Through-hole forming method and substrate for flexible printed wiring board

Abstract

The present application provides a via hole forming method and a substrate for a flexible printed wiring board, the method comprising the following steps: forming a laminate, the laminate having a fluororesin layer, a first adhesive layer, a first reinforcing resin layer, a first conductor layer, a second adhesive layer, a second reinforcing resin layer, and a second conductor layer; curing the first and second adhesive layers by heating the laminate to form first and second cured adhesive layers, respectively; forming an opening in the first conductor layer, and removing the first reinforcing resin layer, the first cured adhesive layer, the fluororesin layer, the second cured adhesive layer, and the second reinforcing resin layer by irradiating laser light onto the opening to form a bottomed via hole having the second conductor layer exposed on a bottom surface; and electrically connecting the first conductor layer and the second conductor layer by forming a plating layer on an inner wall of the via hole, the pyrolysis temperature of the second cured adhesive layer being lower than the pyrolysis temperatures of the first and second reinforcing resin layers, and the thickness of the second cured adhesive layer being 10 μm or more and 200 μm or less.

Description

[0001] Cross-reference of related applications

[0002] This application claims priority to Japanese Patent Application No. 2020-171937, filed with the Japan Patent Office on October 12, 2020, the entire contents of which are hereby incorporated by reference. Technical Field

[0003] This invention relates to a method for forming through-holes and a substrate for flexible printed wiring boards. More specifically, it relates to a method for forming through-holes on a substrate for flexible printed wiring boards, the substrate of which includes an inner fluoropolymer layer. Background Technology

[0004] In recent years, with the development of fifth-generation mobile communication systems (5G), there has been a demand for printed circuit boards (PCBs) with low transmission loss in the high-frequency range. To reduce the transmission loss of PCBs, it is desirable to reduce the relative permittivity of the insulating substrate and the dielectric loss tangent (tanδ).

[0005] To date, films composed of polyimide (PI) or liquid crystal polymer (LCP) have been primarily used as insulating substrates in flexible printed circuit boards (FPCs). However, these materials have relatively high relative permittivity and dielectric loss tangent in the high-frequency region, making it difficult to sufficiently reduce transmission losses for high-frequency signals. Therefore, research has focused on applying fluoropolymers with low relative permittivity and dielectric loss tangent to the insulating layer of flexible printed circuit boards.

[0006] International Publication No. 2017 / 069217 discloses a method for manufacturing a wiring substrate, the wiring substrate comprising: an electrical insulating layer including a fluoropolymer layer; a first conductor layer disposed on a first surface of the electrical insulating layer; and a second conductor layer disposed on a second surface of the electrical insulating layer, having a hole extending from the first conductor layer to the second conductor layer, and having a plating layer formed on the inner wall surface of the hole.

[0007] International Publication No. 2019 / 188611 describes a multilayer film with alternating layers of fluoropolymer resin and polyimide.

[0008] Typically, fluoropolymer layers have high heat resistance and high transmittance to lasers such as carbon dioxide lasers and UV lasers. Therefore, when irradiating a substrate including a fluoropolymer layer with a laser to form a through-hole (conductive via), prolonged laser irradiation (or multiple laser pulses) is required. The prolonged exposure of the insulating layer, such as the polyimide layer adjacent to the fluoropolymer layer to the laser results in the formation of deep recesses (reamed holes) in the fluoropolymer layer of the insulating layer. If such recesses form, during the plating process of forming a plating layer on the inner wall of the conductive via, voids are generated because the plating metal cannot fill the recesses formed on the inner wall of the conductive via. These voids expand during subsequent heat treatment processes, causing substrate deformation. Since deeper recesses are more prone to void formation, it is desirable to suppress the formation of deep voids.

[0009] Based on the inventors' continued research, it has been determined that the aforementioned depression is formed deep within the insulating layer located below the fluoropolymer layer along the direction of laser irradiation. Figure 16 This is a cross-sectional photograph of a through-hole formed on a known substrate with a fluoropolymer (PFA) layer, specifically a substrate with a three-layer structure of PFA / PI / PFA stacked vertically along a central copper foil. The substrate was irradiated with a carbon dioxide laser from above and below, thereby forming a conductive through-hole (100 μm in diameter). Subsequently, a desmearing process and electroless copper plating were performed to form the through-hole. The result is as follows: Figure 16 The cross-sectional photographs show that a deep depression formed in the polyimide layer (PI) near the boundary between the fluoropolymer layer (PFA) and the polyimide layer (PI), creating a void that could not be filled by the copper plating. Because the fluoropolymer layer has high laser transmittance, the polyimide layer was exposed to the laser for an extended period until the fluoropolymer layer penetrated through. Furthermore, the polyimide layer has high heat resistance (high pyrolysis temperature). Therefore, it is believed that the deep depression in the polyimide layer near the boundary between the fluoropolymer layer and the polyimide layer was formed due to the melting and decomposition of the polyimide. Summary of the Invention

[0010] This invention is based on the inventor's unique technical understanding described above. The purpose of this invention is to provide a method for forming vias that can suppress void formation when forming vias in a flexible printed wiring board substrate with an inner layer including a fluoropolymer layer, and a flexible printed wiring board substrate.

[0011] exist Figure 16In the case of the substrate with a three-layer structure consisting of PFA / PI / PFA, the polyimide layer (PI) with a high pyrolysis temperature is not directly bonded to the fluoropolymer layer (PFA) through the adhesive layer with a lower pyrolysis temperature. Therefore, according to the inventors' research according to the present invention, it is believed that during the period until the fluoropolymer layer (PFA) is penetrated by laser irradiation, the polyimide layer (PI) undergoes local melting and decomposition, resulting in the formation of deep depressions in the polyimide layer.

[0012] The inventors of this invention conducted dedicated research and determined that by placing an adhesive layer with a pyrolysis temperature lower than the reinforcing resin layer and a thickness of 10 μm or more between the fluororesin layer and the reinforcing resin layer such as the polyimide layer, a generally uniform, gently sloping, shallow, arched depression is formed across the thickness direction of the adhesive layer during laser processing. It was thus determined that since the depression can be filled with plated metal, the formation of voids can be suppressed. This invention is based on the above-mentioned insights of the inventors.

[0013] The present invention provides a method for forming through-holes, comprising the following steps: forming a laminate comprising: a fluoropolymer layer having a first main surface and a second main surface; a first adhesive layer disposed on the first main surface; a first reinforcing resin layer disposed on the first adhesive layer, having a coefficient of linear expansion less than that of the fluoropolymer layer; a first conductor layer disposed directly or indirectly on the first reinforcing resin layer; a second adhesive layer disposed on the second main surface; a second reinforcing resin layer disposed on the second adhesive layer, having a coefficient of linear expansion less than that of the fluoropolymer layer; and a second conductor layer disposed directly or indirectly on the second reinforcing resin layer; and heating the laminate to cause the first adhesive layer to... The first conductor layer and the second adhesive layer are cured to form a first cured adhesive layer and a second cured adhesive layer, respectively. An opening is formed in the first conductor layer. By irradiating the opening with a laser, the first reinforcing resin layer, the first cured adhesive layer, the fluororesin layer, the second cured adhesive layer, and the second reinforcing resin layer are removed, forming a bottomed conductive hole with the second conductor layer exposed on the bottom surface. The first conductor layer and the second conductor layer are electrically connected by forming a plating layer on the inner wall of the conductive hole. The pyrolysis temperature of the second cured adhesive layer is lower than that of the first reinforcing resin layer and the second reinforcing resin layer. The thickness of the second cured adhesive layer is more than 10 μm and less than 200 μm.

[0014] In addition, in the through-hole forming method, the thickness of the second cured adhesive layer can be more than 20 μm and less than 100 μm.

[0015] Additionally, the via forming method may further include the following steps: patterning the second conductor layer to form a conductive pattern; forming a second laminate, the second laminate comprising: a second fluoropolymer layer having a third main surface and a fourth main surface; a third adhesive layer disposed on the third main surface; a third reinforcing resin layer disposed on the third adhesive layer, having a coefficient of linear expansion less than that of the second fluoropolymer layer; and a third conductor layer disposed directly or indirectly on the third reinforcing resin layer; curing the third adhesive layer by heating the second laminate to form a third cured adhesive layer; and stacking the first laminate and the second layer with a fourth adhesive layer such that the conductive pattern of the heat-treated second laminate is opposite to the second fluoropolymer layer of the heat-treated second laminate. A third layer is formed by stacking the materials; the fourth adhesive layer is cured by heating the third layer to form a fourth cured adhesive layer; an opening is formed in the third conductor layer, and the third reinforcing resin layer, the third cured adhesive layer, the second fluororesin layer, and the fourth cured adhesive layer are removed by irradiating the opening in the third conductor layer with a laser, forming a bottomed second conductive hole with the conductive pattern exposed on the bottom surface; the third conductor layer is electrically connected to the conductive pattern by forming a plating layer on the inner wall of the second conductive hole, the pyrolysis temperature of the fourth cured adhesive layer is lower than the pyrolysis temperatures of the first reinforcing resin layer, the second reinforcing resin layer, and the third reinforcing resin layer, and the thickness of the fourth cured adhesive layer, excluding the thickness of the conductive pattern, is 10 μm to 200 μm.

[0016] This invention provides a substrate for a flexible printed wiring board, comprising: a fluoropolymer layer having a first main surface and a second main surface opposite to the first main surface; a first curing adhesive layer disposed on the first main surface of the fluoropolymer layer; a first reinforcing resin layer disposed on the first curing adhesive layer, having a coefficient of linear expansion less than that of the fluoropolymer layer; a first conductor layer disposed directly or indirectly on the first reinforcing resin layer; a second curing adhesive layer disposed on the second main surface of the fluoropolymer layer; a second reinforcing resin layer disposed on the second curing adhesive layer, having a coefficient of linear expansion less than that of the fluoropolymer layer; a second conductor layer disposed directly or indirectly on the second reinforcing resin layer; and a through-hole formed by the through-hole forming method described above, electrically connecting the first conductor layer and the second conductor layer.

[0017] In addition, in the substrate for the flexible printed wiring board, the first conductor layer can be directly disposed on the first reinforcing resin layer without the adhesive layer.

[0018] In addition, in the substrate for the flexible printed wiring board, the second conductor layer can be directly disposed on the second reinforcing resin layer without the adhesive layer.

[0019] In addition, in the substrate for the flexible printed wiring board, the thickness of the second cured adhesive layer can be more than 20 μm and less than 100 μm.

[0020] In addition, in the substrate for the flexible printed wiring board, the coefficient of linear expansion of the first reinforcing resin layer and the second reinforcing resin layer can be less than 30 ppm / ℃, and the elastic constant can be greater than 3 GPa.

[0021] The present invention also provides a substrate for a flexible printed wiring board, comprising: a first fluoropolymer layer having a first main surface and a second main surface opposite to the first main surface; a first curing adhesive layer disposed on the first main surface of the first fluoropolymer layer; a first reinforcing resin layer disposed on the first curing adhesive layer, having a coefficient of linear expansion less than that of the first fluoropolymer layer; a first conductor layer disposed directly or indirectly on the first reinforcing resin layer; a second curing adhesive layer disposed on the second main surface of the first fluoropolymer layer; a second reinforcing resin layer disposed on the second curing adhesive layer, having a coefficient of linear expansion less than that of the first fluoropolymer layer; a conductive pattern disposed directly or indirectly on the second reinforcing resin layer; and a second fluoropolymer layer having a third main surface opposite to the conductive pattern and a side opposite to the third main surface. The second fluoropolymer layer comprises: a fourth main surface; a third cured adhesive layer, in which the conductive pattern is embedded, bonding the second reinforcing resin layer and the second fluoropolymer layer; a fourth cured adhesive layer disposed on the fourth main surface of the second fluoropolymer layer; a third reinforcing resin layer disposed on the fourth cured adhesive layer, having a coefficient of linear expansion less than that of the second fluoropolymer layer; and a second conductor layer disposed directly or indirectly on the third reinforcing resin layer. The pyrolysis temperatures of the second and third cured adhesive layers are lower than those of the first, second, and third reinforcing resin layers. The thickness of the second cured adhesive layer is 10 μm to 200 μm, and the thickness of the third cured adhesive layer, excluding the thickness of the conductive pattern, is 10 μm to 200 μm.

[0022] In addition, in the substrate for the flexible printed wiring board, the first conductor layer can be directly disposed on the first reinforcing resin layer without the adhesive layer.

[0023] In addition, in the substrate for the flexible printed wiring board, the second conductor layer can be directly disposed on the third reinforcing resin layer without the adhesive layer.

[0024] In addition, in the substrate for the flexible printed wiring board, the conductive pattern can be directly applied to the second reinforcing resin layer without the adhesive layer.

[0025] Furthermore, in the substrate for the flexible printed wiring board, the thickness of the second cured adhesive layer and the thickness of the third cured adhesive layer other than the thickness of the conductive pattern can be more than 20 μm and less than 100 μm.

[0026] In addition, in the substrate for the flexible printed wiring board, the coefficient of linear expansion of the first reinforcing resin layer, the second reinforcing resin layer and the third reinforcing resin layer can be less than 30 ppm / ℃ and the elastic constant can be more than 3 GPa.

[0027] According to the present invention, a via-hole forming method and a flexible printed wiring board substrate are provided that can suppress void formation when a via is formed in a flexible printed wiring board substrate including a fluoropolymer layer in the inner layer. Attached Figure Description

[0028] Figure 1 This is a cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the first embodiment.

[0029] Figure 2 It continues Figure 1 A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the first embodiment.

[0030] Figure 3 This is a cross-sectional view of the substrate for the flexible printed wiring board according to the first embodiment.

[0031] Figure 4 It continues Figure 3 A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the second embodiment.

[0032] Figure 5 It continues Figure 4 A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the second embodiment.

[0033] Figure 6 It continues Figure 5 A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the second embodiment.

[0034] Figure 7 It continues Figure 6 A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the second embodiment.

[0035] Figure 8 It continues Figure 7A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the second embodiment.

[0036] Figure 9 It continues Figure 8 A cross-sectional view illustrating the process of forming the substrate for the flexible printed wiring board of the second embodiment.

[0037] Figure 10 This is a cross-sectional view of the substrate for the flexible printed wiring board according to the second embodiment.

[0038] Figure 11 This is a cross-sectional view used to illustrate the process of forming through holes in a substrate for a flexible printed wiring board according to the second embodiment.

[0039] Figure 12 It continues Figure 11 The cross-sectional view is used to illustrate the process of forming through holes in the substrate of the flexible printed wiring board in the second embodiment.

[0040] Figure 13 It continues Figure 12 The cross-sectional view is used to illustrate the process of forming through holes in the substrate of the flexible printed wiring board in the second embodiment.

[0041] Figure 14 This is a cross-sectional photograph of a through-hole formed on a flexible printed wiring board substrate according to an embodiment.

[0042] Figure 15 This is a cross-sectional photograph of a comparative example of a through-hole formed on a substrate for a flexible printed wiring board.

[0043] Figure 16 This is a known cross-sectional photograph of a through-hole formed in a flexible printed wiring board substrate having an inner fluoropolymer layer. Detailed Implementation

[0044] In the following detailed description, numerous specific details are presented for illustrative purposes and to provide a thorough understanding of the disclosed embodiments. However, it will be apparent that one or more embodiments can be implemented without these specific details. In other instances, well-known structures and apparatuses are shown schematically for the purpose of simplifying the drawings.

[0045] Hereinafter, the first embodiment of the present invention will be described with reference to the accompanying drawings. Furthermore, in each drawing, the same reference numerals are used for constituent elements having equivalent functions. Also, the drawings are schematic; since they are shown centered on the characteristic portions of the embodiment, the relationship between thickness and planar dimensions, as well as the ratio of the thicknesses of each layer, may differ from actual values.

[0046] <Flexible Printed Wiring Board Substrate FPCB1>

[0047] Reference Figure 3 The flexible printed circuit board substrate FPCB1 according to the first embodiment will be described. The core insulating substrate of this flexible printed circuit board substrate FPCB1 is composed of a fluoropolymer layer. The flexible printed circuit board substrate FPCB1 is suitable, for example, for flexible printed circuit boards for high-frequency signal transmission with microstrip lines.

[0048] like Figure 3 As shown, the flexible printed wiring board substrate FPCB1 includes: a fluoropolymer layer 11; a curing adhesive layer 12h disposed on the lower surface of the fluoropolymer layer 11; a reinforcing resin layer 13 disposed on the curing adhesive layer 12h; a conductor layer 14 disposed directly or indirectly on the reinforcing resin layer 13; a curing adhesive layer 16h disposed on the upper surface of the fluoropolymer layer 11; a reinforcing resin layer 17 disposed on the curing adhesive layer 16h; and a conductor layer 18 disposed directly or indirectly on the reinforcing resin layer 17.

[0049] Furthermore, the adhesive layer may or may not be located between the reinforcing resin layer 13 and the conductor layer 14. In this embodiment, the conductor layer 14 is directly bonded to the reinforcing resin layer 13. Similarly, for both the reinforcing resin layer 17 and the conductor layer 18, the adhesive layer may or may not be located between the reinforcing resin layer 17 and the conductor layer 18. In this embodiment, the conductor layer 18 is directly bonded to the reinforcing resin layer 17.

[0050] like Figure 3 As shown, when drilling through a via for forming a through-hole that electrically connects conductor layer 14 and conductor layer 18, a laser is irradiated toward conductor layer 14. Figure 3 In this case, the laser beam is directed from bottom to top.

[0051] Viewed from the direction of laser irradiation, the cured adhesive layer 16h is located inside the fluoropolymer layer 11. Figure 3 (On the upper surface side of the fluoropolymer layer 11). The pyrolysis temperature of the cured adhesive layer 16h is lower than that of the reinforcing resin layers 13 and 17. Furthermore, the thickness of the cured adhesive layer 16h is 10 μm or more. Therefore, when using laser-guided universal holes (general-purpose holes H1 and H2 described later), deep depressions on the cured adhesive layer 16h can be suppressed. As a result, during the plating process, the plating metal can be filled into the depressions, thus suppressing the formation of voids.

[0052] Next, the various components of the flexible printed wiring board substrate FPCB1 will be described in detail.

[0053] The fluoropolymer layer 11 has a first main surface opposite to the cured adhesive layer 12h and a second main surface opposite to the first main surface. This fluoropolymer layer 11 is an insulating film containing fluoropolymer. Examples of fluoropolymers include tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinylidene fluoride (PVdF).

[0054] The thickness of the fluoropolymer layer 11 is, for example, 12.5 to 200 μm, preferably 25 to 100 μm. When the thickness of the fluoropolymer layer 11 is thinner than the lower limit of 12.5 μm, the dielectric properties of the flexible printed wiring board substrate FPCB1 deteriorate. On the other hand, when the thickness of the fluoropolymer layer 11 is thicker than the upper limit of 200 μm, it is difficult to ensure the processability and dimensional stability of the flexible printed wiring board substrate FPCB1.

[0055] Furthermore, the fluoropolymer layer 11 is not limited to a single-layer membrane. The fluoropolymer layer 11 can be a multilayer structure composed of multiple membrane layers stacked together. For example, the fluoropolymer layer 11 can be a two-layer structure of PFA and PTFE stacked together. Alternatively, for example, if the adhesion between PTFE and the adhesive is insufficient, it can also be a three-layer structure (PFA / PTFE / PFA) with PTFE sandwiched between PFA.

[0056] In addition, the fluororesin layer 11 may also contain inorganic fillers with low dielectric constant and low dielectric loss tangent. Examples of inorganic fillers include silica, clay, talc, calcium carbonate, mica, diatomaceous earth, alumina, zinc oxide, titanium oxide, calcium oxide, magnesium oxide, boron nitride, iron oxide, tin oxide, antimony oxide, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, basic magnesium carbonate, magnesium carbonate, zinc carbonate, barium carbonate, sodium aluminum oxide, hydrotalcite, calcium sulfate, barium sulfate, calcium silicate, montmorillonite, bentonite, activated clay, sepiolite, epigealite, sericite, glass fiber, glass beads, silica-based hollow spheres, carbon black, carbon nanotubes, carbon nanotubes, graphite, carbon fiber, glass fiber, silicon fiber, LCP fiber, glass hollow spheres, carbon hollow spheres, wood flour, zinc borate, etc. The fluororesin layer 11 may contain one or more inorganic fillers.

[0057] The content of inorganic filler relative to the fluoropolymer layer 11 is preferably 0.1 to 100% by mass, more preferably 0.1 to 60% by mass. When the inorganic filler is porous, the dielectric constant and dielectric loss tangent of the fluoropolymer layer 11 can be further reduced. Furthermore, the dispersibility of the inorganic filler in the fluoropolymer can be improved by surface-treating the inorganic filler with surface-treatment agents such as silane coupling agents or titanate coupling agents.

[0058] In addition, in order to reduce the linear expansion coefficient of the fluororesin, the fluororesin layer 11 may contain aramid fiber fabric, aramid fiber nonwoven fabric, aramid paper, aramid film, glass fiber fabric, cotton fabric, paper, etc.

[0059] In addition, as the fluororesin layer 11, a surface-modified fluororesin layer or a fluororesin layer copolymerized with adhesive components can be used.

[0060] Next, the curing times of the adhesive layer at 12h and 16h will be explained.

[0061] The 12h and 16h cured adhesive layers are insulation layers that are cured by heating the thermosetting adhesive layers 12 and 16 (described later).

[0062] The thickness of the cured adhesive layers 12h and 16h is preferably 10 μm to 200 μm, more preferably 20 μm to 100 μm. In particular, the cured adhesive layer 16h preferably has a thickness of 10 μm or more, more preferably 20 μm or more. When the cured adhesive layer 16h is thinner than 10 μm, the depressions formed in the cured adhesive layer 16h become deeper when using laser-guided vias, making it easier to generate voids during the plating process. The thicker the cured adhesive layer 16h, the smaller the depth (length) of the formed depressions. Furthermore, when the cured adhesive layers 12h and 16h are thicker than 200 μm, the dielectric properties and flexibility of the flexible printed wiring board substrate FPCB1 deteriorate. Therefore, the thickness of the cured adhesive layers 12h and 16h is preferably 200 μm or less.

[0063] Next, the reinforcing resin layers 13 and 17 will be explained.

[0064] The coefficient of linear expansion of the reinforcing resin layers 13 and 17 is less than that of the fluoropolymer layer 11. The smaller the coefficient of linear expansion, the less it will impair the dimensional stability of the flexible printed wiring board substrate FPCB1, and it also allows for a thinner reinforcing resin layers 13 and 17.

[0065] The coefficient of linear expansion of the reinforcing resin layers 13 and 17 is preferably 30 ppm / ℃ or less, and more preferably 25 ppm / ℃ or less. Here, "the value of the coefficient of linear expansion" refers to the value at room temperature (similarly, the values ​​of the coefficient of linear expansion below are all values ​​at room temperature). By using the reinforcing resin layers 13 and 17 with a coefficient of linear expansion of 30 ppm / ℃ or less, warping and dimensional instability reduction of the flexible printed wiring board substrate FPCB1 can be suppressed.

[0066] The elastic constant of the reinforcing resin layers 13 and 17 is preferably 3 GPa or higher, and more preferably 6 GPa or higher. Here, "elastic constant value" refers to the value at room temperature (similarly, the elastic constant values ​​below are all values ​​at room temperature). As a result, during cooling after the integration process (described later) or during patterning of the conductor layer 18, the shrinkage of the flexible printed wiring board substrate FPCB1 can be reduced, thereby improving the dimensional stability of the flexible printed wiring board substrate FPCB1.

[0067] The reinforcing resin layers 13 and 17 are composed, for example, of aromatic polyimides such as polyimides (PI), liquid crystal polymers (LCP), and polyetheretherketones (PEEK). Since the coefficient of linear expansion of polyimides is lower than that of LCP and PEEK, dimensional stability is easily ensured. The water absorption rates of LCP and PEEK are lower than those of polyimides, thus suppressing the increase in dielectric constant and dielectric loss tangent, and the deterioration of transport characteristics due to moisture absorption. Furthermore, as an LCP film, Vecstar (registered trademark) from Kuraray Co., Ltd. can be used.

[0068] In addition to the materials mentioned above, materials used to reinforce resin layers 13 and 17 may include polyarylate, polysulfone, polyarylsulfone (polyethersulfone, etc.), aromatic polyamide, aromatic polyetheramide, polyphenylene sulfide, polyaryl ether ketone, polyamide imide, liquid crystal polyester, etc.

[0069] The thicknesses of the reinforcing resin layers 13 and 17 are, for example, 7.5 to 200 μm, preferably 12.5 to 100 μm. When the thicknesses of the reinforcing resin layers 13 and 17 are thinner than the lower limit of 7.5 μm, it is difficult to ensure processability and dimensional stability. On the other hand, when the thicknesses of the reinforcing resin layers 13 and 17 are thicker than the upper limit of 200 μm, the dielectric properties and flexibility of the flexible printed wiring board substrate FPCB1 deteriorate.

[0070] Furthermore, each reinforcing resin layer 13, 17 is not limited to a single film. The reinforcing resin layers 13, 17 can be a multilayer structure composed of multiple film layers stacked together. For example, the reinforcing resin layers 13, 17 can be formed by bonding thermoplastic resin and non-thermoplastic resin. As a result, reinforcing resin layers 13, 17 with high flexibility can be obtained.

[0071] Next, conductor layers 14 and 18 will be described.

[0072] Conductor layers 14 and 18 are metal foils such as copper or copper alloy, stainless steel, nickel or nickel alloy (including alloy 42), or aluminum or aluminum alloy. In this embodiment, conductor layers 14 and 18 are copper foils such as rolled copper foil or electrolytic copper foil.

[0073] To improve adhesion, chemical or mechanical surface treatments can be applied to the surfaces of conductor layers 14 and 18 opposite to reinforcing resin layers 13 and 17. Examples of chemical surface treatments include nickel plating, copper-zinc alloy plating, or treatments using surface treatment agents such as aluminum alkyd, aluminum chelates, or silane coupling agents. Among chemical surface treatments, surface treatment using silane coupling agents is preferred. Silane coupling agents containing amino groups are suitable as silane coupling agents. On the other hand, examples of mechanical surface treatments include roughening treatments.

[0074] The thickness of conductor layers 14 and 18 is not particularly limited, as long as it is sufficient to perform the function according to the application of the flexible printed wiring board. Considering the flexibility of the flexible printed wiring board, the thickness of conductor layers 14 and 18 is preferably 6 to 70 μm, and more preferably 9 to 35 μm.

[0075] The ten-point average roughness (Rz) of at least one conductor layer forming the signal line in conductor layers 14 and 18 is preferably 2.0 μm or less, more preferably 1.3 μm or less. Therefore, even if the current of the high-frequency signal flows only in the surface portion of the conductor layer due to the skin effect, the transmission distance of the signal can be suppressed from increasing. As a result, when transmitting high-frequency signals, the transmission speed can be maintained, and the increase in transmission loss can be suppressed.

[0076] like Figure 3 As shown, in this embodiment, the conductor layer 14 is directly disposed on the reinforcing resin layer 13 without an adhesive layer. Similarly, the conductor layer 18 is directly disposed on the reinforcing resin layer 17 without an adhesive layer. Because no adhesive is used, the thickness of the flexible printed circuit board substrate FPCB1 can be reduced, thereby ensuring sufficient flexibility. Alternatively, when the thickness of the flexible printed circuit board substrate FPCB1 is constant, the curing adhesive layers 12h and 16h or the reinforcing resin layers 13 and 17 can be thickened by not using an adhesive. As a result, the dimensional stability of the flexible printed circuit board substrate FPCB1 and the laser processing capability when using laser-guided universal vias can be improved.

[0077] In the flexible printed circuit board substrate FPCB1 described above, the pyrolysis temperature of the cured adhesive layer 16h, which is located inside the fluoropolymer layer 11 when viewed from the laser irradiation direction, is lower than the pyrolysis temperature of the reinforcing resin layers 13 and 17, and the thickness of the cured adhesive layer 16h is 10 μm to 200 μm. Therefore, when using a laser to pass through a via, the formation of deep depressions in the cured adhesive layer 16h can be suppressed. As a result, when plating through the via, the plating metal can be filled into the depressions, thus suppressing the formation of voids.

[0078] <Manufacturing Method of FPCB1, a Substrate for Flexible Printed Wiring Boards>

[0079] Reference Figures 1-3 The manufacturing method of the above-mentioned flexible printed wiring board substrate FPCB1 will be described.

[0080] First, prepare fluororesin layer 11, adhesive layers 12 and 16, and single-sided metal foil laminates 15 and 19.

[0081] The fluoropolymer layer 11 is not limited to a single-layer film. The fluoropolymer layer 11 can be a multilayer structure composed of multiple film layers stacked together. For example, the fluoropolymer layer 11 can be a three-layer structure composed of PFA / PTFE / PFA. Thus, the fluoropolymer layer 11 can be firmly bonded to the adhesive layers 12 and 16.

[0082] Adhesive layers 12 and 16 are preferably composed of adhesives with excellent heat resistance. Examples of such adhesives include thermosetting adhesives with excellent dielectric properties from various resin systems, such as modified polyolefin resin adhesives, epoxy resin adhesives, butyral resin adhesives, bismaleimide adhesives, polyimide adhesives, modified polyphenylene ether adhesives, or modified aromatic polyester adhesives. Among commercially available adhesives, examples include SAFY adhesive sheets from Nikan Kogyo Co., Ltd., ARON MIGHTY AS-700 from Toa Seizo Co., Ltd., adhesives containing PIAD (thermoplastic polyimide varnish) from Arakawa Chemical Co., Ltd., and epoxy and reactive ester resins, as well as ESPANEX NSC-003 from NIPPON STEEL Chemical & Material Co., Ltd.

[0083] The thickness of adhesive layers 12 and 16 is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 100 μm or less. In particular, adhesive layer 16 preferably has a thickness of 10 μm or more, more preferably 20 μm or more.

[0084] The single-sided metal-clad laminate 15 has a structure in which a conductor layer 14, serving as a metal foil, is provided on one side of the reinforcing resin layer 13, which serves as an insulating layer. The single-sided metal-clad laminate 19 has a structure in which a conductor layer 18, serving as a metal foil, is provided on one side of the reinforcing resin layer 17, which serves as an insulating layer. As the single-sided metal-clad laminates 15 and 19, commercially available flexible copper clad laminates (FCCLs) such as the adhesive-free copper clad laminate ESPANEX (registered trademark) from Nippon Steel Technology Co., Ltd. can be used.

[0085] Next, as Figure 1 as well as Figure 2As shown, a laminate LB1 is formed by pressurizing a single-sided metal foil laminate 15, 19 sandwiching a fluororesin layer 11 between adhesive layers 12, 16, which serve as adhesive sheets. Furthermore, this process is preferably performed at a temperature lower than the glass transition temperature of each resin layer (e.g., 60–80°C), known as temporary lamination (temporary stacking). Temporary lamination can be performed by vacuum pressing or a roll-to-roll method. In the case of temporary lamination by a roll-to-roll method, a predetermined area of ​​the sheet released from the metal roller is pressurized and heated. After the pressurization and heating of the predetermined area of ​​the sheet is completed, the metal roller is rotated to pressurize and heat the remaining area.

[0086] Alternatively, without using adhesive sheets as adhesive layers 12 and 16, the single-sided metal foil laminate 15 can be laminated after the adhesive is applied to the lower surface of the fluororesin layer 11 to form adhesive layer 12. Similarly, the single-sided metal foil laminate 19 can be laminated after the adhesive is applied to the upper surface of the fluororesin layer 11 to form adhesive layer 16, thereby forming the laminate LB1. Of course, the adhesive can also be applied to the reinforcing resin layer 13 side in advance to form adhesive layer 12, and then the single-sided metal foil laminate 15 with adhesive layer 12 formed can be laminated to the lower surface of the fluororesin layer 11. Similarly, the adhesive can also be applied to the reinforcing resin layer 17 side in advance to form adhesive layer 16, and then the single-sided metal foil laminate 19 with adhesive layer 16 formed can be laminated to the upper surface of the fluororesin layer 11.

[0087] After forming the laminate LB1 as described above, the adhesive layer 12 and adhesive layer 16 are cured by heating the laminate LB1. Thus, adhesive layer 12 and adhesive layer 16 become cured adhesive layer 12h and cured adhesive layer 16h, respectively (integration process). More specifically, in this process, the laminate LB1 is integrated by heating and pressurizing it at a temperature above the curing temperature of adhesive layers 12 and 16. After this process, a product is obtained... Figure 3 The substrate FPCB1 for the flexible printed wiring board shown is shown.

[0088] Furthermore, the heating temperature of adhesive layers 12 and 16 is, for example, 250°C or lower. Preferably, the heating temperature of adhesive layers 12 and 16 is 200°C or lower, or lower than the glass transition temperature of the fluororesin constituting fluororesin layer 11 plus 90°C (preferably 60°C). If adhesive layers 12 and 16 are heated at a temperature higher than this, residual strain may increase and dimensional stability may deteriorate upon cooling to room temperature. The lower the heating temperature, the slower the curing speed of the adhesive, requiring more time to achieve bonding. On the other hand, spending a longer heating time to cure adhesive layers 12 and 16 can improve dimensional stability.

[0089] Furthermore, if the reinforcing resin layers 13 and 17 contain trace amounts of moisture, and heating at temperatures exceeding 180°C causes rapid evaporation of the moisture, foaming may occur in the reinforcing resin layers 13 and 17. Lower heating temperatures result in slower evaporation, thus suppressing foaming.

[0090] Based on the above, when the fluoropolymer layer 11 is PFA or PTFE and the reinforcing resin layers 13 and 17 are LCP, foaming of the reinforcing resin layers 13 and 17 can be suppressed by heating at a temperature in the range of 130 to 230°C. Therefore, a substrate for flexible printed wiring boards with good dimensional stability can be obtained.

[0091] After the above processes, the following is obtained: Figure 3 The substrate FPCB1 for the flexible printed wiring board shown is shown.

[0092] Furthermore, in the above manufacturing method, a surface-treated fluororesin layer can be used as the fluororesin layer 11. For example, by pre-modifying the surface of the bonding surface (main surface) of the fluororesin layer 11, and then stacking single-sided metal foil laminates 15 and 19 using adhesive layers 12 and 16, the bonding strength after the integration process can be ensured. In this case, a commercially available fluororesin film with surface modification treatment can be used as the fluororesin layer 11.

[0093] Here, surface modification will be explained. Surface modification is performed by forming active sites on the surface of the fluoropolymer layer and bringing any one or a mixture of hydrophilic functional groups, monomers, oligomers, and polymers (hereinafter also referred to as "modifying materials") into contact with the formed active sites. Through surface modification, the surface of the fluoropolymer layer acquires a so-called adhesiveness or tackiness. For example, the fluoropolymer layer 11 to be surface modified uses a fluoropolymer layer that has undergone vacuum plasma treatment. By performing vacuum plasma treatment, hydrophilic functional groups (polar groups) such as NH2 groups, COOH groups, and OH groups are added to the surface of the fluoropolymer layer 11, depending on the type of treatment gas used in the vacuum plasma treatment.

[0094] As a method for forming active sites on the surface of the fluororesin layer 11, there are methods such as irradiating the surface of the fluororesin layer 11 with active light such as ultraviolet (UV) or excimer laser, or using discharge methods such as corona discharge or plasma discharge. In addition to the above methods, another method for forming active sites on the surface of the fluororesin layer 11 is to immerse the fluororesin layer in an alkali metal complex solution. As a contact method for bringing the modified material into contact with the fluororesin layer 11, there are methods for directly contacting the gaseous or liquid modified material with the fluororesin layer 11. In addition to the above methods, another method for contacting the modified material with the fluororesin layer 11 is to contact the fluororesin layer with a mixed gas obtained by diluting the modified material in a carrier gas, an aqueous solution obtained by dissolving the modified material, or an organic solvent solution obtained by dissolving the modified material.

[0095] Furthermore, when irradiating the fluoropolymer layer 11 with active light to form active sites, it is also effective to irradiate the fluoropolymer layer 11 with the gaseous or liquid modified material in direct contact. Alternatively, when irradiating the fluoropolymer layer 11 with active light to form active sites, it is also effective to irradiate the fluoropolymer layer 11 with a mixed gas obtained by diluting the modified material in a carrier gas, an aqueous solution obtained by dissolving the modified material, or an organic solvent solution obtained by dissolving the modified material.

[0096] Vacuum plasma treatment is a powerful method for surface modification. Vacuum plasma treatment is performed by exposing the substrate (in this embodiment, the fluororesin layer 11) to a glow discharge or similar process in a vacuum initiated and sustained by applying a high voltage (DC or AC) between electrodes. In the case of vacuum plasma treatment, there are many options for the treatment gas (modifying material). For example, He, Ne, Ar, N2, O2, carbon dioxide, air, water vapor, ammonia, etc., can be used as the treatment gas. Alternatively, mixtures of these gases can be used. In particular, vacuum plasma treatment using N2 gas, N2+H2 gas (a mixture of nitrogen and hydrogen), N2+O2 gas (a mixture of nitrogen and oxygen), or ammonia as the treatment gas imparts adhesive functional groups to the surface of the fluororesin layer 11, thereby achieving excellent results.

[0097] In addition, while single-sided metal foil laminates 15 and 19 are used in the above manufacturing method, reinforcing resin layer 13 and conductor layer 14 can be sequentially laminated instead of single-sided metal foil laminate 15, and reinforcing resin layer 17 and conductor layer 18 can be sequentially laminated instead of single-sided metal foil laminate 19. In this case, in order to directly bond conductor layers 14 and 18 with low coefficients of linear expansion to reinforcing resin layers 13 and 17 such as PI and LCP, for example, after heating reinforcing resin layers 13 and 17 to near their melting points, conductor layers 14 and 18 are attached to reinforcing resin layers 13 and 17 and pressure is applied. Subsequently, in order to prevent residual strain in reinforcing resin layers 13 and 17, slow cooling is performed. In addition to vacuum pressing, roll-to-roll methods, double-belt presses, Roto presses, etc., can also be used for the lamination and integration of conductor layers 14 and 18 with reinforcing resin layers 13 and 17.

[0098] <Flexible Printed Wiring Board Substrate FPCB2>

[0099] Next, refer to Figure 10 The flexible printed wiring board substrate FPCB2 according to the second embodiment will be described. The core insulating substrate of this flexible printed wiring board substrate FPCB2 is composed of a fluoropolymer layer. The flexible printed wiring board substrate FPCB2 is suitable, for example, for flexible printed wiring boards for high-frequency signal transmission with striplines.

[0100] like Figure 10 As shown, the lower portion of the flexible printed wiring board substrate FPCB2 has the same configuration as the flexible printed wiring board substrate FPCB1 described in the first embodiment. However, in the flexible printed wiring board substrate FPCB2, a conductive pattern 18a is provided instead of the conductor layer 18 of the flexible printed wiring board substrate FPCB1.

[0101] The conductive pattern 18a is disposed directly or indirectly on the reinforcing resin layer 17. In this embodiment, the conductive pattern 18a is directly bonded to the reinforcing resin layer 17 without the adhesive layer.

[0102] like Figure 10 As shown, the flexible printed wiring board substrate FPCB2 also includes a fluoropolymer layer 21; a curing adhesive layer 31h disposed on the lower surface of the fluoropolymer layer 21, with a conductive pattern 18a embedded therein; a curing adhesive layer 22h disposed on the upper surface of the fluoropolymer layer 21; a reinforcing resin layer 23 disposed on the curing adhesive layer 22h; and a conductor layer 24 disposed directly or indirectly on the reinforcing resin layer 23.

[0103] The pyrolysis temperatures of the cured adhesive layers 16h and 31h are lower than those of the reinforcing resin layers 13, 17, and 23. Furthermore, the thickness of the cured adhesive layer 16h is 10 μm to 200 μm. The thickness of the cured adhesive layer 31h (i.e., the length from the upper surface of the conductive pattern 18a to the fluoropolymer layer 21), excluding the thickness of the conductive pattern 18a, is 10 μm to 200 μm. Therefore, when laser-guided through-holes (through-holes H1 and H2 described later), deep depressions in the cured adhesive layer 16h can be suppressed. As a result, when through-hole plating is performed, the plating metal can be filled into the depressions, thus suppressing the formation of voids.

[0104] Next, the various components of the flexible printed wiring board substrate FPCB2 will be described in detail. However, detailed descriptions of components identical to those in the first embodiment will be omitted.

[0105] The fluoropolymer layer 21 has a third main surface opposite to the conductive pattern 18a and a fourth main surface opposite to the third main surface. The fluoropolymer layer 21 is an insulating film containing fluoropolymer. The material of the fluoropolymer layer 21 is the same as that of the aforementioned fluoropolymer layer 11.

[0106] The thickness of the fluoropolymer layer 21 is, for example, 12.5 to 200 μm, preferably 25 to 100 μm. When the thickness of the fluoropolymer layer 21 is thinner than the lower limit of 12.5 μm, the dielectric properties of the flexible printed circuit board substrate FPCB2 deteriorate. On the other hand, when the thickness of the fluoropolymer layer 21 is thicker than the upper limit of 200 μm, it is difficult to ensure the processability and dimensional stability of the flexible printed circuit board substrate FPCB2.

[0107] Similar to the fluoropolymer layer 11, the fluoropolymer layer 21 can be a surface-modified fluoropolymer layer or a fluoropolymer layer copolymerized with adhesive components, etc.

[0108] Furthermore, the fluoropolymer layer 21 is not limited to a single-layer membrane. The fluoropolymer layer 21 can be a multilayer structure consisting of multiple membrane layers stacked together. For example, the fluoropolymer layer 21 can be a two-layer structure consisting of PFA and PTFE stacked together. Alternatively, for example, if the adhesion between PTFE and the adhesive is insufficient, it can also be a three-layer structure (PFA / PTFE / PFA) with PFA sandwiched between PTFE.

[0109] In addition, the fluoropolymer layer 21 may contain inorganic fillers with low dielectric constant and low dielectric loss tangent. The material and content of the inorganic fillers are the same as those of the fluoropolymer layer 11.

[0110] Next, the curing times of the adhesive layer at 22h and 31h will be explained.

[0111] The cured adhesive layers 22h and 31h are insulating layers cured by heating the thermosetting adhesive layers 22 and 31 (described later).

[0112] The cured adhesive layer 31h is an insulating layer in which conductive patterns 18a are embedded and the reinforcing resin layer 17 is bonded to the fluororesin layer 21.

[0113] The thickness of the cured adhesive layer 22h and the thickness of the cured adhesive layer 31h (excluding the thickness of the conductive pattern 18a) are preferably 10 μm to 200 μm, more preferably 20 μm to 100 μm. In particular, the cured adhesive layer 31h is preferably 10 μm or more thick, more preferably 20 μm or more thick. When the cured adhesive layer 31h is thinner than 10 μm, the depressions formed in the cured adhesive layer 31h become deeper when using laser-guided vias, making it easier to generate voids during the plating process. The thicker the cured adhesive layer 31h, the shallower the depth of the depressions formed. Furthermore, when the cured adhesive layers 22h and 31h are thicker than 200 μm, the dielectric properties and flexibility of the flexible printed wiring board substrate FPCB2 deteriorate. Therefore, the thickness of the cured adhesive layers 22h and 31h is preferably 200 μm or less.

[0114] Next, the reinforcing resin layer 23 will be described.

[0115] The coefficient of linear expansion of the reinforcing resin layer 23 is smaller than that of the fluoropolymer layer 21. A smaller coefficient of linear expansion reduces the risk of compromising the dimensional stability of the flexible printed circuit board substrate FPCB2. Furthermore, it allows for a thinner reinforcing resin layer 23. The material of the reinforcing resin layer 23 is the same as that of the aforementioned reinforcing resin layers 13 and 17.

[0116] The thickness of the reinforcing resin layer 23 is, for example, 7.5 to 200 μm, preferably 12.5 to 100 μm. When the thickness of the reinforcing resin layer 23 is thinner than the lower limit of 7.5 μm, it is difficult to ensure processability and dimensional stability. On the other hand, when the thickness of the reinforcing resin layer 23 is thicker than the upper limit of 200 μm, the dielectric properties and flexibility of the flexible printed wiring board substrate FPCB2 deteriorate.

[0117] The coefficient of linear expansion of the reinforcing resin layer 23 is preferably 30 ppm / ℃ or less, and more preferably 25 ppm / ℃ or less. By using the reinforcing resin layer 23 with a coefficient of linear expansion of 30 ppm / ℃ or less, warping and reduced dimensional stability of the flexible printed wiring board substrate FPCB2 can be suppressed.

[0118] The elastic constant of the reinforcing resin layer 23 is preferably 3 GPa or higher, and more preferably 6 GPa or higher. This reduces shrinkage of the flexible printed wiring board substrate FPCB2 during cooling after the integration process, thereby improving the dimensional stability of the flexible printed wiring board substrate FPCB2.

[0119] Furthermore, the reinforcing resin layer 23 is not limited to a single film. The reinforcing resin layer 23 can be a multilayer structure composed of multiple film layers stacked together. For example, a thermoplastic resin and a non-thermoplastic resin can be laminated to form the reinforcing resin layer 23. As a result, a reinforcing resin layer with high flexibility can be obtained.

[0120] Next, conductor layer 24 will be described. As conductor layer 14 is as described above, its description will be omitted. Conductive pattern 18a is the same as conductor layer 18, so its description will also be omitted.

[0121] The conductor layer 24 is a metal foil such as copper or copper alloy, stainless steel, nickel or nickel alloy (including alloy 42), or aluminum or aluminum alloy. In this embodiment, the conductor layer 24 is a copper foil such as rolled copper foil or electrolytic copper foil.

[0122] To improve adhesion, chemical or mechanical surface treatments can be applied to the surface of the conductor layer 24 opposite to the reinforcing resin layer 23.

[0123] The thickness of the conductor layer 24 is not particularly limited, as long as it is sufficient to perform its function according to the application of the flexible printed wiring board. Considering the flexibility of the flexible printed wiring board, the thickness of the conductor layer 24 is preferably 6 to 70 μm, and more preferably 9 to 35 μm.

[0124] like Figure 10 As shown, in this embodiment, the conductor layer 14 is directly disposed on the reinforcing resin layer 13 without an adhesive layer. Similarly, the conductive pattern 18a is directly disposed on the reinforcing resin layer 17 without an adhesive layer. The conductor layer 24 is directly disposed on the reinforcing resin layer 23 without an adhesive layer. Thus, by eliminating the adhesive layer, the thickness of the flexible printed circuit board substrate FPCB2 can be reduced, thereby ensuring sufficient flexibility. Alternatively, given a fixed thickness of the flexible printed circuit board substrate FPCB2, the curing adhesive layers 12h, 16h, 22h, and 31h, as well as the reinforcing resin layers 13, 17, and 23, can be thickened. As a result, the dimensional stability of the flexible printed circuit board substrate FPCB2 and its laser machinability when using laser-guided universal vias can be improved.

[0125] In the flexible printed circuit board substrate FPCB2 described above, the pyrolysis temperature of the cured adhesive layer 16h, which is located inside the fluoropolymer layer 11 when viewed from the laser irradiation direction, is lower than the pyrolysis temperatures of the reinforcing resin layers 13, 17, and 23, and the thickness of the cured adhesive layer 16h is 10 μm to 200 μm. Furthermore, the pyrolysis temperature of the cured adhesive layer 31h, which is located inside the fluoropolymer layer 21 when viewed from the laser irradiation direction, is lower than the pyrolysis temperatures of the reinforcing resin layers 13, 17, and 23, and the thickness of the cured adhesive layer 31h, excluding the thickness of the conductive pattern 18a, is 10 μm to 200 μm. Therefore, when using a laser to pass through vias, the formation of deep depressions in the cured adhesive layers 16h and 31h can be suppressed. As a result, when plating vias, plating metal can be filled into the depressions, thus suppressing the formation of voids.

[0126] <Manufacturing Method of FPCB2, a Substrate for Flexible Printed Wiring Boards>

[0127] Reference Figures 4 to 10 The manufacturing method of the above-mentioned flexible printed wiring board substrate FPCB2 will be described.

[0128] First, a flexible printed wiring board substrate FPCB1 according to the first embodiment is prepared. Furthermore, as... Figure 4 As shown, a conductive pattern 18a is formed by patterning the conductor layer 14 using a known manufacturing method. This conductive pattern 18a may also include signal lines and ground lines. The signal lines and ground lines are electrically connected to signal lines used for high-speed signal transmission and the outer ground layer via vias.

[0129] Next, as Figure 5 As shown, a fluoropolymer layer 21, an adhesive layer 22, and a single-sided metal foil laminate 25 are prepared.

[0130] Similar to the aforementioned fluoropolymer layer 11, the fluoropolymer layer 21 is not limited to a single-layer film. The fluoropolymer layer 21 can be a multilayer structure composed of multiple film layers stacked together. For example, the fluoropolymer layer 21 can be a three-layer structure composed of PFA / PTFE / PFA. This allows the fluoropolymer layer 21 to be firmly bonded to the adhesive layer 22.

[0131] Similar to the aforementioned adhesive layers 12 and 16, adhesive layer 22 is preferably composed of an adhesive with excellent heat resistance. Examples of such adhesives include thermosetting adhesives with excellent dielectric properties from various resin systems, such as modified polyolefin resin adhesives, epoxy resin adhesives, butyral resin adhesives, bismaleimide adhesives, polyimide adhesives, modified polyphenylene ether adhesives, or modified aromatic polyester adhesives.

[0132] The single-sided metal foil laminate 25 has a structure in which a conductor layer 24 as a metal foil is provided on one side of the reinforcing resin layer 23, which serves as an insulating layer. As this single-sided metal foil laminate 25, commercially available flexible copper clad laminates (FCCLs) such as the adhesive-free copper clad laminate ESPANEX (registered trademark) manufactured by Nippon Steel Technology Co., Ltd. can be used.

[0133] Next, as Figure 5 as well as Figure 6 As shown, the laminate LB2 is formed by stacking a single-sided metal foil laminate 25 onto a fluoropolymer layer 21 using an adhesive layer 22 as an adhesive sheet and then applying pressure. Preferably, this process is performed at a temperature lower than the glass transition temperature of each resin layer (e.g., 60–80°C) as a so-called temporary lamination.

[0134] Alternatively, without using an adhesive sheet as the adhesive layer 22, the single-sided metal foil laminate 25 can be laminated after the adhesive layer 22 is formed by applying adhesive to the upper surface of the fluororesin layer 21, thereby forming the laminate LB2. Of course, the adhesive layer 22 can also be formed by applying adhesive to the reinforcing resin layer 23 side in advance, and then the single-sided metal foil laminate 25 with the adhesive layer 22 formed is laminated onto the upper surface of the fluororesin layer 21.

[0135] After forming the laminate LB2 as described above, as Figure 7 As shown, the adhesive layer 22 is cured by heating the laminate LB2, and the adhesive layer 22 is used as the cured adhesive layer 22h (integration process). More specifically, in this process, the laminate LB2 is integrated by heating and pressurizing it at a temperature above the curing temperature of the adhesive layer 22.

[0136] Next, as Figure 8 as well as Figure 9 As shown, a heat-treated (integrated) laminate LB1 (flexible printed wiring board substrate FPCB1) and a heat-treated (integrated) laminate LB2 are stacked together via an adhesive layer 31 to form a laminate LB3. Specifically, the laminate LB1 and laminate LB2 are stacked together via the adhesive layer 31 such that the conductive pattern 18a of the laminate LB1 faces the fluoropolymer layer 21 of the laminate LB2, thereby forming the laminate LB3. The thickness of the adhesive layer 31 is sufficient to embed the conductive pattern 18a, and the thickness of the adhesive layer 31 other than the conductive pattern 18a is ensured to be 10 μm or more.

[0137] Similar to the aforementioned adhesive layers 12, 16, and 22, adhesive layer 31 is preferably composed of an adhesive with excellent heat resistance. Examples of such adhesives include thermosetting adhesives with excellent dielectric properties from various resin systems, such as modified polyolefin resin adhesives, epoxy resin adhesives, butyral resin adhesives, bismaleimide adhesives, polyimide adhesives, modified polyphenylene ether adhesives, or modified aromatic polyester adhesives.

[0138] Alternatively, without using an adhesive sheet as the adhesive layer 31, the adhesive layer 31 can be formed by applying adhesive to the lower surface of the fluoropolymer layer 21 and then bonded to the flexible printed wiring board substrate FPCB1. Alternatively, the adhesive layer 31 can be formed by applying adhesive to the reinforcing resin layer 17 in a manner that embeds conductive patterns 18a, and then bonded. Figure 7 The layered structure LB2 is shown.

[0139] Next, the adhesive layer 31 is cured by heating the laminate LB3, and the adhesive layer 31 is used as the cured adhesive layer 31h. Thus, the desired product is obtained. Figure 10 The second embodiment of the flexible printed wiring board substrate FPCB2 is shown.

[0140] Alternatively, in the above manufacturing method, a surface-treated fluororesin layer 21 can also be used as the fluororesin layer 21. For example, the bonding surface (main surface) of the fluororesin layer 21 can be pre-modified, and then a single-sided metal foil laminate 25 can be stacked through an adhesive layer 22, thereby ensuring the bonding strength after the integration process. In this case, a commercially available fluororesin film with surface modification treatment can be used as the fluororesin layer 21.

[0141] In addition, in the above manufacturing method, a single-sided metal foil laminate 25 is used, but instead of the single-sided metal foil laminate 25, a reinforcing resin layer 23 and a conductor layer 24 can be sequentially laminated on the adhesive layer 22.

[0142] <Methods for forming through holes>

[0143] The following is for reference Figures 11-13 The method for forming through-holes on a flexible printed wiring board substrate FPCB2 is described.

[0144] like Figure 11 As shown, the conductor layer 14 is patterned using a known manufacturing method to form a conformal mask with an opening A1. Similarly, the conductor layer 24 is patterned to form a conformal mask with an opening A2.

[0145] Next, as Figure 11 as well as Figure 12As shown, by irradiating the opening A1 of the conductor layer 14 with a laser, the reinforcing resin layer 13, the cured adhesive layer 12h, the fluororesin layer 11, the cured adhesive layer 16h, and the reinforcing resin layer 17 are removed, forming a conductive via H1. The conductive via H1 is a bottomed hole, with a conductive pattern 18a (conductor layer 18) exposed on the bottom surface. Similarly, by irradiating the opening A2 of the conductor layer 24 with a laser, the reinforcing resin layer 23, the cured adhesive layer 22h, the fluororesin layer 21, and the cured adhesive layer 31h are removed, forming a conductive via H2. The conductive via H2 is a bottomed hole, with a conductive pattern 18a exposed on the bottom surface. Alternatively, the perforation process of the conductive vias H1 and H2 can be performed, for example, by repeatedly irradiating the openings A1 and A2 with laser pulses.

[0146] Next, after the removal of adhesive residue, such as Figure 13 As shown, a through-hole TH1 is formed by forming a plating layer 27 on the inner wall of the conductive through-hole H1, which electrically connects the conductor layer 14 to the conductive pattern 18a (conductor layer 18). Similarly, a through-hole TH2 is formed by forming a plating layer 28 on the inner wall of the conductive through-hole H2, which electrically connects the conductor layer 24 to the conductive pattern 18a.

[0147] As mentioned earlier, the pyrolysis temperature of the cured adhesive layer 16h is lower than that of the reinforcing resin layers 13, 17, and 23, and the thickness of the cured adhesive layer 16h is 10 μm or more. Therefore, in the laser processing step of the through-hole H1, the depression formed in the cured adhesive layer 16h becomes shallower. This is because the volume of the depression formed in the cured adhesive layer 16h (i.e., the amount of cured adhesive melted and decomposed by laser irradiation) is approximately constant regardless of the thickness of the cured adhesive layer 16h (the volume of the depression depends on the number of laser pulses), and because the pyrolysis temperature of the cured adhesive layer 16h is low, the cured adhesive layer 16h melts and decomposes entirely across its thickness. Therefore, the thicker the cured adhesive layer 16h, the shallower the depth (horizontal length) of the depression. Thus, according to this embodiment, the aspect ratio (the ratio of the depth to the width of the depression) of the depression formed in the cured adhesive layer 16h can be reduced.

[0148] As described above, since the aspect ratio of the depressions formed after 16 hours of curing the adhesive layer is small, the depressions are filled by the plated metal during the plating process. As a result, the formation of voids after the formation of the through-hole TH1 can be suppressed.

[0149] The cured adhesive layer 31h is the same as the cured adhesive layer 16h described above. That is, the pyrolysis temperature of the cured adhesive layer 31h is lower than that of the reinforcing resin layers 13, 17, and 23, and the thickness of the cured adhesive layer 31h, excluding the thickness of the conductive pattern 18a, is more than 10 μm. Therefore, the aspect ratio of the depressions generated in the cured adhesive layer 31h is smaller. As a result, the formation of voids after the formation of the through-hole TH2 can be suppressed.

[0150] Alternatively, the flexible printed wiring board substrate FPCB1 of the first embodiment can be used as the object, and through holes that electrically connect the conductor layer 14 and the conductor layer 18 (conductive pattern 18a) can be formed in the same way as described above.

[0151] Figure 14 This is a cross-sectional photograph showing the through-hole actually fabricated as an example. In this example, after forming the through-hole on a substrate having the same configuration as the flexible printed wiring board substrate FPCB1 of the first embodiment, a copper plating layer was formed. A 50 μm thick PFA was used as the fluoropolymer layer 11, a 25 μm thick LCP was used as the reinforcing resin layers 13 and 17, and copper foil with a thickness of approximately 12 μm was used as the conductor layers 14 and 18. Furthermore, the thickness of the cured adhesive layer 12h is 30 μm, and the thickness of the cured adhesive layer 16h is 10 μm. Figure 14 As shown, no deep depressions or cavities were formed on the inner wall of the guide hole.

[0152] Figure 15 This is a cross-sectional photograph of a through-hole that was actually manufactured as a comparative example. Figure 15 The comparison examples shown are Figure 14 The difference in the illustrated embodiments lies in the thickness of the cured adhesive layer. In the comparative example, the thickness of the cured adhesive layer, corresponding to 12h and 16h of curing, is 6μm. In the comparative example, as... Figure 15 As shown, deep depressions formed in the cured adhesive layer beneath the fluororesin layer. These depressions could not be filled with copper plating, resulting in voids. Conversely, the depressions in the cured adhesive layer above the fluororesin layer were shallow, and no voids were formed. This is believed to be because the cured adhesive layer above the fluororesin layer was already penetrated during the perforation of the fluororesin layer at 12h, and therefore was almost unaffected by laser irradiation. In contrast, the cured adhesive layer below the fluororesin layer was exposed to laser light for a longer period until the fluororesin layer was penetrated, resulting in melting and decomposition of the cured adhesive layer.

[0153] Furthermore, assuming that the pyrolysis temperature of the cured adhesive layer 16h is as high as that of the reinforcing resin layers 13 and 17, even if the thickness of the cured adhesive layer 16h is ensured to be 10 μm or more, localized melting and decomposition will occur near the boundary with the fluoropolymer layer during laser treatment. As a result, deep depressions will form only in the upper part of the cured adhesive layer 16h (near the boundary with the fluoropolymer layer). Therefore, for the cured adhesive layers 16h and 31h, it is preferable to ensure a thickness of 10 μm or more and to keep the pyrolysis temperature below the pyrolysis temperature of the reinforcing resin layer. With this configuration, the inner wall of the cured adhesive layer is shaped into a gently sloping arc across the thickness direction. As a result, depressions with large aspect ratios will not form, thus preventing the formation of voids during the plating process.

[0154] Based on the foregoing description, those skilled in the art will likely conceive of additional effects and various modifications of the present invention. However, the present invention is not limited to the embodiments described above. Various additions, modifications, and partial deletions can be made without departing from the scope defined by the claims and their equivalents, and without departing from the technical, conceptual, and spirit aspects of the present invention.

[0155] The detailed description has been given for illustrative and explanatory purposes. Many variations and modifications are possible in accordance with the teachings above. The detailed description is not without omissions or intended to limit the subject matter described herein. Although the subject matter has been described in words with particular structural features and / or methodological processes, it should be understood that the subject matter defined in the claims is not necessarily limited to the specific features or processes described. Rather, the specific features and processes described are illustrated as examples of implementing the claims.

Claims

1. A method for forming a through hole, characterized in that, The process includes the following steps: A laminate is formed, the laminate comprising: a fluoropolymer layer having a first main surface and a second main surface; a first adhesive layer disposed on the first main surface; a first reinforcing resin layer disposed on the first adhesive layer having a coefficient of linear expansion smaller than that of the fluoropolymer layer; and a first conductor layer disposed directly or indirectly on the first reinforcing resin layer. A second adhesive layer is disposed on the second main surface; a second reinforcing resin layer is disposed on the second adhesive layer, and its coefficient of linear expansion is smaller than that of the fluororesin layer. And a second conductor layer, disposed directly or indirectly on the second reinforcing resin layer; By heating the laminate, the first adhesive layer and the second adhesive layer are cured, becoming the first cured adhesive layer and the second cured adhesive layer, respectively. An opening is formed in the first conductor layer. By irradiating the opening with a laser, the first reinforcing resin layer, the first cured adhesive layer, the fluororesin layer, the second cured adhesive layer, and the second reinforcing resin layer are removed, forming a bottomed conductive hole with the second conductor layer exposed on its bottom surface; and The first conductor layer and the second conductor layer are electrically connected by forming a plating layer on the inner wall of the conductive via. The pyrolysis temperature of the second cured adhesive layer is lower than that of the first reinforcing resin layer and the second reinforcing resin layer, and the thickness of the second cured adhesive layer is more than 10 μm and less than 200 μm.

2. The method for forming a through hole according to claim 1, characterized in that, The thickness of the second cured adhesive layer is between 20 μm and 100 μm.

3. The method for forming a through hole according to claim 1 or 2, characterized in that, The second adhesive layer is composed of modified polyolefin resin adhesive, epoxy resin adhesive, butyral resin adhesive, bismaleimide adhesive, polyimide adhesive, modified polyphenylene ether adhesive, or modified aromatic polyester adhesive.

4. The method for forming a through hole according to any one of claims 1 to 3, characterized in that, The through-hole forming method further includes the following steps: The second conductor layer is patterned to form a conductive pattern; A second laminate is formed, the second laminate comprising: a second fluororesin layer having a third main surface and a fourth main surface; and a third adhesive layer disposed on the third main surface; A third reinforcing resin layer is disposed on the third adhesive layer, and its coefficient of linear expansion is less than that of the second fluororesin layer; and a third conductor layer is disposed directly or indirectly on the third reinforcing resin layer. The third adhesive layer is cured by heating the second laminate to form a third cured adhesive layer. A third laminate is formed by laminating the laminate and the second laminate with a fourth adhesive layer in such a way that the conductive pattern of the heat-treated laminate is opposite to the second fluoropolymer layer of the heat-treated second laminate. The fourth adhesive layer is cured by heating the third laminate, thus becoming a fourth cured adhesive layer. An opening is formed in the third conductor layer, and the third reinforcing resin layer, the third cured adhesive layer, the second fluororesin layer, and the fourth cured adhesive layer are removed by irradiating the opening in the third conductor layer with a laser, thereby forming a bottomed second conductive hole with the conductive pattern exposed on the bottom surface. as well as The third conductor layer is electrically connected to the conductive pattern by forming a plating layer on the inner wall of the second conductive via. The pyrolysis temperature of the fourth curing adhesive layer is lower than that of the first reinforcing resin layer, the second reinforcing resin layer, and the third reinforcing resin layer. The thickness of the fourth curing adhesive layer, excluding the thickness of the conductive pattern, is more than 10 μm and less than 200 μm.

5. A substrate for a flexible printed wiring board, characterized in that, include: A fluoropolymer layer having a first main surface and a second main surface opposite to the first main surface; A first cured adhesive layer is disposed on the first main surface of the fluororesin layer; A first reinforcing resin layer is disposed on top of the first cured adhesive layer, and its coefficient of linear expansion is smaller than that of the fluororesin layer. The first conductor layer is disposed directly or indirectly on the first reinforcing resin layer; A second cured adhesive layer is disposed on the second main surface of the fluororesin layer; The second reinforcing resin layer is disposed on the second cured adhesive layer and has a coefficient of linear expansion smaller than that of the fluororesin layer. The second conductor layer is disposed directly or indirectly on the second reinforcing resin layer; as well as A through-hole is formed by the through-hole forming method as described in claim 1, which electrically connects the first conductor layer and the second conductor layer.

6. The substrate for flexible printed wiring boards according to claim 5, characterized in that, The first conductor layer is not directly applied over the first reinforcing resin layer via an adhesive layer.

7. The substrate for flexible printed wiring boards according to claim 5 or 6, characterized in that, The second conductor layer is not directly applied over the second reinforcing resin layer via an adhesive layer.

8. The substrate for flexible printed wiring boards according to any one of claims 5 to 7, characterized in that, The thickness of the second cured adhesive layer is more than 20 μm and less than 100 μm.

9. The substrate for flexible printed wiring boards according to any one of claims 5 to 8, characterized in that, The coefficient of linear expansion of the first reinforcing resin layer and the second reinforcing resin layer is less than 30 ppm / ℃, and the elastic constant is greater than 3 GPa.

10. A substrate for a flexible printed wiring board, characterized in that, include: The first fluororesin layer has a first main surface and a second main surface opposite to the first main surface; A first cured adhesive layer is disposed on the first main surface of the first fluororesin layer; A first reinforcing resin layer is disposed on top of the first cured adhesive layer, and its coefficient of linear expansion is smaller than that of the first fluororesin layer. The first conductor layer is disposed directly or indirectly on the first reinforcing resin layer; A second cured adhesive layer is disposed on the second main surface of the first fluororesin layer; The second reinforcing resin layer is disposed on the second cured adhesive layer and has a coefficient of linear expansion that is smaller than that of the first fluororesin layer. Conductive patterns are disposed directly or indirectly on the second reinforcing resin layer; The second fluoropolymer layer has a third main surface opposite to the conductive pattern and a fourth main surface opposite to the third main surface; The third curing adhesive layer, in which the conductive pattern is embedded, bonds the second reinforcing resin layer to the second fluororesin layer. A fourth cured adhesive layer is disposed on the fourth main surface of the second fluororesin layer; The third reinforcing resin layer is disposed on the fourth cured adhesive layer and has a coefficient of linear expansion that is smaller than that of the second fluororesin layer. as well as The second conductor layer is disposed directly or indirectly on top of the third reinforcing resin layer. The pyrolysis temperatures of the second and third cured adhesive layers are lower than those of the first, second, and third reinforcing resin layers. The thickness of the second cured adhesive layer is 10 μm to 200 μm, and the thickness of the third cured adhesive layer, excluding the thickness of the conductive pattern, is 10 μm to 200 μm.

11. The substrate for flexible printed wiring boards according to claim 10, characterized in that, The first conductor layer is not directly applied over the first reinforcing resin layer via an adhesive layer.

12. The substrate for flexible printed wiring boards according to claim 10 or 11, characterized in that, The second conductor layer is not directly applied over the third reinforcing resin layer via an adhesive layer.

13. The substrate for flexible printed wiring boards according to any one of claims 10 to 12, characterized in that, The conductive pattern is not directly applied to the second reinforcing resin layer via an adhesive layer.

14. The substrate for flexible printed wiring boards according to any one of claims 10 to 13, characterized in that, The thickness of the second cured adhesive layer and the thickness of the third cured adhesive layer, excluding the thickness of the conductive pattern, are 20 μm to 100 μm.

15. The substrate for flexible printed wiring boards according to any one of claims 10 to 14, characterized in that, The linear expansion coefficients of the first reinforcing resin layer, the second reinforcing resin layer, and the third reinforcing resin layer are below 30 ppm / ℃, and the elastic constants are above 3 GPa.

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