Conductive film and manufacturing method therefor

Abstract

Provided is a conductive film that has a conductive coating film formed by plating using a fluororesin film as a base material.　The peel strength of the conductive coating film formed by plating on the fluororesin film is 5-15 N / cm. Accordingly, peeling of the conductive coating film from the fluororesin film due to heat in a use environment is suppressed, and the conductive film can be used for various substrate materials such as a layer-constituting material of a build-up substrate. Moreover, since the conductive coating film is directly plated on the fluororesin film, the characteristics of the fluororesin film such as a low dielectric loss tangent of less than 0.001 can be utilized, and the conductive film is useful as a material for a next-generation mobile communication system represented by 5G / 6G.

Description

Conductive film and method for producing the same

[0001] The present invention relates to a conductive film in which a conductive coating is formed on a fluororesin film by plating, and a method for producing the same.

[0002] The present applicant has proposed a technology relating to a flexible circuit film in which a conductive coating is formed by plating on a functional resin film as a substrate in Patent Document 1. Patent Document 1 also proposes that a flexible circuit film having a fluororesin film as a substrate be used as a flexible circuit board for high-speed transmission, a flexible circuit board for antenna cables, a flexible circuit board for semiconductors, and an actuator.

[0003] However, fluororesin films have extremely poor adhesion to other materials. Therefore, when using fluororesin films as substrates, it is difficult to bond conductive coatings with high strength by plating, leaving room for improvement in terms of practicality. Other known conductive films include those made by impregnating fluororesin films with glass cloth, polyimide varnish, epoxy resin, etc. to reduce thermal expansion and improve adhesion to other materials, and then laminating the resulting copper foil by thermocompression bonding. However, the inclusion of impurities such as glass cloth or polyimide varnish diminishes the fluororesin film's inherently low dielectric loss tangent. Furthermore, the backside of the copper foil used for thermocompression lamination is already roughened, and the unevenness causes loss in transmission characteristics due to the skin effect. Furthermore, the thermocompression bonding process places significant thermal stress on the substrate.

[0004] Furthermore, in recent years, in the field of semiconductor packages and the like, in order to achieve high functionality and miniaturization, a semi-additive process has been adopted in which insulating layers are laminated using a build-up method and circuits are then built up only in necessary areas using electrolytic copper plating. For example, as shown in Non-Patent Document 1, an insulating resin film containing a thermosetting resin such as epoxy resin as a main component is used as such an insulating layer. This insulating resin film is provided as a three-layer structure formed on a PET film and combined with a protective film. The protective film and PET film are peeled off and thermally cured, and then vias and other processes are performed, followed by plating to form a predetermined conductive coating.

[0005] WO2018 / 225760 publication

[0006] "Current Status and Trends of Interlayer Insulation Materials for Semiconductor Package Substrates," Hirohisa Narahashi, Shigeo Nakamura, Journal of the Japan Institute of Electronics Packaging, Vol. 14, No. 5 (2011)

[0007] Materials for next-generation mobile communication systems, such as 5G / 6G, require excellent dielectric properties in high-frequency bands, such as the millimeter-wave band (28 GHz or higher), and substrate materials with low dielectric loss tangents are particularly important. The insulating resin film primarily composed of a thermosetting resin, such as epoxy resin, as described in Non-Patent Document 1, for example, the commercially available "Ajinomoto Build-Up Film (registered trademark)" (Ajinomoto Fine-Techno Co., Ltd.), has a dielectric loss tangent as low as approximately 0.005. On the other hand, fluororesin films have an advantageous dielectric loss tangent of less than 0.001 for high-purity films. However, due to the poor adhesiveness of fluororesin films as described above, impurities must be mixed into them for practical use. Currently, the build-up substrates used are those laminated with the above-mentioned copper foil, and have a dielectric loss tangent exceeding 0.001.

[0008] The present invention has been made in consideration of the above, and an object of the present invention is to provide a conductive film and a method for producing the same, which has a base material of a fluororesin film, to which plating is firmly adhered, has excellent dielectric properties, and is suitable for use as a layer component of a build-up substrate having a conductive coating as a conductive layer.

[0009] In order to solve the above problems, the present invention provides a conductive film comprising a fluororesin film having a conductive coating formed on at least one surface thereof by plating, and the peel strength of the conductive coating relative to the fluororesin film is in the range of 5 to 15 N / cm.

[0010] It is preferable that the fluororesin film be used as a layer constituent material for a build-up substrate in which the fluororesin film serves as an insulating layer and the conductive coating serves as a conductive layer.The layer constituent material can be provided in a form in which the conductive coating is formed on one side of the fluororesin film and the other side is a surface-modified surface.The layer constituent material can be provided in a form in which the conductive coating is formed on both sides of the fluororesin film.The layer constituent material can be provided in a form in which the conductive coating is formed on one side of the fluororesin film and a bonding sheet and a protective film are laminated in that order on the other side.

[0011] The fluororesin forming the fluororesin film is preferably selected from polytetrafluoroethylene (tetrafluoride) (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (4.6fluoride) (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polyvinylidene fluoride (difluoride) (PVDF), polychlorotrifluoroethylene (trifluoride) (PCTFE), or chlorotrifluoroethylene-ethylene copolymer (ECTFE). In this case, it is more preferable that the dielectric loss tangent is less than 0.001. The fluororesin film preferably does not contain impurities to improve adhesion.

[0012] The conductive film of the present invention can be suitably used for flexible circuit boards for high-frequency antenna modules, flexible cables for high-speed transmission, substrates for semiconductor packages, multilayer substrates, SLPs (Substrate-Like PCBs), and actuators.

[0013] The present invention also provides a method for producing a conductive film, which comprises forming a conductive coating on at least one surface of a fluororesin film by plating, the plating being carried out in the following order: surface modification of the fluororesin film, catalyst application, acceleration, electroless copper plating or electroless nickel plating, and electrolytic copper plating or electrolytic nickel plating. The accelerator used in the acceleration is preferably a hot sulfuric acid solution. The hot sulfuric acid solution is preferably a sulfuric acid solution heated to a temperature of 40 to 80°C and having a concentration of 10 to 30% by volume. The immersion time in the sulfuric acid solution is preferably 1 to 5 minutes. In the above-mentioned production method, the fluororesin forming the fluororesin film is preferably selected from polytetrafluoroethylene (tetrafluoride) (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (4.6fluoride) (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polyvinylidene fluoride (difluoride) (PVDF), polychlorotrifluoroethylene (trifluoride) (PCTFE), or chlorotrifluoroethylene-ethylene copolymer (ECTFE).

[0014] According to the present invention, the conductive coating formed on the fluororesin film by plating has a peel strength of 5 to 15 N / cm. As a result, peeling of the conductive coating from the fluororesin film due to heat in the usage environment is suppressed, making it possible to use the conductive coating as a variety of substrate materials, such as layer components in build-up substrates. Furthermore, because the conductive coating is directly plated onto the fluororesin film, the low dielectric loss tangent of the fluororesin film (less than 0.001) can be utilized, making the conductive coating useful as a material for next-generation mobile communication systems, such as 5G / 6G. Furthermore, because the conductive coating is formed by plating, both the plated surface and the plated surface are smooth, eliminating the increased transmission loss caused by unevenness on the backside of the copper foil, as occurs in conventional methods of thermocompression bonding of copper foil. Furthermore, the plating process preferably includes an acceleration treatment using hot sulfuric acid, which also contributes to reducing thermal stress on the conductive film.

[0015] FIGS. 1(a) to 1(c) are diagrams illustrating examples of applications and configurations of the conductive film of the present invention. FIGS. 2(a) to 2(c) are diagrams illustrating another example of the configuration of the conductive film of the present invention. FIGS. 3(a) to 3(d) are diagrams illustrating an example of a manufacturing process for a layer component having one surface subjected to a surface adhesive treatment. FIGS. 4(a) to 4(e) are diagrams illustrating another example of a manufacturing process for a layer component having one surface subjected to a surface adhesive treatment. FIGS. 5(a) to 5(c) are diagrams illustrating an example of a manufacturing process for a layer component having copper plating layers formed on both sides. FIGS. 6(a) to 6(c) are diagrams illustrating an application example to a high-frequency transmission cable flexible circuit board. FIGS. 7(a) to 7(c) are diagrams illustrating the manufacturing process for a build-up board. FIG. 8 is a diagram illustrating an image of a semiconductor package.

[0016] The present invention will be described in more detail below based on the embodiments shown in the drawings. Figures 1(a) to 1(c) are schematic diagrams illustrating the use of the conductive film of the present invention and its configuration example, with polytetrafluoroethylene (PTFE) as the base material. Of these, Figures 1(a) and 1(b) illustrate examples of materials for flexible printed circuits (FPCs) for radio frequency (RF) modules. Figure 1(a) shows PTFE with copper plating on one side, and Figure 1(b) shows PTFE with copper plating on both sides. Figure 1(c) illustrates an example of a configuration suitable for a metal-coated bonding film for build-up substrates and an impedance-matching shielding film for flexible flat cables (FFCs). In Figure 1(c), one side is copper-plated, and a bonding sheet and a protective film are laminated in order on the other side.

[0017] Figures 2(a) to 2(c) are schematic diagrams illustrating further examples of the configuration of the conductive film of the present invention. Examples of configurations suitable for layer components (Types A to C) of build-up substrates are shown. Build-up substrates are used, for example, as substrates for semiconductor packages, as shown in Figure 8. As will be described in detail below, such build-up substrates are manufactured by stacking layer components consisting of a laminated structure of a bonding sheet, a fluororesin film (e.g., PTFE), and copper plating, sandwiching a core substrate, forming vias, and forming conductor circuits by etching, as shown in Figures 7(a) to 7(c). Furthermore, Figure 8 shows a PCB mounting a semiconductor package. SLPs (Substrate-Like PCBs) with fine conductor circuits have recently been used for these PCBs. The conductive film of this embodiment, which allows for the formation of fine circuits by plating, is also suitable as a material for forming such SLPs.

[0018] The Type A layer constituent material shown in Figure 2(a) has a fluororesin film (e.g., PTFE or tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA)) as a base material, with a copper plating layer formed on one side thereof. The other side has been treated (referred to as a "surface adhesion treatment" in this specification) to enable lamination of an adhesive layer such as a bonding sheet when producing a build-up substrate using a fluororesin film with very poor bonding properties.

[0019] 3(a) to 3(d) show an example of a manufacturing process for a layer component having one surface subjected to a surface adhesive treatment. First, both surfaces of a substrate (e.g., PTFE) are subjected to a surface modification treatment such as plasma treatment or sodium treatment (described later) (FIG. 3(a)). Next, both surfaces are electrolessly plated with copper (FIG. 3(b)). The electroless copper plating on one surface is masked, and the other surface is etched (FIG. 3(c)). Next, the mask is removed, and electrolytic copper plating is applied to one surface (FIG. 3(d)). As a result, the electroless copper plating layer on the other surface is etched, exposing the surface modified by plasma treatment, sodium treatment, or the like. This exposed surface-modified surface becomes the surface subjected to the surface adhesive treatment, allowing for the lamination of an adhesive layer.

[0020] Figures 4(a) to 4(e) show another example of the manufacturing process for a layer component in which one side has been subjected to a surface adhesive treatment. The surface modification treatment and electroless copper plating processes (Figures 4(a) and 4(b)) on both sides of the substrate are the same as those shown in Figures 3(a) to 3(b). The process of masking the electroless copper plating on one side and etching the electroless copper plating on the other side shown in Figure 4(c) is also basically the same as that shown in Figure 3(c). However, Figure 4(c) assumes that the surface modified in step (a) does not remain as a result of etching. In this case, after electrolytic copper plating on one side (Figure 4(d)), the other side is again subjected to surface modification treatment (Figure 4(e)). This allows for lamination of an adhesive layer on the other side.

[0021] The Type B layer component shown in Fig. 2(b) has a fluororesin film (e.g., PTFE or PFA) as a substrate, with copper plating layers formed on both sides. Figures 5(a) to 5(c) show the manufacturing process, in which both sides of the substrate are subjected to a surface modification treatment (Fig. 5(a)), then electroless copper plating is applied to both sides (Fig. 5(b)), and then electrolytic copper plating is applied to both sides (Fig. 5(c)).

[0022] Type C shown in Figure 2(c) is a structure equivalent to Type A in Figure 2(a), which has a fluororesin film (e.g., PTFE or PFA) as a base material and a copper plating layer formed on one side, but with a bonding sheet laminated on the other side and a protective film further laminated on that surface. As shown in Figures 3 and 4, Type A in Figure 2(a) has a surface that has been subjected to a surface adhesive treatment that allows a bonding sheet to be attached to the side opposite the copper plating layer. Therefore, if the surface is provided with a bonding sheet and a protective film laminated on it in advance, it can be easily used in build-up substrates. Although fluororesin films have a high linear expansion coefficient, laminating a bonding sheet has the advantage of suppressing the thermal expansion of the fluororesin film.

[0023] Figures 6(a) to 6(c) show examples of configurations used as flexible circuit boards for high-frequency transmission cables and flexible circuit boards for high-frequency antenna modules. Figure 6(a) is an example using Type A of Figure 2(a), Figure 6(b) is an example using Type B of Figure 2(b), and Figure 6(c) is an example using Type C of Figure 2(c) stacked on Type B of Figure 2(b).

[0024] In the case of the build-up board shown in Figures 7(a)-(c), for example, a layer component of Type A (see Figure 2(a)) or Type C (see Figure 2(c)), in which one side of PTFE is copper-plated, is bonded to a core substrate (Figure 7(a)). Next, vias are formed by laser processing, and electroless copper plating is applied to the via holes to establish electrical continuity between the core substrate and the copper-plated PTFE (Figure 7(b)). Because the copper layer is formed on the PTFE by plating, the thickness of the copper plating on the PTFE can be reduced to, for example, 0.5 μm or less, enabling the use of semi-additive processing methods to accommodate even finer-pitch circuits (Figure 7(c)). Conventional RCCs use copper foil coated with an epoxy resin (adhesive), but copper foil, even the thinnest, is 1-2 μm thick, which is thicker than the copper-plated layer formed in this embodiment and is significantly more expensive. Therefore, even if a semi-additive process is performed with this material, when the Cu (1 to 2 μm) of the seed layer is finally dissolved by etching, the circuit portion is also removed by about 2 μm, making it impossible to use for fine patterns. In this respect, the present embodiment is suitable for forming fine patterns as described above. Furthermore, the fluororesin film used in this embodiment does not contain impurities such as glass cloth, as discussed in the prior art section, so it is easy to drill holes using a laser.

[0025] The fluororesin forming the fluororesin film used as the substrate is preferably selected from polytetrafluoroethylene (tetrafluoride) (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (4.6fluoride) (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polyvinylidene fluoride (difluoride) (PVDF), polychlorotrifluoroethylene (trifluoride) (PCTFE), or chlorotrifluoroethylene-ethylene copolymer (ECTFE). Furthermore, the fluororesin film preferably does not contain impurities for improving adhesion, such as glass cloth or polyimide varnish. The presence of these impurities increases the dielectric loss tangent. In particular, polytetrafluoroethylene (tetrafluoride) (PTFE) and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), which have a dielectric loss tangent of less than 0.001, are more preferred.

[0026] The manufacturing process for the layer constituent material shown in Figures 2 to 5 has been outlined above. Although some of the explanation will be repeated, the conductive film of this embodiment used in this layer constituent material is manufactured in more detail as follows. First, a fluororesin film as a substrate is degreased, followed by a surface modification treatment. Next, an ionic or colloidal catalyst application treatment and an acceleration treatment are performed, followed by reduction precipitation using electroless copper plating or electroless nickel plating, and then electrolytic copper plating or electrolytic nickel plating to form a conductive coating of a predetermined thickness.

[0027] Here, the surface modification treatment can be a plasma treatment, a UV treatment, a sodium treatment, a chemical treatment, etc. As the plasma treatment, a low-temperature plasma is used, but the type of vacuum plasma or atmospheric pressure plasma is not limited, and the type of gas used to generate plasma is also not limited, and oxygen, nitrogen, helium, argon, ammonia, etc. can be used. As the chemical treatment, a treatment using a surfactant to make the surface potential cationic or anionic can be used.

[0028] The catalyst application treatment is carried out, for example, by immersing the surface-modified fluororesin film in a catalyst solution, preferably a palladium-based catalyst solution, followed by an acceleration treatment. The acceleration treatment preferably uses a hot sulfuric acid solution as the accelerator. A more preferred accelerator is a sulfuric acid solution with a concentration of 10 to 30% by volume heated to a temperature of 40 to 80°C. The immersion time in the sulfuric acid solution is preferably in the range of 1 to 5 minutes. By carrying out such a catalyst application treatment and acceleration treatment, it is possible to increase the reliability of catalyst adhesion and activation on the surface of the fluororesin film. Furthermore, by carrying out an acceleration treatment using hot sulfuric acid, thermal stress on the conductive film is also suppressed.

[0029] (Peel Test) A fluororesin film whose surface had been modified by plasma treatment was treated with palladium as a catalyst. Subsequently, an acceleration treatment was performed using a 20% by volume sulfuric acid solution heated to 55°C as an accelerator for 1 minute of immersion. Then, electroless plating and electrolytic plating were performed to form a conductive coating on the surface of the fluororesin film, producing conductive film samples. The thickness of the conductive coating for each sample was 5 to 12 μm. For each sample, the peel strength of the conductive coating formed by plating on the substrate (fluororesin film) was measured. Peel strength was measured using a 180-degree peel test (head speed 50 mm / min, stroke 10 mm) in accordance with JIS C 6471. Despite the use of a fluororesin film as the substrate, all peel strengths were in the range of 5 to 15 N / cm.

[0030] Specifically, the results were as follows: Substrate Conductive coating Peel strength (N / cm) PFA Copper 10-14 PTFE Copper 6-8 PVDF Nickel 4-6

[0031] If the peel strength is 5 N / cm or more, practical problems such as peeling due to heat will hardly occur, but it is preferably 6 N / cm or more, and more preferably 8 N / cm or more.

[0032] Among fluororesins, perfluoroalkoxy fluororesin (PFA) and polytetrafluoroethylene (PTFE), which have a dielectric dissipation factor of less than 0.001, are preferably used for flexible circuit boards for high-frequency antenna modules, flexible cables for high-speed transmission, substrates for semiconductor packages, multilayer substrates, and SLPs (Substrate-Like PCBs), as described above. Polyvinylidene fluoride resin (PVDF) has a relatively high dielectric dissipation factor of 0.01 or more, but because it has a piezoelectric function, it is preferably used as various actuators that utilize the piezoelectric function.

Claims

1. A conductive film having a conductive coating formed on at least one surface of a fluororesin film by plating, the peel strength of the conductive coating relative to the fluororesin film being in the range of 5 to 15 N / cm.

2. The conductive film according to claim 1, which is used as a layer component of a build-up substrate, in which the fluororesin film serves as an insulating layer and the conductive coating serves as a conductive layer.

3. The conductive film according to claim 2, which is provided as said layer constituent material in a form in which said conductive coating is formed on one side of said fluororesin film and the other side is a surface that has been subjected to a surface modification treatment.

4. The conductive film according to claim 2, which is provided as said layer constituent material in the form of said conductive coating formed on both sides of said fluororesin film.

5. The conductive film according to claim 2, wherein the conductive coating is formed on one side of the fluororesin film, and a bonding sheet and a protective film are laminated in this order on the other side of the fluororesin film, and the conductive coating is provided as the layer constituent material.

6. The conductive film according to claim 1, wherein the fluororesin forming the fluororesin film is selected from polytetrafluoroethylene (tetrafluoride) (PTFE), tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (4.6 fluoride) (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polyvinylidene fluoride (difluoride) (PVDF), polychlorotrifluoroethylene (trifluoride) (PCTFE), or chlorotrifluoroethylene-ethylene copolymer (ECTFE).

7. The conductive film according to claim 6, which has a dielectric loss tangent of less than 0.

001.

8. The conductive film according to claim 6, wherein the fluororesin film does not contain impurities for improving adhesiveness.

9. The conductive film according to any one of claims 1 to 8, which is used for a flexible circuit board for a high-frequency antenna module, a flexible cable for high-speed transmission, a substrate for a semiconductor package, a multilayer substrate, an SLP (Substrate-Like PCB), or an actuator.

10. A method for producing a conductive film in which a conductive coating is formed on at least one surface of a fluororesin film by plating, the plating being carried out in the following order: surface modification treatment of the fluororesin film, catalyst application treatment, acceleration treatment, electroless copper plating or electroless nickel plating, and electrolytic copper plating or electrolytic nickel plating.

11. The method for producing a conductive film according to claim 10, wherein the accelerator used in the acceleration treatment is a solution of hot sulfuric acid.

12. The method for producing a conductive film according to claim 11, wherein the hot sulfuric acid solution is a sulfuric acid solution having a concentration of 10 to 30% by volume heated to a temperature of 40 to 80°C.

13. The method for producing a conductive film according to claim 12, wherein the immersion time in the sulfuric acid solution is 1 to 5 minutes.

14. The method for producing a conductive film according to claim 10, wherein the fluororesin forming the fluororesin film is selected from polytetrafluoroethylene (tetrafluoride) (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (4.6 fluoride) (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polyvinylidene fluoride (difluoride) (PVDF), polychlorotrifluoroethylene (trifluoride) (PCTFE), or chlorotrifluoroethylene-ethylene copolymer (ECTFE).

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