A method for manufacturing a flexible circuit board for a wearable device

Abstract

The application discloses a manufacturing method of a flexible circuit board for a wearable device in the technical field of circuit board manufacturing, and the base material of the circuit board adopts a polyimide-spandex composite base material, wherein the spandex is a twisted short fiber, the twist degree is 550-600 T / m, the length is 3-6 mm, and the weight dosage is 8-12% of the weight of the polyimide; the calendered copper foil is hot-pressed and connected on the composite base material, and then laser direct imaging and etching are performed to obtain a circuit board with copper lines; subsequently, the covering film is bonded on the surface of the circuit board through hot pressing, and the circuit board is completed. The application adopts a low-temperature process, adds the spandex twisted short fiber with good elasticity into the polyimide, so that the spandex twisted short fiber can keep the twist degree from being excessively lost, the bending resistance of the flexible circuit board can be significantly improved, the polyimide base material and the flexible circuit board are softer, and the flexible circuit board is not easy to deform when repeatedly worn.

Description

Technical Field

[0001] This invention relates to the field of flexible circuit board manufacturing technology, and in particular to a flexible circuit board manufacturing process based on rolled copper foil and polyimide substrate. Background Technology

[0002] A method for one-time forming of inner layer circuits in a multilayer flexible circuit board is disclosed in the Chinese patent database. Its publication number is CN103079365A, and its publication date is 20130501. This method includes the following steps: applying dry film to a roll; fabricating the inner layer circuit board of the multilayer flexible circuit board; fabricating the inner layer circuit board layer; fabricating the inner layer circuit board assembly layer; and fabricating the inner layer circuits of the multilayer flexible circuit board. The process involves applying dry film, exposure, development, etching, film removal, micro-etching, and lamination to produce the inner layer circuit board, inner layer circuit board layer, inner layer circuit board assembly layer, and multilayer flexible circuit board. This method avoids the height difference between the glued and non-glue areas after lamination of the inner layer flexible board and the expansion and contraction problems caused by multiple laminations of the inner layer cover film, thus improving the quality of the multilayer flexible circuit board, achieving a pass rate of over 92%, and shortening the process flow and increasing productivity. When applying dry film to the entire roll, the dry film resist is applied to the copper surface of the copper substrate. The copper substrate is formed by stacking and hot-pressing a polyimide film and a copper cladding film. The polyimide film has a thickness of 12.5 μm, and the copper cladding film has a thickness of 12 μm.

[0003] Traditional flexible circuit board manufacturing often uses electrolytic copper foil (ED) and polyimide substrate, forming circuits through etching. This method has the following problems: the high rigidity of the polyimide substrate makes it prone to breakage at joints; insufficient flexibility of the circuit board restricts human movement when worn, causing discomfort; and polyimide has poor elastic recovery, easily resulting in permanent deformation after repeated wear. Summary of the Invention

[0004] This invention proposes a method for manufacturing flexible circuit boards for wearable devices, which makes the polyimide substrate more flexible and less prone to deformation after repeated wear.

[0005] Therefore, the technical solution of the present invention is: a method for manufacturing a flexible circuit board for wearable devices, comprising the following steps:

[0006] S1. Cut calendered copper foil and polyimide-spandex composite substrate to appropriate sizes, perform surface treatment, and then coat the opposing surfaces of the calendered copper foil and composite substrate with polyetheramine-modified epoxy resin adhesive; wherein the spandex in the polyimide-spandex composite substrate is twisted short fiber with a twist of 550-600 T / m, a length of 3-6 mm, and a weight of 8-12% of the weight of polyimide;

[0007] S2. The pre-coated adhesive composite substrate is bonded to the adhesive-coated surface of the rolled copper foil and then hot-pressed in a hot roll press. The surface temperature of the roll is 180-200°C, the roll pressure is 1.0-1.5MPa, and the roll pressing is carried out in a vacuum environment.

[0008] Avoid mixing in air;

[0009] S3. Forming circuit patterns on the surface of rolled copper foil using direct laser imaging technology: First, apply photoresist, then perform laser exposure according to the designed circuit pattern. After exposure, dissolve the photoresist in the unexposed area in the developer to expose the rolled copper foil in the unexposed area. Then, use an acidic etching solution to etch and remove the exposed copper layer. Finally, peel off the photoresist to obtain a circuit substrate with copper circuits.

[0010] S4. A cover film with a surface layer is hot-pressed onto the surface of a copper circuit. The cover film is made of a polyimide film. A high-strength polyurethane surface layer is provided on the surface of the polyimide film to support the polyimide film. Several through holes corresponding to the positions of the solder pads on the copper circuit are pre-etched on the polyimide film by laser. During hot pressing, a polyetheramine-modified epoxy resin adhesive is first applied to the surface of the cover film. Then, the through holes are aligned with the solder pads on the copper circuit and bonded to expose the solder pads. Vacuum hot pressing is then performed to bond the cover film to the surface of the circuit board.

[0011] S5. Peel off the surface layer on the cover film to complete the circuit board fabrication.

[0012] Furthermore, the thickness of the rolled copper foil is 12-35 μm, the thickness of the polyimide-spandex composite substrate is 30-70 μm, the thickness of the cover film is 15-25 μm, and the thickness of the surface layer on the cover film is 25-35 μm.

[0013] Further, the surface treatment step in step S1 includes:

[0014] S1-1. Roughening treatment of rolled copper foil: Pickling is used to form fine particles with a roughness Ra≤0.13μm on the surface of rolled copper foil to enhance the adhesion with the composite substrate;

[0015] S1-2. Degreasing and activation of rolled copper foil: The rolled copper foil is sequentially degreased by electrolysis and activated by copper sulfate to remove surface contaminants. The electrolysis degreasing is carried out in sodium hydroxide solution with an electrolysis current density of 38A / dm².

[0016] S1-3. Surface treatment of polyimide-spandex composite substrate: Plasma cleaning with a power of 300-500W and a cleaning time of 30-60s is used to control the surface roughness of the composite substrate to Ra 0.8-1.2μm, thereby improving adhesion.

[0017] Furthermore, step S3 specifically includes the following sub-steps:

[0018] S3-1. Photoresist Coating: Coat the substrate surface with a high-resolution photosensitive dry film or liquid photoresist with a thickness of 5-10μm; the photoresist must have high sensitivity (suitable for low-energy exposure) and high etch resistance.

[0019] S3-2. Laser Exposure: Utilizing an ultraviolet laser source with a wavelength of 355-405nm, the laser beam is modulated using a digital micromirror device, controlling the spot diameter to below 10μm; achieving high-precision pattern transfer; exposure energy is 50-200mJ / cm²; too low an energy level will result in incomplete development, while too high an energy level will cause light scattering, affecting edge sharpness. Line-by-line scanning exposure is employed, with a spot overlap rate of 50-70%; to balance efficiency and line quality, multi-beam parallel exposure can be used to increase speed.

[0020] S3-3. Development and Etching: After exposure, the photoresist in the unexposed areas dissolves in the developer to form circuit patterns. The developer is a sodium carbonate solution. Then, an acidic etching solution, either ferric chloride or sulfuric acid-hydrogen peroxide solution, is used to remove the exposed copper layer, leaving the circuit parts covered by the photoresist. After etching, the photoresist is stripped off to obtain a circuit board with copper circuits.

[0021] Furthermore, step S4 specifically includes the following steps:

[0022] S4-1. Surface treatment: Clean the surface of the circuit layer with a lint-free cloth and isopropyl alcohol to remove oxide layer and contaminants, and then preheat at 80-100℃ to improve adhesion; at the same time, perform plasma cleaning on the surface of the cover film; then, apply polyetheramine modified epoxy resin adhesive to the side of the cover film without surface layer.

[0023] S4-2. Positioning and Alignment: Through mechanical or laser positioning, the through-hole windows on the cover film are precisely aligned with the solder pads on the copper circuit, so that the adhesive on the cover film comes into contact with the copper circuit, with the deviation controlled within ±30μm; vacuum adsorption is used to transfer to the hot press bonding device to prevent displacement during the bonding process.

[0024] S4-3. Hot pressing: Using a stainless steel hot press roller with several spherical protrusions on the surface, at a temperature of 145±5℃ and a pressure of 0.4-0.6kgf / cm², the cover film is subjected to local dot matrix hot pressing for pre-fixation; function: to partially melt the adhesive, initially fix the cover film and the circuit layer, reduce the gap and reduce the risk of air bubbles during subsequent full-area hot pressing.

[0025] S4-4. Full-surface hot pressing: Vacuum hot pressing is performed using a flat-surface hot pressing roller. The hot pressing temperature is 180±5℃, the pressure is 0.7-0.9kgf / cm², and the time is 30-90 seconds. The adhesive melts and fills the gaps in the circuit, forming a uniform and bubble-free adhesive layer. Vacuum hot pressing further removes air and improves the uniformity of adhesive layer filling.

[0026] S4-5. Curing and hardening

[0027] Preliminary curing: Bake in a hot air circulating oven at 120-140℃ for 3-10 minutes to achieve preliminary curing of the polyetheramine modified epoxy resin adhesive;

[0028] Segmented ripening:

[0029] First stage: Bake at 80±5℃ for 10-30 minutes to allow the solvent in the adhesive to evaporate slowly;

[0030] Second stage: Bake at 120±5℃ for 30-60 minutes to promote cross-linking of the adhesive;

[0031] Third stage: Bake at 160±5℃ for 60-120 minutes to allow the adhesive to fully cure.

[0032] The purpose of staged curing is to avoid stress concentration by gradually increasing the temperature, thus ensuring the bonding strength (peel strength ≥ 0.8 N / mm).

[0033] Furthermore, the preparation method of the polyimide-spandex composite substrate includes the following steps:

[0034] 1) Polyamic acid (PAA) is synthesized by polycondensation of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether in the aprotic polar solvent DMF (N,N-dimethylformamide) at a reaction temperature of 10-25℃. The molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is 1:(1.01-1.1); the amount of DMF used is 5-6 times the weight of the sum of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether.

[0035] 2) Spray nano aluminum hydroxide dispersion onto the surface of spandex twisted short fibers. The average particle size of aluminum hydroxide is 50nm and the solid content in the aluminum hydroxide dispersion is 15-20%, which improves the heat resistance of spandex (short-term resistance to 200℃). The twist of the spandex twisted short fibers is 550-600 T / m and the length of the short fibers is 3-6mm.

[0036] 3) Add surface-treated spandex twisted staple fiber to the polyamic acid solution, wherein the weight of the spandex twisted staple fiber is 6-9% of the weight of the polyamic acid in the solution; at the same time, add 0.1-0.3% of KH-550 silane coupling agent by weight of polyamic acid to promote the subsequent interfacial bonding between polyimide and spandex.

[0037] 4) The polyamic acid solution is rapidly cyclized through a screw extruder. The screw extrusion temperature is 180-200℃, and the polyamic acid solution stays in the screw extruder for 2-5 minutes. In the cooling zone of 160-130℃ after extrusion, a multi-stage vacuum port is set with a vacuum pressure of 0.05-0.09MPa to remove the volatilized DMF vapor in real time. The polyamic acid is cyclized into polyimide, resulting in weight loss. This makes the weight content of spandex twisted short fibers in the polyimide 8-12%, and the solvent content in the polyimide is less than 5%. The extrusion die of the screw extruder is slit-shaped, and a polyimide-spandex thick film is formed after extrusion.

[0038] 5) Polyimide-spandex thick film is calendered into polyimide-spandex thin film by calendering machine. The calendering roll temperature is 55±5℃, the pressure is 8-10 MPa, and the calendered film thickness is 30-70μm.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] 1. This invention enhances toughness through composite substrate. In the polyimide-spandex composite substrate, the addition of spandex short fibers (twist of 550-600 T / m, length of 3-6mm) increases the elongation at break of the substrate by more than 30%, and the dynamic bending life can reach more than 300,000 cycles (curvature radius of 1mm), which meets the frequent bending requirements of wearable devices and improves mechanical properties and durability.

[0041] 2. High-strength surface layer support: The polyurethane surface layer of the cover film provides temporary rigid support during hot pressing, reducing bonding deformation, minimizing offset, and ensuring line accuracy and reliability.

[0042] 3. Polyetheramine modified epoxy resin adhesive has both high bonding strength and flexibility. It has low-temperature curing properties, which avoids the loss of twist of spandex twisted short fibers.

[0043] 4. Laser precision processing: Laser direct imaging technology (LDI) can achieve high-precision processing, reduce wastewater discharge, and meet environmental protection requirements.

[0044] 5. The dielectric constant of the rolled copper foil (RA copper) and polyimide composite substrate can be reduced to below 3.5, resulting in low insertion loss. Laser ablation of the cover film creates a one-to-one correspondence between the vias and pads, avoiding deviations inherent in traditional windowing processes and improving soldering yield. This enhances electrical performance and signal integrity.

[0045] In summary, this invention comprehensively addresses the core requirements of wearable devices for flexible circuit boards, including high bend resistance, thinness, high-frequency performance, and environmentally friendly manufacturing, demonstrating clear industrialization advantages. The challenge lies in the fact that spandex is prone to decomposition at high temperatures. Current polyimide preparation processes often employ temperatures exceeding 260°C. This invention utilizes a low-temperature process, incorporating well-elastic twisted spandex staple fibers into the polyimide. This allows the spandex staple fibers to maintain their twist without excessive loss, significantly improving the bend resistance of the flexible circuit board. This results in a more flexible polyimide substrate and flexible circuit board that are less prone to deformation after repeated wear. Detailed Implementation

[0046] A method for manufacturing a flexible circuit board for wearable devices includes the following steps:

[0047] S1. Cut calendered copper foil and polyimide-spandex composite substrate to appropriate sizes, perform surface treatment, and then coat the opposing surfaces of the calendered copper foil and composite substrate with polyetheramine-modified epoxy resin adhesive; wherein the polyimide-spandex composite substrate contains twisted short fibers with a twist of 550-600 T / m, a length of 3-6 mm, and a weight of 8-12% of the polyimide weight; the thickness of the calendered copper foil is 12-35 μm, and the thickness of the polyimide-spandex composite substrate is 30-70 μm;

[0048] The manufacturers and product models of polyetheramine-modified epoxy resin adhesives are as follows:

[0049] AD EKA (Japan) Co., Ltd., product model: EP-4000, is a polyetheramine-modified bisphenol A type epoxy resin, temperature resistant up to 200℃, suitable for metal bonding and structural adhesives. EP-4100HF: Low-halogen polyetheramine-modified epoxy resin, high temperature resistant, thermal decomposition temperature greater than 300℃, used for electronic packaging.

[0050] Huai'an Chenhua New Materials Co., Ltd., Product Model: M2070 / M2005 Polyetheramine Modified Epoxy Resin.

[0051] Wuxi Akoli Technology Co., Ltd.; Product Model: Modified Epoxy Resin Series, compatible with polyetheramine D230 / D400 curing agent.

[0052] The polyetheramine-modified epoxy resin adhesive EP-4100HF used in this invention can be replaced by products from other manufacturers.

[0053] Surface treatment may specifically include the following steps:

[0054] S1-1. Roughening treatment of rolled copper foil: Pickling is used to form fine particles with a roughness Ra≤0.13μm on the surface of rolled copper foil to enhance the adhesion with the composite substrate;

[0055] S1-2. Degreasing and activation of rolled copper foil: The rolled copper foil is sequentially degreased by electrolysis and activated by copper sulfate to remove surface contaminants. The electrolysis degreasing is carried out in sodium hydroxide solution with an electrolysis current density of 38A / dm².

[0056] S1-3. Surface treatment of polyimide-spandex composite substrate: Plasma cleaning with a power of 300-500W and a cleaning time of 30-60s is used to control the surface roughness of the composite substrate to Ra 0.8-1.2μm, thereby improving adhesion.

[0057] S2. The pre-coated adhesive composite substrate is bonded to the adhesive-coated surface of the rolled copper foil and then hot-pressed in a hot roll press. The surface temperature of the roll is 180-200°C, the roll pressure is 1.0-1.5MPa, and the rolling is carried out in a vacuum environment to avoid air entrainment.

[0058] S3. Forming circuit patterns on the surface of rolled copper foil using direct laser imaging technology: First, apply photoresist, then perform laser exposure according to the designed circuit pattern. After exposure, dissolve the photoresist in the unexposed area in the developer to expose the rolled copper foil in the unexposed area. Then, use an acidic etching solution to etch and remove the exposed copper layer. Finally, peel off the photoresist to obtain a circuit substrate with copper circuits.

[0059] Specifically, it includes the following steps:

[0060] S3-1. Photoresist Coating: Coat the substrate surface with a high-resolution photosensitive dry film or liquid photoresist with a thickness of 5-10μm; the photoresist must have high sensitivity (suitable for low-energy exposure) and high etch resistance.

[0061] S3-2. Laser Exposure: Utilizing an ultraviolet laser source with a wavelength of 355-405nm, the laser beam is modulated using a digital micromirror device, controlling the spot diameter to below 10μm; achieving high-precision pattern transfer; exposure energy is 50-200mJ / cm²; too low an energy level will result in incomplete development, while too high an energy level will cause light scattering, affecting edge sharpness. Line-by-line scanning exposure is employed, with a spot overlap rate of 50-70%; to balance efficiency and line quality, multi-beam parallel exposure can be used to increase speed.

[0062] S3-3. Development and Etching: After exposure, the photoresist in the unexposed areas dissolves in the developer to form circuit patterns. The developer is a sodium carbonate solution. Then, an acidic etching solution, either ferric chloride or sulfuric acid-hydrogen peroxide solution, is used to remove the exposed copper layer, leaving the circuit parts covered by the photoresist. After etching, the photoresist is stripped off to obtain a circuit board with copper circuits.

[0063] S4. A cover film with a surface layer is hot-pressed onto the surface of a copper circuit. The cover film is made of polyimide film, and a high-strength polyurethane surface layer is provided on the surface of the polyimide film to support the polyimide film. Several through holes corresponding to the positions of the solder pads on the copper circuit are pre-etched on the polyimide film by laser. During hot pressing, a polyetheramine-modified epoxy resin adhesive is first applied to the surface of the cover film. Then, the through holes are aligned with the solder pads on the copper circuit and bonded to expose the solder pads. Vacuum hot pressing is then performed to bond the cover film to the surface of the circuit board. The thickness of the cover film is 15-25 μm, and the thickness of the surface layer on the cover film is 25-35 μm.

[0064] Specifically, it includes the following steps:

[0065] S4-1. Surface treatment: Clean the surface of the circuit layer with a lint-free cloth and isopropyl alcohol to remove oxide layer and contaminants, and then preheat at 80-100℃ to improve adhesion; at the same time, perform plasma cleaning on the surface of the cover film; then, apply polyetheramine modified epoxy resin adhesive to the side of the cover film without surface layer.

[0066] S4-2. Positioning and Alignment: Through mechanical or laser positioning, the through-hole windows on the cover film are precisely aligned with the solder pads on the copper circuit, so that the adhesive on the cover film comes into contact with the copper circuit, with the deviation controlled within ±30μm; vacuum adsorption is used to transfer to the hot press bonding device to prevent displacement during the bonding process.

[0067] S4-3. Hot pressing: Using a stainless steel hot press roller with several spherical protrusions on the surface, at a temperature of 145±5℃ and a pressure of 0.4-0.6kgf / cm², the cover film is subjected to local dot matrix hot pressing for pre-fixation; function: to partially melt the adhesive, initially fix the cover film and the circuit layer, reduce the gap and reduce the risk of air bubbles during subsequent full-area hot pressing.

[0068] S4-4. Full-surface hot pressing: Vacuum hot pressing is performed using a flat-surface hot pressing roller. The hot pressing temperature is 180±5℃, the pressure is 0.7-0.9kgf / cm², and the time is 30-90 seconds. The adhesive melts and fills the gaps in the circuit, forming a uniform and bubble-free adhesive layer. Vacuum hot pressing further removes air and improves the uniformity of adhesive layer filling.

[0069] S4-5. Curing and hardening

[0070] Preliminary curing: Bake in a hot air circulating oven at 120-140℃ for 3-10 minutes to achieve preliminary curing of the polyetheramine modified epoxy resin adhesive;

[0071] Segmented ripening:

[0072] First stage: Bake at 80±5℃ for 10-30 minutes to allow the solvent in the adhesive to evaporate slowly;

[0073] Second stage: Bake at 120±5℃ for 30-60 minutes to promote cross-linking of the adhesive;

[0074] Third stage: Bake at 160±5℃ for 60-120 minutes to allow the adhesive to fully cure.

[0075] Objective: To avoid stress concentration by using a stepped heating method and to ensure bonding strength (peel strength ≥ 0.8 N / mm).

[0076] S5. Peel off the surface layer on the cover film to complete the circuit board fabrication.

[0077] The polyimide-spandex composite substrate of this invention is prepared by the following steps:

[0078] 1) Polyamic acid (PAA) is synthesized by polycondensation of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether in the aprotic polar solvent DMF (N,N-dimethylformamide) at a reaction temperature of 10-25℃. The molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is 1:(1.01-1.1); the amount of DMF used is 5-6 times the weight of the sum of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether.

[0079] 2) Spray nano aluminum hydroxide dispersion onto the surface of spandex twisted short fibers. The average particle size of aluminum hydroxide is 50nm and the solid content in the aluminum hydroxide dispersion is 15-20%, which improves the heat resistance of spandex (short-term resistance to 200℃). The twist of the spandex twisted short fibers is 550-600 T / m and the length of the short fibers is 3-6mm.

[0080] 3) Add surface-treated spandex twisted staple fiber to the polyamic acid solution, wherein the weight of the spandex twisted staple fiber is 6-9% of the weight of the polyamic acid in the solution; at the same time, add 0.1-0.3% of KH-550 silane coupling agent by weight of polyamic acid to promote the subsequent interfacial bonding between polyimide and spandex.

[0081] 4) The polyamic acid solution is rapidly cyclized through a screw extruder. The screw extrusion temperature is 180-200℃, and the polyamic acid solution stays in the screw extruder for 2-5 minutes. In the cooling zone of 160-130℃ after extrusion, a multi-stage vacuum port is set with a vacuum pressure of 0.05-0.09MPa to remove the volatilized DMF vapor in real time. The polyamic acid is cyclized into polyimide, resulting in weight loss. This makes the weight content of spandex twisted short fibers in the polyimide 8-12%, and the solvent content in the polyimide is less than 5%. The extrusion die of the screw extruder is slit-shaped, and a polyimide-spandex thick film is formed after extrusion.

[0082] 5) The polyimide-spandex thick film is calendered into a polyimide-spandex thin film using a calender. The calender roll temperature is 55±5℃, the pressure is 8-10 MPa, and the calendered film thickness is 30-70μm. A polyimide-spandex composite substrate is obtained.

[0083] In polyimide-spandex composite materials, the physical form of the twisted short fibers of spandex locks the twist, giving it high elasticity and high strength when stretched and bent. During the manufacturing process, the twist retention rate is high, and the twist loss after molding is less than 10%.

[0084] Table 1: Performance Comparison of Polyimide-Spandex Composite Substrate and Polyimide

[0085]

[0086] The table above shows that using polyimide-spandex composite substrates results in better elastic recovery and bending resistance, making them more suitable for manufacturing soft wearable devices.

[0087] Table 2: Performance Test Table of the Flexible Circuit Board of the Present Invention

[0088]

[0089] The table above shows that the flexible circuit board of the present invention has a long bending life, high peel strength of rolled copper foil, and the low dielectric constant material can reduce the signal propagation time in the transmission line and improve the circuit response speed.

[0090] Table 3: Comparison of Spandex and Commonly Used Fibers

[0091]

[0092] The table above shows that aluminum hydroxide-coated spandex material has ultra-high elasticity and low density, which is beneficial for its fabrication into highly elastic wearable flexible circuit boards. Basalt fiber and alkali-free glass fiber, on the other hand, are brittle and not suitable for composite with polyimide to form flexible circuit boards.

[0093] This invention uses aluminum hydroxide to spray spandex, thus producing flame-retardant spandex. After fabricating a flexible circuit board using flame-retardant spandex, ordinary spandex, and polyimide, its twist loss at different temperatures was tested.

[0094] Table 4: Twist loss of composite substrates made of flame-retardant spandex and ordinary spandex at different temperatures (initial twist: 600 T / m)

[0095]

[0096] The table above shows that the flexible circuit board produced has a small twist loss below 170°C. Aluminum hydroxide begins to decompose above 200°C, releasing water vapor. The water vapor produced by the decomposition dilutes the concentration of oxygen and combustible gases, inhibits the combustion chain reaction, and forms an inert gas barrier, which can slow down the thermal degradation and combustion rate of spandex.

[0097] Table 5: Comparative Analysis of Technical Effects of Spandex Twisted Short Fibers with Lengths in the 1-10mm Range in Polyimide-Spandex Composite Substrates (Based on Process Parameters: Twist 550T / m, Aluminum Hydroxide Spraying Treatment)

[0098]

[0099] When the short fiber length is less than 3mm, the KH-550 coupling agent can completely coat the fiber surface, but the short fiber leads to low stress transfer efficiency. When the length is greater than 6mm, the fiber is easy to entangle in the screw extrusion, requiring higher shear force for dispersion, which will damage the aluminum hydroxide coating. 3-6mm short fibers are preferred. Through the twisting structure (550-600 T / m), an elastic network is formed. After high-temperature cyclization (180-200℃), the twist loss is less than 10%, which is better than untwisted fibers.

[0100] Table 6: Analysis of the amount of twisted staple fiber used in polyimide-spandex composite substrates

[0101]

[0102] The table above shows that the polyimide-spandex composite matrix exhibits the best overall performance when the amount of twisted short spandex fibers in the polyimide is 8-12 wt% of the weight of the polyimide.

[0103] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A method for manufacturing a flexible circuit board for wearable devices, characterized in that, Includes the following steps: S1. Cut calendered copper foil and polyimide-spandex composite substrate to appropriate sizes, perform surface treatment, and then coat the opposing surfaces of the calendered copper foil and composite substrate with polyetheramine-modified epoxy resin adhesive; wherein the spandex in the polyimide-spandex composite substrate is twisted short fiber with a twist of 550-600 T / m, a length of 3-6 mm, and a weight of 8-12% of the weight of polyimide; S2. The pre-coated adhesive composite substrate is bonded to the adhesive-coated surface of the rolled copper foil and then hot-pressed in a hot roll press. The surface temperature of the roll is 180-200°C, the roll pressure is 1.0-1.5MPa, and the roll pressing is carried out in a vacuum environment. S3. Forming circuit patterns on the surface of rolled copper foil using direct laser imaging technology: First, apply photoresist, then perform laser exposure according to the designed circuit pattern. After exposure, dissolve the photoresist in the unexposed area in the developer to expose the rolled copper foil in the unexposed area. Then, use an acidic etching solution to etch and remove the exposed copper layer. Finally, peel off the photoresist to obtain a circuit substrate with copper circuits. S4. A cover film with a surface layer is hot-pressed onto the surface of a copper circuit. The cover film is made of a polyimide film. A high-strength polyurethane surface layer is provided on the surface of the polyimide film to support the polyimide film. Several through holes corresponding to the positions of the solder pads on the copper circuit are pre-etched on the polyimide film by laser. During hot pressing, a polyetheramine-modified epoxy resin adhesive is first applied to the surface of the cover film. Then, the through holes are aligned with the solder pads on the copper circuit and bonded to expose the solder pads. Vacuum hot pressing is then performed to bond the cover film to the surface of the circuit board. S5. Peel off the surface layer on the cover film to complete the circuit board fabrication.

2. The method for manufacturing a flexible circuit board for wearable devices according to claim 1, characterized in that, The thickness of the rolled copper foil is 12-35 μm, the thickness of the polyimide-spandex composite substrate is 30-70 μm, the thickness of the cover film is 15-25 μm, and the thickness of the surface layer on the cover film is 25-35 μm.

3. The method for manufacturing a flexible circuit board for wearable devices according to claim 1, characterized in that, The surface treatment step in step S1 includes: S1-1. Roughening treatment of rolled copper foil: Acid pickling is used to form fine particles with a roughness Ra≤0.13μm on the surface of rolled copper foil; S1-2. Degreasing and activation of rolled copper foil: The rolled copper foil is sequentially degreased by electrolysis and activated by copper sulfate to remove surface contaminants. The electrolysis degreasing is carried out in sodium hydroxide solution with an electrolysis current density of 38A / dm². S1-3. Surface treatment of polyimide-spandex composite substrate: Plasma cleaning with a power of 300-500W and a cleaning time of 30-60s is used to control the surface roughness of the composite substrate to Ra 0.8-1.2μm.

4. The method for manufacturing a flexible circuit board for wearable devices according to claim 1, characterized in that, Step S3 specifically includes the following sub-steps: S3-1. Photoresist coating: Coating the substrate surface with high-resolution photosensitive dry film or liquid photoresist, with a thickness of 5-10μm; S3-2. Laser Exposure: An ultraviolet laser source with a wavelength of 355-405nm is used. The laser beam is modulated by a digital micromirror device, and the spot diameter is controlled below 10μm. The exposure energy is 50-200mJ / cm². Line-by-line scanning exposure is used, and the spot overlap rate is 50-70%. S3-3. Development and Etching: After exposure, the photoresist in the unexposed areas dissolves in the developer to form circuit patterns. The developer is a sodium carbonate solution. Then, an acidic etching solution of ferric chloride or sulfuric acid-hydrogen peroxide is used to remove the exposed copper layer, leaving the photoresist-covered circuit area intact. After etching, the photoresist is removed to obtain a circuit board with copper lines.

5. A method for manufacturing a flexible circuit board for wearable devices according to claim 1, characterized in that, Step S4 specifically includes the following steps: S4-1. Surface treatment: Clean the surface of the circuit layer with a lint-free cloth and isopropyl alcohol to remove oxide layer and contaminants, and then preheat at 80-100℃ to improve adhesion; at the same time, perform plasma cleaning on the surface of the cover film; then, apply polyetheramine modified epoxy resin adhesive to the side of the cover film without surface layer. S4-2. Positioning and Alignment: Through mechanical or laser positioning, the through-hole windows on the cover film are precisely aligned with the solder pads on the copper circuit, so that the adhesive on the cover film comes into contact with the copper circuit, with the deviation controlled within ±30μm; vacuum adsorption is used to transfer to the hot press bonding device to prevent displacement during the bonding process. S4-3. Hot pressing: Using a stainless steel hot press roller with several spherical arc protrusions on the surface, at a temperature of 145±5℃ and a pressure of 0.4-0.6kgf / cm², the covering film is pre-fixed by local dot matrix hot pressing. S4-4. Full-surface hot pressing: Vacuum hot pressing is performed using a flat-surface hot pressing roller. The hot pressing temperature is 180±5℃, the pressure is 0.7-0.9kgf / cm², and the time is 30-90 seconds. S4-5. Curing and hardening Preliminary curing: Bake in a hot air circulating oven at 120-140℃ for 3-10 minutes to achieve preliminary curing of the polyetheramine modified epoxy resin adhesive; Segmented ripening: First stage: Bake at 80±5℃ for 10-30 minutes to allow the solvent in the adhesive to evaporate slowly; Second stage: Bake at 120±5℃ for 30-60 minutes to promote cross-linking of the adhesive; Third stage: Bake at 160±5℃ for 60-120 minutes to allow the adhesive to fully cure.

6. A method for manufacturing a flexible circuit board for wearable devices according to claim 1, characterized in that, The preparation method of the polyimide-spandex composite substrate includes the following steps: 1) Polyamic acid is synthesized by polycondensation of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether in the aprotic polar solvent DMF (N,N-dimethylformamide) at a reaction temperature of 10-25℃; the molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is 1:(1.01-1.1); the amount of DMF used is 5-6 times the weight of the sum of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether. 2) Spray a nano aluminum hydroxide dispersion onto the surface of spandex twisted short fibers. The average particle size of the aluminum hydroxide is 50 nm, and the solid content in the aluminum hydroxide dispersion is 15-20%. The twist of the spandex twisted short fibers is 550-600 T / m, and the length of the short fibers is 3-6 mm. 3) Add surface-treated spandex twisted staple fiber to the polyamic acid solution, wherein the weight of the spandex twisted staple fiber is 6-9% of the weight of the polyamic acid in the solution; at the same time, add 0.1-0.3% of KH-550 silane coupling agent by weight of polyamic acid to promote the subsequent interfacial bonding between polyimide and spandex. 4) The polyamic acid solution is rapidly cyclized through a screw extruder. The screw extrusion temperature is 180-200℃, and the polyamic acid solution stays in the screw extruder for 2-5 minutes. In the cooling zone of 160-130℃ after extrusion, a multi-stage vacuum port is set with a vacuum pressure of 0.05-0.09MPa to remove the volatilized DMF vapor in real time. The polyamic acid is cyclized into polyimide, resulting in weight loss. This makes the weight content of spandex twisted short fibers in the polyimide 8-12%, and the solvent content in the polyimide is less than 5%. The extrusion die of the screw extruder is slit-shaped, and a polyimide-spandex thick film is formed after extrusion. 5) The polyimide-spandex thick film is calendered into a polyimide-spandex thin film by a calender. The calender roll temperature is 55±5℃, the pressure is 8-10 MPa, and the calendered film thickness is 30-70μm, thus obtaining a polyimide-spandex composite substrate.

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