Flexible circuit board

Abstract

To provide a flexible circuit board which exhibits 90-degree bending resistance required when a flexible circuit board having a strip line structure is applied to an RF cable for a millimeter-wave antenna.SOLUTION: A flexible circuit board is stored in a housing of an electronic apparatus and is bendable by substantially 90 degrees has such a structure that a first ground layer, a first resin layer, a conductive pattern, a second resin layer, and a second ground layer are sequentially stacked. Thickness (L1) of the first resin layer and thickness of the second resin layer (L2) are each independently within ranges of 50 μm or more and 150 μm or less, and a ratio (L2 / L1) is within a range of 0.8 or more and 1.2 or less. Tensile elastic modulus (M1) of the first resin layer and tensile elastic modulus (M2) of the second resin layer are each independently within ranges of 0.9 GPa or more and 7 GPa or less, and a ratio (M2 / M1) is within a range of 0.7 or more and 1.8 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a flexible circuit board. [Background technology]

[0002] One of the important applications of flexible circuit boards (FPCs) for high-frequency applications is RF cables for millimeter-wave antennas. While various structures are known for flexible circuit boards (FPCs) for such RF cables, including coplanar structures with excellent low transmission loss, stripline structures with excellent noise immunity, and microstrip structures with intermediate characteristics, stripline flexible circuit boards with excellent noise immunity are attracting particular attention in order to prevent malfunctions in densely mounted electronic equipment and to achieve a high level of operational stability in electronic devices (see, for example, Figure 1 in Patent Document 1).

[0003] A stripline FPC has a structure in which a laminate consisting of a first dielectric resin layer, a signal conductor pattern formed thereon, and a second resin layer laminated on the signal conductor pattern-forming surface of the first resin layer via an adhesive layer as needed is sandwiched between a pair of ground copper foils. Generally, it is manufactured by processing the copper foil on one side of a double-sided copper-clad laminate (double-sided CCL) into a signal conductor pattern, and then laminating a single-sided copper-clad laminate (single-sided CCL) onto the signal conductor pattern-forming surface of the double-sided copper-clad laminate from the dielectric resin layer side.

[0004] In recent years, attempts have been made to house stripline-structured FPCs, which are used in RF cables for millimeter-wave antennas, within the casings of mobile electronic devices. However, as mobile electronic devices have become smaller and thinner, situations have arisen where the FPCs must be folded at approximately a 90-degree angle when being housed within the casings of mobile electronic devices. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6537172 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, in the case of FPCs with a stripline structure, in order to improve high-frequency transmission characteristics, the total thickness of the first and second resin layers, which are dielectrics on both sides of the signal conductor pattern, must be thicker than the dielectric thickness of FPCs with a coplanar or microstrip structure. As a result, there is a problem that the flexibility is relatively inferior and it is difficult to obtain good 90-degree bending resistance. Furthermore, Patent Document 1 (especially Figure 1) only examines the dielectric properties of a flexible circuit board with a stripline structure, and does not examine at all the 90-degree bending resistance required when such an FPC is applied as an RF cable for a millimeter-wave antenna.

[0007] The object of the present invention is to provide an FPC with a stripline structure that exhibits the necessary 90-degree bending resistance when applied as an RF cable for a millimeter-wave antenna. [Means for solving the problem]

[0008] In investigating the 90-degree bending resistance of a stripline FPC board, the inventors focused on the following points (1) to (3) regarding a pair of first and second resin layers that sandwich a signal conductor pattern between a pair of ground layers.

[0009] (1) The former is derived from bilateral CCL, while the latter is derived from unilateral CCL; (2) In the first resin layer derived from double-sided CCL, a pair of polyimide layers exhibiting relatively higher rigidity than the core layer described later are typically arranged on the copper foil side of both sides to improve the dimensional accuracy of the FPC, and a core layer with lower rigidity and lower dielectric loss tangent than the polyimide layers is arranged between them to improve the high-frequency transmission and flexibility of the FPC, resulting in a three-layer structure. Therefore, in particular, by adjusting the thickness and rigidity of any of the layers, such as the pair of polyimide layers constituting the first resin layer and the core layer sandwiched between them, to a predetermined range, it may be possible to optimize the stripline structure of the FPC for RF cables for millimeter-wave antennas; and (3) In the second resin layer derived from one-sided CCL, a polyimide layer exhibiting relatively high rigidity is usually placed on the copper foil side, and an adhesive layer with lower rigidity and lower dielectric loss tangent than the polyimide layer is placed on the first resin layer side, resulting in a two-layer structure. Since the number of layers to be adjusted is less than that of the first resin layer, it is possible to adjust it to match the rigidity and dielectric properties of the first resin layer relatively easily.

[0010] As a result of diligent research, the inventors have discovered that the objective of the present invention can be achieved by adjusting the following to a predetermined range for the first resin layer derived from double-sided CCL: the respective resin layer thicknesses of the first and second resin layers and their relative ratio, the layer thickness of the core layer and its ratio to the thickness of the first resin layer, and the tensile modulus of the core layer and the respective tensile moduli of the pair of first and second polyimide layers. This led to the completion of the present invention.

[0011] In other words, the present invention relates to a flexible circuit board that is housed within the casing of an electronic device and can be bent at approximately 90 degrees, A conductive pattern formed on one side of the first resin layer, A first ground layer laminated on the other side of the first resin layer, A second resin layer laminated on the conductor pattern forming surface side of the first resin layer, The second resin layer comprises a second gland layer laminated on the side of the second resin layer opposite to the first resin layer, The first resin layer has a structure in which a first polyimide layer, a core layer, and a second polyimide layer are laminated in this order from the side of the first ground layer. The layer thickness of the core layer is in the range greater than 0.4 and less than or equal to 0.8 with respect to the thickness of the first resin layer. The tensile elastic modulus of the core layer is in the range of 0.5 GPa or more and 2 GPa or less. The tensile elastic moduli of the first polyimide layer and the second polyimide layer are each independently in the range of 5 GPa or more and 9 GPa or less. The thickness (L1) of the first resin layer and the thickness (L2) of the second resin layer are each independently in the range of 50 μm or more and 150 μm or less, and A flexible circuit board is provided, characterized in that the ratio (L2 / L1) of the thickness (L2) of the second resin layer to the thickness (L1) of the first resin layer is in the range of 0.8 or more and 1.2 or less.

[0012] In the flexible circuit board of the present invention, the tensile elastic modulus (M1) of the first resin layer and the tensile elastic modulus (M2) of the second resin layer are each independently in the range of 0.9 GPa or more and 7 GPa or less, and it is preferable that the ratio (M2 / M1) of the tensile elastic modulus (M2) of the second resin layer to the tensile elastic modulus (M1) of the first resin layer is in the range of 0.7 or more and 1.8 or less.

[0013] Further, in the flexible circuit board of the present invention, the second resin layer includes a polyimide insulating layer including a single layer or a plurality of polyimide layers and an adhesive layer. The adhesive layer is located between the polyimide insulating layer and the conductor pattern forming surface of the first resin layer. It is preferable that the tensile elastic modulus of the adhesive layer is in the range of 0.2 GPa or more and 2 GPa or less.

[0014] Further, in the flexible circuit board of the present invention, it is preferable that the second ground layer can be bent approximately 90 degrees so that it is on the outside.

Advantages of the Invention

[0015] In the FPC of the present invention having a stripline structure, between a pair of ground layers, with respect to a pair of first resin layers and second resin layers sandwiching a signal conductor pattern, in addition to the relative ratio of the layer thicknesses of the first resin layer and the second resin layer, particularly the layer thickness of the core layer sandwiched between a pair of first polyimide layers and second polyimide layers constituting the first resin layer and the relative ratio to the first resin layer thickness, and the tensile elastic moduli of the first polyimide layer, the second polyimide layer, and the core layer are each limited to a specific range. Therefore, the FPC of the present invention can exhibit bend resistance at 90 degrees, which is useful as a millimeter-wave antenna RF cable.

Brief Description of the Drawings

[0016] [Figure 1] It is a schematic diagram showing the configuration of an FPC according to an embodiment of the present invention. [Figure 2] It is a cross-sectional layer configuration diagram of a material model simulating an FPC. [Figure 3] It is a schematic diagram showing the arrangement and size of rigid bodies in a calculation model. [Figure 4] It is a schematic diagram showing the arrangement of an FPC in the model of FIG. 3. [Figure 5] It is an explanatory diagram of a 45-degree bending state of an FPC on simulation software. [Figure 6] It is an explanatory diagram of a 90-degree bending state of an FPC on simulation software. [Figure 7] It is an explanatory diagram of a state where an FPC is bent at 90 degrees and a pressing plate is pressed against it on simulation software. [Figure 8] It is an explanatory diagram of simulation steps. [Figure 9] It is a stress-strain curve of the polyimide constituting 1c, 1a of the first resin layer and 2a of the second resin layer. [Figure 10] It is a stress-strain curve of the adhesive constituting 1b of the first resin layer. [Figure 11] It is a stress-strain curve of the adhesive constituting 2b of the second resin layer. [Figure 12] It is a stress-strain curve of copper foil. [Figure 13] This is a conductor wiring diagram of a test circuit board sample. [Modes for carrying out the invention]

[0017] Embodiments of the present invention will be described with reference to the drawings as appropriate.

[0018] [Flexible Circuit Board Configuration] The flexible circuit board (FPC) of the present invention is housed within the casing of an electronic device and can be bent at approximately 90 degrees. Examples of its configuration include those shown in Figures 1 and 2. The FPC 10 in Figure 1 has a first ground layer G1, a first resin layer 1 laminated thereon, a conductor pattern S formed on one side thereof, a second resin layer 2 laminated on the conductor pattern forming surface of the first resin layer 1, and a second ground layer G2 further laminated thereon. Here, the laminate of the first ground layer G1 / first resin layer 1 / conductor pattern S is preferably derived from a so-called double-sided copper-clad laminate (double-sided CCL), and the laminate of the second resin layer 2 / second ground layer G2 is preferably derived from a so-called single-sided copper-clad laminate (single-sided CCL) from a manufacturing standpoint. Figure 2 is a cross-sectional layer configuration diagram of a material model simulating the FPC.

[0019] (Basic structure of the first resin layer 1) The first resin layer 1 has a structure in which the first polyimide layer 1a, the core layer 1b, and the second polyimide layer 1c are laminated in this order from the first ground layer G1 side. The first polyimide layer 1a, the core layer 1b, and the second polyimide layer 1c may each be composed of a polyimide insulating layer having one or more polyimide layers. Here, the first polyimide layer 1a and the second polyimide layer 1c exhibit relatively high rigidity and flame retardancy compared to the core layer 1b, mainly to improve the dimensional accuracy of the flexible circuit board, improve the accuracy of interlayer connection processing by drilling and laser processing, and prevent a decrease in the flame retardancy of the FPC. On the other hand, the core layer 1b mainly exhibits a relatively low dielectric loss tangent compared to the first polyimide layer 1a and the second polyimide layer 1c, and is intended to improve the high-frequency transmission characteristics of the FPC. Furthermore, between the first ground layer G1 and the first polyimide layer 1a, and between the conductor pattern S and the second polyimide layer 1c, known adhesive layers such as polyimide adhesives may be provided as needed, to the extent that they have little effect on the dielectric properties and tensile modulus of the first resin layer 1.

[0020] (Thickness of the core layer) The thickness of the core layer 1b is greater than 0.4 and less than or equal to 0.8 of the total thickness of the first resin layer 1, preferably between 0.5 and 0.75. If the thickness of the core layer 1b is less than or equal to 0.4 of the thickness of the first resin layer 1, the dielectric loss tangent of the first resin layer 1 will be insufficient, and sufficient dielectric properties will not be obtained. If it exceeds 0.8, problems such as a decrease in the dimensional stability of the first resin layer 1 tend to occur.

[0021] (Tensile modulus of the core layer) It is known that the tensile modulus is a suitable indicator of the 90-degree bending resistance of FPC10. Generally, when the tensile modulus of the resin layer is high, the resin layer becomes hard and the 90-degree bending resistance decreases, and conversely, when it is low, the 90-degree bending resistance improves. From this viewpoint, the tensile modulus of the core layer 1b constituting the first resin layer 1 is specifically within the range of 0.5 GPa to 2 GPa, preferably 0.75 GPa to 1.5 GPa. If it is less than 0.5 GPa, the rigidity of the FPC will be low and the handling performance when mounting electronic components will decrease, and if it exceeds 2 GPa, there is a concern that the rigidity of the FPC will be high and the 90-degree bending resistance will decrease.

[0022] (Tensile modulus of the first and second polyimide layers) Furthermore, the tensile modulus of the first polyimide layer 1a and the second polyimide layer 1c are independently 5 GPa or more and 9 GPa or less, preferably 6 GPa or more and 8 GPa or less. If it is less than 5 GPa, it means that the rigidity of the first or second polyimide layer in contact with the first ground layer G1 or the conductor pattern S will be low, which will reduce the dimensional accuracy of the FPC 10. If it exceeds 9 GPa, the rigidity will be high, and there is a concern that the 90-degree bending resistance of the FPC 10 will decrease.

[0023] (Measurement of tensile modulus) The tensile modulus of resin layers such as the first polyimide layer 1a, core layer 1b, and second polyimide layer 1c can be measured using a commercially available tensile elasticity tester (for example, the Strograph R-1 from Toyo Seiki Seisakusho Co., Ltd.) in an environment of 23°C and 50% relative humidity. These layers can be composed of a single layer or multiple layers. Generally, when a resin layer is composed of multiple polyimide layers or adhesive layers, the tensile modulus M0 of the entire resin layer can be calculated according to the following formula, taking the case where the resin layer is composed of polyimide layers a, b...n as an example. Here, the total thickness of the resin layer is L0, the thickness of polyimide layer a is La and its individual tensile modulus is Ma, the thickness of polyimide layer b is Lb and its individual tensile modulus is Mb, and the thickness of polyimide layer n is Ln and its individual tensile modulus is Mn.

[0024]

number

[0025] (Basic structure of the second resin layer) On the other hand, as shown in Figure 1, the second resin layer 2 may be a single layer or a multi-layer structure, as long as it matches the characteristics of the first resin layer 1, but it is preferable to have a structure in which a polyimide insulating layer 2a containing one or more polyimide layers and an adhesive layer 2b are laminated. In this case, it is preferable that the adhesive layer 2b is located on the conductor pattern forming surface side of the first resin layer 1. Such a second resin layer 2 is combined with the first resin layer 1 described above to constitute a stripline structure FPC, and its rigidity and dielectric properties are selected to match the thickness, rigidity and dielectric properties of the first resin layer 1 so that the FPC can achieve good 90-degree bending resistance without impairing the dielectric properties of the FPC. In other words, if the thickness, rigidity and dielectric properties of the first resin layer 1 derived from double-sided CCL are determined, the thickness, rigidity and dielectric properties of the second resin layer 2 can be determined accordingly, and the material and layer structure used for the second resin layer 2 can be determined accordingly. Therefore, when manufacturing an FPC with a stripline structure for RF cables for millimeter-wave antennas, it is important to first identify the first resin layer 1 derived from double-sided CCL. If necessary, a known adhesive layer, such as a polyimide-based adhesive, may be provided between the second ground layer G2 and the polyimide insulating layer 2a, to the extent that it has little effect on the dielectric properties and tensile modulus of the second resin layer 2.

[0026] (Thickness of polyimide insulating layer 2a) The thickness of the polyimide insulating layer 2a is preferably greater than 0.25 and less than or equal to 0.5, and preferably between 0.3 and 0.5, relative to the total thickness of the second resin layer 2. If the thickness of the polyimide insulating layer 2a is less than or equal to 0.25 relative to the thickness of the second resin layer 2, the dielectric loss tangent of the second resin layer 2 will be insufficient, and sufficient dielectric properties will not be obtained. If it exceeds 0.5, problems such as a decrease in the dimensional stability of the second resin layer 2 tend to occur.

[0027] (Tensile modulus of polyimide insulating layer 2a) The tensile modulus of the polyimide insulating layer 2a constituting the second resin layer 2 is preferably in the range of 5 GPa to 9 GPa, more preferably 6 GPa to 8 GPa. If it is less than 5 GPa, the rigidity of the FPC 10 will be excessively low, reducing the handling performance when mounting electronic components, and if it exceeds 9 GPa, the rigidity of the FPC 10 will be excessively high, raising concerns about a decrease in its resistance to bending at 90 degrees.

[0028] (Thickness of adhesive layer 2b) The thickness of the adhesive layer 2b is the total thickness of the second resin layer 2 minus the thickness of the polyimide insulating layer 2a, and is greater than 0.5 and less than or equal to 0.75, preferably between 0.5 and 0.7, relative to the thickness of the second resin layer 2. The ratio of the thickness of the adhesive layer 2b to the thickness of the polyimide insulating layer 2a is preferably between 0.25 and 0.5, more preferably between 0.3 and 0.5. If it is less than 0.25, the dielectric loss tangent reduction will be insufficient, and sufficient dielectric properties will not be obtained. If it exceeds 0.5, the rigidity of the FPC 10 will increase, and there is a concern that the resistance to bending at 90 degrees will decrease.

[0029] (Tensile modulus of adhesive layer 2b) Furthermore, the tensile modulus of the adhesive layer 2b is preferably 0.2 GPa or more and 2 GPa or less, more preferably 0.5 GPa or more and 1.5 GPa or less, and even more preferably 0.75 GPa or more and 1.5 GPa or less. If it falls below this range, the handling properties of the adhesive in the manufacturing process of the FPC 10 tend to decrease, and if it exceeds this range, it affects the rigidity of the FPC 10 itself and may contribute to a decrease in its flexibility.

[0030] (Thickness of the first resin layer 1 and the second resin layer 2) The thickness (L1) of the first resin layer 1 and the thickness (L2) of the second resin layer 2 constituting the FPC 10 of the present invention are 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 layer 1 and the second resin layer falls below this range, sufficient transmission characteristics will not be exhibited, and if it exceeds this range, the overall thickness of the FPC will increase, resulting in reduced bending resistance.

[0031] In the FPC 10 of the present invention, the ratio (L2 / L1) of the thickness of the second resin layer 2 to the thickness (L1) of the first resin layer 1 is 0.8 or more and 1.2 or less, preferably 0.9 or more and 1.1 or less. If this ratio (L2 / L1) is below this range, when the FPC is bent by approximately 90 degrees, the neutral plane of the bend (see Japanese Patent Application Publication No. 2021-9997, paragraphs 0043-0045) shifts from the center in the thickness direction of the copper foil toward the first resin layer 1, resulting in a larger maximum stress on the copper foil. If it is above this range, the neutral plane shifts toward the second resin layer 2, resulting in a larger maximum stress on the copper foil.

[0032] (Tensile modulus of the first resin layer 1 and the second resin layer 2) The tensile modulus (M1) of the first resin layer 1 and the tensile modulus (M2) of the second resin layer 2 are each independently within the range of 0.9 GPa to 7 GPa, preferably 1 GPa to 7 GPa, and more preferably 1 GPa to 6 GPa. If the modulus falls below this range, the rigidity of the FPC itself decreases, reducing its handling performance when mounted on electronic equipment. If the modulus exceeds this range, the rigidity of the FPC itself increases, which is one of the factors that reduces its bending resistance.

[0033] In the FPC10 of the present invention, the ratio (M2 / M1) of the tensile modulus of the second resin layer 2 to the tensile modulus of the first resin layer 1 (M1) is 0.7 or more and 1.8 or less, preferably 0.8 or more and 1.2 or less. If it is below this range, when the FPC is bent by approximately 90 degrees, the neutral plane of the bend moves from the center in the thickness direction of the copper foil towards the first resin layer 1, increasing the maximum stress on the copper foil. If it is above this range, the neutral plane moves towards the second resin layer 2, increasing the maximum stress on the copper foil.

[0034] (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 common ground layer materials for FPCs can be used independently. Examples include metal materials such as copper, stainless steel, iron, nickel, beryllium, aluminum, zinc, indium, silver, gold, tin, zirconium, tantalum, titanium, lead, magnesium, and manganese, as well as alloys thereof. Among these, copper or copper alloys are particularly preferred.

[0035] The thickness of the first gland layer G1 and the second gland layer G2 is not particularly limited, but is preferably 5 μm to 30 μm, more preferably 10 μm to 20 μm, from the viewpoint of production stability, handling, and shielding performance of the FPC. When copper foil is used as the first gland layer G1 and the second gland layer G2, it may be rolled copper foil or electrolytic copper foil, and commercially available copper foil can also be used.

[0036] Furthermore, the metal and alloy materials used in the first and second ground layers G1 and G2 may be subjected to surface treatments, such as rust prevention treatment or improved adhesion, using siding agents, aluminum alkoxides, aluminum chelating agents, silane coupling agents, etc. The first and second ground layers G1 and G2 may also be subjected to conductive patterning using known methods as needed.

[0037] (Conductor pattern (signal copper wiring)) In the FPC10 of the present invention, the conductor pattern S functions as a signal line, and can be appropriately selected and applied from materials and configurations conventionally used as signal lines in FPCs, and can be composed of the same materials as the first ground layer G1 and the second ground layer G2. Patterning can be performed by known methods. The layer thickness of the conductor pattern S is not particularly limited, but is preferably 5 μm to 20 μm from the viewpoint of production stability, handling, and transmission performance.

[0038] (Materials for the first resin layer 1 and the second resin layer 2) In the FPC10 of the present invention, in addition to polyimide, which has electrical insulating properties, other resins that have electrical insulating properties can be used or used in combination with polyimide as constituent materials for the first resin layer 1 and the second resin layer 2. Examples of such resins include polyamide, epoxy resin, phenolic resin, polyethylene, polypropylene, polytetrafluoroethylene, silicone, and ethylenetetrafluoroethylene. Furthermore, when polyimide layers are used in the first resin layer 1 and the second resin layer 2, they are not limited to a single layer, but may consist of multiple polyimide layers laminated together. In this invention, "polyimide" refers to resins made of polymers having imide groups in their molecular structure, including polyimide, polyamideimide, polyetherimide, polyesterimide, polysiloxaneimide, and polybenzimidazoleimide.

[0039] The following describes the first polyimide layer 1a, core layer 1b, second polyimide layer 1c, and the polyimide insulating layer 2a and adhesive layer 2b, which are composed of a single or multiple polyimide layers and constitute the first resin layer 1. Here, the polyimides that make up these polyimide layers contain tetracarboxylic acid residues derived from tetracarboxylic dianhydride components and diamine residues derived from diamine compound components. From the viewpoint of storage modulus, they are generally divided into "thermoplastic polyimides" in which the glass transition temperature (Tg) can be clearly confirmed, and "non-thermoplastic polyimides" in which they generally do not soften or exhibit adhesive properties like thermoplastic polyimides even when heated. Specifically, in the present invention, the "storage modulus at 30°C" measured using a dynamic viscoelasticity measuring device (DMA) is 1.0 × 10⁻⁶. 8 Pa or higher and the "storage modulus at 300°C" is 3.0 × 10⁻⁶ 7 "Thermoplastic polyimide" has a Pa of less than 1.0 × 10⁻¹⁰ and a "storage modulus at 30°C" of 1.0 × 10⁻¹⁰. 9 Pa or higher and the "storage modulus at 300°C" is 3.0 × 10⁻⁶ 8They can be broadly divided into "non-thermoplastic polyimides" with a rigidity of Pa or higher and "thermoplastic polyimides." These "thermoplastic polyimides" generally have lower rigidity than non-thermoplastic polyimides but exhibit relatively good adhesion.

[0040] Therefore, it is preferable to apply thermoplastic polyimide to the first polyimide layer 1a and second polyimide layer 1c of the first resin layer 1, which are layers that are in direct contact with the first ground layer G1, the second ground layer G2, or the conductor pattern S, and to the polyimide insulating layer 2a of the second resin layer 2. If they are composed of multiple polyimide layers, thermoplastic polyimide is applied to the layers that are in direct contact with the first ground layer G1, the second ground layer G2, or the conductor pattern S. In this case, non-thermoplastic polyimide, which exhibits relatively better dielectric properties than thermoplastic polyimide, can be applied to the layers that are not in direct contact. For example, as shown in Figure 2, if the polyimide insulating layer 2a is composed of a polyimide layer 2a1 and a polyimide layer 2a2, it is preferable to apply thermoplastic polyimide to the polyimide layer 2a1 that is in contact with the second ground layer G2. In that case, thermoplastic polyimide may also be applied to the inner polyimide layer 2a2, but non-thermoplastic polyimide may also be applied. While a non-thermoplastic polyimide can be used for the core layer 1b of the first resin layer 1, when the core layer 1b is used as a bonding sheet, it can be composed of an adhesive polyimide, which has enhanced film-forming and adhesive properties, among thermoplastic polyimides, as described later.

[0041] (Non-thermoplastic polyimide) In the FPC10 of the present invention, the non-thermoplastic polyimide applicable to the core layer 1b of the first resin layer 1, the polyimide layer 2a2 of the second resin layer, etc. preferably has an acid dianhydride residue derived from an acid 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, etc. As the acid dianhydride component and the diamine component, monomers commonly used in the synthesis of non-thermoplastic polyimides (see paragraphs 0025-0026 of Japanese Patent Publication No. 2014-141083 and paragraphs 0019-0020 of Japanese Patent Publication No. 2016-72405) can be used. By selecting the types of acid dianhydride component and the diamine component, and the molar ratio of each when two or more acid dianhydrides or diamines are used, the thermal expansion properties, adhesion, glass transition temperature, etc. of the polyimide can be controlled.

[0042] Non-thermoplastic polyimides preferably have an imide group concentration of 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 the increase in polar groups.

[0043] Non-thermoplastic polyimides are preferably characterized by a glass transition temperature (Tg) of 280°C or higher, and more preferably 300°C or higher, from the viewpoint of heat resistance.

[0044] Furthermore, 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.

[0045] Furthermore, non-thermoplastic polyimide may be appropriately blended with optional components, such as plasticizers, other curing resin components like epoxy resins, curing agents, curing accelerators, coupling agents, fillers, solvents, and flame retardants, as long as the effects of the invention are not impaired.

[0046] (Thermoplastic polyimide) In the FPC10 of the present invention, a thermoplastic polyimide that can be preferably applied to the first polyimide layer 1a, the second polyimide layer 1c, the polyimide layer 2a1, the adhesive layer 2b, etc., has an acid dianhydride residue derived from an acid 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 acid dianhydride component and the diamine component, monomers commonly used in the synthesis of thermoplastic polyimides can be used (see paragraphs 0025-0026 of Japanese Patent Application Publication No. 2014-141083 and paragraphs 0019-0020 of Japanese Patent Application Publication No. 2016-72405).

[0047] Thermoplastic polyimides preferably have an imide group concentration of 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 the increase in polar groups.

[0048] From the viewpoint of suppressing warping, the thermal expansion coefficient of thermoplastic polyimide is preferably 30 ppm / K or higher, more preferably 30 ppm / K to 100 ppm / K, and particularly preferably 30 ppm / K to 80 ppm / K.

[0049] Furthermore, thermoplastic polyimide may be appropriately blended with optional components, such as plasticizers, other curing resin components like epoxy resins, curing agents, curing accelerators, inorganic fillers, coupling agents, fillers, solvents, and flame retardants, as long as the effects of the invention are not impaired.

[0050] (adhesive layer) In the FPC10 of the present invention, the adhesive layer 2b of the second resin layer 2 may be formed from the thermoplastic polyimide described above, but it can also be formed from an adhesive polyimide that can be used as a bonding sheet with enhanced adhesion and film-forming properties. Similarly, the core layer 1b can be formed not only from a non-thermoplastic polyimide, but also from an adhesive polyimide that can be used as a bonding sheet.

[0051] (Tetracarboxylic acid residues of adhesive polyimides) Adhesive polyimides can generally contain tetracarboxylic acid residues derived from tetracarboxylic dianhydrides used in thermoplastic polyimides without particular limitations. However, it is preferable that the polyimide contains a total of 90 moles or more of tetracarboxylic acid residues derived from tetracarboxylic dianhydrides represented by the following general formula (1) (hereinafter sometimes referred to as "tetracarboxylic acid residue (1)") per 100 moles of tetracarboxylic acid residues. Including a total of 90 moles or more of tetracarboxylic acid residue (1) per 100 moles of tetracarboxylic acid residues is preferable because it facilitates achieving both flexibility and heat resistance in the adhesive polyimide. If the total amount of tetracarboxylic acid residue (1) is less than 90 moles, the solvent solubility of the adhesive polyimide tends to decrease.

[0052] [ka]

[0053] In general formula (1), X represents a single bond or a divalent group selected from the following formulas.

[0054] [ka]

[0055] In the above formula, Z is -C6H4-, -(CH2) n - or -CH2-CH(-OC(=O)-CH3)-CH2-, where n is an integer from 1 to 20.

[0056] Examples of tetracarboxylic dianhydrides used to induce 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).

[0057] The adhesive polyimide may contain tetracarboxylic acid residues derived from acid dianhydrides other than the tetracarboxylic acid dianhydride represented by the general formula (1) above, to the extent that it does not impair the effects of the invention. There are no particular limitations on such tetracarboxylic acid residues, but examples include pyromellitic acid dianhydride, 1,4-phenylenebis(trimellitic acid monoester) dianhydride, 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 2,2',3,3'- or 2,3,3',4'-benzophenonetetracarboxylic acid dianhydride, 2,3',3,4'-diphenylethertetracarboxylic acid dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, 3,3',4,4'-, 2,3,3',4'- or 2,2',3,3'-p-terphenyltetracarboxylic acid dianhydride, 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-anthracenetetracarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)tetrafluoropropane dianhydride, 2,3,5,6-cyclohexane dianhydride, 1,2,5,6-naphthalenetetracarboxylic acid dianhydride, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid 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 dianhydride, 2,3,8,9-,3,4,9,10-,4,5,10,11- or 5,6,11,12-perylene-tetracarboxylic dianhydride, cyclopentane-1,2,3,4-tetracarboxylic dianhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, pyrrolidine-2,3,4,5-tetracarboxylic dianhydride, thiophene-2,3,4,5-tetracarboxylic dianhydride, 4,4'-bis(2,3-dicarboxyphenoxy)diphenylmethane dianhydride, ethylene glycol Examples include tetracarboxylic acid residues derived from aromatic tetracarboxylic dianhydrides such as bisuanhydrotrimellitates.

[0058] (Diamine residues of adhesive polyimides) The adhesive polyimide preferably contains 20 moles or more, more preferably 40 moles or more, and particularly preferably 60 moles or more, of dimer acid type diamine residues derived from dimer acid type diamines per 100 moles of diamine residues. By including dimer acid type diamine residues in the above amounts, the dielectric properties of the adhesive layer 2b, etc., can be improved, and the thermocompression bonding properties can be improved by lowering the glass transition temperature (low Tg) of the adhesive layer, and internal stress can be relaxed by lowering the elastic modulus. If the amount of dimer acid type diamine residues is less than 20 moles per 100 moles of diamine residues, sufficient adhesion as an adhesive layer may not be obtained, and the elastic modulus of the adhesive layer, which has high thermal expansion properties, will be high, which may worsen the dimensional change rate after the formation of the conductor pattern S.

[0059] Here, a dimer acid-type diamine refers to a diamine in which the two terminal carboxylic acid groups (-COOH) of a dimer acid are replaced with primary aminomethyl groups (-CH2-NH2) or amino groups (-NH2). Dimer acid is a known dibasic acid obtained by the intermolecular polymerization reaction of unsaturated fatty acids, and its industrial production process is almost standardized in the industry. It is obtained by dimerizing unsaturated fatty acids with 11 to 22 carbon atoms using a clay catalyst or the like. Industrially obtained dimer acid mainly consists of a dibasic acid with 36 carbon atoms obtained by dimerizing unsaturated fatty acids with 18 carbon atoms, such as oleic acid and linoleic acid. However, depending on the degree of purification, it contains arbitrary amounts of monomeric acids (18 carbon atoms), trimer acids (54 carbon atoms), and other polymeric fatty acids with 20 to 54 carbon atoms. In this invention, it is preferable to use dimer acid in which the 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 this invention, dimer acids obtained by further hydrogenation to reduce the degree of unsaturation are also included.

[0060] A key characteristic of dimer acid-type diamines is their ability to impart properties derived from the dimer acid skeleton to polyimide. Specifically, because dimer acid-type diamines are macromolecules with a molecular weight of approximately 560-620 and are aliphatic, they can increase the molar volume of the molecule and relatively reduce the polar groups of polyimide. This characteristic of dimer acid-type diamines is thought to contribute to improving dielectric properties by reducing the dielectric constant and dielectric loss tangent while suppressing the decrease in the heat resistance of polyimide. Furthermore, because they possess two freely moving hydrophobic chains with 7-9 carbon atoms and two chain-like aliphatic amino groups with lengths close to 18 carbon atoms, they not only provide flexibility to polyimide but can also create asymmetrical and non-planar chemical structures, thus enabling the reduction of the dielectric constant and dielectric loss tangent of polyimide.

[0061] Dimer acid type diamines are commercially available, such as PRIAMINE 1073 (trade name), PRIAMINE 1074 (trade name), and PRIAMINE 1075 (trade name) from Croda Japan, and Versamin 551 (trade name) and Versamin 552 (trade name) from BASF Japan.

[0062] Furthermore, the adhesive polyimide preferably contains a total of 20 to 80 mole parts, and more preferably 20 to 60 mole parts, of diamine residues derived from at least one diamine compound selected from the diamine compounds represented by the following general formulas (B1) to (B7), per 100 mole parts of the total diamine components. Since the diamine compounds represented by general formulas (B1) to (B7) have a flexible molecular structure, using at least one diamine compound selected from these in the above range can improve the flexibility of the polyimide molecular chain and impart thermoplasticity. If the total amount of diamine compounds represented by general formulas (B1) to (B7) exceeds 80 mole parts per 100 mole parts of the total diamine components, the flexibility of the polyimide will be insufficient, and the glass transition temperature (Tg) will rise, leading to an increase in residual stress due to thermocompression bonding and a deterioration in the dimensional change rate after etching.

[0063] [ka]

[0064] In the above formulas (B1) to (B7), R1 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-, -SO2-, -COO-, -CH2-, -C(CH3)2-, -NH-, or -CONH-, and n1 independently represents an integer from 0 to 4. However, any overlap between formula (B2) and formula (B5) in formula (B4) shall be excluded.

[0065] Furthermore, "independently" means that in one or more of the above formulas (B1) to (B7), the multiple linking groups A, multiple R1, or multiple n1 may be the same or different. Also, in formulas (B1) to (B7), the hydrogen atoms in the two terminal amino groups may be substituted, for example, -NR2R3 (where R2 and R3 independently represent any substituent such as an alkyl group).

[0066] The diamine represented by the above formula (B1) (hereinafter sometimes referred to as "diamine (B1)") is an aromatic diamine having two benzene rings. This diamine (B1) has an amino group directly attached to at least one benzene ring and a divalent linking group A at the meta position, which increases the degree of freedom of the polyimide molecular chain and gives it high flexibility, and is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, using diamine (B1) increases the thermoplasticity of the polyimide. Here, -O-, -CH2-, -C(CH3)2-, -CO-, -SO2-, -S-, and -COO- are preferred as linking group A.

[0067] 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.

[0068] The diamine represented by the above formula (B2) (hereinafter sometimes referred to as "diamine (B2)") is an aromatic diamine having three benzene rings. This diamine (B2) has an amino group directly attached to at least one benzene ring and a divalent linking group A at the meta position, which increases the degree of freedom of the polyimide molecular chain and gives it high flexibility, and is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, using diamine (B2) increases the thermoplasticity of the polyimide. Here, -O- is preferred as the linking group A.

[0069] Examples of diamines (B2) include 1,4-bis(3-aminophenoxy)benzene, 3-[4-(4-aminophenoxy)phenoxy]benzeneamine, and 3-[3-(4-aminophenoxy)phenoxy]benzeneamine.

[0070] The diamine represented by the above formula (B3) (hereinafter sometimes referred to as "diamine (B3)") is an aromatic diamine having three benzene rings. In this diamine (B3), the two divalent linking groups A directly attached 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, and is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, using diamine (B3) increases the thermoplasticity of the polyimide. Here, -O- is preferred as the linking group A.

[0071] 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.

[0072] The diamine represented by the above formula (B4) (hereinafter sometimes referred to as "diamine (B4)") is an aromatic diamine having four benzene rings. This diamine (B4) has high flexibility because the amino group directly attached to at least one benzene ring and the divalent linking group A are in the meta position, and it is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the thermoplasticity of the polyimide is increased by using diamine (B4). Here, -O-, -CH2-, -C(CH3)2-, -SO2-, -CO-, and -CONH- are preferred as linking group A.

[0073] 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.

[0074] The diamine represented by the above formula (B5) (hereinafter sometimes referred to as "diamine (B5)") is an aromatic diamine having four benzene rings. This diamine (B5) has two divalent linking groups A directly attached to at least one benzene ring, each in a meta position relative to the other. This increases the degree of freedom of the polyimide molecular chain, resulting in high flexibility, which is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, using diamine (B5) increases the thermoplasticity of the polyimide. Here, -O- is preferred as the linking group A.

[0075] Examples of diamines (B5) include 4-[3-[4-(4-aminophenoxy)phenoxy]phenoxy]aniline and 4,4'-[oxybis(3,1-phenyleneoxy)]bisaniline.

[0076] The diamine represented by the above 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 thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, the thermoplasticity of the polyimide is increased by using diamine (B6). Here, -C(CH3)2-, -O-, -SO2-, and -CO- are preferred as linking group A.

[0077] 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).

[0078] The diamine represented by formula (B7) (hereinafter sometimes referred to as "diamine (B7)") is an aromatic diamine having four benzene rings. Because this diamine (B7) has highly flexible divalent linking groups A on both sides of the diphenyl skeleton, it is thought to contribute to improving the flexibility of the polyimide molecular chain. Therefore, using diamine (B7) increases the thermoplasticity of the polyimide. Here, -O- is preferred as the linking group A.

[0079] Examples of diamines (B7) include bis[4-(3-aminophenoxy)]biphenyl and bis[4-(4-aminophenoxy)]biphenyl.

[0080] The adhesive polyimide may contain diamine residues derived from diamine compounds other than the dimer acid type diamine and diamines (B1) to (B7), to the extent that it does not impair the effects of the invention. The diamine residues derived from the dimer acid type diamine and diamine compounds other than diamines (B1) to (B7) can be any diamine compounds commonly used in thermoplastic polyimides, without limitation.

[0081] In adhesive polyimides, the coefficient of thermal expansion, tensile modulus, glass transition temperature, etc., can be controlled by selecting the types of tetracarboxylic acid residues and diamine residues, and the molar ratio of each when two or more tetracarboxylic acid residues or diamine residues are applied. Furthermore, in adhesive polyimides, if there are multiple polyimide structural units, they may exist as blocks or randomly, but random arrangement is preferable.

[0082] (Imide group concentration of adhesive polyimides) The imide group concentration of adhesive polyimides is preferably 20% or less. Here, "imide group concentration" refers to 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%, the molecular weight of the resin itself decreases, and the low hygroscopicity deteriorates due to the increase in polar groups, while the Tg and elastic modulus increase.

[0083] (Weight-average molecular weight of adhesive polyimides) The weight-average molecular weight of the adhesive polyimide is preferably 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 tends to decrease and it becomes prone to embrittlement. 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 2b tend to occur during the coating process.

[0084] (Imidification rate of adhesive polyimides) Adhesive polyimides are most preferably fully imidized. However, a portion of the polyimide may be amide acid. The imidization rate can be determined by measuring the infrared absorption spectrum of the polyimide thin film using a Fourier transform infrared spectrophotometer (commercially available: FT / IR620, manufactured by JASCO Corporation) by the single-reflection ATR method, where the imidization rate is 10¹⁵ cm⁻¹. -1 Using a nearby benzene ring absorber as a reference, 1780 cm -1 It can be calculated from the absorbance of the C=O stretching originating from the imide group.

[0085] (Crosslinking in adhesive polyimides) When an adhesive polyimide has a ketone group, a crosslinked structure can be formed by reacting the ketone group with the amino group of an amino compound having at least two primary amino groups as functional groups to form a C=N bond. The formation of a crosslinked structure can improve the heat resistance of the adhesive polyimide. Preferred tetracarboxylic anhydrides for forming adhesive polyimides having a ketone group include, for example, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), and examples of diamine compounds include aromatic diamines such as 4,4'-bis(3-aminophenoxy)benzophenone (BABP) and 1,3-bis[4-(3-aminophenoxy)benzoyl]benzene (BABB).

[0086] Examples of amino compounds that can be used for crosslinking adhesive polyimides include dihydrazide compounds, aromatic diamines, and aliphatic amines. Among these, dihydrazide compounds are preferred. Aliphatic amines other than dihydrazide compounds tend to form crosslinked structures even at room temperature, raising concerns about the storage stability of the varnish, while aromatic diamines require high temperatures to form crosslinked structures. Thus, using dihydrazide compounds allows for both storage stability of the varnish and a reduction in curing time. Examples of dihydrazide compounds include dihydrazide oxalate, dihydrazide malonic acid, dihydrazide succinate, dihydrazide glutarate, dihydrazide adipic acid, dihydrazide pimelic acid, dihydrazide suberic acid, dihydrazide azelaic acid, dihydrazide sebacate, dihydrazide dodecanediic acid, dihydrazide maleate, dihydrazide fumarate, and diglyceride. Dihydrazide compounds such as cholic acid dihydrazide, tartrate dihydrazide, malate dihydrazide, phthalate dihydrazide, isophthalate dihydrazide, terephthalate dihydrazide, 2,6-naphthoenioate dihydrazide, 4,4-bisbenzene dihydrazide, 1,4-naphthoate dihydrazide, 2,6-pyridinedioate dihydrazide, and itaconic acid dihydrazide are preferred. These dihydrazide compounds may be used individually or in combination of two or more.

[0087] Adhesive polyimides can be produced by reacting the above-mentioned tetracarboxylic dianhydride with a diamine compound in a solvent to produce polyamic acid, followed by heating and ring closure. For example, polyamic acid, a precursor of polyimide, can be obtained by dissolving the tetracarboxylic dianhydride and the diamine compound in approximately equimolar amounts in an organic solvent and allowing a polymerization reaction to occur by stirring at a temperature in the range of 0 to 100°C for 30 minutes to 24 hours. In the reaction, the reactants are dissolved in the organic solvent in a range of 5 to 50% by weight, preferably 10 to 40% by weight, to produce the precursor. Examples of organic solvents used in polymerization reactions include N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N,N-diethylacetamide, N-methyl-2-pyrrolidone (NMP), 2-butanone, dimethyl sulfoxide (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. There are no particular restrictions on the amount of such organic solvents used, but it is preferable to adjust the amount used so that the concentration of the polyamic acid solution obtained by the polymerization reaction is about 5 to 50% by weight.

[0088] The synthesized polyamic acid is usually advantageous to use as a reaction solvent solution, but it can be concentrated, diluted, or replaced with other organic solvents as needed. Furthermore, polyamic acid is advantageous to use because it generally has excellent solvent solubility. The viscosity of the polyamic acid solution is preferably in the range of 500 cps to 100,000 cps. If it falls outside this range, defects such as uneven thickness and streaks are more likely to occur in the film during coating operations, for example, with a coater.

[0089] The method for imidizing polyamic acid to form adhesive polyimide is not particularly limited, and a heat treatment such as heating in the solvent at a temperature in the range of 80 to 400°C for 1 to 24 hours is preferably employed.

[0090] When crosslinking the adhesive polyimide obtained as described above, the above amino compound is added to a resin solution containing the adhesive polyimide having a ketone group, and the ketone group in the adhesive polyimide and the primary amino group of the amino compound are subjected to a condensation reaction. By this condensation reaction, the resin solution is cured to form a cured product. In this case, the addition amount of the amino compound is such that the total amount of the 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 the ketone group. When the addition amount of the amino compound is less than 0.004 mol in total per 1 mol of the ketone group, the crosslinking of the adhesive polyimide by the amino compound is not sufficient, so the heat resistance tends not to be exhibited in the adhesive layer after curing. When the addition amount of the amino compound exceeds 1.5 mol, the unreacted amino compound acts as a plasticizer and tends to reduce the heat resistance of the adhesive layer 2a and the polyimide layer 1a.

[0091] The conditions for the condensation reaction for crosslink formation are not particularly limited as long as the ketone group in the adhesive polyimide and the primary amino group of the amino compound react to form an imine bond (C=N bond). The temperature for heat condensation is preferably in the range of 120 to 220 °C, more preferably in the range of 140 to 200 °C, for reasons such as releasing the water generated by condensation out of the system or simplifying the condensation step when the heat condensation reaction is subsequently carried out after the synthesis of the adhesive polyimide. The reaction time is preferably about 30 minutes to 24 hours. The end point of the reaction can be confirmed by measuring the infrared absorption spectrum using, for example, a Fourier transform infrared spectrophotometer (commercially available product: FT / IR620 manufactured by JASCO Corporation) and observing the decrease or disappearance of the absorption peak derived from the ketone group in the polyimide resin near 1670 cm -1 and the appearance of the absorption peak derived from the imine group near 1635 cm -1

[0092] ​The thermal condensation of the ketone group of the adhesive polyimide and the primary amino group of the amino compound can be carried out by, for example, (a) adding the amino compound and heating it following the synthesis (imidization) of the adhesive polyimide; (b) pre-charging an excess amount of the amino compound as a diamine component and then heating the adhesive polyimide together with the remaining amino compound that does not participate in imidization or amidation following the synthesis (imidization) of the adhesive polyimide; or (c) heating the adhesive polyimide composition to which the amino compound has been added after it has been processed into a predetermined shape (for example, after being applied to an arbitrary substrate or formed into a film).

[0093] While the formation of imine bonds through the formation of a crosslinked structure was described as a way to impart heat resistance to adhesive polyimides, the method is not limited to this. For example, adhesive polyimides can also be cured by incorporating epoxy resin, epoxy resin curing agents, etc.

[0094] By using the adhesive polyimide obtained as described above, the adhesive layer 2b and core layer 1b will have excellent flexibility and dielectric properties (low dielectric constant and low dielectric loss tangent). Furthermore, as will be described later, since the storage modulus of adhesive polyimide is sufficiently low in the temperature range of around 100°C, the bonding temperature can be significantly lowered compared to other adhesive resins such as fluororesins.

[0095] (Glass transition temperature (Tg) of adhesive polyimides) Adhesive polyimide preferably has a glass transition temperature (Tg) of 250°C or lower, and more preferably within the range of 40°C to 200°C. A Tg of 250°C or lower allows for thermocompression bonding at low temperatures, thereby mitigating internal stresses generated during lamination and suppressing dimensional changes after circuit processing. If the Tg of the adhesive polyimide exceeds 250°C, the temperature required for lamination onto other polyimide layers becomes high, potentially impairing dimensional stability after circuit processing.

[0096] (Storage modulus of adhesive polyimide) The adhesive polyimide satisfies the storage modulus of the thermoplastic polyimide defined in this specification, and further has a temperature range in which the storage modulus decreases steeply with the increase in temperature in the range of 40 to 250 °C. Such characteristics of the adhesive polyimide are considered to be factors for relaxing the internal stress during thermocompression bonding and maintaining the dimensional stability after circuit processing. The storage modulus of the adhesive polyimide at the upper limit temperature of the above temperature range is preferably 5×10 7 Pa or less, more preferably 1×10 5 ~5×10 7 Pa. By setting the storage modulus like this, even if it is the upper limit of the above temperature range, thermocompression bonding at 250 °C or less becomes possible, the adhesion can be ensured, and the dimensional change after circuit processing can be suppressed.

[0097] <Dielectric Dissipation Factor of the First Resin Layer and the Second Resin Layer> In the first resin layer 1 and the second resin layer 2, in order to suppress the deterioration of dielectric loss, the dielectric dissipation factor (Tanδ) at 10 GHz is preferably 0.02 or less, more preferably 0.01 or less, and still more preferably 0.008 or less. When the dielectric dissipation factor exceeds 0.02, when applied to a circuit board, inconveniences such as loss of electrical signals on the transmission path of high-frequency signals are likely to occur. The lower limit value of the dielectric dissipation factor at 10 GHz is not particularly limited, but it can be determined in consideration of the physical property control of the insulating resin layer of the circuit board.

[0098] <Relative Dielectric Constant of the First Resin Layer and the Second Resin Layer> In the first resin layer 1 and the second resin layer 2, as a whole, the relative dielectric constant at 10 GHz is preferably 4.0 or less, more preferably 3.5 or less, and still more preferably 3.2 or less. When the dielectric constant at 10 GHz exceeds 4.0, when applied to a circuit board, it leads to deterioration of dielectric loss, and inconveniences such as loss of electrical signals on the transmission path of high-frequency signals are likely to occur.

[0099] <Flexibility of the FPC against 90-degree bending> In this invention, the 90-degree bending resistance of an FPC can, in principle, be evaluated by repeatedly bending the FPC at 90 degrees and measuring the number of bends until the FPC breaks. However, in this invention, the 90-degree bending simulation of the FPC is performed on a conductor pattern set in the inner layer of the FPC model, and the maximum strain generated in the conductor pattern of the FPC model is calculated 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. A specific example of the simulation of the 90-degree bending resistance of an FPC will be described in the embodiments described later.

[0100] [Manufacturing of flexible circuit boards] The FPC of the present invention can be manufactured according to conventional methods. For example, two identical or different laminates of metal layer / polyimide layer are prepared by coating a polyimide resin composition onto a metal layer such as copper foil and drying it, and these are bonded together with a bonding sheet (BS) made of adhesive polyimide so that the polyimide layers face each other to form a double-sided CCL. A conductive pattern is formed by patterning the metal layer on one side of this double-sided CCL using conventional methods. Separately, a laminate consisting of a metal layer / polyimide insulating layer is prepared by coating and drying one or more polyamide solutions onto a metal layer such as copper foil, and a single-sided CCL is formed by bonding this polyimide insulating layer with a bonding sheet (BS) made of adhesive polyimide. Subsequently, the single-sided CCL is bonded from its BS side to the conductive pattern-forming surface of the double-sided CCL to manufacture an FPC as shown in Figure 1. Note that the FPC of the present invention can also be manufactured by methods other than those described above. [Examples]

[0101] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way by these examples. In the following examples and comparative examples, a simulation model of the FPC was created, and the simulation was performed using the finite element method (FEM) and analyzed using shell elements. This will be explained in detail below.

[0102] The abbreviations used in this example indicate the following compounds. BPDA: 3,3',4,4'-biphenyltetracarboxylic acid dianhydride PMDA: Pyromelit acid dianhydride m-TB: 2,2'-dimethyl-4,4'-diaminobiphenyl TPE-R: 1,3-bis(4-aminophenoxy)benzene Bisaniline-M:1,3-bis[2-(4-aminophenyl)-2-propyl]benzene NMP:N-methyl-2-pyrrolidone DMAc: N,N-dimethylacetamide BTDA:3,3',4,4'-benzophenonetetracarboxylic dianhydride DDA: A 36-carbon aliphatic diamine (manufactured by Croda Japan Co., Ltd., product name: PRIAMINE1074, amine value: 205 mg KOH / g, mixture of cyclic and chain-like dimer amines, dimer component content: 95% by weight or more) N-12: Dodecane dioxide dihydrazide OP935: Organic aluminum phosphinate salt (manufactured by Clariant Japan, product name: Exolit OP935)

[0103] (Synthesis Example 1) <Preparation of resin solution for adhesive layer> A 500 mL four-necked flask equipped with a nitrogen inlet tube, stirrer, thermocouple, Dean-Stark trap, and condenser was charged with 44.92 g of BTDA (0.139 mol), 75.08 g of DDA (0.141 mol), 168 g of NMP, and 112 g of xylene. The mixture was stirred at 40°C for 30 minutes to prepare a polyamic acid solution. This polyamic acid solution was heated to 190°C and stirred for 4 hours, and the distilled water and xylene were removed from the system. The mixture was then cooled to 100°C, 112 g of xylene was added and stirred, and the mixture was further cooled to 30°C to complete the imidation process and prepare polyimide solution 1 (solids content: 29.5% by weight, weight-average molecular weight: 75,700).

[0104] (Synthesis Example 2) <Preparation of polyamic acid solution for insulating resin layer> Under a nitrogen atmosphere, 64.20 g of m-TB (0.302 mol), 5.48 g of bisaniline-M (0.016 mol), and an amount of DMAc such that the solid content concentration after polymerization would be 15% by weight were added to the reaction vessel and stirred at room temperature to dissolve. Next, 34.20 g of PMDA (0.157 mol) and 46.13 g of BPDA (0.157 mol) were added, and the polymerization reaction was carried out by continuing to stir at room temperature for 3 hours to prepare polyamic acid solution 1 (viscosity: 26,500 cps).

[0105] (Synthesis Example 3) <Preparation of polyamic acid solution for insulating resin layer> Polyamic acid solution 2 (viscosity: 2,650 cps) was prepared in the same manner as in Synthesis Example 2, except that the raw material composition consisted of 69.56 g of m-TB (0.328 mol), 542.75 g of TPE-R (1.857 mol), DMAc in an amount that resulted in a solid content concentration of 12% by weight after polymerization, 194.39 g of PMDA (0.891 mol), and 393.31 g of BPDA (1.337 mol).

[0106] (Example 1) <Preparation of resin sheet for adhesive layer> Polyimide varnish 1 was prepared by adding 1.8 g of N-12 (0.0036 mol) and 12.5 g of OP935 to 169.49 g (50 g as solids) of polyimide solution 1, and then diluting it with 6.485 g of NMP and 19.345 g of xylene.

[0107] [Example 1] <Preparation of resin sheets> A resin sheet 1 was prepared by applying polyimide varnish 1 to the silicone-treated surface of a release substrate (length × width × thickness = 320 mm × 240 mm × 25 μm) so that the thickness after drying was 60 μm, then heating and drying at 80°C for 15 minutes, and peeling it off the release substrate. A resin sheet 2 with a thickness of 50 μm after drying was prepared using the same method.

[0108] <Preparation of single-sided metal-clad laminate> Polyamic acid solution 1 was uniformly applied to copper foil 1 (electrolytic copper foil, thickness: 12 μm, surface roughness Rz on the resin layer side: 0.6 μm) so that the thickness after curing would be approximately 2-3 μm, and then heated and dried at 120°C to remove the solvent. Next, polyamic acid solution 2 was uniformly applied on top so that the thickness after curing would be approximately 16 μm, and then heated and dried at 120°C to remove the solvent. Furthermore, polyamic acid solution 1 was uniformly applied on top so that the thickness after curing would be approximately 2-3 μm, and then heated and dried at 120°C to remove the solvent. In addition, stepwise heat treatment was performed from 120°C to 360°C to complete the imidation, and a single-sided metal-clad laminate 1 with a total polyimide thickness of 20 μm was prepared. A single-sided metal-clad laminate 2 with a total polyimide thickness of 50 μm was prepared using the same method.

[0109] <Preparation of double-sided metal-clad laminates> Two single-sided metal-clad laminates 1 were prepared, and the insulating resin layer side of each was placed on both sides of a resin sheet 1. They were then pressed together at 180°C for 2 hours under a pressure of 3.5 MPa to prepare a double-sided metal-clad laminate 1.

[0110] <Wiring pattern formation> A test specimen (test circuit board piece 1) was prepared using a double-sided metal-clad laminate 1. The metal foil on one side was etched to form 10 rows of copper wiring, each 40 mm long, with a line width of 100 μm and a space width of 100 μm along its longitudinal direction. As shown in Figure 13, which shows only the conductor wiring in the test specimen 40, the 10 rows of copper wiring 51 in the test specimen 40 are all continuously connected via U-shaped sections 52, and electrode sections (not shown) for resistance measurement are provided at both ends.

[0111] <Substrate formation simulating a stripline> Using a test circuit board piece 1, a resin sheet 2 (thickness: 50 μm) and a single-sided metal-clad laminate 2 (polyimide thickness: 50 μm) were superimposed on the surface on which the circuit pattern was formed, and the materials were pressed together at 180°C for 2 hours under a pressure of 3.5 MPa to prepare a test metal-clad laminate 1.

[0112] <90-degree bending resistance test> A 90-degree bending resistance test was performed using a test metal-clad laminate 1 according to the method described in Japanese Patent Application No. 2021-142359 for a bending resistance test apparatus and bending resistance test method. The bending test was repeated while constantly monitoring the resistance value of the wiring in the test metal-clad laminate 1. When the resistance reached a predetermined value (500Ω), it was determined that the wiring had broken, and the number of bending cycles up to that point was taken as the crimping measurement value. In addition, as setting parameters for this test, both ends of the test metal-clad laminate 1 were clamped with two pairs of clamps, and a tensile stress of 1,000gf was constantly applied between these clamps. The stress applied by pressing the pressure plate was also 1,000gf. The test metal-clad laminate 1 was subjected to 85 tests.

[0113] [Example 2] <Preparation of resin sheets> A resin sheet 3 was prepared by applying polyimide varnish 1 to the silicone-treated surface of a release substrate (length × width × thickness = 320 mm × 240 mm × 25 μm) so that the thickness after drying was 60 μm, then heating and drying at 80°C for 15 minutes, and peeling it off the release substrate. A resin sheet 4 with a thickness of 75 μm after drying was prepared using the same method.

[0114] <Preparation of single-sided metal-clad laminate> Polyamic acid solution 1 was uniformly applied to copper foil 1 (electrolytic copper foil, thickness: 12 μm, surface roughness Rz on the resin layer side: 0.6 μm) so that the thickness after curing would be approximately 2-3 μm, and then heated and dried at 120°C to remove the solvent. Next, polyamic acid solution 2 was uniformly applied on top so that the thickness after curing would be approximately 16 μm, and then heated and dried at 120°C to remove the solvent. Furthermore, polyamic acid solution 1 was uniformly applied on top so that the thickness after curing would be approximately 2-3 μm, and then heated and dried at 120°C to remove the solvent. Furthermore, stepwise heat treatment was performed from 120°C to 360°C to complete the imidation, and a single-sided metal-clad laminate 3 with a total polyimide thickness of 20 μm was prepared. A single-sided metal-clad laminate 4 with a total polyimide thickness of 25 μm was prepared using the same method.

[0115] <Preparation of double-sided metal-clad laminates> Two single-sided metal-clad laminates 3 were prepared, and the insulating resin layer side of each was placed on both sides of a resin sheet 3. They were then pressed together at 180°C for 2 hours under a pressure of 3.5 MPa to prepare a double-sided metal-clad laminate 2.

[0116] <Wiring pattern formation> A test specimen (test circuit board piece 2) was prepared using a double-sided metal-clad laminate 2, by etching the metal foil on one side to form 10 rows of copper wiring with a line width of 100 μm and a space width of 100 μm, each 40 mm long, along its longitudinal direction. As shown in Figure 13, which shows only the conductor wiring diagram of this test specimen, the 10 rows of copper wiring 51 in the test specimen 40 are all continuously connected via U-shaped sections 52, and electrode sections (not shown) for resistance measurement are provided at both ends.

[0117] <Substrate formation simulating a stripline> Using a test circuit board piece 2, a resin sheet 4 (thickness: 75 μm) and a single-sided metal-clad laminate 4 (polyimide thickness: 25 μm) were superimposed on the surface on which the circuit pattern was formed, and the materials were pressed together at 180°C for 2 hours under a pressure of 3.5 MPa to prepare a test metal-clad laminate 2.

[0118] <90-degree bending resistance test> A 90-degree bending resistance test was performed using a test circuit board piece 2 according to the method described in Japanese Patent Application No. 2021-142359. The bending test was repeated while constantly monitoring the resistance value of the wiring on the test metal-clad laminate 1. Breakage of the wiring was determined when the resistance reached a predetermined value (500Ω), and the number of bending cycles up to that point was taken as the crimping measurement value. In addition, as setting parameters for this test, both ends of the test metal-clad laminate 1 were clamped with two pairs of clamps, and a tensile stress of 1,000gf was constantly applied between the clamps. The stress applied by pressing the pressure plate was also 1,000gf. The test metal-clad laminate 1 was tested 110 times.

[0119] In both Examples 1 and 2, by optimizing the thickness configuration, it was confirmed that the test piece had sufficient bending performance. Therefore, an analysis by simulation using a finite element analysis tool was performed regarding this result. In the test model, not only Examples 1 and 2, but also calculations for Examples 3 and 4 and Comparative Example 1 in which the ratios of the core layer and the first and second polyimide layers in the first resin layer were changed were performed.

[0120] <90 - degree bending resistance of FPC> In the present invention, the 90 - degree bending resistance of an FPC can, in principle, be evaluated by repeating the 90 - degree bending of the FPC and measuring the number of bending times until the FPC breaks according to the content described in the bending resistance test apparatus and the bending resistance test method (Japanese Patent Application No. 2021 - 142359). However, in the present invention, regarding the conductor pattern set in the inner layer of the FPC model, a 90 - degree bending simulation of the FPC is performed, and it can be evaluated by 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 non - linear analysis solver Marc). It can be said that the lower the maximum strain, the better the 90 - degree bending resistance. In the examples described later, a specific example of the simulation of the 90 - degree bending resistance of the FPC will be explained.

[0121] [Mechanical simulation in the elastic - plastic region] In the finite element analysis tool, a finite element analysis model simulating the 90 - degree bending test of the FPC was constructed, and non - linear analysis was performed using the Young's modulus of each material and the plastic strain - stress relationship obtained from the stress - strain relationship of each material obtained from a uniaxial tensile test, and it was evaluated by calculating the maximum plastic strain generated in the conductor pattern of the FPC model.

[0122] The finite element analysis model consists of a material model simulating the FPC, a rigid body for sandwiching and fixing the FPC, a circular rigid body (φ1 mm) for bending the FPC, and a plate-shaped rigid body for pressing against the FPC after bending (Figure 3). The FPC is positioned relative to these rigid bodies as shown in Figure 4. In the calculation, the rigid body sandwiching and fixing the FPC was rotated 90 degrees to find the solution for the strain field generated in the FPC after it has come into contact with a circular rigid body with a constant curvature and been bent 90 degrees (Figure 5: rotated 45 degrees, Figure 6: rotated 90 degrees), and then the pressing plate has moved and pressed against it (Figure 7). In this calculation, a tensile stress of 1,000 gf was always applied to both ends of the FPC, and the stress applied by the pressing plate was also 1,000 gf.

[0123] In the material model simulating a stripline FPC structure, the dimensional elements were as follows, as shown in Figure 2: the height of the signal copper wiring was 12 μm, the height of the ground copper wiring on both the top and bottom sides was 12 μm each, and the signal wiring was located in the center of the FPC's width direction. In this model, the width direction of the wiring was not specified and calculations were performed assuming an infinitely distant structure.

[0124] Figure 8 shows the simulation flow applied to this embodiment. This flow includes at least the following steps in this order: creating a simulation model, inputting material property data, performing a 90-degree bending simulation, and calculating the maximum strain occurring in the signal copper wiring.

[0125] In the "Simulation Model Creation Step," an FPC model to be applied to the simulation and a model of the equipment used for bending the FPC at 90 degrees (FPC clamping jig, FPC bending plate, etc.) are created. In the "Material Property Data Input Step," material property values ​​for each model necessary for the simulation calculation, such as elastic modulus, yield stress, strain-stress curve, etc., are input into the simulation software. In the "90-Degree Bending Simulation Execution Step," as shown in Figures 3 to 7, the FPC is bent at 90 degrees using the actual simulation software. As a result of this execution, in the subsequent "Maximum Strain Calculation Step for 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.

[0126] In the simulation, the calculation model was constructed using the mechanical properties of the resin sheets prepared in Examples 1 and 2, and the polyimide film in the single-sided metal-clad laminates. Resin sheets 1-4 obtained in Examples 1 and 2 were designated as Type A, and the polyimide film obtained by etching off copper foil 1 from single-sided metal-clad laminates 1-4 was designated as Type B. The mechanical properties were calculated based on measured values ​​from tensile tests. Type A is an adhesive-type resin film with a relatively low modulus of elasticity, while Type B is a high-modulus, high-rigidity resin film. The data used for inputting modulus of elasticity, yield stress, strain-stress curves, etc., into the simulation software was extracted from Figures 10 and 11 for Type A, Figure 9 for Type B, and Figure 12 for copper foil 1.

[0127] [Configuration of the simulated FPC in Example 1] The simulated FPC of Example 1 has the structure shown in Figure 1, and the thickness and type of each constituent layer are as follows. Layer name, code, thickness [μm], type Second ground layer G2 12 copper foil 1 Polyimide insulating layer 2a 50 Type B derived from single-sided metal-clad laminate Adhesive layer 2b 50 Type A Derived from resin sheet Conductor pattern S 12 copper foil 1 Second polyimide layer 1c 20 Type B Derived from single-sided metal-clad laminate Core layer 1b 60 Type A Resin sheet derived First polyimide layer 1a 20 Type B Derived from single-sided metal-clad laminate First ground layer G1 12 copper foil 1

[0128] [Configuration of the simulated FPC in Example 2] The simulated FPC of Example 2 has the structure shown in Figure 1, and the thickness and type of each constituent layer are as follows. Layer name, code, thickness [μm], type Second ground layer G2 12 copper foil 1 Polyimide insulating layer 2a 25 Type B Derived from single-sided metal-clad laminate Adhesive layer 2b 75 Type A Derived from resin sheet Conductor pattern S 12 copper foil 1 Second polyimide layer 1c 20 Type B Derived from single-sided metal-clad laminate Core layer 1b 60 Type A Resin sheet derived First polyimide layer 1a 20 Type B Derived from single-sided metal-clad laminate First ground layer G1 12 copper foil 1

[0129] [Configuration of the simulated FPC in Example 3] The simulated FPC of Example 3 has the structure shown in Figure 1, and the thickness and type of each constituent layer are as follows. Layer name, code, thickness [μm], type Second ground layer G2 12 copper foil 1 Polyimide insulating layer 2a 50 Type B derived from single-sided metal-clad laminate Adhesive layer 2b 50 Type A Derived from resin sheet Conductor pattern S 12 copper foil 1 Second polyimide layer 1c 12 Type B Derived from single-sided metal-clad laminate Core layer 1b 75 Type A Resin sheet derived First polyimide layer 1a 12 Type B Derived from a single-sided metal-clad laminate First ground layer G1 12 copper foil 1

[0130] [Configuration of the simulated FPC in Example 4] The simulated FPC of Example 4 has the structure shown in Figure 1, and the thickness and type of each constituent layer are as follows. Layer name, code, thickness [μm], type Second ground layer G2 12 copper foil 1 Polyimide insulating layer 2a 50 Type B derived from single-sided metal-clad laminate Adhesive layer 2b 50 Type A Derived from resin sheet Conductor pattern S 12 copper foil 1 Second polyimide layer 1c 25 Type B Derived from single-sided metal-clad laminate Core layer 1b 50 Type A Resin sheet derived First polyimide layer 1a 25 Type B Derived from a single-sided metal-clad laminate First ground layer G1 12 copper foil 1

[0131] [Configuration of the simulated FPC in Comparative Example 1] The simulated FPC of Comparative Example 1 has the structure shown in Figure 1, and the thickness and type of each constituent layer are as follows. Layer name, code, thickness [μm], type Second ground layer G2 12 copper foil 1 Polyimide insulating layer 2a 50 Type B derived from single-sided metal-clad laminate Adhesive layer 2b 50 Type A Derived from resin sheet Conductor pattern S 12 copper foil 1 Second polyimide layer 1c 38 Type B Derived from single-sided metal-clad laminate Core layer 1b 25 Type A Derived from resin sheet First polyimide layer 1a 38 Type B Derived from a single-sided metal-clad laminate First ground layer G1 12 copper foil 1

[0132] [Measurement of stress-strain curves] The tensile modulus required for the simulation calculation was measured using a Strograph R-1 manufactured by Toyo Seiki Seisakusho Co., Ltd., under conditions of 23°C and 50% relative humidity. The sample used for measurement was MD: 250 mm × TD: 12.7 mm, and the measurement was performed under the following conditions: load cell: 500 N, tensile speed: 50 mm / min, and chuck distance: 50 mm.

[0133] [Calculation of average tensile modulus] The tensile modulus of each single layer of insulator is measured, and the average tensile modulus is calculated using the thickness ratio of the resin laminate composed of dissimilar insulators. In the range of 5 μm, if n types of dissimilar insulators are present, the elastic modulus M1 [GPa] and thickness T1 [μm] are calculated as follows: elastic modulus M2 [GPa] and thickness T2 [μm], and further, the elastic modulus M n [GPa], Thickness T n Using the value in [μm], the average modulus of elasticity is calculated using the following formula.

[0134]

number

[0135] Examples 1-4, Comparative Example 1 Table 1 shows the tensile modulus and yield stress for each layer constituting the FPC shown in Figure 1, along with the layer thickness mentioned earlier. The tensile modulus [GPa] and yield stress [MPa] values ​​entered into the simulation software were obtained from the stress-strain curves in Figures 8-11.

[0136] [Table 1]

[0137] <List of results for examples and comparative examples> The results obtained in Examples 1-4 and Comparative Example 1 are summarized in Table 2.

[0138] [Table 2]

[0139] <Summary> Comparing the FPCs of Examples 1-4 with the FPC of Comparative Example 1, the ratio of the core layer thickness to the first polyimide layer thickness (L1b / L1) was less than 0.4. Therefore, compared to the FPCs of Examples 1-4, where this ratio was in the range of 0.4 to 0.8 or less, the absolute value of the maximum strain (compression mode) was larger, indicating a decrease in 90-degree bending resistance.

[0140] Although embodiments of the present invention have been described in detail above for illustrative purposes, the present invention is not limited to the above embodiments and various modifications are possible. [Explanation of Symbols]

[0141] 1 1st resin layer 1a First polyimide layer 1b Core Layer 1c Second polyimide layer 2 Second resin layer 2a Polyimide insulating layer 2a1, 2a2 Polyimide Layers 2b Adhesive layer 10 Flexible circuit boards G1 First Grand Layer G2 Second Grand Layer S Conductor Pattern

Claims

1. A flexible circuit board housed within the casing of an electronic device and capable of being bent at approximately 90 degrees, A conductive pattern formed on one side of the first resin layer, A first ground layer laminated on the other side of the first resin layer, A second resin layer laminated on the conductor pattern forming surface side of the first resin layer, The second resin layer comprises a second gland layer laminated on the side of the second resin layer opposite to the first resin layer, The first resin layer has a structure in which a first polyimide layer, a core layer, and a second polyimide layer are laminated in this order from the first ground layer side. The thickness of the core layer is greater than 0.4 and less than or equal to 0.8 relative to the thickness of the first resin layer. The tensile modulus of the core layer is in the range of 0.5 GPa or more and 2 GPa or less. The tensile modulus of the first polyimide layer and the second polyimide layer are independently within the range of 5 GPa to 9 GPa. The thickness of the first resin layer (L1) and the thickness of the second resin layer (L2) are each independently within the range of 50 μm or more and 150 μm or less, and A flexible circuit board characterized in that the ratio (L2 / L1) of the thickness of the second resin layer to the thickness (L1) of the first resin layer is within the range of 0.8 to 1.

2.

2. The flexible circuit board according to claim 1, wherein the tensile modulus of the first resin layer (M1) and the tensile modulus of the second resin layer (M2) are each independently within the range of 0.9 GPa or more and 7 GPa or less, and the ratio of the tensile modulus of the second resin layer (M2 / M1) to the tensile modulus of the first resin layer (M1) is within the range of 0.7 or more and 1.8 or less.

3. The second resin layer comprises a polyimide insulating layer including one or more polyimide layers, and an adhesive layer. The adhesive layer is located between the polyimide insulating layer and the conductive pattern forming surface of the first resin layer. The flexible circuit board according to claim 1 or 2, wherein the tensile modulus of the adhesive layer is in the range of 0.2 GPa or more and 2 GPa or less.

4. A flexible circuit board according to claim 1 or 2, which can be bent at approximately 90 degrees so that the second ground layer faces outward.

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