A flexible circuit board that is easy to bend

Abstract

This utility model relates to the technical field of circuit boards, and more particularly to a flexible circuit board that is easy to bend. It includes an electromagnetic shielding film composed of a conductive metal layer and a nano-carbon coating; a flexible reinforcing layer partially disposed beneath the electromagnetic shielding film, covering the critical signal line area of ​​the signal transmission layer; a third adhesive layer located between the electromagnetic shielding film and the flexible reinforcing layer; a first insulating layer made of a modified polyimide material with a low dielectric constant; a first adhesive layer bonding the first insulating layer to the signal transmission layer; and a signal transmission layer formed by etching rolled copper foil to create impedance matching lines. Through the combination of a locally flexible reinforcing layer and an LCP dielectric layer, high-frequency insertion loss is reduced by more than 40%, making it particularly suitable for 5G millimeter-wave communication. The localized design of the polyimide flexible reinforcing layer ensures an overall bending radius ≤1mm, while simultaneously increasing the fatigue strength of critical areas by 30%. No impedance linewidth adjustment is required; a tolerance of ±5μm can be achieved using conventional etching processes, improving yield by more than 15%.

Description

Technical Field

[0001] This utility model relates to the technical field of circuit boards, and in particular to a flexible circuit board that is easy to bend. Background Technology

[0002] As electronic products evolve towards higher integration, miniaturization, and higher reliability, flexible printed circuit boards (FPCs) are increasingly widely used in aerospace, military, mobile communications, and portable electronic devices (such as laptops and digital cameras). To suppress electromagnetic interference (EMI), existing FPCs typically have an electromagnetic shielding film on their surface. However, this design presents several technical challenges: the introduction of the shielding film reduces line impedance, and to meet the target impedance value, traditional solutions require narrowing the signal linewidth. However, thinning the linewidth increases the difficulty of the etching process, imposes stringent linewidth tolerance control requirements, and significantly reduces production yield. Due to the relatively thin overall thickness of the FPC, the thickness of its cover film is limited, resulting in an excessively small spacing between the shielding film and the signal lines. This structure creates a near-field coupling effect, drastically increasing the insertion loss of signal transmission. Simply increasing the cover film thickness to increase the spacing sacrifices the bending performance of the FPC, failing to meet the requirements of applications such as wearable devices. Therefore, a new type of FPC structure is needed that achieves the following simultaneously without increasing the overall thickness or reducing the linewidth: Utility Model Content

[0003] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the present invention.

[0004] In view of the problems existing in the current flexible circuit boards that are easily bent, this utility model is proposed.

[0005] Therefore, the purpose of this utility model is to provide an easily bendable flexible circuit board, which aims to improve bending performance.

[0006] To solve the above-mentioned technical problems, this utility model provides the following technical solution: an electromagnetic shielding film, which is composed of a conductive metal layer and a nano-carbon coating;

[0007] A flexible reinforcing layer is partially disposed below the electromagnetic shielding film, covering the key signal line area of ​​the signal transmission layer;

[0008] The third adhesive layer is located between the electromagnetic shielding film and the flexible reinforcing layer;

[0009] The first insulating layer is made of a modified polyimide material with a low dielectric constant.

[0010] The first adhesive layer bonds the first insulating layer to the signal transmission layer;

[0011] The signal transmission layer is formed by etching rolled copper foil to create impedance matching lines;

[0012] The dielectric layer is made of liquid crystal polymer material;

[0013] The reference formation is a complete copper plane;

[0014] The second adhesive layer and the second insulating layer are sequentially stacked below the reference ground layer.

[0015] As a preferred embodiment of the flexible circuit board of this utility model, the thickness of the electromagnetic shielding film is 15-25μm, the conductive metal layer is electrolytic copper foil, and the sheet resistance of the nano-carbon coating is ≤1Ω / sq.

[0016] As a preferred embodiment of the flexible circuit board of this utility model, the flexible reinforcing layer is made of polyimide film with a thickness of 50-75μm, and only covers the area directly above the high-speed differential line in the signal transmission layer, with a width 200μm wider than the signal line.

[0017] As a preferred embodiment of the flexible circuit board of this utility model, the thickness of the dielectric layer is 40-60μm, the dielectric constant Dk≤2.9, and the dielectric loss Df≤0.002@10GHz.

[0018] As a preferred embodiment of the flexible circuit board of this utility model, the third adhesive layer is an acrylic adhesive with a thickness of 10-15μm and a dielectric constant Dk≤2.8.

[0019] As a preferred embodiment of the flexible circuit board of this utility model, the single-ended impedance of the signal transmission layer is 50Ω, the differential impedance is 100Ω, and the line width tolerance is controlled within ±5μm.

[0020] As a preferred embodiment of the flexible circuit board of this utility model, the flexible circuit board is provided with a protective mechanism, the protective mechanism including an outer protective layer and metal wires disposed inside therein, and the protective mechanism including a sealing sheet.

[0021] The beneficial effects of this invention are as follows: By combining a localized flexible reinforcement layer with an LCP dielectric layer, high-frequency insertion loss is reduced by more than 40%, making it particularly suitable for 5G millimeter-wave communication. The localized design of the polyimide flexible reinforcement layer ensures an overall bending radius of ≤1mm, while simultaneously improving fatigue strength in key areas by 30%. No adjustment of impedance linewidth is required (compared to traditional solutions that require linewidth reduction to 30μm); a tolerance of ±5μm can be achieved using conventional etching processes, improving yield by more than 15%. The electromagnetic shielding film uses a copper foil + nano-carbon coating composite structure, reducing cost by 20% compared to pure silver paste shielding films while improving shielding effectiveness by 10dB. It is adaptable to high-frequency flexible electronics (such as wearable devices and automotive radar modules) and compatible with existing SMT assembly processes. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of this utility model, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Among them:

[0023] Figure 1 A schematic diagram of the overall structure of this utility model.

[0024] Figure 2 This is a schematic diagram of the layered structure provided by this utility model. Detailed Implementation

[0025] To make the above-mentioned objectives, features and advantages of this utility model more apparent and understandable, the specific embodiments of this utility model will be described in detail below with reference to the accompanying drawings.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0027] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that excludes other embodiments.

[0028] Secondly, this utility model is described in detail with reference to the schematic diagrams. When describing the embodiments of this utility model, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of this utility model. In addition, actual manufacturing should include the three-dimensional spatial dimensions of length, width, and depth.

[0029] Example 1

[0030] Reference Figures 1-2 This is the first embodiment of the present utility model.

[0031] Specifically, the flexible circuit board includes the following structures stacked from top to bottom: an electromagnetic shielding film 1, composed of a conductive metal layer and a nano-carbon coating; a flexible reinforcing layer 2, partially disposed below the electromagnetic shielding film 1, covering the key signal line area of ​​the signal transmission layer 6; a third adhesive layer 3, located between the electromagnetic shielding film 1 and the flexible reinforcing layer 2; a first insulating layer 4, made of a modified polyimide material with a low dielectric constant; a first adhesive layer 5, bonding the first insulating layer 4 to the signal transmission layer 6; the signal transmission layer 6, formed by etching rolled copper foil to create impedance matching lines; a dielectric layer 7, made of liquid crystal polymer (LCP) material; a reference ground layer 8, a complete copper plane; and a second adhesive layer 9 and a second insulating layer 10, sequentially stacked below the reference ground layer 8.

[0032] The electromagnetic shielding film 1 has a thickness of 15-25μm, wherein the conductive metal layer is electrolytic copper foil, the sheet resistance of the nano-carbon coating is ≤1Ω / sq, the flexible reinforcing layer 2 is made of polyimide film with a thickness of 50-75μm, and only covers the area directly above the high-speed differential line in the signal transmission layer 6, and its width is 200μm wider than the signal line, the dielectric layer 7 has a thickness of 40-60μm, a dielectric constant Dk≤2.9, a dielectric loss Df≤0.002@10GHz, the third adhesive layer 3 is an acrylic adhesive with a thickness of 10-15μm, a dielectric constant Dk≤2.8, the single-ended impedance of the signal transmission layer 6 is 50Ω, the differential impedance is 100Ω, the line width tolerance is controlled within ±5μm, and a protection mechanism 11 is provided on the flexible circuit board. The protection mechanism 11 includes an outer protective layer 12 and a metal wire 13 disposed inside it, and the protection mechanism 11 includes a sealing sheet 14.

[0033] Preferably, the electromagnetic shielding film (conductive copper foil + nano-carbon coating) has a 12μm electrolytic copper foil (ED copper) as the top layer, which has high conductivity (resistivity ≤1.72×10⁻⁻⁻⁻⁶). 8The bottom layer has a sheet resistance of ≤1Ω / sq, making it easy to etch and process; the lower 10μm nano-carbon coating has a sheet resistance of ≤1Ω / sq, providing lightweight and resistance to bending fatigue. The skin effect of nano-carbon is superior to that of pure metals, reducing electromagnetic leakage in frequency bands above 10GHz. Compared to full silver paste shielding films (which are expensive and brittle), the composite structure reduces costs by 20% and increases bending life by 3 times.

[0034] Preferably, a localized flexible reinforcement layer (polyimide PI film, 50-75μm) is used, employing DuPont Kapton® HN: tensile modulus 2.5GPa, coefficient of thermal expansion (CTE) 12ppm / ℃, and glass transition temperature (Tg) ≥400℃. This layer is placed only directly above the high-speed signal lines, using high-modulus PI to absorb bending stress and avoid the increase in hardness caused by global reinforcement (the dynamic bending radius remains at 1mm). The CTE is close to that of the copper layer (17ppm / ℃), reducing the risk of delamination during high-temperature lamination (tested at 288℃ reflow soldering with no warping).

[0035] Preferably, the dielectric layer (liquid crystal polymer (LCP, 40-60μm) has a dielectric constant Dk=2.85@10GHz, a dielectric loss Df=0.0018, and a water absorption rate <0.02%. Compared to traditional PI (Df≈0.008), LCP reduces Df by 77% and reduces insertion loss from 0.5dB / cm to 0.28dB / cm (@10GHz). Its water absorption rate is extremely low, and high-frequency impedance fluctuation is <±2Ω (compared to traditional PI impedance deviation of ±5Ω at 85%RH).

[0036] Preferably, the signal transmission layer (rolled copper RA copper, 18-35μm) uses rolled copper foil with a tensile strength ≥300MPa and a surface roughness Rz ≤2μm (electrolytic copper Rz ≥3μm). Low roughness reduces signal skin loss (measured 15% higher transmission efficiency at 10GHz compared to electrolytic copper). Rolled copper has a ductility >20%, making it more suitable for dynamic bending scenarios than electrolytic copper (ductility ≈10%).

[0037] Preferably, the adhesive (acrylic adhesive layer, 10-15μm) has a dielectric constant Dk ≤ 2.8, a curing temperature of 150℃, and a peel strength ≥ 1.2N / mm. A low Dk reduces the interference of the adhesive layer on impedance (traditional epoxy adhesives with Dk ≥ 4.0 can lead to impedance deviation > 5%). Curing at 150℃ avoids high-temperature damage to the LCP dielectric layer (LCP melting point ≈ 315℃).

[0038] Material Traditional solution This optimization scheme Improvement effect Shielding film Silver paste (50μm) Copper foil + nano-carbon (22μm) Costs reduced by 20%, bending life increased by 3 times Dielectric layer PI (Df=0.008) LCP (Df=0.0018) Insertion loss ↓77% Signal layer copper foil Electrolytic copper (Rz=3μm) Rolled copper (Rz=2μm) High-frequency efficiency increased by 15% Adhesive layer curing temperature 180℃ (epoxy resin) 150℃ (acrylic acid) Compatible with heat-sensitive materials

[0039] Table 1: Comparison of Advantages

[0040] In summary, the combination of a local flexible reinforcement layer and an LCP dielectric layer reduces high-frequency insertion loss by more than 40% (0.28 dB / cm in Example 1 vs. 0.5 dB / cm in the traditional solution), making it particularly suitable for 5G millimeter-wave communication. The local design of the polyimide flexible reinforcement layer (2) makes the overall bending radius ≤1mm, while improving the fatigue strength of key areas by 30%. There is no need to adjust the impedance linewidth (compared to the traditional solution which requires shrinking the linewidth to 30μm), and the ±5μm tolerance can be achieved using conventional etching processes, improving the yield by more than 15%. The electromagnetic shielding film (1) adopts a copper foil + nano carbon coating composite structure, which reduces the cost by 20% compared to pure silver paste shielding film, and improves the shielding effectiveness by 10dB. It can be adapted to high-frequency flexible electronics (such as wearable devices and automotive radar modules) and is compatible with existing SMT assembly processes.

[0041] It is important to note that the constructions and arrangements of this application shown in several different exemplary embodiments are merely illustrative. Although only a few embodiments are described in detail in this disclosure, those who consult this disclosure will readily understand that many modifications are possible without substantially departing from the novelty and advantages of the subject matter described in this application. For example, variations in the size, dimensions, structure, shape, and proportions of various elements, as well as parameter values ​​such as temperature, pressure, etc., installation arrangements, use of materials, color, orientation, etc. For instance, an element shown as integrally formed may be composed of multiple parts or elements, the position of elements may be inverted or otherwise altered, and the nature or number or position of discrete elements may be changed or altered. Therefore, all such modifications are intended to be included within the scope of this utility model. The order or sequence of any process or method steps may be changed or rearranged according to alternative embodiments. In the claims, any "device plus function" clause is intended to cover the structure performing the function described herein, and not only structural equivalents but also equivalent structures. Other substitutions, modifications, alterations, and omissions may be made in the design, operation, and arrangement of the exemplary embodiments without departing from the scope of this utility model. Therefore, this invention is not limited to the specific embodiments, but extends to various modifications that still fall within the scope of the appended claims. Furthermore, for the purpose of providing a concise description of exemplary embodiments, not all features of the actual embodiments may be omitted, i.e., those features not relevant to the currently considered best mode for carrying out this invention, or those features not relevant to implementing this invention.

[0042] It should be noted that the above embodiments are only used to illustrate the technical solution of this utility model and are not intended to limit it. Although this utility model has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solution of this utility model without departing from the spirit and scope of the technical solution of this utility model, and all such modifications or substitutions should be covered within the scope of the claims of this utility model.

Claims

1. A flexible wiring board which is easy to bend, characterized by: This includes the following structures stacked from top to bottom: The electromagnetic shielding film (1) is composed of a conductive metal layer and a nano-carbon coating; A flexible reinforcing layer (2) is partially disposed below the electromagnetic shielding film (1) and covers the key signal line area of ​​the signal transmission layer (6); The third adhesive layer (3) is located between the electromagnetic shielding film (1) and the flexible reinforcing layer (2); The first insulating layer (4) is made of a modified polyimide material with a low dielectric constant; The first adhesive layer (5) bonds the first insulating layer (4) to the signal transmission layer (6). The signal transmission layer (6) is formed by etching rolled copper foil to create an impedance matching circuit; The dielectric layer (7) is made of liquid crystal polymer (LCP) material; Reference stratum (8) is a complete copper plane; The second adhesive layer (9) and the second insulating layer (10) are sequentially stacked below the reference ground layer (8).

2. The flexible circuit board according to claim 1, wherein: The thickness of the electromagnetic shielding film (1) is 15-25μm, wherein the conductive metal layer is electrolytic copper foil and the sheet resistance of the nano carbon coating is ≤1Ω / sq.

3. The flexible circuit board according to claim 1, wherein: The flexible reinforcing layer (2) is made of polyimide film with a thickness of 50-75μm, and only covers the area directly above the high-speed differential line in the signal transmission layer (6), with a width 200μm wider than the signal line.

4. The flexible circuit board according to claim 3, wherein: The dielectric layer (7) has a thickness of 40-60 μm, a dielectric constant Dk≤2.9, and a dielectric loss Df≤0.002@10GHz.

5. The flexible circuit board according to claim 4, characterized in that: The third adhesive layer (3) is an acrylic adhesive with a thickness of 10-15 μm and a dielectric constant Dk≤2.

8.

6. The flexible circuit board according to claim 5, characterized in that: The single-ended impedance of the signal transmission layer (6) is 50Ω, the differential impedance is 100Ω, and the line width tolerance is controlled within ±5μm.

7. The flexible circuit board according to claim 6, characterized in that: The flexible circuit board is provided with a protective mechanism (11), which includes an outer protective layer (12) and a metal wire (13) disposed inside it, and the protective mechanism (11) includes a sealing sheet (14).

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