A high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate, its preparation method and sintering process

Abstract

This invention provides a high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate, its preparation method, and sintering process. The high-temperature conductive silver paste for the microcrystalline glass thick-film heating plate, by mass percentage, comprises: silver powder: 75-85%; composite glass powder: 1-5%; organic carrier: 10-24%. The composite glass powder comprises component A and component B in a mass ratio of 1-3:1. Component B is silicon micropowder. Component A, by mass percentage, comprises: silicon oxide: 35-50%; boric acid: 20-30%; magnesium oxide: 10-15%; aluminum oxide: 5-10%; titanium oxide: 3-5%; and zirconium oxide: 1-2%. The high-temperature conductive silver paste for the microcrystalline glass thick-film heating plate prepared by this invention has high solderability, strong adhesion, a maximum operating temperature exceeding 450℃, strong resistance to high current impact, high re-firing stability, and excellent overall performance.

Description

Technical Field

[0001] This invention relates to the field of conductive silver paste technology, and in particular, to a high-temperature conductive silver paste for a microcrystalline glass thick film heating plate, its preparation method, and sintering process. Background Technology

[0002] Microcrystalline glass substrates are characterized by high strength, light weight, high insulation strength, good weather resistance and durability, good corrosion resistance, and high cost-effectiveness, making them suitable for mass production. They are particularly advantageous due to their smooth surface, low coefficient of thermal expansion, and strong resistance to thermal shock. Utilizing the properties of microcrystalline glass, thick-film circuits can be fabricated on the surface using thick-film technology. These circuits offer unparalleled advantages such as fast thermal response, small size, environmental friendliness, energy efficiency, and hygiene, and are widely used in applications such as health-preserving kettles, barbecue grills, medical devices, and industrial appliances, leading to rapid market growth.

[0003] The thick-film heating plate on a microcrystalline glass substrate is manufactured using a screen printing process to print resistive paste, conductive paste, and encapsulating paste onto the microcrystalline glass substrate. The resistive paste, after high-temperature firing, forms a thick-film resistive layer, which provides heating. The conductive paste, after high-temperature firing, forms a thick-film conductive layer, which acts as electrodes or pads, connecting to the power supply via welding or mechanical pressing. The encapsulating paste, after high-temperature firing, forms a glass encapsulation layer on the surfaces of the resistive and conductive layers, primarily serving to isolate them from water and air, ensuring that the resistive and conductive layers are not oxidized or corroded.

[0004] In the fabrication of a thick-film heating plate on a microcrystalline glass substrate, the first step is to print conductive silver paste, which is then sintered at 850℃ to form a thick-film conductor. The second step is to print resistive paste, which is also sintered at 850℃ to form a thick-film resistive layer. The third step is to print encapsulation paste, which is then sintered at 600℃ to form a thick-film encapsulation layer. The conductive layer needs to overlap with the resistive layer to form a circuit with a certain power. The encapsulation layer needs to cover the conductor layer and the resistive layer, and there is also an overlapping interface.

[0005] Because the coefficient of thermal expansion of microcrystalline glass substrates is small, typically only 0~0.5. 10 -6 / ℃. Conductor pastes include several types such as pure silver conductors, silver-palladium conductors, and silver-platinum conductors. Their main components are various metal powders or machine-made alloy powders of silver, palladium, platinum, etc., with small amounts of glass powder and organic carriers. Because the coefficient of thermal expansion of metallic silver is generally 19.5... 10 -6The temperature difference between the microcrystalline substrate and the conductive silver paste is enormous. Therefore, preparing conductive paste on microcrystalline glass substrates is extremely prone to defects such as blistering and cracking due to thermal expansion mismatch. Adding glass powder with a low coefficient of thermal expansion can alleviate the problem to some extent, but it's difficult to fundamentally solve it. Furthermore, the preparation of glass with a low coefficient of thermal expansion is challenging, requiring a strong theoretical foundation and extensive practical experience.

[0006] After years of research, commercially available medium-temperature microcrystalline glass conductor pastes are now available. These pastes are characterized by a sintering temperature generally around 500℃ and the use of low-melting-point glass powder with a softening temperature of 420-430℃, along with high-specific-surface-area microcrystalline silver powder. This is because glass powder with higher softening temperatures is less likely to soften at 500℃, thus failing to effectively bond the silver powder to the substrate. However, when using this low-softening-temperature glass bonding phase, once the operating temperature reaches above 300℃, the conductive silver layer will rearrange under thermal stress. Simultaneously, the glass bonding phase will continue to extend deeper into the contact interface of the microcrystalline glass plate, causing more microcracks at the interface. Ultimately, this leads to a decrease in the power of the microcrystalline glass thick-film heating plate, resulting in failure.

[0007] For alumina ceramic substrates, the most commonly used substrate for thick-film heating elements, fabricating one that meets application requirements—simultaneously satisfying technical demands such as appearance, solderability, solder resistance, and adhesion—is not particularly difficult. This is because the coefficient of thermal expansion of alumina ceramic is approximately 7.2. With a thermal expansion rate of 10⁻⁶ / ℃, it is easily compatible with thick-film pastes. However, for microcrystalline glass substrates, while their low thermal expansion characteristics provide excellent thermal stability, they also pose challenges to the series of thick-film pastes that can be matched with them. Conventional high-temperature conductive silver pastes, when applied to microcrystalline substrates, often lead to cracking, blistering, and peeling due to thermal expansion mismatch, or cause open circuits during the multi-cycle sintering process required for thick-film components, or result in significant power attenuation of thick-film components after multiple current surges during testing and service. Therefore, there is an urgent need for a new technology for high-temperature conductive silver paste for microcrystalline glass thick-film heating plates. Summary of the Invention

[0008] To address the appearance defects such as cracking, blistering, and peeling that occur in the conductive paste of microcrystalline glass thick film heating plates under high-temperature preparation conditions, as well as problems such as open circuits, breakage at the overlap between the conductive and resistive layers, and decreased solderability and adhesion under multi-sintering cycle manufacturing conditions, this invention provides a high-temperature conductive silver paste for microcrystalline glass heating plates. By rationally selecting silver powder with low sintering shrinkage characteristics, preparing composite glass powder with low shrinkage and low permeability characteristics, and employing a rapid sintering process, better thermal expansion matching between the conductive silver paste and the microcrystalline glass substrate is achieved. This improves the high-temperature creep resistance and current surge resistance of the conductive silver paste in the microcrystalline glass plate, enabling it to meet the application requirements of high-performance microcrystalline glass thick film heating plates.

[0009] To achieve the above objectives, the present invention provides a high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate, comprising, by weight percentage, […].

[0010] Silver powder: 75-85%; Composite glass powder: 1-5%; Organic carrier: 10-24%;

[0011] The composite glass powder comprises component A and component B in a mass ratio of 1 to 3:1;

[0012] Component B is silicon micropowder, or similar substances with high melting point properties such as alumina, titanium dioxide, zirconium oxide, silicon carbide, etc.

[0013] Component A, by mass percentage, comprises 35-50% silicon dioxide, 20-30% boric acid, 10-15% magnesium oxide, 5-10% aluminum oxide, 3-5% titanium oxide, and 1-2% zirconium oxide.

[0014] Furthermore, the silver powder is spherical with an average particle size of 1.5–3 micrometers, a tap density of 4.5–6.5 g / ml, and a specific surface area of ​​0.4–0.7 m². 2 / g.

[0015] Furthermore, the silicon micropowder is composed of silicon dioxide with a purity of 99.9% and an average particle size of 0.5 to 1 micrometer; the maximum particle size does not exceed 2 micrometers.

[0016] Furthermore, the coefficient of thermal expansion of the composite glass powder is 2.8. 10 -6 The temperature range is 755~762℃, the softening point is 755~762℃, the D50 is 0.8~1.3 micrometers, and the D99 is less than 10 micrometers.

[0017] Furthermore, the organic carrier comprises, by mass percentage: 30-40% hexadecyl alcohol ester, 10-20% tributyl citrate, 10-20% divalent ester, 5-10% butyl carbitol acetate, 5-10% ethylene glycol phenyl ether, and 3-6% ethyl cellulose Dow Chemical STD100; each component is boiled in a water bath at 80°C for 4-6 hours to form a uniform and transparent solvent, which is then filtered through a 300-mesh polyester screen.

[0018] Furthermore, the viscosity of the conductive silver paste is 140~180 Pa·s, and the fineness is no greater than 15 micrometers.

[0019] This invention also provides a method for preparing the above-mentioned high-temperature conductive silver paste for a microcrystalline glass thick film heating plate, comprising the following steps:

[0020] 1) First, mix component A in the composite glass powder evenly with a mixer, use a platinum crucible, melt in a high-temperature melting furnace, hold at 300℃ for 15 minutes, raise the temperature to 900℃, hold for 15 minutes, raise the temperature to 1550℃, hold for 1 hour, and then quench in water.

[0021] 2) Components A and B were mixed in a certain proportion and then ball-milled. The ball-milled mixture yielded a glass-ceramic composite glass powder with a low coefficient of thermal expansion and a high softening point. The coefficient of thermal expansion of the composite glass powder was 2.8. 10 -6 The glass powder has a temperature range of 755~762℃, a softening point of 755~762℃, a D50 of 0.8~1.3 micrometers, a D99 of less than 10 micrometers, and a specific gravity of 2.2 g / mm³. 3 ;

[0022] 3) The composite glass powder, silver powder and organic carrier are mixed evenly in a centrifugal degassing machine at a speed of 1200 rpm, and then rolled 5 to 6 times by a three-roll mill to obtain the high-temperature conductive silver paste for the microcrystalline glass thick film heating plate with a viscosity between 140 and 180 Pa·s and a fineness of no more than 15 microns.

[0023] This invention also provides a sintering process for the aforementioned high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising: printing the aforementioned high-temperature conductive silver paste for a thick-film microcrystalline glass heating plate onto the surface of a microcrystalline glass heating substrate using a 200-250 mesh stainless steel composite mesh; then drying it sequentially at 150°C for 6-8 minutes; and finally sintering it for 4-6 minutes at a peak sintering temperature of 800-850°C in a mesh belt sintering furnace with dry air circulation. The time from when the sample enters the sintering furnace from one end to when sintering is completed from the other end is 28-32 minutes, forming a conductive layer on the surface of the microcrystalline glass heating substrate. This conductive layer is silvery-white in color, has a dense and smooth surface, does not bubble or crack, and its main properties, such as solderability and adhesion, do not decrease after multiple high-temperature re-firing processes.

[0024] In principle, sintering is a densification process of various slurry powders under thermal action. The sintering of thick-film slurries is essentially a thermal imbalance sintering process, mostly driven by solid-phase diffusion reactions. Different sintering curves are set according to different mesh belt sintering furnaces to provide a stable and repeatable sintering environment for the microcrystalline glass thick-film heating plate. While heating and cooling rates are important, the most crucial factors are the peak sintering temperature (i.e., the highest sintering temperature) and the peak holding time (i.e., the holding time at the highest sintering temperature), which have a critical impact on the performance of the conductive layer. Adjustments are made according to the size of the workpiece being sintered. Larger workpieces require higher peak sintering temperatures and longer holding times. Compared to commonly used thick-film sintering processes, this sintering process features shorter sintering times and faster belt speeds. By employing this rapid sintering process, the diffusion depth of the glass binder phase into the glass-ceramic is effectively controlled by reducing the diffusion time of the glass binder phase into the glass-ceramic interface. Consequently, the stress between the conductive layer and the glass-ceramic is reduced. This appropriate diffusion depth also helps to improve the conductive film layer's resistance to current surges.

[0025] In this rapid sintering process, the conductor paste contains three main components: a functional phase (such as silver powder, palladium powder, platinum powder, nickel powder, etc.), a binder phase (such as silicate glass, oxides, additives, etc.), and an organic carrier (a mixture of resin and solvent). During the drying process at 150℃, the organic solvent in the organic carrier is basically evaporated, but most of the resin remains in the dried film, serving to fix and connect the powder particles. At around 500℃, the resin in the conductor paste is basically evaporated. At around 750℃, the glass phase in the conductor paste begins to soften, on the one hand tightening the silver powder particles to allow solid-phase diffusion between them, gradually eliminating pores; on the other hand, it undergoes a solid-phase diffusion reaction with the microcrystalline substrate, penetrating into the surface layer of the microcrystalline glass substrate. As the temperature gradually increases, especially at 850℃, the solid-phase diffusion reaction between the main functional phase component, silver powder, and the glass phase accelerates, the porosity gradually decreases, and a dense film layer is formed. A suitable sintering temperature will yield a more ideal conductive layer with superior conductivity, adhesion, solderability, and solder resistance. If the workpiece is over-sintered, the glass phase in the conductor paste will float to the surface, reducing the solderability of the silver paste, meaning the conductor paste will be difficult to tin. If the workpiece is under-sintered, the conductor paste will not be fully densified, and the diffusion between various powders in the conductor paste and between the conductor paste and the microcrystalline glass substrate will be incomplete, resulting in insufficient adhesion and the conductive layer being easily eroded by tin.

[0026] Because the sintering of thick film slurries is a non-equilibrium sintering process dominated by solid-phase reactions, unlike the liquid-phase sintering in metal smelting, the heat and mass transfer of the powder occurs within a very small range. Therefore, the properties of the functional phase powder, such as silver powder, have a significant impact on the resulting film. Generally speaking, the performance indicators of silver powder include morphology, particle size, tap density, and specific surface area. There are generally two main methods for producing silver powder: one is the liquid-phase reduction method, which involves adding a reducing agent and a dispersant to an aqueous solution of silver nitrate, causing silver ions to crystallize, nucleate, and grow in the aqueous solution. By controlling factors such as the reaction temperature and stirring speed, silver powder with different properties can be obtained; this is a chemical synthesis method. The other is the gas atomization method, which uses a high-speed gas flow to break up molten liquid silver into small droplets and solidify them into powder. This method produces powder with high sphericity and high purity; it is a physical production method. Thick-film pastes typically use silver powder produced by liquid-phase reduction due to its higher cost-effectiveness, especially since its surface is coated with an organic layer, which facilitates the formation of a fluid with superior rheological properties suitable for printing with the organic carrier in the thick-film paste. Silver powder produced by gas atomization, due to its particle size distribution, is mostly used in applications such as 3D printing, except for the ultrafine silver powder with a finer particle size used in thick-film pastes. Selecting the appropriate silver powder for different application conditions is often the core technology in developing conductive or resistive pastes.

[0027] The present invention has the following beneficial effects:

[0028] 1. This invention provides a high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate, comprising, by mass percentage: silver powder: 75-85%; composite glass powder: 1-5%; and organic carrier: 10-24%. The silver powder has a spherical or near-spherical morphology, an average particle size of 1.5-3 micrometers, a tap density of 4.5-6.5 g / ml, and a specific surface area of ​​0.4-0.7 m². 2 / g. The composite glass powder comprises component A and component B in a mass ratio of 1~3:1. Component B is silica powder. Component A, by mass percentage, comprises 35~50% silicon dioxide, 20~30% boric acid, 10~15% magnesium oxide, 5~10% aluminum oxide, 3~5% titanium oxide, and 1~2% zirconium oxide. The coefficient of thermal expansion of the composite glass powder is 2.8. 10 -6 The silicon micropowder has a softening point of 755~762℃, a D50 of 0.8~1.3 micrometers, and a D99 of less than 10 micrometers. The main component of the silicon micropowder is silicon dioxide, with a purity of 99.9%, an average particle size of about 0.5 to 1 micrometer, and a maximum particle size of no more than 2 micrometers.

[0029] Silver powder, as a functional phase in the high-temperature silver paste of the microcrystalline glass thick-film heating plate, mainly plays a role in conductivity and welding. Higher silver powder content results in better conductivity after sintering into a thick-film conductor layer, a thicker silver film layer, and better solderability and weldability of the conductor layer. Using silver powder with large particle size, high tap density, and low specific surface area results in low shrinkage during high-temperature sintering, reducing the likelihood of cracks or bubbles. Composite glass powder, as a binder phase in the thick-film conductive silver paste, forms a bonding layer with the microcrystalline glass substrate through solid-phase diffusion and also "bonds" the functional phase silver powder particles together. The silica micropowder in the composite glass powder acts as a "pinning" agent for the glass phase during high-temperature sintering, reducing shrinkage and regulating thermal stress during sintering. Higher glass phase content leads to stronger adhesion between the conductor sintered film and the microcrystalline glass substrate, but gradually worsens solderability and reduces welding adhesion. If the proportion of the glass phase is too small, the solderability of the conductor paste is excellent, but the adhesion to the microcrystalline glass substrate deteriorates, and the solderability is relatively worse. A reasonable glass phase content balances various performance indicators such as conductivity, substrate adhesion, solderability, solderability, and weld adhesion. The organic carrier mainly affects the printing characteristics of the conductor paste. Generally, the higher the organic carrier content, the lower the viscosity, all other things being equal. Too high a viscosity hinders the conductor paste from passing through the screen and results in a rough conductive layer surface, affecting the appearance; too low a viscosity leads to smeared edges after printing. Controlling the viscosity of the conductor paste within a reasonable range improves its printing characteristics, resulting in a smooth and dense conductive sintered film.

[0030] 2. The high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate prepared by this invention is applied to microcrystalline glass using an 850℃ high-temperature sintering process with a holding time of 5 to 10 minutes. It exhibits excellent resistance to current surges, exceeding 10,000 cycles, and after a 1000-hour lifespan test, power attenuation is less than 5%. Furthermore, this conductive silver paste demonstrates strong adhesion, exceeding 40N (2mm). (2mm test block), solderability greater than 95%, solderability greater than 3 times, maximum working temperature can reach over 450℃, excellent performance.

[0031] 3. The high-temperature conductive silver paste for the microcrystalline glass thick film heating plate prepared by this invention has good compatibility with the microcrystalline glass substrate. The conductive paste cannot be scraped off with a utility knife, has high strength and hardness, and is free of bubbles and cracks. Moreover, the conductive silver paste of this invention does not decrease in solderability, solderability, and adhesion under multiple sintering conditions at 850℃, which can meet the application requirements of multiple sintering cycles of microcrystalline glass plates.

[0032] In addition to the objectives, features and advantages described above, the present invention has other objectives, features and advantages. Detailed Implementation

[0033] The embodiments of the present invention will be described in detail below, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0034] Example 1

[0035] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, spherical silver powder (tap density 6.5 g / ml, average particle size 1.8 micrometers, specific surface area 0.4 m²). 2 / g): 82%; Composite glass powder: 3%; Organic carrier: 15%.

[0036] The coefficient of thermal expansion of the composite glass powder is 2.8. 10 -6 The softening point is 755~762℃, and it includes component A and component B in a mass ratio of 1:1. Component B is silicon micro powder. Component A includes, by mass percentage, 35~50% silicon oxide, 20~30% boric acid, 10~15% magnesium oxide, 5~10% aluminum oxide, 3~5% titanium oxide, and 1~2% zirconium oxide.

[0037] The organic carrier comprises, by mass percentage: 30-40% hexadecyl alcohol ester, 10-20% tributyl citrate, 10-20% divalent ester, 5-10% butyl carbitol acetate, 5-10% ethylene glycol phenyl ether, and 3-6% ethyl cellulose Dow Chemical STD100. Each component is boiled in a water bath at 80°C for 4-6 hours to form a uniform and transparent solvent, which is then filtered through a 300-mesh polyester screen.

[0038] The specific preparation method is as follows:

[0039] 1) First, mix component A in the composite glass powder evenly with a mixer, use a platinum crucible, melt it in a high-temperature melting furnace, hold at 300℃ for 15 minutes, raise the temperature to 900℃, hold for 15 minutes, raise the temperature to 1550℃, hold for 1 hour, and then quench in water.

[0040] 2) After mixing component A and component B in a certain proportion, the mixture is ball-milled to obtain a glass-ceramic composite glass powder with a low coefficient of thermal expansion and a high softening point. The composite glass powder is measured.

[0041] 3) Mix the composite glass powder, silver powder and organic carrier evenly in a centrifugal degassing machine at a speed of 1200 rpm, and then roll it through a three-roll mill 5 to 6 times to obtain the high-temperature conductive silver paste for the microcrystalline glass thick film heating plate.

[0042] The conductive paste prepared in this embodiment has a viscosity of 165 Pa·s and a fineness of less than 10 micrometers.

[0043] Example 2:

[0044] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, microcrystalline silver powder (taper density 4.5 g / ml, average particle size 2.1 micrometers, specific surface area 0.42 m²). 2 / g): 82%; Composite glass powder: 3%; Organic carrier: 15%;

[0045] The coefficient of thermal expansion of the composite glass powder is 2.8. 10 -6 The softening point is 755~762℃, and it includes component A and component B in a mass ratio of 1:1. Component B is silicon micro powder. Component A includes, by mass percentage, 35~50% silicon oxide, 20~30% boric acid, 10~15% magnesium oxide, 5~10% aluminum oxide, 3~5% titanium oxide, and 1~2% zirconium oxide.

[0046] The organic carrier, by mass percentage, comprises: 30-40% hexadecyl alcohol ester, 10-20% tributyl citrate, 10-20% divalent ester, 5-10% butyl carbitol acetate, 5-10% ethylene glycol phenyl ether, and 3-6% ethyl cellulose Dow Chemical STD100. Each component is boiled in a water bath at 80°C for 4-6 hours to form a uniform and transparent solvent, which is then filtered through a 300-mesh polyester screen.

[0047] The specific preparation method is the same as in Example 1. The viscosity of the obtained conductive paste is 168 Pa·S and the fineness is less than 10 micrometers.

[0048] Example 3:

[0049] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, microcrystalline silver powder (taper density 4.5 g / ml, average particle size 2.1 micrometers, specific surface area 0.42 m²). 2 / g): 75%; Composite glass powder: 3%; Organic carrier: 22%.

[0050] The coefficient of thermal expansion of the composite glass powder is 2.8. 10 -6 The temperature range is 755-762℃, and the softening point is 755-762℃. It includes components A and B in a mass ratio of 3:1. The specific composite glass powder used, including components A and B, and the organic carrier, is the same as in Example 1. The conductive paste prepared in this example has a viscosity of 175 Pa·s and a fineness of less than 10 micrometers.

[0051] Example 4:

[0052] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, microcrystalline silver powder (taper density 6.5 g / ml, average particle size 1.8 micrometers, specific surface area 0.4 m²).2 / g): 85%; Composite glass powder: 1%; Organic carrier: 14%.

[0053] The coefficient of thermal expansion of the composite glass powder is 2.8. 10 -6 The temperature range is 755-762℃, and the softening point is 755-762℃. It includes components A and B in a mass ratio of 2:1. The specific composite glass powder used, including components A and B, and the organic carrier, is the same as in Example 1. The conductive paste prepared in this example has a viscosity of 180 Pa·s and a fineness of less than 10 micrometers.

[0054] Comparative Example 1: (using microcrystalline silver powder)

[0055] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, microcrystalline silver powder (taper density 2.78 g / m³, average particle size 0.8 μm, specific surface area 3.17 m²). 2 / g): 82%; Composite glass powder: 3%; Organic carrier: 15%;

[0056] Comparative Example 1 is the same as Example 1 except for the silver powder used.

[0057] The specific preparation method is the same as in Example 1. The viscosity of the obtained conductive paste is 185 Pa·S and the fineness is less than 10 micrometers.

[0058] Comparative Example 2: (using high-temperature calcium aluminum silicate glass powder)

[0059] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, spherical silver powder (tap density 6.5 g / ml, average particle size 1.8 micrometers, specific surface area 0.4 m²). 2 / g): 82%; Glass powder: 3%; Organic carrier: 15%;

[0060] The glass powder is high-temperature calcium aluminum silicate glass powder with a coefficient of thermal expansion of 7.8. 10 -6 The temperature is ℃, the softening point is 750~760℃, and by mass percentage, it includes: calcium oxide 32%; aluminum oxide 25%; silicon oxide 20%; boric acid 15%; barium oxide 3%; titanium oxide 3%; zirconium oxide 1%; and strontium oxide 1%.

[0061] Comparative Example 2 is the same as Example 1 except that the glass powder used is different.

[0062] The specific preparation method is the same as in Example 1. The viscosity of the obtained conductive paste is 169 Pa·S and the fineness is less than 10 micrometers.

[0063] Comparative Example 3: (using microcrystalline silver powder and high-temperature calcium aluminum silicon glass powder)

[0064] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, microcrystalline silver powder (taper density 2.78 g / m³, average particle size 0.8 μm, specific surface area 3.17 m²). 2 / g): 82%; Glass powder: 3%; Organic carrier: 15%;

[0065] The glass powder is high-temperature calcium aluminum silicon glass powder with a coefficient of thermal expansion of 7.8. 10 -6 The temperature is ℃, the softening point is 750~760℃, and by mass percentage, it includes: calcium oxide 32%; aluminum oxide 25%; silicon oxide 20%; boric acid 15%; barium oxide 3%; titanium oxide 3%; zirconium oxide 1%; and strontium oxide 1%.

[0066] The difference between Comparative Example 3 and Comparative Example 2 is that the silver powder used is different, but everything else is the same.

[0067] The specific preparation method is the same as in Example 1. The viscosity of the obtained conductive paste is 175 Pa·S and the fineness is less than 10 micrometers.

[0068] Comparative Example 4 (The composite glass powder contains only component A, without the addition of silica fume)

[0069] A high-temperature conductive silver paste for a microcrystalline glass heating plate, comprising, by mass percentage, spherical silver powder (tap density 6.5 g / ml, average particle size 1.8 micrometers, specific surface area 0.4 m²). 2 / g), 82%; Component A in composite glass powder: 3%; Organic carrier: 15%.

[0070] The difference between Comparative Example 4 and Example 1 is that the amount of composite glass powder used is the same, but it only contains component A and does not contain silicon micropowder; otherwise, it is the same as Example 1.

[0071] The other preparation methods are the same as in Example 1. The viscosity of the conductive paste prepared in this example is 160 Pa·S and the fineness is less than 10 micrometers.

[0072] The conductive silver pastes prepared in Examples 1-2 and Comparative Examples 1-4 were subjected to the same sintering process to form a conductive layer. Specifically, the conductive paste was printed onto the surface of a microcrystalline glass heating substrate using a 200-250 mesh stainless steel composite mesh. The substrate was then dried at 150°C for 8-15 minutes, and finally sintered in a mesh belt sintering furnace with dry air at a peak sintering temperature of 850°C for 5-10 minutes. The surface was then observed, and solderability, solder resistance, adhesion, and resistance to current surges were tested. Considering that the conductive paste often requires multiple sintering processes in practical applications, a re-sintering test was added. The sample that had undergone the first test was placed back into the mesh belt furnace and sintered again at 850°C. The appearance was observed, and the solderability, solder resistance, and adhesion were tested to determine if there were any changes.

[0073] All surface condition observations were conducted under a 200x optical microscope.

[0074] Solderability test: SnAgCu solder was used in a constant temperature solder pot at 260℃. The solder was immersed at 90° perpendicular to the solder surface for 10 seconds, and the solder coverage ratio was measured.

[0075] Solderability test: SnAgCu solder was used in a constant temperature solder pot at 260℃. The solder was immersed at 90° perpendicular to the solder surface for 10 seconds, and the solder pad was completely counted once.

[0076] Adhesion test: using 2mm 2mm solder pads, 0.8mm diameter tinned copper wire, L-shaped bend, 400℃ constant temperature soldering;

[0077] Current surge resistance test: Under the same power condition of 1000W, the circuit is subjected to 1000 pulses of continuous power supply for 30 seconds followed by 30 seconds of cooling. The rate of change of resistance is measured and calculated using the formula ΔR = (Rfinal - Rinitial) / Rinitial. 100%.

[0078] The specific experimental results are shown in Table 1:

[0079] Table 1:

[0080]

[0081] As shown in Table 1, comparing Examples 1 and 2, the components are identical except for the silver powder used. The silver powder used in Example 1 has a higher tap density and smaller specific surface area, resulting in better adhesion and stronger resistance to current surges. This is because the higher tap density silver powder produces a denser silver layer after sintering, leading to better bonding with the substrate and stronger resistance to current surges. Although the silver powder in Example 2 has a lower tap density than that in Example 1, its larger particle size and smaller specific surface area result in less shrinkage during sintering, which also basically meets the product application requirements.

[0082] Comparing Examples 1, 2, and 1, the glass powder used in all three examples was the same, and the proportions of the organic carrier were similar. Comparative Example 1 used microcrystalline silver powder with small particle size, high specific surface area, and low tap density. However, this silver powder exhibited a larger shrinkage ratio during sintering, resulting in greater surface roughness and significantly lower sintered film density compared to Examples 1 and 2. Although no obvious defects were visible on the surface, its solderability, solderability, adhesion, and current resistance were all inferior. Particularly in re-sintering tests simulating multiple sintering cycles in product applications, localized detachment occurred at the conductor layer edges, potentially leading to cracks and open circuits at the junction with the resistive paste in actual applications. Therefore, silver powder with large particle size, small specific surface area, and high tap density is more suitable for applications in microcrystalline glass conductor pastes due to its smaller sintering shrinkage ratio.

[0083] Comparing Example 1 and Comparative Example 2, both used the same silver powder and organic carrier. Example 1 used a carrier with a thermal expansion coefficient of 2.8. 10 -6 The glass powder used in Comparative Example 2 had a low coefficient of thermal expansion of 7.8 at a temperature of / ℃. 10 -6 High thermal expansion coefficient glass powder ( / ℃). Since the thermal expansion coefficient of the glass-ceramic substrate is close to zero, the high thermal expansion coefficient glass powder exhibits a more severe thermal expansion mismatch with the glass-ceramic compared to composite glass powder with a low thermal expansion coefficient. Although the difference may not be particularly obvious visually, the actual thermal stress is greater. During the re-firing process, localized detachment of the conductor layer edge in Comparative Example 2 occurred because the thermal expansion mismatch between the conductor layer and the glass-ceramic substrate was further exacerbated by another high-temperature sintering. With each additional sintering cycle, the delamination of the conductor layer intensifies. Therefore, for the glass phase of the conductor paste in glass-ceramic plates, a glass with a low thermal expansion coefficient is more suitable.

[0084] Comparing Comparative Examples 2 and 3, although both used glass powder with a high coefficient of thermal expansion, Comparative Example 2 used silver powder with large particle size, low specific surface area, and high tapping, while Comparative Example 3 used silver powder with small particle size, large specific surface area, and low tapping. During high-temperature sintering, the conductor paste in Comparative Example 3 exhibited relatively poorer surface quality and a more pronounced thermal expansion mismatch due to the larger shrinkage ratio of the silver powder, resulting in relatively worse performance. However, for conventional ceramic substrates, Comparative Example 3 fully met the application requirements. The reason it could not meet the application requirements on microcrystalline boards is due to the significant difference in thermal expansion coefficients, leading to various defects such as cracking and blistering, making densification during sintering impossible.

[0085] Comparing Comparative Example 4 with Example 1, the difference is that Comparative Example 4 only used component A of the composite glass powder and did not add silica powder. Otherwise, they were identical. Test results showed that the conductor paste in Comparative Example 4 was almost identical to that in Example 1 in terms of appearance, solderability, solder resistance, and adhesion. The difference lay in resistance to current surges; the resistance change rate in Comparative Example 4 was 7.5%, significantly higher than the 1.6% change rate in Example 1. Furthermore, during the reheating process, the solderability and adhesion of the conductor layer in Comparative Example 4 decreased to some extent. This is most likely because, during the reheating process, the glass powder in Comparative Example 4, lacking high-melting-point silica powder, floated during sintering, forming a very thin glass layer on the surface of the conductor paste. During soldering, tin could not easily penetrate this thin glass layer to form a silver-tin alloy with silver, thus reducing solderability and affecting solder adhesion. The composite glass powder in Example 1, containing high-melting-point silicon micropowder, has two advantages: firstly, it increases the softening point of the composite glass powder, making the conductor paste more resistant to high-temperature sintering during re-firing; secondly, the silicon micropowder disperses in the glass binder phase during high-temperature sintering, slowing down the viscosity decrease of the glass binder phase during softening, thus slowing down its solid-phase diffusion reaction with the microcrystalline glass substrate and preventing deeper diffusion that could generate greater thermal stress. This improves resistance to current surges and re-firing. In the fabrication of microcrystalline glass thick-film heating elements, multiple sintering of the conductor paste is a necessary process. Therefore, Example 1 is more suitable for application than Comparative Example 4. The results of Examples 3 and 4 are similar to those of Example 1 and will not be repeated here.

[0086] In summary, this invention, through the selection of specific components and corresponding proportions, achieves synergistic effects, resulting in a high-temperature conductive silver paste for microcrystalline glass thick-film heating plates. This paste exhibits high solderability, strong adhesion, a maximum operating temperature exceeding 450℃, strong resistance to high-current impacts, high re-firing stability, and excellent overall performance. Silver powder, as the functional phase in the high-temperature silver paste, primarily functions as an conductive and soldering agent. Higher silver powder content leads to better conductivity after sintering into a thick-film conductor layer, resulting in a thicker silver film and improved solderability and weldability. Using silver powder with large particle size, high tap density, and low specific surface area minimizes shrinkage during high-temperature firing, reducing the likelihood of cracks or bubbles. Composite glass powder, acting as the binder phase in the thick-film conductive silver paste, forms an adhesive layer with the microcrystalline glass substrate through solid-phase diffusion and simultaneously "bonds" the functional phase silver powder particles together. The silica fume in the composite glass powder acts as a "pinning" agent for the glass phase during high-temperature sintering, reducing shrinkage and regulating thermal stress. Higher glass phase content strengthens the adhesion between the conductor sintered film and the microcrystalline glass substrate, but gradually reduces solderability and welding adhesion. Conversely, too low a glass phase content results in excellent solderability of the conductor paste, but poorer adhesion to the microcrystalline glass substrate and relatively lower solderability. A reasonable glass phase content balances conductivity, substrate adhesion, solderability, solderability, and welding adhesion. The organic carrier primarily affects the printing characteristics of the conductor paste. Generally, higher organic carrier content results in lower viscosity, all other things being equal. High viscosity hinders the conductor paste from passing through the screen and creates a rough conductive layer surface, affecting appearance; low viscosity leads to smudged edges after printing. Maintaining the viscosity of the conductor paste within a reasonable range improves its printing characteristics, resulting in a smooth and dense conductive sintered film.

[0087] This invention provides a high-temperature conductive silver paste for a thick-film heating plate on microcrystalline glass. When applied to microcrystalline glass using an 850℃ high-temperature sintering process with a holding time of 5 to 10 minutes, it remains effective even after multiple sintering cycles. It exhibits excellent resistance to current surges, exceeding 10,000 cycles, and after a 1000-hour lifespan test, power attenuation is less than 5%. Furthermore, this conductive silver paste demonstrates strong adhesion, exceeding 40N (2mm). (2mm test block), solderability greater than 95%, solderability greater than 3 times, and maximum operating temperature above 450℃, exhibiting excellent performance. The high-temperature conductive silver paste of this invention matches well with the microcrystalline glass substrate; the conductive paste cannot be scraped off with a utility knife, exhibiting high strength and hardness, and showing no bubbles or cracking. Furthermore, the conductive silver paste of this invention does not decrease in solderability, solderability, or adhesion under multiple sintering conditions at 850℃, meeting the application requirements of multi-sintering cycles for microcrystalline glass panels.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate, characterized in that, By weight percentage, including Silver powder: 75-85%; Composite glass powder: 1-5%; Organic carrier: 10-24%; The composite glass powder comprises component A and component B in a mass ratio of 1 to 3:1; Component B is silicon micro powder, and component A, by mass percentage, includes 35-50% silicon oxide, 20-30% boric acid, 10-15% magnesium oxide, 5-10% aluminum oxide, 3-5% titanium oxide, and 1-2% zirconium oxide. The silver powder is spherical with an average particle size of 1.5–3 micrometers, a tap density of 4.5–6.5 g / ml, and a specific surface area of ​​0.4–0.7 m². 2 / g; The coefficient of thermal expansion of the composite glass powder is 2.

8. 10 -6 The temperature range is 755~762℃, the softening point is 755~762℃, the D50 is 0.8~1.3 micrometers, and the D99 is less than 10 micrometers.

2. The high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate according to claim 1, characterized in that, The silicon micropowder is composed of silicon dioxide with a purity of 99.9% and an average particle size of 0.5 to 1 micrometer; the maximum particle size does not exceed 2 micrometers.

3. The high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate according to claim 1, characterized in that, The organic carrier comprises, by mass percentage: 30-40% hexadecyl alcohol ester, 10-20% tributyl citrate, 10-20% divalent ester, 5-10% butyl carbitol acetate, 5-10% ethylene glycol phenyl ether, and 3-6% ethyl cellulose Dow Chemical STD100. Each component is boiled in a water bath at 80°C for 4-6 hours to form a uniform and transparent solvent, which is then filtered through a 300-mesh polyester screen.

4. The high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate according to any one of claims 1 to 3, characterized in that, The conductive silver paste has a viscosity of 140~180 Pa·s and a fineness of no more than 15 micrometers.

5. The method for preparing high-temperature conductive silver paste for a microcrystalline glass thick-film heating plate according to any one of claims 1-4, characterized in that, Includes the following steps: 1) First, mix component A in the composite glass powder evenly with a mixer, melt it in a high-temperature melting furnace, keep it at 300℃ for 15 minutes, raise the temperature to 900℃, keep it at 900℃ for 15 minutes, raise the temperature to 1550℃, keep it at 1 hour, and then quench it in water. 2) After mixing component A and component B in a certain proportion, the mixture is ball-milled to obtain a composite glass powder with a low coefficient of thermal expansion and a high softening point. The composite glass powder, silver powder and organic carrier are mixed evenly in a centrifugal degassing machine at a speed of 1200 rpm, and then rolled 5 to 6 times by a three-roll mill to obtain the high-temperature conductive silver paste for the microcrystalline glass thick film heating plate.

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