Method for liquid metal direct-write printing of conductive circuits and applications thereof

Abstract

The application discloses a kind of liquid metal direct writing printing conductive circuit method and its application, the method includes: substrate surface is activated and is coated anchor layer material to form nano anchor layer;Liquid metal is mixed with conductive reinforcing particles, antioxidant and thixotropic agent to prepare composite conductive ink;Under the condition of local atmosphere protection, direct writing printing is carried out;Energy irradiation is used to cure and shape in situ to conductive circuit;Deposited insulation solder resist layer and regulated impedance characteristic;And post-processing step.The application realizes the real-time oxidation resistance of liquid metal by adding antioxidant in ink and combining with reducing gas curtain protection in printing process;Through the construction of double anchoring interface by nano anchor layer, the adhesion of the circuit is improved;Through energy irradiation, rapid shaping is realized, high-precision and curved conformal printing are supported;And liquid metal recycling process is established.The application can be applied to the preparation of rigid PCB, flexible PCB, wearable device conformal PCB and high-frequency high-speed circuit board.

Description

Technical Field

[0001] This invention relates to the field of liquid metal printing technology, and more specifically, to a method for direct-write printing of conductive circuits using liquid metal and its application. Background Technology

[0002] Printed circuit boards (PCBs) are the core basic components of electronic products. Traditional PCB manufacturing mainly uses subtractive processes such as etching, electroplating, and developing. Although these processes are mature, they generally have defects such as long process flow, serious chemical pollution, large material consumption, and limited processing accuracy. In particular, it is difficult to fabricate conformal circuits on curved or irregular surfaces, which cannot meet the needs of emerging applications such as flexible electronics, wearable devices, and conformal antennas.

[0003] Liquid metals (such as gallium-based alloys) are considered an ideal alternative to traditional rigid conductive materials due to their excellent conductivity and room-temperature fluidity. Direct-write printing technology based on liquid metals provides a new technological path for PCB additive manufacturing. However, existing liquid metal direct-write printing technologies still face a series of insurmountable technical bottlenecks in industrial applications: gallium-based liquid metals are easily oxidized in air to form insulating oxide films, leading to line resistance drift and conduction failure; liquid metals have high surface tension, resulting in poor wettability with substrates such as FR-4 and PI, weak adhesion, and easy detachment and breakage; they exhibit a fluid dynamic state at room temperature, making rapid shaping difficult and hindering the achievement of narrow linewidths, high density, and three-dimensional conformal structures; there is a lack of reliable solder mask packaging methods, making it difficult to meet high-frequency, high-speed, and automotive-grade requirements; liquid metals are difficult to recycle after disposal, causing resource waste and environmental risks; traditional printing lacks local atmosphere protection and relies on large vacuum equipment, which is not conducive to industrial mass production.

[0004] To address some of the aforementioned issues, researchers have attempted improvements from the perspective of material modification. For example, Chinese invention patent application CN202411718474.X discloses a liquid metal composite conductive ink for flexible circuits and its preparation method. This method modifies liquid metal nanoparticles with polyvinyl alcohol and adds citric acid to remove the oxide layer, giving the prepared flexible circuit initial conductivity. However, this approach primarily focuses on modifying the ink material; the oxide layer removal relies on the effect of citric acid during the drying process, which is a post-processing method. Circuit shaping mainly relies on polymer film formation and does not address the rapid curing mechanism during printing. Furthermore, this approach does not address aspects such as strengthening interfacial adhesion or recovering the liquid metal.

[0005] Therefore, how to suppress liquid metal oxidation in real time during the printing process, enhance the interfacial adhesion between the circuit and the substrate, achieve rapid shaping of printed circuits, and realize recyclable green manufacturing have become urgent technical problems to be solved. Summary of the Invention

[0006] Therefore, it is necessary to address the above-mentioned technical problems by providing a method for direct writing conductive circuits using liquid metal and its application.

[0007] The first aspect of this invention provides a method for direct writing conductive circuits using liquid metal, comprising the following steps: Substrate pretreatment step: The substrate surface is activated and coated with anchoring layer material to form a nano-anchoring layer; Ink formulation steps: Liquid metal is mixed with conductive reinforcing particles, antioxidants and thixotropic agents to form a composite conductive ink with shear-thinning properties and antioxidant capabilities; Printing steps: Under local atmosphere protection conditions, a multi-axis linkage printing system is used to directly write and print composite conductive ink onto the substrate surface to form conductive lines; The molding process involves using energy irradiation to cure and shape the printed conductive lines in situ. Solder mask encapsulation steps: deposit an insulating solder mask layer on the surface of the shaped conductive lines and adjust the impedance characteristics of the lines; Post-processing steps.

[0008] Furthermore, in the substrate pretreatment step, the thickness of the nano-anchoring layer is 100~300nm, and the anchoring layer material comprises nanoporous SiO2 and KH550 coupling agent, wherein the particle size of the nanoporous SiO2 is 30~100nm, and the KH550 content is 1.5~2.5wt%.

[0009] Furthermore, in the ink preparation step, the liquid metal is gallium-based liquid metal, the conductive reinforcing particles are nano-Cu particles and / or nano-Ag particles, and the addition amount is 5~15wt%; the antioxidant is a reducing antioxidant, and the addition amount is 0.5~2wt%; the thixotropic agent is fumed silica, and the addition amount is 1~3wt%.

[0010] Furthermore, in the printing step, the local atmosphere protection adopts an annular air curtain to introduce a protective gas, which includes a mixture of nitrogen and reducing gas; the inner diameter of the printing needle is 30~80μm, and the printing line width is 30~80μm.

[0011] Furthermore, in the shaping step, the energy irradiation is near-infrared light irradiation, the irradiation temperature is 60~90℃, and the irradiation time is 20~40 seconds.

[0012] Furthermore, in the solder mask encapsulation step, the insulating solder mask layer is a nano-SiO2-AlN composite solder mask layer, wherein the mass ratio of SiO2 to AlN is 2.5:1 to 4:1, the thickness of the solder mask layer is 8 to 12 μm, and the characteristic impedance of the line is 50 to 90 Ω.

[0013] Furthermore, the post-processing steps include online testing of the finished product and / or recycling of liquid metals in the waste product.

[0014] Furthermore, the substrate is a rigid substrate or a flexible substrate.

[0015] Furthermore, the method is applicable to planar substrates, curved substrates, and substrates with blind holes and / or buried holes.

[0016] The second aspect of this invention proposes the application of the above-described method in the fabrication of rigid PCBs, flexible PCBs, conformal PCBs for wearable devices, or high-frequency and high-speed circuit boards.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for direct-write printing conductive circuits using liquid metal and its application. By adding a reducing antioxidant to the ink and using a reducing gas curtain composed of nitrogen and trace amounts of formic acid for local atmosphere protection during the printing process, dual antioxidant protection is achieved for both the ink interior and the printing environment. This method can suppress liquid metal oxidation in real time during printing, resulting in a circuit resistance drift rate of less than 2% and significantly improved electrical performance stability. Through the synergistic effect of atmospheric pressure plasma activation and a nanoporous SiO2-KH550 coupling agent anchoring layer, a dual mechanical and chemical bonding structure is constructed on the substrate surface. The re-anchored interface significantly improves the adhesion between the liquid metal circuitry and the substrate, achieving a cross-cut adhesion test rating of 4B or higher. Near-infrared light-thermal coupling irradiation enables rapid in-situ curing and shaping of the printed circuitry, effectively preventing circuitry flow and displacement, and supporting precise molding of narrow linewidths, high density, and curved conformal structures. A nano-SiO2-AlN composite ceramic solder resist layer integrates insulation protection and impedance regulation, meeting impedance matching requirements of 50~90Ω. Simultaneously, a non-destructive recycling process for liquid metal has been established, achieving a comprehensive recycling rate of over 98%, realizing green and low-carbon manufacturing.

[0018] Compared with traditional subtractive PCB processes and existing liquid metal printing technologies, this invention has significant advantages in terms of oxidation resistance, adhesion, molding accuracy, environmental friendliness, and recyclability. Detailed Implementation

[0019] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention. Example 1

[0020] This embodiment provides a method for direct writing conductive circuits using liquid metal, and fabricates high-density conductive circuits on a rigid FR-4 substrate.

[0021] S1. Preparation of the nano-anchoring layer at the substrate interface: FR-4 copper-clad laminate was selected as the substrate. First, it underwent atmospheric pressure plasma cleaning at a power of 500W for 30 seconds. Subsequently, a nano-mesoporous SiO2-KH550 coupling agent composite anchoring solution was uniformly coated onto the substrate surface using a spin-coating method at a speed of 1500 rpm for 30 seconds. The anchoring solution contained 65 nm nano-SiO2 particles and 2 wt% KH550. After coating, the substrate was dried in an 80℃ oven for 20 minutes to form a 200 nm thick nano-anchoring layer.

[0022] S2. Formulation of Antioxidant Thixotropic Composite Liquid Metal Ink: Using GaInSn liquid metal as the matrix, the mass ratio of Ga:In:Sn is 68.5:21.5:10. 10 wt% nano-Ag particles, 1 wt% reducing antioxidant, and 2 wt% fumed silica are added to the matrix. The average particle size of the nano-Ag particles is 50 nm. The reducing antioxidant is selected from a vitamin E and propyl gallate complex (mass ratio 1:1). The fumed silica is hydrophilic with a specific surface area of ​​200 m². 2 / g. The mixture was stirred for 15 minutes at 2000 rpm using a planetary centrifugal mixer to obtain a uniform, paste-like composite ink.

[0023] S3. Micro-atmosphere protected five-axis linkage microfluidic field direct writing printing: The prepared composite ink is loaded into the syringe of the five-axis linkage microfluidic field direct writing printing system. The printhead is equipped with an annular air curtain, through which nitrogen (flow rate 5L / min) and a trace amount of formic acid reducing gas (flow rate 50mL / min) are introduced. The inner diameter of the printhead is set to 50μm, the printing pressure to 250kPa, and the printing speed to 10mm / s. Direct writing is performed on the FR-4 substrate according to the preset lines to form conductive lines with a line width of 50μm and a line spacing of 40μm.

[0024] S4. Near-infrared light-thermal coupling in-situ rheological setting: Near-infrared lamps are synchronously mounted at the rear of the printhead to irradiate the lines immediately after printing. The irradiation temperature is 80℃ and the irradiation time is 30 seconds, which allows the liquid metal ink to quickly solidify and set.

[0025] S5. Deposition and Impedance Control of Nano-Ceramic Solder Mask Layer: A layer of nano-SiO2-AlN composite solder mask (SiO2 to AlN mass ratio of 3:1) is deposited on the shaped circuit using inkjet printing. It is then cured at 120℃ for 30 minutes to form a 10μm thick solder mask layer. By controlling the dielectric constant and thickness of the solder mask layer, the characteristic impedance of the circuit is precisely matched to 50Ω.

[0026] S6. Online Inspection and Recycling: 100% online inspection of finished products is carried out using a laser profilometer and a four-probe resistance tester. Example 2

[0027] This embodiment provides a method for direct writing conductive circuits using liquid metal, and fabricates high-density conductive circuits on a rigid FR-4 substrate.

[0028] S1. Preparation of the nano-anchoring layer at the substrate interface: FR-4 copper-clad laminate was selected as the substrate. First, it underwent atmospheric pressure plasma cleaning at a power of 500W for 30 seconds. Subsequently, a nano-mesoporous SiO2-KH550 coupling agent composite anchoring solution was uniformly coated onto the substrate surface using a spin-coating method at a speed of 1200 rpm for 40 seconds. The anchoring solution contained 35 nm nano-SiO2 particles and 1.5 wt% KH550. After coating, the substrate was dried in a 70℃ oven for 25 minutes to form a 100 nm thick nano-anchoring layer.

[0029] S2. Formulation of Antioxidant Thixotropic Composite Liquid Metal Ink: Using GaInSn liquid metal as the matrix, the mass ratio of Ga:In:Sn is 68.5:21.5:10. 5 wt% nano-Ag particles, 0.5 wt% reducing antioxidant, and 1 wt% fumed silica are added to the matrix. The average particle size of the nano-Ag particles is 50 nm. The reducing antioxidant is selected from a complex of vitamin E and propyl gallate (mass ratio 1:1). The fumed silica is hydrophilic with a specific surface area of ​​200 m². 2 / g. The mixture was stirred for 20 minutes at 1500 rpm using a planetary centrifugal mixer to obtain a uniform, paste-like composite ink.

[0030] S3. Micro-atmosphere protected five-axis linkage microfluidic field direct writing printing: The prepared composite ink is loaded into the syringe of the five-axis linkage microfluidic field direct writing printing system. The printhead is equipped with an annular air curtain, through which nitrogen (flow rate 5L / min) and a trace amount of formic acid reducing gas (flow rate 50mL / min) are introduced. The inner diameter of the printhead is set to 80μm, the printing pressure to 200kPa, and the printing speed to 8mm / s. Direct writing is performed on the FR-4 substrate according to the preset lines to form conductive lines with a line width of 80μm and a line spacing of 60μm.

[0031] S4. Near-infrared light-thermal coupling in-situ rheological setting: Near-infrared lamps are synchronously mounted at the rear of the printhead to irradiate the lines immediately after printing. The irradiation temperature is 60℃ and the irradiation time is 40 seconds, which allows the liquid metal ink to quickly solidify and set.

[0032] S5. Deposition and Impedance Control of Nano-Ceramic Solder Mask Layer: A layer of nano-SiO2-AlN composite solder mask (SiO2 to AlN mass ratio of 4:1) is deposited on the shaped circuit using inkjet printing. It is then cured at 110℃ for 40 minutes to form an 8μm thick solder mask layer. By controlling the dielectric constant and thickness of the solder mask layer, the characteristic impedance of the circuit is precisely matched to 90Ω.

[0033] S6. Online Inspection and Recycling: 100% online inspection of finished products is carried out using a laser profilometer and a four-probe resistance tester. Example 3

[0034] This embodiment provides a method for direct writing conductive circuits using liquid metal, and fabricates high-density conductive circuits on a rigid FR-4 substrate.

[0035] S1. Preparation of the nano-anchoring layer at the substrate interface: FR-4 copper-clad laminate was selected as the substrate. First, it underwent atmospheric pressure plasma cleaning at a power of 500W for 30 seconds. Then, a nano-mesoporous SiO2-KH550 coupling agent composite anchoring solution was uniformly coated onto the substrate surface using a spin-coating method at 1800 rpm for 20 seconds. The anchoring solution contained 95 nm nano-SiO2 particles and 2.5 wt% KH550. After coating, the substrate was dried in a 90℃ oven for 15 minutes to form a 300 nm thick nano-anchoring layer.

[0036] S2. Formulation of Antioxidant Thixotropic Composite Liquid Metal Ink: Using GaInSn liquid metal as the matrix, the mass ratio of Ga:In:Sn is 68.5:21.5:10. 15 wt% nano-Ag particles, 2 wt% reducing antioxidant, and 3 wt% fumed silica are added to the matrix. The average particle size of the nano-Ag particles is 50 nm. The reducing antioxidant is selected from a vitamin E and propyl gallate complex (mass ratio 1:1). The fumed silica is hydrophilic with a specific surface area of ​​200 m². 2 / g. The mixture was stirred for 10 minutes at 2500 rpm using a planetary centrifugal mixer to obtain a uniform, paste-like composite ink.

[0037] S3. Micro-atmosphere protected five-axis linkage microfluidic field direct writing printing: The prepared composite ink is loaded into the syringe of the five-axis linkage microfluidic field direct writing printing system. The printhead is equipped with an annular air curtain, through which nitrogen (flow rate 5L / min) and a trace amount of formic acid reducing gas (flow rate 50mL / min) are introduced. The inner diameter of the printhead is set to 30μm, the printing pressure to 300kPa, and the printing speed to 12mm / s. Direct writing is performed on the FR-4 substrate according to the preset lines to form conductive lines with a line width of 30μm and a line spacing of 30μm.

[0038] S4. Near-infrared light-thermal coupling in-situ rheological setting: Near-infrared lamps are synchronously mounted at the rear of the printhead to irradiate the lines immediately after printing. The irradiation temperature is 90℃ and the irradiation time is 20 seconds, which rapidly solidifies and sets the liquid metallic ink.

[0039] S5. Deposition and Impedance Control of Nano-Ceramic Solder Mask Layer: A layer of nano-SiO2-AlN composite solder mask (SiO2 to AlN mass ratio of 3.5:1) is deposited on the shaped circuit using inkjet printing. It is then cured at 130℃ for 20 minutes to form a 9μm thick solder mask layer. By controlling the dielectric constant and thickness of the solder mask layer, the characteristic impedance of the circuit is precisely matched to 75Ω.

[0040] S6. Online Inspection and Recycling: 100% online inspection of finished products is carried out using a laser profilometer and a four-probe resistance tester. Example 4

[0041] This embodiment provides a method for direct writing conductive circuits using liquid metal, and fabricates conformal conductive circuits for wearable devices on a flexible PI substrate.

[0042] S1. Preparation of Nano-Anchoring Layer at Substrate Interface: A 50μm thick PI flexible film was selected as the substrate. First, it was subjected to atmospheric pressure plasma cleaning at a power of 500W for 30 seconds. Subsequently, a nanoporous SiO2-KH550 coupling agent composite anchoring liquid was uniformly coated onto the substrate surface using a spraying method. After coating, it was placed in an 80℃ oven for low-temperature drying for 20 minutes to form a 150nm thick nano-anchoring layer.

[0043] S2. Antioxidant thixotropic composite liquid metal ink formulation: Using GaInSn liquid metal as the matrix, add 8wt% nano-Cu particles, 1wt% reducing antioxidant and 2wt% fumed silica, and mix evenly using a planetary centrifugal mixer.

[0044] S3. Micro-atmosphere protected five-axis linkage microfluidic direct writing printing: A PI substrate is bonded to a curved mold mimicking a hand and ankle joint. The prepared composite ink is loaded into the syringe of the five-axis linkage microfluidic direct writing printing system. The printhead is equipped with an annular air curtain that introduces nitrogen and a trace amount of formic acid reducing gas. The five-axis linkage system performs conformal printing according to the path generated by the curved surface model, with the printhead inner diameter set to 50μm and the linewidth controlled at 80μm.

[0045] S4. Near-infrared light-thermal coupling in-situ rheological shaping: Near-infrared lamps are synchronously mounted at the rear of the print head, with an irradiation temperature of 70℃ and an irradiation time of 30 seconds.

[0046] S5. Deposition and impedance control of nano-ceramic solder resist layer: A nano-SiO2-AlN composite solder resist layer is deposited on the shaped circuit and cured at 120℃ for 30 minutes.

[0047] S6. Online Inspection: The finished product is inspected using a laser profilometer and a four-probe resistance tester. Example 5

[0048] This embodiment provides a green recycling method for liquid metals in PCBs prepared using the method of the present invention.

[0049] S1. Recycling: Take the waste FR-4 PCB prepared in Example 2, immerse it in a weakly alkaline stripping solution, and let it stand at room temperature for 5 minutes. The conductive lines will completely detach from the substrate surface and remain suspended in the solution. The weakly alkaline stripping solution has a pH of 9 to 10 and its main component is an aqueous solution of triethanolamine.

[0050] S2. Purification and Regeneration: The detached liquid metal complex was collected by filtration, washed three times with deionized water, and then dried in a vacuum oven at 60°C for 2 hours. The recovered material was then replenished with 1 wt% fumed silica according to step S2 of Example 2 and mixed thoroughly again.

[0051] Comparative Example 1 This comparative example provides a method for direct writing conductive circuits using liquid metal. The only difference between this method and Example 1 is that in step S1, the FR-4 substrate is cleaned by atmospheric pressure plasma before printing, and the nanoporous SiO2-KH550 coupling agent composite anchoring liquid is not sprayed.

[0052] Comparative Example 2 This comparative example provides a method for direct writing conductive circuits using liquid metal, which differs from Example 1 in that: in step S2, no nano-Ag particles, reducing antioxidants, or fumed silica thixotropic agents are added; pure GaInSn liquid metal is used directly for printing.

[0053] Comparative Example 3 This comparative example provides a method for direct writing conductive circuits using liquid metal, which differs from Example 1 in that: in step S2, the amount of nano Ag particles added is 20wt%, the amount of reducing antioxidant added is 0.1wt%, and the amount of fumed silica thixotropic agent added is 0.5wt%.

[0054] Comparative Example 4 This comparative example provides a method for direct writing of conductive circuits using liquid metal. The difference between this method and Example 1 is that in step S3, the annular air curtain is closed, and nitrogen and formic acid reducing gases are not introduced. Direct writing is performed in a normal atmospheric environment.

[0055] Comparative Example 5 This comparative example provides a method for direct writing conductive circuits using liquid metal, which differs from Example 1 in that step S4 is omitted, and the printed circuits are not subjected to near-infrared irradiation but are naturally shaped at room temperature.

[0056] Comparative Example 6 This comparative example provides a method for fabricating FR-4 PCBs using a traditional subtractive method (photolithography + etching), with the specific steps as follows: S1. Preparation of copper clad laminate: Select FR-4 copper clad laminate (copper foil thickness of 35μm) as the substrate, and perform degreasing, pickling, water washing and drying treatment in sequence.

[0057] S2. Film lamination: A dry film (photosensitive resin film) is laminated onto the surface of the copper-clad laminate. A hot press roller is used to tightly adhere the dry film to the copper foil surface at 110°C.

[0058] S3. Exposure: The designed circuit pattern mask is placed on the dry film and exposed using a UV exposure machine (exposure energy 80mJ / cm²). 2 This causes the dry film in the exposed area to cross-link and solidify.

[0059] S4. Development: Immerse the exposed copper-clad laminate in the developer (1wt% Na2CO3 solution). The dry film in the unexposed areas is dissolved and removed, exposing the copper foil that needs to be etched.

[0060] S5. Etching: Immerse the developed copper-clad laminate in an acidic etching solution (main components are CuCl2, HCl, and H2O2) and etch for 5 minutes at 50°C to remove the exposed copper foil and retain the circuit pattern covered by the dry film.

[0061] S6. Film Removal: Immerse the etched copper-clad board in the film removal solution (3wt% NaOH solution) and soak at 50℃ for 3 minutes to remove the dry film on the circuit surface.

[0062] S7. Solder resist layer preparation: Photosensitive solder resist ink is screen-printed on the circuit surface, and then exposed, developed, and heat-cured (150℃, 30 minutes) to form a solder resist layer.

[0063] S8. Surface treatment: Chemically plate nickel-gold onto the exposed pads to complete the PCB fabrication.

[0064] Verification Example 1 The conductive lines or PCBs prepared in Examples 1-3 and Comparative Examples 1-6 were subjected to performance tests. The test methods and results for each performance index are as follows.

[0065] 1. Adhesion Test: The adhesion test is conducted according to GB / T 9286-1998. 100 small squares of 1mm×1mm are drawn on the circuit surface using a cross-cutting tool. 3M 600 tape is applied and then quickly peeled off. The rating standards are as follows: 5B (Grade 0, the cut edges are completely smooth and no squares fall off), 4B (Grade 1, there is a small amount of peeling at the intersection of the cuts, and the peeling area is <5%), 3B (Grade 2, the peeling area is 5%-15%), 2B (Grade 3, the peeling area is 15%-35%), 1B (Grade 4, the peeling area is 35%-65%), 0B (Grade 5, the peeling area is >65%).

[0066] 2. Antioxidant test: The double 85 test is adopted. The sample is placed in a constant temperature and humidity chamber at 85℃ and 85% relative humidity for 500 hours. The circuit resistance before and after aging is tested, and the resistance drift rate is calculated: Drift rate = (R2 - R1) / R1 ×100%.

[0067] 3. Electrical performance testing: The resistivity of the circuit is measured using a four-probe resistance tester; the characteristic impedance is tested using a vector network analyzer or a time domain reflectometer.

[0068] 4. Flame retardant rating test: Vertical burning test conducted according to UL94 standard.

[0069] 5. Yield test: For each scheme, 50 pieces are produced continuously. The number of products that are qualified in appearance (no broken lines, no bridging short circuits, no excessive tolerances) and whose resistance values ​​are within ±10% of the nominal value is counted, and the yield rate is calculated.

[0070] Table 1. Performance test results of Examples 1-3 and Comparative Examples 1-6 As shown in Table 1, Examples 1-3 performed excellently in all test items: adhesion reached 4B-5B, resistivity drift was controlled between 1.5% and 1.8%, and resistivity was 2.9-3.8 × 10⁻⁶. -8With a characteristic impedance that can be precisely matched to 50Ω~90Ω, all flame retardant ratings reach V-0, and the yield rate is as high as 96.2%~98.5%. This indicates that the present invention achieves the comprehensive advantages of high adhesion, high oxidation resistance, high conductivity, high yield, and customizable impedance of liquid metal conductive circuits through the synergistic effect of nano-anchoring layer, anti-oxidation thixotropic ink, atmosphere-protected printing, photothermal setting, and nano-ceramic solder resist layer.

[0071] Comparative Example 1, lacking a nano-anchoring layer, suffered from adhesion reduced to 1B, circuit detachment and breakage, resulting in a yield of less than 20% and inability to measure impedance. Comparative Example 2, using pure liquid metal without additives, exhibited severe oxidation, flow deformation, a resistance drift exceeding 50%, and a resistivity as high as 100 × 10⁻⁶. -8 The resistance drift rate was less than 30% in Comparative Example 3 due to excessive nano-Ag and insufficient antioxidants and thixotropic agents, resulting in a resistance drift rate of 25% and a yield of only 55%, making it impossible to measure the impedance stably. Comparative Example 4 lacked atmosphere protection, resulting in a resistance drift rate of 15% and a yield rate of 75%. Comparative Example 5 lacked photothermal shaping, leading to circuit flow and a yield rate of 70%. Comparative Example 6 used the traditional subtractive method, which, although having a better resistivity than this invention, had a long process, high pollution, and could not prepare curved conformal structures.

[0072] Verification Example 2 The performance of the flexible PI conformal conductive circuit prepared in Example 4 above was tested, and the test results of various performance indicators are shown in Table 2.

[0073] 1. Bending Reliability Test: Using a bending tester, the flexible circuit is repeatedly bent at a bending angle of 180°, a bending radius of 5mm, and a bending frequency of 1Hz. The circuit is observed for any abnormal phenomena such as cracking, detachment, or oxidation, and the circuit resistance is tested before and after bending.

[0074] 2. Surface conformal accuracy test: The three-dimensional shape of the printed circuit is scanned using a three-dimensional optical profilometer and compared with the CAD design model to evaluate the printing accuracy.

[0075] 3. Adhesion test: The test method is the same as in verification example 1.

[0076] Table 2. Performance test results of Example 4 (flexible PI substrate) As shown in Table 2, the flexible conformal circuit prepared in Example 4, after being bent 1000 times at 180°, exhibited no cracking, peeling, or oxidation, with a resistance change rate of +2.8%, maintaining stable electrical performance. The linewidth deviation of the conformal printing on the curved surface was ±5μm, meeting the design requirements for printing accuracy. The adhesion reached 4B. This indicates that the method of the present invention also possesses good bending reliability and interface adhesion on flexible substrates.

[0077] Verification Example 3 The performance of the recycled liquid metal ink and the reprinted circuits from Example 5 were tested, and the test results of each performance index are shown in Table 3.

[0078] 1. Metal recovery rate test: Weigh the total mass of the recovered and purified metal, divide it by the total mass of the metal in the original circuit, and calculate the overall recovery rate.

[0079] 2. Performance comparison test of recycled ink: The circuit was reprinted using recycled ink according to the method in Example 2, and its resistivity, adhesion and oxidation resistance were tested and compared with the newly made ink.

[0080] Table 3. Performance test results of Example 5 (liquid metal recovery and regeneration) As shown in Table 3, the method of this invention can achieve efficient recovery of liquid metal, with a comprehensive recovery rate of 98.2%. The lines reprinted using recycled ink have an adhesion of 4B and a resistivity of 3.9 × 10⁻⁶. -8 The resistivity shift rate was 1.6% (Ω·m), which is consistent with the test results of the newly prepared ink in Example 2 (adhesion 4B, resistivity 3.8×10). -8 The resistance (Ω·m and resistivity drift rate 1.5%) are basically the same, indicating that the performance of the recycled ink is comparable to that of the newly made ink. The recycling method provided by this invention has the characteristics of high efficiency and high performance retention, and can realize the recycling of liquid metal.

[0081] Obviously, the embodiments described above are merely some embodiments of this application, not all embodiments, and do not limit the patent scope of this application. This application can be implemented in many different forms; on the contrary, the purpose of providing these embodiments is to make the disclosure of this application more thorough and comprehensive. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or make equivalent substitutions for some of the technical features. Any equivalent structures made using the content of this application's specification, directly or indirectly applied to other related technical fields, are similarly within the patent protection scope of this application.

Claims

1. A method for direct-write printing conductive circuits using liquid metal, characterized in that, Includes the following steps: Substrate pretreatment step: The substrate surface is activated and coated with anchoring layer material to form a nano-anchoring layer; Ink formulation steps: Liquid metal is mixed with conductive reinforcing particles, antioxidants and thixotropic agents to form a composite conductive ink with shear-thinning properties and antioxidant capabilities; Printing steps: Under local atmosphere protection conditions, a multi-axis linkage printing system is used to directly write and print composite conductive ink onto the substrate surface to form conductive lines; The molding process involves using energy irradiation to cure and shape the printed conductive lines in situ. Solder mask encapsulation steps: deposit an insulating solder mask layer on the surface of the shaped conductive lines and adjust the impedance characteristics of the lines; Post-processing steps.

2. The method according to claim 1, characterized in that, In the substrate pretreatment step, the thickness of the nano-anchoring layer is 100~300nm, and the anchoring layer material comprises nanoporous SiO2 and KH550 coupling agent, wherein the particle size of the nanoporous SiO2 is 30~100nm, and the KH550 content is 1.5~2.5wt%.

3. The method according to claim 1, characterized in that, In the ink preparation step, the liquid metal is gallium-based liquid metal, the conductive reinforcing particles are nano-Cu particles and / or nano-Ag particles, and the addition amount is 5~15wt%; the antioxidant is a reducing antioxidant, and the addition amount is 0.5~2wt%; the thixotropic agent is fumed silica, and the addition amount is 1~3wt%.

4. The method according to claim 1, characterized in that, In the printing step, the local atmosphere protection is achieved by introducing a protective gas through an annular air curtain. The protective gas contains a mixture of nitrogen and reducing gas. The inner diameter of the printing needle is 30~80μm, and the printing line width is 30~80μm.

5. The method according to claim 1, characterized in that, In the shaping step, the energy irradiation is near-infrared light irradiation, the irradiation temperature is 60~90℃, and the irradiation time is 20~40 seconds.

6. The method according to claim 1, characterized in that, In the solder mask encapsulation step, the insulating solder mask layer is a nano-SiO2-AlN composite solder mask layer, wherein the mass ratio of SiO2 to AlN is 2.5:1 to 4:1, the thickness of the solder mask layer is 8 to 12 μm, and the characteristic impedance of the line is 50 to 90 Ω.

7. The method according to claim 1, characterized in that, The post-processing steps include online testing of the finished product and / or recycling of liquid metals from waste products.

8. The method according to claim 1, characterized in that, The substrate can be a rigid substrate or a flexible substrate.

9. The method according to claim 1, characterized in that, The method is applicable to planar substrates, curved substrates, and substrates with blind holes and / or buried holes.

10. The application of the method according to any one of claims 1 to 9 in the fabrication of rigid PCBs, flexible PCBs, conformal PCBs for wearable devices, or high-frequency and high-speed circuit boards.

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