A method for preparing a flexible multi-layer circuit board

Abstract

A method for preparing a flexible multilayer circuit board and its application in a flat flexible vibration-resistant circuit board and a high thermal conductivity curved conformal circuit board, specifically: using 3D printing technology to directly form a curved flexible substrate with a conductive circuit, a heat dissipation structure, an electromagnetic shielding structure, and a through-hole portion of the interlayer interconnection channel, and then forming a multilayer flexible circuit board with high conductivity, good heat dissipation, and excellent electromagnetic shielding by pouring a multifunctional slurry. The present invention achieves the preparation of large-area conductive layer, thermal conductive layer, and electromagnetic shielding layer channels by designing a micro-support structure. The hydrophilic group hydroxyl group on the channel surface is greatly increased by ozone or oxygen plasma treatment, which significantly improves the wettability of the substrate and the slurry; the preparation method of the present invention can take into account both the thermal conductivity and electromagnetic shielding performance of the circuit board, forming an integrated packaging structure, and greatly improving the preparation efficiency of the flexible circuit board.

Description

Technical Field

[0001] The present invention relates to a method for preparing a flexible multi-layer circuit board. In particular, the method for preparing a flexible circuit board can realize integrated interlayer interconnection of multi-layer circuits and integrated packaging of circuit boards by means of substrate additive manufacturing and conductive paste injection, without the need for lamination, etching, punching, electroplating and other processes in the preparation of traditional circuit boards. Background Art

[0002] Circuit boards are the carriers of electronic devices and the cornerstones of their functions. They have the functions of electrical signal transmission, mechanical support, and heat dissipation, and are an indispensable part of electronic products. In recent years, flexible electronic devices have continued to develop, and the circuit interconnection and mechanical interconnection structures between different flexible devices have become increasingly complex. For example, the continuous integration of wearable devices has put forward the demand for multi-layer and flexible circuit boards, which has brought about the difficulty of preparing multi-layer flexible circuit boards.

[0003] At present, flexible circuit boards mainly use flexible and bendable materials such as polyimide as substrates, which can be bent to a certain extent. However, the substrates are mostly molded by mold casting, which makes it difficult to prepare circuit boards with complex shapes and has poor conformality. There are two main methods for preparing conductive circuits: copper etching and conductive paste screen printing. Both methods can prepare large-area conductive circuits. Among them, the circuits obtained by copper etching have good conductivity but low efficiency. Screen printing has low cost and high efficiency, but it cannot prepare conductive circuits for curved circuit boards. As the line width and line spacing of conductive lines become smaller and smaller, both methods are difficult to meet the preparation requirements of multi-layer circuit boards for high-precision printing and alignment technology. In addition, the interlayer interconnection of multi-layer circuit boards often adopts conductive through holes, conductive blind holes, and conductive buried holes, which are generally punched by mechanical drilling or laser drilling, but the above methods are difficult to accurately prepare small-aperture holes, and it is also difficult to achieve high-reliability interconnection between layers of small-line width and line spacing conductive circuits. Moreover, the subsequent metallization of interconnection holes also requires multiple complex processes such as cleaning, activation, and electroplating. In addition to the preparation process of flexible circuit boards, the application of flexible devices is further restricted due to the poor thermal conductivity and electromagnetic shielding performance of flexible substrate materials.

[0004] In summary, it is necessary to develop more efficient methods for preparing multi-layer flexible circuit boards, especially high-precision methods for preparing multi-layer conductive circuits and high-reliability interlayer interconnection technologies. At the same time, the thermal conductivity and electromagnetic shielding performance of flexible circuit boards must be further optimized. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a flexible multilayer circuit board, which aims to use 3D printing technology to directly integrate a flexible substrate with a conductive circuit, a heat dissipation structure, an electromagnetic shielding structure, and a through-hole portion of an interlayer interconnection, and then realize the integrated packaging of the conductive circuit, the heat dissipation structure, and the electromagnetic shielding structure by pouring a multifunctional slurry, and finally form a multilayer flexible circuit board with high conductivity, good heat dissipation, and excellent electromagnetic shielding.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a flexible multilayer circuit board, the method comprising:

[0008] (1) Determination of the circuit board shape: The three-dimensional structure design of the curved or flat multi-layer flexible circuit board is carried out according to the circuit board curvature radius, thickness, size, etc. The three-dimensional structure design refers to the shape and thickness of the circuit board, and irregular shaped circuit boards can be manufactured;

[0009] (2) Interlayer circuit design: Graphic and structural design of the conductive layer, thermal conductive layer, and electromagnetic shielding layer, with 2 to 20 layers. Graphic and structural design refers to the shape of each layer and how the layers are connected.

[0010] (3) Circuit channel conversion: converting the structure designed in (2) into a channel and embedding it into the three-dimensional structure of the circuit board in (1), wherein the channel cross section is circular or rectangular and the cross-sectional size is variable;

[0011] (4) Circuit board opening design: design the slurry injection hole, outflow hole and vent hole based on (1) the shape of the circuit board substrate and (3) the channel structure and distribution;

[0012] (5) 3D file conversion: converting the 3D structure obtained by (1)-(4) into a file format suitable for 3D printing and slicing it;

[0013] (6) Preparation of 3D printing photocurable materials: mixing a photoinitiator, a crosslinking agent, and one of polyimide, polyurethane, and acrylic resin to prepare a 3D printing ink;

[0014] (7) Circuit board substrate molding: Use UV-curing 3D printer to mold flexible circuit board substrate;

[0015] (8) Channel cavity molding: Ventilate the holes on the surface of the substrate to discharge the residual printing ink in the channel, and use ultraviolet light to irradiate for secondary curing for 10min-30min to improve the fatigue resistance of the substrate and prevent cracking;

[0016] (9) Post-treatment of substrate: After the substrate is secondary cured in (8), it is dried and placed in an ozone environment for 6 h to 18 h, or treated with oxygen plasma for 3 h to 6 h, so as to significantly improve the wettability of the substrate surface to the slurry;

[0017] (10) Slurry preparation: using organic solvents and surfactants, and one of nano-metal conductive particles, liquid metal and conductive polymer to prepare high-performance and high-wettability conductive slurry, thermal conductive slurry and electromagnetic shielding slurry;

[0018] (11) interlayer circuit formation: injecting the slurry prepared in (10) into the channel of the flexible circuit board substrate to realize interlayer circuit preparation and interlayer interconnection; specifically, injecting the conductive slurry prepared in (10) into the conductive channel of the flexible circuit board substrate to realize conductive circuit preparation and interlayer interconnection; injecting the thermal conductive slurry into the thermal conductive structure channel to form a thermal conductive layer; injecting the electromagnetic shielding slurry into the electromagnetic shielding structure channel to form an electromagnetic shielding layer;

[0019] (12) Post-processing of interlayer circuits: The flexible multilayer circuit board initially prepared in (11) is treated at 100°C-200°C for 1-2h to sinter and solidify the slurry to improve electrical conductivity, thermal conductivity and electromagnetic shielding properties; infrared light is used to further densify the slurry and improve various performances.

[0020] Furthermore, in step (1), the radius of curvature of the circuit board at each location should be greater than 30 mm, and the thickness of the circuit board should be 0.5 mm-10 mm; in step (3), the cross-sectional diameter or side length should be 100 μm-1000 μm; when the length of the channel exceeds 20 mm, a micro support column structure should be designed to prevent collapse during subsequent 3D printing.

[0021] Furthermore, in step (2), the pattern and structure design should not only meet the electrical performance requirements, but also facilitate slurry pouring and ensure that at least two vertical channels are connected between the two connected layers; the thermal conductive layer should be as close to the conductive layer as possible and more distributed near the conductive layer with a larger current carrying capacity or a higher frequency. The thermal conductive layer is wired according to the heat generation, and the wiring is denser in places with more heat generation.

[0022] Furthermore, in step (4), the cross-sectional diameter or side length of the opening should be 1.2-1.5 times that of the channel, and the surface should be chamfered to facilitate the inflow and outflow of the slurry; the slurry injection hole is located on the upper surface of the substrate, the slurry outflow hole is located on the lower surface of the substrate, and the air outlet can be selected from a slurry injection hole on the upper surface; for a planar substrate, the holes should be distributed near the center and edge of the substrate; for a curved substrate, the injection holes are distributed at a higher height and the outflow holes are distributed at a lower height, and the air outlet is selected from the highest point of the substrate; in addition, the surface openings should be as far away from the interlayer interconnection channel as possible to facilitate the slurry to better fill the channel.

[0023] Furthermore, in step (5), the file format is one of .stl, .step, and .ctb; the slicing direction is the Z direction, and the slicing thickness is 0.02 mm-0.10 mm.

[0024] Furthermore, in step (6), the polyimide is composed of hydroxyethyl methacrylate grafted polyamino acid (PAA-xHEMA), trimethylolpropane triacrylate (TMPTA), and N,N-dimethylformamide (DMF). The stiffness and modulus of the flexible circuit board substrate are achieved by adjusting the ratio of polyimide. The greater the proportion of hydroxyethyl methacrylate grafted polyamino acid, the greater the stiffness and modulus of the flexible substrate after UV curing.

[0025] Furthermore, in step (7), the ultraviolet wavelength used in the photocuring 3D printing is 200nm-405nm, the exposure time of each layer is 2s-5s, the exposure time of the bottom layer is 10s-40s, and the printing starting point is the upper right corner of the XY plane.

[0026] Furthermore, in step (8), in the ink discharge method, for the interconnected channels, any surface hole is selected as the air outlet, and air is injected into the other holes at the same time, with a pressure of 0.15MPa-0.40MPa, until no ink is discharged from the air outlet; before the secondary curing, the substrate is cleaned and dried with anhydrous ethanol, and during the secondary curing, the substrate is immersed in deionized water to reduce the residual stress of the substrate and prevent the substrate from discoloring.

[0027] Furthermore, in step (10), the nano-metal conductive particles are one of nano-silver, nano-copper, nano-gold, copper-silver core-shell or gold-silver solid solution; the liquid metal is various gallium-based and indium-based composite materials; the conductive polymer is one of polyaniline, polypyrrole, poly (3,4-ethylenedioxythiophene) / polystyrene sulfonate (PEDOT:PSS); the organic solvent can be selected from one of isopropyl alcohol, ethylene glycol, polyethylene glycol, diethylene glycol, pinene alcohol or xylene according to the affinity with the substrate and viscosity requirements; the ratio of nanoparticles can be adjusted in each slurry according to the requirements of conductivity, thermal conductivity and electromagnetic shielding; the viscosity range of the slurry is 10-3000cp; the surfactant is non-ionic, and the added mass fraction is 0.1%-0.5%. For example, the thermal conductive paste is made by mixing silver nanoparticles and polyethylene glycol. Nano copper paste, nano gold paste or alloy pastes such as copper-silver core-shell and gold-silver solid solution particles can also be used. The solvent used in the conductive paste is isopropyl alcohol, diethylene glycol or xylene.

[0028] Furthermore, in step (11), in the conductive slurry pouring process, first, a rubber plug is used to seal all the conductive slurry outlets at the bottom of the circuit board, and a conductive slurry inlet is reserved at the top for air discharge. A syringe is used to inject the conductive slurry at the remaining top slurry inlets at the same time, and the injection pressure is 0.15MPa-0.25MPa until the conductive slurry flows out continuously from the reserved holes; the rubber plug at the bottom of the circuit board is removed, and the conductive slurry is injected into all the top conductive slurry inlets at an injection pressure of 0.25MPa-0.30MPa. At the same time, use a syringe to apply a negative pressure of 0.01MPa-0.03MPa to all the bottom conductive paste outlets until the conductive paste flows out continuously from all the bottom conductive paste outlets; due to the excellent wettability of the paste and the substrate, the surface circuit will be automatically filled with the paste, or the paste can be coated on the surface after pouring, and then a scraper can be used to print the paste into the surface channel; the pouring of thermal conductive paste and electromagnetic shielding paste is similar to that of conductive paste. Due to their large viscosity, the outlet negative pressure should be appropriately increased to 0.03MPa-0.05MPa.

[0029] Compared with the existing method for preparing a flexible multilayer circuit board, the present invention has the following advantages:

[0030] (1) The preparation method of the present invention realizes the preparation of a flexible multilayer circuit board through 3D printing integrated substrate molding and integrated slurry infusion, which can take into account both the thermal conductivity and electromagnetic shielding performance of the circuit board, forming an integrated packaging structure, and greatly improving the preparation efficiency of the flexible circuit board.

[0031] (2) The present invention has wide compatibility. In terms of structural compatibility, flexible circuit boards of various complex shapes and various functional structures can be prepared as needed through 3D printing, and the device has strong conformality and functionality. In terms of material compatibility, the functional structure is prepared by injection, which is compatible with slurries with a wide viscosity range, and the functional slurry can be selected or modified as needed. In terms of equipment compatibility, this method is compatible with various types of photocuring printers and has low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the structure and principle of a flat flexible vibration-resistant circuit board and a flexible multi-layer circuit board;

[0033] Figure 2 It is a schematic diagram of the slurry channel structure and principle of the present invention;

[0034] Figure 3 This is a schematic diagram of the structure and principle of a curved conformal high thermal conductivity multi-layer circuit board;

[0035] Among them, 1-slurry injection hole, 2-surface channel, 3-flexible substrate, 4-slurry outflow hole, 5-internal channel, 6-interlayer interconnection channel, 7-micro support column, 8-air vent, 9-fixed metal part, 10-thermal conductive slurry injection hole, 11-conductive slurry injection hole, 12-thermal conductive slurry channel, 13-conductive slurry channel, 14-conductive slurry outflow hole, 15-thermal conductive slurry outflow hole. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0037] The principle of the present invention is: to realize the integrated molding and packaging of flexible multi-layer circuit boards by means of 3D printing additive manufacturing and slurry infusion; to realize the preparation of large-area conductive layer, thermal conductive layer and electromagnetic shielding layer channels by designing micro-support structures. The hydrophilic group hydroxyl group on the channel surface is greatly increased by ozone or oxygen plasma treatment, which significantly improves the wettability of the substrate and the slurry; by adding non-ionic surfactants to the functional slurry, according to the principle of polar opposites repel each other, its hydrophilic group is attracted by water and dissolved in water, and its lipophilic group is repelled by water and leaves water, so that the surfactant molecules are adsorbed on the interface between the two phases, and the interfacial tension between the slurry and the substrate is significantly reduced; finally, under the joint action of gravity, air pressure and wetting force, the slurry fills the narrow channel to form a conductive layer, a thermal conductive layer and an electromagnetic shielding layer.

[0038] Embodiment 1:

[0039] An application scheme of a flexible multilayer circuit board method in a flat flexible vibration-resistant circuit board, the specific implementation steps are as follows:

[0040] (1) Circuit board design

[0041] The flexible circuit board has a size of 100mm×100mm, is designed as 5 layers, has a circuit board thickness of 6mm, and a layer spacing of 0.8mm; the conductive channel cross-section is designed to be rectangular with a cross-sectional size of 200μm×100μm; the interlayer interconnection channel cross-section is designed to be circular with a cross-sectional diameter of 300μm.

[0042] (2) Preparation of 3D printing photocurable materials:

[0043] 50 g of hydroxyethyl methacrylate grafted polyamino acid was weighed with an electronic balance and then dissolved in 300 ml of a solvent in which trimethylolpropane triacrylate and N,N-dimethylformamide were mixed in a volume ratio of 1:1; the mixture was stirred with a magnetic stirrer in a 50°C water bath for 1 hour to obtain the desired 3D printing photocurable material.

[0044] (3) 3D printing of flat flexible vibration-resistant circuit board substrates:

[0045] The designed CAD model was sliced ​​along the Z direction with a slice thickness of 0.03 mm. A 405 nm UV-curing 3D printer was used. The exposure time for each layer was set to 5 s and the exposure time for the bottom layer was 40 s. The obtained circuit board substrate was partially as follows Figure 1 shown.

[0046] (4) Cleaning and post-processing of substrates

[0047] For each connected channel, a bottom slurry outflow hole is reserved, and the remaining holes are passed through 0.2MPa compressed air until no 3D printing ink flows out of the bottom reserved hole. The substrate is soaked in anhydrous ethanol for 1 hour and then dried. Finally, the substrate is soaked in deionized water and placed in a 405nm ultraviolet curing box for another 10 minutes to improve the fatigue resistance of the substrate and prevent cracking. The substrate is placed in an ozone environment for 8 hours to greatly improve the wettability of the substrate surface to the slurry.

[0048] (5) Prepare conductive paste:

[0049] 14 g of silver nanoparticles with a particle size of 60 nm were weighed with an electronic balance, added to a mixed solution of 6 g of diethylene glycol and isopropanol in a mass ratio of 1:1, and stirred at 600 r / min with a magnetic stirrer for 10 min to evenly disperse the nanoparticles in the solution; then 30 mg of a non-ionic fluorocarbon surfactant was added and stirred with a magnetic stirrer, and finally ultrasonicated for 5 min to obtain the desired conductive slurry.

[0050] (6) Conductive slurry pouring:

[0051] by Figure 2 The conductive paste pouring process is described. First, use a rubber plug to seal all the conductive paste outflow holes 4 at the bottom of the circuit board, reserve an air outlet 8 at the top for air discharge, and use a syringe with a sealing ring to inject the conductive paste at the paste injection hole 1. The injection pressure is 0.15MPa, until the conductive paste flows out continuously from the air outlet 8; remove the rubber plug at the bottom of the circuit board, inject the conductive paste at all the top conductive paste inlets, and the injection pressure is 0.25MPa. At the same time, use a syringe to apply a negative pressure of 0.01MPa at all the bottom conductive paste outlets, until the conductive paste flows out continuously from all the bottom conductive paste outlets. The chamfer at the interlayer interconnection channel 6 is for the slurry to flow down better. Due to the excellent wettability of the slurry and the substrate, the surface circuit will be automatically filled with the slurry. The slurry can also be applied on the surface after pouring, and then a scraper can be used to print the slurry into the surface channel.

[0052] (7) Slurry sintering:

[0053] The circuit board filled with conductive paste was placed in a hot air oven at 200°C for 1 hour to form sintering necks between the nanoparticles. The circuit board was then placed in an infrared drying oven for 20 minutes to further densify the slurry sintered body. The test showed that the average line resistance of the conductive line of the flat flexible vibration-resistant circuit board was 0.8Ω / cm, and the resistance increase did not exceed 15% when the deformation was 10%.

[0054] Embodiment 2:

[0055] An application scheme of a flexible multi-layer circuit board method in a high thermal conductivity curved surface conformal circuit board, the specific implementation steps are as follows:

[0056] (1) Circuit board design

[0057] The dimensions of the flexible circuit board are arc length × width of 250mm × 250mm. It is designed with 6 layers, including 4 layers of conductive circuits and 2 layers of thermal conductive circuits. The circuit board thickness is 5mm and the interlayer spacing is 0.6mm. The cross-section of the conductive channel is designed to be circular with a cross-sectional diameter of 150μm. The cross-sectional size of the interlayer interconnection channel is designed to be rectangular with a cross-sectional size of 200μm × 200μm.

[0058] (2) Preparation of 3D printing photocurable materials:

[0059] 45 g of hydroxyethyl methacrylate grafted polyamino acid was weighed with an electronic balance and then dissolved in 600 ml of a solvent of trimethylolpropane triacrylate and N,N-dimethylformamide mixed in a volume ratio of 1:1; the mixture was stirred with a magnetic stirrer in a 50°C water bath for 1 hour to obtain the desired 3D printing photocurable material.

[0060] (3) 3D printing of curved conformal circuit board substrates:

[0061] According to the designed CAD model, slices were sliced ​​along the Z direction with a slice thickness of 0.02 mm. A 350nm UV-curing 3D printer was used. The exposure time for each layer was set to 3s and the exposure time for the bottom layer was 30s. The schematic diagram of the circuit board substrate structure was as shown below: Figure 3 shown.

[0062] (4) Cleaning and post-processing of substrates

[0063] For each connected channel, a bottom slurry outflow hole is reserved, and the remaining holes are passed through 0.25MPa compressed air until no 3D printing ink flows out of the bottom reserved hole. The substrate is soaked in anhydrous ethanol for 1 hour and then dried. Finally, the substrate is soaked in deionized water and placed in a 350nm ultraviolet curing box for another 10 minutes to improve the fatigue resistance of the substrate and prevent cracking. The substrate is placed in oxygen plasma for 4 hours to greatly improve the wettability of the substrate surface to the slurry.

[0064] (5) Prepare conductive paste:

[0065] 14 g of silver nanoparticles with a particle size of 20 nm were weighed with an electronic balance, added to a mixed solution of 6 g of xylene and isopropanol in a mass ratio of 1:1, and stirred at 500 r / min with a magnetic stirrer for 10 min to evenly disperse the nanoparticles in the solution; then 30 mg of a non-ionic fluorocarbon surfactant was added and stirred with a magnetic stirrer, and finally ultrasonicated for 3 min to obtain the desired conductive slurry.

[0066] (6) Configure thermal conductive slurry:

[0067] 16g of silver nanoparticles were weighed with an electronic balance, 4ml of polyethylene glycol was added thereto with a pipette, and a magnetic stirrer was used to stir the nanoparticles so that they were evenly dispersed in the solution to form a paste slurry. The thermal conductive slurry here can also be selected from nano copper slurry, nano gold slurry, or alloy slurry such as copper-silver core-shell, gold-silver solid solution particles, etc.

[0068] (7) Slurry pouring:

[0069] by Figure 3 The cross section of the circuit board is used as an example to illustrate the conductive paste pouring process. First, a rubber plug is used to seal the conductive paste outflow hole 14 at the bottom of the circuit board, and the opening at the highest point is used as the air outlet. The conductive paste is injected into the slurry injection holes distributed on both sides of the upper surface of the circuit board using a syringe, and the injection pressure is 0.12MPa, until the conductive paste flows out continuously from the top air outlet 3; the rubber plug at the bottom of the circuit board is removed, and the conductive paste is injected into all the top conductive paste inlets, and the injection pressure is 0.20MPa. At the same time, a negative pressure of 0.02MPa is applied to all the bottom conductive paste outlets using a syringe, until the conductive paste flows out continuously from all the bottom conductive paste outlets. The pouring of thermal conductive paste is similar to that of conductive paste. In particular, the air outlet of thermal conductive paste is also located at the highest point of the upper surface of the substrate, but it is not in the cross section of this schematic diagram; due to the high viscosity of thermal conductive paste, the outlet negative pressure is appropriately increased to 0.04MPa; finally, both sides of the conductive paste are in contact with the metal fixings, which improves the heat dissipation performance.

[0070] (8) Slurry sintering:

[0071] The circuit board filled with conductive paste was placed in a hot air oven at 150°C for 1.5 hours to form sintering necks between the nanoparticles. The circuit board was then placed in an infrared drying oven for 20 minutes to further densify the slurry sintered body.

[0072] The above are only preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a flexible multilayer circuit board, characterized in that: The method is: Step (1): Determine the appearance of the circuit board: Design the three-dimensional structure of the multi-layer flexible circuit board as required; Step (2): Interlayer circuit design: Conduct graphic and structural design of the conductive layer, thermal conductive layer, and electromagnetic shielding layer, with the number of layers ranging from 2 to 20; Step (3): Circuit channel conversion: converting the structure designed in step (2) into a channel and embedding it into the three-dimensional structure of the circuit board in step (1), wherein the cross section of the channel is circular or rectangular; Step (4): Circuit board opening design: design the slurry injection hole, outflow hole and vent hole based on the circuit board substrate shape in step (1) and the channel structure and distribution in step (3); Step (5): 3D file conversion: converting the 3D structure obtained in steps (1) to (4) into a file format suitable for 3D printing and slicing it; Step (6): Preparation of 3D printing photocurable materials: mixing a photoinitiator, a crosslinking agent, and one of polyimide, polyurethane, and acrylic resin to prepare a 3D printing ink; Step (7): Circuit board substrate molding: using a UV-curing 3D printer to mold the flexible circuit board substrate; Step (8): channel cavity forming: ventilate the holes on the surface of the substrate to discharge the residual printing ink in the channel, and perform secondary curing by ultraviolet irradiation for 10 min to 30 min; Step (9): Post-treatment of the substrate: drying the substrate after secondary curing in step (8), placing it in an ozone environment for 6 h to 18 h, or using oxygen plasma treatment for 3 h to 6 h; Step (10): slurry preparation: using an organic solvent, a surfactant and nano-metal conductive particles to prepare a slurry; the proportion of the nano-metal conductive particles in each slurry is adjusted according to the requirements of electrical conductivity, thermal conductivity and electromagnetic shielding; Step (11): interlayer circuit formation: inject the slurry prepared in step (10) into the groove of the flexible circuit board substrate to achieve interlayer circuit preparation and interlayer interconnection; the conductive slurry injection process is as follows: first, use a rubber plug to seal all the conductive slurry outlets at the bottom of the circuit board, reserve a conductive slurry inlet at the top for air discharge, and use a syringe to inject the conductive slurry at the remaining top slurry inlets at the same time, with an injection pressure of 0.15 MPa-0.25 MPa, until the conductive slurry flows out continuously from the reserved holes; remove the rubber plug at the bottom of the circuit board, inject the conductive slurry at all the top conductive slurry inlets at an injection pressure of 0.25 MPa-0.30 MPa, and use a syringe to apply a negative pressure of 0.01 MPa-0.03 MPa to all the bottom conductive slurry outlets until the conductive slurry flows out continuously from all the bottom conductive slurry outlets; due to the excellent wettability of the slurry and the substrate, the surface circuits are automatically filled with the slurry, or the slurry is coated on the surface after injection, and then the slurry is leaked into the surface grooves using a scraper; The pouring of thermal conductive slurry and electromagnetic shielding slurry is similar to that of conductive slurry. Due to the high viscosity of these two slurries, the negative pressure at the outlet should be appropriately increased to 0.03 MPa-0.05 MPa. Step (12): Post-processing of interlayer circuits: The flexible multilayer circuit board initially prepared in step (11) is treated at 100°C-200°C for 1-2 h.

2. The method for preparing a flexible multilayer circuit board according to claim 1, characterized in that: In step (1), the radius of curvature of the circuit board should be greater than 30 mm at all locations, and the thickness of the circuit board should be 0.5 mm-10 mm; in step (3), the cross-sectional diameter or side length should be 100 μm-1000 μm; when the length of the channel exceeds 20 mm, a micro support column should be designed.

3. The method for preparing a flexible multilayer circuit board according to claim 1, characterized in that: In step (2), in addition to meeting the electrical performance requirements, the graphic and structural design should also facilitate slurry injection and ensure that there are at least two vertical channels connecting the two connected layers; the thermal conductive layer should be as close to the conductive layer as possible and more distributed near the conductive layer with a larger current carrying capacity or a higher frequency.

4. The method for preparing a flexible multi-layer circuit board according to claim 1, characterized in that: In step (4), the cross-sectional diameter or side length of the opening should be 1.2-1.5 times that of the channel, and the surface should be chamfered to facilitate the inflow and outflow of the slurry; The slurry injection hole is located on the upper surface of the substrate, the slurry outflow hole is located on the lower surface of the substrate, and the air outlet hole can be selected from a slurry injection hole on the upper surface; For a flat substrate, the holes should be distributed in the center and near the edge of the substrate; for a curved substrate, the injection holes are distributed at higher positions and the outflow holes are distributed at lower positions, and the outlet holes are opened at the highest point of the substrate; in addition, the surface openings should be as far away from the interlayer interconnection channels as possible to facilitate the slurry to better fill the channels.

5. The method for preparing a flexible multi-layer circuit board according to claim 1, characterized in that: In step (5), the file format is one of .stl, .step, and .ctb; the slicing direction is the Z direction, and the slicing thickness is 0.02 mm-0.10 mm.

6. The method for preparing a flexible multi-layer circuit board according to claim 1, characterized in that: In step (6), the polyimide is composed of hydroxyethyl methacrylate grafted polyamino acid, trimethylolpropane triacrylate, and N,N-dimethylformamide.

7. The method for preparing a flexible multi-layer circuit board according to claim 1, characterized in that: In step (7), the ultraviolet wavelength used in UV curing 3D printing is 200 nm-405 nm, the exposure time of each layer is 2 s-5 s, the exposure time of the bottom layer is 10 s-40 s, and the printing starting point is the upper right corner of the XY plane.

8. The method for preparing a flexible multi-layer circuit board according to claim 1, characterized in that: In step (8), the ink discharge method, for the interconnected channels, arbitrarily selects a surface hole as an air outlet, and simultaneously injects air into the other holes at a pressure of 0.15 MPa-0.40 MPa until no ink is discharged from the air outlet; before the secondary curing, the substrate is cleaned and dried with anhydrous ethanol, and during the secondary curing, the substrate is immersed in deionized water.

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