Liquid dispersion assisted large area low pressure sintering of silver interconnects and sintered substrates

Abstract

The application provides a liquid dispersant assisted large-area low-pressure sintering silver interconnection process and a sintered substrate, and belongs to the technical field of electronic power devices. The application solves the technical problem of a complex application process in the prior art and reduces industrial cost. The process comprises the following steps: a steel screen printing control thickness method is adopted, a scraper is used to uniformly pre-coat 80-120 mu m thick nano-silver solder paste on the surface of a substrate, and then the substrate is dried at 120 DEG C for 30 min; a certain amount of liquid dispersant is uniformly formed on another substrate, and then the substrate is butted against the substrate coated with the nano-silver solder paste; the two butted substrates are placed on a preheated hot press, heated from T1 DEG C to 275 DEG C at a heating rate of 5 DEG C / min, and then kept at 275 DEG C and P1 MPa for 50 min, and then cooled to room temperature, so that a sintered connection sample is finally prepared.

Description

Technical Field

[0001] This invention relates to the field of electronic power device technology, and in particular to a liquid dispersant-assisted large-area low-pressure sintering silver interconnect process and sintering substrate. Background Technology

[0002] With the advancement of electronic power technology, third-generation semiconductor devices, represented by SiC and GaN, are continuously developing towards higher power density and higher integration. Interconnect layers, as key channels for heat transfer in power modules, have a significant impact on the reliable operation of power modules at high temperatures. For high-temperature, high-power electronic power packaging systems, heat dissipation is a critical issue determining the overall system efficiency and reliability. Thermal interface materials can effectively fill the air gaps at the contact surfaces between the substrate and cooling devices (heat sinks, heat plates, etc.), ensuring an effective heat transfer path and playing a connecting and auxiliary heat dissipation role. Traditional thermal interface materials, such as greases, gels, and phase change materials, are limited in high-temperature, high-power electronic packaging due to their bottlenecks in heat dissipation and long-term reliability.

[0003] Nano-silver solder paste, as a novel green thermal interface material, possesses excellent performance and is favored by various parties. Utilizing a novel interface interconnection technology—sintered silver technology—stable electrical and mechanical interconnections are achieved between chips and insulating substrates (chip-level, small area) and substrates and heat dissipation devices (substrate-level, large area) through low-temperature sintering of nano / micro silver particles. Sintering can be achieved at temperatures below 300℃, with a theoretical application temperature as high as 961℃ after sintering. The thermal resistance is less than 2mm²*K / W, and the thermal conductivity is 3-5 times that of traditional thermal interface materials. It has been commercialized not only in the field of chip-level small-area interconnection but also has broad application prospects in the field of substrate-level large-area interconnection.

[0004] For substrate-level large-area (>1000mm²) sintered silver interconnect technology, achieving effective interface contact and reliable sintering under certain pressure assistance has become an industry consensus. To form reliable adhesion, large-area welding generally requires high pressure of 20-40MPa. However, high-pressure sintering can lead to warping, deformation, or even breakage of the power module substrate, resulting in reduced device yield and significantly increased costs. High pressure is the bottleneck limiting the widespread application of substrate-level large-area sintered silver interconnect technology.

[0005] To address the critical issue of reducing the sintering pressure over large areas, current efforts have focused on optimizing the bonding process and improving the silver paste. By introducing double-layer printing and rapid drying methods, the pressure has been reduced to 1.8 MPa. While this ensures good contact between the solder paste and the substrate interface, the sintering curve's applicability to factors such as solder paste thickness and bonding area is generally limited. The sintering curve needs to be adjusted according to specific requirements, complicating the application process, increasing industrial costs, and hindering large-scale production. Summary of the Invention

[0006] The purpose of this invention is to provide a liquid dispersant-assisted large-area low-pressure sintering silver interconnect process and sintering substrate, which solves the technical problems of complex application process in the prior art and reduces industrial costs.

[0007] In a first aspect, the present invention provides a liquid dispersant-assisted large-area low-pressure sintering silver interconnect process, comprising the following steps:

[0008] The thickness is controlled by stencil printing. A squeegee is used to uniformly pre-coat an 80-120μm thick nano silver solder paste onto the substrate surface, and then it is dried at 120℃ for 30 minutes.

[0009] A fixed amount of liquid dispersant is uniformly formed on another substrate;

[0010] Connect another substrate to the substrate coated with nano silver solder paste;

[0011] The two mated substrates were placed on a preheated hot press and heated from T1℃ to 275℃ at a heating rate of 5℃ / min. After holding at 275℃ and P1MPa for 50 minutes, the temperature was cooled to room temperature, and the sintered bonded sample was finally obtained.

[0012] Furthermore, the total mass fraction of nano-silver particles or micron-sized silver flakes in the nano-silver solder paste is 85.3%.

[0013] Furthermore, the substrate is a copper substrate with a purity of 99.9%, electroplated with a 4μm silver layer and a 0.4μm nickel layer.

[0014] Furthermore, the liquid dispersant includes α-terpineol, polyethylene glycol, and polyvinylpyrrolidone.

[0015] Furthermore, the step of uniformly forming a measured amount of liquid dispersant on the dried solder paste layer includes:

[0016] A quantitative liquid dispersant is uniformly formed on another substrate using a spraying or printing process, and then the other substrate is joined to the substrate coated with nano silver solder paste.

[0017] Furthermore, the liquid dispersant is α-terpineol, and the amount of α-terpineol used is 0.5 mg / cm³. 2 .

[0018] Furthermore, T1 is 15–25, and P1 is 2.0.

[0019] In a second aspect, the present invention also provides a sintered substrate, comprising:

[0020] First substrate;

[0021] Nano silver solder paste formed on the first substrate, the thickness of the nano silver solder paste is 80-120μm;

[0022] A liquid dispersant layer formed on the second substrate;

[0023] A second substrate sintered with nano-silver solder paste;

[0024] The first substrate and the second substrate are heated from 15 to 25°C to 275°C at a heating rate of 5°C / min, and then held at 275°C and 2.0 MPa for 50 min before being cooled to room temperature and sintered.

[0025] Furthermore, the total mass fraction of nano-silver particles or micron-sized silver flakes in the nano-silver solder paste is 85.3%.

[0026] Furthermore, the liquid dispersant includes at least one of α-terpineol, polyethylene glycol, and polyvinylpyrrolidone.

[0027] The liquid dispersant-assisted large-area low-pressure sintering silver interconnect process and sintered substrate provided by this invention achieve large-area bonding under pressure not exceeding 5 MPa. This achieves good contact at the sintering interface while reducing sintering pressure, and minimizes the impact of solder paste thickness and bonding area on sintering quality. This makes the industrial sintering process more controllable, avoids the hazards of high-pressure sintering, and effectively reduces industrial costs. The liquid dispersant-assisted silver sintering bonding process of this invention ensures that the solder paste layer is free of macroscopic cracks and delamination after sintering, exhibiting high bonding strength. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 Large-area sintering process curves and flowcharts provided for embodiments of the present invention;

[0030] Figure 2 The mechanism of liquid dispersant-assisted large-area silver sintering in this embodiment of the invention;

[0031] Figure 3 Ultrasonic scanning imaging of the sintered structure in an embodiment of the present invention;

[0032] Figure 4 The shear strength distribution of the sintered structure in an embodiment of the present invention;

[0033] Figure 5 The cross-sectional microstructure of the sintered structure in an embodiment of the present invention is shown below.

[0034] Figure 6 This is a temperature cycling ultrasonic scanning imaging and peeling degree data diagram of the sintered structure according to an embodiment of the present invention;

[0035] Figure 7 This is a schematic diagram of a sintered substrate provided in an embodiment of the present invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] The terms "comprising" and "having," and any variations thereof, used in the embodiments of this invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the steps or units listed, but may optionally include other steps or units not listed, or may optionally include other steps or units inherent to such processes, methods, products, or devices.

[0038] This invention provides a liquid dispersant-assisted large-area low-pressure sintering silver interconnect process, comprising the following steps:

[0039] Printing: The thickness is controlled by stencil printing. A squeegee is used to uniformly pre-coat the substrate surface with 80-120μm thick nano silver solder paste.

[0040] Drying: Dry at 120℃ for 30 minutes.

[0041] Spraying: A quantitative amount of liquid dispersant is uniformly formed on another substrate.

[0042] Docking: Connect another substrate to the substrate coated with nano silver solder paste.

[0043] Sintering: The two mated substrates are placed on a preheated hot press and heated from T1℃ to 275℃ at a heating rate of 5℃ / min. After holding at 275℃ and P1MPa for 50min, they are cooled to room temperature to obtain the sintered joint sample.

[0044] The liquid dispersant provided in this embodiment of the invention assists in large-area (>1000mm) 2The low-pressure (<5MPa) sintering silver interconnect process achieves large-area bonding under pressure not exceeding 5MPa. This ensures good contact at the sintering interface while reducing sintering pressure, and minimizes the impact of solder paste thickness and bonding area on sintering quality. This makes the industrial sintering process more controllable and avoids the hazards of high-pressure sintering, thereby effectively reducing industrial costs. The liquid dispersant-assisted silver sintering bonding process of this invention ensures that the solder paste layer is free of macroscopic cracks and delamination after sintering, exhibiting high bonding strength.

[0045] The following are specific embodiments provided by the present invention:

[0046] Taking a solder paste thickness of 100μm, a bonding area of ​​25.4mm*50.8mm, and a sintering pressure P1 of 2.0MPa at room temperature as an example, the specific sintering process is explained in detail:

[0047] (1) A substrate made of 99.9% pure copper was cut into 25.4mm*50.8mm*1.0mm pieces, and after cleaning, a 4μm silver layer and a 0.4μm nickel layer were electroplated.

[0048] (2) Clean the silver-plated copper substrate in an ultrasonic cleaner for 10 minutes, and wipe it with lint-free paper after cleaning. Place the cleaned substrate on the mold base, and fix the matching steel mesh on the base containing the heat dissipation device, such as... Figure 1 As shown in the middle printing section, the thickness is controlled by screen printing, and a 100μm thick silver solder paste is uniformly pre-coated on the substrate surface using a squeegee.

[0049] (3) such as Figure 1 After the solder paste is printed as shown in the drying section, remove the stencil from the mold base, place the substrate with the solder paste on the heating table, and proceed as follows. Figure 1 The drying curve shown indicates drying at 120°C for 30 min. The total mass fraction of nano-silver particles or micron-sized silver flakes in the nano-silver solder paste is 85.3%. The nano-silver solder paste is printed on the substrate as a thermal interface material. Stable interconnection is achieved by spraying a liquid dispersant onto the surface of the heat dissipation device to assist in the sintering of silver and connecting the large-area substrate and the heat dissipation device.

[0050] (4) such as Figure 1 As shown in the spraying section, a heat dissipation device measuring 25.4mm*50.8mm*1.0mm is placed in the mold base. A liquid dispersant (hydroxyl and lactam dispersants such as α-terpineol, polyethylene glycol, and polyvinylpyrrolidone) is evenly sprayed onto the heat dissipation device using a spraying device, with a uniform dosage of 0.5mg / cm³. 2 The amount of liquid dispersant is precisely controlled using an electronic balance with an accuracy of 0.1 mg.

[0051] These dispersants can directionally control the rheology of solder paste, prevent particle agglomeration, promote particle rearrangement, create a protective gas sintering atmosphere, thereby promoting sintering and significantly improving sintering quality.

[0052] (5) After spraying the liquid dispersant, connect the heat dissipation device to the first substrate with solder paste, such as... Figure 1 The sintering section is shown in the diagram.

[0053] (6) Place the assembled structure into the hot press and press it according to the following... Figure 1 The sintering curve shown is obtained by heating from room temperature (15-25℃) to 275℃ at a rate of 5℃ / min under a pressure of 2.0MPa, sintering under pressure for 50min and then cooling to room temperature.

[0054] This invention also provides a sintered substrate, such as... Figure 7 As shown, the sintered substrate includes a first substrate 1, a nano-silver solder paste 2 formed on the first substrate 1, a liquid dispersant layer 3 formed on a second substrate, and a second substrate 4 sintered with the nano-silver solder paste 2.

[0055] The nano-silver solder paste has a thickness of 80-120 μm, and the total mass fraction of nano-silver particles or micron-sized silver flakes in the nano-silver solder paste is 85.3%. The liquid dispersant includes at least one of α-terpineol, polyethylene glycol, and polyvinylpyrrolidone. The first substrate and the second substrate are heated from 15-25°C to 275°C at a heating rate of 5°C / min, held at 275°C and 2.0 MPa for 50 min, and then cooled to room temperature in the furnace for sintering.

[0056] The mechanism of liquid dispersant-assisted large-area low-pressure sintering in this embodiment of the invention is as follows:

[0057] The mechanism by which the three dispersants prevent aggregation is to increase steric hindrance. Due to their different dispersion and flow behaviors, they improve the bonding quality to varying degrees. Taking α-terpineol as an example, firstly, the wet paste at the interface is easily deformed under low pressure. The liquid dispersant accelerates the rearrangement of silver particles / flakes and generates rearrangement torque, making the planar contact as... Figure 2 As shown in section a, the capillary force applied by the liquid dispersant accelerates the diffusion of silver molecules from the slurry to the top of the matrix, increasing the flow deformation and contact deformation capacity of silver particles at the interface. Figure 2 As shown in section b, the interfacial bonding is improved. α-terpineol adsorbs onto the surface of silver particles, preventing particle aggregation and significantly increasing the stability of the nano-silver by creating a solid-liquid environment. Figure 2 As shown in section c. The evaporation and combustion of α-terpineol produce an organic protective gas atmosphere, preventing low-temperature non-dense sintering, such as... Figure 2As shown in section d. The use of a liquid dispersant to assist sintering is crucial for forming a uniform, dense, and highly bonded sintered silver layer. Compared to polyethylene glycol and polyvinylpyrrolidone, spraying α-terpineol at a dosage of 0.5 mg / cm³ is more effective. 2 The optimal sintering effect is achieved at the specified time. The hydroxyl groups adsorb onto the surface of the silver nanoparticles, resulting in an electrically neutral outer layer that prevents particle agglomeration. With a viscosity of 6.4 MPa·s, it promotes contact and deformation between the solder paste and the substrate. Its boiling point of 215-217℃ provides a protective gas atmosphere during rapid evaporation above the boiling point, preventing low-temperature non-densification sintering.

[0058] The effect of liquid dispersant-assisted large-area low-pressure sintering in this embodiment of the invention is characterized as follows:

[0059] This section characterizes the sintering effect of the novel sintering process mentioned in the embodiments of the present invention by examining the macroscopic structure, microstructure, and bond strength. Taking an interconnect structure with a solder paste thickness of 100 μm and a bonding area of ​​25.4 mm * 50.8 mm as an example, firstly, the bonded sample using three liquid dispersants (α-terpineol, polyethylene glycol, and polyvinylpyrrolidone) was subjected to non-destructive testing using ultrasound. Then, the sintered structure (25.4 mm * 50.8 mm) was cut into two equal parts along its length. One part was mounted, sanded, and polished, and the microstructure of the cross-section was observed using an electron scanning microscope. The other part was further cut into 16 equal parts (4.8 * 5 mm) for measuring its spatial shear strength distribution. Figure 3 The image shown is an ultrasonic scanning image of the sintered structure of the sintered sample after non-destructive testing. Figure 4 The diagram shows the spatial distribution of the shear strength of the sample. It can be seen that using α-terpineol as the dispersant resulted in no cracking or delamination, and the shear strength of the sample ranged from 35.1 MPa to 46.8 MPa, with an average shear strength as high as 41.4 MPa. Compared to α-terpineol, the dispersant polyethylene glycol showed a smaller delamination area and a higher average shear strength of 49.8 MPa, but its shear strength varied significantly between 20 and 70 MPa. When polyvinylpyrrolidone was used for sintering, large-area delamination occurred, with an average shear strength of only 36.8 MPa. Figure 5 The figure shows the results of using different amounts of α-terpineol (a=0 mg / cm). 2 b = 0.5 mg / cm 2 c=1.0mg / cm 2 The microstructure of the cross-section of the sintered sample, when the amount of α-terpineol is 0 mg / cm³ 2 and 1.0 mg / cm 2 At that time, the interfacial bonding between the sintered silver film and the coated silver film was weak, with large voids. In contrast, when the α-terpineol dosage was 0.5 mg / cm³, the bonding was much stronger. 2During the sintering process, a strong interfacial bond was formed between the sintered silver film and the silver-plated film, resulting in optimal interfacial bonding quality. The reliability of interconnects with and without α-terpineol was evaluated, such as... Figure 6 The temperature cycling ultrasonic scanning imaging and peeling degree data of the sintered structure are shown in the figure. (a)-(e) do not have α-terpineol; (f)-(j) have terpineol; (k) is the relationship between peeling area and number of cycles. It can be seen that using α-terpineol to assist sintering can improve the long-term reliability of the sintered interconnect layer.

[0060] Optimizing the large-area silver sintering process by changing the pressing temperature, pressure and dispersant type shows that: (1) the bonding quality of the interconnected silver layer decreases with increasing pressing temperature; (2) the density and average shear strength of the interconnected silver layer increase with increasing pressure; (3) under the same other sintering conditions, all three liquid dispersants (α-terpineol, polyethylene glycol and polyvinylpyrrolidone) can improve the sintering quality, but the bonding quality is the best when α-terpineol is used as the dispersant.

[0061] Therefore, using the liquid-assisted sintering process of this invention, with α-terpineol as a dispersant for large-area sintering, the suitable dosage is 0.5 mg / cm³. 2 The sintering process was optimized. With the assistance of a liquid dispersant, a large area (>1000 mm²) between the power module substrate and the heat dissipation device can be achieved at a lower pressure (<5 MPa). 2 The sintered interconnect silver layer has no cracks or delamination on a macroscopic scale, and is uniform and dense on a microscopic scale, with good interface connection quality and high average bonding strength.

[0062] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the scope of the technology disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. All should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A liquid dispersant-assisted large-area low-pressure sintering silver interconnect process, characterized in that, Includes the following steps: The thickness is controlled by stencil printing. A squeegee is used to uniformly pre-coat an 80-120μm thick nano silver solder paste onto the substrate surface, and then it is dried at 120℃ for 30 minutes. A fixed amount of liquid dispersant is uniformly formed on another substrate; Connect another substrate to the substrate coated with nano silver solder paste; The two mated substrates were placed on a preheated hot press and heated from T1°C to 275°C at a heating rate of 5°C / min. After holding at 275°C and P1MPa for 50 minutes, the substrates were cooled to room temperature to obtain the sintered bonded sample.

2. The liquid-assisted large-area low-pressure sintering silver interconnect process according to claim 1, characterized in that, The total mass fraction of nano-silver particles or micron-sized silver flakes in the nano-silver solder paste is 85.3%.

3. The liquid-assisted large-area low-pressure sintering silver interconnect process according to claim 1, characterized in that, The substrate is a copper substrate with a purity of 99.9%, electroplated with a 4μm silver layer and a 0.4μm nickel layer.

4. The liquid-assisted large-area low-pressure sintering silver interconnect process according to claim 1, characterized in that, The liquid dispersant includes at least one of α-terpineol, polyethylene glycol, and polyvinylpyrrolidone.

5. The liquid-assisted large-area low-pressure sintering silver interconnect process according to claim 1, characterized in that, The step of uniformly forming a measured amount of liquid dispersant on a dried solder paste layer includes: A quantitative liquid dispersant is uniformly formed on another substrate using a spraying or printing process, and then bonded to a substrate containing dry solder paste.

6. The liquid-assisted large-area low-pressure sintering silver interconnect process according to claim 4, characterized in that, The liquid dispersant is α-terpineol, and the amount of α-terpineol used is 0.5 mg / cm³. 2 .

7. The liquid-assisted large-area low-pressure sintering silver interconnect process according to claim 1, characterized in that, T1 is 15~25, P1 is 2.

0.

8. A sintered substrate formed using the interconnect process of claim 1, characterized in that, include: First substrate; Nano silver solder paste formed on the first substrate, the thickness of the nano silver solder paste is 80-120μm; A liquid dispersant layer formed on the second substrate; A second substrate sintered with nano-silver solder paste; The first substrate and the second substrate are heated from 15~25°C to 275°C at a heating rate of 5°C / min, and then held at 275°C and 2.0MPa for 50 min before being cooled to room temperature and sintered.

9. The sintered substrate according to claim 8, characterized in that, The total mass fraction of nano-silver particles or micron-sized silver flakes in the nano-silver solder paste is 85.3%.

10. The sintered substrate according to claim 8, characterized in that, The liquid dispersant includes at least one of α-terpineol, polyethylene glycol, and polyvinylpyrrolidone.

Images