Flexible multilayer circuit board

Abstract

To provide a flexible multilayer substrate that can suppress fluctuations in the electrical characteristics of the wiring section. [Solution] A flexible multilayer circuit board 10 comprising an insulating layer 21, 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 wiring portion 5, wherein the insulating layer has a porous insulating layer 2 and a non-porous insulating layer 3, and the wiring portion is embedded in the porous insulating layer.

Description

Technical Field

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

Background Art

[0002] In recent years, the development of so-called "fifth-generation (5G)" standard wireless communication has been underway. With the "fifth-generation (5G)" standard wireless communication, a large amount of data can be transmitted at high speed. In the "fifth-generation (5G)" standard wireless communication, high frequencies including millimeter waves are used. As a substrate for a high-frequency antenna that emits such millimeter waves, a substrate with a low dielectric constant (low-dielectric substrate) is required. Also, as a flexible printed circuit board (FPC), a high-speed transmission FPC that transmits data at high speed is required, and a low-dielectric substrate is also required as a substrate for this high-speed transmission FPC.

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

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] It may be necessary to manufacture a flexible multilayer circuit board by laminating two or more layer precursors. An example thereof will be described below. Figure 10 is a schematic diagram of an example of a conventional flexible multilayer circuit board. The flexible multilayer circuit board 60 shown in Figure 10 comprises a porous insulating layer 52 having holes P, a first conductor layer 51 disposed on one side in the thickness direction of the porous insulating layer 52, a second conductor layer 53 disposed on the other side in the thickness direction of the porous insulating layer 52, and wiring portions 54 embedded in the porous insulating layer 52. The flexible multilayer circuit board 60 shown in Figure 10 is manufactured by the method shown in Figures 11A to 11F. The method is described below. First, a laminate of the wiring precursor 54A and the porous insulating layer first precursor 52A is prepared (Figure 11A). The porous insulating layer first precursor 52A has pores P. Next, the wiring portion precursor 54A in the prepared laminate is selectively etched to form the wiring portion 54, thereby obtaining the first circuit board precursor 70A (Figure 11B). Next, a second circuit board precursor 70B is prepared, comprising a second conductor layer 53 and a porous insulating layer second precursor 52B arranged on one side in the thickness direction of the second conductor layer 53 (Figure 11C). The porous insulating layer second precursor 52B has pores P. Next, the first circuit board precursor 70A and the second circuit board precursor 70B are arranged so that the wiring section 54 and the porous insulating layer second precursor 52B face each other (Figure 11D). Then, the first circuit board precursor 70A and the second circuit board precursor 70B are bonded together. By doing so, the first porous insulating layer precursor 52A and the second porous insulating layer precursor 52B become one, forming the porous insulating layer 52 (Figure 11E). Next, the first conductor layer 51 is bonded to one side of the porous insulating layer 52 in the thickness direction (Figure 11F). This yields the flexible multilayer circuit board 60 shown in Figure 10. Here, when the first circuit board precursor 70A and the second circuit board precursor 70B are bonded together, the resulting porous insulating layer 52 deforms under pressure. This deformation of the porous insulating layer 52 changes its insulating properties (e.g., dielectric properties), and as a result, the electrical properties of the wiring section 54 may deviate from the desired electrical properties.

[0006] The present invention aims to provide a flexible multilayer circuit board that can suppress fluctuations in the electrical characteristics of the wiring section. [Means for solving the problem]

[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. In other words, the present invention encompasses the following:

[0008] [1] Insulating layer and, A first conductor layer is arranged on one side in the thickness direction of the insulating layer, A second conductor layer is arranged on the other side in the thickness direction of the insulating layer, Wiring section and A flexible multilayer circuit board comprising, The insulating layer comprises 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 arranged on one side in the thickness direction of the porous insulating layer, The second non-porous insulating layer is located on the other side in the thickness direction of the porous insulating layer. [1] The flexible multilayer circuit board described above. [3] The flexible multilayer circuit board according to [1] or [2], wherein the porous insulating layer comprises a liquid crystal polymer, a cycloolefin polymer, or a polyimide. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a flexible multilayer circuit board that can suppress fluctuations in the electrical characteristics of the wiring section. [Brief explanation of the drawing]

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

[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 section. The first conductor layer is positioned on one side in the thickness direction of the insulating layer. The second conductor layer is positioned on the opposite side in the thickness direction of the insulating layer. The insulating layer comprises a porous insulating layer and a non-porous insulating layer. The wiring is embedded in a porous insulating layer.

[0012] In a flexible multilayer circuit board having wiring embedded in a porous insulating layer, the presence of a non-porous insulating layer suppresses deformation of the porous insulating layer due to pressure during the manufacturing of the flexible multilayer circuit board. As a result, fluctuations in the electrical characteristics of the wiring in the flexible multilayer circuit board can be suppressed. Furthermore, having a non-porous insulating layer offers advantages in terms of reliability and adhesion. The reasons are as follows: While porous insulating layers are susceptible to moisture and ions due to the presence of pores, non-porous insulating layers suppress the intrusion of moisture and ions into the porous insulating layer. As a result, degradation of the porous insulating layer can be prevented. Furthermore, degradation of the porous insulating layer and the intrusion of moisture into the porous insulating layer lead to a deterioration of its dielectric properties. In addition, degradation of the porous insulating layer leads to a decrease in adhesion between the porous insulating layer and adjacent layers. When adhesion decreases, interfacial delamination is more likely to occur during mounting and bending processes. Interfacial delamination can lead to short circuits. Moreover, decreased adhesion makes it easier for wiring to collapse when forming the wiring sections of flexible multilayer circuit boards using photolithography. A non-porous insulating layer can prevent such a decrease in reliability and adhesion.

[0013] An example of the flexible multilayer circuit board of the present invention will be described below with reference to Figure 1. Figure 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 length direction of the wiring section. The flexible multilayer circuit board 10 shown in Figure 1 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring section 5. The insulating layer 21 comprises a porous insulating layer 2 and a non-porous insulating layer 3. The porous insulating layer 2 has numerous 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 positioned on one side in the thickness direction of the insulating layer 21. The first conductor layer 1 is in contact with the porous insulating layer 2 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 in Figure 1. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 5 transmits electrical signals, for example.

[0014] In the flexible multilayer circuit board 10 shown in Figure 1, the first conductor layer 1 and the second conductor layer 4 extend in the longitudinal direction of the wiring section 5. In Figure 1, the longitudinal direction of the wiring section 5 is the direction perpendicular to the vertical and horizontal directions of the paper.

[0015] In Figure 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 are sandwiched by an insulating layer 21.

[0016] The materials of the first and second conductor layers are not particularly limited, and examples include metallic materials. Examples of metallic materials include copper, nickel, gold, solder, and alloys of two or more of these materials. 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 also, for example, 50 μm or less, preferably 30 μm or less. The thicknesses of the first conductor layer and the second conductor layer may be the same or different. In this invention, "thickness" refers to the length of the flexible multilayer circuit board in the thickness direction. The thickness direction of the flexible multilayer circuit board refers, for example, to the direction perpendicular to the plane direction of the first conductor layer and the second conductor layer.

[0017] The material of the wiring section is not particularly limited, and examples 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, but is, for example, 3 μm or more, preferably 5 μm or more, and also, for example, 50 μm or less, preferably 30 μm or less. 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 section (the length of the wiring section in a direction perpendicular to the thickness and length directions of the wiring section) is not particularly limited, but is usually shorter than the width of the first and second conductor layers (the length of the first and second conductor layers in a direction perpendicular to the thickness and length directions of the first and second conductor layers), for example, 1 / 5 to 1 / 2 of the width of the first and second conductor layers. Also, since the wiring section is embedded in the porous insulating layer, the width of the wiring section is usually shorter than the width of the porous insulating layer.

[0018] The wiring portion is embedded in a porous insulating layer, but the wiring portion may also have a non-porous insulating film covering its surface. Examples of materials that constitute the non-porous insulating film include those listed in the description of the materials that constitute the insulating layer, which will be discussed later.

[0019] Examples of materials constituting the insulating layer include resins. In other words, the insulating layer includes, for example, a resin. 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, cycloolefin polymer, and liquid crystal polymer. From the viewpoint of high insulating properties, high heat resistance, and high mechanical strength, polyimide resin, cycloolefin polymer, and liquid crystal polymer are preferred. 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 also, for example, 1000 μm or less, preferably 600 μm or less.

[0020] The porous insulating layer (including the second porous insulating layer and the third porous insulating layer described later) is porous. The porous insulating layer has many fine pores (air pockets) as holes. Examples of cellular structures in porous insulating layers include closed-cell structures and open-cell structures. A porous insulating layer with an open-cell structure is more susceptible to deformation under pressure during the manufacturing 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 if the porous insulating layer has an open-cell structure. For this reason, in the present invention, it is preferable that the porous insulating layer has an open-cell structure. Here, a closed-cell structure refers to a structure in which there are no pores or other openings in the resin portion between pores that connect them, and gas does not flow between adjacent pores. In contrast, a continuous-cell structure refers to a structure in which adjacent pores are connected by pores, and gas flows between them.

[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 also, 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. Furthermore, the porosity of the porous insulating layer is, for example, less than 100%, and even more preferably 99% or less. When the material of the porous insulating layer is polyimide resin, the porosity of the porous insulating layer can be determined by calculation based on the following formula. The dielectric constant of a 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 a 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 also, for example, greater than 1.0. The dielectric constant of the porous insulating layer is measured by a resonator method using a frequency of 60 GHz.

[0024] The dielectric loss tangent of a porous insulating layer at a frequency of 60 GHz is, for example, 0.006 or less, and also, for example, greater than 0. The dielectric loss tangent of a porous insulating layer is measured using a resonator method at a frequency of 60 GHz.

[0025] The non-porous insulating layer (including the second, third, and fourth non-porous insulating layers described later) is not porous. Therefore, the porosity of the non-porous insulating layer is approximately 0%. Furthermore, the dielectric constant of the non-porous insulating layer is consistent with the dielectric constant determined from the material constituting the non-porous insulating layer. For example, if a non-porous insulating layer is composed solely of resin, the dielectric constant of the non-porous insulating layer will match the dielectric constant of the resin constituting the non-porous 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 also, for example, 150 μm or less, preferably 100 μm or less.

[0027] Furthermore, the non-porous insulating layer and the non-porous insulating film may be skin layers. 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 the pores on the surface of the porous insulating layer or porous insulating layer precursor and to densify the surface.

[0028] One embodiment of the manufacturing method for the flexible multilayer circuit board shown in Figure 1 will be explained using Figures 2A to 2D. First, a laminate of the wiring precursor 5A and the porous insulating layer first precursor 2A is prepared (Figure 2A). Both the wiring 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 with the porous insulating layer second precursor 2B, thereby integrating with the porous insulating layer second precursor 2B to form a porous insulating layer. Next, the prepared laminated wiring precursor 5A is selectively etched to form the wiring portion 5, thereby obtaining the first circuit board precursor 20A (Figure 2B). Next, a second circuit board precursor 20B and a third circuit board precursor 20C are prepared. The second circuit board precursor 20B comprises a first conductor layer 1 and a porous insulating layer second precursor 2B arranged on one side in the thickness direction of the first conductor layer 1. The porous insulating layer second precursor 2B has pores P. The third circuit board precursor 20C comprises a second conductor layer 4 and a non-porous insulating layer 3 arranged on one side in the thickness direction 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 and the porous insulating layer second precursor 2B face each other. Furthermore, the second circuit board precursor 20B and the third circuit board precursor 20C are then arranged so that the side of the porous insulating layer first precursor 2A opposite to the wiring portion 5 side faces the non-porous insulating layer 3 (Figure 2C). Then, the second circuit board precursor 20B, the first circuit board precursor 20A, and the third circuit board precursor 20C are bonded together. There are no particular restrictions on the pressure and temperature during bonding. By doing so, the first porous insulating layer precursor 2A and the second porous insulating layer precursor 2B become integrated to form a porous insulating layer 2, and further, the wiring section 5 is embedded in the porous insulating layer 2 (Figure 2D). As a result, the flexible multilayer circuit board 10 shown in Figure 1 is obtained.

[0029] Figure 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 length direction of the wiring section. The flexible multilayer circuit board 10 shown in Figure 3 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring section 5. The insulating layer 21 comprises a second non-porous insulating layer 6, a porous insulating layer 2, and a non-porous insulating layer 3 in that order. The porous insulating layer 2 has numerous 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 thickness and material 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 positioned 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 in Figure 3. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 5 transmits electrical signals, for example.

[0030] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in Figure 3, 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. Because the porous insulating layer 2 is sandwiched between the non-porous insulating layer 3 and the second non-porous insulating layer 6, deformation of the porous insulating layer due to pressure during the manufacturing of the flexible multilayer circuit board can be further suppressed. In addition, because the porous insulating layer 2 is sandwiched between the non-porous insulating layer 3 and the second non-porous insulating layer 6, it becomes easier to make the distance between the wiring section 5 and the first conductor layer 1 equal to the distance between the wiring section 5 and the second conductor layer 4. Here, if the wiring section 5 is a signal line and the first conductor layer 1 and the second conductor layer 4 are ground layers, if the distance between the wiring section 5 and the first conductor layer 1 and the distance between the wiring section 5 and the second conductor layer 4 differ significantly, only the conductor layer closer to the wiring section 5 will function as a ground layer. On the other hand, if the distance between the wiring section 5 and the first conductor layer 1 and the distance between the wiring section 5 and the second conductor layer 4 are equal, both the first conductor layer 1 and the second conductor layer 4 can function as ground layers.

[0031] Figure 4 is a schematic diagram illustrating one embodiment of the manufacturing method for the flexible multilayer circuit board shown in Figure 3. The flexible multilayer circuit board 10 shown in Figure 3 can be obtained, for example, by bonding together 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 Figure 4.

[0032] The first precursor circuit board 20D has a second conductor layer 4 and a non-porous insulating layer 3 disposed on one side in the thickness direction of the second conductor layer 4. The second precursor circuit board 20E comprises a first porous insulating layer precursor 2C and a wiring portion 5 arranged on one side in the thickness direction of the first porous insulating layer precursor 2C. The first porous insulating layer precursor 2C has pores P. 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 precursor 20F of the circuit board is the second precursor 2D of the porous insulating layer. The second precursor 2D of the porous insulating layer has pores P. The fourth precursor circuit board 20G has a first conductor layer 1 and a second nonporous insulating layer 6 arranged on one side in the thickness direction 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 Figure 4. In this way, the first porous insulating layer precursor 2C and the second porous insulating layer precursor 2D become integrated to form the porous insulating layer 2. As a result, the flexible multilayer circuit board 10 shown in Figure 3 is obtained.

[0034] In Figure 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) were bonded together at once. The number of circuit board precursors bonded together at one time may be two, three, or four or more.

[0035] Figure 5 is a schematic diagram of another embodiment of a flexible multilayer circuit board. This schematic diagram is a cross-sectional view perpendicular to the length direction of the wiring section. The flexible multilayer circuit board 10 shown in Figure 5 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring section 5. The insulating layer 21 comprises a second non-porous insulating layer 6, a porous insulating layer 2, and a non-porous insulating layer 3 in that order. The porous insulating layer 2 has numerous 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 thickness and material 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 positioned 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 in Figure 5. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 5 transmits electrical signals, for example.

[0036] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in Figure 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. Because the porous insulating layer 2 is sandwiched between the non-porous insulating layer 3 and the second non-porous insulating layer 6, deformation of the porous insulating layer due to pressure during the manufacturing of the flexible multilayer circuit board can be further suppressed.

[0037] Furthermore, the non-porous insulating layer 3 and the second non-porous insulating layer 6 in the flexible multilayer circuit board 10 shown in Figure 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 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 the pores on the surface of the porous insulating layer or porous insulating layer precursor and to densify the surface.

[0038] Figure 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 section. The flexible multilayer circuit board 10 shown in Figure 6 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring section 5. The insulating layer 21 comprises 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 in this order. The porous insulating layer 2, the second porous insulating layer 7, and the third porous insulating layer 11 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 thickness and material 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 in 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 positioned 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 in Figure 6. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 5 transmits electrical signals, for example.

[0039] In the insulating layer 21 of the flexible multilayer circuit board 10 shown in Figure 6, the porous insulating layer 2 is sandwiched between the third non-porous insulating layer 8 and the fourth non-porous insulating layer 9. Because the porous insulating layer 2 in which the wiring section 5 is embedded is sandwiched 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 during the manufacturing of the flexible multilayer circuit board can be further suppressed.

[0040] Figure 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 length direction of the wiring section. The flexible multilayer circuit board 10 shown in Figure 7 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, a wiring section 5, and a non-porous insulating film 12. The insulating layer 21 comprises a second non-porous insulating layer 6, a porous insulating layer 2, and a non-porous insulating layer 3 in that order. The porous insulating layer 2 has numerous 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 thickness and material 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 positioned 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 in Figure 7. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 5 transmits electrical signals, for example. The wiring section 5 has a non-porous insulating film 12 covering its surface. In other words, the wiring section 5, whose surface is covered with a 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 constituting 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 the pores on the surface of the porous insulating layer at the interface between the porous insulating layer and the wiring portion, thereby densifying 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 and the second conductor layer 4. Because the porous insulating layer 2 is sandwiched between the non-porous insulating layer 3 and the second non-porous insulating layer 6, deformation of the porous insulating layer due to pressure during the manufacturing of the flexible multilayer circuit board can be further suppressed.

[0042] An embodiment of a flexible multilayer circuit board will be explained with reference to the figures. Figures 8A and 8B are schematic diagrams of an example of a flexible multilayer circuit board. The flexible multilayer circuit boards shown in Figures 8A and 8B are striplines. Figure 8A is a perspective view. Figure 8B is a cross-sectional view taken along line A-A' in Figure 8A. Note that Figures 8A and 8B are schematic diagrams. Therefore, for example, in Figure 8B, the ratio of the length in the longitudinal direction to the length in the thickness direction is not accurate. The flexible multilayer circuit board 10 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring section 5. The insulating layer 21 comprises a porous insulating layer 2 and a non-porous insulating layer 3. The porous insulating layer 2 has numerous 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 positioned on one side in the thickness direction of the insulating layer 21. The first conductor layer 1 is in contact with the porous insulating layer 2 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 in Figure 8B. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 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 constitute the ground. Note that a portion of the conductive portion 101 is formed simultaneously when the wiring portion 5 is formed, so a portion of the conductive portion 101 is made of the same material as the wiring portion 5. Furthermore, as shown in Figures 8A and 8B, the two conductive portions 101 are positioned on either side of the wiring portion 5 that extends in the longitudinal direction. The conductive portions 101 are also spaced apart along the wiring portion 5.

[0043] An embodiment of a flexible multilayer circuit board will be explained with reference to the figures. Figures 9A and 9B are schematic diagrams of an example of a flexible multilayer circuit board. The flexible multilayer circuit boards shown in Figures 9A and 9B are striplines. Figure 9A is a perspective view. Figure 9B is a cross-sectional view taken along line A-A' in Figure 9A. Note that Figures 9A and 9B are schematic diagrams. Therefore, for example, in Figure 9B, the ratio of the length in the longitudinal direction to the length in the thickness direction is not accurate. The flexible multilayer circuit board 10 comprises an insulating layer 21, a first conductor layer 1, a second conductor layer 4, and a wiring section 5. The insulating layer 21 comprises a porous insulating layer 2 and a non-porous insulating layer 3. The porous insulating layer 2 has numerous 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 positioned on one side in the thickness direction of the insulating layer 21. The first conductor layer 1 is in contact with the porous insulating layer 2 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 in Figure 9B. The second conductor layer 4 is positioned 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 from the one side. The wiring section 5 is embedded in the porous insulating layer 2. The wiring section 5 is a signal line that transmits electrical signals. In the flexible multilayer circuit board shown in Figures 9A and 9B, the wiring section 5 has two signal lines (first signal line 501 and 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 constitute the ground. Note that a portion of the conductive portion 101 is formed simultaneously when the wiring portion 5 (first signal line 501 and second signal line 502) is formed, so a portion of the conductive portion 101 is made of the same material as the wiring portion 5. Furthermore, as shown in Figures 9A and 9B, the two conductive parts 101 are positioned on either side of the wiring section 5 (first signal line 501 and second signal line 502) which extends in the longitudinal direction. The conductive parts 101 are also spaced apart along the wiring section 5 (first signal line 501 and second signal line 502).

[0044] Flexible multilayer circuit boards are used in electronic devices such as mobile phones, smartphones, tablet devices, and digital cameras, for example, as flexible multilayer circuit boards for high-speed transmission, in response to the need for miniaturization, weight reduction, and high functionality. [Explanation of Symbols]

[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 section 5A Wiring Section 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 boards 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 section 54A Wiring section precursor 60 Flexible Multilayer Circuit Boards 70A circuit board first precursor 70B Circuit board second precursor 101 Conductive section 501 First signal line 502 Second signal line P hole

Claims

1. Insulating layer and, A first conductor layer is arranged on one side in the thickness direction of the insulating layer, A second conductive layer is arranged on the other side in the thickness direction of the insulating layer, A wiring section that is a signal line and not a waveguide, A flexible multilayer circuit board comprising, The insulating layer comprises a porous insulating layer and a non-porous insulating layer. The porous insulating layer and the non-porous insulating layer are bonded together. 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 arranged on one side in the thickness direction of the porous insulating layer, The second non-porous insulating layer is located on the other side in the thickness direction of the porous insulating layer. 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.

Images