Lead-free environmental protection silver paste suitable for high-temperature co-fired ceramic and preparation method thereof

Abstract

The application relates to the technical field of conductive paste, and discloses a lead-free environment-friendly silver paste suitable for high-temperature co-fired ceramic and a preparation method, aiming to solve the technical problem that the components introduced for inhibiting silver ion migration of the lead-free silver paste in the prior art can easily cause the interface electric contact performance and weldability of the lead-free silver paste and a ceramic substrate to be deteriorated, a composite glass phase containing a specific interface buffer unit and a migration inhibition unit is designed, the mass ratio of the two is controlled, the buffer unit preferentially forms a benign interface layer, a stable working environment is provided for the inhibition unit, the silver paste can realize excellent silver ion migration resistance reliability, low resistance ohmic contact between the silver paste and the ceramic substrate and good welding wettability under the premise of being lead-free and environment-friendly, and the comprehensive performance meets the application requirements of the high-temperature co-fired ceramic.

Description

Technical Field

[0001] This invention relates to the field of conductive paste technology, and in particular to a lead-free and environmentally friendly silver paste suitable for high-temperature co-fired ceramics and its preparation method. Background Technology

[0002] High-temperature co-fired ceramics (HTCC) are a key technology for manufacturing high-performance electronic components and multilayer substrates. In this process, conductive metal pastes need to be co-sintered with ceramic green bodies at temperatures exceeding 850°C to form interconnects and electrodes. Silver-based pastes are widely used due to their conductivity. To ensure sintering density and adhesion to the ceramic substrate, traditional silver pastes often add lead oxide to the glass phase. Given the environmental and human health hazards of lead, the development of lead-free silver pastes has become a major future direction in this field. For example, Chinese patent CN112185605A discloses a silver paste for ceramic substrates and its preparation method, which explicitly adopts a lead-free design.

[0003] In practical applications of lead-free silver paste, it has been found that due to the differences in chemical properties between the glass systems that replace lead, silver ions are prone to migration and diffusion into the ceramic matrix under high temperature or electric field conditions. This leads to a decrease in the insulation performance of the ceramic and affects the long-term reliability of the device. To address this issue, existing technologies have proposed solutions to suppress silver ion migration. For example, Chinese Patent No. CN119361210A discloses a conductive silver paste for co-fired ceramic internal electrodes and its preparation method. By optimizing the silver powder and adding specific additives, the penetration of silver ions into the ceramic matrix is ​​suppressed, thereby improving insulation reliability.

[0004] However, experimental verification and practice have revealed that the highly active components added to bind silver ions are prone to excessive interfacial reactions with the alumina ceramic substrate during the high-temperature sintering process required for HTCC. This results in the formation of an interface layer with excessive resistance between the metal electrode and the ceramic substrate, which increases the ohmic contact resistance and degrades the solderability of the metallization layer surface. This technical contradiction limits the application of lead-free silver paste in high-performance HTCC devices. Summary of the Invention

[0005] The technical problem to be solved by this invention is that the existing technology, in improving the anti-migration performance, fails to take into account the shortcomings of the interface electrical contact quality and welding reliability. To this end, we propose a lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics and its preparation method.

[0006] To achieve the above objectives, this application adopts the following technical solution: a lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics, which, by mass percentage, consists of the following components: 49.5%-72.5% conductive phase, 3.46%-17.4% glass binder phase, and the balance being an organic carrier; the glass binder phase includes a matrix, magnesium oxide, lanthanum oxide, and zirconium dioxide, with magnesium oxide and lanthanum oxide serving as interface buffer units, and zirconium dioxide serving as migration inhibition units, the mass ratio of interface buffer units to migration inhibition units being 1.1:1-1.2:1.

[0007] Preferably, the matrix comprises bismuth oxide, boron oxide, and silicon dioxide.

[0008] Preferably, the conductive phase includes spherical silver powder and flake silver powder, wherein the mass of the spherical silver powder accounts for 40.0%-62.0% of the total mass of the silver paste, and the mass of the flake silver powder accounts for 2.5%-10.5% of the total mass of the silver paste.

[0009] Preferably, the aspect ratio of the flake silver powder is greater than 10.

[0010] Preferably, the median particle size D50 of the spherical silver powder is 1.0-1.3 mm.

[0011] Preferably, the organic carrier comprises a solvent, a resin, and an additive, wherein the solvent is a mixture of terpineol and butyl carbitol acetate in a mass ratio of 1.1:1 to 2.0:1.

[0012] Preferably, the resin is a compound of ethyl cellulose with different molecular weights; the additives include thixotropic agents, polymeric dispersants and silane coupling agents.

[0013] Preferably, the thixotropic agent is hydrogenated castor oil, the polymeric dispersant is a polyurethane or polyacrylate dispersant, and the silane coupling agent is γ-glycidoxypropyltrimethoxysilane.

[0014] A method for preparing lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics includes the following steps: S1: Bismuth oxide, boron oxide, silicon dioxide, magnesium oxide and lanthanum oxide are mixed, melted, water-quenched and ball-milled to obtain basic glass powder; a zirconium salt precursor solution is subjected to hydrolysis and condensation reaction to obtain nano-zirconia sol; the basic glass powder and nano-zirconia sol are mixed, spray-dried and heat-treated to obtain the composite glass powder; S2: Solvent, resin and additives are mixed to prepare the organic carrier; S3: The composite glass powder, spherical silver powder and flake silver powder are mixed with the organic carrier, and kneaded, dispersed and rolled in sequence to obtain the silver paste.

[0015] Preferably, the fineness of the silver paste is ≤10 mm, and the viscosity is stable at 25-45 Pa·s at 25°C.

[0016] Technical effects and advantages of the present invention:

[0017] In this invention, an interface buffer unit composed of magnesium oxide and lanthanum oxide and a migration inhibition unit composed of zirconium dioxide are constructed in the glass binder phase. By controlling a specific mass ratio between the two, the former preferentially forms a dense, low-resistance transition layer during sintering, while the latter is stably supported in the glass network to effectively bind silver ions. Combined with the corresponding coating preparation process, the two processes of interface reaction and ion binding are essentially coordinated, so that the final silver paste can simultaneously achieve high resistance to silver ion migration, low interfacial contact resistance, and good solderability under the premise of being environmentally friendly and lead-free. This effectively solves the problem of performance being difficult to balance in the prior art. Attached Figure Description

[0018] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings, the same reference numerals are used to refer to the same parts:

[0019] Figure 1 This is a flow chart of the preparation process of the lead-free environmentally friendly silver paste of the present invention. Detailed Implementation

[0020] It is readily understood that, based on the technical solution of this invention, those skilled in the art can propose various interchangeable structural methods and implementations without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention.

[0021] This invention provides a lead-free, environmentally friendly silver paste suitable for high-temperature co-fired ceramics. By mass percentage, the silver paste consists of the following components: 49.5%-72.5% conductive phase, 3.46%-17.4% glass binder phase, and the balance being an organic carrier. The specific composition and mass percentage range of each component are shown in Table 1 below.

[0022] Table 1. Composition and mass percentage of each component in lead-free environmentally friendly silver paste: ;

[0023] The main function of the conductive phase is to form a continuous conductive network after sintering. This invention uses a system of spherical micron-sized silver powder and flake-shaped silver powder. Among them, the spherical silver powder is used as the conductive host, and its median particle size D50 is preferably 1.0-1.3 mm, which is conducive to forming a highly conductive path.

[0024] The aspect ratio of the flake silver powder should be >10. Its flat structure during the sintering process helps to stack with the spherical powder, thereby improving the density of the sintered film and reducing the porosity. At the same time, the flake structure forms multiple physical barriers inside the conductive film, which can effectively extend the diffusion path of silver ion migration and play an auxiliary role in inhibiting silver ion migration.

[0025] The glass binder phase is key to achieving performance balance under lead-free conditions. It includes a matrix and functional additives. The matrix is ​​composed of Bi2O3, B2O3 and SiO2, which respectively provide sintering activity, regulate melting temperature and stabilize network structure.

[0026] The core design of this glass binder phase lies in the introduction of functionally complementary interface buffer units and migration suppression units. The interface buffer unit is composed of MgO and La2O3. During high-temperature sintering, MgO preferentially reacts with the Al2O3 ceramic substrate to form a low-resistivity magnesium-aluminum spinel interface layer; La2O3 helps to densify the structure of this interface layer. The migration suppression unit is ZrO2, which can utilize the interaction between high-valence zirconium ions and silver ions to bind the activity of silver ions and suppress their migration.

[0027] To achieve the optimal balance between anti-migration properties and interfacial conductivity, the ratio of the two units must be precisely controlled. Experiments have verified that the mass ratio of the interface buffer unit to the migration suppression unit, i.e., the sum of the masses of MgO and La2O3 to the mass of ZrO2, should be controlled at 1.1-1.2:1. Within this range, the buffer unit can form a complete and moderately benign interface, while providing a stable environment for the suppression unit to exert its effectiveness. If the ratio is too low, the interface will deteriorate; if it is too high, the overall performance may be impaired due to excessive interfacial reaction or insufficient suppression ability.

[0028] The organic carrier is used to support the solid component and provide suitable rheological properties. The solvent is preferably a mixture of terpineol and butyl carbitol acetate, with a mass ratio of 1.1-2.0:1.

[0029] The resin is compounded using ethyl cellulose of different molecular weights to simultaneously satisfy the requirements of low viscosity flowability of the paste under high shear force and high viscosity anti-settling property under low shear force, thereby ensuring the accuracy of the printed pattern and the storage stability of the paste.

[0030] The additives mainly include thixotropic agents, dispersants, and coupling agents. The preferred thixotropic agent is hydrogenated castor oil, which is used to adjust the thixotropic properties of the slurry and prevent the printed pattern from collapsing. Polyurethane and other polymeric dispersants are used, mainly to utilize their steric hindrance effect to prevent the agglomeration of high surface energy nano and micron-sized powders. Silane coupling agents are selected to improve the compatibility between inorganic powders and organic phases through their amphiphilic molecular structure.

[0031] Reference Figure 1 As shown, based on the above-mentioned component composition and their established functional synergistic relationships, in order to achieve the expected performance of the silver paste, this invention also provides a method for preparing a lead-free and environmentally friendly silver paste suitable for high-temperature co-fired ceramics, specifically including the following steps:

[0032] S1: Weigh Bi2O3, B2O3, SiO2, MgO and La2O3 raw materials according to the formula, mix them evenly, and place them in a high-temperature furnace at 1150-1250℃ to melt and keep them at that temperature for more than 1 hour to ensure that each component reacts fully to form a homogeneous glass phase. Pour the glass melt into deionized water with high-speed stirring for water quenching to obtain crushed glass. After drying the crushed glass, use a zirconia ball mill jar and anhydrous ethanol as the medium for wet ball milling for 36-48 hours. After ball milling, dry and sieve to obtain basic glass powder with a particle size D50 <2 mm.

[0033] S2: Using zirconium oxychloride as a precursor, it is dissolved in a mixed solution of ethanol and deionized water. Dilute ammonia solution is added dropwise under stirring to adjust the pH of the reaction system to 8-9, so that the precursor is hydrolyzed and condensed to form a stable and translucent nano-zirconia sol.

[0034] S3: The base glass powder is uniformly dispersed in the sol. The sol is fully coated on the surface of the base glass particles by mechanical stirring and ultrasonic dispersion. The suspension is dried and granulated by spray dryer to obtain precursor powder. The powder is placed in a muffle furnace and heat-treated at 500-600℃ for 1-2 hours to stabilize the amorphous coating layer and obtain composite glass powder.

[0035] S4: Mix terpineol and butyl carbitol acetate according to the ratio as a solvent, add the compounded ethyl cellulose resin and heat and stir to completely dissolve it, then add hydrogenated castor oil, polyurethane or polyacrylate polymeric dispersant and silane coupling agent in sequence, and continue stirring until a homogeneous and transparent solution is formed to obtain an organic carrier.

[0036] S5: Weigh the composite glass powder, spherical silver powder and flake silver powder, mix for 1-2 hours, so that the powders of different densities and particle sizes can be initially uniformly distributed in the dry state to obtain the premixed powder;

[0037] S6: Slowly add the premixed powder to the organic carrier. Use a planetary mixer to knead the powder at a low speed for 45-55 minutes by combining revolution and rotation to fully wet the powder with the organic carrier. Then increase the speed and disperse at high speed for 1-1.5 hours to break up the powder agglomerates and form a paste.

[0038] S7: The paste is transferred to a three-roll mill for repeated rolling, 5-9 times, with the gap between the rollers decreasing each time. Extremely high shear force is applied to the slurry to completely crush the residual agglomerates and achieve the required fineness. The slurry fineness is rolled until it is ≤10 mm and the viscosity is stable at 25-45 Pa·s at 25°C, thus obtaining the silver paste.

[0039] To clearly and completely illustrate the technical solution of the present invention and verify its technical effects, the following specific embodiments and comparative experiments are used to further illustrate the invention. Each embodiment is for illustrative purposes only and does not constitute a limitation on the scope of protection of the present invention.

[0040] Example 1: This example provides a method for preparing lead-free, environmentally friendly silver paste suitable for high-temperature co-fired ceramics, specifically including:

[0041] S1: Weigh 41.0g of Bi2O3, 18.5g of B2O3, 12.0g of SiO2, 6.6g of MgO, and 3.4g of La2O3, mix them evenly, melt them at 1200℃ and hold for 1.5h, quench them with water, dry them, and then ball mill them until D50 < 2mm to obtain basic glass powder; Weigh 21.0g of zirconium oxychloride (ZrOCl2·8H2O), which corresponds to the generation of about 9.1g of ZrO2, and prepare nano-zirconia sol. Disperse the basic glass powder in the sol, and after spray drying and heat treatment at 550℃, obtain composite glass powder; The ratio of the sum of the masses of MgO and La2O3 to the mass of ZrO2 is 1.1:1;

[0042] S2: Mix terpineol and butyl carbitol acetate at a mass ratio of 1.5:1, add compounded ethyl cellulose resin, dissolve, then add hydrogenated castor oil, polyurethane dispersant and silane coupling agent KH-560, stir evenly to obtain a uniform and transparent organic carrier.

[0043] S3: Weigh 55.0g of spherical silver powder, 10.0g of flake silver powder and 8.5g of the above composite glass powder and premix for 1.5h. Add the premixed powder to 26.5g of organic carrier, knead and disperse at high speed in a planetary mixer, and then transfer to a three-roll mill for rolling 7 times to obtain a silver paste with a fineness of 8㎛ and a viscosity of 35Pa·s.

[0044] Example 2: The only difference between this example and Example 1 is that the ratio of the glass binder raw materials is adjusted so that the ratio of the sum of the masses of MgO and La2O3 to the mass of ZrO2 in the prepared composite glass powder is 1.15:1. Specifically, the amount of MgO is adjusted to 6.8g, the amount of La2O3 is 3.2g, and the corresponding amount of ZrO2 is adjusted to about 8.7g.

[0045] Example 3: The only difference between this example and Example 1 is that the ratio of the glass binder raw materials is adjusted so that the ratio of the sum of the masses of MgO and La2O3 to the mass of ZrO2 in the prepared composite glass powder is 1.2:1. Specifically, the amount of MgO is adjusted to 7.0g, the amount of La2O3 is 3.0g, and the corresponding amount of ZrO2 is adjusted to about 8.3g.

[0046] Comparative Example 1: The only difference between this comparative example and Example 1 is that the ratio of the glass binder raw materials is adjusted so that the ratio of the sum of the masses of MgO and La2O3 to the mass of ZrO2 in the prepared composite glass powder is 0.8:1. Specifically, the amount of MgO is adjusted to 5.0g, the amount of La2O3 is 3.0g, and the corresponding amount of ZrO2 is adjusted to about 10.0g.

[0047] Comparative Example 2: The only difference between this comparative example and Example 1 is that the raw material ratio of the glass binder phase is adjusted so that the ratio of the sum of the masses of MgO and La2O3 to the mass of ZrO2 in the prepared composite glass powder is 1.0:1. Specifically, the amount of MgO is adjusted to 6.0g, the amount of La2O3 is 3.0g, and the corresponding amount of ZrO2 is adjusted to about 10.0g.

[0048] Comparative Example 3: The only difference between this comparative example and Example 1 is that the raw material ratio of the glass binder phase is adjusted so that the ratio of the sum of the masses of MgO and La2O3 to the mass of ZrO2 in the prepared composite glass powder is 2.0:1. Specifically, the amount of MgO is adjusted to 10.0g, the amount of La2O3 is 5.0g, and the corresponding amount of ZrO2 is adjusted to about 7.0g.

[0049] Comparative Example 4: In this comparative example, traditional lead-containing glass powder was used instead of the composite glass powder of the present invention. 8.5g of lead borosilicate glass powder was weighed, which included 55% PbO, 30% B2O3, and 15% SiO2. The composition and amount of the conductive phase and organic carrier, as well as the preparation process, were consistent with those of Example 1.

[0050] Comparative Example 5: This comparative example aims to verify the role of MgO in forming a benign interface. The only difference from Example 1 is that the MgO in the glass binder phase is replaced with calcium oxide (CaO) in equal molar amounts. Specifically, 41.0 g of Bi2O3, 18.5 g of B2O3, 12.0 g of SiO2, 5.9 g of CaO, and 3.4 g of La2O3 are weighed and composite glass powder is prepared in the same way. At this time, the ratio of the sum of the masses of CaO and La2O3 to the mass of ZrO2 is 1.1:1.

[0051] Comparative Example 6: This comparative example aims to verify the effect of La2O3 in refining the interfacial structure. The only difference from Example 1 is that the equimolar amount of La2O3 in the glass binder phase is replaced with yttrium oxide (Y2O3). Specifically, 41.0 g of Bi2O3, 18.5 g of B2O3, 12.0 g of SiO2, 6.6 g of MgO, and 3.4 g of Y2O3 are weighed and composite glass powder is prepared in the same way. At this time, the ratio of the sum of the masses of MgO and Y2O3 to the mass of ZrO2 is 1.1:1.

[0052] Comparative Example 7: The only difference between this comparative example and Example 1 is that the sol preparation and spray drying steps are omitted and replaced with direct mechanical mixing. Specifically, the base glass powder and the corresponding amount of commercially available nano-zirconia powder are directly placed in a three-dimensional mixer and physically mixed for 4 hours. This mixed powder is used as the glass binder phase.

[0053] Comparative Example 8: The only difference between this comparative example and Example 1 is that the polyurethane polymeric dispersant in the organic carrier is replaced with an equal mass of a traditional small molecule dispersant. Specifically, when preparing the organic carrier, a phosphate ester dispersant is used instead of a polyurethane dispersant.

[0054] To verify whether the silver paste described in this invention can suppress silver ion migration while simultaneously ensuring the quality of interfacial electrical contact and welding reliability, the aforementioned examples and comparative samples were subjected to the following performance tests.

[0055] All test samples were prepared according to a unified standard. The paste was screen-printed onto a 96% alumina ceramic green body, and then debinded at 850°C and sintered in air at 885°C for 30 minutes to form a sintered film with a thickness of about 12μm.

[0056] Experimental Example 1: This experimental example aims to verify the ability of the silver paste to inhibit the electrochemical migration of silver ions. Examples 1-3 and Comparative Examples 1-6 were selected as samples for testing.

[0057] The sample with comb-shaped electrodes was placed in a constant temperature and humidity chamber. Under the conditions of 130℃ and 85% relative humidity, a DC bias voltage of 50V was applied between the electrodes for 96 hours. After the test, the sample was taken out and allowed to recover for 2 hours in a standard laboratory environment. The insulation resistance between the same electrodes was measured using a high resistance meter, and its retention rate relative to the test was calculated. The test results are shown in Table 2 below.

[0058] Table 2. Test results of the anti-silver ion migration performance of each sample: ;

[0059] The insulation resistance retention rates of Examples 1-3 were all above 97.5%, and the data were concentrated and stable, indicating that the synergistic effect of the migration suppression unit and the glass phase network reached its optimal level within this specific ratio range. The performance of Comparative Examples 1-3 decreased. In Comparative Example 1, the excess ZrO2 disrupted the continuity of the glass phase and introduced stress concentration points, which accelerated migration. In Comparative Example 3, the excess MgO and La2O3 excessively consumed the components used to form a stable glass network, weakening the overall binding force of the glass phase on silver ions.

[0060] The performance of Comparative Examples 5-6 was lower than that of Example 1. In Comparative Example 5, after CaO replaced MgO, Ca... 2+ The calcium aluminate phase formed by the reaction with Al2O3 has a higher ionic conductivity than the magnesium aluminum spinel phase, which is not conducive to inhibiting migration. In Comparative Example 6, after Y2O3 replaces La2O3, its regulatory effect on the microstructure of the glass phase is inferior to that of La2O3, resulting in a decrease in anti-migration performance.

[0061] Experimental Example 2: This experimental example aims to verify the ohmic contact quality and conductivity of the interface formed between the silver paste and the ceramic substrate after sintering. Examples 1-3 and Comparative Examples 1-6 were selected as samples for testing.

[0062] A circular silver electrode with a diameter of 2 mm was fabricated on a ceramic substrate. Using a four-probe tester, two probes were used as current input terminals, and a constant small current was applied to the center of the circular electrode. The other two probes were used as voltage measurement terminals, and the voltage was measured on the surface of the ceramic substrate outside the electrode at a fixed interval. The measured voltage was compared with the applied voltage to calculate the spreading resistance. The smaller the value, the easier it is for the current to diffuse from the electrode through the interface to the substrate, that is, the better the interfacial electrical contact performance. Five points were tested for each sample and the average value was taken. The test results are shown in Table 3 below.

[0063] Table 3. Test results of the interfacial electrical contact performance of each sample: ;

[0064] The extended resistance of Examples 1-3 is lower than that of the other comparative examples. The extended resistance of Comparative Example 1 is extremely high, indicating that excessive ZrO2 will seriously hinder the formation of the interfacial conductive pathway. The extended resistance of Comparative Example 5 is much higher than that of Example 1, proving that MgO has an irreplaceable role in forming a highly conductive magnesium aluminum spinel interfacial layer.

[0065] Experimental Example 3: This experimental example aims to verify the weld wettability of the sintered silver layer surface of the silver paste. Example 1 and Comparative Examples 3-6 were selected as samples for testing.

[0066] Standard lead-free solder balls were placed on the surface of the sintered silver layer and melted in a nitrogen-protected reflow soldering atmosphere at 255°C for 60 seconds. Then, the solder was cooled and solidified. The static contact angle between the solder and the silver layer was measured using an optical contact angle meter. Five different locations were measured for each sample and the average value was taken. The test results are shown in Table 4 below.

[0067] Table 4. Weldability test results for each sample: ;

[0068] The wetting angle of Example 1 is comparable to that of the lead-containing reference, while the wetting angles of Comparative Examples 3 and 5 are increased. This indicates that excessive interfacial reaction caused by excessive MgO / La2O3 may form compounds or microstructures on the silver layer surface that are not easily wetted by solder, thereby impairing solderability.

[0069] Experimental Example 4: The freshly prepared slurry was sealed and stored in a light-proof environment at 25°C. On the 1st and 30th day of storage, samples were taken to measure its viscosity and fineness. The test results are shown in Table 5 below.

[0070] Table 5. Results of storage stability tests for each sample slurry: ;

[0071] Example 1 exhibited good storage stability with minimal changes in viscosity and fineness, while Comparative Example 8 showed severe agglomeration. This demonstrates that the steric stabilization provided by polyurethane polymeric dispersants is indispensable for the complex multiphase system containing high surface energy nanoparticles and micron-sized silver powders in this invention, and small molecule dispersants cannot provide long-term stability.

[0072] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A lead-free, environmentally friendly silver paste suitable for high-temperature co-fired ceramics, characterized in that, By mass percentage, it consists of the following components: 49.5%-72.5% conductive phase, 3.46%-17.4% glass binder phase, and the balance being an organic carrier. The conductive phase is composed of spherical silver powder and flake silver powder. The glass binder phase comprises a matrix, magnesium oxide, lanthanum oxide, and zirconium dioxide. The matrix includes bismuth oxide, boron oxide, and silicon dioxide. The magnesium oxide and lanthanum oxide serve as interface buffer units, and the zirconium dioxide serves as a migration inhibition unit. The mass ratio of the interface buffer unit to the migration inhibition unit is 1.1:1 to 1.2:

1.

2. The lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 1, characterized in that: The spherical silver powder accounts for 40.0%-62.0% of the total mass of the silver paste, and the flake silver powder accounts for 2.5%-10.5% of the total mass of the silver paste.

3. The lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 2, characterized in that: The aspect ratio of the flake silver powder is greater than 10.

4. The lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 2, characterized in that: The median particle size D50 of the spherical silver powder is 1.0-1.3 mm.

5. The lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 1, characterized in that: The organic carrier includes a solvent, a resin, and an additive, wherein the solvent is a mixture of terpineol and butyl carbitol acetate in a mass ratio of 1.1:1 to 2.0:

1.

6. The lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 5, characterized in that: The resin is a compound of ethyl cellulose with different molecular weights; the additives include thixotropic agents, polymeric dispersants and silane coupling agents.

7. The lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 6, characterized in that: The thixotropic agent is hydrogenated castor oil, the polymeric dispersant is a polyurethane or polyacrylate dispersant, and the silane coupling agent is γ-glycidoxypropyltrimethoxysilane.

8. A method for preparing lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: Bismuth oxide, boron oxide, silicon dioxide, magnesium oxide and lanthanum oxide are mixed, melted, water-quenched and ball-milled to obtain basic glass powder. The zirconium salt precursor solution is subjected to hydrolysis and condensation reaction to obtain nano-zirconia sol. The basic glass powder and nano-zirconia sol are mixed, spray-dried and heat-treated to obtain composite glass powder. S2: Prepare an organic carrier by mixing solvent, resin and additives; S3: The composite glass powder, spherical silver powder and flake silver powder are mixed with an organic carrier, and kneaded, dispersed and rolled in sequence to obtain the silver paste.

9. The method for preparing a lead-free environmentally friendly silver paste suitable for high-temperature co-fired ceramics according to claim 8, characterized in that: The silver paste has a fineness of ≤10㎛ and a viscosity that is stable at 25-45 Pa·s at 25℃.

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