A conductive copper paste and its laser sintering process

Abstract

This invention belongs to the field of printed electronics technology and relates to a conductive copper paste and its laser sintering process. The conductive copper paste contains micron-sized copper powder, copper ligands, and a solvent; the copper ligands are formed by the coordination of anhydrous copper formate with amino ligands; the amino ligands are one or more selected from hexylamine, 2-ethylhexylamine, butylamine, β-alanine, glycine, 2-amino-2-methyl-1-propanol, octylamine, and oleylamine. The laser sintering process involves printing the conductive copper paste onto the surface of a substrate, then sintering it using a picosecond or femtosecond pulsed laser system under an atmosphere of air, nitrogen, inert gas, or reducing gas. During sintering, the laser scanning program is controlled by software to perform multi-path scanning of the sample surface. This invention solves the problems of existing technologies that require additional atmosphere control equipment, increase production costs, and that a closed atmosphere limits process openness and large-scale continuous production.

Description

Technical Field

[0001] This invention belongs to the field of printed electronics technology and relates to a conductive copper paste and its laser sintering process. Background Technology

[0002] In the field of printed electronics, the conductivity and processing cost of metal interconnects are the core concerns. Copper, due to its excellent conductivity and low cost, has become the key conductive phase in low-silver / silver-free conductive pastes.

[0003] In the prior art, patent application CN119634731A has proposed applying low-temperature laser sintering technology to low-silver or silver-free conductive pastes. By precisely controlling the laser frequency, power density and wavelength, efficient and uniform forming of the conductive layer can be achieved at a lower temperature. This avoids the damage to the material structure caused by traditional high-temperature sintering and reduces the amount of silver used, thereby reducing production costs to a certain extent and providing a feasible path for the application of low-silver / silver-free conductive pastes.

[0004] However, the aforementioned patent application still faces a key technical bottleneck: to avoid oxidation of the metal powder (especially copper powder) surface during sintering, the sintering operation must be carried out under a nitrogen or reducing gas atmosphere. Without this protective atmosphere, a copper oxide or cuprous oxide layer will rapidly form on the surface of the copper powder. This oxide layer will significantly hinder the diffusion and migration of copper atoms, preventing the formation of effective sintering necks between copper particles. Consequently, the resistance of the conductive layer after sintering will increase significantly, or even the conductive function will be lost.

[0005] This limitation directly leads to practical application problems. On the one hand, the preparation, transportation, and atmosphere control of nitrogen or reducing gases require additional specialized equipment, increasing the initial investment and maintenance costs of the production line. On the other hand, the closed atmosphere environment restricts the openness and flexibility of the process, making it difficult to adapt to large-scale, continuous, open production scenarios, thus hindering the industrial promotion efficiency of low-silver / silver-free copper-based conductive pastes in the field of printed electronics. Summary of the Invention

[0006] The purpose of this invention is to solve the problems existing in the prior art and to provide a conductive copper paste and its laser sintering process.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A conductive copper paste contains micron-sized copper powder, copper ligands (copper-containing chemical substances or combinations thereof that can be converted into metallic copper during laser sintering / heating), and a solvent; wherein the content of micron-sized copper powder is not less than 50 wt%, and the content of copper ligands is not less than 2 wt%; the copper ligands are formed by coordination of anhydrous copper formate with amino ligands, and the molar ratio of anhydrous copper formate to amino ligands is 1:2-4; the amino ligands are one or more of hexylamine, 2-ethylhexylamine, butylamine, β-alanine, glycine, 2-amino-2-methyl-1-propanol, octylamine, and oleylamine, and these amino ligands have the common characteristic of being able to decompose and release reducing or neutral volatile products when heated.

[0009] As a preferred technical solution:

[0010] The conductive copper paste described above has a content of no more than 80 wt% for micron-sized copper powder, a content of no more than 10 wt% for copper ligands, and a mass ratio of copper ligands to solvent of 1:3-6.

[0011] The conductive copper paste described above uses an organic solvent, which is one or more of methanol, ethanol, ethylene glycol, ethylene glycol methyl ether, and 1-methyl-2-pyrrolidone. The copper ligand and the organic solvent form a copper ligand solution. The organic solvent is used to dissolve the copper ligand and adjust the dispersibility and viscosity of the paste.

[0012] The preparation steps of the copper ligand solution for the conductive copper paste described above include:

[0013] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0014] (b) After adding the mixed powder to the organic solvent, the mixture is slowly heated to the reaction temperature and kept at that temperature under stirring to completely dissolve the mixed powder, thus obtaining the copper ligand solution.

[0015] In the conductive copper paste described above, in step (b), the stirring speed is 200-1500 rpm, the heating rate is 0.5-3℃ / min, the reaction temperature is 90-120℃, and the holding time is 2-8h.

[0016] The conductive copper paste described above has a D50 particle size of 1-2 μm for the micron-sized copper powder.

[0017] The conductive copper paste described above also contains organic additives, the content of which is no more than 10 wt%. The organic additives are one or more of the following: terpineol, dihydroterpineol, ethylene glycol butyl ether, diethylene glycol butyl ether, diethylene glycol butyl ether acetate, tributyl citrate, dibutyl phthalate, ethyl cellulose, cellulose acetate butyrate, and acrylic resin. The organic additives mainly exist as carriers, including binders and plasticizers. Their core function is to make the copper paste easy to print, easy to form a film, and have fewer defects, and to maintain structural stability before and after sintering, and to remove it quickly during sintering with as little residue as possible.

[0018] The present invention also provides a process for laser sintering a conductive copper paste as described in any of the preceding claims. After the conductive copper paste is printed on the surface of a substrate, it is sintered in an atmosphere of air, nitrogen, inert gas (e.g., argon) or reducing gas using a picosecond or femtosecond pulsed laser system. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser wavelength is 1064 nm, the power is 0.1-10 W, the scanning rate is 10-200 mm / s, and the pulse frequency is 500-1000 kHz.

[0019] As a preferred technical solution:

[0020] As described above, the substrate can be a flexible substrate, a ceramic substrate, a glass substrate, or an epoxy resin substrate. There are no restrictions on the type of substrate, and it can be selected according to actual needs.

[0021] Invention principle:

[0022] In existing technologies, the core of laser sintering of conductive copper paste lies in using a high-energy laser beam to rapidly heat copper particles, allowing them to reach or approach their melting point in a short time, thereby achieving metallurgical bonding between particles. From an atomic perspective, after absorbing laser energy, the surface temperature of copper particles in the conductive copper paste rises sharply to thousands of degrees Celsius, forming localized high-temperature zones. Because the laser action time is extremely short, only in the femtosecond to picosecond range, the heat does not have time to diffuse, thus preventing the copper particles from completely melting. Instead, bonding is achieved through surface melting or solid-state diffusion. Specifically, if the surface temperature of the copper particles exceeds the melting point, a liquid copper film forms on its surface. Driven by surface tension, atoms migrate towards the particle contact point (i.e., the neck), forming a liquid-phase bridge. If the temperature does not reach the melting point, solid copper atoms migrate to the neck under thermal excitation through surface diffusion, grain boundary diffusion, or vacancy mechanisms, forming a solid-state connection. The neck radius increases exponentially over time, and its growth rate is affected by temperature, particle size, and diffusion coefficient. In high-temperature environments, the atomic diffusion rate accelerates, and neck growth is more rapid. As the neck continues to grow, the pores between the particles gradually shrink, eventually forming a dense metallurgical bond.

[0023] The rapid heating characteristic of laser sintering allows copper particles to be joined in a solid or semi-solid state, effectively avoiding grain coarsening or oxidation problems caused by prolonged high temperatures in traditional sintering processes. However, this sintering mechanism cannot be carried out in air because when copper is heated in air, copper oxide (CuO) or cuprous oxide (Cu2O) will rapidly form on its surface.

[0024] On the one hand, the oxide layer covers the surface of the copper particles, forming a dense barrier with a diffusion coefficient several orders of magnitude lower than that of pure copper. This directly leads to a reduction of more than 90% in the neck growth rate. On the other hand, the resistivity of the oxide layer is much higher than that of pure copper, which blocks the current conduction path. Once the current conduction path is blocked, the current can only pass through the narrow metal contact points during the sintering process of the conductive copper paste. This leads to an increase in resistivity and may even cause the conductive copper paste to completely lose its conductivity.

[0025] The laser sintering of the conductive copper paste of the present invention can be carried out in any atmosphere of air, nitrogen, inert gas, or reducing gas, and does not necessarily have to be carried out in nitrogen, inert gas, or reducing gas because:

[0026] Under the transient high temperature generated by the laser, copper ligands undergo an auto-oxidation-reduction reaction. Amino ligands donate electrons to copper ions in anhydrous copper formate via charge transfer, reducing them to zero-valent metallic copper. This process rapidly generates in-situ copper nanonuclei under the transient high temperature of the laser. Due to their extremely small particle size and large specific surface area, these copper nanonuclei possess extremely high surface energy and diffusion activity, readily undergoing surface migration and aggregation. This leads to rapid diffusion and aggregation of the copper nanonuclei in the particle contact area, forming initial neck connections. As laser irradiation continues, the surface of the micron-sized copper powder partially melts, and the neck size gradually expands under the bridging effect of the copper nanonuclei. Ultimately, the copper nanonuclei and the micron-sized copper powder work together to form a dense and continuous metallurgical network. Throughout the process, the pyrolysis of copper and amino ligands releases reducing or neutral volatile products such as carbon monoxide and amino radicals. These products create a localized reducing / inert environment on the surfaces of the nano-copper nuclei and micron-sized copper powder, preventing surface oxidation during sintering and thus ensuring the smooth progress of the sintering process in air. Furthermore, the amino ligands volatilize in gaseous form after thermal decomposition, leaving only a small amount of metallic copper and trace oxides, significantly reducing the impact of impurities on conductivity.

[0027] This invention controls the molar ratio of anhydrous copper formate to amino ligands to be 1:2-4. This ensures a high proportion of organic components (formate and amino ligands) in the copper ligands, thereby guaranteeing the smooth progress of the "absorption-reduction-removal" process. If the proportion of organic components is too low, the copper ligands will not absorb enough laser energy, resulting in incomplete reaction and insufficient release of reducing or neutral volatile products during decomposition. This prevents the formation of an effective local reducing / inert environment on the particle surface, thus failing to meet the sintering requirements under air atmosphere. Conversely, if the proportion of organic components is too high, excessive organic byproducts and gases will be released during decomposition, leading to blistering, cracking, or excessive organic residues in the film, ultimately damaging the film's density and conductivity.

[0028] This invention controls the content of copper ligands in the conductive copper paste to be no less than 2 wt%. When the content of copper ligands is too low, on the one hand, the gas and heat generated by decomposition are insufficient, and a stable reaction environment cannot be formed on the particle surface, which leads to the inhibition of copper ion reduction and neck formation, thereby destroying the sintering conditions under air atmosphere; on the other hand, it will reduce the densification degree of the material and at the same time lead to poor printing stability.

[0029] This invention controls the content of micron-sized copper powder in the conductive copper paste to be no less than 50 wt%. This is because when the content of micron-sized copper powder is too low, the system lacks sufficient scattering and energy interception effects, and cannot provide a stable energy environment for the decomposition and reduction of copper ligands. Therefore, the nano-copper crystal nuclei are easily oxidized by oxygen in the air, making sintering in an air atmosphere difficult.

[0030] The micron-sized copper powder in conductive copper paste can also improve laser energy utilization because it possesses a high specific surface area structure. This structure generates strong scattering and shielding effects, significantly reducing laser transmittance and thus enhancing the overall absorption capacity and energy retention effect of the conductive copper paste. Specifically, its mechanism of action is as follows:

[0031] First, within the membrane, the incident laser is not only absorbed by the copper ligands but also subjected to strong scattering by the micron-sized copper powder. Through multiple scattering, the effective path of the laser within the membrane becomes longer, resulting in a greater number of laser irradiations to the copper ligands.

[0032] Secondly, multiple scattering can reduce the direct penetration energy of the laser and decrease the heat absorption of the substrate, thus retaining the heat mainly within the film layer. At the same time, micron-sized copper powder has high thermal conductivity, which can homogenize the local temperature gradient at the microscale, which is beneficial for the uniform pyrolysis and synergistic densification of copper ligands.

[0033] This invention controls the content of micron-sized copper powder to be no less than 50 wt%, and can also avoid underburning and increased sheet resistance caused by insufficient scattering effect and imperfect skeleton structure.

[0034] The laser sintering parameters of this invention (laser wavelength of 1064nm, power of 0.1-10W, pulse frequency of 500-1000kHz) are matched with the optical properties of conductive copper paste, forming a more refined parameter-structure-performance coupling path, effectively supporting large-scale applications of industrial flexible circuits or electromagnetic shielding patterns. The essence of this matching is to ensure that the effective penetration depth δ is on the same order of magnitude as the film thickness t, while simultaneously allowing the energy per unit length to cross the thermal threshold of "copper ligand decomposition / nucleation + particle neck growth" while remaining below the damage threshold. The 1064nm laser wavelength matches the NIR absorption band of the copper ligands, allowing micron-sized copper powder to enhance scattering, reduce transmittance, and amplify thermal deposition. If any parameter—wavelength, power, or pulse frequency—is too large or too small, it will affect the energy absorption effect, disrupt air sintering conditions, lead to oxidation, prevent sintering in air, and push the system towards the failure zone of "only sintering the surface / under-sintering / over-sintering," resulting in increased sheet resistance, increased porosity / cracking, or substrate damage.

[0035] Beneficial effects:

[0036] (1) The conductive copper paste of the present invention contains micron-sized copper powder, copper ligands and solvent. Combined with a specific laser sintering process, it can be sintered in an air atmosphere without relying on nitrogen or reducing gas protection. This solves the problems in the prior art that require additional atmosphere control equipment, increase production costs, and restrict the openness of the process and large-scale continuous production due to the closed atmosphere. It reduces the investment and maintenance costs of the production line, is suitable for open and continuous production scenarios, and is conducive to industrial promotion.

[0037] (2) By controlling the composition and content of copper ligands, the present invention can generate nano-copper crystal nuclei by self-oxidation and reduction under laser transient high temperature, which can assist micron-sized copper powder to form a dense and continuous metallurgical network. It can also release reducing or neutral volatile products to form a local reduction / inert environment on the surface of copper powder and nano-copper crystal nuclei, effectively preventing the oxidation of copper during sintering, avoiding the problem that the oxide layer hinders the diffusion of copper atoms, leading to increased resistance of the conductive layer or loss of conductivity, and ensuring the conductivity of the copper film after air sintering.

[0038] (3) The present invention controls the content of micron-sized copper powder to be no less than 50wt%. Its high specific surface area structure can enhance the absorption capacity of copper paste to laser and the energy retention effect, reduce the heat absorption of the substrate and homogenize the local temperature gradient, help the copper ligand to uniformly pyrolyze and synergistically densify, avoid the problems of underburning and increased sheet resistance caused by insufficient scattering effect and imperfect skeleton structure, and further ensure the conductivity and structural performance of copper film. Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0040] The following are the test methods for the relevant performance indicators in each embodiment and comparative example. All indicator tests were performed in parallel 10 times, and the average value was taken as the final test result:

[0041] Sheet resistance, sheet resistance change rate after baking at 200℃ for 2 hours, and sheet resistance change rate after exposure to air for 720 hours: Sheet resistance was tested using an ST2258C digital four-probe tester with a test current of 10mA, a test voltage range of 0-200mV, and a probe spacing of 2mm; test environment conditions: temperature 25±2℃, relative humidity 60±5%; the sheet resistance change rate was calculated using the formula: sheet resistance change rate = [(treated sheet resistance - initial sheet resistance) / initial sheet resistance] × 100%, where treated sheet resistance refers to sheet resistance after baking at 200℃ for 2 hours or sheet resistance after exposure to air for 720 hours.

[0042] Electromagnetic shielding effectiveness at 5.5 GHz: Referring to GB / T 12190-2006, a vector network analyzer (Agilent N5247A) was used in conjunction with a coaxial transmission line test fixture to test and record the reflection coefficient and transmission coefficient at the 5.5 GHz frequency point. The electromagnetic shielding effectiveness was then calculated according to the calculation method specified in the standard.

[0043] Example 1

[0044] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0045] (1) Preparation of materials;

[0046] Anhydrous copper formate;

[0047] Amino ligand: hexylamine;

[0048] Organic solvent: methanol;

[0049] Micron-sized copper powder: D50 particle size is 1μm;

[0050] Organic additive: terpineol, CAS number 8000-41-7;

[0051] Substrate: PET flexible substrate, manufactured by DuPont, brand name Melinex ST506, thickness 0.125mm;

[0052] (2) Prepare copper ligand solution;

[0053] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0054] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0055] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:2; in step (b), the stirring speed is 200 rpm, the heating rate is 0.5℃ / min, the reaction temperature is 90℃, the holding time is 2h, and the mass ratio of mixed powder to organic solvent is 1:3.

[0056] (3) Prepare conductive copper paste;

[0057] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 50 wt% micron-sized copper powder, 40 wt% copper ligand solution, and 10 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 10 wt%.

[0058] (4) Laser sintering;

[0059] After conductive copper paste is printed onto the substrate surface, it is sintered in air using a picosecond pulsed laser system to form a copper film with a thickness of 0.05 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 30 μm, the scanning interval is 15 μm, the number of repeated scans is 3, the laser wavelength is 1064 nm, the power is 0.1 W, the scanning rate is 10 mm / s, and the pulse frequency is 500 kHz.

[0060] Tests show that the sheet resistance of the copper film is 15mΩ / □, the sheet resistance change rate of the copper film is 3% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 7% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 35dB.

[0061] Example 2

[0062] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0063] (1) Preparation of materials;

[0064] Anhydrous copper formate;

[0065] Amino ligand: a mixture of 2-ethylhexylamine and butylamine in a molar ratio of 1:3;

[0066] Organic solvent: a mixture of ethanol and ethylene glycol methyl ether in a molar ratio of 1:4;

[0067] Micron-sized copper powder: D50 particle size is 1.5μm;

[0068] Organic additive: diethylene glycol butyl ether acetate;

[0069] Substrate: Ceramic substrate, manufactured by Kyocera Corporation, mainly made of alumina (alumina accounts for 96% of the mass), with a thickness of 0.635mm;

[0070] (2) Prepare copper ligand solution;

[0071] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0072] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0073] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:3; in step (b), the stirring speed is 800 rpm, the heating rate is 1℃ / min, the reaction temperature is 110℃, the holding time is 5h, and the mass ratio of mixed powder to organic solvent is 1:4.

[0074] (3) Prepare conductive copper paste;

[0075] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 65 wt% micron-sized copper powder, 30 wt% copper ligand solution, and 5 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 6 wt%.

[0076] (4) Laser sintering;

[0077] After conductive copper paste is printed onto the substrate surface, it is sintered under a nitrogen atmosphere using a femtosecond pulsed laser system to form a copper film with a thickness of 0.03 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 30 μm, the scanning interval is 15 μm, the number of repeated scans is 2, the laser wavelength is 1064 nm, the power is 5 W, the scanning rate is 100 mm / s, and the pulse frequency is 800 kHz.

[0078] Tests show that the sheet resistance of the copper film is 8mΩ / □, the sheet resistance change rate of the copper film is 2% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 6% after exposure to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 40dB.

[0079] Example 3

[0080] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0081] (1) Preparation of materials;

[0082] Anhydrous copper formate;

[0083] Amino ligand: a mixture of β-alanine and glycine in a molar ratio of 1:4;

[0084] Organic solvent: 1-methyl-2-pyrrolidone;

[0085] Micron-sized copper powder: D50 particle size is 1.2μm;

[0086] Organic additive: acrylic resin, CAS number 9003-01-4;

[0087] Substrate: Glass substrate, manufactured by Corning Incorporated, brand name EAGLE XG, thickness 0.7mm;

[0088] (2) Prepare copper ligand solution;

[0089] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0090] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0091] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:4; in step (b), the stirring speed is 1500 rpm, the heating rate is 3℃ / min, the reaction temperature is 120℃, the holding time is 8h, and the mass ratio of mixed powder to organic solvent is 1:6.

[0092] (3) Prepare conductive copper paste;

[0093] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 80 wt% micron-sized copper powder, 15 wt% copper ligand solution, and 5 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 2.1 wt%.

[0094] (4) Laser sintering;

[0095] After conductive copper paste is printed onto the substrate surface, it is sintered in air using a picosecond pulsed laser system to form a copper film with a thickness of 0.02 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 25 μm, the scanning interval is 12 μm, the number of repeated scans is 2, the laser wavelength is 1064 nm, the power is 10 W, the scanning rate is 200 mm / s, and the pulse frequency is 1000 kHz.

[0096] Tests show that the sheet resistance of the copper film is 5mΩ / □, the sheet resistance change rate of the copper film is 5% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 10% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 30dB.

[0097] Example 4

[0098] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0099] (1) Preparation of materials;

[0100] Anhydrous copper formate;

[0101] Amino ligand: a mixture of 2-amino-2-methyl-1-propanol and octylamine in a molar ratio of 1:2.5;

[0102] Organic solvent: a mixture of ethylene glycol and ethanol in a molar ratio of 1:5;

[0103] Micron-sized copper powder: D50 particle size is 1.8μm;

[0104] Organic additive: dibutyl phthalate;

[0105] Substrate: PI flexible substrate, manufactured by DuPont, brand name Kapton HN, thickness 0.05mm;

[0106] (2) Prepare copper ligand solution;

[0107] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0108] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0109] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:2.5; in step (b), the stirring speed is 500 rpm, the heating rate is 2℃ / min, the reaction temperature is 100℃, the holding time is 4h, and the mass ratio of mixed powder to organic solvent is 1:5.

[0110] (3) Prepare conductive copper paste;

[0111] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 70 wt% micron-sized copper powder, 25 wt% copper ligand solution, and 5 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 4.2 wt%.

[0112] (4) Laser sintering;

[0113] After conductive copper paste is printed onto the substrate surface, it is sintered in air using a femtosecond pulsed laser system to form a copper film with a thickness of 0.04 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 30 μm, the scanning interval is 20 μm, the number of repeated scans is 3, the laser wavelength is 1064 nm, the power is 3 W, the scanning rate is 20 mm / s, and the pulse frequency is 600 kHz.

[0114] Tests show that the sheet resistance of the copper film is 12mΩ / □, the sheet resistance change rate of the copper film after baking at 200℃ for 2h is 4.5%, and the sheet resistance change rate of the copper film after exposure to air for 720h is 8%; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 33dB.

[0115] Example 5

[0116] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0117] (1) Preparation of materials;

[0118] Anhydrous copper formate;

[0119] Amino ligand: a mixture of oleylamine and octylamine in a molar ratio of 1:5;

[0120] Organic solvent: ethylene glycol butyl ether;

[0121] Micron-sized copper powder: D50 particle size is 2μm;

[0122] Organic additive: Ethyl cellulose, CAS No. 9004-57-3;

[0123] Substrate: FR-4 epoxy resin substrate, manufactured by Kingboard Laminates Holdings Limited, with a thickness of 1mm;

[0124] (2) Prepare copper ligand solution;

[0125] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0126] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0127] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:3.5; in step (b), the stirring speed is 1000 rpm, the heating rate is 1.5℃ / min, the reaction temperature is 95℃, the holding time is 6h, and the mass ratio of mixed powder to organic solvent is 1:5.

[0128] (3) Prepare conductive copper paste;

[0129] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 75 wt% micron-sized copper powder, 20 wt% copper ligand solution, and 5 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 3.3 wt%.

[0130] (4) Laser sintering;

[0131] After conductive copper paste is printed onto the substrate surface, it is sintered under an argon atmosphere using a picosecond pulsed laser system to form a copper film with a thickness of 0.02 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 25 μm, the scanning interval is 10 μm, the number of repeated scans is 2, the laser wavelength is 1064 nm, the power is 8 W, the scanning rate is 150 mm / s, and the pulse frequency is 900 kHz.

[0132] Tests show that the sheet resistance of the copper film is 7mΩ / □, the sheet resistance change rate of the copper film is 3% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 8% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 32dB.

[0133] Example 6

[0134] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0135] (1) Preparation of materials;

[0136] Anhydrous copper formate;

[0137] Amino ligand: a mixture of 2-ethylhexylamine and butylamine in a molar ratio of 1:4;

[0138] Organic solvent: 1-methyl-2-pyrrolidone;

[0139] Micron-sized copper powder: D50 particle size is 1.41μm;

[0140] Organic additives: a mixture of dihydroterpineol, ethylene glycol butyl ether and diethylene glycol butyl ether in a molar ratio of 1:1:1;

[0141] Substrate: PET flexible substrate, manufactured by DuPont, brand name Melinex ST506, thickness 0.125mm;

[0142] (2) Prepare copper ligand solution;

[0143] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0144] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0145] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:2; in step (b), the stirring speed is 600 rpm, the heating rate is 2.5℃ / min, the reaction temperature is 120℃, the holding time is 6h, and the mass ratio of mixed powder to organic solvent is 1:4.

[0146] (3) Prepare conductive copper paste;

[0147] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 60 wt% micron-sized copper powder, 35 wt% copper ligand solution, and 5 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 7 wt%.

[0148] (4) Laser sintering;

[0149] After conductive copper paste is printed onto the substrate surface, it is sintered in air using a picosecond pulsed laser system to form a copper film with a thickness of 0.04 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 35 μm, the scanning interval is 15 μm, the number of repeated scans is 3, the laser wavelength is 1064 nm, the power is 6 W, the scanning rate is 80 mm / s, and the pulse frequency is 700 kHz.

[0150] Tests show that the sheet resistance of the copper film is 10mΩ / □, the sheet resistance change rate of the copper film is 5% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 11% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 35dB.

[0151] Example 7

[0152] A laser sintering process for conductive copper paste, the specific steps of which are as follows:

[0153] (1) Preparation of materials;

[0154] Anhydrous copper formate;

[0155] Amino ligand: a mixture of 2-amino-2-methyl-1-propanol and octylamine in a molar ratio of 1:3;

[0156] Organic solvent: ethylene glycol butyl ether;

[0157] Micron-sized copper powder: D50 particle size is 1.65μm;

[0158] Organic additive: a mixture of tributyl citrate and cellulose acetate butyrate (CAS No. 9004-36-8) in a mass ratio of 1:1;

[0159] Substrate: Glass substrate, manufactured by Corning Incorporated, brand name EAGLE XG, thickness 0.7mm;

[0160] (2) Prepare copper ligand solution;

[0161] (a) Anhydrous copper formate and amino ligands are ground and mixed to obtain a mixed powder;

[0162] (b) After adding the mixed powder to the organic solvent, the temperature is slowly raised to the reaction temperature and kept warm under stirring conditions so that the mixed powder is completely dissolved, thus obtaining the copper ligand solution.

[0163] In step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:4; in step (b), the stirring speed is 1200 rpm, the heating rate is 1.2℃ / min, the reaction temperature is 105℃, the holding time is 3h, and the mass ratio of mixed powder to organic solvent is 1:5.

[0164] (3) Prepare conductive copper paste;

[0165] The conductive copper paste is composed of micron-sized copper powder, copper ligand solution, and organic additives. The conductive copper paste contains 70 wt% micron-sized copper powder, 25 wt% copper ligand solution, and 5 wt% organic additives. Calculations show that the copper ligand content in the conductive copper paste is 4.2 wt%.

[0166] (4) Laser sintering;

[0167] After conductive copper paste is printed onto the substrate surface, it is sintered in air using a femtosecond pulsed laser system to form a copper film with a thickness of 0.02 mm, thus obtaining a copper-clad substrate. During the sintering process, the laser scanning program is controlled by software to perform multi-path scanning on the sample surface. The laser spot diameter is 25 μm, the scanning interval is 10 μm, the number of repeated scans is 1, the laser wavelength is 1064 nm, the power is 9 W, the scanning rate is 160 mm / s, and the pulse frequency is 850 kHz.

[0168] Tests show that the sheet resistance of the copper film is 14 mΩ / □, the sheet resistance change rate of the copper film after baking at 200℃ for 2 hours is 3.8%, and the sheet resistance change rate of the copper film after exposure to air for 720 hours is 5%; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5 GHz band is 38 dB.

[0169] Comparative Example 1

[0170] The laser sintering process for conductive copper paste differs from that in Example 1 in that the amount of micron-sized copper powder added is reduced when preparing the conductive copper paste. The reduced portion of micron-sized copper powder is replaced by an equal mass of organic solvent, and the content of micron-sized copper powder in the conductive copper paste is 45 wt%.

[0171] Tests show that the sheet resistance of the copper film is 19 mΩ / □, the sheet resistance change rate of the copper film is 7% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 12% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5 GHz band is 28 dB.

[0172] Compared with Comparative Example 1 and Example 1, the performance indicators of the copper film and copper-clad substrate are significantly worse. This is because the amount of micron-sized copper powder added is too low. On the one hand, the system lacks sufficient scattering and energy retention, which cannot provide stable conditions for the decomposition and reduction of copper ligands. As a result, the nano-copper crystal nuclei are easily oxidized by oxygen in the air, which destroys the sintering conditions under the air atmosphere. On the other hand, the insufficient scattering effect and imperfect framework structure lead to under-burning and increased sheet resistance. At the same time, the utilization rate of laser energy by micron-sized copper powder decreases, and the densification degree of the film layer is insufficient, which ultimately leads to the deterioration of various performance indicators.

[0173] Comparative Example 2

[0174] A laser sintering process for conductive copper paste differs from Example 1 in that: in step (a), the molar ratio of anhydrous copper formate to amino ligand is 1:1.

[0175] Tests show that the sheet resistance of the copper film is 22mΩ / □, the sheet resistance change rate of the copper film is 6% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 14% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 25dB.

[0176] Compared with Example 1, the performance indicators of the copper film and the copper-clad substrate were significantly worse. This is because the proportion of organic components in the copper ligands was insufficient, resulting in insufficient absorption of laser energy by the copper ligands, incomplete reaction, and insufficient amount of reducing or neutral volatile products released during decomposition. As a result, an effective local reducing / inert environment could not be formed on the particle surface, which disrupted the air sintering conditions. The copper particles were easily oxidized and the neck growth was hindered, leading to a decrease in the density and conductivity of the film layer, which in turn resulted in a deterioration in various performance indicators.

[0177] Comparative Example 3

[0178] The laser sintering process for conductive copper paste differs from that in Example 3 in that the molar ratio of anhydrous copper formate to amino ligand in step (a) is 1:5.

[0179] Tests show that the sheet resistance of the copper film is 18mΩ / □, the sheet resistance change rate of the copper film after baking at 200℃ for 2 hours is 11%, and the sheet resistance change rate of the copper film after exposure to air for 720 hours is 20%; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 23dB.

[0180] Compared with Comparative Example 3 and Example 3, the performance indicators of the copper film and the copper-clad substrate are significantly worse. This is because the proportion of organic components in the copper ligand is too high, which will release excessive organic by-products and gases during the decomposition process, resulting in blistering, cracking or excessive residue of the film layer, which damages the compactness of the film layer. At the same time, excessive impurities will also affect conductivity, and excessive gas may interfere with the normal growth and metallurgical bonding of the neck, ultimately causing the performance indicators to deteriorate.

[0181] Comparative Example 4

[0182] The laser sintering process for conductive copper paste differs from that in Example 1 in that the laser wavelength in step (4) is 1065nm.

[0183] Tests show that the sheet resistance of the copper film is 16mΩ / □, the sheet resistance change rate of the copper film is 8% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 13.5% after exposure to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 26dB.

[0184] Compared with Example 1, the performance indicators of the copper film and the copper-clad substrate are significantly worse. This is because the laser wavelength shifted to 1065nm. The wavelength mismatch prevents it from effectively matching the NIR absorption band of the copper ligand. The scattering enhancement and thermal deposition amplification effect of the micron-sized copper powder are weakened, making the effective penetration depth and film thickness no longer on the same order of magnitude. The energy per unit length is difficult to accurately cross the thermal threshold of "copper ligand decomposition / nucleation + particle neck growth". This not only fails to ensure that the sintering reaction is fully carried out, but may also lead to the oxidation of copper particles, ultimately resulting in performance degradation such as increased sheet resistance and decreased shielding effectiveness.

[0185] Comparative Example 5

[0186] The laser sintering process for conductive copper paste differs from that in Example 1 in that the laser wavelength in step (4) is 1063 nm.

[0187] Tests show that the sheet resistance of the copper film is 17mΩ / □, the sheet resistance change rate of the copper film is 4% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 11% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 30dB.

[0188] Compared with Comparative Example 5 and Example 1, the performance indicators of the copper film and copper-clad substrate are significantly worse. This is because the laser wavelength shifted to 1063nm, and the wavelength mismatch reduced the fit between the laser and the NIR absorption band of the copper ligand. The scattering and energy interception effect of the micron-sized copper powder on the laser was weakened, and the thermal deposition was insufficient. This resulted in incomplete decomposition of the copper ligand, hindered the generation of nano-copper crystal nuclei and neck growth, and made it difficult to form a stable local reduction environment. The copper particles were easily oxidized, ultimately leading to poor performance indicators.

[0189] Comparative Example 6

[0190] The laser sintering process for conductive copper paste differs from that in Example 1 in that the power in step (4) is 0.05W.

[0191] Tests show that the sheet resistance of the copper film is 25mΩ / □, the sheet resistance change rate of the copper film after baking at 200℃ for 2 hours is 10%, and the sheet resistance change rate of the copper film after exposure to air for 720 hours is 12%; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 29dB.

[0192] Compared with Comparative Example 6 and Example 1, the performance indicators of the copper film and copper-clad substrate are significantly worse. This is because the laser power is too low, so the energy per unit length does not reach the thermal threshold of "copper ligand decomposition / nucleation + particle neck growth". Insufficient energy leads to insufficient decomposition and reduction of copper ligands, resulting in a small amount of nano-copper crystal nuclei generated with low activity. The surface of micron-sized copper powder is difficult to melt effectively, the neck growth is slow and insufficient, and the film layer has a low degree of densification. At the same time, the release of reducing volatile products is insufficient, which cannot resist oxidation. Ultimately, this leads to performance degradation problems such as a significant increase in sheet resistance and a decrease in shielding effectiveness.

[0193] Comparative Example 7

[0194] The laser sintering process for conductive copper paste differs from that in Example 3 in that the power in step (4) is 12W.

[0195] Tests show that the sheet resistance of the copper film is 16mΩ / □, the sheet resistance change rate of the copper film is 8% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 15% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 26dB.

[0196] Compared with Comparative Example 7 and Example 3, the performance indicators of the copper film and the copper-clad substrate are significantly worse. This is because the laser power is too high, causing the energy per unit length to exceed the damage threshold of the system. Excessive energy will not only cause excessive decomposition of copper ligands and violent volatilization of organic components, leading to blistering and cracking of the film, but may also cause excessive melting of copper particles or grain coarsening, or even damage to the substrate. At the same time, excessive energy may disrupt the stability of the local reducing / inert environment, exacerbate impurity residue, and ultimately significantly reduce the conductivity, stability and shielding effectiveness of the copper film.

[0197] Comparative Example 8

[0198] The laser sintering process for conductive copper paste differs from that in Example 1 in that the pulse frequency in step (4) is 400 kHz.

[0199] Tests show that the sheet resistance of the copper film is 20mΩ / □, the sheet resistance change rate of the copper film is 6% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 13% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 27dB.

[0200] Compared with Comparative Example 8 and Example 1, the performance indicators of the copper film and copper-clad substrate are significantly worse. This is because the pulse frequency is too low. On the one hand, the low-frequency pulse cannot provide a continuous and sufficient transient high-temperature environment for the copper ligands, resulting in incomplete decomposition of the copper ligands, hindering the charge transfer reaction of the amino ligands, and insufficient generation and uneven dispersion of nano-copper crystal nuclei, making it difficult to effectively bridge the micron-sized copper powder to form a neck connection. On the other hand, the discontinuous energy input of the low-frequency pulse causes excessive local temperature fluctuations in the film layer, which cannot maintain a stable local reduction / inert environment, making the copper particles easy to be oxidized, and also hindering the uniform growth and densification of the neck, ultimately leading to performance degradation problems such as increased sheet resistance and decreased shielding effectiveness.

[0201] Comparative Example 9

[0202] The laser sintering process for conductive copper paste differs from that in Example 3 in that the pulse frequency in step (4) is 1100 kHz.

[0203] Tests show that the sheet resistance of the copper film is 15mΩ / □, the sheet resistance change rate of the copper film is 7% after baking at 200℃ for 2 hours, and the sheet resistance change rate of the copper film is 14% after being exposed to air for 720 hours; the electromagnetic shielding effectiveness of the copper-clad substrate in the 5.5GHz band is 24dB.

[0204] Compared with Comparative Example 9 and Example 3, the performance indicators of the copper film and the copper-clad substrate were significantly worse. This is because the pulse frequency was too high, resulting in excessive overlap of laser pulses and excessive energy accumulation per unit time. The high-frequency pulses caused the thermal effects of adjacent pulses to overlap, forming local overheated areas. This not only caused excessive pyrolysis of copper ligands, releasing a large amount of organic byproducts and gases, and causing film blistering and cracking, but also caused excessive melting and grain coarsening of the surface of micron-sized copper powder, damaging the microstructure of the copper film. At the same time, the overheated environment may accelerate the volatilization of amino ligands that have not participated in the charge transfer reaction, weakening their reducing protective effect. Furthermore, excessive energy can easily damage the substrate surface, further deteriorating the conductivity and stability of the copper film, ultimately leading to a significant decrease in various performance indicators.

Claims

1. A process for laser sintering of conductive copper paste, characterized in that, After the conductive copper paste is printed on the surface of the substrate, sintering is performed in an air, nitrogen, inert gas or reducing gas atmosphere using a picosecond or femtosecond pulsed laser system, and during the sintering process, the sample surface is scanned by multiple paths through software control of the laser scanning program, wherein the laser wavelength is 1064 nm, the power is 0.1-10 W, and the pulse frequency is 500-1000 kHz; The conductive copper paste is composed of micron copper powder, copper ligand, solvent and organic additive; wherein the content of micron copper powder is not less than 50wt%, and the content of copper ligand is not less than 2wt%; the copper ligand is formed by coordination of copper formate and amino ligand, and the molar ratio of copper formate to amino ligand is 1:2-4; the amino ligand is one or more of hexylamine, 2-ethylhexylamine, butylamine, beta-alanine, glycine, 2-amino-2-methyl-1-propanol, octylamine and oleylamine.

2. A process for laser sintering of electrically conductive copper paste as claimed in claim 1, wherein, The substrate includes a flexible substrate, a ceramic substrate, a glass substrate or an epoxy resin substrate.

3. The process for laser sintering of electrically conductive copper paste as claimed in claim 1 wherein, The content of micron copper powder is not more than 80wt%, and the content of copper ligand is not more than 10wt%, and the mass ratio of copper ligand to solvent is 1:3-6.

4. The process for laser sintering of electrically conductive copper paste as claimed in claim 1 wherein, The solvent is an organic solvent, and the copper ligand and the organic solvent form a copper ligand solution.

5. A process for laser sintering of electrically conductive copper paste as claimed in claim 4, wherein, The configuration steps of the copper ligand solution include: (a) grinding and mixing the copper formate and the amino ligand to obtain a mixed powder; (b) adding the mixed powder to the organic solvent, then under stirring, heating to a reaction temperature and keeping the temperature, so that the mixed powder is completely dissolved, and the copper ligand solution is obtained.

6. A process for laser sintering of electrically conductive copper paste as claimed in claim 5 wherein, In step (b), the stirring speed is 200-1500 rpm, the heating rate is 0.5-3 ℃ / min, the reaction temperature is 90-120 ℃, and the holding time is 2-8 h.

7. The process of claim 1, wherein the conductive copper paste is a mixture of copper powder, glass frit, organic vehicle, and a fluxing agent. The D50 particle size of the micron copper powder is 1-2 μm.

8. The process for laser sintering of electrically conductive copper paste as claimed in claim 1 wherein, The content of the organic additive is not more than 10wt%, and the organic additive is one or more of terpineol, dihydromyrcenol, ethylene glycol butyl ether, diethylene glycol butyl ether, diethylene glycol butyl ether acetate, tributyl citrate, dibutyl phthalate, ethyl cellulose, cellulose acetate butyrate and acrylic resin.