Flexible multilayer circuit board

Abstract

A flexible multilayer circuit board comprises: an insulation layer; a first conductor layer that is disposed on one side in the thickness direction of the insulation layer; a second conductor layer that is disposed on the other side in the thickness direction of the insulation layer; and a conduction part that electrically connects the first conductor layer and the second conductor layer. The insulation layer has a porous insulator region and a non-porous insulator region, and the non-porous insulator region covers the lateral face of the conduction part.

Description

Flexible multilayer circuit board

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

[0002] Since a porous polymer film can obtain a low dielectric constant by being made porous, for example, its use as an insulating layer of an FPC (Flexible printed circuits) has been promoted.

[0003] For example, a laminate for a wiring board has been proposed, which includes a porous film formed of a resin containing at least one of polyamic acid, polyimide, polyamideimide, polyamide, polyvinylidene fluoride, polybenzoxazole resin, polybenzimidazole resin, polysulfone, polyarylsulfone, and polyethersulfone, and having a plurality of voids in a spherical or continuous spherical shape, and a conductive film laminated on at least one surface of the porous film (see Patent Document 1). Further, a wiring board has been proposed in which a wiring pattern is formed on at least a part of the conductive film of the laminate for the wiring board (see Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2020-124925

[0005] Flexible multilayer circuit boards typically have conductive portions (e.g., conductive vias) that penetrate the porous insulating layer for electrical connection between two conductive layers sandwiching the porous insulating layer. An example of its manufacturing will be explained using Figures 11A to 11C. First, a laminate is prepared having a first conductive layer 102, a porous insulating layer 101, and a second conductive layer 103 in that order (Figure 11A). Next, a through-hole a is formed in the laminate from the second conductive layer 103 side, penetrating the second conductive layer 103 and the porous insulating layer 101 and reaching the first conductive layer 102 (Figure 11B). The through-hole a can be formed, for example, by laser processing. Next, the through-hole a is plated and filled with copper to form a conductive portion 104 (Figure 11C). Here, when forming the conductive portion, the plating solution may penetrate into the fine pores (pores that constitute the porous structure) on the side of the through-hole in the porous insulating layer. In particular, in the case of porous insulating layers with an open-cell structure, the penetration of the plating solution into the pores becomes significant. If the plating solution remains in the porous insulating layer, there is a risk that the electrical properties of the insulating layer (relative permittivity, dielectric loss tangent, etc.) will deteriorate.

[0006] Therefore, the present invention aims to provide a flexible multilayer circuit board that, when a porous insulator is used for the insulating layer, can prevent liquid from entering the fine pores constituting the porous structure when forming conductive parts, thereby suppressing a decrease in the electrical properties of the insulating layer.

[0007] The inventors of the present invention conducted diligent research to solve the above problems and, as a result, found that they could solve the above problems, and completed the present invention having the following gist.

[0008] In other words, the present invention encompasses the following: [1] A flexible multilayer circuit board comprising: an insulating layer; a first conductor layer disposed on one side in the thickness direction of the insulating layer; a second conductor layer disposed on the other side in the thickness direction of the insulating layer; and a conductive portion electrically connecting the first conductor layer and the second conductor layer, wherein the insulating layer has a porous insulating region and a non-porous insulating region, and the non-porous insulating region covers the side surface of the conductive portion. [2] The flexible multilayer circuit board according to [1], wherein the width of the non-porous insulating region is 1 μm to 100 μm. [3] The flexible multilayer circuit board according to [1] or [2], further comprising a wiring portion, wherein the wiring portion is embedded in the porous insulating region.

[0009] According to the present invention, when a porous insulator is used for the insulating layer, it is possible to provide a flexible multilayer circuit board that prevents liquid from entering the fine pores constituting the porous structure when forming conductive parts, thereby suppressing a decrease in the electrical properties of the insulating layer.

[0010] Figure 1 is a schematic cross-sectional view of one embodiment of a flexible multilayer circuit board. Figure 2 is a schematic cross-sectional view of another embodiment of a flexible multilayer circuit board. Figure 3 is a schematic cross-sectional view of another embodiment of a flexible multilayer circuit board. Figure 4 is a schematic cross-sectional view of another embodiment of a flexible multilayer circuit board. Figure 5A is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 1 (part 1). Figure 5B is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 1 (part 2). Figure 5C is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 1 (part 3). Figure 5D is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 1 (part 4). Figure 5E is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 1 (part 5). Figure 5F is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 1 (part 6). Figure 6A is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 4 (part 1). Figure 6B is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board of Figure 4 (part 2). Figure 6C is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board shown in Figure 4 (part 3). Figure 6D is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board shown in Figure 4 (part 4). Figure 6E is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board shown in Figure 4 (part 5). Figure 6F is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board shown in Figure 4 (part 6). Figure 6G is a diagram illustrating one embodiment of the manufacturing method of the flexible multilayer circuit board shown in Figure 4 (part 7). Figure 7A is a schematic cross-sectional view of another embodiment of the flexible multilayer circuit board. Figure 7B is a cross-sectional view of the flexible multilayer circuit board shown in Figure 7A along line A-A'. Figure 8A is a schematic cross-sectional view of another embodiment of the flexible multilayer circuit board. Figure 8B is a cross-sectional view of the flexible multilayer circuit board shown in Figure 8A along line A-A'. Figure 9 is a schematic cross-sectional view of another embodiment of the flexible multilayer circuit board. Figure 10 is a schematic cross-sectional view of another embodiment of the flexible multilayer circuit board.Figure 11A is a schematic cross-sectional view illustrating a conventional flexible multilayer circuit board (Part 1). Figure 11B is a schematic cross-sectional view illustrating a conventional flexible multilayer circuit board (Part 2). Figure 11C is a schematic cross-sectional view illustrating a conventional flexible multilayer circuit board (Part 3).

[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 conductive portion. The first conductor layer is arranged on one side in the thickness direction of the insulating layer. The second conductor layer is arranged on the other side in the thickness direction of the insulating layer. The conductive portion electrically connects the first conductor layer and the second conductor layer. The insulating layer has a porous insulating region and a non-porous insulating region. The non-porous insulating region covers the side surface of the conductive portion.

[0012] The non-porous insulating region prevents liquid from entering the fine pores (pores that make up the porous structure) of the porous insulating region when forming conductive areas. As a result, the deterioration of the electrical properties of the insulating layer of the flexible multilayer circuit board can be suppressed.

[0013] In a flexible multilayer circuit board, one non-porous insulating region may cover the side surface of one conductive portion, or it may cover the side surfaces of two or more conductive portions. For example, if the distance between two adjacent conductive portions is short, one non-porous insulating region may cover the side surfaces of two or more conductive portions. For example, if the distance between two adjacent conductive portions is long, one non-porous insulating region may cover the side surface of one conductive portion. Here, a short distance between two adjacent conductive portions refers to, for example, a case where the distance between the two conductive portions is 150 μm or less. The distance between two conductive portions is the shortest distance between the ends of the two conductive portions. The distance between two conductive portions may be, for example, 50 μm to 150 μm.

[0014] The width of the non-porous insulator region is not particularly limited, for example, 1 μm to 100 μm, and preferably 1 μm to 50 μm. If the width of the non-porous insulator region is 1 μm or more, the penetration of liquid (e.g., plating solution) into the porous insulator region can be sufficiently suppressed. On the other hand, if the width of the non-porous insulator region is too thick relative to the spacing between adjacent conductive parts, the proportion of the porous insulator region in the insulating layer decreases, and the dielectric constant of the insulating layer increases. In this respect, the width of the non-porous insulator region is preferably 50 μm or less. The width of the non-porous insulator region is the thickness of the non-porous insulator region in a direction perpendicular to the thickness direction of the flexible multilayer circuit board. Note that, as shown in Figure 1 later, if the conductive part 4 has a tapered shape, and the width of the non-porous insulator region 12 differs depending on the position due to the influence of this tapered shape, the width (T) of the non-porous insulator region 12 refers to the shortest distance between the conductive part 4 and the porous insulator region 11. Furthermore, as shown in Figure 8B later, when one non-porous insulator region covers the sides of two or more conductive parts, the width (T) of the non-porous insulator region 12 refers to the shortest distance between the conductive part 4 and the porous insulator region 11.

[0015] The following describes each component.

[0016] <Insulating Layer> Examples of materials for the insulating layer include resin. The type of resin is not limited. Examples of resins include polycarbonate resin, polyimide resin, fluorinated polyimide resin, epoxy resin, phenolic resin, urea resin, melamine resin, diallyl phthalate resin, silicone resin, thermosetting urethane resin, fluororesin, and liquid crystal polymer. Polyimide resin and liquid crystal polymer are preferred.

[0017] The insulating layer has a porous insulating region and a non-porous insulating region. The porous insulating region may be, for example, a porous insulating layer. The insulating layer may have, for example, a non-porous insulating layer and a second non-porous insulating layer. The insulating layer may have, for example, an adhesive insulating layer. The insulating layer may have a second non-porous insulating region.

[0018] Examples of materials for the porous insulating region, non-porous insulating region, second non-porous insulating region, non-porous insulating layer, and second non-porous insulating layer include the resin exemplified as the material for the insulating layer.

[0019] The porous insulator region is porous. The porous insulator region has closed cells and / or open cells. The porosity in the porous insulator region is, for example, 50% or more, preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. The porosity in the porous insulator region is, for example, less than 100%, and even more preferably 99% or less. When the material of the porous insulator region is polyimide resin, the porosity of the porous insulator region can be determined by calculation based on the following formula.

[0020] The relative permittivity of the porous insulator region = relative permittivity of air × porosity + relative permittivity of polyimide × (1 - porosity) Here, the relative permittivity of air is 1 and the relative permittivity of polyimide resin is 3.5, so the following is obtained: Relative permittivity of the porous insulator region = porosity + 3.5(1 - porosity) Porosity (%) = [(3.5 - relative permittivity of the porous insulator region) / 2.5] × 100

[0021] The relative permittivity in the porous insulator region at a frequency of 10 GHz is, for example, 2.5 or less, preferably 1.9 or less, more preferably 1.6 or less, and also, for example, greater than 1.0. The relative permittivity in the porous insulator region is measured by a resonator method using a frequency of 10 GHz.

[0022] The dielectric loss tangent in the porous insulator region at a frequency of 10 GHz is, for example, 0.006 or less, and also, for example, greater than 0. The dielectric loss tangent in the porous insulator region is measured using a resonator method with a frequency of 10 GHz.

[0023] The material for the adhesive insulating layer is not particularly limited, and various types of adhesives can be used, such as hot-melt adhesives and thermosetting adhesives. Specifically, examples include acrylic adhesives, epoxy adhesives, and silicone adhesives.

[0024] The thickness of the insulating layer is not particularly limited, but is, for example, 5 μm or more, preferably 10 μm or more, and also, for example, 150 μm or less, preferably 100 μm or less.

[0025] <Conducting Layer> The material of the conductor layer (first conductor layer, second conductor layer) is not particularly limited and includes, for example, copper, iron, silver, gold, aluminum, nickel, and their alloys (e.g., stainless steel, bronze). Copper is preferred. The conductor layer may be in the shape of a pattern. The thickness of the conductor layer is not particularly limited and is, for example, 5 μm or more, preferably 10 μm or more, and also, for example, 100 μm or less, preferably 80 μm or less.

[0026] <Conductive Part> The material of the conductive part is not particularly limited, and examples include conductors. Examples of conductors include copper, iron, silver, gold, aluminum, nickel, and their alloys (e.g., stainless steel, bronze). Copper is preferred. The size of the conductive part is not particularly limited, but the diameter is preferably 50 μm or more, more preferably 75 μm or more, and preferably 300 μm or less, and more preferably 200 μm or less.

[0027] <Wiring Section> A flexible multilayer circuit board may have a wiring section. The material of the wiring section is not particularly limited, and examples include copper, iron, silver, gold, aluminum, nickel, and their alloys (e.g., stainless steel, bronze). Copper is preferred. The wiring section is, for example, a signal line.

[0028] Figure 1 shows one embodiment of a flexible multilayer circuit board. The flexible multilayer circuit board shown in Figure 1 has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, and a conductive portion 4. The first conductor layer 2 is arranged on one side in the thickness direction of the insulating layer 1. The second conductor layer 3 is arranged on the other side in the thickness direction of the insulating layer 1. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulating region 11 and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The porous insulating region 11 is a porous insulating layer having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is arranged within the through holes of the porous insulating layer. The conductive portion 4 has a tapered shape that narrows from the second conductor layer 3 side to the first conductor layer 2 side. The shape of the conductive portion 4 is not particularly limited and may be cylindrical, for example. Here, "side surface" refers to a surface different from the surface in the thickness direction. The side surface of the conductive portion is a surface different from the surface in the thickness direction of the conductive portion. The side surface of the through-hole in the porous insulating layer is a surface different from the surface of the opening of the through-hole.

[0029] As shown in Figure 1, the presence of a non-porous insulator region 12 between the conductive portion 4 and the porous insulator region 11 (in other words, the conductive portion 4 and the porous insulator region 11 are not in contact) prevents liquid from entering the fine pores (pores constituting the porous structure) of the porous insulator region 11 when the conductive portion 4 is formed. As a result, the deterioration of the electrical properties of the insulating layer of the flexible multilayer circuit board can be suppressed.

[0030] A flexible multilayer circuit board may have an adhesive insulating layer. One embodiment of a flexible multilayer circuit board having an adhesive insulating layer is, for example, the same flexible multilayer circuit board as the one shown in Figure 1, except that the adhesive insulating layer is arranged between the porous insulating region 11 and the second conductor layer 3.

[0031] A flexible multilayer circuit board may have a non-porous insulating layer. One embodiment of a flexible multilayer circuit board having a non-porous insulating layer is, for example, the same flexible multilayer circuit board as the one shown in Figure 1, except that the non-porous insulating layer is arranged between the porous insulating region 11 and the first conductor layer 2.

[0032] A flexible multilayer circuit board may have an adhesive insulating layer and a non-porous insulating layer. One embodiment of a flexible multilayer circuit board having an adhesive insulating layer and a non-porous insulating layer is, for example, the same flexible multilayer circuit board as the one shown in Figure 1, except that the adhesive insulating layer and the non-porous insulating layer are arranged between the porous insulating region 11 and the second conductor layer 3. In this case, for example, the non-porous insulating layer is in contact with the porous insulating region 11 and the adhesive insulating layer. The adhesive insulating layer is in contact with the non-porous insulating layer and the second conductor layer 3.

[0033] Figure 2 shows another embodiment of a flexible multilayer circuit board. The flexible multilayer circuit board shown in Figure 2 has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, and a conductive portion 4. The first conductor layer 2 is located on one side of the insulating layer 1 in the thickness direction. The second conductor layer 3 is located on the other side of the insulating layer 1 in the thickness direction. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulating region 11 and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The porous insulating region 11 is a porous insulating layer having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is located within the through holes of the porous insulating layer. The conductive portion 4 has a tapered shape that narrows from the second conductor layer 3 side to the first conductor layer 2 side. The insulating layer 1 further comprises an adhesive insulating layer 13, a second non-porous insulating layer 14, and a non-porous insulating layer 15. The non-porous insulating layer 15 is located on the second conductor layer 3 side of the porous insulating region 11. The adhesive insulating layer 13 is located on the porous insulating region 11 side of the second conductor layer 3. The non-porous insulating layer 15 is in contact with the porous insulating region 11 and the adhesive insulating layer 13. The adhesive insulating layer 13 is in contact with the non-porous insulating layer 15 and the second conductor layer 3. The second non-porous insulating layer 14 is located on the first conductor layer 2 side of the porous insulating region 11. The second non-porous insulating layer 14 is in contact with the porous insulating region 11 and the first conductor layer 2.

[0034] Figure 3 shows another embodiment of a flexible multilayer circuit board. The flexible multilayer circuit board shown in Figure 3 differs from the flexible multilayer circuit board shown in Figure 2 in that it does not have an adhesive insulating layer 13. The flexible multilayer circuit board shown in Figure 3 has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, and a conductive portion 4. The first conductor layer 2 is arranged on one side in the thickness direction of the insulating layer 1. The second conductor layer 3 is arranged on the other side in the thickness direction of the insulating layer 1. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulating region 11 and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The porous insulating region 11 is a porous insulating layer having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is arranged within the through holes of the porous insulating layer. The conductive portion 4 has a tapered shape that narrows from the second conductor layer 3 side towards the first conductor layer 2 side. The insulating layer 1 further includes a second non-porous insulating layer 14 and a non-porous insulating layer 15. The non-porous insulating layer 15 is located on the second conductor layer 3 side of the porous insulating region 11. The non-porous insulating layer 15 is in contact with the porous insulating region 11 and the second conductor layer 3. The second non-porous insulating layer 14 is located on the first conductor layer 2 side of the porous insulating region 11. The second non-porous insulating layer 14 is in contact with the porous insulating region 11 and the first conductor layer 2.

[0035] Figure 4 shows another embodiment of a flexible multilayer circuit board. The flexible multilayer circuit board shown in Figure 4 has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, and a conductive portion 4. The first conductor layer 2 is located on one side of the insulating layer 1 in the thickness direction. The second conductor layer 3 is located on the other side of the insulating layer 1 in the thickness direction. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulating region 11 and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The porous insulating region 11 is a porous insulating layer having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is located within the through holes in the porous insulating layer. The conductive portion 4 is cylindrical in shape. The insulating layer 1 further has a second non-porous insulating layer 14. The second non-porous insulating layer 14 is positioned on the first conductor layer 2 side of the porous insulating region 11. The second non-porous insulating layer 14 is in contact with the porous insulating region 11 and the first conductor layer 2. The insulating layer 1 further has a second non-porous insulating region 16. The second non-porous insulating region 16 covers the side surface of the porous insulating region 11 at the widthwise end of the flexible multilayer circuit board. When forming the porous insulating region 11 by coating during the manufacturing of the flexible multilayer circuit board, forming the porous insulating region 11 by coating after providing the second non-porous insulating region 16 prevents the coating liquid from flowing out from the edges and makes the thickness of the porous insulating region 11 uniform.

[0036] Next, an example of the manufacturing of a flexible multilayer circuit board will be described. Figures 5A to 5F are diagrams illustrating an example of the manufacturing of the flexible multilayer circuit board shown in Figure 1. First, a laminate is prepared having a first conductor layer 2, a porous insulator region 11 (porous insulating layer), and a second conductor layer 3 in that order (Figure 5A). Next, a through-hole a is formed in the laminate, penetrating the second conductor layer 3 and the porous insulator region 11 and reaching the first conductor layer 2 (Figure 5B). The through-hole a can be formed, for example, by laser processing. Examples of lasers include YAG lasers and carbon dioxide lasers. Next, a non-porous insulator 12a is formed inside the through-hole a to fill the through-hole a, and a non-porous insulating layer is formed on the second conductor layer 3 (Figure 5C). The non-porous insulator 12a and the non-porous insulating layer can be formed, for example, by applying a resin-containing solution and drying it. Next, the non-porous insulating layer and a part of the non-porous insulator 12a filling the through-hole a are removed by etching (Figure 5D). Next, a through-hole a is formed in the non-porous insulator 12a, penetrating the non-porous insulator 12a and reaching the first conductor layer 2, such that the outer periphery of the non-porous insulator 12a remains (Figure 5E). The through-hole a can be formed, for example, by laser processing. The remaining outer periphery becomes the non-porous insulator region 12. Next, copper plating is applied to the through-hole a, filling the inside of the through-hole a with copper to form the conductive portion 4 (Figure 5F). The copper plating is, for example, electrolytic copper plating. Before performing electrolytic copper plating, a seed layer may be formed. As a result, the flexible multilayer circuit board shown in Figure 1 (Figure 5F) is obtained.

[0037] Next, we will describe another example of the manufacturing of a flexible multilayer circuit board. Figures 6A to 6G are diagrams illustrating the manufacturing example of the flexible multilayer circuit board shown in Figure 4. First, a first conductor layer 2 is prepared (Figure 6A). Next, a layered non-porous insulator 14A is formed on the first conductor layer 2 (Figure 6B). The non-porous insulator 14A can be formed, for example, by applying a photosensitive resin solution containing a photosensitive resin and drying it. Next, the non-porous insulator 14A is processed so that a second non-porous insulating layer 14 is formed on the first conductor layer 2, and a cylindrical non-porous insulator 12A for forming a non-porous insulating region 12 and a second non-porous insulating region 16 covering the side surface of the porous insulating region 11 at the widthwise end of the flexible multilayer circuit board are formed on the second non-porous insulating layer 14 (Figure 6C). The processing can be performed, for example, by exposure and development of the photosensitive non-porous insulator 14A for patterning. Next, a porous insulating region 11 is formed on the second non-porous insulating layer 14 (Figure 6D). Methods for forming the porous insulating region 11 include, for example, applying a solution containing a resin and a porosizing agent onto the second non-porous insulating layer 14, drying it, and then performing porosization. Methods for porosization include, for example, extracting the porosizing agent from the resin film using supercritical carbon dioxide. Another method for forming the porous insulating region 11 is, for example, applying a solution containing a resin and hollow particles onto the second non-porous insulating layer 14 and drying it. Next, a second conductor layer 3 is formed on the porous insulating region 11, the non-porous insulator 14A, and the second non-porous insulating region 16 (Figure 6E). The second conductor layer 3 can be formed, for example, by thermocompressing copper foil as the second conductor layer 3 onto the porous insulating region 11, the non-porous insulator 14A, and the second non-porous insulating region 16. The conditions for thermocompression are not particularly limited. Next, through-holes a are formed in the second conductor layer 3 and the non-porous insulator 12A, penetrating the non-porous insulator 12A and reaching the first conductor layer 2, such that the outer periphery of the non-porous insulator 12A remains (Figure 6F). The through-holes a can be formed, for example, by laser processing. The remaining outer periphery becomes the non-porous insulator region 12.Next, copper plating is applied to the through-hole a, filling the through-hole a with copper to form the conductive portion 4 (Figure 6G). The copper plating is, for example, electrolytic copper plating. Before performing electrolytic copper plating, a seed layer may be formed. As a result, the flexible multilayer circuit board shown in Figure 4 (Figure 6G) is obtained.

[0038] Figures 7A and 7B show another embodiment of the flexible multilayer circuit board. Figure 7A is a schematic cross-sectional view of another embodiment of the flexible multilayer circuit board. Figure 7B is a cross-sectional view taken along line A-A' of the flexible multilayer circuit board of Figure 7A. The flexible multilayer circuit board shown in Figures 7A and 7B has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, and a conductive portion 4. The first conductor layer 2 is located on one side of the insulating layer 1 in the thickness direction. The second conductor layer 3 is located on the other side of the insulating layer 1 in the thickness direction. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulating region 11 and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The porous insulating region 11 is a porous insulating layer having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is located within a through-hole in the porous insulating layer. The conductive portion 4 is cylindrical in shape. The insulating layer 1 further includes a second non-porous insulating layer 14. The second non-porous insulating layer 14 is located on the first conductor layer 2 side of the porous insulating region 11. The second non-porous insulating layer 14 is in contact with both the porous insulating region 11 and the first conductor layer 2.

[0039] In the flexible multilayer circuit board shown in Figures 7A and 7B, as shown in Figure 7B, the two non-porous insulating regions 12 covering the sides of two adjacent conductive portions 4 are separate and independent. The width (T) of the non-porous insulating region 12 covering the sides of the conductive portions 4 is, for example, 1 μm to 100 μm, and preferably 1 μm to 50 μm.

[0040] Figures 8A and 8B show another embodiment of the flexible multilayer circuit board. Figure 8A is a schematic cross-sectional view of another embodiment of the flexible multilayer circuit board. Figure 8B is a cross-sectional view taken along line A-A' of the flexible multilayer circuit board of Figure 8A. The flexible multilayer circuit board shown in Figures 8A and 8B has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, and a conductive portion 4. The first conductor layer 2 is located on one side of the insulating layer 1 in the thickness direction. The second conductor layer 3 is located on the other side of the insulating layer 1 in the thickness direction. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulating region 11 and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The porous insulating region 11 is a porous insulating layer having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is located within a through-hole in the porous insulating layer. The conductive portion 4 is cylindrical in shape. The insulating layer 1 further includes a second non-porous insulating layer 14. The second non-porous insulating layer 14 is located on the first conductor layer 2 side of the porous insulating region 11. The second non-porous insulating layer 14 is in contact with both the porous insulating region 11 and the first conductor layer 2.

[0041] In the flexible multilayer circuit board shown in Figures 8A and 8B, as shown in Figure 8B, the sides of three adjacent conductive portions 4 are covered by a single non-porous insulating region 12. The distance (L) between two conductive portions 4 covered by a single non-porous insulating region 12 is, for example, 150 μm or less, and between 50 μm and 150 μm. The distance (L) between two conductive portions 4 is the shortest distance between the ends of the two conductive portions 4.

[0042] Figure 9 shows another embodiment of a flexible multilayer circuit board. The flexible multilayer circuit board shown in Figure 9 has an insulating layer 1, a first conductor layer 2, a second conductor layer 3, a conductive portion 4, and a wiring portion 5. The first conductor layer 2 is arranged on one side in the thickness direction of the insulating layer 1. The second conductor layer 3 is arranged on the other side in the thickness direction of the insulating layer 1. The conductive portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a first porous insulating region 21, an adhesive insulating layer 22, a second porous insulating region 23, and a non-porous insulating region 12. The non-porous insulating region 12 covers the side surface of the conductive portion 4. The first porous insulating region 21 and the second porous insulating region 23 are porous insulating layers having through holes that penetrate in the thickness direction. The non-porous insulating region 12 covers the side surface of the through holes in the porous insulating layer. The conductive portion 4 is arranged within the through holes of the porous insulating layer. The conductive portion 4 is columnar. The wiring portion 5 is embedded in the insulating layer 1. Specifically, the wiring portion 5 is embedded in the adhesive insulating layer 22. The wiring portion 5 is not in contact with the first conductor layer 2, the second conductor layer 3, and the conductive portion 4. The wiring portion 5 is, for example, a signal line that transmits electrical signals, and preferably a signal line for high-speed transmission. An example of high-speed transmission is a fifth-generation mobile communication system (5G).

[0043] The manufacturing method for the flexible multilayer circuit board shown in Figure 9 is as follows. First, a first laminate is prepared by stacking a first conductor layer 2, a porous insulating layer which is a first porous insulating region 21, a first adhesive insulating layer precursor layer, and a conductor layer for creating wiring sections in this order. The wiring sections 5 and conductors are formed by patterning the conductor layer for creating wiring sections of the first laminate by photolithography using a photoresist (for example, subtractive method). A second laminate is prepared by stacking a second conductor layer 3, a porous insulating layer which is a second porous insulating region 23, and a second adhesive insulating layer precursor layer in this order. The first laminate with the wiring sections 5 formed on it and the second laminate are placed facing each other and bonded together to obtain a third laminate. In this way, the first adhesive insulating layer precursor layer and the second adhesive insulating layer precursor layer become integrated to form an adhesive insulating layer 22. In the obtained third laminate, through holes are formed that penetrate the first conductor layer 2, the first porous insulator region 21, the adhesive insulator layer 22, the conductor embedded in the adhesive insulator layer 22, the second porous insulator region 23, and the second conductor layer 3. Then, non-porous insulator regions 12 are formed on the sides of the formed through holes. Finally, conductive portions 4 are formed. As a result, the flexible multilayer circuit board shown in Figure 9 is obtained.

[0044] FIG. 10 shows another embodiment of a flexible multilayer circuit board. The flexible multilayer circuit board shown in FIG. 10 includes an insulating layer 1, a first conductor layer 2, a second conductor layer 3, a conduction portion 4, and a wiring portion 5. The first conductor layer 2 is disposed on one side in the thickness direction of the insulating layer 1. The second conductor layer 3 is disposed on the other side in the thickness direction of the insulating layer 1. The conduction portion 4 electrically connects the first conductor layer 2 and the second conductor layer 3. The insulating layer 1 has a porous insulator region 31 and a non-porous insulator region 12. The non-porous insulator region 12 covers the side surface of the conduction portion 4. The porous insulator region 31 is a porous insulating layer having through holes penetrating in the thickness direction. The non-porous insulator region 12 covers the side surface of the through holes of the porous insulating layer. The conduction portion 4 is disposed in the through holes of the porous insulating layer. The conduction portion 4 has a columnar shape. The insulating layer 1 further has a non-porous insulating layer 32. The non-porous insulating layer 32 is disposed on the second conductor layer 3 side of the porous insulator region 31. The non-porous insulating layer 32 contacts the porous insulator region 11 and the second conductor layer 3. The second conductor layer 3 has a pattern shape. The wiring portion 5 is disposed on the non-porous insulating layer 32. The wiring portion 5 does not contact the first conductor layer 2, the second conductor layer 3, and the conduction portion 4. The wiring portion 5 is, for example, a signal line for transmitting an electrical signal, and preferably a signal line for high-speed transmission. Examples of high-speed transmission include the fifth-generation mobile communication system (5G). The wiring portion 5 and the patterned second conductor layer 3 exist on the same plane.

[0045] 1 Insulating layer 2 First conductor layer 3 Second conductor layer 4 Conductive section 5 Wiring section 11 Porous insulating region 12 Non-porous insulating region 12a Non-porous insulator 12A Non-porous insulator 13 Adhesive insulating layer 14 Second non-porous insulating layer 14A Non-porous insulator 15 Non-porous insulating layer 16 Second non-porous insulating region 21 First porous insulating region 22 Adhesive insulating layer 23 Second porous insulating region 31 Porous insulating region 32 Non-porous insulating layer 101 Porous insulating layer 102 First conductor layer 103 Second conductor layer 104 Conductive section a Through hole

Claims

1. A flexible multilayer circuit board comprising: an insulating layer; a first conductor layer disposed on one side in the thickness direction of the insulating layer; a second conductor layer disposed on the other side in the thickness direction of the insulating layer; and a conductive portion electrically connecting the first conductor layer and the second conductor layer, wherein the insulating layer has a porous insulating region and a non-porous insulating region, and the non-porous insulating region covers the side surface of the conductive portion.

2. The flexible multilayer circuit board according to claim 1, wherein the width of the non-porous insulating region is 1 μm to 100 μm.

3. The flexible multilayer circuit board according to claim 1, further comprising a wiring portion, wherein the wiring portion is embedded in the porous insulator region.

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