Pressureless sintering high-density micro-nano silver soldering paste and preparation method and application thereof

Abstract

The invention discloses pressureless sintering high-density micro-nano silver soldering paste and a preparation method and application thereof. According to the pressureless sintering high-density micro-nano silver soldering paste, the solvent capable of improving the surface activation energy of the micron silver sheets, the nano silver sheets and the nano silver balls is selected and mixed with the micro-nano silver particles with the proper size and the proper proportion according to a certain proportion, and therefore uniform dispersion of the micro-nano silver particles can be achieved; the specific solvent is also beneficial to tight combination of the micro-nano silver particles, and at a lower temperature (lt; the method is characterized in that a welding joint with relatively high shear strength and relatively high density can be obtained without applying extra pressure, an extra binder and the like are not needed, the energy consumption and the production cost can be reduced, and meanwhile, the welding joint is high in high-temperature service reliability and high in heat cycle resistance and can be used as a good welding material of a packaging module; and moreover, a large amount of harmful substances cannot be generated during sintering, so that the material is an environment-friendly material.

Description

Technical Field

[0001] This invention belongs to the field of electronic packaging interconnect materials technology, specifically relating to a pressureless sintering high-density micro / nano silver solder paste, its preparation method, and its application. Background Technology

[0002] With the increasing operating temperature of power electronic devices, the silver particle sintering method has broad application prospects in the packaging field due to its excellent comprehensive performance. The size and morphology of silver particles and the composition of organic solvents in solder paste all affect the performance of the sintered silver solder paste. Among these, the solvent composition has a significant impact on the physical properties of silver solder paste, including viscosity and flowability. The evaporation and removal of the solvent during sintering affects the post-sintering performance of the silver solder paste. All of these factors play a crucial role in the sintering of the solder paste, but related research is currently limited. Based on the current research and development progress of micron-sized silver solder paste systems, it has been confirmed that sintering systems using only micron-sized silver particles have a significant densification bottleneck (porosity ~30%, see Chinese patent CN112207481A). However, there are still key technical deficiencies in the synergistic enhancement mechanism for composite multi-scale silver particle systems. For example, in research on mixed micro / nano particle silver solder paste systems containing micron-sized silver flakes, nano-sized silver flakes, and nano-sized silver spheres, not only has the multi-level filling effect of mixed micro / nano particles not been clarified, but quantitative correlation models between geometric parameters and density are also lacking. Furthermore, compared with single-micron silver foil system solder paste, hybrid micro-nano particle silver solder paste has higher high-temperature service and thermal cycling reliability, which can improve the lifespan of electronic devices. Summary of the Invention

[0003] The technical problem to be solved by this invention is to overcome the shortcomings of silver solder paste, such as high porosity, low density, poor reliability in high-temperature service and thermal cycling, etc. The invention provides a pressureless sintering high-density micro-nano silver solder paste, which requires a lower sintering temperature, does not require additional pressure, does not contain harmful additives, does not produce a large amount of harmful substances during the sintering process, and the welded joint after sintering has good shear strength, high density and service reliability.

[0004] Another objective of this invention is to provide a method for preparing pressureless sintering high-density micro / nano silver solder paste.

[0005] Another objective of this invention is to provide an application of pressureless sintering high-density micro / nano silver solder paste.

[0006] The above-mentioned objective of this invention is achieved through the following technical solution: A pressureless sintering high-density micro / nano silver solder paste comprises 85%–90% micro / nano silver particles and 10%–15% organic solvent by weight percentage. The micro / nano silver particles include: micron-sized silver flakes with a diameter of 1–10 μm and a thickness of 0.1–2 μm; nano-sized silver flakes with a diameter of 100–1000 nm and a thickness of 10–200 nm; nano-sized silver spheres with a diameter of 50–100 nm; and any mixture of the above three components in any proportion. The organic solvent is a mixture of 2–4 of the following: ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol 400, terpineol, glycerol, hexanediol, diethylene glycol dimethyl ether, diethylene glycol monoethyl ether, tripropylene glycol methyl ether, and tetraethylene glycol dimethyl ether.

[0007] This invention provides a pressureless sintering high-density micro / nano silver solder paste. By selecting suitable micro / nano silver particle shapes and sizes, and by optimizing solvents that enhance the surface activity of the micro / nano silver particles, not only can the micro / nano silver particles be evenly dispersed, but it also facilitates tight bonding between the micro / nano silver particles. On one hand, ether-based reagents have low viscosity, and when mixed with alcohol solvents with higher viscosity, the viscosity of the silver solder paste is adjusted to a suitable level, resulting in good printability. Simultaneously, the low surface tension of ethers gives them excellent adhesion properties, which is beneficial for forming an effective connection between the solder joint and the substrate / silicon wafer. On the other hand, ultraviolet spectroscopy characterization reveals that the organic layer on the surface of the silver particles contains -COOH or -COOM functional groups. Therefore, the alcohol solvent can react with or dissolve in the organic shell layer, which is beneficial for close contact between the silver particles and thus densification during sintering. Mixing organic solvents can adjust the volatility of the original single-solvent silver solder paste, allowing the organic solvent to evaporate gradually and layered, significantly reducing the porosity of the joint microstructure. Meanwhile, silver solder paste containing multi-scale silver particles composed of micron-sized silver flakes, nano-sized silver flakes, and nano-sized silver spheres can further reduce the porosity of the sintered silver layer, increase density and strength, and improve service reliability. The micro / nano silver solder paste of this invention requires no additional adhesive or pressure, and the sintering temperature required is relatively low to achieve good shear strength, low porosity, and high reliability.

[0008] Preferably, the mass percentage of the micro / nano silver particles is 85% to 90%.

[0009] Preferably, the organic solvent has a mass percentage of 10% to 15%.

[0010] Preferably, the micro / nano silver particles are a mixture of 70% to 100% by mass of micron-sized silver flakes with a diameter of 1-6 μm and a thickness of 0.1-1 μm and nano-silver particles with a mass percentage of 0% to 30%.

[0011] Preferably, the silver nanoparticles are silver nanosheets with a diameter of 100-1000 nm and a thickness of 10-200 nm, or silver nanospheres with a diameter of 50-100 nm, or a mixture of both.

[0012] Preferably, the solvent is a mixture of 2 to 4 of the following: ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol 400, terpineol, glycerol, hexanediol, diethylene glycol dimethyl ether, diethylene glycol monoethyl ether, tripropylene glycol methyl ether, and tetraethylene glycol dimethyl ether.

[0013] This invention protects the preparation method of the above-mentioned pressureless sintering high-density micro / nano silver solder paste, comprising the following steps: First, the surface of micro / nano silver particles is pretreated by alternately cleaning the surface of silver particles with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid. Next, one of sodium citrate, oleic acid, and povidone K15 is added to form an organic coating layer on the surface of the silver particles to ensure the storage stability and low-temperature sintering activity of the micro / nano silver particles. Finally, the pretreated micro / nano silver particles are dried to obtain the pretreated micro / nano silver particles. After mixing 2 to 4 solvents evenly, one or more of the pretreated micron-sized silver sheets, nano-sized silver sheets, and nano-sized silver balls are added. After vacuum stirring evenly, the pressureless sintering high-density micro / nano silver solder paste is obtained.

[0014] This invention also protects the application of the above-mentioned pressureless sintering high-density micro / nano silver solder paste in the field of power device packaging. It includes the following steps: The pressureless sintering high-density micro-nano silver solder paste was placed on the material to be soldered and sintered at 240~280 ℃ for 30 min, with a heating rate of 21~36 ℃ / min.

[0015] Preferably, the materials to be welded are a silver-plated copper substrate and a silver-plated silicon chip.

[0016] The porosity of the welded joint of the present invention is 20.5~27.4%; the shear strength is 30.1~44.4MPa; and no obvious cracking occurred at the welded joint after 1000 thermal cycles of the micro-nano silver paste.

[0017] Compared with the prior art, the beneficial effects of the present invention are: The pressureless sintering high-density micro / nano silver solder paste of the present invention is made of a certain proportion of micron-sized silver flakes, nano-sized silver flakes, nano-sized silver spheres, and various mixed solvents. By selecting a solvent that can improve the surface activation energy of micro / nano silver particles and adjust the viscosity of the solder paste, it is mixed with micro / nano silver particles of appropriate size and proportion in a certain ratio. In this way, the uniform dispersion of micro / nano silver particles can be achieved. The specific solvent also facilitates the tight bonding of micro / nano silver particles. Sintering can be carried out at a relatively low temperature (<300 ℃) without the application of additional pressure, resulting in a weld joint with high shear strength, high density, and high reliability. No additional binders are required, which can reduce energy consumption and production costs. It can be used as a good soldering material for power electronic device packaging, and it does not produce a large amount of harmful substances during sintering, making it an environmentally friendly material. Attached Figure Description

[0018] Figure 1 The microscopic cross-sectional morphology of the low-temperature pressureless sintering micro / nano silver solder joints of this invention is shown in the figure.

[0019] Figure 2 The images show the cross-sectional microstructure of the welded joints after sintering of the pressureless sintered high-density micro / nano silver solder paste prepared in the examples and comparative examples.

[0020] Figure 3 The mechanical properties of the welded joint after sintering of the pressureless sintered high-density micro / nano silver solder paste prepared for the example are shown in the figure after service at a high temperature of 300 °C.

[0021] Figure 4 The image shows the X-ray image of the welded joint after sintering of the pressureless sintered high-density micro / nano silver solder paste prepared for the example, after thermal cycling at -55~125℃. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0023] The pretreatment steps for the micro-nano silver particles in the following embodiments are as follows: First, the surface of the micro-nano silver particles is pretreated by alternately cleaning the surface of the silver particles with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid. Finally, one of sodium citrate, oleic acid and polyvinylpyrrolidone K15 (medium to low molecular weight polyvinylpyrrolidone) is added to form an organic coating layer on the surface of the silver particles. Example

[0024] A pressureless sintering high-density micro / nano silver solder paste comprises 90% micro / nano particles, 5% ethylene glycol, and 5% diethylene glycol dimethyl ether (DGE) by weight percentage. The micro / nano particles consist of 70% by weight of 5 μm diameter and 0.1 μm thickness micron-sized silver flakes and 30% by weight of 500 nm diameter and 80 nm thickness nano-sized silver flakes. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0025] The preparation method of the above-mentioned pressureless sintering high-density micro / nano silver solder paste includes the following steps: First, mix ethylene glycol and diethylene glycol dimethyl ether evenly. Then, place the micron-sized silver sheet, nano-sized silver sheet and mixed solvent in a planetary mixer and stir in vacuum mode for 6 minutes. Stir until evenly mixed to obtain pressureless sintering high-density micro / nano silver solder paste. Example

[0026] A pressureless sintering high-density micro / nano silver solder paste comprises 90% micro / nano particles, 5% terpineol (an organic solvent), and 5% diethylene glycol dimethyl ether (DGE). The micro / nano particles consist of 70% by weight of 5 μm diameter and 0.1 μm thickness micron-sized silver flakes and 30% by weight of 500 nm diameter and 80 nm thickness nano-sized silver flakes. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0027] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0028] A pressureless sintering high-density micro / nano silver solder paste comprises 88% micro / nano particles, 4% organic solvents (terpineol, ethylene glycol, and diethylene glycol dimethyl ether), and 4% by weight. The micro / nano particles consist of 70% by weight of 5 μm diameter and 0.1 μm thickness micron-sized silver flakes and 30% by weight of 500 nm diameter and 80 nm thickness nano-sized silver flakes. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0029] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0030] A pressureless sintering high-density micro / nano silver solder paste comprises 88% micron-sized silver flakes, 3% terpineol, 3% ethylene glycol, 3% diethylene glycol, and 3% diethylene glycol dimethyl ether (by weight percentage). The micro / nano particles consist of 70% micron-sized silver flakes with a diameter of 5 μm and a thickness of 0.1 μm and 30% silver nanospheres with a diameter of 50 nm. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at a temperature of 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0031] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0032] A pressureless sintering high-density micro / nano silver solder paste comprises 89% by weight of micron-sized silver flakes and 11% by weight of ethylene glycol as solvent. The micro / nano particles consist of 70% by weight of micron-sized silver flakes with a diameter of 5 μm and a thickness of 0.1 μm, and 30% by weight of silver nanospheres with a diameter of 50 nm. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and finally, 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at a temperature of 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0033] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0034] A pressureless sintering high-density micro / nano silver solder paste comprises 90% micron-sized silver flakes, 5% ethylene glycol, and 5% diethylene glycol monoethyl ether by weight percentage. The micro / nano particles consist of 70% micron-sized silver flakes with a diameter of 5 μm and a thickness of 0.1 μm, 25% silver nano-flakes with a diameter of 500 nm and a thickness of 80 nm, and 5% silver nanospheres with a diameter of 50 nm by weight percentage. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at a temperature of 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0035] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0036] A pressureless sintering high-density micro / nano silver solder paste comprises 90% micron-sized silver flakes, 5% terpineol solvent, and 5% diethylene glycol dimethyl ether by weight percentage. The micro / nano particles consist of 70% micron-sized silver flakes with a diameter of 5 μm and a thickness of 0.1 μm, 20% silver nano-flakes with a diameter of 500 nm and a thickness of 80 nm by weight percentage, and 10% silver nanospheres with a diameter of 50 nm by weight percentage. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at a temperature of 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0037] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0038] A pressureless sintering high-density micro / nano silver solder paste comprises 90% micron-sized silver flakes, 5% ethylene glycol, and 5% tetraethylene glycol dimethyl ether by weight percentage. The micro / nano particles consist of 70% micron-sized silver flakes with a diameter of 5 μm and a thickness of 0.1 μm, 15% silver nano-flakes with a diameter of 500 nm and a thickness of 80 nm, and 15% silver nanospheres with a diameter of 50 nm. First, the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% sodium citrate is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is screen-printed onto a silver-plated copper substrate. A silver-plated silicon chip is then mounted on the surface of the silver paste, and sintering is performed at a temperature of 250°C, a heating rate of 25°C / min, and a sintering time of 30 min.

[0039] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0040] A pressureless sintering high-density micro / nano silver solder paste is the same as in Example 2, except that the subsequent sintering temperature is 280°C.

[0041] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1. Example

[0042] A pressureless sintering high-density micro / nano silver solder paste is the same as in Example 7, except that the subsequent sintering temperature is 280 °C.

[0043] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1.

[0044] A pressureless sintering high-density micro / nano silver solder paste is the same as in Example 7, except that the solvent is replaced with a single terpineol, which accounts for 10% by mass.

[0045] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1.

[0046] A pressureless sintering high-density micro / nano silver solder paste is the same as in Example 7, except that the micro / nano silver particles are replaced with micron-sized silver flakes with a diameter of 5 μm and a thickness of 0.1 μm.

[0047] The preparation method of the pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1.

[0048] A pressureless sintering high-density micro / nano silver solder paste is prepared similarly to that in Example 7, except that the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and finally, 0.8 wt% oleic acid is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is then screen-printed onto a silver-plated copper substrate, and a silver-plated silicon chip is mounted on the surface of the paste. The preparation method of the above pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1.

[0049] A pressureless sintering high-density micro / nano silver solder paste is prepared similarly to that in Example 7, except that the surface of the micro / nano silver particles is pretreated by alternately cleaning the surface with deionized water and ethanol to remove free ions and excess reducing agent. Then, the surface is activated with citric acid, and 0.8 wt% of polyvinylpyrrolidone K15 (medium-low molecular weight polyvinylpyrrolidone) is added to form an organic coating layer on the surface of the silver particles. The pressureless sintering high-density micro / nano silver solder paste is then screen-printed onto a silver-plated copper substrate, and a silver-plated silicon chip is mounted on the surface of the silver paste. The preparation method of the above pressureless sintering high-density micro / nano silver solder paste is the same as that in Example 1.

[0050] The micro / nano silver solder paste and micron silver solder paste prepared in each embodiment and comparative example are placed between the silver-plated silicon chip and the silver-plated copper substrate, and then placed on a heating table for pressureless sintering.

[0051] 1. Testing Method Shear strength of welded joints: Shear strength was tested at room temperature using a Try Precision MFM1200 push-pull testing machine at a test speed of 100 μm / s.

[0052] Conductivity: 40×10×0.12 mm strip samples were printed on glass slides for conductivity testing. The conductivity of the samples was tested using a four-probe resistance meter (Loresta-GX MCP-T700, Nittoseiko Analytech).

[0053] 2. Test Results Table 1. Conductivity test results of micro / nano silver solder joints prepared in some embodiments and comparative examples. <![CDATA[Conductivity (×10 7 S / m)]]> Example 1 1.63 Example 2 1.66 Example 3 1.59 Example 4 1.42 Example 5 1.23 Example 6 1.82 Example 7 1.78 Example 8 1.74 Example 9 1.76 Example 10 1.94 Comparative Example 1 0.88 Comparative Example 2 1.48 Comparative Example 3 1.35 Comparative Example 4 1.22 The welded joints obtained by the pressureless sintering high-density micro / nano silver solder paste of the present invention exhibit high shear strength and good density after sintering. In the above embodiments and comparative examples, different silver particles or different organic solvents were used, resulting in certain differences in the obtained welded joints, indicating that both silver particles and organic solvents play a role in the sintering process.

[0054] In Examples 1 to 3, the silver solder paste used a composite multi-scale silver particle system of micron-sized silver flakes and nano-sized silver flakes, while Comparative Example 2 used only one type of micron-sized silver flake. Its joint shear strength was slightly lower than that of Examples 1 to 3, and its porosity increased accordingly. In Examples 4 and 5, the silver solder paste used a composite multi-scale silver particle system of micron-sized silver flakes and nano-sized silver spheres. Its shear strength was lower than that of the silver solder paste in Comparative Example 2, indicating that the introduction of nano-sized silver spheres increased the sintering difficulty. During sintering, nano-sized silver spheres are in point contact. Although the intrinsic driving force is high, without pressure reduction, they can only provide point-like sintering necks, requiring higher temperatures or longer holding times to eliminate porosity. In Examples 6 to 10, the silver solder paste used a composite multi-scale silver particle system of micron-sized silver flakes, nano-sized silver flakes, and nano-sized silver spheres. Compared with Comparative Example 2, the shear strength increased by 23.6% to 43.4%, the porosity decreased significantly, and the sintered structure was more dense. By using a composite multi-scale silver particle system instead of single-scale micron-sized silver particles, the multi-level filling effect of the composite scale silver particle system can be utilized to effectively improve the packing density. On the other hand, the different diffusion mechanisms of silver particles of different scales can be used to reduce the sintering activation energy and optimize the sintering process.

[0055] Compared to Examples 1 to 4, which used mixed organic solvent formulations, Example 5 used only one organic solvent. The joint shear strength of Example 5 was significantly lower than that of Examples 1 to 4, and the porosity increased accordingly. Mixed organic solvents can adjust the volatility and wetting ability of the silver solder paste, which is beneficial for improving the mechanical properties of the sintered silver joint and slowing down the formation of large pores. In Example 5, ethylene glycol was used as the solvent. Due to the high surface tension and viscosity of ethylene glycol, the micron-sized silver sheets could not be well dispersed in ethylene glycol alone, nor could it promote tight bonding between the micron-sized silver sheets. At the same sintering temperature, the shear strength of the welded joint was low. Therefore, the choice of organic solvent is crucial.

[0056] Examples 2 and 9, and 7 and 10, were silver solder pastes prepared with the same formula and sintered at different temperatures. As the sintering temperature increased from 250 °C to 280 °C, the shear strength and conductivity of the silver solder joints both increased. The conductivity of Example 10 reached 1.94 × 10⁻⁶. 7 S / m, approximately the conductivity of pure silver (6.3 × 10⁻⁶). 7 The shear strength (S / m) is 30%. For different silver solder pastes sintered at the same temperature, Example 10 also exhibited the best shear strength and optimal conductivity, followed by Example 7. The conductivity of Comparative Example 1 is less than 14% of that of pure silver, significantly lower than other examples. Comparative Example 2 differs from Example 7 only in that the micro / nano hybrid silver particles are replaced with micron-sized silver flakes, but both its shear strength and conductivity are significantly lower than Example 7, indicating that the micro / nano silver solder paste has better electrical and mechanical properties. Using a composite multi-scale silver particle system instead of single-scale micron-sized silver particles effectively utilizes their respective geometric and energy advantages to sequentially complete the construction of a three-level conductive network ("point-line-surface") under low-temperature, pressureless conditions, while minimizing porosity. Nano-silver flakes can be embedded in the triangular gaps of micron-sized silver flakes, forming a continuous "surface-line-surface" conductive channel, while nano-silver spheres fill the pores, providing "point-point" support, further increasing the number of conductive channels, thereby effectively improving the conductivity of sintered silver.

[0057] In Example 7, the micro / nano silver particles were pretreated with 0.8 wt% sodium citrate, while Comparative Examples 3 and 4 were pretreated with 0.8 wt% oleic acid and povidone K15, respectively. The solder joint strength and conductivity of the sample prepared in Example 7 were significantly higher than those in Comparative Examples 3 and 4, indicating that different organic coatings have a significant impact on the performance of sintered silver. The coating layer must not only prevent agglomeration and provide rheological compatibility during storage, but also completely decompose and desorb during sintering. Sodium citrate has a low decomposition temperature (160-200℃), and its -COOH or -COOM functional groups can be dissolved or reacted by alcohol solvents, which is beneficial for close contact between silver particles and thus densification during sintering. In contrast, the oleic acid used in Comparative Example 3 has a high decomposition temperature (230-280℃), and cannot completely decompose during low-temperature sintering, hindering the densification process of the silver particles. The povidone K15 used in Comparative Example 4 has a high decomposition temperature (250-300℃) and its main functional group is a pentamer (─NH─C=O), which has poor compatibility with the added solvent and cannot be completely decomposed. The residual carbon film significantly increases electrical resistance and reduces shear strength. Therefore, the organic coating layer on the surface of silver particles is also an important factor affecting the performance of micro / nano silver solder paste. The organic coating layer can ensure the storage stability of silver particles, improve low-temperature sintering activity, and provide a material basis for subsequent pressureless / low-temperature interconnects.

[0058] The above results indicate that the pressureless sintering high-density micro / nano silver solder paste of the present invention requires a lower sintering temperature, can achieve good interconnection effect, reduce energy consumption, and does not produce harmful substances.

[0059] Cross-section of interconnected sintered structures using silver solder paste, such as Figure 1 As shown, the upper side is a silver-plated silicon chip, the lower side is a silver-plated copper substrate, and the middle side is a porous silver sintered body. Figure 2 The images show the microscopic cross-sections of sintered bodies prepared with silver solder paste using different organic solvents, corresponding to (a) Example 1; (b) Example 10; (c) Comparative Example 1; and (d) Comparative Example 2, respectively. Figure 2As can be seen, the sintered joints of the embodiments all exhibited reliable interconnect structures and good density. In particular, the sintered structure of Example 10 showed significantly higher density compared to the other examples, indicating that a reliable sintered joint was obtained after sintering. A composite multi-scale silver particle system composed of micron-sized silver sheets, nano-sized silver sheets, and nano-sized silver spheres was used instead of single-scale micron-sized silver particles. Utilizing the multi-level filling effect of the composite-scale silver particle system, the micron-sized silver sheets constructed the macroscopic framework, the nano-sized silver sheets bridged the gaps, and the nano-sized silver spheres filled the pores. Related simulations showed that this effectively improved the packing density. Comparative Examples 1 and 2 were silver solder pastes prepared from single-scale micron-sized silver sheets. During sintering, they could not fully fill the pores, resulting in higher porosity within the sintered body. Secondly, the organic solvents have an impact: First, terpineol and diethylene glycol dimethyl ether have relatively low surface tension, thus providing better spreading performance for silver; second, alcohols such as terpineol, ethylene glycol, and diethylene glycol can react with the organic shell on the surface of silver particles, which is beneficial to the tight bonding between silver particles; thirdly, the evaporation temperature of the mixed solvents has a gradient, with diethylene glycol dimethyl ether, ethylene glycol, terpineol, and diethylene glycol evaporating in stages from low to high, making the overall evaporation rate of the silver solder paste more gradual, which is conducive to reducing the formation of pores. Figure 3 The graph shows the shear strength change of Example 10 after high-temperature aging at 300℃. As can be seen from the graph, the mechanical properties of the welded joint can still maintain 41.8MPa after 1000h of high-temperature aging, with the strength decreasing by only 5.6%. Figure 4 The X-ray non-destructive testing results of Example 10 after thermal cycling at -55℃ to 125℃ correspond to (a) 250 cycles; (b) 500 cycles; (c) 750 cycles; and (d) 1000 cycles, respectively. Figure 4 It can be seen that no significant cracking occurred at the weld joint after 1000 thermal cycles of the micro / nano silver paste. These results demonstrate that the pressureless sintering high-density micro / nano silver solder paste of the present invention has high reliability.

[0060] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A pressureless sintered high density micro-nano silver solder paste, characterized in that, The micro-nano silver particles comprise 80-90% by mass of one or more of micron silver flakes with a diameter of 1-10 μm and a thickness of 0.1-2 μm, nano silver flakes with a diameter of 100-1000 nm and a thickness of 10-200 nm, and nano silver spheres with a diameter of 50-100 nm, and 10-20% by mass of organic solvents.

2. The pressureless sintered high density micro-nano silver solder paste according to claim 1, wherein, The micro-nano silver particles are mixed silver particles comprising 70-100% by mass of micron silver flakes with a diameter of 1-6 μm and a thickness of 0.1-1 μm and 0-30% by mass of nano silver particles.

3. The pressureless sintered high density micro-nano silver solder paste according to claim 1, wherein, The nano silver particles are nano silver flakes with a diameter of 100-1000 nm and a thickness of 10-200 nm, nano silver spheres with a diameter of 50-100 nm, or a mixture of both.

4. The pressureless sintered high density micro-nano silver solder paste according to claim 1, wherein, The mass percentage of the micro-nano silver particles is 85-90%.

5. The pressureless sintered high density micro-nano silver solder paste according to claim 1, wherein, The total mass percentage of the organic solvents is 10-15%, and the proportion of each solvent in the mixed organic solvents differs by no more than 2%.

6. The method for preparing the high-density micro-nano silver solder paste sintered without pressure according to any one of claims 1-5, characterized in that, The method comprises the following steps: first, pre-treating the surface of the micro-nano silver particles, cleaning the surface of the silver particles alternately with deionized water and ethanol to remove free ions and excess reducing agent, then activating the surface with citric acid, adding one of sodium citrate, oleic acid, and povidone K15 to form an organic coating layer on the surface of the silver particles to ensure the storage stability and low-temperature sintering activity of the micro-nano silver particles, and finally drying to obtain pre-treated micro-nano silver particles; mixing 2-4 kinds of solvents uniformly, then adding one or more of the pre-treated micron silver flakes, nano silver flakes, and nano silver spheres, and stirring uniformly under vacuum to obtain the pressureless sintering high-density micro-nano silver paste.

7. Use of the pressureless sintering high-density micro-nano silver paste of any one of claims 1-5 in the field of packaging and interconnection of wide-bandgap semiconductor devices.

8. Use according to claim 7, characterized in that, The method comprises the following steps: The pressureless sintering high-density micro-nano silver paste is placed on the material to be welded, and sintered at 240-300 ℃ for 30 min with a heating rate of 21-36 ℃ / min.

9. Use according to claim 7, characterized in that, The material to be welded is a silver-plated copper substrate and a silver-plated silicon chip.

10. Use according to claim 7, characterized in that, The porosity of the welded joint is 20.5-27.4%, the shear strength is 30.1-44.4 MPa, and the micro-nano silver paste does not cause obvious cracking at the welded joint after 1000 cycles of thermal cycling.

Images