Solder with multi-layer structure and welding assembly

Abstract

The invention relates to a welding flux with a multilayer structure and a welding assembly. The solder includes a first layer, a second layer, and a third layer disposed between and connecting the first and second layers; wherein each of the first, second and third layers includes metal particles of first, second and third sizes, and wherein the median diameter D501 of the metal particles of the first size satisfies: 10 [mu] m < = D501 < = 50 [mu] m; the second size D502 meets the condition that D502 is more than or equal to 500 nm and less than or equal to 5 microns The third size D503 meets the condition that D503 is more than or equal to 30nm and less than or equal to 300nm; wherein the content of the metal particles of the third size in each of the first layer and the second layer is 60-90% by mass, and the content of the metal particles of the third size is reduced layer by layer and the content of the metal particles of the first size is increased layer by layer from each of the first layer and the second layer to the third layer.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging technology, and more specifically to a solder having a multilayer structure and a solder assembly formed therefrom. Background Technology

[0002] Driven by the electrification trend of modern electronic devices and transportation, the performance and packaging technology of semiconductor devices are facing unprecedented challenges and development opportunities. In particular, the continuous increase in the power density of semiconductor devices has led to a shift from traditional Si to third-generation wide-bandgap SiC, posing a significant challenge to currently used tin-based solders.

[0003] To address these challenges, the industry has begun seeking novel low-temperature sintering solders, such as nano-silver paste and nano-copper paste. These nano-solders exhibit advantages such as high temperature resistance, high thermal conductivity, and high electrical conductivity after sintering, and are considered strong candidates to replace tin-based solders. However, nano-solder pastes still face many challenges in practical applications, including high porosity, complex nanoparticle fabrication processes, and high costs.

[0004] To address the aforementioned issues, patent CN108526751B proposes a pressureless sintering micro / nano hybrid solder paste. This paste is obtained by mixing a mixture A containing micron-sized particles and a mixture B containing nano-sized particles in a mass ratio of 1:9 to 3:1. It is then applied to the welding surface via dispensing or screen printing and sintered using overall heating, ultrasonic-assisted heating, or laser heating. While this method can increase the packing density and suppress sintering shrinkage, the sintered structure still contains numerous voids, posing a potential reliability risk during subsequent service life of the welded component.

[0005] WO2020199638A1 discloses a multi-size nanoparticle hybrid metal film for chip packaging interconnects. This metal film comprises at least two sizes of nano-metal particles, wherein the large-size metal particles have an average diameter of 1 nm to 10 μm, and the small-size metal particles have an average diameter of 0.5 nm to 20 nm. The small-size nanoparticles are implanted into the gaps between the large-size metal film particles through physical impact, forming a multi-size hybrid metal film. While this method can reduce the porosity of the sintered metal layer and improve electrical and thermal conductivity, the porosity (10%–15%) is still relatively high, and the shear strength (9.5–14.5 MPa) is relatively low, indicating room for further improvement.

[0006] In summary, while existing technologies have made some progress in improving the performance of interconnect solders for semiconductor devices, there are still needs to reduce porosity, increase shear strength, and / or reduce sintering shrinkage. Summary of the Invention

[0007] This invention addresses the aforementioned problems in the prior art, and one of its objectives is to provide a solder with a multilayer structure to meet the demand for high-performance packaging materials in next-generation power semiconductor devices. The solder comprises:

[0008] The first layer has a first surface that forms the first outer surface to be welded of the solder;

[0009] The second layer has a second surface forming the second outer surface to be soldered, the second surface being opposite to the first surface; and

[0010] The third layer is disposed between and connects the first and second layers;

[0011] Each of the first, second, and third layers comprises metal particles of a first, second, and third size, wherein the median particle size D501 of the first-sized metal particles satisfies: 10μm≤D501≤50μm; the median particle size D502 of the second-sized metal particles satisfies: 500nm≤D502≤5μm; and the median particle size D503 of the third-sized metal particles satisfies: 30nm≤D503≤300nm.

[0012] Specifically, by mass percentage, the content of third-size metal particles in each of the first and second layers is 60-90%, and from each of the first and second layers to the third layer, the content of third-size metal particles decreases layer by layer while the content of first-size metal particles increases layer by layer.

[0013] D50 is the median particle size, which is the particle size corresponding to a cumulative particle size distribution percentage of 50% in a sample. The particles can be selected from the group consisting of: copper, silver, gold, nickel, titanium, and any combination of two or more of the above.

[0014] In one embodiment, by mass percentage, both the first layer and the second layer comprise 3-10% of metal particles of the first size, 70-90% of metal particles of the third size, and the balance being metal particles of the second size, and the third layer comprises 20-50% of metal particles of the first size, 15-45% of metal particles of the third size, and the balance being metal particles of the second size.

[0015] In one embodiment, by mass percentage, both the first layer and the second layer comprise 3-5% of metal particles of the first size, 80-90% of metal particles of the third size, and the balance being metal particles of the second size, and the third layer comprises 30-50% of metal particles of the first size, 20-35% of metal particles of the third size, and the balance being metal particles of the second size.

[0016] In one embodiment, the D503 of the third-sized metal particle is less than or equal to 1 / 3 of the D502 of the second-sized metal particle, and the D502 of the second-sized metal particle is less than or equal to 1 / 3 of the D501 of the first-sized metal particle.

[0017] In one embodiment, the total average thickness of the solder after pre-sintering is 15 to 100 μm, wherein the average thickness of each layer is not less than 5 μm.

[0018] Optionally, the content of the third-sized metal particles in the first and / or second layers is 2 to 5 times that in the third layer, by mass percentage.

[0019] Optionally, in one embodiment, at least one intermediate layer is further included between the first layer and / or the second layer and the third layer, wherein, by mass percentage, the content of the third-sized metal particles decreases layer by layer from the first layer and / or the second layer of the solder to the at least one intermediate layer to the third layer, and the content of the first-sized metal particles increases layer by layer.

[0020] In one embodiment, each layer of solder further includes metal particles of at least another size that are larger than a third size and smaller than a first size and different from a second size.

[0021] Another object of the present invention is to provide a welding assembly comprising:

[0022] First component;

[0023] The second component; and

[0024] The welded section between the first component and the second component, which welds the two together.

[0025] The welded portion includes at least three layers, including a first outer layer adjacent to the first welding interface formed with the first component, a second outer layer adjacent to the second welding interface formed with the second component, and a first inner layer inside the first and second outer layers relative to the first and second welding interfaces.

[0026] Each layer of the welded part has metal particles of a first size, a second size, and a third size, wherein the median particle size D501 of the first size metal particles satisfies: 10μm≤D501≤50μm; the median particle size D502 of the second size metal particles satisfies: 500nm≤D502≤5μm; and the median particle size D503 of the third size metal particles satisfies: 30nm≤D503≤300nm.

[0027] Specifically, by mass percentage, the content of third-size metal particles in each of the first outer layer and the second outer layer is 60-90%, and from each of the first outer layer and the second outer layer to the first inner layer, the content of third-size metal particles decreases layer by layer and the content of first-size metal particles increases layer by layer.

[0028] In one embodiment, the welded portion comprises a five-layer structure, namely a first outer layer and a second outer layer adjacent to the first welding interface and the second welding interface, a first inner layer adjacent to the first outer layer, a second inner layer adjacent to the second outer layer, and a third inner layer between the first inner layer and the second inner layer; wherein, by mass percentage, the content of the third-sized metal particles in the first and second outer layers is 70-90% and correspondingly 1.25-2 times that in the first and second inner layers, and the content of the third-sized metal particles in the first or second inner layer is 1.25-2 times that in the third inner layer.

[0029] The solder according to the present invention has the following advantages:

[0030] 1. The porosity of the welded part formed by the solder after welding shall not exceed 3%, and the shear strength shall not be less than 40 MPa;

[0031] 2. By making the content of third-size metal particles account for 60% to 90% by mass percentage in the first and second layers of the solder (i.e., the outer layer of the solder that bonds to the component to be soldered), the solder can achieve the soldering of semiconductor devices at low temperature compared to solder that only contains coarse particles.

[0032] 3. By making the content of first-size metal particles account for 20-50% by mass percentage in the inner layer (i.e., the third layer) of the solder and having a smaller content of third-size metal particles than the outer layer of the solder, the heat ratio of the third-size metal particles is reduced, thereby saving the cost of material preparation. Attached Figure Description

[0033] Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description and drawings.

[0034] Figure 1 A first embodiment of the solder according to the present invention is illustrated schematically.

[0035] Figure 2 A second embodiment of the solder according to the invention is illustrated schematically. Detailed Implementation

[0036] The present invention will now be described in more detail with reference to exemplary embodiments. Those skilled in the art should understand that the following embodiments or experimental data are intended to illustrate the invention by way of example and do not constitute a limitation thereof. All descriptions of chemical element content (%) herein refer to weight percentages. Unless otherwise specified, preferred embodiments can be freely combined as needed. Unless explicitly stated otherwise, all ranges include end values.

[0037] In the field of electronic packaging, solder paste is typically prepared using single or mixed particle sizes. However, because smaller nanoparticles cannot effectively fill the gaps between larger metal particles, high porosity remains a problem. Furthermore, using too many smaller nanoparticles significantly increases costs, and their outer surface requires more organic solvent coating, further increasing costs and exacerbating porosity during sintering. Conversely, reducing the use of smaller nanoparticles relatively increases the content of larger particles, raising the sintering temperature and making low-temperature sintering impossible.

[0038] To at least alleviate one of the above problems, the present invention provides a solder having at least a three-layer structure, wherein the number of nano-metal particles decreases from the outer layer to the inner layer of the solder.

[0039] Figure 1 A solder 10 according to a first embodiment of the present invention is schematically shown. The solder 10 has a first layer 12, a second layer 14, and a third layer 16 disposed between and connecting the first layer 12 and the second layer 14. The first layer and the second layer are the outermost layers of the solder, forming the first outer surface 121 and the second outer surface 141 of the solder to be soldered, respectively, which will be bonded to the surfaces of the two parts to be soldered during the soldering process; and the third layer is the inner layer of the solder.

[0040] Each of the first layer 12, the second layer 14, and the third layer 16 includes metal particles 21 of a first size, metal particles 22 of a second size, and metal particles 23 of a third size. The median particle size D501 of the first-size metal particles satisfies: 10 μm ≤ D501 ≤ 50 μm; the median particle size D502 of the second-size metal particles satisfies: 500 nm ≤ D502 ≤ 5 μm; and the median particle size D503 of the third-size metal particles satisfies: 30 nm ≤ D503 ≤ 300 nm. Optionally, 10 μm ≤ D501 ≤ 30 μm; 500 nm ≤ D502 ≤ 2.5 μm; and 30 nm ≤ D503 ≤ 150 nm. The particles can be selected from the group consisting of copper, silver, gold, nickel, titanium, and any combination of two or more of the aforementioned materials. Optionally, each layer also includes metal particles of at least a fourth size that is larger than the third size but smaller than the first size; more medium-sized metal particles can be selected as needed.

[0041] By weight percentage, the content of third-size metal particles 23 in the first and second layers is 60-90%, and from each of the first and second layers to the third layer, the content of third-size metal particles decreases layer by layer and the content of first-size metal particles increases layer by layer.

[0042] Optionally, both the first and second layers comprise 3-10% of first-sized metal particles, 70-90% of third-sized metal particles, and the remainder being second-sized metal particles; and the third layer comprises 20-50% of first-sized metal particles, 15-45% of third-sized metal particles, and the remainder being second-sized metal particles, such that the content of third-sized metal particles decreases layer by layer from the outermost to the innermost layer of the solder, and the content of first-sized metal particles increases layer by layer from the outermost to the innermost layer of the solder. In one embodiment, both the first and second layers comprise 3-5% of first-sized metal particles, 80-90% of third-sized metal particles, and the remainder being second-sized metal particles by mass percentage; and the third layer comprises 30-50% of first-sized metal particles, 20-35% of third-sized metal particles, and the remainder being second-sized metal particles by mass percentage.

[0043] For example, both the first and second layers comprise 8% first-size metal particles, 90% third-size metal particles, and the remainder being second-size metal particles; and the third layer comprises 40% first-size metal particles, 18% third-size metal particles, and the remainder being second-size metal particles. Alternatively, both the first and second layers comprise 8% first-size metal particles, 70% third-size metal particles, and the remainder being second-size metal particles; and the third layer comprises 35% first-size metal particles, 35% third-size metal particles, and the remainder being second-size metal particles. Or, both the first and second layers comprise 4% first-size metal particles, 84% third-size metal particles, and the remainder being second-size metal particles; and the third layer comprises 45% first-size metal particles, 25% third-size metal particles, and the remainder being second-size metal particles.

[0044] In one embodiment, the content of the third-sized metal particles, by mass percentage, is 70-90% in the first / second layer and 2-5 times the content in the third layer; alternatively, the content of the third-sized metal particles is 80-90% in the first / second layer and 2.5-4 times the content in the third layer.

[0045] Figure 2 Solder 20 according to a second embodiment of the present invention is schematically shown. Where possible, the same reference numerals have been used to denote the same or similar parts. In this embodiment, solder 20 has a first layer 12, a second layer 14, a third layer 16 disposed between the first layer 12 and the second layer 14, and a fourth layer 18 disposed between the second layer 14 and the third layer 16, all connected together. The first and second layers are the outermost layers of the solder, forming the first and second outer surfaces of the solder to be soldered, which will bond to the surfaces of the two components to be soldered during the soldering process. Each of the first layer 12, the second layer 14, the third layer 16, and the fourth layer 18 includes metal particles 21 of a first size, metal particles 22 of a second size, and metal particles 23 of a third size, wherein the material and size selection of the metal particles are the same as in the first embodiment and will not be repeated.

[0046] By weight percentage, the content of third-size metal particles in the first and second layers is 60-90%, both greater than the content of third-size metal particles in the third and fourth layers. The content of third-size metal particles in the fourth layer is greater than that in the third layer, resulting in a gradual decrease in the content of third-size metal particles from the second (outermost) to the fourth and then back to the third (innermost) layer of the solder. Correspondingly, the content of first-size metal particles increases gradually from the outermost to the innermost layer of the solder.

[0047] Alternatively, a fourth layer may be disposed between the first and third layers, or a fourth layer may be present both between the first and third layers and between the second and third layers. In one embodiment, the content of third-sized metal particles in the first / second layer is 70-90% and 1.25-2 times that in the fourth layer, and the content of third-sized metal particles in the fourth layer is 1.25-2 times that in the third layer.

[0048] In other embodiments, at least one additional size of metal particles is present in each layer of solder, for example, the additional size is larger than a third size and smaller than a first size and different from a second size.

[0049] The method for preparing the solder according to the present invention is as follows:

[0050] 1. Provide conventional micron-sized metal powders for industrial use (e.g., copper powder of approximately 50μm in D501);

[0051] 2. Metal particles of the first size (10μm≤D501≤50μm), the second size (500nm≤D502≤5μm), and the third size (30nm≤D503≤300nm) are prepared from micron-sized metal powder by physical methods, and then subjected to acid washing, centrifugation, and drying.

[0052] 3. Mix the granules separately according to the following proportions by mass percentage:

[0053] a) Mixture A is formed by 3 to 10% of first-size metal particles, 70 to 90% of third-size metal particles, and the remainder is second-size metal particles, and is used as the outermost layer of solder; for example, first-size metal particles account for 6%, third-size metal particles account for 82%, and the remainder is second-size metal particles.

[0054] b) The first-size metal particles account for 20-50%, the third-size metal particles account for 15-45%, and the balance is the second-size metal particles, forming mixture B, which is used as the inner layer of the solder; for example, the first-size metal particles account for 35%, the third-size metal particles account for 38%, and the balance is the second-size metal particles.

[0055] c) Optionally, the first-size metal particles account for 11-19%, the third-size metal particles account for 46-65%, and the balance is the second-size metal particles, forming a mixture C, which is used as an intermediate layer between the outermost and inner layers of the solder.

[0056] 4. Mixtures A and B, and optionally mixture C, are respectively mixed with organic additives and stirred until homogeneous to obtain mixture paste A, mixture paste B, and optionally mixture paste C, respectively; the organic additives include organic dispersants and reducing agents, wherein the mass percentage of the organic dispersant in each mixture paste is 0.1-2%, and the mass percentage of the reducing agent is 0.1-1%. The organic dispersant is at least one of polyalkylene amides, polyalkylene salts, alkyl salts, etc.; the reducing agent is at least one of organic acids;

[0057] 5. The mixture is screen-printed sequentially in the order of ABA to form a stacked structure in which the size of the metal particles decreases from the outside to the inside. Optionally, the mixture is screen-printed sequentially in the order of ACBCA to form a five-layer stacked structure. It will be understood that a four-layer stacked structure can also be formed in the order of ACBA, or there can be more intermediate layers between layers A and B of the mixture, as long as the size of the metal particles decreases from the outside to the inside. In one embodiment, the thickness ratio of layer A to layer B of the mixture is in the range of 1:3 to 1:20, for example, 1 to 10.

[0058] 6. The multilayer structure is pre-sintered at 100-150°C for 5-30 minutes, during which the organic additives will volatilize, thereby obtaining a solder with a three-layer structure / optionally five or more layers, wherein the total average thickness of the solder after pre-sintering is 15-100 μm, wherein the average thickness of each layer is not less than 5 μm.

[0059] The solder obtained according to the above method can be used to weld semiconductor devices. In one embodiment, the solder according to the invention is placed between the surfaces to be welded of a first component and a second component (e.g., an insulated gate bipolar transistor (IGBT) chip and a copper-clad ceramic substrate) to form a stacked structure. Then, under helium protection, pressure of 10–30 MPa is applied from both the top and bottom of the stacked structure at a temperature of 200–300°C for 10–30 minutes to perform welding. Finally, the welded first and second components are air-cooled to room temperature to obtain a welded assembly.

[0060] Therefore, the present invention also provides a welding assembly comprising:

[0061] First component;

[0062] The second component; and

[0063] The welded section between the first component and the second component, which welds the two together.

[0064] The welded portion is formed by the solder according to the invention, and therefore possesses the aforementioned structure and advantages.

[0065] In one embodiment, the welded portion comprises a five-layer structure, namely: a first outer layer and a second outer layer adjacent to a first welding interface formed with a first component and a second welding interface formed with a second component, a first inner layer adjacent to the first outer layer, a second inner layer adjacent to the second outer layer, and a third inner layer between the first inner layer and the second inner layer; wherein, by mass percentage, the metal particles of the third size constitute 70-90% in the first outer layer and the second outer layer and are correspondingly 1.25-2 times more in the first inner layer and the second inner layer, and the metal particles of the third size constitute 1.25-2 times more in the first inner layer or the second inner layer than in the third inner layer.

[0066] It will be understood that the semiconductor devices that can be soldered by the solder according to the present invention include, but are not limited to, direct copper-clad ceramic substrates, active metal hard solder ceramic substrates, or copper-clad metal heat dissipation structures, and functional chips with one or more metal layers disposed on the front and back surfaces of the chip, using silicon, silicon carbide, or gallium nitride as the main materials. The outermost metal layer may be a copper layer, a gold layer, an aluminum layer, a silver layer, a nickel layer, or a titanium layer. For example, the chip may be an IGBT chip, a metal-oxide-semiconductor field-effect transistor (MOSFET) chip, a bipolar transistor (BJT) chip, or a fast recovery diode (FRD) chip.

[0067] In this invention, porosity is defined as the ratio of the total pore area to the sampling area. The porosity of the welded assembly is measured as follows: On the cross-section of the obtained welded assembly, samples are taken within the weld portion within a range of 1 μm width and any length of 10 μm along the length direction of the weld interface. The total area of ​​all visible pores within the sampling range at each location is measured using ImageJ image processing software. Then, the porosity at each sampling location is calculated based on the measurement results, and the average value is taken. The average thickness of the weld portion is measured as follows: On the cross-section of the obtained welded assembly, measurements are taken at at least 5 locations between the two weld interfaces, and the average value is taken.

[0068] In this invention, the shear fracture test method is as follows: the bottom of the second component of the welded assembly is fixed to the test platform with resin, and a movable mechanical component is used to press against the side of the first component of the welded assembly, applying a shear force parallel to the sample at a moving speed of 20 μm / min. Five samples of each product are tested and the average value is taken.

[0069] In this invention, the shrinkage rate is calculated by measuring the printed area A0 of the solder before welding and the welding area A1 after welding, that is, shrinkage rate = (A0-A1) / A0.

[0070] Specific embodiments of the welding assembly according to the present invention are given below.

[0071] First embodiment of welding assembly

[0072] First, conventional copper powder (D50 = 50 μm) is provided. Then, metal particles of the first size D501 = 10 μm, the second size D502 = 1 μm, and the third size D503 = 30 nm are prepared from the conventional copper powder by ball milling, followed by acid washing, centrifugation, and drying. The D502 of the second-size metal particles is 10% of the D501 of the first-size metal particles, and the D503 of the third-size metal particles is 3% of the D502 of the second-size metal particles. In other embodiments, the first, second, and third-size metal particles can also be prepared from conventional copper powder using physical techniques such as gas condensation, electrospraying, photolithography, and thermal decomposition. In other embodiments, the first, second, and third-size metal particles can be the same or different metals, for example, selected from copper, silver, gold, nickel, and titanium. In other embodiments, fourth, fifth, and more sizes of metal particles, which are between the first and third sizes and different from the second size, may also be included.

[0073] Then, metal particles of the first, second, and third sizes are mixed in a mass ratio of 5:5:90, and a dispersant (polyalkylene amide) and a reducing agent (organic acid) are added and stirred until homogeneous to form mixed paste A, which serves as the outermost layer of the solder (i.e., the first and second layers). Next, metal particles of the first, second, and third sizes are mixed in a mass ratio of 30:50:20, and a dispersant (polyalkylene amide) and a reducing agent (organic acid) are added and stirred until homogeneous to form mixed paste B, which serves as the inner layer of the solder (i.e., the third layer). In mixed pastes A and B, the mass percentage of the organic dispersant is 0.6%, and the mass percentage of the reducing agent is 0.3%.

[0074] Next, a 10μm thick mesh is used to apply the mixed paste A onto the surface of the silicone-coated support plane to form the first layer of solder. Then, a 30μm thick mesh is used to apply mixed paste B over the first layer to form the inner layer of solder. Finally, a 10μm thick mesh is used to apply mixed paste A over the inner layer to form the second layer of solder, thus obtaining a laminated structure. It will be understood that other mesh thicknesses can also be used for each layer, such as a 15μm mesh for the first layer, a 30μm mesh for the inner layer, and a 15μm mesh for the second layer.

[0075] The resulting laminated structure was pre-sintered at 150°C for 10 minutes to obtain a solder with a three-layer structure. The average thicknesses of the first, inner, and second layers of the solder are 9.5 μm, 30 μm, and 9.5 μm, respectively, with variances within an acceptable range in the art, such as ±5%. Furthermore, the content of the third-sized metal particles in the first and second layers is 4.5 times that in the inner layer. It will be understood that the thickness of each layer can be adjusted as needed.

[0076] A three-layer solder structure is sandwiched between a copper-clad ceramic substrate and a gold-plated IGBT chip to obtain a stacked structure. Then, under helium protection, the stacked structure is sintered by applying pressure (20 MPa) from both the top and bottom for 15 minutes at 280°C. Finally, the soldered IGBT chip and copper-clad ceramic substrate are air-cooled to room temperature to obtain a welded assembly.

[0077] The obtained welding assembly includes an IGBT chip, a copper-clad ceramic substrate, and a welding part between the IGBT chip and the copper-clad ceramic substrate, which welds the two together. The welding part includes a three-layer structure formed by the solder of the above-mentioned three-layer structure. Each layer has metal particles of a first size, a second size, and a third size with gradually decreasing particle size. The third size metal particles decrease layer by layer and the first size metal particles increase layer by layer from the first outer layer and the second outer layer adjacent to the first and second welding interfaces of the welding part with the IGBT chip and the copper-clad ceramic substrate to the inner layer of the welding part, by mass percentage.

[0078] Experimental analysis revealed that the average thickness of the welded portion of the first embodiment welding assembly (i.e., the average thickness between the first and second welding interfaces) was 49 μm, the shrinkage rate was 3%, the average porosity was 2%, and the average shear strength was 50 MPa.

[0079] Second embodiment of welding assembly

[0080] Similar to the first embodiment, except as follows:

[0081] 1. The first size of the metal particles is D501 = 50 μm, the second size of the metal particles is D502 = 5 μm, and the third size of the metal particles is D503 = 300 nm;

[0082] 2. Mix metal particles of the first, second, and third sizes in a mass percentage ratio of 10:20:70, add organic additives, and stir evenly to form mixed paste A; and mix metal particles of the first, second, and third sizes in a mass percentage ratio of 40:40:20, add organic additives, and stir evenly to form mixed paste B.

[0083] The obtained three-layer solder structure was sandwiched between the surface of a copper-clad ceramic substrate and an IGBT chip with a gold surface layer to obtain a stacked structure. The content of the third-sized metal particles in the first and second layers was 3.5 times that in the inner layer. Sintering was performed in a helium-protected environment at 280°C, pressurized from both the top and bottom of the stacked structure at a pressure of 20 MPa for 15 minutes. The soldered IGBT chip and copper-clad ceramic substrate were then air-cooled to room temperature to obtain a soldered assembly.

[0084] Experimental analysis revealed that the average thickness of the welded portion of the second embodiment welding assembly was 47 μm, the shrinkage rate was 1%, the average porosity was 3%, and the average shear strength was 46 MPa.

[0085] First comparison of welding components

[0086] Similar to the first embodiment, except as follows:

[0087] 1. Prepare only one type of mixed paste, that is, mix metal particles of the first size, the second size, and the third size in a mass percentage ratio of 30:50:20, add an organic dispersant and physically stir until uniform to form a mixed paste;

[0088] 2. Apply the mixed paste to the support plane using a 50μm thick screen to form a pre-layer of single-layer solder.

[0089] 3. The obtained pre-formed layer is pre-sintered at 150℃ for 10 minutes to obtain a single layer of solder, wherein the average thickness of the solder is 45μm.

[0090] A single layer of solder is sandwiched between the surface of a copper-clad ceramic substrate and an IGBT chip with a gold surface layer to obtain a stacked structure. Sintering is performed in a helium-protected environment at 280°C, pressurized from both the top and bottom of the stacked structure at a pressure of 20 MPa for 15 minutes. The soldered IGBT chip and copper-clad ceramic substrate are then air-cooled to room temperature to obtain a soldered assembly.

[0091] Experimental analysis revealed that the average thickness of the welded portion of the first comparative welding assembly was 45 μm, the shrinkage rate was 10%, the average porosity was 15%, and the average shear strength was 10 MPa.

[0092] Second comparison of welding components

[0093] Similar to the first embodiment, but with a different proportion of metal particles in the mixed paste. Specifically, metal particles of the first, second, and third sizes are mixed in a mass percentage ratio of 30:30:40, and an organic dispersant is added and physically stirred until homogeneous to form mixed paste A, which is used as the outermost layer of the solder (i.e., the first and second layers); and metal particles of the first, second, and third sizes are mixed in a mass percentage ratio of 30:20:50, and an organic dispersant is added and physically stirred until homogeneous to form mixed paste B, which is used as the inner layer of the solder (i.e., the third layer).

[0094] Subsequently, similar to the first embodiment, mixed paste A and mixed paste B were coated with screens of 10 μm and 40 μm thickness, respectively, to obtain a laminated structure of solder. The resulting laminated structure was then pre-sintered at 150°C for 10 minutes to obtain a solder with a three-layer structure. The average thicknesses of the first, second, and third layers of the solder were 9.5 μm, 28 μm, and 9.5 μm, respectively, with variances within an acceptable range in the art, such as ±5%, and the content of the third-sized metal particles in the first and second layers was 0.8 times that in the inner layers.

[0095] The solder was sandwiched between the surface of a copper-clad ceramic substrate and an IGBT chip with a gold surface to obtain a stacked structure. Sintering was performed in a helium-protected environment at 280°C, pressurized from both the top and bottom of the stacked structure at a pressure of 20 MPa for 15 minutes. The soldered IGBT chip and copper-clad ceramic substrate were then air-cooled to room temperature to obtain a welded assembly.

[0096] Experimental analysis revealed that the average thickness of the welded portion of the second comparative welding assembly was 45 μm, the shrinkage rate was 10%, the average porosity was 5%, and the average shear strength was 20 MPa.

[0097] Table 1. Parameter list for each embodiment and comparative example.

[0098]

[0099] Compared to the single-layer structure in the first comparative example, the three-layer structure in the second comparative example reduced the average porosity to some extent, but it remained at a high level, and the average shear strength was still low. This demonstrates that simply increasing the number of solder layers does not achieve the desired performance improvement.

[0100] Compared to the three-layer structure of the second comparative example, the welded portions formed by the solders of the first and second embodiments of the present invention exhibit lower shrinkage and average porosity, as well as significantly improved average shear strength. This is because the solders of these embodiments employ a high content of small metal particles (i.e., nanoscale third-size particles) in the outer layer, enhancing the filling effect between the gaps between large (first-size) metal particles, thereby improving the density of the welded portion after sintering, reducing porosity, and increasing shear strength. Simultaneously, this high content of small metal particles also allows for low-temperature sintering, reducing the large shrinkage caused by large temperature differences. On the other hand, the inner layer of these solders employs a low content of small metal particles (third-size) and a high content of large metal particles (first-size), resulting in a lower heat percentage of small metal particles in the inner layer, thereby reducing the cost of material preparation.

[0101] In summary, the solder according to the present invention achieves low-temperature sintering while obtaining a welded part with low porosity and shrinkage and high shear strength, thereby improving the service performance of the welded assembly formed therefrom.

[0102] The above embodiments and experimental data are intended to illustrate the present invention by way of example. Those skilled in the art should understand that the present invention is not limited to these embodiments, and various modifications can be made without departing from the scope of protection of the present invention.

Claims

1. A solder having a multilayer structure, the solder comprising: The first layer has a first surface that forms the first outer surface to be welded of the solder; The second layer has a second surface that forms the second outer surface to be welded of the solder, the second surface being opposite to the first surface; as well as The third layer is disposed between and connects the first and second layers; Each of the first, second, and third layers comprises metal particles of a first, second, and third size, wherein the median particle size D501 of the first-sized metal particles satisfies: 10μm≤D501≤50μm; the median particle size D502 of the second-sized metal particles satisfies: 500nm≤D502≤5μm; and the median particle size D503 of the third-sized metal particles satisfies: 30nm≤D503≤300nm. Specifically, by mass percentage, the content of third-size metal particles in each of the first and second layers is 60-90%, and from each of the first and second layers to the third layer, the content of third-size metal particles decreases layer by layer while the content of first-size metal particles increases layer by layer.

2. The solder according to claim 1, wherein, By mass percentage, the first layer and the second layer each comprise 3 to 10% of first-size metal particles, 70 to 90% of third-size metal particles, and the balance being second-size metal particles, and the third layer comprises 20 to 50% of first-size metal particles, 15 to 45% of third-size metal particles, and the balance being second-size metal particles.

3. The solder according to claim 1, wherein, By mass percentage, the first layer and the second layer each comprise 3-5% of first-size metal particles, 80-90% of third-size metal particles, and the balance being second-size metal particles, and the third layer comprises 30-50% of first-size metal particles, 20-35% of third-size metal particles, and the balance being second-size metal particles.

4. The solder according to any one of claims 1 to 3, wherein, The D503 of the third-sized metal particle is less than or equal to 1 / 3 of the D502 of the second-sized metal particle, and the D502 of the second-sized metal particle is less than or equal to 1 / 3 of the D501 of the first-sized metal particle.

5. The solder according to any one of claims 1 to 3, wherein, The total average thickness of the solder after pre-sintering is 15 to 100 μm, wherein the average thickness of each layer is not less than 5 μm.

6. The solder according to any one of claims 1 to 3, wherein, The metal particles are selected from the group consisting of: copper, silver, gold, nickel, titanium, and any combination of two or more of the foregoing.

7. The solder according to any one of claims 1 to 3, wherein, The solder also includes at least one intermediate layer between the first layer and / or the second layer and the third layer, wherein, by mass percentage, the content of the third-sized metal particles decreases layer by layer from the first layer and / or the second layer to the at least one intermediate layer to the third layer, and the content of the first-sized metal particles increases layer by layer; or, each layer of the solder also includes metal particles of at least another size that are larger than the third size and smaller than the first size and different from the second size.

8. The solder according to any one of claims 1 to 3, wherein, The content of the third-sized metal particles in the first and / or second layers is 2 to 5 times that in the third layer, by mass percentage.

9. A welding assembly comprising: First component; Second component; as well as The welded section between the first component and the second component, which welds the two together. The welded portion includes at least three layers, including a first outer layer adjacent to the first welding interface formed with the first component, a second outer layer adjacent to the second welding interface formed with the second component, and a first inner layer inside the first and second outer layers relative to the first and second welding interfaces. Each layer of the welded part has metal particles of a first size, a second size, and a third size, wherein the median particle size D501 of the first size metal particles satisfies: 10μm≤D501≤50μm; the median particle size D502 of the second size metal particles satisfies: 500nm≤D502≤5μm; and the median particle size D503 of the third size metal particles satisfies: 30nm≤D503≤300nm. Specifically, by mass percentage, the content of third-size metal particles in each of the first outer layer and the second outer layer is 60-90%, and from each of the first outer layer and the second outer layer to the first inner layer, the content of third-size metal particles decreases layer by layer and the content of first-size metal particles increases layer by layer.

10. The welding assembly according to claim 9, wherein, The welded portion comprises a five-layer structure, namely a first outer layer and a second outer layer adjacent to the first welding interface and the second welding interface, a first inner layer adjacent to the first outer layer, a second inner layer adjacent to the second outer layer, and a third inner layer between the first inner layer and the second inner layer; wherein, by mass percentage, the content of the third-sized metal particles in the first and second outer layers is 70-90% and correspondingly 1.25-2 times that in the first and second inner layers, and the content of the third-sized metal particles in the first or second inner layer is 1.25-2 times that in the third inner layer.

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