Two kinds of nano-particle size copper powder dense coating micro copper sheet copper powder and its preparation method and the application of preparing copper paste

Abstract

The application discloses two kinds of copper powder with micron copper sheet densely coated by nano-particle copper powder, a preparation method of the copper powder and application of the copper paste, and belongs to the technical field of copper powder. The two kinds of copper powder with micron copper sheet densely coated by nano-particle copper powder are composed of micron copper sheets with a length of 1-2 microns and nano copper particles with two particle diameters of 5-15 nm and 40-100 nm. The micron copper sheet is completely and tightly wrapped by the nano copper particles with the two particle diameters, and the nano copper particles with the particle diameter of 5-15 nm are tightly coated around the nano copper particles with the particle diameter of 40-100 nm. The copper powder is prepared by a two-step process one-time synthesis method, the copper powder is mixed with an organic solvent and stirred to prepare a copper paste, the obtained copper paste can promote copper particles to realize sintering and organization densification under low-temperature sintering and obtain a high-strength interconnection structure, and is suitable for low-temperature pressureless sintering interconnection packaging of power chips and power devices.

Description

Technical Field

[0001] This invention relates to copper powder for copper paste, specifically to a copper powder with two nano-sized copper powders densely coating micron-sized copper sheets, its preparation method, and its application in preparing copper paste; this copper paste is applicable to the field of power chip and power device packaging materials technology. Background Technology

[0002] With the increasing demand for high-performance, high-reliability energy conversion and high-frequency operation in fields such as electric vehicles, 5G communications, aerospace, industrial automation, and new energy, power semiconductors are playing an increasingly important role in power conversion and efficient energy utilization. However, the performance of traditional silicon-based semiconductor devices under harsh conditions such as high temperature, high pressure, and high frequency is gradually showing its limitations. Third-generation semiconductor materials, such as silicon carbide (SiC) and gallium nitride (GaN), are gradually leading the new development of power semiconductor technology with their superior characteristics of high frequency, high power, high temperature resistance, and lower energy loss.

[0003] The emergence of third-generation semiconductor materials and devices has placed higher demands on packaging materials. However, traditional packaging materials cannot meet the stringent requirements of high-temperature operation of the packaged structure. Currently, low-temperature sintering technology using nano-silver paste for high-power device packaging has been widely researched and applied. However, the high electron and ion migration rates of sintered silver connectors can easily lead to short circuits and failures in small-pitch silver interconnects within chips or devices. Furthermore, the high price of silver significantly limits the application of nano-silver paste in high-power device packaging. Since copper costs approximately one percent of silver and possesses good electrical and thermal conductivity as well as strong resistance to electromigration and ion migration, nano-copper paste sintering interconnect technology has attracted considerable attention from researchers due to its enormous development potential and broad application prospects.

[0004] Currently, copper paste is mostly prepared using copper powder with a single particle size. However, copper paste prepared from copper powder with a single particle size has many inter-particle voids, resulting in a porous sintered body with low density. Researchers often mechanically mix separate nano-copper powders with micron-copper powders to increase the packing density of the copper powder, thus achieving higher density in the sintered copper paste. However, this method requires preparing two or more copper powders of different particle sizes separately, increasing the complexity of the preparation process and easily causing uneven mixing of the two powder sizes. Furthermore, copper is prone to oxidation, and the resulting oxides hinder inter-particle atomic diffusion and sintering, deteriorating interconnect performance and making it difficult for copper paste to form reliable interconnects at lower temperatures below 200°C. To obtain a dense sintered structure and high-strength interconnects, significant pressure must be applied during sintering, which significantly increases process complexity and production costs, and may even damage power chips.

[0005] Chinese invention patent CN109926577B discloses a copper paste that can be used for low-temperature sintering and achieves low porosity. This copper paste comprises: spherical copper particles, flake copper particles, flake indium particles, and a high-linkage resin; the two types of copper particles account for more than 80%, the indium particles account for 10-20%, and the high-linkage resin accounts for 0-10%. Under pressureless conditions, the sintering temperature of this copper paste can be as low as approximately 180-250℃, and the density after sintering reaches over 95%. The addition of 10-20% indium particles is beneficial for lowering the sintering temperature, but its high price limits its practicality and makes large-scale application difficult.

[0006] Chinese invention patent CN 114054746 B discloses a copper powder with a three-peaked particle size distribution ranging from nanometers to micrometers, composed of copper nanoparticles with particle sizes of 5–15 nm, submicron copper particles with particle sizes of 120–210 nm, and micron-sized flake copper powder with particle sizes of 1–2 μm; the submicron and micron-sized flake copper powders are coated with nanoparticle copper powder. The copper powder in this technology is obtained by reacting a reducing agent with a reaction solution at 80–100°C under stirring, followed by washing the reaction product after centrifugation; the reaction solution is obtained by mixing a composite dispersant formed from copper salts, organic acids, and organic amines with ethylene glycol. However, based on the description in the patent specification and the appendix… Figure 1 It is evident that in the three-peaked copper powder distribution, nano-particle copper powder coats the surface of submicron and micron-sized copper powder, while submicron-sized copper powder does not coat the surface of micron-sized copper powder. In particular, the content of small nano-particle copper powder that is easy to sinter is relatively low. The amount of nano-particle copper powder on both the surface of submicron and micron-sized copper powder is small, and no dense coating is formed. The overall packing density of the copper powder is low, resulting in poor interfacial bonding ability for power chip packaging. Therefore, this patented technology is mainly used for direct printing circuits on flexible substrates, and it is difficult to apply it to power chip and power device packaging. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides a copper powder with two nano-sized copper powders densely packed and exhibiting a tri-peaked particle size distribution, which can be produced in one step using a two-step process, has good repeatability, low cost, and is conducive to large-scale production, and is densely packed with copper powders of two nano-sized particles, densely coating micron-sized copper sheets, and the preparation method thereof.

[0008] Another objective of this invention is to provide the application of copper powder with the two nano-sized copper powders densely coating micron-sized copper sheets in the preparation of copper paste, overcoming the difficulty in obtaining dense sintered structure and high-strength interconnect joints under low-temperature and pressureless sintering conditions in existing copper pastes, while simplifying the sintering process. This copper paste can be used in power chip and power device packaging.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] The copper powder, which is a dense coating of micron-sized copper flakes with two different nano-sized copper powders, consists of micron-sized copper flakes with a length of 1–2 μm and nano-sized copper particles with diameters of 5–15 nm and 40–100 nm. The micron-sized copper flakes are completely and tightly wrapped by the nano-sized copper particles of the two different sizes, and the copper particles with a diameter of 5–15 nm are tightly wrapped around the nano-sized copper particles with a diameter of 40–100 nm. The three types of copper powder exhibit a three-peak distribution of particle size and a densely packed aggregate.

[0011] The preparation method of the two types of copper powder with dense coating of micron-sized copper flakes using nano-sized copper powder is as follows: A weak reducing agent is added to the first pre-prepared liquid in a reaction vessel to obtain an initial reaction liquid. The initial reaction liquid is continuously stirred at a temperature of 80–120°C to obtain a suspension of flake-shaped copper powder. A strong reducing agent is first added to the flake-shaped copper powder suspension, followed by the addition of a second pre-prepared liquid to form a final reaction liquid. The temperature is maintained at 80–120°C and the mixture is continuously stirred to obtain three suspensions of copper powder with different particle sizes. The suspensions are then cooled, centrifuged, and washed to obtain copper powder with dense coating of micron-sized copper flakes using two types of nano-sized copper powder.

[0012] The first pre-prepared liquid is obtained by mixing inorganic copper salt, organic acid, amine compound and polyol solvent, and continuously stirring at a temperature of 80–100°C.

[0013] The second pre-prepared liquid is obtained by mixing organic copper salt, organic acid and polyol solvent, and stirring continuously at a temperature of 80–100°C.

[0014] Preferably, the inorganic copper salt is one or more of copper carbonate, basic copper carbonate, copper sulfate, copper hydroxide, copper chloride, and copper nitrate trihydrate;

[0015] The organic acid is one or more selected from oxalic acid, glycine, citric acid, tartaric acid, lactic acid, propionic acid, and oleic acid;

[0016] The amine compounds mentioned are one or more selected from oleylamine, ethylenediamine, methanolamine, triethanolamine, N,N-dimethylethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, and diethylenetriamine;

[0017] The polyol solvent is one or more selected from ethylene glycol, diethylene glycol, propylene glycol, glycerol, isopropanol, and n-propanol;

[0018] The organic copper salt is one or more of copper acetate, copper amino acid, copper rosinate, copper citrate, and copper tyrosine.

[0019] Preferably, in the first pre-prepared solution, the concentration of the inorganic copper salt is 30–80 g / L, and the total concentration of organic acid and amine compounds is 300–600 g / L, wherein the mass concentration ratio of organic acid to amine compounds is 1:1–5:1.

[0020] In the second pre-prepared solution, the concentration of the organic copper salt is 30–80 g / L, and the concentration of the organic acid is 400–1200 g / L.

[0021] Preferably, the weak reducing agent is one or more of sodium citrate, sodium hypophosphite, sodium phosphite, potassium tartrate, hydrogen peroxide, and glucose;

[0022] The strong reducing agent is one or more of hydrazine hydrate, hydrazine sulfate, sodium borohydride, ascorbic acid, and tetrabutylammonium borohydride.

[0023] Preferably, the concentration of the weak reducing agent in the initial reaction solution is 400–800 g / L; and the concentration of the strong reducing agent in the final reaction solution is 200–500 g / L.

[0024] Preferably, in the preparation of the flake copper powder suspension, the three different particle size copper powder suspensions, the first pre-prepared solution and the second pre-prepared solution, the continuous stirring rate is 400–600 r / min, the stirring time in the preparation of the first pre-prepared solution and the second pre-prepared solution is 5–20 min, the stirring time in the preparation of the flake copper powder suspension is 10–60 min, and the stirring time in the preparation of the three different particle size copper powder suspensions is 10–30 min.

[0025] The cooling mentioned refers to cooling to room temperature;

[0026] The centrifugation is performed by centrifuging the cooled reaction product at a speed of 3000–6000 r / min for 3–10 minutes using a centrifuge.

[0027] The washing process involves repeatedly washing 2–5 times with one or more of ethanol, deionized water, and acetone.

[0028] The application of copper powder preparation of copper paste by densely coating micron-sized copper flakes with the two types of nano-sized copper powders.

[0029] Preferably, copper powder with two nano-sized copper powders densely coating micron-sized copper sheets is uniformly stirred with an organic solvent and degassed to prepare copper paste; the organic solvent is one or more of ethylene glycol, propylene glycol, glycerol, diethylene glycol, terpineol, and polyethylene glycol.

[0030] Preferably, the uniform mixing is performed using a planetary gravity mixer;

[0031] By mass percentage, the copper powder of the two types of nano-sized copper powder densely coating the micron-sized copper sheets accounts for 75–90%, and the organic solvent accounts for 25–10%.

[0032] The application of the copper paste in the preparation of sintered joints: The copper paste is printed on the surface of a pure copper substrate using a screen printing method, with a thickness of 10–400 μm. The power chip or power device is placed on the printed copper paste surface and a bonding pressure of 0–0.5 MPa is applied and maintained for 1–5 min to obtain a sandwich structure assembly of "chip / copper paste / copper substrate" to be sintered. Under a nitrogen atmosphere and without pressure assistance, the sintered joint is formed at a temperature of 160–240 °C for 10–40 min.

[0033] Compared with the prior art, the advantages of this invention are:

[0034] 1) The copper powder of the present invention, which is densely coated with two types of nano-sized copper powder, consists of micro-sized copper flakes with a larger size of 1–2 μm and two types of nano-sized copper particles with smaller sizes of 5–15 nm and 40–100 nm. The micro-sized copper flakes are tightly wrapped by the two types of nano-sized copper particles, while the 5–15 nm nano-sized copper particles tightly wrap around the 40–100 nm nano-sized copper particles to form aggregates. The copper powder as a whole exhibits the characteristics of a three-peaked particle size distribution and dense packing. Micron-sized copper sheets, acting as a reinforcing "skeleton," possess higher strength, superior oxidation resistance, and stability. High-surface-energy nano-copper particles exhibit greater sintering driving force and excellent low-temperature sintering performance. The prepared copper powder combines micron-sized copper sheets and nano-copper particles, leveraging the advantages of both. More importantly, the abundant nano-copper particles of both sizes fill the gaps between the micron-sized copper sheets, increasing the density of the copper powder structure and achieving complete encapsulation without significant voids. This ensures that after low-temperature sintering and diffusion fusion, the nano-copper particles of both sizes connect the micron-sized copper sheets, forming the "skeleton," resulting in a dense sintered structure. This ensures that the copper powder of this invention is suitable for the field of power chip and power device packaging materials, with interconnect strength exceeding 20 MPa after sintering under low-temperature, pressureless sintering conditions at 200°C.

[0035] 2) This invention utilizes copper paste made from copper powder with a three-peaked particle size distribution and dense particle packing. This paste can be sintered at low temperature and without pressure in a nitrogen atmosphere to obtain high-strength interconnects. During the chip sintering interconnection process using this copper paste, the organic acids adsorbed on the surface of the copper powder and the reducing alcohol organic solvents in the copper paste can both inhibit the oxidation of the copper powder. This eliminates the need to introduce a reducing atmosphere, avoiding the use of reducing atmospheres (such as formic acid) and their damage to sintering equipment and the hazards to human health and safety. Simultaneously, the large particle size difference between micron-sized copper flakes and nano-sized copper particles in this copper powder drives the rapid fusion of nano-sized copper particles on the micron-sized copper flakes. The surface energy released during fusion causes a local temperature increase at the sintering neck, thus enabling low-temperature rapid sintering. The sintering temperature can be as low as 200°C, significantly reducing electricity costs.

[0036] 3) The copper paste prepared by this invention does not require pressure during the sintering process, which can further simplify the sintering process and reduce production costs.

[0037] 4) This invention employs a two-step, one-time synthesis method to prepare three different copper powders with densely packed particles. This two-step, one-time synthesis method first prepares a suspension containing micron-sized flake-like copper powder, then nucleates and grows two sizes of copper nanoparticles on the micron-sized flake-like copper powder, ultimately obtaining copper powder with a three-peaked particle size distribution and dense particle packing. This process is simple, efficient, and reproducible. Compared to traditional mechanical mixing methods of copper powders with different particle sizes and one-time synthesis methods, the two-step, one-time synthesis method allows for precise control of the copper powder ratio and ensures a more uniform distribution and dense packing of the copper powder.

[0038] 5) This invention allows for the control of the shape and particle size distribution of copper powder by adjusting the amounts of copper salts, organic acids, and amine compounds as needed. When the content of organic acids is low, amine compounds play a major role in controlling the shape and particle size distribution of copper powder. They can react with inorganic copper salts to form complexes, hindering the redox reaction. Furthermore, amine compounds exhibit good selective adsorption of copper nanocrystals, which helps promote the formation of micron-sized copper flakes. When the content of organic acids is high, organic acids play a major role in controlling the shape and particle size distribution of copper powder. Organic acids accelerate the redox reaction rate, generating more copper crystal nuclei per unit time, which helps promote the formation of smaller nanoparticles. Attached Figure Description

[0039] Figure 1 The image shows scanning electron microscope (SEM) images of copper powder with two nanoparticle sizes densely coating micron-sized copper flakes prepared in Example 1.

[0040] Figure 2 The images are transmission electron microscope (TEM) images of copper powder with two nanoparticle sizes densely coated with micron-sized copper flakes prepared in Example 1.

[0041] Figure 3 The image shows a scanning electron microscope (SEM) image of the fracture morphology of the copper paste sintered interconnect joint prepared in Example 1.

[0042] Figure 4 The image shows a scanning electron microscope (SEM) image of small copper powder-coated copper microsheets prepared for Comparative Example 2. Detailed Implementation

[0043] To better understand the present invention, the invention will be further described below with reference to the accompanying drawings and embodiments, but the implementation of the present invention is not limited thereto.

[0044] Compared to the copper powder with a three-peaked particle size distribution from nanometer to micrometer in Chinese invention patent CN 114054746 B, the copper powder of this invention appears to have a similar particle size, mainly differing in the particle size of the 120–210nm submicron copper powder from the 40–100nm nanoparticle copper powder of this invention. However, the substantial difference lies in the quantity of nanoparticle copper powder and the combination of the three types of copper powder. In Chinese invention patent CN 114054746 B, nanoparticle copper powder is coated on the surface of submicron and micron flake copper powder. The amount of nanoparticle copper powder on both the surface of submicron and micron flake copper powder is small, failing to form a dense coating. The overall packing density of the copper powder is low, resulting in poor interfacial bonding ability for power chip packaging. Therefore, this patented technology is mainly used for direct printing circuits on flexible substrates and is difficult to apply to power chip and power device packaging. The three types of densely packed copper powder with different particle sizes in this invention are composed of micron-sized copper flakes of a larger size of 1–2 μm and nano-sized copper particles of two sizes of smaller size: 5–15 nm and 40–100 nm. The micron-sized copper flakes are tightly wrapped by the nano-sized copper particles of the two sizes, while the 5–15 nm nano-sized copper particles tightly coat the larger 40–100 nm nano-sized copper particles to form aggregates. The synthesized copper powder as a whole exhibits a three-peaked particle size distribution and dense packing characteristics. The copper powder of the two sizes of nano-copper particles in this invention truly achieves complete coating of the micron-sized copper flakes.

[0045] The preparation method of the two types of copper powder with dense coating of micron-sized copper flakes in this invention is a two-step process in one synthesis. In the same reaction vessel, a suspension of larger-scale flake copper powder is first prepared, followed by two process steps to prepare suspensions of copper powder with three different particle sizes. After centrifugation and washing, three densely packed copper powders with different particle size characteristics are obtained. Specifically, the preparation method of copper powder with two nano-sized copper powders densely coating micron-sized copper flakes involves adding a weak reducing agent to a first pre-prepared liquid in a reaction vessel to obtain an initial reaction liquid. The initial reaction liquid is continuously stirred at a temperature of 80–120°C to obtain a suspension of flake-shaped copper powder. A strong reducing agent is first added to the flake-shaped copper powder suspension, followed by the addition of a second pre-prepared liquid to form a final reaction liquid. The temperature is maintained at 80–120°C and the mixture is continuously stirred to obtain three suspensions of copper powder with different particle sizes. After cooling, centrifugation, and washing, copper powder with two nano-sized copper powders densely coating micron-sized copper flakes is obtained. The first pre-prepared liquid is obtained by mixing inorganic copper salts, organic acids, amine compounds, and polyol solvents and continuously stirring at a temperature of 80–100°C. The second pre-prepared liquid is obtained by mixing organic copper salts, organic acids, and polyol solvents and continuously stirring at a temperature of 80–100°C. In this preparation method, the specific composition of the first and second pre-prepared solutions is a key technical measure that distinguishes this invention from the prior art. The inorganic copper salts, organic acids, amine compounds, polyol solvents, and organic copper salts can all be selected and adjusted by those skilled in the art according to the purpose of the invention, as detailed below:

[0046] The preparation of the first pre-prepared liquid in this method can be appropriately referenced from the preparation method of Chinese Invention Patent CN 114054746 B. The main advantage is that the shape and particle size distribution of the copper powder can be controlled by adjusting the amounts of copper salt, organic acid, and amine compounds as needed. When the content of organic acid is low, amine compounds play a major role in controlling the shape and particle size distribution of the copper powder. They can react with inorganic copper salt to form complexes, hindering the redox reaction. Furthermore, amine compounds exhibit good selective adsorption of copper nanocrystals, which helps promote the formation of micron-sized copper flakes. When the content of organic acid is high, organic acid plays a major role in controlling the shape and particle size distribution of the copper powder. Organic acid accelerates the redox reaction rate, producing more copper crystal nuclei per unit time, which helps promote the formation of smaller nanoparticles. Although this is an important control method in the preparation method of this invention, the specific results of the two types of copper powder with densely coated micron-sized copper flakes can be fully defined by their composition and coating method.

[0047] Example 1

[0048] 1) Mix 2g of copper hydroxide, 8g of oleic acid and 5g of triisopropanolamine in 40ml of ethylene glycol solution, stir at a rotation speed of 600r / min and heat to 90℃ to form a uniformly mixed first pre-prepared solution. Then add 20g of sodium hypophosphite and continue to stir at the same rotation speed for 20min to obtain a suspension containing micron-sized flake copper powder.

[0049] 2) In the suspension prepared above, 20g of ascorbic acid and a newly prepared second pre-solution consisting of 2g copper acetate, 25g oleic acid, and 40ml ethylene glycol were added sequentially. The mixture was reacted for 15 minutes under constant temperature and continuous stirring, then cooled to room temperature. The cooled reaction product was centrifuged at 4000 rpm for 3 minutes, and then washed twice with ethanol to obtain copper powder with a three-peaked particle size distribution and dense particle packing. Figure 1 and Figure 2 As shown, statistical analysis using image analysis software reveals that the copper powder consists of relatively large 1–2 μm micron-sized copper flakes and smaller 5–15 nm and 40–100 nm nano-sized copper particles. The micron-sized copper flakes are tightly wrapped by the two types of nano-sized copper particles, while the 5–15 nm nano-sized copper particles tightly coat the 40–100 nm nano-sized copper particles, forming aggregates. The copper powder as a whole exhibits a tri-peaked particle size distribution and dense particle packing. The tri-peaked copper powder obtained in this embodiment was synthesized through a two-step, one-step oxidation-reduction method, where the two types of nano-sized copper particles tightly and uniformly coat the micron-sized copper flakes.

[0050] Polyethylene glycol and glycerol were mixed in a 1:1 mass ratio. After thorough stirring, the mixture was allowed to stand for 30 minutes to allow air bubbles to escape, yielding an organic solvent for preparing copper paste. Subsequently, 80% by mass of the prepared copper powder with a tri-peaked particle size distribution and dense particle packing was uniformly stirred with 20% by mass of the organic solvent using a planetary gravity mixer, and the mixture was degassed to prepare copper paste. The organic solvent used in this invention exhibits good reducing and adhesive properties; it ensures uniform dispersion of copper powder in the copper paste and can adjust the viscosity, adhesion, and printability of the copper paste. Its good reducing properties also inhibit the oxidation of copper powder during sintering. The organic solvents used in the copper paste preparation in the following examples serve the same purpose and will not be described individually.

[0051] A pure copper substrate was immersed in a 3 vol.% dilute sulfuric acid solution for 2 minutes, followed by cleaning with ethanol and carefully wiping with a lint-free cloth to ensure the substrate surface was clean, dust-free, and dry before use. Next, a 150 μm thick layer of copper paste prepared in this embodiment was printed onto the pure copper substrate using a screen printing machine. The analog power chip was then placed on the printed copper paste, and a pressure of 0.5 MPa was applied and held for 1 minute to ensure full and tight contact between the analog chip, the printed copper paste, and the pure copper substrate, thus preparing the assembly to be sintered. The assembly was then placed in a sintering furnace and heated to 200°C at a rate of 10°C / min under a nitrogen atmosphere and held for 10 minutes. After the holding period, the furnace was cooled, and the sintered interconnect was finally prepared. The shear strength of the sintered interconnect was measured to be 23.2 MPa using a mechanical property testing machine. The fracture morphology of the sintered interconnect was observed using a scanning electron microscope (SEM), as shown below. Figure 3 As shown. From Figure 3 A large number of bulk sintered copper structures can be clearly observed in the fracture surface. This is because the nano-copper particles rapidly sinter and fuse to grow, connecting the micron-sized copper sheets and achieving a good sintered connection. Moreover, there are many dimples and obvious plastic deformation traces in the sintered structure, which fully demonstrates that the sintered interconnect joint has good mechanical properties such as strength and plasticity.

[0052] The copper powder obtained in this embodiment, which densely coats micron-sized copper sheets with two different nano-sized copper powders, exhibits a tri-peaked particle size distribution and dense particle packing. A large number of these two-scale copper nanoparticles fill the gaps between the micron-sized copper sheets, increasing the density of the copper powder structure. After low-temperature sintering, the two-scale copper nanoparticles fuse together to connect the micron-sized copper sheets, forming a dense sintered structure. In this embodiment, the sintered joint obtained after pressureless sintering at 200°C for 10 minutes in a nitrogen atmosphere exhibits a shear strength exceeding 20 MPa.

[0053] It should be noted that the organic acid used in this embodiment, as well as the polyethylene glycol and glycerol used to prepare the copper paste, all have reducing properties. They can inhibit the oxidation of copper particles during sintering. Moreover, the organic acid can react with copper oxides, and the resulting organic acid copper decomposes into extremely small nano-copper particles at higher temperatures. This further promotes the sintering of the copper paste matrix and the element diffusion between the sintered copper paste matrix and the chip and pure copper substrate. This can improve the performance of the sintered interconnect to a certain extent. However, the core function is the complete coating of the micron-sized copper sheet by the two sizes of nano-copper particles in the copper powder.

[0054] Furthermore, the copper paste made from copper powder with two nano-sized copper powders densely coating micron-sized copper sheets used in this embodiment can obtain high-strength sintered interconnects under low-temperature and pressureless sintering conditions, greatly reducing production costs and avoiding damage to the chip.

[0055] Example 2

[0056] 1) Mix 2g of basic copper carbonate, 9g of citric acid and 5g of N,N-dimethylethanolamine in 40ml of propylene glycol solution, stir at 400r / min and heat to 95℃ to form a uniformly mixed first pre-prepared solution, then add 18g of sodium citrate and stir continuously at the same speed for 30min to obtain a suspension containing micron-sized flake copper powder.

[0057] 2) In the suspension prepared above, 22g of sodium borohydride and the newly prepared second pre-solution, which is a mixture of 2g of copper citrate, 35g of citric acid and 40ml of propylene glycol, were added sequentially. The reaction was continued for 10 minutes under constant temperature and continuous stirring, and then cooled to room temperature. The cooled reaction product was centrifuged at 3000r / min for 10 minutes, and then washed repeatedly with acetone 3 times to obtain copper powder with a three-peaked particle size distribution and dense particle packing.

[0058] Ethylene glycol and propylene glycol were mixed in a 3:1 mass ratio and stirred thoroughly. The mixture was then allowed to stand for 30 minutes to remove air bubbles, yielding an organic solvent for preparing copper paste. Next, 75% (by mass) of the prepared copper powder with a tri-peaked particle size distribution and dense particle packing was uniformly stirred with 25% (by mass) of the organic solvent using a planetary gravity mixer. After degassing, copper paste was prepared.

[0059] A pure copper substrate was immersed in a 5 vol.% dilute sulfuric acid solution for 2 minutes, followed by cleaning with ethanol and carefully wiping with a lint-free cloth to ensure a clean and dust-free surface. The substrate was then dried before use. Next, a 300 μm thick layer of copper paste was printed onto the pure copper substrate using a screen printing machine. An analog chip was then placed on the printed copper paste, and a pressure of 0.1 MPa was applied and held for 5 minutes to ensure full and tight contact between the analog chip, the printed copper paste, and the pure copper substrate, thus preparing the assembly to be sintered. The assembly was then placed in a sintering furnace and heated to 160°C at a rate of 10°C / min under a nitrogen atmosphere, and held for 20 minutes. After the holding period, the furnace was cooled, resulting in the sintered interconnect. The shear strength of the sintered interconnect was measured to be 16.2 MPa using a mechanical property testing machine.

[0060] Example 3

[0061] 1) Mix 2g of copper nitrate trihydrate, 15g of lactic acid and 5g of triethanolamine in 40ml of diethylene glycol solution, stir at 500r / min and heat to 85℃ to form a uniformly mixed first pre-prepared solution. Add 18g of sodium phosphite and stir continuously at the same speed for 20min to obtain a suspension containing micron-sized flake copper powder.

[0062] 2) In the suspension prepared above, 22g of hydrazine sulfate and the newly prepared second pre-solution, which is a mixture of 2g of copper oxychloride, 30g of lactic acid and 40ml of diethylene glycol, were added sequentially. The reaction was continued for 30 minutes under constant temperature and continuous stirring, and then cooled to room temperature. The cooled reaction product was centrifuged at 6000r / min for 5 minutes, and then washed repeatedly with deionized water 4 times to obtain copper powder with a three-peaked particle size distribution and dense particle packing.

[0063] Ethylene glycol, propylene glycol, and glycerol were mixed in a 1:1:1 mass ratio. After thorough stirring, the mixture was allowed to stand for 30 minutes to remove air bubbles, yielding an organic solvent for preparing copper paste. Next, 85% (by mass) of the prepared copper powder with a tri-peaked particle size distribution and dense particle packing was uniformly stirred with 15% (by mass) of the organic solvent using a planetary gravity mixer to degas and prepare copper paste.

[0064] A pure copper substrate was immersed in a 5 vol.% dilute sulfuric acid solution for 2 minutes, followed by cleaning with ethanol and carefully wiping with a lint-free cloth to ensure a clean and dust-free surface. The substrate was then dried before use. Next, a 200 μm thick layer of copper paste was printed onto the pure copper substrate using a screen printing machine. The analog chip was then placed on the printed copper paste, and a pressure of 0.1 MPa was applied and held for 5 minutes to ensure full and tight contact between the analog chip, the printed copper paste, and the pure copper substrate, thus preparing the assembly to be sintered. The assembly was then placed in a sintering furnace and heated to 240°C at a rate of 10°C / min under a nitrogen atmosphere, and held for 10 minutes. After holding, the furnace was cooled, resulting in the sintered interconnect. The shear strength of the sintered interconnect was measured to be 26.7 MPa using a mechanical property testing machine.

[0065] Example 4

[0066] 1) Mix 2g of copper chloride, 15g of glycine and 5g of oleylamine in 40ml of n-propanol solution, stir at 400r / min and heat to 90℃ to form a uniformly mixed first pre-prepared solution. Add 20g of glucose and stir continuously at the same speed for 50min to obtain a suspension containing micron-sized flake copper powder.

[0067] 2) In the suspension prepared above, 20g of hydrazine hydrate and the newly prepared second pre-solution, which is a mixture of 2g of copper amino acid, 40g of glycine and 40ml of n-propanol, were added sequentially. The reaction was continued for 20min under constant temperature and continuous stirring, and then cooled to room temperature. The cooled reaction product was centrifuged at 5000r / min for 5 minutes, and then washed repeatedly with acetone 5 times to obtain copper powder with a three-peaked particle size distribution and dense particle packing.

[0068] Terpineol and diethylene glycol were mixed in a 2:1 mass ratio and stirred thoroughly. The mixture was then allowed to stand for 30 minutes to remove air bubbles, yielding an organic solvent for preparing copper paste. Next, 80% by mass of the prepared copper powder with a tri-peaked particle size distribution and dense particle packing was uniformly stirred and degassed using a planetary gravity mixer to produce copper paste.

[0069] A pure copper substrate was immersed in a 3 vol.% dilute sulfuric acid solution for 2 minutes, followed by cleaning with ethanol and carefully wiping with a lint-free cloth to ensure a clean and dust-free surface. The substrate was then dried before use. Next, a 100 μm thick layer of copper paste was printed onto the pure copper substrate using a screen printing machine. An analog chip was then placed on the printed copper paste, and a pressure of 0.5 MPa was applied and held for 1 minute to ensure full and tight contact between the analog chip, the printed copper paste, and the pure copper substrate, thus preparing the assembly to be sintered. The assembly was then placed in a sintering furnace and heated to 180°C at a rate of 10°C / min under a nitrogen atmosphere, and held for 30 minutes. After the holding period, the furnace was cooled, resulting in the sintered interconnect. The shear strength of the sintered interconnect was measured to be 18.3 MPa using a mechanical property testing machine.

[0070] Comparative Example 1

[0071] Non-densely coated flake copper powder was prepared by centrifugation and washing of a suspension containing micron-sized copper flakes: 2g copper hydroxide, 8g oleic acid, and 5g triisopropanolamine were mixed in 40ml ethylene glycol solution, stirred at 600r / min, and heated to 90℃ to form a homogeneous premix. 20g sodium hypophosphite was added, and the mixture was stirred continuously at the same speed for 20min. After cooling to room temperature, the cooled reaction product was centrifuged at 4000r / min for 3 minutes, and then washed twice with ethanol to obtain non-densely coated flake copper powder, wherein the particle size of the small nano-copper particles was 5–15nm, and the particle size of the micron-sized copper flakes was 1–2μm.

[0072] Polyethylene glycol and glycerol were mixed in a 1:1 mass ratio and stirred thoroughly. The mixture was then allowed to stand for 30 minutes to remove air bubbles, yielding an organic solvent for preparing copper paste. Next, 80% by mass of the prepared non-dense coated flake copper powder and 20% by mass of the organic solvent were uniformly stirred and degassed using a planetary gravity mixer to prepare the copper paste.

[0073] A pure copper substrate was immersed in a 3 vol.% dilute sulfuric acid solution for 2 minutes, then cleaned with ethanol and carefully wiped with a lint-free cloth to ensure the substrate surface was clean, dust-free, and dry before use. Next, a 150 μm thick layer of copper paste was printed onto the pure copper substrate using a screen printing plate. The analog chip was then placed on the printed copper paste, and a pressure of 0.5 MPa was applied and held for 1 minute to ensure full and tight contact between the analog chip, the printed copper paste, and the pure copper substrate, thus preparing the assembly to be sintered. The assembly was then placed in a sintering furnace and heated to 200°C at a rate of 10°C / min under a nitrogen atmosphere and held for 10 minutes. After holding, the furnace was cooled. Inspection revealed that the sintered interconnects showed virtually no interfacial bonding, and the shear strength of the sintered interconnects, measured using a mechanical property testing machine, was only 0.3 MPa.

[0074] Compared to Example 1, Comparative Example 1 showed a significant reduction in the number of nano-copper particles coating the surface of the micron-sized copper sheet, resulting in a low initial packing density of copper powder. This led to insufficient sintering and a less dense sintered structure. The reduction in the number of nano-copper particles with high sintering driving force weakened the diffusion driving force between the copper paste and the chip and the pure copper substrate, ultimately making it difficult to achieve a good metallurgical connection in the sintered joint.

[0075] Comparative Example 2

[0076] This comparative example is provided exactly according to Example 1 of Chinese Invention Patent CN 114054746 B.

[0077] Copper powder with a three-peaked particle size distribution ranging from nanometers to micrometers was prepared. 2g of copper acetate, 10g of lactic acid, and 6g of triethanolamine were added to 40ml of ethylene glycol solution as the reaction solution. The reaction solution was heated to 85℃, and then 20g of sodium hypophosphite was added and stirred at 600rpm to dissolve. After reacting for 10min, the mixture was centrifuged at 4000rpm and washed twice with ethanol to obtain copper powder with a coated structure and a three-peaked particle size distribution. The nano-copper particles had a particle size of 5–9nm, the submicron copper particles had a particle size of 120–180nm, and the micron copper flakes had a particle size of 1–2μm.

[0078] Polyethylene glycol and glycerol were mixed in a 1:1 mass ratio and stirred thoroughly. The mixture was then allowed to stand for 30 minutes to remove air bubbles, yielding an organic solvent for preparing copper paste. Next, 80% by mass of the prepared copper powder and 20% by mass of the organic solvent were uniformly stirred and degassed using a planetary gravity mixer to produce copper paste.

[0079] A pure copper substrate was immersed in a 3 vol.% dilute sulfuric acid solution for 2 minutes, followed by cleaning with ethanol and carefully wiping with a lint-free cloth to ensure the substrate surface was clean, dust-free, and dry before use. Next, a 150 μm thick layer of copper paste was printed onto the pure copper substrate using a screen printing plate. The analog chip was then placed on the printed copper paste, and a pressure of 0.5 MPa was applied and held for 1 minute to ensure full and tight contact between the analog chip, the printed copper paste, and the pure copper substrate, thus preparing the assembly to be sintered. The assembly was then placed in a sintering furnace and heated to 200°C at a rate of 10°C / min under a nitrogen atmosphere and held for 10 minutes. After holding, the furnace was cooled. Inspection revealed that no interfacial bonding had occurred in the sintered interconnect joints, and the shear strength of the sintered interconnect joints was measured to be 0 MPa using a mechanical property testing machine.

[0080] Figure 4 This is a scanning electron microscope (SEM) image of small-sized copper powder-coated copper microsheets obtained in Example 1 of Chinese Invention Patent CN 114054746 B. (Comparison) Figure 1 and Figure 4 It is evident that, compared to Example 1, Comparative Example 2's copper powder contains micron-sized copper flakes, submicron-sized copper particles, and nano-sized copper particles. The submicron-sized copper particles do not coat the surface of the micron-sized copper flakes, and the number of easily sinterable small nano-sized copper particles coating the surface of the micron-sized copper flakes is relatively small. This results in a low overall packing density of the copper powder, leading to an insufficiently dense sintered structure. Furthermore, the limited number of nano-sized copper particles with high sintering driving force results in poor interfacial bonding between the copper paste and the chip and pure copper substrate, preventing metallurgical bonding. Therefore, the copper powder obtained in Comparative Example 2 cannot be used for power chip and power device packaging.

[0081] The embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing copper powder with two nano-sized copper powders densely coating micron-sized copper flakes, characterized in that, A weak reducing agent was added to the first pre-prepared liquid in the reactor to obtain the initial reaction liquid. The initial reaction liquid was continuously stirred at a temperature of 80–120℃ to obtain a suspension of flake copper powder. A strong reducing agent was first added to the flake copper powder suspension, followed by the second pre-prepared liquid to form the final reaction liquid. The temperature was maintained at 80–120℃ and the mixture was continuously stirred to obtain three copper powder suspensions with different particle sizes. After cooling, centrifugation, and washing, copper powder with two nano-sized copper powders densely coated with micron-sized copper flakes was obtained. The first pre-prepared liquid is obtained by mixing inorganic copper salt, organic acid, amine compound and polyol solvent, and continuously stirring at a temperature of 80–100°C. The second pre-prepared liquid is obtained by mixing organic copper salt, organic acid and polyol solvent, and stirring continuously at a temperature of 80–100 °C. In the first pre-prepared solution, the concentration of the inorganic copper salt is 30–80 g / L, and the total concentration of organic acid and amine compounds is 300–600 g / L, wherein the mass concentration ratio of organic acid to amine compounds is 1:1–5:

1. In the second pre-prepared solution, the concentration of the organic copper salt is 30–80 g / L, and the concentration of the organic acid is 400–1200 g / L. The weak reducing agent is one or more of sodium citrate, sodium hypophosphite, sodium phosphite, potassium tartrate, hydrogen peroxide, and glucose. The strong reducing agent is one or more of hydrazine hydrate, hydrazine sulfate, sodium borohydride, ascorbic acid, and tetrabutylammonium borohydride.

2. The method for preparing copper powder with two nano-sized copper powders densely coated with micron-sized copper sheets according to claim 1, characterized in that, The inorganic copper salt is one or more of copper carbonate, basic copper carbonate, copper sulfate, copper hydroxide, copper chloride, and copper nitrate trihydrate; The organic acid is one or more selected from oxalic acid, glycine, citric acid, tartaric acid, lactic acid, propionic acid, and oleic acid; The amine compounds mentioned are one or more selected from oleylamine, ethylenediamine, methanolamine, triethanolamine, N,N-dimethylethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, and diethylenetriamine; The polyol solvent is one or more selected from ethylene glycol, diethylene glycol, propylene glycol, glycerol, isopropanol, and n-propanol; The organic copper salt is one or more of copper acetate, copper amino acid, copper rosinate, copper citrate, and copper tyrosine.

3. The method for preparing copper powder with two nano-sized copper powders densely coated with micron-sized copper sheets according to claim 1, characterized in that, The concentration of the weak reducing agent in the initial reaction solution is 400–800 g / L; the concentration of the strong reducing agent in the final reaction solution is 200–500 g / L.

4. The method for preparing copper powder with two nano-sized copper powders densely coated with micron-sized copper sheets according to claim 1, characterized in that, In the preparation of flake copper powder suspension, three copper powder suspensions with different particle sizes, first prepreg and second prepreg, the continuous stirring rate is 400–600 r / min, the stirring time in the preparation of first prepreg and second prepreg is 5–20 min, the stirring time in the preparation of flake copper powder suspension is 10–60 min, and the stirring time in the preparation of three copper powder suspensions with different particle sizes is 10–30 min. The cooling mentioned refers to cooling to room temperature; The centrifugation is performed by centrifuging the cooled reaction product in a centrifuge at a speed of 3000–6000 r / min for 3–10 minutes; The washing process involves repeatedly washing 2–5 times with one or more of ethanol, deionized water, and acetone.

5. Copper powder with two nano-sized copper powders densely coating micron-sized copper flakes, characterized in that... It is prepared by the method described in claim 1; it consists of micron-sized copper sheets with a length of 1–2 μm and nano-sized copper particles with diameters of 5–15 nm and 40–100 nm; wherein the micron-sized copper sheets are completely and tightly wrapped by nano-sized copper particles of the two diameters, and the copper particles with a diameter of 5–15 nm are tightly wrapped around the nano-sized copper particles with a diameter of 40–100 nm, and the three copper powders exhibit a three-peak distribution of particle size and a densely packed aggregate.

6. The application of copper powder preparation of copper paste by densely coating micron-sized copper sheets with the two types of nano-sized copper powders as described in claim 5.

7. The application of copper powder preparation of copper paste by densely coating micron-sized copper flakes with two types of nano-sized copper powder according to claim 6, characterized in that, Copper powder with two nano-sized copper powders densely coating micron-sized copper sheets is mixed with an organic solvent and then degassed to prepare copper paste; the organic solvent is one or more of ethylene glycol, propylene glycol, glycerol, diethylene glycol, terpineol and polyethylene glycol.

8. The application of copper powder preparation of copper paste by densely coating micron-sized copper flakes with two types of nano-sized copper powder according to claim 7, characterized in that, The uniform mixing is achieved by mixing using a planetary gravity mixer; By mass percentage, the copper powder of the two types of nano-sized copper powder densely coating the micron-sized copper sheets accounts for 75–90%, and the organic solvent accounts for 25–10%. The application of the copper paste in the preparation of sintered joints: The copper paste is printed on the surface of a pure copper substrate using a screen printing method. The thickness of the copper paste is 10–400 μm. The power chip or power device is placed on the printed copper paste surface and a bonding pressure of 0–0.5 MPa is applied and the pressure is maintained for 1–5 min to obtain a chip / copper paste / copper substrate sandwich structure assembly to be sintered. Sintered joints are formed by sintering at 160–240 °C for 10–40 min under nitrogen atmosphere and without pressure assistance.

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