JP7793678B2 - Flexible Multilayer Circuit Board - Google Patents

Description

The present invention relates to a flexible multilayer circuit board.

In recent years, development of so-called "fifth generation (5G)" wireless communications has been progressing. "5th generation (5G)" wireless communications will enable the transmission of large volumes of data at high speeds. "5th generation (5G)" wireless communications will use high frequencies, including millimeter waves. Substrates for high-frequency antennas that emit these millimeter waves require substrates with a low dielectric constant (low-dielectric substrates). Furthermore, there is a demand for high-speed transmission FPCs (flexible printed circuit boards) that can transmit data at high speeds, and low-dielectric substrates are also required as substrates for these high-speed transmission FPCs.

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

As a method for making an insulating film porous, for example, a porous film manufacturing method has been proposed that includes the steps of: (1) 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 that penetrate both sides of the film; and (2) washing the resulting porous film with a second solvent (see Patent Document 2).

Japanese Patent Application Laid-Open No. 2022-182956 Japanese Patent Application Laid-Open No. 2001-342282

The porous films disclosed in Patent Documents 1 and 2 are manufactured by forming voids in the film. When pressure is applied to such porous films, the porous film deforms. Therefore, when wiring portions are embedded in the porous film, when pressure is applied to the porous film, the wiring portions are likely to deform in conjunction 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, which results in greater deformation.

The objective of the present invention is to provide a flexible multilayer circuit board whose wiring section is less likely to deform even when pressure is applied.

That is, the present invention includes the following.
[1] A porous insulating layer containing hollow particles;
a first conductor layer disposed on one side of the porous insulating layer in a thickness direction;
a second conductor layer disposed on the other side of the porous insulating layer in the thickness direction;
a wiring portion embedded in the porous insulating layer;
A flexible multilayer circuit board comprising:
[2] The flexible multilayer circuit board according to [1], wherein the hollow particles are organic hollow particles.
[3] The porous insulating layer has a first porous insulating layer and a second porous insulating layer disposed on one side in a 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 size of the first hollow particles is smaller than the average particle size of the second hollow particles;
The wiring portion is embedded in the first porous insulating layer.
The flexible multilayer circuit board according to [1] or [2].

The present invention provides a flexible multilayer circuit board in which the wiring portion is less likely to deform even when pressure is applied.

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

(flexible multilayer circuit board)
The flexible multilayer circuit board of the present invention comprises 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 disposed on the other side in the thickness direction of the porous insulating layer.
The wiring portion is embedded in the porous insulating layer.
The porous insulating layer contains hollow particles.

The porous insulating layer contains a large number of hollow particles, thereby forming a porous structure.
Compared to a porous structure formed by forming voids in a film, a porous structure formed by incorporating hollow particles exhibits smaller thickness fluctuations when pressure is applied. Wiring portions embedded in such a porous structure are suppressed from deforming when pressure is applied. Therefore, wiring portions embedded in the porous insulating layer of the flexible multilayer circuit board of the present invention are suppressed from deforming when pressure is applied.

An example of the flexible multilayer circuit board of the present invention will be described below with reference to FIG.
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 longitudinal direction of the wiring portion.
The flexible multilayer circuit board 1 shown in FIG. 1 includes a porous insulating layer 3 , a first conductor layer 2 , a second conductor layer 4 , and a wiring portion 5 .
The first conductor layer 2 is disposed on one side in the thickness direction of the porous insulating layer 3. The first conductor layer 2 is in contact with the porous insulating layer 3 on one side in the thickness direction of the porous insulating layer 3. The thickness direction of the porous insulating layer 3 is the vertical direction on the paper surface in FIG.
The second conductor layer 4 is disposed 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 opposite to the one side.
The wiring portion 5 is embedded in the porous insulating layer 3. The wiring portion 5 transmits, for example, an electrical signal.
The porous insulating layer 3 contains a large number of hollow particles 3a.

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 length direction of the wiring section 5. Note that the length direction of the wiring section 5 in Figure 1 is a direction perpendicular to the up-down direction and left-right direction of the paper.

<First Conductive Layer and Second Conductive Layer>
The material of the first conductor layer and the second conductor layer is not particularly limited and may be, for example, a metal material, such as copper, nickel, gold, solder, or an alloy of two or more of these.
The materials of the first conductor layer and the second conductor layer may be the same or different.
The thickness of the first conductor layer and the second conductor layer is not particularly limited and is, for example, 3 μm or more, preferably 5 μm or more, and for example, 50 μm or less, preferably 30 μm or less.
The thickness of the first conductor layer and the second conductor layer may be the same or different.
In the present invention, the term "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 to, for example, a direction perpendicular to the surface direction of the first conductor layer and the second conductor layer.

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

<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 having a shell made of an inorganic material. In other words, inorganic hollow particles have a shell made of an inorganic material and a hollow portion surrounded by the shell.
The organic hollow particles are hollow particles having a shell made of an organic resin. In other words, the organic hollow particles have a shell made of an organic resin and a hollow portion surrounded by the shell.

As the hollow particles, organic hollow particles are preferred because they are excellent in laser processability.
When manufacturing a flexible multilayer circuit board, laser processing may be performed on a porous insulating layer. When a porous insulating layer containing inorganic hollow particles is laser processed (for example, by forming through holes using a laser), inorganic shell fragments of the inorganic hollow particles may be generated. In contrast, when a porous insulating layer containing organic hollow particles is laser processed, no shell fragments are generated. This is because the organic resin shells of the organic hollow particles are fused together during laser processing.

Examples of inorganic materials that form the shell 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 aluminosilicates. Among these, silica is particularly preferred as an inorganic hollow particle due to its low dielectric tangent.

The organic resin that constitutes the shell of the organic hollow particles is not particularly limited, but it is preferable that the organic resin have an ether bond, as this allows the organic hollow particles to be well dispersed in the porous insulating layer.

The organic resin used as the shell of the organic hollow particles may be a thermoplastic resin or a cured product of a thermosetting resin.
The organic resin serving as the shell of the organic hollow particles may or may not have a crosslinked structure.

The organic resin may be, for example, a polymer of a monomer component containing a divinyl aromatic compound.
The organic resin is, for example, a resin having at least one of a urea bond and a urethane bond, and is obtained, for example, by reacting an isocyanate compound having multiple isocyanate groups with an active hydrogen compound having multiple amino groups or hydroxy groups and/or water.
Examples of organic hollow particles include hollow particles described in Japanese Patent Nos. 6,924,533 and 7,396,735.

The hollow 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, by the following formula.
Hollow ratio of hollow particles (%)=(R ₁ /R ₂ ) ³ ×100
R ₁ : inner diameter of hollow particle R ₂ : outer diameter of hollow particle For example, the hollow ratio of any 10 hollow particles is determined by observation under an electron microscope, and the arithmetic mean value thereof is taken as the hollow ratio.

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

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

The resin preferably has at least one of an ester bond and an ether bond in the main chain.
When the resin for dispersing the organic hollow particles has at least one of an ester bond and an ether bond in the main chain, the dispersibility of the organic hollow particles is improved.

The polyimide resin may or may not contain fluorine, but fluorine-free polyimide resin is preferred.

Polyimide resins are obtained, for example, by imidizing polyamic acid.

Note that polyimide resins with an imidization rate that is not 100% may contain carboxy groups, but the -COO- and -O- groups that are part of such carboxy groups are not included in the ester bonds (-COO-) and ether bonds (-O-) of the present invention.

An example of a method for forming a porous insulating layer when the resin is a polyimide resin will be described.
For example, a diamine component solution is prepared by blending a diamine component with an organic solvent. The organic solvent is not particularly limited, and examples thereof include polar aprotic solvents such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethyl sulfoxide, as well as ether solvents, ester solvents, aliphatic hydrocarbon solvents, and aromatic hydrocarbon solvents. Polar aprotic solvents are preferred. The number of parts by mass of the organic solvent per 100 parts by mass of the diamine component is, for example, 100 parts by mass or more and, for example, 1,000 parts by mass or less. The percentage of the 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 blended to prepare a mixture, and at this time, an appropriate amount of organic solvent can be added to the mixture, if necessary.
The mixture is then heated to a temperature of, for example, 50° C. or higher and 100° C. or lower, whereby a polyamic acid solution is prepared by a ring-opening polyaddition reaction between the diamine component and the acid dianhydride component.
Subsequently, the polyamic acid solution and the hollow particles are mixed.
The polyamic acid solution containing the hollow particles is then applied to a substrate, the organic solvent is then removed, and the substrate is then heated. This causes a dehydration cyclization reaction of the polyamic acid, which imidizes the polyamic acid to obtain a polyimide resin. At the same time, a porous insulating layer is obtained.

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 in, for example, JP 2020-147670 A and JP 2004-189867 A.

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

The melting point of the liquid crystal polymer is not limited. It is, for example, 200°C or higher, preferably 220°C or higher, more preferably 250°C or higher, and 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 temperature is increased at a rate of 10°C/min, for example, over a range of 25°C to 400°C, and the liquid crystal polymer is heated in a nitrogen atmosphere. If the liquid crystal polymer is commercially available, the catalog value for the commercially available product can be used as is.

The glass transition temperature of the liquid crystal polymer is not limited. For example, the glass transition temperature of the liquid crystal polymer is 80°C or higher and 125°C or lower. The glass transition temperature of the liquid crystal polymer is determined by differential scanning calorimetry performed at a heating rate of 10°C/min.

The relative dielectric constant of the resin at 10 GHz is not particularly limited, but is preferably 4.0 or less. The lower limit of the relative dielectric constant is not particularly limited, but the relative dielectric constant 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 the dielectric loss tangent is, for example, 0.0005 or more.
In the present invention, the relative dielectric constant and the dielectric loss tangent can be measured at 10 GHz, for example, by the SPDR method (split post dielectric resonance method) in accordance with ASTM D150 using a "10 GHz SPDR resonator" manufactured by QWED.

There is no particular lower limit for 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 amount of hollow particles contained in the porous insulating layer, but the amount 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.

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.

The relative dielectric constant of the porous insulating layer at 10 GHz is not particularly limited, but is preferably, for example, 3.00 or less.
The lower limit of the relative dielectric constant of the porous insulating layer at 10 GHz is not limited, and for example, the lower limit of the relative dielectric constant of the porous insulating layer at 10 GHz is 1.00.

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 the porous insulating layer at 10 GHz is not limited.

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

The method for producing the porous insulating layer is not particularly limited.
For example, hollow particles may be dispersed in a resin solution containing a resin and an organic solvent, and then the resin solution may be applied and dried.
In addition, when the resin is a polyimide resin, hollow particles may be dispersed in a polyamic acid solution containing polyamic acid and an organic solvent, and then the polyamic acid solution may be applied, dried, and then thermally imidized.
Alternatively, the resin and hollow particles may be kneaded at a temperature equal to or higher than the melting temperature of the resin, and then the resulting kneaded product may be pressed into a sheet.

The porous insulating layer may be composed of two or more porous insulating layers containing hollow particles with different average particle diameters (median diameters).
For example, the porous insulating layer includes a first porous insulating layer and a second porous insulating layer.
The second porous insulating layer is 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 wiring portion is embedded in the first porous insulating layer.
Here, the average particle size of the first hollow particles is preferably smaller than the average particle size of the second hollow particles.
The average particle size of the hollow particles in the first porous insulating layer in which the wiring portions are embedded is smaller than the average particle size of the hollow particles in the second porous insulating layer, so that pressure is more easily dispersed by the small hollow particles around the wiring portions, thereby further suppressing deformation of the wiring portions.
The average particle size of the first hollow particles is, for example, 0.1 μm to 1 μm.
The average particle size of the second hollow particles is, for example, 1 μm to 10 μm.

<Connection via>
The flexible multi-layer circuit board may include connecting vias.
The connection via connects the first conductor layer and the second conductor layer, penetrates the insulating layer in the thickness direction, and serves to ground, for example, weak currents that affect the wiring portion.
The connection vias are arranged in rows, for example, in the longitudinal direction of the wiring portion. There are two rows each made up of a plurality of connection vias, and the wiring portion is sandwiched between these two rows.

The material of the connection via is not particularly limited, and examples thereof include metal materials such as copper, nickel, gold, solder, and alloys of two or more of these.
The material of the connection 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 via is not particularly limited, and examples thereof include a square, rectangle, circle, oval, etc. in a cross section perpendicular to the thickness direction of the flexible multilayer circuit board.
The ratio ( _X1 : _Y1 ) of the length ( _X1 ) of the connection via in the length direction of the wiring portion to the length ( _Y1 ) of the connection via in the width direction of the wiring portion may be, for example, 3:1 to 1:3, or 1:2 to 2:1, or may be 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.

An embodiment of a method for manufacturing the flexible multilayer circuit board shown in FIG. 1 will be described with reference to FIGS. 2A to 2F.
A laminate (first laminate) of a first conductor layer 2 and a porous insulating layer 31A is prepared (FIG. 2A). The porous insulating layer 31A contains hollow particles 3a.
Also, a laminate (second laminate) of a conductor layer 5A and a porous insulating layer 31B is prepared ( FIG. 2B ). The conductor layer 5A in the second laminate is a conductor layer for forming a wiring portion by patterning. The porous insulating layer 31B contains hollow particles 3a. The porous insulating layer 31B can be thermally 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 with each other, and are thermally laminated (FIG. 2C).
Next, the wiring portion 5 is formed from the conductor layer 5A to obtain a laminate (third laminate) shown in Fig. 2D. The wiring portion 5 is formed by, for example, patterning by photolithography (for example, a subtractive method) using a photoresist.
Next, a laminate (fourth laminate) is prepared, which includes 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 can be thermally laminated. The third laminate and the fourth laminate are then arranged so that the wiring portion 5 and the porous insulating layer 31D face each other ( FIG. 2E ).
The third laminate and the fourth laminate are then bonded together. There are no particular limitations on the pressure and temperature used during bonding. By doing so, porous insulating layer 31A, porous insulating layer 31B, porous insulating layer 31C, and porous insulating layer 31D are integrated to form porous insulating layer 3 (FIG. 2F). At this time, the wiring portion is embedded in the porous insulating layer, and deformation of the wiring portion can be suppressed by the porous structure formed by the porous insulating layer containing hollow particles.
As a result of the above, the flexible multilayer circuit board 1 shown in FIG. 1 is obtained.

Another embodiment of the method for manufacturing the flexible multilayer circuit board shown in FIG. 1 will be described with reference to FIGS. 3A to 3E.
A laminate (first laminate) of a first conductor layer 2 and a porous insulating layer 32A is prepared (FIG. 3A). The porous insulating layer 32A contains hollow particles 3a. The porous insulating layer 32A can be thermally laminated.
Next, a conductor layer 5A is formed on the porous insulating layer 32A (FIG. 3B). The conductor layer 5A is a conductor layer for forming wiring portions by patterning. The conductor layer 5A can be formed, for example, by thermally laminating a metal layer onto the porous insulating layer 32A.
Next, the wiring portion 5 is formed from the conductor layer 5A to obtain the laminate (second laminate) shown in Fig. 3C. The wiring portion 5 is formed by, for example, patterning by photolithography (for example, a subtractive method) using a 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 can be thermally laminated. The second laminate and the third laminate are then arranged so that the wiring portion 5 and the porous insulating layer 32B face each other ( FIG. 3D ).
The second laminate and the third laminate are then bonded together. The pressure and temperature during bonding are not particularly limited. By doing so, the porous insulating layer 32A and the porous insulating layer 32B are integrated to form the porous insulating layer 3 (FIG. 3E).
As a result of the above, the flexible multilayer circuit board 1 shown in FIG. 1 is obtained.

Another embodiment of the method for manufacturing the flexible multilayer circuit board shown in FIG. 1 will be described with reference to FIGS. 4A to 4H.
A laminate (first laminate) of a first conductor layer 2 and a porous insulating layer 33A is prepared (FIG. 4A). The porous insulating layer 33A contains hollow particles 3a.
Next, a seed layer 5B is formed on the porous insulating layer 33A (FIG. 4B).
Next, a patterned resist layer 5C is formed on the seed layer 5B (FIG. 4C). The patterned resist layer 5C has spaces where wiring portions are to be formed.
Next, the spaces in the patterned resist layer 5C are plated to form the wiring portion 5 (FIG. 4D).
Next, the patterned resist layer 5C and the unnecessary seed layer 5B are removed (FIG. 4E). In FIG. 4E, the plated object and the underlying seed layer are integrated to form the wiring portion 5.
Next, a porous insulating layer 33B is formed on the porous insulating layer 33A and the wiring portion 5 (FIG. 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 a resin and hollow particles onto the porous insulating layer 33A and the wiring portion 5.
Next, by drying, the porous insulating layer 33A and the porous insulating layer 33B are integrated together to form the porous insulating layer 3 (FIG. 4G).
Next, a second conductor layer 4 is formed on the porous insulating layer 3 (FIG. 4H). If the porous insulating layer 3 is thermally laminable, the second conductor layer 4 can be formed by laminating a metal layer onto the porous insulating layer 3. If the porous insulating layer 3 is not thermally laminable, the second conductor layer 4 may be formed on the porous insulating layer 3 by sputtering or plating.
As a result of the above, the flexible multilayer circuit board 1 shown in FIG. 1 is obtained.

Another example of the flexible multilayer circuit board of the present invention will be described below with reference to FIG.
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 portion.
The flexible multilayer circuit board 1 shown in FIG. 5 includes a porous insulating layer 3 , a first conductor layer 2 , a second conductor layer 4 , and a wiring portion 5 .
The first conductor layer 2 is disposed on one side in the thickness direction of the porous insulating layer 3. The first conductor layer 2 is in contact with the porous insulating layer 3 on one side in the thickness direction of the porous insulating layer 3. The thickness direction of the porous insulating layer 3 is the vertical direction on the paper surface in FIG.
The second conductor layer 4 is disposed 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 opposite to the one side.
The porous insulating layer 3 includes a second porous insulating layer 34A, a first porous insulating layer 34B, and a third porous insulating layer 34C in this 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 size of the hollow particles 3b is smaller than the average particle size of the hollow particles 3c.
The wiring portion 5 is embedded in the first porous insulating layer 34B. The wiring portion 5 transmits, for example, an electrical signal.

Reference examples are shown below. Note that the present invention is in no way limited to these examples. In these examples, the dispersibility, dielectric constant, and processability of porous insulating layers (porous polymer sheets) were tested.

The meanings and structures of the abbreviations of 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

Priamine 1075: Dimer diamine, manufactured by Croda Japan Co., Ltd. Priamine 1075 is a dimer diamine containing a compound represented by the following structure.

<Tetracarboxylic acid dianhydride component>
s-BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride

The organic hollow particles used in the reference examples were the following manufactured by Sansui Co., Ltd.
・Average particle diameter: 100nm to 300nm
・Hollow rate: 40% to 60%
・Relative dielectric constant: 1.3
・Dielectric tangent: 0.0005
・Thermal decomposition temperature: 390℃
The shell of the organic hollow particles used has an ether bond.

The hollow silica particles used in the reference examples were as follows:
Silica hollow particles: manufactured by EpoChemical Corporation Average particle size: 35 μm

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

<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 the tetracarboxylic dianhydride component shown in Table 1.

<Reference example 1>
To the polyamic acid solution obtained in Synthesis Example 1, 3.56 g of organic hollow particles were added, and the mixture was stirred at 35 rpm for 30 minutes with a Three-One Motor (BL300, manufactured by Shinto Scientific Co., Ltd.).
The polyamic acid solution containing the organic hollow particles was applied to a copper foil (BHY-82F-HA-V2 manufactured by JX Metals Corporation) having a thickness of 12 μm using an applicator (SA-201 manufactured by Tester Sangyo Co., Ltd.), dried in a fan dryer at 80°C for 15 minutes, then at 120°C for 25 minutes, and further heated in a vacuum heating furnace at 390°C for 185 minutes for imidization.
This resulted in a copper clad laminate having a 50 μm thick porous polymer sheet and copper foil in that order.
After that, the copper foil was dissolved using _FeCl3 solution to obtain a porous polymer sheet.
The mass ratio of the organic hollow particles to the resin (polyimide resin) in the porous polymer sheet was 20 mass %.

<Reference examples 2 to 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).

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

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

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

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

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

[test]
<Dispersibility>
The dispersibility of the hollow particles in the porous polymer sheet was evaluated by the following method.
The cross section of the porous polymer sheet was observed under an electron microscope at a magnification of 1,800 times.
If there were agglomerations of hollow particles in the porous polymer sheet, the result was rated as "agglomerations present," and if there were no agglomerations, the dispersibility was rated as "good." The results are shown in Tables 2-1 and 2-2.

<Dielectric constant>
The (porous) polymer sheets of Reference Examples 1 to 9 were subjected to measurement of the dielectric constant.
The dielectric constant (Dk) and dielectric loss tangent (Df) of the polymer sheet at 10 GHz were measured using a "10 GHz SPDR resonator" manufactured by QWED Inc. using the SPDR method (split post dielectric resonance method) in accordance with ASTM D150. The results are shown in Tables 2-1 and 2-2.

<Workability>
The polymer sheet was irradiated with a YAG laser to form through-holes. The formed through-holes were observed under an electron microscope. The magnification during observation was 1,000 times. Cases where fragments of the shell of hollow particles were observed on the side or bottom surface of the through-hole were rated as "fragments present," and cases where no shell fragments were observed were rated as "no fragments present." The results are shown in Tables 2-1 and 2-2.

REFERENCE SIGNS LIST 1 Flexible multilayer circuit board 2 First conductor layer 3 Porous insulating layer 3a Hollow particles 3b Hollow particles 3c Hollow particles 4 Second conductor layer 5 Wiring portion 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 (2)

a porous insulating layer containing hollow particles;
a first conductor layer disposed on one side of the porous insulating layer in a thickness direction;
a second conductor layer disposed on the other side of the porous insulating layer in the thickness direction;
a wiring portion embedded in the porous insulating layer;
Equipped with
the porous insulating layer has a first porous insulating layer and a second porous insulating layer disposed on one side in a 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 size of the first hollow particles is smaller than the average particle size of the second hollow particles;
The wiring portion is embedded in the first porous insulating layer.
Flexible multilayer circuit board. the first hollow particles are organic hollow particles,
the second hollow particles are organic hollow particles;
The flexible multilayer circuit board according to claim 1 .