Flexible multilayer circuit board

Abstract

To provide a flexible multilayer circuit board in which a wiring part is hardly deformed even when pressure is applied.SOLUTION: A flexible multilayer circuit board comprising: a porous insulating layer containing hollow particles; a first conductor layer disposed on one side in a thickness direction of the porous insulating layer; a second conductor layer disposed on the other side in the thickness direction of the porous insulating layer; and a wiring portion embedded in the porous insulating layer.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] In recent years, development of wireless communication complying with the so-called "fifth generation (5G)" standard has been advanced. Wireless communication complying with the "fifth generation (5G)" standard allows large-capacity data to be transmitted at high speed. In wireless communication complying with the "fifth generation (5G)" standard, 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. In addition, as an FPC (flexible printed circuit board), a high-speed transmission FPC that transmits data at high speed is in demand, and a low-dielectric substrate is also required as the substrate for this high-speed transmission FPC.

[0003] As a flexible multilayer circuit board, for example, a wiring circuit board has been proposed which includes a porous insulating layer and a conductor layer sequentially toward one side in the thickness direction, wherein 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] As a method for making an insulating film porous, for example, a method for producing a porous film has been proposed, which includes (1) a step of washing a film made of a liquid crystalline polymer composition containing an inorganic compound soluble in a first solvent with the first solvent to elute the inorganic compound and form holes penetrating both surfaces of the film, and (2) a step of washing the obtained porous film with a second solvent (see Patent Document 2).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of Invention

[0006] In the porous films disclosed in Patent Documents 1 and 2, the porous film is manufactured by forming voids in the film. In such porous films, the porous film deforms when pressure is applied. Therefore, if wiring is embedded in the porous film, when pressure is applied to the porous film, the wiring is more likely to deform along with the deformation of the porous film. In particular, the porous structure of the porous film disclosed in Patent Document 2 is an open-cell structure, so the deformation is even greater.

[0007] The present invention aims to provide a flexible multilayer circuit board in which the wiring section is less likely to deform even when pressure is applied. [Means for solving the problem]

[0008] In other words, the present invention encompasses the following: [1] A porous insulating layer containing hollow particles, A first conductive layer is disposed on one side in the thickness direction of the porous insulating layer, A second conductive layer is disposed on the other side in the thickness direction of the porous insulating layer, A wiring section embedded in the porous insulating layer, A flexible multilayer circuit board equipped with the following features. [2] The flexible multilayer circuit board according to [1], wherein the hollow particles are organic hollow particles. [3] The porous insulating layer comprises a first porous insulating layer and a second porous insulating layer disposed on one side in the thickness direction of the first porous insulating layer, The first porous insulating layer contains first hollow particles, The second porous insulating layer contains second hollow particles, The average particle diameter of the first hollow particle is smaller than the average particle diameter of the second hollow particle. The aforementioned wiring section is embedded in the first porous insulating layer. A flexible multilayer circuit board as described in [1] or [2]. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a flexible multilayer circuit board in which the wiring portion is less likely to deform even when pressure is applied. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic cross-sectional view of an example of the flexible multilayer circuit board of the present invention. [Figure 2A] Figure 2A is a schematic cross-sectional view illustrating an example of a manufacturing method for the flexible multilayer circuit board shown in Figure 1 (Part 1). [Figure 2B] Figure 2B is a schematic cross-sectional view illustrating an example of a manufacturing method for the flexible multilayer circuit board shown in Figure 1 (part 2). [Figure 2C] Figure 2C is a schematic cross-sectional view illustrating an example of a manufacturing method for the flexible multilayer circuit board shown in Figure 1 (part 3). [Figure 2D] Figure 2D is a schematic cross-sectional view illustrating an example of a manufacturing method for the flexible multilayer circuit board shown in Figure 1 (part 4). [Figure 2E] Figure 2E is a schematic cross-sectional view illustrating an example of a manufacturing method for the flexible multilayer circuit board shown in Figure 1 (part 5). [Figure 2F] Figure 2F is a schematic cross-sectional view illustrating an example of a manufacturing method for the flexible multilayer circuit board shown in Figure 1 (part 6). [Figure 3A] Figure 3A is a schematic cross-sectional view illustrating another example of the manufacturing method of the flexible multilayer circuit board shown in Figure 1 (Part 1). [Figure 3B] Figure 3B is a schematic cross-sectional view illustrating another example of the manufacturing method of the flexible multilayer circuit board shown in Figure 1 (part 2). [Figure 3C] Figure 3C is a schematic cross-sectional view illustrating another example of the manufacturing method of the flexible multilayer circuit board shown in Figure 1 (part 3). [Figure 3D] Figure 3D is a schematic cross-sectional view illustrating another example of the manufacturing method of the flexible multilayer circuit board shown in Figure 1 (part 4). [Figure 3E] FIG. 3E is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 5). [Figure 4A] FIG. 4A is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 1). [Figure 4B] FIG. 4B is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 2). [Figure 4C] FIG. 4C is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 3). [Figure 4D] FIG. 4D is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 4). [Figure 4E] FIG. 4E is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 5). [Figure 4F] FIG. 4F is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 6). [Figure 4G] FIG. 4G is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 7). [Figure 4H] FIG. 4H is a schematic cross-sectional view for explaining another example of the method for manufacturing the flexible multilayer circuit board of FIG. 1 (Part 8). [Figure 5] FIG. 5 is a schematic cross-sectional view of another example of the flexible multilayer circuit board of the present invention. [Modes for Carrying Out the Invention]

[0011] (Flexible Multilayer Circuit Board) The flexible multilayer circuit board of the present invention includes a porous insulating layer, a first conductor layer, a second conductor layer, and a wiring portion. The first conductor layer is disposed on one side in the thickness direction of the porous insulating layer. The second conductor layer is positioned on the other side of the thickness direction of the porous insulating layer. The wiring is embedded in a porous insulating layer. The porous insulating layer contains hollow particles.

[0012] The porous insulating layer forms a porous structure by containing a large number of hollow particles. Compared to porous structures formed by creating voids in a film, porous structures formed by containing hollow particles exhibit less variation in thickness when pressure is applied. Wiring embedded in such a porous structure is less prone to deformation when pressure is applied. Therefore, the wiring embedded in the porous insulating layer of the flexible multilayer circuit board of the present invention is less prone to deformation when pressure is applied.

[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 1. This schematic diagram is a cross-sectional view perpendicular to the length direction of the wiring section. The flexible multilayer circuit board 1 shown in Figure 1 comprises a porous insulating layer 3, a first conductor layer 2, a second conductor layer 4, and a wiring section 5. The first conductor layer 2 is positioned on one side of the porous insulating layer 3 in the thickness direction. The first conductor layer 2 is in contact with the porous insulating layer 3 on one side in the thickness direction. The thickness direction of the porous insulating layer 3 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 porous insulating layer 3. The second conductor layer 4 is in contact with the porous insulating layer 3 on the other side in the thickness direction of the porous insulating layer 3. The other side is the opposite side from the one side. The wiring section 5 is embedded in the porous insulating layer 3. The wiring section 5 transmits electrical signals, for example. The porous insulating layer 3 contains a large number of hollow particles 3a.

[0014] In the flexible multilayer circuit board 1 shown in Figure 1, the first conductor layer 2 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] <First conductor layer and second conductor layer> 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.

[0016] <Wiring section> 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.

[0017] <Porous insulating layer> The porous insulating layer contains hollow particles. The hollow particles may be inorganic hollow particles or organic hollow particles. Inorganic hollow particles are hollow particles that have a shell made of inorganic material. In other words, inorganic hollow particles have a shell made of inorganic material and a hollow part surrounded by the shell. Organic hollow particles are hollow particles that have an organic resin shell. In other words, organic hollow particles have a shell made of organic resin and a hollow part surrounded by the shell.

[0018] As hollow particles, organic hollow particles are preferred due to their excellent laser processability. When manufacturing flexible multilayer circuit boards, laser processing is sometimes performed on the porous insulating layer. When a porous insulating layer containing inorganic hollow particles is laser processed (for example, by creating through holes with a laser), fragments of the inorganic shell of the inorganic hollow particles may be generated. In contrast, when a porous insulating layer containing organic hollow particles is laser processed, no fragments of the shell are generated. This is because the organic resin shell of the organic hollow particles fuses together during laser processing.

[0019] Examples of inorganic materials that constitute the shells of inorganic hollow particles include silica, alumina, glass, cordierite, silicon oxide, barium sulfate, barium carbonate, talc, clay, mica powder, zinc oxide, hydrotalcite, boehmite, aluminum hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate, magnesium oxide, boron nitride, aluminum nitride, manganese nitride, aluminum borate, strontium carbonate, strontium titanate, calcium titanate, magnesium titanate, bismuth titanate, titanium oxide, zirconium oxide, barium titanate, barium zirconate titanate, barium zirconate, calcium zirconate, zirconium phosphate, zirconium tungstate phosphate, and aluminosilicate. Among these, silica is particularly excellent in low dielectric loss tangent, so hollow silica is preferred as inorganic hollow particles.

[0020] The organic resin constituting the shell of the organic hollow particles is not particularly limited, but it is preferable that the organic resin has ether bonds in order to ensure good dispersion of the organic hollow particles in the porous insulating layer.

[0021] The organic resin used as the shell in the hollow organic particles may be a thermoplastic resin or a cured product of a thermosetting resin. The organic resin serving as the shell in the hollow organic particles may or may not have a cross-linked structure.

[0022] Examples of organic resins include polymers of monomer components containing divinyl aromatic compounds. Organic resins are, for example, resins having at least one of urea bonds and urethane bonds. Such resins are obtained, for example, by the reaction of an isocyanate compound having multiple isocyanate groups with an active hydrogen compound having multiple amino groups or hydroxyl groups and / or water. Examples of organic hollow particles include the hollow particles described in Japanese Patent Publication No. 6924533 and Japanese Patent Publication No. 7396735.

[0023] The hollowness ratio of the hollow particles is not particularly limited, but is preferably 5% to 80%, more preferably 10% to 75%, and particularly preferably 30% to 70%. The hollow ratio can be calculated, for example, using the following formula. Hollow fraction of a hollow particle (%) = (R1 / R2) 3 ×100 R1: Inner diameter of the hollow particle R2: Outer diameter of hollow particle For example, electron microscope observation is performed to determine the hollowness ratio of any 10 hollow particles, and the arithmetic mean of these ratios is taken as the hollowness ratio.

[0024] The average particle diameter (median diameter) of the hollow particles is not particularly limited, but 50 nm to 500 nm is preferred. The average particle size (median diameter) of the hollow particles is not particularly limited, but it is preferably 1 / 20 to 1 / 5 of the thickness of the porous insulating layer.

[0025] The porous insulating layer contains, for example, a resin and hollow particles. The resin contributes to maintaining the layer state (sheet state) in the porous insulating layer. In a porous insulating layer, hollow particles are dispersed in the resin. Examples of resins include polycarbonate resin, polyimide resin, epoxy resin, phenolic resin, urea resin, melamine resin, diallyl phthalate resin, silicone resin, thermosetting urethane resin, fluororesin, and liquid crystal polymer. From the viewpoint of high insulation, high heat resistance, and high mechanical strength, polyimide resin and liquid crystal polymer are preferred.

[0026] The resin preferably has at least one of ester bonds and ether bonds in its main chain. The resin used to disperse organic hollow particles has at least one of ester bonds and ether bonds in its main chain, which improves the dispersibility of the organic hollow particles.

[0027] The polyimide resin may or may not contain fluorine, but a polyimide resin that does not contain fluorine is preferred.

[0028] Polyimide resins can be obtained, for example, by imidizing polyamic acid.

[0029] Furthermore, polyimide resins with an imidization rate of less than 100% may contain carboxyl groups, but in this invention, -COO- and -O-, which are some of these carboxyl groups, are not included in the ester bond (-COO-) and ether bond (-O-).

[0030] An example of a method for forming a porous insulating layer when the resin is polyimide resin is described below. For example, a diamine component solution is prepared by combining a diamine component with an organic solvent. The organic solvent is not particularly limited and includes, for example, polar aproton solvents such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethyl sulfoxide, such as ether solvents, ester solvents, aliphatic hydrocarbon solvents, and aromatic hydrocarbon solvents. Polar aproton solvents are preferred. The amount of organic solvent per 100 parts by mass of diamine component is, for example, 100 parts by mass or more, and for example, 1,000 parts by mass or less. The percentage of diamine component in the diamine component solution is, for example, 1% by mass or more, and for example, 10% by mass or less. Next, the diamine component solution and the acid dianhydride component are combined to prepare a mixture. If necessary, an appropriate amount of organic solvent can be added to the mixture at this stage. Subsequently, this mixture is heated. This causes a ring-opening polyaddition reaction between the diamine component and the acid dianhydride component, thereby preparing a polyamic acid solution. The heating temperature is, for example, between 50°C and 100°C. Next, the polyamic acid solution and hollow particles are mixed. Subsequently, a polyamic acid solution containing hollow particles is applied to the substrate, the organic solvent is removed, and then the mixture is heated. This causes a dehydration cyclization reaction of the polyamic acid, resulting in imidation and obtaining a polyimide resin. Simultaneously, a porous insulating layer is obtained.

[0031] Liquid crystal polymers are liquid crystalline thermoplastic resins. Examples of liquid crystal polymers include liquid crystal polyesters, preferably aromatic liquid crystal polyesters. Liquid crystal polymers are specifically described, for example, in Japanese Patent Publication No. 2020-147670 and Japanese Patent Publication No. 2004-189867.

[0032] Commercially available liquid crystal polymers can be used. Examples of commercially available products include UENO LCP (registered trademark, hereinafter the same) 8100 series (low melting point type, manufactured by Ueno Pharmaceutical Co., Ltd.) and UENO LCP 5000 series (high melting point type, manufactured by Ueno Pharmaceutical Co., Ltd.). Preferably, the UENO LCP 5000 series is used.

[0033] The melting point of the liquid crystal polymer is not limited. For example, the melting point of the liquid crystal polymer is 200°C or higher, preferably 220°C or higher, more preferably 250°C or higher, and also, for example, 370°C or lower. The melting point of the liquid crystal polymer is determined by differential scanning calorimetry. In differential scanning calorimetry, the heating rate is 10°C / min, and the liquid crystal polymer is heated in a nitrogen atmosphere, for example, within a range of 25°C to 400°C. Furthermore, if the liquid crystal polymer is a commercially available product, the catalog value of the commercially available product can be used directly.

[0034] The glass transition temperature of a liquid crystal polymer is not limited. For example, it may be above 80°C, or below 125°C. The glass transition temperature of a liquid crystal polymer is determined by differential scanning calorimetry performed at a heating rate of 10°C / min.

[0035] The relative permittivity of the resin at 10 GHz is not particularly limited, but is preferably 4.0 or less. The lower limit of the relative permittivity is not particularly limited, but is, for example, 2.5 or more. The dielectric loss tangent of the resin at 10 GHz is not particularly limited, but is preferably 0.005 or less. The lower limit of the dielectric loss tangent is not particularly limited, but is, for example, 0.0005 or more. In this invention, the relative permittivity and dielectric loss tangent can be measured at 10 GHz using a QWED 10 GHz SPDR resonator, for example, in accordance with the SPDR method (split post dielectric resonant method) compliant with ASTMD150.

[0036] There is no particular lower limit to the content of hollow particles in the porous insulating layer, but the content of hollow particles is preferably 1 part by mass or more, more preferably 5 parts by mass or more, and particularly preferably 10 parts by mass or more, per 100 parts by mass of resin. There is no particular upper limit to the content of hollow particles in the porous insulating layer, but the content of hollow particles is preferably 300 parts by mass or less, more preferably 200 parts by mass or less, and particularly preferably 100 parts by mass or less, per 100 parts by mass of resin.

[0037] The total content of resin and hollow particles in the porous insulating layer is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 99% by mass or more.

[0038] The relative permittivity of the porous insulating layer at 10 GHz is not particularly limited, but for example, 3.00 or less is preferred. There is no lower limit to the relative permittivity of a porous insulating layer at 10 GHz. For example, the lower limit to the relative permittivity of a porous insulating layer at 10 GHz is 1.00.

[0039] The dielectric loss tangent of the porous insulating layer at 10 GHz is not particularly limited, but is preferably 0.0025 or less, and more preferably 0.0020 or less. The lower limit of the dielectric loss tangent of a porous insulating layer at 10 GHz is not limited.

[0040] The thickness of the porous insulating layer is not particularly limited, for example, 0.01 mm to 1 mm, preferably 0.05 mm to 1 mm, and more preferably 0.1 mm to 1 mm.

[0041] The method for manufacturing the porous insulating layer is not particularly limited. For example, this method involves dispersing hollow particles in a resin solution containing a resin and an organic solvent, then coating the resin solution and drying it. Furthermore, if the resin is a polyimide resin, the hollow particles are dispersed in a polyamic acid solution containing polyamic acid and an organic solvent, the polyamic acid solution is applied, dried, and then thermal imidization is performed. Another method involves kneading the resin and hollow particles at a temperature above the melting point of the resin, and then pressing the resulting mixture into a sheet.

[0042] Furthermore, the porous insulating layer may be composed of two or more porous insulating layers containing hollow particles of different average particle sizes (median diameters). For example, the porous insulating layer has a first porous insulating layer and a second porous insulating layer. The second porous insulating layer is positioned on one side of the thickness direction of the first porous insulating layer. The first porous insulating layer contains the first hollow particles. The second porous insulating layer contains second hollow particles. The wiring section is embedded in the first porous insulating layer. Here, it is preferable that the average particle diameter of the first hollow particle is smaller than the average particle diameter of the second hollow particle. Because the average particle size of the hollow particles in the first porous insulating layer in which the wiring is embedded is smaller than the average particle size of the hollow particles in the second porous insulating layer, the pressure around the wiring is more easily dispersed by the smaller hollow particles. As a result, deformation of the wiring is further suppressed. The average particle size of the first hollow particle can be, for example, 0.1 μm to 1 μm. The average particle size of the second hollow particle can be, for example, 1 μm to 10 μm.

[0043] <Connecting via> Flexible multilayer circuit boards may have connecting vias. The connecting vias connect to the first and second conductor layers. The connecting vias penetrate the insulating layer in the thickness direction. For example, the connecting vias serve to ground weak currents that may affect the wiring. The connection vias are arranged in rows along the length of the wiring section, for example. There are two rows of connection vias, and the wiring section is sandwiched between these two rows.

[0044] The material of the connecting vias is 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. The material of the connecting via may be the same as or different from the material of the first conductor layer and the second conductor layer. The shape of the connection vias is not particularly limited; for example, in a cross-section perpendicular to the thickness direction of the flexible multilayer circuit board, examples include square, rectangular, circular, and elliptical shapes. The ratio (X1:Y1) of the length of the connection via in the longitudinal direction of the wiring section (X1) to the length of the connection via in the width direction of the wiring section (Y1) may be, for example, 3:1 to 1:3, 1:2 to 2:1, or 1:1. When the ratio (X1:Y1) is 1:1, the shape of the connection via in a cross-section perpendicular to the thickness direction of the flexible multilayer circuit board is, for example, square or circular.

[0045] One embodiment of the manufacturing method for the flexible multilayer circuit board shown in Figure 1 will be explained using Figures 2A to 2F. A laminate (first laminate) of a first conductor layer 2 and a porous insulating layer 31A is prepared (Figure 2A). The porous insulating layer 31A contains hollow particles 3a. Furthermore, a laminate of a conductor layer 5A and a porous insulating layer 31B (second laminate) is prepared (Figure 2B). In the second laminate, the conductor layer 5A is a conductor layer for patterning to form wiring sections. The porous insulating layer 31B contains hollow particles 3a. The porous insulating layer 31B is heat-laminated. Next, the first laminate and the second laminate are bonded together so that the porous insulating layer 31A and the porous insulating layer 31B are in contact, and then heat-laminated (Figure 2C). Next, wiring sections 5 are formed from the conductive layer 5A to obtain the laminate (third laminate) shown in Figure 2D. The wiring sections 5 are formed, for example, by patterning using photolithography (e.g., subtractive method) with photoresist. Next, a laminate (fourth laminate) is prepared consisting of the second conductor layer 4, the porous insulating layer 31C, and the porous insulating layer 31D. The porous insulating layer 31C and the porous insulating layer 31D contain hollow particles 3a. The porous insulating layer 31D is heat-laminated. Then, the third laminate and the fourth laminate are arranged so that the wiring section 5 and the porous insulating layer 31D face each other (Figure 2E). Then, the third laminate and the fourth laminate are bonded together. There are no particular restrictions on the pressure and temperature during bonding. By doing so, the porous insulating layer 31A, porous insulating layer 31B, porous insulating layer 31C, and porous insulating layer 31D are integrated to form the porous insulating layer 3 (Figure 2F). At this time, the wiring portion is embedded in the porous insulating layer, but because the porous insulating layer is a porous structure formed by containing hollow particles, deformation of the wiring portion can be suppressed. Based on the above, the flexible multilayer circuit board 1 shown in Figure 1 is obtained.

[0046] Another embodiment of the manufacturing method for the flexible multilayer circuit board shown in Figure 1 will be explained using Figures 3A to 3E. A laminate (first laminate) of a first conductor layer 2 and a porous insulating layer 32A is prepared (Figure 3A). The porous insulating layer 32A contains hollow particles 3a. The porous insulating layer 32A is heat-laminated. Next, a conductor layer 5A is formed on the porous insulating layer 32A (Figure 3B). The conductor layer 5A is a conductor layer for patterning to form wiring sections. The conductor layer 5A can be formed, for example, by heat laminating a metal layer onto the porous insulating layer 32A. Next, wiring sections 5 are formed from the conductive layer 5A to obtain the laminate (second laminate) shown in Figure 3C. The wiring sections 5 are formed, for example, by patterning using photolithography (e.g., subtractive method) with photoresist. Next, a laminate (third laminate) of the second conductor layer 4 and the porous insulating layer 32B is prepared. The porous insulating layer 32B contains hollow particles 3a. The porous insulating layer 32B is heat-laminated. Then, the second laminate and the third laminate are arranged so that the wiring section 5 and the porous insulating layer 32B face each other (Figure 3D). Then, the second laminate and the third laminate are bonded together. There are no particular restrictions on the pressure and temperature during bonding. In this way, the porous insulating layer 32A and the porous insulating layer 32B become one, forming the porous insulating layer 3 (Figure 3E). Based on the above, the flexible multilayer circuit board 1 shown in Figure 1 is obtained.

[0047] Another embodiment of the manufacturing method for the flexible multilayer circuit board shown in Figure 1 will be explained using Figures 4A to 4H. A laminate (first laminate) of a first conductor layer 2 and a porous insulating layer 33A is prepared (Figure 4A). The porous insulating layer 33A contains hollow particles 3a. Next, a seed layer 5B is formed on the porous insulating layer 33A (Figure 4B). Next, a patterned resist layer 5C is formed on the seed layer 5B (Figure 4C). The patterned resist layer 5C has spaces where the wiring sections are formed. Next, plating is applied to the spaced areas of the patterned resist layer 5C to form the wiring section 5 (Figure 4D). Next, the patterned resist layer 5C and the unnecessary seed layer 5B are removed (Figure 4E). In Figure 4E, the plated object and the seed layer beneath it form a single unit that constitutes the wiring section 5. Next, a porous insulating layer 33B is formed on the porous insulating layer 33A and the wiring section 5 (Figure 4F). The porous insulating layer 33B contains hollow particles 3a. The porous insulating layer 33B can be formed, for example, by applying a solution containing resin and hollow particles onto the porous insulating layer 33A and the wiring section 5. Next, by drying, the porous insulating layer 33A and the porous insulating layer 33B become one, forming the porous insulating layer 3 (Figure 4G). Next, a second conductive layer 4 is formed on the porous insulating layer 3 (Figure 4H). If the porous insulating layer 3 is heat-laminated, the second conductive layer 4 can be formed by bonding a metal layer to the porous insulating layer 3. If the porous insulating layer 3 is not heat-laminated, the second conductive layer 4 may be formed on the porous insulating layer 3 by sputtering or plating. Based on the above, the flexible multilayer circuit board 1 shown in Figure 1 is obtained.

[0048] Another example of the flexible multilayer circuit board of the present invention will be described below with reference to Figure 5. Figure 5 is a schematic diagram of another embodiment of the flexible multilayer circuit board 1. This schematic diagram is a cross-sectional view perpendicular to the longitudinal direction of the wiring section. The flexible multilayer circuit board 1 shown in Figure 5 comprises a porous insulating layer 3, a first conductor layer 2, a second conductor layer 4, and a wiring section 5. The first conductor layer 2 is positioned on one side of the porous insulating layer 3 in the thickness direction. The first conductor layer 2 is in contact with the porous insulating layer 3 on one side in the thickness direction. The thickness direction of the porous insulating layer 3 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 porous insulating layer 3. The second conductor layer 4 is in contact with the porous insulating layer 3 on the other side in the thickness direction of the porous insulating layer 3. The other side is the opposite side from the one side. The porous insulating layer 3 comprises a second porous insulating layer 34A, a first porous insulating layer 34B, and a third porous insulating layer 34C in that order. The second porous insulating layer 34A contains hollow particles 3c. The first porous insulating layer 34B contains hollow particles 3b. The third porous insulating layer 34C contains hollow particles 3c. The average particle diameter of hollow particle 3b is smaller than the average particle diameter of hollow particle 3c. The wiring section 5 is embedded in the first porous insulating layer 34B. The wiring section 5 transmits electrical signals, for example. [Examples]

[0049] The following are reference examples. However, the present invention is not limited to these reference examples. In the reference examples, the dispersibility, dielectric constant, and processability of a porous insulating layer (porous polymer sheet) were tested.

[0050] The meanings and structures of the abbreviations for the components used in the synthesis of polyamic acid are as follows: <Diamine component> • ODA: 4,4'-oxydianiline • PDA: p-phenylenediamine APAB: 4-aminophenyl-4-aminobenzoate [ka]

[0051] • Priamine 1075: Dimer amine, manufactured by Croda Japan Co., Ltd. Priamine 1075 is a dimer amine containing the compound represented by the following structure. [ka]

[0052] <Tetracarboxylic acid dianhydride component> • s-BPDA: 3,3',4,4'-biphenyltetracarboxylic acid dianhydride [ka]

[0053] The organic hollow particles used in the reference example were those manufactured by Sansui Co., Ltd., as follows: ·Average particle size: 100nm~300nm ·Hollow rate: 40%~60% • Relative permittivity: 1.3 • Dielectric loss tangent: 0.0005 ·Thermal decomposition temperature: 390℃ Furthermore, the shells of the hollow organic particles used possess ether bonds.

[0054] The silica hollow particles used in the example were as follows: • Silica hollow particles: Manufactured by EpoChemical. ·Average particle size: 35μm

[0055] <Synthesis Example 1> Under a nitrogen atmosphere, the diamine components shown in Table 1 and 100 g of anhydrous NMP (N-methyl-2-pyrrolidone) were added to a 300 mL separable flask and stirred at 25°C for 20 minutes. Subsequently, the tetracarboxylic dianhydride components shown in Table 1 and 10 g of anhydrous NMP were added and stirred at 60°C. The stirring was stopped, and the mixture was allowed to cool to prepare a brown polyamic acid solution.

[0056] <Synthesis Example 2> A polyamic acid solution was prepared in the same manner as in Synthesis Example 1, except that the diamine component and the tetracarboxylic dianhydride component were changed to the diamine component and tetracarboxylic dianhydride component shown in Table 1.

[0057] [Table 1]

[0058] <Reference example 1> 3.56 g of organic hollow particles were added to the polyamic acid solution obtained in Synthesis Example 1, and the mixture was stirred at 35 rpm for 30 minutes using a BL300 motor (manufactured by Shinto Kagaku Co., Ltd.). A polyamic acid solution containing organic hollow particles was applied to a 12 μm thick copper foil (BHY-82F-HA-V2, manufactured by JX Metals Co., Ltd.) using an applicator (SA-201, manufactured by Tester Industries Co., Ltd.), dried in a forced-air dryer at 80°C for 15 minutes followed by 120°C for 25 minutes, and then heated in a vacuum heating furnace at 390°C for 185 minutes to imide it. This resulted in a copper-clad laminate comprising a porous polymer sheet with a thickness of 50 μm and copper foil in that order. Subsequently, a porous polymer sheet was obtained by dissolving the copper foil using an FeCl3 solution. The mass ratio of organic hollow particles in the porous polymer sheet is 20% by mass relative to the resin (polyimide resin).

[0059] <Reference examples 2~4> A porous polymer sheet was obtained in the same manner as in Reference Example 1, except that the amount of organic hollow particles added to the polyamic acid solution obtained in Synthesis Example 1 was changed to the amount shown in Table 2-1 relative to the resin (polyimide resin).

[0060] <Reference example 5> A porous polymer sheet was obtained in the same manner as in Reference Example 1, except that 3.56 g of organic hollow particles were replaced with 3.56 g of silica hollow particles, and the thickness of the resulting porous polymer sheet was changed to 150 μm.

[0061] <Reference example 6> A polymer sheet was obtained in the same manner as in Reference Example 1, except that organic hollow particles were not added.

[0062] <Reference example 7> A porous polymer sheet was obtained in the same manner as in Reference Example 1, except that the polyamic acid solution was replaced with the polyamic acid solution obtained in Synthesis Example 2.

[0063] <Reference example 8> A porous polymer sheet was obtained in the same manner as in Reference Example 7, except that 3.56 g of organic hollow particles were replaced with 3.56 g of silica hollow particles, and the thickness of the resulting porous polymer sheet was changed to 150 μm.

[0064] <Reference example 9> A polymer sheet was obtained in the same manner as in Reference Example 7, except that organic hollow particles were not added.

[0065] [test] <Dispersibility> The dispersibility of hollow particles in porous polymer sheets was evaluated using the following method. An electron microscope was used to observe the cross-section of a porous polymer sheet. The magnification during observation was 1,800x. If aggregates of hollow particles were present in the porous polymer sheet, it was classified as "aggregates present." If aggregates were absent, the dispersibility was classified as "good." The results are shown in Tables 2-1 and 2-2.

[0066] <Dielectric constant> The dielectric constant was measured for the (porous) polymer sheets of Reference Examples 1-9. The relative permittivity (Dk) and dielectric loss tangent (Df) of polymer sheets at 10 GHz were measured using the SPDR method (split-post dielectric resonant method) compliant with ASTMD150, with a QWED 10 GHz SPDR resonator. The results are shown in Tables 2-1 and 2-2.

[0067] <Workability> Through-holes were formed in a polymer sheet by irradiating it with a YAG laser. The formed through-holes were observed using an electron microscope. The magnification during observation was 1,000x. If fragments of hollow particle shells were observed on the side or bottom surface of the through-hole, it was classified as "fragments present," and if no shell fragments were observed, it was classified as "fragments absent." The results are shown in Tables 2-1 and 2-2.

[0068] [Table 2-1]

[0069] [Table 2-2] [Explanation of Symbols]

[0070] 1 Flexible multilayer circuit board 2. First Conductor Layer 3. Porous insulating layer 3a Hollow particle 3b Hollow particle 3c hollow particle 4. Second Conductor Layer 5 Wiring section 5A Conductor layer 5B Seed Layer 5C patterned resist layer 31A Porous insulating layer 31B Porous insulating layer 31C Porous insulating layer 31D Porous Insulating Layer 32A Porous insulating layer 32B Porous insulating layer 33A Porous insulating layer 33B Porous insulating layer 34A Second porous insulating layer 34B First porous insulating layer 34C Third porous insulating layer

Claims

1. A porous insulating layer containing organic hollow particles, A first conductive layer is disposed on one side in the thickness direction of the porous insulating layer, A second conductive layer is disposed on the other side in the thickness direction of the porous insulating layer, A wiring section embedded in the porous insulating layer, Equipped with, The organic resin constituting the shell of the aforementioned organic hollow particle has an ether bond. Flexible multilayer circuit board.

2. In the porous insulating layer, the organic hollow particles are dispersed in the resin. The resin has at least one of ester bonds and ether bonds in its main chain. A flexible wiring circuit board according to claim 1.

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