A composite metal material and a high thermal conductivity low-temperature solder paste containing the same

Abstract

The present invention relates to a composite metal material and a high thermal conductivity low-temperature solder paste containing the same. The composite metal material is a Cu core / Ag intermediate layer / Sn shell type metal powder (Cu@Ag@Sn) or a Cu core / Ni intermediate layer / Sn shell type metal powder (Cu@Ni@Sn), with a particle size ranging from 20 μm to 60 μm. After being mixed with Sn-Bi series alloy powder and soldering flux paste, the composite metal material forms a high thermal conductivity low-temperature solder paste, which can be used for welding various substrates, is suitable for low-temperature welding, and the welding temperature is below 150°C. The process of the composite metal material of the present invention is simple, the cost is low, and the practicability is strong, solving the problems such as poor heat dissipation after the power device chip is pasted due to the low thermal conductivity of the current solder paste on the market.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic soldering, and a composite metal material and a high thermal conductivity low-temperature solder paste containing the same. Background Art

[0002] With the development of electronic products towards ultra-large scale integration and miniaturization, the chip interconnection density has increased sharply. Under this background, solder paste has become the most important connection material in surface mount technology (SMT). However, due to the large heat generation and high working temperature of high-power devices, new challenges have been posed to the thermal conductivity of the solder used for chip bonding. Therefore, developing a solder with a high thermal conductivity coefficient has become an important research issue.

[0003] Traditional solder paste generally uses tin-silver-copper (SAC) series alloys as welding materials, and the welding temperature usually needs to be not less than 240°C. During the welding process of electronic devices, problems such as device deformation are likely to occur. Therefore, solder paste using Sn-Bi series alloys as low-temperature solders is mostly used in the current market. Among them, the eutectic point Sn-58Bi alloy has a melting point of 138°C. The content of Bi in the Sn-Bi series alloy has a great influence on the melting point. When the Bi content is less than 58wt%, the melting point of the alloy is higher than 138°C; otherwise, the melting point of the alloy is lower than or equal to 138°C.

[0004] On the one hand, when the existing series of alloys solidify, segregated brittle Bi will appear, resulting in low thermal conductivity and tensile strength, and poor resistance to mechanical drop impact. On the other hand, reducing the Bi content can improve the thermal conductivity, but it will cause the melting point of the alloy to increase. When brazing copper tubes, it needs to be carried out below 140°C. As Figure 1 shown, brazing the copper tube and the heat dissipation device with solder. In order to conduct the heat of the copper tube to the heat dissipation device faster for cooling, it is necessary to increase the thermal conductivity of the solder. At the same time, affected by the thermal expansion of the copper tube, the welding temperature of the solder is required not to exceed 150°C. In order to braze the copper tube to the heat dissipation device, the solder needs to melt below 140°C. Otherwise, the liquid in the copper tube has the risk of explosion.

[0005] Therefore, it is of great significance to develop a high thermal conductivity low-temperature solder.

[0006] Composite solder is a new type of solder with enhanced properties, which is made by adding materials such as micro-nano particles, rare elements, and porous metals to traditional solder. In recent years, due to its important properties in traditional solder joints, especially in terms of shear strength, thermal stability, intermetallic compound growth, and phase nucleation, composite solder joints have attracted great attention in both academia and industry. Yang Liu et al. from Yangzhou University found through research (Liu, Yang, et al. Journal of Materials Science: Materials in Electronics. 2020, 31, 8258 - 8267.) that adding porous foam Cu sheets to the Sn58Bi alloy can increase the thermal conductivity of the Sn58Bi alloy to 41.32 W / (m·K). However, the cost of the foam Cu used in this method is relatively high, and the resulting sheet structure cannot be directly used to prepare solder paste. Hao Zhang et al. from Harbin University of Science and Technology (Hao Zhang, et al. Journal of Materials Science: Materials in Electronics. 2019, 30: 340–347) added 5 wt.% of Cu particles with a diameter of 5 μm to the Sn58Bi solder paste, which can increase the thermal conductivity of the Sn58Bi solder layer from 18.89 W / (m·K) to 26.60 W / (m·K), showing good thermal performance in LED packaging. Hongtao Chen et al. from Harbin Institute of Technology (Hongtao Chen, Tianqi Hu, et al. IEEE Transactions on Power Electronics. 2017, 32(1): 441 - 51.) proposed a solder with a Cu@Sn core-shell structure. By plating Sn on the surface of Cu particles to form Cu@Sn core-shell metal particles, and then pressing the Cu@Sn core-shell metal particles into a composite solder sheet with a thickness of 400 ± 20 μm under a pressure of 30 MPa for Cu-Cu connection. Due to the presence of a large number of Cu cores, the thermal conductivity of the composite solder sheet can reach 127.99 - 154.26 W / (m·K). However, this composite solder sheet needs to be welded at 250 °C to melt the Sn layer to connect the Cu balls in the solder, and the outer Sn is transformed into a Cu-Sn intermetallic compound with a high remelting temperature, finally making the weld joint resistant to high temperature and having the ability to resist thermal cycling, and it is not suitable for low-temperature connection at 150 °C. Therefore, it is very important to develop a low-melting-point and high-thermal-conductivity solder that can be used in solder paste. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies of the prior art and provide a composite metal material, which is a Cu@Ag@Sn core-shell metal powder or a Cu@Ni@Sn core-shell metal powder. During the reflux process, the Sn outer shell layer does not melt or melts only to a limited extent, thereby protecting the Cu core and the Ag and Ni intermediate layers from not dissolving or dissolving only slightly, and reducing the formation of the inner layer of high thermal resistance intermetallic compound Cu6Sn5. It can increase the thermal conductivity of the Sn-Bi series solder from 21.4 W / (m·K) to 50.82 W / (m·K) at 85 °C, and the melting point of the solder is only 138.9 °C, which is suitable for low-temperature soldering, solving the contradiction between the high thermal conductivity and low melting point of the solder, and realizing the advantageous combination of high thermal conductivity and low-temperature melting.

[0008] The present invention also provides a high thermal conductivity and low-temperature solder paste containing the above composite metal material, which has strong practicability and solves the problems such as poor heat dissipation after the power device chip is pasted due to the low thermal conductivity of the solder paste currently on the market.

[0009] The specific solutions are as follows:

[0010] A composite metal material, the composite metal material is spherical, with a diameter of 20 μm to 60 μm; it is a three-layer core-shell structure, the inner core is Cu, the intermediate layer is Ag or Ni, and the shell layer is Sn. In a specific embodiment, the composite metal material is a perfect sphere, and its diameter can be 25 to 55 μm, for example, it can be 30 μm, 35 μm, 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, 52 μm, 54 μm. The composite metal material with the above structure has good structural stability and plays a role in improving the thermal conductivity of the solder in the solder, thereby improving the thermal performance of the product.

[0011] Furthermore, the diameter of the inner core is 15 μm to 60 μm, preferably 20 - 50 μm, and can be 25 μm, 30 μm, 33 μm, 35 μm, 38 μm, 40 μm, 45 μm, 48 μm; the thickness of the intermediate layer is 0.5 μm to 2 μm, preferably 0.8 - 1.8 μm, and can be 1.0 μm, 1.2 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm; the thickness of the shell layer is 0.5 μm to 2 μm, preferably 0.8 - 1.8 μm, and can be 0.9 μm, 1.0 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm. Using a composite metal material with an optimized thickness of three layers can better play the role of protecting the inner core and effectively prevent the formation of Cu6Sn5 when heated, thereby significantly increasing the thermal conductivity of the solder.

[0012] Further, in the composite metal material, by weight, Cu accounts for 60%-98% of the total weight, preferably 65%-95%, and can be 70%, 73%, 75%, 76%, 78%, 80%, 82%, 85%, 88%, 90%; Ag or Ni accounts for 1%-20% of the total weight, preferably 3%-18%, and can be 4%, 6%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%; Sn accounts for 1%-20% of the total weight, preferably 3%-18%, and can be 4%, 6%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%. By adjusting the proportions of the three elements, while improving the thermal conductivity, the solder can still complete welding at a low temperature of 139°C, achieving an improvement in the thermal conductivity of the solder.

[0013] The present invention also protects a method for preparing the composite metal material. The composite metal material is Cu@Ag@Sn core-shell metal powder, and the preparation process includes:

[0014] S1: Take Cu powder for pickling, and then disperse it in distilled water to form a mixed solution A;

[0015] S2: Add a reducing agent and a stabilizer to the solution A, disperse it ultrasonically, and then add oleylamine to obtain a mixed solution B; Pour the mixed solution B into a reaction kettle for heating reaction, collect the reaction product, rinse the reaction product, and then disperse it in distilled water to obtain a mixed solution C;

[0016] S3: Add a complexing agent, a brightening agent, silver nitrate, and a pH regulator to the mixed solution C, mix evenly to obtain a mixed solution D; Carry out a heat preservation reaction on the mixed solution D, collect the reaction product, carry out rinsing, and then disperse it in distilled water to obtain a mixed solution E;

[0017] S4: Add a dispersant, a complexing agent, and Sn powder to the mixed solution E, stir and react, collect the product after the reaction, carry out rinsing and drying to obtain Cu@Ag@Sn core-shell metal powder.

[0018] Further, in S1, Cu powder with a particle size distribution of 15μm - 60μm is selected by a sieve, subjected to ultrasonic pickling, and then evenly dispersed in distilled water to form a mixed solution A, so that the concentration of Cu powder is 1g / mL - 3g / mL, for example, it can be 1.5g / mL, 2.0g / mL, 2.5g / mL; Preferably, the pickling uses any one of nitric acid, hydrochloric acid or sulfuric acid, and the mass concentration is 2% - 5%;

[0019] Optionally, the reducing agent described in S2 is formate, selected from one of sodium formate, potassium formate, and calcium formate. The concentration of the reducing agent in the mixed solution B is 0.1 g / mL to 0.2 g / mL; the stabilizer is selected from one of N,N-dimethylformamide, glycerol, and propylene glycol. The addition amount of the stabilizer accounts for 60 to 80% of the volume of the mixed solution B; the addition amount of oleylamine accounts for 3 to 6% of the volume of the mixed solution B; preferably, the heating reaction is carried out at 120°C to 200°C for 10 hours to 20 hours, and magnetic stirring is applied during the heat preservation, with a rotation speed of 100 revolutions per minute to 500 revolutions per minute; the rinsing is carried out by alternately rinsing with distilled water and ethanol 2 to 3 times; the concentration of the mixed solution C is 5 g / mL to 10 g / mL;

[0020] Optionally, the complexing agent described in S3 is selected from at least one of N-β-hydroxyethyl ethylenediamine triacetic acid, imidazole, citric acid, tartaric acid, and gluconic acid. The concentration of the complexing agent in the mixed solution D is 0.03 g / mL to 0.05 g / mL; the brightening agent is selected from one of glycerol, ethylene glycol, and glycine. The concentration of the brightening agent in the mixed solution D is 0.003 g / mL to 0.005 g / mL; the concentration of silver nitrate in the mixed solution D is 0.004 g / mL to 0.006 g / mL; the pH regulator is selected from at least one of nitric acid and hydrochloric acid, and the pH of the mixed solution D is adjusted to 3 - 4; preferably, the heat preservation reaction is carried out at 40°C to 70°C for 2 minutes to 8 minutes, and magnetic stirring is applied during the heat preservation, with a rotation speed of 100 revolutions per minute to 200 revolutions per minute; the concentration of the mixed solution E is 0.1 g / mL to 0.3 g / mL;

[0021] Optionally, in S4, the dispersant is selected from at least one of paraffin, polyvinylpyrrolidone, and polyethylene glycol. The concentration of the dispersant in the mixed solution E is 0.1 g / mL to 0.2 g / mL; the complexing agent is selected from at least one of ammonium thiocyanate and thiourea. The concentration of the complexing agent in the mixed solution E is 0.1 g / mL to 0.2 g / mL; the concentration of Sn powder in the mixed solution E is 0.03 g / mL to 0.06 g / mL; preferably, the stirring reaction is carried out at room temperature for 5 - 10 minutes, with a rotation speed of 100 revolutions per minute to 200 revolutions per minute; the product is collected and rinsed with distilled water 2 - 3 times, and dried at 50 - 70°C for 30 - 60 minutes to obtain the Cu@Ag@Sn core-shell metal powder.

[0022] As another case of the present invention, the composite metal material is a Cu@Ni@Sn core-shell metal powder, and the preparation process includes:

[0023] P1: Take Cu powder for pickling, and then disperse it in distilled water to form a mixed solution A;

[0024] P2: Add a reducing agent and a stabilizer to the solution A, add oleylamine after ultrasonic dispersion to obtain a mixed solution B; pour the mixed solution B into a reaction kettle for heating reaction, collect the reaction product, and rinse the reaction product to obtain product C;

[0025] P3: Add a nickel salt, a reducing agent, a stabilizer, a promoter, and a pH regulator to a deep eutectic solvent, and dissolve to obtain a plating solution D. The deep eutectic solvent is composed of a hydrogen bond acceptor and a hydrogen bond donor mixed;

[0026] P4: Place the product C in the plating solution D for electroless nickel plating. After the plating is completed, collect the product, rinse it, and then disperse it in distilled water to obtain a mixed solution F;

[0027] P5: Add a dispersant, a complexing agent, and Sn powder to the mixed solution F, stir and react. After the reaction is completed, collect the product, rinse and dry it to obtain the Cu@Ni@Sn core-shell metal powder.

[0028] Further, in P1, Cu powder with a particle size distribution of 15 μm to 60 μm is selected by a sieve, ultrasonically pickled, and then uniformly dispersed in distilled water to form a mixed solution A, so that the concentration of Cu powder is 1 g / mL to 3 g / mL; preferably, the pickling is carried out with any one of nitric acid, hydrochloric acid or sulfuric acid, and the mass concentration is 2% to 5%;

[0029] Optionally, in P2, the reducing agent is formate, selected from one of sodium formate, potassium formate, and calcium formate, and the concentration of the reducing agent in the mixed solution B is 0.1 g / mL to 0.2 g / mL; the stabilizer is selected from one of N,N-dimethylformamide, glycerol, and propylene glycol, and the addition amount of the stabilizer accounts for 60 to 80% of the volume of the mixed solution B; the addition amount of oleylamine accounts for 3 to 6% of the volume of the mixed solution B; preferably, the heating reaction is carried out at 120 °C to 200 °C for 10 hours to 20 hours, and magnetic stirring is applied during the heat preservation, and the rotation speed is 100 revolutions per minute to 500 revolutions per minute; the rinsing is carried out by alternately rinsing with distilled water and ethanol 2 to 3 times; the concentration of the mixed solution C is 5 g / mL to 10 g / mL;

[0030] Optionally, the nickel salt described in P3 is one or a combination of nickel chloride, nickel sulfate, nickel acetate, and nickel sulfamate, with a concentration range of 5 to 50 g / L; the reducing agent is selected from one of hydrazine, sodium hypophosphite, sodium borohydride, potassium borohydride, and dimethylamine borane, with a concentration range of 1 to 30 g / L; the stabilizer is one or a combination of boric acid, citric acid, lactic acid, potassium sodium tartrate, and disodium ethylenediaminetetraacetate, with a concentration range of 0.5 to 15 g / L; the accelerator is one or a combination of ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, and N,N-dimethylethanolamine, with a concentration range of 5 to 20 g / L; the pH regulator is a 5 wt% sodium hydroxide-ethylene glycol solution, with a pH range of 7.0 to 14.0; preferably, in the deep eutectic solvent, the hydrogen bond acceptor is one or a combination of choline chloride, tetramethylammonium chloride, tetrabutylammonium chloride, tetraethylammonium chloride, and their derivatives, the hydrogen bond donor is one or two combinations of polyols, amides, and carboxylic acids, and the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:5. After mixing the two, stirring is carried out at 60 to 90 °C for 2 to 4 h to obtain the deep eutectic solvent;

[0031] Optionally, the plating temperature for the electroless nickel plating treatment described in P4 is 60 °C to 150 °C, the plating pH is 7.0 to 14.0, and the plating time is 0.5 to 4.0 h; during this period, magnetic stirring is applied at a rotation speed of 100 to 200 revolutions per minute; after the reaction ends, the product is collected and rinsed 2 - 3 times with distilled water. Then, the rinsed product is dispersed in distilled water to obtain a mixed solution F, and the concentration of the mixed solution F is 0.1 g / mL to 0.3 g / mL;

[0032] Optionally, the dispersant described in P5 is selected from at least one of paraffin, polyvinylpyrrolidone, and polyethylene glycol, and the concentration of the dispersant in the mixed solution F is 0.1 g / mL to 0.2 g / mL; the complexing agent is selected from at least one of ammonium thiocyanate and thiourea, and the concentration of the complexing agent in the mixed solution F is 0.1 g / mL to 0.2 g / mL; the concentration of Sn powder in the mixed solution E is 0.03 g / mL to 0.06 g / mL; the stirring reaction is carried out at room temperature for 5 - 10 minutes at a rotation speed of 100 to 200 revolutions per minute; the product is collected and rinsed 2 - 3 times with distilled water, and dried at 50 - 70 °C for 30 - 60 minutes to obtain the Cu@Ni@Sn core-shell metal powder.

[0033] The present invention also protects a high thermal conductivity and low temperature solder paste, which contains the composite metal material. The high thermal conductivity and low temperature solder paste includes the composite metal material, Sn-Bi series alloy powder and flux paste. The composite metal material accounts for 10% - 30% of the total weight, preferably 12 - 28%, for example, it can be 14%, 16%, 18%, 19%, 20%, 21%, 23%, 25%, 26%; the Sn-Bi series alloy powder accounts for 10% - 30% of the total weight, preferably 12 - 28%, for example, it can be 14%, 16%, 18%, 19%, 20%, 21%, 23%, 25%, 26%.

[0034] Further, the flux paste is a medium and low temperature rosin-based flux paste. Here, the low temperature means that the soldering temperature of the solder is lower than 150°C, specifically, the melting point of the solder is between 138 - 140°C. The rosin-based flux paste refers to a paste made by dissolving rosin and organic acids with an organic solvent using a thixotropic agent, and it can be a common rosin-based flux paste in the art;

[0035] Optionally, the Sn-Bi series alloy powder is selected from at least one of hypoeutectic Sn-Bi alloy powder with a particle size distribution of 20μm - 60μm, eutectic Sn-58Bi alloy powder, and hypereutectic Sn-Bi alloy powder; preferably, the particle size distribution is 25μm - 55μm, for example, it can be 28μm, 30μm, 32μm, 35μm, 38μm, 40μm, 41μm, 42μm, 45μm, 48μm, 50μm, 53μm.

[0036] Optionally, the starting melting point of the high thermal conductivity and low temperature solder paste is 138 - 140°C, and the peak temperature is 140 - 145°C; the thermal conductivity is 50 - 55W / (m·K). In specific embodiments, the starting melting point of the high thermal conductivity and low temperature solder paste is 138.5°C, 139.0°C, 139.5°C, or 139.8°C, and the peak temperature is 140.5°C, 141.0°C, 141.5°C, 142.0°C, 142.5°C, 143.0°C, 143.5°C, or 144°C; the thermal conductivity is 50.5W / (m·K), 51.0W / (m·K), 51.5W / (m·K), 52.0W / (m·K), 52.5W / (m·K), 53.0W / (m·K), 54.0W / (m·K), or 54.5W / (m·K).

[0037] The present invention also protects a device obtained by soldering using the high thermal conductivity and low temperature solder paste.

[0038] Beneficial effects:

[0039] 1. The present invention first adopts a multi-layer electroless plating process to coat an Ag or Ni intermediate layer and a Sn outer layer on the surface of Cu powder, obtaining Cu@Ag@Sn core-shell structure particles and Cu@Ni@Sn core-shell structure particles with good morphology. This not only solves the problem that current Cu powder is prone to oxidation and difficult to preserve in air, but also the Ag and Ni intermediate layers and the Sn outer layer can effectively protect the Cu core, Ag and Ni intermediate layers from not dissolving or slightly dissolving and reduce the formation of the inner high-thermal-resistance intermetallic compound Cu6Sn5, significantly improving the thermal conductivity of the solder;

[0040] 2. Compared with Sn-Bi series solder pastes, the high-thermal-conductivity low-temperature solder paste made by the present invention has its thermal conductivity increased from 21.4 W / (m·K) to 50.82 W / (m·K) at 85 °C, and the melting point of the solder is only 138.9 °C, which is suitable for low-temperature welding, solving the problems such as poor heat dissipation after the power device chip is pasted due to the low thermal conductivity of the current solder pastes on the market;

[0041] 3. The high-thermal-conductivity low-temperature solder paste made by the present invention can form a connection during the reflow process, with a simple process, greatly reducing the large-area chip pasting time;

[0042] 4. The process of the present invention is simple, with low cost and strong practicability. Metal powders with different particle sizes and different coating amounts can be selected to make the solder according to the actual use environment.

[0043] In summary, the present invention can obtain Cu@Ag@Sn core-shell metal particles and Cu@Ni@Sn core-shell metal particles with good morphology. The Ag and Ni intermediate layers and the Sn outer layer are coated relatively evenly, which can effectively protect the Cu core, Ag and Ni intermediate layers from not dissolving or slightly dissolving and reduce the formation of the inner high-thermal-resistance intermetallic compound Cu6Sn5, thereby significantly improving the thermal conductivity of the solder. Brief Description of the Drawings

[0044] In order to more clearly illustrate the technical solutions of the present invention, the drawings will be briefly introduced below. Obviously, the drawings in the following description only relate to some embodiments of the present invention and do not limit the present invention.

[0045] Figure 1 is the welding physical diagram provided by the background technology of the present invention;

[0046] Figure 2 is the physical diagram and X-ray diffraction pattern of Cu@Ag@Sn core-shell metal powder provided by Embodiment 3 of the present invention;

[0047] Figure 3 is the scanning electron microscope image of Cu@Ag@Sn core-shell metal particles provided by Embodiment 3 of the present invention;

[0048] Figure 4It is the energy spectrum surface scanning diagram of the Cu@Ag@Sn core-shell metal particles provided by Embodiment 3 of the present invention;

[0049] Figure 5 It is the cross-sectional energy spectrum line scanning diagram of the Cu@Ag@Sn core-shell metal particles provided by Embodiment 3 of the present invention;

[0050] Figure 6 It is the physical diagram of the high thermal conductivity low-temperature solder provided by Embodiment 7 of the present invention;

[0051] Figure 7 It is the DSC curve diagram of the high thermal conductivity low-temperature solder provided by Embodiment 7 of the present invention;

[0052] Figure 8 It is the thermal conductivity diagram of the high thermal conductivity low-temperature solder prepared in Embodiment 7 of the present invention; Detailed implementation manners

[0053] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. For those not specified in the examples regarding specific techniques or conditions, the techniques or conditions described in the literature in the relevant field or according to the product specifications are followed. For reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained through commercial purchase. In the following examples, unless otherwise specified, "%" all refers to weight percentage.

[0054] The following low-temperature lead-free rosin-based solder paste used is the zero-halogen solder paste S8 of Xiamen Jishiyu Solder Co., Ltd.

[0055] Embodiment 1 Preparation of Cu@Ag@Sn core-shell metal powder

[0056] S1: Select 20 g of Cu powder with a particle size of 20 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass for 5 minutes, and then uniformly disperse it in 20 mL of distilled water to form a mixed solution A;

[0057] S2: Add 1.2 g of sodium formate and 72 mL of N,N-dimethylformamide to A, ultrasonically disperse for 3 minutes, then add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reaction kettle, keep it warm at 150 °C for 18 hours, and apply magnetic stirring during the heat preservation period at a rotation speed of 200 revolutions per minute. Collect the reaction product, and alternately rinse it 3 times with distilled water and ethanol. Disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution C;

[0058] S3: Add 6 g of N-β-hydroxyethyl ethylenediaminetriacetic acid, 6 g of ethylene glycol, and 8 g of silver nitrate to C, adjust the pH to 4 with 38% hydrochloric acid by mass fraction, and obtain a mixed solution D after mixing evenly. Keep D at 50 °C for 8 minutes, and apply magnetic stirring during the insulation period at a rotation speed of 100 revolutions per minute. After the insulation ends, collect the reaction product, rinse it 3 times with distilled water, and disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution E;

[0059] S4: Add 20 g of paraffin, 20 g of ammonium thiocyanate, and 6 g of pure Sn powder to E, and perform magnetic stirring at room temperature for 8 minutes at a rotation speed of 100 revolutions per minute to make it react fully. After the reaction ends, collect the product and rinse it 3 times with distilled water. Finally, dry it at 60 °C for 40 minutes to obtain the Cu@Ag@Sn core-shell metal powder.

[0060] Example 2 Preparation of Cu@Ag@Sn Core-Shell Metal Powder

[0061] S1: Select 20 g of Cu powder with a particle size of 20 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass fraction for 5 minutes, and then disperse it evenly in 20 mL of distilled water to form a mixed solution A;

[0062] S2: Add 1.2 g of sodium formate and 72 mL of N,N-dimethylformamide to A, ultrasonically disperse for 3 minutes, and then add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reactor, keep it at 150 °C for 18 hours, and apply magnetic stirring during the insulation period at a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it 3 times alternately with distilled water and ethanol. Disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution C;

[0063] S3: Add 10 g of N-β-hydroxyethyl ethylenediaminetriacetic acid, 10 g of ethylene glycol, and 12 g of silver nitrate to C, adjust the pH to 4 with 38% hydrochloric acid by mass fraction, and obtain a mixed solution D after mixing evenly. Keep D at 50 °C for 8 minutes, and apply magnetic stirring during the insulation period at a rotation speed of 100 revolutions per minute. After the insulation ends, collect the reaction product, rinse it 3 times with distilled water, and disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution E;

[0064] S4: Add 20 g of paraffin, 20 g of ammonium thiocyanate, and 6 g of pure Sn powder to E, and perform magnetic stirring at room temperature for 8 minutes at a rotation speed of 100 revolutions per minute to make it react fully. After the reaction ends, collect the product and rinse it 3 times with distilled water. Finally, dry it at 60 °C for 40 minutes to obtain the Cu@Ag@Sn core-shell metal powder.

[0065] Example 3 Preparation of Cu@Ag@Sn Core-Shell Metal Powder

[0066] S1: Select 20 g of Cu powder with a particle size of 20 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass for 5 minutes, and then uniformly disperse it in 20 mL of distilled water to form a mixed solution A;

[0067] S2: Add 1.2 g of sodium formate and 72 mL of N,N-dimethylformamide to A, ultrasonically disperse for 3 minutes, then add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reactor, keep it warm at 150 °C for 18 hours, and apply magnetic stirring during the heat preservation with a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it alternately with distilled water and ethanol 3 times. Disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution C;

[0068] S3: Add 6 g of N-β-hydroxyethyl ethylenediamine triacetic acid, 6 g of ethylene glycol, and 8 g of silver nitrate to C, adjust the pH to 4 with 38% hydrochloric acid by mass, and mix evenly to obtain a mixed solution D. Keep D warm at 50 °C for 8 minutes, and apply magnetic stirring during the heat preservation with a rotation speed of 100 revolutions per minute. After the heat preservation ends, collect the reaction product, rinse it 3 times with distilled water, and disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution E;

[0069] S4: Add 40 g of paraffin, 40 g of ammonium thiocyanate, and 12 g of pure Sn powder to E, and magnetically stir at room temperature for 8 minutes with a rotation speed of 100 revolutions per minute to make it react fully. After the reaction ends, collect the product and rinse it 3 times with distilled water. Finally, dry it at 60 °C for 40 minutes to obtain the Cu@Ag@Sn core-shell metal powder.

[0070] Example 4 Preparation of Cu@Ni@Sn Core-Shell Metal Powder

[0071] P1: Select 20 g of Cu powder with a particle size of 20 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass for 5 minutes, and then uniformly disperse it in 20 mL of distilled water to form a mixed solution A;

[0072] P2: Add 1.2 g of sodium formate and 72 mL of N,N-dimethylformamide to A, ultrasonically disperse for 3 minutes, then add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reactor, keep it warm at 150 °C for 18 hours, and apply magnetic stirring during the heat preservation with a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it alternately with distilled water and ethanol 3 times to obtain product C;

[0073] P3: Dissolve 25 g / L of nickel chloride, 15 g / L of sodium hypophosphite, 10 g / L of citric acid, and 10 g / L of triethanolamine in a eutectic solvent with a molar ratio of choline chloride to ethylene glycol of 1:2, and adjust the pH to 9.0 with a 5 wt% NaOH-ethylene glycol solution. Stir and dissolve to obtain an electroless plating solution D.

[0074] P4: Place C in plating solution D, with the plating temperature at 60 °C and the plating time at 2 h. During this period, apply magnetic stirring at a rotation speed of 100 to 200 revolutions per minute to ensure full reaction. After the reaction, collect the product and rinse it 3 times with distilled water. Then disperse the rinsed product in distilled water to obtain a mixed solution F with a concentration of 0.2 g / mL.

[0075] P5: Add 40 g of paraffin, 40 g of ammonium thiocyanate, and 12 g of pure Sn powder to F, and perform magnetic stirring at room temperature for 8 minutes at a rotation speed of 100 revolutions per minute to ensure full reaction. After the reaction, collect the product and rinse it 3 times with distilled water. Finally, dry it at 60 °C for 40 minutes to obtain Cu@Ni@Sn core-shell metal powder.

[0076] Preparation of Solder Paste in Example 5

[0077] Mix the Cu@Ag@Sn core-shell metal powder prepared in the previous Example 1 with commercially available eutectic Sn-58Bi alloy powder with a diameter of 45 μm at a mass ratio of 1:8, and then mix it with a low-temperature lead-free rosin-based soldering flux paste. The mixing process is carried out at room temperature, and the metal powder is added gradually while stirring until a paste is formed. The mixing ratio is that the Cu@Ag@Sn core-shell metal powder accounts for 10% of the total mass of the solder paste, and the soldering flux paste accounts for 10%, thus forming a high thermal conductivity low-temperature solder paste.

[0078] Preparation of Solder Paste in Example 6

[0079] Mix the Cu@Ag@Sn core-shell metal powder prepared in the previous Example 2 with commercially available eutectic Sn-58Bi alloy powder with a diameter of 45 μm at a mass ratio of 2:7, and then mix it with a low-temperature lead-free rosin-based soldering flux paste. The mixing process is carried out at room temperature, and the metal powder is added gradually while stirring until a paste is formed. The mixing ratio is that the Cu@Ag@Sn core-shell metal powder accounts for 20% of the total mass of the solder paste, and the soldering flux paste accounts for 10%, thus forming a high thermal conductivity low-temperature solder paste.

[0080] Preparation of Solder Paste in Example 7

[0081] Mix the Cu@Ag@Sn core-shell metal powder prepared in the previous Example 3 with commercially available eutectic Sn-58Bi alloy powder with a diameter of 45 μm at a mass ratio of 3:6, and then mix it with a low-temperature lead-free rosin-based soldering flux paste. The mixing process is carried out at room temperature, and the metal powder is added gradually while stirring until a paste is formed. The mixing ratio is that the Cu@Ag@Sn core-shell metal powder accounts for 30% of the total mass of the solder paste, and the soldering flux paste accounts for 10%, thus forming a high thermal conductivity low-temperature solder paste.

[0082] Example 8

[0083] The Cu@Ni@Sn core-shell metal powder prepared in the aforementioned Example 4 was mixed with a commercially available eutectic Sn-58Bi alloy powder with a diameter of 45 μm at a mass ratio of 1:8, and then mixed with a low-temperature lead-free rosin-based soldering paste. The mixing process was carried out at room temperature, and the metal powder was added gradually while stirring until a paste was formed. The mixing ratio was that the Cu@Ni@Sn core-shell metal powder accounted for 10% of the total mass of the solder paste, and the soldering paste accounted for 10%, thus forming a high thermal conductivity low-temperature solder paste.

[0084] Example 9 Preparation of Cu@Ag@Sn Core-Shell Metal Powder

[0085] S1: Select 20 g of Cu powder with a particle size of 50 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass for 5 minutes, and then uniformly disperse it in 20 mL of distilled water to form a mixed solution A;

[0086] S2: Add 1.5 g of sodium formate and 72 mL of N,N-dimethylformamide to A, ultrasonically disperse for 3 minutes, then add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reaction kettle, keep it warm at 200 °C for 10 hours, and apply magnetic stirring during the heat preservation, with a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it alternately with distilled water and ethanol 3 times. Disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution C;

[0087] S3: Add 5 g of imidazole, 7 g of ethylene glycol, and 7 g of silver nitrate to C, adjust the pH to 4 with 38% hydrochloric acid by mass, and mix evenly to obtain a mixed solution D. Keep D warm at 50 °C for 8 minutes, and apply magnetic stirring during the heat preservation, with a rotation speed of 100 revolutions per minute. After the heat preservation ends, collect the reaction product, rinse it with distilled water 3 times, and disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution E;

[0088] S4: Add 25 g of polyethylene glycol, 25 g of ammonium thiocyanate, and 5 g of pure Sn powder to E, and magnetically stir at room temperature for 8 minutes with a rotation speed of 100 revolutions per minute to make it react fully. After the reaction ends, collect the product and rinse it with distilled water 3 times. Finally, dry it at 60 °C for 40 minutes to obtain the Cu@Ag@Sn core-shell metal powder.

[0089] Example 10 Preparation of Cu@Ag@Sn Core-Shell Metal Powder

[0090] S1: Select 20 g of Cu powder with a particle size of 30 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass for 5 minutes, and then uniformly disperse it in 20 mL of distilled water to form a mixed solution A;

[0091] S2: Add 1.2 g of sodium formate and 80 mL of N,N-dimethylformamide to A. After ultrasonic dispersion for 3 minutes, add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reactor and keep it at 120 °C for 20 hours with magnetic stirring during the heat preservation at a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it alternately with distilled water and ethanol 3 times. Disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution C;

[0092] S3: Add 6 g of tartaric acid, 6 g of glycerol, and 8 g of silver nitrate to C. Adjust the pH to 4 with 38% hydrochloric acid by mass fraction. After mixing evenly, obtain a mixed solution D. Keep D at 50 °C for 8 minutes with magnetic stirring during the heat preservation at a rotation speed of 100 revolutions per minute. After the heat preservation ends, collect the reaction product, rinse it with distilled water 3 times, and disperse the rinsed product in 200 mL of distilled water to obtain a mixed solution E;

[0093] S4: Add 20 g of polyvinylpyrrolidone, 20 g of thiourea, and 8 g of pure Sn powder to E, and stir magnetically at room temperature for 8 minutes at a rotation speed of 100 revolutions per minute to make it react fully. After the reaction ends, collect the product and rinse it with distilled water 3 times. Finally, dry it at 60 °C for 40 minutes to obtain Cu@Ag@Sn core-shell metal powder.

[0094] Example 11 Preparation of Cu@Ni@Sn Core-Shell Metal Powder

[0095] P1: Select 20 g of Cu powder with a particle size of 50 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass fraction for 5 minutes, and then evenly disperse it in 20 mL of distilled water to form a mixed solution A;

[0096] P2: Add 1.0 g of sodium formate and 80 mL of propylene glycol to A. After ultrasonic dispersion for 3 minutes, add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reactor and keep it at 200 °C for 10 hours with magnetic stirring during the heat preservation at a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it alternately with distilled water and ethanol 3 times to obtain product C;

[0097] P3: Dissolve 15 g / L of nickel acetate, 15 g / L of dimethylamine borane, 10 g / L of citric acid, and 10 g / L of N - methyldiethanolamine in a eutectic solvent with a molar ratio of choline chloride to ethylene glycol of 1:2, and adjust the pH to 9.0 with a 5 wt% NaOH-ethylene glycol solution. After stirring and dissolving, obtain electroless plating solution D.

[0098] P4: Place C in plating solution D at a plating temperature of 120 °C for a plating time of 2 h. During this period, apply magnetic stirring at a rotation speed of 100 to 200 revolutions per minute to allow sufficient reaction. After the reaction, collect the product and rinse it 3 times with distilled water. Then disperse the rinsed product in distilled water to obtain a mixed solution F with a concentration of 0.1 g / mL;

[0099] P5: Add 50 g of polyethylene glycol, 40 g of ammonium thiocyanate, and 16 g of pure Sn powder to F, and magnetically stir at room temperature for 8 minutes at a rotation speed of 100 revolutions per minute to allow sufficient reaction. After the reaction, collect the product and rinse it 3 times with distilled water. Finally, dry it at 60 °C for 40 minutes to obtain the Cu@Ni@Sn core-shell metal powder.

[0100] Preparation of Cu@Ni@Sn Core-Shell Metal Powder in Example 12

[0101] P1: Select 20 g of Cu powder with a particle size of 40 μm using a sieve, ultrasonically clean it with 2% hydrochloric acid by mass for 5 minutes, and then uniformly disperse it in 20 mL of distilled water to form a mixed solution A;

[0102] P2: Add 1.5 g of calcium formate and 70 mL of propylene glycol to A, ultrasonically disperse for 3 minutes, then add 8 mL of oleylamine to obtain a mixed solution B. Then pour B into a high-pressure reaction kettle, keep it at 120 °C for 20 hours, and apply magnetic stirring during the heat preservation at a rotation speed of 200 revolutions per minute. Collect the reaction product and rinse it 3 times alternately with distilled water and ethanol to obtain product C;

[0103] P3: Dissolve 20 g / L of nickel sulfate, 12 g / L of sodium borohydride, 15 g / L of boric acid, and 15 g / L of diethanolamine in a deep eutectic solvent with a molar ratio of choline chloride to ethylene glycol of 1:2, and adjust the pH to 9.0 with a 5 wt% NaOH-ethylene glycol solution. After stirring and dissolving, obtain the electroless plating solution D.

[0104] P4: Place C in plating solution D at a plating temperature of 100 °C for a plating time of 2 h. During this period, apply magnetic stirring at a rotation speed of 100 to 200 revolutions per minute to allow sufficient reaction. After the reaction, collect the product and rinse it 3 times with distilled water. Then disperse the rinsed product in distilled water to obtain a mixed solution F with a concentration of 0.3 g / mL;

[0105] P5: Add 45 g of polyvinylpyrrolidone, 35 g of thiourea, and 15 g of pure Sn powder to F, and magnetically stir at room temperature for 8 minutes at a rotation speed of 100 revolutions per minute to allow sufficient reaction. After the reaction, collect the product and rinse it 3 times with distilled water. Finally, dry it at 60 °C for 40 minutes to obtain the Cu@Ni@Sn core-shell metal powder.

[0106] Adding porous foam Cu sheets to the Sn58Bi alloy in Comparative Example 1

[0107] The porous foam Cu was impregnated with the molten Sn58Bi solder, and referring to Liu, Yang, et al. Journal of Materials Science: Materials in Electronics. 2020, 31, 8258 - 8267., a composite solder sheet was prepared, and then its thermal conductivity was tested. The specific steps were as follows:

[0108] S1: Select porous foam Cu with 500 pores per inch (500 ppi), a porosity of 85%, and a thickness of 0.05 mm;

[0109] S2: The porous foam Cu was impregnated in the molten Sn58Bi solder at 250 °C for 7 s, taken out and cooled to room temperature to obtain a composite solder sheet;

[0110] S3: The composite solder sheet was made into a circular specimen with a diameter of 12.7 mm and a thickness of 2 mm, and then its thermal conductivity was tested using NETZSCH LFA 447.

[0111] The thermal conductivity of the composite solder sheet impregnated with Sn58Bi by the porous foam Cu sheet prepared by this method was 41.32 W / (m·K). Compared with this existing method, the thermal conductivity of the high - thermal - conductivity low - temperature solder paste prepared in Example 7 of this application can reach 50.82 W / (m·K).

[0112] Adding Cu particles to the Sn58Bi solder paste in Comparative Example 2

[0113] 5 wt.% Cu particles were added to the Sn58Bi solder paste, and referring to Hao Zhang, et al. Journal of Materials Science: Materials in Electronics. 2019, 30: 340–347, a composite solder paste was prepared, and then its thermal conductivity was tested. The specific steps were as follows:

[0114] S1: Select Cu particles with an average diameter of 5 μm and Sn58Bi particles of 45 μm, mix them and add a flux, and mechanically stir for 30 minutes to obtain a composite solder paste;

[0115] S2: The composite solder paste was brushed on an aluminum substrate with copper pads and circuits, then 3535 LED lights were placed, heated, the peak temperature was 170 °C, and the 3535 LED lights were soldered on the substrate to form solder joints;

[0116] S3: T3Ster was used to evaluate the thermal behavior of the soldered LED lights and the thermal conductivity of the Sn58Bi - 5 wt.% Cu solder layer.

[0117] The peak welding temperature of the Sn58Bi-5wt.%Cu composite solder prepared by this method is 170 °C, and the thermal conductivity of the Sn58Bi-5wt.%Cu solder layer is 26.60 W / (m·K). Compared with this existing method, the melting point of the high thermal conductivity low-temperature solder paste prepared in Example 7 of this application is 138.9 °C, the welding temperature is not higher than 150 °C, and the thermal conductivity can reach 50.82 W / (m·K).

[0118] Composite solder sheet of Cu@Sn in Comparative Example 3

[0119] Sn is plated on the surface of Cu particles to form Cu@Sn core-shell metal particles, and then the Cu@Sn core-shell metal particles are pressed into a 400±20 μm composite solder sheet under a pressure of 30 MPa (Hongtao Chen, Tianqi Hu, et al. IEEE Transactions on Power Electronics. 2017, 32(1): 441-51.), and then its thermal conductivity is tested. The specific steps are as follows:

[0120] S1: At room temperature, thiourea (0.65 mol / L), EDTA (0.0014 mol / L), hydroquinone (0.0036 mol / L) and sodium hypophosphite (0.2 mol / L) are dissolved in 80 mL of deionized water, and then methanesulfonic acid (0.0042 mol / L) and ethylene glycol (15 mL / L) are added, and the solution is gently stirred until the materials are completely dissolved to obtain a mixed solution A;

[0121] S2: 2 grams of stannous chloride are dissolved in 1 mL of hydrochloric acid solution to obtain a mixed solution B;

[0122] S3: 2 grams of Cu particle powder with an average diameter of 35 μm are dropped into an ethanol solution containing 5% hydrochloric acid, and the solution is ultrasonically cleaned to remove surface contaminants and oxide layers outside the Cu particles; finally, the Cu particles are washed four times with deionized water;

[0123] S4: Solution B is poured into solution A, and continuous stirring is applied until the solutions are mixed evenly. Then the pickled Cu particles are quickly added to the mixed solution. Stir continuously at room temperature for 3 hours and filter to obtain Cu@Sn core-shell particles;

[0124] S5: The Cu@Sn core-shell metal particles are kept under a pressure of 30 MPa for 1 minute and pressed into a 400±20 μm composite solder sheet;

[0125] S6: The thermal diffusivity and specific heat capacity of the composite brazing sheet were tested using NETZSCH 477 and NETZSCH STA 449F3 respectively. Finally, the thermal conductivity of the composite brazing sheet can be obtained by multiplying the obtained diffusivity, specific heat capacity, and density.

[0126] The thermal conductivity of the Cu@Sn composite brazing sheet prepared by this method is 154.26 W / (m·K) at 30 °C, 130.64 W / (m·K) at 150 °C, and 127.99 W / (m·K) at 250 °C; the reflow temperature for welding is 250 °C and it is not applicable to low-temperature welding at 150 °C. Compared with this existing method, the melting point of the high-thermal-conductivity low-temperature brazing paste prepared in Example 7 of this application is 138.9 °C, the welding temperature is not higher than 150 °C, and the thermal conductivity can reach 50.82 W / (m·K).

[0127] Performance detection

[0128] (1) Morphology of the composite material

[0129] The metal composite material prepared in Example 3 was taken for analysis. Figure 2 Shown are the Cu@Ag@Sn metal powder and its XRD pattern. It can be seen that the Ag element, Sn element, and the compound Ag3Sn formed by Ag atoms and Sn atoms are present, indicating that the surface of the Cu particles was successfully coated with Ag and Sn layers.

[0130] Figure 3 and Figure 4 are the scanning electron microscope image and the energy dispersive spectrometer surface scanning image of the Cu@Ag@Sn core-shell metal particles respectively. The results show that after electroless plating of Ag and Sn on the Cu particles, a relatively thick coating formed by Ag and Sn atoms was deposited on the surface of the Cu particles, and the surface of the formed Cu@Ag@Sn core-shell metal particles was rough and pitted.

[0131] Figure 5 is the local energy dispersive spectrometer line scanning image of the cross-section of the Cu@Ag@Sn core-shell metal particles. It can be clearly observed from the figure that the Ag intermediate layer and the Sn outer layer after two electroless plating coatings are closely attached to the Cu inner core without pores; the energy spectrum line scan results show the alternating fluctuations of the intensities of the Ag element and the Sn element at different coatings, confirming that the coatings are the Ag layer and the Sn layer respectively.

[0132] (2) Melting point

[0133] Take the brazing paste prepared in Example 7. The physical picture is shown in Figure 6, its melting point was tested. The specific method was as follows: The solder paste was placed on an Al3O2 ceramic substrate and heated and refluxed at 150 °C for 5 minutes; since the molten solder did not wet the Al3O2 ceramic, the solder shrank into a spherical shape under the action of surface tension, and a solder alloy ball was prepared after cooling and solidification; then the solder ball was polished to make a solder alloy sheet with a diameter of 12.50 - 12.90 mm and a thickness of 1 - 5 mm. Then a DSC204F1 differential scanning calorimeter was used to test the DSC curve of the solder alloy. The test results are as Figure 7 shown. The peak temperature of the solder alloy was 146.0 °C. According to the regulations of the standardization committee ICTA, the intersection of the extended line of the front baseline and the tangent line at the maximum slope of the front edge of the peak on the DSC curve represents the melting point, that is, the melting point of this solder alloy was 138.9 °C.

[0134] (3) Thermal conductivity

[0135] The solder paste prepared in Example 7 was taken for thermal conductivity testing. The specific method was as follows: The solder paste was placed on an Al3O2 ceramic substrate and heated and refluxed at 150 °C for 5 minutes; since the molten solder did not wet the Al3O2 ceramic, the solder shrank into a spherical shape under the action of surface tension, and a solder alloy ball was prepared after cooling and solidification; then the solder ball was polished to make a solder alloy sheet with a diameter of 12.50 - 12.90 mm and a thickness of 1 - 5 mm. Then a BS210S electronic density balance, an LFA447 laser internal injection method thermal conductivity analyzer, and a DSC 204F1 differential scanning calorimeter were used to test the density, thermal diffusivity, and specific heat capacity of the solder alloy sheet respectively. Finally, the thermal diffusivity, specific heat capacity, and density obtained were multiplied to obtain the thermal conductivity of the composite solder sheet. The test results are as Figure 8 shown. The thermal conductivity of this solder alloy was 40.59 W / (m·K) at 25 °C, 42.39 W / (m·K) at 40 °C, 43.84 W / (m·K) at 55 °C, 45.44 W / (m·K) at 70 °C, 50.82 W / (m·K) at 85 °C, and 52.51 W / (m·K) at 100 °C.

[0136] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0137] In addition, it should be noted that in the above specific embodiments, the various specific technical features described can be combined in any suitable way without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0138] In addition, any combination can be made among various different embodiments of the present invention, as long as it does not violate the idea of the present invention, and it should equally be regarded as the content disclosed by the present invention.

Claims

1. A high thermal conductivity low-temperature solder paste, characterized in that: The high thermal conductivity low-temperature solder paste includes a composite metal material, Sn-Bi series alloy powder, and a soldering flux paste. The composite metal material accounts for 10% - 30% of the total weight, and the Sn-Bi series alloy powder accounts for 10% - 30% of the total weight. The composite metal material is spherical with a diameter of 30μm - 54μm; it has a three-layer core-shell structure, with the inner core being Cu, the middle layer being Ag or Ni, and the shell layer being Sn. The diameter of the inner core is 25 - 50μm, the thickness of the middle layer is 0.5μm - 2μm, and the thickness of the shell layer is 0.5μm - 2μm. In the composite metal material, by weight, Cu accounts for 60% - 98% of the total weight; Ag or Ni accounts for 1% - 20% of the total weight; Sn accounts for 1% - 20% of the total weight. The starting melting point of the high thermal conductivity low-temperature solder paste is 138 - 140°C, and the peak temperature is 140 - 145°C.

2. The high thermal conductivity low-temperature solder paste according to claim 1, wherein: The thickness of the middle layer is 0.8 - 1.8μm; the thickness of the shell layer is 0.8 - 1.8μm.

3. The high thermal conductivity low-temperature solder paste according to claim 1 or 2, wherein: In the composite metal material, by weight, Cu accounts for 65 - 95% of the total weight; Ag or Ni accounts for 3 - 18% of the total weight; Sn accounts for 3 - 18% of the total weight.

4. The high thermal conductivity low-temperature solder paste according to claim 1, wherein: The composite metal material is Cu@Ag@Sn core-shell metal powder, and the preparation process includes: S1: Take Cu powder for pickling, and then disperse it in distilled water to form a mixed solution A. S2: Add a reducing agent and a stabilizer to the solution A, disperse it ultrasonically, and then add oleylamine to obtain a mixed solution B; pour the mixed solution B into a reaction kettle for heating reaction, collect the reaction product, rinse the reaction product, and then disperse it in distilled water to obtain a mixed solution C. S3: Add a complexing agent, a brightening agent, silver nitrate, and a pH regulator to the mixed solution C, mix evenly to obtain a mixed solution D; carry out a heat preservation reaction on the mixed solution D, collect the reaction product, rinse it, and then disperse it in distilled water to obtain a mixed solution E. S4: Add a dispersing agent, a complexing agent, and Sn powder to the mixed solution E, stir and react. After the reaction ends, collect the product, rinse it, and dry it to obtain Cu@Ag@Sn core-shell metal powder.

5. The high thermal conductivity low-temperature solder paste according to claim 4, characterized in that: In S1, Cu powder with a particle size distribution of 15μm - 60μm is selected by a sieve, ultrasonically pickled, and then evenly dispersed in distilled water to form a mixed solution A, with the concentration of Cu powder being 1g / mL - 3g / mL.

6. The high thermal conductivity low-temperature solder paste according to claim 5, wherein: In S1, the pickling uses any one of nitric acid, hydrochloric acid, or sulfuric acid, with a mass concentration of 2% - 5%.

7. The high thermal conductivity low-temperature solder paste according to claim 5, wherein: In S2, the reducing agent is selected from one of sodium formate, potassium formate, and calcium formate, and the concentration of the reducing agent in the mixed solution B is 0.1g / mL - 0.2g / mL; the stabilizer is selected from one of N,N-dimethylformamide, glycerol, and propylene glycol, and the addition amount of the stabilizer accounts for 60 - 80% of the volume of the mixed solution B; the addition amount of oleylamine accounts for 3 - 6% of the volume of the mixed solution B.

8. The high thermal conductivity low-temperature solder paste according to claim 7, wherein: The heating reaction described in S2 is carried out at 120°C to 200°C for 10 hours to 20 hours, with magnetic stirring applied during the holding period at a rotation speed of 100 revolutions per minute to 500 revolutions per minute; the rinsing is carried out by alternately rinsing with distilled water and ethanol 2 - 3 times; the concentration of the mixed solution C is 5 g / mL to 10 g / mL.

9. The high thermal conductivity and low temperature solder paste according to claim 5, characterized in that: The complexing agent described in S3 is selected from at least one of N-β-hydroxyethyl ethylenediamine triacetic acid, imidazole, citric acid, tartaric acid, gluconic acid, and the concentration of the complexing agent in the mixed solution D is 0.03 g / mL to 0.05 g / mL; the brightening agent is selected from one of glycerol, ethylene glycol, glycine, and the concentration of the brightening agent in the mixed solution D is 0.003 g / mL to 0.005 g / mL; the concentration of silver nitrate in the mixed solution D is 0.004 g / mL to 0.006 g / mL; the pH regulator is selected from at least one of nitric acid and hydrochloric acid, and the pH of the mixed solution D is adjusted to 3 - 4.

10. The high thermal conductivity low-temperature solder paste according to claim 9, characterized in that: The holding reaction described in S3 is carried out at 40°C to 70°C for 2 minutes to 8 minutes, with magnetic stirring applied during the holding period at a rotation speed of 100 revolutions per minute to 200 revolutions per minute; the concentration of the mixed solution E is 0.1 g / mL to 0.3 g / mL.

11. The high thermal conductivity low-temperature solder paste according to claim 5, characterized in that: In S4, the dispersant is selected from at least one of paraffin, polyvinylpyrrolidone, polyethylene glycol, and the concentration of the dispersant in the mixed solution E is 0.1 g / mL to 0.2 g / mL; the complexing agent is selected from at least one of ammonium thiocyanate and thiourea, and the concentration of the complexing agent in the mixed solution E is 0.1 g / mL to 0.2 g / mL; the concentration of Sn powder in the mixed solution E is 0.03 g / mL to 0.

12. The high thermal conductivity low-temperature solder paste according to claim 11, wherein: ​ 13. The high thermal conductivity low-temperature solder paste according to claim 1, wherein: ​ ​ ​ ​ ​ ​ 14. The high thermal conductivity low-temperature solder paste according to claim 13, characterized in that: In P1, Cu powder with a particle size distribution ranging from 15 μm to 60 μm is selected by a sieve, ultrasonically pickled, and then uniformly dispersed in distilled water to form a mixed solution A with a Cu powder concentration of 1 g / mL to 3 g / mL.

15. The high thermal conductivity and low temperature solder paste according to claim 14, characterized in that: In P1, the pickling is carried out using any one of nitric acid, hydrochloric acid, or sulfuric acid, with a mass concentration of 2% to 5%.

16. The high thermal conductivity low-temperature solder paste according to claim 14, characterized in that: In P2, the reducing agent is selected from one of sodium formate, potassium formate, and calcium formate, and the concentration of the reducing agent in the mixed solution B is 0.1 g / mL to 0.2 g / mL; the stabilizer is selected from one of N,N-dimethylformamide, glycerol, and propylene glycol, and the addition amount of the stabilizer accounts for 60 to 80% of the volume of the mixed solution B; the addition amount of oleylamine accounts for 3 to 6% of the volume of the mixed solution B.

17. The high thermal conductivity and low temperature solder paste according to claim 16, characterized in that: In P2, the heating reaction is carried out at 120 °C to 200 °C for 10 hours to 20 hours, and magnetic stirring is applied during the holding period at a rotation speed of 100 revolutions per minute to 500 revolutions per minute; the rinsing is carried out by alternately rinsing with distilled water and ethanol 2 - 3 times; the concentration of the mixed solution C is 5 g / mL to 10 g / mL.

18. The high thermal conductivity and low temperature solder paste according to claim 14, characterized in that: In P3, the nickel salt is one or a combination of nickel chloride, nickel sulfate, nickel acetate, and nickel sulfamate, with a concentration range of 5 to 50 g / L; the reducing agent is selected from one of hydrazine, sodium hypophosphite, sodium borohydride, potassium borohydride, and dimethylamine borane, with a concentration range of 1 to 30 g / L; the stabilizer is one or a combination of boric acid, citric acid, lactic acid, sodium potassium tartrate, and disodium ethylenediaminetetraacetate, with a concentration range of 0.5 to 15 g / L; the accelerator is one or a combination of ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, and N,N-dimethylethanolamine, with a concentration range of 5 to 20 g / L; the pH regulator is a 5 wt% sodium hydroxide-ethylene glycol solution, with a pH range of 7.0 to 14.

0.

19. The high thermal conductivity low-temperature solder paste according to claim 18, wherein: In P3, in the deep eutectic solvent, the hydrogen bond acceptor is one or a combination of choline chloride, tetramethylammonium chloride, tetrabutylammonium chloride, tetraethylammonium chloride, and their derivatives, the hydrogen bond donor is one or two combinations of polyols, amides, and carboxylic acids, and the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:

5. After mixing the two, stirring is carried out at 60 to 90 °C for 2 to 4 h to obtain the deep eutectic solvent.

20. The high thermal conductivity low-temperature solder paste according to claim 14, wherein: In P4, the plating temperature for the electroless nickel plating treatment is 60 °C to 150 °C, the plating pH is 7.0 to 14.0, and the plating time is 0.5 to 4.0 h; during this period, magnetic stirring is applied at a rotation speed of 100 revolutions per minute to 200 revolutions per minute; after the reaction ends, the product is collected, rinsed 2 - 3 times with distilled water, and the rinsed product is dispersed in distilled water to obtain a mixed solution F with a concentration of 0.1 g / mL to 0.3 g / mL.

21. The high thermal conductivity and low temperature solder paste according to claim 14, characterized in that: The dispersant described in P5 is selected from at least one of paraffin, polyvinylpyrrolidone, and polyethylene glycol, and the concentration of the dispersant in the mixed solution F is 0.1 g / mL to 0.2 g / mL; the complexing agent is selected from at least one of ammonium thiocyanate and thiourea, and the concentration of the complexing agent in the mixed solution F is 0.1 g / mL to 0.2 g / mL; the concentration of Sn powder in the mixed solution E is 0.03 g / mL to 0.06 g / mL; the stirring reaction is carried out at room temperature for 5 - 10 minutes with a rotation speed of 100 revolutions per minute to 200 revolutions per minute; the collected product is rinsed 2 - 3 times with distilled water and dried at 50 - 70 °C for 30 - 60 minutes to obtain the Cu@Ni@Sn core-shell metal powder.

22. The high thermal conductivity low-temperature solder paste according to claim 1, wherein: The soldering paste is a medium- and low-temperature lead-free rosin-based soldering paste.

23. The high thermal conductivity low-temperature solder paste according to claim 22, wherein: The Sn-Bi series alloy powder is selected from at least one of hypoeutectic Sn-Bi alloy powder with a particle size distribution of 20 μm to 60 μm, eutectic Sn-58Bi alloy powder, and hypereutectic Sn-Bi alloy powder.

24. The high thermal conductivity low-temperature solder paste according to claim 22, wherein: The thermal conductivity of the high thermal conductivity low-temperature solder paste is 50 - 55 W / (m·K).

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