Antioxidant nano-copper powder, preparation method thereof and nano-copper solder paste

Abstract

The application provides an anti-oxidation nano copper powder, a preparation method of the anti-oxidation nano copper powder and a nano copper solder paste. The preparation method of the anti-oxidation nano copper powder comprises the following steps: dissolving copper salt in a solvent, adding soluble metal hydroxide to prepare a precursor solution; wherein the molar ratio of the soluble metal hydroxide to the copper salt is 0.5-3:1; dissolving L-ascorbic acid in the solvent, stirring at 40-80 DEG C for 10-20 min, and cooling to obtain a reaction solution; mixing the precursor solution and the reaction solution to obtain a mixed solution, wherein the molar ratio of the copper salt to the L-ascorbic acid is 1:2-4; adding the mixed solution into a reducing alcohol, stirring uniformly, heating to 155-195 DEG C, and keeping the reaction for more than 15 min, and then washing, centrifuging and drying the product to obtain the anti-oxidation nano copper powder. The nano copper powder obtained by the technical scheme has small size, good anti-oxidation and good stability in air, realizes low-temperature sintering interconnection of a chip, and has high joint reliability.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging materials technology, and in particular to an antioxidant nano-copper powder and its preparation method, as well as nano-copper solder paste. Background Technology

[0002] In the field of microelectronic packaging, third-generation semiconductor materials, represented by silicon carbide (SiC) and gallium nitride (GaN), possess properties such as wide bandgap (>2.2eV), high breakdown voltage, good radiation interference resistance, and chemical stability. They can maintain good conversion characteristics and operating capabilities at temperatures exceeding 300°C, and are therefore widely used in high-temperature and high-pressure environments such as aerospace, smart grids, new energy vehicles, rail transportation, deep-sea exploration, and nuclear reactions. However, these extremely harsh service conditions pose a severe challenge to the packaging materials of devices. With the development trend of high-density interconnects and miniaturization of packaging, packaging materials must be able to achieve packaging interconnects at lower temperatures while ensuring high service reliability under long-term high-temperature and high-pressure conditions.

[0003] Traditionally used encapsulation materials include tin-based solders and thermally conductive adhesives. However, tin-based solders generally have low melting points; the commonly used tin-silver-copper solder SAC305 has a melting point of only 217℃. Thermally conductive adhesives, due to their high resin content, have poor heat resistance and thermal conductivity far lower than metal solders. These factors severely limit their application in third-generation semiconductor devices. In contrast, due to nanoscale effects and high surface energy, nanoscale metal powders exhibit higher activity and diffusion rates at low temperatures, enabling solid-state interconnection at low temperatures. Furthermore, after sintering, the theoretical service temperature and thermal conductivity of nanoscale metal powders are far higher than those of tin-based solders, offering significant advantages. Among these, nanoscale Cu powder is favored for its low cost, excellent electrical and thermal conductivity, and strong resistance to electromigration compared to nanoscale Ag powder. However, Cu itself has weak oxidation resistance; the CuO formed after oxidation severely reduces the electrical and thermal conductivity and reliability of interconnects, and the oxidation rate of nanoscale Cu is further accelerated. Although current research focuses on introducing high-molecular-weight organic coatings, core-shell structures, or acidic atmospheres to further improve the antioxidant properties of nano-Cu powder, the antioxidant effect is not significant and may even affect the subsequent sintering quality. Therefore, there is an urgent need for a novel synthetic method that is simple and rapid to prepare antioxidant, uniformly sized, and fine nano-Cu powder. Summary of the Invention

[0004] To address the above technical problems, this invention discloses an antioxidant nano-copper powder and its preparation method, as well as nano-copper solder paste. The obtained copper powder is antioxidant, has good stability in air, reduces the risk of interface embrittlement, enables low-temperature sintering interconnection of chips, and has high reliability of interconnect joints. It can be widely used in power electronic devices, especially in the packaging of third-generation semiconductor devices.

[0005] The technical solution adopted by this invention is as follows:

[0006] A method for preparing antioxidant nano-copper powder includes the following steps:

[0007] Step S1: Dissolve the copper salt in a solvent, add a soluble metal hydroxide, and prepare a precursor solution; wherein the molar ratio of the soluble metal hydroxide to the copper salt is 0.5-2:1.

[0008] Step S2: Dissolve L-ascorbic acid in a solvent and stir at 40-80°C for 10-20 minutes until the color turns orange or reddish-brown. Cool to room temperature to obtain the reaction solution.

[0009] Step S3: The precursor solution and the reaction solution are mixed to obtain a mixture, wherein the molar ratio of copper salt to L-ascorbic acid is 1:2-4; the mixture is added to a reducing alcohol, stirred evenly, heated to a reaction temperature of 155-195°C, and kept at this temperature for at least 15 minutes; after cooling to room temperature, the product is obtained; the use of a reducing alcohol can prevent the generated nano-Cu from being oxidized again to copper oxide or cuprous oxide;

[0010] Step S4: After washing, centrifuging and drying the product, antioxidant nano copper powder is obtained.

[0011] The solvents mentioned in steps S1 and S2 are one or a mixture of two or more of 9-octadeceneamine, N,N-dimethyldodecyl tertiary amine, N,N-dimethyldodecylamine, and N,N-diethyltrimethylsilylamine.

[0012] Among them, 9-octadeceneamine, N,N-dimethyldodecyl tertiary amine, N,N-dimethyldodecylamine, and N,N-diethyltrimethylsilylamine are long-chain alkylamines with dual functions as strong reducing agents and solvents. The long-chain alkyl portion of these long-chain organic amine molecules can form a hydrophobic layer on the surface of nano-Cu, hindering the aggregation of nano-Cu; simultaneously, the amino portion at the other end can charge the nano-Cu, generating electrostatic repulsion, further reducing particle aggregation, ultimately ensuring the small particle size of nano-Cu and appropriate printing viscosity. Secondly, long-chain alkylamines are liquid at room temperature. As a common reducing agent, compared to powdered reducing agents such as L-ascorbic acid, the liquid state is more conducive to the occurrence of redox reactions. At the same time, the reducing ionic environment formed by the amino structure can further inhibit the oxidation of Cu. Thirdly, from a sintering perspective, although the above-mentioned long-chain alkylamines are oily coating layers, they can rapidly decompose at a temperature of 150–160°C, promoting the rapid diffusion of subsequent nano-Cu, making them very suitable for the preparation of nano-Cu for sintering interconnection in this invention.

[0013] Furthermore, the long-chain alkylamine used in the above technical solution is liquid at room temperature, directly acting as both a stabilizer and a solvent. This prevents the reduced Cu atoms from being re-oxidized to copper oxide or cuprous oxide, or from the products directly generated during the reduction process containing a large amount of copper oxide, effectively ensuring the uniformity of the fine Cu nanoparticles. It also avoids the use of large amounts of solvent and maintains a sufficient chemical reaction rate even at low or room temperature. Thus, a homogeneous solution can be formed by stirring at room temperature when preparing the precursor solution. Room temperature stirring creates a more stable precursor solution, allowing for controlled release of Cu ions.

[0014] Third, from the perspective of sintering, the long-chain alkylamine selected in the technical solution of this invention has a certain degree of volatility, and the initial boiling or decomposition temperature is 150-160℃. In this way, during the heating stage of the sintering process, they gradually decompose on the surface of Cu nanoparticles, allowing the exposed Cu and the surface of Cu particles to come into contact with each other, promoting the rapid diffusion of Cu atoms and improving the strength of the sintered joint.

[0015] This technical solution involves adding alkalis such as hydroxides to a copper salt solution, which provides abundant hydroxide ions in solution. These hydroxide ions then react with Cu... 2+ The precipitate phase formed inhibits the ionization and reduction process of Cu, and combines with long-chain organic amines, which have strong reducing agents and solvents, to produce fine-sized organic amine-coated nano-Cu particles, giving the obtained nano-Cu particles excellent antioxidant properties.

[0016] In the above scheme, without the addition of soluble metal hydroxides, the resulting nano-Cu particles are relatively large (most exceeding 300 nm). Excessively large particle sizes will prevent the full utilization of the high surface energy and rapid diffusion advantages of nanomaterials, thus requiring an increased sintering temperature to achieve the same sintering density. Appropriate addition of soluble metal hydroxides can control the particle size and morphology.

[0017] Furthermore, the soluble metal hydroxide is sodium hydroxide or potassium hydroxide.

[0018] As a further improvement of the present invention, the copper salt is one or a mixture of two or more of copper sulfate, copper chloride, copper nitrate, and copper acetate. More preferably, the copper salt is copper acetate.

[0019] As a further improvement of the present invention, the concentration of copper salt in the precursor solution is 0.1–1 mol / L, and the molar ratio of the soluble metal hydroxide to the copper salt is 1–2:1. Using this technical solution, a large number of hydroxide ions can more significantly suppress the ionization and reduction process of Cu ions, allowing the organic amine sufficient time to uniformly coat Cu atoms or clusters, producing fine-sized nano-Cu particles. The resulting nano-Cu particles are more uniform in size, which is beneficial for densification during sintering.

[0020] As a further improvement of the present invention, the concentration of L-ascorbic acid in the reaction solution is 0.4 to 2 mol / L.

[0021] As a further improvement of the present invention, in step S3, the precursor solution and the reaction solution are mixed in equal volumes, and the volume ratio of the mixture to the reducing alcohol is 1:2 to 1:4.

[0022] As a further improvement of the present invention, the reducing alcohol is one or a mixture of two or more of ethylene glycol, thioethanol, isopropanol, hydroxypropanol, and tert-butanol.

[0023] As a further improvement of the present invention, in step S3, the heating rate is 20-40°C / min.

[0024] As a further improvement of the present invention, in step S3, the reaction time is 30-60 min. Using this technique, the gradually accumulated energy achieves nucleation work, promoting the nucleation and subsequent growth of Cu, and refining the particle size to approximately 80-100 nm with uniform distribution. This fully utilizes the nanoscale effect, thereby reducing the sintering temperature in subsequent use. However, if the reaction time is further increased to 120 min, the particle size actually increases. If the reaction time is too long, reaching 150 min, the generated nanoparticles will adsorb onto each other under continuous heat and agglomerate significantly during cooling, ultimately resulting in Cu nanoparticle sizes larger than 500 nm or even micrometers, failing to meet the low-temperature sintering requirements.

[0025] As a further improvement of the present invention, the reaction vessel in step S3 is glass or quartz. Besides possessing the same high transparency and good optical properties as conventional glass, which facilitates the observation of the chemical reaction process and results, quartz glass containers also exhibit superior high-temperature resistance and chemical inertness. Due to the strong alkaline environment (the addition of a large amount of soluble metal hydroxides, the ionization of acetate ions, and the reducing alcohol conditions) and the volatilization of organic vapors in this experiment, quartz glass containers can better withstand the strong alkaline environment, fully absorb the exothermic reaction, and withstand high gas pressure, thus ensuring experimental safety.

[0026] As a further improvement of the present invention, in step S4, the solvent used for washing is methanol, dichloromethane, or cyclohexane; furthermore, the solvent used for washing is a mixture of methanol and dichloromethane in a volume ratio of 1:2-3. The double bonds and amino groups in the long-chain organic amine structure are highly reactive functional groups. After the reaction at the reaction temperature in step S3, the double bonds undergo oxidation and the amines undergo acylation, subsequently transforming into carboxylic acids, alcohols, or amides (the polarity of these substances increases significantly). Methanol itself has high polarity, making it suitable for cleaning long carbon chains, especially alcohols. Dichloromethane itself has a relatively symmetrical structure and lacks individual hydrogen atoms, exhibiting low polarity, making it very suitable for cleaning substances with similarly symmetrical structures, such as amides. Considering that the amine group at the tail end has higher reactivity than the intermediate carbon-carbon double bond, its acylation reaction will inevitably occur earlier and faster, resulting in a higher amide content. Therefore, when considering the relative content of methanol and dichloromethane, a higher volume ratio of dichloromethane is selected. Moreover, using a 1:3 volume ratio of methanol to dichloromethane, after one wash, the residual organic matter around the particles is significantly reduced, and after two washes, almost no residual organic matter remains around the particles, demonstrating better cleaning effect. In addition, these washing solvents are all highly volatile substances, so there is no need to worry about introducing other organic matter again after washing away the residual organic amines, thus reducing the quality of subsequent sintering.

[0027] As a further improvement of the present invention, in step S3, the centrifugal washing is performed three or more times at a centrifugal speed of 4000 to 6000 rpm.

[0028] As a further improvement of the present invention, the particle diameter of the antioxidant nano-copper powder is 25-400 nm; the surface of the antioxidant nano-copper powder is uniformly covered with a dense organic amine coating layer.

[0029] This invention also discloses an antioxidant nano-copper powder, which is prepared by any one of the methods described above, wherein the surface of the antioxidant nano-copper powder is covered with an organic amine coating layer. Further, the particle diameter of the antioxidant nano-copper powder is 25–400 nm.

[0030] The present invention also discloses a nano-copper solder paste, which is obtained by uniformly mixing the antioxidant nano-copper powder as described above with solvent and flux; the mass ratio of the antioxidant nano-copper powder, solvent and flux is (7-9):(1-3):(0.02-0.06).

[0031] As a further improvement of the present invention, the solvent is terpineol, and the flux comprises ethyl cellulose and cellulose acetate. Using this technical solution, on the one hand, terpineol can effectively help the active Cu powder to disperse uniformly in the solder paste, thereby improving the chemical stability of the solder paste. This is crucial for ensuring the smooth progress of the sintering process. On the other hand, terpineol has a low decomposition temperature and low residual carbon content, which means that during the sintering process, the terpineol in the solder paste can fully volatilize, leaving fewer residues and reducing the residual carbon content in the sintered structure, which is beneficial for improving the density and reliability of the sintered joint. The main components of the flux are ethyl cellulose and cellulose acetate. Ethyl cellulose can be adjusted to achieve a suitable viscosity to achieve uniform printing of the solder paste on the chip; cellulose acetate can act as a plasticizer and antioxidant, increasing the viscosity of the solder paste and reducing the surface tension of the chip back or substrate. In addition, both types of cellulose are used as binders and film-forming agents for particles. They only begin to decompose at around 200°C. They can adhere to the surface of nano-Cu, isolate water and oxygen in the air, and prevent nano-Cu from oxidizing and deteriorating during the heating process. At the same time, they can decompose during the heat preservation process without hindering the contact diffusion of Cu, which is beneficial to improving the density of Cu nanoparticles after sintering.

[0032] As a further improvement of the present invention, the stirring speed is 1800-2200 r / s and the stirring time is 180-300 s.

[0033] As a further improvement of the present invention, the prepared solder paste is stored in a freezing environment of -5 to 0°C. Before soldering the upper and lower chips, the solder paste is thawed at room temperature for 1 to 2 hours before use.

[0034] The present invention also discloses a method for low-temperature sintering interconnection of components, wherein the nano-copper solder paste described above is applied to the surface of the components to be soldered, and after preheating, vacuum hot pressing is performed for sintering.

[0035] As a further improvement of the present invention, the nano-copper solder paste is uniformly applied to the surface of the component to be connected by screen printing 2 to 3 times. After the first preheating, the component is placed in the position to be connected. After the second preheating, the interconnection is achieved by vacuum hot pressing sintering.

[0036] As a further improvement of this invention, the screen material is nylon filament. Nylon screen has high strength and abrasion resistance, which allows it to withstand greater pressure and friction during the printing process. Its good elastic tension and chemical stability further ensure the quality and durability of the print. Furthermore, the uniform filament diameter and smooth surface of the nylon screen allow solder paste to pass through smoothly, reducing clogging and ink dripping, thus improving printing efficiency and quality.

[0037] As a further improvement of the present invention, the screen printing uses a screen with a thickness of 20-50 μm and a mesh size of 200-300 mesh. The relatively small mesh size of the 200-300 mesh screen allows for better control of ink distribution, resulting in clearer edges of the printed pattern and more refined printing, making it suitable for small chip packaging structures.

[0038] As a further improvement of the present invention, the distance between the screen and the chip surface is set to 0.4-0.6 mm before printing, the squeegee movement speed is 0.5-1 cm / s, and the squeegee angle is 5-15°.

[0039] Using this technical solution, a distance of 0.4-0.6mm is used to ensure that the solder paste passing through the screen has a certain thickness, while also having an appropriate spreading effect, and is evenly printed on the entire surface.

[0040] The squeegee operates at a speed of 0.5–1 cm / s, ensuring the solder paste flows steadily into the mesh openings to form a fine printed pattern. Due to the high viscosity of nano-Cu paste, at higher squeegee speeds (above 1 cm / s), the solder paste enters the mesh too quickly and cannot pass through all the openings, resulting in poor filling performance. Furthermore, at higher speeds, any fluctuations in the screen can lead to uneven printing thickness. Conversely, squeegee speeds below 0.5 cm / s result in low printing efficiency, increased time costs, and solder paste buildup on the screen also contributes to uneven printing thickness.

[0041] Regarding the squeegee angle, when the preset squeegee angle is too large, the deformation and wear at the squeegee side edges are more severe, resulting in insufficient extrusion pressure through the mesh. This causes solder paste to easily accumulate in front of the squeegee side edges during transfer, leading to leakage, especially for high-viscosity nano-Cu paste. To address this, a smaller squeegee angle (5–15°) is needed to apply greater downward extrusion pressure to the solder paste, creating an approximately straight line at the start of printing. This, combined with a pre-setting screen-to-surface distance of 0.4–0.6 mm, increases the amount of solder paste printed and reduces the leakage rate.

[0042] As a further improvement of this invention, the squeegee is made of polyurethane. The squeegee's function is to scrape and squeeze the solder paste from the screen, allowing it to be printed onto the chip or substrate, forming a uniformly thick intermediate sintering layer. The polyurethane squeegee maintains the elongation and resilience of rubber under high hardness, reducing damage to the squeegee, screen, and substrate surfaces. Furthermore, due to its excellent chemical resistance, it maintains its appearance and performance after multiple screen printings, reducing replacement frequency.

[0043] As a further improvement of the present invention, the first preheating temperature is 80-100℃, and the preheating time is 2-3 minutes; the second preheating temperature is 60-80℃, and the preheating time is 5-6 minutes. The purpose of the first preheating is to remove organic substances such as terpineol from the solder paste without damaging the organic amine coating layer on the surface of the nano-Cu. The preheating time should not be too long to prevent the nano-Cu from agglomerating before sintering. The second preheating operation is performed immediately before vacuum hot pressing sintering, and its purpose is to make the preheated nano-Cu layer more easily reach the temperature required for sintering, improve sintering efficiency, and reduce energy consumption.

[0044] More preferably, the vacuum degree of the vacuum hot pressing sintering is <5×10⁻⁶. -2 The pressure is 5–15 MPa, and the sintering temperature is 200–350 °C. More preferably, the sintering temperature is 200–350 °C, and the sintering time is 15–40 min; the shear strength of the interconnect joint after sintering can reach 48 MPa. Vacuum conditions can suppress Cu oxidation during sintering, and hot pressing conditions can increase the rapid diffusion between Cu particles and sintering densification. Since the nano-Cu powder prepared by the technical solution of this invention has only a thin organic amine coating layer on its surface, when the particle size is as low as 90 nm, the interconnect joint strength obtained by sintering at a temperature of 250 °C, a pressure of 10 MPa, and a time of 40 min can reach 48 MPa.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] First, the technical solution of this invention improves upon the traditional method for preparing nano-Cu powder. By selecting appropriate additives and controlling reaction conditions (reaction temperature, alkali concentration, and reaction time, etc.), the coating effect of long-chain amines is regulated, generating a uniform, dense, and extremely thin low-molecular-weight organic amine coating layer on the surface of nano-Cu powder. This significantly reduces the particle size of nano-Cu powder, inhibits Cu oxidation during synthesis, improves the stability of nano-copper in air, and reduces the risk of interfacial embrittlement during subsequent sintering. Furthermore, the entire reaction process in this technical solution is carried out in an atmospheric environment, with a short reaction time (as low as 15 minutes), and is simple to operate. Compared to the complex synthesis process of existing technologies, it avoids the need for high-temperature reactors and inert gas environments, significantly reducing manufacturing costs and improving production efficiency. Simultaneously, the solvents used in the reaction are all non-toxic or low-toxic reagents, meeting the production concepts of green chemistry and low-carbon manufacturing.

[0047] Secondly, the organic components in the nano-solder paste of this invention can volatilize and decompose under the set two preheating conditions, allowing the nano-Cu powder to fully contact and thus promoting the sintering of the nano-copper powder. In addition, the extremely thin low-molecular-weight organic amine coating layer on the surface of the nano-Cu powder can rapidly decompose at a sintering temperature of about 160°C, thereby promoting the rapid sintering and densification of Cu particles during the sintering process, further improving the reliability of the sintered joint.

[0048] Third, the technical solution of this invention adopts solder paste components and screen printing process that match the nano-Cu powder, and establishes a corresponding sintering process window, giving full play to the characteristics of low cost, anti-electromigration, high surface energy and low temperature sintering of nano-Cu powder. In particular, it effectively reduces the sintering temperature and time, and obtains interconnect connectors with high printing accuracy (thickness 20μm), good oxidation resistance and high reliability interface, which meets the packaging requirements of third-generation semiconductor devices. Attached Figure Description

[0049] Figure 1 The images shown are scanning electron microscope / secondary electron mode (SEM / SE) images of Cu nanoparticles obtained at different reaction temperatures in Example 1 of this invention, where (a) 155℃, (b) 175℃, and (c) 195℃.

[0050] Figure 2 This is the X-ray diffraction pattern of Cu nanopowder in Example 1 of the present invention.

[0051] Figure 3 The images shown are transmission electron microscope bright-field (TEM-BF) images and energy dispersive spectroscopy (EDS) surface scan elemental distribution maps of Cu nanopowder obtained in Example 1 of this invention, wherein (a) is a TEM-BF image, (b) is a Cu elemental distribution map, (c) is a C elemental distribution map, and (d) is an O elemental distribution map.

[0052] Figure 4 The images show SEM / SE images of Cu nanoparticles obtained under different molar ratios of sodium hydroxide to copper salt in Example 2 of this invention, where (a) no sodium hydroxide was added, (b) 0.5:1, (c) 1:1, and (d) 2:1.

[0053] Figure 5 The images shown are SEM / SE images of Cu nanoparticles obtained at different holding times in Example 3 of this invention, where (a) 15 min, (b) 30 min, (c) 60 min, and (d) 120 min.

[0054] Figure 6 The curves show the strength variation of the sintered interconnect joints obtained in Examples 2 and 3 of this invention during shear testing, with the peak value representing the shear strength.

[0055] Figure 7 These are comparative photographs of the solutions after the reaction in Example 1 and Comparative Example 1 of the present invention. The reaction temperatures of a, b, and c are 155℃, 135℃, and 115℃, respectively.

[0056] Figure 8 This is a photograph of the morphology of Cu nanoparticles obtained under the heat preservation reaction condition of 150 min in Comparative Example 2 of the present invention.

[0057] Figure 9 These are comparative photos of the cleaning effects of Examples 1-3 and Comparative Example 3 of the present invention, wherein (a) water + ethanol (Comparative Example 3), (b) methanol + dichloromethane 1:2 (Comparative Example 3), and (c) methanol + dichloromethane 1:3 (Example 1).

[0058] Figure 10 This is a microstructure diagram of the sintered joint of the "chip-Cu nano solder paste layer-substrate" after sintering, as shown in Comparative Example 4 of this invention. Detailed Implementation

[0059] To better explain and facilitate understanding of the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, the scope of protection of the present invention is not limited to the contents described herein.

[0060] A method for preparing antioxidant nano-copper powder includes the following steps:

[0061] Step (a): Dissolve the copper salt in solvent A and add a certain amount of sodium hydroxide to prepare precursor solution B;

[0062] Step (b): Dissolve L-ascorbic acid in solvent A and stir magnetically at 40-80°C for 10-20 minutes until the color turns orange or reddish-brown. Cool to room temperature to form reaction solution C.

[0063] Step (c): Mix the precursor solution B and the reaction solution C, and then immediately add the mixture to the reducing alcohol D. Stir until homogeneous to obtain the reaction system.

[0064] Step (d): The reaction system from step (c) is placed in a reaction vessel and heated to the specified reaction temperature at a heating rate of 20-40℃ / min, and held at that temperature for more than 15 minutes. After cooling to room temperature, a dark red product E is obtained.

[0065] In step (e), product E is washed, centrifuged and vacuum dried to obtain nano copper powder, which is then sealed for later use.

[0066] The concentration of copper salt in precursor solution B is 0.1–1 mol / L, and the molar ratio of sodium hydroxide concentration to copper salt is 0:1–2:1, preferably 1:1–2:1. The concentration of L-ascorbic acid in reaction solution C is 0.4–2 mol / L; precursor solution B and reaction solution C are mixed in equal volumes, and the mixing ratio of this mixed solution to reducing alcohol D is 1:2–1:4.

[0067] In step (c), the reducing alcohol D is selected as one or a mixture of several of ethylene glycol, thioethanol, isopropanol, hydroxypropanol, and tert-butanol.

[0068] In step (d), the reaction vessel is selected to be glass or quartz.

[0069] Preferably, the optimal reaction temperature range in step (d) is 155–195°C, and the optimal reaction time is 30–60 min.

[0070] In step (e), the solvent used for washing is methanol, dichloromethane or cyclohexane. Preferably, the solvent is a mixture of methanol and dichloromethane in a volume ratio of 1:3, and the washing is performed more than 3 times. The centrifugation speed is 4000 to 6000 rpm.

[0071] The obtained nano-Cu powder has a particle diameter of 25–400 nm and an average particle diameter of 80 nm; the surface of the nano-Cu is uniformly covered with a dense organic amine coating layer.

[0072] The following description uses specific examples and comparative models for illustration.

[0073] Example 1

[0074] A method for preparing antioxidant nano-copper powder includes the following steps:

[0075] Step (a): Dissolve 0.01 mol of copper acetate in 50 ml of 9-octadeceneamine and add 0.01 mol of sodium hydroxide to prepare a precursor solution;

[0076] Step (b): Dissolve 0.025 mol L-ascorbic acid in 50 ml of 9-octadeceneamine and stir magnetically at 40–80 °C for 10–20 min until the color turns orange or reddish-brown. Cool to room temperature to form a reaction solution.

[0077] Step (c): Mix the precursor solution and the reaction solution in equal volumes, and then immediately add the mixture to 120 ml of ethylene glycol. Stir until homogeneous to obtain the reaction system.

[0078] In step (d), the reaction system from step (c) is placed in a glass reaction vessel and heated to 155-195°C at a heating rate of 20-40°C / min. In this step, three different reaction temperatures are used in parallel in this embodiment: 155°C, 175°C, and 195°C, respectively, and the temperature is maintained for 30 min. After cooling to room temperature, a dark red product is obtained in all cases.

[0079] Step (e) involves washing, centrifuging, and vacuum drying the dark red product to obtain nano-copper powder, which is then sealed for later use. The washing solvent is a 1:3 volume ratio mixture of methanol and dichloromethane, and the product is washed three times. The centrifugation speed is 4000–6000 rpm. The SEM / SE images of the obtained nano-Cu powder are shown below. Figure 1 As shown, the XRD pattern is as follows Figure 2 As shown (the XRD patterns obtained at the three reaction temperatures are identical), the transmission electron microscope bright-field (TEM-BF) images and their energy dispersive spectroscopy (EDS) surface scan elemental distribution maps are as follows. Figure 3 As shown.

[0080] like Figures 1-3 As shown, the particle diameter of the nano-Cu powder obtained at different reaction temperatures ranges from 120 to 400 nm. The nano-Cu exhibits a polycrystalline structure. Furthermore, the elemental distribution diagrams for C and O show that C and O exhibit the same distribution trend, indicating that the surface of the nano-Cu is not oxidized CuO, but rather a thin and dense organic amine coating uniformly covering the Cu surface. Figure 5 bd). Additionally Figure 2 The XRD pattern shown also indicates that the nano-Cu powder does not have any CuO diffraction peaks, further proving that this method yields nano-Cu powder with extremely strong antioxidant properties.

[0081] Using the nano-Cu powder prepared above, nano-copper solder paste was further prepared. The steps are as follows: 0.9 parts of terpineol, 0.05 parts of ethyl cellulose and 0.05 parts of cellulose acetate were added to 9 parts of nano-Cu powder, and the mixture was placed in a mixing tank and stirred thoroughly at a stirring speed of 1800 r / s for 300 s to obtain nano-copper solder paste. The prepared solder paste was stored in a freezing environment of -5 to 0℃. Before soldering the upper and lower chips, the solder paste needed to be thawed at room temperature for 1 to 2 hours before use.

[0082] The solder paste is evenly applied to the back of one or both sides of the chip through 2-3 screen printing processes. The squeegee is made of polyurethane, the screen is made of nylon filament with a thickness of 20μm and a mesh size of 300. Before printing, the distance between the screen and the chip surface is set to 0.6mm, the squeegee movement speed is 0.5cm / s, and the squeegee angle is 5°.

[0083] Furthermore, after the first preheating (preheating temperature 80℃, preheating time 3min), the upper and lower chips to be interconnected are placed opposite each other using a pick-and-place machine and fixture. After a second preheating (preheating temperature 60℃, preheating time 6min), vacuum thermoforming is used to achieve the mechanical and electrical interconnection of the upper and lower chips, wherein the vacuum degree is <5×10 - 2 The welding pressure applied to the chip is 10 MPa, the sintering temperature is 250-300℃, and the sintering time is 30 min.

[0084] This method controls the reaction temperature to regulate the coating effect of long-chain amines, generating a uniform, dense, and extremely thin low-molecular-weight organic amine coating layer on the surface of nano-Cu powder. This reduces the particle size of the nano-Cu powder and significantly inhibits Cu oxidation. A matching screen printing process was designed, and a corresponding sintering process window was established, effectively reducing the sintering temperature and time. This resulted in interconnects with high printing precision (20 μm thickness), good oxidation resistance (no CuO diffraction peaks), and a highly reliable interface. After sintering, the shear strength of the interconnects reached 36 MPa, achieving the same level as commonly used Sn-based solders in the market, basically meeting the requirements of third-generation semiconductor packaging.

[0085] Example 2

[0086] A method for preparing antioxidant nano-copper powder, which differs from Example 1, is as follows:

[0087] In step (a), the copper salt is copper nitrate. Different amounts of sodium hydroxide are added to prepare a precursor solution. The molar ratio of sodium hydroxide to copper salt is 0.5:1, 1:1, and 2:1, respectively. A blank control group is set up, i.e. no sodium hydroxide is added.

[0088] In step (d), the temperature is raised to 175°C;

[0089] The solvent used in steps (a) and (b) is N,N-dimethyldodecyl tertiary amine, and the reducing alcohol D in step (c) is tert-butanol.

[0090] The SEM / SE image of the Cu nanoparticles obtained in this embodiment is shown below. Figure 4 As shown, the nano-Cu powder obtained by this method has a particle diameter of 80–300 nm. Especially when the molar mass ratio of sodium hydroxide to copper acetate is in the range of 1:1 to 2:1, the nano-Cu powder is even finer and more uniformly distributed, with an average particle diameter as low as 140 nm (e.g., ...). Figure 4 (c)-4(d)) Smaller nanoscale sizes can exert stronger size effects and high surface energy advantages, enabling sintering at lower temperatures. At the same time, a layer of organic amine coating can be seen uniformly covering the Cu surface.

[0091] In this embodiment, the nano solder paste is composed of 1.4 parts terpineol, 0.05 parts ethyl cellulose, and 0.05 parts cellulose acetate added to 8.5 parts nano Cu powder. Furthermore, in this scheme, the viscosity of the nano Cu paste is reduced, the screen mesh size is adjusted to 200 mesh, the distance between the screen and the chip surface is adjusted to 0.4 mm before printing, the squeegee movement speed is increased to 1 cm / s, and the squeegee angle is adjusted to 15°.

[0092] The sintering temperature in the vacuum hot pressing sintering step is 200-250℃, and the sintering time is 30 minutes.

[0093] This method, based on the optimal reaction temperature, regulates the coating effect of long-chain amines by controlling the concentration of alkali, further reducing the particle size of nano-Cu powder. Smaller particle size, lower viscosity, and a suitable screen printing process further reduce the sintering temperature to 200–250°C. The shear strength of the interconnects after sintering can reach 43 MPa, and its strength change during shear testing is as follows: Figure 6 As shown. This strength exceeds the equivalent level of commonly used Sn-based solders on the market, meeting the requirements of third-generation semiconductor packaging.

[0094] Example 3

[0095] A method for preparing antioxidant nano-copper powder, which differs from Examples 1 and 2, is as follows:

[0096] In step (a), the copper salt is copper chloride, and 0.01 mol of sodium hydroxide is added to prepare a precursor solution;

[0097] In step (d), the temperature is raised to 175°C and held for 15–120 min. After cooling to room temperature, a dark red product is obtained. In this embodiment, different holding times, i.e., reaction times, are set as follows: (a) 15 min, (b) 30 min, (c) 60 min, and (d) 120 min.

[0098] The solvent used in steps (a) and (b) is N,N-dimethyldodecylamine, and the reducing alcohol D in step (c) is isopropanol.

[0099] SEM / SE images of Cu nanoparticles obtained at different reaction times are shown below. Figure 5 As shown, the particle diameter of the nano-Cu powder obtained in this embodiment is 20–240 nm. Especially when the reaction time is in the range of 30–60 min, the size of the nano-Cu powder is even smaller and the distribution is more uniform, with an average particle diameter as low as 80 nm. Figure 5 (b)-5(c)) It is precisely because of such small particle size that ultra-high strength sintered joints can be prepared at lower sintering temperatures in the future.

[0100] The formulation and steps for preparing solder paste using the nano-Cu powder prepared in this embodiment are the same as in Example 1. However, due to the smaller particle size compared to Example 1, the screen printing parameters are different:

[0101] The screen mesh size is 300 mesh. Before printing, the distance between the screen and the chip surface is set to 0.4 mm, the squeegee movement speed is 1 cm / s, and the squeegee angle is 10°.

[0102] The sintering temperature in the vacuum hot pressing sintering step is 200-350℃, and the sintering time is 40 minutes.

[0103] In this embodiment, the coating effect of long-chain amines is further controlled by adjusting the reaction time. Figure 5 The SEM / SE images of the obtained copper powder show that the particle size first decreases and then increases with increasing reaction time, with the smallest particle size and an average grain size of 90 nm observed at a reaction temperature of 30 min. This method fully utilizes the low cost, anti-electromigration, high surface energy, and low-temperature sintering characteristics of nano-Cu powder, while still achieving interconnects with high printing accuracy, good oxidation resistance, and high-reliability interfaces. The shear strength of the sintered interconnects can reach 48 MPa. Figure 6 As shown, its strength far exceeds that of commonly used Sn-based solders in the market, fully meeting the requirements of third-generation semiconductor device packaging.

[0104] Comparative Example 1

[0105] Based on Examples 1-3, the reaction temperature in this comparative example differs, mainly in the following aspects:

[0106] The reaction temperatures were 115°C and 135°C (two different reaction temperatures). At lower reaction temperatures, the molecular motion of the reactants is slower, and the reaction energy is insufficient for nucleation, resulting in a slower crystal nucleus formation rate and a relatively smaller number of crystal nuclei. Under these conditions, the resulting crystals, due to the lack of sufficient crystal nuclei and the slower formation rate, tend to have larger crystal sizes, or even fail to form at all, which does not meet the requirements for nanoparticles needed for low-temperature sintering of chips.

[0107] Comparative photos of the solutions after reaction in Example 1 and Comparative Example 1 are shown below. Figure 7 As shown, the final reaction solutions obtained at reaction temperatures of 135℃ and 115℃ are as follows: Figure 7 As shown in b and 7c, the solution remains pale yellow or white, indicating that Cu ions were not completely reduced or even not reduced at the low reaction temperature. However, as... Figure 7As shown in Figure a (Examples 1 to 3 are the same, only the reaction temperature of 155°C in Example 1 is listed for comparison), the solutions after the reaction in Examples 1 to 3 at the appropriate reaction temperature are dark red, indicating that the increase in reaction temperature promotes the rapid formation and growth of crystal nuclei, thus generating a large number of nano-Cu particles. The bottom of the centrifuge tube contains the generated nano-Cu powder.

[0108] Comparative Example 2

[0109] The reaction time in this comparative example differs from that in Examples 1-3, mainly in the following aspects:

[0110] The reaction time was 150 min. Due to the excessively long reaction time, a large number of nano-Cu particles agglomerated together, resulting in the morphology of the final nano-Cu powder as shown in the image. Figure 8 As shown, the formed nano-Cu particles are large and irregular in size, with some particles reaching the micrometer level. This significantly reduces the surface energy and the number of surface active sites. To achieve the joint strength shown in Examples 1-3 after sintering, the sintering temperature, pressure, and sintering time all need to be significantly increased. Furthermore, excessively long reaction times lead to large-area cross-linking of the organic matter, such as... Figure 8 As shown, even after 4-5 cycles of washing with a mixture of methanol and dichloromethane, the organic matter adhering to the particle surface could not be removed. This also reduced the density and reliability of the sintered joint.

[0111] Comparative Example 3

[0112] Based on Examples 1-3, the composition of the cleaning solvent and the number of cleaning cycles differ in this comparative example, mainly in the following aspects:

[0113] The cleaning solvents were water and ethanol, and the cleaning was performed three times. The nano-Cu powder obtained after three cleaning cycles with water and ethanol is shown below. Figure 9 As shown in (a), the cleaning effect is far from ideal. The nano-Cu particles are coated with a large amount of residual organic matter, which cannot be removed and hinders the contact of nano-Cu during subsequent sintering. This is extremely detrimental to reliable sintering.

[0114] The cleaning solvent was methanol and dichloromethane in a 1:2 volume ratio, and the cleaning was performed once. The resulting copper powder image is shown below. Figure 9 As shown in (b), it can be seen that after cleaning once, the amount of organic matter remaining around the nano-Cu powder is significantly reduced.

[0115] In Example 1, the cleaning solvent was methanol and dichloromethane in a 1:3 volume ratio, and the cleaning was performed twice. The resulting copper powder is shown in the image below. Figure 9 As shown in (c), after two cleanings, there is almost no residual organic matter left around the nano-Cu powder particles.

[0116] It is evident that the cleaning solvent used in Example 1 is more efficient. However, multiple washes with a 1:2 volume ratio of methanol and dichloromethane can also achieve the same effect.

[0117] In summary, the technical solution of this invention improves upon the traditional preparation method of nano-Cu powder. By controlling reaction conditions (reaction temperature, alkali concentration, and reaction time, etc.), the coating effect of long-chain amines is modulated, generating a uniform, dense, and extremely thin low-molecular-weight organic amine coating layer on the surface of nano-Cu powder. This significantly reduces the particle size of nano-Cu powder (average particle size can reach 80 nm) and inhibits Cu oxidation during synthesis. Furthermore, a solder paste composition and screen printing process matching the nano-Cu powder were designed, and a corresponding sintering process window was established. This fully leverages the low cost, electromigration resistance, high surface energy, and low-temperature sintering characteristics of nano-Cu powder, especially effectively reducing the sintering temperature and time. This results in interconnects with high printing accuracy (20 μm thickness), good oxidation resistance, and high-reliability interfaces, meeting the packaging requirements of third-generation semiconductor devices.

[0118] Comparative Example 4

[0119] Based on Example 1, the difference in this comparative example is that hexadecylamine, which is not easily volatile at low temperatures, was used as the long-chain alkylamine. Furthermore, only terpineol was used as a component in the solder paste preparation, while keeping the sintering process parameters unchanged.

[0120] The sintered joint of the "chip-Cu nano-solder paste layer-substrate" exhibits a strength of less than 20 MPa or even almost no strength, showing a significant decrease in strength. The microstructure is as follows: Figure 10 As shown, the white area in the middle represents sintered Cu, while the surrounding black area represents residual organic matter and unsintered nanoparticles. Because the long-chain alkylamines in this method are not easily volatilized, the organic amines coating the Cu surface were not removed before sintering. Furthermore, the solder paste provided by terpineol alone has poor flowability, leaving a large amount of residual organic matter around the nanoparticles. Both of these factors hinder the formation of a dense sintered structure between Cu and Cu nanoparticles and cause large cracks at the interface, thus significantly reducing the strength of the sintered joint. This also illustrates the importance of using ethyl cellulose and cellulose acetate in the solder paste.

[0121] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing antioxidant nano-copper powder, characterized in that: Includes the following steps: Step S1: Dissolve the copper salt in a solvent, add a soluble metal hydroxide, and prepare a precursor solution; wherein the molar ratio of the soluble metal hydroxide to the copper salt is 0.5-2:

1. Step S2: Dissolve L-ascorbic acid in a solvent and stir at 40-80°C for 10-20 minutes until the color turns orange or reddish-brown. Cool to room temperature to obtain the reaction solution. Step S3: Mix the precursor solution and the reaction solution to obtain a mixture, wherein the molar ratio of copper salt to L-ascorbic acid is 1:2-4; add the mixture to a reducing alcohol, stir evenly, heat to a reaction temperature of 155-195°C, keep the reaction at this temperature for more than 15 minutes, and cool to room temperature to obtain the product. Step S4: After washing, centrifuging and drying the product, antioxidant nano copper powder is obtained. The solvents mentioned in steps S1 and S2 are one or a mixture of two or more of 9-octadeceneamine, N,N-dimethyldodecyl tertiary amine, N,N-dimethyldodecylamine, and N,N-diethyltrimethylsilylamine.

2. The method for preparing antioxidant nano-copper powder according to claim 1, characterized in that: The copper salt is one or a mixture of two or more of copper sulfate, copper chloride, copper nitrate, and copper acetate.

3. The method for preparing antioxidant nano-copper powder according to claim 2, characterized in that: In the precursor solution, the concentration of copper salt is 0.1–1 mol / L, and the molar ratio of the soluble metal hydroxide to copper salt is 1–2:1; in the reaction solution, the concentration of L-ascorbic acid is 0.4–2 mol / L; in step S3, the precursor solution and the reaction solution are mixed in equal volumes, and the volume ratio of the mixture to the reducing alcohol is 1:2–1:

4.

4. The method for preparing antioxidant nano-copper powder according to claim 3, characterized in that: The reducing alcohol is one or a mixture of two or more of ethylene glycol, thioethanol, isopropanol, hydroxypropanol, and tert-butanol.

5. The method for preparing antioxidant nano-copper powder according to claim 1, characterized in that: In step S3, the reaction time is 30-60 min; in step S4, the solvent used for washing is methanol, dichloromethane or cyclohexane; the washing is centrifuged three times or more at a speed of 4000-6000 rpm; the particle diameter of the antioxidant nano copper powder is 25-400 nm.

6. An antioxidant nano-copper powder, characterized in that: It is prepared by the method of preparing antioxidant nano-copper powder as described in any one of claims 1 to 5, wherein the surface of the antioxidant nano-copper powder is covered with an organic amine coating layer.

7. A nano-copper solder paste, characterized in that: The antioxidant nano-copper powder as described in claim 6 is obtained by mixing it with solvent and flux evenly; the mass ratio of the antioxidant nano-copper powder, solvent and flux is (7-9):(1-3):(0.02-0.06).

8. The nano-copper solder paste according to claim 7, characterized in that: The solvent is terpineol, and the flux contains ethyl cellulose and cellulose acetate; the stirring speed is 1800-2200 r / s, and the stirring time is 180-300 s.

9. A method for low-temperature sintering interconnection of electronic components, characterized in that: The nano-copper solder paste of claim 7 is applied to the surface of the component to be soldered, and after preheating, it is vacuum hot-pressed and sintered.

10. The method for low-temperature sintering interconnection of components according to claim 9, characterized in that: The nano-copper solder paste is evenly applied to the surface of the component to be connected by screen printing 2 to 3 times. After the first preheating, the component is placed in the position to be connected. After the second preheating, the interconnection is achieved by vacuum hot pressing sintering. The screen printing uses a screen thickness of 20–50 μm and a mesh size of 200–300 mesh. Before printing, the distance between the screen and the chip surface is set to 0.4–0.6 mm, the squeegee movement speed is 0.5–1 cm / s, and the squeegee angle is 5–15°. The first preheating temperature is 80–100°C, and the preheating time is 2–3 min. The second preheating temperature is 60–80°C, and the preheating time is 5–6 min. The vacuum degree of the vacuum hot pressing sintering is <5×10 -2 MPa, pressure of 5-15 MPa, sintering temperature of 200-350℃.

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