A low temperature sintering silver paste with no carbon residue eutectic solvent carrier

Abstract

This invention discloses a low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier. The paste contains a eutectic solvent as a dispersion medium, modified composite silver powder, and glass powder. This invention uses a eutectic solvent composed of a low-melting-point quaternary ammonium salt and a metal chloride, which is liquid at room temperature, replacing organic solvents such as terpineol and dibutylcarbidol. It is combined with ultrafine layered aluminosilicates to form an inorganic thixotropic system, eliminating the need for ethyl cellulose, organic dispersants, and organic thixotropic waxes. No high-molecular-weight carbon-forming components are introduced during the sintering process, and the carbon residue in the silver layer after sintering is less than 5 ppm.

Description

Technical Field

[0001] This invention belongs to the field of electronic conductive pastes, and particularly relates to a low-temperature sintering silver paste with a carbon-residue-free, low-eutectic solvent carrier. Background Technology

[0002] Traditional organic carrier-based silver pastes consist of silver powder, lead-free / lead-containing glass powder, and an organic carrier. After sintering, the pyrolysis of resins and other organic materials produces residual carbon, resulting in a silver film porosity of 8%–25%, high resistivity, and poor thermal conductivity. Simultaneously, the VOCs generated during resin decomposition during sintering not only pollute the environment and increase waste gas treatment processes, but the released gases can also easily cause film bulging and cracking, weakening adhesion to ceramic substrates such as aluminum nitride and alumina. During long-term high-temperature service, organic decomposition products may also induce electrochemical migration of silver, leading to device leakage failure. Summary of the Invention

[0003] The purpose of this invention is to provide a low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier, in order to solve the problems of high carbon residue, high sintering temperature, and insufficient stability and adhesion of traditional silver pastes.

[0004] The present invention adopts the following technical solution: a low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier, wherein the paste contains a eutectic solvent as a dispersion medium, modified composite silver powder, and glass powder.

[0005] The beneficial effects of this invention are:

[0006] This invention is 100% carbon-free and VOC-free; the formula does not contain heavy metals such as lead and cadmium.

[0007] This invention uses a low-melting-point quaternary ammonium salt combined with a metal chloride as a eutectic solvent, which is liquid at room temperature, to replace organic solvents such as terpineol and dibutylcarbidol; it is combined with ultrafine layered aluminosilicate to form an inorganic thixotropic system, which does not require ethyl cellulose, organic dispersants, or organic thixotropic waxes, and does not introduce high-molecular carbon-forming components during sintering. After sintering, the carbon residue in the silver layer is less than 5 ppm.

[0008] The present invention coats the surface of silver powder with nano-silver oxalate inorganic precursor, and the silver oxalate is heated to 420°C to decompose in situ to generate active nano-silver crystal nuclei, which catalyze the diffusion of micron-sized silver solid phase, reduce the sintering activation energy, and reduce the sintering temperature by more than 180°C compared with conventional inorganic silver paste.

[0009] When the glass powder of this invention melts, TiO2 nanocrystals precipitate in situ. TiO2 forms Ti-O and Ti-N covalent bonds with the surface of aluminum nitride / alumina ceramics, which improves adhesion. On this basis, Ba2SiO4 insulating microcrystals are dispersed inside the glass to block the migration channels of silver ions and improve resistance to electrochemical migration.

[0010] The nano CePO4 filler of this invention has a density between that of silver powder and inorganic liquid phase, thus constructing a suspension buffer layer and solving the problem of high-density silver powder sedimentation.

[0011] This invention employs a combination of spherical, flake-shaped, and submicron-sized silver powders to construct modified composite silver powder. The different particle sizes of silver powders are combined with each other. Small-sized silver powders can fill the gaps between large-sized silver powders, effectively improving the packing density of silver powders and reducing the porosity of the sintered silver layer, thereby improving conductivity. Meanwhile, flake-shaped silver powders can form a stable surface-to-surface contact structure, and submicron-sized silver powders fill the microscopic gaps. The two work synergistically to improve the rheological properties of the paste and enhance the overall printability of the paste.

[0012] This invention utilizes a lead-free glass powder system of B2O3-SiO2-BaO-TiO2-ZnO, where B2O3 and SiO2 form the basic glass network framework; BaO acts as a network modifier to lower the glass softening temperature and regulate the overall coefficient of thermal expansion; and TiO2 and ZnO work together to improve the glass crystallization characteristics and enhance the chemical stability of the glass phase after sintering. This glass composition exhibits suitable melt viscosity and spreading fluidity in the low-temperature sintering range, effectively wetting silver particles and the substrate surface. The low residual glass phase content after sintering is beneficial for preparing a low-resistance conductive silver layer. Compared to the Bi2O3-based glass system commonly used in existing low-temperature silver pastes, this invention is completely free of bismuth. Bismuth-based glasses suffer from high-temperature crystallization and poor thermal stability, which can lead to silver film resistivity drift and performance degradation with long-term use. Furthermore, this glass system is lead-free, meeting the environmental protection standards for electronic pastes.

[0013] Existing technologies generally employ organic carrier systems composed of organic solvents such as terpineol and butyl carbitol, combined with ethyl cellulose resin. These systems contain high-molecular-weight organic components. This invention, however, does not use organic components such as ethyl cellulose and terpineol, but instead uses a eutectic solvent as the sole dispersion medium. Since this solvent itself does not contain a high-molecular-weight carbon skeleton, no resin pyrolysis-related carbon residues are generated during sintering. Therefore, the carbon residue in the silver layer mainly originates from trace impurities introduced by the raw materials, rather than from carrier decomposition.

[0014] Existing technologies for achieving low-temperature sintering mostly rely on the physical sintering effect of added nano-silver powder. This invention utilizes a chemical in-situ catalytic sintering mechanism, pre-coating the surface of modified composite silver powder with a silver oxalate inorganic precursor layer. During the sintering stage, the precursor decomposes to generate highly active nano-silver crystal nuclei, which are deposited in situ on the surface of the silver powder. This effectively reduces the solid-phase diffusion activation energy of silver particles, promotes the formation of sintering necks and particle bridging, and achieves low-temperature dense sintering.

[0015] Existing technologies often suppress silver migration by physically doping with nanoparticles such as SiO2 and Al2O3. However, the added powders suffer from poor dispersion uniformity in the slurry, resulting in limited long-term reliability. This invention improves performance through in-situ modification of the glass components. During sintering, rutile TiO2 nanocrystals and Ba2SiO4 insulating microcrystals precipitate in situ. The TiO2 nanocrystals can covalently bond with the aluminum nitride and aluminum oxide substrate surfaces, enhancing the interfacial shear strength of the silver film. The Ba2SiO4 insulating microcrystals can effectively fill micropores, blocking silver ion electromigration and improving durability. Attached Figure Description

[0016] Figure 1 This is a process flow diagram of the present invention;

[0017] Figure 2 This is a sintering mechanism diagram of the in-situ silver oxalate coating of silver powder according to the present invention. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0019] This invention discloses a low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier. The paste contains a eutectic solvent as a dispersion medium, modified composite silver powder, and glass powder, such as... Figure 1 As shown.

[0020] Preferably, the eutectic solvent is prepared from choline chloride and zinc chloride in a ratio of 1:2.

[0021] Preferably, the eutectic solvent is prepared from choline chloride and aluminum chloride in a ratio of 1:0.2.

[0022] Preferably, the eutectic solvent is prepared by choline chloride and silver chloride in a ratio of 1:2.

[0023] Preferably, the modified composite silver powder is composed of 30-55 wt% spherical silver powder with a D50 of 2.5 μm, 25-40 wt% flake silver powder with a D50 of 0.8 μm, and 15-30 wt% submicron silver powder with a D50 of 0.3 μm.

[0024] By blending silver powders of different particle sizes in a gradient, small particles fill the gaps between large particles, resulting in dense packing of silver powder and reduced gaps between silver particles. After sintering, the voids in the silver are significantly reduced, and the conductivity of silver is higher.

[0025] Preferably, the modified composite silver powder is in situ coated with a 3-6 nm silver oxalate inorganic precursor coating layer, with the coating amount accounting for 1.5 wt% to 5.5 wt% of the total mass of the silver powder.

[0026] like Figure 2As shown, silver powder coated with silver oxalate begins to decompose at 190℃ and is completely decomposed at 240-250℃. At the same time, dense silver nanoparticles are generated on the surface of the silver powder particles. The silver nanoparticles melt above 350℃ and bridge adjacent silver powder particles to form a conductive path. As the temperature rises further, the surface of the silver powder particles begins to melt. These molten silver nanoparticles and the molten silver on the surface of the silver powder have a certain fluidity, filling some of the gaps formed by the accumulation of silver powder and reducing the residual porosity of the silver layer after sintering.

[0027] Preferably, the glass powder is composed of the following components by mass percentage: B2O3 32wt%, SiO2 28wt%, BaO 24wt%, TiO2 12wt%, and ZnO 4wt%.

[0028] Preferably, the titanium barium borosilicate lead-free microcrystalline glass powder has a D50 of 0.4~0.7μm and a softening point of 505℃. During the sintering process, rutile TiO2 and barium silicate (Ba2SiO4) microcrystals are precipitated in situ. In subsequent use, TiO2 can form Ti-O and Ti-N covalent bonds with the surface of aluminum nitride / alumina ceramic substrates, which greatly improves the adhesion between the sintered silver and the substrate. The insulating Ba2SiO4 microcrystals dispersed inside the glass block the migration channels of silver ions, improve the resistance to electrochemical migration, and help extend the service life of electronic devices.

[0029] Preferably, the slurry further includes layered magnesium aluminum silicate to induce thixotropy in the slurry and nano-CePO4 filler to inhibit the sedimentation of modified composite silver powder and glass powder. The layered magnesium aluminum silicate has a sheet thickness of 20-50 nm and a particle size of 1-3 μm, while the nano-CePO4 buffer filler has a D50 particle size of 20-60 nm.

[0030] The slurry consists of the following components by mass percentage: 8-16 wt% eutectic solvent, 5-14 wt% glass powder, and the remainder is modified composite silver powder.

[0031] Preferably, the slurry comprises, by mass percentage: 8-16 wt% eutectic solvent, 5-14 wt% glass powder, 1.2-3.5 wt% magnesium aluminum silicate, 0.8-2.6 wt% nano CePO4 filler, with the balance being modified composite silver powder.

[0032] Preparation method of the present invention:

[0033] Step 1: Preparation of modified composite silver powder

[0034] The composite silver powder was added to a deionized water-based inorganic suspension and stirred until homogeneous. Simultaneously, 0.5 mol / L silver nitrate solution and 0.4 mol / L ammonium oxalate aqueous solution were added dropwise, and the mixture was reacted at room temperature for 30 min. During the reaction, silver nitrate provided silver ions, and ammonium oxalate provided oxalate ions, resulting in a metathesis reaction to generate water-insoluble silver oxalate microcrystals, which were deposited on the surface of the composite silver powder to form a silver oxalate coating layer, achieving in-situ coating of the inorganic precursor. No organic surfactant was added during the coating process. The mixture was then filtered, washed with pure water until the filtrate was clear and free of impurities, and dried under vacuum at 120℃ to obtain the modified composite silver powder. No organic surfactant was added throughout the entire process.

[0035] Step 2: Preparation of eutectic solvent

[0036] Weigh out the quaternary ammonium salt and metal chloride separately by molar ratio, place them in a flask to complete the mixing of solid materials, and heat at 70-170℃ with continuous magnetic stirring. As the solid gradually melts and forms a clear and transparent liquid, the reaction is complete.

[0037] Step 3: Silver paste preparation (completely sealed and anhydrous)

[0038] Layered aluminosilicate and nano CePO4 buffer filler were added sequentially to a eutectic solvent and mechanically stirred for 30 min to form a uniform inorganic gel substrate. Modified composite silver powder and glass powder were added in three batches, and stirred for 25 min after each batch. The mixture was then ground with a three-roll mill until the fineness was less than 10 micrometers. Finally, it was filtered through a 200-mesh stainless steel filter to obtain the finished inorganic silver paste, which was then sealed and stored at low temperature.

[0039] The slurry sintering process of this invention:

[0040] Step 1: Screen printing on an alumina / aluminum nitride / LTCC green ceramic substrate, with a wet film thickness of 12~25μm;

[0041] Step 2: Stepwise heating sintering in air atmosphere, no nitrogen protection required, room temperature → 400℃ (heating 3℃ / min, slow evaporation of the eutectic solvent phase, no bulging); hold at 400℃ for 10min (silver oxalate coated on the surface of the composite silver powder is converted into carbon dioxide upon heating, with no volatile organic compound emissions, and active crystal nuclei that promote sintering are generated, precipitating in situ on the surface of the composite silver powder and in the interparticle spaces); 400℃ → 520℃ (heating 2℃ / min), hold at 520℃ for 20min (glass melting, solid-phase sintering and densification of silver particles); naturally cool to room temperature to complete metallization.

[0042] Step 3: Print a wet film with a thickness of 12~25μm; heat to 400℃ at 2~4℃ / min and hold for 8~12min; then heat to 500~540℃ at 1~3℃ / min and hold for 15~25min; allow to cool naturally to complete metallization; during the 400℃ holding stage, silver oxalate decomposes to generate nano-silver catalytic crystal nuclei, and in the 500~540℃ range, microcrystalline glass melts and combines with nano-silver to achieve dense sintering of the silver layer.

[0043] Example 1 (for Low Temperature Co-fired Ceramic Devices (LTCC))

[0044] Take 40g of spherical silver powder with a D50 of 2.5μm, 15g of flake silver powder with a D50 of 0.8μm, and 21.4g of submicron silver powder with a D50 of 0.3μm (total modified composite silver powder is 76.4g), and mix them with 11g of eutectic solvent, 8g of glass powder, 2.4g of aluminosilicate, and 2.2g of nano CePO4 filler.

[0045] The porosity of the prepared silver electrode layer was 1.8%; the porosity of conventional organic silver paste was 10%~22%; and the resistivity was 2.6 μΩ·cm.

[0046] After sintering, the interfacial shear strength between the silver electrode and the alumina substrate is 27 MPa, which is much higher than the existing silver paste of 10~18 MPa; the leakage current is only 0.02 μA after 1000h of 85℃ / 85RH damp heat cycling, and there is no silver dendrite growth.

[0047] Example 2 (for aluminum nitride power devices)

[0048] Take 38.5g of spherical silver powder with a D50 of 2.5μm, 17.5g of flake silver powder with a D50 of 0.8μm, 14g of submicron silver powder with a D50 of 0.3μm, 12g of eutectic solvent, 13g of glass powder, 3.2g of aluminosilicate, 1.8g of CePO4, and 70g of modified composite silver powder.

[0049] The porosity of the prepared silver electrode layer was 2.1%; the resistivity was 2.8 μΩ·cm.

[0050] After sintering, the interfacial shear strength between the silver electrode and the aluminum nitride substrate is 32 MPa; after 1000 hours of 85℃ / 85RH damp heat cycling, the leakage current is only 0.01 μA with no silver migration.

[0051] Example 3 (Specifically for 5G ceramic filters)

[0052] Take 24g of spherical silver powder with a D50 of 2.5μm, 32g of flake silver powder with a D50 of 0.8μm, 24g of submicron silver powder with a D50 of 0.3μm, 9.5g of eutectic solvent, 6g of glass powder, 1.5g of aluminosilicate, 3.0g of CePO4, and 80g of modified composite silver powder.

[0053] The prepared silver electrode layer had a porosity of 2.0% and a resistivity of 2.7 μΩ·cm. After sintering, the interfacial shear strength between the silver electrode and the ceramic substrate was 24 MPa; after 1000 h of 85℃ / 85RH damp heat cycling, the leakage current was only 0.04 μA, with no silver migration.

[0054] Comparative Example 1 (Commercially available silver paste for 5G ceramic filters containing organic carrier)

[0055] Commercially available standard ceramic silver paste (containing ethyl cellulose, terpineol, and bismuth glass):

[0056] The D50 consists of 37g of 2.5µm spherical silver powder, 42g of 2µm flake silver powder, 12g of organic carrier, 6g of glass powder, and 3g of additives. The peak sintering temperature is 820℃.

[0057] The prepared silver electrode layer had a porosity of 16.3% and a resistivity of 6.9 μΩ·cm.

[0058] After sintering, the interfacial shear strength between the silver electrode and the aluminum nitride ceramic is 14 MPa; after 200 h of 85℃ / 85RH damp heat cycling, the leakage current is 25 mA, and silver migration occurs.

[0059] Comparative Example 2: Silver paste for low-temperature co-fired ceramic devices:

[0060] The D50 consists of 45g of 2.0µm spherical silver powder, 31g of 1µm flake silver powder, 15g of organic carrier, 7g of glass powder, and 2g of additives. The peak sintering temperature is 750℃.

[0061] The prepared silver electrode layer had a porosity of 9.7 wt% and a resistivity of 4.2 μΩ·cm.

[0062] After sintering, the interfacial shear strength between the silver electrode and the alumina substrate is 17 MPa; after 1000 hours of humid heat cycling at 85℃ / 85RH, the leakage current is 50 mA, and there is obvious silver migration.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier, characterized in that, The slurry contains a eutectic solvent as a dispersion medium, modified composite silver powder, and glass powder.

2. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The eutectic solvent is prepared from choline chloride and zinc chloride in a ratio of 1:

2.

3. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The eutectic solvent is prepared from choline chloride and aluminum chloride in a ratio of 1:0.

2.

4. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The eutectic solvent is prepared by choline chloride and silver chloride in a ratio of 1:

2.

5. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The modified composite silver powder is composed of 30-55 wt% spherical silver powder with a D50 of 2.5 μm, 25-40 wt% flake silver powder with a D50 of 0.8 μm, and 15-30 wt% submicron silver powder with a D50 of 0.3 μm.

6. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The modified composite silver powder is in situ coated with a 3-6 nm silver oxalate inorganic precursor coating layer, with the coating amount accounting for 1.5 wt% to 5.5 wt% of the total mass of the silver powder.

7. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The glass powder has the following composition by mass percentage: B2O3 32wt%, SiO2 28wt%, BaO 24wt%, TiO2 12wt%, ZnO 4wt%.

8. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The slurry also includes layered magnesium aluminum silicate for inducing thixotropy of the slurry and nano CePO4 filler for inhibiting the sedimentation of modified composite silver powder and glass powder.

9. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 1, characterized in that, The slurry comprises, by mass percentage: 8-16 wt% eutectic solvent, 5-14 wt% glass powder, and the balance being modified composite silver powder.

10. The low-temperature sintering silver paste with a carbon-residue-free eutectic solvent carrier according to claim 9, characterized in that, The slurry also includes, by weight percentage: 1.2~3.5wt% magnesium aluminum silicate and 0.8~2.6wt% nano CePO4 filler.

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