JP2024072480A - Laminate for electronic circuit board and method for producing same - Google Patents

Abstract

[Problem] To provide a laminate for electronic circuit boards that can produce flexible circuit boards with small transmission loss of high frequency electric signals. [Solution] A laminate for electronic circuit boards 10 in which a metal foil 17 is laminated on one or both sides of a base film 15 via an adhesive layer 16, the base film being mainly composed of syndiotactic polystyrene, being biaxially oriented, having a melting point of 260-290°C, a glass transition temperature of 230-260°C, a relative dielectric constant of 2.6 or less at a frequency of 10 GHz, and a dielectric loss tangent of 0.002 or less, and the peel strength when the metal foil is peeled off is 3 N/10 mm or more before and after a heating test at 260°C for 30 seconds. [Selected Figure] Figure 1

Description

The present invention relates to a laminate for manufacturing flexible circuit boards.

In order to manufacture flexible circuit boards and the like, flexible copper-clad laminates (FCCLs) are used, which are made by laminating a synthetic resin film with copper foil adhered to one or both sides of the synthetic resin film. The transmission loss of an electric signal consists of conductor loss and dielectric loss, and the higher the signal frequency, the greater the dielectric loss. In recent years, as the frequency of electric signals has rapidly increased, reducing the dielectric loss caused by materials adjacent to the copper foil has become an issue. Patent documents 1 to 5 describe FCCLs that aim to reduce dielectric loss, and are three- or five-layer copper-clad laminates made of a synthetic resin film, an adhesive layer, and copper foil. Patent document 6 describes a laminate for electronic circuit boards in which a metal layer is formed by plating on the surface of a syndiotactic polystyrene-based resin film. Patent document 7 describes a film containing a syndiotactic polystyrene-based resin as a base film for flat cables for efficiently transmitting high-frequency signals.

JP 2021-038281 A International Publication No. 2018/030026 JP 2017-121807 A Patent No. 6539404 International Publication No. 2016/017473 JP 2015-002334 A International Publication No. 2019/049922

The present invention was made in consideration of the above circumstances, and aims to provide a laminate for electronic circuit boards that can be used to manufacture flexible circuit boards with low transmission loss of high-frequency electrical signals.

The laminate for electronic circuit boards of the present invention is a laminate for electronic circuit boards in which a metal foil is laminated on one or both sides of a base film via an adhesive layer. The base film is mainly composed of syndiotactic polystyrene, is biaxially oriented, has a melting point of 260 to 290°C, a glass transition temperature of 230 to 260°C, a relative dielectric constant of 2.6 or less at a frequency of 10 GHz, and a dielectric loss tangent of 0.002 or less. The peel strength when peeling off the metal foil is 3 N/10 mm or more before and after a heating test at 260°C for 30 seconds.

Here, the glass transition temperature Tg is a value measured by thermomechanical analysis (TMA). Specifically, it can be measured in accordance with JIS C6481-1996. Furthermore, the peel strength refers to the 90-degree peel strength as specified in JIS C5016-1994.

This configuration results in a laminate for electronic circuit boards that can be used to manufacture flexible circuit boards that have high strength and heat resistance and low transmission loss for high-frequency electrical signals.

Preferably, the base film is substantially composed of syndiotactic polystyrene and a styrene-based thermoplastic elastomer, and more preferably, the styrene-based thermoplastic elastomer is a polystyrene-poly(ethylene/butylene)-polystyrene copolymer. This makes it possible to increase the peel strength while keeping the transmission loss of the laminate for electronic circuit boards low.

The laminate for electronic circuit boards preferably has an S21 parameter, which indicates the transmission loss when the metal foil is pattern-etched to produce a microstrip line, of -5 dB/100 mm or more and 0 or less at 40 GHz.

Preferably, the maximum height roughness Rz of the interface between the metal foil and the adhesive layer is 2.0 μm or less. Here, the maximum height roughness Rz is the maximum height roughness specified in JIS B0601-2013. This makes it possible to further reduce the transmission loss of the flexible circuit board.

The method for producing a laminate for electronic circuit boards of the present invention includes the steps of melting and kneading a resin composition mainly composed of syndiotactic polystyrene having a melting point of 260°C or higher to form a precursor film, biaxially stretching the precursor film and subjecting it to a relaxation heat treatment at a temperature of 230°C or higher to produce a base film, forming an adhesive layer on at least one side of the base film, and laminating a metal foil on the adhesive layer.

The laminate for electronic circuit boards of the present invention uses a base film whose main component is syndiotactic polystyrene as the base material, so flexible circuit boards with low transmission loss even for high-frequency electrical signals can be manufactured at lower cost than when liquid crystal polymers or polyimides are used. In addition, because syndiotactic polystyrene has low water absorption, the transmission characteristics of the flexible circuit board are less likely to deteriorate even when used in a humid environment.

1A and 1B are diagrams showing the layer structure of a laminate for an electronic circuit board according to one embodiment of the present invention, A: 5-layer structure, B: 3-layer structure. FIG. 2 is a diagram showing the measurement results of A: relative dielectric constant Dk, B: dielectric loss tangent Df of a base film used in a laminate for electronic circuit boards in an example. FIG. 13 is a diagram showing measurement results of the S21 parameter of laminates for electronic circuit boards of Examples and Comparative Examples.

Referring to FIG. 1A, the electronic circuit board laminate 10 of this embodiment has a five-layer structure in which metal foil 17 is laminated on both sides of the base film 15 via an adhesive layer 16.Referring to FIG. 1B, another electronic circuit board laminate 11 of this embodiment has a three-layer structure in which metal foil 17 is laminated on one side of the base film 15 via an adhesive layer 16.A flexible circuit board can be manufactured by a subtractive method in which the metal foil 17 of the electronic circuit board laminates 10 and 11 is etched into a circuit pattern.In addition, the electronic circuit board laminates 10 and 11 can be combined with other resin films, glass cloth, etc. to manufacture a build-up board.Note that the characteristics and materials of each layer will be described below with reference to the five-layer electronic circuit board laminate 10, but the description also applies to the three-layer electronic circuit board laminate 11.

The base film 15 is primarily composed of syndiotactic polystyrene (SPS) and is biaxially oriented.

Biaxial orientation means that the polymer is oriented in two different directions in the plane, for example, the extrusion direction of the film (MD) and the direction perpendicular thereto (TD). By biaxially orienting the substrate film 15, the required strength and heat resistance can be imparted. Biaxial orientation can be achieved by biaxially stretching an unstretched precursor film.

SPS is a styrene-based polymer having a syndiotactic structure. The syndiotactic structure means a three-dimensional structure in which side chains of phenyl groups or substituted phenyl groups are alternately positioned in opposite directions relative to the main chain formed from carbon-carbon bonds. The degree of stereoregularity (tacticity) of SPS can be quantified by nuclear magnetic resonance (13C-NMR) using carbon isotopes. The tacticity of SPS-based resins measured by 13C-NMR can be indicated by the proportion of chains consisting of several monomer units, for example, dyads in the case of two units, triads in the case of three units, and pentads in the case of five units, in which the configuration of the constituent units is reversed and syndiotactic (racemic dyads, etc.). The SPS in this embodiment is usually a styrene-based polymer having a syndiotacticity of 75% or more, preferably 85% or more, or racemic dyads in the case of 60% or more, preferably 75% or more, or racemic triads in the case of 30% or more, preferably 50% or more. In addition, two or more different types of SPS may be mixed together for the base film 15.

Examples of styrene-based polymers as SPS include polystyrene, poly(alkylstyrene), poly(halogenated styrene), poly(halogenated alkylstyrene), poly(alkoxystyrene), poly(vinyl benzoate ester), hydrogenated polymers thereof, and mixtures thereof, or copolymers mainly composed of these. Examples of poly(alkylstyrene) include poly(methylstyrene), poly(ethylstyrene), poly(isopropylstyrene), poly(tertiary butylstyrene), poly(phenylstyrene), poly(vinylnaphthalene), poly(vinylstyrene), and the like. Examples of poly(halogenated styrene) include poly(chlorostyrene), poly(bromostyrene), poly(fluorostyrene), and the like. Examples of poly(halogenated alkylstyrene) include poly(chloromethylstyrene), and the like. Examples of poly(alkoxystyrene) include poly(methoxystyrene), poly(ethoxystyrene), and the like. As a styrene-based polymer as SPS, polystyrene is preferred.

The weight average molecular weight of SPS is 10,000 to 3,000,000, preferably 30,000 to 1,500,000, and particularly preferably 50,000 to 500,000.

The melting point of SPS is 260°C or higher. This increases the glass transition temperature Tg of the base film, thereby improving the heat resistance of the electronic circuit board laminate 10. On the other hand, there is no particular problem if the melting point of SPS is high, but the melting point of SPS does not usually exceed 290°C.

Preferably, the base film 15 is substantially made of SPS and a styrene-based thermoplastic elastomer (TPS). TPS is a thermoplastic elastomer (TPE) whose hard segment is made of polystyrene. "Substantially made of SPS and TPS" means that even if resins other than SPS and TPS are contained, the content is within a range that provides the required transmission loss and heat resistance for the laminate 10 for electronic circuit boards. Specifically, the sum of the proportions of SPS and TPS in the total resin of the base film 15 is 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 100% by mass. By including TPS in the base film 15, the peel strength can be increased while suppressing deterioration of the dielectric properties of the base film.

Various commercially available TPSs can be used. It is also preferable to use TPS that has been hydrogenated. This improves the heat resistance of the TPS and prevents unexpected reactions from occurring during the melting and extrusion process of the base film raw material, which is carried out at high temperatures.

As hydrogenated TPS, various types with different soft segments can be used, such as polystyrene-poly(ethylene/butylene)-polystyrene (TPS-SEBS), polystyrene-poly(ethylene/propylene)-polystyrene (TPS-SEPS), polystyrene-poly(ethylene-ethylene/propylene)-polystyrene (TPS-SEEPS), and polystyrene-poly(ethylene/propylene)-polystyrene (TPS-SEP). Of these, it is particularly preferable to use TPS-SEBS, whose soft segment is made of poly(ethylene/butylene). The TPS contained in the base film 15 may be a mixture of two or more different resins.

The amount of TPS blended is preferably such that the weight ratio of SPS (a) to TPS (b) is (a)/(b) = 97/3 to 60/40, more preferably 95/5 to 70/30, or 90/10 to 80/20. This preferred blending ratio is also applicable when TPS-SEBS is used as all or part of the TPS. If the amount of TPS blended is too small, the effect of improving the peel strength of the laminate 10 for electronic circuit boards is small. On the other hand, if the amount of TPS blended is too large, the deterioration of the dielectric properties of the base film 15 cannot be ignored.

In addition to the polymer, the base film 15 may contain additives such as plasticizers, antioxidants, UV absorbers, light stabilizers, lubricants, antistatic agents, inorganic fillers, colorants, crystal nucleating agents, and flame retardants.

The melting point of the base film 15 is 260°C or higher, regardless of whether the base film is made of only SPS or substantially made of SPS and TPS. This increases the glass transition temperature Tg of the base film, thereby improving the heat resistance of the electronic circuit board laminate 10. On the other hand, although there is no particular problem if the base film 15 has a high melting point, a base film whose main component is SPS will not normally exceed 290°C.

The glass transition temperature Tg of the substrate film 15 is 230°C or higher, preferably 240°C or higher. This can increase the heat resistance of the laminate 10 for electronic circuit boards. On the other hand, although there is no particular problem if the glass transition temperature Tg is high, the substrate film mainly composed of SPS usually does not exceed 260°C. In this specification, the glass transition temperature Tg refers to a temperature measured by thermomechanical analysis (TMA). The glass transition temperature Tg can be measured by several methods, but the TMA method is superior as an index of practical heat resistance. Specifically, the glass transition temperature Tg by TMA can be obtained from a TMA curve measured in accordance with JIS C6481-1996. Although the SPS material itself has a low glass transition temperature Tg, the glass transition temperature Tg can be increased by biaxial orientation, thereby improving heat resistance.

The thermal expansion coefficient of the base film 15 is preferably 80 ppm/°C or less, more preferably 70 ppm/°C or less, in both MD and TD. The smaller the thermal expansion coefficient, the better, but for base films whose main component is SPS, it usually does not fall below 10 ppm/°C. The absolute value of the difference between the thermal expansion coefficients in MD and TD is preferably 50 ppm/°C or less, more preferably 20 ppm/°C or less.

The thickness of the base film 15 is preferably 10 to 100 μm, and more preferably 12 to 50 μm. This allows the electronic circuit board laminate 10 to have a good balance between strength and flexibility.

The dielectric constant Dk of the base film 15 is 2.6 or less at a frequency of 10 GHz. The dielectric loss tangent Df of the base film 15 is 0.002 or less, preferably 0.001 or less, at a frequency of 10 GHz. The transmission loss of the laminate 10 for electronic circuit boards is affected by all layers, the base film 15, the adhesive layer 16, and the metal foil 17. However, since the dielectric constant Dk and the dielectric loss tangent Df of the base film 15 are low, it is possible to keep the transmission loss of high-frequency electrical signals low even when using commercially available adhesives, copper foil, etc. In addition, a base film mainly composed of SPS usually has a dielectric constant of 2.0 or more and a dielectric loss tangent of 0.00001 or more.

The relative dielectric constant Dk of the adhesive layer 16 is preferably 2.6 or less at a frequency of 10 GHz. The dielectric tangent Df of the adhesive layer is preferably 0.005 or less, more preferably 0.003 or less, at a frequency of 10 GHz. Such an adhesive layer can be formed using a commercially available adhesive such as AF-700 manufactured by Toa Gosei Co., Ltd. or SAFY manufactured by Nikkan Kogyo Co., Ltd.

The components of the adhesive layer 16 are not particularly limited as long as they have the required adhesive strength to the base film 15 and the metal foil 17, and various known adhesives, for example, the adhesives described in Patent Documents 2, 3, or 5, can be used. The adhesive layer may be formed by applying a liquid adhesive to the surface of the base film 15 or the metal foil 17, or may be formed using an adhesive formed into a film (hereinafter referred to as an "adhesive film"). The adhesive layer is preferably formed using an adhesive film.

The thickness of the adhesive layer 16 is preferably 3 to 50 μm, and more preferably 5 to 30 μm. If the adhesive layer is too thin, sufficient adhesive performance may not be obtained. On the other hand, if the adhesive layer is too thick, the solvent is likely to remain and may foam during the manufacturing process of the flexible circuit board.

The type of metal foil 17 is not particularly limited, and for example, foil of copper, gold, aluminum, or an alloy containing these as the main components can be used, and copper foil is preferably used. As the copper foil, various commercially available copper foils can be used.

The smoother the interface between the metal foil 17 and the adhesive layer 16, the lower the transmission loss of the flexible circuit board. For this reason, it is preferable that the surface of the metal foil 17 that is to be joined to the adhesive layer 16 is smooth, and the maximum height roughness Rz of the metal foil surface is preferably 2.0 μm or less, more preferably 1.0 μm or less. Here, the maximum height roughness Rz is the maximum height roughness specified in JIS B0601-2013. On the other hand, the maximum height roughness Rz of the surface of the metal foil that is to be joined to the adhesive layer is preferably 0.2 μm or more. This ensures sufficient adhesive strength with the adhesive layer. The maximum height roughness Rz of the surface of the metal foil that is to be joined to the adhesive layer is the maximum height roughness Rz of the interface between the metal foil 17 and the adhesive layer 16 in the laminate 10 for electronic circuit boards.

The thickness of the metal foil 17 can be determined within a range that provides the required conductivity depending on the type of metal. When the metal foil is copper foil, the thickness is preferably 1 to 100 μm, and more preferably 1 to 40 μm. A thickness within these ranges can provide both sufficient conductivity and a high level of flexibility required of a flexible circuit board.

The laminate 10 for electronic circuit boards is required to have various properties, but the most important ones are transmission loss, peel strength, heat resistance, and thermal shock resistance.

The transmission loss of the laminate 10 for electronic circuit boards can be evaluated by the S21 parameter when the metal foil 17 is pattern-etched to create a microstrip line, and is preferably -5 dB/100 mm or more and 0 or less at 40 GHz. A negative S21 parameter indicates that there is a transmission loss, and the smaller the absolute value of the S21 parameter, the smaller the transmission loss.

The peel strength of the laminate 10 for electronic circuit boards is the peel strength when the metal foil 17 is peeled off from the laminate 10, and the 90 degree peel strength as specified in JIS C5016-1994 is 3 N/10 mm or more, preferably 5 N/10 mm or more, and more preferably 6 N/10 mm or more.

The heat resistance of the laminate 10 for electronic circuit boards can be evaluated by the peel strength after a heating test. The laminate 10 for electronic circuit boards is typically required to have a heat resistance of 260°C or more, assuming reflow processing of flexible circuit boards. Specifically, the heating test can be performed by passing a test piece through a reflow furnace and heating it at 260°C for 30 seconds. Heating at 260°C for 30 seconds may also be repeated. The test piece is passed through the reflow furnace at least once, preferably six or more times, and the peel strength after the heating test is 3N/10mm or more, preferably 5N/10mm or more, and more preferably 6N/10mm or more.

The thermal shock resistance of the laminate 10 for electronic circuit boards can be evaluated based on the change in electrical resistance before and after a thermal shock (high temperature immersion) test specified in JIS C5016-1994. Specifically, the thermal shock test involves subjecting a test piece to a specified number of thermal cycles from 260°C to 20°C. The change in electrical resistance is preferably 10% or less before and after 100 thermal cycles. Details of the test method will be described later in the examples.

Next, a method for manufacturing the electronic circuit board laminate 10 of this embodiment will be described.

The resin composition that is the raw material for the base film 15 is melted and kneaded to form a precursor film. The precursor film can be formed, for example, by extrusion molding, calendar molding, or casting, and is preferably formed by extrusion molding.

The molded unstretched precursor film is biaxially oriented, for example, by a simultaneous biaxial stretching method or a sequential biaxial stretching method, preferably by a simultaneous biaxial stretching method. The stretching ratio, stretching temperature, and stretching speed of the biaxial stretching can be selected according to the thermal properties of the resin, the desired thermal expansion coefficient, and the nominal tensile break strain. In this embodiment, the stretching ratio is preferably 2.0 to 5.0 times in both MD and TD, and more preferably 2.2 to 4.0 times. It is preferable that the stretching ratios in MD and TD are similar. Specifically, the difference between the stretching ratio in MD and the stretching ratio in TD is preferably 0.6 or less, more preferably 0.3 or less.

The biaxially stretched film is further subjected to a relaxation heat treatment. This is to reduce the absolute value of the thermal shrinkage rate, increase the glass transition temperature Tg, and improve the heat-resistant dimensional stability. The relaxation treatment is performed below the melting point of the stretched film, preferably below (melting point - 10°C). The relaxation treatment temperature is 230°C or higher, preferably 240°C or higher. This allows the glass transition temperature Tg of the base film to be 230°C or higher. The relaxation ratio is preferably 0.80 to 1.00 times, more preferably 0.85 to 1.00 times, and most preferably 0.90 to 0.98 times, in both MD and TD. It is preferable that the relaxation ratios in MD and TD are similar. Specifically, the difference between the relaxation ratio in MD and the relaxation ratio in TD is preferably 0.1 or less, more preferably 0.05 or less, and most preferably 0.02 or less.

Next, an adhesive layer 16 and metal foil 17 are laminated onto the substrate film 15 manufactured as described above. In the five-layered electronic circuit board laminate 10, the adhesive layer 16 and metal foil 17 are laminated onto both sides of the substrate film 15. In the three-layered electronic circuit board laminate 11, the adhesive layer 16 and metal foil 17 are laminated onto only one side of the substrate film 15.

Since the SPS constituting the base film 15 has poor adhesiveness, the surface of the base film is first activated. The activation method is not particularly limited, and methods such as corona discharge treatment, ozone oxidation treatment, UV/ozone treatment, plasma discharge treatment, and electron beam irradiation can be used. Preferably, the surface of the base film is treated with atmospheric pressure plasma, and more preferably, the surface of the base film is treated with atmospheric pressure nitrogen plasma. For base films mainly composed of SPS, atmospheric pressure plasma, and especially atmospheric pressure nitrogen plasma treatment, is preferable because it can reduce damage to the substrate.

When a liquid adhesive is used to form the adhesive layer 16, the adhesive is applied to both sides of the base film 15, metal foil 17 is laid on the applied surfaces, and the whole is sandwiched between a press or the like and the adhesive is cured by heating. Alternatively, the adhesive may be applied to one side of the base film 15, metal foil 17 is laid on top of it, the whole is sandwiched between a press or the like and the adhesive is cured by heating, and then the adhesive layer 16 and metal foil 17 may be laminated on the other side in the same manner.

When an adhesive film is used to form the adhesive layer 16, the adhesive film and metal foil are laminated on both sides of the base film 15, and the whole is sandwiched between a press or the like, and the adhesive is cured by heating. Alternatively, an adhesive film and metal foil may be laminated on one side of the base film 15, and then an adhesive film and metal foil may be laminated on the other side, or an adhesive film may be laminated on both sides of the base film, and then metal foil may be laminated on the adhesive film on both sides.

As a result of the above, all layers are integrated to produce the electronic circuit board laminate 10. According to the method for producing an electronic circuit board laminate of this embodiment, the base film and the metal foil are bonded with an adhesive, making production easier than forming a conductive layer by plating.

The laminate for electronic circuit boards of the above embodiment and the laminate for electronic circuit boards of the comparative example were produced and their performance was evaluated.

First, we prepared the base film SF-1 used in the examples and the base films SF-2 and SF-3 used in the comparative examples.

(Base film SF-1)
A full compound in which 90% by mass of SPS (Idemitsu Kosan Co., Ltd., Zarec, glass transition point 100 ° C., melting point 270 ° C.) and 10% by mass of TPS-SEBS (Kuraray Co., Ltd., Septon) were pre-kneaded was melt-extruded at 320 ° C. using an extruder equipped with a T-die at the tip, and then cooled to obtain a precursor film. This precursor film was simultaneously biaxially stretched at 110 ° C. with a stretching speed of about 500% / min and a stretching ratio of 3.4 x 3.4 (MD x TD), and then subjected to a relaxation heat treatment at 250 ° C. with a relaxation ratio of 0.95 x 0.95 (MD x TD) to produce a substrate film with a thickness of 50 μm. The glass transition temperature Tg of this substrate film SF-1 was 240 ° C.

(Base film SF-2)
SPS (Idemitsu Kosan Co., Ltd., Zarec, glass transition point 95°C, melting point 247°C) was melt-extruded at 320°C using an extruder equipped with a T-die at the tip, and cooled to obtain a precursor film (about 500 μm). This precursor film was simultaneously biaxially stretched at 110°C with a stretching speed of 500%/min and a stretching ratio of 3.3 x 3.4 (MD x TD), and then subjected to a relaxation heat treatment at 230°C with a relaxation ratio of 0.94 x 0.96 (MD x TD) to produce a substrate film with a thickness of 50 μm. The glass transition temperature Tg of this substrate film SF-2 was 200°C.

(Base film SF-3)
A full compound in which 80% by mass of SPS (Idemitsu Kosan Co., Ltd., Zarec, glass transition point 95 ° C., melting point 247 ° C.) and 20% by mass of TPS-SEEPS (Kuraray Co., Ltd., Septon) were pre-kneaded was melt-extruded at 320 ° C. using an extruder equipped with a T-die at the tip, and then cooled to obtain a precursor film. This precursor film was simultaneously biaxially stretched at 110 ° C. with a stretching speed of about 500% / min and a stretching ratio of 3.4 x 3.4 (MD x TD), and then subjected to a relaxation heat treatment at 210 ° C. with a relaxation ratio of 0.95 x 0.95 (MD x TD) to produce a substrate film with a thickness of 50 μm. The glass transition temperature Tg of this substrate film SF-3 was 200 ° C.

Example 1
Both sides of the base film SF-1 were activated with atmospheric nitrogen plasma at a line speed of 10 m/min and an output of 4 kW/width 500 mm, and then the adhesive film (Toa Gosei Co., Ltd., AF-700, thickness 5 μm) with the separate film on one side peeled off was stacked and attached using a vacuum press at 120 ° C. × 0.4 MPa × 30 seconds. Next, the separate film on the other side of the adhesive film was peeled off, and copper foil (maximum height roughness Rz = 1.3 μm on the adhesive film side surface, thickness 18 μm) was stacked and attached using a vacuum press at 120 ° C. × 0.4 MPa × 30 seconds. Further, it was pressed with a heat press at 180 ° C. × 3 MPa × 30 minutes, and after the press was released, it was placed in a heating oven and heated at 180 ° C. × 30 minutes to produce a laminate for electronic circuit boards (5-layer structure) of Example 1.

Example 2
Example 2 of the electronic circuit board laminate (five-layer structure) was produced in the same manner as in Example 1, except that the copper foil used had a maximum height roughness Rz of 0.85 μm on the adhesive film side surface.

Example 3
The same method as in Example 1 was used, except that both sides of the base film were activated with vacuum plasma at a line speed of 10 m/min and an output of 4 kW/width of 500 mm, to produce a laminate for electronic circuit boards (five-layer structure) of Example 3.

(Comparative Example 1)
The same method as in Example 3 was used, except that SF-2 was used as the base film and the copper foil had a maximum height roughness Rz of 0.85 μm on the adhesive film side surface, to produce a laminate for electronic circuit boards (five-layer structure) of Comparative Example 1.

(Comparative Example 2)
The same method as in Example 3 was used, except that SF-3 was used as the base film and the copper foil had a maximum height roughness Rz of 0.85 μm on the adhesive film side surface, to produce a laminate for electronic circuit boards (five-layer structure) of Comparative Example 2.

(Comparative Example 3)
A laminate for electronic circuit boards (five-layer structure) of Comparative Example 3 was produced in the same manner as in Example 3, except that SF-3 was used as the base film.

(Comparative Example 4)
A commercially available liquid crystal polymer (LCP) film (thickness 50 μm) was used as the base film, and the same adhesive film and copper foil as in Examples 1 and 3 were used to prepare a laminate for electronic circuit boards (five-layer structure) of Comparative Example 4.

(Comparative Example 5)
A commercially available polyimide (PI) film (thickness 50 μm) was used as the base film, and the same adhesive film and copper foil as in Examples 1 and 3 were used to prepare a laminate for electronic circuit boards (five-layer structure) of Comparative Example 5.

Table 1 shows the dielectric constant Dk and dielectric loss tangent Df of the base film and adhesive layer. In Table 1, the dielectric properties of the base film were measured by the cavity resonator method specified in ASTM D2520. The dielectric properties of the adhesive layer are the values in the manufacturer's catalog. Figure 2 shows the dielectric constant Dk and dielectric loss tangent Df of the base film SF-1 used in the examples in the range above 10 GHz. The Dk and Df in Figure 2 are the results of three measurements using the balanced disk resonator method.

The transmission loss of the laminates for electronic circuit boards of the examples and comparative examples was evaluated. In addition, peel strength, heat resistance, and thermal shock resistance were evaluated for some samples.

The transmission loss of each sample was evaluated by creating a microstrip line with a line width of approximately 0.2 mm and a length of 100 mm by pattern-etching the copper foil, and measuring the S21 parameter at a characteristic impedance of 50 Ω at frequencies up to 40 GHz using a network analyzer (Keysight Technologies, E8363B) and a probe (Form Factor, ACP40-GSG250). A negative S21 parameter indicates the presence of transmission loss, and the smaller the absolute value of the S21 parameter, the smaller the transmission loss.

The peel strength was measured by cutting the laminate for electronic circuit boards to 20 mm x 100 mm, etching the copper foil to a width of 10 mm, and measuring the peel strength in a 90-degree direction in accordance with JIS C5016-1994 by pulling the copper foil at a speed of 50 mm/min.

For heat resistance, a test piece of the same shape as in the peel test above was passed through a reflow furnace and heated at 270°C for 30 seconds, and the peel strength was measured after six cycles. Note that heating tests are often performed at 260°C, but in this example, evaluation was performed under slightly stricter conditions.

For thermal shock resistance, a high-temperature immersion test was conducted in accordance with JIS C5016-1994, but with different thermal cycle conditions, and the conductive resistance was measured before and after the test. Specifically, it is as follows. A daisy-chain board for thermal shock resistance testing of copper-plated through holes was prepared similar to that shown in Figure 5 of the JIS, and a voltage of 100 V was applied to measure the initial conductive resistance value between two specified points. The board was then immersed in a 260°C silicon oil bath for 15 seconds, removed and transferred within 15 seconds, immersed in a 20°C cooling bath for 15 seconds, and removed and transferred within 15 seconds. This thermal cycle was repeated 100 times, and the conductive resistance was measured again in the same manner. The standard for thermal shock resistance was that the conductive resistance should not change by more than 10% due to the thermal cycle.

Table 2 shows the layer structure of each sample along with the evaluation results. Figure 3 shows the S21 parameters of several samples. All samples have a five-layer structure, with the same adhesive layer and copper foil laminated on both sides of the base film. In Table 2, the maximum height roughness Rz of the copper foil is the value of the surface on the adhesive layer side. The peel strength is the average of the results of two to four tests. Thermal shock resistance was rated "OK" when the change in conductive resistance before and after the thermal cycle was less than 10%.

From the S21 parameters shown in Table 2 and Figure 3, it was confirmed that the transmission loss in Examples 1 to 3 was smaller than that in Comparative Examples 4 and 5. Furthermore, the transmission loss was almost the same in Example 2 and Comparative Example 2, which have the same maximum height roughness Rz of the adhesive film side surface of the copper foil. However, as shown in Table 2, Comparative Example 2 had poor heat resistance and thermal shock resistance.

As can be seen from Table 2, in Examples 1 to 3, the peel strength did not decrease even after the heating test at 270°C. Comparing Example 1 and Example 3, Example 1 had a higher peel strength. This is thought to be because in Example 1, damage to the base film was suppressed by atmospheric pressure plasma treatment. In Comparative Examples 1 to 3, deformation due to the heating test was severe, and the peel strength after the heating test could not be measured. The failure mode in the peel test was material failure of the surface layer of the base film for all samples in the Examples and Comparative Examples.

In addition, in Examples 1 to 3, no deformation of the test pieces was observed after the thermal shock test, and the rate of change in electrical resistance was 10% or less. In Comparative Examples 1 to 3, the deformation due to the thermal shock test (thermal cycle) was so severe that the electrical resistance after the thermal shock test could not be measured.

The present invention is not limited to the above-mentioned embodiments and examples, and various modifications are possible within the scope of the technical concept.

10 Laminate for electronic circuit board (5-layer structure)
11 Laminate for electronic circuit board (three-layer structure)
15 Base film 16 Adhesive layer 17 Metal foil

Claims (6)

A laminate for electronic circuit boards, comprising a base film and a metal foil laminated on one or both sides of the base film via an adhesive layer,
the base film is composed mainly of syndiotactic polystyrene, is biaxially oriented, has a melting point of 260 to 290°C, a glass transition temperature of 230 to 260°C, a relative dielectric constant at a frequency of 10 GHz of 2.6 or less, and a dielectric loss tangent of 0.002 or less;
The peel strength when peeling off the metal foil is 3 N/10 mm or more before and after a heating test at 260 ° C. for 30 seconds.
Laminate for electronic circuit boards. The base film is substantially composed of syndiotactic polystyrene and a styrene-based thermoplastic elastomer.
The laminate for an electronic circuit board according to claim 1 . The styrene-based thermoplastic elastomer is a polystyrene-poly(ethylene/butylene)-polystyrene copolymer.
The laminate for an electronic circuit board according to claim 2 . When the metal foil is pattern-etched to produce a microstrip line, the S21 parameter is −5 dB/100 mm or more and 0 or less at 40 GHz.
The laminate for an electronic circuit board according to claim 1 . The maximum height roughness Rz of the interface between the metal foil and the adhesive layer is 2.0 μm or less;
The laminate for an electronic circuit board according to claim 1 . A step of melting and kneading a resin composition containing syndiotactic polystyrene as a main component and having a melting point of 260° C. or more, and forming the resin composition into a precursor film;
a step of biaxially stretching the precursor film and subjecting it to a relaxation heat treatment at a temperature of 230° C. or higher to produce a base film;
forming an adhesive layer on at least one surface of the base film;
laminating a metal foil on the adhesive layer;
The present invention relates to a method for producing a laminate for an electronic circuit board.

Images