JP7584274B2 - Multilayer adhesive film, flexible metal-clad laminate and flexible printed circuit board - Google Patents

Description

The present invention relates to a multilayer adhesive film in which an adhesive layer containing a thermoplastic polyimide is provided on at least one side of a non-thermoplastic polyimide film. More specifically, the present invention relates to a multilayer adhesive film that can be suitably used for flexible printed circuit boards that are expected to be used under harsh conditions from low to high temperatures, and a flexible metal-clad laminate and a flexible printed circuit board in which a metal layer is provided on at least one side of the multilayer adhesive film.

Polyimide films are widely used as electronic circuit board materials due to their excellent mechanical strength, heat resistance, electrical insulation, and chemical resistance. For example, flexible copper-clad laminates (hereafter also referred to as FCCLs) are manufactured by laminating copper foil on at least one side of polyimide film as a circuit board material, and flexible printed circuit boards (hereafter also referred to as FPCs) are manufactured by further creating circuits, and are used in various electronic devices.

In recent years, the demand for low dielectric constants and low dielectric dissipation factor for electronic circuit board materials has increased as electrical signals propagate through circuits at higher frequencies in electronic devices for high-speed signal transmission. This is because lowering the dielectric constant and dielectric dissipation factor is an effective way to suppress transmission loss of electrical signals. In recent years, as we are in the dawn of the IoT society, there has been a trend toward higher frequencies, and there is a demand for circuit board materials, FCCLs, and FPCs that can suppress transmission loss even in the range of 10 GHz and above.

As a representative technology of IoT, autonomous driving technology for automobiles and drones is currently under development. Electronic devices used in these devices, such as camera modules and millimeter wave antenna modules, require large volumes of acquired data and high-speed transmission. In particular, when used in vehicles, they are expected to be exposed to high temperatures due to exhaust heat from the engine and drive unit, or to be used in cold regions, so in addition to high-speed transmission characteristics, they are required to be able to perform stable high-speed transmission over a wide temperature range. Furthermore, if high-speed transmission can be achieved over a wide temperature range, it will be possible to increase the freedom of where signal circuits are installed inside the vehicle. Therefore, there is a need for the development of substrate materials that can be used appropriately for FPCs, which are expected to be used under such harsh conditions.

Incidentally, liquid crystal polyester (LCP) is sometimes used as a substrate material other than polyimide. Although LCP generally has poor processability, it is a material with excellent low moisture absorption and low dielectric tangent due to its crystallinity. In order to achieve an even lower dielectric tangent, a method is known in which a wholly aromatic liquid crystal polyester that meets certain conditions is heat-treated to improve the dielectric properties, and it is known that this can achieve a low dielectric tangent in the range of 30°C to 100°C (Patent Document 1).

WO2020/003690

Attempts have been made to develop polyimide films with low dielectric constants and dielectric dissipation factors, but little attention has been paid to the temperature dependence of the dielectric properties of polyimide films. However, in recent years, the locations and applications in which FPCs are used have become more diverse, and it is expected that FPCs will be exposed to high temperatures as well as at extremely low temperatures. Therefore, materials that exhibit excellent dielectric properties over a wide temperature range from low to high temperatures are desired. The objective of the present invention is to provide a multilayer adhesive film that has an adhesive layer containing thermoplastic polyimide on at least one side of a non-thermoplastic polyimide film, which can exhibit excellent dielectric properties, particularly a low dielectric dissipation factor, over a wide temperature range.

The present invention can solve the above problems by providing the following novel multilayer adhesive film, flexible metal-clad laminate, and flexible printed circuit board.

1) A multilayer adhesive film having an adhesive layer containing a thermoplastic polyimide on at least one side of a non-thermoplastic polyimide film, characterized in that the dielectric tangents of the multilayer adhesive film at 10 GHz measured at every 50°C increase in temperature (including -50°C) in the temperature range of -50°C to 150°C are Df-50 at -50°C, Df0 at 0°C, Df5 at 50°C, Df100 at 100°C, and Df150 at 150°C, respectively, and the difference between the maximum and minimum values is 0.0005 or less, and all of Df-50, Df0, Df5, Df100, and Df150 are 0.0020 or less.

2) The multilayer adhesive film described in 1), characterized in that the difference between the maximum and minimum of Df-50, Df0, Df5, Df100 and Df150 is 0.0004 or less.

3) The polyimide film according to 1) or 2), characterized in that Df-50, Df0, Df5, Df100 and Df150 are all in the range of 0.0013 to 0.0020.

4) The multilayer adhesive film according to any one of 1) to 3), characterized in that the non-thermoplastic polyimide film is a film made from diamine components of at least 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene, and an acid dianhydride component as raw materials.

5) The multilayer adhesive film described in 4) above, characterized in that the non-thermoplastic polyimide film further uses 3,3',4,4'-biphenyltetracarboxylic dianhydride as an acid dianhydride component.

6) The multilayer adhesive film described in 5) is characterized in that the non-thermoplastic polyimide film uses 4,4'-oxydiphthalic dianhydride in addition to 3,3',4,4'-biphenyltetracarboxylic dianhydride as the acid dianhydride component.

7) A flexible metal-clad laminate having a metal layer on at least one side of the multilayer adhesive film described in any one of 1) to 6).

8) A flexible printed circuit board obtained by forming a circuit on the metal layer of the flexible metal-clad laminate according to claim 7.

The multilayer adhesive film of the present invention is a multilayer adhesive film in which an adhesive layer containing a thermoplastic polyimide is provided on at least one side of a non-thermoplastic polyimide film. Therefore, a flexible copper-clad laminate (FCCL) can be manufactured by forming a metal layer on the surface of the adhesive layer, and a flexible printed circuit board (FPC) can be manufactured by forming a circuit on the metal layer. The obtained FCCL and FPC have a low dielectric tangent over a wide temperature range from low to high temperatures, and the dielectric tangent is within a certain range in all temperature ranges, so that they can be suitably used for special applications under severe conditions, such as in-vehicle parts. Specific examples include camera modules, image processing ECUs, millimeter-wave radar parts, inverters, and motors in advanced driving assistance systems, and even when used for the above applications, stable high-speed transmission can be achieved over a wide temperature range.

1 is a plot of the dielectric tangents at temperatures from −50° C. to 150° C. for the multilayer adhesive films of the examples and the films of the comparative examples.

The following describes an embodiment of the present invention, but the present invention is not limited thereto. All academic and patent documents described in this specification are incorporated herein by reference. Unless otherwise specified in this specification, "A to B" representing a numerical range means "A or more (including A and greater than A) and B or less (including B and smaller than B)."

The multilayer film of the present invention is a multilayer adhesive film having an adhesive layer containing a thermoplastic polyimide on at least one side of a non-thermoplastic polyimide film, and when the dielectric tangents of the multilayer adhesive film at 10 GHz measured at every 50°C increase in temperature (including -50°C) in the temperature range of -50°C to 150°C are Df-50 at -50°C, Df0 at 0°C, Df5 at 50°C, Df100 at 100°C, and Df150 at 150°C, respectively, the difference between the maximum and minimum of these values is 0.0005 or less, and all of Df-50, Df0, Df5, Df100, and Df150 are 0.0020 or less.

Although liquid crystal polyester (LCP) as disclosed in Patent Document 1 exhibits a relatively low dielectric constant and dielectric dissipation factor in cold to room temperatures, the dielectric dissipation factor tends to increase in high temperature environments. This is presumably due to the low heat resistance of LCP and the large permanent dipole moment of the molecules that make up LCP. Therefore, LCP is disadvantageous for use in vehicles where severe temperature changes may occur. Furthermore, Patent Document 1 does not suggest whether the dielectric constant and dielectric dissipation factor are low and within a constant range in the wider temperature range of -50°C to 150°C as in the present invention.

On the other hand, multilayer adhesive films in which an adhesive layer containing a thermoplastic polyimide is provided on at least one side of a non-thermoplastic polyimide film have been widely used in FPC applications in recent years, but no attention has been paid to the temperature dependence of their dielectric tangent.

In response to this conventional knowledge, the present inventors focused on the temperature dependence of the dielectric tangent of a multilayer adhesive film in which an adhesive layer containing a thermoplastic polyimide is provided on at least one side of a non-thermoplastic polyimide film, and discovered that by using a polyimide resin with a certain structure in the multilayer adhesive film, it is possible to keep the dielectric tangent of the electronic circuit substrate constant and low, which is generally thought to fluctuate in harsh temperature environments, especially in high temperature regions. FPCs manufactured using this multilayer adhesive film can be used even under harsh conditions.

Here, the dielectric tangent at 10 GHz measured every 50°C increase in temperature, including -50°C, in the temperature range of 50°C to 150°C refers to the dielectric tangent measured as follows:

<Equipment used>
1. Network analyzer N5290A (Keysight Technologies)
2. Split cylinder resonator 10GHz CR710 (EM Lab)
3. Split Cylinder Resonator Dielectric Constant Measurement Software CRMA (EM Lab)
4. Environmental test chamber SH662 (manufactured by Espec Corporation)

<Measurement method>
The sample was set in a split cylinder resonator connected to a network analyzer and left to stand for 2 hours in an environmental tester maintained at a constant temperature. After standing, the dielectric constant and dielectric loss tangent were measured. The temperature measurements were performed in order from the highest to the lowest: 150°C, 100°C, 50°C, 0°C, and -50°C.
The dielectric loss tangents thus obtained are designated as Df-50 at -50°C, Df0 at 0°C, Df5 at 50°C, Df100 at 100°C, and Df150 at 150°C, respectively, and the maximum and minimum are determined. The difference between the maximum and minimum is preferably 0.0005 or less, more preferably 0.0004 or less. Df-50, Df0, Df5, Df100, and Df150 are all preferably 0.0020 or less, and more preferably 0.0018 or less.

The multilayer adhesive film of the present invention is a multilayer polyimide film having at least one non-thermoplastic polyimide resin layer.

The total thickness of the multilayer adhesive film is preferably 7 μm or more from the viewpoint of handling when the film is processed into FCCL, etc., and more preferably 10 μm to 100 μm.

The thickness of the non-thermoplastic polyimide film and the adhesive layer is preferably in the range of 75:25 to 60:40.

Below, we will explain the non-thermoplastic polyimide film, the adhesive layer containing thermoplastic polyimide, the production of the multilayer adhesive film, the flexible metal-clad laminate, and the flexible printed circuit board in that order.

(Non-thermoplastic polyimide film)
An example of a method for producing the non-thermoplastic polyimide film of the present invention will be described in detail. Polyamic acid (hereinafter, also referred to as polyamic acid), which is a precursor of polyimide used in the production of the non-thermoplastic polyimide film of the present invention, is obtained by mixing and reacting at least one diamine and at least one acid dianhydride in an organic solvent so that the mixture is substantially equimolar.

The diamine used in the polyamic acid of the non-thermoplastic polyimide film of the present invention is not particularly limited, but examples include 2,2-bis{4-(4-aminophenoxy)phenyl}propane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenylpropane, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 2,2'-dimethyl-4,4'-diaminobiphenyl, 3,3'-dihydroxy-4,4'-diamino-1,1'-biphenyl, 4,4'-diamino-3,3'-dimethylbiphenyl, 4,4'-diamino-3,3'-hydroxybiphenyl, 1,4-diaminobenzene, 1,3-diaminobenzene, 4,4'-diaminodiphenyl ether, and 1,3-bis(3-aminophenoxy)benzene.

The easiest way to obtain a multilayer film in which the maximum and minimum differences in Df-50, Df0, Df5, Df100, and Df150 are 0.0005 and all of these are 0.0020 or less is to control these properties of the non-thermoplastic polyimide film. This is because the non-thermoplastic polyimide film is the main component of the multilayer adhesive film. From this perspective, it is preferable to use 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene in combination. Furthermore, the amount of 1,4-diaminobenzene used is preferably 70 mol% or more relative to 100 mol% of the diamine component, more preferably in the range of 70 mol% to 95 mol%, and even more preferably in the range of 80 mol% to 90 mol%, and it is preferable to use 5 mol% or more of 1,3-bis(4-aminophenoxy)benzene relative to 100 mol% of the diamine component, more preferably in the range of 5 mol% to 30 mol%, and even more preferably in the range of 10 mol% to 20 mol%. If a non-thermoplastic polyimide film manufactured using 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene as raw materials within this range is used for a multilayer adhesive film, the difference between the maximum and minimum of Df-50, Df0, Df5, Df100, and Df150 of the multilayer adhesive film will be small, and all of these values will tend to be small.

The acid dianhydride is not particularly limited, but specific examples include pyromellitic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propanoic dianhydride, p-phenylenebis(trimellitic monoester acid anhydride), 4,4'-oxydiphthalic dianhydride, and 3,4'-oxydiphthalic dianhydride.

From the viewpoint that the difference between the maximum and minimum values of Df-50, Df0, Df5, Df100 and Df150 of the multilayer adhesive film becomes small and all of these values can be made small, it is preferable to use 3,3',4,4'-biphenyltetracarboxylic dianhydride among the above. The amount of 3,3',4,4'-biphenyltetracarboxylic dianhydride used is preferably 40 mol% or more relative to 100 mol% of the acid dianhydride component, and more preferably in the range of 50 mol% to 70 mol%. Furthermore, it is preferable to use 4,4'-oxydiphthalic dianhydride in addition to the use of 3,3',4,4'-biphenyltetracarboxylic dianhydride. It is preferable to use 20 mol% or more of 4,4'-oxydiphthalic dianhydride relative to 100 mol% of the acid dianhydride component, and more preferably in the range of 30 mol% to 50 mol%.

In addition to the above dianhydride monomers, other dianhydrides such as pyromellitic dianhydride may be added as long as the dielectric properties are not reduced. The amount of other dianhydrides added is preferably 10 mol% or less, more preferably 6 mol% or less, and even more preferably 3 mol% or less, relative to 100 mol% of the dianhydride component.

From the above, when 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene are used as the diamine components and 3,3',4,4'-biphenyltetracarboxylic dianhydride is used as the acid dianhydride component, or when 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene are used as the amine components and 4,4'-oxydiphthalic dianhydride is used as the acid dianhydride component, it becomes easier to reduce the differences between the maximum and minimum values of Df-50, Df0, Df5, Df100 and Df150 of the final multilayer adhesive film and to reduce all of these values. Furthermore, when 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene are used as the diamine components, and 3,3',4,4'-biphenyltetracarboxylic dianhydride and 4,4'-oxydiphthalic dianhydride are used as the acid dianhydride components, it becomes easier to reduce the difference between the maximum and minimum values of Df-50, Df0, Df5, Df100, and Df150 of the final multilayer adhesive film, and to reduce all of these values.

Polyamic acid, which is a precursor of non-thermoplastic polyimide, is obtained by mixing and reacting the above diamine and acid dianhydride in an organic solvent so that they are substantially equimolar. Any organic solvent that dissolves polyamic acid can be used, but amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone are preferred, with N,N-dimethylformamide and N,N-dimethylacetamide being particularly preferred. There are no particular limitations on the solids concentration of polyamic acid, and if it is within the range of 5% to 35% by weight, a polyamic acid that has sufficient mechanical strength when made into polyimide can be obtained.

There are no particular limitations on the order in which the raw materials diamine and dianhydride are added, but by controlling not only the chemical structure of the raw materials but also the order in which they are added, it is possible to control the dielectric tangent of the non-thermoplastic polyimide film, thereby reducing the difference between the maximum and minimum values of Df-50, Df0, Df5, Df100, and Df150 of the final multilayer adhesive film, and thus reducing all of these values.

A filler can be added to the polyamic acid to improve the film's properties such as sliding properties, thermal conductivity, electrical conductivity, corona resistance, and loop stiffness. Any filler can be used, but preferred examples include silica, titanium oxide, alumina, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, and mica.

In addition, thermosetting resins such as epoxy resins and phenoxy resins, and thermoplastic resins such as polyether ketone and polyether ether ketone may be mixed in as long as the overall properties of the resulting non-thermoplastic polyimide film are not impaired. Methods for adding these resins include adding them to the polyamic acid if they are soluble in a solvent. If the polyimide is also soluble, they may be added to the polyimide solution. If they are insoluble in a solvent, they may be first imidized by the polyamic acid and then compounded by melt kneading. However, it is preferable to use a soluble resin to be mixed with the polyimide.

To obtain the non-thermoplastic polyimide film of the present invention, the following steps are preferably included.
i) reacting an aromatic diamine with an aromatic tetracarboxylic dianhydride in an organic solvent to obtain a polyamic acid solution;
ii) casting a membrane-forming dope containing the polyamic acid solution onto a support;
iii) peeling the gel film from the support after heating on the support;
iv) further heating to imidize the remaining amic acid and drying.

ii) The processes after this can be broadly divided into thermal imidization and chemical imidization. Thermal imidization is a method in which a polyamic acid solution is poured onto a support as a film-forming dope without using a dehydrating ring-closing agent, and the imidization is promoted by simply heating. On the other hand, chemical imidization is a method in which a polyamic acid solution to which at least one of a dehydrating ring-closing agent and a catalyst has been added as an imidization promoter is used as a film-forming dope to promote imidization. Either thermal imidization or chemical imidization can be used, but chemical imidization is more productive.

As the dehydration ring-closing agent, an acid anhydride such as acetic anhydride can be preferably used. As the catalyst, a tertiary amine such as an aliphatic tertiary amine, an aromatic tertiary amine, or a heterocyclic tertiary amine can be preferably used.

In the steps after ii), a glass plate, aluminum foil, an endless stainless steel belt, a stainless steel drum, etc. can be suitably used as a support for casting the film-forming dope. The heating conditions are set according to the thickness of the final film to be obtained and the production speed, and the film is partially imidized and/or dried, and then peeled off from the support to obtain a polyamic acid film (hereinafter also referred to as a gel film or gel membrane).

In the subsequent steps of iv) the ends of the gel film are fixed to avoid shrinkage during curing and then dried, water, residual solvent, and imidization accelerator remaining in the film are removed, and the remaining amic acid is completely imidized to obtain a film containing polyimide. It is preferable that the ends of the gel film are not only fixed but also stretched in the conveying direction or perpendicular to the conveying direction.

The polyimide film obtained in this manner has a small and constant dielectric tangent at -50°C, 0°C, 50°C, 100°C, and 150°C, making it suitable for use as a high-frequency circuit board for in-vehicle applications.

(Adhesive layer containing thermoplastic polyimide)
The diamine and dianhydride used in the thermoplastic polyimide used in the adhesive layer of the present invention can be the same as those used in the non-thermoplastic polyimide resin layer. The thickness of the adhesive layer is thinner than that of the non-thermoplastic polyimide film, so its influence is not as large as that of the non-thermoplastic polyimide film, but by using a preferable component as the thermoplastic polyimide contained in the adhesive layer, it is possible to control the maximum and minimum differences of Df-50, Df0, Df5, Df100 and Df150 of the multilayer adhesive film finally obtained, as well as these values. From this viewpoint, as a diamine that can be preferably used, it is preferable to use at least one selected from 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 2,2-bis(4-aminophenoxyphenyl)propane, and 2,2'-dimethyl-4,4'-diaminobiphenyl.

As an acid dianhydride that can be suitably combined with these diamines, it is preferable to use at least one selected from pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydiphthalic dianhydride.

The method for producing the polyamic acid solution, which is the precursor of the thermoplastic polyimide used in the adhesive layer of the present invention, can be any known method, so long as the thermoplastic polyimide obtained by imidizing the resulting polyamic acid has the properties required for conventional flexible printed circuit board materials.

The resulting polyamic acid solution can be used to produce a multilayer film by the multilayer film production method described below. In addition to the thermoplastic polyimide, the adhesive layer may contain silica, calcium phosphate, etc., to the extent that the smoothness of the adhesive layer is not impaired.

(Manufacture of multi-layer adhesive film)
The multilayer adhesive film of the present invention is a multilayer adhesive film having two or more layers, including at least one non-thermoplastic polyimide resin layer as described above as a core film and an adhesive layer containing a thermoplastic polyimide.

In the present invention, the method for producing the multilayer adhesive film includes a method (coating method) in which a polyamic acid solution, which is a precursor of the thermoplastic polyimide, is applied to at least one side of the non-thermoplastic polyimide film obtained as described above, and then dried and imidized.

Alternatively, the process for obtaining a non-thermoplastic polyimide film exemplified in the above section (Non-thermoplastic polyimide film),
i) reacting an aromatic diamine with an aromatic tetracarboxylic dianhydride in an organic solvent to obtain a polyamic acid solution;
ii) casting a membrane-forming dope containing the polyamic acid solution onto a support;
iii) peeling the gel film from the support after heating on the support;
iv) further heating to imidize the remaining amic acid and drying;
In step ii) of the above, a co-extrusion die having a plurality of flow paths may be used to simultaneously form multiple resin layers. That is, in order to obtain a multi-layer polyimide film having an adhesive layer containing a thermoplastic polyimide on at least one side of the above-mentioned non-thermoplastic polyimide film, a precursor solution of the non-thermoplastic polyimide and a precursor solution of the thermoplastic polyimide are cast onto a support using a co-extrusion die, and the steps from iii) onwards are carried out to obtain the film.

Because very high temperatures are required for imidization, when forming a resin layer other than polyimide, it is preferable to use a coating method to suppress thermal decomposition. When forming an adhesive layer containing a thermoplastic polyimide by coating, a precursor of the thermoplastic polyimide may be applied and then imidized, or a solution of the thermoplastic polyimide may be applied and dried.

(Flexible metal-clad laminates and flexible printed circuit boards)
The multi-layer polyimide film thus obtained can be made into a flexible metal-clad laminate by providing a metal layer on at least one side of the multi-layer polyimide film.
a) a method of obtaining a multilayer polyimide film as described above, and then laminating a metal foil by heating and pressing to obtain a flexible metal-clad laminate; b) a method of casting an organic solvent solution containing a polyamic acid onto a metal foil, removing the solvent by heating, and carrying out imidization to obtain a flexible metal-clad laminate. Details of a) and b) are described below.

In the method of a), a metal foil is laminated to the obtained multilayer polyimide film by heating and pressing (lamination), thereby obtaining the flexible metal foil laminate of the present invention. The method and conditions for laminating the metal foil may be appropriately selected from those conventionally known.

In the method of b), the method for casting the organic solvent solution containing polyamic acid (a solution containing a precursor of a thermoplastic polyimide or an amide acid solution which is a precursor of a non-thermoplastic polyimide) onto the metal foil is not particularly limited, and conventionally known methods such as a die coater, a comma coater (registered trademark), a reverse coater, or a knife coater can be used. Conventionally known methods can also be used for heating the material to remove the solvent and perform imidization, such as a hot air oven or a far-infrared oven.

When a chemical imidization method is used to form a non-thermoplastic polyimide film or adhesive layer by means of (b), an acid is generated from the acid anhydride, which is the dehydrating ring-closing agent, during the imidization process, and oxidation may progress depending on the type of metal foil. It is preferable to select the dehydrating ring-closing agent appropriately depending on the type of metal foil and heating conditions.

When providing an adhesive layer containing a non-thermoplastic polyimide film and a thermoplastic polyimide, the above-mentioned casting and heating steps can be repeated multiple times, or a method of forming multiple cast layers by co-extrusion or continuous casting and heating them all at once can be suitably used.

In the method of b), the flexible metal foil laminate of the present invention is obtained at the same time as the imidization is completed. When metal foil layers are provided on both sides of the resin layer, the metal foil can be bonded to the opposite resin layer surface by applying heat and pressure.

The metal foil that can be used in the present invention is not particularly limited, but when the flexible metal-clad laminate of the present invention is used for electronic and electrical equipment applications, examples of the metal foil that can be used include foils made of copper or copper alloys, stainless steel or its alloys, nickel or nickel alloys (including alloy 42), and aluminum or aluminum alloys. In general flexible laminates, copper foils such as rolled copper foils and electrolytic copper foils are often used, but they can also be preferably used in the present invention. The surfaces of these metal foils may be coated with an anti-rust layer, a heat-resistant layer, or an adhesive layer. The thickness of the metal foil is not particularly limited, and may be any thickness that can provide sufficient functionality depending on the application.

The metal layer of the flexible metal-clad laminate thus obtained is then etched using a known method to obtain a flexible printed circuit board.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The methods for evaluating the relative dielectric constant and dielectric loss tangent of the laminates in the synthesis examples, examples, and comparative examples are as follows.

(Measurement of dielectric tangent)
A multilayer polyimide film cut to 50 mm x 70 mm was placed on a split cylinder resonator CR710 (10 GHz, EM Lab) connected to a network analyzer N5290A (Keysight Technologies), and left to stand for 2 hours in an environmental tester kept at a constant temperature. After standing, the dielectric constant and dielectric loss tangent were measured. The measurements were performed at 150°C, 100°C, 50°C, 0°C, and -50°C, starting from the highest temperature. In addition, each time the temperature inside the environmental tester was set to a specific temperature, the film was left to stand for 2 hours.

As a comparative example, the dielectric tangent of an LCP film (FELIOS R-F705T, manufactured by Panasonic) was measured using the same method as described above in "Measurement of dielectric tangent".

(Synthesis Example 1)
In a 2000 ml glass flask, 656.66 g of N,N-dimethylformamide (hereinafter also referred to as DMF), 12.12 g of 1,3-bis(4-aminophenoxy)benzene (hereinafter also referred to as TPE-R), 25.44 g of 1,4-diaminobenzene (hereinafter also referred to as PDA) were added, and 48.86 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter also referred to as BPDA) was added, followed by stirring under a nitrogen atmosphere. After confirming that BPDA had dissolved, 31.77 g of 4,4'-oxydiphthalic dianhydride (hereinafter also referred to as ODPA) was added and stirred for 30 minutes. Finally, 1.8 g of pyromellitic dianhydride (hereinafter, also referred to as PMDA) was dissolved in DMF to prepare a solution having a solid content of 7.2%. This solution was gradually added to the above reaction solution while paying attention to the increase in viscosity. When the viscosity at 23° C. reached 2000 poise, the addition and stirring were stopped, and a polyamic acid solution was obtained.

(Preparation of non-thermoplastic polyimide film)
The polyamic acid solution of Synthesis Example 1 was added with an imidization promoter consisting of acetic anhydride/isoquinoline/DMF (weight ratio 2.1/1.1/0.8) at a weight ratio of 40% to the polyamic acid solution, and was continuously stirred with a mixer, extruded from a T-die, and cast onto a stainless steel endless belt. This resin film was heated at 100°C x 330 seconds, and then the self-supporting gel film was peeled off from the endless belt and fixed to a tenter clip, and dried and imidized in a hot air oven at 250°C x 41 seconds, a hot air oven at 350°C x 37 seconds, a hot air oven at 380°C x 32 seconds, and a far infrared heater oven at 380°C x 97 seconds to obtain a non-thermoplastic polyimide film with a thickness of 33 μm.

(Synthesis Example 2)
642.12g of DMF, 53.02g of TPE-R and 38.16g of BPDA were added to a 2000ml glass flask, and the mixture was stirred for 30 minutes under a nitrogen atmosphere. Next, 8.49g of PMDA was added and stirred for 30 minutes. Then, 16.52g of 2,2'-dimethyl-4,4'-diaminobiphenyl (hereinafter also referred to as m-TB) and 18.11g of PMDA were added and stirred for 30 minutes. Finally, a solution was prepared by dissolving 1.7g of PMDA in DMF to a solid content concentration of 7.2%, and this solution was gradually added to the above reaction solution while paying attention to the increase in viscosity, and when the viscosity at 23°C reached 1000 poise, the addition and stirring were stopped, and a polyamic acid solution was obtained.

Example 1
The polyamic acid solution, which is a precursor of the thermoplastic polyimide prepared in Synthesis Example 2, was diluted with DMF until the solid content concentration reached 10% by weight, and then the polyamic acid was applied to one side of the non-thermoplastic polyimide film obtained in Synthesis Example 1 with a comma coater so that the final thickness on one side was 8.5 μm, and the film was heated in a drying oven set at 120° C. for 4 minutes. The other side was also similarly coated with polyamic acid so that the final thickness was 8.5 μm, and the film was heated in a drying oven set at 120° C. for 4 minutes. Subsequently, the film was heated for 15 seconds in a hot air oven at an atmospheric temperature of 380° C. to obtain a multilayer adhesive film with a total thickness of 50 μm.

(Comparative Example 1)
An LCP film (FELIOS R-F705T, manufactured by Panasonic Corporation) having a thickness of 50 μm was prepared. The results of measuring the dielectric tangent for the examples and comparative examples are shown in Table 1 and FIG.

The multilayer adhesive film of the present invention has a difference between the maximum and minimum dielectric tangent values at -50°C, 0°C, 50°C, 100°C, and 150°C of 0.0005 or less, and all of the dielectric tangent values measured at the aforementioned temperatures are 0.0020 or less, indicating that the multilayer adhesive film exhibits a low and constant dielectric tangent over a wide temperature range.

Therefore, it can be said to be an ideal substrate for FPC boards for in-vehicle applications that require operational reliability in temperatures ranging from below zero to over 100°C. Specific applications include camera modules in advanced driver assistance systems, image processing ECUs, millimeter wave radar parts, inverters, and motors. This allows for greater freedom in the installation location of FPCs with excellent high-speed transmission characteristics.

Claims (6)

A multi-layer adhesive film having an adhesive layer containing a thermoplastic polyimide on at least one side of a non-thermoplastic polyimide film,
the dielectric dissipation factor at 10 GHz of the multilayer adhesive film measured at every 50°C increase in temperature, including -50°C, in a temperature range of -50°C to 150°C is Df-50 at -50°C, Df0 at 0°C, Df5 at 50°C, Df100 at 100°C, and Df150 at 150°C, respectively, the difference between the maximum and minimum of these values is 0.0005 or less, and all of Df-50, Df0, Df5, Df100, and Df150 are 0.0020 or less;
The non-thermoplastic polyimide film is a multi-layer adhesive film characterized in that it is a film using, as raw materials, at least diamine components of 1,4-diaminobenzene and 1,3-bis(4-aminophenoxy)benzene, and acid dianhydride components of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 4,4'-oxydiphthalic dianhydride . The multilayer adhesive film according to claim 1, characterized in that the difference between the maximum and minimum of Df-50, Df0, Df5, Df100 and Df150 is 0.0004 or less. 3. The multilayer adhesive film according to claim 1, wherein all of Df-50, Df0, Df5, Df100 and Df150 are in the range of 0.0013 to 0.0020. the diamine used in the thermoplastic polyimide is at least one selected from 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 2,2-bis(4-aminophenoxyphenyl)propane, and 2,2'-dimethyl-4,4'-diaminobiphenyl;
The multilayer adhesive film according to any one of claims 1 to 3, characterized in that the acid dianhydride used in the thermoplastic polyimide is one or more selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydiphthalic dianhydride. A flexible metal-clad laminate comprising the multi-layer adhesive film according to any one of claims 1 to 4 and a metal layer on at least one surface of the multi-layer adhesive film. A flexible printed circuit board obtained by forming a circuit on the metal layer of the flexible metal-clad laminate according to claim 5 .

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