIG. 2B is a schematic diagram (part 2) for explaining one embodiment of a method for manufacturing a flexible multilayer circuit board. FIG. 2C is a schematic diagram (part 3) for explaining one embodiment of a method for manufacturing a flexible multilayer circuit board. FIG. 2D is a schematic diagram (part 4) for explaining one embodiment of a method for manufacturing a flexible multilayer circuit board. FIG. 3 is a schematic diagram of another embodiment of a flexible multilayer circuit board. FIG. 4 is a schematic diagram for explaining another embodiment of a method for manufacturing a flexible multilayer circuit board. FIG. 5 is a schematic diagram of another embodiment of a flexible multilayer circuit board. FIG. 6 is a schematic diagram of another embodiment of a flexible multilayer circuit board. FIG. 7 is a schematic diagram of another embodiment of a flexible multilayer circuit board. FIG. 8A is a perspective view of an example of a flexible multilayer circuit board. FIG. 8B is a cross-sectional view taken along line A-A' of FIG. 8A. FIG. 9A is a perspective view of an example of a flexible multilayer circuit board. FIG. 9B is a cross-sectional view taken along line A-A' of FIG. 9A. FIG. 10 is a schematic view of an example of a conventional flexible multilayer circuit board. Fig. 11A is a schematic diagram for explaining a method for manufacturing an example of a conventional flexible multilayer circuit board (part 1). Fig. 11B is a schematic diagram for explaining a method for manufacturing an example of a conventional flexible multilayer circuit board (part 2). Fig. 11C is a schematic diagram for explaining a method for manufacturing an example of a conventional flexible multilayer circuit board (part 3). Fig. 11D is a schematic diagram for explaining a method for manufacturing an example of a conventional flexible multilayer circuit board (part 4). Fig. 11E is a schematic diagram for explaining a method for manufacturing an example of a conventional flexible multilayer circuit board (part 5). Fig. 11F is a schematic diagram for explaining a method for manufacturing an example of a conventional flexible multilayer circuit board (part 6).

[0011] (Flexible multilayer circuit board) The flexible multilayer circuit board of the present invention comprises an insulating layer, a first conductor layer, a second conductor layer, and a wiring portion. The first conductor layer is disposed on one side of the insulating layer in the thickness direction. The second conductor layer is disposed on the other side of the insulating layer in the thickness direction. The insulating layer has a porous insulating layer and a non-porous insulating layer. The wiring portion is embedded in the porous insulating layer.

[0012] In a flexible multilayer circuit board having wiring portions embedded in a porous insulating layer, the inclusion of a nonporous insulating layer can suppress deformation of the porous insulating layer due to pressure during manufacturing of the flexible multilayer circuit board. As a result, fluctuations in the electrical properties of the wiring portions of the flexible multilayer circuit board can be suppressed. Furthermore, the inclusion of a nonporous insulating layer is advantageous in terms of reliability and adhesion. The reasons are as follows. While the porous insulating layer is susceptible to moisture and ions due to the presence of pores, a nonporous insulating layer suppresses the intrusion of moisture and ions into the porous insulating layer. As a result, deterioration of the porous insulating layer can be prevented. Furthermore, deterioration of the porous insulating layer and the intrusion of moisture into the porous insulating layer can lead to deterioration of the dielectric properties of the porous insulating layer. Furthermore, deterioration of the porous insulating layer can lead to a decrease in adhesion between the porous insulating layer and adjacent layers. Decreased adhesion can easily cause interfacial delamination during mounting, bending, and other processes. Interfacial delamination can lead to short circuits. Furthermore, decreased adhesion can easily cause the wiring portions of a flexible multilayer circuit board to collapse when formed by photolithography. The nonporous insulating layer can prevent such a decrease in reliability and adhesion.

[0013] An example of a flexible multilayer circuit board of the present invention will be described below with reference to FIG. 1 . FIG. 1 is a schematic diagram of one embodiment of a flexible multilayer circuit board. This schematic diagram is a cross-sectional view perpendicular to the longitudinal direction of the wiring portion. The flexible multilayer circuit board 10 shown in FIG. 1 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring portion 5. The insulating layer 21 includes a porous insulating layer 2 and a non-porous insulating layer 3. The porous insulating layer 2 has a large number of pores P. The insulating layer 21 is a laminate of the porous insulating layer 2 and the non-porous insulating layer 3. The first conductor layer 1 is disposed on one side of the insulating layer 21 in the thickness direction. The first conductor layer 1 contacts the porous insulating layer 2 of the insulating layer 21 on one side of the insulating layer 21 in the thickness direction. The thickness direction of the insulating layer 21 is the vertical direction on the paper surface in FIG. 1 . The second conductor layer 4 is disposed on the other side of the insulating layer 21 in the thickness direction. The second conductor layer 4 is in contact with the non-porous insulating layer 3 of the insulating layer 21 on the other side in the thickness direction of the insulating layer 21. The other side is opposite to the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 transmits, for example, an electrical signal.

[0014] In the flexible multilayer circuit board 10 shown in Fig. 1, the first conductor layer 1 and the second conductor layer 4 extend in the length direction of the wiring portion 5. In Fig. 1, the length direction of the wiring portion 5 is a direction perpendicular to the up-down direction and the left-right direction of the paper.

[0015] 1, the first conductor layer 1 and the second conductor layer 4 are the outermost layers constituting the flexible multilayer circuit board 10. The first conductor layer 1 and the second conductor layer 4 sandwich an insulating layer 21 therebetween.

[0016] The material of the first conductor layer and the second conductor layer is not particularly limited, and examples thereof include metal materials. Examples of metal materials include copper, nickel, gold, solder, and alloys of two or more of these. The materials of the first conductor layer and the second conductor layer may be the same or different. The thickness of the first conductor layer and the second conductor layer is not particularly limited, and is, for example, 3 μm or more, preferably 5 μm or more, and, for example, 50 μm or less, preferably 30 μm or less. The thickness of the first conductor layer and the second conductor layer may be the same or different. In the present invention, "thickness" refers to the length in the thickness direction of the flexible multilayer circuit board. The thickness direction of the flexible multilayer circuit board refers, for example, to the direction perpendicular to the surface direction of the first conductor layer and the second conductor layer.

[0017] The material of the wiring portion is not particularly limited, and examples thereof include metal materials. Examples of metal materials include copper, nickel, gold, solder, and alloys of two or more of these. The thickness of the wiring portion is not particularly limited, and is, for example, 3 μm or more, preferably 5 μm or more, and for example, 50 μm or less, preferably 30 μm or less. Note that, since the wiring portion is embedded in the porous insulating layer, the thickness of the wiring portion is usually thinner than the thickness of the porous insulating layer. The width of the wiring portion (the length of the wiring portion in the thickness direction and the direction perpendicular to the length direction of the wiring portion) is not particularly limited, but is usually shorter than the width of the first conductor layer and the second conductor layer (the length of the first conductor layer and the second conductor layer in the thickness direction and the direction perpendicular to the length direction of the first conductor layer and the second conductor layer), for example, 1 / 5 to 1 / 2 of the width of the first conductor layer and the second conductor layer. Note that, since the wiring portion is embedded in the porous insulating layer, the width of the wiring portion is usually shorter than the width of the porous insulating layer.

[0018] The wiring portion is embedded in the porous insulating layer, and the wiring portion may have a non-porous insulating film covering its surface. Examples of materials constituting the non-porous insulating film include the materials listed in the description of the materials constituting the insulating layer, which will be described later.

[0019] Examples of materials constituting the insulating layer include resins. In other words, the insulating layer contains, for example, a resin. 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, cycloolefin polymers, and liquid crystal polymers. From the viewpoints of high insulation properties, high heat resistance, and high mechanical strength, polyimide resins, cycloolefin polymers, and liquid crystal polymers are preferred. Note that the resins constituting the porous insulating layer and the non-porous insulating layer may be the same resin or different resins. The thickness of the insulating layer is not particularly limited and is, for example, 10 μm or more, preferably 20 μm or more, and, for example, 1000 μm or less, preferably 600 μm or less.

[0020] The porous insulating layer (including the second porous insulating layer and third porous insulating layer described below) is porous. The porous insulating layer has a large number of fine pores (air pores). Examples of the air bubble structure of the porous insulating layer include a closed-cell structure (closed-cell structure) and an open-cell structure (open-cell structure). A porous insulating layer with an open-cell structure is more susceptible to deformation due to pressure during the manufacture of a flexible multilayer circuit board than a porous insulating layer with a closed-cell structure. Therefore, since the flexible multilayer circuit board of the present invention has a non-porous insulating layer, deformation of the porous insulating layer due to pressure can be further suppressed when the porous insulating layer has an open-cell structure. Therefore, in the present invention, the porous insulating layer preferably has an open-cell structure. Here, the closed-cell structure refers to a structure in which the resin portion existing between the pores does not have holes that connect the pores, and gas does not flow between adjacent pores. In contrast, the open-cell structure refers to a structure in which adjacent pores are connected by pores, allowing gas to flow between the pores.

[0021] The thickness of the porous insulating layer is not particularly limited and is, for example, 5 μm or more, preferably 10 μm or more, and for example, 150 μm or less, preferably 100 μm or less.

[0022] The porosity of the porous insulating layer is, for example, 50% or more, preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. The porosity of the porous insulating layer is, for example, less than 100%, or even 99% or less. When the material of the porous insulating layer is a polyimide resin, the porosity of the porous insulating layer is determined by calculation based on the following formula: Dielectric constant of porous insulating layer = Dielectric constant of air × Porosity + Dielectric constant of polyimide × (1 - Porosity) Here, the dielectric constant of air is 1, and the dielectric constant of polyimide resin is 3.5. Dielectric constant of porous insulating layer = Porosity + 3.5(1 - Porosity) Porosity (%) = [(3.5 - Dielectric constant of porous insulating layer) / 2.5] × 100

[0023] The dielectric constant of the porous insulating layer at a frequency of 60 GHz is, for example, 2.5 or less, preferably 1.9 or less, more preferably 1.6 or less, and is, for example, greater than 1.0. The dielectric constant of the porous insulating layer is actually measured by a resonator method using a frequency of 60 GHz.

[0024] The dielectric loss tangent of the porous insulating layer at a frequency of 60 GHz is, for example, not more than 0.006, and is, for example, greater than 0. The dielectric loss tangent of the porous insulating layer is actually measured by a resonator method using a frequency of 60 GHz.

[0025] The nonporous insulating layer (including the second nonporous insulating layer, third nonporous insulating layer, and fourth nonporous insulating layer described below) is not porous. Therefore, the porosity of the nonporous insulating layer is approximately 0%. Furthermore, the dielectric constant of the nonporous insulating layer matches the dielectric constant determined from the material that constitutes the nonporous insulating layer. For example, if the nonporous insulating layer is composed only of resin, the dielectric constant of the nonporous insulating layer matches the dielectric constant of the resin that constitutes the nonporous insulating layer.

[0026] The thickness of the non-porous insulating layer is not particularly limited and is, for example, 1 μm or more, preferably 5 μm or more, and for example, 150 μm or less, preferably 100 μm or less.

[0027] The nonporous insulating layer and the nonporous insulating film may also be a skin layer. The skin layer is a dense film made of the same resin as the resin constituting the adjacent porous insulating layer or porous insulating layer precursor. The skin layer can be obtained, for example, by heating the porous insulating layer or porous insulating layer precursor to eliminate pores on the surface of the porous insulating layer or porous insulating layer precursor and densify the surface.

[0028] One embodiment of a method for manufacturing the flexible multilayer circuit board shown in FIG. 1 will be described with reference to FIGS. 2A to 2D. First, a laminate of a wiring portion precursor 5A and a porous insulating layer first precursor 2A is prepared ( FIG. 2A ). Both the wiring portion precursor 5A and the porous insulating layer first precursor 2A are layered. The porous insulating layer first precursor 2A has pores P. The porous insulating layer first precursor 2A is bonded to a porous insulating layer second precursor 2B, thereby integrating them into a porous insulating layer. Next, the wiring portion precursor 5A of the prepared laminate is selectively etched to form the wiring portion 5, thereby obtaining a circuit board first precursor 20A ( FIG. 2B ). Next, a circuit board second precursor 20B and a circuit board third precursor 20C are prepared. The circuit board second precursor 20B includes a first conductor layer 1 and a porous insulating layer second precursor 2B disposed on one side of the first conductor layer 1 in the thickness direction. The porous insulating layer second precursor 2B has pores P. The third circuit board precursor 20C includes a second conductor layer 4 and a non-porous insulating layer 3 disposed on one thickness-wise side of the second conductor layer 4. The first circuit board precursor 20A and the second circuit board precursor 20B are then arranged so that the wiring portion 5 faces the second porous insulating layer precursor 2B. The second circuit board precursor 20B and the third circuit board precursor 20C are then arranged so that the non-porous insulating layer 3 faces the surface of the first porous insulating layer precursor 2A opposite the wiring portion 5 ( FIG. 2C ). The second circuit board precursor 20B, the first circuit board precursor 20A, and the third circuit board precursor 20C are then bonded together. The pressure and temperature during bonding are not particularly limited. By doing so, the first porous insulating layer precursor 2A and the second porous insulating layer precursor 2B are integrated to form the porous insulating layer 2, and the wiring portion 5 is embedded in the porous insulating layer 2 ( FIG. 2D ). This process yields the flexible multilayer circuit board 10 shown in FIG. 1 .

[0029] FIG. 3 is a schematic diagram of another embodiment of a flexible multilayer circuit board. This schematic diagram is a cross-sectional view perpendicular to the longitudinal direction of the wiring portion. The flexible multilayer circuit board 10 shown in FIG. 3 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring portion 5. The insulating layer 21 includes a second non-porous insulating layer 6, a porous insulating layer 2, and a non-porous insulating layer 3, in this order. The porous insulating layer 2 has a large number of pores P. The insulating layer 21 is a laminate of the second non-porous insulating layer 6, the porous insulating layer 2, and the non-porous insulating layer 3. The thicknesses and materials of the non-porous insulating layer 3 and the second non-porous insulating layer 6 may be the same or different. The first conductor layer 1 is disposed on one side of the insulating layer 21 in the thickness direction. The first conductor layer 1 contacts the second non-porous insulating layer 6 of the insulating layer 21 on one side of the insulating layer 21 in the thickness direction. The thickness direction of the insulating layer 21 is the vertical direction of the paper in FIG. 3. The second conductor layer 4 is disposed on the other side in the thickness direction of the insulating layer 21. The second conductor layer 4 contacts the non-porous insulating layer 3 of the insulating layer 21 on the other side in the thickness direction of the insulating layer 21. The other side is opposite to the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 transmits, for example, an electrical signal.

[0030] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in FIG. 3 , the porous insulating layer 2 is sandwiched between the nonporous insulating layer 3 and the second nonporous insulating layer 6. The porous insulating layer 2 is not in contact with the first conductor layer 1 or the second conductor layer 4. By sandwiching the porous insulating layer 2 between the nonporous insulating layer 3 and the second nonporous insulating layer 6, deformation of the porous insulating layer due to pressure during manufacturing of the flexible multilayer circuit board can be further suppressed. In addition, by sandwiching the porous insulating layer 2 between the nonporous insulating layer 3 and the second nonporous insulating layer 6, it becomes easier to make the distance between the wiring portion 5 and the first conductor layer 1 and the distance between the wiring portion 5 and the second conductor layer 4 equal. Here, when the wiring portion 5 is used as a signal line and the first conductor layer 1 and the second conductor layer 4 are used as ground layers, if the distance between the wiring portion 5 and the first conductor layer 1 and the distance between the wiring portion 5 and the second conductor layer 4 are significantly different, only the conductor layer closest to the wiring portion 5 functions as the ground layer. On the other hand, since the distance between the wiring portion 5 and the first conductor layer 1 is equal to the distance between the wiring portion 5 and the second conductor layer 4, both the first conductor layer 1 and the second conductor layer 4 can function as ground layers.

[0031] Fig. 4 is a schematic diagram for explaining one embodiment of a method for manufacturing the flexible multilayer circuit board shown in Fig. 3. The flexible multilayer circuit board 10 shown in Fig. 3 is obtained by bonding together, for example, a first circuit board precursor 20D, a second circuit board precursor 20E, a third circuit board precursor 20F, and a fourth circuit board precursor 20G, as shown in Fig. 4.

[0032] The first circuit board precursor 20D has a second conductor layer 4 and a non-porous insulating layer 3 arranged on one thickness-wise side of the second conductor layer 4. The second circuit board precursor 20E has a porous insulating layer first precursor 2C and a wiring portion 5 arranged on one thickness-wise side of the first porous insulating layer precursor 2C. The first porous insulating layer precursor 2C has holes P. Note that the first porous insulating layer precursor 2C is bonded to the second porous insulating layer precursor 2D, thereby integrating with the second porous insulating layer precursor 2D to form a porous insulating layer. The third circuit board precursor 20F is the second porous insulating layer precursor 2D. The second porous insulating layer precursor 2D has holes P. The fourth circuit board precursor 20G has a first conductor layer 1 and a second non-porous insulating layer 6 arranged on one thickness-wise side of the first conductor layer 1.

[0033] The first circuit board precursor 20D, the second circuit board precursor 20E, the third circuit board precursor 20F, and the fourth circuit board precursor 20G are placed opposite each other and bonded together as shown in Fig. 4. By doing so, the first porous insulating layer precursor 2C and the second porous insulating layer precursor 2D are integrated to form the porous insulating layer 2. As a result, the flexible multilayer circuit board 10 shown in Fig. 3 is obtained.

[0034] 4, four circuit board precursors (first circuit board precursor 20D, second circuit board precursor 20E, third circuit board precursor 20F, and fourth circuit board precursor 20G) are bonded together at one time. The number of circuit board precursors bonded together at one time may be two, three, or four or more.

[0035] FIG. 5 is a schematic view of another embodiment of a flexible multilayer circuit board. This schematic view is a cross-sectional view orthogonal to the length direction of the wiring portion. The flexible multilayer circuit board 10 shown in FIG. 5 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring portion 5. The insulating layer 21 has a second non-porous insulating layer 6, a porous insulating layer 2, and a non-porous insulating layer 3 in this order. The porous insulating layer 2 has a large number of holes P. The insulating layer 21 is a laminate of the second non-porous insulating layer 6, the porous insulating layer 2, and the non-porous insulating layer 3. The thicknesses and materials of the non-porous insulating layer 3 and the second non-porous insulating layer 6 may be the same or different. The first conductor layer 1 is disposed on one side in the thickness direction of the insulating layer 21. The first conductor layer 1 is in contact with the second non-porous insulating layer 6 of the insulating layer 21 on one side in the thickness direction of the insulating layer 21. The thickness direction of the insulating layer 21 is the vertical direction of the paper surface in FIG. 5. The second conductor layer 4 is disposed on the other side in the thickness direction of the insulating layer 21. The second conductor layer 4 is in contact with the non-porous insulating layer 3 of the insulating layer 21 on the other side in the thickness direction of the insulating layer 21. The other side is the opposite side of the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 transmits, for example, an electrical signal.

[0036] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in FIG. 5, the porous insulating layer 2 is sandwiched between the non-porous insulating layer 3 and the second non-porous insulating layer 6. Note that the porous insulating layer 2 is not in contact with the first conductor layer 1 and the second conductor layer 4. Since the porous insulating layer 2 is sandwiched between the non-porous insulating layer and the second non-porous insulating layer 6, deformation of the porous insulating layer due to pressure during manufacturing of the flexible multilayer circuit board can be further suppressed.

[0037] Also, the non-porous insulating layer 3 and the second non-porous insulating layer 6 in the flexible multilayer circuit board 10 shown in FIG. 5 are so-called skin layers. The skin layer is a dense film made of the same resin as the resin constituting the adjacent porous insulating layer or the porous insulating layer precursor. The skin layer can be obtained, for example, by heating the porous insulating layer or the porous insulating layer precursor to eliminate the holes on the surface of the porous insulating layer or the porous insulating layer precursor and densify the surface.

[0038] FIG. 6 is a schematic diagram of another embodiment of a flexible multilayer circuit board. This schematic diagram is a cross-sectional view perpendicular to the longitudinal direction of the wiring portion. The flexible multilayer circuit board 10 shown in FIG. 6 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring portion 5. The insulating layer 21 includes, in this order, a second non-porous insulating layer 6, a second porous insulating layer 7, a third non-porous insulating layer 8, a porous insulating layer 2, a fourth non-porous insulating layer 9, a third porous insulating layer 11, and a non-porous insulating layer 3. The porous insulating layer 2, the second porous insulating layer 7, and the third porous insulating layer 11 each have a large number of pores P. The insulating layer 21 is a laminate of the second non-porous insulating layer 6, the second porous insulating layer 7, the third non-porous insulating layer 8, the porous insulating layer 2, the fourth non-porous insulating layer 9, the third porous insulating layer 11, and the non-porous insulating layer 3. The thicknesses and materials of the porous insulating layer 2, the second porous insulating layer 7, and the third porous insulating layer 11 may be the same or different. The size, structure (closed-cell structure or open-cell structure), and porosity of the pores P of the porous insulating layer 2, the second porous insulating layer 7, and the third porous insulating layer 11 may be the same or different. The first conductor layer 1 is disposed on one side in the thickness direction of the insulating layer 21. The first conductor layer 1 contacts the second non-porous insulating layer 6 of the insulating layer 21 on one side in the thickness direction of the insulating layer 21. The thickness direction of the insulating layer 21 is the up-and-down direction on the paper in FIG. 6. The second conductor layer 4 is disposed on the other side in the thickness direction of the insulating layer 21. The second conductor layer 4 contacts the non-porous insulating layer 3 of the insulating layer 21 on the other side in the thickness direction of the insulating layer 21. The other side is opposite to the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 transmits, for example, electrical signals.

[0039] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in Fig. 6, the porous insulating layer 2 is sandwiched between a third non-porous insulating layer 8 and a fourth non-porous insulating layer 9. By sandwiching the porous insulating layer 2 with the wiring portion 5 embedded therein between the third non-porous insulating layer 8 and the fourth non-porous insulating layer 9, deformation of the porous insulating layer due to pressure when manufacturing the flexible multilayer circuit board can be further suppressed.

[0040] FIG. 7 is a schematic diagram of another embodiment of a flexible multilayer circuit board. This schematic diagram is a cross-sectional view perpendicular to the longitudinal direction of the wiring portion. The flexible multilayer circuit board 10 shown in FIG. 7 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, a wiring portion 5, and a non-porous insulating film 12. The insulating layer 21 includes a second non-porous insulating layer 6, a porous insulating layer 2, and a non-porous insulating layer 3, in this order. The porous insulating layer 2 has a large number of pores P. The insulating layer 21 is a laminate of the second non-porous insulating layer 6, the porous insulating layer 2, and the non-porous insulating layer 3. The thicknesses and materials of the non-porous insulating layer 3 and the second non-porous insulating layer 6 may be the same or different. The first conductor layer 1 is disposed on one side of the insulating layer 21 in the thickness direction. The first conductor layer 1 contacts the second non-porous insulating layer 6 of the insulating layer 21 on one side of the insulating layer 21 in the thickness direction. The thickness direction of the insulating layer 21 is the vertical direction of the paper in FIG. 7. The second conductor layer 4 is arranged on the other side in the thickness direction of the insulating layer 21. The second conductor layer 4 contacts the non-porous insulating layer 3 of the insulating layer 21 on the other side in the thickness direction of the insulating layer 21. The other side is opposite to the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 transmits, for example, electrical signals. The wiring portion 5 has a non-porous insulating film 12 covering its surface. In other words, the wiring portion 5, whose surface is covered with the non-porous insulating film 12, is embedded in the porous insulating layer 2. The non-porous insulating film 12 is, for example, a skin layer. The skin layer is a dense film made of the same resin as the resin that constitutes the adjacent porous insulating layer. The skin layer can be obtained, for example, by heating the wiring portion and the porous insulating layer to eliminate pores in the surface of the porous insulating layer at the interface between the porous insulating layer and the wiring portion and densify the surface.

[0041] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in Figure 7, the porous insulating layer 2 is sandwiched between the non-porous insulating layer 3 and the second non-porous insulating layer 6. The porous insulating layer 2 is not in contact with the first conductor layer 1 or the second conductor layer 4. By sandwiching the porous insulating layer 2 between the non-porous insulating layer 3 and the second non-porous insulating layer 6, deformation of the porous insulating layer due to pressure when manufacturing the flexible multilayer circuit board can be further suppressed.

[0042] An embodiment of a flexible multilayer circuit board will be described with reference to the drawings. FIGS. 8A and 8B are schematic diagrams of an example of a flexible multilayer circuit board. The flexible multilayer circuit board shown in FIGS. 8A and 8B is a stripline. FIG. 8A is a perspective view. FIG. 8B is a cross-sectional view taken along the line A-A' in FIG. 8A. Note that FIGS. 8A and 8B are schematic views. Therefore, for example, in FIG. 8B, the ratio between the longitudinal length and the thickness length is not accurate. The flexible multilayer circuit board 10 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring portion 5. The insulating layer 21 includes a porous insulating layer 2 and a non-porous insulating layer 3. The porous insulating layer 2 has a large number of pores P. The insulating layer 21 is a laminate of the porous insulating layer 2 and the non-porous insulating layer 3. The first conductor layer 1 is disposed on one side of the insulating layer 21 in the thickness direction. The first conductor layer 1 contacts the porous insulating layer 2 of the insulating layer 21 on one side of the insulating layer 21 in the thickness direction. The thickness direction of the insulating layer 21 is the vertical direction of the paper in Figure 8B. The second conductor layer 4 is arranged on the other side of the thickness direction of the insulating layer 21. The second conductor layer 4 is in contact with the non-porous insulating layer 3 of the insulating layer 21 on the other side of the thickness direction of the insulating layer 21. The other side is opposite to the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 is a signal line that transmits electrical signals. The flexible multilayer circuit board 10 further has a conductive portion 101. The conductive portion 101 electrically connects the first conductor layer 1 and the second conductor layer 4. The first conductor layer 1, the second conductor layer 4, and the conductive portion 101 form a ground. Note that a portion of the conductive portion 101 is formed simultaneously when the wiring portion 5 is formed, and therefore a portion of the conductive portion 101 is made of the same material as the wiring portion 5. 8A and 8B, the two conductive parts 101 are arranged on either side of the wiring part 5 extending in the longitudinal direction. The conductive parts 101 are also arranged along the wiring part 5 at a distance from each other.

[0043] An embodiment of a flexible multilayer circuit board will be described with reference to the drawings. FIGS. 9A and 9B are schematic diagrams of an example of a flexible multilayer circuit board. The flexible multilayer circuit board shown in FIGS. 9A and 9B is a stripline. FIG. 9A is a perspective view. FIG. 9B is a cross-sectional view taken along the line A-A' of FIG. 9A. Note that FIGS. 9A and 9B are schematic views. Therefore, for example, in FIG. 9B, the ratio between the longitudinal length and the thickness length is not accurate. A flexible multilayer circuit board 10 includes an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring portion 5. The insulating layer 21 includes a porous insulating layer 2 and a non-porous insulating layer 3. The porous insulating layer 2 has a large number of pores P. The insulating layer 21 is a laminate of the porous insulating layer 2 and the non-porous insulating layer 3. The first conductor layer 1 is disposed on one side of the insulating layer 21 in the thickness direction. The first conductor layer 1 contacts the porous insulating layer 2 of the insulating layer 21 on one side of the insulating layer 21 in the thickness direction. The thickness direction of the insulating layer 21 is the vertical direction of the paper in FIG. 9B . The second conductor layer 4 is disposed on the other side of the insulating layer 21 in the thickness direction. The second conductor layer 4 contacts the non-porous insulating layer 3 of the insulating layer 21 on the other side of the thickness direction of the insulating layer 21. The other side is opposite to the one side. The wiring portion 5 is embedded in the porous insulating layer 2. The wiring portion 5 is a signal line that transmits electrical signals. In the flexible multilayer circuit board shown in FIGS. 9A and 9B , the wiring portion 5 has two signal lines (a first signal line 501 and a second signal line 502). The first signal line 501 and the second signal line 502 are arranged in parallel. The first signal line 501 and the second signal line 502 constitute differential wiring for differential signal transmission. The flexible multilayer circuit board 10 further has a conductive portion 101. The conductive portion 101 electrically connects the first conductor layer 1 and the second conductor layer 4. The first conductor layer 1, the second conductor layer 4, and the conductive portion 101 form a ground. Note that a portion of the conductive portion 101 is formed simultaneously with the formation of the wiring portion 5 (the first signal line 501 and the second signal line 502), and therefore the portion of the conductive portion 101 is made of the same material as the wiring portion 5. Also, as shown in Figures 9A and 9B, the two conductive portions 101 are arranged to sandwich the wiring portion 5 (the first signal line 501 and the second signal line 502) extending in the longitudinal direction.The conductive portions 101 are arranged along the wiring portion 5 (the first signal line 501 and the second signal line 502) at intervals.

[0044] BACKGROUND ART Flexible multilayer circuit boards are used as flexible multilayer circuit boards for high-speed transmission in electronic devices such as mobile phones, smartphones, tablet terminals, and digital cameras, as they become smaller, lighter, and more functional.

[0045] 1 First conductor layer 2 Porous insulating layer 2A Porous insulating layer first precursor 2B Porous insulating layer second precursor 2C Porous insulating layer first precursor 2D Porous insulating layer second precursor 3 Non-porous insulating layer 4 Second conductor layer 5 Wiring portion 5A Wiring portion precursor 6 Second non-porous insulating layer 7 Second porous insulating layer 8 Third non-porous insulating layer 9 Fourth non-porous insulating layer 10 Flexible multilayer circuit board 11 Third porous insulating layer 12 Non-porous insulating film 20A Circuit board first precursor 20B Circuit board second precursor 20C Circuit board third precursor 20D Circuit board first precursor 20E Circuit board second precursor 20F Circuit board third precursor 20G Circuit board fourth precursor 21 Insulating layer 51 First conductor layer 52 Porous insulating layer 52A Porous insulating layer first precursor 52B Porous insulating layer second precursor 53 Second conductor layer 54 Wiring portion 54A Wiring portion precursor 60 Flexible multilayer circuit board 70A First circuit board precursor 70B Second circuit board precursor 101 Conductive portion 501 First signal line 502 Second signal line P Hole

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 that is a signal line and is not a waveguide; A flexible multilayer circuit board comprising: the insulating layer has a porous insulating layer and a non-porous insulating layer, the wiring portion is embedded in the porous insulating layer; Flexible multilayer circuit board.

2. the insulating layer further comprises a second non-porous insulating layer; the non-porous insulating layer is disposed on one side of the porous insulating layer in a thickness direction, the second porous insulating layer is disposed on the other side of the porous insulating layer in the thickness direction; The flexible multilayer circuit board according to claim 1 .

3. The flexible multilayer circuit board according to claim 1 , wherein the porous insulating layer comprises a liquid crystal polymer, a cycloolefin polymer, or a polyimide.