Polyimide flexible circuit board and method of making the same

Abstract

This invention discloses a polyimide flexible circuit board and its fabrication method, relating to the fields of micro-nano fabrication and electronic materials technology. The fabrication method includes: coating a photosensitive polyamic acid solution containing halogen bonds onto a substrate to form a film; exposing the film using a grating mask to obtain a polyamic acid thin film substrate with an M-type micro-nano structure; immersing the substrate in a palladium ion activation solution, removing it, washing, drying, and thermal imidizing to obtain a polyimide substrate; and immersing the polyimide substrate in a chemical copper plating solution for chemical plating to form micro-nano fine circuitry with an M-type embedded mechanical interlocking effect. The embedded mechanical interlocking of this invention completely encapsulates the conductive circuitry within the substrate. The substrate material itself has excellent barrier properties, effectively isolating external corrosive media such as oxygen, moisture, and salt spray, significantly slowing down the oxidation and aging process of the circuitry, extending its service life, and meeting the processing requirements of high-density flexible circuit boards.

Description

Technical Field

[0001] This invention relates to micro-nano fabrication and the manufacturing process of printed circuit boards, specifically to a polyimide flexible circuit board and its preparation method. The polyimide flexible circuit board prepared by this method has micro-nano fine circuitry with an M-type embedded mechanical interlocking effect. Background Technology

[0002] With the large-scale commercialization of 5G communication technology and the continuous evolution of next-generation mobile communication technologies, the requirements for signal integrity and flexibility of high-frequency microelectronic devices are constantly increasing. Electronic devices are evolving towards miniaturization, high-density integration, and flexibility. Against this backdrop, the fabrication of high-precision, high-adhesion fine conductive lines on flexible substrates has become one of the key technological bottlenecks restricting industrial development.

[0003] Currently, the fabrication processes for conductive lines on polymer substrates mainly include photolithography, screen printing, inkjet printing, and electroless plating. Among these, electroless plating, with its advantages of simplicity, low cost, no need for vacuum equipment, and suitability for large-area production, has been widely used in the metallization of flexible circuit boards. However, the bonding methods between conductive lines and the substrate formed by existing fabrication processes all share common limitations, resulting in conductive lines primarily existing as surface-attached lines. These traditional surface-attached lines rely mainly on physical adsorption or chemical bonding to fix the metal layer to the substrate surface, resulting in limited interfacial bonding strength. During long-term use, especially under harsh conditions such as repeated bending, damp heat aging, and thermal shock, the interface between the metal layer and the substrate is highly susceptible to fatigue damage. When the accumulated interfacial stress exceeds the bonding strength threshold, it will lead to line peeling, detachment, or even open-circuit failure. This problem is particularly prominent in flexible electronic devices and has become a key bottleneck restricting product lifespan and reliability.

[0004] Embedding conductive lines into polymer substrates to form mechanically interlocked structures has long been a goal in this field. However, existing technologies face the following insurmountable technical challenges in achieving high-yield embedded line fabrication: 1) Catalyst selective loading challenge: In traditional electroless plating processes, catalyst particles (palladium) are loaded through physical adsorption or SnCl2 reduction, resulting in random distribution. For micro- and nano-scale recessed structures, insufficient contact between the bottom of the recess and the activation solution leads to a much lower density of catalyst particles at the bottom compared to planar areas, making it easy to form voids during filling. 2) Uniform photolithography morphology challenge: In traditional photolithography, the cross-sectional morphology of the recesses formed after exposure and development is usually U-shaped or V-shaped. U-shaped recesses have vertical sidewalls, and the mechanical interlocking formed after filling is only a one-sided latch, limiting the line fixation capability; while V-shaped recesses are funnel-shaped, making it easier for metal lines to detach from the substrate. Existing technologies lack means to control the photolithography morphology, making it difficult to obtain recess morphologies more conducive to mechanical interlocking. 3) Problem of removing byproducts: Chemical copper plating with formaldehyde as a reducing agent will produce hydrogen byproducts. Hydrogen bubbles are difficult to escape in the micron-level depressions and are prone to forming pinholes or voids in the filling layer.

[0005] Therefore, developing an embedded structure circuit fabrication method that can simultaneously solve the above-mentioned problems—namely, achieving precise control of the concave morphology and selective loading of catalysts, breaking away from the surface adhesion mode, and realizing the integrated combination of circuits and substrates—is of great significance for promoting the development of flexible electronics technology. Summary of the Invention

[0006] This invention aims to solve the technical problems in the prior art, such as the single morphological features of photolithography, the difficulty in selectively loading catalysts, insufficient adhesion of conductive lines, easy aging, and limited fine processing capabilities. It provides a method for forming M-type micro-nano structures by controlling photolithographic morphology using halogen bonds and preparing polyimide flexible circuit boards with embedded fine lines.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a polyimide flexible circuit board, comprising the following steps:

[0008] S1. Prepare a photosensitive polyamic acid solution containing halogen bonds, the raw materials of which include halogenated diamine monomer, photosensitive diamine monomer, 1,6-hexanediamine and dianhydride monomer;

[0009] The halogenated diamine monomer is selected from... , , Any one of them, wherein the X group is -Cl, -Br or -I;

[0010] The photosensitive diamine monomer contains a 1,4-dihydropyridine photosensitizing unit, preferably , , Any one of the following, wherein the Y group linked by an ether bond has a 1,4-dihydropyridine (DHP) photosensitive unit; more preferably, the structural formula of the Y group is: (Wave lines represent the connection points with the main skeleton), n is an integer from 1 to 40, preferably n=1 to 8;

[0011] S2. The photosensitive polyamic acid solution containing halogen bonds is coated onto a substrate to form a film, and exposed under ultraviolet light through a grating mask to obtain a polyamic acid thin film substrate with an M-type micro / nano structure; the formation of the M-type micro / nano structure depends on one or more of the following effects induced by halogen bonds: halogen bond-induced molecular chain orientation, photochemical reaction kinetic regulation, and differential swelling during the development process.

[0012] S3. The polyamic acid film substrate is immersed in an activation solution containing palladium ions to selectively anchor the palladium ions in the grooves of the M-type micro / nano structure. Then, it is subjected to thermal imidization treatment to obtain a photosensitive polyimide substrate containing halogen bonds. The activation solution is a palladium ion system. The palladium ions are selectively anchored in the grooves of the M-type micro / nano structure by utilizing the coordination effect between the pyridine groups in the substrate and the palladium ions.

[0013] S4. The photosensitive polyimide substrate is immersed in a chemical copper plating solution for chemical plating, so that metallic copper fills the groove, thereby forming micro-nano fine circuits with M-type embedded mechanical interlocking effect on the surface of the polyimide substrate, and finally obtaining a polyimide flexible circuit board.

[0014] As a further preferred technical solution of the present invention, the molar ratio of the halogenated diamine monomer, the photosensitive diamine monomer, and the 1,6-hexanediamine is (0.2~4):(0.1~2):(0.7~14), wherein the molar ratio of all diamine monomers to dianhydride monomers is 1:1~1.15.

[0015] As a further preferred embodiment of the present invention, the dianhydride monomer is pyromellitic tetracarboxylic anhydride.

[0016] As a further preferred technical solution of the present invention, the morphology of the M-type micro / nano structure is composed of two prominent main peaks and a central groove; the groove depth of the M-type micro / nano structure is 10~100 nm, and the linewidth of the micro / nano fine circuit is 2~40 μm.

[0017] As a further preferred technical solution of the present invention, the wavelength of the ultraviolet light source is 320~400nm and the exposure time is 10~300s.

[0018] As a further preferred technical solution of the present invention, the thermal imidization is carried out in an inert environment by segmented heating, in the following order: 30~80℃, holding for 15~30 minutes; 80~150℃, holding for 15~30 minutes; 150~250℃, holding for 15~30 minutes.

[0019] As a further preferred embodiment of the present invention, the electroless copper plating solution comprises copper sulfate pentahydrate, a complexing agent, a stabilizer, and a reducing agent; the complexing agent is disodium EDTA and / or potassium sodium tartrate; the stabilizer is 2,2'-bipyridine and / or potassium ferrocyanide; and the reducing agent is glyoxylic acid. More preferably, the mass fractions of each component in the electroless copper plating solution are: 8-10 parts copper sulfate pentahydrate, 20-30 parts disodium EDTA, 10-15 parts potassium sodium tartrate, 0.09-0.12 parts 2,2'-bipyridine, 0.02-0.07 parts potassium ferrocyanide, and 5-10 parts glyoxylic acid.

[0020] According to a second aspect of the present invention, a flexible polyimide circuit board is also provided, which is prepared by the method of the first aspect described above. This micro / nano fine circuit includes a photosensitive polyimide substrate containing halogen bonds and conductive circuitry; the polyimide substrate utilizes the halogen bonds to form an M-shaped micro / nano structure through photolithography; the conductive circuitry (made of copper) completely fills the grooves of the M-shaped micro / nano structure and forms an embedded mechanical interlocking structure with the inner wall of the grooves of the M-shaped micro / nano structure, thereby achieving a physical interlocking effect on the circuitry.

[0021] Mechanism of Action: Halogen bonds regulate molecular arrangement and development behavior, forming a bimodal M-shaped micro / nano structure. After UV lithography, the photosensitive group is converted from 1,4-dihydropyridine to 2,6-dimethylpyridine. Activated by palladium ion activating solution, the chemical inertness and hydrophobicity of the halogen bonds prevent the pyridine group from being destroyed during lithography / activation, preserving high-density pyridine-Pd²⁺ anchoring coordination sites. The catalyst is only loaded on the inner wall and bottom of the recess, and its electron-withdrawing effect regulates the electron cloud density of the pyridine nitrogen atoms, enhancing the stability of the pyridine-Pd²⁺ coordination bonds. This allows copper to grow densely from the bottom up, thus completely filling the recessed copper wires. The residual stress generated by the difference in the thermal expansion coefficients of the substrate is dispersed by the M-shaped micro / nano structure rather than concentrated at the interface. This invention increases and protects the activation sites through the regulation of halogen bonds, allowing copper to grow densely within the recessed grooves of the M-shaped micro / nano structure, enhancing the adhesion between the copper wires and the substrate.

[0022] According to a third aspect of the invention, the invention also provides an electronic device comprising a polyimide flexible circuit board with micro / nano fine lines having an M-type embedded mechanical interlocking effect as described in the second aspect. This electronic device can be applied in fields such as flexible displays, advanced semiconductor packaging, aerospace, or microelectronic device manufacturing.

[0023] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0024] The method for fabricating polyimide flexible circuit boards provided by this invention utilizes the M-shaped micro / nano structure recessed pattern formed by halogen bonds and ultraviolet lithography to precisely confine the metal deposition position and morphology. The line edges are determined by the pattern boundaries, resulting in no side etching, no plating penetration, and no diffusion. The linewidth accuracy can reach ±1μm, and fine lines with linewidths of 2~40μm can be stably fabricated, meeting the processing requirements of high-density flexible circuit boards. Furthermore, the embedded structure completely encapsulates the conductive lines within the substrate. The substrate material itself has excellent barrier properties, effectively isolating external corrosive media such as oxygen, moisture, and salt spray, significantly slowing down the oxidation and aging process of the lines and extending their service life.

[0025] This invention utilizes the M-shaped embedded mechanical interlocking effect formed by halogen bonds and ultraviolet lithography to create micro-nano fine circuits that are more resistant to detachment under repeated bending. One of its core features lies in its geometrically induced stress dispersion mechanism. This design effectively avoids stress concentration, making the bond between the coating and the substrate more stable. The M-shaped micro-nano structure, through its periodic undulating morphology, decomposes external forces onto multiple slopes and vertices, achieving multi-point stress distribution. This not only reduces local strain but also enhances the mechanical interlocking effect, making it more difficult for the coating to slip off the substrate. Furthermore, the slopes of the M-shaped structure generate tiny slip buffers during bending, absorbing some deformation energy, equivalent to a "flexible hinge" effect. Combined with the embedded circuits formed by electroless copper plating, a "mortise and tenon"-like interlocking structure is formed between the metal layer and the polyimide substrate, improving fatigue resistance. Experimental data shows that after hundreds of bending cycles, the M-shaped structure coating still maintains complete conductivity, while conventional U-shaped structures often exhibit sudden increases in resistance or breakage after only a few dozen cycles. Attached Figure Description

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1 This is a periodic diagram of the M-type micro / nano structure formed on the halogen-containing polyimide polymer substrate in Example 1;

[0028] Figure 2 The image shows an AFM 2D diagram of the M-type micro / nano structure formed on the halogen-containing polyimide polymer substrate in Example 1.

[0029] Figure 3 This is a cross-sectional schematic diagram of the embedded M-type micro / nano fine circuit structure of Example 1. The shaded area in the figure represents the embedded copper.

[0030] Figure 4 This is a periodic diagram of the M-type micro / nano structure formed on the halogen-containing polyimide polymer substrate in Example 2;

[0031] Figure 5 The image shows an AFM 2D diagram of the M-type micro / nano structure formed on the halogen-containing polyimide polymer substrate in Example 2.

[0032] Figure 6 This is a periodic diagram of the U-shaped micro / nano structure formed on the halogen-free polyimide polymer substrate in Comparative Example 1.

[0033] Figure 7 AFM 2D image of the U-shaped micro / nano structure formed on the halogen-free polyimide polymer substrate in Comparative Example 1;

[0034] Figure 8 This is an optical microscope image of the embedded fine circuit of the present invention;

[0035] Figure 9 This is a comparison of the adhesion curves of the M-type embedded fine circuit in Example 1 and the ordinary U-type polyimide copper-clad circuit in Comparative Example 1.

[0036] Figure 10 This is a comparison of the adhesion curves of the M-type embedded fine circuit in Example 2 and the ordinary U-type polyimide copper-clad circuit in Comparative Example 1.

[0037] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0038] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0039] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0040] Example 1:

[0041] This embodiment provides a polyimide flexible circuit board with micro-nano fine circuitry featuring an M-type embedded mechanical interlocking effect, the specific fabrication method of which is as follows:

[0042] (1) Under nitrogen protection, 1,6-hexanediamine (5.58 g, 0.048 mol), 2-bromo-1,4-phenylenediamine (8.98 g, 0.048 mol) and photosensitive diamine monomer (26.62 g, 0.024 mol) were dissolved in a three-necked flask containing 606.15 g of N,N-dimethylacetamide. After magnetic stirring and light protection for 1 h, pyromellitic anhydride (26.17 g, 0.12 mol) was added and reacted fully in an ice bath at 0-5 °C for 24 h to obtain a photosensitive polyamic acid solution containing halogen bonds.

[0043] (2) A polyamic acid solution was coated onto the substrate. After removing the solvent by heating at 70°C for 120 min, the substrate was exposed to a 365 nm ultraviolet light source for 150 s through a mask with a linewidth of 2 μm. After exposure, an M-type micro / nano structure with a depth of approximately 100 nm and a linewidth of approximately 2 μm was formed on the substrate surface (e.g., Figure 1 and 2 The polyamic acid film substrate (as shown) is a polyamic acid film substrate.

[0044] (3) The polyamic acid film substrate with M-type micro-nano structure was immersed in 0.05 mol / L PdCl2 ethanol solution for 15s, then washed with deionized water and dried with nitrogen; the substrate was then placed in a hot press and heated to 30°C for 15 minutes under nitrogen protection, then to 100°C for 15 minutes, then to 200°C for 30 minutes, and finally to 300°C for 60 minutes to obtain the reduced photosensitive polyimide substrate containing halogen bonds.

[0045] (4) Immerse the photosensitive M-type micro / nano structured polyimide substrate containing halogen bonds in a water bath chemical copper plating solution at 55°C. The copper plating solution, based on 1L of deionized water, contains 9 parts copper sulfate pentahydrate, 25 parts disodium EDTA, 12 parts potassium sodium tartrate, 0.01 parts 2,2'-bipyridine, and 0.05 parts potassium ferrocyanide. Adjust the pH to 12, then add 8 parts glyoxylic acid. The plating time is 15 minutes. Remove the copper-plated substrate, rinse it three times with deionized water for 2 minutes each time, and then dry it with nitrogen gas to obtain the desired result. Figure 3 The polyimide flexible circuit board shown has a micro-nano fine circuit with an M-type embedded mechanical interlocking effect.

[0046] The structural formula of the photosensitive diamine monomer in Example 1 is as follows:

[0047] .

[0048] Example 2:

[0049] This embodiment provides a polyimide flexible circuit board with micro-nano fine circuitry featuring an M-type embedded mechanical interlocking effect, the specific fabrication method of which is as follows:

[0050] (1) Under nitrogen protection, 1,6-hexanediamine (3.72 g, 0.032 mol), 4,4'-diamino-3,3'-dichlorodiphenylmethane (29.92 g, 0.112 mol) and photosensitive diamine monomer (17.75 g, 0.016 mol) were dissolved in a three-necked flask containing 488.97 g of N,N-dimethylacetamide. After magnetic stirring and light protection for 1 h, pyromellitic anhydride (34.90 g, 0.16 mol) was added and the mixture was reacted fully in an ice bath at 0-5 °C for 24 h to obtain a photosensitive polyamic acid solution containing halogen bonds.

[0051] (2) A polyamic acid solution was coated onto the substrate. After removing the solvent by heating at 80°C for 90 min, the substrate was exposed to a 400 nm ultraviolet light source for 60 s through a mask with a linewidth of 2 μm. This formed an M-type micro / nano structure on the substrate surface with a depth of approximately 70 nm and a linewidth of approximately 2 μm (e.g., Figure 4 and 5 The polyamic acid film substrate (as shown);

[0052] (3) The M-type micro / nano structured polyamic acid film substrate was immersed in a 0.08 mol / L PdCl2 ethanol solution for 10 seconds, then rinsed with deionized water and dried with nitrogen. The substrate was then placed in a hot press and heated under nitrogen protection at 50°C for 20 minutes, 150°C for 20 minutes, 200°C for 30 minutes, and 280°C for 60 minutes. The reduced halogen-bonded photosensitive polyimide substrate was obtained.

[0053] (4) Immerse the photosensitive polyimide substrate containing halogen bonds in a water bath chemical copper plating solution at a temperature of 45°C. The copper plating solution contains 10 parts of copper sulfate pentahydrate, 30 parts of disodium EDTA, 15 parts of potassium sodium tartrate, 0.012 parts of 2,2'-bipyridine and 0.02 parts of potassium ferrocyanide, based on 1L of deionized water. Adjust the pH to 12.5, and then add 13 parts of glyoxylic acid. The plating time is 10 minutes. Take out the copper-plated substrate, rinse it with deionized water 5 times, 1 minute each time, and then blow it dry with nitrogen to obtain a polyimide flexible circuit board with micro-nano fine lines having an M-type embedded mechanical interlocking effect.

[0054] The structural formula of the photosensitive diamine monomer in Example 2 is as follows:

[0055] .

[0056] Comparative Example 1:

[0057] As a control experiment for Example 1, the difference lies in the absence of a halogen-containing diamine monomer, meaning the substrate does not contain halogen bonds. The specific preparation method is as follows:

[0058] (1) Under nitrogen protection, 1,6-hexanediamine (5.58 g, 0.048 mol), 1,4-phenylenediamine (5.19 g, 0.048 mol) and photosensitive diamine monomer (same structure as in Example 1, 26.62 g, 0.024 mol) were dissolved in a three-necked flask containing 572.04 g of N,N-dimethylacetamide. After magnetic stirring and light protection for 1 h, pyromellitic anhydride (26.17 g, 0.12 mol) was added and reacted fully in an ice bath at 0-5 °C for 24 h to obtain a photosensitive polyamic acid solution.

[0059] (2) A polyamic acid solution was coated onto the substrate. After removing the solvent by heating at 70°C for 120 min, the substrate was exposed to a 365 nm ultraviolet light source for 150 s through a mask with a linewidth of 2 μm. After exposure, a U-shaped micro / nano structure with a depth of approximately 70 nm and a linewidth of approximately 2 μm was formed on the substrate surface (e.g., Figure 6 and 7 The polyamic acid film substrate (as shown);

[0060] (3) The U-shaped micro / nano structured polyamic acid film substrate was immersed in a 0.05 mol / L PdCl2 ethanol solution for 15 seconds, then rinsed with deionized water and dried with nitrogen. The substrate was then placed in a hot press and heated under nitrogen protection at a programmed temperature of 30°C for 15 minutes, 100°C for 15 minutes, 200°C for 30 minutes, and 300°C for 60 minutes to obtain the reduced photosensitive polyimide substrate.

[0061] (4) Immerse the photosensitive polyimide substrate in a chemical copper plating solution at a temperature of 55°C. The copper plating solution, based on 1L of deionized water, contains 9 parts of copper sulfate pentahydrate, 25 parts of disodium EDTA, 12 parts of potassium sodium tartrate, 0.01 parts of 2,2'-bipyridine, and 0.05 parts of potassium ferrocyanide. Adjust the pH to 12, then add 8 parts of glyoxylic acid. The plating time is 15 minutes. Remove the copper-plated substrate, rinse it three times with deionized water for 2 minutes each time, and then dry it with nitrogen to obtain a polyimide flexible circuit board with U-shaped micro-nano circuitry.

[0062] Comprehensive test:

[0063] Compared with Comparative Example 1, both systems in Examples 1 and 2 used halogen-containing diamine monomers. The halogen bonds in the resulting halogen-containing photosensitive polyimide can regulate photochemical reaction kinetics and differentiate swelling behavior through mechanisms such as intermolecular halogen bond interactions, electron-withdrawing effects, and differential swelling behavior, thereby changing the morphology after photolithography from the conventional U-shaped / V-shaped ( Figure 6 ) transforms into an M-shape (two peaks sandwiching a valley) Figure 1 and Figure 4 ).

[0064] Example 1 utilizes M-shaped micro / nano-scale recessed patterns formed by halogen bonds and ultraviolet lithography to precisely confine the location and morphology of metal deposition. The line edges are determined by the pattern boundaries, resulting in no side etching, no plating penetration, and no diffusion. Linewidth accuracy can reach ±1μm, and fine lines with linewidths of 2~40μm can be stably fabricated. Figure 8 This meets the processing requirements of high-density flexible circuit boards.

[0065] Example 1 utilizes the groove in the middle of the M-shaped structure to form an embedded mechanical interlock after filling, such as... Figure 3 As shown. Example 1's M-type embedded circuit (such as...) Figure 9 The red line (in the center) performed significantly better than traditional U-shaped embedded lines (such as...) in adhesion tests. Figure 9 (Midpoint dashed line), the bonding strength can reach over 6MPa, which is more than twice that of traditional surface-attached circuits, such as... Figure 10 As shown, the M-type embedded circuit of Embodiment 2 is also superior to the U-type embedded circuit.

[0066] The samples of Example 1 and Comparative Example 1 were aged in an 85℃ / 85%RH constant temperature and humidity chamber. The peel strength was tested at different aging times, and the results are summarized in Table 1.

[0067] Table 1

[0068]

[0069] Test results show that Example 1 retained 92% of its peel strength after aging for 168 hours, while Comparative Example 1's peel strength retention rate had decreased to 64%. This indicates that the embedded structure of the present invention effectively isolates external humid and hot air, slowing down interface aging.

[0070] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is defined only by the appended claims.

Claims

1. A method for preparing a polyimide flexible circuit board, characterized in that, Includes the following steps: S1. Prepare a photosensitive polyamic acid solution containing halogen bonds, the raw materials of which include halogenated diamine monomer, photosensitive diamine monomer, 1,6-hexanediamine and dianhydride monomer; The halogenated diamine monomer is selected from... , , Any one of them, wherein the X group is -Cl, -Br or -I; The photosensitive diamine monomer contains a 1,4-dihydropyridine photosensitizing unit; S2. The photosensitive polyamic acid solution containing halogen bonds is coated onto a substrate to form a film, and exposed under ultraviolet light through a grating mask to obtain a polyamic acid thin film substrate with an M-type micro-nano structure. S3. The polyamic acid film substrate is immersed in an activation solution containing palladium ions to selectively anchor the palladium ions in the grooves of the M-type micro / nano structure. After washing and drying, it is then subjected to thermal imidization treatment to obtain a polyimide substrate. S4. The polyimide substrate is immersed in a chemical copper plating solution for chemical plating, so that metallic copper fills the groove, thereby forming micro-nano fine circuits with M-type embedded mechanical interlocking effect on the surface of the polyimide substrate, and finally obtaining a polyimide flexible circuit board.

2. The preparation method according to claim 1, characterized in that, The molar ratio of the halogenated diamine monomer, the photosensitive diamine monomer, and 1,6-hexanediamine is (0.2~4):(0.1~2):(0.7~14), wherein the molar ratio of all diamine monomers to dianhydride monomers is 1:1~1.

15.

3. The preparation method according to claim 1, characterized in that, The diacid anhydride monomer is pyromellitic anhydride.

4. The preparation method according to claim 1, characterized in that, The photosensitive diamine monomer is selected from... , , Any one of them, wherein the structure of the Y group linked by an ether bond is: n is an integer from 1 to 40.

5. The method for fabricating micro / nano fine circuits with M-type embedded mechanical interlocking effect according to claim 1, characterized in that, The morphology of the M-type micro / nano structure is composed of two prominent main peaks and a central groove; the groove depth of the M-type micro / nano structure is 10~100 nm, and the linewidth of the micro / nano fine circuit is 2~40 μm.

6. The preparation method according to claim 1, characterized in that, The wavelength of the ultraviolet light source is 320~400nm, and the exposure time is 10~300s.

7. The preparation method according to claim 1, characterized in that, The thermal imidization is carried out in an inert environment by segmented heating: 30~80℃ for 15~30 minutes; 80~150℃ for 15~30 minutes; 150~250℃ for 15~30 minutes; and 250~300℃ for 30~60 minutes.

8. The preparation method according to claim 1, characterized in that, The electroless copper plating solution comprises copper sulfate pentahydrate, a complexing agent, a stabilizer, and a reducing agent; the complexing agent is disodium EDTA and / or sodium potassium tartrate; the stabilizer is 2,2'-bipyridine and / or potassium ferrocyanide; and the reducing agent is glyoxylic acid.

9. A polyimide flexible circuit board, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. An electronic device, characterized in that, The invention comprises the polyimide flexible circuit board as described in claim 9.

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