WO2015182696A1 - Fluororesin base material and flexible printed circuit board - Google Patents

Abstract

　The purpose of the present invention is to provide a fluororesin base material with which it is possible to achieve a low dielectric constant and a low coefficient of thermal expansion while maintaining a smooth outside surface, and with which transmission loss in high-frequency bands can be reduced. A fluororesin base material according to one embodiment of the present invention has one or multiple dielectric layers of which a fluororesin is the main component, wherein at least a portion of the dielectric layers contain air bubbles, the average air bubble density in proximity to the outside surface of the air-bubble-containing dielectric layer being lower than the average air bubble density in the interior. In preferred practice, the air-bubble-containing dielectric layer has a first dielectric layer which contains air bubbles, and a second dielectric layer which does not contain air bubbles, the second dielectric layer being arranged on the outside surface side of the first dielectric layer. Additionally, the air bubbles may be configured from hollow particles or porous particles.

Description

Fluororesin substrate and flexible printed wiring board

The present invention relates to a fluororesin substrate and a flexible printed wiring board.

In recent years, the amount of information communication has been increasing, and in order to respond to this, communication in a high-frequency region such as microwaves and millimeter waves has become popular in devices such as IC cards and mobile phone terminals. For this reason, a printed wiring board for high frequency with a small transmission loss when used in a high frequency region is required.

Here, the transmission loss αd satisfies the following relationship (formula (1)) with the frequency f, the dielectric constant εr of the dielectric layer, and the dielectric loss tangent tanδ.

That is, in order to reduce the transmission loss αd, it is desirable to reduce the relative dielectric constant εr of the dielectric layer. Moreover, in order to prevent plating breakage, such as copper plating laminated | stacked on the base material of a flexible printed wiring board, it is desired that the base material has a low coefficient of thermal expansion. For this reason, a fluororesin substrate containing hollow glass beads in the dielectric layer has been proposed (see Patent Document 1). In this fluororesin base material, the relative permittivity of air in the hollow portion of the glass bead is as low as 1.0. Therefore, by controlling the volume ratio of the hollow portion, the relative permittivity εr can be reduced and the hollow portion is hollow. The thermal expansion coefficient can be reduced by the portion.

JP 2013-201344 A

However, in the conventional fluororesin base material, the glass beads may cause unevenness on the outer surface of the fluororesin base material. Since the high-frequency signal travels on the conductor surface due to the skin effect, this unevenness may cause a transmission loss in the high-frequency signal.

The present invention has been made based on the above circumstances, and is a fluororesin base that enables low dielectric constant and low thermal expansion coefficient while maintaining smoothness of the outer surface, and can reduce transmission loss in a high frequency region. The purpose is to provide materials.

A fluororesin substrate according to an aspect of the present invention made to solve the above problems is a fluororesin substrate having one or more dielectric layers mainly composed of a fluororesin, and at least a part of the fluororesin substrate. The dielectric layer contains bubbles, and the average bubble density in the vicinity of the outer surface of the bubble-containing dielectric layer is smaller than the internal average bubble density.

The fluororesin substrate of the present invention enables a low dielectric constant and a low thermal expansion coefficient while maintaining the smoothness of the outer surface. Therefore, the fluororesin substrate can effectively reduce transmission loss in high-frequency signals and can be suitably used as a high-frequency double-sided printed wiring board.

FIG. 1 is a schematic partial cross-sectional view showing a fluororesin substrate according to one embodiment of the present invention. FIG. 2 is a schematic partial cross-sectional view illustrating a flexible printed wiring board according to an aspect of the present invention.

[Description of Embodiment of the Present Invention]
A fluororesin substrate according to an aspect of the present invention made to solve the above problems is a fluororesin substrate having one or more dielectric layers mainly composed of a fluororesin, and at least a part of the fluororesin substrate. The dielectric layer contains bubbles, and the average bubble density in the vicinity of the outer surface of the bubble-containing dielectric layer is smaller than the internal average bubble density.

Since the fluororesin base material has a dielectric layer containing bubbles, the relative permittivity can be reduced by controlling the volume ratio (void ratio) of the bubbles, and the transmission loss can be reduced. In addition, since the average bubble density in the vicinity of the outer surface of the bubble-containing dielectric layer is smaller than the inner average bubble density, the outer surface of the fluororesin substrate is less likely to be uneven, and transmission loss in high-frequency signals due to the unevenness can be suppressed. Therefore, the fluororesin substrate can effectively reduce transmission loss in high frequency signals. Moreover, the said fluororesin base material has a thermal expansion coefficient by having a bubble containing dielectric material layer. Therefore, for example, plating breakage due to heating such as copper plating laminated on the fluororesin substrate can be prevented.

The bubble-containing dielectric layer may include a first dielectric layer containing bubbles and a second dielectric layer disposed on the outer surface side of the first dielectric layer and containing no bubbles. In this way, the bubble-containing dielectric layer has a first dielectric layer containing bubbles, and a second dielectric layer disposed on the outer surface side of the first dielectric layer and containing no bubbles, While maintaining a low dielectric constant, the interface between the conductive pattern laminated on the outer surface of the fluororesin substrate and the fluororesin substrate can be easily smoothed, and transmission loss due to the skin effect can be further effectively reduced.

It is preferable that the bubbles are composed of hollow particles or porous particles. Thus, by comprising the said bubble by a hollow particle or a porous particle, a dielectric material layer can contain a bubble easily and reliably, and can suppress the dimensional change of a fluororesin base material at the time of processes, such as cutting.

It is preferable to further include a reinforcing dielectric layer laminated on the bubble-containing dielectric layer directly or via another layer and impregnated with a fluororesin in a reinforcing material. By further including a reinforcing dielectric layer in which a reinforcing material is impregnated with a fluororesin in this way, the elastic modulus, Poisson's ratio, and thermal expansion coefficient of the fluororesin substrate are lowered. Therefore, the occurrence of warpage and dimensional change of the fluororesin base material during heat treatment such as reflow can be suppressed, and the plating distortion at the time of heat cycle test such as copper plating laminated on the outer surface of the fluororesin base material is reduced, and the plating breakage is reduced. It becomes difficult to occur.

It is preferable that a pair of the bubble-containing dielectric layers is provided on both surfaces of the reinforcing dielectric layer. Thus, by having the pair of bubble-containing dielectric layers on both sides of the reinforcing dielectric layer, a high-frequency double-sided printed wiring board having a circuit pattern that can effectively reduce transmission loss in high-frequency signals is obtained on both sides. be able to.

The reinforcing dielectric layer should not contain bubbles. As described above, since the reinforcing dielectric layer does not contain bubbles, metal migration is suppressed and it is difficult to cause insulation deterioration.

The average porosity of the bubble-containing dielectric layer is preferably 1% by volume to 50% by volume. By setting the average porosity of the bubble-containing dielectric layer within the above range, it is possible to promote both the smoothing of the surface of the fluororesin substrate and the reduction of the dielectric constant while maintaining the strength of the fluororesin substrate.

It is preferable that the average bubble density of the bubble-containing dielectric layer monotonously increases in the direction from the outer surface toward the inside. In this way, the average bubble density of the bubble-containing dielectric layer is inclined (monotonically increasing) from the outer surface to the inside, thereby suppressing the occurrence of irregularities on the outer surface and the bubble content of the bubble-containing dielectric layer. Can be increased, and the relative dielectric constant can be effectively reduced. Therefore, transmission loss in a high frequency signal can be further effectively reduced.

The flexible printed wiring board which concerns on 1 aspect of this invention is equipped with the said fluororesin base material and the electrically conductive pattern laminated | stacked on the outer surface of the said fluororesin base material. Since the flexible printed wiring board includes the fluororesin base material, the transmission loss in the high frequency signal is low, and the flexible printed wiring board can be suitably used as a high frequency printed wiring board.

The “main component” is a component having the highest content, for example, a component having a content of 50% by mass or more. Also, “near the outer surface” means the outer surface of the bubble-containing dielectric layer that is the outermost surface of the fluororesin substrate (if the fluororesin substrate is a single layer, any outer surface of the bubble-containing dielectric layer) to the bubble-containing dielectric It means a region up to a thickness corresponding to the average diameter of bubbles contained in the layer, for example, a region within 20 μm from the outer surface. “Inside” means a region excluding the vicinity of the outer surface of the bubble-containing dielectric layer. The “average bubble density” means the integrated distribution of the number of bubbles in the thickness direction of the bubble-containing dielectric layer (the direction from the outer surface to the inside), and the numerical value differentiated in the thickness direction is the value of the bubble-containing dielectric layer. It means a value divided by the planar view area, and is a numerical value determined depending on the position in the thickness direction. “The average bubble density is inclined” means that the average bubble density is monotonously changing (increasing) in the thickness direction of the bubble-containing dielectric layer (the direction from the outer surface toward the inside). A part where the average bubble density does not change may be included in part. The “average porosity of the bubble-containing dielectric layer” means the ratio of voids to the bubble-containing dielectric layer, and is calculated from the content and density of components constituting the bubble-containing dielectric layer. Is V0, and the volume including voids in the bubble-containing dielectric layer is V1, the amount calculated by (V1−V0) / V1 × 100.

[Details of the embodiment of the present invention]
Hereinafter, an embodiment of a fluororesin substrate and a flexible printed wiring board according to the present invention will be described in detail with reference to the drawings.

[Fluororesin substrate]
The fluororesin substrate 1 in FIG. 1 mainly includes a bubble-containing dielectric layer 2 containing a fluororesin as a main component and a reinforcing dielectric layer 3 in which a fluororesin is impregnated in a reinforcing material. Specifically, the fluororesin substrate 1 has a pair of the bubble-containing dielectric layers 2 on both surfaces of the reinforcing dielectric layer 3.

<Bubble-containing dielectric layer>
The bubble-containing dielectric layer 2 has a fluororesin as a main component, a first dielectric layer 21 containing bubbles 2a, and a second dielectric that is disposed on the outer surface side of the first dielectric layer 21 and does not contain bubbles. Layer 22. Therefore, the average bubble density in the vicinity of the outer surface of the bubble-containing dielectric layer 2 is smaller than the inner average bubble density.

The lower limit of the average thickness of the bubble-containing dielectric layer 2 is preferably 15 μm, and more preferably 25 μm. The upper limit of the average thickness of the bubble-containing dielectric layer 2 is preferably 500 μm, and more preferably 200 μm. When the average thickness of the bubble-containing dielectric layer 2 is less than the lower limit, the content of the bubbles 2a is insufficient, and the effect of reducing the relative dielectric constant may be insufficient. On the other hand, when the average thickness of the bubble-containing dielectric layer 2 exceeds the upper limit, the fluororesin substrate 1 may be unnecessarily thick. Here, “average thickness” means an average value of thicknesses measured at arbitrary ten points.

The lower limit of the average thickness of the first dielectric layer 21 is preferably 10 μm, more preferably 15 μm. The upper limit of the average thickness of the first dielectric layer 21 is preferably 400 μm and more preferably 150 μm. When the average thickness of the first dielectric layer 21 is less than the lower limit, the content of the bubbles 2a is insufficient, and the effect of reducing the relative dielectric constant may be insufficient. On the other hand, when the average thickness of the first dielectric layer 21 exceeds the upper limit, the fluororesin substrate 1 may be unnecessarily thick.

The lower limit of the average thickness of the second dielectric layer 22 is preferably 1 μm, and more preferably 10 μm. Further, the upper limit of the average thickness of the second dielectric layer 22 is preferably 50 μm, and more preferably 30 μm. When the average thickness of the second dielectric layer 22 is less than the lower limit, the interface between the conductive pattern laminated on the outer surface of the fluororesin substrate 1 and the fluororesin substrate 1 is not sufficiently smoothed, and the skin Transmission loss due to effects may occur. Moreover, when the bubble 2a is comprised with the hollow particle or the porous particle, the surface roughness of the said fluororesin base material 1 increases by the drop-off | omission of the hollow particle and the porous particle from the outer surface of the said fluororesin base material 1. Transmission loss may also occur. On the other hand, when the average thickness of the second dielectric layer 22 exceeds the upper limit, the average thickness of the first dielectric layer 21 for obtaining the effect of reducing the relative dielectric constant is increased, and the fluororesin substrate 1 May become unnecessarily thick.

(Fluorine resin)
In the fluororesin, at least one hydrogen atom bonded to the carbon atom constituting the repeating unit of the polymer chain is substituted with a fluorine atom or an organic group having a fluorine atom (hereinafter also referred to as “fluorine atom-containing group”). Say things. The fluorine atom-containing group is a group in which at least one hydrogen atom in a linear or branched organic group is substituted with a fluorine atom, and examples thereof include a fluoroalkyl group, a fluoroalkoxy group, and a fluoropolyether group. .

“Fluoroalkyl group” means an alkyl group in which at least one hydrogen atom is substituted with a fluorine atom, and includes a “perfluoroalkyl group”. Specifically, a “fluoroalkyl group” is a group in which all hydrogen atoms of an alkyl group are substituted with fluorine atoms, and all hydrogen atoms other than one hydrogen atom at the end of the alkyl group are substituted with fluorine atoms. Group.

“Fluoroalkoxy group” means an alkoxy group in which at least one hydrogen atom is substituted with a fluorine atom, and includes a “perfluoroalkoxy group”. Specifically, a “fluoroalkoxy group” is a group in which all hydrogen atoms of an alkoxy group are substituted with fluorine atoms, and all hydrogen atoms other than one hydrogen atom at the end of the alkoxy group are substituted with fluorine atoms. Group.

The “fluoropolyether group” is a monovalent group having an oxyalkylene unit as a repeating unit and having an alkyl group or a hydrogen atom at the terminal, and at least one hydrogen of the alkylene oxide chain or the terminal alkyl group A monovalent group in which an atom is substituted with a fluorine atom. The “fluoropolyether group” includes a “perfluoropolyether group” having a plurality of perfluoroalkylene oxide chains as repeating units.

Examples of the fluororesin constituting the bubble-containing dielectric layer 2 include tetrafluoroethylene / hexafluoropropylene copolymer (FEP), polytetrafluoroethylene (PTFE), polytetrafluoroethylene / perfluoroalkyl vinyl ether copolymer ( PFA), tetrafluoro (tetrafluoro) ethylene / ethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene / ethylene copolymer (ECTFE), Examples thereof include polyvinyl fluoride (PVF), fluoroelastomer, and thermoplastic fluororesin (THV) composed of three types of monomers, tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride. Further, a mixture or copolymer containing these compounds can also be used as a material constituting the bubble-containing dielectric layer 2.

Among the above-mentioned fluororesins, the bubble-containing dielectric layer 2 may contain one or more of FEP, PTFE and PFA. As described above, the bubble-containing dielectric layer 2 contains one or more of FEP, PTFE, and PFA, so that the bubble-containing dielectric layer 2 can be easily reduced in dielectric constant. Moreover, PTFE is more preferable from the viewpoint of heat resistance.

In addition, the bubble-containing dielectric layer 2 can contain, as optional components, for example, engineering plastics, flame retardants, flame retardant aids, antioxidants, lubricants, processing stabilizers, plasticizers, and the like.

The engineering plastic can be selected from known ones according to the properties required for the fluororesin substrate 1, and typically an aromatic polyetherketone resin can be used.

This aromatic polyetherketone has a structure in which the benzene rings are bonded to the para position and the benzene rings are connected by a rigid ketone bond (—C (═O) —) or a flexible ether bond (—O—). It is a thermoplastic resin. Examples of the aromatic polyether ketone include polyether ether ketone (PEEK), ether bond, and benzene having structural units in which an ether bond, a benzene ring, an ether bond, a benzene ring, a ketone bond, and a benzene ring are arranged in this order. Examples include polyether ketone (PEK) having a structural unit in which a ring, a ketone bond, and a benzene ring are arranged in this order. Among them, PEEK is preferable as the aromatic polyether ketone. Such an aromatic polyether ketone is excellent in wear resistance, heat resistance, insulation, workability and the like.

Commercial products can be used as aromatic polyether ketones such as PEEK. As aromatic polyether ketones, those of various grades are commercially available, and a single grade of aromatic polyether ketone that is commercially available may be used alone, or multiple grades of aromatic polyether ketone. A ketone may be used in combination, or a modified aromatic polyether ketone may be used.

The lower limit of the ratio of the total engineering plastic content in the fluororesin substrate 1 to the fluororesin is not particularly limited, but is preferably 10% by mass, more preferably 20% by mass, and even more preferably 35% by mass. When the total content of engineering plastics is less than the above lower limit, the characteristics of the fluororesin substrate 1 may not be sufficiently improved. On the other hand, the upper limit of the ratio of the total content of engineering plastics to the fluororesin is not particularly limited, but is preferably 50% by mass and more preferably 45% by mass. When the content of the engineering plastic exceeds the above upper limit, the advantageous characteristics of the fluororesin may not be sufficiently exhibited.

As the flame retardant, various known ones can be used, and examples thereof include halogen flame retardants such as bromine flame retardants and chlorine flame retardants.

As the flame retardant aid, various known ones can be used, and examples thereof include antimony trioxide.

As the antioxidant, various known ones can be used, and examples thereof include phenolic antioxidants.

(Bubbles)
The bubbles 2a are preferably composed of hollow particles or porous particles. Specifically, a plurality of hollow particles or porous particles may be dispersed and contained in the first dielectric layer 21 of the bubble-containing dielectric layer 2. As described above, since the bubbles are composed of hollow particles or porous particles, the dielectric layer can easily and reliably contain bubbles, and the dimensional change of the fluororesin substrate 1 can be suppressed during processing such as cutting. .

The material of the hollow particles or porous particles is not particularly limited, and examples thereof include alumina, titanium oxide, silica, and the like. Among these, silica is preferable from the viewpoint of high insulation and low dielectric constant.

The lower limit of the average particle diameter of the hollow particles or porous particles is preferably 10 nm, and more preferably 100 nm. Moreover, as an upper limit of the average particle diameter of the said hollow particle or porous particle, 10 micrometers is preferable and 1 micrometer is more preferable. When the average particle diameter of the hollow particles or porous particles is less than the lower limit, the hollow particles or porous particles are likely to aggregate when contained in the bubble-containing dielectric layer 2, and the dispersion of the particles becomes non-uniform. There is a fear. On the other hand, when the average particle diameter of the hollow particles or porous particles exceeds the above upper limit, the outer surface of the bubble-containing dielectric layer 2 is likely to be uneven, and as a result, the outer surface of the fluororesin substrate 1 may be uneven. is there. Here, the “average particle diameter” means a value (D50) of an integrated value 50% in the integrated distribution of the volume-based particle diameter. The integrated distribution is created based on a value obtained by converting the value of a circle radius measured by image analysis of 500 particles observed with a scanning electron microscope (SEM) into a volume.

The lower limit of the average porosity of the hollow particles or porous particles (average volume ratio of voids per particle) is preferably 10%, more preferably 40%. Moreover, as an upper limit of the average porosity of the said hollow particle or porous particle, 90% is preferable and 80% is more preferable. When the average porosity of the hollow particles or porous particles is less than the lower limit, the relative dielectric constant of the bubble-containing dielectric layer 2 may not be sufficiently lowered. On the other hand, when the average porosity of the hollow particles or porous particles exceeds the upper limit, the pressure resistance is insufficient, and the hollow particles or porous particles may be damaged.

The bubbles 2a may be directly formed inside the first dielectric layer 21 of the bubble-containing dielectric layer 2 without using hollow particles or porous particles. Such closed cells can be formed by mixing the surfactant in the dispersion in the dispersion preparation step, which will be described later, and adjusting the sintering conditions in the bubble-containing dielectric layer formation step. it can. Further, it can be formed by stretching or gas injection in the bubble-containing dielectric layer forming step.

The lower limit of the average diameter of the closed cells is preferably 10 nm, and more preferably 100 nm. Moreover, as an upper limit of the average diameter of the said closed cell, 10 micrometers is preferable and 1 micrometer is more preferable. When the average diameter of the closed cells is less than the lower limit, when the bubbles are contained in the bubble-containing dielectric layer 2, the distribution of closed cells may be non-uniform. On the other hand, when the average diameter of the closed cells exceeds the upper limit, the closed cells are concentrated on one surface of the bubble-containing dielectric layer 2 due to the buoyancy, and the closed cells may be unevenly distributed. Here, “the average diameter of the bubbles” means a value (D50) of an integrated value 50% in the integrated distribution of the volume-based bubble diameter. The integrated distribution is created based on a value obtained by volume-converting the value of the circle radius measured by image analysis of 500 bubbles observed with a scanning electron microscope (SEM).

The lower limit of the average porosity of the bubble-containing dielectric layer 2 (the total of the first dielectric layer 21 and the second dielectric layer 22) is preferably 1% by volume, and more preferably 10% by volume. Moreover, as an upper limit of the average porosity of the said bubble containing dielectric material layer 2, 50 volume% is preferable and 30 volume% is more preferable. When the average porosity of the bubble-containing dielectric layer 2 is less than the lower limit, the relative dielectric constant of the bubble-containing dielectric layer 2 may not be sufficiently reduced. On the other hand, when the average porosity of the bubble-containing dielectric layer 2 exceeds the above upper limit, the strength of the fluororesin substrate 1 may be reduced, and the surface roughness of the fluororesin substrate 1 may be increased.

The lower limit of the average porosity of the first dielectric layer 21 is preferably 2% by volume, more preferably 20% by volume. Moreover, as an upper limit of the average porosity of the said 1st dielectric material layer 21, 60 volume% is preferable and 50 volume% is more preferable. When the average porosity of the first dielectric layer 21 is less than the lower limit, the relative dielectric constant of the bubble-containing dielectric layer 2 may not be sufficiently lowered. On the other hand, when the average porosity of the first dielectric layer 21 exceeds the upper limit, the strength of the fluororesin substrate 1 may be reduced.

<Dielectric layer for reinforcement>
The reinforcing dielectric layer 3 is a dielectric layer in which a reinforcing material is impregnated with a fluororesin. As the fluororesin impregnated in the reinforcing material of the reinforcing dielectric layer 3, the same fluororesin as the bubble-containing dielectric layer 2 can be used.

Examples of the reinforcing material for the reinforcing dielectric layer 3 include plain weave glass cloth, non-woven fabric such as aramid, polyimide, twill fabric, polyimide, liquid crystal polymer film, ceramics added with inorganic filler, silicon carbide, aluminum nitride, etc. Sheet.

The lower limit of the average thickness of the reinforcing dielectric layer 3 is preferably 10 μm, more preferably 20 μm. The upper limit of the average thickness of the reinforcing dielectric layer 3 is preferably 100 μm and more preferably 80 μm. If the average thickness of the reinforcing dielectric layer 3 is less than the lower limit, the strength and dimensional stability of the fluororesin substrate 1 may be insufficient. On the other hand, when the average thickness of the reinforcing dielectric layer 3 exceeds the upper limit, the fluororesin substrate 1 may be unnecessarily thick.

The reinforcing dielectric layer 3 should not contain bubbles. As described above, since the reinforcing dielectric layer 3 does not contain bubbles, metal migration is suppressed, and insulation deterioration of the fluororesin substrate 1 can hardly occur.

<Method for producing fluororesin substrate>
The fluororesin substrate 1 can be easily and reliably manufactured by a manufacturing method including, for example, a reinforcing dielectric layer forming step, a dispersion preparing step, and a bubble-containing dielectric layer forming step.

(Reinforcing dielectric layer forming process)
First, in the reinforcing dielectric layer forming step, the reinforcing material is impregnated with a fluororesin to form the reinforcing dielectric layer 3. Specifically, for example, when glass cloth is used as a reinforcing material and PTFE is used as a fluororesin, the reinforcing dielectric layer 3 is obtained by immersing the glass cloth in a PTFE dispersion and sintering the glass cloth impregnated with PTFE. Can be formed. The temperature profile of sintering is not particularly limited, but for example, it can be a profile in which the temperature changes in three stages. Specifically, the first stage temperature is 140 ° C. to 160 ° C., the first stage time is 9 minutes to 11 minutes, the second stage temperature is 190 ° C. to 210 ° C., the second stage This time can be from 25 minutes to 35 minutes, the third stage temperature can be from 410 ° C. to 430 ° C., and the third stage time can be from 50 seconds to 70 seconds.

(Dispersion preparation process)
Next, a dispersion for forming the first dielectric layer 21 is prepared in the dispersion preparation step. For example, when the air bubbles 2a are constituted by hollow silica particles, the hollow silica particles and water are stirred, and the first dispersion is prepared by adding PTFE dispersion to the stirring liquid. Further, in order to form the second dielectric layer 22, a second dispersion liquid containing water and the PTFE dispersion and not containing hollow silica particles is further prepared.

(Bubble-containing dielectric layer forming process)
Finally, in the bubble-containing dielectric layer forming step, the first dielectric layer 21 is formed by applying the first dispersion to one surface of the reinforcing dielectric layer 3 and performing heat treatment. Specifically, the first dispersion liquid is applied to one surface of the reinforcing dielectric layer 3 and subjected to heat treatment to form the first dielectric layer 21. Next, the second dispersion is applied to the outer surface of the first dielectric layer 21, and heat treatment is performed to form the second dielectric layer 22 on the outer surface of the first dielectric layer 21. The temperature profile of the heat treatment at the time of forming the first dielectric layer 21 and the second dielectric layer 22 is not particularly limited. For example, the temperature profile can be the same as the temperature profile at the time of forming the reinforcing dielectric layer 3. . In this way, the bubble-containing dielectric layer 2 can be formed.

Similarly, the first dispersion liquid and the second dispersion liquid are applied to the other surface of the reinforcing dielectric layer 3 and subjected to heat treatment, whereby the bubble-containing dielectric layer 2 is formed on both surfaces of the reinforcing dielectric layer 3. Can be formed. The bubble-containing dielectric layer 2 may be simultaneously formed on both surfaces of the reinforcing dielectric layer 3.

<Advantages>
Since the fluororesin substrate 1 has the bubble-containing dielectric layer 2 containing the bubbles 2a, the relative permittivity can be reduced by controlling the volume ratio (void ratio) of the bubbles 2a, and transmission loss is reduced. Can be reduced. Further, since the average bubble density in the vicinity of the outer surface in the bubble-containing dielectric layer 2 is smaller than the inner average bubble density, the outer surface of the fluororesin substrate 1 is less likely to be uneven, and transmission loss in high-frequency signals due to the unevenness is suppressed. it can. Therefore, the fluororesin substrate 1 can effectively reduce transmission loss in high frequency signals. Moreover, the said fluororesin base material 1 has a bubble containing dielectric material layer 2, and a thermal expansion coefficient is reduced. Therefore, for example, plating breakage due to heating such as copper plating laminated on the fluororesin substrate 1 can be prevented. Furthermore, the fluororesin substrate 1 has a reinforcing dielectric layer 3 in which a fluororesin is impregnated with a reinforcing material, so that the elastic modulus, Poisson's ratio, and thermal expansion coefficient are reduced, and the fluororesin substrate is subjected to heat treatment such as reflow. 1 warpage and dimensional change can be suppressed.

[Flexible printed wiring board]
The flexible printed wiring board of FIG. 2 mainly includes the fluororesin substrate 1 and a pair of conductive patterns 4 laminated on both surfaces (outer surfaces) of the fluororesin substrate 1. The flexible printed wiring board further includes a pair of coverlays 5 stacked on the outer surfaces of the fluororesin substrate 1 and the conductive pattern 4. In FIG. 2, the same elements as those of the fluororesin substrate of FIG.

<Conductive pattern>
The conductive pattern 4 is formed in a desired planar shape (pattern) by etching a metal layer laminated on the outer surface of the fluororesin substrate 1. The conductive pattern 4 can be formed of a conductive material, but is generally formed of copper, for example.

The lower limit of the average thickness of the conductive pattern 4 is preferably 2 μm and more preferably 5 μm. Moreover, as an upper limit of the average thickness of the said conductive pattern 4, 50 micrometers is preferable and 30 micrometers is more preferable. When the average thickness of the conductive pattern 4 is less than the lower limit, the conductivity of the conductive pattern 4 may be insufficient. On the other hand, when the average thickness of the conductive pattern 4 exceeds the upper limit, the flexible printed wiring board may be unnecessarily thick.

<Coverlay>
The coverlay 5 protects the conductive pattern 4 from external force and moisture. The coverlay 5 includes a cover film 51 and an adhesive layer 52.

(Cover film)
The cover film 51 has flexibility and insulation. Examples of the main component of the cover film 51 include polyimide, epoxy resin, phenol resin, acrylic resin, polyester, thermoplastic polyimide, polyethylene terephthalate, fluororesin, and liquid crystal polymer. In particular, polyimide is preferable from the viewpoint of heat resistance. The cover film 51 may contain a resin other than the main component, a weathering agent, an antistatic agent, and the like.

The lower limit of the average thickness of the cover film 51 is not particularly limited, but is preferably 3 μm and more preferably 10 μm. Moreover, as an upper limit of the average thickness of the cover film 51, although it does not specifically limit, 500 micrometers is preferable and 150 micrometers is more preferable. When the average thickness of the cover film 51 is less than the above lower limit, the protection of the conductive pattern 4 may be insufficient, and when the cover film 51 is required to have insulation, the insulation may be insufficient. There is. On the other hand, when the average thickness of the cover film 51 exceeds the above upper limit, the protective effect of the conductive pattern 4 may be reduced and the flexibility of the cover film 51 may be insufficient.

(Adhesive layer)
Although it does not specifically limit as an adhesive agent which comprises the contact bonding layer 52, The thing excellent in the softness | flexibility and heat resistance is preferable. Examples of the adhesive include various resin adhesives such as epoxy resin, polyimide, polyester, phenol resin, polyurethane, acrylic resin, melamine resin, and polyamideimide.

The lower limit of the average thickness of the adhesive layer 52 is preferably 5 μm, and more preferably 10 μm. Moreover, as an upper limit of the average thickness of the contact bonding layer 52, 50 micrometers is preferable and 40 micrometers is more preferable. When the average thickness of the adhesive layer 52 is less than the above lower limit, the adhesive strength of the coverlay 5 to the fluororesin substrate 1 and the conductive pattern 4 may be insufficient. On the other hand, when the average thickness of the adhesive layer 52 exceeds the upper limit, the flexible printed wiring board may be unnecessarily thick.

<Method for producing flexible printed wiring board>
The said flexible printed wiring board can be manufactured easily and reliably by the manufacturing method provided with a conductive pattern formation process and a coverlay lamination process.

(Conductive pattern formation process)
In the conductive pattern forming step, the metal layer is laminated on the outer surface of the fluororesin substrate 1, and the metal layer is processed to form the conductive pattern 4. The method for laminating the metal layer on the outer surface of the fluororesin substrate 1 is not particularly limited. For example, the number of thicknesses formed on the fluororesin substrate 1 by an adhesion method in which a metal foil is bonded with an adhesive, sputtering or vapor deposition. A sputtering / plating method for forming a metal layer by electrolytic plating on a thin conductive layer (seed layer) of nm, a laminating method for attaching a metal foil by hot pressing, or the like can be used. Moreover, the said conductive pattern 4 can be formed by processing by a conventionally well-known method, for example, can be formed by etching the desired location of the metal layer laminated | stacked on the outer surface of the fluororesin base material 1. FIG.

(Coverlay lamination process)
Next, in the cover lay stacking step, the cover lay 5 is stacked on the outer surfaces of the fluororesin substrate 1 and the conductive pattern 4. Specifically, the coverlay 5 can be formed by attaching a cover film 51 to the fluororesin substrate 1 and the conductive pattern 4 via an adhesive layer 52.

<Advantages>
Since the flexible printed wiring board includes the fluororesin substrate 1, the transmission loss in the high frequency signal is low, and the flexible printed wiring board can be suitably used as a high frequency printed wiring board.

[Other Embodiments]
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is not limited to the configuration of the embodiment described above, but is defined by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

In the above embodiment, the case where the fluororesin substrate has the bubble-containing dielectric layers on both sides of the reinforcing dielectric layer has been described. However, the fluororesin substrate has the bubble-containing dielectric layer only on one side of the reinforcing dielectric layer. You may have.

In the above embodiment, the case where the fluororesin substrate has a reinforcing dielectric layer has been described. However, the reinforcing dielectric layer is not an essential component, and the average bubble density near the outer surface is the average bubble density inside. A fluororesin substrate composed of a smaller pair of bubble-containing dielectric layers and a fluorine resin substrate having only one bubble-containing dielectric layer are also within the intended scope of the present invention.

In the above embodiment, the bubble-containing dielectric layer having the first dielectric layer containing bubbles and the second dielectric layer not containing bubbles has been described. However, the bubble-containing dielectric layer may not be divided into two layers. . That is, the average cell density of one cell-containing dielectric layer may be inclined from the outer surface to the inside. As described above, the average bubble density of the bubble-containing dielectric layer is inclined from the outer surface to the inside, thereby suppressing the occurrence of irregularities on the outer surface and increasing the bubble content of the bubble-containing dielectric layer. And the relative dielectric constant can be effectively reduced. Therefore, transmission loss in a high frequency signal can be further effectively reduced.

Further, the second dielectric layer may have bubbles. When the second dielectric layer has bubbles, the average porosity of the second dielectric layer is less than the average porosity of the first dielectric layer, and can be, for example, 1 volume% or more and 10 volume% or less.

Furthermore, the bubble-containing dielectric layer may have three or more dielectric layers. For example, the bubble-containing dielectric layer can be formed into a multilayer structure by sequentially applying and laminating a plurality of dispersions having different hollow silica particle contents.

Further, the fluororesin base material may have a multilayer structure having another dielectric layer. For example, a bubble-free dielectric layer that does not include bubbles may be provided between the reinforcing dielectric layer and the bubble-containing dielectric layer. Thus, by having a bubble-free dielectric layer between the reinforcing dielectric layer and the bubble-containing dielectric layer, it is possible to prevent hollow particles or the like from entering the reinforcing dielectric layer. Thereby, metal migration is suppressed in the dielectric layer for reinforcement, and insulation deterioration of the fluororesin substrate can be made difficult to occur.

Furthermore, although the case of the double-sided flexible printed wiring board has been described in the above embodiment, a single-sided flexible printed wiring board may be used, and in that case, the bubble-containing dielectric layer is provided only on the surface on which the conductive pattern is laminated. A fluororesin base material can be used.

Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

[No. 1]
First, a plain weave glass cloth (CGN-500NF from Chukoh Chemical Industry Co., Ltd.) having an average thickness of 27 μm and a basis weight of 17 g / m ² , PTFE 60 mass%, surfactant 6 mass% or less, water 30 mass % PTFE dispersion (“D-210C” from Daikin Industries, Ltd.) containing from 40% to 40% by weight. The glass cloth impregnated with PTFE was sintered in the air to form a reinforcing dielectric layer. The temperature profile of sintering was set at a temperature of 150 ° C. for 10 minutes, followed by a temperature of 200 ° C. for 30 minutes, and a temperature of 420 ° C. for 1 minute.

Next, 0.12 g of hollow silica particles and 2.8 g of water were stirred, and 1.12 g of the above PTFE dispersion was added to the stirred liquid to prepare a first dispersion. The hollow silica particles have an average particle diameter of 80 nm to 130 nm, a shell thickness of 5 nm to 15 nm, a specific surface area of 100 m ² / g to 500 m ² / g, and a porosity of 50%. “Silinax” was used.

The first dispersion was applied to one surface of the reinforcing dielectric layer using a doctor knife. The average thickness of the coating was 25 μm. Thereafter, the reinforcing dielectric layer coated with the first dispersion was sintered under the same conditions as the temperature profile when the reinforcing dielectric layer was formed in the atmosphere. Furthermore, only 1.12 g of the above PTFE dispersion (second dispersion) not containing hollow silica particles was additionally applied at an average thickness of 25 μm and sintered under the same sintering conditions. Thus, the bubble-containing dielectric layer having the first dielectric layer and the second dielectric layer was formed. A bubble-containing dielectric layer was formed on the other surface of the reinforcing dielectric layer in the same manner to obtain a fluororesin substrate.

Next, after performing plasma treatment on one surface of the fluororesin base material, electroless copper plating treatment was performed. 1 printed wiring board substrates were prepared. The average thickness of the printed wiring board substrate was 81 μm, and the average porosity of the bubble-containing dielectric layer was 19% by volume.

[No. 2]
On one surface of an aluminum foil having an average thickness of 50 μm, A bubble-containing dielectric layer having a first dielectric layer and a second dielectric layer similar to 1 was formed. Next, the aluminum foil was removed from the bubble-containing dielectric layer. As in 1, a bubble-containing dielectric layer having a first dielectric layer and a second dielectric layer was formed. Then, no. No. 1 was subjected to plasma treatment and electroless copper plating treatment. 2 printed wiring board substrates were prepared. The average thickness of the printed wiring board substrate was 54 μm, and the average porosity of the bubble-containing dielectric layer was 26% by volume.

[No. 3]
No. 1 except that the amount of hollow silica particles in the first dispersion was 0.35 g in the formation of the first dielectric layer. No. 1 as in No. 1. 3 printed wiring board substrates were prepared. The average thickness of the printed wiring board substrate was 50 μm, and the average porosity of the bubble-containing dielectric layer was 52% by volume.

[No. 4]
No. 1 except that the amount of hollow silica particles in the first dispersion was 0.36 g in the formation of the first dielectric layer. No. 2 in the same manner. 4 printed wiring board substrates were prepared. The average thickness of the printed wiring board substrate was 50 μm, and the average porosity of the bubble-containing dielectric layer was 53% by volume.

[No. 5]
No. 1 except that both the first dielectric layer and the second dielectric layer were formed using the first dispersion containing hollow silica particles. No. 1 as in No. 1. 5 was prepared as a comparative example. The average thickness of the printed wiring board substrate was 51 μm, and the average porosity of the bubble-containing dielectric layer was 15% by volume.

[No. 6]
No. 1 except that both the first dielectric layer and the second dielectric layer were formed using the first dispersion containing hollow silica particles. No. 2 in the same manner. Six printed wiring board substrates were prepared as comparative examples. The average thickness of the printed wiring board substrate was 51 μm, and the average porosity of the bubble-containing dielectric layer was 26% by volume.

[No. 7]
The first dielectric layer and the second dielectric layer are both No. 1 except that the second dispersion containing no hollow silica particles is used. No. 1 as in No. 1. 7 was prepared as a comparative example. The average thickness of the printed wiring board substrate was 50 μm.

[No. 8]
The first dielectric layer and the second dielectric layer are both No. 1 except that the second dispersion containing no hollow silica particles is used. No. 2 in the same manner. Eight printed wiring board substrates were prepared as comparative examples. The average thickness of the printed wiring board substrate was 50 μm.

[Evaluation]
No. above. 1-No. For the printed wiring board substrate of 8, the relative dielectric constant, the thermal expansion coefficient, and the average surface roughness (Ra) were measured.

<Relative permittivity>
The relative dielectric constant was measured in accordance with a measurement method defined in JIS-C-2138: 2007. The results are shown in Table 1.

<Coefficient of thermal expansion>
The coefficient of thermal expansion was measured by thermomechanical analysis (TMA) according to JIS-K-7197: 2012. The results are shown in Table 1.

<Average surface roughness>
The average surface roughness (Ra) was calculated by measuring 10 points of surface roughness in the range of 1000 μm ² using a laser microscope and obtaining the average value. The results are shown in Table 1.

From the results in Table 1, No. 1-No. No. 4 printed wiring board substrate is No.4. 5 and no. As compared with the printed wiring board substrate of No. 6, the average surface roughness is small while the dielectric constant is the same. Therefore, no. 1-No. In the printed wiring board substrate No. 4, the dielectric constant can be lowered while maintaining the smoothness of the outer surface, and the transmission loss in the high frequency region can be reduced. On the other hand, no. 5 and no. Since the printed wiring board substrate of No. 6 contains hollow particles up to the outer surface side (second dielectric layer) of the dielectric layer, the average surface roughness is large and the smoothness of the outer surface cannot be maintained. Therefore, transmission loss in the high frequency region cannot be sufficiently reduced. From this, it is considered that the transmission loss in the high frequency region can be reduced by making the average bubble density near the outer surface in the bubble-containing dielectric layer smaller than the average bubble density inside.

Also, No. 1-No. In the printed wiring board substrate of No. 4, the dielectric layer does not contain bubbles. 7 and no. As compared with the printed wiring board substrate of No. 8, the thermal expansion coefficient can be reduced while maintaining the same relative dielectric constant and average surface roughness.

Furthermore, No. 1 and no. 2 and No. 3 and no. 4 and No. 4 1 and no. No. 2 has a smaller surface roughness. 1 and no. The surface roughness of No. 2 does not include hollow particles. 7 and no. This is equivalent to a surface roughness of 8. This indicates that the surface of the fluororesin substrate can be further smoothed by setting the average porosity of the bubble-containing dielectric layer to 1% by volume or more and 50% by volume or less.

As described above, the fluororesin substrate of the present invention enables a low dielectric constant and a low thermal expansion coefficient while maintaining the smoothness of the outer surface. Therefore, the fluororesin substrate can effectively reduce transmission loss in high-frequency signals and can be suitably used as a high-frequency double-sided printed wiring board.

DESCRIPTION OF SYMBOLS 1 Fluororesin base material, 2 Bubble containing dielectric layer, 2a Bubble, 21 1st dielectric layer, 22 2nd dielectric layer, 3 Reinforcing dielectric layer, 4 Conductive pattern, 5 Coverlay, 51 Cover film, 52 Adhesive layer

Claims (9)

1. A fluororesin substrate having one or more dielectric layers mainly composed of fluororesin,
   At least a portion of the dielectric layer contains bubbles,
   A fluororesin base material having an average cell density in the vicinity of an outer surface of the bubble-containing dielectric layer smaller than an average cell density inside.
2. The said bubble containing dielectric layer has the 1st dielectric layer containing a bubble, and the 2nd dielectric layer which is arrange | positioned on the outer surface side of the said 1st dielectric layer, and does not contain a bubble. Fluororesin substrate.
3. The fluororesin substrate according to claim 1 or 2, wherein the bubbles are constituted by hollow particles or porous particles.
4. 4. The fluororesin according to claim 1, further comprising a reinforcing dielectric layer laminated directly on the bubble-containing dielectric layer or through another layer and impregnated with a fluororesin in a reinforcing material. Base material.
5. The fluororesin substrate according to claim 4, wherein the reinforcing dielectric layer has a pair of the bubble-containing dielectric layers on both surfaces.
6. The fluororesin substrate according to claim 4 or 5, wherein the reinforcing dielectric layer does not contain bubbles.
7. The fluororesin substrate according to any one of claims 1 to 6, wherein an average porosity of the bubble-containing dielectric layer is 1% by volume or more and 50% by volume or less.
8. The fluororesin substrate according to any one of claims 1 to 7, wherein an average cell density of the cell-containing dielectric layer monotonously increases in a direction from the outer surface toward the inside.
9. A flexible printed wiring board comprising the fluororesin base material according to any one of claims 1 to 8 and a conductive pattern laminated on an outer surface of the fluororesin base material.

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