Preparation process of superfine and ultrathin nano silver paste

Abstract

The application discloses a kind of superfine ultra-thin nano silver paste preparation process, it is related to nano silver paste preparation technical field, first in silver amine complex system introduction trace interface control agent and low-temperature reduction formation silver base nucleus, again through secondary silver shell growth make non-silver interface control component with silver particle synchronous distribution and be diluted, subsequently on the surface of nano silver particle construction homologous reducible silver precursor shell layer, with sol or nano precursor form introduction non-continuous interface stationary phase, finally by compounding grinding defoaming obtains nano silver paste, by printing, segmented thermal activation and low-temperature sintering form conductive silver film.The application makes protective layer convert into conductive silver bridge in heat treatment stage, makes interface stationary phase with dot or island mode participate in adhesion, suitable for ultra-thin electrode, microcircuit and low-temperature conductive connection scene.

Description

Technical Field

[0001] This invention relates to the field of nano-silver paste preparation technology, specifically to a process for preparing ultrafine and ultrathin nano-silver paste. Background Technology

[0002] Nano silver paste is a commonly used conductive material in flexible electronics, touch circuits, RFID tags, photovoltaic grids, and low-temperature interconnects. It is usually composed of silver powder, organic carrier, and a small amount of additives. After application, it is dried and sintered to form a conductive silver film. As the line width and film thickness continue to decrease, the particle size, dispersion state, sintering start temperature, and interface fixation method between the silver particles in the paste and the substrate directly affect the printing process and film continuity.

[0003] Existing ultrafine silver pastes often use nano-silver powder and polymeric dispersants directly compounded. Although the dispersant can reduce storage agglomeration, it needs to be burned off during the sintering stage, which can easily leave pores in the ultrathin film layer. When glass powder or inorganic binder phase is directly added externally, it may form local insulation enrichment. In addition, the surface energy increases after the silver particle size is reduced, and hard agglomeration and edge shrinkage are more obvious. Simply relying on increasing the silver content or extending the sintering time is not enough to meet the requirements of low-temperature sintering, ultrathin continuous conductivity and interface adhesion. Summary of the Invention

[0004] The purpose of this invention is to provide a process for preparing ultrafine and ultrathin silver nanoparticle paste. This process combines interface control of the silver-based core, secondary silver shell growth, homologous reducible silver precursor shell, ultrathin silver nanosheet bridging, and discontinuous interface fixation, so that the silver nanoparticle paste has an interconnected structural evolution path during the storage, coating, and heat treatment stages.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a process for preparing ultrafine and ultrathin nano-silver paste, comprising the following steps: S1. An interface regulator is added to the silver salt complexation system and reduced to nucleation to obtain a silver-based nucleus dispersion containing interface regulation sites. S2. Add silver salt complex solution to the silver-based core dispersion and perform secondary silver shell growth to obtain silver core-shell nanoparticles; S3. Construct a homologous reducible silver precursor shell on the surface of silver core-shell nanoparticles; S4. The surface-treated ultrathin silver nanosheets are combined with silver core-shell nanoparticles, organic carrier and interfacial stationary phase precursor liquid, and then ground and degassed to obtain nano-silver paste. S5. The nano-silver paste is coated onto the substrate and then subjected to segmented thermal activation and sintering to form a conductive silver film.

[0006] In one exemplary embodiment of this disclosure, an interface modifier is added to a silver salt complexation system and reduced to a nucleus to obtain a silver-based nucleus dispersion containing interface-modifying sites, comprising: Silver nitrate was added to a mixture of deionized water and ethanolamine to form a silver amine complex solution, with the silver ion concentration controlled at 0.05-0.30 mol / L and the system pH at 9.5-11.5. Add at least one of rare earth salts, bismuth salts, or alkali-free borate precursors to the silver amine complex solution so that the mass fraction of non-silver inorganic elements in the interface modifier relative to silver is 0.10%-0.60%; A reduction system consisting of ascorbic acid, glucose, ethylene glycol, or borohydride is added dropwise at 5-15℃ to nucleate silver ions and obtain a silver-based nucleus dispersion with a particle size of 3-12 nm.

[0007] In one exemplary embodiment of this disclosure, adding a silver salt complex solution to a silver-based core dispersion and performing a secondary silver shell growth includes: Add silver amine complex solution to the silver-based nucleus dispersion so that the mass of added silver ions is 5-30 times the mass of silver in the silver-based nucleus. Add ascorbic acid or glucose reducing solution dropwise at 20-45℃ to deposit the newly generated silver on the surface of the silver-based parent nucleus to form a secondary silver shell; Through secondary silver shell growth, the mass fraction of non-silver interface-regulated components in the final silver core-shell nanoparticles is reduced to 0.005%-0.08%, and the non-silver interface-regulated components are distributed at the nucleation interface, local sites on the particle surface, and sintering migration interface.

[0008] In one exemplary embodiment of this disclosure, a homologous reducible silver precursor shell is constructed on the surface of silver core-shell nanoparticles, including: The silver core-shell nanoparticles after secondary silver shell growth are centrifuged and washed or filtered through a membrane to remove free salts, residual reducing agents and small molecule byproducts. The washed silver core-shell nanoparticles were transferred to ethylene glycol butyl ether, terpineol, diethylene glycol monobutyl ether, propylene glycol methyl ether acetate or a mixture thereof. During the phase inversion process, at least one of short-chain silver carboxylic acid, silver lactate, silver citrate, or silver ethanolamine complex is added to adsorb the silver precursor onto the surface of the silver core-shell nanoparticles.

[0009] In one exemplary embodiment of this disclosure, the silver precursor is adsorbed onto the surface of silver core-shell nanoparticles, comprising: The thickness of the shell layer of the homologous reducible silver precursor was controlled to be 1-5 nm, and the mass of convertible silver in the shell layer of the homologous reducible silver precursor was 1.0%-8.0% of the mass of the silver core-shell nanoparticles. During the storage and coating stages of the nano-silver paste, the shell layer of the homologous reducible silver precursor isolates adjacent silver core-shell nanoparticles. During the segmented thermal activation stage, the shell of the homologous reducible silver precursor is decomposed to release active silver and form silver bridges between particles.

[0010] In one exemplary embodiment of this disclosure, before compounding the surface-treated ultrathin silver nanosheets with silver core-shell nanoparticles, the organic carrier, and the interfacial stationary phase precursor solution, the method further includes: Using some of the silver core-shell nanoparticles grown from secondary silver shells as a silver source, two-dimensional growth is carried out in the presence of polyvinylpyrrolidone, citrate, or ethanolamine to obtain ultrathin silver nanosheets with a thickness of 5-25 nm and a lateral dimension of 80-600 nm. Alternatively, the sheet-like silver precursor is wet-sheared and thinned to obtain silver nanosheets with a thickness of no more than 30 nm.

[0011] In one exemplary embodiment of this disclosure, the surface-treated ultrathin silver nanosheets include The surface of ultrathin silver nanosheets is treated with a homologous reducible silver precursor shell that is the same as or similar to the silver core-shell nanoparticles, so that the surface of the ultrathin silver nanosheets has an organic-silver complex interface. The surface-treated ultrathin silver nanosheets are added to the nanosilver paste at 0.5%-6.0% of the total amount of silver solids, so that it provides bridging pathways in the plane of the ultrathin film layer.

[0012] In one exemplary embodiment of this disclosure, the interfacial stationary phase precursor solution is prepared by at least one of alkali-free borosilicate sol, bismuth borate sol, zinc borate sol or rare earth oxide sol with an organic solvent, and a thermally degradable complexing agent is added. The solid content of the interface stationary phase precursor liquid is 1%-10%, and the non-silver inorganic solid content after adding nano-silver paste is 0.02%-0.20% of the silver solid mass. During the drying and sintering process, the interface stationary phase precursor liquid forms discontinuous fixation points at the interface between the film layer and the substrate, at the edge of the lines and near the micropores.

[0013] In one exemplary embodiment of this disclosure, surface-treated ultrathin silver nanosheets are compounded with silver core-shell nanoparticles, an organic carrier, and an interfacial stationary phase precursor solution, and then ground and degassed to obtain a nano-silver paste, comprising: By mass percentage, 70%-88% of silver solid component, 8%-25% of organic support, 1%-8% of homologous reducible silver precursor, 0.02%-0.20% of interfacial stationary phase precursor solid and 0.1%-2.0% of rheology modifier are mixed. Vacuum planetary mixing, low-shear pre-wetting, three-roll milling or centrifugal degassing are employed, and the parts in contact with the silver paste are made of polytetrafluoroethylene, zirconium oxide, alumina, silver-coated stainless steel or inert ceramic materials.

[0014] In one exemplary embodiment of this disclosure, the process of coating a substrate with nano-silver paste and subjecting it to segmented thermal activation and low-temperature sintering includes: Nano silver paste is applied to glass, polyimide, PET, ceramic, silicon wafers, or metal substrates coated with insulating layers by means of screen printing, gravure printing, microgravure coating, inkjet printing, or blade coating. The silver film was pre-dried at 60-90℃ for 1-10 min, thermally activated at 110-160℃ for 3-20 min, and sintered at 180-260℃ for 5-40 min to obtain an ultrathin conductive silver film with a thickness of 0.3-5.0 μm.

[0015] The technical effects and advantages provided by the present invention in the above technical solution are as follows: 1. Compared to existing ultrafine silver pastes made directly from nano-silver powder, polymeric dispersants, and glass powder, this application does not simply add the protective and adhering components as foreign powders. Instead, it sets interface control sites during the formation of the silver-based nucleus and dilutes them into the relevant interfaces of the silver particles after the secondary silver shell growth. The process control device first reduces the nucleation, then adds silver salt complexing liquid to allow the silver shell to grow a second time, and then forms a homologous reducible silver precursor shell. Next, ultrathin silver nanosheets and interface stationary phase precursor liquid are added and ground. Finally, coating and segmented thermal activation sintering are performed. Since the silver precursor shell can be transformed into a silver bridge during the heat treatment stage, the particle isolation function and the conductive necking function are taken over at different stages, thereby reducing the voids formed after the protective layer is burned off, which is beneficial for the ultrathin film layer to maintain a continuous conductive path.

[0016] 2. Compared with the existing practice of adding inorganic binder phase in the form of large-particle glass powder, this application introduces the interface stationary phase precursor liquid in the form of sol or nano-precursor and controls its content in silver solid. First, the slurry is dispersed by organic carrier and rheology modifier. Second, the precursor liquid migrates with the solvent to the interface between the film layer and the substrate, the edge of the line and the vicinity of the micropore. Then, during the sintering process, dot-shaped, island-shaped or thin boundary-shaped fixing points are formed. Third, the ultrathin silver nanosheets provide bridging pathways on the film plane. Finally, an ultrathin conductive silver film suitable for micro-circuits is obtained. Since the stationary phase does not cover the silver particles with a continuous insulating layer, the conflict between interface adhesion, line edge shaping and conductive network is reduced. This is beneficial to obtain printable, sinterable nano-silver paste that can be used for low-temperature conductive bonding under lower thermal budget conditions.

[0017] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure. It is obvious that the drawings described below are merely some embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0019] Figure 1 This is a flowchart of the nano-silver paste preparation process of the present invention.

[0020] Figure 2 A system architecture diagram for this invention is provided.

[0021] Figure 3 This is a diagram illustrating the application scenario of the nano-silver paste of the present invention.

[0022] Figure 4 This is a schematic diagram of the formation of the silver-based nucleus interface regulatory site according to the present invention.

[0023] Figure 5 This is a schematic diagram of the secondary silver shell growth and interface regulation component dilution of the present invention.

[0024] Figure 6 This is a schematic diagram of the formation of the silver bridge according to the present invention.

[0025] Figure 7 This is a schematic diagram of the planar bridging of ultrathin silver nanosheets according to the present invention.

[0026] Figure 8 This is a schematic diagram of the discontinuous interface stationary phase distribution of the present invention.

[0027] Figure 9 This is a schematic diagram of the nano-silver paste compounding, grinding and degassing process of the present invention.

[0028] Figure 10 This is a schematic diagram of the segmented thermal activation and low-temperature sintering temperature curves of the present invention.

[0029] Figure 11 This is a schematic diagram illustrating the fabrication scenario of a low-temperature circuit based on the flexible substrate of the present invention.

[0030] Figure 12 This is a schematic diagram of the preparation process detection of the present invention. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments. The names of raw materials and process conditions in the embodiments are used to illustrate the possible implementation methods of the present invention. Those skilled in the art can make equivalent substitutions for solvent types, stirring equipment, filtration methods and heat treatment equipment without departing from the technical concept of the present invention.

[0032] Please see Figure 1 As shown, the core of this invention lies in first introducing trace amounts of non-silver inorganic elements into the relevant interfaces of silver particles through a silver-based core containing interface control sites, and then reducing the final content of these elements to a low level through secondary silver shell growth, so that they do not exist as randomly added powder but are mainly located at the nucleation interface, local particle sites, and subsequent sintering migration interfaces, such as... Figure 2 As shown.

[0033] In the silver-based nucleus preparation stage, silver nitrate and ethanolamine form a silver amine complex solution. Ethanolamine keeps the silver ions in a complexed state. After the reducing agent is added dropwise at low temperature, the silver ions undergo rapid nucleation. The reaction system should be kept uniformly stirred. The stirring speed can be controlled between 200-800 rpm depending on the volume of the reactor, and the dropping time can be controlled between 5-40 min. Figure 4 As shown.

[0034] The interface modifier can be a combination of lanthanum nitrate, bismuth citrate and alkali-free borosilicate sol. Lanthanum nitrate is used to form rare earth oxide anchoring sites during subsequent heat treatment, bismuth citrate is used to provide low-temperature sintering activation aids, and alkali-free borosilicate sol is used to form a discontinuous stationary phase at the film interface.

[0035] When the mass of silver in the silver-based core is 1.0 kg and the mass of silver in the silver salt complex solution is 12.0 kg, the total amount of silver after the secondary silver shell growth is 13.0 kg. If the non-silver inorganic elements in the interface regulator are 3.9 g, then the final mass fraction of the non-silver interface regulator component relative to silver is approximately 0.03%. This calculation method is used to determine the feeding dilution result and is not a human scoring method.

[0036] like Figure 5 As shown, during the secondary silver shell growth, the newly generated silver preferentially deposits on the surface of the silver-based parent core. The particles gradually grow from the highly active parent core into silver core-shell nanoparticles. If too few silver ions are added, the interface control components will not be diluted sufficiently. If too many silver ions are added, the particle size will grow too quickly, which may reduce the necking speed of low-temperature sintering.

[0037] The washing and phase inversion steps are used to reduce free salt, residual reducing agent and small molecule byproducts. Centrifugal washing can be performed by centrifuging at 3000-9000 rpm for 5-20 min and repeated 2-5 times. Membrane filtration can be performed using solvent-resistant ultrafiltration membranes or ceramic membranes. The phase inversion solvent can be ethylene glycol butyl ether, terpineol, diethylene glycol monobutyl ether and propylene glycol methyl ether acetate.

[0038] like Figure 6 As shown, the shell of the homologous reducible silver precursor is not a traditional polymer dispersion layer. The short-chain silver carboxylic acid, silver lactate, silver citrate, or silver ethanolamine complex in the shell can decompose and release active silver when heated. The active silver fills the gaps between nanoparticles and participates in conductive necking.

[0039] The shell thickness can be controlled by the amount of silver precursor added, the adsorption time, and the degree of washing. The adsorption time can be controlled between 0.5 and 4 hours, and the system temperature can be controlled between 20 and 50 degrees Celsius. If the shell thickness is less than 1 nm, the amount of silver used for storage isolation and bridging will be insufficient. If the shell thickness exceeds 5 nm, the risk of shrinkage and local accumulation during the drying stage will increase.

[0040] Ultrathin silver nanosheets can be obtained by further two-dimensional growth of silver core-shell nanoparticles after partial secondary growth. In the presence of polyvinylpyrrolidone or citrate, by controlling the silver source replenishment rate, reducing agent concentration and shear strength, silver can be grown in the sheet-like direction to obtain sheet-like bridging components with controlled thickness and lateral dimensions.

[0041] The amount of ultrathin silver nanosheets added is 0.5%-6.0% based on the total amount of silver solids. The ultrathin film layer, such as... Figure 7 As shown, spherical silver core-shell nanoparticles are used for filling and low-temperature sintering, while ultrathin silver nanosheets are used for planar bridging. The combination of the two can reduce the probability of breakpoints appearing in simple spherical particles under low film thickness conditions.

[0042] The interface stationary phase precursor should preferably exist in the form of a sol or nano-precursor, and coarse glass powder should be avoided from being directly mixed in. During the slurry drying process, the precursor will migrate with the solvent, forming discontinuous fixing points at the substrate interface and line edges after sintering, making it difficult to form a complete coating layer. Specifically, as shown below... Figure 8 As shown.

[0043] like Figure 9 As shown, when compounding the slurry, the silver core-shell nanoparticles and some organic carriers can be pre-wetted with low shear first, then the homologous reducible silver precursor and ultrathin silver nanosheets can be added, followed by the addition of the interfacial stationary phase precursor liquid and rheology modifier, and finally three-roll milling and vacuum degassing are performed to avoid the breakage of nanosheets caused by long-term high shear.

[0044] The inner walls of containers, grinding rollers, filters, and storage containers that come into contact with the silver paste should preferably be made of polytetrafluoroethylene, zirconium oxide, alumina, silver-coated stainless steel, or inert ceramic materials to prevent iron, nickel, copper, and other ions from entering the silver paste and affecting storage stability and the sintering process.

[0045] Low-temperature sintering employs a segmented thermal activation method. The 60-90℃ pre-drying stage is primarily used for slow solvent escape and edge shaping; the 110-160℃ stage is mainly used for the decomposition of homologous reducible silver precursors and the formation of silver bridges; and the 180-260℃ stage is mainly used for necking connections between spherical silver core-shell nanoparticles and ultrathin silver nanosheets. Figure 10 As shown.

[0046] like Figure 11As shown, for flexible substrates with low temperature resistance such as polyimide or PET, pulsed light sintering or near-infrared rapid sintering can be combined after heat treatment at 120-160℃ to first transform the silver precursor shell into a silver bridge, and then promote the necking of the nano-silver surface through instantaneous energy input. The energy density should be controlled according to the heat distortion temperature of the substrate during the treatment.

[0047] The nano-silver paste prepared by this invention can be used for screen printing, gravure printing, microgravure coating, inkjet printing, or blade coating. The wet film thickness can be adjusted according to the target dry film thickness and silver solid content. The silver film thickness after drying can be controlled to be 0.3-5.0 μm, preferably 0.5-2.0 μm. Figure 3 As shown.

[0048] To facilitate industrial scale-up, electronic-grade or analytical-grade silver nitrate should be selected as the silver salt raw material, and the conductivity of deionized water should not exceed 2 μS / cm. Ethanolamine can be dehydrated by molecular sieve or degassed under reduced pressure before use to reduce the disturbance of free water and dissolved oxygen on the nucleation rate.

[0049] When preparing the silver amine complex solution, first completely dissolve silver nitrate in deionized water, then slowly add ethanolamine and control the system temperature below 30°C. If the local temperature rises rapidly, an ice-water bath or jacket cooling can be used to avoid local reduction or color change in the complex system.

[0050] The reducing agent solution should be prepared before use. Ascorbic acid and glucose can be dissolved in deionized water or a water-alcohol mixture. The dripping pipe should be made of polytetrafluoroethylene or inert plastic. The dripping port should extend into the main circulation zone formed by stirring to avoid local accumulation of the reducing agent on the liquid surface.

[0051] After low-temperature nucleation is completed, the color, sedimentation rate and particle size distribution of the mother nucleus dispersion can be observed by centrifugation or membrane filtration. This inspection is used to determine whether there are coarse particles or flocs. If significant sedimentation occurs, the addition rate of the interface regulator can be reduced or the stirring cycle intensity can be increased.

[0052] During the secondary silver shell growth process, the silver amine complexing solution and reducing solution should be added simultaneously through two channels. The dropping rate should be adjusted according to the volume of the reactor and the target particle size. The temperature fluctuation of the reaction system should be controlled near the set value to avoid silver from forming independent nucleation in the solution and increasing the fine powder tail end.

[0053] The size of the nano-silver particles after secondary growth can be confirmed by laser particle size analyzer, transmission electron microscope or centrifugal sedimentation method. The purpose of the test is to check the D50 and D90 range. If D90 is too large, washing and grading can be increased or the subsequent grinding load can be reduced. If D50 is too low, the proportion of silver source in secondary growth can be appropriately increased.

[0054] The conductivity of the washing liquid can be used as a process indicator for removing free salt. In actual production, the conductivity of the supernatant of the last wash can be compared with the conductivity of the blank solvent. When the difference decreases steadily, the phase inversion step is initiated to avoid residual salt affecting the continuous formation of silver bridges during sintering.

[0055] During the phase inversion process, a small amount of compatible co-solvent can be used to gradually replace the aqueous phase. First reduce the water content and then add the main solvent. After the phase inversion is completed, the water content in the slurry precursor dispersion should be controlled to prevent premature decomposition of short-chain silver carboxylic acid or silver ethanolamine complex.

[0056] The formation of the homologous reducible silver precursor shell can be achieved by controlling pH, precursor concentration and adsorption time. After adsorption, the unadsorbed free precursor can be removed by washing 1-2 times with the same main solvent, thereby reducing spontaneous crystallization during slurry storage.

[0057] Two-dimensional growth of ultrathin silver nanosheets can be carried out at a low silver source acceleration rate. Polyvinylpyrrolidone or citrate is used in the system to limit vertical thickening. After the reaction, strong ultrasonic treatment for a long time should be avoided to prevent the nanosheets from breaking into irregular fragments.

[0058] The sol particle size in the interface stationary phase precursor solution should be kept at the nanoscale and filtered before adding silver paste. The filtration pore size can be selected from 0.2 to 1.0 μm according to the stability of the sol. The purpose of filtration is to remove gel particles, rather than to change the solid content of the precursor solution.

[0059] The solvent system of the organic carrier can be adjusted according to the printing method. For screen printing, terpineol and glycol ether solvents with slower evaporation rates are preferred. For inkjet printing, low-viscosity acetate solvents can be appropriately increased. For squeegee or dispensing, the ratio of resin and thixotropic modifier can be increased.

[0060] Rheology modifiers can be cellulose derivatives, polyacrylate auxiliaries, or polyamide wax auxiliaries. When selecting them, attention should be paid to their thermal decomposition residues and compatibility with the silver precursor shell to avoid the formation of organic residues that are difficult to be discharged during the thermal activation stage.

[0061] When using three-roll milling, a larger gap should be used to complete the pre-dispersion first, and then the gap should be gradually reduced. After a single pass of milling, large particles can be checked with a scraper fineness gauge. If the fineness has reached the target range, it is not advisable to continue excessive milling to avoid damaging the planar structure of the ultrathin silver nanosheets.

[0062] Nano silver paste can be stored in light-proof sealed containers at a temperature of 5-25℃. Before use, it should be stirred at low speed or rolled to re-mix. High-power ultrasonic treatment should not be used to directly process the finished paste to prevent the silver precursor shell from being heated or partially desorbed.

[0063] Before printing, plasma cleaning, ultraviolet ozone treatment, or solvent wiping can be performed on the substrate surface. The purpose of the treatment is to remove surface oil and adsorbed water, and enhance the migration and adhesion of the interfacial stationary phase precursor liquid at the bottom of the film layer.

[0064] For patterns with a line width of less than 20 μm, the wet film thickness should not be too high. During the pre-drying stage, slow heating or multi-stage low-temperature insulation should be used to prevent coffee ring-like accumulation at the edges of the lines due to rapid solvent evaporation.

[0065] For large-area thin film coating, pre-drying can be divided into two stages: low-speed surface drying and high-temperature slow drying. Low-speed surface drying is used to maintain the uniformity of the line and surface, while slow drying is used to remove the internal solvent, so that the silver bridges formed in the subsequent thermal activation stage are more evenly distributed.

[0066] like Figure 12 As shown, the sintered silver film can be evaluated by four-probe sheet resistance test, microscopic morphology observation, tape peeling inspection, and resistance change inspection after bending. The above evaluation items are used to confirm the conductive continuity and adhesion state of the film layer and are not intended to limit the scope of the claims of this invention.

[0067] When it is necessary to prepare a conductive interconnect layer for chip interconnection, the proportion of solid silver components can be increased and the proportion of organic carriers can be reduced. At the same time, the proportion of homologous reducible silver precursors can be appropriately increased so that more migratable silver can be generated during the low-temperature thermal activation stage to fill the interface.

[0068] When it is necessary to prepare silver paste for transparent conductive peripheral fine lines, the thickness of the silver film can be reduced and the planar orientation stability of the ultrathin silver nanosheets can be improved. After printing, a milder pre-drying condition is used so that the nanosheets can play a bridging role in the film plane.

[0069] In mass production equipment, the amount of interface regulator added, the amount of silver source added, the amount of precursor shell adsorption, and the amount of discontinuous stationary phase added can be recorded as batch records. The records should be based on the actual weighing and the results of the online flow meter, avoiding the use of subjective human judgment.

[0070] The process route of this invention can also be used in conjunction with a nitrogen-protected drying oven, roll-to-roll printing equipment, or pulsed light sintering equipment. The differences in equipment mainly affect the coating speed, heating curve, and energy input method, without changing the silver particle structure and silver bridge formation mechanism.

[0071] Before the parent nucleus dispersion enters the secondary growth stage, a short aging step can be set. The aging temperature can be 5-20℃ and the time can be 10-60min. Aging is used to stabilize the adsorption state of the interface control sites and the silver-based parent nucleus surface, so as to avoid local aggregation when the silver source is added later.

[0072] If lanthanum nitrate is used as a rare earth salt, it is advisable to prepare a low-concentration aqueous solution first and then add the silver amine complex solution. If bismuth citrate is used, it is advisable to first form a dispersible complex with a small amount of ethanolamine or citrate, and then mix it with the silver amine complex solution to reduce the hydrolysis and precipitation of bismuth salt.

[0073] Alkali-free borosilicate sol should not be added under strong acid conditions. After addition, observe whether milky white flocculent matter appears in the system. If flocculent matter appears, it indicates that the sol is not stable enough. This can be adjusted by reducing the solid content, changing the order of addition, or increasing the proportion of complexing agent.

[0074] After the secondary silver shell growth is completed, the dispersion can be kept at 20-30℃ and stirred for 20-60 minutes to allow the reduction reaction on the particle surface to end. Then, washing and separation can be performed to reduce the precipitation of unreacted silver complexes during subsequent phase inversion.

[0075] Silver core-shell nanoparticles should not be completely dried into powder before phase inversion, as complete drying will increase the risk of hard agglomeration. The better approach is to introduce them into the organic phase replacement step with a wet filter cake or concentrated dispersion, so that the particles are always in a liquid phase isolated state.

[0076] The silver source of the shell of the homologous reducible silver precursor is the same silver element as the main silver particles. Therefore, no new non-conductive framework will be introduced after heat treatment. The organic ligands of the shell should be short-chain ligands that can volatilize or decompose within the target heat treatment temperature range.

[0077] In low-temperature flexible substrate scenarios, the silver precursor shell should not be too thick, and the convertible silver mass can be controlled at 1.0%-4.0%. In high-temperature substrate or interconnect filling scenarios, the convertible silver mass can be controlled at 4.0%-8.0% to meet different bridging silver requirements.

[0078] The thickness and lateral dimensions of ultrathin silver nanosheets can be confirmed by sampling using atomic force microscopy, transmission electron microscopy, or scanning electron microscopy. Sampling inspection is used to determine whether the two-dimensional bridging components meet the film thickness requirements. If the sheet diameter is too large, it can be graded by short-time low-energy shearing.

[0079] When adding silver nanosheets to a slurry, it is advisable to pre-disperse them in a small amount of organic carrier before mixing them with the dispersion of spherical silver core-shell nanoparticles. It is not advisable to directly add the dried silver nanosheets to the high-viscosity slurry to avoid local accumulation of the sheet-like components.

[0080] The discontinuous state of the interfacial stationary phase can be achieved by controlling the amount of precursor liquid added and the solvent migration rate. If the amount added is too low, there will be insufficient interfacial stationary points. If the amount added is too high, the stationary phase may form a local insulating region on the surface of the silver particles. Therefore, it should be kept within the range of 0.02%-0.20%.

[0081] When the substrate is glass or ceramic, the proportion of borosilicate component in the interface stationary phase precursor solution can be appropriately increased. When the substrate is polyimide or PET, the inorganic solid content and the pre-drying temperature can be appropriately reduced to reduce the surface stress of the flexible substrate.

[0082] The thixotropic index of nano-silver paste can be characterized by the ratio of low-shear to high-shear viscosity; in practical control, a value of 1s can be selected. -1 and 10s -1 The viscosity at shear rate is compared to allow the paste to flow under the action of the printing blade and maintain the pattern boundary after leaving the blade.

[0083] If inkjet printing is used, the slurry solids content and particle size need to be further reduced, which can be achieved by diluting the organic carrier and improving the filtration accuracy. However, the homologous reducible silver precursor shell and a small amount of ultrathin silver nanosheets should still be retained to ensure low-temperature silver bridges and in-plane connections.

[0084] If screen printing is used, the paste should have high thixotropy and low stringing tendency, which can be achieved by adjusting the ratio of cellulose derivatives to acrylic resin. At the same time, the number of large particles after three-roll milling should be kept under control to avoid screen clogging and burrs on the line edges.

[0085] If gravure or microgravure coating is used, solvent evaporation rate and cell release are the main control factors. A combination of glycol ethers and ester solvents can be used to keep the slurry wet during the transfer of the coating roller and to set it on the substrate surface in time.

[0086] Segmented heat treatment can be carried out in an air, nitrogen, or inert gas environment. An air environment is conducive to the oxidation and removal of some organic components, while a nitrogen or inert gas environment is conducive to reducing the oxidation of silver particle surface and thermal oxidation of substrate. The specific atmosphere is determined according to the substrate and equipment.

[0087] The heating rate can be selected according to the film thickness. Thick films can use a lower heating rate to facilitate solvent discharge, while thin films can use a faster heating rate to shorten the process time. However, it is not advisable to skip the thermal activation stage and directly enter the high-temperature sintering stage.

[0088] Sudden cooling should be avoided during the cooling stage after sintering, especially for continuous fine lines on glass and ceramic substrates. Slow cooling helps reduce thermal stress concentration. Flexible substrates can be removed from the stage after the temperature drops below 60°C.

[0089] If the viscosity of the finished silver paste increases after long-term storage, it can be first rolled at low speed in a sealed container to re-mix and test the fineness. If the fineness does not change significantly, the application viscosity can be adjusted with a small amount of compatible solvent. If the fineness increases, it should be determined whether hard agglomeration has occurred and the paste should be filtered or discarded.

[0090] The mass percentage and temperature range in this invention can be slightly adjusted according to the equipment volume, heat transfer efficiency and target film thickness, but the sequential relationship between each step should remain unchanged, that is, the core interface regulation precedes the secondary silver shell growth, the silver precursor shell layer precedes the slurry compounding, and the segmented thermal activation precedes the final sintering.

[0091] The silver solid components in this invention can be prepared into concentrated dispersions before compounding. The solid content of the concentrated dispersions should be higher than the solid content of the corresponding components in the finished slurry. When using, they should be added according to the silver content to facilitate batch weighing and reduce contamination caused by dry powder transfer.

[0092] To avoid changes in printing state caused by differences in the particle size of silver core-shell nanoparticles from different batches, the solid content of the silver core-shell nanoparticle dispersion can be determined before compounding. The solid content can be determined by weighing and drying, and the drying temperature should be lower than the temperature that causes significant decomposition of the silver precursor.

[0093] When nano silver paste is used for narrow linewidth patterns, vacuum standing degassing can be performed after compounding. The degassing time can be 5-30 minutes, and the vacuum degree can be adjusted according to the viscosity of the paste. Vigorous shaking should not be performed after degassing to avoid reintroducing air bubbles and affecting the continuity of the lines.

[0094] If the substrate surface contains an organic coating, the solvent resistance and heat resistance of the coating should be confirmed first. Before coating, a small area test print can be used to observe the wetting boundary. If pinholes appear, the solvent polarity can be adjusted or a small amount of wetting agent can be added.

[0095] For multilayer circuit fabrication scenarios, after the previous silver film is sintered, the surface can be cleaned and the next layer can be printed. The subsequent heat treatment temperature should not exceed the tolerance range of the previous substrate and insulating layer. At the same time, the silver precursor shell should be able to complete the secondary bridging.

[0096] If the silver film needs to be bonded to solder or conductive adhesive, a low level of organic residue can be maintained after the final sintering to avoid interface residues hindering subsequent bonding. Specifically, the removal status of the organic carrier can be confirmed by thermogravimetric analysis or infrared detection.

[0097] This invention does not require the use of specific brands of reactors, centrifuges, three-roll mills or sintering furnaces. As long as the equipment can provide the required temperature, stirring, grinding, degassing and heat treatment conditions, the same process logic can be achieved.

[0098] In actual production, if the sintering temperature is limited due to the type of substrate, it can be compensated by increasing the proportion of homologous reducible silver precursor, extending the thermal activation time, or introducing pulsed light-assisted sintering. However, it is important to avoid simply increasing the content of the interfacial stationary phase to replace the formation of silver bridges.

[0099] Thus, the nano-silver paste forms a continuous process chain from silver particle structure, shell transformation, sheet bridging, interface fixation and segmented heat treatment, which facilitates parameter migration between different substrates and different printing methods.

[0100] Example 1 This embodiment provides a process for preparing ultrafine and ultrathin nano-silver paste suitable for micro-circuits on polyimide substrates. First, silver nitrate is dissolved in a mixture of deionized water and ethanolamine, and the silver ion concentration is controlled at 0.12 mol / L and the pH is 10.4. Lanthanum nitrate, bismuth citrate and alkali-free borosilicate sol are added as interface regulators to make the mass fraction of non-silver inorganic elements relative to silver 0.35%.

[0101] The above system was cooled to 8°C, and ascorbic acid aqueous solution was added dropwise under stirring for 20 min. After reacting for another 30 min, a silver-based core dispersion was obtained. The particle size of the silver-based core in the dispersion was detected by dynamic light scattering, and the particle size distribution was controlled to be concentrated in the range of 3-12 nm.

[0102] Silver amine complex solution was added to the silver-based core dispersion. The mass of silver ions added was 12 times the mass of silver in the silver-based core. The system was heated to 32°C and glucose reducing solution was added dropwise to allow silver to grow a secondary silver shell on the surface of the core. After the reaction was completed, silver core-shell nanoparticles with a D50 of about 28 nm and a D90 of no more than 80 nm were obtained.

[0103] The silver core-shell nanoparticles were centrifuged and washed at 6000 rpm for 10 min each time. After repeating this process three times, they were transferred to a mixed solvent of ethylene glycol butyl ether and terpineol. Silver lactate and silver ethanolamine complex were added to control the shell thickness of the homologous reducible silver precursor to 2-3 nm.

[0104] Another portion of the silver core-shell nanoparticles were used as the silver source and subjected to two-dimensional growth in the presence of polyvinylpyrrolidone to obtain ultrathin silver nanosheets with a thickness of 8-18 nm and a lateral dimension of 120-350 nm. The surface of these nanosheets was then treated with the same homologous reducible silver precursor shell.

[0105] Silver core-shell nanoparticles, ultrathin silver nanosheets, terpineol, ethylene glycol butyl ether, acrylic resin, cellulose derivatives, interfacial stationary phase precursor solution and thixotropic modifier were mixed. The mass fraction of silver solid component was controlled at 82%, the amount of ultrathin silver nanosheets added was 2.5% of the total silver solid, and the content of interfacial stationary phase precursor solid was 0.08% of the silver solid mass.

[0106] The mixture was subjected to vacuum planetary stirring for 20 minutes, low-shear pre-wetting for 30 minutes, three-roll milling three times, and centrifugation for degassing. The milling rollers were made of zirconium oxide, and the filter element was made of polytetrafluoroethylene. The mixture was then subjected to centrifugation at 25°C for 10 seconds.-1 The shear rate corresponds to the viscosity of the nano-silver paste with a viscosity of 120-180 Pa·s.

[0107] Nano-silver paste was screen-printed onto a polyimide substrate. The wet film was pre-dried at 80°C for 5 min, then kept at 140°C for 10 min to convert the homologous reducible silver precursor and form silver bridges between particles. Finally, it was kept at 220°C for 20 min to obtain an ultrathin conductive silver film with a thickness of about 1.2 μm.

[0108] Example 2 This embodiment provides a process for preparing ultrafine and ultrathin nano-silver paste suitable for transparent peripheral electrodes on glass substrates. Unlike Example 1, the silver ion concentration is controlled at 0.20 mol / L, the pH is controlled at 10.8, the interface regulator is lanthanum nitrate and alkali-free borosilicate sol, and the mass fraction of non-silver inorganic elements relative to silver is 0.25%.

[0109] The nucleation temperature was controlled at 12℃, and the reducing agent was a mixed reducing system of ascorbic acid and glucose. The dropping time was 30 min. After obtaining the silver-based nucleus dispersion, the mass of silver ions added was 20 times the mass of silver in the silver-based nucleus. The secondary silver shell growth temperature was controlled at 38℃.

[0110] The silver core-shell nanoparticles after secondary growth were washed by ceramic membrane filtration and then transferred to a mixed solvent of diethylene glycol monobutyl ether and terpineol. Silver citrate and short-chain silver carboxylic acid were added to make the mass of convertible silver in the silver precursor shell 4.0% of the mass of the silver core-shell nanoparticles.

[0111] Ultrathin silver nanosheets were prepared by wet shearing and peeling. The sheet-like silver precursor was mildly sheared in the presence of ethanolamine and citrate to obtain silver nanosheets with a thickness of no more than 30 nm and a lateral size of 150-500 nm. The amount added was 1.5% of the total amount of silver solids.

[0112] The interface stationary phase precursor solution is prepared from bismuth borate sol and rare earth oxide sol, with a solid content controlled at 4%. After adding silver paste, the non-silver inorganic solid content is 0.05% of the silver solid mass. This content is used to form discontinuous stationary points instead of a continuous insulating phase.

[0113] When the paste is compounded, the mass fraction of silver solids is controlled at 76%, the organic carrier is terpineol, propylene glycol methyl ether acetate and polycarbonate resin, and the amount of rheology modifier added is 0.6%. After planetary stirring, three-roll milling and vacuum degassing, a nano silver paste suitable for fine line printing is obtained.

[0114] Nano-silver paste is coated onto a glass substrate using a microgravure coating method. It is first pre-dried at 70°C for 8 minutes, then thermally activated at 150°C for 12 minutes, and then sintered at 240°C for 15 minutes to form a conductive silver film with a thickness of approximately 0.8 μm. The heat treatment process can be adjusted according to the thickness of the glass substrate and the line width.

[0115] Example 3 This embodiment provides a process for preparing ultrafine and ultrathin nano-silver paste suitable for low-temperature circuits on PET substrates. The concentration of silver ions in the silver amine complex solution is controlled at 0.08 mol / L, the pH is controlled at 9.8, the interface regulator is bismuth citrate and alkali-free borosilicate sol, and the mass fraction of non-silver inorganic elements relative to silver is 0.18%.

[0116] A weak reducing system consisting of glucose and trace amounts of borohydride was added dropwise at 5℃ to nucleate silver ions, resulting in a silver-based nucleus dispersion with a particle size of 3-10 nm. Subsequently, a silver amine complex solution was added to make the mass of added silver ions 8 times the mass of silver in the nucleus, and a secondary silver shell growth was carried out at 25℃.

[0117] After secondary growth, the silver core-shell nanoparticles were centrifuged and washed, and then transferred to a mixed solvent of propylene glycol methyl ether acetate and ethylene glycol butyl ether. Ethanolamine silver complex was added to make the shell thickness of the homologous reducible silver precursor 1-2 nm, so as to reduce volatilization and shrinkage during low-temperature treatment.

[0118] The amount of ultrathin silver nanosheets added is controlled to be 3.0% of the total amount of silver solids. The surface of the silver nanosheets is treated with a homologous reducible silver precursor to make it have an organic silver complex interface similar to that of spherical silver core-shell nanoparticles, thereby reducing compatibility differences during the compounding stage.

[0119] In the nano-silver paste formulation, the mass fraction of silver solid component is 72%, the mass fraction of organic carrier is 22%, the mass fraction of homologous reducible silver precursor is 4.5%, the mass fraction of interfacial stationary phase precursor solid relative to silver solid is 0.03%, and the mass fraction of rheology modifier is 1.0%.

[0120] The compounding process involves premixing in a polytetrafluoroethylene-lined container, followed by three-roll milling with an alumina ceramic grinding roller twice, and then filtration with an inert ceramic filter. The final fineness is controlled to be no more than 1.5 μm, and the mixture is degassed under vacuum for 15 min.

[0121] Nano-silver paste was coated onto a PET substrate by a blade coating method. It was first pre-dried at 65°C for 6 min, and then heat-treated at 130°C for 15 min to transform the silver precursor shell into a silver bridge. Subsequently, a short-time treatment was performed by pulsed light sintering to obtain a low-temperature conductive silver film with a thickness of about 1.5 μm.

[0122] Example 4 This embodiment provides a process for preparing ultrafine and ultrathin nano-silver paste suitable for low-temperature interconnection of ceramic substrates. The silver ion concentration is controlled at 0.28 mol / L, the pH is controlled at 11.2, and the interface regulators are lanthanum nitrate, bismuth citrate and zinc borate sol. The mass fraction of non-silver inorganic elements relative to silver is 0.50%.

[0123] The nucleation stage temperature was controlled at 15℃, and a silver-based core dispersion was obtained using an ascorbic acid reduction system. During the secondary silver shell growth, the mass of silver ions added was 25 times the mass of silver in the silver-based core, and the reaction temperature was controlled at 42℃, resulting in silver core-shell nanoparticles with a D50 in the range of 35-45nm.

[0124] Silver core-shell nanoparticles were washed and transferred into a system of terpineol and diethylene glycol monobutyl ether. Short-chain silver carboxylate and silver citrate were added, and the mass of convertible silver in the silver precursor shell was controlled to be 7.0% of the mass of the silver core-shell nanoparticles. This was used to increase the amount of silver bridges formed in thicker wet films on ceramic substrates.

[0125] Ultrathin silver nanosheets were prepared by two-dimensional growth, with a thickness controlled at 15-25 nm and a lateral dimension controlled at 200-600 nm. The amount added was 4.0% of the total amount of silver solids. Together with spherical silver nanoparticles, they formed a silver solid component that combined planar bridging and vertical necking.

[0126] The interface stationary phase precursor solution was prepared from alkali-free borosilicate sol and zinc borosilicate sol, with a solid content of 8%. The amount added was controlled to be 0.15% of the mass of silver solid. After compounding, it was subjected to vacuum planetary stirring, low-shear pre-wetting, three-roll milling and degassing to obtain a nano silver paste suitable for dispensing or coating.

[0127] Nano-silver paste is coated onto a ceramic substrate, pre-dried at 90°C for 8 minutes, thermally activated at 160°C for 15 minutes, and sintered at 260°C for 30 minutes to form a conductive silver film with a thickness of approximately 3.0 μm. The heat treatment temperature in this embodiment is relatively high and is mainly used for heat-resistant substrates such as ceramic substrates or silicon wafers.

[0128] Example 5 This embodiment is used to illustrate the coordination relationship between the interface regulator and the secondary silver shell growth. First, it is calculated that the silver in the silver-based core is 1.0 kg. Then, 3.0 g of non-silver inorganic elements in the interface regulator are added. If the silver added for the secondary silver shell growth is 5.0 kg, the final total silver amount is 6.0 kg. The mass fraction of non-silver inorganic elements relative to silver is 0.05%.

[0129] If 20.0 kg of silver is added under the same parent nucleus conditions, the final total amount of silver will be 21.0 kg. The mass fraction of non-silver inorganic elements relative to silver is approximately 0.014%. This value shows that secondary silver shell growth not only determines the particle size increase, but also the dilution level and final distribution state of the interface-regulated components.

[0130] The above calculations are used for material feeding verification and quality balance during process scale-up. They do not involve subjective scoring. In actual production, the amount of silver to be added should also be determined in conjunction with particle size detection, electrical conductivity path formation, sintering shrinkage state, and substrate adhesion requirements.

[0131] Example 6 This embodiment is used to illustrate the process window for segmented thermal activation. When the temperature is too low during the pre-drying stage, the solvent escapes slowly and the edge is not sufficiently shaped. When the temperature is too high, the surface layer dries first and forms a shell structure. Therefore, it is advisable to select the temperature within the range of 60-90℃ based on the solvent boiling point and wet film thickness.

[0132] The optimal temperature for the thermal activation stage is 110-160℃. This stage allows the homologous reducible silver precursor to gradually decompose and release active silver. If the temperature is directly raised to the sintering temperature, the organic components and solvents in the shell may evaporate rapidly, leading to micropores or edge accumulation.

[0133] The temperature for the low-temperature sintering stage should be 180-260℃. For heat-resistant substrates, higher temperatures and longer holding times can be used, while for flexible substrates, lower temperatures can be used in conjunction with pulsed light or near-infrared rapid sintering to allow silver bridges to form first and then promote the necking of particles on the surface.

[0134] Example 7 In continuous preparation scenarios, silver-based nucleus preparation vessel, secondary growth vessel, washing and phase inversion device, precursor shell adsorption tank, slurry compounding equipment and segmented heat treatment equipment can be connected in process sequence, and pH, temperature, dropping amount, stirring speed, viscosity and fineness can be recorded online.

[0135] When the slurry fineness is detected to be greater than 3μm, one more pass of three-roll milling can be added or the pre-wetting time can be adjusted. At 25℃, 10 seconds is recommended. -1 When the shear rate corresponds to a viscosity below 50 Pa·s, the proportion of resin or thixotropic modifier can be appropriately increased. When the viscosity is above 300 Pa·s, the compatibility solvent can be increased or the mass fraction of silver solids can be reduced.

[0136] The process record can also retain samples for inspection of silver film thickness, linewidth, drying shrinkage morphology and substrate adhesion. The inspection results are used for fine-tuning of the formulation in the next batch, but do not change the technical route of this invention for preparing nano-silver paste through core interface regulation, secondary silver shell dilution, homologous silver bridge formation and discontinuous interface fixation.

[0137] The above embodiments are only used to illustrate the process implementation of the present invention. Those skilled in the art can make adaptations and adjustments to the type of reducing agent, the type of silver precursor, the solvent system, the printing method, and the sintering equipment without departing from the principle of the present invention. These adjustments should still fall within the protection scope defined by the claims of the present invention.

Claims

1. A process for preparing ultrafine and ultrathin nano-silver paste, characterized in that, Includes the following steps: S1. An interface regulator is added to the silver salt complexation system and reduced to nucleation to obtain a silver-based nucleus dispersion containing interface regulation sites. S2. Add silver salt complex solution to the silver-based core dispersion and perform secondary silver shell growth to obtain silver core-shell nanoparticles; S3. Construct a homologous reducible silver precursor shell layer on the surface of the silver core-shell nanoparticles; S4. The surface-treated ultrathin silver nanosheets are combined with the silver core-shell nanoparticles, organic carrier and interface stationary phase precursor liquid, and then ground and degassed to obtain nano silver paste. S5. The nano-silver paste is coated onto the substrate and then subjected to segmented thermal activation and sintering to form a conductive silver film.

2. The preparation process of ultrafine and ultrathin nano-silver paste according to claim 1, characterized in that, The process of adding an interface regulator to a silver salt complexing system and reducing it to a nucleus to obtain a silver-based core dispersion containing interface regulation sites includes: Silver nitrate was added to a mixture of deionized water and ethanolamine to form a silver amine complex solution, with the silver ion concentration controlled at 0.05-0.30 mol / L and the system pH at 9.5-11.

5. At least one of rare earth salts, bismuth salts, or alkali-free borate precursors is added to the silver amine complex solution to make the mass fraction of non-silver inorganic elements in the interface modifier relative to silver 0.10%-0.60%; A reduction system consisting of ascorbic acid, glucose, ethylene glycol, or borohydride is added dropwise at 5-15℃ to nucleate silver ions and obtain a silver-based nucleus dispersion with a particle size of 3-12 nm.

3. The preparation process of ultrafine and ultrathin nano-silver paste according to claim 2, characterized in that, Adding silver salt complex solution to the silver-based core dispersion and performing secondary silver shell growth includes: Add silver amine complex solution to the silver-based core dispersion, such that the mass of added silver ions is 5-30 times the mass of silver in the silver-based core; Add ascorbic acid or glucose reducing solution dropwise at 20-45℃ to deposit the newly generated silver on the surface of the silver-based parent nucleus to form a secondary silver shell; Through the secondary silver shell growth, the mass fraction of non-silver interface regulating components in the final silver core-shell nanoparticles is reduced to 0.005%-0.08%, and the non-silver interface regulating components are distributed at the nucleation interface, local sites on the particle surface, and sintering migration interface.

4. The preparation process of ultrafine and ultrathin nano-silver paste according to claim 3, characterized in that, Constructing a homologous reducible silver precursor shell layer on the surface of the silver core-shell nanoparticles includes: The silver core-shell nanoparticles after secondary silver shell growth are centrifuged and washed or filtered through a membrane to remove free salts, residual reducing agents and small molecule byproducts. The washed silver core-shell nanoparticles were transferred to ethylene glycol butyl ether, terpineol, diethylene glycol monobutyl ether, propylene glycol methyl ether acetate or a mixture thereof. During the phase inversion process, at least one of short-chain silver carboxylic acid, silver lactate, silver citrate, or silver ethanolamine complex is added to adsorb the silver precursor onto the surface of the silver core-shell nanoparticles.

5. The preparation process of ultrafine and ultrathin nano-silver paste according to claim 4, characterized in that, The process of adsorbing the silver precursor onto the surface of silver core-shell nanoparticles includes: The thickness of the shell layer of the homologous reducible silver precursor is controlled to be 1-5 nm, and the mass of convertible silver in the shell layer of the homologous reducible silver precursor is 1.0%-8.0% of the mass of the silver core-shell nanoparticles; During the storage and coating stage of the nano-silver paste, the homologous reducible silver precursor shell layer isolates adjacent silver core-shell nanoparticles. During the segmented thermal activation stage, the shell of the homologous reducible silver precursor is decomposed to release active silver and form silver bridges between particles.

6. The process for preparing ultrafine and ultrathin nano-silver paste according to any one of claims 1-5, characterized in that, Before compounding the surface-treated ultrathin silver nanosheets with the silver core-shell nanoparticles, the organic carrier, and the interfacial stationary phase precursor solution, the process further includes: Using some of the silver core-shell nanoparticles grown from secondary silver shells as a silver source, two-dimensional growth is carried out in the presence of polyvinylpyrrolidone, citrate, or ethanolamine to obtain ultrathin silver nanosheets with a thickness of 5-25 nm and a lateral dimension of 80-600 nm. Alternatively, the sheet-like silver precursor is wet-sheared and thinned to obtain silver nanosheets with a thickness of no more than 30 nm.

7. The process for preparing ultrafine and ultrathin nano-silver paste according to claim 6, characterized in that, The surface-treated ultrathin silver nanosheets include The surface of the ultrathin silver nanosheets is treated with a homologous reducible silver precursor shell layer that is the same as or similar to the silver core-shell nanoparticles, so that the surface of the ultrathin silver nanosheets has an organic-silver complex interface. The surface-treated ultrathin silver nanosheets are added to the nanosilver paste at 0.5%-6.0% of the total amount of silver solids, so that they provide bridging pathways in the plane of the ultrathin film layer.

8. The preparation process of ultrafine and ultrathin nano-silver paste according to claim 7, characterized in that, The interface stationary phase precursor solution is prepared by mixing at least one of alkali-free borosilicate sol, bismuth borate sol, zinc borate sol or rare earth oxide sol with an organic solvent and adding a thermally degradable complexing agent. The solid content of the interface stationary phase precursor liquid is 1%-10%, and the non-silver inorganic solid content after adding nano-silver paste is 0.02%-0.20% of the silver solid mass. During the drying and sintering process, the interface stationary phase precursor liquid forms discontinuous fixing points at the interface between the film layer and the substrate, at the edge of the lines and near the micropores.

9. The process for preparing ultrafine and ultrathin nano-silver paste according to any one of claims 1-5, characterized in that, Surface-treated ultrathin silver nanosheets are compounded with silver core-shell nanoparticles, an organic carrier, and an interfacial stationary phase precursor solution, and then ground and degassed to obtain a nano-silver paste, comprising: By mass percentage, 70%-88% of silver solid component, 8%-25% of organic support, 1%-8% of homologous reducible silver precursor, 0.02%-0.20% of interfacial stationary phase precursor solid and 0.1%-2.0% of rheology modifier are mixed. Vacuum planetary mixing, low-shear pre-wetting, three-roll milling or centrifugal degassing are employed, and the parts in contact with the silver paste are made of polytetrafluoroethylene, zirconium oxide, alumina, silver-coated stainless steel or inert ceramic materials.

10. The preparation process of ultrafine and ultrathin nano-silver paste according to claim 9, characterized in that, The process involves coating the nano-silver paste onto a substrate and then subjecting it to segmented thermal activation and low-temperature sintering, including: Nano silver paste is applied to glass, polyimide, PET, ceramic, silicon wafers, or metal substrates coated with insulating layers by means of screen printing, gravure printing, microgravure coating, inkjet printing, or blade coating. The silver film was pre-dried at 60-90℃ for 1-10 min, thermally activated at 110-160℃ for 3-20 min, and sintered at 180-260℃ for 5-40 min to obtain an ultrathin conductive silver film with a thickness of 0.3-5.0 μm.

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