Reinforcing structure for flexible circuit board

Abstract

The utility model discloses a reinforcing structure for a flexible circuit board in the field of circuit board design, which comprises a reinforcing steel sheet, the reinforcing steel sheet is connected with the flexible circuit board through a thermosetting adhesive, the thickness of the thermosetting adhesive is twice of that of a base material of the flexible circuit board, and the thickness of the reinforcing steel sheet is larger than that of the base material of the flexible circuit board. And a plurality of conductive adhesives are arranged between the reinforcing steel sheet and the flexible circuit board. The utility model solves the problems that the impedance of a reinforcing area is greatly reduced and return loss is deteriorated due to the fact that the distance between an existing surface transmission line and a reinforcing steel sheet is close, and the requirement of high-speed signal transmission cannot be met due to insertion loss resonance caused by coupling of the transmission line and the reinforcing steel sheet. According to the utility model, the distance between the surface transmission line and the reinforcing steel sheet can be increased, the capacitance between the transmission line and the reinforcing steel sheet can be weakened, the impedance of a reinforcing area can be improved, the return loss can be improved, the insertion loss resonance problem caused by the backflow difference of the transmission line can be improved, and the transmission line can meet the requirement of high-speed signal transmission.

Description

Technical Field

[0001] The utility model relates to the technical field of circuit board design, in particular to a reinforcement structure for a flexible circuit board. Background Art

[0002] Flexible Printed Circuit (FPC) is a printed circuit made of polyimide with high reliability and excellent flexibility. By embedding the circuit design on a bendable thin plastic sheet, a large number of precision components can be stacked in a narrow and limited space to form a bendable flexible circuit. This circuit can be bent and folded at will, is light in weight, small in size, has good heat dissipation, and is easy to install, breaking through the traditional interconnection technology.

[0003] With the development of the information industry, the signal rate is constantly increasing, 10Gbps, 25Gbps, 56Gbps, 112Gbps. As the signal rate increases, electronic products continue to be miniaturized and lightweight. Chips and connectors are getting smaller and smaller following the trend. Flexible circuit boards, which are widely used in miniaturized devices, are also facing higher speed requirements. In the structure of flexible circuit boards, the components are substrates, cover films, copper foils, adhesives, and reinforcing steel sheets, among which reinforcing steel sheets are used as materials for local reinforcement of flexible circuit boards.

[0004] Usually, the reinforcing steel sheet is directly connected to the flexible circuit board through a thermosetting adhesive. However, the transmission line in the reinforcing steel sheet is coupled with the reinforcing steel sheet, which enhances the capacitance of the transmission line and causes impedance mutation of the transmission line in the reinforcing area, thereby adversely affecting the quality of high-speed signals. When the impedance mutation of high-speed signals is large, it will cause delays, distortion or signal loss in high-speed signals. The appearance of these problems will affect the performance and use of electronic products. In addition, the conventional thickness of the thermosetting adhesive is 1 mil, resulting in a close distance between the surface transmission line and the reinforcing steel sheet. According to the flat plate capacitor formula C = ε × A / d (C is capacitance, ε is dielectric constant of the medium, A is plane area, d is plane distance.) It can be seen that for planes of the same size with a constant dielectric constant, the smaller the distance between the two planes, the larger the flat plate capacitance. Therefore, the spacing between the traces of the flexible circuit board and the steel sheet is too small, which introduces a large amount of capacitance, causing the impedance of the reinforced area to be greatly reduced and the return loss to deteriorate. At the same time, due to the coupling between the transmission line and the reinforced steel sheet, the energy coupled to the reinforced steel sheet has no smooth return channel, resulting in insertion loss resonance, which cannot meet the requirements of high-speed signal transmission.

[0005] The above defects need to be solved urgently. Utility Model Content

[0006] In order to solve the problems that the existing surface transmission line and the reinforcing steel sheet are close to each other, resulting in a significant reduction in the impedance of the reinforcement area, worsening the return loss, and the coupling between the transmission line and the reinforcing steel sheet, resulting in insertion loss resonance, and the requirement of high-speed signal transmission cannot be met, the utility model provides a reinforcement structure for a flexible circuit board.

[0007] The technical solution of the utility model is as follows:

[0008] A reinforcing structure for a flexible circuit board includes a reinforcing steel sheet connected to the flexible circuit board via a thermosetting adhesive, the thickness of the thermosetting adhesive being twice the thickness of a substrate of the flexible circuit board, and a plurality of conductive adhesives being provided between the reinforcing steel sheet and the flexible circuit board.

[0009] According to the above-mentioned solution of the utility model, the thermosetting adhesive is provided with a mounting opening, and the conductive glue is provided on the mounting opening.

[0010] According to the utility model of the above solution, a plurality of slots are arranged on one side of the flexible circuit board close to the reinforcing steel sheet, and a plurality of the conductive adhesives are matched with the corresponding slots.

[0011] According to the above-mentioned solution of the utility model, the orthographic projection area of ​​the slot is the same as that of the mounting opening.

[0012] According to the above-mentioned solution of the utility model, a transmission line is arranged inside the flexible circuit board, and the length direction of the conductive adhesive is arranged along the transmission line direction of the flexible circuit board.

[0013] According to the above solution of the utility model, the portion of the flexible circuit board located below the reinforcing steel sheet is the reinforcement area, and the line width of the transmission line located in the reinforcement area is smaller than the line width of the transmission line located outside the reinforcement area.

[0014] According to the above-mentioned solution of the utility model, the line width of the transmission line located in the reinforcement area is 0.02 mm smaller than the line width of the transmission line located outside the reinforcement area.

[0015] According to the utility model of the above solution, the conductive adhesive is in the shape of a long strip.

[0016] According to the above-mentioned solution of the utility model, the length of the conductive glue is 0.8 mm to 1.1 mm.

[0017] According to the above-mentioned solution of the utility model, there are two conductive adhesives, and the two conductive adhesives are symmetrically arranged on both sides of the reinforcing steel sheet.

[0018] The utility model according to the above scheme has the following beneficial effects:

[0019] In the above reinforcement structure for flexible circuit boards, the reinforcing steel sheet is connected to the flexible circuit board through a thermosetting adhesive, and the thickness of the thermosetting adhesive is twice the thickness of the substrate of the flexible circuit board, that is, the thickness of the thermosetting adhesive is determined according to the thickness of the substrate of the flexible circuit board, which can increase the distance between the surface transmission line and the reinforcing steel sheet, reduce the capacitance between the transmission line and the reinforcing steel sheet, reduce the influence of the reinforcing steel sheet on the transmission line, increase the impedance of the reinforcement area, and increase the return loss. In addition, a number of conductive adhesives are provided between the reinforcing steel sheet and the flexible circuit board, so that the reinforcing steel sheet and the flexible circuit board form a complete reflux, improve the insertion loss resonance problem caused by the reflux difference of the transmission line, and make the transmission line meet the requirements of high-speed signal transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 It is a schematic diagram of the structure of the utility model;

[0022] Figure 2 It is a cross-sectional view of the utility model;

[0023] Figure 3 This is a disassembled diagram of the utility model;

[0024] Figure 4 is a schematic diagram of the structure of the transmission line;

[0025] Figure 5 This is a comparison chart of impedance simulation test between the utility model and the traditional method;

[0026] Figure 6 This is a comparison chart of the insertion loss simulation test of the utility model and the traditional method;

[0027] Figure 7 This is a comparison chart of return loss simulation test between the utility model and the traditional method.

[0028] In the figure, 1. reinforcing steel sheet; 2. thermosetting adhesive; 21. mounting opening; 3. conductive glue; 4. transmission line; 5. flexible circuit board; 6. substrate; 7. GND plane. DETAILED DESCRIPTION

[0029] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] In the structure of flexible circuits, the materials are substrate, cover film, copper foil, adhesive and reinforcing steel sheet. The substrate is made of polyimide core board; the cover film is made of polyimide protective layer, which is located on the upper and lower surfaces of the flexible circuit; copper foil is used to form lines and patterns; adhesive is used to adhere copper foil and cover film, and also to adhere reinforcing steel sheet; reinforcing steel sheet is used as a material for local reinforcement of flexible circuit boards.

[0031] In the prior art, the reinforcing steel sheet is directly connected to the flexible circuit board through a thermosetting adhesive, and the thickness of the thermosetting adhesive is usually 1 mil, which makes the distance between the surface transmission line on the flexible circuit board and the reinforcing steel sheet relatively narrow. According to the basic formula of flat plate capacitor C = ε × A / d (where C represents capacitance, ε is the dielectric constant of the medium, A is the plane area, and d is the plane spacing), when the dielectric constant and area are fixed, the reduction in spacing will directly lead to an increase in capacitance, which not only introduces additional capacitance, but also greatly reduces the impedance of the reinforced area, thereby worsening the return loss. In addition, due to the close coupling between the transmission line and the reinforcing steel sheet, the energy that should have been coupled to the steel sheet causes insertion loss resonance due to the lack of an effective return channel, further exacerbating the trouble of signal transmission.

[0032] like Figure 1 , Figure 2 As shown, in order to solve the above technical problems, the utility model provides a reinforcement structure for a flexible circuit board, including a reinforcement steel sheet 1, which is connected to a flexible circuit board 5 through a thermosetting adhesive 2, and the thickness of the thermosetting adhesive 2 is twice the thickness of a substrate 6 of the flexible circuit board 5, that is, the thickness of the thermosetting adhesive 2 is determined according to the thickness of the substrate 6 of the flexible circuit board 5, which can increase the distance between the surface transmission line 4 and the reinforcement steel sheet 1, reduce the capacitance between the transmission line 4 and the reinforcement steel sheet 1, reduce the influence of the reinforcement steel sheet 1 on the transmission line 4, and improve the impedance of the reinforcement area, so that the flexible circuit board 5 can better resist external interference and signal attenuation, thereby ensuring the integrity and accuracy of the signal, and improving the return loss, thereby further improving the overall performance of the circuit board.

[0033] In this embodiment, the material of the reinforcing steel sheet 1 can be designed to be stainless steel. The stainless steel sheet has extremely high strength and can effectively enhance the overall mechanical strength of the flexible circuit board 5, so that it has higher resistance to bending and stretching when facing external forces. At the same time, stainless steel has excellent thermal conductivity and can effectively dissipate the heat generated on the flexible circuit board 5 to avoid performance degradation or damage caused by overheating. In addition, stainless steel also has excellent corrosion resistance and can maintain stable performance even under harsh environmental conditions, thereby extending the service life of the flexible circuit board 5. Of course, the material of the reinforcing steel sheet 1 can also be designed to be FR4. FR4 is a glass fiber reinforced epoxy resin-based composite material with excellent electrical properties and mechanical strength. FR4 can also enhance the mechanical strength of the flexible circuit board 5, and its electrical properties are stable, with little impact on signal transmission. In actual design, the material of the reinforcing steel sheet 1 can be designed according to actual needs.

[0034] like Figure 2 , Figure 3 As shown, in this embodiment, a plurality of conductive adhesives 3 are further provided between the reinforcing steel sheet 1 and the flexible circuit board 5 to enhance the electrical connection between the reinforcing steel sheet 1 and the flexible circuit board 5, so that the reinforcing steel sheet 1 and the flexible circuit board 5 form a complete reflux, improve the insertion loss resonance problem caused by the reflux difference of the transmission line 4, and enable the transmission line 4 to meet the requirements of high-speed signal transmission. Specifically, a mounting opening 21 is provided on the thermosetting adhesive 2, and the conductive adhesive 3 is provided on the mounting opening 21, so that the reinforcing steel sheet 1 and the flexible circuit board 5 are tightly connected together, effectively improving the insertion loss resonance problem caused by the reflux difference of the transmission line 4. In the process of high-frequency signal transmission, the reflux difference often leads to signal distortion and energy loss, thereby affecting the overall performance of the circuit board. The introduction of the conductive adhesive 3 forms a complete reflux path between the reinforcing steel sheet 1 and the flexible circuit board 5, reduces the reflux difference, and thus reduces the probability of the occurrence of insertion loss resonance.

[0035] In this embodiment, a plurality of slots are provided on one side of the flexible circuit board 5 close to the reinforcing steel sheet 1. After the flexible circuit board 5 is slotted, a plurality of conductive adhesives 3 are matched with the corresponding slots. After the conductive adhesive 3 is placed in the mounting opening 21, the bottom of the conductive adhesive 3 is set in the slot, and the conductive adhesive 3 is connected to the GND plane 7 of the flexible circuit board 5, so that the reinforcing steel sheet 1 and the flexible circuit board 5 can form a complete reflux, and improve the insertion loss resonance problem caused by the reflux difference of the transmission line 4. In addition, the orthographic projection area of ​​the slot is the same as that of the mounting opening 21, that is, the area of ​​the slot is the same as the bottom area of ​​the mounting opening 21, and the shape and size are the same, so as to realize a stable connection and a complete reflux path between the reinforcing steel sheet 1 and the flexible circuit board 5, which not only improves the electrical performance and stability of the circuit board, but also provides a strong guarantee for the reliable operation of the entire system.

[0036] like Figure 2 , Figure 3 As shown, in this embodiment, the conductive adhesive 3 is in the shape of a long strip, a transmission line 4 is arranged inside the flexible circuit board 5, and the length direction of the conductive adhesive 3 is arranged along the direction of the transmission line 4 of the flexible circuit board 5, that is, the conductive adhesive 3 extends along the path of the transmission line 4, thereby providing a more stable and efficient electrical connection for it. When the current flows on the transmission line 4, the conductive adhesive 3 can ensure the stable transmission of the current and reduce the insertion loss resonance problem caused by the poor reflux, which not only improves the electrical performance of the flexible circuit board 5, but also enhances its stability and reliability. Of course, in actual design, the shape of the conductive adhesive 3 can be designed according to actual needs.

[0037] like Figure 2 , Figure 3 As shown, in this embodiment, the length of the conductive glue 3 is 0.8mm-1.1mm, and the length of the conductive glue 3 is less than the length of the reinforcing steel sheet 1. The length of the conductive glue 3 can be designed to be any value in the range of 0.82mm, 0.84mm, 0.85mm, 0.87mm, 0.88mm, 0.89mm, 0.9mm, 0.92mm, 0.95mm, 0.97mm, 0.99mm, 1.0mm, 1.1mm, etc. In actual design, the length of the conductive glue 3 can be designed according to actual needs. In addition, there are two conductive glues 3, and the two conductive glues 3 are symmetrically arranged on both sides of the reinforcing steel sheet 1 to ensure that the current can be evenly distributed during the transmission process, avoiding the thermal effect or electrical performance degradation caused by current concentration, and the two conductive glues 3 are respectively located on both sides of the reinforcing steel sheet 1, providing two parallel return paths for the current, thereby effectively reducing the risk of insertion loss resonance.

[0038] like Figure 2 , Figure 3 As shown, in this embodiment, the portion of the flexible circuit board 5 located below the reinforcing steel sheet 1 is the reinforcement area, and the line width of the transmission line 4 located in the reinforcement area is smaller than the line width of the transmission line 4 located outside the reinforcement area, that is, the line width of the impedance line in the reinforcement area is adjusted to adapt to different plates and impedance schemes. The portion of the transmission line 4 located outside the reinforcement area is a microstrip line, and the portion of the transmission line 4 located inside the reinforcement area is a stripline. Therefore, the transmission line 4 in the reinforcement area needs to use a stripline model for impedance calculation, and the stripline model is selected for calculation using the impedance calculation software. It should be noted that the stripline model and the impedance calculation software are prior art, and the present utility model has not made any improvements to this part, so its principles and processes will not be described in detail.

[0039] like Figure 3 , Figure 4As shown, in this embodiment, the line width of the transmission line 4 located in the reinforcement area is 0.02 mm smaller than the line width of the transmission line 4 located outside the reinforcement area, that is, the performance of the transmission line 4 in the reinforcement area is further optimized by adjusting the line width. In addition, the line width of the transmission line 4 located in the reinforcement area can be designed to be 0.05 mm, and the line width of the transmission line 4 located outside the reinforcement area can be designed to be 0.07 mm. Of course, in actual design, the line width of the transmission line 4 can be designed according to actual needs.

[0040] like Figure 5 As shown, compared with the impedance simulation test of the conventional method, the impedance of the reinforcement area of ​​the present invention is higher, so that the impedance of the transmission line 4 in the reinforcement area is increased by 27 ohms. The solid line is the simulation test result of the present invention, and the dotted line is the simulation test result of the conventional method.

[0041] like Figure 6 As shown, compared with the insertion loss simulation test of the conventional method, the insertion loss of the present invention has no resonance. The solid line is the simulation test result of the present invention, and the dotted line is the simulation test result of the conventional method.

[0042] like Figure 7 As shown, compared with the return loss simulation test of the traditional method, the return loss of the utility model is smaller, and the return loss is improved by 32dB. Among them, the solid line is the simulation test result of the utility model, and the dotted line is the simulation test result of the traditional method.

[0043] It should be noted that the indicated orientation or position relationship is based on the orientation or position relationship shown in the drawings, or is the orientation or position relationship in which the application product is usually placed when used, or is the orientation or position relationship commonly understood by technical personnel in this field, or is the orientation or position relationship in which the application product is usually placed when used. It is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application.

[0044] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the claims attached to the utility model.

[0045] The above is an exemplary description of the utility model patent in conjunction with the accompanying drawings. It is obvious that the implementation of the utility model patent is not limited to the above-mentioned method. As long as various improvements are made by adopting the method concept and technical solution of the utility model patent, or the concept and technical solution of the utility model patent are directly applied to other occasions without improvement, they are all within the protection scope of the utility model.

Claims

1. A reinforcement structure for a flexible circuit board, characterized in that: It includes a reinforcing steel sheet, which is connected to a flexible circuit board through a thermosetting adhesive. The thickness of the thermosetting adhesive is twice the thickness of the base material of the flexible circuit board, and some conductive glue is also arranged between the reinforcing steel sheet and the flexible circuit board.

2. The reinforcement structure for a flexible circuit board according to claim 1, characterized in that: The thermosetting adhesive is provided with a mounting opening, and the conductive glue is arranged on the mounting opening.

3. The reinforcement structure for a flexible circuit board according to claim 2, characterized in that: A plurality of slots are arranged on one side of the flexible circuit board close to the reinforcing steel sheet, and a plurality of the conductive adhesives are matched with the corresponding slots.

4. The reinforcement structure for a flexible circuit board according to claim 3, characterized in that: The orthographic projection area of ​​the slot is the same as that of the mounting opening.

5. The reinforcement structure for a flexible circuit board according to claim 1, characterized in that: A transmission line is arranged inside the flexible circuit board, and the length direction of the conductive adhesive is arranged along the transmission line direction of the flexible circuit board.

6. The reinforcement structure for a flexible circuit board according to claim 5, characterized in that: The portion of the flexible circuit board located below the reinforcing steel sheet is a reinforcing area, and the line width of the transmission line located in the reinforcing area is smaller than the line width of the transmission line located outside the reinforcing area.

7. The reinforcement structure for a flexible circuit board according to claim 6, characterized in that: The line width of the transmission line located in the reinforcement area is 0.02 mm smaller than the line width of the transmission line located outside the reinforcement area.

8. The reinforcement structure for a flexible circuit board according to claim 1, characterized in that: The conductive adhesive is in the shape of a long strip.

9. The reinforcement structure for a flexible circuit board according to claim 1, characterized in that: The length of the conductive adhesive is 0.8 mm to 1.1 mm.

10. The reinforcement structure for a flexible printed circuit board according to claim 1, characterized in that: There are two conductive adhesives, and the two conductive adhesives are symmetrically arranged on both sides of the reinforcing steel sheet.

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