Conductive copper paste suitable for precision 3D printing and preparation method and application thereof

Abstract

The application relates to the field of conductive paste, in particular to a conductive copper paste suitable for precise 3D printing and a preparation method and application thereof. The conductive copper paste comprises nano copper particles and a dispersing preparation; the surface of the nano copper particles is coated with ligands; the particle size of the nano copper particles is 50-500 nm; and the dispersing preparation is composed of a binder, a protective agent and an organic solvent. The copper nano particles are prepared by adopting a liquid phase reduction method, the copper precursor is added into a ligand-containing reducing agent solution, the non-synchronous growth of the copper precursor nucleation in the initial stage of the reaction can be controlled, the obtained nano copper particles are small in size, high in uniformity, excellent in dispersity and stability, and the particle surface effectively forms a coating body, the contact probability of the nano particle surface and oxygen is reduced, and the oxidation resistance of the particles is improved.

Description

Technical Field

[0001] This invention relates to the field of conductive pastes, and more particularly to a conductive copper paste suitable for precision 3D printing, its preparation method, and its application. Background Technology

[0002] Precision 3D printing technology is a cutting-edge advanced manufacturing technology worldwide. Through printing control in different dimensions, it can realize the preparation of complex micro- and nano-scale three-dimensional structures. It has attracted widespread attention in fields such as precision optics, micro- and nano-chips, and printed electronics, and has great industrial potential.

[0003] This technology, applied in the microelectronics industry, places high demands on the performance of conductive pastes. The pastes used must not only ensure smooth discharge from fine nozzles but also possess excellent shape retention and conductivity. Currently, the market primarily uses conductive metal pastes prepared from nano-silver particles. These pastes not only possess excellent oxidation resistance but also high conductivity, attracting attention in the field of 3D integrated electronics. However, traditional conductive silver pastes still have some problems, such as poor electrochemical migration tolerance and high raw material costs, significantly limiting capacity expansion. In large-scale production, copper's price is far lower than silver's, and its process compatibility is better. With its strong anti-electromigration ability and high conductivity (only 6% lower than silver), copper has enormous potential to replace expensive traditional conductive silver pastes.

[0004] Existing methods for preparing conductive copper paste mainly include two steps: (1) obtaining elemental copper particles using methods such as exploding wire method, gas phase synthesis method, vapor phase deposition method, and liquid phase reduction method; (2) mixing elemental copper particles with a dispersant to prepare conductive copper paste. Because nanoscale metal particles have high electrostatic adsorption capacity, they are prone to agglomerate to the micrometer scale, making it difficult to control the size of copper nanoparticles to maintain a finer and more uniform level according to production requirements.

[0005] For example, patent CN 104505137 A discloses a conductive copper paste, its preparation method, and its application. The conductive copper paste is prepared by mixing and dispersing ultrafine copper powder with organic binders, reducing agents, surfactants, and other reagents. The copper powder used has 1-5μm particles, making it only suitable for screen printing processes with low raw material utilization efficiency and unable to achieve high-precision 3D printing below 30μm.

[0006] For example, patent CN 113362984 A uses copper powder as a protective agent to formulate copper nanoparticles into an oil-soluble slurry. Lacking strong polar functional group ligands, it has weak interfacial affinity when applied to polar substrates such as glass and silicon wafers, and is prone to detachment after sintering. Furthermore, its adhesion performance has not yet been characterized.

[0007] Patent CN 109467984 A discloses a self-assembled copper sphere, conductive ink, its preparation method, and its application. The method employs a liquid-phase reduction process, simultaneously mixing and reacting a reducing agent with a copper precursor to obtain composite copper spheres coated with the oxidation products of the reducing agent. However, this approach fails to effectively control the adsorption and aggregation between the original elemental copper particles, leading to the continuous self-assembly of nano-sized particles into 2-10 μm scale composite copper spheres during synthesis. Consequently, the resulting conductive ink cannot be applied to precision 3D printing. Summary of the Invention

[0008] The present invention aims to overcome the defects of existing conductive copper pastes as described above, and therefore provides a conductive copper paste, its preparation method, and its application.

[0009] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a conductive copper paste suitable for precision 3D printing. This includes copper nanoparticles and dispersing agents; among which, The surface of the copper nanoparticles is coated with ligands; The particle size of the copper nanoparticles is 50-500 nm; The dispersion formulation consists of a binder, a protective agent, and an organic solvent.

[0010] In existing conductive copper pastes, the nano-copper particles are generally prepared via liquid-phase reduction, primarily by adding a reducing agent to a copper precursor solution. In this method, the reduction of the copper precursor and the nucleation and growth of nano-copper elements occur simultaneously. Due to the high concentration of the copper precursor in the initial stage of the reaction, the addition of the reducing agent easily causes localized agglomeration and growth of the elemental particles, resulting in particles that are often large in size and irregular in shape. This significantly affects the dispersibility and stability of the conductive copper paste. Furthermore, because the copper particles in the conductive copper paste are generally large, mostly at the micrometer or submicrometer level, it is only suitable for screen printing processes with low raw material utilization efficiency. However, this process is limited to printing patterns with linewidths greater than 30μm, making it difficult to achieve higher precision printing processes.

[0011] The conductive copper paste of this invention has ligands coated on the surface of the copper nanoparticles. This prevents the nanoparticles from adsorbing and agglomerating, despite their nanoparticle size remaining at the nanometer scale. Consequently, the copper nanoparticles exhibit excellent dispersibility and stability. Therefore, the conductive copper paste of this invention is suitable for precision 3D printing with a scale of ≥1μm, and its material utilization rate is higher than that of existing screen printing processes.

[0012] Meanwhile, because the surface of the nano-copper particles is coated with ligands, the elemental copper will not come into direct contact with moisture and oxygen in the air, thus effectively preventing the oxidation of the nano-copper particles. Therefore, its sintering performance will not decrease in the subsequent sintering process, and the storage stability of the slurry will not decrease significantly.

[0013] Furthermore, due to the coating of ligands, the copper nanoparticles can have good interfacial affinity with polar substrates such as glass and silicon wafers, thus exhibiting good adhesion properties after sintering.

[0014] Finally, in order to improve the stability of the conductive copper paste, the present invention also selectively introduces a protective agent into the dispersion formulation, which can further improve the stability of the nano-copper particles, thereby further improving the stability and sintering conductivity of the paste.

[0015] Further preferably, the particle size of the copper nanoparticles is 50-150 nm.

[0016] Preferably, the adhesive comprises any one or more combinations of cellulose derivatives, epoxy resins, phenolic resins, acrylic resins, and polyamide resins.

[0017] Further preferably, the cellulose derivative may be ethyl cellulose, hydroxyethyl cellulose, etc.

[0018] Preferably, the protective agent includes any one or more combinations of thiols, primary alcohol amines with the general chemical formula OH-X-NH2, where X is 2 ≤ carbon number ≤ 6, secondary alcohol amines with the general chemical formula NH-(Y-OH)2, where Y is 2 ≤ carbon number ≤ 6, ascorbic acid, oxalic acid, and formic acid.

[0019] Preferably, the organic solvent includes any one or more combinations of mono / polyols with 2 ≤ carbon number ≤ 12, tertiary alcohol amines with the chemical formula N-(Z-OH)3 where Z is 2 ≤ carbon number ≤ 6, ethylene glycol monomethyl ether, dipropylene glycol methyl ether, and triethylene glycol dimethyl ether.

[0020] Further preferably, the mono / polyol is terpineol, ethanol, ethylene glycol, propylene glycol, or glycerol.

[0021] Preferably, the viscosity range of the conductive copper paste is 10 Pa·s to 1000 Pa·s; The resistivity after sintering is <50 μΩ·cm; The resistivity change was maintained at less than 8% after being stored at 0~5℃ for 1 year. The adhesion properties after sintering meet the ASTM D3359-2017 test standard 5B performance.

[0022] Secondly, the present invention also provides a method for preparing conductive copper paste suitable for precision 3D printing as described above. Includes the following steps: (1) The copper precursor solution is gradually added to the reducing agent solution containing ligands so that the nucleation and growth of the copper precursor are asynchronous during the reduction process, and finally a copper nanoparticle dispersion is obtained. (2) After separation, washing and drying, copper nanoparticle dispersion is used to obtain copper nanoparticles; (3) The conductive copper paste is obtained by mixing the nano-copper particles with the dispersing agent.

[0023] Existing technologies generally prepare copper nanoparticles through liquid-phase reduction, primarily by adding a reducing agent to a copper precursor solution or by mixing the reducing agent and copper precursor solution in a single step. In this mode, the reduction of the copper precursor and the nucleation and growth of elemental copper nanoparticles occur simultaneously. Due to the high concentration of the copper precursor in the initial stage of the reaction, the addition of the reducing agent can easily cause local agglomeration and growth of elemental particles. The resulting particles are often large in size, irregular in shape, and lack effective ligand coating on the surface, which significantly affects the dispersibility and stability of the conductive copper paste.

[0024] Furthermore, according to recent domestic and international literature, some technologies have been developed to prepare nanoscale conductive copper paste. However, as the particle size in the copper paste decreases, the surface area and surface free energy of the nanoscale copper particles increase rapidly, resulting in high activity. Specifically, this manifests as: 1) easy adsorption and aggregation between particles; 2) easy oxidation upon exposure to air, moisture, and oxygen, reducing sintering performance. This leads to decreased storage stability of the paste and reduced high-precision direct-write 3D printing capabilities, limiting its development and application in the field of precision electronics.

[0025] The difference between this invention and existing technologies lies in the fact that, in the liquid-phase reduction method for preparing copper nanoparticles, this invention controls the gradual addition of the copper precursor to the reducing agent solution containing ligands. This allows for asynchronous nucleation and growth of the copper precursor in the initial stage of the reaction, preventing localized aggregation of copper nanoparticles during growth. This results in effectively reduced particle size, more regular shapes, and improved dispersion stability of the prepared copper nanoparticles. Therefore, this is more conducive to high-precision direct-write 3D printing, expanding its potential for development and application in the field of precision electronics.

[0026] Furthermore, due to the preparation method of this invention, the nucleation and growth of the copper precursor are asynchronous. After the copper particles grow to the nanoscale, their surface can be effectively coated with ligands, which further reduces the mutual adsorption between particles. In addition, the ligands can effectively separate the copper nanoparticles from the moisture and oxygen in the air, effectively reducing their oxidation possibility. This has a significant impact on the dispersibility and stability of the conductive copper paste, thereby improving its thermal sintering performance.

[0027] Preferably, the concentration of the copper precursor solution in step (1) is 1-7.5 mol / L.

[0028] Preferably, the copper precursor solution is added at a rate of 0.2-10 ml / min.

[0029] After testing by the inventors of this application, it was found that when the copper precursor solution was added at a rate of 0.2-10 ml / min, the resulting copper nanoparticles had more uniform particle size and morphology, and the final printing accuracy was also higher.

[0030] Preferably, the copper precursor solution is added in one or more of the following methods: dropwise addition via a separatory funnel, dropwise addition via a constant-pressure dropping funnel, injection via a syringe pump, or injection via a peristaltic pump.

[0031] Preferably, in step (1), the molar ratio of copper precursor to reducing agent is 1:(1-6).

[0032] This invention, by controlling the molar ratio of copper precursor to reducing agent, achieves good reduction of the copper precursor while conserving the amount of reducing agent used. Furthermore, under this ratio, the generated copper nanoparticles exhibit more uniform morphology and size, which is beneficial for high-precision direct-write 3D printing capabilities.

[0033] Preferably, the copper precursor includes one or more of the following: copper sulfate pentahydrate, copper formate, copper hydroxide, copper acetate, copper chloride, copper nitrate, copper sulfide, copper citrate, and copper acetylacetonate.

[0034] Preferably, the reducing agent includes one or more combinations of hydrazine hydrate, hypophosphite, sodium hypophosphite, sodium phosphite, sodium borohydride, potassium borohydride, sodium dithionite, formaldehyde, ascorbic acid, citric acid, and oxalic acid.

[0035] Preferably, the ligand comprises any one or a combination of two of polyacrylic acid and polyvinylpyrrolidone.

[0036] Preferably, the method for preparing conductive copper paste further includes the step of dispersing a reducing agent and a ligand in a solvent to form a mixed solution.

[0037] Further preferably, the dispersion method can be one or more of ultrasonic, magnetic stirring, and mechanical stirring. In some specific embodiments of the present invention, the dispersion method is preferably magnetic stirring, and the stirring speed is further preferably 200-1500 r / min.

[0038] Further optimization involves gradually adding the copper precursor solution to the reducing agent solution containing the ligand, followed by stirring and dispersing the solution under heating conditions to induce a redox reaction in the solution, ultimately obtaining a copper nanoparticle dispersion.

[0039] Preferably, the heating conditions can be one or more of water bath, oil bath, and electric heating plate, and the heating temperature is 60-150℃.

[0040] Preferably, the stirring and dispersing method can be one or more of magnetic stirring and mechanical stirring. In some specific embodiments of the present invention, the dispersion method is preferably magnetic stirring, and the stirring speed is further preferably 200-1500 r / min.

[0041] Preferably, the separation step in step (2) can be performed by high-speed centrifugation to separate the layers, and the precipitate is obtained after filtration.

[0042] Preferably, the solution used in the washing step (2) can be any one or a combination of deionized water, ethanol, methanol, acetone, and ethyl acetate. After washing the precipitate with the above solution, it is separated by centrifugation, and this step is repeated several times.

[0043] Preferably, the drying process in step (2) is as follows: the washed precipitate is dried in a nitrogen, argon or vacuum environment for ≥1h to obtain nanoscale copper elemental particles.

[0044] Preferably, in step (3), the nano-copper particles are mixed with the dispersing agent and then subjected to multiple mixing and degassing processes before being ground using a three-roll mill to finally obtain nano-conductive copper paste.

[0045] Further preferably, the parameters for the slurry degassing treatment are set as follows: rotation speed 800-2000 rpm, vacuum degree ≥0.2 kPa.

[0046] Thirdly, the conductive copper paste described above is also provided, or the conductive copper paste prepared by the above preparation method can achieve high-precision 3D printing at a scale of ≥1μm, thereby enabling its application in the fields of precision 3D printing, precision optics, micro-nano chips, and printed electronics.

[0047] Preferably, the substrate materials suitable for the conductive copper paste include, but are not limited to, glass, silicon wafers, PI, stainless steel, ceramics, etc.

[0048] Therefore, the present invention has the following beneficial effects: (1) Beneficial effects: The method for preparing conductive copper paste described in this invention uses a liquid-phase reduction method to prepare copper nanoparticles. By adding the copper precursor to a reducing agent solution containing ligands, the nucleation and growth of the copper precursor can be controlled to proceed asynchronously in the early stage of the reaction, and an effective coating is formed on the particle surface, reducing the probability of contact between the nanoparticle surface and oxygen, and improving the oxidation resistance of the particles. The resulting copper nanoparticles are small in size (50-150 nm), highly uniform, and have excellent dispersibility and stability.

[0049] (2) Beneficial effects: The method for preparing conductive copper paste according to the present invention selectively introduces a protective agent as one of the components of the dispersant, thereby improving the stability and sintering conductivity of the paste. The resulting copper paste has a viscosity range of 10 Pa·s to 1000 Pa·s, and its resistivity is low after sintering, reaching <50 μΩ·cm. Furthermore, it maintains a resistivity change of <8% after being stored at 0~5℃ for one year, meeting the needs of most applications.

[0050] (3) Beneficial effects: The preparation method of conductive copper paste described in this invention has a simple operation process, low cost (only 1 / 3 of silver paste), and green and environmentally friendly production process. It can replace the acid copper electroplating process with high environmental pollution in the electronics manufacturing industry and has extremely high replacement value.

[0051] (4) Beneficial effects: The conductive copper paste of the present invention is suitable for precision 3D printing with a scale of ≥1μm. The raw material utilization rate is higher than that of the existing screen printing process. It has good adhesion performance on substrates such as glass, PI, ceramics, silicon wafers, and stainless steel, with an adhesion performance of 5B, and is suitable for mass production. Attached Figure Description

[0052] Figure 1 This is a flowchart illustrating the preparation process of the conductive copper paste of the present invention.

[0053] Figure 2 An image of the conductive copper paste prepared according to the present invention.

[0054] Figure 3 This is a SEM image of copper nanoparticles from Example 1.

[0055] Figure 4 This is a statistical diagram of the particle size distribution of copper nanoparticles in Example 1.

[0056] Figure 5 This is a SEM image of copper nanoparticles from Example 3.

[0057] Figure 6 This is a statistical analysis of the particle size distribution of copper nanoparticles in Example 3.

[0058] Figure 7 This is a SEM image of copper nanoparticles for Comparative Example 1.

[0059] Figure 8 The particle size distribution of copper nanoparticles is shown in Comparative Example 1.

[0060] Figure 9 The image shows a comparative example 2 SEM image of copper nanoparticles.

[0061] Figure 10 The graph shows the change in resistivity of the conductive copper paste after sintering as a function of storage time for Examples 1-5 and Comparative Examples 1-2 and 4.

[0062] Figure 11 This is the morphology of a single-layer circuit printed with slurry in Example 2.

[0063] Figure 12 This is the morphology of the multilayer circuit printed with paste in Example 2. Detailed Implementation

[0064] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0065] Example 1: (1) Dissolve 0.15 mol sodium hypophosphite, 0.3 mmol polyacrylic acid (Mw=3000) and 0.1 mmol polyacrylic acid (Mw=10,000) in 80 ml of deionized water and disperse them at 35 kHz using an ultrasonic instrument at room temperature until the particles are completely dissolved. (2) 45g of 40wt% copper formate aqueous solution was slowly added dropwise to the above solution using a separatory funnel at a flow rate of 0.2ml / min. The mixture was then stirred magnetically at 200r / min for 3h and heated in an oil bath at 80℃ for 1h while still being stirred to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 7000 rpm in a Beckman high-speed centrifuge for 10 min, washed twice with 20 ml of deionized water, and dried at 40℃ in a nitrogen atmosphere for 3 h to obtain copper elemental particles with a particle size of 100-150 nm. (4) A dispersion formulation consisting of 20 wt% ethyl cellulose, 75 wt% terpineol and 5 wt% ascorbic acid was prepared. The dispersion was mixed with nano-copper particles at a ratio of 1:4. The mixture was degassed and stirred for 15 min at 1000 rpm and 0.2 kPa vacuum using a Thinky planetary mixer. Finally, the mixture was thoroughly ground and dispersed using a three-roll mill to obtain a conductive copper paste with a viscosity of 500 Pa·s.

[0066] Example 2: (1) Dissolve 0.08 mol anhydrous citric acid, 0.15 mol hypophosphoric acid and 0.4 mmol polyvinylpyrrolidone (K23-27) in 100 ml of deionized water and stir at 200 r / min with a magnetic stirrer at room temperature until the particles are completely dissolved. (2) 40 ml of 1.875 mol / L copper hydroxide solution was slowly added to the above solution at a flow rate of 0.5 ml / min using a constant pressure dropping funnel. The mixture was then stirred magnetically at 1200 r / min for 3 h and heated in an oil bath at 60 °C for 1 h to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 4000 rpm in a Beckman high-speed centrifuge for 10 min, washed three times with 20 ml of ethanol, and vacuum dried at 25 °C for 5 h to obtain copper elemental particles with a particle size of 50-120 nm. (4) A dispersion formulation composed of 60 wt% phenolic resin, 30 wt% ethylene glycol and 10 wt% ethylenedithiol was prepared. The dispersion formulation and nano-copper particles were mixed in a ratio of 1:9. The mixture was degassed and stirred for 10 min at 800 rpm and 0.4 kPa vacuum using a Thinky planetary mixer. The mixture was then thoroughly ground and dispersed using a three-roll mill to obtain a conductive copper paste with a viscosity of 1000 Pa·s.

[0067] Example 3: (1) Dissolve 0.48 mol of hydrazine hydrate and 0.3 mmol of polyvinylpyrrolidone (K30) in 60 ml of deionized water and stir at 800 r / min with a mechanical stirrer at room temperature until the particles are completely dissolved. (2) Dissolve 20g of copper sulfate pentahydrate in 80ml of water, adjust the pH value to 9-10 with ammonia, and after mixing evenly, slowly inject it into the above solution at a flow rate of 10ml / min using a syringe pump. Stir the reaction with magnetic force at 1500r / min for 3h, and then keep stirring and heat at 60℃ for 1h with an electric heating plate to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 9000 rpm in a Beckman high-speed centrifuge for 10 min, washed 4 times with 20 ml of methanol, and dried at 50℃ under an argon atmosphere for 1 h to obtain copper elemental particles with a particle size of 80-130 nm. (4) Prepare a dispersion formulation consisting of 70wt% epoxy resin, 20wt% ethanol and 10wt% isopropanolamine. Mix the dispersion formulation with the nano-copper particles at a ratio of 2:3. Degas and stir for 8 minutes using a Thinky planetary mixer at 1500 rpm and 0.8 kPa vacuum. After thorough grinding and dispersion using a three-roll mill, a conductive copper paste with a viscosity of 10 Pa·s is obtained.

[0068] Example 4: (1) Dissolve 0.48 mol of hydrazine hydrate and 0.1 mmol of polyacrylic acid (Mw=50,000) in 80 ml of ethylene glycol and stir at 1200 r / min with a magnetic stirrer at room temperature until the particles are completely dissolved. (2) Add ammonia to 30g of 60wt% copper acetate solution to adjust the pH value to 9-10. After mixing evenly, use a peristaltic pump to slowly inject the solution into the above solution at a flow rate of 2ml / min. Stir the reaction with magnetic force at 500r / min for 3h and then keep stirring. Heat in an oil bath at 120℃ for 30min to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 12000 rpm in a Beckman high-speed centrifuge for 10 min, washed three times with 20 ml of ethyl acetate, and vacuum dried at 25 °C for 5 h to obtain copper elemental particles with a particle size of 90-130 nm. (4) A dispersion formulation composed of 28 wt% hydroxyethyl cellulose, 70 wt% ethylene glycol monomethyl ether and 2 wt% formic acid was prepared. The dispersion formulation and nano-copper particles were mixed in a ratio of 3:7. The mixture was degassed and stirred for 5 min at 2000 rpm and 1.0 kPa vacuum using a Thinky planetary mixer. After thorough grinding and dispersion by a three-roll mill, a conductive copper paste with a viscosity of 400 Pa·s was obtained.

[0069] Example 5: (1) Dissolve 0.48 mol of hydrazine hydrate and 0.1 mmol of polyacrylic acid (Mw=10,000) in 80 ml of ethylene glycol and stir at 1200 r / min with a magnetic stirrer at room temperature until the particles are completely dissolved. (2) Add ammonia to 30g of 60wt% copper acetate solution to adjust the pH value to 9-10. After mixing evenly, use a peristaltic pump to slowly inject the solution into the above solution at a flow rate of 2ml / min. Stir the reaction with magnetic force at 500r / min for 3h and then keep stirring. Heat in an oil bath at 150℃ for 30min to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 12000 rpm in a Beckman high-speed centrifuge for 10 min, washed three times with 20 ml of ethyl acetate, and vacuum dried at 25 °C for 5 h to obtain copper elemental particles with a particle size of 90-130 nm. (4) A dispersion formulation composed of 28 wt% hydroxyethyl cellulose, 70 wt% ethylene glycol monomethyl ether and 2 wt% formic acid was prepared. The dispersion formulation and nano-copper particles were mixed in a ratio of 3:7. The mixture was degassed and stirred for 5 min at 2000 rpm and 1.0 kPa vacuum using a Thinky planetary mixer. After thorough grinding and dispersion by a three-roll mill, a conductive copper paste with a viscosity of 500 Pa·s was obtained.

[0070] Comparative Example 1: (1) Dissolve 0.3 mmol polyacrylic acid (Mw=3000) and 0.1 mmol polyacrylic acid (Mw=10,000) in 57 ml of deionized water, add 0.08 mol copper formate, and disperse and stir at 35 kHz using an ultrasonic instrument at room temperature until the particles are completely dissolved. (2) Dissolve 0.15 mol of sodium hypophosphite in 50 ml of deionized water and add it directly to the above solution. Stir the mixture magnetically at 200 r / min for 3 h and then heat it in an oil bath at 80 °C for 1 h while stirring. After the reaction is complete, cool the mixture to obtain a nano-copper dispersion. (3) The nano copper dispersion was centrifuged at 7000 rpm in a Beckman high-speed centrifuge for 10 min, washed twice with 20 ml of deionized water, and dried at 40℃ in a nitrogen atmosphere for 3 h to obtain copper elemental particles with a particle size of 200-350 nm. (4) A dispersion formulation composed of 20 wt% ethyl cellulose, 75 wt% terpineol and 5 wt% ascorbic acid was prepared and mixed with the nano-copper particles at a ratio of 1:4. The mixture was degassed and stirred for 15 min at 1000 rpm and 0.2 kPa vacuum using a Thinky planetary mixer. After thorough grinding and dispersion by a three-roll mill, a conductive copper paste with a viscosity of 900 Pa·s was obtained.

[0071] Comparative Example 2: (1) Dissolve 20g of copper sulfate pentahydrate and 0.3mmol of polyvinylpyrrolidone (K30) in 80ml of water, adjust the pH value to 9-10 with ammonia, and stir with a mechanical stirrer at 800r / min at room temperature until the particles are completely dissolved. (2) Add 60 ml of deionized water to 0.48 mol of hydrazine hydrate, mix thoroughly, and add directly to the above solution. Stir magnetically at 1500 r / min for 3 h, and then keep stirring and heat at 60 °C for 1 h to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 9000 rpm in a Beckman high-speed centrifuge for 10 min, washed 4 times with 20 ml of methanol, and dried at 50℃ under an argon atmosphere for 1 h to obtain copper elemental particles with a particle size of 200-350 nm. (4) A dispersion formulation composed of 70 wt% epoxy resin, 20 wt% ethylene glycol and 10 wt% ethanolamine was prepared. The dispersion formulation and nano-copper particles were mixed in a ratio of 2:3. The mixture was degassed and stirred for 8 min at 1500 rpm and 0.8 kPa vacuum using a Thinky planetary mixer. After thorough grinding and dispersion by a three-roll mill, a conductive copper paste with a viscosity of 85 Pa·s was obtained.

[0072] Comparative Example 3: The corresponding copper paste was prepared according to the preparation steps of Example 1 in patent CN 113362984 A.

[0073] Comparative Example 4: (1) Dissolve 0.15 mol sodium hypophosphite, 0.3 mmol polyacrylic acid (Mw=3000) and 0.1 mmol polyacrylic acid (Mw=10,000) in 80 ml of deionized water and disperse them at 35 kHz using an ultrasonic instrument at room temperature until the particles are completely dissolved. (2) 45g of 40wt% copper formate aqueous solution was slowly added dropwise to the above solution using a separatory funnel at a flow rate of 0.2ml / min. The mixture was then stirred magnetically at 200r / min for 3h and heated in an oil bath at 80℃ for 1h while still being stirred to obtain a nano copper dispersion. (3) The nano copper dispersion was centrifuged at 7000 rpm in a Beckman high-speed centrifuge for 10 min, washed twice with 20 ml of deionized water, and dried at 40℃ in a nitrogen atmosphere for 3 h to obtain copper elemental particles with a particle size of 100-150 nm. (4) Prepare a dispersion formulation composed of 20wt% ethyl cellulose and 80wt% terpineol. Mix the dispersion formulation and nano-copper particles at a ratio of 1:4. Degas and stir for 15 minutes using a Thinky planetary mixer at a speed of 1000 rpm and a vacuum of 0.2 kPa. Finally, grind and disperse thoroughly using a three-roll mill to obtain a conductive copper paste with a viscosity of 500 Pa·s.

[0074] [Performance Testing] The detection method is as follows: (1) Particle size observation: The size and morphology of copper nanoparticles were characterized using a Phenom scanning electron microscope.

[0075] The specific method is as follows: ① Take a small amount of copper paste and dilute it in deionized water or organic solvent at a ratio of 1:1000; ② Disperse the diluted solution thoroughly, take a small amount with a dropper and add it to the surface of the carrier, and heat it to remove excess solvent; ③ After preparing the sample and spraying it with gold, send the sample for observation to obtain image information on the size and morphology of copper nanoparticles.

[0076] (2) Resistivity test: The resistivity of copper paste was measured using the Ruikeweiye FT-340 four-probe square resistance tester.

[0077] The specific method is as follows: ① The prepared conductive copper paste is coated onto the glass substrate using a coating machine and sintered at 300℃ for 60 minutes under a nitrogen atmosphere to obtain a conductive copper film; ② The thickness of the copper film sample after high-temperature sintering is measured using a step meter; ③ The corresponding parameters are set in the sheet resistance meter, and the four probes are adjusted to be above the sample to be tested and then pressed down vertically to touch it; ④ After the readings stabilize, the sheet resistance, resistivity and other data are recorded.

[0078] (3) Viscosity test: The viscosity of copper paste was measured using a Brookfield DVNextCP cone-plate viscometer.

[0079] The specific steps are as follows: ① Zero the torque and adjust the cone-plate spacing to 0.0005 inches; ② Place 0.5 ml of conductive copper paste in the center of the sample cup, set the rotation speed and test time, with the rotation speed set to 0.5 rpm; ③ Start the test to obtain and record the material viscosity information.

[0080] (4) Adhesion test ① The prepared conductive copper paste is coated onto the glass substrate using a coating machine and sintered at 300°C for 10 min under a nitrogen atmosphere to obtain a conductive copper film; ② Using ASTM D3359-2017 as the test standard, use a cross-cutting tool to cut the surface of the test piece at a cutting speed of 20-50 mm / s; ③ Rotate the specimen 90° and repeat the above operation on the cut to form a grid pattern; ④ Use a soft brush to brush the diagonals on both sides of the grid pattern, brushing lightly 5 times each way. ⑤ Use 3M 600-1PK test tape to perform a peel test and evaluate the material's adhesion performance.

[0081] Characterization of experimental results: The prepared conductive copper paste was stored in a sealed container at 0-5℃, and its sintering stability was monitored during 360 days of storage. The particle size distributions of Examples 1, 3, and Comparative Examples 1-2 are shown in the figure. Figure 3-10 The particle morphology and resistivity changes after sintering of Examples 1-4 and Comparative Examples 1-2 and 4 are shown in Table 1. Figure 10 As shown.

[0082] Table 1. Particle morphology and resistivity changes after sintering in Examples 1-5 and Comparative Examples 1-2 and 4 Example 1 Spherical D50: 100-150nm 11.6±2.0 12.1±3.0 12.3±2.0 12.6±2.0 12.6±5.0 Example 2 Spherical D50: 50-120nm 18.9±7.0 18.2±3.0 18.5±2.0 19.1±1.0 19.9±3.0 Example 3 Spherical D50: 80-130nm 14.8±4.0 15.1±2.0 15.0±5.0 15.6±6.0 15.9±4.0 Example 4 Spherical D50: 90-130nm 25.5±7.0 25.8±5.0 26.4±4.0 27.1±7.0 27.7±8.0 Example 5 Spherical D50: 400-500nm 26.7±5.0 26.9±4.0 28.2±3.0 28.7±6.0 29.4±5.0 Comparative Example 1 Octahedral D50: 200-350nm 36.5±7.0 38.7±3.0 42.9±5.0 50.2±11.0 76.3±15.0 Comparative Example 2 Flaky, cross-section D50: 200-350nm 47.8±2.0 53.7±4.0 58.2±5.0 67.3±8.0 79.7±12.0 Comparative Example 4 Spherical D50: 100-150nm 15.7±3.0 21.9±5.0 35.2±5.0 48.7±11.0 61.3±12.0

[0083] According to Table 1 and Figure 10 Experimental test results show that the conductive copper paste obtained by the preparation method described in this invention has high particle morphology uniformity, low resistivity after sintering (only 11.6±2.0μΩ·cm), and maintains a resistivity change of <8% after being stored at 0~5℃ for 1 year.

[0084] Table 2 Adhesion of slurries from Examples 1-5 and Comparative Example 3 after sintering in different media Example 1 5B 5B 5B 5B 5B Example 2 5B 5B 5B 5B 5B Example 3 5B 5B 5B 5B 5B Example 4 5B 5B 5B 5B 5B Example 5 5B 5B 5B 5B 5B Comparative Example 3 3B 5B 5B 3B 5B

[0085] According to the experimental test results in Table 2, the conductive copper paste obtained by the preparation method of the present invention has sintering adhesion performance on substrates such as glass, silicon wafers, PI, ceramics, and stainless steel that meets the ASTM D3359-2017 test standard 5B performance, which is better than the scheme in Comparative Example 3.

[0086] also, Figure 11 This is the printing state of a single-layer circuit using the paste in Example 2. Figure 12 The conductive copper paste described in Example 4 is used for multi-layer circuit printing. It can achieve precision 3D printing with a scale of ≥1μm and has good shape retention performance, which can meet the needs of most precision electronic applications.

Claims

1. A conductive copper paste suitable for precision 3D printing, characterized in that, This includes copper nanoparticles and dispersing agents; among which, The surface of the copper nanoparticles is coated with ligands; The particle size of the copper nanoparticles is 50-500 nm; The dispersion formulation consists of a binder, a protective agent, and an organic solvent; The protective agent includes any one or more combinations of thiols, primary alcohol amines with the general chemical formula OH-X-NH2, where X is 2 ≤ carbon number ≤ 6, secondary alcohol amines with the general chemical formula NH-(Y-OH)2, where Y is 2 ≤ carbon number ≤ 6, ascorbic acid, oxalic acid, and formic acid. The method for preparing copper nanoparticles includes: gradually adding a copper precursor solution to a reducing agent solution containing ligands, so that the nucleation and growth of the copper precursor occur asynchronously during the reduction process.

2. The conductive copper paste suitable for precision 3D printing according to claim 1, characterized in that, The adhesive includes any one or more combinations of cellulose derivatives, epoxy resins, phenolic resins, acrylic resins, and polyamide resins.

3. The conductive copper paste suitable for precision 3D printing according to claim 1, characterized in that, The organic solvent includes any one or more combinations of mono / polyols with 2 ≤ carbon number ≤ 12, tertiary alcohol amines with the general chemical formula N-(Z-OH)3 where Z is 2 ≤ carbon number ≤ 6, ethylene glycol monomethyl ether, dipropylene glycol methyl ether, and triethylene glycol dimethyl ether.

4. The conductive copper paste suitable for precision 3D printing according to claim 1, characterized in that, The viscosity range of the conductive copper paste is 10 Pa·s to 1000 Pa·s; The resistivity after sintering is <50 μΩ·cm; The resistivity change was maintained at less than 8% after being stored at 0~5℃ for 1 year. The adhesion properties after sintering meet the ASTM D3359-2017 test standard 5B performance.

5. The method of claim 1-4 for preparing the conductive copper paste for precision 3D printing, Its features are, Includes the following steps: (1) The copper precursor solution is gradually added to the reducing agent solution containing ligands so that the nucleation and growth of the copper precursor are asynchronous during the reduction process, and finally a copper nanoparticle dispersion is obtained. (2) After separation, washing and drying, copper nanoparticle dispersion is used to obtain copper nanoparticles; (3) The conductive copper paste is obtained by mixing the nano-copper particles with the dispersing agent.

6. The method for preparing conductive copper paste suitable for precision 3D printing according to claim 5, characterized in that, In step (1), the concentration of the copper precursor solution is 1-7.5 mol / L; The copper precursor solution was added at a rate of 0.2-10 ml / min.

7. The method for preparing conductive copper paste suitable for precision 3D printing according to claim 5, characterized in that, In step (1), the molar ratio of copper precursor to reducing agent is 1:(1-6).

8. The method for preparing conductive copper paste suitable for precision 3D printing according to claim 5, 6, or 7, characterized in that, In step (1): The copper precursor includes one or more of the following: copper sulfate pentahydrate, copper formate, copper hydroxide, copper acetate, copper chloride, copper nitrate, copper sulfide, copper citrate, and copper acetylacetonate. The reducing agent includes one or more of the following: hydrazine hydrate, hypophosphite, sodium hypophosphite, sodium phosphite, sodium borohydride, potassium borohydride, sodium dithionite, formaldehyde, ascorbic acid, citric acid, and oxalic acid. The ligand includes any one or a combination of two of polyacrylic acid and polyvinylpyrrolidone.

9. The conductive copper paste suitable for precision 3D printing as described in any one of claims 1 to 4, or the conductive copper paste suitable for precision 3D printing prepared by the preparation method described in any one of claims 5 to 8, can achieve high-precision 3D printing at a scale of ≥1μm, and can be used in the fields of precision 3D printing, precision optics, micro-nano chips, and printed electronics.

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