Flexible multilayer circuit board

Abstract

This flexible multilayer circuit board comprises an insulating layer, a first layer disposed on one side, in a thickness direction, of the insulating layer, a second layer disposed on the other side, in the thickness direction, of the insulating layer, and a wiring portion, wherein: the first layer includes a first ground conductor portion in which a plurality of first conductor pads are arranged in a pattern, and a first electromagnetic wave shielding layer; the second layer includes a second ground conductor portion in which a plurality of second conductor pads are arranged in a pattern; the plurality of first conductor pads are embedded in the first electromagnetic wave shielding layer; and when the first ground conductor portion and the second ground conductor portion are projected onto a plane orthogonal to the thickness direction, the patterns of the first ground conductor portion and the second ground conductor portion overlap.

Description

Flexible Multilayer Circuit Board

[0001] The present invention relates to a flexible multilayer circuit board.

[0002] One example of a wired circuit board is an electromagnetically shielded flexible circuit board, which is formed by electrically connecting a conductive shielding layer formed on an electromagnetic wave shielding material and a ground circuit formed on a flexible circuit board via a conduction hole formed in an insulating material of the flexible circuit board. The electromagnetically shielded flexible circuit board has a conduction hole with a diameter of 2 mm or less in at least one of an insulating coating layer and an insulating base material layer on the ground circuit formed on the flexible circuit board, and has a conductive metal pillar inside the conduction hole, so that the conductive shielding layer is electrically connected to the ground circuit via the conductive metal pillar (see Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2005-109101

[0004] In recent years, with the miniaturization of electronic devices, wired circuit boards are sometimes bent and arranged within the electronic devices. Furthermore, in foldable phones with foldable displays, wired circuit boards are sometimes arranged in the bendable portion (hinge portion). In such cases, the wired circuit board is required to have flexibility (bending resistance) so that it will not break even when bent.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a flexible multilayer circuit board having good bending resistance.

[0006] The present inventors have conducted extensive research to solve the above problems, and as a result have found that the above problems can be solved, and have completed the present invention having the following gist.

[0007] That is, the present invention includes the following: [1] A flexible multilayer circuit board including an insulating layer, a first layer disposed on one side of the insulating layer in a thickness direction, a second layer disposed on the other side of the insulating layer in the thickness direction, and a wiring portion, wherein the first layer has a first ground conductor portion including a plurality of first conductor pads arranged in a pattern and a first electromagnetic wave shielding layer, the second layer has a second ground conductor portion including a plurality of second conductor pads arranged in a pattern, the plurality of first conductor pads are embedded in the first electromagnetic wave shielding layer, and when the first ground conductor portion and the second ground conductor portion are projected onto a plane orthogonal to the thickness direction, the patterns of the first ground conductor portion and the second ground conductor portion overlap. [2] The flexible multilayer circuit board according to [1], wherein the wiring portion is embedded in the insulating layer. [3] The flexible multilayer circuit board according to [1] or [2], wherein the second layer further includes a second electromagnetic wave shielding layer, and the plurality of second conductor pads are embedded in the second electromagnetic wave shielding layer. [4] The flexible multilayer circuit board according to any one of [1] to [3], wherein the insulating layer includes a porous insulating layer. [5] The flexible multilayer circuit board according to any one of [1] to [4], wherein the insulating layer has a tensile modulus of elasticity of 2.0 GPa or less. [6] The flexible multilayer circuit board according to any one of [1] to [5], wherein the surface roughness of the surface of the first conductor pad of the first ground conductor that contacts the first electromagnetic wave shielding layer is greater than the surface roughness of the wiring portion.

[0008] According to the present invention, a flexible multilayer circuit board having good bending resistance can be provided.

[0009] FIG. 1 is a schematic cross-sectional view of an example of a flexible multilayer circuit board. FIG. 2A is a schematic cross-sectional view (part 1) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2B is a schematic cross-sectional view (part 2) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2C is a schematic cross-sectional view (part 3) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2D is a schematic cross-sectional view (part 4) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2E is a schematic cross-sectional view (part 5) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2F is a schematic cross-sectional view (part 6) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2G is a schematic cross-sectional view (part 7) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2H is a schematic cross-sectional view (part 8) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2I is a schematic cross-sectional view (part 9) illustrating an example of a method for manufacturing the flexible multilayer circuit board of FIG. 1. FIG. 2J is a cross-sectional view taken along the line A-A' in FIG. 2I. FIG. 2K is a cross-sectional view taken along the line B-B' in FIG. 2I. Fig. 3 is a schematic cross-sectional view of another example of a flexible multilayer circuit board. Fig. 4 is a schematic cross-sectional view of another example of a flexible multilayer circuit board. Fig. 5 is a schematic cross-sectional view of another example of a flexible multilayer circuit board. Fig. 6 is a schematic cross-sectional view of another example of a flexible multilayer circuit board.

[0010] (Flexible multilayer circuit board) A flexible multilayer circuit board of the present invention includes an insulating layer, a first layer, a second layer, and a wiring portion. The first layer is disposed on one side of the insulating layer in the thickness direction. The second layer is disposed on the other side of the insulating layer in the thickness direction. The first layer has a first ground conductor portion formed by a plurality of first conductor pads arranged in a pattern. The first layer further has a first electromagnetic wave shielding layer. The second layer has a second ground conductor portion formed by a plurality of second conductor pads arranged in a pattern. The plurality of first conductor pads are embedded in the first electromagnetic wave shielding layer. When the first ground conductor portion and the second ground conductor portion are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor portion and the second ground conductor portion overlap.

[0011] The electromagnetic wave shielding layer has better flexibility than a conductor layer (e.g., copper foil). In the flexible multilayer circuit board of the present invention, instead of using two conductor layers (e.g., copper foil) as the two layers sandwiching the insulating layer, the first layer has a first electromagnetic wave shielding layer in which a first ground conductor having a plurality of first conductor pads arranged in a pattern is embedded, and the second layer has a second ground conductor having a plurality of second conductor pads arranged in a pattern. This makes it possible to obtain a flexible multilayer circuit board with better flexibility and bending resistance than a conductor layer (e.g., copper foil).

[0012] The wiring portion is, for example, a signal line, and is, for example, buried in an insulating layer.

[0013] The second layer may further include a second electromagnetic wave shielding layer, and the second conductive pads may be embedded in the second electromagnetic wave shielding layer. The inclusion of the second electromagnetic wave shielding layer improves the shielding performance for the wiring portion.

[0014] The insulating layer may have a single layer structure or a multi-layer structure. The insulating layer preferably has a porous insulating layer, in that the electrical properties of the wiring portion are excellent.

[0015] The insulating layer preferably has a tensile modulus of 2.0 GPa or less. This allows for a flexible multilayer circuit board with better bending resistance to be obtained. The lower limit of the tensile modulus is not particularly limited, but for example, the tensile modulus is 0.1 GPa or more. The tensile modulus can be measured, for example, by the following method. A 100 μm thick film (insulating layer) is cut to a size of 100 mm length and 25 mm width, and pulled using a precision universal testing machine (manufactured by Shimadzu Corporation, device name "Autograph AG-IS") at a chuck distance of 50 mm and a pulling speed of 300 mm / min, and the change in stress until the film undergoes plastic deformation is recorded to obtain a stress-strain curve. The tensile modulus is determined by the strain ε at two specified points. １ = 1 and ε ２The tensile modulus is determined by linear regression of the curve between σ = 2. The above measurement is performed using three test pieces cut out from different locations, and the average value of the measurements is taken as the tensile modulus. The above measurement is performed in accordance with JIS K 7161 at 23°C and 50% RH.

[0016] In the flexible multilayer circuit board, when the first ground conductor and the second ground conductor are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor and the second ground conductor overlap. In other words, the first conductor pads of the first ground conductor and the second conductor pads of the second ground conductor are arranged opposite each other. This reduces misalignment of electrical coupling. Regarding the overlap of the patterns of the first ground conductor and the second ground conductor, it is sufficient for the patterns to overlap partially. An overlap of 50% or more of the pattern area is preferred, an overlap of 75% or more is more preferred, and an overlap of 90% or more is particularly preferred. Note that if the patterns do not overlap at all, the overlap is 0%, and if they overlap completely, the overlap is 100%. A 50% overlap means that 50% of the area of ​​the pattern of the first ground conductor overlaps with the pattern of the second ground conductor.

[0017] One of the first conductive pads and one of the second conductive pads, which are arranged opposite to each other, are electrically connected by, for example, a conductive via that penetrates the insulating layer.

[0018] The surface roughness of the first conductor pad of the first ground conductor that contacts the first electromagnetic shielding layer is preferably rougher than the surface roughness of the wiring portion. The roughness of the surface of the first conductor pad of the first ground conductor that contacts the first electromagnetic shielding layer improves adhesion between the first electromagnetic shielding layer and the first conductor pad, resulting in better bending resistance. The surface roughness (arithmetic mean roughness: Ra) of the wiring portion is preferably 0.15 μm or less. The surface roughness (arithmetic mean roughness: Ra) of the surface of the first conductor pad of the first ground conductor that contacts the first electromagnetic shielding layer is preferably greater than 0.15 μm. The surface roughness of the wiring portion and the surface roughness of the first conductor pad of the first ground conductor that contacts the first electromagnetic shielding layer can be changed, for example, by roughening treatment. Examples of roughening treatment include etching, plating, and polishing.

[0019] <Wiring portion> The material of the wiring portion is not particularly limited, and examples thereof include copper, iron, silver, gold, aluminum, nickel, and alloys thereof (e.g., stainless steel, bronze). Copper is preferred. The width of the wiring portion is not particularly limited, and is, for example, 0.05 mm or more, preferably 0.1 mm or more, and for example, 3 mm or less, preferably 2 mm or less. The width of the wiring portion corresponds to the length in the direction (width direction) perpendicular to the thickness direction of the insulating layer and the length direction of the wiring portion. The thickness of the wiring portion is not particularly limited, and is, for example, 5 μm or more, preferably 10 μm or more, and for example, 100 μm or less, preferably 80 μm or less. The thickness of the wiring portion corresponds to the length in the thickness direction of the insulating layer.

[0020] <Insulating Layer> Examples of materials for the insulating layer include resins. The type of resin is not limited. Examples of resins include polycarbonate resins, polyimide resins, fluorinated polyimide resins, epoxy resins, phenolic resins, urea resins, melamine resins, diallyl phthalate resins, silicone resins, thermosetting urethane resins, fluororesins, and liquid crystal polymers. Preferred examples include polyimide resins and liquid crystal polymers.

[0021] The insulating layer may have a single layer structure or a multi-layer structure. The insulating layer preferably has a porous insulating layer, in that the electrical properties of the wiring portion are excellent.

[0022] The porous insulating layer is porous. The porous insulating layer has closed cells and / or open cells. The porosity of the porous insulating layer is, for example, 50% or more, preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. The porosity of the porous insulating layer is, for example, less than 100%, or even 99% or less. When the material of the porous insulating layer is a polyimide resin, the porosity of the porous insulating layer can be calculated based on the following formula.

[0023] Relative permittivity of porous insulating layer = relative permittivity of air × porosity + relative permittivity of polyimide × (1 - porosity) Here, the relative permittivity of air is 1, and the relative permittivity of polyimide resin is 3.5, so Relative permittivity of porous insulating layer = porosity + 3.5 (1 - porosity) Porosity (%) = [(3.5 - relative permittivity of porous insulating layer) / 2.5] × 100

[0024] The porous insulating layer has a relative dielectric constant at a frequency of 60 GHz of, for example, 2.5 or less, preferably 1.9 or less, more preferably 1.6 or less, and for example, greater than 1.0. The relative dielectric constant of the porous insulating layer is actually measured by a resonator method using a frequency of 60 GHz.

[0025] The dielectric loss tangent of the porous insulating layer at a frequency of 60 GHz is, for example, not more than 0.006, and is, for example, greater than 0. The dielectric loss tangent of the porous insulating layer is actually measured by a resonator method using a frequency of 60 GHz.

[0026] The insulating layer may have an adhesive layer. The material of the adhesive layer is not particularly limited, and examples thereof include an acrylic adhesive, an epoxy adhesive, and a silicone adhesive.

[0027] The thickness of the insulating layer is not particularly limited, and is, for example, 5 μm or more, preferably 10 μm or more, and is, for example, 100 μm or less, preferably 80 μm or less.

[0028] <First Conductive Pads and Second Conductive Pads> The material of the first conductive pads in the first ground conductor is not particularly limited and may be, for example, copper, iron, silver, gold, aluminum, nickel, or an alloy thereof (for example, stainless steel or bronze). Copper is preferred. The material of the second conductive pads in the second ground conductor is not particularly limited and may be, for example, copper, iron, silver, gold, aluminum, nickel, or an alloy thereof (for example, stainless steel or bronze). Copper is preferred.

[0029] The first and second contact pads may have a single-layer structure or a multi-layer structure.

[0030] For example, when the wiring portion and the first conductor pads are projected onto a plane perpendicular to the thickness direction, the plurality of first conductor pads form two rows aligned in the longitudinal direction of the wiring portion, with the wiring portion sandwiched between the two rows in the width direction, and the first conductor pads in each of the two rows are aligned at a predetermined interval. The predetermined interval is not particularly limited, but may be, for example, 0.5 mm to 3.0 mm as the distance between the centers of two adjacent first conductor pads.

[0031] For example, when the wiring portion and the second conductor pads are projected onto a plane perpendicular to the thickness direction, the second conductor pads form two rows aligned in the longitudinal direction of the wiring portion, with the wiring portion sandwiched between the two rows in the width direction, and the second conductor pads in each of the two rows are aligned at a predetermined interval. The predetermined interval is not particularly limited, but may be, for example, 0.5 mm to 3.0 mm as the distance between the centers of two adjacent second conductor pads.

[0032] The first and second conductor pads are, for example, circular when viewed in the thickness direction.

[0033] <First Electromagnetic Shielding Layer and Second Electromagnetic Shielding Layer> The electromagnetic shielding layers (first electromagnetic shielding layer and second electromagnetic shielding layer) may each have a single-layer structure or a multi-layer structure. The electromagnetic shielding layer has, for example, a laminated structure including a conductive adhesive layer and a metal layer. The laminated structure may further include a protective layer.

[0034] <<Conductive Adhesive Layer>> The conductive adhesive layer has adhesiveness and conductivity. Examples of the conductive adhesive layer include the conductive adhesive layer described in JP 2021-028985 A.

[0035] The thickness of the conductive adhesive layer is not particularly limited, but is preferably 3 μm to 20 μm, and more preferably 5 μm to 15 μm.

[0036] <<Metal Layer>> The metal layer may have a single-layer structure or a multi-layer structure. Examples of metals constituting the metal layer include gold, silver, copper, aluminum, nickel, tin, palladium, chromium, titanium, zinc, and alloys thereof. Among these, a copper layer and a silver layer are preferred, and a silver layer is more preferred, from the viewpoint of excellent shielding performance against high-frequency electromagnetic waves.

[0037] The thickness of the metal layer is not particularly limited, but is preferably 0.05 to 1 μm, and more preferably 0.1 to 0.5 μm.

[0038] The metal layer can be formed by a method such as vapor deposition or sputtering.

[0039] <<Protective Layer>> The protective layer has the function of protecting the electromagnetic wave shielding layer. Examples of the protective layer include an insulating resin layer. The protective layer may have a single-layer structure or a multi-layer structure. Examples of the protective layer include the insulating layer described in JP 2021-028985 A. The thickness of the insulating layer is not particularly limited, but is preferably 1 μm to 15 μm, and more preferably 3 μm to 10 μm.

[0040] <Conductive via> The material of the conductive via that electrically connects one of the first conductive pads and one of the second conductive pads that are arranged opposite each other is not particularly limited, and examples thereof include copper, iron, silver, gold, aluminum, nickel, and alloys thereof (e.g., stainless steel, bronze), etc. Copper is preferred.

[0041] The flexible multilayer circuit board may have an adhesive layer, an insulating cover layer, etc. The insulating cover layer covers, for example, the second ground conductor. The insulating cover layer may cover, for example, the second ground conductor via an adhesive layer. Examples of materials for the insulating cover layer include the materials for the insulating layer described above.

[0042] The material of the adhesive layer is not particularly limited, and various types of adhesives such as hot melt adhesives and thermosetting adhesives can be used. Specific examples include acrylic adhesives, epoxy adhesives, and silicone adhesives. Acrylic adhesives are preferred. The thickness of the adhesive layer is, for example, 2 μm or more, preferably 5 μm or more, and, for example, 50 μm or less, preferably 25 μm or less.

[0043] An embodiment of a flexible multilayer circuit board of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram (cross-sectional view) of an embodiment of a flexible multilayer circuit board of the present invention. The flexible multilayer circuit board includes an insulating layer 1, a first layer 2, a second layer 3, and a wiring portion 4. The first layer 2 is disposed on one thickness-wise side of the insulating layer 1. The second layer 3 is disposed on the other thickness-wise side of the insulating layer 1. The first layer 2 includes a first ground conductor portion having a plurality of first conductor pads 2a arranged in a pattern. The first layer 2 also includes a first electromagnetic wave shielding layer 2b. The second layer 3 includes a second ground conductor portion having a plurality of second conductor pads 3a arranged in a pattern. The plurality of first conductor pads 2a are embedded in the first electromagnetic wave shielding layer 2b. When the first ground conductor portion and the second ground conductor portion are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor portion and the second ground conductor portion overlap.

[0044] Furthermore, in the flexible multilayer circuit board shown in Fig. 1, the insulating layer 1 has a first resin layer 1a, an adhesive layer 1b, and a second resin layer 1c, in this order. The wiring portion 4 is embedded in the insulating layer 1. More specifically, the wiring portion 4 is embedded in the adhesive layer 1b. Furthermore, the second layer 3 has a second electromagnetic wave shielding layer 3b. A plurality of second conductor pads 3a are embedded in the second electromagnetic wave shielding layer 3b. The first conductor pad 2a and the second conductor pad 3a are electrically connected by conductor vias 5 that penetrate the insulating layer 1. The wiring portion 4 is not in contact with the plurality of first conductor pads 2a, the plurality of second conductor pads 3a, or the conductor vias 5.

[0045] Next, an example of a method for manufacturing the flexible multilayer circuit board shown in FIG. 1 will be described using the drawings. FIGS. 2A to 2I are diagrams illustrating one embodiment of a method for manufacturing the flexible multilayer circuit board shown in FIG. 1. First, a laminate is prepared in which a second conductor layer 3A, a second resin layer 1c, an adhesive layer 1b-1, and a conductor layer 4A for forming a wiring portion are laminated in this order ( FIG. 2A ). Examples of materials for the second conductor layer 3A include the same material as the conductor used to form the conductor vias 5. Copper is a preferred example of such a material. The second resin layer 1c may be, for example, a porous resin layer or a laminate structure of a porous resin layer and a non-porous resin layer. The non-porous resin layer may be, for example, a skin layer. Next, the conductor layer 4A for forming a wiring portion is patterned by photolithography (e.g., a subtractive method) using a photoresist to form the wiring portion 4 and the conductor 5a ( FIG. 2B ). The wiring portion 4 is formed to extend in a direction perpendicular to the cross section (longitudinal direction). The conductor 5a is formed in a circular shape so as to become part of the conductor via 5 in a later process. Next, a laminate is prepared in which a first conductor layer 2A, a first resin layer 1a, and an adhesive layer 1b-2 are laminated in this order ( FIG. 2C ). Examples of the material for the first conductor layer 2A include the same material as the material of the conductor used to form the conductor via 5. Copper is a preferred example of such a material. Furthermore, the first resin layer 1a may be, for example, a porous resin layer, or a laminate structure of a porous resin layer and a non-porous resin layer. The non-porous resin layer may be, for example, a skin layer. Next, the laminate of FIG. 2B and the laminate prepared in FIG. 2C are arranged so that the adhesive layer 1b-1 and the adhesive layer 1b-2 face each other, and the two adhesive layers are bonded together ( FIG. 2D ). By doing so, the two adhesive layers are integrated to form adhesive layer 1b, and a laminate is obtained having, from the other side of the thickness direction to one side, the second conductor layer 3A, the second resin layer 1c, the adhesive layer 1b, the first resin layer 1a, and the first conductor layer 2A in this order (FIG. 2E). At this time, the wiring portion 4 and the conductor 5a are buried in the adhesive layer 1b. Next, holes 5c (via holes) reaching the conductor 5a are drilled in the second conductor layer 3A, the second resin layer 1c, and the adhesive layer 1b from the other side of the thickness direction.Additionally, holes 5b (via holes) reaching the conductors 5a are drilled from one side in the thickness direction through the first conductor layer 2A, the first resin layer 1a, and the adhesive layer 1b ( FIG. 2F ). Examples of methods for forming the holes 5b and 5c include drilling. Examples of drilling methods include laser processing, drilling, and blasting. Laser processing is preferred. The formed holes 5c may have a tapered shape in which the cross-sectional area gradually decreases from the surface of the second conductor layer 3A toward the conductors 5a. The formed holes 5b may have a tapered shape in which the cross-sectional area gradually decreases from the surface of the first conductor layer 2A toward the conductors 5a. Next, plating 3B is applied to the holes 5c in the second conductor layer 3A, the second resin layer 1c, and the adhesive layer 1b, as well as to the surface of the second conductor layer 3A. This fills the holes 5c in the second conductor layer 3A, the second resin layer 1c, and the adhesive layer 1b with conductors. Furthermore, plating 2B is applied to the first conductor layer 2A, the first resin layer 1a, the holes 5b in the adhesive layer 1b, and the surface of the first conductor layer 2A. This fills the holes 5b in the first conductor layer 2A, the first resin layer 1a, and the adhesive layer 1b with a conductor. As a result, the filled conductor and the conductor 5a combine to form a conductive via 5 that electrically connects the portion that will become the first conductor pad 2a and the portion that will become the second conductor pad 3a (FIG. 2G). Next, plating 3B on the second conductor layer 3A is removed so as to leave plating 3B on the second conductor layer 3A over and around hole 5c. Furthermore, plating 2B on the first conductor layer 2A is removed so as to leave plating 2B on the first conductor layer 2A over and around hole 5b (FIG. 2H). Examples of removal methods include patterning by photolithography (e.g., subtractive processing) using a photoresist. This results in a first conductor pad 2a being formed over the hole 5b filled with the conductive via 5. A second conductive pad 3a is formed on the hole 5c filled with the conductive via 5. Next, a first electromagnetic wave shielding layer 2b is laminated so as to cover the first conductive pad 2a and the first resin layer 1a. A second electromagnetic wave shielding layer 3b is laminated so as to cover the second conductive pad 3a and the second resin layer 1c. The first electromagnetic wave shielding layer 2b has a conductive adhesive layer, a metal layer, and a protective layer in this order, with the conductive adhesive layer in contact with the first conductive pad 2a and the first resin layer 1a.The second electromagnetic wave shielding layer 3b has a conductive adhesive layer, a metal layer, and a protective layer in this order, and the conductive adhesive layer is in contact with the second conductor pads 3a and the second resin layer 1c. In this way, the flexible multilayer circuit board shown in Figure 1 is obtained (Figure 2I).

[0046] Here, Figure 2J shows an A-A' cross-sectional view of the flexible multilayer circuit board of Figure 2I. The wiring portion 4 projected onto this cross-section is indicated by a dashed line. In the cross-sectional view of Figure 2J, a plurality of first conductor pads 2a form two rows aligned in the longitudinal direction of the wiring portion 4. The two rows sandwich the wiring portion 4 in the width direction, and in each of the two rows, the first conductor pads 2a are aligned at a predetermined interval.

[0047] 2K shows a cross-sectional view taken along the line B-B' of the flexible multilayer circuit board of FIG. 2I. The wiring portion 4 projected onto this cross section is indicated by a dashed line. In the cross-sectional view of FIG. 2K, a plurality of second conductor pads 3a are arranged in two rows aligned in the longitudinal direction of the wiring portion 4. The two rows sandwich the wiring portion 4 in the width direction, and in each of the two rows, the second conductor pads 3a are aligned at a predetermined interval.

[0048] The A-A' cross-sectional view in Fig. 2J and the B-B' cross-sectional view in Fig. 2K are both cross-sectional views perpendicular to the thickness direction. When the A-A' cross-sectional view and the B-B' cross-sectional view are overlapped, the first conductor pads 2a and the second conductor pads 3a completely overlap. Therefore, when the first ground conductor and the second ground conductor are projected onto a plane perpendicular to the thickness direction in the flexible multilayer circuit board shown in Fig. 2I, the patterns of the first ground conductor and the second ground conductor completely overlap.

[0049] Next, another example of a flexible multilayer circuit board is shown. FIG. 3 is a schematic diagram (cross-sectional view) of another embodiment of a flexible multilayer circuit board of the present invention. The flexible multilayer circuit board of FIG. 3 has the same structure as the flexible multilayer circuit board of FIG. 1 except that the first resin layer and the second resin layer are porous resin layers. The flexible multilayer circuit board includes an insulating layer 1, a first layer 2, a second layer 3, and a wiring portion 4. The first layer 2 is disposed on one side of the insulating layer 1 in the thickness direction. The second layer 3 is disposed on the other side of the insulating layer 1 in the thickness direction. The first layer 2 includes a first ground conductor portion having a plurality of first conductor pads 2a arranged in a pattern. The first layer 2 further includes a first electromagnetic wave shielding layer 2b. The second layer 3 includes a second ground conductor portion having a plurality of second conductor pads 3a arranged in a pattern. The plurality of first conductor pads 2a are embedded in the first electromagnetic wave shielding layer 2b. When the first ground conductor and the second ground conductor are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor and the second ground conductor overlap.

[0050] Furthermore, in the flexible multilayer circuit board shown in Fig. 3, the insulating layer 1 has a first porous resin layer 1d, an adhesive layer 1b, and a second porous resin layer 1e, in this order. The wiring portion 4 is embedded in the insulating layer 1. More specifically, the wiring portion 4 is embedded in the adhesive layer 1b. Furthermore, the second layer 3 has a second electromagnetic wave shielding layer 3b. A plurality of second conductor pads 3a are embedded in the second electromagnetic wave shielding layer 3b. The first conductor pad 2a and the second conductor pad 3a are electrically connected by conductor vias 5 that penetrate the insulating layer 1. The wiring portion 4 is not in contact with the plurality of first conductor pads 2a, the plurality of second conductor pads 3a, or the conductor vias 5.

[0051] Fig. 4 is a schematic diagram (cross-sectional view) of another embodiment of the flexible multilayer circuit board of the present invention. The flexible multilayer circuit board of Fig. 4 differs from the flexible multilayer circuit board of Fig. 3 in the following two points: the second layer 3 does not have a second electromagnetic wave shielding layer and has an adhesive layer 6 and a cover insulating layer 7; and the wiring portion 4 is not embedded in the adhesive layer 1b but is embedded in the second layer 3 (specifically, the adhesive layer 6) and in contact with the second porous resin layer 1e. The flexible multilayer circuit board includes an insulating layer 1, a first layer 2, a second layer 3, and a wiring portion 4. The first layer 2 is disposed on one side of the insulating layer 1 in the thickness direction. The second layer 3 is disposed on the other side of the insulating layer 1 in the thickness direction. The first layer 2 has a first ground conductor portion formed by a plurality of first conductor pads 2a arranged in a pattern. The first layer 2 further has a first electromagnetic wave shielding layer 2b. The second layer 3 has a second ground conductor having a plurality of second conductor pads 3 a arranged in a pattern. The plurality of first conductor pads 2 a are embedded in the first electromagnetic wave shielding layer 2 b. When the first and second ground conductors are projected onto a plane perpendicular to the thickness direction, the patterns of the first and second ground conductors overlap.

[0052] Furthermore, in the flexible multilayer circuit board shown in Figure 4, the insulating layer 1 has a first porous resin layer 1d, an adhesive layer 1b, and a second porous resin layer 1e, in this order. Furthermore, the second layer 3 has an adhesive layer 6 and a cover insulating layer 7. The adhesive layer 6 is in contact with the second porous resin layer 1e. A plurality of second conductor pads 3a are embedded in the adhesive layer 6. The first conductor pads 2a and the second conductor pads 3a are electrically connected by conductive vias 5 that penetrate the insulating layer 1. The wiring portion 4 is embedded in the second layer 3 (specifically, the adhesive layer 6) and in contact with the second porous resin layer 1e. The wiring portion 4 is not in contact with the plurality of first conductor pads 2a, the plurality of second conductor pads 3a, or the conductive vias 5.

[0053] Fig. 5 is a schematic diagram (cross-sectional view) of another embodiment of a flexible multilayer circuit board of the present invention. The flexible multilayer circuit board of Fig. 5 differs from the flexible multilayer circuit board of Fig. 4 in the following respect: two conductor vias 5 are connected at a location where they are embedded in the adhesive layer 1b. The flexible multilayer circuit board includes an insulating layer 1, a first layer 2, a second layer 3, and a wiring portion 4. The first layer 2 is disposed on one side of the insulating layer 1 in the thickness direction. The second layer 3 is disposed on the other side of the insulating layer 1 in the thickness direction. The first layer 2 includes a first ground conductor portion having a plurality of first conductor pads 2a arranged in a pattern. The first layer 2 further includes a first electromagnetic wave shielding layer 2b. The second layer 3 includes a second ground conductor portion having a plurality of second conductor pads 3a arranged in a pattern. The plurality of first conductor pads 2a are embedded in the first electromagnetic wave shielding layer 2b. When the first ground conductor and the second ground conductor are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor and the second ground conductor overlap.

[0054] Furthermore, in the flexible multilayer circuit board shown in FIG. 5 , the insulating layer 1 has a first porous resin layer 1d, an adhesive layer 1b, and a second porous resin layer 1e, in this order. Furthermore, the second layer 3 has an adhesive layer 6 and a cover insulating layer 7. The adhesive layer 6 is in contact with the second porous resin layer 1e. A plurality of second conductor pads 3a are embedded in the adhesive layer 6. The first conductor pads 2a and the second conductor pads 3a are electrically connected by conductor vias 5 that penetrate the insulating layer 1. The wiring portion 4 is embedded in the second layer 3 (specifically, the adhesive layer 6) and in contact with the second porous resin layer 1e. The wiring portion 4 is not in contact with the plurality of first conductor pads 2a, the plurality of second conductor pads 3a, or the conductor vias 5. Parts of the two conductor vias 5 are connected at locations embedded in the adhesive layer 1b.

[0055] 6 is a schematic diagram (cross-sectional view) of another embodiment of a flexible multilayer circuit board according to the present invention. The flexible multilayer circuit board includes an insulating layer 1, a first layer 2, a second layer 3, and a wiring portion 4. The first layer 2 is disposed on one thickness-wise side of the insulating layer 1. The second layer 3 is disposed on the other thickness-wise side of the insulating layer 1. The first layer 2 includes a first ground conductor having a plurality of first conductor pads 2a arranged in a pattern. The first layer 2 also includes a first electromagnetic wave shielding layer 2b. The second layer 3 includes a second ground conductor having a plurality of second conductor pads 3a arranged in a pattern. The plurality of first conductor pads 2a are embedded in the first electromagnetic wave shielding layer 2b. When the first ground conductor and the second ground conductor are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor and the second ground conductor overlap.

[0056] Furthermore, in the flexible multilayer circuit board shown in Figure 6, the insulating layer 1 has a porous resin layer 1f and an adhesive layer 1g in this order. Furthermore, the second layer 3 has an adhesive layer 6 and a cover insulating layer 7. The adhesive layer 6 is in contact with the porous resin layer 1f. A plurality of second conductor pads 3a are embedded in the adhesive layer 6. The first conductor pads 2a and the second conductor pads 3a are electrically connected by conductive vias 5 that penetrate the insulating layer 1. The wiring portion 4 is embedded in the second layer 3 (specifically, the adhesive layer 6) and in contact with the porous resin layer 1f. The wiring portion 4 is not in contact with the plurality of first conductor pads 2a, the plurality of second conductor pads 3a, or the conductive vias 5.

[0057] 1 Insulating layer 1a First resin layer 1b Adhesive layer 1b-1 Adhesive layer 1b-2 Adhesive layer 1c Second resin layer 1d First porous resin layer 1e Second porous resin layer 1f Porous resin layer 1g Adhesive layer 2 First layer 2a First conductor pad 2A First conductor layer 2b First electromagnetic wave shielding layer 2B Plating 3 Second layer 3a Second conductor pad 3A Second conductor layer 3b Second electromagnetic wave shielding layer 3B Plating 4 Wiring portion 4A Conductor layer for forming wiring portion 5 Conductor via 5a Conductor 5b Hole 5c Hole 6 Adhesive layer 7 Cover insulating layer

Claims

1. A flexible multilayer circuit board comprising an insulating layer, a first layer disposed on one side of the insulating layer in the thickness direction, a second layer disposed on the other side of the insulating layer in the thickness direction, and a wiring section, wherein the first layer has a first ground conductor section comprising a plurality of first conductor pads arranged in a pattern, and a first electromagnetic wave shielding layer, the second layer has a second ground conductor section comprising a plurality of second conductor pads arranged in a pattern, the plurality of first conductor pads are embedded in the first electromagnetic wave shielding layer, and when the first ground conductor section and the second ground conductor section are projected onto a plane perpendicular to the thickness direction, the patterns of the first ground conductor section and the second ground conductor section overlap.

2. The flexible multilayer circuit board according to claim 1, wherein the wiring portion is embedded in the insulating layer.

3. The flexible multilayer circuit board according to claim 1, wherein the second layer further comprises a second electromagnetic wave shielding layer, and the plurality of second conductive pads are embedded in the second electromagnetic wave shielding layer.

4. The flexible multilayer circuit board of claim 1, wherein the insulating layer comprises a porous insulating layer.

5. The flexible multilayer circuit board according to claim 1, wherein the insulating layer has a tensile modulus of elasticity of 2.0 GPa or less.

6. A flexible multilayer circuit board as described in claim 1, wherein the surface roughness of the surface of the first ground conductor portion that contacts the first electromagnetic wave shielding layer in the first conductor pad is rougher than the surface roughness of the wiring portion.

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