JP7633071B2 - Flexible Circuit Board - Google Patents

Description

The present invention relates to a flexible circuit board.

One important application of high-frequency flexible circuit boards is known as an RF cable for a millimeter-wave antenna. Flexible circuit boards (FPCs) for such RF cables generally have a structure (so-called stripline structure) in which a laminate consisting of a first resin film, which is a dielectric, a signal conductor pattern formed thereon, and a second resin film laminated via an adhesive layer on the signal conductor pattern forming surface of the first resin film is sandwiched between a pair of ground copper foils (see FIG. 1 of Patent Document 1).

In recent years, the housings of mobile electronic devices have become smaller and thinner, and when an RF cable for a millimeter-wave antenna is housed in the housing of a mobile electronic device, the RF cable is bent 90 degrees to house it. For this reason, there is a demand for good resistance to 90-degree bending for RF cables for millimeter-wave antennas with a stripline structure.

Patent No. 6537172

However, in Patent Document 1 (particularly FIG. 1), only the dielectric properties of the flexible circuit board with a stripline structure are considered, and there is no consideration at all of the resistance to 90-degree bending that is required when such a flexible circuit board is used as an RF cable for a millimeter-wave antenna.

The object of the present invention is to provide a flexible circuit board with a stripline structure that exhibits the necessary 90-degree bending resistance when used as an RF cable for a millimeter-wave antenna, etc.

As a result of intensive research, the inventors have discovered that in a flexible circuit board with a stripline structure, the film thickness and film thickness ratio of a pair of first and second resin films sandwiching a signal conductor pattern between a pair of ground layers, as well as the tensile elastic modulus and tensile elastic modulus ratio of the first and second resin films, are closely related to the 90-degree bending resistance, and have completed the present invention.

That is, the present invention provides a flexible circuit board that is folded at approximately 90 degrees and stored in a housing of an electronic device,
A first resin film;
A conductive pattern formed on one surface of the first resin film;
a first ground layer laminated on the other surface of the first resin film;
a second resin film laminated on the conductive pattern forming surface side of the first resin film;
a second ground layer laminated on a surface of the second resin film opposite to the first resin film,
a thickness (L1) of the first resin film is within a range of 50 μm or more and 150 μm or less, a thickness (L2) of the second resin film is within a range of 50 μm or more and 150 μm or less, and a ratio (L2/L1) of the thickness (L2) of the second resin film to the thickness (L1) of the first resin film is within a range of 0.8 or more and 1.2 or less,
The flexible circuit board is characterized in that the tensile modulus of elasticity (M1) of the first resin film is in the range of 0.9 GPa or more and 7 GPa or less, the tensile modulus of elasticity (M2) of the second resin film is in the range of 0.9 GPa or more and 7 GPa or less, and the ratio (M2/M1) of the tensile modulus of elasticity (M2) of the second resin film to the tensile modulus of elasticity (M1) of the first resin film is in the range of 0.7 to 1.3.

In the flexible circuit board of the present invention, the second resin film has a polyimide insulating layer including a single or multiple polyimide layers and an adhesive layer,
the adhesive layer is located between the polyimide insulating layer and a surface of the first resin film on which a conductor pattern is formed,
The adhesive layer preferably has a tensile modulus of elasticity in the range of 0.2 GPa or more and 2 GPa or less.

In addition, in the flexible circuit board of the present invention, it is preferable that the first resin film has a polyimide layer (A) in the center layer, and the thickness of the polyimide layer (A) is in the range of 0.5 to 0.96 relative to the total thickness of the first resin film.

In the flexible circuit board of the present invention, it is preferable that the second ground layer is bent at approximately 90 degrees so that it faces outward.

The flexible circuit board of the present invention having a stripline structure has a pair of first and second resin films sandwiching a signal conductor pattern between a pair of ground layers, and the film thickness and film thickness ratio of each of the first and second resin films, as well as the tensile elastic modulus and tensile elastic modulus ratio of each of the first and second resin films, each limited to a specific range. Therefore, the flexible circuit board of the present invention can exhibit good resistance to 90-degree bending. Therefore, it is useful as an RF cable for a millimeter wave antenna.

1 is a schematic diagram showing a configuration of a flexible circuit board according to an embodiment of the present invention; FIG. 4 is a schematic diagram showing a configuration of a flexible circuit board according to another embodiment of the present invention. FIG. 4 is a schematic diagram showing a configuration of a flexible circuit board according to another embodiment of the present invention. FIG. 4 is a schematic diagram showing a configuration of a flexible circuit board according to another embodiment of the present invention. FIG. 1 is a cross-sectional layer configuration diagram of a material model simulating an FPC. FIG. 13 is a diagram illustrating the size of a material model simulating an FPC. FIG. 13 is an explanatory diagram of a state in which an FPC is sandwiched between rigid bodies. FIG. 13 is an explanatory diagram of the state in which the FPC is bent at 90 degrees using a roller. FIG. 13 is an explanatory diagram of a 90-degree bent state on an FPC simulation software. FIG. 1 is a diagram illustrating a simulation step. 1 is a schematic cross-sectional view of an FPC model applied to the simulation of Example 1. FIG. 11 is a schematic cross-sectional view of an FPC model applied to the simulation of Example 2. FIG. 11 is a schematic cross-sectional view of an FPC model applied to the simulation of Example 3. FIG. 13 is a schematic cross-sectional view of an FPC model applied to the simulation of Example 4. FIG. 13 is a schematic cross-sectional view of an FPC model applied to the simulation of Example 5. 1 is a stress-strain curve of insulating resin type A. 1 is a stress-strain curve of insulating resin type B. 1 is a stress-strain curve of insulating resin type C. 1 is a stress-strain curve of copper foil.

The embodiment of the present invention will be explained with reference to the drawings as appropriate.

[Configuration of flexible circuit board]
The flexible circuit board (FPC) of the present invention is folded at approximately 90 degrees and stored inside the housing of an electronic device, and examples of its configuration include those shown in Figures 1 to 4. The FPC 10 in Figure 1 has a first ground layer G1, a first resin film 1 laminated thereon, a conductor pattern S formed on the surface of the first resin film, a second resin film 2 laminated on the conductor pattern forming surface of the first resin film 1, and a second ground layer G2 laminated thereon.

The second resin film 2 may be composed of an adhesive layer 2a and a polyimide insulating layer 2b containing a single layer or multiple polyimide layers, as shown in FIG. 2. In this case, it is preferable that the adhesive layer 2a is disposed on the conductive pattern forming surface side of the first resin film 1. In addition, when the polyimide insulating layer 2b is formed from multiple polyimide layers, it can be composed of a polyimide layer 2b1 and a polyimide layer 2b2 as shown in FIG. 3, but is not limited to this structure. Here, it is preferable that the polyimide layer 2b1 and the polyimide layer 2b2 have different characteristics in terms of blocking properties with the conductive pattern S and adhesion with the second ground layer, but they may have the same characteristics.

On the other hand, the first resin film 1 may also be composed of a polyimide insulating layer having a single layer or multiple polyimide layers. For example, as shown in FIG. 4, the first resin film 1 may be composed of a polyimide layer (A) 1a in the center layer and a polyimide layer 1b and a polyimide layer 1c on both sides. In this case, the layer thickness of the polyimide layer (A) 1a is preferably 0.5 to 0.96, more preferably 0.5 to 0.8, relative to the total thickness of the first resin film 1. If the ratio of the thickness of the polyimide layer (A) 1a to the thickness of the first resin film 1 is below this range, the first resin film 1 tends to have insufficient low dielectric tangent and insufficient dielectric properties, and if it exceeds this range, the first resin film 1 tends to have problems such as a decrease in dimensional stability. Here, the central layer refers to the polyimide layer located exactly in the middle when the first resin film is composed of an odd number of polyimide layers (three or more); when the first resin film is composed of an even number of polyimide layers (two or more), it is either of the two adjacent central polyimide layers, but it is preferably the layer closest to the first ground layer G1.

(Thickness of First Resin Film 1 and Second Resin Film 2)
The thickness (L1) of the first resin film 1 and the thickness (L2) of the second resin film 2 constituting the FPC 10 of the present invention are each independently 50 μm to 150 μm, preferably 70 μm to 125 μm, in order to make the FPC 10 applicable to high frequency applications. If the thickness of the first resin film 1 is below this range, sufficient transmission characteristics are not exhibited, and if it exceeds this range, the thickness of the entire FPC becomes thick, resulting in low bending resistance. Also, if the thickness of the second resin film 2 is below this range, sufficient transmission characteristics are not exhibited, and if it exceeds this range, the thickness of the entire FPC becomes thick, resulting in low bending resistance.

In the FPC 10 of the present invention, the ratio (L2/L1) of the thickness (L2) of the second resin film 2 to the thickness (L1) of the first resin film 1 is 0.8 or more and 1.2 or less, and preferably 0.9 or more and 1.1 or less. If this ratio (L2/L1) is below this range, when the FPC is bent 90 degrees, the neutral plane of the bend shifts from the center of the copper foil in the thickness direction toward the first resin film side, increasing the maximum stress applied to the copper foil, and if it exceeds this range, the neutral plane shifts toward the second resin film side, increasing the maximum stress applied to the copper foil.

(Tensile Modulus of First Resin Film and Second Resin Film)
In the FPC 10 of the present invention, the tensile modulus (M1) of the first resin film 1 and the tensile modulus (M2) of the second resin film 2 are each independently 0.9 GPa to 7 GPa, preferably 1 GPa to 7 GPa, and more preferably 1 GPa to 6 GPa. If the modulus is below this range, the rigidity of the FPC itself is reduced, and the handling properties when mounted on an electronic device are reduced, whereas if the modulus is above this range, the rigidity of the FPC itself is increased, which is one factor in reducing bending resistance.

In the FPC 10 of the present invention, the ratio (M2/M1) of the tensile modulus of elasticity (M2) of the second resin film 2 to the tensile modulus of elasticity (M1) of the first resin film 1 is 0.7 to 1.3, preferably 0.8 to 1.2. If it is below this range, when the FPC is bent 90 degrees, the neutral plane of the bend moves from the center of the copper foil in the thickness direction to the first resin film side, increasing the maximum stress applied to the copper foil, and if it is above this range, the neutral plane moves to the second resin film side, increasing the maximum stress applied to the copper foil.

When the second resin film 2 has an adhesive layer 2a as shown in FIG. 2, the tensile modulus of the adhesive layer is preferably 0.2 GPa to 2 GPa, more preferably 0.5 GPa to 1.5 GPa. If it is below this range, the adhesive tends to be difficult to handle in the FPC manufacturing process, and if it is above this range, it may affect the rigidity of the FPC itself and be a factor in reducing the flexibility of the FPC.

The tensile modulus of the first resin film 1 and the second resin film 2 can be measured using a commercially available tensile elasticity tester (for example, Strograph R-1 by Toyo Seiki Seisakusho Co., Ltd.) in an environment of 23°C temperature and 50% relative humidity. The tensile modulus (M1 or M2) when the first resin film 1 and the second resin film 2 are composed of multiple polyimide layers and adhesive layers can be calculated according to the following formula, for example, when the first resin film 1 is composed of polyimide layers 1a, 1b, . . . 1n. Here, the total thickness of the first resin film 1 is L1 μm, the thickness of the polyimide layer 1a is L1a and the tensile modulus of the polyimide layer 1a alone is M1a, the thickness of the polyimide layer 1b is L1b and the tensile modulus of the polyimide layer 1a alone is M1b, and the thickness of the polyimide layer 1b is L1n and the tensile modulus of the polyimide layer 1n alone is M1n.

(First Ground Layer and Second Ground Layer)
In the FPC 10 of the present invention, the first ground layer G1 and the second ground layer G2 are not particularly limited and may be made of any common FPC ground layer material, such as copper, stainless steel, iron, nickel, beryllium, aluminum, zinc, indium, silver, gold, tin, zirconium, tantalum, titanium, lead, magnesium, manganese, or other metal materials or alloy materials thereof. Among these, copper or copper alloys are particularly preferred.

The thickness of the first ground layer G1 and the second ground layer G2 is not particularly limited, and is preferably 5 μm to 30 μm, more preferably 10 μm to 20 μm, from the viewpoints of production stability, handling, FPC shielding performance, etc. In addition, when copper foil is used as the first ground layer G1 and the second ground layer G2, it may be rolled copper foil or electrolytic copper foil, or commercially available copper foil may be used.

The metal or alloy material used for the first ground layer G1 and the second ground layer G2 may be subjected to a surface treatment using, for example, a siding agent, an aluminum alcoholate agent, an aluminum chelating agent, a silane coupling agent, etc., for the purpose of, for example, rust prevention or improving adhesion. The first ground layer G1 and the second ground layer G2 may be subjected to conductor patterning by a known method, if necessary.

(Conductor pattern)
In the FPC 10 of the present invention, the conductor pattern S functions as a signal line and can be appropriately selected from materials and configurations conventionally used as signal lines of FPCs, and can be made of the same material as the first ground layer G1 and the second ground layer G2. Patterning can be performed by a known method. The layer thickness of the conductor pattern S is not particularly limited, and is preferably 5 μm or more and 20 μm or less from the viewpoints of production stability, handling properties, transmission performance, etc.

(Materials of the First Resin Film and the Second Resin Film)
In the FPC 10 of the present invention, the first resin film 1 and the second resin film 2 are not particularly limited as long as they are made of a resin having electrical insulation properties, and examples thereof include polyimide, epoxy resin, phenolic resin, polyethylene, polypropylene, polytetrafluoroethylene, silicone, and ethylene tetrafluoroethylene, but are preferably made of polyimide. The first resin film 1 and the second resin film 2 are not limited to a single layer, and may be a laminate of multiple resin layers, preferably multiple polyimide layers. In addition, when polyimide is used in the present invention, it means a resin made of a polymer having an imide group in the molecular structure, such as polyamideimide, polyetherimide, polyesterimide, polysiloxaneimide, and polybenzimidazoleimide, in addition to polyimide.

Hereinafter, the case where the first resin film and the second resin film are composed of a single layer or a plurality of polyimide layers will be described. Here, the polyimide constituting the polyimide layer contains a tetracarboxylic acid residue derived from a tetracarboxylic dianhydride component and a diamine residue derived from a diamine compound component, and from the viewpoint of storage modulus, it is generally divided into "thermoplastic polyimides" whose glass transition temperature (Tg) can be clearly confirmed, and "non-thermoplastic polyimides" whose softening and adhesiveness do not generally occur even when heated. Specifically, in the present invention, it is roughly divided into "thermoplastic polyimides" whose "storage modulus at 30°C" is 1.0×10 ⁸ Pa or more and whose "storage modulus at 300°C" is less than 3.0×10 ⁷ Pa, as measured using a dynamic mechanical analyzer (DMA), and "non-thermoplastic polyimides" whose "storage modulus at 30°C" is 1.0×10 ⁹ Pa or more and whose "storage modulus at 300°C" is 3.0×10 ⁸ Pa or more.

When the first resin film 1 and the second resin film 2 are each a single polyimide layer, they may be made of non-thermoplastic polyimide, but it is preferable to make them of thermoplastic polyimide, which exhibits relatively better adhesion to the first ground layer G1, the second ground layer G2, and the conductor pattern S than non-thermoplastic polyimide.

In addition, when the first resin film 1 and the second resin film 2 are each composed of a plurality of polyimide layers, it is preferable to apply a thermoplastic polyimide to the layer that is in direct contact with the first ground layer G1, the second ground layer G2, or the conductor pattern S, and a non-thermoplastic polyimide that exhibits relatively better dielectric properties than a thermoplastic polyimide can be applied to the layer that is not in direct contact. For example, in the case of FIG. 2, a thermoplastic polyimide can be preferably applied to the adhesive layer 2a and the single-layer polyimide insulating layer 2b, and when the polyimide insulating layer 2b is composed of a plurality of polyimide layers 2b1 and a polyimide layer 2b2 as shown in FIG. 3, it is preferable to apply a thermoplastic polyimide to the polyimide layer 2b2 that is in contact with the second ground layer G2. In that case, a thermoplastic polyimide may be applied to the inner polyimide layer 2b1, but a good non-thermoplastic polyimide may also be applied. In the case of FIG. 4, it is preferable to use thermoplastic polyimide for polyimide layers 1b and 1c, and non-thermoplastic polyimide can be used for polyimide layer (A) 1a. However, when polyimide layer (A) 1a is used as a bonding sheet, it can be made of adhesive polyimide, which has enhanced film-forming properties and adhesiveness among thermoplastic polyimides, as described below.

(Non-thermoplastic polyimide)
In the FPC 10 of the present invention, the non-thermoplastic polyimide applicable to the polyimide layer (A) 1a, the polyimide insulating layer 2b1, etc. preferably has a dianhydride residue derived from a dianhydride component containing an aromatic tetracarboxylic dianhydride component, and a diamine residue derived from a diamine component containing an aliphatic diamine and/or an aromatic diamine. As the dianhydride component and the diamine component, a monomer generally used in the synthesis of non-thermoplastic polyimides (see paragraphs [0025] to [0026] of JP 2014-141083 A and paragraphs [0019] to [0020] of JP 2016-72405 A) can be used. By selecting the type of the dianhydride component and the diamine component, and the respective molar ratios when two or more types of dianhydrides or diamines are used, the thermal expansion, adhesiveness, glass transition temperature, etc. of the polyimide can be controlled.

Non-thermoplastic polyimides preferably have an imide group concentration of 33% or less, more preferably 32% or less. If the imide group concentration exceeds 33%, the flame retardancy of the polyimide decreases, and the dielectric properties also deteriorate due to an increase in polar groups.

The thickness of the non-thermoplastic polyimide layer is preferably in the range of 6 μm to 100 μm, more preferably in the range of 9 μm to 50 μm, from the viewpoint of ensuring the function as a base layer and transportability during production and during thermoplastic polyimide coating. If the thickness of the non-thermoplastic polyimide layer is less than the lower limit above, electrical insulation and handling properties will be insufficient, and if the thickness exceeds the upper limit, productivity will decrease.

From the viewpoint of heat resistance, the non-thermoplastic polyimide preferably has a glass transition temperature (Tg) of 280°C or higher, and more preferably 300°C or higher.

In addition, from the viewpoint of suppressing warping, the thermal expansion coefficient of the non-thermoplastic polyimide is preferably 1 ppm/K or more and 30 ppm/K or less, more preferably 1 ppm/K or more and 25 ppm/K or less, and particularly preferably 15 ppm/K or more and 25 ppm/K or less.

In addition, non-thermoplastic polyimides can be appropriately blended with optional components, such as plasticizers, other curable resin components such as epoxy resins, curing agents, curing accelerators, coupling agents, fillers, solvents, and flame retardants, within the scope of the invention, without impairing the effects of the invention.

(Thermoplastic polyimide)
In the FPC 10 of the present invention, the thermoplastic polyimide preferably applicable to the polyimide layers 1b and 1c, the polyimide insulating layer 2b, the adhesive layer 2a, etc. has an acid dianhydride residue derived from an acid anhydride component containing an aromatic tetracarboxylic dianhydride component, and a diamine residue derived from a diamine component containing an aliphatic diamine and/or an aromatic diamine. As the acid dianhydride component and the diamine component, monomers generally used in the synthesis of thermoplastic polyimides (see JP2014-141083A, paragraphs [0025] to [0026], and JP2016-72405A, paragraphs [0019] to [0020]) can be used.

The imide group concentration of the thermoplastic polyimide is preferably 33% or less, and more preferably 32% or less. If the imide group concentration exceeds 33%, the flame retardancy of the polyimide decreases, and the dielectric properties also deteriorate due to an increase in polar groups.

From the viewpoint of ensuring adhesive function, the thickness of the thermoplastic polyimide layer is preferably in the range of 1 μm to 10 μm, and more preferably in the range of 1 μm to 5 μm. If the thickness of the thermoplastic polyimide layer is less than the lower limit value, the adhesiveness becomes insufficient, and if the thickness exceeds the upper limit value, the dimensional stability tends to deteriorate.

The thermal expansion coefficient of the thermoplastic polyimide is preferably 30 ppm/K or more, more preferably 30 ppm/K or more and 100 ppm/K or less, and particularly preferably 30 ppm/K or more and 80 ppm/K or less, from the viewpoint of suppressing warping.

In addition, optional components such as plasticizers, other curable resin components such as epoxy resins, curing agents, curing accelerators, inorganic fillers, coupling agents, bulking agents, solvents, and flame retardants can be appropriately blended into the thermoplastic polyimide within the scope of the invention, so long as the effects of the invention are not impaired.

(Adhesive Layer)
In the FPC 10 of the present invention, the adhesive layer 2a may be formed from the above-mentioned thermoplastic polyimide, but it can also be formed from an adhesive polyimide that can be used as a bonding sheet with improved adhesion and film-forming properties. The polyimide layer (A) 1a can also be formed from not only a non-thermoplastic polyimide but also an adhesive polyimide that can be used as a bonding sheet.

(Tetracarboxylic acid residue of adhesive polyimide)
The adhesive polyimide may contain, without particular limitation, tetracarboxylic acid residues derived from tetracarboxylic anhydrides generally used in thermoplastic polyimides, but preferably contains a total of 90 molar parts or more of tetracarboxylic acid residues derived from tetracarboxylic anhydrides represented by the following general formula (1) (hereinafter, sometimes referred to as "tetracarboxylic acid residues (1)") relative to 100 molar parts of the tetracarboxylic acid residues. By containing a total of 90 molar parts or more of tetracarboxylic acid residues (1) relative to 100 molar parts of the tetracarboxylic acid residues, it is easy to achieve both flexibility and heat resistance of the adhesive polyimide, which is preferable. If the total amount of tetracarboxylic acid residues (1) is less than 90 molar parts, the solvent solubility of the adhesive polyimide tends to decrease.

In general formula (1), X represents a single bond or a divalent group selected from the following formulas:

In the above formula, Z represents —C ₆ H ₄ —, —(CH ₂ ) _n —, or —CH ₂ —CH(—O—C(═O)—CH ₃ )—CH ₂ —, where n represents an integer of 1 to 20.

Examples of tetracarboxylic dianhydrides for deriving the tetracarboxylic acid residue (1) include 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA), 4,4'-oxydiphthalic anhydride (ODPA), 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA), p-phenylenebis(trimellitic acid monoester anhydride) (TAHQ), and ethylene glycol bisanhydrotrimellitate (TMEG).

The adhesive polyimide may contain tetracarboxylic acid residues derived from acid anhydrides other than the tetracarboxylic acid anhydride represented by the above general formula (1) within the scope of the invention. There are no particular limitations on such tetracarboxylic acid residues, but examples thereof include pyromellitic dianhydride, 1,4-phenylene bis(trimellitic acid monoester) dianhydride, 2,3',3,4'-biphenyl tetracarboxylic acid dianhydride, 2,2',3,3'- or 2,3,3',4'-benzophenone tetracarboxylic acid dianhydride, 2,3',3,4'-diphenyl ether tetracarboxylic acid dianhydride, bis(2,3-dicarboxyphenyl) ether dianhydride, 3,3',4,4'-, 2,3,3',4'- or 2,2',3,3'-p-terphenyl Tetracarboxylic acid dianhydrides, 2,2-bis(2,3- or 3,4-dicarboxyphenyl)-propane dianhydride, bis(2,3- or 3,4-dicarboxyphenyl)methane dianhydride, bis(2,3- or 3,4-dicarboxyphenyl)sulfone dianhydride, 1,1-bis(2,3- or 3,4-dicarboxyphenyl)ethane dianhydride, 1,2,7,8-, 1,2,6,7- or 1,2,9,10-phenanthrene tetracarboxylic acid dianhydride, 2,3,6,7-anthracene tetracarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl) 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylic dianhydride, 2,6- or 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,3,6,7-(or 1,4,5,8-)tetrachloronaphthalene-1 ,4,5,8-(or 2,3,6,7-)tetracarboxylic acid dianhydride, 2,3,8,9-, 3,4,9,10-, 4,5,10,11- or 5,6,11,12-perylene-tetracarboxylic acid dianhydride, cyclopentane-1,2,3,4-tetracarboxylic acid dianhydride, pyrazine-2,3,5,6-tetracarboxylic acid dianhydride, pyrrolidine-2,3,4,5-tetracarboxylic acid dianhydride, thiophene-2,3,4,5-tetracarboxylic acid dianhydride, 4,4'-bis(2,3-dicarboxyphenoxy)diphenylmethane dianhydride, ethylene glycol bisanhydrotrimellitate, and other aromatic tetracarboxylic acid dianhydrides are included.

(Diamine Residue of Adhesive Polyimide)
The adhesive polyimide preferably contains 20 or more mol parts, more preferably 40 or more mol parts, and particularly preferably 60 or more mol parts of dimer acid type diamine residues derived from dimer acid type diamines relative to 100 mol parts of diamine residues. By containing the dimer acid type diamine residues in the above amount, the dielectric properties of the adhesive layer 2a etc. can be improved, and the thermocompression bonding properties can be improved by lowering the glass transition temperature (lowering Tg) of the adhesive layer, and the internal stress can be relaxed by lowering the elastic modulus. If the dimer acid type diamine residues are less than 20 mol parts relative to 100 mol parts of diamine residues, sufficient adhesiveness as an adhesive layer may not be obtained, and the elastic modulus of the adhesive layer, which has high thermal expansion, may increase, which may deteriorate the dimensional change rate after the formation of the conductor pattern S.

Here, the dimer acid type diamine means a diamine in which two terminal carboxylic acid groups (-COOH) of a dimer acid are replaced by a primary aminomethyl group (-CH ₂ -NH ₂ ) or an amino group (-NH ₂ ). Dimer acid is a known dibasic acid obtained by intermolecular polymerization reaction of unsaturated fatty acid, and its industrial production process is almost standardized in the industry, and it is obtained by dimerizing unsaturated fatty acid having 11 to 22 carbon atoms with a clay catalyst or the like. Industrially obtained dimer acid is mainly composed of a dibasic acid having 36 carbon atoms obtained by dimerizing unsaturated fatty acid having 18 carbon atoms such as oleic acid and linoleic acid, but contains arbitrary amounts of monomer acid (having 18 carbon atoms), trimer acid (having 54 carbon atoms), and other polymerized fatty acid having 20 to 54 carbon atoms depending on the degree of purification. In the present invention, it is preferable to use a dimer acid whose dimer acid content has been increased to 90% by weight or more by molecular distillation. Furthermore, although double bonds remain after the dimerization reaction, in the present invention, those which have been further subjected to a hydrogenation reaction to reduce the degree of unsaturation are also included in the dimer acid.

A feature of dimer acid diamines is that they can impart properties derived from the dimer acid skeleton to polyimide. In other words, dimer acid diamines are aliphatic macromolecules with a molecular weight of approximately 560 to 620, so they can increase the molecular molar volume and relatively reduce the polar groups of polyimide. It is believed that such features of dimer acid diamines contribute to improving the dielectric properties by reducing the dielectric constant and dielectric tangent while suppressing the deterioration of the heat resistance of polyimide. In addition, since they have two freely moving hydrophobic chains with 7 to 9 carbon atoms and two chain-like aliphatic amino groups with a length close to 18 carbon atoms, they not only impart flexibility to polyimide, but also allow polyimide to have an asymmetrical or non-planar chemical structure, which is believed to enable the polyimide to have a low dielectric constant and low dielectric tangent.

Dimer acid diamines are commercially available, such as PRIAMINE 1073 (trade name), PRIAMINE 1074 (trade name), and PRIAMINE 1075 (trade name) manufactured by Croda Japan, and VERSAMINE 551 (trade name) and VERSAMINE 552 (trade name) manufactured by BASF Japan.

In addition, the adhesive polyimide preferably contains a diamine residue derived from at least one diamine compound selected from the diamine compounds represented by the following general formulas (B1) to (B7) in a total amount of 20 to 80 mol parts, more preferably 20 to 60 mol parts, relative to 100 mol parts of the total diamine components. Since the diamine compounds represented by the general formulas (B1) to (B7) have a molecular structure with flexibility, the flexibility of the polyimide molecular chain can be improved and thermoplasticity can be imparted by using at least one diamine compound selected from these in an amount within the above range. If the total amount of the diamine compounds represented by the general formulas (B1) to (B7) exceeds 80 mol parts relative to 100 mol parts of the total diamine components, the flexibility of the polyimide becomes insufficient and the Tg increases, which tends to increase the residual stress due to thermocompression bonding and deteriorate the dimensional change rate after etching.

In formulas (B1) to (B7), R ₁ independently represents a monovalent hydrocarbon group or alkoxy group having 1 to 6 carbon atoms, linking group A independently represents a divalent group selected from -O-, -S-, -CO-, -SO-, -SO ₂ -, -COO-, -CH ₂ -, -C(CH ₃ ) ₂ -, -NH- or -CONH-, and n1 independently represents an integer of 0 to 4, with the proviso that formula (B3) overlaps with formula (B2), and formula (B5) overlaps with formula (B4).

Here, "independently" means that in one or more of the above formulae (B1) to (B7), multiple linking groups A, multiple R1s, or multiple n1s may be the same or different. In addition, in formulae (B1) to (B7), the hydrogen atoms in the two terminal amino groups may be substituted, for example, -NR ₂ R ₃ (wherein R ₂ and R ₃ are independently any substituent such as an alkyl group).

The diamine represented by formula (B1) (hereinafter sometimes referred to as "diamine (B1)") is an aromatic diamine having two benzene rings. In this diamine (B1), since an amino group directly bonded to at least one benzene ring and a divalent linking group A are at meta positions, the degree of freedom of the polyimide molecular chain is increased and the polyimide molecular chain has high flexibility, which is considered to contribute to improving the flexibility of the polyimide molecular chain. Therefore, by using diamine (B1), the thermoplasticity of the polyimide is increased. Here, as the linking group A, -O-, -CH ₂ -, -C(CH ₃ ) ₂ -, -CO-, -SO ₂ -, -S-, and -COO- are preferred.

Examples of diamines (B1) include 3,3'-diaminodiphenylmethane, 3,3'-diaminodiphenylpropane, 3,3'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 3,3-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylpropane, 3,4'-diaminodiphenyl sulfide, 3,3'-diaminobenzophenone, and (3,3'-bisamino)diphenylamine.

The diamine represented by formula (B2) (hereinafter sometimes referred to as "diamine (B2)") is an aromatic diamine having three benzene rings. In this diamine (B2), the amino group directly bonded to at least one benzene ring and the divalent linking group A are in the meta position, which increases the degree of freedom of the polyimide molecular chain and gives it high flexibility, which is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the use of diamine (B2) increases the thermoplasticity of the polyimide. Here, -O- is preferable as the linking group A.

Examples of diamines (B2) include 1,4-bis(3-aminophenoxy)benzene, 3-[4-(4-aminophenoxy)phenoxy]benzeneamine, and 3-[3-(4-aminophenoxy)phenoxy]benzeneamine.

The diamine represented by formula (B3) (hereinafter sometimes referred to as "diamine (B3)") is an aromatic diamine having three benzene rings. In this diamine (B3), two divalent linking groups A directly bonded to one benzene ring are in the meta position relative to each other, which increases the degree of freedom of the polyimide molecular chain and gives it high flexibility, which is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the use of diamine (B3) increases the thermoplasticity of the polyimide. Here, -O- is preferred as the linking group A.

Examples of diamines (B3) include 1,3-bis(4-aminophenoxy)benzene (TPE-R), 1,3-bis(3-aminophenoxy)benzene (APB), 4,4'-[2-methyl-(1,3-phenylene)bisoxy]bisaniline, 4,4'-[4-methyl-(1,3-phenylene)bisoxy]bisaniline, and 4,4'-[5-methyl-(1,3-phenylene)bisoxy]bisaniline.

The diamine represented by formula (B4) (hereinafter sometimes referred to as "diamine (B4)") is an aromatic diamine having four benzene rings. This diamine (B4) has high flexibility due to the fact that the amino group directly bonded to at least one benzene ring and the divalent linking group A are at the meta position, and is considered to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the use of diamine (B4) enhances the thermoplasticity of the polyimide. Here, the linking group A is preferably -O-, -CH ₂ -, -C(CH ₃ ) ₂ -, -SO ₂ -, -CO-, or -CONH-.

Examples of diamines (B4) include bis[4-(3-aminophenoxy)phenyl]methane, bis[4-(3-aminophenoxy)phenyl]propane, bis[4-(3-aminophenoxy)phenyl]ether, bis[4-(3-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)]benzophenone, and bis[4,4'-(3-aminophenoxy)]benzanilide.

The diamine represented by formula (B5) (hereinafter sometimes referred to as "diamine (B5)") is an aromatic diamine having four benzene rings. In this diamine (B5), two divalent linking groups A directly bonded to at least one benzene ring are mutually at meta positions, so that the degree of freedom of the polyimide molecular chain is increased and the polyimide has high flexibility, which is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, by using diamine (B5), the thermoplasticity of the polyimide is increased. Here, as the linking group A, -O- is preferable.

Examples of diamines (B5) include 4-[3-[4-(4-aminophenoxy)phenoxy]phenoxy]aniline, 4,4'-[oxybis(3,1-phenyleneoxy)]bisaniline, etc.

The diamine represented by formula (B6) (hereinafter sometimes referred to as "diamine (B6)") is an aromatic diamine having four benzene rings. This diamine (B6) has high flexibility due to having at least two ether bonds, and is considered to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the use of diamine (B6) enhances the thermoplasticity of the polyimide. Here, the linking group A is preferably -C( _CH3 ) _2- , -O-, _-SO2- , or -CO-.

Examples of diamines (B6) include 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), bis[4-(4-aminophenoxy)phenyl]ether (BAPE), bis[4-(4-aminophenoxy)phenyl]sulfone (BAPS), and bis[4-(4-aminophenoxy)phenyl]ketone (BAPK).

The diamine represented by formula (B7) (hereinafter sometimes referred to as "diamine (B7)") is an aromatic diamine having four benzene rings. This diamine (B7) has a highly flexible divalent linking group A on both sides of the diphenyl skeleton, and is therefore thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the use of diamine (B7) enhances the thermoplasticity of the polyimide. Here, -O- is preferred as the linking group A.

Examples of diamines (B7) include bis[4-(3-aminophenoxy)]biphenyl, bis[4-(4-aminophenoxy)]biphenyl, etc.

The adhesive polyimide may contain diamine residues derived from diamine compounds other than the above dimer acid type diamines and diamines (B1) to (B7) within the scope of the invention. As the diamine residues derived from diamine compounds other than the above dimer acid type diamines and diamines (B1) to (B7), any diamine compound generally used in thermoplastic polyimides may be used without limitation.

In adhesive polyimides, the thermal expansion coefficient, tensile modulus, glass transition temperature, etc. can be controlled by selecting the types of tetracarboxylic acid residues and diamine residues, or by selecting the molar ratios of two or more types of tetracarboxylic acid residues or diamine residues. In addition, in adhesive polyimides, when multiple polyimide structural units are present, they may be present as blocks or randomly, but are preferably present randomly.

The imide group concentration of the adhesive polyimide is preferably 20% by weight or less. Here, "imide group concentration" means the value obtained by dividing the molecular weight of the imide group (-(CO) ₂ -N-) in the polyimide by the molecular weight of the entire polyimide structure. If the imide group concentration exceeds 20% by weight, the molecular weight of the resin itself decreases, and the low moisture absorption property deteriorates due to an increase in polar groups, and Tg and elastic modulus increase.

The weight-average molecular weight of the adhesive polyimide is preferably, for example, in the range of 10,000 to 400,000, and more preferably in the range of 20,000 to 350,000. If the weight-average molecular weight is less than 10,000, the strength of the adhesive layer 2a decreases and it tends to become brittle. On the other hand, if the weight-average molecular weight exceeds 400,000, the viscosity increases excessively, and defects such as uneven thickness and streaks in the adhesive layer 2a tend to occur during the coating process.

The adhesive polyimide is most preferably a completely imidized structure. However, a part of the polyimide may be an amic acid. The imidization rate can be calculated from the absorbance of the C=O stretching derived from the imide group at 1780 cm ^- 1, based on the benzene ring absorber at about 1015 cm- ¹ , by measuring the infrared absorption spectrum of the polyimide thin film by the single reflection ATR method using a Fourier transform infrared spectrophotometer (commercial product: FT/IR620 manufactured by JASCO Corporation).

(Cross-linking in adhesive polyimides)
When the adhesive polyimide has a ketone group, the ketone group can be reacted with an amino group of an amino compound having at least two primary amino groups as functional groups to form a C=N bond, thereby forming a crosslinked structure. The formation of the crosslinked structure can improve the heat resistance of the adhesive polyimide. A preferred tetracarboxylic anhydride for forming an adhesive polyimide having a ketone group is, for example, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), and a preferred diamine compound is, for example, an aromatic diamine such as 4,4'-bis(3-aminophenoxy)benzophenone (BABP) or 1,3-bis[4-(3-aminophenoxy)benzoyl]benzene (BABB).

Examples of amino compounds that can be used to crosslink adhesive polyimides include dihydrazide compounds, aromatic diamines, and aliphatic amines. Among these, dihydrazide compounds are preferred. Aliphatic amines other than dihydrazide compounds are prone to forming crosslinked structures even at room temperature, which raises concerns about the storage stability of the varnish, while aromatic diamines require high temperatures to form crosslinked structures. In this way, when a dihydrazide compound is used, it is possible to achieve both the storage stability of the varnish and a shortened curing time. Examples of dihydrazide compounds include oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, pimelic acid dihydrazide, suberic acid dihydrazide, azelaic acid dihydrazide, sebacic acid dihydrazide, dodecanedioic acid dihydrazide, maleic acid dihydrazide, fumaric acid dihydrazide, diglyceryl ester dihydrazide, ... Dihydrazide compounds such as cholic acid dihydrazide, tartaric acid dihydrazide, malic acid dihydrazide, phthalic acid dihydrazide, isophthalic acid dihydrazide, terephthalic acid dihydrazide, 2,6-naphthoedioic acid dihydrazide, 4,4-bisbenzene dihydrazide, 1,4-naphthoic acid dihydrazide, 2,6-pyridine diacid dihydrazide, and itaconic acid dihydrazide are preferred. The above dihydrazide compounds may be used alone or in combination of two or more.

Adhesive polyimide can be produced by reacting the above tetracarboxylic dianhydride and diamine compound in a solvent to produce polyamic acid, which is then heated to close the ring. For example, the tetracarboxylic dianhydride and diamine compound are dissolved in approximately equal molar amounts in an organic solvent, and the mixture is stirred at a temperature in the range of 0 to 100°C for 30 minutes to 24 hours to cause a polymerization reaction, thereby obtaining polyamic acid, which is a precursor of polyimide. In the reaction, the reaction components are dissolved so that the resulting precursor is in the range of 5 to 50% by weight, preferably 10 to 40% by weight, in the organic solvent. Examples of organic solvents used in the polymerization reaction include N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N,N-diethylacetamide, N-methyl-2-pyrrolidone (NMP), 2-butanone, dimethylsulfoxide (DMSO), hexamethylphosphoramide, N-methylcaprolactam, dimethyl sulfate, cyclohexanone, dioxane, tetrahydrofuran, diglyme, triglyme, and cresol. Two or more of these solvents can be used in combination, and aromatic hydrocarbons such as xylene and toluene can also be used in combination. The amount of such organic solvents used is not particularly limited, but it is preferable to adjust the amount so that the concentration of the polyamic acid solution obtained by the polymerization reaction is about 5 to 50% by weight.

The synthesized polyamic acid is usually advantageously used as a reaction solvent solution, but it can be concentrated, diluted, or replaced with another organic solvent if necessary. Polyamic acid is also advantageously used because it generally has excellent solvent solubility. The viscosity of the polyamic acid solution is preferably within the range of 500 cps to 100,000 cps. If it is outside this range, defects such as uneven thickness and streaks are likely to occur in the film during coating operations using a coater, etc.

There are no particular limitations on the method for imidizing the polyamic acid to form the adhesive polyimide, and a suitable method is, for example, heat treatment in the solvent at a temperature in the range of 80 to 400°C for 1 to 24 hours.

When crosslinking the adhesive polyimide obtained in the above manner, the amino compound is added to a resin solution containing an adhesive polyimide having a ketone group, and a condensation reaction is caused between the ketone group in the adhesive polyimide and the primary amino group of the amino compound. This condensation reaction hardens the resin solution to become a hardened product. In this case, the amount of the amino compound added is such that the total amount of primary amino groups is 0.004 mol to 1.5 mol, preferably 0.005 mol to 1.2 mol, more preferably 0.03 mol to 0.9 mol, and most preferably 0.04 mol to 0.5 mol per 1 mol of ketone groups. If the amount of amino compound added is such that the total number of primary amino groups per mole of ketone groups is less than 0.004 moles, the crosslinking of the adhesive polyimide by the amino compound is insufficient, and the adhesive layer after curing tends not to exhibit heat resistance. If the amount of amino compound added exceeds 1.5 moles, the unreacted amino compound acts as a thermoplasticizer, tending to reduce the heat resistance of the adhesive layer 2a and the polyimide layer 1a.

The conditions of the condensation reaction for forming the crosslinks are not particularly limited as long as the ketone group in the adhesive polyimide reacts with the primary amino group of the amino compound to form an imine bond (C=N bond). The temperature of the heat condensation is preferably in the range of, for example, 120 to 220°C, more preferably in the range of 140 to 200°C, for the purpose of releasing water generated by condensation out of the system, or for the purpose of simplifying the condensation step when the heat condensation reaction is carried out subsequently after the synthesis of the adhesive polyimide. The reaction time is preferably about 30 minutes to 24 hours, and the end point of the reaction can be confirmed by, for example, a Fourier transform infrared spectrophotometer (commercially available: FT/IR620 manufactured by JASCO Corporation) by measuring the infrared absorption spectrum, and by the decrease or disappearance of the absorption peak derived from the ketone group in the polyimide resin at around 1670 cm ^-1 , and the appearance of the absorption peak derived from the imine group at around 1635 cm ^-1 .

The thermal condensation of the ketone group of the adhesive polyimide with the primary amino group of the amino compound can be carried out, for example, by (a) adding an amino compound and heating after synthesis (imidization) of the adhesive polyimide, (b) pre-feeding an excess amount of an amino compound as a diamine component, and heating the adhesive polyimide together with the remaining amino compound that is not involved in imidization or amidation after synthesis (imidization) of the adhesive polyimide, or (c) processing the adhesive polyimide composition to which the amino compound has been added into a desired shape (for example, after coating on any substrate or forming into a film), and then heating the composition.

To impart heat resistance to adhesive polyimide, the formation of imine bonds has been described as forming a crosslinked structure, but this is not limiting. For example, adhesive polyimide can be cured by blending an epoxy resin, an epoxy resin curing agent, etc.

By using the adhesive polyimide obtained in the above manner, the adhesive layer 2a and the polyimide layer 1a have excellent flexibility and dielectric properties (low dielectric constant and low dielectric tangent). As described below, the adhesive polyimide has a sufficiently low storage modulus in the temperature range of about 100°C, so the bonding temperature can be significantly lowered compared to other adhesive resins such as fluorine-based resins.

<Glass transition temperature (Tg) of adhesive polyimide>
The adhesive polyimide preferably has a glass transition temperature (Tg) of 250° C. or less, more preferably in the range of 40° C. to 200° C. By making the Tg of the adhesive polyimide 250° C. or less, thermocompression bonding at a low temperature is possible, so that the internal stress generated during lamination can be alleviated and dimensional change after circuit processing can be suppressed. If the Tg of the adhesive polyimide exceeds 250° C., the temperature during lamination to another polyimide layer becomes high, and there is a risk of impairing dimensional stability after circuit processing.

<Storage Modulus of Adhesive Polyimide>
The adhesive polyimide satisfies the storage modulus of thermoplastic polyimide defined in this specification, and further has a temperature range in which the storage modulus decreases steeply with increasing temperature in the range of 40 to 250°C. It is believed that such properties of the adhesive polyimide are the factors that alleviate the internal stress during thermocompression bonding and maintain dimensional stability after circuit processing. The adhesive polyimide preferably has a storage modulus of 5 x 10 ⁷ Pa or less at the upper limit temperature of the above temperature range, more preferably in the range of 1 x 10 ⁵ to 5 x 10 ⁷ Pa. By setting the storage modulus at such a range, even if it is the upper limit of the above temperature range, thermocompression bonding at 250°C or less is possible, and adhesion can be ensured and dimensional change after circuit processing can be suppressed.

<Dielectric tangent of first resin film and second resin film>
In the first resin film 1 and the second resin film 2, in order to suppress deterioration of the dielectric loss, the dielectric loss tangent (Tan δ) at 10 GHz is preferably 0.02 or less, more preferably 0.01 or less, and even more preferably 0.008 or less. If the dielectric loss tangent exceeds 0.02, when applied to a circuit board, problems such as loss of electrical signals on the transmission path of high-frequency signals are likely to occur. The lower limit of the dielectric loss tangent at 10 GHz is not particularly limited, but can be determined in consideration of controlling the physical properties as an insulating resin layer of a circuit board.

<Relative Dielectric Constant of First Resin Film and Second Resin Film>
The first resin film 1 and the second resin film 2 as a whole have a relative dielectric constant at 10 GHz of preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.2 or less. If the dielectric constant at 10 GHz exceeds 4.0, this leads to an increase in dielectric loss when applied to a circuit board, and is likely to cause problems such as loss of electrical signals on the transmission path of high-frequency signals.

<FPC 90 degree bending resistance>
In the present invention, the 90-degree bending resistance of an FPC can be evaluated in principle by repeatedly bending the FPC 90 degrees and measuring the number of bending times until the FPC breaks. In the present invention, however, the 90-degree bending resistance can be evaluated by performing a 90-degree bending simulation of the FPC for the conductor pattern set in the inner layer of the FPC model, and calculating the maximum strain generated in the conductor pattern of the FPC model using a known finite element analysis tool (for example, the general-purpose nonlinear analysis solver Marc). The lower the maximum strain, the better the 90-degree bending resistance. In the following examples, a specific example of a simulation of the 90-degree bending resistance of an FPC will be described.

[Manufacturing of flexible circuit boards]
The flexible circuit board of the present invention can be manufactured according to a conventional method. For example, an adhesive layer is formed on a release film as a resin sheet. Separately, a single-sided metal-clad laminate consisting of a metal layer/polyimide layer is prepared by applying and drying a single or multiple polyamide solutions on a metal layer. Next, a resin sheet is attached to the polyimide layer of the single-sided metal-clad laminate, and then the release sheet is removed, and a metal layer is attached to the exposed resin sheet, which is then patterned to form a conductor pattern. A new resin sheet is attached to the conductor pattern, and a polyimide layer of another single-sided metal-clad laminate is further attached to produce a flexible circuit board as shown in FIG. 1. The flexible circuit board of the present invention can also be manufactured by methods other than those described above.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. In the following examples and comparative examples, a simulation model of an FPC was created, and the simulation was performed using the finite element method (FEM), and analysis was performed using shell elements. A more detailed explanation will be given below.

[Mechanical simulation in the elastic-plastic region]
In a finite element analysis tool (general-purpose nonlinear analysis solver Marc), a finite element analysis model simulating a 90-degree bending test of an FPC is constructed using shell elements from the Young's modulus and plastic strain-stress relationship of the material obtained from the stress-strain relationship of each material obtained from a uniaxial extension tensile test, and a nonlinear structural analysis is performed to find a solution for the strain field generated in the FPC (focusing on the signal copper wiring on the inner layer) when a rigid body present in the finite element analysis model is forcibly displaced.

[Simulation model]
The finite element analysis model includes a material model (Fig. 5) that simulates an FPC created using shell elements, a rectangular parallelepiped rigid body (Fig. 7) for fixing the FPC, and a roll-shaped rigid body (Fig. 8) for bending the FPC. That is, as shown in Fig. 7, the FPC is fixed with a rectangular rigid body for fixing, and as shown in Fig. 8, the roll-shaped rigid body for bending the FPC is moved and brought into contact with the FPC to bend the FPC 90 degrees, and the solution of the strain field generated in the FPC at that time is obtained. Fig. 9 shows the actual 90-degree bent state on the simulation software.

The dimensional elements of the material model simulating an FPC with a stripline structure are as shown in Figure 6: the signal copper wiring has a line width of 2 mm, a height of 20 μm, the ground copper wiring on both the top and bottom sides is 12 μm each, the signal wiring is located in the center of the FPC width direction, and the overall width of the FPC is 10 mm.

[Simulation flow]
The flow of the simulation applied to this embodiment is shown in Fig. 10. This flow includes at least the steps of creating a simulation model, inputting physical property data, performing a 90-degree bending simulation, and calculating the maximum distortion occurring in the signal copper wiring, in this order.

In the "step of creating a simulation model", an FPC model to be applied to the simulation and an equipment model for bending the FPC 90 degrees (FPC clamping jig, FPC bending roller device, etc.) are created. In the "step of inputting physical property data", the physical property values of each model required for the simulation calculation, such as the elastic modulus, yield stress, and strain-stress curve, are input into the simulation software. In the "step of performing a 90-degree bending simulation", as shown in Figure 9, the FPC is bent 90 degrees using the actual simulation software. As a result of this, in the subsequent "step of calculating the maximum strain generated in the signal copper wiring", the relationship between stress and strain in the signal copper wiring of the FPC is calculated, and the maximum strain is calculated.

[Measurement of stress-strain curve]
The tensile modulus required for calculation of the simulation was measured under an environment of a temperature of 23° C. and a relative humidity of 50% using a Strograph R-1 manufactured by Toyo Seiki Seisakusho Co., Ltd. The measurement sample used was 250 mm in MD and 12.7 mm in TD, and the measurement was performed under the conditions of a load cell of 500 N, a tensile speed of 50 mm/min, and a chuck distance of 50 mm.

[Calculation of average tensile elastic modulus]
The tensile modulus of each insulator layer is measured, and the average tensile modulus is calculated using the thickness ratio of the resin laminate composed of different insulators. When n types of different insulators are present within a range of 5 μm, the average modulus of elasticity is calculated by the following formula using the values of the elastic modulus M ₁ [GPa], thickness T ₁ [μm], the elastic modulus M ₂ [GPa], thickness T ₂ [μm], and further the elastic modulus M _n [GPa] and thickness T _n [μm].

[Comparative Example 1]
11, the thicknesses of the insulating resins were (A) 25 μm, (B) 25 μm, and (C) 50 μm, and the resin types were (A): Type A, (B): Type B, and (C): Type A. The thicknesses of the ground and signal copper wiring were 12 μm and 20 μm, respectively, and the copper foil type was copper foil 1 for all.

[Example 1]
12, the thicknesses of the insulating resins were (A) 50 μm, (B) 25 μm, (C) 25 μm, (D) 25 μm, and (E) 25 μm, and the resin types were (A): Type A, (B): Type B, (C): Type A, (D): Type B, and (E): Type A. The thicknesses of the ground and signal copper wiring were 12 μm and 20 μm, respectively, and the copper foil type was copper foil 1 for all.

[Example 2]
13, the thicknesses of the insulating resins were (A) 50 μm, (B) 25 μm, (C) 25 μm, (D) 37.5 μm, (E) 25 μm, and (F) 37.5 μm, and the resin types were (A): Type B, (B): Type C, (C): Type B, (D): Type B, (E): Type C, and (F): Type B. The thicknesses of the ground and signal copper wiring were 12 μm and 20 μm, respectively, and the copper foil type was copper foil 1 for all.

[Example 3]
14, the thicknesses of the insulating resins were (A) 50 μm, (B) 50 μm, (C) 25 μm, (D) 25 μm, and (E) 50 μm, and the resin types were (A): Type A, (B): Type B, (C): Type A, (D): Type B, and (E): Type A. The thicknesses of the ground and signal copper wiring were 12 μm and 20 μm, respectively, and the copper foil type was copper foil 1 for all.

[Example 4]
15, the thicknesses of the insulating resins were (A) 50 μm, (B) 25 μm, (C) 50 μm, (D) 50 μm, (E) 25 μm, and (F) 50 μm, and the resin types were (A): Type B, (B): Type C, (C): Type B, (D): Type B, (E): Type C, and (F): Type B. The thicknesses of the ground and signal copper wiring were 12 μm and 20 μm, respectively, and the copper foil type was copper foil 1 for all.

<Parameters entered into the simulation software>
The tensile modulus [GPa] and yield stress [MPa] of the parameters input into the simulation software for the insulating resin and copper foil 1 are shown in the following Table 1. The yield stress values were obtained from the stress-strain curves in Figures 16 to 19.

<List of results of Examples and Comparative Examples>
The results obtained in Examples 1 to 4 and Comparative Example 1 are summarized in Table 2.

<Summary>
When comparing the FPCs of Examples 1 to 4 with the FPC of Comparative Example 1, it can be seen that the tensile modulus of the first resin film in the FPC of Comparative Example 1 exceeds 7 GPa, and therefore the absolute value of the maximum strain (compression mode) is larger than in the FPCs of Examples 1 to 4, in which the tensile modulus of the first resin film is within the range of 0.9 to 7 GPa, and it can be determined that the 90-degree bending resistance is reduced.

The above describes in detail the embodiment of the present invention for illustrative purposes, but the present invention is not limited to the above embodiment and various modifications are possible.

1 First resin film 1a Polyimide layer (A)
1b Polyimide layer 1c Polyimide layer 2 Second resin film 2a Adhesive layer 2b Polyimide insulating layer 2b1 Polyimide layer 2b2 Polyimide layer 10 FPC
G1 First ground layer G2 Second ground layer S Conductor pattern

Claims (3)

A flexible circuit board that is folded at approximately 90 degrees and stored in a housing of an electronic device,
A first resin film;
A conductive pattern formed on one surface of the first resin film;
a first ground layer laminated on the other surface of the first resin film;
a second resin film laminated on the conductive pattern forming surface side of the first resin film;
a second ground layer laminated on a surface of the second resin film opposite to the first resin film,
a thickness (L1) of the first resin film is within a range of 50 μm or more and 150 μm or less, a thickness (L2) of the second resin film is within a range of 50 μm or more and 150 μm or less, and a ratio (L2/L1) of the thickness (L2) of the second resin film to the thickness (L1) of the first resin film is within a range of 0.8 or more and 1.2 or less,
the first resin film has a polyimide layer (A) in a central layer, and the thickness of the polyimide layer (A) is in the range of 0.5 to 0.96 with respect to the total thickness of the first resin film;
A flexible circuit board characterized in that the tensile modulus of elasticity (M1) of the first resin film is in the range of 0.9 GPa or more and 7 GPa or less, the tensile modulus of elasticity (M2) of the second resin film is in the range of 0.9 GPa or more and 7 GPa or less, and the ratio (M2/M1) of the tensile modulus of elasticity (M2) of the second resin film to the tensile modulus of elasticity (M1) of the first resin film is in the range of 0.7 or more and 1.3 or less. the second resin film has a polyimide insulating layer including a single or multiple polyimide layers and an adhesive layer;
the adhesive layer is located between the polyimide insulating layer and a surface of the first resin film on which a conductor pattern is formed,
2. The flexible circuit board according to claim 1, wherein the adhesive layer has a tensile modulus of elasticity in the range of 0.2 GPa or more and 2 GPa or less. 3. The flexible circuit board according to claim 1, wherein the second ground layer is bent at approximately 90 degrees so that the second ground layer faces outward.

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