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

Abstract

The invention discloses a flexible circuit board. The flexible circuit board 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 stacked; the second dielectric layers are located on the two opposite surfaces of the first dielectric layer and do not coincide with the projection of the circuit layer on the first dielectric layer; the protection layer covers the circuit layer and the surfaces of the first dielectric layer and the second dielectric layer exposed out of the circuit layer; wherein the second dielectric layer is made of a mixture of thermoplastic polyurethane elastomer rubber and a liquid crystal polymer, and the liquid crystal polymer is dispersed in the thermoplastic polyurethane elastomer rubber in a fibrous form. The invention 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] During use, the wearable device is usually bent repeatedly. 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 , a cavity 60' is usually set in the circuit substrate 10' of the flexible circuit board 100', and an air layer and a dielectric layer 65' are used to bear deformation, bending stress and friction stress. During the bending process of the cavity 60', relative sliding occurs between the two dielectric layers 65' surrounding the cavity 60', resulting in a large change in the thickness of the air layer, which makes the dynamic impedance difference large. 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, comprising 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; 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 dielectric layer and the second dielectric layer exposed to the circuit layer; wherein 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.

[0005] In some embodiments of the present application, along the direction in which the flexible circuit board is stacked, 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 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 also 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, and the electromagnetic shielding layer is located on a surface of the protective layer 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 which are stacked; arranging a second dielectric layer 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 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.

[0013] In some embodiments of the present application, before the step of covering the protective layer, the method for manufacturing a flexible circuit board also includes: setting a third dielectric layer on one of the surfaces 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 set 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 signal impedance can be minimized to achieve dynamic stability of impedance. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0020] Figure 5 Schematic cross-sectional views of flexible printed circuit boards provided in some further embodiments of the present application.

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

[0022] Figure 7 For etching Figure 6 A schematic cross-sectional view of the circuit substrate obtained after the copper layer is shown.

[0023] Figure 8 For Figure 7 The schematic cross-sectional view shows a circuit substrate after second dielectric layers are disposed on two opposite surfaces thereof.

[0024] Fig. 9 For Figure 8 The schematic cross-sectional view shows a circuit substrate and a surface of the second dielectric layer covered with a protective layer.

[0025] Fig.10 A cross-sectional schematic diagram of a circuit substrate after a third dielectric layer is disposed on one surface of the circuit substrate provided in some other embodiments of the present application.

[0026] Main component symbols

[0027] Wearable devices 200 Flexible circuit board 100, 100a, 100b, 100' Circuit board 10、10b、10’ The first dielectric layer 11 Circuit layer 13 Copper Layer 15 Adhesive layer 17b Second dielectric layer 20 The third dielectric layer 25a Protective layer 30 Adhesive layer 31 Covering 33 Electromagnetic shielding layer 40 Double-sided copper clad laminate 50 Cavity 60’ Dielectric layer 65’ DETAILED DESCRIPTION

[0028] In order to more clearly understand the above-mentioned purposes, features and advantages of the present application, the present application is described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, in the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other. In the following description, many specific details are set forth to facilitate a full understanding of the present application, and the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used in the specification of this article are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used herein includes all and any combination of one or more related listed items.

[0030] In the embodiments of the present application, for the convenience of description and not to limit 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", etc. are only used to indicate relative position relationship. When the absolute position of the described object changes, the relative position relationship also changes accordingly.

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

[0032] See also 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 two 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 where the second dielectric layer 20 is provided is a bending area, and the area where the second dielectric layer 20 is not provided is a non-bending area. The flexible circuit board 100 can be dynamically bent multiple times in the area where the second dielectric layer 20 is provided.

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

[0035] The second dielectric layer 20 is located on two 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 circuit layer 13 on the first dielectric layer 11. The material of the second dielectric layer 20 is a mixture of thermoplastic polyurethanes (TPU) and liquid crystal polymers (LCP), and the LCP is dispersed in the TPU in a fibrous form. Among them, TPU has good tensile properties, flexural properties, and wear resistance; LCP has low ductility, and LCP is dispersed in the TPU in a fibrous form, which is beneficial to improve the bending performance of the second dielectric layer 20, thereby improving the service life of the flexible circuit board 100.

[0036] The protective layers 30 are located on two opposite surfaces of the circuit substrate 10, and each protective layer 30 may include an adhesive layer 31 and a covering layer 33. One adhesive layer 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. The covering layer 33 is located on the surface of the adhesive layer 31 away from the circuit substrate 10. The adhesive layer 31 may be a semi-cured film, and the material may be epoxy resin, acrylic acid, or polyimide resin.

[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 of less than 3.5, a Df of less than 0.02, a thermal expansion coefficient of 17-20 PPM / °C, a hygroscopicity of less than 1%, and a glass transition temperature of 220°C. The bonding 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 of less than 3.5, a Df of less than 0.02, a thermal expansion coefficient of 20-30 PPM / °C, a hygroscopicity of less than 0.6%, a peel strength of 0.95 kgf / cm, and a glass transition temperature of 65-75°C. 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%, and a thermal expansion coefficient of 11.6×10 -6mm / ℃, hygroscopicity is less than 0.1%, and peel strength is 1.4-6.1kgf / cm. Among them, the adhesive layer 31 and the second dielectric layer 20 respectively select special tensile strength, Young's modulus, elongation and other relationships, which is conducive to improving the local stretching and flexural performance of the flexible circuit board 100, avoiding stress concentration on other layers during bending, and is conducive to improving the reliability of the bending area. In addition, the position of the neutral axis can be adjusted by matching the selection of the thickness of each layer, so that the design of the flexible circuit board 100 meets the use conditions of stretching and bending.

[0038] The flexible circuit board 100 provided in the embodiment of the present application has a second dielectric layer 20 disposed in the bending area. During the bending process of the flexible circuit board 100, each interface is relatively fixed and no relative sliding occurs, thereby reducing the difference in thickness change and further reducing the difference in dynamic impedance, which is beneficial to maintaining the impedance stability of the flexible circuit board 100 during the bending process.

[0039] Along the direction in which the flexible circuit board 100 is stacked, the thickness of the second dielectric layer 20 is greater than the thickness of the circuit layer 13 or the 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-50 μm. Providing a thicker second dielectric layer 20 is conducive to transferring the neutral axis of the bending radius of the flexible circuit board 100 (i.e., the line where the positive stress values ​​of each point on the intersection line of the cross section and the stress plane are zero in the case of plane bending and oblique bending) to the circuit layer 13 or the copper layer 15. The closer to the neutral axis, the smaller the force, so as to increase 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 protection layer 30 away from the circuit substrate 10 . The electromagnetic shielding layer 40 is used to shield the flexible circuit board 100 from the influence of external electromagnetic signals.

[0041] See also Figure 4Another embodiment of the present application provides a flexible circuit board 100a. Different from the flexible circuit board 100, the flexible circuit board 100a of this embodiment further includes a third dielectric layer 25a. The third dielectric layer 25a is located on one of the surfaces in the same area of ​​the circuit substrate 10, that is, when there are multiple third dielectric layers 25a, 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 a thermoplastic material, such as TPU, and LCP fiber can also be doped in the TPU. In the process of manufacturing 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 be in a static bending shape in the area 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 bonding layer 31 at the same time to increase the coverage area of ​​the copper layer 15 by the third dielectric layer 25a and improve the stress tolerance of the flexible circuit board 100a; the third dielectric layer 25a can also penetrate the copper layer 15, and the two opposite surfaces of the third dielectric layer 25a are respectively connected to the first dielectric layer 11 and the bonding layer 31; the third dielectric layer 25a can also be arranged on the surface of the copper layer 15 and buried in the bonding layer 31.

[0043] In some embodiments, the third dielectric layer 25 a may also be disposed 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 may be 1.2 mm-5 mm, the thickness of the flexible circuit board 100a may be 0.15 mm-0.4 mm, and the bending angle of the flexible circuit board 100a in the third dielectric layer 25a may be 45°-180°.

[0045] The third dielectric layer 25a may be one or more. When the number of the third dielectric layer 25a is more than one, the anti-bending and bending capabilities of the flexible circuit board 100a can be further enhanced to facilitate stress release in the static bending zone and avoid stress concentration on the same material on the same bending axis.

[0046] See also Figure 5 Another embodiment of the present application provides a flexible circuit board 100b. Different from the flexible circuit board 100, the circuit substrate 10b of this embodiment includes a plurality of first dielectric layers 11 and a plurality of circuit layers 13. Adjacent dielectric layers can be bonded by a glue layer 17b, and the circuit layer 13 located between two adjacent dielectric layers can be buried in the glue layer 17b.

[0047] See also Figures 6 to 9 as well as Figure 3 The present application also provides a method for manufacturing a flexible circuit board 100, which may include the following steps:

[0048] Step S1: Please refer to Figure 6 , a double-sided copper-clad laminate 50 is provided, including a first dielectric layer 11 and copper layers 15 located on two opposite surfaces of the first dielectric layer 11 .

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

[0050] Step S2: Please refer to Figure 7 , etching partial areas of the two copper layers 15 to expose two opposite surfaces of the first dielectric layer 11 in partial areas, and performing circuit manufacturing on one of the copper layers 15 to form a circuit layer 13, thereby forming a circuit substrate 10.

[0051] In other embodiments, the steps of forming the circuit substrate are not limited to the above steps, and the manufacturing steps can be adjusted according to the structural adaptability of the actual circuit substrate 10 .

[0052] Step S3: Please refer to Figure 8 A second dielectric layer 20 is disposed on two 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 projections of the circuit layer 13 and the copper layer 15 on the first dielectric layer 11 .

[0053] The second dielectric layer 20 is spaced apart from the copper layer 15 and the circuit layer 13. In this embodiment, the thickness of the second dielectric layer 20 is greater than the thickness of the circuit 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: Please refer to Fig. 9 , a protective layer 30 is covered on the surfaces of the circuit substrate 10 and the second dielectric layer 20 .

[0055] The protective layers 30 are located on two opposite surfaces of the circuit substrate 10, and each protective layer 30 may include an adhesive layer 31 and a covering layer 33. One adhesive layer 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. The covering layer 33 is located on the surface of the adhesive layer 31 away from the circuit substrate 10.

[0056] Step S5: Please refer to Figure 3 The electromagnetic shielding layer 40 is pressed onto the surface of the protection layer 30 facing away from the circuit substrate 10 .

[0057] See also Figure 8as well as Fig.10 In other embodiments, before the step of covering 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 covering 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 may be heated and cured to allow the flexible circuit board 100a to be bent 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, and the flexible circuit board 100 (or 100a, 100b) can minimize the rate of change of signal impedance during the bending process to achieve dynamic stability of impedance.

[0059] The above implementation modes 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 implementation modes, a person skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing 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 is located on two opposite surfaces of the first dielectric layer and does not overlap 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, characterized in that: 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.

3. The flexible circuit board according to claim 1 or 2, characterized in that: The circuit substrate further comprises a copper layer, which is located on a surface of the first dielectric layer 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.

4. The flexible circuit board according to claim 3, characterized in that: 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.

5. The flexible circuit board according to claim 3, characterized in that: The flexible circuit board also 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, characterized in that: The flexible circuit board further includes an electromagnetic shielding layer, and the electromagnetic shielding layer is located on a surface of the protective layer away from the circuit substrate.

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

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; A second dielectric layer is disposed on two opposite surfaces of the first dielectric layer, and 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, characterized in that: 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, characterized in that: 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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