Flexible circuit board, wearable device, and method for manufacturing flexible circuit board

Abstract

The application discloses a flexible circuit board, which comprises a circuit substrate, a second dielectric layer and a protective layer. The circuit substrate comprises a first dielectric layer and a circuit layer which are arranged in a stack; the second dielectric layer is located on opposite surfaces of the first dielectric layer and does not overlap with the projection of the circuit layer on the first dielectric layer; and the protective layer covers the circuit layer, the surface of the first dielectric layer exposed to the circuit layer and the second dielectric layer. The material of the second dielectric layer is a mixture of thermoplastic polyurethane elastomer rubber and liquid crystal polymer, and the liquid crystal polymer is dispersed in the thermoplastic polyurethane elastomer rubber in a fibrous form. The application further provides a wearable device and a manufacturing method of the flexible circuit board.

Description

Technical Field

[0001] The present application relates to the field of flexible circuit boards, and in particular to a flexible circuit board, a wearable device, and a method for manufacturing a flexible circuit board. Background Art

[0002] Wearable devices are usually bent repeatedly during use. The number of times the flexible circuit board 100' in the wearable device is bent affects the service life of the wearable device. Figure 1 Typically, a cavity 60' is defined within the circuit substrate 10' of a flexible printed circuit board 100'. An air layer and a dielectric layer 65' are used to withstand deformation, bending stress, and frictional stress. During the bending process, relative sliding occurs between the two dielectric layers 65' surrounding the cavity 60', causing significant variations in the thickness of the air layer and, consequently, significant differences in dynamic impedance. Summary of the Invention

[0003] In view of this, it is necessary to provide a flexible circuit board with small dynamic impedance difference to solve the above problems.

[0004] A flexible circuit board includes a circuit substrate, a second dielectric layer, and a protective layer. The circuit substrate comprises a first dielectric layer and a circuit layer, which are stacked one on top of the other. The second dielectric layer is located on two opposite surfaces of the first dielectric layer and does not overlap with the projection of the circuit layer on the first dielectric layer. The protective layer covers the circuit layer and the surfaces of the first and second dielectric layers exposed to the circuit layer. The second dielectric layer is made of a mixture of thermoplastic polyurethane elastomer rubber and liquid crystal polymer, with the liquid crystal polymer being dispersed in the thermoplastic polyurethane elastomer rubber in a fibrous form.

[0005] In some embodiments of the present application, along the stacking direction of the flexible circuit board, the thickness of the second dielectric layer is greater than the thickness of the circuit layer located on the same side of the first dielectric layer as the second dielectric layer.

[0006] In some embodiments of the present application, the circuit substrate further includes a copper layer, which is located on a surface of the first dielectric layer facing away from the circuit layer, and a projection of the copper layer on the first dielectric layer does not overlap with a projection of the second dielectric layer on the first dielectric layer.

[0007] In some embodiments of the present application, along the stacking direction of the flexible circuit board, the thickness of the second dielectric layer is greater than the thickness of the copper layer located on the same side of the first dielectric layer as the second dielectric layer.

[0008] In some embodiments of the present application, the flexible circuit board further includes a third dielectric layer, which is located on one surface of the same area of ​​the circuit substrate and between the copper layer and the protective layer; the material of the third dielectric layer includes a thermoplastic material.

[0009] In some embodiments of the present application, the third dielectric layer further penetrates the copper layer and is connected to the first dielectric layer.

[0010] In some embodiments of the present application, the flexible circuit board further includes an electromagnetic shielding layer, which is located on a surface of the protective layer facing away from the circuit substrate.

[0011] A wearable device includes a flexible circuit board.

[0012] A method for manufacturing a flexible circuit board includes providing a circuit substrate, the circuit substrate including a first dielectric layer and a circuit layer stacked in layers; disposing second dielectric layers on two opposite surfaces of the first dielectric layer, wherein the projection of the second dielectric layer on the first dielectric layer does not overlap with the projection of the circuit layer on the first dielectric layer; and covering the surfaces of the circuit substrate and the second dielectric layer with a protective layer; wherein the second dielectric layer is made of a mixture of thermoplastic polyurethane elastomer rubber and liquid crystal polymer, and the liquid crystal polymer is dispersed in the thermoplastic polyurethane elastomer rubber in a fibrous form.

[0013] In some embodiments of the present application, before the step of covering the protective layer, the method for manufacturing a flexible circuit board further includes: setting a third dielectric layer on one surface of the same area of ​​the flexible circuit board, the third dielectric layer being located between the circuit substrate and the protective layer, wherein the material of the third dielectric layer includes a thermoplastic material.

[0014] In some embodiments of the present application, before the step of covering the protective layer, the method for manufacturing a flexible circuit board further includes: a step of laminating an electromagnetic shielding layer on the surface of the protective layer.

[0015] The flexible circuit board provided in the embodiment of the present application has a mixed second dielectric layer provided in the dynamic bending area, which is beneficial to improving the bending performance of the flexible circuit board; the second dielectric layer is located between the protective layer and the first dielectric layer. During the bending process of the flexible circuit board, the rate of change of the signal impedance can be minimized to the greatest extent to achieve dynamic stabilization of the impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic cross-sectional view of a flexible circuit board with a cavity provided in the related art of this application.

[0017] Figure 2 A schematic diagram of the structure of a wearable device provided in an embodiment of the present application.

[0018] Figure 3 A schematic cross-sectional view of a flexible circuit board provided in some embodiments of the present application.

[0019] Figure 4 Schematic cross-sectional views of flexible circuit boards provided in some other embodiments of the present application.

[0020] Figure 5 A cross-sectional view of a flexible circuit board provided for some embodiments of the present application.

[0021] Figure 6 A cross-sectional view of a double-sided copper clad board provided for some embodiments of the present application.

[0022] Figure 7 A cross-sectional view of a circuit board after etching Figure 6 A cross-sectional view of a circuit board after etching

[0023] Figure 8 A cross-sectional view of a circuit board after etching Figure 7 A cross-sectional view of a circuit board after etching

[0024] Figure 9 A cross-sectional view of a circuit board after etching Figure 8 A cross-sectional view of a circuit board after etching

[0025] Figure 10 A cross-sectional view of a circuit board after etching

[0026] Main component symbol explanation

[0027] Wearable device 200 Flexible circuit board 100, 100a, 100b, 100' Circuit substrate 10、10b、10’ First dielectric layer 11 Circuit layer 13 Copper layer 15 Adhesive layer 17b Second dielectric layer 20 Third dielectric layer 25a Protective layer 30 Bonding layer 31 Cover layer 33 Electromagnetic shielding layer 40 Double-sided copper clad board 50 Cavity 60’ Dielectric layer 65’ DETAILED DESCRIPTION

[0028] In order to more clearly understand the above objectives, features and advantages of the present application, the following will provide a detailed description of the present application with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict. In the following description, a large number of specific details are set forth in order to facilitate a full understanding of the present application. The described embodiments are only some of the embodiments of the present application, but not all the embodiments.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used herein, the term "and / or" includes all and any combinations of one or more of the associated listed items.

[0030] In the embodiments of the present application, for the convenience of description but not limitation of the present application, the term "connection" used in the patent application specification and claims of the present application is not limited to physical or mechanical connection, whether direct or indirect. "Up", "down", "above", "below", "left", "right" and the like are only used to indicate relative positional relationship, which changes accordingly when the absolute position of the described object changes.

[0031] Referring to Figure 2 The present application provides a wearable device 200, such as a smart watch, smart glasses, smart bracelet, smart helmet, etc. In the embodiment, the wearable device 200 is a smart bracelet. The wearable device 200 is provided with a flexible circuit board 100.

[0032] Referring to Figure 3 The flexible circuit board 100 includes a circuit substrate 10, a second dielectric layer 20 and a protective layer 30. The second dielectric layer 20 is located on the opposite surfaces of the circuit substrate 10, and the protective layer 30 covers the circuit substrate 10 and the second dielectric layer 20. The area provided with the second dielectric layer 20 is a bending area, and the area not provided with the second dielectric layer 20 is a non-bending area. The flexible circuit board 100 can be dynamically bent multiple times in the area provided with the second dielectric layer 20.

[0033] The circuit substrate 10 includes a first dielectric layer 11 and a circuit layer 13 stacked, and the number of layers of the first dielectric layer 11 and the circuit layer 13 can be one or more. In the embodiment, the number of layers of the first dielectric layer 11 and the circuit layer 13 is one, and the circuit layer 13 is located on one surface of the first dielectric layer 11. The circuit substrate 10 can also include a copper layer 15, which is located on the surface of the first dielectric layer 11 away from the circuit layer 13. The copper layer 15 has good ductility, which can play a certain supporting role in the bending process, which is conducive to reducing the deformation amount of the circuit layer 13; and the cross-sectional area of the copper layer 15 is large, and the resistance is small. In some embodiments, the copper layer 15 can also be made into a circuit layer 13 according to the needs of the circuit layer 13.

[0034] The material of the first dielectric layer 11 is one of flexible materials such as polyimide (PI), liquid crystal polymer (LCP) and modified polyimide (MPI), so that the flexible circuit board 100 has bending performance.

[0035] The second dielectric layer 20 is located on opposite surfaces of the first dielectric layer 11, and a projection of the second dielectric layer 20 on the first dielectric layer 11 does not coincide with a projection of the circuit layer 13 on the first dielectric layer 11. The second dielectric layer 20 is made of a mixture of thermoplastic polyurethane elastomer rubber (TPU) and liquid crystal polymer (LCP), and the LCP is dispersed in the TPU in a fibrous form. The TPU has good tensile properties, flexural properties, and wear resistance, and the LCP has low ductility, and the LCP dispersed in the TPU in a fibrous form is beneficial to improving the bending properties of the second dielectric layer 20, thereby improving the service life of the flexible circuit board 100.

[0036] The protective layer 30 is located on opposite surfaces of the circuit substrate 10, and each protective layer 30 can include an adhesive layer 31 and a cover layer 33. One of the adhesive layers 31 covers the circuit layer 13, the second dielectric layer 20, and the first dielectric layer 11 exposed to the circuit layer 13, and the other adhesive layer 31 covers the copper layer 15, the second dielectric layer 20, and the first dielectric layer 11 exposed to the copper layer 15, and the cover layer 33 is located on the surface of the adhesive layer 31 away from the circuit substrate 10. The adhesive layer 31 can be a semi-cured sheet, and the material can be epoxy, acrylic, or polyimide resin, etc.

[0037] In some embodiments, the first dielectric layer 11 has a tensile strength of 100-300 MPa, a Young's modulus of 1.3-10 GPa, an elongation of 20%-40%, a Dk less than 3.5, a Df less than 0.02, a thermal expansion coefficient of 17-20 PPM / ℃, a moisture absorption less than 1%, and a glass transition temperature of 220℃. The adhesive layer 31 has a tensile strength of 100-300 MPa, a Young's modulus of 1.3-10 GPa, an elongation of 30%-120%, a Dk less than 3.5, a Df less than 0.02, a thermal expansion coefficient of 20-30 PPM / ℃, a moisture absorption less than 0.6%, a peel strength of 0.95 kgf / cm, and a glass transition temperature of 65-75℃. The second dielectric layer 20 has a tensile strength of 2000-6000 MPa, a Young's modulus greater than 10 GPa, an elongation of 1%-10%, a thermal expansion coefficient of 11.6x10 -6mm / °C, hygroscopicity less than 0.1%, and a peel strength of 1.4-6.1 kgf / cm. The adhesive layer 31 and the second dielectric layer 20 are each designed with specific properties such as tensile strength, Young's modulus, and elongation. This improves the localized stretching and flexural properties of the flexible circuit board 100, preventing stress concentration on other layers during bending and enhancing the reliability of the bending zone. Furthermore, the neutral axis position can be adjusted by adjusting the thickness of each layer, ensuring that the design of the flexible circuit board 100 meets the requirements of stretching and bending.

[0038] The flexible circuit board 100 provided in the embodiment of the present application has a second dielectric layer 20 provided in the bending region. During the bending process of the flexible circuit board 100, the interfaces are relatively fixed and no relative sliding occurs, thereby reducing the difference in thickness variation and, in turn, the difference in dynamic impedance, which is conducive to maintaining the impedance stability of the flexible circuit board 100 during the bending process.

[0039] Along the stacking direction of the flexible circuit board 100, the thickness of the second dielectric layer 20 is greater than the thickness of the circuit layer 13 or copper layer 15 located on the same side of the first dielectric layer 11 as the second dielectric layer 20. In some embodiments, the thickness of the second dielectric layer 20 is 10 μm to 50 μm. The thicker second dielectric layer 20 helps shift the neutral axis of the bending radius of the flexible circuit board 100 (i.e., the line where the normal stress at each point on the intersection of the cross section and the stress plane is zero in both planar and oblique bending) to the circuit layer 13 or copper layer 15. Positions closer to the neutral axis experience less stress, thereby increasing the fatigue life of the flexible circuit board 100.

[0040] The flexible circuit board 100 further includes an electromagnetic shielding layer 40 . The electromagnetic shielding layer 40 is located on a surface of the protective layer 30 facing away from the circuit substrate 10 . The electromagnetic shielding layer 40 is used to shield the flexible circuit board 100 from the effects of external electromagnetic signals.

[0041] See also Figure 4In another embodiment of the present application, a flexible circuit board 100a is provided, which is different from the flexible circuit board 100 in that the flexible circuit board 100a further comprises a third dielectric layer 25a. The third dielectric layer 25a is located on one surface of the same region of the circuit substrate 10, i.e. when the third dielectric layer 25a has multiple layers, the multiple third dielectric layers 25a can be located on the same surface or different surfaces of the circuit substrate 10, and the projections of the multiple third dielectric layers 25a on the circuit substrate 10 do not overlap with each other. In this embodiment, the third dielectric layer 25a is located on the surface of the copper layer 15, and the protective layer 30 covers the third dielectric layer 25a. The material of the third dielectric layer 25a includes thermoplastic materials, such as TPU, and LCP fibers can also be doped in the TPU. During the manufacturing process of the flexible circuit board 100a, the thermoplastic material can be heated to melt and then solidify, so that the manufactured flexible circuit board 100a can have a static bending shape in the region where the third dielectric layer 25a is located.

[0042] In some embodiments, the third dielectric layer 25a can penetrate the copper layer 15 and the adhesive layer 31 at the same time, so as to increase the coverage area of the third dielectric layer 25a on the copper layer 15 and improve the stress resistance of the flexible circuit board 100a. The third dielectric layer 25a can also penetrate the copper layer 15, and the opposite surfaces of the third dielectric layer 25a are connected with the first dielectric layer 11 and the adhesive layer 31, respectively. The third dielectric layer 25a can also be arranged on the surface of the copper layer 15 and embedded in the adhesive layer 31.

[0043] In some embodiments, the third dielectric layer 25a can also be arranged on the side where the circuit layer 13 is located.

[0044] In some embodiments, the width of the third dielectric layer 25a along the extension direction of the flexible circuit board 100a can be 1.2-5 mm, the thickness of the flexible circuit board 100a can be 0.15-0.4 mm, and the bending angle of the flexible circuit board 100a at the third dielectric layer 25a can be 45°-180°.

[0045] The third dielectric layer 25a can be one or multiple, and when the number of the third dielectric layer 25a is multiple, the flexibility and bending resistance of the flexible circuit board 100a can be further improved, so as to release the stress in the static bending region and avoid stress concentration on the same material along the same bending axis.

[0046] Please refer to Figure 5 In another embodiment of the present application, a flexible circuit board 100b is provided, which is different from the flexible circuit board 100 in that the circuit substrate 10b of this embodiment comprises multiple first dielectric layers 11 and multiple circuit layers 13, the adjacent dielectric layers can be bonded by the adhesive layer 17b, and the circuit layer 13 located between the adjacent two dielectric layers can be embedded in the adhesive layer 17b.

[0047] Please refer toFigure 6 to Figure 9 and Figure 3 The embodiment of the present application further provides a manufacturing method of the flexible circuit board 100, which can comprise the following steps:

[0048] Step S1: refer to Figure 6 provides a double-sided copper-clad plate 50, which comprises a first dielectric layer 11 and copper layers 15 located on opposite surfaces of the first dielectric layer 11.

[0049] The material of the first dielectric layer 11 is flexible material.

[0050] Step S2: refer to Figure 7 etches part of the two copper layers 15 to expose the opposite surfaces of the first dielectric layer 11 in the part, and performs line manufacturing on one of the copper layers 15 to form a line layer 13, thereby forming a line substrate 10.

[0051] In other embodiments, the step of forming the line substrate is not limited to the above step, and the manufacturing step can be adaptively adjusted according to the structure of the actual line substrate 10.

[0052] Step S3: refer to Figure 8 a second dielectric layer 20 is arranged on the opposite surfaces of the first dielectric layer 11, and the projection of the second dielectric layer 20 on the first dielectric layer 11 does not overlap with the projection of the line layer 13 and the copper layer 15 on the first dielectric layer 11.

[0053] The second dielectric layer 20 is arranged in a spaced manner with the copper layer 15 and the line layer 13. In this embodiment, the thickness of the second dielectric layer 20 is greater than the thickness of the line layer 13 or the copper layer 15 located on the same side of the first dielectric layer 11 as the second dielectric layer 20, which is beneficial to improve the service life of the flexible circuit board 100.

[0054] Step S4: refer to Figure 9 a protective layer 30 is arranged on the surface of the line substrate 10 and the second dielectric layer 20.

[0055] The protective layer 30 is arranged on the opposite surfaces of the line substrate 10, and each protective layer 30 can comprise an adhesive layer 31 and a covering layer 33. One of the adhesive layers 31 covers the line layer 13, the second dielectric layer 20 and the first dielectric layer 11 exposed to the line layer 13, and the other adhesive layer 31 covers the copper layer 15, the second dielectric layer 20 and the first dielectric layer 11 exposed to the copper layer 15, and the covering layer 33 is located on the surface of the adhesive layer 31 away from the line substrate 10.

[0056] Step S5: refer to Figure 3 The electromagnetic shielding layer 40 is pressed on the surface of the protective layer 30 away from the line substrate 10.

[0057] refer to Figure 8as well as Figure 10 In other embodiments, before applying the protective layer 30, the manufacturing method may further include providing a third dielectric layer 25a on one surface of the circuit substrate 10. In this embodiment, the third dielectric layer 25a is located on the surface of the copper layer 15. After applying the protective layer 30, the third dielectric layer 25a is located between the circuit substrate 10 and the protective layer 30. The third dielectric layer 25a can be heated and cured to allow the flexible circuit board 100a to bend in the area where the third dielectric layer 25a is provided.

[0058] The flexible circuit board 100 (or 100a, 100b) provided in the embodiment of the present application is provided with a second dielectric layer 20 of a mixture of LCP and TPU in the dynamic bending area, which is beneficial to improving the bending performance of the flexible circuit board 100 (or 100a, 100b); the second dielectric layer 20 is located between the protective layer 30 and the first dielectric layer 11. During the bending process of the flexible circuit board 100 (or 100a, 100b), the rate of change of the signal impedance can be minimized to achieve dynamic stabilization of the impedance.

[0059] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application should not depart from the spirit and scope of the technical solutions of the present application.

Claims

1. A flexible circuit board, characterized in that: The flexible circuit board comprises: A circuit substrate, comprising a first dielectric layer and a circuit layer stacked in layers; a second dielectric layer, located on two opposite surfaces of the first dielectric layer and not overlapping with a projection of the circuit layer on the first dielectric layer; and a protective layer covering the circuit layer, the first dielectric layer exposed to the circuit layer, and the surface of the second dielectric layer; The material of the second dielectric layer is a mixture of thermoplastic polyurethane elastomer rubber and liquid crystal polymer, and the liquid crystal polymer is dispersed in the thermoplastic polyurethane elastomer rubber in a fibrous form.

2. The flexible circuit board according to claim 1, wherein: Along the stacking direction of the flexible circuit boards, the thickness of the second dielectric layer is greater than the thickness of the circuit layer located on the same side of the first dielectric layer as the second dielectric layer.

3. The flexible circuit board according to claim 1 or 2, characterized in that: The circuit substrate further includes a copper layer, which is located on a surface of the first dielectric layer facing away from the circuit layer. The projection of the copper layer on the first dielectric layer does not overlap with the projection of the second dielectric layer on the first dielectric layer.

4. The flexible circuit board according to claim 3, characterized in that: Along the stacking direction of the flexible circuit boards, the thickness of the second dielectric layer is greater than the thickness of the copper layer located on the same side of the first dielectric layer as the second dielectric layer.

5. The flexible circuit board according to claim 3, wherein: The flexible circuit board further includes a third dielectric layer, which is located on one surface of the same area of ​​the circuit substrate and between the copper layer and the protective layer. The material of the third dielectric layer includes a thermoplastic material.

6. The flexible circuit board according to claim 5, characterized in that: The third dielectric layer also penetrates the copper layer and is connected to the first dielectric layer.

7. The flexible circuit board according to claim 1, wherein: The flexible circuit board further includes an electromagnetic shielding layer, which is located on a surface of the protective layer facing away from the circuit substrate.

8. A wearable device, characterized in that: The wearable device comprises the flexible circuit board according to any one of claims 1 to 7.

9. A method for manufacturing a flexible circuit board, characterized in that: include: Providing a circuit substrate, the circuit substrate comprising a first dielectric layer and a circuit layer stacked; Disposing second dielectric layers on two opposite surfaces of the first dielectric layer, wherein a projection of the second dielectric layer on the first dielectric layer does not overlap with a projection of the circuit layer on the first dielectric layer; as well as Covering the surfaces of the circuit substrate and the second dielectric layer with a protective layer; The material of the second dielectric layer is a mixture of thermoplastic polyurethane elastomer rubber and liquid crystal polymer, and the liquid crystal polymer is dispersed in the thermoplastic polyurethane elastomer rubber in a fibrous form.

10. The method for manufacturing a flexible circuit board according to claim 9, wherein: Before the step of covering the protective layer, the method for manufacturing the flexible circuit board further includes: A third dielectric layer is disposed on one surface of the same area of ​​the flexible circuit board, and the third dielectric layer is located between the circuit substrate and the protective layer, wherein the material of the third dielectric layer includes a thermoplastic material.

11. The method for manufacturing a flexible circuit board according to claim 9, wherein: Before the step of covering the protective layer, the method for manufacturing the flexible circuit board further includes: The step of laminating an electromagnetic shielding layer on the surface of the protective layer.

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